Cutting-edge development in waste-recycled nanomaterials for energy storage and conversion applications

2.1.1 AC

AC is a nongraphitic material and is one of the most famous carbonaceous materials because of its high specific surface area, distinctive pore size distribution, high degree of surface-active sites, and physicochemical stability [21,22,23]. Three recycled source materials that produce AC are biomass, industrial waste, and agro-industrial waste. Numerous biomass wastes have been explored for AC production. The greatest advantage of biomass waste is creating AC via various plant types, cellulose, hemicellulose, and lignin constituents. Hemicellulose in biomass partially hydrolyzes at low temperatures, resulting in the formation of AC out of polymerization [21].

Because of the ban on environmentally harmful palm waste disposal mechanisms, especially in the palm milling industry, challenges of palm waste should be managed by converting it into valuable materials, like AC [23,24,25]. Porous AC (PAC), associated with mesoporous and microporous properties, is also strongly dependent on the utilization of the biomass type and synthesis methodology [26,27,28]. The conversion of fruit peel wastes into PAC is interesting to activate alkalis like H₃PO₄ [29]. Interestingly, PAC nanosheets are derived from leaves, such as Syzygium oleana leaves, via pretreatment with KOH or NaOH before the pyrolysis process [30,31]. For the development of 3D biomass PAC, three standard biomass waste precursors have been chosen as carbon sources: bagasse, wheat straw, and wood shavings [32]. Mangrove and waste palm trunk (WPT) have been used as low-cost and sustainable precursor materials for the development of AC, which can be used in various applications under different carbonization and activation conditions. The surface area of AC derived from WPT was 1,402 m²g⁻¹, whereas that derived from mangrove was 2,131 m²g⁻¹ at the same activation conditions [33].

Coal as waste material has been used for the synthesis of AC. Recent research investigated the possibility of producing alternative and cheap triggering agents to improve the adsorption properties of AC using mine coal [34]. Rubber tire wastes are non-biodegradable and harm climate and land management. It was suggested that this material could be used in a management strategy for diverse applications, like energy and carbon production [35]. In one study, plastic waste (PW) was separated from the dry waste of an institute, which primarily consisted of packaging and laboratory waste. Polypropylene (PP), polyethylene terephthalate (PET), high-density polyethylene (HDPE), and low-density polyethylene (LDPE) are among the thermoplastic polymers used in PW [36]. End-of-life flexible printed circuit boards (PCBs) typically have a polyamide (PA)-based substrate with metal foils, such as copper, interconnecting the circuits.

Several resources are available for this process, including sawdust, wood–plastic, and leather solid wastes (LSWs). Sawdust is ideal for fabricating carbonaceous NMs because the carbon content from sawdust varies from 77.51 to 93.59%, with ash content as low as 0.08%. Agro-industrial wastes are also used as alternatives for the production of AC. Extrusion, hot pressing, and injection are used to create wood–plastic composite materials [37] from plastics like PP, PE, and polystyrene (PS), and biomass fibers like rice husk (RH), corn stalk, and peanut shell. By increasing the carbonization temperature, the surface area of biochar is increased, reaching a maximum value of 518.72 m²g⁻¹ [38].

Nagaraju et al. [39] successfully used naturally available pine cone flowers, which have abundant carbon contents, as biomass to prepare honeycomb-like porous AC powder with meso/macropore structure and porous 3D properties. Nagaraju used chemical activation with KOH and pyrolysis processes under Ar inert gas atmosphere. The synthesized sample was uniformly coated on fluorine-doped tin oxide (FTO) glass using a smooth brush. Compared with commercially available AC, the obtained results were improved (Figure 8).

Figure 8

Schematic of (a–d) porous AC fabrication and (e–f) dye-sensitized solar cell preparation process with porous AC-coated FTO glass. Copied with permission from ref. [39]; Elsevier, 2017.

2.1.2 CNFs

CNFs resemble CNTs in structure but have more advantageous properties, such as decreased cost and ease of synthesis. The properties of CNFs make them the principal substituting blocks for the synthesis of CNFs at the nanoscale [40,41]. CNFs structures appear hollow, porous, smooth, helical, stacked with a high specific area, with good electrical and thermal conductivity with low weight, and are primarily used in energy and environmental sciences [42].

CNFs biomass waste is considered one of the most valuable CNM sources. It can be extracted from biomass waste sources, like bamboo, palm kernel shell, and pine nut shell-derived char. The process for determining extracted species is pyrolysis, wherein the process temperature is important to release different constituents of the CNMs [43,44].

CNFs are generated from pyrolytic bio-oil and biogas, depending on the heat extraction method [45,46]. The bio-oil microwave method’s production of phenols, methoxy phenols, substituted methoxyphenols, naphthalene, benzene, and alkenes leads to the fabrication of CNFs. Additionally, the biogas microwave-assisted pyrolysis (MAP) process, i.e., CO and CO₂, leads to hollow CNFs, like those extracted from pine nut shells and palm kernel shells, at low temperatures via isothermal oxidation [46].

The extraction of cellulose nanofibrils is usually applied for various biomass, such as soft and hard woods, corn husk, and banana peel [47,48]. Waste-tire pyrolysis oil (TPO) and coal fly ash (CFA) are used to synthesize CNFs/CNTs [49]. TPO acts as a carbon precursor and is one of the most important byproducts of waste-tire thermal degradation in an oxygen-free environment [49]. The presence of iron (in the form of oxide) in the CFA is a catalyst that aids in growth [50].

Nitrogen-doped CNFs with open channels can be prepared using a simple electrospinning method with subsequent two-step carbonization using polyacrylonitrile, waste poly(vinyl butyral), and urea [51]. Also, using waste HDPE, self-prepared micron-sized silicon/CNFs/carbon (Si/CNFs/C) composite has been fabricated with pyrolyzed carbon and CNFs [52].

Coal liquefaction residue waste has been used to prepare CNFs films via electrospinning [53]. Also, coal-derived CNFs can be obtained from powder River Basin coal via electrospinning; the raw coal is depolymerized to form coal chars that are then combined with polymeric precursors (polyacrylonitrile [PAN]/polyvinylpyrrolidone [PVP]) to form a homogeneous solution for fabrication [54,55,56].

Zhang et al. [57] synthesized CNFs decorated with a carbonized loofah from loofah sheets using in situ chemical vapor deposition (CVD). The CNFs were used for water purification via solar energy due to the high lighttrapping capability of the CNFs. The loofah sheets were first carbonized at 500°C, washed with alcohol, dried, and then soaked in a dopamine-based solution. The polydopamine-coated carbonized loofah was further carbonized at 800°C and then impregnated with Ni(NO₃)₂ as a catalyst. Lastly, the sample was placed in a CVD furnace with a gas flow of N₂ and ethanol. The CNFs showed a degree of crystallization with a lattice spacing of 0.34 nm and a diameter of 20 nm. X-ray diffraction (XRD) analysis confirmed the crystal structure of the carbonized loofah CNFs.

Cao et al. [58] used alkali lignin as a renewable carbon source, mixing it with PAN in a solution as a carbon precursor for CNFs preparation. It was blended with Sn-based PVP solution and then electro-spun at 23 kV at fixed temperature and humidity. Afterward, the spun nanofibers were preheated before being carbonized at 800°C. The result was porous CNFs with SnO _x nanonodules distributed homogenously throughout the composite. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the composite morphology (Figure 9), whereas XRD and Brunauer–Emmett–Teller (BET) surface area analyses were utilized to study the crystal and pore structure of the composite, respectively. Additionally, two- and three-symmetric electrode systems were built to test the electrochemical performance of the composite for supercapacitor applications.

Figure 9

(a) SEM and (b) TEM images of CNFs. Copied with permission from ref. [58]; Elsevier, 2021.

2.1.3 CNTs

CNTs have attracted attention because of their superior electrical, thermal, mechanical, and optical properties. Synthesis of CNTs from renewable resources will reduce costs and make the world more sustainable [59]. CNT feedstocks include biopolymers or bioderived chemical compounds, such as essential oils. CVD has completed the fabrication of CNTs from essential oils in the presence of ferrocene as a catalyst. Many precursors act as bio-feedstocks to produce CNTs. Palm oil is not a good carbon precursor for the production of CNTs [60]. However, poor quality CNTs are obtained from sunflower, palm, and sesame oils (I _D/I _G ratio of 1.00), which indicates that these precursors are not ideal for CNT synthesis.

There are other essential oils available to produce high-quality CNTs. The production of multiwalled carbon nanotubes (MWCNTs) by turpentine oil was discovered by Afre et al. [61]. High-quality MWCNTs were fabricated on quartz rather than silicon via CVD [62]. MWCNTs can easily be created using camphor oil in the presence of ferrocene [63]. According to Awasthi and coworkers, MWCNTs with 20 and 60 nm diameters were fabricated with castor oil [64]. Also, bamboo-shaped N-doped MWCNTs were produced by castor oil in the presence of ammonia [65].

Cobalt catalyzed carbonization of biomass chitosan results in the formation of cobalt/nitrogen-doped CNTs. The presence of cobalt causes a transition from graphene-like carbon nanosheets to tubular graphitic carbon [66]. Biochar is a carbon-rich porous material made from thermochemically treated biomass. Microwave-assisted heating makes CNTs from a mixture of biochar and ferrocene [67]. The coconut shell is an example of a mineral-rich biomass natural resource. By using the mineral content in the source material as catalysts for CNT growth, MWCNTs have been synthesized over coconut shell-derived charcoal pyrolyzed at 900°C [68].

Plastic wastes are considered as a main source of CNMs because of their high amounts of carbon. The most common waste plastics are PE and PP. Plastics are first decomposed in a pyrolysis reactor, and then CNTs are synthesized in a separate reactor using a catalyst. Catalyst growth is a major challenge for increasing CNT amount and quality [69].

High quality and quantity of CNTs can be obtained using 5 wt% of Mn in Fe-based catalyst via catalytic pyrolysis of PP rather than a Fe-based catalyst [69]. Therefore, selecting catalysts, especially bimetallic catalysts, is important for the fabrication of high-quality CNTs. MWCNTs have been produced by single-stage chemical vapor decomposition using PP in the presence of bimetallic Fe–Mn/Al₂O₃ catalyst [70].

CNTs can be manufactured from PE wastes using heavy metal-support interaction, e.g., La_0.8Ni _x Fe_1−x O_3−δ , at high temperatures [71]. Also, CNTs can be manufactured from PE waste at 800°C using a spherical alumina-supported catalyst, the diameter and yield increasing significantly as the Ni content increases [72].

A mixture of PP and LDPE was also used as the main source of CNTs using a two-stage fluidized catalytic bed reactor system with a fraction of 48 wt%. In one study, temperatures of the first- and second-stage reactors were regulated at 600 and 800°C, respectively [73]. MWCNTs can be obtained by using a pure mixture of LDPE, PP, and mixed plastics (MPs) over a Ni-based catalyst at two different temperatures, 500 and 800°C [74].

Coproduction of H₂ with high yield and CNTs with high quality can be prepared from the pyrolytic product of waste tires using Ni supported on AC as a catalyst [75]. Single-walled carbon nanotubes (SWCNTs) with very efficient yield have been synthesized using vulcanized scrap rubber via thermal CVD at 850°C on suitable catalytic systems, such as bimetallic oxides of Fe and Ni supported on zeolites [76]. Also, well-defined CNTs can be obtained from scrap rubber by thermal aging the rubber at 90°C for 14 days before CVD using Fe–Ni–Cu/MgO as a catalyst and a growth temperature of 750°C applied for 60 min [77]. The stress buffer layer shell that occurs on the surface of micron-sized silicon waste is a CNT. A high-performance Si/nano-Cu/CNT/C porous structure was obtained from photovoltaic silicon waste by integrating nanocopper-assisted chemical etching with graphite and CNT coating technology [78]. CNTs with a high degree of graphitization can be obtained using a red mud sample by alkali treatment followed by fabrication of bio-composite films of PVA and, finally, the pyrolysis of the composite film at 500°C [79]. The floating catalyst CVD approach offers a continuous single-step process to synthesize aerogel CNTs using waste engine oil as a carbon source in a ferrocene catalyst at a temperature of 1,150°C [80].

Wu et al. [81] have shown that the same process from which hydrogen is produced from the plastic can be utilized to create cost-effective CNTs as a byproduct in NiMnAl-based catalysts. The examination of the resulting mixture showed that the carbon yield depends on the molar ratios of the NiMnAl catalyst used. NiMnAl with a molar ratio of 4:4:4 (Ni:Mn:Al) had a higher carbon yield (57.7 wt%) and a slightly higher CNT yield (91.2 wt%). However, CNTs formed using NiMnAl with a molar ratio of 4:2:4 had better uniformity and higher crystallinity overall. CNTs produced this way were shown to improve the mechanical strength of LPDE significantly. The CNT–LPDE composite had more stiffness than virgin LPDE and improved dynamic load handling abilities. The overall process is also cost-effective, which opens the door to potentially wide industrial applications.

Biomass-based methods for CNT formation have also undergone considerable developments over the last few years. Hidalgo et al. used biochar as a precursor for synthesizing CNTs by microwave irradiation [82]. Their method involves heating a mixture of biochar and ferrocene in a microwave reactor operating at 2.45 GHz, 80°C, 200 W, and 17 psi. Different kinds of biomass from different agricultural wastes were pyrolyzed to form different biochar samples, then used as carbon precursors in the synthesis process (Figure 10). The different biochar samples had diverse surface areas, pore volumes, aromaticity, etc., depending on the agricultural waste used and the pyrolysis conditions, which affected the characteristics of the CNTs produced. Biochar samples obtained from hazelnut hull and wheat straw pyrolyzed at 600°C yielded CNTs with overall higher quality. Biomass gasification also generates a carbon-rich mixture of gases, such as CO, CO₂, and CH₄ [83].

