Perspective on advanced nanomaterials used for energy storage and conversion

Physical properties

It is commonly agreed that the nanomaterials showed an ultra-high surface effect, ultrahigh volume effect, and quantum size effect. Many atoms on the surface are the major reason for the ultra-high surface effect. Based on the calculation, the surface atoms account for about 20 % of the total atoms within “perfect” nanomaterials, with the size decreased to 10 nm. With future size decreased to 1 nm, the surface atoms account for up to 99 % of the total number [20]. This observation suggested the existence of the dangling atoms, which show high potency to form chemical bonds with other atoms to stabilize the system energy. Our photocatalysts (rGO-TiO₂) used in water remediation displayed ultrahigh chemical reactivities due to the surface effect. We also found that the particle size deduction will decrease the bandgap due to the quasi-discrete energy of electron configurations. Our previous study indicated that the bandgap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) was tunable by varying the synthesis variables (Fig. 3). Nanoclusters of decreased dimensions impact the electronic structure through energy changes on the valence band between the HUMO and the conduction band of the LUMO [21]. Optical properties like absorption and emission transitions and line strength are connected to changes between these bands. Other changes in the size of nanoparticles will alter the physical and electronic properties, such as color and conductivity in semiconductors and metals [22].

Magnetic properties

One family of metal oxide, iron metal oxide (Fe₃O₄), showed ferromagnetic property due to unpaired electrons in the d-orbitals and their sensitivity to magnetic fields. Those materials are intrinsically magnetically ordered and develop spontaneous magnetization even though an external magnetic field is not applied. The ordering mechanism is the quantum mechanical exchange interaction. One of the main concerns in producing magnetic particles is to control their sizes and size distribution. Magnetic particles with ultrafine tend to agglomerate, which will lower their vitality because of their high surface tension. However, this vitality is directly linked with these nanoparticles’ high accessible surface region and volume degree. On the other side, these metallic or magnetic nanoparticles are synthetically dynamic; can be oxidized in the air resulting in loss of the magnetic property and dispersibility [23]. To widen the applications of these magnetic nanoparticles, the author treated these particles with natural product extracts, which behave like surfactants, polymers, and other inorganic layers (such as silica or carbon). This treatment also prevents the nanoparticles from debasement and protects them from losing attractive properties (Fig. 4). It is also an important outer layer, or the shell of these nanoparticles must be chosen based on the functions, and the applications as the shell can be made from different ligands [24]. The functionalized nanoparticles can be used in diversified fields, as mentioned above. This functionalization enhances the high scattering, high reactivity, and simple detachments. Now, these magnetic nanomaterials should be blended using different techniques which do not involve oxidation and corrosive disintegration as the security of the particles is essential [25]. The bioencapsulated Fe₃O₄ magnetic nanoparticles functionalized by natural plants were found to show high removal efficiency of As (V) and Cr(IV). These bioencapsulated MNPs (0.5–5.0 mg) will be introduced into an aliquot of 20 mL Na₂HAsO₄ and Na₂CrO₄ solutions with different concentrations. The mixtures will be under agitation using an orbital shaker for 24 h to reach an equilibrium. The adsorption will be performed under different pH values (5–9). The As(V) and Cr(VI) concentrations in the remaining solutions will be determined by an inductively coupled plasma-optical emission spectroscopy (Optima 5300DV). The adsorption isotherm will be obtained by varying the initial As(V) and Cr(VI) concentrations. The relationship between the MNPs’ removal ability and the solution concentration of the contaminants Ar(V) and Cr(VI) was improved from 0.68 to 12.6 mg g⁻¹ in comparison with the reported data (Table 1) [26].

Electronic properties

The Bohr model of the atom (H as an example) depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus. Within the atoms, nuclei provide electrostatic attraction to the electrons, while electrons repel from each other. According to the principal quantum number, the shells in a Bohr-type atom occupy three-dimensional space, corresponding spectroscopic labels K, L, M, N [27]. which was used to describe chemical stability across the periodic table in terms of half-full or complete shells. Normally, this phenomenon will be observed if the particle sizes are comparable to de Broglie’s wavelength of electrons [28]. Below a critical size length, conductors become insulators due to the separation of conduction and valence bands. When the system length scale is reduced, changes in electronic properties are related mainly to the increasing influence of the wave-like property and the scarcity of scattering centers based on quantum mechanical effects [29]. When the particle size is comparable with the de Broglie wavelength (λ = h/mν, h represents Planks constant; m the mass of the particle and ν the velocity) of the electrons, the apparent differences between the energy states can be observed, where the electrons are confined in all three dimensions [30]. The electrons can move freely between adjacent nanostructures, defined as the quantum tunneling effect, since the barriers are lower than expected under classical mechanics of larger bodies, enabling movement. The purpose of this electron configuration analysis is to understand the likelihood of co-alloying metals to lower energy levels and enable catalysis to occur at lower input energies. A common example is the incorporation of Gd, where the f-orbitals of the metal are available for bonding from oxygen with cobalt oxide, for example, in a semiconductor-based catalyst. In our example, boron is used as an electron-deficient ‘sink,’ which also has excellent stability under alkaline conditions, with vanadium oxide as the capacitor storage material. The de Broglie effect enables efficient charge separation and twinned ion channel paths for both charge species and oxygen diffusion. Under external voltage, the discrete energy levels are aligned due to the propensity of dative bond formation and charge transport to enhance charge storage. This perspective demonstrated the electron density (Fig. 5) of V₂O₅ used for supercapacitor and TiB₂ for microwave absorption.

