Electrochemical Water Oxidation Research Articles

Molecular complex inspired design of an efficient copper(II)-containing robust porous polymers for electrochemical water oxidation

Homogeneous copper-bipyridine complexes have been reported as effective catalysts for water oxidation, demonstrating significant potential as alternatives to precious metal-based complexes. However, these complexes face long-term stability and product separation challenges similar to many homogeneous catalysts. Meanwhile, recent studies have underscored the potential of metalated polymer networks, which integrate the structural advantages of polymers with the functional benefits of metal species, making them highly attractive for various applications. Herein, we integrated copper-bipyridine units into porous polymer networks to overcome the limitations of copper-bipyridine complexes. We prepared a series of copper-incorporated polymers (TpBpy-Cux) for electrochemical water oxidation. The electrochemical properties of these polymers were tuned by varying the copper content, with TpBpy-Cu3 presenting the best performance among the samples studied. TpBpy-Cu3 demonstrated a low Tafel slope of 69 mV/decade, achieved a high Faradaic efficiency (FE) of 94 %, and exhibited exceptional stability over 1000 cyclic scans. This study offers insights into the design and optimization of metal-incorporated porous polymer networks building on the foundational understanding of their molecular counterparts for advanced catalytic applications.

Transforming Prussian Blue Analogues: The Future-Centric Pathway to Rise of Thin Layered Double Hydroxides for Superior Water Oxidation: An Overview

Prussian blue analogues (PBAs)[1–5] have exhibited great performance towards electrochemical water oxidation because of their high activity and stability. However, there were some challenges because of their limited surface area and less accessible active sites. Herein this work, we have explored the synthesis of a Prussian blue analogue-based thin layered double hydroxide (PBA-TLDH)[6–9] through a reconstruction method. The (PBA-TLDH) possessed high surface area and a large number of active sites for enhanced electrochemical water oxidation. These PBA-LDH materials were well characterized by X-ray diffraction, Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), and electrochemical techniques[1,7,10–12]. The PBA-TLDH materials had a well-defined thin layered structure with the PBAs uniformly distributed in the interlayer spaces of the LDHs. The PBA-LDH catalyst showed enhanced electrocatalytic activity towards water oxidation, with a very low overpotentials up to 240 mV and current density of 10 mA cm-2,[9,13–16] have been achieved which is significantly lower than that of the pristine PBAs. Moreover, these thin layered double hydroxides material showed excellent stability over large electrochemical cycle. This work provides a new strategy for the design and synthesis of highly active and stable electrocatalysts for water oxidation.

Carbonate-carbonate coupling on platinum surface promotes electrochemical water oxidation to hydrogen peroxide

Water electro-oxidation to form H2O2 is an important way to produce H2O2 which is widely applied in industry. However, its mechanism is under debate and HO(ads), hydroxyl group adsorbed onto the surface of the electrode, is regarded as an important intermediate. Herein, we study the mechanism of water oxidation to H2O2 at Pt electrode using in-situ Raman spectroscopy and differential electrochemical mass spectroscopy and find peroxide bond mainly originated from the coupling of two CO32- via a C2O62- intermediate. By quantifying the 18O isotope in the product, we find that 93% of H2O2 was formed via the CO32- coupling route and 7% of H2O2 is from OH(ads)-CO3•− route. The OH(ads)-OH(ads) coupling route has a negligible contribution. The comparison of various electrodes shows that the strong adsorption of CO3(ads) at the electrode surface is essential. Combining with a commercial cathode catalyst to produce H2O2 during oxygen reduction, we assemble a flow cell in which the cathode and anode simultaneously produce H2O2. It shows a Faradaic efficiency of 150% of H2O2 at 1 A cm−2 with a cell voltage of 2.3 V.

