Mixed Oxide Electrodes Research Articles

Assessment of Ti, Ir, Ta and Ru influence on mixed metal oxide electrodes for photoelectrochemical generation of persulfate: Impact on sulfamethoxazole degradation

The presence of persulfate (S2O82−) in decontamination processes favors the oxidation of organic pollutants due to its strong oxidation power. In this research we study the photoelectrochemical generation of persulfate using five mixed metal oxides electrodes (MMO) with different compositions and its effect on the degradation of sulfamethoxazole antibiotic (SMX) by photoelectrocatalysis (PEC) and electro-oxidation (EO).By PEC, all anodes generated a higher concentration of S2O82− than those not exposed to light. The high S2O82−concentration obtained by PEC was 0.150 mM using MMO[Ti/Ir/Ta] in a solution with Na2SO4 100 mM applying a current density of 2 mA/cm2. On the other hand, the maximum concentration obtained was 0.250 mM at 30 min of electrolysis for MMO[Ti/Ir/Ta] using Na2SO4 50 mM and applying current density of 5 mA/cm2. S2O82−production by EO was between 0.005 and 0.089 mM. It is observed that MMO based in Ta2O5 showed the best S2O82− production.The effect of S2O82− electro-generation (using the anode with the highest and the anode with the lowest S2O82− production) on the degradation of sulfamethoxazole by PEC and EO was studied using the experimental conditions with the best production of this oxidant. MMO[Ti/Ir/Ta] and MMO[Ti/Ru] were used as anodes, and it was observed that by PEC, 100% of SMX was degraded after 30 min of electrolysis using MMO[Ti/Ir/Ta] and 60 min using MMO[Ti/Ru]. By EO, the degradation of SMX was partial, demonstrating that the electrophotocatalytic effect favors the generation of S2O82−, enhancing the degradation of SMX at short electrolysis times.

Electrochemical oxidation of Florfenicol in aqueous solution with mixed metal oxide electrode: Operational factors, reaction by-products and toxicity evaluation

Veterinary antibiotics have become an emerging pollutant in water and wastewater sources due to excess usage, toxicity and resistance to traditional water and wastewater treatment. The present study explored the degradation of a model antibiotic- Florfenicol (FF) using electrochemical oxidation (EO) with Ti–RuO2/IrO2 anode. The anode material was characterized using SEM-EDS studies expressing stable structure and optimal interaction of the neighboring metal oxides with each other. The EDS results showed the presence of Ru, Ir, Ti, O and C elements with 6.44%, 2.57%, 9.61%, 52.74% and 28.64% atomic weight percentages, respectively. Optimization studies revealed pH 5, 30 mA cm−2 current density and 0.05 M Na2SO4 for 5 mg L−1 FF achieved 90% TOC removal within 360 min treatment time. The degradation followed pseudo-first order kinetics. LC-Q-TOF-MS studies revealed six predominant byproducts illustrating hydroxylation, deflourination, and dechlorination to be the major degradation mechanisms during the electrochemical oxidation of FF. Ion chromatography studies revealed an increase in Cl−, F− and NO3− ions as treatment time progressed with Cl− decreasing after the initial phase of the treatment. Toxicity studies using Zebrafish (Danio rerio) embryo showed the treated sample to be toxic inducing developmental disorders such as pericardial edema, yolk sac edema, spinal curvature and tail malformation at 96 h post fertilization (hpf). Compared to control, delayed hatching and coagulation were observed in treated embryos. Overall, this study sets the stage for understanding the effect of mixed metal oxide (MMO) anodes on the degradation of veterinary antibiotic-polluted water and wastewater sources using electrochemical oxidation.

Nickel-copper-cobalt mixed oxide electrode material for high performance asymmetric supercapacitor

Nickel copper cobalt oxide (NiCuCoO) ternary metal oxide nanoparticles were synthesized by employing the hydrothermal method. NiCuCoO electrode demonstrates a specified capacity of 596 C g−1 at 1 A g−1, high capacitance retaining of 99% even if 1000 sequences at the density of current 10 A g−1, and significant extended cyclic strength over 1000 sequences. The gathered asymmetric supercapacitor (ASC) tool via NiCuCoO as the cathode and activated carbon as anode materials achieve a specified capacity of 168 C g−1 at a current density of 1 Ag−1, an excellent capacity retaining of 95% even later than 5000 sequences at a density of current 10 A g−1. The fabricated device exhibits a high density of energy and power is 96 Wh kg−1 and 841 W kg−1. The prepared material confirms an excellent capacitance routine, so this work represents for a next-generation energy storage device.

