Catalysts For Electrochemical Water Oxidation Research Articles

Stabilization of Ni-containing Keggin-type Polyoxometalates with Variable Oxidation States as Novel Catalysts for Electrochemical Water Oxidation

The development of new recyclable and inexpensive electrochemically active species for water oxidation catalysis is the most crucial step for future utilization of renewables. Particularly, transition metal complexes containing internal...

A cauliflower-like Ni metal-organic phosphate modified carbon paste electrode as an efficient catalyst for electrochemical water oxidation in neutral media

We fabricated the cauliflower-like Ni metal-organic phosphates (Ni-MOPh) with different synthesis time (Ni-MOPh-Ts, Ts = Times) as a carbon paste electrode material (Ni-MOPh-Ts/C) for oxygen evolution reaction (OER) in neutral media. The as-synthesized Ni-MOPh-12 h/C shows a better OER ability than that of Ni-MOPh-6 h/C and Ni-MOPh-2 h/C, which also shows conspicuous performance with the overpotential of 420 mV to obtain 10 mA cm−2. Reaction kinetics studies exhibit that Ni-MOPh-12 h/C can provide higher electrochemical ability, mainly due to the small charge transfer resistance, and a high number of catalytic active sites.

Revealing electronic structure of nanostructured cobalt titanate via a combination of optical and electrochemical approaches toward water splitting and CO2 reduction

AbstractBackgroundThe shortage of clean energy has become a serious problem due to the rapid development of societies and the increasing consumption of fossil fuels. Metal oxide semiconductor nanomaterials have been studied as photo‐/electrocatalysts for water splitting in terms of clean energy generation. Applications of semiconductors depend on their electronic structures. Therefore, elucidating the electronic diagram is essential for determining the specific applications of novel semiconductors.ResultsHerein, we demonstrate using a combination of UV–visible diffuse reflectance spectroscopy (DRS) and Mott–Schottky analysis via electrochemical impedance spectroscopy for sketching the electronic diagram of nanostructured cobalt titanate (CoTiO3) prepared by the sol–gel method. UV–visible DRS studies reveal a band gap of 2.5 and 2.1 eV for direct and indirect transitions of prepared nanostructured materials, respectively. Mott–Schottky analysis shows a 0.8 V versus Ag/AgCl value for the flat band potential for CoTiO3. We further show the application of this diagram toward the interpretation of the electrochemical behavior of nanostructured CoTiO3 for electrochemical water splitting reactions and the electrochemical CO2 reduction reaction (eCO2RR).ConclusionThe presented electrochemical and photoelectrochemical studies demonstrate nanostructured CoTiO3 as an effective catalyst for electrochemical water oxidation and the eCO2RR. Moreover, our results provide valuable information for further investigation of water splitting and photovoltaic energy conversion. © 2023 Society of Chemical Industry (SCI).

Size‐Controlled Synthesis of IrO2 Nanoparticles at High Temperatures for the Oxygen Evolution Reaction

AbstractIridium oxide is the state‐of‐the‐art catalyst for electrochemical water oxidation in an acidic medium. Despite being one of the rarest elements in the Earth's crust, there is a pressing need to maximize the utilization and longevity of active iridium centers. While conventional low‐temperature synthesis can yield nanostructures with high mass‐specific activity, they are often insufficiently stable during water oxidation. Structurally ordered iridium oxide is one of the most stable electrocatalysts utilized in polymer electrolyte membrane water electrolysis that benefits from the chemically ordered structure. However, its preparation requires thermal treatment at high temperatures, which improves its durability but can also result in reduced surface area and altered particle morphology. In this study, the challenge of controlling nanoparticle size during the preparation of structurally ordered iridium oxide is successfully addressed, which typically requires high‐temperature thermal treatment. By utilizing a silica nanoreactor as a hard template, a precise control is achieved over the nanoparticle size during high‐temperature thermal treatment. This approach maintains high durability while avoiding the common problem of reduced surface area and altered particle morphology. Specifically, this study is able to synthesize iridium oxide nanoparticles at temperatures up to 800 °C, while keeping their dimensions below 10 nm.