Figure 10

SEM images of carbon products from various sources. Copied with permission from ref. [84]; Elsevier, 2018. (a) CNTs from CH₄ decomposition process (P _CH4 = 0.2 atm), (b) CNTs from CO decomposition process (P _CO = 0.2 atm), (c) CNTs from CO/CH₄ decomposition process (P _CO/P _CH4 = 0.5), and (d) CNTs from CO/CH₄ decomposition process (P _CO/P _CH4 = 2).

2.1.4 Graphene

Graphene and its derivatives can be synthesized from unused waste material to produce novel materials. There are four main categories of recycled waste material sources to synthesize graphene and its derivatives: biomass, plastic, industrial, and other domestic wastes. In the following sections, each source will be discussed individually.

Graphene and its derivatives can be synthesized using different biomass wastes, such as RH, hemp fiber, sugarcane bagasse, glucose, and chitosan. The merits of using recycled agriculture wastes are their simplicity, environmental friendliness, high crystallinity, and the lack of toxic gas emissions during synthesis. RH contains more than 70% of carbonaceous components besides its silica (15–20%), lignin (25–30%), and cellulose (50%) constituents [85]. For high-purity graphene materials, a pretreatment process with a strong alkali, like KOH or NaOH, is required to eliminate SiO₂ impurities [86].

Farmers burn hemp fibers every year; therefore, it is important to decrease this biowaste by benefiting from the availability and eco-friendliness of hemp fibers to produce distinctive materials, like graphene and its derivatives [87]. Hemp has been hydrothermally treated and accompanied by KOH activation to create extremely porous graphene materials [87].

Glucose is considered the only renewable monosaccharide plant-based source of carbon in nature produced simultaneously in plants’ photosynthesis process. Researchers used glucose as a raw material for the processing of graphene and its derivatives because of its uniqueness in nature [13,88].

Sugarcane bagasse is one of the main sources of glucose for the production of graphene and its derivatives [89]. The most common fabrication technique for graphene and its derivatives from sugarcane bagasse is the direct oxidation under a muffled atmosphere with ferrocene as a catalyst. Using a pyrolysis technique combined with a simple treatment, different forms of graphene, such as N-doped graphene, have been manufactured from chitosan [90].

Plastic bags and plastic water bottles (PET) are massively used today. All packaging materials formed of different polymeric composites are collected together as layers to form plastic materials with an inner layer of aluminum [91]. PE has been recycled to prepare promising graphene/mesoporous carbon composites for energy storage devices through the monitoring of graphene oxide (GO) during a lower temperature carbonization process [92]. High-quality graphene flakes have been prepared using PP waste while organically catalyzed with montmorillonite [93]. High-quality single-layered graphene can be made from PS and other carbon-containing wastes. Additionally, multilayer graphene sheets were prepared from waste expanded PS using FeCl₃ as a catalyst [94].

High-quality monolayer and multilayer graphene was successfully fabricated using synthetic polymers, such as polymethylmethacrylate (PMMA), polyimide, and waste plastics, as the carbon precursor, whereas hydrogen gas acts as a reducing agent [94]. PMMA film was also used as a starting medium for CVD-grown graphene sheets on sapphire substrates with Cu catalysts [95].

High-quality graphene from PET waste was developed without any catalyst [96]. The treatment of PET wastes involves two main steps before forming carbon-based materials: grinding and crushing. Pyrolysis-generated mixed PWs are a major source of bulk graphene nanosheets [97]. In one study, PE, PP, and PS combined PWs were obtained from a local city council and second-hand markets, cleaned, and dried. The properties of the produced GO sheets varied from the starting raw materials such that the characteristic properties of the GO sheets were nearly identical to those of highly pure graphene [98,99].

Graphene films were developed from pure coal through a graphitization process that has been identified as simple and cost-effective. Since coal is a carbon-rich material for developing CNTs, fullerenes, and amorphous carbon thin films, it is also used to construct graphene, graphene derivatives, and anthracite coal [100,101]. Graphene materials have been recycled from lead and zinc–manganese batteries, besides some types of transportable batteries. This recycling method was carried out in a heating system where NPs of various morphologies were formed, such as hexagonal prisms, fibers, and sheets [16,102].

A significant number of tires, approximately one billion, are produced globally per year. Studies investigating the control of rubber tire disposal as an economically friendly sustainable source for carbonaceous materials composed of 81.2 wt% carbon [103] are in great demand. The high-value carbon graphene NPs resulting from these studies were derived from waste-tire rubber and exhibited good thermal stability and conductivity [104].

Electronic gadgets (phones, laptops, iPads, etc.) have become essential to everyday life. These electronic equipment comprise diverse mixtures of metals, plastics, glass, and even nanomaterials [105]. PCBs are considered the main constituents of such electronics, and they have been considered a good source for graphene synthesis. The recycling process of valuable and high-quality graphene NMs from high pollutant wastes offers a more energy-saving manufacturing process than extraction from ores [106].

Graphene has been prepared using different low-cost carbon-containing materials from daily use, such as cookies, chocolate, leftover bread, salads, grass, cockroaches, and even animal feces [107]. Although different elements, such as O, N, Fe, S, and P, were present in the primary material sources, the graphene was free of those elements. Thus, the synthesis of graphene from such sources is preferred [108].

Several natural waste materials for the synthesis of graphene and GO, such as tea leaves; coconut shells; peanut shells; fruit peels, i.e., orange, banana, and mango peels [109,110,111]; wood; bagasse; leaves; vegetation waste; fruit waste; and powder soot collected from the exhaust of diesel engine and waste newspapers have been used in graphene sheet synthesis [98]. Tea waste contains various components, such as polyphenol, caffeine, amino acids, and tannins, whereas tea leaf constituents include cellulose, hemicelluloses, lignin, tannins, and proteins [112]. Graphitic carbon can be produced because of the presence of these cellulosic materials by treatment at high pyrolysis temperatures [109].

Nut shell waste is globally used in animal feed or construction materials because of its novelty, low cost, and existence as a useful biological resource. Nut shells are of high carbon content and first produce biochar that can be easily converted into graphene [113]. Plenty of application fields encourage recycling such a low-cost graphene resource as it is used as a bio-adsorber or in metal ion removal and wastewater treatment [114].

Fungus can be treated hydrothermally to produce porous graphene materials; for example, the use of a fungus (Auricularia) with a KOH pretreatment and then a carbonization process. The resultant porous graphene material net structure is used in supercapacitor electrode production [115]. The pulping industry is a rich source of alkaline lignin, which has been used to successfully prepare graphene sheets thousands of nanometers in size [116].

Ding et al. [116] prepare 3D graphene from the precursor of black liquor (Figure 11). The black liquor was collected and filtrated to remove impurities. A 15% weight percentage of black liquor was dissolved in deionized water and heated at 180°C in an autoclave. Then, it was cooled at room temperature and washed with deionized water. Finally, graphene was obtained by centrifugation of suspension.

Figure 11

Three-dimensional graphene schematic. Copied with permission from ref. [116]; Elsevier, 2020.

Three-dimensional graphene comprises tightly combined micron-sized graphene sheets, and the graphene sheets have dihedral angles. Both sp² and sp³ carbon atoms, with a ratio of 8:1, are present. Three-dimensional graphene with porous carbon, which consists of multilayers of graphene, is of considerable interest in many applications because of its distinctive properties, such as self-supporting, high surface area, high mechanical properties, high electrical conductivity, and a high degree of crystallization [117].

Researchers have also used agricultural waste as biowaste precursors to synthesize graphene to reduce toxic chemicals. Agriculture produces millions of tons of RH per year. Therefore, it is of great interest for the large-scale production of graphene. Kumar et al. [118] have synthesized graphene using RH as the precursor via the microwave method with ferrocene as a catalyst. The RH was washed and dried and, after drying, was mechanically ground to powder and mixed with ethanol using ferrocene as a catalyst [119]. Yeleuov et al. [120] synthesized graphene with RH as a precursor using a two-step chemical method. The graphene was then modified with nickel hydroxide through a simple chemical precipitation method. The properties of graphene provided it with high performance in energy storage applications.

2.1.5 Inorganic quantum dots

2.1.5.1 CQDs

CQDs are a fascinating type of carbon NPs with diameters of approximately 10 nm. From a synthesis point of view, CQDs have been produced from several natural carbon sources or waste organic products; the benefits of using green carbon sources are their cost-effectiveness, eco-friendliness, and wide availability in nature [121,122]. Figure 12 shows the types of waste used to produce CQDs, which will be discussed in detail.

Figure 12

Wastes used as precursors for synthesizing CQDs.

The raw materials needed to produce CQDs from food waste are abundant, such as cucumber peel, pineapple peel, sugarcane bagasse, garlic peel, and taro peel. The type of food waste utilized in the synthesis of CQDs can affect its quality. For example, there were significant variations in essential properties of CQDs obtained from cucumber and pineapple peels using the same synthesis approach [123]. The CQDs obtained from pineapple peel were fully degraded after a few weeks of storage, whereas those from cucumber peel remained stable. Additionally, it was found that CQDs prepared from sugarcane bagasse, garlic peel, and taro peel using the same synthesis process (the ultrasonic-assisted wet-chemical oxidation method) had very different quantum yields (QYs) (4.5, 13.8, and 26.2%, respectively) [124].

From the waste management perspective, researchers have synthesized CQDs from industrial waste to dispose of the waste. There has been a significant advancement in plastic technology in several fields, including electronics, packaging, etc. Because of the negative health and environmental impacts of the inadequate management of PW and the large amounts of waste from plastic industries, it is urgent to convert this waste into CQDs [125,126]. These conversions are obtained from the pyrolysis of wastes of polyolefins. Plastic bottles, cups, and PE bags have been used for CQDs by simple hydrothermal carbonization and have notable QY values of 64, 65, and 62%, respectively, with sizes ranging from 5 to 30 nm [126].

Whey is the liquid remaining after curds have been removed during the cheese-making process [127]. The discharge of watery portions after separating fat and caseins from whole milk creates serious environmental problems. Scientists utilized whey waste, a major dairy, and cheese industry waste product, to resolve these problems into CQDs. Using a facile and environmentally friendly synthetic method and underoptimized synthesis parameters, the CQDs prepared from whey waste exhibited a notable QY (11.4%) and excitation-dependent emission behavior [128].

The paper industrial sector creates large quantities of pulp residual fiber waste that needs further treatment before discharge. In this context, the transformation of such waste into CQD materials has piqued interest in several applications [129]. The formation of tiny CQDs with excellent physicochemical characteristics, such as a QY of 2.7%, particle size of 17.5 nm, and steady-state and lifetime fluorescence, has been achieved by facile microwave-assisted protocol [129].

Additionally, printed office paper has been successfully converted into fluorescent CQDs associated with small particle size, good photostability, high photoluminescence QY (10.8%), and low toxicity [130]. Also, N-doped CQDs from office paper have demonstrated perfect optoelectronic properties, faster response, and better sensitivity in the visible range than undoped CQDs [131]. Interestingly, CQDs fabricated from paper waste from a supermarket showed a significantly higher photoluminescence QY (5.1%) than CQDs prepared from the lignocellulosic residue, with a particle size of approximately 4.8 nm and a high response rate for trinitrotoluene [132].

In recent decades, dumped sugarcane bagasse is soil and earth waste that poses major environmental and health problems, especially in developing countries [133]. Therefore, researchers have utilized industrial sugarcane waste as a carbon precursor to produce highly fluorescent CQDs with an effective QY of 17.98% [134,135].

Discharged batteries, the main component of electronic waste, were used as a precursor for synthesizing uniform spherical CQDs with a particle size of 5 nm and QY of ∼15.3% [136]. White wine lees have also been used as raw material for producing CQDs, exhibiting a QY of 2.53% and a particle size of 10 nm [137].

Biomass is a biodegradable, bio-organic, and abundant element derived from various sources, including agricultural, fishery, animal, and forestry wastes. High-quality and green CQDs have been successfully synthesized using potential valuable byproducts of plant refineries, like hemicellulose and ammonium hydroxide, as solvents. The highest QY of N-doped CQDs produced with these byproducts was up to 16%, higher than undoped CQDs (only 2%) [138]. Watermelon peel is another precursor used to fabricate CQDs and has a particle size of 2.0 nm, strong blue luminescence, satisfactory fluorescence lifetime, and high stability across a wide pH range and at high salt concentrations [139]. Wheat bran has also been used as a precursor for preparing CQDs via hydrothermal treatment. According to fluorescence emission studies, wheat bran CQDs showed the highest fluorescence emission at a wavelength of 500 nm, and the QY of these prepared CQDs was 33.23% [140]. Rice residue and glycine have been used as carbon and nitrogen sources to synthesize N-CQDs using a one-step hydrothermal approach. These N-CQDs showed excellent results as a probe to detect Fe³⁺ and tetracycline (TC) antibiotics, with a maximum emission at a wavelength of 440 nm and a high QY percentage (23.48%) [141,142]. A single-step hydrothermal carbonization method has been used for the green synthesis of water-soluble monodisperse CQDs with coconut husks as the carbon precursor. Hydroxyl and carboxyl functionals, which play crucial roles in surface passivation and result in more stable NP dispersion, were confirmed by Fourier transform infrared (FTIR) spectroscopy on the surface of the CQDs [143]. Waste tea leaves and peanut shells were also carbon sources used to synthesize CQDs by the one-step hydrothermal method. These CQDs showed high stability and high QY. There was no discernible morphological distinction between tea CQDs and peanut shell CQDs; both had diameters of 7–9 nm, uniform dispersion, and spherical shape, indicating that they were more stable in water [144].

Recently, numerous attempts have been made to upcycle polymeric waste to reduce polymer waste buildup. Chaudhary et al. [145] have reported a sustainable process to convert PW from bottles, cups, and PE bags through facile heating to create FL-CQDs (Figure 13). The XRD of the CQDs showed crystalline characteristics. Also, the CQDs possessed crystallite sizes ranging from 7.5 to 28.3 nm.

Figure 13

Schematic displaying the production of FL-CQDs from various types of plastic waste. Copied with permission from ref. [145]; Elsevier, 2020.

