Recent advances on hydrogen generation based on inorganic metal oxide nano-catalyst using water electrolysis approach

4.2.1 Noble-metal based electrocatalysts

PGMs, which include Pt, Pd, Ru, Ir, and Rh, are examples of noble metals that exhibit exceptional HER catalytic activity. In Figure 3, Pt is positioned near the peak of the volcanic curve. However, the limited storage and high price of these noble metal based catalysts prevent their practical utilization. A thoughtful design and shape of catalysts with low metal loading, good metal utilization is required to meet this problem. When Pt is alloyed with transition metals, it can be used much more effectively. The synergistic effects of these alloys can change the electrical environment, which in turn increases the electroactivity of HERs. At a current density of 10 mA cm⁻², Sun et al. demonstrated excellent HER activity with a modest over-potential of 37 mV in 0.1 M KOH, using carbon cloth and an ultralow loading Pt concentration of 7.7 % to create an in-situ array of ultrafine Pt–Ni nanoparticles decorated with Ni nanosheets.

The results of the long-term durability test up to 90 h of catalytic activity are quite remarkable. It is reasonable to assume that the improved HER performance of PtNi–Ni NA/CC is related to the downshift relative to Pt’s d-band center, which lowers the adsorption energy of oxygenated species such (OH) on the surface Pt atom. ^{4 , 45} As a general rule, platinum electrocatalysts have decreased HER activity in acidic media compared to alkaline ones. Hydrolysis on the surface of Pt is inefficient, so the HER activity is modest. One common method for increasing the alkaline HER activity is to combine Pt with water dissociation promoters. ⁴⁰ Improving the electrocatalytic activity of different platinum-based electrocatalysts for HER requires the capacity to modify the surface metal composition. ^{46 , 47 , 48}

Figure 10:

Mechanism of electrocatalysis in HER. (A) Depiction of the nanoscales mechanism of the HER process Ni(OH)₂ on the surface of a Pt electrode, (B) annealing PtNi/C to PtNi/–O/C using air with HER mass activities 0.07 versus HER causes illustrations. Copyright from Advance Material (2019).

Markovic et al. ⁴⁹ recently shown an eight-fold increase in HER activity compared to Pt of the highest quality by controlling the formation of nanometer-scale Ni(OH)₂ clusters on Pt electrode surfaces. Figure 4A shows the HER mechanism. The Ni(OH)₂ cluster’s periphery on Pt promotes water dissociation, leading to the formation of M–H_ad intermediates.

H₂ is produced by the reaction of the adsorbate hydrogen intermediates. Drawing inspiration from the synergy between Ni(OH)₂ and Pt(111), Huang et al. developed the concept of surface-engineered PtNi–O nanoparticles with an enhanced NiO/PtNi interface. In conditions with a high pH, this surface makeup changes to Ni(OH)₂, resulting in an interface that resembles Ni(OH)₂/Pt(111). With just 5.1 gpt cm² of Pt added, the catalyst displayed a small HER overpotential of 39.8 mV at 10 mA cm². The structural change from PtNi/C to Pt–Ni–O/C is seen in Figure 4(a) throughout the annealing process. Pt–Ni–O/C exhibits the greatest mass activity at an overpotential of 70 mV against the reversible hydrogen electrode (RHE), surpassing both Pt–Ni (5.35 mA/gpt) and commercial Pt/C (0.92 mA/gpt), as shown in Figure 4(b). The graphic shows that the Tafel-slope. Another efficient method to increase the HER catalytic capacity with less Pt is to dope platinum-based materials with other metal components. Wang et al. ⁴⁰ created a catalyst with Pt–Ni nanowires that were modified with N. An extraordinarily low overpotential of 13 mV at 10 mA cm² in acidic conditions was achieved as a consequence of nitrogen-induced orbital alteration, which improved hydrolysis kinetics. Pt–Ni, Pt–Ni(N), Pt–Ni/Ni4N, and retail Pt/C linear remove voltamogram (LSV) curves (a) are shown in Figure 5(a)–(d) to illustrate the HER catalytic evidence of PtNi(N) NWs in an alkaline medium. Pt–Ni(N) NWs exhibit the most favorable overpotential, measuring 13 mV, when the current density is 10 mA cm² s can be used to investigate the rate-determining phases of the alkaline HER process. Data on the intrinsic activity of the catalysts are also uncovered by studying TOF data, as shown in Figure 11(c).