Construction of nickel selenides heterointerfaces with electron redistribution for solar-driven water splitting

Developing heterostructures electrocatalysts was a promising method to improve the water splitting efficiency. However, due to the difficulty of synthesis, such low-cost and high-activity heterostructures electrocatalysts have not been developed. In this work, Ni3Se2/NiSe heterostructure nanosheets with rich-phase boundaries were synthesized by soaking at room temperature and annealing treatment using nickel foam as both Ni source and substrate. Such a nanosheet-like Ni3Se2/NiSe heterostructure could expose more active sites and efficient mass transfer at solid-liquid-gas three-phase interfaces, which exhibits a lower overpotential of 336 mV at 100 mA cm−2 and exceptional stability over 80 h at the current density of 300 mA cm−2 for OER in alkaline solution. Furthermore, the heterostructures electrode implements solar-driven water with a high solar-to-hydrogen efficiency of 18.3%. This heterostructure strategy might be a major breakthrough for improving the transition metal selenides and designing high-active and stable catalysts for electrochemical water oxidation.

A hollow cage-like layered double hydroxide-modified carbon paste electrode as an efficient catalyst for electrochemical water oxidation in neutral media

We fabricated a hollow cage-like layered double hydroxide-modified carbon paste electrode as an efficient catalyst for electrochemical water oxidation in neutral media.

Stabilization and activation of polyoxometalate over poly(vinyl butylimidazolium) cations towards electrocatalytic water oxidation in alkaline media.

Sandwich polyoxometalate [WCo3(H2O)2(CoW9O34)2]12- (Co-WCo3) heterogenized with poly(vinyl butyl imidazolium) cations (PVIM+) acts as a stable electrochemical water oxidation catalyst in 1 M KOH with 0.28 V@10 mA cm-2 overpotential and TOF of 6.16 s-1. Various in situ spectroelectrochemical studies and control experiments provide the details of the catalytic activities and long-term stability of the catalyst.

Sol-Gel Synthesized High Entropy Metal Oxides as High-Performance Catalysts for Electrochemical Water Oxidation.

Hexanary high-entropy oxides (HEOs) were synthesized through the mechanochemical sol-gel method for electrocatalytic water oxidation reaction (WOR). As-synthesized catalysts were subjected to characterization, including X-ray diffraction (XRD), Fourier transforms infrared (FTIR) analysis, and scanning electron microscopy (SEM). All the oxide systems exhibited sharp diffraction peaks in XRD patterns indicating the defined crystal structure. Strong absorption between 400–700 cm−1 in FTIR indicated the formation of metal-oxide bonds in all HEO systems. WOR was investigated via cyclic voltammetry using HEOs as electrode platforms, 1M KOH as the basic medium, and 1M methanol (CH3OH) as the facilitator. Voltammetric profiles for both equiatomic (EHEOs) and non-equiatomic (NEHEOs) were investigated, and NEHEOs exhibited the maximum current output for WOR. Moreover, methanol addition improved the current profiles, thus leading to the electrode utility in direct methanol fuel cells as a sequential increase in methanol concentration from 1M to 2M enhanced the OER current density from 61.4 to 94.3 mA cm−2 using NEHEO. The NEHEOs comprising a greater percentage of Al, ([Al0.35(Mg, Fe, Cu, Ni, Co)0.65]3O4) displayed high WOR catalytic performance with the maximum diffusion coefficient, D° (10.90 cm2 s−1) and heterogeneous rate constant, k° (7.98 cm s−1) values. These primary findings from the EC processes for WOR provide the foundation for their applications in high-energy devices. Conclusively, HEOs are proven as novel and efficient catalytic platforms for electrochemical water oxidation.

Water oxidation reaction in the presence of a dinuclear Mn(II)-semicarbohydrazone coordination compound.