Nanohybrids of B- and N-codoped reduced graphene oxide/CeO2–Co3O4 nanocomposite for efficient water oxidation

Transition metal oxides are well known as highly active and durable catalysts for the oxygen evolution reaction (OER). In this work, to enhance the electrocatalytic activity of CeO2 nanostructure in the water oxidation reaction, CeO2/Co3O4 and boron-nitrogen co-doped reduced graphene oxide incorporated (B,N-rGO) into CeO2/Co3O4 with a weight ratio of 3:1 of nanoparticles to graphene oxide were synthesized using co-precipitation and hydrothermal methods, respectively. The prepared materials were studied in detail regarding their crystal structure and morphological properties. The as-prepared materials were used as modified carbon paste electrodes for electrochemical water oxidation in an alkaline solution. The results showed that incorporating graphene oxide network into the nanoparticles structure enhances their OER performance. The material based on CeO2/Co3O4@B,N-rGO hybrid nanocomposite showed excellent OER activity with an overpotential of 410@10 mA cm−2, which is 240 and 140 mV less than that of the single CeO2 and CeO2/Co3O4 samples. Also, CeO2/Co3O4@B,N-rGO displayed the lowest Tafel slope and charge transfer resistance among the other catalysts. Moreover, the proposed catalyst displayed high stability and durability during OER. The enhanced OER performance of the CeO2/Co3O4@B,N-rGO is attributed to several catalytically active sites created by B and N doped rGO in the nanocomposite structure. The high performance of this catalyst results from the synergy of combining CeO2/Co3O4 nanoparticles with a three-dimensional co-doped rGO network.

Heterobimetallic Synergism in Triple-Redox 2D Framework for Largely Boosted Water Oxidation and Flanked Carboxylic-Acid-Triggered Unconventional Tandem Catalysis.

A fish-bone-shaped and thermochemically stable 2D metal-organic framework (MOF) with multimodal active center-decked pore-wall is devised. Redox-active [Co2(COO)4] node and thiazolo[5,4-d]thiazole functionalization benefit this mixed-ligand MOF exhibiting electrochemical water oxidation with 375mV overpotential at 10mA cm-2 current density and 78mV per dec Tafel slope in alkaline medium. Pair of oppositely oriented carboxylic acids aids postmetalation with transition metal ions to engineer heterobimetallic materials. Notably, overpotential of Ni2+ grafted triple-redox composite reduces to 270mV with twofold declined Tafel slope than the parent MOF, ranking among the best-reported values, and outperforming majority of related catalysts. Significantly, turnover frequency and charge transfer resistance display 35.5 and 1.4-fold upsurge, respectively, with much uplifted chronopotentiometric stability and increase active surface area owing to synergistic Co(II)-Ni(II) coupling. The simultaneous presence of ─COOH and nitrogen-rich moieties renders this hydrogen-bonded MOF as acid-base synergistic catalyst for recyclable deacetalization-Knoevenagel reaction with >99% product yield under solvent-free mild condition. Besides control experiments, unique role of ─COOH as hydrogen-bond donor site in substrate activation is validated from comparing the performances of molecular-shearing approach-derived structurally similar unfunctionalized MOF, and the heterobimetallic composite. To the best of tandem Knoevenagel condensation, larger-sized acetal exhibits poor yield of α,β-unsaturated dicyanides, and demonstrates pore-fitting-mediated size-selectivity.

Promoting effect of interfacial hole accumulation on photoelectrochemical water oxidation in BiVO4 and Mo-doped BiVO4

Hole transfer at the semiconductor-electrolyte interface is a key elementary process in (photo)electrochemical (PEC) water oxidation. However, up to now, a detailed understanding of the hole transfer and the influence of surface hole density on PEC water oxidation kinetics is lacking. In this work, we propose a model for the first time in which the surface accumulated hole density in BiVO4 and Mo-doped BiVO4 samples during water oxidation can be acquired via employing illumination-dependent Mott-Schottky measurements. Based on this model, some results are demonstrated as below: (1) Although the surface hole density increases when increasing light intensity and applied potential, the hole transfer rate remains linearly proportional to surface hole density on a log-log scale. (2) Both water oxidation on BiVO4 and Mo-doped BiVO4 follow first-order reaction kinetics at low surface hole densities, which is in good agreement with literature. (3) We find that water oxidation active sites in both BiVO4 and Mo-doped BiVO4 are very likely to be Bi5+, which are produced by photoexcited or/and electro-induced surface holes, rather than VOx species or Mo6+ due to their insufficient redox potential for water oxidation. (4) Introduction of Mo doping brings about higher OER activity of BiVO4, as it suppresses the recombination rate of surface holes and increases formation of Bi5+. This surface hole model offers a general approach for the quantification of surface hole density in the field of semiconductor photoelectrocatalysis.

