Solar powered hydrogen evolution reaction (HER) is one of the key reactions in solar-to-chemical energy conversion. It is desirable to develop photocathodic materials that exhibit high activity toward photoelectrochemical (PEC) HER at more positive potentials because a higher potential means a lower overpotential for HER. In this work, the Cu₂O/CuO bilayered composites were prepared by a facile method that involved an electrodeposition and a subsequent thermal oxidation. The resulting Cu₂O/CuO bilayered composites exhibited a surprisingly high activity and good stability toward PEC HER, expecially at high potentials in alkaline solution. The photocurrent density for HER was 3.15 mA·cm⁻²at the potential of 0.40 Vvs. RHE, which was one of the two highest reported at the same potential on copper-oxide-based photocathode. The high photoactivity of the bilayered composite was ascribed to the following three advantages of the Cu₂O/CuO heterojunction: (1) the broadened light absorption band that made more efficient use of solar energy, (2) the large space-charge-region potential that enabled a high efficiency for electron-hole separation, and (3) the high majority carrier density that ensured a faster charge transportation rate. This work reveals the potential of the Cu₂O/CuO bilayered composite as a promising photocathodic material for solar water splitting.

Measuring experimentally the activation energy (E_a) of the oxygen evolution reaction (OER) is very important to understand the OER mechanism and to develop efficient electrocatalysts. However, few studies concerning E_a measurements are reported to support theoretical predictions. Herein, we prepared nickel foam (NF) supported β-NiFeOOH, β-NiOOH, γ-NiFeOOH, and γ-NiOOH and compared their OER performances. β-NiFeOOH exhibits an exceptionally high activity, with an overpotential (η) of 196, 245 and 257 mV to yield a current density of 10, 50, and 100 mA cm⁻², respectively. We measured E_a of the OER on different catalysts and established the E_avs. η relationship. The activation energy at zero-overpotential and the overpotential at zero-activation-energy on β-NiFeOOH/NF are the lowest (89 kJ mol⁻¹ and 350 mV), revealing the key origin of its high activity. This work provides the first experimental evidence for the high OER activity of Fe-doped NiOOH catalysts from the aspect of electrochemical activation energy.