Biowaste Aegle marmelos leaf powder synthesized CQDs via a hydrothermal carbonization process. The obtained CQDs exhibited a high photoluminescence QY of 22% and, in the presence of a wide range of ionic (KCl) concentrations, revealed their suitability for in vivo and in vitro biosensing applications [146]. Using a pulsed laser ablation method, n-doped microspore CQDs (NM-CQDs) were produced from waste Platanus biomass. The prepared NM-CQDs exhibited a high QY and fluorescence lifetime (32.4% and 6.56 ns, respectively). Furthermore, stable emission behaviors have been obtained for NM-CQDs under various conditions, including varying ionic salt concentrations, temperatures, irradiation times, pH values, and different excitation wavelengths. NM-CQDs are ideal for cellular staining images because of their strong and stable PL emission [147,148]. Additionally, the green synthesis of carbon NPs using a hydrothermal process from cow manure is becoming more prevalent. These CQDs have an approximately spherical morphology and a narrow distribution, with most particle sizes varying between 10 and 15 nm. Photocatalytic activity of the CQDs was observed by the degradation of methylene blue under visible light, which decreased up to 40% after 12 h [149].

Waste crab shells and three different transition metal ions, Gd³⁺, Mn²⁺, and Eu³⁺ were used in a one-pot MAP procedure to fabricate magnetofluorescent CQDs (MFCQDs). Waste crab shells served as carbon sources and ligands to form complexes with transition metal ions. Consequently, MFCQDs exhibit good stability at different ionic strengths and pH values, making them useful for biomedical applications in vivo. Additionally, intense fluorescence, perfect aqueous dispersibility, excellent magnetic resonance response using various transition metal ions, and a fluorescent QY of 19.84, 12.86, and 14.97% were observed for Gd-, Mn-, and Eu-CQDs, respectively [150,151]. Also, prawn shells have been used to synthesize fluorescent carbon dots. Results revealed that these CQDs (with an average diameter of 4 nm) have several desirable characteristics, including high monodispersity, good stability, the ability to remove blue fluorescence under UV light (365 nm), high QY (9%), and excellent water solubility [152].

With a green synthesis route, several synthesis methods have been developed to produce CQDs, including hydrothermal or solvothermal synthesis, microwave-assisted synthesis, laser ablation methods, and chemical oxidation methods. However, toxic chemical reagents or organic solvents are widely used as precursors in these traditional synthesis processes. Hydrothermal or solvothermal methods were used to prepare CQDs with citric acid and l-histidine as precursors; these synthesis techniques require large heat energy of 200°C for 5 h. The fluorescence QY of the obtained CQDs was approximately 22%, with a maximum emission wavelength of 414 nm [153]. Using these methods, CQDs obtained from rice residue and glycine has shown a high QY percentage (23.48%) [141]. Furthermore, highly fluorescent amphiphilic CQDs have been prepared via MAP using citric acid and 4,7,10-trioxa-1,13-tridecanediamine (TTDDA), which served as an A3 and B2 polyamidation-form monomer collection. TTDDA-based CQDs have exhibited a fluorescence QY of 29%, which is higher than CQDs prepared via waste crab shells [154]. Thus, the MAP technique is affected by uncontrollable reaction conditions and high energy demand.

Chemical oxidation, which is one of the most common synthesis methods, typically necessitates using a strong oxidizing agent (strong acid or alkali) during the synthesis process. This poses environmental concerns because it is difficult to eliminate excess oxidizing agents fully. Also, a serious drawback of this approach is the absence of homogeneity in the size distribution of the resulting particles [155].

2.1.5.2 GQDs

GQDs are zero-dimensional (0D) derivatives of graphene [156,157]. They have unique physical and biological properties that are interesting for energy applications. The renewable, available, and inexpensive routes toward GQDs from biomass and industrial wastes are discussed. Figure 14 shows some types of waste used as precursors for producing GQDs. Natural biomass represents cost-effective and renewable resources for the fabrication of GQDs. High-quality GQDs with an average size of 3.9 nm have been obtained using the byproducts of rice milling [158]. RHs undergo pyrolysis to produce RH carbon. Strong acids were used to activate the RH carbon under a hydrothermal treatment at 200°C for 10 h to produce GQDs. The product achieved a yield of 15% and showed intense photoluminescence and high biocompatibility, exhibiting high functionality for biomedical fields [159]. Spent tea represents an inexpensive, renewable, and green biomass waste for GQDs. As a carbon precursor, spent tea has been used to synthesize GQDs with high yields of over 84% via simple microwave treatment [157].

Figure 14

Wastes used as precursors for synthesizing GQDs.

The green synthesis of GQDs by microwave treatment has been confirmed using grape seeds as the carbon source. The GQDs are easily collected and organized in an aqueous medium without any external direction; therefore, they are called “self-assembled” GQDs or (sGQDs). These sGQDs exhibited a QY of 31.79% and a production yield of 53.6%, with sizes in the range of 50–60 nm, which is large compared with other GQDs, i.e., sizes typically 1–8 nm [160].

Plant leaves have been used for the green production of GQDs without oxidation, reduction, passivation agents, or organic solvents [159]. It has been confirmed that biomass pyrolysis treatments can produce a disordered, highly porous form of carbon on the nanometer scale. The thermal treatment of this disordered carbon at 90°C can lead to GQDs at a large scale, which is favorable for inexpensive mercantile production [161]. Neem leaves (Azadirachta indica) have been treated for the fabrication of GO sheets at 300°C for 2 h. After 8 h of treatment, these GO sheets were subsequently broken down into GQDs. The process used green and facile one-pot hydrothermal synthesis method with water as a solvent [162]. The as-produced GQDs were highly photostable and suitable for white light-emitting diodes (LEDs); they had an average size of 5 nm and a high QY of 41.2% [163]. Fenugreek leaves (Trigonella foenum-graecum), a type of green plant, can also be used to produce GQDs. Fenugreek extracts are highly carbonaceous with many hydrocarbons, making them an ideal precursor for GQDs synthesis [163].

Ugly food (any food considered imperfect to sell) is one type of food waste; rice grains are included in this group. According to the FAO of the United Nations, these waste grains contain a significant carbon footprint [164]. Rice grains have been used as a carbon source in a facile and green approach to synthesize monodispersed GQDs with sizes of 2–6.5 nm. The QY of the as-produced GQDs in water depends on their size; it increases from 16% to 24% with a decrease in GQDs’ size (from 6.5 to 2.0 nm) [165].

GQDs have been synthesized by the electrochemical scissoring of wood charcoal, a type of biomass. Electrochemical oxidation has been used to cut charcoal graphene sheets into very small particles called E-GQDs [159]. The size of the product was uniform and ∼5 nm, with optical and structural properties. Wood charcoal is a cheap and widely available source for synthesizing E-GQDs [166].

Miscanthus or silver grass is a genus of African, Eurasian, and Pacific Island plants in the grass family. It is a biorefinery waste comprising sugars and depolymerized lignin. A straightforward, effective, and general strategy has been developed for fabricating GQDs from Miscanthus (M-GQDs). As-produced M-GQDs show many advantages, such as few-layer graphene-like single crystalline structure, sulfur and nitrogen codoping, bright fluorescence, excitation-dependent photoluminescence, and long fluorescence lifetime (11.95 ns). Moreover, M-GQDs exhibit notable fluorescence reduction with good linearity (≤0.995) toward a trace amount of Fe³⁺. M-GQDs are built from well-dispersed NPs with a uniform size of 4.05 ± 0.61 nm [167].

Green production of highly fluorescent GQDs with a high yield of more than 80 wt% was obtained using coal tar pitch, a thick liquid byproduct from coke and coal gas production. The as-produced GQDs exhibit a tight size distribution of 1.7 ± 0.4 nm and high solubility in aqueous solutions [168]. Additionally, gram-scale GQDs with a yield of 75 wt% and high purity (99.96 wt%) were obtained using Vulcan XC-72 carbon black from battery and fuel cell manufacturing [169]. These GQDs have exhibited multicolor photoluminescence, from green to light red. Bituminous coal is one of the largest types of industrial waste, especially in power plants, and has been used to synthesize GQDs with a uniform size of 2.96 ± 0.96 nm [170]. Anthracite coal is another coal type used to synthesize GQDs, similar to bituminous coal. The average diameter of the GQDs produced from anthracite coal was 29 ± 11 nm [171,172]. GO is a typical starting material for nanosized GQDs when in the presence of other facilitating functional groups [173]. A new path of transforming industrial waste, particularly car bumper waste, into CNMs has also been developed. Reduced GO (rGO) was obtained as a catalyst for the upcycling of waste bumper via an economical thermal decomposition method [174]. GQDs have been synthesized using GO as a precursor through acidic oxidation. The as-produced GQDs exhibited photoluminescence features and the same properties as peroxide catalytic functionality, which can be used to detect H₂O₂ with a detection limit of 87 nm [175]. GQDs have been obtained from GO by hydrothermal reaction. GQDs with 40 nm uniform lateral dimensions, a yield of 32%, and a QY of 3.6% have been obtained by the hydrothermal cutting method. This simple method shows the notable advantages of a one-step reaction and short time consumption [176]. This method was also used to synthesize GQDs from RH, neem leaf, and fenugreek leaf waste. GQDs were produced with QYs of 41.2, 38.9, and 15% from neem leaves, fenugreek leaves, and RH, respectively. A facile synthesis method of high-quality GQDs from 3D graphene grown by chemical vapor decomposition has been used via a new, highly efficient, and green electrochemical strategy. The as-produced GQDs possess a uniform distribution in diameter (3 nm) and thickness and are mostly single-layered [177]. Also, GQDs have been synthesized using wood charcoal in the same technique. This product is called E-GQDs and is uniform in size (∼5 nm) but is not a big different from this precursor and 3D graphene.

Wang and his team [178] reported a green fractionation bottom-up (two-step) approach to convert lignin into glowing carbon nanocrystal GQDs, as shown in Figure 15. Alkali lignin is fractionated using an acid hydrotrope, which can be readily recovered for sustainability. The GQDs had a nanoscale few-layer structure, were water-soluble, and exhibited long-term photostability, bright fluorescence (ultrahigh UV transmission ≥ 305 nm), outstanding biocompatibility, and ultralow cytotoxicity. With these features, lignin-based GQDs were utilized as probes for detecting hydrogen peroxide in biological systems. The researchers succeeded in detecting low concentrations of hydrogen peroxide (reaching 0.13 nM). Further theoretical validation for GQDs’ ultrasensitivity was confirmed using density functional theory. Synthesis methods of different carbon-based nanomaterials derived from different biomass/waste sources are listed in Table 1.

Figure 15

Schematic for the green bottom-up synthesis of lignin-based GQDs via a two-step method. Copied with permission from ref. [178]; Royal Society of Chemistry, 2019.

Table 1

Synthesis of different carbon-based nanomaterials derived from different biomass/waste sources

Carbon nanomaterials	Biomass source	Synthesis method/technique	Reagent	Conditions	Result/yield morphology and structure	Ref.
Activated carbon	Tamarind fruit shell	Pyrolysis process/CVD technique	KOH/Ar gas	Muffle (800°C for 140 min) and Ar (5°C/min and 100 SCCM)	Activated carbon nanosheets	[179]
	Coconut shells	Concurrent activation and magnetization processes on the biochars of coconut shells	FeCl3.6H2O/N2 gas	Mixing (2 h), oven (120°C), muffle (700°C), and pH (∼6.5–7.0)	Magnetic activated carbon	[180]
	RHs and straw	Pulping processes (pyrolysis)/biological process	Sodium hydroxide/sodium sulfite/sodium sulfite–sodium carbonate blend/NaOH/ammonia thermodesorption/cellulose and peroxidase enzymes	Pulping processes: Na2O (140°C for 2 h), NaOH (60°C for 1 h)Biological process: activation (27°C for 14 days)	Microporous activated carbons	[181]
Carbon nanotubes	Palm kernel shell biomass	Fast microwave pyrolysis	N2 gas	Microwave (200°C), N2 (30 min), and microwave (1,200, 1,300, and 1,400°C for 30 min)	CNTs	[182]
	Waste polypropylene	Pyrolysis	Waste polypropylene/nickel nitrate	Thermal treatment (H2 at 10 SCCM and Ar at 90 SCCM; ramp temperature [30°C/min], and reaction temperature [600°C, 700°C, and 800°C]), and cooling (RT)	MWCNTs	[183]
	Plastic wastes (low-density polyethylene, high-density polyethylene, polypropylene, polyethylene terephthalate, and polystyrene)	Pyrolysis	Ni–Mo/Al2O3 catalyst/H2/N2 gases	Catalytic conditions (600°C for 1 h of H2 [50 SCCM] and N2 [30 SCCM], and then 650°C/N2)	MWCNTs	[184]
				CNTs (650°C then RT, N2 at 100 SCCM)
	Pomelo peel biomass	Microwave-assisted pyrolysis	Tantalum pentoxide, melamine, trithiocyanuric acid, potassium hydroxide, sulfuric acid, n-Hexane, and surfactant	Pyrolysis (N2 at 400 ml/min and 600°C for 30 min)	Ta-decorated and N, S-doped, CNT enriched mesoporous electrocatalyst	[185]
	Liquefied larch sawdust	In situ polymerization, foaming, and carbonization	Formaldehyde solution/sodium carbonate/sulfuric acid/tween 80/n-hexane	Carbonization (N2 at 800°C for 2 h)	MWCNTs/carbon foam nanocomposites	[186]
Carbon nanofiber	Loofah biomass	Calcination/CVD	Dopamine hydrochloride/Tris-Cl solution/N2/Ni (NO3)2 solution/ethanol	Carbonization (N2 at 500°C for 2 h), drying (80°C), soaking (1 day), and further treatment (N2 at 800°C for 2 h)	CNFs	[57]
	Rubber fruit shell	Hydrothermal process	H2SO4/H3PO4/NaOH	Rinsing (4 h), drying (60°C for 24 h), centrifugation (10,000 rpm for 30 min), stirring (300 rpm and 60°C for 30 h), and drying (90°C)	CNFs	[187]
	Sapindus trifoliatus nut shells	High-temperature carbonization followed by physical activation method	N2/Alumina boat/CO2	Carbonization (N2 at 700°C for 3 h), furnace under N2 flow (80 cm3 min−1) at a heating rate of 5°C min−1 up to 700°C, and then CO2 for 2 h	CNFs	[188]
	Pithecellobium Jiringa shell waste	Pyrolysis	KOH/ZnCl2/N2 gas/CO2 gas	Precarbonization (250°C), carbonization (N2 at 600°C), and activation (CO2 for 850°C)	Carbon nanofiber/nanosheet	[189]
	Bamboo waste materials	Pyrolysis	KOH/N2 gas/CO2 gas	Carbonization (N2 for 1 h at 600–900°C) and activation (CO2 at 900°C for 2.5 h).	Highly porous activated carbon nanofibers	[190]
Graphene	Pomelo peels	Ultralight microwave absorption	Hydrogen peroxide (H2O2) and acetic acid (HAc)	Autoclave (120°C for 3 h) and pyrolysis (Ar at 800°C for 3 h)	Graphene-like porous carbon nanosheets	[191]
	Sawdust (from Betula platyphylla)	Shear exfoliation and carbothermal redox process	FeCl3·6H2O and hydrochloric acid	Precarbonization (N2 at 450°C for 2 h), annealing (N2 for 1,000°C for 3 h at 10°C min−1), and centrifugation (3,000 rpm for 20 min)	Graphene sheets	[192]
	Orange peel wastes	Ball-milled/the carbonization/activation process	KOH/N2/HCl	Precarbonization (N2 at 400°C for 2 h), stirring (RT for 12 h), and drying (90°C overnight)	Honeycomb-like architecture with a 3D hierarchically ordered pore size distribution	[193]
	Biomass film (kraft lignin [KL] and cellulose nanofibers)	Ultrafast laser writing technique	CNFs/KL/NaOH	—	Three-dimensional interconnected porous graphene network with defect-rich boundaries	[194]
Quantum Dots	Spent black tea	Hydrothermal treatment	Sodium hydroxide (NaOH), nitric acid (HNO3 > 67%) and sulfuric acid (H2SO4 > 97%) were purchased from Fisher Scientific, UK. The metal salts include CoCl2, CaCl2, AlCl3, AgNO3, CrCl3, FeCl2, CuCl2, ZnCl2, SrCl2, FeCl3, PbCl2, NiCl2, MoCl2, LiCl, NaCl, MnCl2, and MgCl2	Autoclave (200°C for 12 h) and dialysis (0.1 µm PVDF for 1 day)	NPs/Nanospheres (NSs)/GQDs	[195]
	Corn stalk shell	Hydrothermal approach in near-critical water pyrolysis	—	Reaction (270°C and 5 MPa for 10 min), cooling (RT), and dialysis bag (1,000 Da for 24 h)	CQDs	[196]
	Lemon peel waste	Hydrothermal process	Waste lemon peels, titanium isopropoxide, methyl, 6-aminohexanoic acid, and sodium hypo chloride	Oven (100°C for 10 h), drying (100°C for 4 h), pH = ∼7, autoclave (200°C for 12 h), centrifugation (10,000 rpm for 30 min), and drying (100°C)	CQDs	[197]

