Carbonate Ions Induce Highly Efficient Electrocatalytic Water Oxidation by Cobalt Oxyhydroxide Nanoparticles

Macrocyclic cyanocobalamin (vitamin B12) as a homogeneous electrocatalyst for water oxidation under neutral conditions

Homogeneous electrochemical water oxidation under neutral conditions using impressively stable vitamin B₁₂.

Promotive Effect of Bicarbonate Ion on Two-Electron Water Oxidation to Form H2 O2 Catalyzed by Aluminum Porphyrins

Two-electron water oxidation initiated by one-electron oxidation of aluminum porphyrins (AlTMPyP) is an alternative water oxidation to the conventional four-electron pathway and could help to avoid the bottleneck subject of photon-flux density in artificial photosynthesis. Here, a dramatic enhancement of the reactivity by bicarbonate ion in the two-electron water oxidation to form H₂ O₂ is reported. An addition of sodium carbonate (Na₂ CO₃ ) controlled both catalytic current and product selectivity of the two-electron water oxidation to enhance the activity of AlTMPyP at pH≈10-11. Controlled potential electrolysis experiments at different concentrations of Na₂ CO₃ (10-100 mm) showed that peroxide selectivity was improved up to approximately 73 % by the increase of [Na₂ CO₃ ] added to the system. The promotion of the reaction cycle was induced by an enhanced dynamic capturing of H₂ O₂ from the hydroperoxy complex of AlTMPyP through an attack of a bicarbonate ion. The detailed electrochemical studies and product selectivity indicated that the bicarbonate ion served as a good cofactor for producing H₂ O₂ from water. At stronger alkaline conditions (pH 12.5), however, a retardative effect of the addition of Na₂ CO₃ on the catalytic reactivity was observed.

Modelling the (Essential) Role of Proton Transport by Electrolyte Bases for Electrochemical Water Oxidation at Near-Neutral pH

The oxygen-evolution reaction (OER) in the near-neutral pH-regime is of high interest, e.g., for coupling of OER and CO2-reduction in the production of non-fossil fuels. A simple model is proposed that assumes equal proton activities in the catalyst film and the near-surface electrolyte. Equations are derived that describe the limitations relating to proton transport mediated by fluxes of molecular “buffer bases” in the electrolyte. The model explains (1) the need for buffer bases in near-neutral OER and (2) the pH dependence of the catalytic current at high overpotentials. The latter is determined by the concentration of unprotonated buffer bases times an effective diffusion constant, which can be estimated for simple cell geometries from tabulated diffusion coefficients. The model predicts (3) a macroscopic region of increased pH close to the OER electrode and at intermediate overpotentials, (4) a Tafel slope that depends on the reciprocal buffer capacity; both predictions are awaiting experimental verification. The suggested first-order model captures and predicts major trends of OER in the near-neutral pH, without accounting for proton-transport limitations at the catalyst–electrolyte interface and within the catalyst material, but the full quantitative agreement may require refinements. The suggested model also may be applicable to further electrocatalytic processes.

H/D Isotope Effects Reveal Factors Controlling Catalytic Activity in Co-Based Oxides for Water Oxidation

Understanding the mechanism for electrochemical water oxidation is important for the development of more efficient catalysts for artificial photosynthesis. A basic step is the proton-coupled electron transfer, which enables accumulation of oxidizing equivalents without buildup of a charge. We find that substituting deuterium for hydrogen resulted in an 87% decrease in the catalytic activity for water oxidation on Co-based amorphous-oxide catalysts at neutral pH, while ¹⁶O-to-¹⁸O substitution lead to a 10% decrease. In situ visible and quasi-in situ X-ray absorption spectroscopy reveal that the hydrogen-to-deuterium isotopic substitution induces an equilibrium isotope effect that shifts the oxidation potentials positively by approximately 60 mV for the proton coupled Co^II/III and Co^III/IV electron transfer processes. Time-resolved spectroelectrochemical measurements indicate the absence of a kinetic isotope effect, implying that the precatalytic proton-coupled electron transfer happens through a stepwise mechanism in which electron transfer is rate-determining. An observed correlation between Co oxidation states and catalytic current for both isotopic conditions indicates that the applied potential has no direct effect on the catalytic rate, which instead depends exponentially on the average Co oxidation state. These combined results provide evidence that neither proton nor electron transfer is involved in the catalytic rate-determining step. We propose a mechanism with an active species composed by two adjacent Co^IV atoms and a rate-determining step that involves oxygen-oxygen bond formation and compare it with models proposed in the literature.

Structural and functional role of anions in electrochemical water oxidation probed by arsenate incorporation into cobalt-oxide materials

Arsenate ions are incorporated in amorphous cobalt oxide catalysts at the periphery of the lattice or substituting cobalt ions.

Superior Inorganic Ion Cofactors of Tetraborate Species Attaining Highly Efficient Heterogeneous Electrocatalysis for Water Oxidation on Cobalt Oxyhydroxide Nanoparticles

A heterogeneous catalyst incorporating an inorganic ion cofactor for electrochemical water oxidation was exploited using a CoO(OH) nanoparticle layer-deposited electrode. The significant catalytic current for water oxidation was generated in a Na₂B₄O₇ solution at pH 9.4 when applying 0.94 V versus Ag/AgCl in contrast to no catalytic current generation in the K₂SO₄ solution at the same pH. HB₄O₇^- and B₄O₇^2- ions were indicated to act as key cofactors for the induced catalytic activity of the CoO(OH) layer. The Na₂B₄O₇ concentration dependence of the catalytic current was analyzed based on a Michaelis-Menten-type kinetics to provide an affinity constant of cofactors to the active sites, K_m = 28 ± 3.6 mM, and the maximum catalytic current density, I_max = 2.3 ± 0.13 mA cm^-2. The I_max value of HB₄O₇^- and B₄O₇^2- ions was 1.4 times higher than that (1.3 mA cm^-2) for the previously reported case of CO₃^2- ions. This could be explained by the shorter-range proton transfer from the active site to the proton-accepting cofactor because of the larger size and more flexible conformation of HB₄O₇^- and B₄O₇^2- ions compared with that of CO₃^2- ions.