NiCeO[subscript x] OER co-catalyst on hematite photoanode for photoelectrochemical water oxidation
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There has been debate on whether Ni(OH)₂ is truly catalytically active for the photo/-electrocatalytic oxygen evolution reaction. In this report, we have synthesized a Ni(OH)₂ co-catalyst 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 photo-induced capacitive charging effect. We have discovered that this photo-induced charging of Ni²⁺/³⁺ can be overcome by mixing cerium nitrate into the Ni precursor solution. Under illumination, the NiCeO[subscript x] co-catalyst 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 compared to its bare counterpart. Material characterization by electrochemical impedance spectroscopy revealed that the Ni species create a p-n junction across the space charge region, which facilitates the collection of the photo-generated holes by the co-catalyst layer, and core level X-ray photoelectron spectroscopy showed that Ce incorporated into the Ni-based co-catalyst layer may induce the oxidation of the Ni species. In addition, we have observed a decrease in binding energies of Ni species after photoelectrochemical water splitting reactions, which suggests that the lattice oxygen of the NiCeO[subscript x] is consumed in the catalytic cycle, forming oxygen vacancies. The NiCeO[subscript x] co-catalyst, 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.