Mixed metal oxide (Ru, Ir, Sn) electrodes in the anodic activation of peroxydisulfate for a sustainable degradation of rifampicin

Electrochemical advanced oxidation processes (EAOPs) are promising approaches to wastewater purification. Anodic oxidation (AO), due to its ease and cost-efficiency, has attracted rising attention in this research area. Herein, we employ Mixed Metal Oxide (MMO) electrodes, including RuIrSnOx@Ti, RuIrOx@Ti, and RuOx@Ti, to activate peroxydisulfate (PDS) in the anodic activation process. The Rifampicin (RIF) pharmaceutical was selected as the model pollutant to be treated in the proposed electrochemical system. The surface and electrochemical characteristics of the electrodes were characterized to analyze and determine their properties and effectiveness. Also, the various parameters affecting the efficiency of the process, such as the different metal oxide electrodes, PDS/electrolyte/RIF concentration, and the current flow, are optimized. The optimal values for these parameters were determined to be [RIF]0 = 40 mg/L, I = 80 mA, [PDS] = 1 mmol/L, and [Na2SO4] = 50 mmol/L. The LC-MS and COD techniques were also used to identify electrochemical-induced intermediates and describe a plausible degradation pathway. Finally, the proposed electrochemical system can be considered a simple wastewater purification method through this research.

Treatment of Organics in Wastewater Using Electrogenerated Gaseous Oxidants.

This work focuses on the comparison of the performance of direct electrochemical oxidation with indirect electrolysis mediated by gaseous oxidants in the treatment of diluted wastewater. To do this, energy consumptions of the electrolysis using mixed metal oxide (MMO) electrodes are compared with those required for the production and use of chlorine dioxide in the degradation of methomyl contained in aqueous solutions. Results demonstrate the feasibility of the mediated oxidation process and that this process is competitive with direct oxidation. The oxidants are produced under optimized conditions using the same anodic material applied for the direct degradation of organics, thus avoiding efficiency losses associated with mass transfer limitations in the degradation of dilute organic solutions. Thus, using the ClO2 gaseous oxidant, a concentration of 0.1 mM of methomyl from a solution containing 500 mL is completely removed with an energy consumption as low as 50 Wh. The application of the same energy to a direct electrolytic process for treating the same wastewater can only reach less than half of this removal. These findings may have a very important application in the use of electrochemical technology to achieve the remediation of persistent pollutants in wastewater, where their low concentrations typically make direct processes very inefficient.

Groundwater Bioremediation through Reductive Dechlorination in a Permeable Bioelectrochemical Reactor

A new membrane-less bioelectrochemical reactor configuration was developed for contaminated groundwater remediation. The new bioelectrochemical reactor configuration was inspired by the utilisation of a permeable reactive barrier (PBR) configuration with no separation membrane. The corresponding reactive zones were created by using graphite granules and mixed metal oxide (MMO) electrodes to stimulate the reductive and oxidative biological degradation of chlorinated aliphatic hydrocarbons. In the present study, the PBR-like bioelectrochemical reactor has been preliminarily operated with synthetic contaminated groundwater, testing the reductive dechlorination activity on cis-dichloroethylene (cisDCE). Moreover, to assess the effects of competing anions presence for the electron donor (i.e., the cathode), the synthetic wastewater contained sulphate and nitrate anions. In the PBR-like reactor operation, nearly all cisDCE was removed in the initial sampling port, with only VC detected as the observable RD product. During the same biotic test of the PRB reactor, the presence of both the reductive dechlorination and anions reduction was confirmed by the complete nitrate reduction in the cathodic chamber of the PRB reactor. On the contrary, sulphate reduction showed a lower activity; indeed, only 25% of the influent sulphate was removed by the PRB reactor.