Figure 11:

Design of noble-metal-based electrocatalyst. (a) Synthetic route of Ru-based catalysts with Ru–N–S coordination. (b) HAADF-STEM image of Ru–N–S–Ti₃C₂T _x . (c) HER curves of Ru-based catalysts. Copyright 2019 Wiley-VCH. (d) Synthetic route of Pt–Ti₃C₂T _x with O vacancy. (e) EPR of catalysts. (f) HER activity comparison of catalysts. Reprinted from reference. ⁴⁹ Copyright 2022 American Chemical Society. (g) Synthetic route of Pt@CoOx. (h) Intrinsic activity comparison of Pt-based catalysts. ⁵⁰ Copyright 2022 Wiley-VCH GmbH.

A greater TOF is achieved by Pt–Ni(N) compared to Pt/C and Pt–Ni alone. Lastly, the chrono-potentiometric durability test of Pt–Ni(N) shows that after 10 h at a current density of 40 mA cm², no noticeable change in potential is seen. Through the use of density function theory (DFT) calculations, the modulation essence of nitrogen in the Pt–Ni nanowire was examined (Figure 12(e)–(g)). Nitrogen may cause a decrease in electron density surrounding the Ni site as a consequence of strong interaction between the two elements, as shown by analysis of the surface electron density difference in (Figure 12(e)). The dz² empty orbital with high N–Ni connection and has ideal orientation for the dissociation of water is confirmed by the orbital analysis.

Figure 12:

Electrochemical analysis of HER. (a) Shows the results of infrared correction applied to the HER presentation, with LSV curves plotted against different catalysts in 1.0 M KOH. The picture (b) shows Tafel plots, and curves c and d represent total organic solvents and Pt–Ni(N) NWs, respectively. Results from the insets e-DFT calculation: By comparing the electron densities of Pt–Ni and Pt–Ni(N) slices observed from the top and side views of the orbital above the fermi level, we were able to calculate the difference in electron densities. ⁴⁰ Copyright from Advance Material (2019).

4.2.2 Platinum group metals-based electrocatalysts

It is commonly known that, according to the Sabatier principle, the binding energy between the intermediate Had and the catalyst establishes the rate-determining stage of the reaction process. ^{51 , 52} Reaction speed decreases due to excessively strong Had binding that restricts H₂ release. Conversely, had is more difficult to adsorb due to its lower binding energy. Consequently, the electrocatalytic reaction process benefits from the mild adsorption and desorption energy (Figure 13). ^{53 , 54 , 55}

Figure 13:

Metal based electrocatalysts. (a) Synthesis routes of N-NiCoP _x . (b) SEM of N–NiCoP _x . (c) Selected area electron diffraction (SAED) of N–NiCoP _x . (d) EDS mapping of N–NiCoP _x . (e) HER curves of catalysts. (f) Comparison of N–NiCoP _x with previous reported catalysts. ⁵⁶ Copyright 2020 Elsevier B.V. (g) Synthesis of Co(OH)₂@PANI. (h) SEM of Co(OH)₂@PANI. (i) HER curves of Co(OH)₂@PANI. ⁵⁷ Copyright 2015 Wiley-VCH.

The electronic structure of the active centre can be efficiently controlled by transition metal catalysts that are doped with non-metal atoms, such as N, P, S, or Cl.67–69 N-doped Ni–Co phosphide catalyst was produced by using hydrothermal and plasma treatment techniques (Figure 14(a)). The shape of the NiCo hydroxide nanosheet is visible in the SEM image (Figure 14(b)). Large surface areas and an abundance of catalytic active sites are features of this type of morphology. Energy-dispersive spectroscopy (EDS) has been used to characterise the component (Figure 6(d)). Additionally, the HER curves demonstrate that NiCoP (111) activity (Figure 6(c)) is comparable to commercial Pt/C and earlier studies (Figure 14(e) and (f)). Consequently, it makes sense to populate an electrocatalyst that is efficient, clean, and beneficial to the environment. Additionally, in order to address the issue of high stability, composite catalysts have been produced because (Figure 14(h) and (i)).