Activating Lattice Oxygen Oxidation Mechanism in Asymmetric [IrO6] Octahedra of Ir‐Based Oxides Toward Superior Acidic Electrochemical Water Oxidation

Activating Lattice Oxygen Oxidation Mechanism in Asymmetric [IrO₆] Octahedra of Ir‐Based Oxides Toward Superior Acidic Electrochemical Water Oxidation

Structure Properties Correlations on Nickel‐Iron Oxide Catalysts Deposited by Atomic Layer Deposition for the Oxygen Evolution Reaction in Alkaline Media

Thermal atomic layer deposition (ALD) is used for the first time to deposit iron‐nickel oxides onto carbon nanotubes in a ternary process to produce a wide range of mixed oxide thin films. When using ferrocene (FeCp2) and nickelocene (NiCp2) with ozone (O3) as metals and oxygen sources, respectively, a competition between the metal precursors and the growth modes is observed. Indeed, while ferrocene promotes a 2D‐growth, nickelocene prefers a 3D‐growth. Although both precursors are homoleptic metallocenes, they behave differently in the ALD of their respective metal oxide, leading to unexpected atomic ratios and films morphologies of the iron‐nickel oxides. The 2Fe:1Ni sample displays the best performances in the electrochemical water oxidation (oxygen evolution reaction) exhibiting an overpotential of 267 mV at a current density of 10 mA cm−1, a Tafel slope of 36.8 mV dec−1, as well as a good stability after 15 h of continuous operation.

Electrochemically Created Active Centers in a Bimetallic CoNi-Triazole Metal-Organic Framework for Enhanced Oxygen Evolution Reaction Activity.

Electrochemical water oxidation utilizing bimetallic CoNi-Tz (Tz = 1,2,4-triazole) framework is explored. Initially, CoNi-Tz possesses active tetrahedral Co center and electron-mediated octahedral Ni chain. After performing an electrochemical activation, the partial structural transformation on the Ni center occurs. This leads to the generation of excessive active centers which can promote catalytic activity of the framework. The activated CoNi-Tz catalyst displays a remarkably low OER overpotential of 293 mV at a current density of 10 mA cm-2 with a small Tafel slope of 49.98 mV dec-1, outperforming the single metal Co-Tz and benchmark IrO2 catalysts.

Bioinspired octanuclear copper cubane complex as an efficient molecular electrocatalyst for water oxidation

Water oxidation catalysts (WOCs) with excellent activity and stability are highly desired. Numerous efforts have been dedicated to develop WOCs mimicking the cubic structure and activity of Mn4CaO5 cluster in oxygen-evolving center (OEC) of photosynthesis system II (PSII). Herein, a octanuclear Cu cluster [Cu8(dpy{OH}O)8(OAc)4]4+ (1, dpy{OH}O− = the monoanion of the hydrated, gem-diol form of di-2-pyridyl ketone) that contains two Cu4O4 cubic fragments with bioinspired structure mimicking the Mn4CaO5 cluster of OEC of natural PSII is developed as efficient homogeneous catalyst for electrochemical water oxidation under near neutral condition for the first time. It exhibits outstanding water oxidation catalytic activity via accelerating oxygen evolution with onset overpotential of 730 mV and appreciable turnover frequency of 20.1 s−1. Collective experiments together confirmed the homogeneity and stability of cluster 1 under long-term water oxidation catalytic cycle. Kinetic study reveals the water nucleophilic attack (WNA) catalytic mechanism occurs on the coordination unsaturated Cu center of the cluster. This work provides the inspiration of the development of robust bioinspired catalyst for electrochemical water oxidation.