Water splitting, producing of oxygen, and hydrogen molecules, is an essential reaction for clean energy resources and is one of the challenging reactions for artificial photosynthesis. The Mn4Ca cluster in photosystem II (PS-II) is responsible for water oxidation in natural photosynthesis. Due to this, water oxidation reaction by Mn coordination compounds is vital for mimicking the active core of the oxygen-evolving complex in PS-II. Here, a new dinuclear Mn(II)-semicarbohydrazone coordination compound, [Mn(HL)(µ-N3)Cl]2 (1), was synthesized and characterized by various methods. The structure of compound 1 was determined by single crystal X-ray analysis, which revealed the Mn(II) ions have distorted octahedral geometry as (MnN4OCl). This geometry is created by coordinating of oxygen and two nitrogen donor atoms from semicarbohydrazone ligand, two nitrogen atoms from azide bridges, and chloride anion. Compound 1 was used as a catalyst for electrochemical water oxidation, and the surface of the electrode after the reaction was investigated by scanning electron microscopy, energy dispersive spectrometry, and powder X-ray diffraction analyses. Linear sweep voltammetry (LSV) experiments revealed that the electrode containing 1 shows high activity for chemical water oxidation with an electrochemical overpotential as low as 377mV. Although our findings showed that the carbon paste electrode in the presence of 1 is an efficient electrode for water oxidation, it could not withstand water oxidation catalysis under bulk electrolysis and finally converted to Mn oxide nanoparticles which were active for water oxidation along with compound 1.

Fe0.5Co0.5-Co1.15Fe1.15O4/carbon composite nanofibers prepared by solution blow spinning: Structure, morphology, Mössbauer spectroscopy, and application as catalysts for electrochemical water oxidation

In this work, we report the synthesis of Fe0.5Co0.5-Co1.15Fe1.15O4/carbon composite nanofibers by solution blow spinning (SBS) and study their structure, morphology, and catalytic activity toward the oxygen evolution reaction (OER, electrochemical water oxidation) in an alkaline medium. The solution blow spun fibers, prepared using nitrate precursors (Fe and Co) and poly(vinyl pyrrolidone, PVP), were calcined at 620 °C for 1 h in argon atmosphere. The thermogravimetric test shows that the stabilization of mass loss and the formation of the compound occur at ∼600 °C. X-ray diffraction revealed the presence of two crystalline phases, Fe0.5Co0.5 and Co1.15Fe1.15O4, with crystallite sizes of 16.5 nm and 18.1 nm, respectively. Scanning and transmission electron microscopies show that the average fiber and nanoparticle diameters are 362 nm and 28.41 nm, respectively. The quantification of Fe0.5Co0.5 and Co1.15Fe1.15O4 phases determined by the Rietveld refinement agrees well with the relative absorption area (RAA) values obtained by Mössbauer spectroscopy. Electrochemical analyses of Fe0.5Co0.5-Co1.15Fe1.15O4/carbon composite nanofibers supported by commercial Ni foam reveal a low overpotential value of η = 308 mV at 10 mA cm−2 and only 400 mV to generate 450 mA cm−2. The chronopotentiometry test over 15 h indicates that Fe0.5Co0.5-Co1.15Fe1.15O4/carbon nanofibers have excellent chemical and mechanical stability for their use as electrodes for water oxidation.

Boosting Activity and Stability of Electrodeposited Amorphous Ce‐Doped NiFe‐Based Catalyst for Electrochemical Water Oxidation

AbstractNiFe‐based hydroxides are considered as promising nonprecious catalysts for water oxidation due to their low cost and easy preparation. However, the rational design of NiFe‐based electrocatalysts remains a great challenge to address the sluggish reaction kinetics and severe deactivation problems for oxygen evolution reaction (OER). Here, the authors report a facile approach to fabricate an amorphous Ce‐doped NiFe hydroxide catalyst, which enables high activity and outstanding stability toward OER in alkaline media. The overpotential of electrodeposited amorphous Ce‐doped NiFe is only 195 mV at 10 mA cm−2. Meanwhile, the durability of the amorphous Ce‐doped NiFe is maintained for 300 h at 100 mA cm−2. The comprehensive characterization results reveal that the improved electrochemical performance of the amorphous Ce‐doped NiFe catalyst is originated from the favorable oxidation transition of active sites enabled by Ce‐doping. It is a very good strategy to introduce highly oxidized state ions to regulate the NiFe‐based catalyst to improve the catalytic activity and stability.

In Situ Fabrication of Nickel-Iron Oxalate Catalysts for Electrochemical Water Oxidation at High Current Densities.