Platinum-Modified Mixed Metal Oxide Electrodes for Efficient Chloralkaline-Based Energy Storage

In this work, a series of novel mixed metal oxide (MMO) electrodes with the composition Ti/RuO2Sb2O4Ptx (0 ≤ x ≤ 10.0) were developed, envisaging their application in a reversible electrochemical cell based on the chloralkaline process as an energy storage system. These electrodes were synthesized via the ionic liquid method. Comprehensive physical, chemical, and electrochemical characterizations were conducted to evaluate their performance. The feasibility of employing these electrodes within reversible processes was explored, using the products generated during the electrolytic operation of the system for fuel cell operation. During the electrolyzer operation, higher current densities resulted in enhanced current efficiencies for the production of oxidized chlorine species. Notably, the presence of platinum in the catalyst exhibited a negligible impact on the coulombic efficiency at low current densities where water oxidation predominates. However, at higher current densities, the presence of platinum significantly improved coulombic efficiency, approaching values of approximately 60%. Transitioning to a fuel cell operation, despite the improved kinetic performance associated with a higher platinum content, the process efficiency was predominantly governed by ohmic losses. Curiously, the MMO electrode made without platinum (Ti/(RuO2)70-(Sb2O4)30) displayed the lowest ohmic losses. This study establishes optimal conditions for future investigations into this promising possibility, which holds great potential for energy storage via chloralkaline-based reversible reactions.

Application of electro oxidation process for treating wastewater from petrochemical with mixed metal oxide electrode

Petrochemicals require a large volume of water for their operation, which results in the production of a large volume of wastewater. Treatment of petrochemical wastewater is an important process before discharging it into the environment. This research examines the treatment of real petrochemical wastewater using the electrochemical oxidation process. Direct anodic oxidation is an effective advanced electrochemical oxidation process (AEOP), with different electrodes using a parallel plate electrochemical reactor. Four types of real wastewater were received from different petrochemical units were treated by AEOP. Real wastewater samples with chemical oxygen demand (COD) concentrations ranging from 20,450 to 52,300 mg/l. The main goal of this research is to make electrodes of Mixed Metal Oxide (MMO), which can reduce the treatment time and electricity consumption for oxidation, greater stability of the surface of the electrodes. Investigation of the rate constant kinetics shows that high COD removal efficiency can be achieved following the pseudo-second order reaction rate (R2 > 98%). When the wastewater pH is less than 5, COD removal efficiency is higher and the treatment process will be successful, which succeeded in removing 79% COD, but in alkaline wastewater, COD reduction efficiency was not satisfactory. The electricity consumption for 79% removal during 6 min was 117 kWh/m3. As a result, due to the very short time of the process (6 min), it can be used as one of the pre-treatment steps of petrochemical wastewater with acidic pH.

Design of Bi-functional mixed oxide electrodes for selective oxidative C–C cleavage of glycerol to formate and synchronized green hydrogen production

NiCoMn oxide nanoneedles with a cactus-like morphology are shown as bifunctional electrocatalysts that selectively oxidize glycerol to HCOOH and concurrently produce H2.

Efficient degradation of propranolol with electrochemically activated peracetic acid process: Mechanisms, degradation pathway, and toxicity evaluation

Electrochemical oxidation has attracted considerable attention in advanced oxidation processes (AOPs) due to its clean and green advantages. In this work, mixed metal oxide (MMO) electrodes were applied for the first time in electrochemically activated peracetic acid (EC/PAA) to investigate the systematic degradation of propranolol (PPL). The results showed that the EC/PAA-MMO/MMO system significantly promoted the removal of PPL from water, achieving a remarkable removal rate of 87.05% within 20 min, which was better than that of electrochemically activated peroxymonosulfate (PMS) or H2O2. Hydroxyl radical (OH), organic radical (RO), singlet oxygen (1O2), and superoxide radical (O2−) were confirmed to be the primary reactive species. Apart from OH, the contribution rate of the other reactive species was up to 41.92%, which played a crucial role in the degradation of PPL. Notably, RO exhibited a specific effect on the degradation of PPL, while 1O2, and O2− were crucial in initiating the chain reaction of the reactive species. With the increasing of PAA concentration and current density, and the decreasing of electrode distance, an acceleration of PPL degradation efficiency was observed. Additionally, maintaining a neutral pH was found to be beneficial for the degradation of PPL. Furthermore, the degradation pathways of PPL were elucidated based on density-functional theory and LC-MS detection. Quantitative structure–activity relationship (QSAR) analyses revealed a significant reduction in the toxicity of PPL products after the degradation by EC/PAA. The results suggest that EC/PAA-MMO/MMO is a promising technology for the removal of naphthyl-containing pollutants that are difficult to be degraded by OH.