Investigation of the Activity and Stability of Manganese-Based Electrocatalysts for the Electrochemical Oxidation of Water and Small Organic Molecules in Acidic Electrolyte

Water electrolysis is a crucial process for hydrogen production, providing a clean and sustainable method for H2 production. Iridium-based catalysts are particularly effective in promoting the oxygen evolution reaction (OER) that takes place at the anode of electrolyzers. The unique properties of iridium make it well-suited for this purpose, but its high cost becomes a bottleneck in making water electrolysis economically viable on a large scale. Manganese-based electrocatalysts have emerged as promising candidates for the OER, mainly due to their earth abundance, low toxicity, and activity. It has been shown that manganese oxide can operate stably in acid, in a restricted potential window, where the formation of permanganate ions is avoided. Thus, for practical application, the stability must be improved, so a larger potential window can be employed. To address this challenge, some approaches have emerged from combining a catalytically active, but instable oxide, with the one that is stable, but inactive for the OER. In the presentation, we will show the results of activity, stability, and faradaic efficiency (FE) for O2 formation for water electro-oxidation on manganese oxide, and on manganese oxide combined with antimony, tin, titanium, niobium, and silicon oxide. The faradaic efficiencies, determined via EC-MS (Electrochemistry coupled to mass spectrometry) measurements revealed that manganese antimonate and pure manganese oxide have the highest values for O2 formation, with the former presenting higher stability at high overpotentials. We will discuss the obtained trends in stability and FEs for the other synthesized oxides. Additionally, for these two most efficient electrocatalysts, EC-MS measurements were conducted for the electrochemical oxidation of some small organic molecules, such as formic acid, methanol, ethanol, and ethylene glycol (electrochemical reforming for H2 production). The results showed that the selectivity for the electro-oxidation of water and/or of the organic molecule (monitoring the formation of CO2) strongly depends on the catalyst composition and on the number of carbon atoms of the molecule. We will further discuss, in the presentation, the factors that govern the selectivity and stability for each case. The obtained results of this study may be helpful for developing more efficient, selective and, mainly, more stable earth-abundant electrocatalysts for electrolyzers.

Stable production of hydrogen peroxide over zinc oxide @ zeolitic imidazolate Framework-8 composite catalysts

A promising method of producing hydrogen peroxide (H2O2) is the electrochemical two-electron water oxidation reaction (2e− WOR). In this process, it is important to design electrocatalysts that are both earth abundant and environmentally friendly, as well as offering high stability and production rates. The research of WOR catalysts, such as the extensively used transition metal oxides, is mainly focused on the modification of transition metal elements. Few studies pay attention to the protective heterostructure of metal oxides. Here, we demonstrate for the first time an organometallic skeleton protection strategy to develop highly stable WOR catalysts for H2O2 generation. Unlike the pure ZnO and zeolite imidazole framework-8 (ZIF-8) catalysts, ZnO@ZIF-8 enabled the production of hydrogen peroxide at high voltages. The experimental results demonstrate that the ZnO@ZIF-8 catalyst stably generates H2O2 even under a high voltage of 3.0 V vs. RHE, with a yield reaching 2845.819 μmolmin−1 g−1. ZnO@ZIF-8 shows a relatively low overpotential, with a current density of 10 mA cm−2 and an overpotential of 110 mV. The ZnO@ZIF-8 catalyst’s maximal FE value was 4.72 %. Moreover, the ZnO@ZIF-8 catalyst exhibits remarkable durability even after an extended 60-hour stability test. Operando Raman and theoretic calculation analyses reveal that the metal–organic skeleton being encapsulated on the metal oxide surface synergizes with each other, not only expanding the electrochemical surface area, but also adjusting the catalyst metal sites’ adsorption capacity. A novel approach to the modification of 2e− WOR metal oxide catalyst is presented in this work.

Advanced Nanocarbons Toward two-Electron Oxygen Electrode Reactions for H2O2 Production and Integrated Energy Conversion.