Ni-Fe-based electrode materials are promising candidates for the oxygen evolution reaction (OER). The synergy between Fe and Ni atoms is crucial in modulating the electronic structure of the active site to enhance electrochemical performance. Herein, a simple chemical immersion technique was used to grow Ni-Fe oxalate nanowires directly on a porous nickel foam substrate. The as-prepared Ni-Fe oxalate electrode exhibited an excellent electrochemical performance of the OER with ultralow overpotentials of 210 and 230 mV to reach 50 and 100 mA cm-2 current densities, respectively, in a 1 M KOH aqueous solution. The excellent OER performance of this Ni-Fe oxalate electrode can be attributed to its bimetallic composition and nanowire structure, which leads to an efficient ionic diffusion, high electronic conductivity, and fast electron transfer. The overall analysis indicates a suitable approach for designing electrocatalysts applicable in energy conversion.

Noncovalent Immobilization of Pentamethylcyclopentadienyl Iridium Complexes on Ordered Mesoporous Carbon for Electrocatalytic Water Oxidation

The attachment of molecular catalysts to conductive supports for the preparation of solid‐state anodes is important for the development of devices for electrocatalytic water oxidation. The preparation and characterization of three molecular cyclopentadienyl iridium(III) complexes, Cp*Ir(1‐pyrenyl(2‐pyridyl)ethanolate‐κO,κN)Cl (1) (Cp* = pentamethylcyclopentadienyl), Cp*Ir(diphenyl(2‐pyridyl)methanolate‐κO,κN)Cl (2), and [Cp*Ir(4‐(1‐pyrenyl)‐2,2′‐bipyridine)Cl]Cl (3), as precursors for electrochemical water oxidation catalysts, are reported. These complexes contain aromatic groups that can be attached via noncovalent π‐stacking to ordered mesoporous carbon (OMC). The resulting iridium‐based OMC materials (Ir‐1, Ir‐2, and Ir‐3) were tested for electrocatalytic water oxidation leading to turnover frequencies (TOFs) of 0.9–1.6 s−1 at an overpotential of 300 mV under acidic conditions. The stability of the materials is demonstrated by electrochemical cycling and X‐ray absorption spectroscopy analysis before and after catalysis. Theoretical studies on the interactions between the molecular complexes and the OMC support provide insight onto the noncovalent binding and are in agreement with the experimental loadings.

Co-Cu-P nanosheet-based open architecture for high-performance oxygen evolution reaction

The exploration of earth-abundant electrocatalysts and insight on the relationship between their activity and structure is challenging for large-scale electrochemical water oxidation. Herein, 2D Co-Cu-P nanosheets on Ni foam are fabricated by electrodeposition and subsequent phosphorization process. The Co-Cu-P nanosheets are highly interconnected to form an open-structured hierarchical network structure, which endows the product large surface area, efficient mass transportation, and fast gas releasing. In addition, the surface of Co-Cu-P would rapidly convert into hydroxides layer in the earlier oxygen evolution reaction (OER) process, which preserves the Co-Cu-P phosphide inside and serves as the real active sites for OER process. Benefiting from the rationally designed open-structured hierarchical network and the synergistic effect from the two kinds of metal cations with high conductivity, the resultant Co-Cu-P bimetal phosphide open-structured network delivers superb OER performance with an overpotential of 240 mV to drive a current density of 10 mA cm−2 in alkaline medium, outperforming the performance of the Co-P and Cu3P monometallic phosphide counterpart. This work provides a simple synthesis approach to construct bimetal phosphide with a rationally designed open-structured hierarchical network toward boosting electrocatalytic OER performance. The open-structured hierarchical Co-Cu-P/NF network would serve as a robust catalyst for electrochemical water oxidation. The remarkable OER electrocatalytic performance of Co-Cu-P/NF can be mainly attributed to the large surface area, efficient mass transportation and fast gas releasing, as well as the metallic character of Co-Cu-P/NF with a high conductivity.