Electrosynthesis of semitransparent Ni-Co2-Cux mixed oxide electrode film for novel solid state asymmetric planar supercapattery application

Electrosynthesis of semitransparent Ni-Co2-Cux mixed oxide electrode film for novel solid state asymmetric planar supercapattery application

Cation Incorporation into Copper Oxide Lattice at Highly Oxidizing Potentials.

Electrolyte cations can have significant effects on the kinetics and selectivity of electrocatalytic reactions. We show an atypical mechanism through which electrolyte cations can impact electrocatalyst performance─direct incorporation of the cation into the oxide electrocatalyst lattice. We investigate the transformations of copper electrodes in alkaline electrochemistry through operando X-ray absorption spectroscopy in KOH and Ba(OH)2 electrolytes. In KOH electrolytes, both the near-edge structure and extended fine-structure agree with previous studies; however, the X-ray absorption spectra vary greatly in Ba(OH)2 electrolytes. Through a combination of electronic structure modeling, near-edge simulation, and postreaction characterization, we propose that Ba2+ cations are directly incorporated into the lattice and form an ordered BaCuO2 phase at potentials more oxidizing than 200 mV vs the normal hydrogen electrode (NHE). BaCuO2 formation is followed by further oxidation to a bulk Cu3+-like BaxCuyOz phase at 900 mV vs NHE. Additionally, during reduction in Ba(OH)2 electrolyte, we find both Cu-O bonds and Cu-Ba scattering persist at potentials as low as -400 mV vs NHE. To our knowledge, this is the first evidence for direct oxidative incorporation of an electrolyte cation into the bulk lattice to form a mixed oxide electrode. The oxidative incorporation of electrolyte cations to form mixed oxides could open a new route for the in situ formation of active and selective oxidation electrocatalysts.

Fabrication of novel Ti/SnO2-Nb2O5 electrode in comparison with traditional doping metal oxides for electrochemical textile wastewater treatment

Textile wastewater contains a number of hazardous and refractory organic contaminants, especially various dyes such as cibacron, cycafix, monozol, and procion, that contribute to the negative effects of textile wastewater on aquatic creatures due to a limitation of light penetration and the consumption of oxygen. The selection of electrode materials is always a primary concern for electrochemical methods. SnO2-based electrodes are now being pursued by researchers due to their low cost. As a result, adding another stable oxide is recommended to enhance the efficiency and stability of SnO2-based electrodes. This study aims to fabricate the Ti/SnO2-Nb2O5 electrodes in comparison with different traditional doping metal oxides, Ti/SnO2-Sb2O5, Ti/SnO2-TiO2, and Ti/SnO2-Ta2O5, using the sol–gel method for electrochemical textile wastewater treatment. The electrode’s surface morphologies, crystal structures, and electrochemical characteristics were determined using scanning electron microscopy (SEM), X-ray diffraction (XRD), and cyclic voltammetry (CV), respectively. The SEM images illustrated that the surfaces of the four electrodes had typical cracking structures of mixed metal oxide electrodes. The presence of crystalline structures of SnO2, Sb2O5, Nb2O5, TiO2, and Ta2O5 on the electrodes was clearly determined. The direct experimental comparisons indicated that the Ti/SnO2-Nb2O5 electrode performed the best among the four oxides in the treatment of textile wastewater, with 87.0% and 84.3% efficiency in removing COD and color, respectively. A current density of 5.8 mA/cm 2 and a NaCl concentration of 2 g/L were found to be optimal operating conditions. The electrochemical properties of the fabricated electrodes showed a high oxygen evolution potential, stability, and the generation of numerous free radicals and oxidants such as •OH, H2O2, O3, and Cl2. These findings serve to inform the future selection of appropriate metal oxides to reinforce SnO2 electrodes for textile wastewater treatment.

