Building upon recent study of cobalt-oxide electrocatalysts in fluoride-buffered electrolyte at pH 3.4, we have undertaken a mechanistic investigation of cobalt-catalyzed water oxidation in aqueous buffering electrolytes from pH 0-14. This work includes electrokinetic studies, cyclic voltammetric analysis, and electron paramagnetic resonance (EPR) spectroscopic studies. The results illuminate a set of interrelated mechanisms for electrochemical water oxidation in alkaline, neutral, and acidic media with electrodeposited Co-oxide catalyst films (CoO(x)(cf)s) as well as for a homogeneous Co-catalyzed electrochemical water oxidation reaction. Analysis of the pH dependence of quasi-reversible features in cyclic voltammograms of the CoO(x)(cf)s provides the basis for a Pourbaix diagram that closely resembles a Pourbaix diagram derived from thermodynamic free energies of formation for a family of Co-based layered materials. Below pH 3, a shift from heterogeneous catalysis producing O(2) to homogeneous catalysis yielding H(2)O(2) is observed. Collectively, the results reported here provide a foundation for understanding the structure, stability, and catalytic activity of aqueous cobalt electrocatalysts for water oxidation.

Nickel-iron oxides/hydroxides are among the most active electrocatalysts for the oxygen evolution reaction. In an effort to gain insight into the role of Fe in these catalysts, we have performed operando Mössbauer spectroscopic studies of a 3:1 Ni:Fe layered hydroxide and a hydrous Fe oxide electrocatalyst. The catalysts were prepared by a hydrothermal precipitation method that enabled catalyst growth directly on carbon paper electrodes. Fe(4+) species were detected in the NiFe hydroxide catalyst during steady-state water oxidation, accounting for up to 21% of the total Fe. In contrast, no Fe(4+) was detected in the Fe oxide catalyst. The observed Fe(4+) species are not kinetically competent to serve as the active site in water oxidation; however, their presence has important implications for the role of Fe in NiFe oxide electrocatalysts.

Earth-abundant materials capable of catalyzing the electrochemical decomposition of water into molecular hydrogen and oxygen are necessary components of many affordable water-splitting technologies. However, water oxidation catalysts that facilitate sustained oxygen evolution at device-relevant current densities in strongly acidic electrolytes have been limited almost exclusively to precious metal oxides. Here, we show that nanostructured films of cobalt oxide (Co 3 O 4 ) on fluorine-doped tin oxide (FTO) substrates, made by first depositing Co onto FTO and heating in air at 400 °C to produce films having a robust electrical and mechanical Co 3 O 4 /FTO interface, function as active electrocatalysts for the oxygen evolution reaction (OER) in 0.5 M H 2 SO 4 . The Co 3 O 4 /FTO electrodes evolve oxygen with near-quantitative Faradaic yields and maintain a current density of 10 mA/cm 2 for over 12 h at a moderate overpotential of 570 mV. At lower current densities that require lower overpotentials, sustained oxygen production for several days and weeks can be achieved.

Ni:Fe:Al mixed oxides were identified as highly active water oxidation electrocatalysts. A systematic investigation of these materials has led to the characterization of a well-defined NiFeAlO 4 inverse spinel catalyst. Electrochemical characterization of NiFeAlO 4 shows activity exceeding previously reported catalysts of similar composition and/or structure, including NiO, NiFe (9 : 1), and NiFe 2 O 4 .

Electrochemical water oxidation is a major focus of solar energy conversion efforts. This anodic half-reaction affords electrons and protons needed to achieve fuel production by reduction of water, CO 2 , or other abundant feedstocks at the cathode of a photoelectrochemical cell (PEC; Figure 1). [1][2][3] High kinetic barriers associated with the oxidation of water to O 2 [4,5] and the common use of high-cost electrocatalytic materials are among the challenges that limit the utility of photoelectrochemical energy storage. Improved electrocatalysts that operate at lower overpotential and avoid the use of expensive precious-metal or rare-earth elements are needed. While valuable progress is being made toward this goal, [1,[4][5][6][7][8][9][10]

No abstract

Electrochemical water oxidation is a major focus of solar energy conversion efforts. This anodic half-reaction affords electrons and protons needed to achieve fuel production by reduction of water, CO 2 , or other abundant feedstocks at the cathode of a photoelectrochemical cell (PEC; Figure 1). [1][2][3] High kinetic barriers associated with the oxidation of water to O 2 [4,5] and the common use of high-cost electrocatalytic materials are among the challenges that limit the utility of photoelectrochemical energy storage. Improved electrocatalysts that operate at lower overpotential and avoid the use of expensive precious-metal or rare-earth elements are needed. While valuable progress is being made toward this goal, [1,[4][5][6][7][8][9][10]