2.2.1 Manganese oxide

More than 60 billion alkaline manganese-based batteries are manufactured annually. The development of technology to recover or isolate Mn from Co, Ni, and Li is critical as Mn concentration of 3 g L⁻¹ greatly reduces the selective separation of Co and Ni [203]. Since industrial LiB wastes typically contain both active materials and impurities, such as Al, Fe, and Cu, few major recycling technologies are available for recovering Mn [204]. Mn can be extracted in different forms, such as powder, metal, metal oxide, and NM, from electronic waste, battery waste, etc., [5,16,204,205]. Additionally, nanoscale MnO₂ can be extracted from the waste of portable batteries [206–211], whereas porous flower-like MnO₂-NiO has been recovered from spent Zn–Mn batteries [212], and MnCo₂O₄ and Li _x MnO _x+1 have been recovered from spent LiBs [213].

Interconnected spheroidal MnO _x NPs, in nanorods (NRs) and nanoflowers have been recovered using Urginea sanguinea [214]. A green synthesis method for MnO₂ NPs, with sizes of approximately 32 nm, has been achieved using Yucca gloriosa leaf extract and was stabilized using turmeric extract [215]. As a reducing and stabilizing agent, clove (i.e., Syzygium aromaticum extract) was used to prepare MnO NPs [216]. The successful biosynthesis of MnO₂ NPs from rhizophytic bacteria Paenibacillus polymyxa strain is achieved. Utilization of KMnO₄ as a precursor with an aqueous leaf extract of Kalopanax pictus for the production of MnO₂ NPs, with an average particle size of 19.2 nm and a particle diameter ranging from 1 to 60 nm, was also achieved [217]. MnO₂ NRs with an average size of 40–50 nm were successfully fabricated using a leaf extract of Phyllanthus amarus [218]. Mn₃O₄ NPs with an average crystallite size of 44 nm were recovered using manganese sulfate monohydrate as the precursor salt and leaf extract of Malabar nut (Adhatoda vasica Nees/Justicia adhatoda) as the reductant [218]. Moreover, Ananas comosus (L.) peel extract was used to fabricate Mn₃O₄ nanospheres with an average size of 10–34 nm [219]. Chen et al. also showed recovery in MnO₂/Fe(0) composites from Li-ion batteries via ferrous sulfate, lithium manganite, and the hydrothermal technique. XRD peaks confirmed the presence of the β-MnO₂ phase (Figure 17) [220]. Min et al. [221] have also reported the recovery of MnO₂ from spent LiBs, which can fundamentally be formed from three steps: (1) pretreatment, (2) leaching, and (3) catalyst preparation (Figure 18).

Figure 17

(a) TEM, (b) SEM, and (c–e) mapping images of MnO₂/Fe(0) composites. Copied with permission from ref. [220]; Elsevier, 2021.

Figure 18

Schematic with photographs of MnO₂ fabrication from spent LiBs. Copied with permission from ref. [221]; Elsevier, 2021.

2.2.2 Cobalt oxide

Electronic waste (e-waste) generation now exceeds 50 million metric tons per year globally [2]. The bulk of this waste does not get recycled and ends up in landfills in developing countries. Heavy metal residues, common in e-waste landfills, leach into soil and rivers, posing a health risk to residents. Sulfuric and citric acid systems have been used to extract Co from cobalt-bearing waste with a Co leaching efficiency of over 99% [222]. The recovery of 70% of Co from LiBs in cell phones was accomplished by a hydrometallurgical route [223]. Moreover, the extraction of Co and LiCoO₂ from LiB wastes, using the green glycine-based method, has been reported [224]. Co₃O₄ nanocatalysts have been produced from spent LiBs, Ni–Cd batteries, and LCD panels using a hydrometallurgical technique [225]. Co has been recovered from spent LiBs by using supercritical carbon dioxide extraction with sulfuric acid and H₂O₂ as reagents [226,227]. Interestingly, the electrochemical-hydrothermal process produced cobalt oxide from waste LiCoO₂ [228]. The removal of Co from mixed and polymetallic wastes was achieved via biohydrometallurgy [229], and the highest leach yields (53.2% cobalt) have been achieved using biogenic ferric leaching augmented with 100 mM H₂SO₄. For long-term recovery of Co from spent LiBs, a green process with potential environmental and economic benefits has been suggested [230].

Bioreagents are a feasible and environmentally sustainable alternative to conventional mineral processing that could be improved even further with proper LiB wastes pretreatment [231]. Biohydrometallurgy could be used to remove and recycle metals from mixed and polymetallic wastes. LiB wastes were removed using noncontact bioleaching with biogenic ferric iron and sulfuric acid [229]. The highest leach, providing 53.2% cobalt, was achieved using biogenic ferric leaching augmented with 100 mM H₂SO₄ [230]. Compared with the chemical leaching process, bacterial communities recovered 100% of Li(i) and 99.3% of Co(ii) after 72 h (91.4 and 94.2%, respectively) [232].

Cobalt ions were collected of waste LiCoO₂ cathodes of LiBs waste using a nitric acid leaching solution. For cobalt recovery, solvent extraction and chemical precipitation extracted Co(ii) from a chloride medium [233]. Ni and Co concentrations, conversely, have not exceeded 2.5 and 0.15%, respectively. To recover this material, a leaching phase must first occur. High-pressure acid leaching, which is commonly used, is costly because of its high energy consumption. However, there are now many alternatives to atmospheric leaching [234].

Ni and Co are chemically close, making separation impossible. Solvent extraction is considered the only method for extracting high-rate outputs from a mixture of these elements [235] and is also the only successful method regardless of the metal content in treated solutions [236]. Arsenic is considered the most known contaminant in soil, especially in former mining areas, but if present as arsenide, it is considered a new possibility for exploitable metals. In the presence of citric acid, bleached cobalt produces up to 92% but, with noninoculated and chemical controls, produces only 4 and 10%, respectively. Although the addition of citric acid enhances cobalt liberation, which has appeared during a more stable operation, excluding it has resulted in cobalt liberation ranging from 35 to 82%, based on the ability of the arsenide to deal with [237]. Recently, Dubey et al. [238] used Calotropis procera to assist in synthesizing Co₃O₄ NPs (Figure 19). XRD data and TEM analysis confirmed that the Co₃O₄ NPs ranged in size from 3 to 5 nm and were spherical in shape without displaying aggregation.

Figure 19

Synthesis steps of Co₃O₄ NPs. Copied with permission from ref. [238]; Elsevier, 2018.

2.2.3 Iron oxide

Recycled resources are one of the main precursors in producing iron oxides [239–241]. Every year, the TiO₂ industry produces massive amounts of ferrous sulfate with excess sulfuric acid, stored or disposed as solid waste in landfills. Also, vast amounts of iron ore tailings are collected in the steel industry, in iron ore plants, and from the bottom (BA) and fly (FA) ashes in MSW incineration, which can liberate Fe-bearing minerals during incinerator operation, management, and landfilling [242].

Iron has been recovered from iron ore tailings with a total iron content of nearly 72.36%. This iron was cubic or spheroidal in shape, ranging from 8.3 to 23 nm [243]. The TiO₂ industry produces a large amount of ferrous sulfate with excess sulfuric acid, which is harmful to the environment and urgently needs to be recycled. Nanosphere α-Fe₂O₃ red pigment powders with an average particle size of 45 nm were produced using waste ferrous sulfate as the secondary raw material via Acidithiobacillus ferrooxidans with high-purity Fe₂O₃ of 98.24 wt% [244–246]. Rusted iron collected from scraps (iron waste) can be used to synthesize an interconnected hematite phase (α-Fe₂O₃ NPs) [247]. The synthesis of iron oxides, goethite (α-FeO(OH)), magnetite (Fe₃O₄), and/or maghemite (γ-Fe₂O₃) from the razor blade and bottle cap wastes has been achieved with iron being the predominant element (at percentages above 70%). Iron oxides in magnetite and impure forms (maghemite, titanomagnetite, titanohematite, and ferrite) can be produced from ash from incinerator operations [242].

Iron nanocomposites (T-Fe₃O₄) have been synthesized with mesoporous hexagonal nanocrystallinity and with dimensions of less than 100 nm [248]. Green tea leaves (Camellia sinensis) were used to synthesize mesoporous α-Fe₂O₃ particles at a large scale; the particles were well crystallized, highly pure, and had an average particle size of 60 nm [249]. Iron NPs prepared by this method were used to degrade malachite green. Iron-based NPs can also be sensitized using oolong tea extracts, resulting in spherical particles ranging from 40 to 50 nm [250,251]. Fruit waste materials can produce Fe₃O₄ mesoporous spherical magnetic NPs with a surface area of 3.517 m²g⁻¹ and an average pore size relative to single-point adsorption total volume (P/P _o) equal to 0.9905 cm³g⁻¹ [252]. Yadav et al. [253] have reported the synthesis of iron oxide NPs from incense stick waste ash (Figure 20). XRD data confirmed the amorphous character of iron oxide NPs and SEM images verified they possess a polyhedral form. Furthermore, the NPs have an amorphous nature, as presented in the TEM images in Figure 21.

Figure 20

Synthesis of iron oxide NPs. Copied with permission from ref. [253]; Elsevier, 2020.

Figure 21

(a) and (b) TEM images of iron oxide NPs, (c) d-spacing, and (d) selected area electron diffraction pattern. Copied with permission from ref. [253]; Elsevier, 2020.

2.2.4 Titanium oxide

Despite the high quality of TiO₂ fabricated by chemical methods, synthesis from recycled wastes is a promising inexpensive alternative to chemically synthesized TiO₂ NPs [254,255]. TiO₂ nanocrystals with a size range between 50 and 150 nm can be synthesis-biomediated using tangerine peel waste [256]. Echinacea purpurea extract can be used as a bioreductant to synthesize 120 nm TiO₂ NPs [257]. Spherical TiO₂ NPs can be synthesized using Murraya koenigii leaf extract without any surfactant, catalyst, or template; these NPs have a particle size of nearly 10 nm [258]. TiO₂ NPs with an average size of 19 nm have been prepared using neem leaf extract in a convenient and biodegradable procedure [259].

Alumina-coated TiO₂ pigment was produced using a thermal recycling process from a paint waste matrix with a TiO₂ content of 30.47 wt% [260]. Furthermore, paint manufacturing industry waste (paint waste sludge) can be recycled into TiO₂ powders by sintering the waste containing TiO₂ [261]. Boron enrichment waste (BEW) was used to synthesize (TiO₂-BEW) photocatalysts, and the TiO₂-BEW contained nearly 13.7 wt% TiO₂ [262]. Nanocomposite K₂Ti₆O₁₃/TiO₂ has been prepared from water extracted from wood ash using calcination–precipitation method; particle size varied with calcination temperature [263]. Based on oxalic acid leaching, WO₃/TiO₂ can be produced from the spent V₂O₅–WO₃/TiO₂ catalyst, containing approximately 80–90 wt% TiO₂ [264].

