The development of active, cost-effective and stable oxygen-evolving catalysts is one of the major challenges for solar-to-fuel conversion towards sustainable energy generation. Iridium oxide exhibits the best available compromise between catalytic activity and stability in acid media, but it is prohibitively expensive for large-scale applications. Therefore, preparing oxygen-evolving catalysts with lower amounts of the scarce but active and stable iridium is an attractive avenue to overcome this economical constraint. Here we report on a class of oxygen-evolving catalysts based on iridium double perovskites which contain 32 wt% less iridium than IrO2 and yet exhibit a more than threefold higher activity in acid media. According to recently suggested benchmarking criteria, the iridium double perovskites are the most active catalysts for oxygen evolution in acid media reported until now, to the best of our knowledge, and exhibit similar stability to IrO2.

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Iridium-based double perovskites for efficient water oxidation in acid media

The surface composition of p‐type GaAs etched in HCl or Br2 in methanol, and n‐type InP etched in HCl, H2SO4, HNO3, or Br2 in methanol has been studied by means of x‐ray photoelectron spectroscopy (XPS). The surface compositions of GaAs and the binding energy of the surface As atoms vary with the etching solution and with the extent of oxidation of the surface. The full width at half‐maximum of the Ga(3p) photoelectron peak increases upon exposure of etched GaAs to air. The XPS results are compared with Schottky barrier heights previously measured for similarly prepared surfaces with Pb contacts. The amount of oxidized P on InP surfaces is higher after an HNO3 etch than after HCl, H2SO4, or Br2/methanol treatments. An HCl etch leaves an unoxidized slightly In‐rich surface.

XPS study of chemically etched GaAs and InP

To probe the active sites of nitrogen-doped carbon nanostructures (CNx), the effect of dihydrogen phosphate (H2PO4–) anion on their oxygen reduction reaction (ORR) performance was investigated by adding increasing concentrations of phosphoric acid in half-cell measurements. A linear decrease in specific kinetic current at 0.7 V was noted with increasing phosphate anion concentration. It was also found that the adsorption of phosphate species on CNx was strong and the corresponding ORR activity was not recovered when the catalyst was reintroduced to a fresh HClO4 solution. Trends similar to those noted upon addition of H3PO4 to the half-cell were observed when CNx catalysts were soaked in phosphoric acid. Adsorption of dihydrogen phosphate ions on the surface of CNx exposed to phosphoric acid was verified by transmission infrared (IR) and Raman spectroscopy as well as X-ray photoelectron spectroscopy (XPS). XPS results also showed a decrease in the surface concentration of pyridinic-N species accompanied b...

Peter Mc Breen

Papers

X-ray photoelectron spectroscopy (XPS) and FTIR studies of vanadium barium phosphate glasses