Electrode catalyst

This section will discuss the ABO₃ and ABO₃-A₂BO₄ series of electrode catalysts to demonstrate their synthesis, structural characterization, and electrochemical performances. A facile colloidal synthesis was used to prepare La_1-xSr_xCo_1-yFe_yO_3-δ precursor with tuneable the polymeric frameworks (Fig. 6a-1), which will be subject to calcination for crystallization. When the chemical process incorporates these principles to manipulate the structure and property of the nanomaterials, the “green” outcomes are achieved in reducing the production of hazardous wastes and increasing great energy efficiency [32]. This approach has additional benefits of generating minimal waste, offering low manufacturing costs, and ease of operation. This strategy has the advantage of controlling materials preparation from the molecular/atomic level and guiding the development of nanoscale materials to provide the maximum benefit of these products for society and the environment [33]. The second method, sputter deposition (Fig. 6a–2), was used to prepare ABO₃-A₂BO₄ (La_0.6Sr_0.4CoO₃-(SrLa)CoO₄) and followed by the calcination at various temperatures. The sputtering variables were optimized and controlled: the power to initiate plasma at 90 W, power to deposit film at 20 W, Ar flow at 8 mL/min, and the partial pressure of the gaseous atmosphere at 1.0 atm. The deposition time of the ABO₃-A₂BO₄ thin films could vary according to the thickness, and it was controlled at 16 h in the current experimental set-up. The sputtered samples were then calcinated at various temperatures, ranging from 400 to 900 °C with an increment of 100 °C under the heating rate of 10 °C/min. Heat-treatment can be conducted under the oxygen (O₂) or argon (Ar) atmosphere for both family’s perovskite materials. The calcination duration was varied significantly to understand the influence of the diffusion co-efficiency of the cations [34].

Fig. 6a:

The preparation of cathode catalysts used in solid-state chemistry using colloidal wet-chemistry and sputtering deposition, followed by calcination for crystallization.

The purpose of the fabrication techniques is to give more control over the selective deposition of perovskite ceramic material, including the predefined thickness of the electrode, to promote facile ion transport at higher temperatures. The literature summarizes multiple synthetic approaches, which all have a similar aim. We applied facile non-stoichiometric syntheses to introduce A-site vacancies, which caused lattice strain and enhanced the degree of covalency between the metal and oxygen. This covalency is attributed to electron filling in the metal cation and overlapping between 3d (transition) or 4f (inner transition) to 2p orbital overlap to increase the catalytic ability of catalyst on the oxygen reduction reaction. Our approach was to couple facile colloidal synthesis with sputtering to introduce the catalyst-electrode assembly for chemical conversion to electricity at the SOFC device level of integration.

Structural evaluation

The spectroscopy, electron microscopy, and X-ray diffraction were systematically used to evaluate the crystalline phase, morphology, and elemental composition of different cathode catalysts and composites. The Ultima IV X-ray powder diffractometer, with high-temperature features, was used to determine the crystalline phase of the SOFC cathodic catalyst. The parallel beam geometry was applied for all tests under various temperatures to eliminate the systematic errors caused by the sample thermal expansion, surface displacement, and deformation. The temperature range was controlled from ambient conditions to 900 °C with the increment of 100 °C under a heating rate of 10 °C/min (Fig. 6b-1). Quantitatively, the average crystalline size was calculated according to the Scherer equation. The William-Hall equation was also used to determine the micro-stress of the cathode catalysts with the temperature changes. Both scanning and transmission electron microscopic analyses were employed to determine material characteristics at the micro- to the nanoscale. This multiscale and multi-modal analysis of morphology and defects allows for reliable and informed decisions for process control of materials preparation and design improvement.

Fig. 6b:

The X-ray powder diffraction analysis of Pt-modified perovskite cathode catalysts (LaSrCoFeO₃) used in solid oxide fuel cells under different temperature gradients.