Figure 14:

Shows the solution of Mo₂C–R in 0.5 M H₂SO₄, we find the following: (a) The process of creating molybdenum carbide (Mo₂C) nano-particles that attach to graphene nano-ribbons (GNRs); and (b–d) the effects of acidic, basic, and neutral conditions on the activity and longevity of Mo₂C-GNR. Copyright from Appl. Catal. (2014), and Chem. Eng. (2016).

4.2.3 Electrocatalysts based on non-noble metals

TMCs are transition metal-carbides. The development of electrocatalysts based on non-noble metals has generated significant interest in TMCs. For instance, it has been demonstrated that molybdenum carbide micro particles (Mo₂C) has strong catalytic activity towards HER. Their hydrogen adsorption properties and d-band electronic density state that similar to that of Platinum also demonstrate an ideal grouping, which is thought the primary cause of the high HER activity that has been found. In addition to their elevated electrical conductivity. In 1973, Levy and Boudart made the initial discovery that tungsten carbide had platinum-like catalytic behavior because it has d-band electronic thickness states that were comparable to those of Pt specialties. ^{58 , 59} Additionally, Chen et al. used DFT calculations to examine a series of transition metals’ physical, chemical, and electronic structure features. According to their findings, adding carbon atoms to the lattice interstitials gives them d-band electronic density states that are comparable to the Pt benchmark. Experimental data first confirmed the theoretical prediction in 2012. In acidic as well as alkaline conditions, commercially available molybdenum carbide micro particles (com–Mo₂C) have well HER catalytic activity, according to Hu et al. To achieve a cathodic current of 10 mA cm², there is a relatively high overpotential (190–230 mV), though. Researchers followed various strategies for the Mo₂C catalyst optimization by nano-engineering the materials to expose additional active areas in response to the groundbreaking investigations. By easily carburizing anilinium-molybdate effectively synthesized Mo₂C nanorods having a permeable structure.

Transmission electron microscopy (TEM) and field emission scanning electron microscope (FE-SEM) pictures of nanorods morphological features having a even surface and a spongy structure are shown in Figure 14A(a) and (b) of this article. Mo₂C nanorods catalyst has improved HER electrocatalytic performance, as demonstrated. As can be seen from Figure 14A(c), the Mo₂C nanorods outperform the commercial Mo₂C in terms of activity, according to the results of the linear scan voltammetry (LSV) in 0.5 M H₂SO₄. The stability test results for the Mo₂C are also shown in Figure 14A(d). Based on the results, it appears that the activity remains constant even after 2,000 cycles, suggesting a longer cycling life. The Mo₂C nano-rod outperforms commercial Mo₂C in 1 M KOH, according to additional studies conducted in acidic environments.

Due to their high conductivity and clearly defined, porous architecture, the Mo₂C nanorods exhibit very competitive presentation for HER in the acidic as well as alkaline media. Ni nanoparticles addition could enhance the catalytic activity even more. Another technique to enhance HER’s efficiency as a hybrid nano-electrocatalyst is the accumulation of molybdenum carbides (Mo₂C) on the surface of materials having carbon based properties.

Molybdenum carbide (Mo₂C) materials nano-particles attached on the graphene nano-ribbons (GNRs) were produced by hydrothermally forming carbides on a GNR template in situ, followed by high temperature calcinations process, as illustrated in Figure 14B(a). In acidic and basic, as well as neutral environments, the Mo₂C-GNR hybrid demonstrates exceptional electrocatalytic action and durability (Figure 14B(b)–(d)). Phosphide of transitional metals one of the quickly expanding fields in the improvement of electrocatalysts with high catalytic activity and stability in both acid as well as basic (pH universal) environments is the study of transition metal phosphides (TMP). It has been suggested that the P atom’s strong conductivity and distinctive electrical structure contribute significantly to TMP. Ni₂P catalysts were first identified as one of the finest real-world HER catalysts in 2005. A number of electrocatalysts were investigated by Liu and Rodriguez using density functional theory (DFT). ⁶⁰

