Activation of a Nickel-Based Oxygen Evolution Reaction Catalyst on a Hematite Photoanode via Incorporation of Cerium for Photoelectrochemical Water Oxidation

Abstract

There has been debate on whether Ni(OH)₂ is truly catalytically active for the photo/electrocatalytic oxygen evolution reaction. In this report, we synthesized a Ni(OH)₂ cocatalyst on a hematite photoanode and showed that, as has been proposed in other studies, the current density varies as a function of scan rate, which arises due to a photoinduced capacitive charging effect. We discovered that this photoinduced charging of Ni^2+/3+ can be overcome by mixing cerium nitrate into the Ni precursor solution. Under illumination, the NiCeO_x cocatalyst on a hematite photoanode exhibited an approximately 200 mV cathodic shift in onset potential and a ∼53% enhancement in photocurrent at 1.23 V vs RHE. Material characterization by electrochemical impedance spectroscopy revealed that the Ni species create a p-n junction across the charge space region, which facilitates collection of the photogenerated holes by the cocatalyst layer, and core level X-ray photoelectron spectroscopy showed that Ce incorporated into the Ni-based cocatalyst layer may possibly induce the oxidation of the Ni species. In addition, we observed a reduction in binding energies of Ni after photoelectrochemical water splitting reactions, which suggests that the lattice oxygen of the NiCeO_x is consumed in the catalytic cycle, forming oxygen vacancies. The NiCeO_x cocatalyst, however, was incapable of passivating the surface recombination centers of the hematite photoanode, as indicated by the unaltered flat-band potential determined with Mott-Schottky analysis.