The La_0.8Sr_0.2Co_0.8Fe_0.2O₃ (LSCF) and cathode on Sm-doped Ceria (Sm_0.2Ce_0.8O₂) were obtained after heating at 900 °C for 4 h at air atmosphere. The typical XRD pattern (Fig. 6b–2) aligns well with the standard perovskite LSCF (unit cell of tetragonal, JCPSD 89-1264). Our results are consistent with the report by Riza and his co-worker [35], whose study showed a degree of distortion from tetragonal to orthorhombic. The fine scanning at 2θ of 31.5° indicated a peak splitting due to the elongation along the z-axis. This elongation was the major reason for non-uniform micro stress, in this case, lattice distortion. Dr. Wang and his group [36] reported the phase transition of LSCF from rhombohedral to cubic, suggesting the deformation of the crystalline phase may depend on the dopant amount of Fe ion and its charge density. Our study also suggested that this lattice distortion is associated with the isostructural atomic re-ordering. The Waller group [37] observed that a quantity of strontium is not incorporated into the perovskite lattice. The symmetry perovskite has an A-site deficiency. The deficiency at the A-site may result in a certain amount of oxygen vacancies, consequently, improve the current exchange density. Two different phases of Fe₃O₄ and a-Sr from LSCF perovskite seem to be formed at >600 °C, which also more or less hindered the LSCF growth. The metal oxide formation acts as a double-edged sword, preventing particle growth while decreasing the electrical conductivity. The crystalline size of the sol-gel derived LSCF, and its composite was distributed from 20–40 nm. Due to this high specific surface area, the current density was improved by one order in magnitude. In short, our study on phase transition, crystallite growth, and thermal expansion indicated that the parallel beam geometry eliminates the systematic errors caused by the sample thermal expansion and surface displacement and deformation.

The XRD pattern at the different temperatures indicated that crystalline phases composite ABO₃-AA′BO₄ (La_0.6Sr_0.4CoO₃-(SrLa)CoO₄) corresponds to the International Centre for Diffraction Data (ICDD) standards. Two distinctive phases were obtained, rhombohedral La_0.6Sr_0.4CoO₃ (00-036-1393, a = 5.427 Å, c = 13.218 Å, a = 90°, Space group: R 3 ‾ m   166 ) and tetragonal (SrLa)CoO₄, (00-050-0093, a = 3.806 Å, b = 12.487 Å, space group I4/mmm 139). However, the sample crystalline phases were subject to changes when heat-treatment was applied after sputtering. The crystallization temperature was found to be at 600 °C or higher, and crystallinity incrementally increased with calcination temperature [38]. Compared with the standard XRD patterns, the measurement XRD results suggested forming the Ruddlesden-Popper type structure. With the change of the heat-treatment duration, the crystalline structure was kept no significant changes, but the crystallinity was increased from the comparison of the intensities [39]. The analysis indicates that the catalyst is stable across a temperature regime commonly employed by SOFC without lattice distortion and charge ‘leakage.’ The analysis confirms the crystalline structure and stability of the catalysis and electrode up to 800 °C for efficient ORR.

Electrochemical performances

The electrochemistry performance of the LSCF catalysts was determined using cyclic voltammetry and AC impedance (Fig. 6c). The IR compensated I/E data (Fig. 6c-1) were obtained using the positive feedback method collaborating with Dr. V. Birss. The exchange currents, i ₀ , obtained from these low field measurements has increased from ca. 0.25–250 mA cm⁻² by increasing the temperature from 400 to 700 °C, which corresponded with the data obtained from AC impedance measurements (Fig. 6c-2) [40]. The charge transfer resistance (R _ct ) value of 0.31 Ω cm² of LSCF cathode at 700 °C is one order less than that of 2.03 Ω cm² reported by Yuan Ji and his colleagues and similar to the area-specific resistance of 0.287 Ω cm² by Steele [41]. The results showed that sol-gel technology has the instinct advantage of forming cathodic materials with large specific surface area and good homogeneity and high porosity. Therefore, after heat treatment, nanoscale cathodic material with a stable structure can be obtained, which will result in stable electrochemical property and reproducibility. It was also found that with the increasing cathodic applied polarization potential, there was no significant change in R _ct of LSCF, which implied that homogeneity material had been obtained. For efficient fuel-cell operation, a value of R _c  < 1 Ω cm⁻² is desired [42]. The LSCF cathode formed from the sol-gel method displays lower resistance that ca. 0.30 Ω cm² at 700 °C. However, at a lower temperature range of 600–400 °C, the cathode exhibited higher resistance with a value of ca. 2, 16, and 200 Ω cm² respectively. Dr. Kim and his co-workers reported that cathodic composite of LSM and LSCF formed by traditional solid-state powder reaction exhibited good performance of 375 mA cm⁻² at 750 °C [42]. Our study of pure LSCF formed by sol-gel technology displayed high exchange current density with the value of ca. 250 mA cm⁻² at a temperature of 700 °C. The noble metal Pt-modified LSCF showed significant improvement in both current density and charge transfer resistance. However, the in-situ exsolution was found, which hindered the long-term stability of the SOFC devices [43].

Fig. 6c:

LSCF perovskite cathode catalysts, (1): CV studies of ORR at SG LSCF cathode treated at high temperatures, ranging from 400 to 700 °C, 20 mV/s, Air, IR compensated; (2): AC impedance analysis of the catalyst at a different frequency, 0.05–10⁶ Hz; and (3): The ANN simulated current density as a function of cathode thickness and particle size.