The hydrogen species would require more energy to desorbs from the metal surface due to the tight connection. Because P atoms dilute the amount of highly active Ni sites, a reasonable bonding to intermediates and products is achieved, or what is called the “ensemble effect.” The first direct experimental evidence in favor of the catalytic synergy was shown, who demonstrated that a Ti foil substrate bearing a Ni₂P nanoparticle exhibited outstanding HER activity, with an exchange density of 3.3 × 10–5 mA cm² and a Tafel slope of 46 m V dec1. ³⁶ Having said that, the stability of the Ni₂P/Ti electrode was lacking, especially in an electrolyte with a high pH. Hu et al.’s follow-up research involved the creation of NiCo₂P _x , as a bimetal ordered phosphide electrocatalyst. It has been shown that this specific catalyst performs well as a pH-universal catalyst for HER and shows great durability and long-term stability in various electrolytes.

On commercial carbon felt (CF), the self support NiCo₂P _x nanowire arrangements were created using a wet chemical-hydrothermal process and an in-place phosphorization reaction. Figure 15(a) demonstrates NiCo₂P _x ’s superior HER performance in neutral, alkaline, and acidic environments. NiCo₂P _x , as opposed to CoP _x , NiP _x , and commercial Pt, had the lowest overpotential in the alkaline electrolyte at 58 mV at a current density of 10 mA cm² Figure 15(b) illustrates the shape of NiCo₂P _x following in term of HER activity test and indicates that, even after 5,000 cycles under various circumstances, the catalyst structure of NiCo2P _x is still well-preserved. According to Figure 15(c), NiCo₂P _x in an alkaline electrolyte is thought to have a synergistic impact. ^{2 , 61 , 62}

Figure 15:

Shows the NiCo₂P _X catalyst in action: (a) The catalytic reaction (HER) in 1 M KOH, PBS, and 0.5 M H₂SO₄; (b) scanning electron micrographs taken before and after the HER long-term stability test in various media; and (c) a simplified representation of the water dissociation process on NiCo₂P _X in an alkaline solution, as described in reference. ⁶³ Copyright from Advance Material (2017).

There are two significant interactions: the one between atom (P) and the H atom, and the other between the under coordinated metal centre (M⁺, M = Ni, Co) as well as the oxygen atom. The H–OH bond is weakened by the combination of these two interactions, which causes the water molecule to split into hydrogen atom and an OH molecule. The (H) atom is subsequently moved to a close by open metal site, where it anchors as adsorbed H. Molecular H₂ is created by the combination of the adsorbed H atoms. Chalcogenides of transition metals (selenides and sulphides) in 2005, ⁶² et al. [54] grown triangular MoS₂ single crystals on the active site of the MoS₂ structure ⁵⁴ demonstrated that the free energy of the atomic hydrogen bonding to the MoS₂ edging is similar to Pt. This finding suggests that MoS₂ could be used as an electrocatalyst for HER.

They proved that the amount of corner points on the MoS₂ catalyst linearly affects the electrocatalytic HER activity. MoS₂ has extremely active catalytic sites at its edges. This understanding has led to the development of numerous methods, such as nanostructure tuning and morphology tuning, to expose the active areas and increase HER activity. An approach to design flaws in MoS₂ ultra thin nano-sheets, for instance, was disclosed by Xie et al. and was demonstrated to significantly enhance MoS₂’s electrocatalytic HER performance. This high activity was attributable to the high fault structure’s additional active edge sites, which were created by partially breaking the basal plane, which is catalytically inert. CoS₂ with tunable film, micro-wire (MW), and nano-wire (NW) shape was successfully synthesized ^{60 , 64} (Figure 16A(b)–(d)).

