Ni(OH)₂ as Hole Mediator for Visible Light-Induced Urea Splitting

Urea is a small molecule produced in millions of tons per day and is ubiquitous in nature. Biological treatment is commonly used to oxidize the urea wastewater produced each day across the world, which produces additional solid waste and eliminates any potential for utilizing the stored chemical energy within. A solar waste-to-fuels concept is presented to synergistically produce hydrogen fuel from visible sunlight while remediating urea wastewaters. A cascade semiconductor-catalyst electrode assembly was designed to drive the photoconversion of urea to hydrogen. Proper band energy alignment facilitates catalyst activation via hole transfer across the semiconductor-catalyst interface. Specifically CdS-sensitized TiO₂ with Ni(OH)₂ urea electrocatalyst on fluorine-doped tin oxide coated glass was employed as photoanode. The steady-state response of the semiconductor-catalyst electrode is investigated in a photoelectrochemical cell, and charge transfer and recombination kinetics are elucidated to identify limiting charge-transfer reactions within the electrode architecture. Back electron transfer from semiconductor to catalyst is found to be competitive with urea oxidation reaction, which hinders steady-state photoconversion efficiency. Furthermore, the photoanode rapidly decomposes in urea electrolyte solutions as a result of the water-mediated photocorrosion of chalcogenide electrodes. Passivation of CdS with ZnS prior to catalyst deposition significantly improves open-circuit potential and photostability.