To mitigate the drawbacks of the traditional error-and-trial method, we implemented the artificial neural network (ANN, Fig. 6c-3) in the SOFC cathode screening. Based on the “fit” trials, we propose to combine the ANN modeling and SOFC cathode design to increase the accuracy of our experimental results. ANNs are a class of computer algorithms that attempts to mimic the inference logic of the human brain. ANNs have been used widely in image processing and data classification applications. We found that the most reliable ANN model assists us in predicting the performance of the fuel cell materials [44]. Function_Approx.m and simulation were implemented on the Matlab™ workspace in our previous study. The inputs of the M_1 model included concentration of starting materials (Fe(NO₃)₃), viscosity of LSCF colloid, catalyst heating temperature, and uncontrollable fabrication variables, and the outputs were catalyst characteristics including particle size (D), the thickness of LSCF (d), and zeta-potential (ζ). M_2 mapped from catalyst microstructure and single-cell measurement variables to current density to predict SOFCs single cell performance. M_2 used catalyst microstructural characteristics (D, d, ζ), oxygen flow rate, operating temperature, and uncontrollable cell test variables as inputs and current density (i _o ) as outputs. The trained ANN model was applied to simulate the SOFC cathode performance using microstructural characteristics, O₂ flow rate, and performance measurement temperature as inputs and current density (i _o ) as outputs. ANN model mapping inputs denoted as I _p , to output denoted as O _p of performance evaluation was constructed based on the data of particle size (I _p -1), thickness of cathode (I _p -2), zeta-potential (I _p -3), oxygen flow rate (I _p -4), measurement temperature (I _p -5) and current density as the output. These values are close to the state-of-the-arts values for the microdevice at approximately 250 mA cm⁻² [45].

Ball-milling preparation of perovskite catalyst

Different formations of non-stoichiometric cathodic powders (La_0.33Sr_0.67Cr_1-xFe_xO_3-δ) were prepared by an optimized ball milling approach ( OBMA ). The ratio of reactants vs. milling balls, ball diameters, rotation rates, and pre-post low-temperature heat-treatment were optimized in this study to obtain homogeneous particles, although thermodynamically metastable [46]. The advantage of this technique is the ability to tailor size both in wet and dry conditions to obtain particles with rough surfaces to increase oxygen adsorption. OBMA produced materials have better catalysis due to improved oxygen reduction through O²⁻ anion diffusion and surface exchange. These processes led to better-distributed atoms catalysis than traditional wet sol-gel synthesis. These nanostructured green powders were then used to construct half-cell and solid oxide electrolysis cells (SOECs). The half-cells were subject to test for their electrochemical reactivity to facilitate NH₃ production. Our study chose two different-sized marble balls to provide high impact forces, frequencies, and acceleration. Those balls were found to be projected into the milling chamber and then “cascading” down at different rates, resulting in efficient crushing of all fed reactants, LSCrF, and GDC powders. The quantity of these reactants was controlled at a maximum, 100 % to improve the rate of OBMA milling. The speed of rotation and the coefficient of friction of the fed reactants were also optimized based on our previous experiments to ensure the high efficiency of the milling operation.

Structural characterization

The X-ray powder diffraction results showed that the formation of A-B sites dual-doped La_xSr_1-xCr_yFe_1-yO_3-δ (LSCrF) powders and corresponding LSCrF-GDC composites (LaCrO₃, (PDF): 75–0288, a = 3.88–3.91 Å, c = 7.76 Å, α = 90°) polycrystalline phases with an acceptable frequency of merit. The major diffraction peaks appearing at about 32.50°corresponded to the (101) plane, which showed peak splitting (Fig. 7a), indicating the lattice micro-distortion occurred. It was found that the Fe³⁺ amount increase resulted in more evidential peak splitting and the diffraction moved toward a smaller angle, indicating an elongation of crystal lattice along the c-axis. This phenomenon resulted from the large ionic size of Fe³⁺ than Cr³⁺. The high-resolution transmission electron microscopic (TEM) images (Fig. 7b) were collected under steady incident multi-beam, indicating a well-crystallized particle and pore size uniformity of the LSCrF nanocatalyst with four different formulations. It was found that the molar ratio of Fe³⁺ (0.67 Å) and 11 mass % of PMMA was the optimal amount of porogen, from which all catalysts were prepared under this synthesis condition. The LSCrF has an average of 10.5 nm (±0.15 nm) with high integrity and well-defined domains. In the composite samples, both LSCrF and GDC were detected with high crystallinity. Upon turning the pore structure using PMMA-5, the LSCrF was well-distinctive, while some of their edges and corners were subject to polarization and distortion, which can act as the reactive sites to enhance NH₃ production. It is anticipated that, with the Fe³⁺ amount increase, the secondary phase of SrCrO₄ may occur, as shown in the XRD analysis. However, the impact of this observation will be evaluated to optimize the synthesis variables further and maximize the NH₃ production efficiency in future studies. In general, the electrode surface roughness was proposed to provide a large triple phase boundary, favorable for the gaseous reactants’ adsorption and further production formation.