Figure 16:

A shows polarization curves for the CoS₂ film, MW array, and NW array electrode for HER. Then, from b to d, there are scanning electron micrographs of the film, array, and graphite. Finally, from e to f, there are schematic representations of the surface of the film, NWs, and MWs, as well as the produced H₂(g) bubbles. This figure was reprinted with permission from reference. ^{65 , 66 , 67 , 68} The XRD pattern of the products obtained at different temperatures and the HER performance of the catalysts with varied oxygen-incorporated MoS₂ ultrathin nanosheets are shown in graph B. (c, d) two-dimensional structural models of 2H–MoS₂ and oxygen-incorporated MoS₂ with increased interlayer gap. (e, f) visual depiction of the disorganized structure in ultrathin MoS₂ nanosheets with oxygen incorporation. The purple shadings show the enrichment impact of active sites caused by the disordered structure, while the blue lines show the fast electron transport between the nano-domains that are quasi-periodically aligned. In reference. ⁶⁹ Copyright from J. Am. Chem. Soc. (2013).

They established the shape dependent improvement of both activity as well as stability by methodically studying their structures, activities, and stabilities. Due to their large efficient electrode surface area and quick release of gas bubbles from the electrode surface (Figure 16A(e)), CoS₂ NWs exhibit the best HER catalytic performance, stability among the three distinct morphologies (Figure 16A(a)). In order to maximize the HER electrocatalytic activity, significant work has been put towards tailoring metal chalcogenides’ electronic conductivity in addition to their active sites. The use of heteroatom doping to increase HER activity is efficient. According to, ⁴⁶ adding oxygen atoms and engineering adjustable disorder can successfully manage the electrical structure of MoS₂ ultrathin nano-sheets, increasing conductivity.

A variety of XRD patterns for catalysts grown at various temperatures are displayed in Figure 8B(a), revealing that MoS₂ ultrathin nano-sheets made at 220 °C have numerous flaws. A novel structure with an enlarged interlayer spacing of 9.5 instead of 6.15 in ideal 2H–MoS₂ is suggested by the appearance of two new peaks at the low-diffraction angle area when the synthesis temperature decreases to 200 °C or lower. In Figure 8B(c) and (d), the suggested structure models are displayed. The MoS₂ structures from the various temperatures can be controlled to have varying degrees of disorder, as seen by HRTEM pictures and associated FFT patterns. The ultrathin MoS₂ nanosheet with oxygen incorporates was subjected to electrochemical tests with varying degrees of disorder.

5.3.1 Electrocatalysts based on nobel metals

The mainly effective electrode resources in OER have long been thought to be noble-metal and M–O electrocatalysts. Examples include the cutting-edge electrocatalysts for OER, RuO₂ and IrO₂, which are frequently cited as examples. However, the significant dissolution of RuO₂ and IrO₂ and their expensive cost are the main issues, which focus a lot of attention on the change of the catalysts’ chemical composition and configuration as well as shape. Numerous methods have suggested increasing the activity and constancy of electrocatalysts as well as lower their high cost. There is a lot of interest in heteroatom doping as a means of modification the chemical composition of IrO₂ base OER electrocatalysts. However, host systems diverse guest atoms produced unique energy domains. By thermally decomposing derivatives of Ru exchanged metal organic framework (MOF) materials, Chen created Cu-doped RuO₂ with empty porous polyhedral shape made up of extremely small nanocrystals. ^{65 , 78 , 85 , 86} Figure 10(a) shows the high index surface facet of the Cu-doped RuO₂ phase, which was created by incorporating Cu into the RuO₂ lattice. This information was collected by high resolution transmission electron microscopy (TEM) and XRD statistics. Figure 10(b) shows that the synthesis of OOH on RuO₂(110) is caused by the RDS. Cu-dopant RuO₂ can enhance OER activity by changing the electronic arrangement, which results in a wide binding area closer to the Fermi level of the p-band center. Additionally, Cu dopant generates O positions on the surface and induces the formation of unsaturated Ru sites. An innovative method to create OER catalysts that can successfully change the electrical structure and optimize the chemical reaction (Figure 18). ^{2 , 26 , 87 , 88 , 89}

Figure 18:

Electrocatalysis of nobel metals. (a) Shows a high-resolution transmission electron microscopy (HR-TEM) image of Cu-doped RuO₂ showing raised surface features, and Figure 10(b) shows the free energy profile of optical emission spectra (OER) on surfaces (110) and (111) according to reference. ⁹⁰ Copyright from Advance Materials (2018).

