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

Abstract

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.

Solar-assisted urea oxidation at silicon photoanodes promoted by an amorphous and optically adaptive Ni–Mo–O catalytic layer

“…These tests were performed at a low overpotential, a crucial parameter for photoelectrochemical UOR that has been overlooked. 36–42 Generally speaking, as previously discussed and shown in Fig. 3b, UOR and OER start at distinct potentials, therefore, to ensure that UOR (and not OER) is actually occurring, it is important to perform stability tests at a low overpotential where OER does not occurs.…”

“…73,74 Our photoanodes present, by far, the best performance and stability for Si-based photoanodes (Table S2†). 41,42 Generally speaking, one can hardly compare the performance of our photoanodes with that reported on other semiconductors 36–40 because, as discussed before, in all cases, too high overpotentials are employed to specifically assess UOR (see Table S2†).…”

“…For instance, it has been used on plasmonic Ag nanoparticles for fuel cells 34 and with TiO 2 for medical dialysis. 35 In the field of H 2 -producing PECs, UOR has been explored on Co phosphate-modified BiVO 4 36 and Ni(OH) 2 -modified: TiO 2 , 37 α-Fe 2 O 3 , 38,39 CdS-sensitized TiO 2 , 40 and Si photoanodes. 41,42 If these works underline the strong potential of UOR to couple solar H 2 production to water depollution, long-term photoelectrochemical UOR remains to be demonstrated.…”

Designing Advanced Catalysts for Energy Conversion Based on Urea Oxidation Reaction

“…In this work, Ni(OH) 2 plays both the roles of hole storage layer and UOR catalysts, which enhances both charge separation and catalytic performance in PEC urea oxidation. In Zhao et al's work, Ni(OH) 2 is used for the modification of CdS‐sensitized TiO 2 . Apart from the study on PEC performance, the authors also point out the issue of back electron transfer from semiconductor to catalyst, which results in lowered photoconversion efficiency and reduced catalytic activity.…”

Solar‐Driven Catalytic Urea Oxidation for Environmental Remediation and Energy Recovery

“…Furthermore, to overcome TiO 2 limitations in practical solar conversion due to its poor visible light absorption resulting from its wide bandgap ( � 3.5 eV), Radich and coworkers fabricated a cascade semiconductor-catalyst electrode to drive the photoconversion of urea for hydrogen harvest. [47] They simply introduced CdS nanocrystals into Ni(OH) 2 -TiO 2 through photosensitization to construct straddled band structures relative to the water splitting potentials. Since the VB position of CdS lies below that of Ni 2 + oxidation, the photogenerated holes were thermodynamically favored to transfer across the CdSÀ Ni(OH) 2 interface.…”