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In water splitting, anode catalysts for the oxygen evolution reaction (OER), which is the rate‐determining step, are more critical than cathode catalysts. Herein, the authors prepare Ni‐IrOx composite catalysts consisting of NiO and IrO2 for the OER by a solid‐state reaction with different ratios of NiO to IrO2 and reaction temperatures. In particular, Ni‐IrOx‐400 with a molar ratio of NiO/IrO2 = 1:1 heated at 400 °C shows the best OER performance. In the overall water splitting test using an anion exchange membrane (AEM) water electrolyzer, the single cell with Ni‐IrOx‐400 as the anode catalyst shows current densities of 1454.8 mA cm−2, respectively, measured at 1.8 V. Furthermore, the stability tests of the AEM single cells are carried out at 50 °C under a constant current density of 500 mA cm−2. The single cell with Ni‐IrOx‐400 shows only a slight increase in the overpotential (rate: 2.0 mV h−1) for 100 h owing to the enhanced stability of Ni‐IrOx‐400 compared to IrO2 (12.5 mV h−1). The improved OER performance of the Ni‐IrOx‐400 may be attributed to a composite structure that can prevent particle agglomeration and thus preserve the active sites during the OER. [ABSTRACT FROM AUTHOR]

The design of Pt-based cathode catalyst for the oxygen reduction reaction (ORR) is critical for fabricating high-performance polymer electrolyte membrane fuel cells. In this study, hollow PtNi alloy nanostructures (h-PtNi/C-X) deposited on a carbon black support are prepared using a facile synthesis method at various pH value (X). The degree of alloying of the hollow nanostructures is modulated with increasing pH. Among these h-PtNi/C catalysts, h-PtNi/C-11 exhibits superior ORR activity (five-fold mass activity and higher power density) and improved durability both in half and unit cells, compared to a commercial Pt/C catalyst. [Display omitted] • Hollow PtNi alloy nanostructures were formed by the Kirkendall effect. • With increasing pH, the degree of alloying were gradually increased. • h-PtNi/C-11 cathode catalyst exhibited the best ORR performance. • The enhanced performance is attributed to a hollow PtNi alloy nanostructure. • The enhanced performance can result from an electronic modification. [ABSTRACT FROM AUTHOR]

Water electrolysis (WE) is an attractive advanced technology for the production of H 2. A key issue in the WE is to develop suitable catalysts for the oxygen evolution reaction (OER), where a relatively large energy loss occurs. The Ir-doped NiCo 2 O 4 catalysts exhibit the formation of a solid solution and a porous nanostructure. The oxygen vacancies formed by Ir doping can enhance the electrocatalytic activity for the OER owing to increased electrical conductivity and tunable adsorption energies of OH- and H 2 O. In comparison to IrO 2 as a representative OER catalyst, the Ir(10 at%) doped NiCo 2 O 4 catalyst (10-Ir-NCO) exhibits the better OER performance (overpotential of 303 mV at 10 mA cm−2) and its stability for 20,000 s. In addition, in the overall water splitting test, the 10-Ir-NCO || Pt/C exhibits the excellent OER activity (a low overpotential of 0.5 V at 100 mA cm−2) and stability (20 h at 50 mA cm−2). [Display omitted] • Ir-doped NiCo 2 O 4 catalysts were synthesized with various amounts of Ir dopant. • The catalysts exhibited a spinel structure consisting of NiCo 2 O 4 host and Ir dopant. • The catalyst exhibited the enhanced activity and stability for the OER compared to IrO 2. • The enhanced activity may result from the replacement of Ni and Co with Ir in octahedral sites. • The enhanced activity may be attributed to the oxygen vacancies. [ABSTRACT FROM AUTHOR]