Succinate-assisted synthesis of ZnO-CuO heterostructure electrocatalysts for tunable hydrogen evolution reaction

Developing noble-metal-free electrocatalysts for the alkaline hydrogen evolution reaction (HER) requires strategies that overcome the intrinsically poor electronic conductivity and suboptimal hydrogen adsorption properties of metal oxide semiconductors. Herein, ZnO-CuO n-p heterojunction composites were synthesized via a succinate-assisted co-precipitation route to systematically investigate the influence of Cu incorporation (6, 12 and 24 at%) on HER performance in 1 M KOH. FTIR analysis confirms that succinate ligands coordinate both Zn²⁺ and Cu²⁺ precursor species, while X-ray diffraction reveals biphasic wurtzite ZnO/monoclinic CuO composites with progressively increasing heterojunction density as the Cu content increases. Among all compositions, the 24 at% Cu sample (ZCu-C) exhibits the best HER activity, requiring an overpotential of only 301 mV to achieve a current density of 10 mA cm^-2, representing an improvement of 238 mV compared with pristine ZnO. Furthermore, its electrochemically active surface area (ECSA)-normalized specific activity is more than three times higher than that of ZnO. Electrochemical impedance spectroscopy was analysed using six candidate equivalent-circuit models. Comparison based on the reduced chi-squared criterion identified the two-time-constant circuit, R _s + (R _ct‖CPE) + (R _ct‖CPE), as the most appropriate representation of all impedance spectra, consistently yielding reduced chi-squared values one to two orders of magnitude lower than conventional single-time-constant Randles-type circuits. The two distributed relaxation processes are consistent with charge-transfer phenomena occurring at ZnO grain boundaries and at the ZnO-CuO heterojunction interfaces, respectively. These findings demonstrate that engineering oxide-oxide heterojunctions through a coordination-chemistry-assisted synthesis strategy provides an effective, scalable and noble-metal-free approach for enhancing alkaline HER electrocatalysis.