Hydrogen peroxide (H2O2) plays a pivotal role in advancing sustainable technologies due to its eco-friendly oxidizing capability. The electrochemical two-electron (2e-) oxygen reduction reaction and water oxidation reaction present an environmentally green method for H2O2 production. Over the past three years, significant progress is made in the field of carbon-based metal-free electrochemical catalysts (C-MFECs) for low-cost and efficient production of H2O2 (H2O2EP). This article offers a focused and comprehensive review of designing C-MFECs for H2O2EP, exploring the construction of dual-doping configurations, heteroatom-defect coupling sites, and strategic dopant positioning to enhance H2O2EP efficiency; innovative structural tuning that improves interfacial reactant concentration and promote the timely release of H2O2; modulation of electrolyte and electrode interfaces to support the 2e- pathways; and the application of C-MFECs in reactors and integrated energy systems. Finally, the current challenges and future directions in this burgeoning field are discussed.

Tailoring Electrochemical Water Oxidation Activity from the Isostructural Series of Alkaline‐Stable Bimetallic Fe,Ni‐Azolate Metal–Organic Frameworks

AbstractIncorporating electrocatalytic active sites into the reticular framework allows for the design of novel catalytic structures promoting desired catalytic reactions with efficient mass transfer. However, to fully exploit the advantages of such a framework, it is crucial to ensure the electrochemical stability of the material. In this context, alkaline‐stable bimetallic MxNi2‐xCl2BTDD (M = Fe, Co) metal–organic frameworks (MOFs) are employed as electrocatalysts for alkaline water oxidation reaction (WOR) to understand the correlation between secondary metal incorporation and catalytic activity changes. For a specific Fe ratio within the MOF, the optimal catalytic activity is achieved at an overpotential of 315 mV at a WOR current density of 10 mA cm−2geo. Based on its well‐defined crystal structure and uniformly arranged metal nodes, which are not limited to specific facets, the cooperative electrocatalytic mechanism during the faradaic WOR process is investigated through operando Raman spectroscopy and density functional theory calculations. These results enable the interpretation of the electrochemical WOR mechanism based on MOF, providing a further fundamental understanding of electrocatalytic activity in bimetallic systems.

Continuous Production of Ethylene and Hydrogen Peroxide from Paired Electrochemical Carbon Dioxide Reduction and Water Oxidation (Adv. Energy Mater. 18/2024)

Continuous Production of Ethylene and Hydrogen Peroxide from Paired Electrochemical Carbon Dioxide Reduction and Water Oxidation (Adv. Energy Mater. 18/2024)

Solar enhanced oxygen evolution reaction with transition metal telluride.

The photo-enhanced electrocatalytic method of oxygen evolution reaction (OER) shows promise for enhancing the effectiveness of clear energy generation through water splitting by using renewable and sustainable source of energy. However, despite benefits of photoelectrocatalytic (PEC) water splitting, its uses are constrained by its low efficiency as a result of charge carrier recombination, a large overpotential, and sluggish reaction kinetics. Here, we illustrate that Nickel telluride (NiTe) synthesized by hydrothermal methods can function as an extremely effective photo-coupled electrochemical oxygen evolution reaction (POER) catalyst. In this study, NiTe was synthesized by hydrothermal method at 145°C within just an hour of reaction time. In dark conditions, the NiTe deposited on carbon cloth substrate shows a small oxygen evolution reaction overpotential (261mV) at a current density of 10mA cm-2, a reduced Tafel slope (65.4mV dec-1), and negligible activity decay after 12h of chronoamperometry. By virtue of its enhanced photo response, excellent light harvesting ability, and increased interfacial kinetics of charge separation, the NiTe electrode under simulated solar illumination displays exceptional photoelectrochemical performance exhibiting overpotential of 165mV at current density of 10mA cm-2, which is about 96mV less than on dark conditions. In addition, Density Functional Theory investigations have been carried out on the NiTe surface, the results of which demonstrated a greater adsorption energy for intermediate -OH on the catalyst site. Since the -OH adsorption on the catalyst site correlates to catalyst activation, it indicates the facile electrocatalytic activity of NiTe owing to favorable catalyst activation. DFT calculations also revealed the facile charge density redistribution following intermediate -OH adsorption on the NiTe surface. This work demonstrates that arrays of NiTe elongated nanostructure are a promising option for both electrochemical and photoelectrocatalytic water oxidation and offers broad suggestions for developing effective PEC devices.