Fig. 7a:

The LSCrF-GDC composites as the SOEC catalysts, (a): The XRD pattern indicating the z-elongation occurrence resulted in lattice distortion; and (b): The simulated LaCoO₃ crystal showing the well-crystallized catalysts formed using ball-milling synthesis.

SOEC used for green synthesis of ammonia

To overcome the challenges of using traditional methods to produce NH₃, our groups invented a SOEC method to produce NH₃ [47]. It was found that the current density significantly dropped from 95 to 80 mA cm⁻² and then stayed at constant values with the increase in reaction time duration for ammonia production using this electrochemical green method. This observation is only seen under 600 °C and 1.6 V, in which the mechanism remains mysterious for our group at this point. High applied voltage essentially generated a high current density, indicating a high NH₃ production yield. The study on the stability of the reaction rate within 3 h was evaluated, suggesting the current density changed by ±0.62 % for majority studies (Fig. 7c). However, spikes were noticed due to the fluctuation of the electrochemical reactivities. As a general tendency, the electrodes are stable over time at OCV, as shown be by the deviations of current density varying by ±0.62 %, except the measurement at 600 °C at 1.6 V. LSCrF electrode appears to be stable under the test conditions without any signs of ongoing phase segregation or decomposition as evaluated by XRD. The secondary Sr-chromite phases that initially were present seem to re-dissolve into the perovskite within 10 h at 1000 °C. The substitution of the La site and Cr site with the other elements can decrease the sintering temperature and increase the electronic conductivity. Because interconnects must separate fuel and oxidant gases, the LaCrO₃-based perovskite should be a dense body. However, alkaline earth-doped or non-doped LaCrO₃ with stoichiometric compositions (La/Cr = 1) are difficult to sinter in an air atmosphere because of the vaporization of chromium vapor-phase transports, which causes grain growth without densification. Doping of alkaline earth with liquid-phase formation can reduce the sintering temperature and improve densification. Doping Sr into LaCrO₃ (La_0.84Sr_0.16CrO₃) markedly increases the sintered density when 3–5 mol% excess SrCO₃ is added as a sintering aid before firing in a reducing atmosphere around 2200 K. Liquid-phase formation was also observed in La_0.76Sr_0.24CrO₃ and Y_0.6Ca_0.4CrO₃. Because of limited alkaline earth solubility, melting chromates, SrCrO₄ and CaCrO₄ precipitated from the perovskite structure at intermediate temperatures. These liquid-phase chromates enhance the densification of doped LaCrO₃ at high temperatures. The major advantage of ammonia synthesis by SOEC is conducting the reaction at ambient pressure compared to the Haber-Bosch process. The high temperature and pressure are required in the Haber process due to the large endothermic energy attributed to the high energy cost of nitrogen bond dissociation enthalpy. In contrast, the redox couple supplied the reducing ‘equivalence’ in the electrolyzer process and conducted it at low pressure (Fig. 7d). Like the Haber-Bosch process, high temperatures are required to make the reaction kinetics acceptable on an industrial scale. The electrochemical production of ammonia at low pressure will reduce engineering costs and make ammonia synthesis more portable in that large engineering stations are not required. While electrochemical synthesis of ammonia is a step forward, it is not comparable to the biosynthesis of ammonia by nitrogenase-containing microorganisms. The catalytic center usually consists of a dinitrogenase reductase (Fe-containing) and dinitrogenase (Fe/Mo containing) domain. Electrons are transferred from reduced ferredoxin (Fe) to molybdoferredoxin (Fe/Mo). Each mole of fixed nitrogen requires 16 mol of adenosine triphosphate (ATP) to be hydrolyzed to adenosine diphosphate (ADP) and inorganic phosphate (P_i). Under ambient conditions, ammonia is biosynthesized; however, the reaction rate is slower than that observed with SOEC. The biosynthesized ammonia is often converted into ammonium and incorporated into proteins, which hinders the application of microbial-assisted biosynthesis of ammonia. The next section briefly outlines how microorganisms convert simple sugars to carbon dioxide or lactates and electrical power through exoelectrogenesis [48].

Fig. 7c:

Electrochemical performance of LSCrF-GDC with different A and B-site doping used as the electrocatalyst; (d): NH₃ production using the solid oxide electrolysis cells, showing high production rate in comparison with traditional methods.

The design criteria between a SOFC and SOEC are similar, but the operational mechanisms are different. The purpose of the anode electrode (e.g., Ni-YSZ) in a SOFC device is to provide mechanical strength and to limit polarization losses. The design of the electrolyte aims to enhance ionic conductivity and material compatibility. The cathode electrode extends the triple-phase boundary and offers low ohmic loss. The purpose of the SOFC is to generate electricity using fuel and oxidants. SOFC can operate at 0.7–1.00 cell voltage (V) using hydrogen as the preferred fuel, which involves a four-electron oxidation reaction. The electrons are transported from the anode to the cathode through an external circuit, while the oxygen anion diffuses through the solid electrolyte in the opposite direction. In SOEC mode, external power input is applied to facilitate the redox reaction. In electrolyzer mode, as reported by Vikrar, the oxygen partial pressure inside the device increased, resulting in delamination of the electrode surface, which needs to be considered at the device level of integration [49]. In our example, ammonia is formed in the cathode chamber using nitrogen and water steam as reactants. The ammonia formation occurs at 550 °C with an applied voltage (1.6 V) without pressure build-up.