Corn stover‐derived porous carbon for asymmetric supercapacitors

AbstractIn this investigation, hydrothermal liquefaction‐derived hydrochar from pulverized corn stover (avg. particle size 1.12 mm and ash content 6.75 wt%) is chemically activated with aqueous KOH in the presence of Pluronic F127 surfactant and thermally treated further up to 800°C to generate porous carbon (POC) with higher specific surface area and porosity. POC obtained under different processing conditions is thoroughly characterized by Brunauer‐Emmett‐Teller surface area analyzer, Fourier transform infrared spectroscopy, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. Metallic impurity present in POC is characterized by inductively coupled plasma mass spectrometry. Higher specific surface area POC is observed with the use of the surfactant during the activation process. Fully characterized POC is used with (Mn, Ti)‐mixed oxide electrode to fabricate asymmetric supercapacitor (ASC). Specific capacitance of ASC is measured by cyclic voltammetry (CV) technique using Gamry G‐300 potentiostat/galvanostat/ZRA. CV plots are obtained with different voltage scan rates whereas galvanostatic charge‐discharge plots are studied by varying the current density. Renewable corn stover‐derived POC prepared using Pluronic F127/KOH activation method is found to be highly suitable as an electrode material because of higher capacitance and electrochemical stability over 100 charging‐discharging cycles.

Elimination of sulfamethoxazole by anodic oxidation using mixed metal oxide anodes

The degradation of sulfamethoxazole (SMX) was studied by anodic oxidation (AO) process using mixed metal oxide (MMO) electrodes with different Ru/Ir ratios. Each electrode was characterized morphological and electrochemically. The electrolyzes were performed in NaCl and Na2SO4 applying two current densities (10 and 50 mA cm−2). The electrode with the highest composition of Ir, Ru/Ir (30/70), showed greater SMX degradation and generation of oxidizing species and was used to treat the antibiotic by AO and AO assisted by solar energy in natural water and actual municipal wastewater effluents.The efficiency in SMX degradation depends on the type of electrode used (MMO) and electrolytic medium. All MMO electrodes, reached almost total degradation of SMX in chloride medium. However, 60 % degradation of SMX in sulfate medium was achieved with anode with lower Ru/Ir ratio. Additionally, degradation of SMX in complex matrices can be successfully carried out by solar-assisted AO and AO processes, without the need to adjust the pH, at room temperature and using anode with lower Ru/Ir ratio. Finally, the AO process assisted by solar energy reduced electrolysis times and the cell's potential, leading to lower energy consumption.

Removal of fluoride and pathogens from water using the combined electrocoagulation-inline-electrolytic disinfection process

Abstract The consecutive removal of fluoride (defluoridation) and pathogens (disinfection) in drinking water through combined electrocoagulation-inline-electrolytic disinfection (EC–ECl2) process with aluminum and dimension-stable mixed oxide electrodes was reported in this study. Laboratory trials were conducted on the effects of flow rate, initial pH, current density, and supporting electrolytes for defluoridation and disinfection processes. The results have shown that with a flow rate of 10 L/h, initial pH of 6, the current density of 9.4 mA/cm2 (EC cell) and 3.1 mA/cm2 (ECl2 cell), supporting electrolyte concentration of 165 mg/L, and electrolysis time of 50 min, a defluoridation rate of 88% (initial concentration of 12.3 mg/L) and complete disinfection (initial fecal coliforms of 19,700 colony-forming units per 100 mL (CFU/100 mL)) can be reached. The final concentration of fluoride and pathogens in treated water was 1.44 mg/L and 0 CFU/100 mL, which are within the acceptable limit of the World Health Organization and the Tanzania Bureau of Standards of 1.5 mg/L and 0 CFU/100 mL, respectively. The EC–ECl2 system is a promising approach for consecutive defluoridation and disinfection of water to save millions from fluorosis and waterborne diseases. However, optimization potential with regard to energetic efficiency and system complexity was identified.

Application of Ti-based mixed metal oxide electrodes towards electrochemical mineralization of Metronidazole: Evaluation of influencing factors and removal by-products