Synthesis of iron and vanadium co‐doped mesoporous cobalt oxide: An efficient and robust catalysts for electrochemical water oxidation

Dual metal doping and optimization are considered as vital approaches for enhancing the electrocatalytic features toward oxygen evolution reaction. Herein, a sequence of Fe and V dual metal-doped mesoporous cobalt oxide (FeV/meso-Co) electrocatalysts was successfully synthesized through citric acid-assisted evaporation-induced self-assembly (EISA) method. The textural, morphological, crystallinity, and electrochemical activities of Fe/V-promoted meso-Co (124 m2/g) are found strongly associated with dual (Fe and V) metal concentration. Benefiting from the combined effect of FeV-doping, the FeV/meso-Co exhibited an extremely lower overpotential of 280 mV to reach 10 mA/cm2 for oxygen evolution reaction (OER) in 1M KOH electrolyte, which was the considerably lowest value among the earlier catalysts, and the FeV/meso-Co showed similar features as IrO2 electrodes. Furthermore, FeV/meso-Co electrodes display highly durable (>30 hours) electrocatalytic performance for OER. This inexpensive approach of producing transition dual metal-doped mesoporous materials offers excellent promise for fabricating efficient catalysts and other electrochemical energy-conversion devices.

Concisely Synthesized FeNiWOx Film as a Highly Efficient and Robust Catalyst for Electrochemical Water Oxidation

The critical bottleneck for water splitting, which is important for sustainable production of hydrogen, has remained in sluggish oxygen evolving reaction (OER) requiring insufficiently low overpote...

Electrochemical water oxidation by a copper complex with an N4-donor ligand under neutral conditions

A mononuclear copper(ii) complex [Cu(H2L)](NO3)2 with an N4-donor redox-active ligand is found to be an efficient homogeneous catalyst for electrochemical water oxidation with the assistance of ligand oxidation.

Spectroelectrochemical Experiment for Studying Water Oxidation with a Nickel Oxide Catalyst

Electrochemical and spectroelectrochemical techniques are very important for characterizing energy materials. In the search for new sources of renewable energy, water splitting for hydrogen production and CO2 reduction is attracting significant attention. These applications require efficient and durable catalysts and a detailed understanding of the underlying catalytic mechanism. Here, we experimentally study electrodeposited nickel oxide as a catalyst for electrochemical water oxidation by combining UV–vis and electrochemical techniques, with emphasis on revealing the interplay between catalytic activity and physical changes in the catalyst. Specifically, the spectroelectrochemical characterization of the catalyst at different pH enables students to establish correlations between catalytic activity, pH, and physical changes in the catalyst. Students are also introduced to fundamental principles of electrocatalysis such as Tafel plots, TOF (turnover frequency), and midpoint potential. Moreover, they are guided toward the reliable interpretation of optical absorption and electrochemical data, along with the conceptual aspects behind these techniques.

Selective Synthesis of Monodisperse CoO Nanooctahedra as Catalysts for Electrochemical Water Oxidation.

Thermal decomposition is a promising route for the synthesis of metal oxide nanoparticles because size and morphology can be tuned by minute control of the reaction variables. We synthesized CoO nanooctahedra with diameters of ∼48 nm and a narrow size distribution. Full control over nanoparticle size and morphology could be obtained by controlling the reaction time, surfactant ratio, and reactant concentrations. We show that the particle size does not increase monotonically with time or surfactant concentration but passes through minima or maxima. We unravel the critical role of the surfactants in nucleation and growth and rationalize the observed experimental trends in accordance with simulation experiments. The as-synthesized CoO nanooctahedra exhibit superior electrocatalytic activity with long-term stability during oxygen evolution. The morphology of the CoO particles controls the electrocatalytic reaction through the distinct surface sites involved in the oxygen evolution reaction.

Functions of MnOx in NaCl Aqueous Solution for Artificial Photosynthesis.

SummaryPhotoelectrochemical water splitting has been intensively investigated as artificial photosynthesis technology to convert solar energy into chemical energy. The use of seawater and salted water has advantages for minimum environmental burden; however, the oxidation of Cl− ion to hypochlorous acid (HClO), which has toxicity and heavy corrosiveness, should occur at the anode, along with the oxygen evolution. Here, O2 and HClO production in aqueous solution containing Cl− on photoanodes modified with various metal oxides was investigated. The modification of MnOx resulted in the promotion of the O2 evolution reaction (OER) specifically without HClO production over a wide range of conditions. The results will contribute not only to the practical application of artificial photosynthesis using salted water but also to the elucidation of substantial function of manganese as the element for OER center in natural photosynthesis.