Operando Analysis of NiFe and Fe Oxyhydroxide Electrocatalysts for Water Oxidation: Detection of Fe4⁺ by Mössbauer Spectroscopy.

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.

Electrochemical water oxidation with cobalt-based electrocatalysts from pH 0-14: the thermodynamic basis for catalyst structure, stability, and activity.

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.

Interface-mediated noble metal deposition on transition metal dichalcogenide nanostructures.

Functionalizing the surfaces of transition metal dichalcogenide (TMD) nanosheets with noble metals is important for electrically contacting them to devices, as well as improving their catalytic and sensing capabilities. Solution-phase deposition provides a scalable approach to the creation of metal-TMD hybrid systems, but controlling such processes remains challenging. Here we elucidate the different pathways by which gold and silver deposit at room temperature onto colloidal 1T-WS2, 2H-WS2, 2H-MoSe2, 2H-WSe2, 1T'-MoTe2 and Td-WTe2 few-layer nanostructures to produce several distinct classes of 0D-2D and 2D-2D metal-TMD hybrids. Uniform gold nanoparticles form on all of the TMDs. By contrast, silver deposits as nanoparticles with a bimodal size distribution on the disulfides and diselenides, and as atomically thin layers on the ditellurides. The various sizes and morphologies of these surface-bound metal species arise from the relative strengths of the interfacial metal-chalcogen bonds during the reduction of Au3+ or Ag+ by the TMDs.