Single-cell assembly

The MFC system’s power output and long-term stabilities will be affected by a strain of microorganism, type of substrate, oxygen tension, microbial growth rate, substrate affinity, kinetic and biomass transfer, and thermodynamics of the anodic reaction. A series of interactive biofilms were prepared by enabling an attachment between select extroelectrogenic microbe (bacteria and yeast) to the nanostructured anode surface via an active “sucking” attractive interaction. Upon nano-modification, these biofilm anodes are anticipated to respond to the external environmental changes and self-regulate their structures. To better understand how microorganisms such as sulfur-reducing function on the anode surface, an anodic compartment was assembled using the select yeast (Saccharomyces cerevisiae). The media, composed of glucose enriched phosphate-buffered saline (PBS), was used, and yeast was harvested in the late exponential phase by centrifugation under anaerobic conditions. The cell pellets were suspended in a freshwater medium without an electron donor or acceptor and inoculated into the anode chambers. In addition, a suspension composed of graphene and other inorganic catalysts was introduced into the above anodic media. The resultant biofilm anodes (Fig. 8) were found to show high conductivity due to the extension of electron transfer by conductive microorganisms.

Fig. 8:

The performance of MFC devices using electrocatalysts to form biofilm using graphene as support.

Device performance

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) will be used to evaluate the MFC performance. A two-chamber MFC power system (Fig. 8b) was constructed using the above anode and a polymeric PTS membrane (developed in Liu’s lab). The configuration, substrates, and measurement conditions have been controlled and optimized to secure high maximum power density. Although the conductivity of prepared anodes is anticipated to improve power output generated by the MFCs, the mixed cell culture shows its potential application for the advancement of MFCs due to its synergistic effects of different components. The nano-emulsion allows for formations of interactive nanomaterials by a simplified feasible wet-chemistry, securing a homogeneous tunability of structure and properties from the molecular or atomic level. The current density was highly dependent on the microbial surface coverage, suggesting the use of multilayer biofilm increasing anode current. Production efficiency per cell can be correlated with biofilm thickness even when the outer layer is significantly distant from the anode surface. Viability staining indicates cell viability to biofilm thickness to current as cells are expected to grow at a distance from the electrode surface divide during biofilm development, suggesting if the cells are metabolically active and involved in electron transfer from microbes to the anodic electrode [54].

Mechanism study

The proposed mechanism of oxygen reduction reaction (ORR) involves a multielectron transfer process with five elementary steps, namely, O₂ adsorption, reduction of O_2ads through two processes; desorption of product of H₂O₂; reduction of reduction H₂O_2ads , and desorption of adsorbed H₂O [55]. Previous data reported by Logan et al. have demonstrated that they can produce micrometer-long proteinaceous. These appendages can extend from their outer surface into the extracellular matrix, involved in extracellular electron transport processes. The biofilm anodes displayed two properties: a conductive solid matrix characterized by the biofilm conductivity and accepting electrons from microbes and conducting these electrons to the anode [56]. The biofilm conductivity strongly influences the electron-donor (ED), current fluxes, and biomass distribution. The conductivity was correlated to an increased ED and current fluxes under different conditions, such as carbon sources, biofilm thickness, and strain of microbes. Our study inferred two possible modes of the current generation: direct and indirect electron transfer. The first mode by direct electron transfer mechanism involves electron transfer via conductive pili, where the microbial nanowires act as conductive channels. The second mode is indirect electron transfer via redox-active protein, where the microbe extrudes redox-active molecules into the extra membrane space. By careful adjustment of the redox potential of the mediators, a charge transfer shuttle enables indirect electron transport. The modified biofilm anodes are anticipated to directly contact the microorganisms, further promoting electron transfer due to graphene’s high conductivity. A microorganism capable of forming nanowires can extend its proteinaceous filaments into the extracellular matrix to facilitate electron transport. The redox-active microbial membrane proteins also allow for short-range electron transfer without physical contact between microbe and electrode. These electrons can be transferred to the electrode surface from the conductive proteins such as C-type cytochromes [57].