5.3.2 Self-supported bi-functional electrocatalysts for overall water splitting

In order to reduce the overpotential during the actual electrolysis of water, the HER and OER catalysts are used in the same electrolyte, which is a strongly acidic or basic solution. The majority of HER catalysts that are based on nonprecious metals, such as carbides, nitrides, phosphides, and chalcogens, exhibit their best catalytic activity in acidic media. Conversely, OER catalysts, like transition metal oxides and oxy/hydroxides, are only effective in alkaline electrolytes. Finding two electrocatalysts that couple well in a single electrolyzers for overall water splitting (OWS) is made challenging by the mismatching of operating conditions. ^{66 , 80 , 91 , 92} It is preferred to develop a bifunctional electrocatalyst that can efficiently catalyze both HER and OER at the same time in order to reduce device costs, simplify the design of the electrolyzer, and prevent the negative crossover effect of different electrocatalysts at two electrodes. Thus, great efforts have been made to create high-performance bifunctional catalysts for the electrolysis of water, primarily concentrating on establishing the bifunctionality of the HER and OER catalysts that are now under development (Figure 19).

Figure 19:

Water electrolysis using a self-supported electrode as a bifunctional catalyst for HER/OER. the following are reproduced with permission: (a) SEM pictures; (b) Electrocatalytic water splitting performance; and (c) Schematic electrolyzer application of nanoporous copyright from Royal Society of Chemistry, Copyright 2016. (d) SEM pictures, (e) Water electrolysis results, and (f) Nitrogen-doped tungsten carbide nanoarrays applied to an electrolyzer. (d–f) Used with authorization. ⁹³ (2018) All rights reserved.

By using a eutectic directed self-templating approach, ⁹⁴ created a group of Ir–M (M = Ni, Co & Fe) type of catalysts with a distinctive system topology made up of interconnecting nonporous nanowires. The findings reveal a property that depends on the transition metal. Ir–Ni display the OER activity in comparison to Ir–Fe and Ir–Co NWs and have the minimum overpotential at 274 mV at 09 mA cm². The DFT calculation results (Figure 11) provided an explanation for the increased activity of IrNi NW. Ir as well as M are oxidized at maximum potential to create Ir–MOx during OER process. IrO₂, IrFeO _x , IrCoO _x , and IrNiO _x had d-band centers of 3.61, 3.72, 4.09, and 4.34 eV, respectively (Figure 20(a)). The density of state (DOS) has changed in a negative way. The d-band electron distribution gone out the Fermi scale is induced by the ligands that effect after alloying, according to the down-shift of Ir d-band centre.

Figure 20:

IrNiCu DNF structures. (A) Illustration of the HAADF-STEM, transmission electron microscope (TEM) as well as HRTEM, and elemental map of preferred IrNiCu DNF structure. ⁹⁵ Copyright ACS Nano (2017) (B) (a, b) transmission electron microscope (TEM) imagery of IrO₂ nano-particles along with IrO₂ nano-needles, (c, d) OER presentation before plus after 2 h galvano-static operation, (c) depiction of LSV curves of IrO₂ NN and amorphous nano-particles, (d) Tafel slope ⁹⁶ Copyright from Advance. Funct. Material (2018).

In order toward expose catalytically active areas and benefit from the interfacial effect, surface structure alterations are crucial. One component of the surface structure change is morphology regulation, which has received. There has been a negative change in the density-of-state (DOS). The down-shift for Ir’s d-band centre indicates that the ligand effect after alloying induces the d-band electron distribution further from the Fermi level. According to Figure 11(b)–(d), the binding powers of various species like O, OH, plus OOH having a prominent impact on OER activity. Surface chemical composition modifications are essential to expose catalytically active regions and gain from the interfacial effect. A single electro-catalyst could be created using straightforward one-step production, and the change in surface structure is.

