Photoelectrocatalytic hydrogen production is crucial to reducing greenhouse gas emissions for carbon neutrality and meeting energy demands. Pivotal advances in photoelectrochemical (PEC) water splitting have been achieved by increasing solar light absorption. P-type Cu-based metal oxide materials have a wide range of energy band gaps and outstanding band edges for PEC water splitting. In this study, we first prepared Cu₂O thin films using electrodeposition and fabricated a heterojunction structure of CuO/Cu₂O by controlling annealing temperatures. The surface morphological, optical, and electrochemical properties were characterized using various analytical tools. X-ray and Raman spectroscopic approaches were used to verify the heterojunction of CuO/Cu₂O, while surface analyses revealed surface roughness changes in thin films as the annealing temperatures increased. Electrochemical impedance spectroscopic measurements in conjunction with the Mott–Schottky analysis confirm that the CuO/Cu₂O heterojunction thin film can boost photocurrent generation (1.03 mA/cm² at 0 V vs. RHE) via enhanced light absorption, a higher carrier density, and a higher flat band potential than CuO and Cu₂O thin films (0.92 and 0.08 mA/cm², respectively).

High-entropy materials have attracted extensive attention as emerging electrode materials in various energy applications due to their flexible tunability, unusual outstanding activities, and cost-effectiveness using multiple earth-abundant elements. We introduce a novel high-entropy composite oxide with the five elements of Cu, Ni, Co, Fe, and Cr (HEO-3CNF) for use in the oxygen evolution reaction (OER) in electrocatalytic water splitting. HEO-3CNF is composed of two phases with a non-equimolar, deficient high-entropy spinel oxide of (Cu_0.2−xNi_0.2Co_0.2Fe_0.2Cr_0.2)₃O₄ and monoclinic copper oxide (CuO). Electrochemical impedance spectroscopy (EIS) with distribution of relaxation times (DRT) analysis validates that the HEO-3CNF-based electrode exhibits faster charge transfer than benchmark CuO. It results in improved OER performance with a lower overpotential at 10 mA/cm² and a Tafel slope than CuO (518.1 mV and 119.7 mV/dec versus 615.9 mV and 131.7 mV/dec, respectively) in alkaline conditions. This work may provide a general strategy for preparing novel, cost-effective, high-entropy electrodes for water splitting.