Zalas et al. proposed a new method for producing renewable energy (via dye-sensitized solar cells, DSSCs) by converting titania-rich waste materials from the paper industry into usable materials. The fabricated DSSCs provided solar electricity but low photon-to-current efficiencies and short circuit photocurrent densities. The fill factor (FF) and open photovoltage in the circuit reached 67% and 719 mV, respectively. A titania paste annealing phase is required to make a compact mesoporous semiconducting layer via the normal working electrode development process. This phase temperature is usually approximately 723–783 K. The obtained semiconducting layer was unstable because of the high cellulose content, in contrast to TiO₂ in waste materials. This compelled a calcination step to acquire cellulose-free materials [265].

Qiang et al. [266] synthesized hierarchically porous TiO₂ using a simple and reproducible approach combining biofuel materials (Figure 22). Acacia mangium tannin extract (BA tannin) and triblock copolymer pluronic (P123) were used as a dual template, and traditional poisonous and expensive organic alkoxide was substituted with nontoxic and low-cost titanium sulfate (Ti(SO₄)₂). The biofuel BA tannin processed sulfonic and phenolic hydroxyl groups, effectively coordinating ligands. The results revealed that TiO₂ has a multiscale porous structure with macropore and mesopore sizes of 4 nm and 13 nm, respectively. The pore structure could be modified by adjusting the P123 and BA tannin ratios.

Figure 22

Synthesis steps for porous TiO₂. Copied with permission from ref. [266]; Elsevier, 2020.

2.2.5 Zinc oxide

Because of their abundance and low cost, recycled wastes are one of the main sources of producing ZnO NPs [15]. Green synthesis of ZnO NPs is an efficient, environmentally friendly, and simple method that aims to minimize the use of toxic chemicals in NPs. In this regard, high-purity ZnO nanospheres with an average particle size of 21 nm have been fabricated from Tragopogon collinus [267]. ZnO nanopowder acquired from orange fruit peel extract has a fluffy shape and different colors, with small spherical particles (10–20 nm) [268]. ZnO NPs from Kappaphycus alvarezii have exhibited a rod shape with a diameter range of 62–74 nm [269], whereas those from Punica granatum (pomegranate) fruit peel have spherical and hexagonal shapes at different annealing temperatures (600–700°C) [270]. Banana peel extract is an interesting precursor that creates ZnO rod-like or sheet-like structures with diameters ranging from 8 to 10 nm [271]. ZnO NPs from Lippia adoensis leaf extract are pure, hexagonal wurtzite, and thermally stable after 400°C [272,273], whereas those from goat slaughter waste are also pure, hexagonal wurtzite, spherical in shape, and have a size of 3–11 nm [274]. ZnO NPs from Phoenix dactylifera waste (date pulp waste [DPW]) are spherical, pure, crystalline, have hexagonal wurtzite phases, and have a mean diameter of 30 nm, and do not agglomerate [275]. ZnO NPs from leaves of the Ocimum tenuiflorum plant have a hexagonal shape with a diameter range of 11–25 nm and an average particle size of 13.86 nm [276].

Commercially, ZnO was synthesized from top submerged lance furnace flue dust (tire grade, purity > 99.0%) [277]. Synthesized ZnO from end-of-life (EOL) Zn–C batteries is a porous nanosheet with a thickness of up to 100 nm and a hexagonal step. Long-term calcination promotes crystal growth, but agglomeration can occur if the process is prolonged [278]. The purity of ZnO synthesized from spent Zn–C batteries is 99.6%, with impurities like SiO₂, and particle size decreases from 50 to 20 nm with a decrease in NaOH concentration. The resultant ZnO particles from this process are hexagonal zincite phases of ZnO [279,280].

ZnO/diatomite hybrid is made of pure Zn, Si, and O with no impurities [11]. CuO/ZnO hybrid nanocatalysts with honeycomb-like structures have been fabricated from solid waste developed in the organosilane industry. CuO and ZnO phases were present in all samples, and there were no other visible crystalline impurities [102]. Pure ZnO particles synthesized from spent Zn–MnO₂ alkaline batteries exhibited a size range of 40–50 nm [16]. Rambabu et al. synthesized ZnO NPs from DPW (Phoenix dactylifera) [275]. A high yield of 89.3% ZnO NPs was obtained (Figure 23). UV-vis absorption was carried out, and a red-shifted characteristic peak at 381 nm was obtained compared to the bulk material. This is due to the plasmonic resonance of the NPs. The NPs possessed a spherical appearance with a smooth distribution and no aggregate, as illustrated in Figure 24.

Figure 23

Synthesis steps of ZnO NPs. Copied with permission from ref. [275]; Elsevier, 2021.

Figure 24

SEM (a and b) and TEM (c and d) for ZnO-NPs at different magnifications. Copied with permission from ref. [275]; Elsevier, 2021.

2.2.6 Copper oxide

CuO NPs are synthesized using various physical, chemical, biological, and hybrid approaches. Physical and chemical methods are commonly used to synthesize CuO-NPs [281]. Control of size on the nanoscale, good stability, and high purity have all been demonstrated via biosynthesis of CuO nanostructures [282] using Lantana camara (flower) [283], Capparis spinosa (leaf) [284], [282,285], Allium cepa L. (peel), banana peel [286], Bauhinia tomentosa leaf [287,288], Madhuca longifolia [121,283,284], silver grass leaf extract [285,289–291], and gum arabic (GA) [292]. Conversely, mixed CuO/Cu₂O nanospheres can be biosynthesized in the presence of GA [292]. By selecting the appropriate extract, the morphology and size of the synthesized CuO can be monitored. For example, the synthesis of highly pure porous CuO NRs is achieved in the presence of Lantana camara flower extract versus CuO NRs in the absence of the extract [293,294].

Cu and CuO NPs from copper scrap have spherical and twisted sphere morphologies, with average particle sizes of 78 and 67 nm, respectively. Copper scrap processed under N₂ gas atmosphere yields single-phase cubic Cu NPs free of impurities and oxide [295]. Additionally, there were no impurity products, such as Cu(OH)₂ or Cu₂O, indicating that the CuO NPs are in a pure phase. Cauliflower morphology was observed in CuO NPs made from waste SIM cards [296]. The crystallinity of the recycled CuO NSs from Cu²⁺ waste is low, with wrinkled nanosheet-like aggregate structures and Cu²⁺ ion recovery performance of 88 and 99% as the pH rises to 11 and 12, respectively [297]. The yield of copper(ii) oxide from lead frame etching waste is 98.0 wt%, with an average particle size of 1.49 mm [15].

Phang et al. [298] managed to produce CuO NPs from papaya peel extract. The characterization studies showed that the CuO NPs were spherical, with sizes ranging from 85 to 140 nm, and the crystalline phase was monoclinic with good purity. Sharma et al. [299] used Ocimum tenuiflorum to synthesize CuO NPs, as illustrated in Figure 25. These CuO NPs possessed different morphologies dependent on copper acetate monohydrate concentrations (Figure 26). Also, unique synthesis methods of different metal oxide-based nanomaterials derived from different waste sources are listed in Table 2.

Figure 25

Synthesis of CuO NPs. Copied with permission from ref. [299]; Elsevier, 2021.

Figure 26

SEM images of CuO NPs created from various copper acetate monohydrate concentrations. Copied with permission from ref. [299]; Elsevier, 2021. (a) [Cu(CH₃COO)₂·H₂O] = 5 mmol kg⁻¹ (b) [Cu(CH₃COO)₂·H₂O] = 10 mmol kg⁻¹ (c) [Cu(CH₃COO)₂·H₂O] = 50 mmol kg⁻¹.

3.1.1 Carbon-based nanomaterials

Carbon-based materials as supercapacitor electrodes are widely studied and used commercially. Applications of carbon-based materials have proven promising for solving current environmental and energy problems. Agriculture waste precursors could be used as target sources for producing carbon-based materials to preserve the environment and maintain low-cost materials. This includes oil palm biomass residues (leaves, fronds, trunks, empty fruit bunches, shells, and fibers), banana fibers, Lablab purpureus, sago bark, Allium cepa peel, RH, waste tires, pineapple leaf fibers, and cellulose [17,19,20,28,370–375]. Several biowaste sources have been used to fabricate CNFs, which act as supercapacitor electrodes. For example, CNFs derived from pineapple leaf fibers exhibited a specific capacitance (C _s) of 175 F g⁻¹ [375]. Also, cellulose-derived CNFs (CCNFs) with a diameter of 200 nm resulted in C _s of 145 F g⁻¹ [376].

Graphene has recently emerged as a powerful and efficient material for electrodes in supercapacitors because of its excellent characteristics, such as large surface area, carrier mobility, and mechanical power. Graphene is distinguished by its properties, making it an excellent choice for supercapacitor electrodes [377]. Supercapacitors currently rely on unsatisfactory AC, which has less than 120 F g⁻¹ and a low rate for charge and discharge ratio [378]. According to Jinag and Pickering, graphene could be recycled from supercapacitor electrodes and reused [369]. Also, graphite waste could be recycled and turned into graphene; waste graphite is collected from batteries and other electronics [368].

Karakoti and his team produced GNs from PW for supercapacitor applications and reported C _s equal to 38.78 F g⁻¹ [379]. Additionally, Sankar et al. used RHs to prepare GNs for supercapacitor electrodes using KOH activation [86]. They recorded a capacitance of 115 F g⁻¹ at a current density of 0.5 mA cm⁻¹ and capacitance retention of 88% after 2,000 cycles, illustrating the excellent stability of GN electrodes [86]. The recycled GNs showed considerable energy and power densities of 36.8 W h kg⁻¹ and 323 W kg⁻¹, respectively, in an aqueous 1 M Na₂SO₄ electrolyte [86]. Krishna et al. studied the effective preparation of porous GO nanosheets from waste rice, resulting in capacitance of 158.6 F g⁻¹ at 1 A g⁻¹. The quasirectangular form of the capacitance–voltage (CV) curve indicates good capacitance activity and significant charge transfer capability at various scan rates. If the CV curve maintains its quasirectangular form, the electrode has good rate capability.

Using a mix of chemical and physical activation techniques, Yuli et al. used cocoa skin to produce AC, showing C _s produced by chemical activation of 90.2 F g⁻¹ [380]. Additionally, corn husk was used as a precursor for AC supercapacitor electrodes with a C _s of 127 F g⁻¹ at 1 A g⁻¹ in 6 M KOH with a small energy density (4.4 W h kg⁻¹) [381]. The galvanostatic charge-discharge (GCD) curves for these electrodes had a triangular outline and linear patterns, indicating that the charge storage is an electrical double layer formation and is highly reversible [381]. In 2021, Ramayani et al. reported AC preparation from carrot juice waste via a one-stage integrated pyrolysis approach and KOH chemical activation [382]. The obtained C _s, power, and energy were 155 F g⁻¹, 77.57 W kg⁻¹, and 21.52 W h kg⁻¹, respectively, in 1 M H₂SO₄ [382]. Cui and Liu prepared AC from industrial pyrolytic tires and achieved a C _s of 190 F g⁻¹ [383].

CNTs have a novel structure, accessible surface area, a narrow size distribution (in the nanometer range), high stability, and low resistivity, suggesting that they are suitable materials for supercapacitor electrodes. Mishra et al. have synthesized MWCNTs from PP PW and used them for supercapacitor applications [384]. The recycled MWCNTs showed C _s of 59 F g⁻¹ in 1 M KOH solution at a scan rate of 5 mV s⁻¹, and it was directly proportional to their surface area. POME was used to produce CNTs [385] by a pyrolysis process at 900°C in a flowing N₂ atmosphere. The electrical conductivity of CNTs is 10⁷ S cm⁻¹, making them suitable for use as a supercapacitor electrode. C _s obtained for POME recycled CNT electrodes were lower than that for commercial CNTs. Throughout the charging and discharging process, all the cells have similar linear curves of voltage vs time [385]. Sunflower seed hulls and sago were pyrolyzed at 800°C to produce CNTs [386]. The capacitances of CNTs obtained from pyrolyzing natural sunflower seed hulls and sago were 86.9 and 26.7 F g⁻¹, respectively.

CQDs can be generated from denatured (spoiled) milk [387]. The CQDs electrode demonstrated C _s of 95 F g⁻¹ over 1,000 cycles with exceptional stability and Coulombic efficiency. The stability of the fabricated CQDs electrode material was tested at a current density of 0.12 A g⁻¹, even after 1,000 cycles, no noticeable change in capacitive behavior was identified, demonstrating excellent long-term cycling stability [387].

Additionally, CQDs were made from durian peel waste using a pyrolysis process [388]. CQDs have also been applied successfully to AC-based electrodes to improve their electrochemical qualities and supercapacitor performance. According to the cyclic voltammogram, the C _s of the AC-CQDs composite electrode was 1.4 times higher (approximately 60 F g⁻¹) than that of the pure AC electrode (43 F g⁻¹) [388]. This increase in capacitance is mainly due to the introduction of high surface area CQDs and the pseudocapacitive behaviors provided by CQDs functional groups.

Porous carbon NPs (PCNs) have been prepared from different biowastes, such as sago bark, Allium cepa peel, Lablab purpureus, and oil palm leaves [17–20,374]. The PCNs obtained from these agricultural wastes have been electrochemically studied in detail, either as single electrodes or complete symmetrical devices in 5 M KOH or 1 M Na₂SO₄. For the full practical supercapacitor fabricated using PCNs obtained from palm oil fronds, the C _s values were 309 F g⁻¹ in KOH [374]. The single electrode fabricated from Lablab purpureus showed 300 F g⁻¹ with high stability of 94% up to 52,000 cycles (as shown in Figure 30). Simultaneously, the symmetrical supercapacitor showed C _s of 129 F g⁻¹ with a high energy density of 17.9 W h kg⁻¹ under a wider potential window of 1.5 V [19].