Ternary metal oxide selection

Among the consortia of energy power systems, the supercapacitors are one class of electrochemical storages that have been systematically studied for several decades due to their rapid charge-discharge properties, improved power density, and enhanced efficiency cycle stability. We demonstrated self-regulatable heterojunction configurations composed of manganese (IV) oxide nanoparticles (MnO₂) grafted vanadium (V) oxide ribbon arrays (V₂O₅) to enhance their super capacitive performance. During the two-step hydrothermal reaction, an endothermic metathesis of NH₄VO₃ resulted in the formation of V₂O₅, while reduction of KMnO₄ leads to the formation of nanoscales MnO₂ under 180 °C (Fig. 9a–d). The surface-bound Mn⁴⁺ cation was nucleated under high pressure to form [MnO₆] octahedra building units, then formed MnO₂ phased NPs during the hydrothermal treatment (Fig. 9c and 9f) under various temperatures and pressures. These surface Mn atoms enable attraction with the negative oxygen end from V₂O₅ single-crystal units through ion-diploe intermolecular forces. This attraction allows for the formation of twinned channels and strong adhesion through which charge transport can occur between two crystalline phases at their interfaces. These attractive forces ensured ion intercalation into a redox-active material without causing crystallographic phase collapse. The XRD data showed orthorhombic V₂O₅ well-indexed with the standard (JCPDS: 41–1426, a = 11.516 Å, b = 3.5656 Å, c = 4.3727 Å, α = 90°, space group: Pmmn (59)) and tetragonal α-MnO₂ (JCPDS: 44–0141, a = 9.7847 Å, c = 2.8630 Å, α = 90°, space group: I4/m (87)). The XRD spectra suggest that both α-MnO₂ and V₂O₅ crystalline phases were maintained well after grafting. These grafted structures were found to be assisting Li-ion insertion, allowing for rapid ionic conduction and an improved supercapacitor performance. The well-organized interleaved structure significantly improved their capacitance (10–20 times) and energy density (>10 times) compared with the published data [58, 59]. Multiple factors have been considered for selecting two metal oxides, MnO₂ and V₂O_5, as the supercapacitor electrodes, particularly their potential windows in an aqueous solution.

Fig. 9:

Hydrothermal synthesis to create bi-metallic oxide electrode heterojunction structure; (a): Nucleation of metal oxides under optimized synthesis variables; (b): Nanografting to form desired crystalline phases and morphology; (c): Crystallization and particle growth due to the thermal motion; (d): Intercalation to form heterojunction structure, offering shortened spontaneous pathways to facilitate ion diffusion and electron conduction; (e): The schematic of hydrothermal synthesis to produce grafted heterojunction structures; and (f): Relationship between reaction temperature and pressure.

Electrochemical performance

The heterojunction materials were used as the anode and cathode electrodes to assemble symmetrical supercapacitors (SC: 1.0 × 1.0 cm² in length and width with a thickness of 0.02 cm) in two configurations, the first using V₂O₅ defined as VSC and the second using MnO₂ grafted V₂O₅ as MVSC. The CV profiles were collected under different applied voltage and scan rates, respectively. The CV spectra indicated a distinct polarization for both devices (Fig. 10) were observed at the voltage window at 2.0 V. The specific capacitances of MVSC were 370, 258, 216, 189, and 162 F g⁻¹ at different scan rates (5, 10, 20, 50, 100 mV s⁻¹), showing an average 15.4 % increase, compared with the VSC systems (Fig. 10a). This improvement of current density corroborated that MVSC systems can effectively storage electrolyte cations, showing potential to be used as effective emerging energy storage devices. The VMSC capacitors were also able to prevent polarization due to a decrease of concentration gradient at a high voltage window. However, we only reported the data up to 1.8 V. The supercapacitors’ galvanostatic discharge curves (GDC) spectra demonstrated similar variations in variations between VSC and VMSCs. The VMSCs exhibited 14.1–22.8 % improvement of specific capacitance (Fig. 10b), averaged at 307, 271, 235, 208, and 180 F g⁻¹ at different current densities (0.5, 1.0, 2.0, 5.0, and 10.0 A g⁻¹), respectively.

Fig. 10:

Electrochemical analyses of select metal oxide electrodes; (a): CV results under different scan rates; (b): GDC results; (c): DPSCA results; and (d): Ragone plots, showing an improved combination of energy and power density (note references numbers are from actual publication [62]).

Both complimentary CV and GDC spectra illustrated that the tetragonal MOPs played multifunctional roles due to their intrinsic intercalation nature. The double potential step chronoamperometry (DPSCA) was implemented to evaluate the current response of the metal oxide electrodes as a function of time (Fig. 10c). Based on the maximum charge and minimum discharge peak current density, the relaxation time will be determined as indicators of quasi-reversible reactions and (un)equal diffusion coefficients of the oxidant and the reductant. It is hypothesized that ion diffusion may be the major pathway of mass transport from an electrolyte to the electrodes when the metal oxide electrode is used. Based on the Cottrell equation, the diffusion coefficient of Li⁺ ions will be calculated to understand the redox kinetics based on the relationship between current density and time. The Ragone plot of the VSC and VMSC (Fig. 10d-1) indicated that the relationship between their gravimetric energy density and power density, which were calculated using the specific capacitive values (Cs, F g⁻¹) [60] based on the GCD curves. The energy densities of the VSC and VMSC devices were determined to be 117 and 138 W h kg⁻¹ at 450 W kg⁻¹, while the energy densities can be retained at 62.6 and 81.0 W h kg⁻¹ at a high-power density of 9000 W kg⁻¹. Compared with the other reported transition metal oxide electrodes (Fig. 10d-2), the hydrothermal derived bimetallic oxide electrodes and their supercapacitors exhibited comparable or superior behaviors [61].