BackgroundThe treatment of Metronidazole (MTZ) as a persistent pharmaceutical waste in the water is a significant challenge. To reduce the adverse effect of MTZ in water, we tested a novel SiO2-incorporated MMO-Ti/IrO2-Ta2O5 electrode via the electrochemical oxidation (EO) method. MethodsThe electrode was characterized by scanning electron microscopy, energy dispersive spectrum, atomic force microscopy, and cyclic voltammetry analyses. Structural characterization results were represented high resistance, compact surface, and better electrocatalytic properties for Ti/IrO2-Ta2O5-SiO2 electrode. The influence of critical operational parameters, including applied current density, initial pH, initial concentration of MTZ, and electrolysis time, was detected on electrocatalytic removal efficiency. Significant findingsIt was recognized that the high electrocatalytic power of SiO2-incorporated MMO-Ti/IrO2-Ta2O5 electrode gives a high amount of •OH during the removal process. At optimum conditions (pH = 7, J = 200 mA/cm2, [MTZ]0 = 40 mg/L, t = 80 min), 98.76% MTZ removal efficiency was achieved. Besides, comparison analysis was performed with SiO2-incorporated MMO-Ti/IrO2-Ta2O5 electrode and Ti/IrO2-Ta2O5, so results point out the higher total organic carbon removal of about 78.90% and 54.30% with SiO2-incorporated MMO-Ti/IrO2-Ta2O5 than Ti/IrO2-Ta2O5, respectively. Furthermore, MTZ removal intermediates formed during the electrocatalytic oxidation process were identified by the gas chromatography–mass spectrometry technique.

Cost-effective electrodes based on mixed iridium-zirconium oxides for vanadium electrolyte rebalancing cell

Developing the mixed metal oxide (MMO) electrodes with high electrocatalytic activity, low cost of materials and long service life is an important and relevant task for various applications including the vanadium electrolyte rebalancing. In this work, the cost-effective electrodes based on mixed iridium-zirconium oxides were fabricated using the Pechini method followed by thermal decomposition. The Raman and XPS analysis of mixed IrO2–ZrO2 electrodes showed a partially crystalline (IrO2 and ZrO2) and partially amorphous (IrOx and ZrOx) structure. Conducted electrochemical tests demonstrated the high electrocatalytic performance of the electrodes with 15–30 mol% of Zr addition comparable to Ir-based electrodes toward the OER. Successful proceeding of the electrochemical reduction in the rebalancing cell was proved using operando absorbance spectra analysis of the reducing vanadium electrolyte. The results of a prolonged vanadium electrolyte electroreduction showed more effective performance of the cell with 70Ir–30Zr electrode than the same cell with 70Ir electrode under a high current density of 250 mA cm−2. This approach allows reducing the electrode cost with reducing the Ir loading without sacrificing the electrocatalytic properties with Zr adding.

Removal of contaminants of emerging concern and Escherichia coli from effluent using an optimized electrochemical reactor

Contaminants of emerging concern (CECs) in wastewater are a matter of great importance as they have a huge environmental impact. This work investigated the electrochemical oxidation (EO) technology as an alternative tertiary treatment in wastewater treatment plants (WWTP) aiming to simultaneous remove CECs and Escherichia coli. Platinum titanium (Pt/Ti) and metal mixed oxide (MMO, RuO2–IrO2/Ti) electrode materials with different shapes (bar, mesh and circular mesh) were tested. The results obtained showed that removal rate not only depends on CECs structure and physicochemical characteristics, but electrode material and shape play a key role in the whole treatment process. Overall, the removal efficiency increased (20–50%) for the recalcitrant CECs when using MMO with the electrode characterized by a higher surface area with a circular-mesh as both anode and cathode. Varying the current intensity had a relatively minor effect on the yield of CECs removal. CECs removals were greater than 90% with simultaneous removal of E. coli within 2 h. Overall, EO can be considered an eco-friendly process since no reagents were added and has a low estimated treatment cost of 1.1 €/m3.

Polymetallic Clusters in Capacitive Deionization

Research and development of water purification technology can effectively alleviate the impact of water shortage. Capacitive deionization technology (CDI) is a novel water treatment technology, by applying low voltage, ions in solution are absorbed by electrodes to form double electric layers to remove pollutants in water. Electrode material is the main factor affecting the effect of CDI treatment. At present, the traditional CDI electrodes mainly focus on the research of carbon materials and carbon-based composites materials. However, their low effective specific surface area, complex preparation process and high resistance limit their application in practical wastewater treatment. Mixed metal oxides have become a new hotspot in the research and application of CDI electrode materials due to their excellent electrochemical properties, such as high reactivity, large specific capacitance and strong conductivity. However, there are few articles on its research and application in CDI. Therefore, this paper reviews the latest research progress, existing problems and application development of mixed metal oxide electrodes and their composite electrodes in the context of the types of polymetallic oxides.