Figure 30

Capacitance retention and Coulombic efficiency of PCNs obtained from Lablab purpureus. Copied with permission from ref. [19]; Royal Society of Chemistry, 2018.

3.1.2 Metal oxide-based nanomaterials

Metal oxides are widely used for energy storage due to their excellent morphologies, high surface area, high theoretical specific capacity, and eco-friendliness [209,210,362,363,389]. Metal oxide electrodes could be a single metal oxide, bimetallic oxides, or metal oxide heterostructures. The recycling process of metal oxide from waste is essential to obtain low-cost electrode materials. Different waste sources, such as Zn–C batteries, LiBs, SIM cards, and red mud, have been reported as effective precursors for metal oxide recovery [209,211,296,390–392]. Various metal oxide materials have been recovered from electronic waste and used as supercapacitor electrodes, including MnO₂, Co₃O₄, ZnO, Fe₃O₄, Fe₂O₃, and CuO [211,366,391,393]. The supercapacitors fabricated from these recovered materials showed electrochemical behavior close to that found using chemically prepared materials.

Recycling household Zn–C batteries into manganese oxide has been studied. Nanoflower MnO₂ was successfully obtained by electrodeposition of Zn–C battery cathode powder [209–211]. A schematic diagram of the recovery of MnO₂ nanoflowers from Zn–C battery is presented in Figure 31 [209]. The charge-discharge curve for the MnO₂ nanoflower electrode shows linear curves and C _s of 294 F g⁻¹ [211]. This high storage efficiency is due to the nanoflower morphology, which is preferable for the diffusion of ions.

Figure 31

Schematic of the recovery of MnO₂ nanoflowers from a Zn–C battery. Copied with permission from ref. [209]; Elsevier, 2017.

Another method to recover MnO₂ from spent batteries is using the sol–gel process with cathode powder leaching solution [365]. The recovered electrode showed 275 F g⁻¹ at 1 A g⁻¹. The electrochemical cycling of Mn₃O₄, produced from the heat treatment of Zn–C batteries, led to nanoflower MnO₂ particles that showed C _s of 309 F g⁻¹ at 0.1 A g⁻¹ [209]. Moreover, nanoflower MnO₂ showed capacitance retention of 93% after 1,650 cycles; therefore, recycled nanoflower MnO₂ is an excellent material for the supercapacitor industry [209].

There are different methods to recover cobalt oxides from spent LiB, and metal oxide electrode capacitance varies according to these methods. The high surface area and microporosity of the as-prepared Co₃O₄ enhance electron transfer and ion diffusion. There is a connection between the number of cycles and the amount of capacitance retained. The Co₃O₄ samples maintained approximately 89.1% of their capacitance after 1,000 cycles [366]. The redox mechanism primarily regulates the capacitance action.

Aboelazm et al. recycled LiBs into hierarchical nanostructures of Co₃O₄ using an electrodeposition technique combined with a magnetic field (MF) [391]. The recovered Co₃O₄ nanostructures had a charge storage capacity of 1,273 F g⁻¹ at 1 A g⁻¹ (four times higher than electrodeposited Co₃O₄ produced in the absence of an MF). A stability of 96% after 5,000 cycles also demonstrates the enhanced high cycling stability of the Co₃O₄ nanostructures. Co₃O₄ recovery from spent LiBs improved in the experiments, suggesting that it could be used as a supercapacitor electrode material. When calculating the diffusion coefficient, the electroactive surface area is essential. The diffusion value for Co₃O₄(MF) was much higher than that of Co₃O₄ as a result of Co₃O₄(MF) hierarchical nanostructures [391]. Figure 32 shows the stability of a supercapacitor fabricated from Co₃O₄ recycled from LiBs [391]. Additionally, a facile solvothermal method was used to recover Co₃O₄ from LiBs [366]. C _s and capacitance retention of the recovered Co₃O₄ were 50.8 F g⁻¹ and 89.1%, respectively, making it a good candidate for supercapacitor devices.

Figure 32

Capacitance retention (stability) for Co₃O₄(MF) compared with Co₃O₄. The Coulombic efficiency and some GCD cycles are shown in the insets. Copied with permission from ref. [391]; American Chemical Society, 2018.

Copper oxide (CuO) NPs were recycled from SIM card waste through a one-pot green process [296]. The supercapacitive behavior of the obtained CuO NPs was investigated because of their extensive range of technological applications for CuO. GCD revealed typical pseudocapacitive behavior for CuO NPs (as shown in Figure 33a), with 542 F g⁻¹. Additionally, cyclic stability of the CuO electrode material was achieved even after 5,000 GCD cycles, with 95.3% retention (as shown in Figure 33b). Moreover, the CuO electrode has lower resistance, a reasonable rate, and good reversibility. The CuO NP electrodes showed excellent electrochemical characteristics; therefore, they are promising candidates for supercapacitors [296].

Figure 33

(a) GCD curves and (b) stability of copper oxide recovered from SIM cards. Copied with permission from ref. [296]; Elsevier, 2020.

Spent Zn–C batteries have been used as a precursor to produce ZnO supercapacitor electrodes [394] using in situ production of ZnO NPs in the form of an ultrathin film. The CV curve at 100 mV s⁻¹ and other scan rates investigated the charge transfer through a surface redox reaction (Figure 34). ZnO NPs showed high C _s of 547 F g⁻¹ at 5 mV s⁻¹ and a low charge transfer resistance of 6.5 Ω [394]. This excellent electrochemical behavior is due to the small size of the ZnO NPs (45 nm) and their spherical morphology.

Figure 34

Electrochemical behavior (CV) of ZnO NPs in 0.6 M KOH at (a) 100 mV s⁻¹ and (b) at different scan rates. Copied with permission from ref. [394]; Springer, 2019.

Fe₂O₃ has piqued interest in supercapacitor applications among various iron oxides. It has many crystal structures (α, β, γ, and ε); however, the most thermodynamically stable phase is α-Fe₂O₃ [395]. To improve its electrochemical properties, Fe₂O₃ nanomaterials, including NPs [396], NRs [397], and NTs [398], with different morphologies, have been explored. Mill scale is a rich source of iron oxide steel industry waste and can be used as a source of iron oxide recovery [399]. Supercapacitor electrode from iron oxide was fabricated by spray deposition onCu with spider silk (SS) as a current collector. The recovered iron oxides (Fe₂O₃ and Fe₃O₄) gave a capacitance of 92 F g⁻¹ in a half-cell design; 25 F g⁻¹ was achieved for the full cell arrangement with a stability of 83% [399]. Compared with commercially available electrodes, the fabricated electrodes significantly differed in prices. The recovered iron oxide-based supercapacitors cost less and are easily fabricated. Comparing mill-scale-based supercapacitors with commercial LiBs and supercapacitors, using their data from other sources, [400] shows that high energy costs are a characteristic of supercapacitors. By contrast, LiBs are characterized by high cost, whereas mill-scale supercapacitors using the data from ref. [399] are broadly competitive with commercial LiBs and supercapacitors. In another work by Fu et al., mill scale was used to produce Fe₂O₃ and then convert it to hollow porous Fe₂O₃ microrods [393]. The hollow and porous Fe₂O₃ microrod-based supercapacitor electrodes showed 346 F g⁻¹ at 2 mV s⁻¹ with 88% capacitance retention over 5,000 cycles. Industrial waste red mud has also been used as a supercapacitor electrode material as it contains a rich mixture of metal oxides. Red mud-based supercapacitor electrodes have C _s of 317 F g⁻¹ at a 10 mV s⁻¹ scan rate with retention of approximately 97% after 5,000 cycles with high electrode stability and performance [392].

3.1.3 Hybrid nanomaterials

Carbon materials possess good stability, but they have low capacitance. By contrast, pseudocapacitor materials show high capacitive values but low stability. Therefore, it is necessary to develop a hybrid energy system comprising both materials and investigate their synergetic effect on capacitive storage. To improve the electrochemical efficiency of transition metal oxides, Co₃O₄ recovered from industrial and domestic wastes has been combined with nanocarbon materials, such as graphene, CNTs, and several mesoporous materials with specific structures. Many hybrid materials have been obtained from waste including carbon/carbon (AC/MWCNTs and CQDs/AC) [388], metal oxide/carbon (MnO₂/graphene, CaO/AC, AC/SrFe₁₂O₁₉, and NiCo₂O₄/AC) [28,210,364], and metal sulfide/carbon (NiS₂/graphene) [401].

CaO/palm kernel shells (ACPKS) are composed of calcium oxide, and AC extracted from the chicken eggshell and ACPKS waste, respectively [28,402]. By comparing CaO/ACPKS and ACPKS electrodes, it was found that CaO/ACPKS has a long discharge time, high charge accumulation, and high capacitance (222 F g⁻¹ at 0.025 A g⁻¹) [28]. CaO/ACPKS electrodes have lower resistances (0.44 Ω) than ACPKS (0.86 Ω). This indicates an improvement in conductivity with the addition of CaO to the AC. CaO/ACPKS has high electrochemical stability of 93% and a high energy density of 27.9 W h kg⁻¹ [28]. This excellent electrochemical behavior could be attributed to the synergistic effects of the hybrid structure of CaO and AC and its unique honeycomb morphology, as shown in Figure 35. Another type of hybrid supercapacitor includes AC from crab shells and SrFe₁₂O₁₉ NPs [364]. At 0.5 and 10 A g⁻¹, AC–CS/SrFe₁₂O₁₉ has C _s of 742.6 and 401.3 F g⁻¹, respectively, suggesting good rate capability.

Figure 35

Schematic of the recovery of CaO from eggshells and AC from palm kernel shells. Copied with permission from ref. [28]; Springer Nature, 2018.

Porous carbon was made from lemon peel waste [403]. A hybrid supercapacitor with NiCo₂O₄ and nanograss@biowaste-derived porous carbon was fabricated and evaluated [403]. At a current of 1 mA, the fabricated supercapacitor showed a C _s of 17.5 F g⁻¹ and an energy density of 6.61 W h kg⁻¹. The presence of regenerative graphene oxide with NiS₂ enhances electrochemical efficiency compared with pure NiS₂ [401]. Figure 36 and Table 5 summarize the main sources and findings of supercapacitors fabricated from different waste precursors.

Figure 36

Schematic of the main waste sources used to fabricate supercapacitor electrode materials.

3.2.1 CNMs

Carbon-based materials have been commonly used as anodes in LiBs. Carbon-based materials, including graphene, AC, CNTs, and CNFs, are promising as advanced electrodes for high-performance batteries. This is because of their high electrical conductivity, structural tunability at the atomic/morphological levels, stability under extreme conditions [414], low cost, variety of forms (composites, fibers, aerogels, sheets, powders, tubes, etc.), electrocatalytically active sites for a wide range of redox reactions, and large specific surface area [415]. Carbon-based materials have been consumed exponentially because they have shown excellent energy storage characteristics. Therefore, they have to be obtained in different ways. Scientists have developed materials with excellent electrochemical properties by converting waste into valuable nanostructured carbon materials. These waste-derived carbon materials have demonstrated promising effects in various applications, particularly energy storage.

CNFs are promising candidates for LiBs due to their large surface-to-volume ratio, fast kinetics, and 3D conductive networks. LiBs can be stored through the following mechanisms: (1) intercalation; (2) vacant space between bundles; (3) adsorption on the outer surface; and (4) defect sites, nanopores, and cavities [416]. CNFs are 1D sp² hybridized carbon nanostructures with aspect ratios (length/diameter) exceeding 100. CNFs are promising alternatives to commonly used graphite-based anode materials.

Since CNFs are derived from nonrenewable precursors, finding other sustainable resources is necessary to develop high-performance CNFs materials and reduce fossil fuel use.

Tao et al. prepared CNFs from waste biomass (walnut shells) and demonstrated a significant possibility for transforming low-cost biomass into high-value carbon materials for energy storage. The electrochemical performance was investigated and showed that CNFs fabricated at 1,000°C have a high capacity of 271.7 mA h g⁻¹ at 30 mAg⁻¹. The charge–discharge curves of the electrodes are shown in Figure 38 [417].

Figure 38

CV curves of CNFs at first cycle (a), and CNFs-1000 at first five cycles (b), charge/discharge curves (c). Adapted with permission from Ref. [419], Royal Society of Chemistry, 2018.

Graphene has only recently been introduced as a LiBs cathode material, although it is considered a promising material because of its excellent electrochemical performance [418]. Stacked platelet graphene NFs have good ion diffusion because of their lateral dimensions, which allow Li⁺ to diffuse into the interlayer space much more easily and with reversibility. Therefore, graphene has improved energy storage capacity (461 mA h g⁻¹).

It has recently become highly desirable to synthesize graphene sheets using biomass sources such as butter, honey, foodstuffs, and gelatin. Chen et al. have developed graphene from useless wheat straw by a hydrothermal and graphitization method [419]. The biomass recovered graphene sheets exhibited superior electrochemical properties when used as anode material in LiBs. The results showed excellent rate capability (463.5, 431.4, and 306.8 mAh g⁻¹ at 1, 2, and 5 C, respectively), high capacity (502 mAh g⁻¹ at 0.1 C), and excellent cycling performance (392.8 mAh g⁻¹ at 1 C after 300 cycles), as shown in Figure 39 [419].

Figure 39

Graphene nanosheet d-spacing and charge capacity relationship. Adapted with permission from ref. [420]; Royal Society of Chemistry, 2011.

AC has been commonly used in different industries (food, petrochemical, environmental protection, and electric power system) because of its excellent conductivity, high surface area, and large pore volume. AC is expensive and efficient, so low cost and readily available alternatives from recycled material are needed. Recently, much attention has been placed on the use of waste, such as banana peel [370], corn stalk [421], tires [371], and RH [372], to construct AC.