According to findings from following instruments HAADF-STEM, TEMas well as HRTEM, and from elemental mapping, the intended Ir–Ni–Cu DNF arrangement (ternary alloy) retains a rhombic dodecahedral morphology following intense acidic etching and has a consistent factor distribution throughout, whole DNF structure. Ir–Ni–Cu catalysts for oxygen evolution reactions have exceptional electrocatalytic activity and durability because its frame structure prevents particles from becoming agglomerated and coarse and allows for the in situ creation of a durable rutile IrO₂ phase during the OER process. Electrical conductance of catalyst is also demonstrated to be impacted by morphology management, which enhances OER electrocatalytic activity.

IrO₂ nano-needles were created by Lee et al. using a scale-able molten salt technique, and they exhibit greater OER activity and stability than IrO₂ nanoparticles Figure 12B(a) and (b). According to Figure 12B(c) and (d), which compares the electro-chemical activity of IrO₂ NPs as well as IrO₂ nano-needles for OER, the IrO₂ nano-needles have higher activity and stability than IrO₂ NPs do. The electrical conductance of ultra-thin IrO₂ nano-needles is 318.3 S cm, which is higher than the electrical conductivity of IrO₂ unshaped nanoparticles, which is 25.9 S cm. ⁹⁶ This suggests that the form of the catalyst is critical for the high activity for OER caused by electron transfer. In addition to Ir and Ru, additional Nobel metals like (Rh, Au, and Pt as well as Pd) have also been showing promising results for OER electrocatalysts. Hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and oxygen evolution reaction (OER) catalyst design entails building bi- or tri-functional electrocatalysts from Pt, Pd, Ru, or Au. It is common practice to study the OER behaviors of Rh, Pt, Au, and Pd in alkaline solutions since their dissolving resistances are lower than Ir’s and Ru’s in acidic electrolytes with a high over-potential. To achieve the appropriate electrocatalytic capabilities, the catalysts’ morphology and composition must be under control.

To achieve the appropriate electrocatalytic capabilities, the catalysts’ morphology and composition must be under control. Structure of Au–Co nano-particles as OER catalysts in basic conditions was demonstrated by. ⁹⁷ The Au–Co nanoparticles have a core shell shape and a homogeneous size distribution.

5.3.3 Electrocatalysts based on non-noble metals

OER electrocatalysts without noble metals have generated a lot of research attention due to their affordability and availability. The search for effective noble-metal free OER electrocatalysts has been become more intense. In the previous few decades, significant progress has been made in exhibiting excellent catalytic activity that is similar to the catalyst constructed of noble metals. For the purpose of logically designing highly effective OER electrocatalysts, we will highlight some recent approaches to produce more active sites by controlling various reaction factors like morphology, chemical composition of intermediates, tuning electronic shape as well as binding energy through elemental doping method an also by inducing hybrid materials into composites. Recently, Zhang et al. ⁹⁸ used sol-gel fabrication to create a gelled version of Fe–Co–W oxyhydroxide with uniform metal allocation. The lowest overpotential was 190 mV for the Fe–Co–W oxyhydroxide at 10 mA cm² current density and 502 h cycle stability. This result shows that of the benchmark catalyst made of NiFe. The outcome demonstrates that on various substrates, G–FeCoW has a much better catalytic activity than anneal A–Fe–Co–W, gelled FeCo rather than W (G–Fe–Co), with LDH Fe–Co. The two dimensional (2D) volcano plot (Figure 14(c)) shows that the computed theoretical OER overpotential has dramatically increased catalytic activity towards OER. The local electrical and geometrical surroundings are modulated to obtain a hypothetical overpotential of just 0.4 V.

