Hierarchically Structured NiFeO x /CuO Nanosheets/Nanowires as an Efficient Electrocatalyst for the Oxygen Evolution Reaction

It is desirable to fabricate hierarchically structured NiFe‐based materials to further boost the performance of these state‐of‐the‐art electrocatalysts for the oxygen evolution reaction (OER). Most of the NiFeO x catalysts in the form of layered double hydroxides feature a nanosheet structure, which limits their spatial extension at the electrode substrate. Herein, we report the fabrication of unique NiFeO x /CuO nanosheets/nanowires and their use as efficient electrocatalysts for the OER. The surface‐bound Cu(OH) 2 nanowires were grown in situ on a copper substrate by a simple solution‐based method, which were then converted to CuO nanowires by calcination in air. The coating of NiFeO x nanosheets was achieved by anodic co‐deposition in concentrated carbonate solution containing Ni II and Fe III . The presence of concentrated carbonate anions allows Fe III to be dissolved in a basic solution by complex ion formation. This strategy therefore avoids the destruction of CuO nanowires caused by the usual cathodic procedure in acidic solutions. At a planar copper foil, a small Tafel slope of 36 mV dec −1 was obtained in 1  m KOH and a current density of 100 mA cm −2 was reached at a very low overpotential of 300 mV from the Tafel plot. Electrolysis experiments showed high stability of the catalyst with nearly no loss in efficiency and morphology change after a prolonged period. The same catalyst could be established on a three‐dimensional copper foam.