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The conversion and storage of clean renewable energy can be achieved using water splitting. However, water splitting exhibits sluggish kinetics because of the high overpotentials of the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) and should therefore be promoted by OER and/or HER electrocatalysts. As the kinetic barrier of the former reaction exceeds that of the latter, high-performance OER catalysts are highly sought after. Herein, K-doped NiCo 2 O 4 (HK-NCO) was hydrothermally prepared from a Prussian blue analog with a metal-organic framework structure and assessed as an OER catalyst. Extensive K doping increased the number of active oxygen vacancies and changed their intrinsic properties ( e.g. , binding energy), thus increasing conductivity. As a result, HK-NCO exhibited a Tafel slope of 49.9 mV dec -1 and a low overpotential of 292 mV at 10 mA cm -2 , outperforming a commercial OER catalyst (Ir) and thus holding great promise as a component of high-performance electrode materials for metal-oxide batteries and supercapacitors., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

We present a polymer-assisted spin coating process used to fabricate high-density p-type CuBi 2 O 4 (CBO) thin films. Polyvinylpyrrolidone (PVP) is introduced in the precursor solutions in order to promote uniform nucleation of CBO and prevent formation of the secondary phase, such as Bi 2 O 3, by Bi 3+ ion hydrolysis. Slow PVP molecule decomposition during the two-step annealing process, with a 1 M/0.5 M (Bi 3+ /Cu 2+ ) metal ion concentration, enables optimum contact at the CBO/substrate interface by avoiding formation of voids. This resulted in the formation of non-porous, compact CBO thin films. The highest current density of the photoelectrochemical (PEC) oxygen reduction reaction is obtained with non-porous, compact CBO thin films due to unimpeded charge transport through the CBO bulk, as well as across the interface. When combined with silicon, the high-density CBO thin film investigated in this work is expected to provide new PEC tandem cell options to use for solar applications.