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.
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