Li et al. prepared mesoporous AC from corn stalk by a simple carbonized-activated process for LiBs anode materials. The mesoporous structure of AC significantly improves its electrochemical performance. After 100 cycles at 0.2 C, the mesoporous AC demonstrated a high reversible capacity of 504 mAh g⁻¹ [421]. Shilpa Kumar and Sharma prepared high surface area carbon material from waste tires [371]. The acid-treated carbon showed excellent electrochemical performance with a high reversible capacity of ∼670 mAh g⁻¹ after 100 cycles [371]. Yu et al. prepared AC derived from RH via carbonization at a low temperature and activation at a high temperature. After 100 cycles at 0.2 C, the capacity remained at 448 mAh g⁻¹ [372]. Fu et al. prepared AC derived from RH for LiBs anode materials. After charging–discharging for 60 cycles, the capacity of LiBs remained at 400 mAh g⁻¹ [422].

CNTs, like many other NMs, are still relatively new in the industrial field. Thus, CNT EOL considerations and the application of recycled waste have still not been widely experimented with or studied [367]. One of the primary considerations is the application of recycled industrial waste in CNTs and their reuse in batteries. As for batteries in general, critical aspects to evaluate or measure performance are capacity, life cycle, charge, discharge rates, and safety. Those are the main properties that should be investigated while studying prospective materials and making recycled CNTs in batteries fabrication worthy. CNTs have proved their applicability in high energy and high-power density LiBs because of their unique electromechanical and chemical properties. One of the critical factors to consider while reusing CNTs in batteries is the energy required to reproduce CNTs from waste compared with the energy needed for virgin material and the energy produced by the recycled CNTs. Recycled SWCNTs have been effectively used for Li⁺ battery coins for more than one life cycle, with promising results. As presented in Figure 40, recycled CNTs perform better than pure SWCNTs [423].

Figure 40

(a) CV and (b) charge–discharge curves of an electrode obtained from rice husk. Adapted with permission from ref. [372]; Elsevier, 2018.

CNTs have a much lower resistance than AC, which allows them to have a much higher power density. If all interstitial sites (intertube channels, inner-shell van der Waals spaces, and inner cores) are accessible for Li intercalation, CNTs might have a higher Li capacity. The reversible lithium-ion capacities of CNT-based anodes are substantially higher than those of conventional graphite anodes. Furthermore, the open structure and enriched chirality of CNTs help improve power and electrical transport in CNT-based LiBs.

Yeast-fermented wheat dough scaffolds were used to prepare CNTs via a simple, green, and sustainable activation method. The recovered CNTs were used as a sulfur host for a lithium-sulfur battery. The Li–S cell demonstrated excellent cyclic performance with a capacity of 450 mAh g⁻¹ [423]. Figure 40 shows the charge–discharge curves of the electrode at different cycles.

3.2.2 Metal oxide-based nanomaterials

Generally, metal oxides are used in batteries as cathode- or anode-like lithium metal oxide, the cathode material for LiBs [424]. Particularly, nanostructured metal oxides, such as MnO₂, Fe₃O₄, and Co₃O₄, have the potential as electrodes for energy storage systems.

MnO₂ has been used as both cathode and anode for LiBs because of its environmentally friendly characteristics, low cost, and high theoretical capacity (1,230 mAh g⁻¹) [425]. MnO₂ is considered an outstanding anode material for LiBs [426] but has disadvantages, such as low specific charge–discharge capacity, poor electrical conductivity, and volume expansion, which inhibit its commercial use [427]. Waste eggshell, considered as a natural hard template with high porosity, can synthesize MnO₂ with different morphologies, limit particle size, and improve electrochemical performance [428]. MnO₂ produced from manganese sulfate using eggshell as a hard template demonstrates the best electrochemical performance (2156.6/1008.2 mA h g⁻¹ at 100 mA g⁻¹ and average Coulombic efficiency of 97.7%) [429]. LiMn₂O₄ could be obtained from waste Zn–Mn batteries and delivered a high discharge capacity of 103.6 mAh g⁻¹ [430].

Using a hydrometallurgical process, nanostructured Co₃O₄ has been successfully recovered from spent LiBs [431]. A GCD between 0.02 and 2 V vs 3 V vs Li/Li⁺ with a constant current density of 125 mA g⁻¹ evaluated the electrochemical performance of Co₃O₄ at 760.9 mAh g⁻¹ with a Coulombic efficiency of 99.7% [431]. A waste Zn–C battery was separated to access the powdered materials and used as a precursor to synthesize ZnO NPs [432]. The formation of new composites has also included using ZnO NPs recycled from alkaline batteries. For example, ZnO NPs synthesized from waste Zn–C batteries have effective optical properties, spherical shape, and a 50 nm size [432].

Conversely, an alternate research paper prepared a ZnO sample using a chemical method (the sol–gel synthetic method) to produce ZnO NPs with high electrochemical performance, calculated by GCD cycling (like anodes for LiBs), and a reversible capacity of 1,652 and 318 mAh g⁻¹ at 1^st and 100th cycles, respectively. Additionally, the methodology can perhaps carry a reversible capacity of 229 mAh g⁻¹ at a high current density of 1.5 C [433]. Therefore, since the recycled ZnO has similar characteristics, it will be helpful for battery applications.

Interconnected α-Fe₂O₃ NPs (30–60 nm) used as anode material for LiBs have been prepared by recycling tin ore tailings [434]. The electrochemical behavior of α-Fe₂O₃ was observed to exhibit the finest performance for LiBs, displaying a capacity of 1,146 mAh g⁻¹ at 0.5 A g⁻¹ after 300 cycles [434]. Additionally, it was hypothesized that an α-Fe₂O₃ electrode with a smaller surface area might maintain a smaller irreversible capacity loss through the cycles than that of γ-Fe₂O₃. Simultaneously, the larger average pore size provides an earlier gain for the electrolyte infiltration into the α-Fe₂O₃ nanoelectrode; rapid infiltration is advantageous for transporting Li⁺ from the electrode substance to the electrolyte [435]. Fe₂O₃ is produced by direct chemical processing using an industrial mill and has a theoretical capacity of 1,007 mAh g⁻¹ [393]. It is also nontoxic, low-cost, and environmentally friendly. Chemical treatment has already transformed mill scale, a waste material from the steel industry widely accessible and composed of a mixture of iron oxides, into hollow and porous Fe₂O₃ microrods [393].

Recently, TiO₂ NPs were effectively biosynthesized by applying remnant water, an ideal kitchen waste [436]. The nanosized TiO₂ enhances cell performance and cycling stability by increasing the surface area, the electrode interaction area, and consequently shortening the path lengths for Li-ion and electron transport. The electrochemical significance of TiO₂ was evaluated in a half-cell configuration, from 1 to 3 V at room temperature via a current density of 33 mA g⁻¹, with a high reversible capacity of 164 mA g⁻¹ following 60 cycles 98% capacity retention was revealed. This technique can be studied further for synthesizing other materials for LiBs.

3.2.3 Hybrid nanomaterials

Recycling spent LiBs make it possible to create LiFePO₄/rGO composites from cathode material and spent graphite anode. Nanostructured LiFePO₄/rGO batteries have high capacities and high Coulombic efficiencies. Using melamine as a nitrogen source, biowaste bagasse was turned into carbon and nitrogen-doped carbon (NBC) [437]. The electrochemical performance of S@NBC (sulfur and NBC) composite cathodes created by assembling Li–S cells exhibited significantly high performance. S@NBC had the highest output with a reversible capacity of 1,169 mAh g⁻¹ at 0.2 C, and capacitance of 8.98 mAh g⁻¹ was retained after 200 cycles with a capacity retention of 77%. Furthermore, the composite had an outstanding rate potential of 454 mAh g⁻¹ at a 4 C rate [437].

Xu et al. constructed 3D porous carbon@graphene composites from waste cigarette filters. Their results showed a high capacity of 1,010 mAh g⁻¹ at 0.2 C after 200 cycles, with only ∼0.04% capacity loss per cycle [438]. Additionally, Wong et al. were able to generate silicon NPs from RH using a magnesiothemic reduction method; these NPs were used to manufacture Si/graphene, which serves as an anode in LiB. The silica composites displayed C _s of 1,000 mAh g⁻¹ at 1,000 mA d⁻¹ [103]. Figure 41 and Table 6 summarize the main sources and findings of batteries fabricated from different waste precursors.

Figure 41

Schematic diagram of the primary waste sources used to fabricate battery electrode materials.

4.1.1 CNMs

Zhou et al. prepared heteroatom-doped porous CNFs via a simple pyrolysis method using natural SS as a precursor for the oxygen reduction reaction (ORR) in MFCs [447]. The prepared CNFs showed good ORR activity in alkaline conditions (0.98 V onset potential versus the reversible hydrogen electrode (RHE) and 0.85 V half-wave potential), which are higher than Pt/C and common metal-free carbon catalysts. The improved catalytic properties were ascribed to the enriched carbon lattice with electronegative N and S atoms and the high surface area due to the porous and nanofibrillar CNFs structure. Additionally, MFCs applying a prepared CNFs cathode exhibited an 1,800 mW m⁻² power density, 1.56 times higher than those applying a Pt/C cathode.

Zhou et al. used eggplant-based biomass with plate-like structures and rich porous networks to prepare N-doped porous graphene (NDPG) catalyst for the ORR in low-temperature fuel cells (LTFCs) [448]. Compared with costly commercial Pt/C catalysts, the synthesized NDPG displayed excellent ORR activity and stability. The measured half-wave potential was approximately 10 mV higher than that of Pt/C in alkaline media, and the stability was higher than Pt/C in alkaline and acidic media. The exceptional catalytic efficiency of the synthesized graphene can be ascribed to its mesoporous structure with an ultra-high surface area (1,969 m²g⁻¹), along with its high electron conductivity and effective nitrogen content.

Zhao et al. produced high yield and quality graphene sheets using disposable paper cups as a carbon source and Fe²⁺ as a catalyst [449]. Moreover, Pt/graphene and Fe/graphene were obtained from the synthesis. Pt/graphene catalysts showed high ORR activity in fuel cells. Additionally, their electrochemical measurements displayed that prepared Pt/graphene catalyst possessed a better electrochemical efficiency for ORR than Pt/C catalyst.

Ma et al. extracted S-doped graphene (SG) from Li–S batteries and used it as a sustainable and economical metal-free electrocatalyst for the ORR [450]. The prepared SG electrocatalyst, with transferred electron number (n ≈ 3.13), exhibited higher electrocatalytic activity than pristine undoped graphene, higher fuel selectivity, and longer ORR durability. Additionally, SG electrocatalyst had more stable electrocatalytic activity than commercial Pt/C.

Through pyrolysis of economic, biobased corn starch, urea, and FeCl₃, Pan et al. synthesized Fe–N-doped graphene (FeNG) electrocatalysts for ORR in fuel cells [451]. Compared with the commercial Pt/C catalyst, the prepared FeNG catalysts showed a high electrocatalytic activity that was approximately 99 mV more positive in KOH, as shown in Figure 42. Additionally, the catalyst was 34 mV more negative in H₂SO₄. By adjusting the form of the doped species, the ORR of the prepared FeNG catalysts could be controlled at various pH levels. Additionally, ORR potential in basic media may be attributed to the sites of quaternary N and Fe/Fe₃C, whereas acidic media ORR performance originated mainly from Fe–N _x complexes.

Figure 42

ORR electrochemical activities of FeNG catalysts in 0.1 M KOH electrolyte: (a) various linear sweep voltammetry (LSV) curves of samples at a 5 mV s⁻¹ scan rate and 1,200 rpm rotation rate; (b) E _1/2 and E _O values obtained from LSV curves in (a); (c) FeNG-1,100 LSV curves at a 5 mV s⁻¹ scan rate and various rotation speeds, and (d) LSV data-based Tafel plots for FeNG catalysts. Copied with permission from ref. [451]; John Wiley and Sons, 2015.

Zhou et al. used a novel, facile, and inexpensive technique to prepare PNDG as a high-performance electrocatalyst (with a very large surface area of 1,152 m²g⁻¹) for ORR in LTFCs using intrinsically porous biomass (soybean shells) as a carbon and nitrogen source [452]. The prepared NDG catalyst showed an onset and half-wave potential of −0.009 and −0.202 V (vs a saturated calomel electrode), respectively, which are similar to the catalytic behavior of common Pt/C catalyst in alkaline media. Compared with Pt/C, the catalyst performed well in acidic media and exhibited higher ORR stability. Furthermore, the NDG catalyst showed a higher ORR stability and a greater CO and CH₃OH tolerance than the Pt/C catalyst in alkaline media.

Liu et al. investigated biomass-derived AC doped with N as a cost-effective catalyst with high-performance ORR performance in an MFC with a maximum output voltage of 544 ± 6 mV and power density of 977 ± 32 mW m⁻². The generated electricity from four of the air cathode MFCs connected in series was able to light a LED. Using iron NPs anchored on graphene as a catalyst and urea as a combined source of nitrogen moieties and depolymerization agent, Sridhar and Park presented an easy and efficient synthesis of iron and nitrogen codoped CNTs (Fe@NCNTs-rGO) from PET waste using microwaves for ORR in fuel cells [453]. The prepared Fe@NCNT-rGO electrocatalyst (n ≈ 4) showed a positive E _1/2 of 0.75 V relative to the RHE. Synthesized Fe@NCNT-rGO exhibited unique electrocatalytic ORR behavior with a good onset potential of 0.896 V vs RHE.

Additionally, Fe@NCNT-rGO electrocatalyst possessed a lower current density of 4.8 mA cm⁻² concerning commercial Pt/C electrodes (current density of 5.7 mA cm⁻²). Furthermore, compared with commercially available 20% Pt/C, Fe@NCNT-rGO had outstanding methanol tolerance in direct methanol fuel cells. This can be attributed to embedding catalytically active Fe–N _x moieties inside the CNT walls. Consequently, oxidation and consequent fouling of catalyst due to the crossover of methanol was hampered.