Electrochemical versus Photoelectrochemical Water Oxidation Kinetics on Bismuth Vanadate (Photo)anodes.

This study reports a comparison of the kinetics of electrochemical (EC) versus photoelectrochemical (PEC) water oxidation on bismuth vanadate (BiVO4) photoanodes. Plots of current density versus surface hole density, determined from operando optical absorption analyses under EC and PEC conditions, are found to be indistinguishable. We thus conclude that EC water oxidation is driven by the Zener effect tunneling electrons from the valence to conduction band under strong bias, with the kinetics of both EC and PEC water oxidation being determined by the density of accumulated surface valence band holes. We further demonstrate that our combined optical absorption/current density analyses enable an operando quantification of the BiVO4 photovoltage as a function of light intensity.

Nanostructured boron-doped diamond electrodes for two-electron water oxidation to produce hydrogen peroxide

Boron-doped diamond (BDD) electrodes have been widely used in water treatment, e.g., degradation of antibiotics and pigments for water purification. Hydrogen peroxide production by two-electron electrochemical oxidation of water is a green and attractive method that has the potential to replace the conventional anthraquinone autoxidation (AO) process. In this study, BDD films were prepared on single-crystal silicon wafers by using hot-filament chemical vapor deposition (HFCVD) technique. Then, dense tubular nanostructure (nanotubes) arrays were prepared on the BDD films employing inductively coupled plasma (ICP) etching. Energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) proved the formation of oxygen-terminated nanostructures, which was ascribed to the oxygen plasma and the self-masking. The length of diamond nanotubes as well as the root diameter increased gradually and non-linearly with the increase of etching time. It was indicated by cyclic voltammetry (CV) analysis that the effective active area of the electrodes increased after etching, and variations of resulting reaction performance were observed. The oxygen-terminated and nanostructured BDD electrode obtained by etching with the optimized parameters features better electrocatalytic performance, with reduced oxidation potential and 46.8 % higher Faraday Efficiency (FE) for hydrogen peroxide production, compared to the original BDD sample. Therefore, the manufacturing of nanostructures is an effective method to improve the efficiency of BDD water oxidation for the production of hydrogen peroxide.

Nanoneedles like FeP engineered on Ni-Foam as an effective catalyst towards overall alkaline freshwater, urea, and seawater splitting

The slow kinetics of electrochemical urea and water oxidation processes at electrode surfaces, which can be recognised as promising resources for pollution-free hydrogen energy production, motivate the scientific community to design and fabricate low-cost, high-efficiency electrocatalysts. Because the performance of electrocatalysts is dependent on their structural, morphological, and electronic properties, herein, the tuning of these properties of Fe3O4@Ni-Foam via phosphorylation and sulfurization, yielding iron phosphide (FeP@Ni-Foam) and iron sulphide (FeS@Ni-Foam), with nanoneedles (along with microstructures) and nanoflower-like morphologies, respectively, is investigated. Among all the prepared samples, FeP is found to be the most effective electrocatalyst for the oxygen evolution process (OER), the urea oxidation reaction (UOR), and seawater electrolysis. The overpotentials observed for OER, UOR, and seawater splitting are significantly reduced when using FeP as compared to other materials, with values of 207 mV, 133 mV, and 287.1 mV, respectively, at a current density of 10 mA/cm2. The enhanced catalytic activity of FeP over FeS and Fe3O4 is attributed to morphological changes, improved electronic conductivity, and exceptional endurance. The theoretical studies reveal that FeP has a better density of states over the fermi level than FeS and Fe3O4, lowering the energy barriers for the OER and UOR processes and demonstrating significant catalytic activity towards these processes. This work demonstrates that phosphorylation and sulfurization treatments can alter morphologies and electrical characteristics, hence improving the catalytic activity of the materials.

Semimetallic hydroxide materials for electrochemical water oxidation

Semimetallic hydroxide materials for electrochemical water oxidation