Another efficient method for controlling the structural and electrical characteristics of electrocatalysts is defect engineering. By varying the intermediate adsorption energy, the OER activity can be increased, which occasionally results in unanticipated active sites. A technique to produce enough oxygen vacancies using a plasma engraving process on a Co₃O₄ nanosheet was described by Dai et al. According to X-ray photoelectron spectroscopy (XPS), Ar plasma treatment partially reduces Co³⁺ to Co²⁺ and creates oxygen vacancies. By manipulating the Co²⁺/Co³⁺ ratio, by this technique we can change the surface area of material but also alters the electronic structure of material, resulting in good OER process catalytic action having over-potential of 154 eV at current density of 10 mA cm². For enhancing OER activity, tuning the electron transit is just as important as the fundamental change for ideal absorption energy of the intermediate materials. On nickel foams, carried out in situ formation of the 3D spongy films made of perpendicularly aligned Ni–Fe–LDH nano-plates in (Figure 21(a)).

Figure 21:

Performances of non-nobel metals in OER. (a) The following potentials were determined using density-functional theory (DFT): oxyhydroxides and W, Fe-doped Co oxyhydroxides, cobalt tungstate, and W oxides; and the surface of Au-(111) and GCE (glass carbon electrode). The measurements were taken at 10 mA cm⁻². ⁹⁶ Copyright from Science (2016).

The catalyst displays a noticeable endurance and a minor over potential of 152 mV at 31 mA cm² better by 20 wt% of Ir/C catalyst. ⁹⁶ The 3D spongy arrangement, which offers with high density of active sites of the large surface area, was said to have a synergistic impact that led to the significant electrocatalytic activity that was found. The porous features and metallic electronic conductivity of Ni foam substrate, as depicted in Figure 22A(b), are thought to make it an ideal substrate for OER because they speed up the electron transport process. The active phase was investigated using in situ Raman technology (Figure 22A(c)). The newly discovered bands at oxygen evolution potential show that NiO₂H was the active part for OER by proving the conversion of LDH into NiO₂H.

Figure 22:

Electrocatalytic activity of OER. (a) A representation of NiFe-LDH nano-plates grown on nickel foam, (b) a scanning electron micrograph of nickel fluff, and (c) an in situ Raman spectrum of Ni–Fe–LDH films with and without oxygen evolution reaction (OER) applied. ⁵⁰ The publication by Chemical Communication in 2014. Potential energy surface polarization curves of Ni(OH)₂ and Ni–Fe LDH and Ni_0.83Fe_0.17 (OH)₂ after correction for infrared light. The number 66 published in ACS Catalysis in 2018 by C (a) SEM pictures of Ni _x Fe₁–xSe₂ (b) pictures of Ni _x Fe₁–xSe₂–Do (c) evaluation of Ni _x Fe₁–xSe₂ and Ni _x Fe₁–xSe₂ in images of elemental mapping. ^{80 , 90} Copyright from the edition of Natural Communication 2016.

By using a simple and universal cation-exchange technique ⁹⁶ created Fe doped Ni(OH)₂ nano-sheets with numerous defects and a nono-porous surface structure. There is an increase in OER activity in these nano-sheets. A defect-rich Ni(0.83), Fe(0.17) nano-sheet exhibits the lowest over-potential of 246 mV at a current density of 10 mA cm² when contrasted with the conventional Ni–Fe layered double hydroxide (LDH) nanosheet. Several factors, such as a high number of flaws, an enhancement in surface wet ability, and an increase in active surface sites, contribute to the remarkable OER activity. A new approach to making highly effective OER catalysts has been unveiled by the successful utilization of the cation exchange method in the fabrication of the active Fe doped Co(OH)₂ nanosheet.

Metal oxides as well as hydroxides are amongst the non-precious metal catalysts that are most commonly used. Recently, numerous more interesting electrocatalysts, which include transition-metal phosphides, sulphides, and selenides, have shown excellent OER catalytic activity. However, these compounds exhibit unsatisfactory stability in an alkaline solution with a high oxidative potential. As a result, a lot of emphasis has been paid to understanding the chemical makeup of real active sites in the development of catalysts for OER. In order to provide an in situ creation of a high active Ni–Fe oxide catalyst, Hu and coworkers synthesized nanostructured nickel iron di-selenide (Ni _x Fe₁xSe₂). The OER activity of this catalyst was outstanding, with over-potential of just 195 mV for 10 mV cm².