Cai et al. synthesized iron and nitrogen codoped CNTs (Fe–N-CNTs) for ORR in fuel cells by catalytic pyrolysis of PP waste plastics over Fe–Al₂O₃ [454]. The prepared Fe–N-CNT850 catalyst outperformed pristine Fe–CNT in terms of catalytic activity, as shown in Figure 43. Compared with Fe–CNT, the onset and half-wave potential of the Fe–N-CNT850 were 137 and 156 mV, respectively. The limiting current density of Fe–N-CNT850 was also higher than that of Fe–CNT. Compared with commercial Pt/C catalyst, the prepared Fe–N-CNTs showed an onset potential of 0.943 V vs RHE, a half-wave potential of 0.811 V vs RHE, and increased stability anti-poisoning properties. Additionally, the prepared Fe–N-CNTs exhibited greater methanol tolerance than that of 20% Pt/C catalyst, and the current of Fe–N-CNT850 was slightly lowered upon methanol addition, continuing to be stable for about 800 s. This indicated that Fe–N-CNT850 had greater methanol tolerance and poison resistance than the reference Pt/C electrocatalyst.

Figure 43

(a) LSV curves of O₂-saturated solutions for all catalysts at a scan rate of 5 mV s⁻¹ and a rotation speed of 1,600 rpm. (b) Fe–NCNT850 LSV curves at various rotation speeds. (c) Fe–NCNT850 K–L plot at various potentials. (d) Fe–NCNT850 catalyst electron transfer number (n). Copied with permission from ref. [454]; John Wiley and Sons, 2020.

Moo et al. transformed waste plastics (PE, PP, PS, and PET) into CNTs for ORR in fuel cells using sequential pyrolysis and catalytic CVD techniques [74]. At temperatures between 500°C and 800°C, there were only small variations in the performance of CNTs based on the plastic source. ORR onset potentials range from −0.18 to −0.21 V for electrode surfaces modified with CNTs synthesized at 800°C. However, electrode surfaces modified with CNTs synthesized at 500°C showed ORR potentials ranging from −0.11 to −0.14 V. The lowest recorded onset ORR potential of CNTs synthesized from MP at 500°C was −0.11 V.

Veksha et al. synthesized MWCNTs by catalytic CVD using 11.8 and 27.5 wt% PET waste plastics for ORR in fuel cells [455]. MWCNTs from PET-12 and PET-28 had heterogeneous electron transfer concentrations of 0.0042 cm s⁻¹, like those observed in Pt/C and commercial MWCNTs (CCNTs), 0.0043 and 0.0040 cm s⁻¹, respectively. GC and Pt/C had onset potentials of −0.19 and −0.096 V, respectively, whereas CCNTs, MWCNTs from PET-28, and MWCNTs from PET-12 exhibited onset potentials of −0.091, −0.057, and −0.033 V, respectively. These overpotentials are smaller than those recorded for graphene-based materials, like pure graphene (−0.18 V), NDG (−0.16 V), GO (−0.18 V), and electrochemically fabricated rGO (−0.11 V). The lower onset potentials of commercial and synthesized MWCNTs indicated their high electrocatalytic activity and confirmed that these materials would substitute noble metal electrocatalysts and graphene-based materials for ORR applications. Interestingly, compared with CCNTs, MWCNTs from PET-28 and PET-12 displayed higher current densities (0.037 and 0.060 mA cm⁻²) at peak potentials, suggesting a higher volume of decreased oxygen molecules per unit of time. Table 7 summarizes common waste recycled carbon materials used for fuel cell applications.

4.1.2 Metals and metal oxide-based nanomaterials

Passaponti et al. recycled chars (the least valuable waste material from waste-tire pyrolysis) to prepare extremely effective ORR catalysts applied in AFCs and metal-air batteries [464]. The chars were derived from MAP and showed −90 mV (RHE) ORR onset potential, and a 4e⁻ mechanism was observed. The obtained activity was attributed to ZnO NPs in the carbon matrix, its porosity, and its large surface area.

4.1.3 Hybrid nanomaterials

Via pyrolysis of a mixture of a naturally abundant source of seaweed biomass (Sargassum tenerrimum) and deep eutectic solvent (a mixture of choline chloride and FeCl₃), Mondal et al. synthesized Fe₃O₄/Fe-doped magnetic GNs that possessed both high electrical conductivity of 2384.6 mS m⁻¹ and a 220 m²g⁻¹ surface area [465]. Compared with catalysts commonly used in proton exchange fuel cells in alkaline media, the prepared GNs showed higher ORR performance. High cathodic current density, positive onset potential, and less than 5% hydrogen peroxide formation were observed. Additionally, more than 80% activity retention after 30,000 cycles was obtained, making Sargassum tenerrimum-based GNs sustainable substitutes for currently available metal-based ORR catalysts.

Elessawy et al. prepared a nonprecious and highly active fullerene iron oxide composite as a promising ORR and oxygen evolution reaction (OER) catalyst in AFCs via thermal catalytic dissociation of PET bottle waste [466]. The prepared catalyst on glass carbon (fullerene/GC) (with n ≈ 4) possessed a high onset potential of −0.1 V and an E _1/2 of −0.50 V as compared with commercial Pt/C catalyst (onset potential and E _1/2 of −0.005 and −0.15 V, respectively). Moreover, over 6 h at −0.45 V, the fullerene/GC electrode showed high stability in alkaline electrolyte, with just 13% decay compared with 52% for a commercial Pt/C electrode. Furthermore, at 0.95 V, an onset potential of 0.61 V and a current density of 5 mA cm⁻² were observed, comparable with a Pt/C electrode. For the OER, the fullerene/GC also displayed high stability. At 0.7 V and 21,600 s, the current density of the fullerene/GC electrode decreased by 18.7% compared with 85.6% for the commercial Pt/C catalyst. These findings indicated that the synthesized composite is an efficient bifunctional electrocatalyst with ORR and OER potential. This can be attributed to its 3D architecture, which provides a wide surface area with small pore size, allowing enhanced electrolyte and oxygen molecule transport during electrochemical reactions. Ma et al. prepared graphitic carbon-based nanosilver/iron oxide (Ag NPs/Fe₃O₄/GC) composite from pomelo skin waste, which was used in MFCs as a catalyst for antibacterial ORR [467]. The prepared composite exhibited a huge power density of 1,712 ± 35 mW m⁻², which decreased by 4.12% after 17 cycles. ORR reactions followed the 4e⁻ pathway, with no toxic impact on anode microorganisms due to inhibiting cathodic biofilm overgrowth.

4.2.1 CNMs

Graphene has received considerable attention due to its versatile applications. Pandey et al. upcycled waste plastics (PP, PE, and PET) into valuable GNs for DSSCs. Bentonite nanoclay was used as a degradation agent for waste plastics via pyrolysis processes in a nitrogen (N₂) atmosphere [92]. Recorded results revealed that DSSCs applying GNs part of the photoanode with polymeric electrolyte exhibited a promising FF of approximately 86.4% and an open-circuit voltage (V _OC) of 0.77 V, as shown in Figure 44. Interestingly, a short circuit current density (J _SC) of 0.302 mA cm⁻² was obtained.

Figure 44

(a) Schematic of a designed DSSC with corresponding (b) J–V curves, (c) EIS curves, and (d) equivalent circuits. Copied with permission from ref. [92]; Nature, 2021.

Wu et al. synthesized green and affordable electrochemically active graphene-encapsulated cobalt NPs embedded in an N, P, and S codoped carbon 3D matrix through direct pyrolysis of kelp biomass [472]. The prepared composite was labeled as Co-KCM-1000 and demonstrated promising activity as a counter electrode (CE) for DSSCs because of its excellent electrocatalytic performance in a triiodide reduction reaction. The exceptional structure of multilayer-graphene-encapsulated cobalt NPs is valuable for the increase in the contact area and the prohibition of dissolution and accumulation of metal NPs. The prepared Co-KCM-1000 and Pt as CEs in DSSCs were compared. The photovoltaic parameters, including open-circuit V _OC, J _SC, FF, and power conversion efficiency (PCE), are recorded in Table 8. The DSSCs assembled by Co-KCM-1000-CE showed a PCE of 7.48%, and the Pt-CE showed 7.64%.

DSSCs are important energy conversion devices as they convert solar energy into electricity. Nagaraju et al. prepared biomass-based 3D-activated porous carbon from fallen pine coneflowers that possessed meso/macropores and led to DSSCs with excellent solar energy conversion characteristics [39]. Their study prepared honeycomb-like activated porous carbon (HPC) via pyrolysis and chemical activation under an inert gas atmosphere. A smooth brush was used to uniformly coat the formed HPC material onto fluorine-doped tin oxide (SnO₂) glass for cost-effective DSSCs. Under AM 1.5 G illumination, a high J _SC of approximately 13.51 mA cm⁻² and a PCE of 4.98% were observed, as shown in Figure 45. The observed activity was ascribed to the prepared HPC sample’s hierarchical porous structure and high specific surface area. Interestingly, the recorded data were better than commercially available AC-based CEs applied in DSSCs (J _SC = 12.11 mA cm⁻² and PCE = 4.45%).

Figure 45

(a) Schematic of a DSSC applying HPC as the CE with corresponding (b) J–V curves and (c) incident photon-to-current efficiency (IPCE) spectra (with a TiO₂ NP photoanode and reference Pt). Copied with permission from ref. [39]; Elsevier, 2017.

GQDs are carbon NMs with fascinating optical properties suitable for solar cell applications. Teymourinia et al. reported the synthesis of biomass-derived GQDs as downconversion materials for DSSCs using corn powder via the hydrothermal method [473]. The GQDs converted UV light into DSSC-favored 450- and 520-nm light. Their results showed a 21% J _SC enhancement in solar cells modified with GQDs concerning reference cells. The recorded enhancement was due to the downconversion effect of GQDs, as shown in Figure 46.

Figure 46

Schematic of the possible mechanism of DSSC enhancement by GQDs. Copied with permission from ref. [473]; Elsevier, 2018.

4.2.2 Metals and metal oxide-based nanomaterials

Ecofriendly and low-cost green synthesized CuO NPs were prepared using Calotropis gigantea plant leaf extract and were applied as the CE in DSSCs [474]. The prepared sample exhibited crystalline nature and good structural, electrochemical, and photovoltaic properties. The results revealed a relatively high solar to electrical energy conversion efficacy of ∼3.4%, a J _SC of ∼8.13 mA, a V _OC of ∼0.676 V, and an FF of approximately 0.62 via CE-based CuO NPs in the fabricated DSSC.

ZnO nanostructures possess promising efficiency for photovoltaics. Ansari et al. reported preparing 3D ZnO hierarchical superstructures (HSs) derived from biomass for enhanced photovoltaic performance [475]. They used a facile one-step hydrothermal method for assembling compacted ZnO NRs to build a ZnO HS through the assistance of anionic polygalacturonic acid as a crystal growth modifier. Their results showed that 3D ZnO HSs exhibited an efficiency (η) of approximately 5.37% compared with only 3.48% for ZnO NRs. This substantial enhancement in ZnO HSs was attributed to good charge separation and collection because of the enhanced electron transfer capability of 3D-compacted ZnO HSs, as shown in Figure 47. Additionally, improved light scattering, higher BET surface area for sensitizer loading, and efficient electron injection from the applied dye (D1) were observed.

Figure 47

(a) J–V for ZnO heterostructure-based photovoltaic devices and corresponding (b) IPCE, (c) Nyquist, and (d) Bode phase plots. Copied with permission from ref. [475]; Elsevier, 2018.

Zhu et al. improved efficiency, reduced cost, and made sustainable and renewable PSCs by recycling FTO/TiO₂ substrates from degraded CsPbIBr₂ PSCs [476]. They found that some residual CsPbIBr₂ decreased the conduction band minimum difference of recycled FTO/TiO₂ from 0.51 to 0.36 eV at the CsPbIBr₂/TiO₂ interface. Work characteristics of the TiO₂ layer decreased from 4.13 to 3.89 eV, whereas defects decreased, and halide phase separation was suppressed in the upper CsPbIBr₂ film. These characteristics help decrease nonradiative recombination in the subsequent PSCs and expand charge transporter extraction. Accordingly, the average efficiency of the CsPbIBr₂ increased, with PSC efficacy enhanced by 20%, from 6.51% ± 0.62% to 8.14% ± 0.63%, with an excellent PSC producing an impressive PCE of 9.12%, as shown in Figure 48. Consequently, recycling FTO/TiO₂ substrates was an eco-friendly method to reduce cost and improve the PSC efficiency.

Figure 48

CsPbIBr₂ PSC performance as synthesized from recycled and pristine FTO/TiO₂. Copied with permission from ref. [476]; American Chemical Society, 2020.

4.2.3 Hybrid nanomaterials

Ahmed et al. reported the green synthesis of GQDs from corn power by a simple eco-friendly method for application in PSCs [477]. They prepared a ZnO/GQDs hybrid photoelectrode to employ as the electron transport layer. Because of the oxygen agent passivation of GQDs on the ZnO surface, electrons were quickly extracted from the PVK film, and electron-hole pair recombination was elongated. Their recorded results indicated that GQDs improved the light-harvesting ability of PVK by 2% and increased its PCE from 10 to 17.67% under AM 1.5 G illumination.

Arshad et al. proposed an efficient strategy for advanced photovoltaics by embedding a mixture of cobalt and manganese oxides in biomass-derived porous carbon (Co/MnO₂–PC) prepared from aloe vera peel [478]. They proposed a unique 3D design to improve the active catalytic sites and increase surface areas, allowing multiple and fast electron transfer channels. The proposed hybrid was applied as an accelerant to reduce triiodide in DSSCs, as shown in Figure 49. Promising cell efficiency and attractive electrochemical stability were observed. Compared with a platinum electrode with an efficiency of 6.44%, higher photovoltaic efficiency of 7.01% was recorded in the hybrid. Additionally, the prepared hybrid showed long-term electrochemical stability in I₃ ⁻/I⁻ electrolyte; it exhibited 97% retention of its original efficiency after going through many cyclic voltammetry scans. Table 9 lists the recycled nanomaterials used for solar cell applications.

Figure 49

J–V curves and PCE values of the prepared Co/MnO₂–PC hybrid; inset illustrates the concept of multiple electron diffusion routes for I₃ ⁻ reduction. Copied with permission from ref. [478]; Elsevier, 2020.