Synthesis of MHAE

The metal-organic framework-complexed hydrogenase active site analogs as electrocatalysts (MHAEs) were designed and prepared using hydro-solvo-chemistry. The purpose of the MHAE study was not to generate hydrogen but to design catalytic materials to mimic microbial hydrogenase. These microorganisms can generate hydrogen under anaerobic and ambient conditions using a binuclear center composed of ferredoxin [Fe-Fe], nickel-iron, or Fe-S catalysts. In our approach, we configured the active center based on highly porous metal-organic frameworks (MOFs) for immobilization and outer coordination sphere interactions that might protect delicate molecular catalysts with enhanced catalytic efficiency. The MOFs simulate an active center in ferredoxin [FeFe]-H₂ase by various chemical modification routes. Our goal was to design MOF/model complex constructs thermodynamically favorable for hydrogen production and catalyst longevity (Fig. 11). Previously Drs. Darensbourg, Zhou, and Liu found a synthetic route (Fig. 11a) to coordinate the zirconium-metalloporphyrin PCN-95 MOF (Zr-TBAPy, PCN-95, the ligand H₄TBAPy: 4,4′,4″,4‴-(pyrene-1,3,6,8-tetryl) tetra benzoic acid (H₄TBAPy)) to the [FeFe]-H₂ase as shown in X-ray powder diffraction. However, the MHAE synthesis and H₂ production variables remain optimized, and the actual moles/h rate was low. The approach demonstrates that a hydrogenase-based mimetic could be formulated if the redox-active site could be constructed with the correction interspaced distances and orientation. Our post-synthetic modification route for PCN-95 MOFs (from Zhou’s group) was extremely robust and had large open cavities [63]. According to well-established synthetic protocol, the 2Fe2S catalyst will be prepared with a pyridine attached at the nitrogen bridgehead of the S to S-linker. The MOFs will be used as a matrix during the reduction of disulfides to thiols. CO/CN can accomplish incorporation of H2ase active site models into MOFs⁻ exchange and insertion of the (µ-SRS)[Fe(CO)₂CN]₂ ²⁻ complex. Alternatively, the CO/CN⁻ exchange may be attempted on the MOF pre-loaded with the (µ-SRS) [Fe(CO)₃]₂ to enhance insertion percent incorporation. The very mild conditions that affect the diatomic ligand exchange are not expected to degrade PCN-95 MOF [64]. Both metal complexes feature di-carboxylic acid units prime for incorporation into MOFs through post-synthetic exchange or the diiron complex as a metallic ligand to synthesize new MOFs. Two strategies are shown, one which incorporates the dicarboxylic acid unit through the carbene ligand, or a second, which places the carboxylic acids at the nitrogen of the S to S-linker. Both complexes were used for the direct synthesis of MOFs (Zr-TBAPy, PCN-95, the ligand H₄TBAPy: 4,4′,4″,4‴-(pyrene-1,3,6,8-tetryl) tetra benzoic acid (H₄TBAPy)). Three new [FeFe]-H₂ase metallo-ligands were synthesized through condensation of Fe₂(SH)₂(CO)₆ with formaldehyde and an appropriate primary amine [65]. The amino substituent features either a pyridinyl group or a phenyl carboxylic acid, which will act as the linker between the MOF and the [FeFe]-H₂ase model complex. The above narrative demonstrates the importance of catalysts used to assemble devices and optimization at each step from nano to micro to macro integration.

Fig. 11:

Synthesis and characterization of MHEAs, (a): The simplified procedure to prepare immobilized catalysts; (b): crystallographic analysis of a selected MHAE, in which data indicated H₂ase synthetic analog was inserted into the MOF pore; (c): Spectroscopic analyses of four MOF motifs complexed H₂ase synthetic analog by a post-modification; and (d): X-ray powder diffraction of these four MOF motifs.

Characterization of MHAE

All complexes were characterized by single-crystal X-ray diffraction (SCXRD, Fig. 11b), Infrared and UV–Vis data (Fig. 11c), and powered X-ray diffraction (Fig. 11d), to evaluate the effects that the MOF has on the geometry and stability of the [FeFe]-H₂ase model complexes [66]. Specifically, the frameworks’ ability to produce a rotated model complex geometry was estimated to provide steric pressure similar to the enzyme active site. The powder D-8 X-ray diffractometer equipped with a Cu-sealed tube and APEX-II diffractometer with a low-temperature device and a fine-focus sealed-tube X-ray source (Mo-K _α radiation) have been used to identify the crystalline structure of electrocatalysts [67]. The electrocatalyst structures were solved by direct methods and refined by the full-matrix least-squares technique against F² with the anisotropic temperature parameters via the SHELXTL software package. In this approach, the size and shape of the MOF-H₂ase catalyst were evaluated. Therefore, the optimized particle size and distribution will be controlled according to the current density of the ORR and HOR, respectively [68] for energy storage applications [69].