Synergistic Approach to the Stable Production of Solar Fuels by Coupling Urea Oxidation with Hydrogen Generation

Abstract

A solar waste-to-fuels concept was presented to synergistically produce hydrogen fuel from visible sunlight while remediating urea wastewater. 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 (FTO) coated glass was employed as the photoanode. The steady-state response of the semiconductor-catalyst electrode was investigated in a photoelectrochemical cell, and charge transfer and recombination kinetics were elucidated to identify limiting charge transfer reactions within the electrode architecture. Back electron transfer from semiconductor to catalyst was found to be competitive with the urea oxidation reaction, which hindered steady-state photoconversion efficiency. Furthermore, the photoanode rapidly decomposed in urea electrolyte solutions due to the water-mediated photocorrosion of chalcogenide electrodes. Photocorrosion of photoelectrodes is an issue limiting the application of semiconductors. Thin dielectric layers ZnS and SiO2 were employed to reduce photocorrosion and improve performance of photoelectrodes. ZnS and SiO2 were deposited onto CdS to study the photostability and performance of this system. ZnS and SiO2 thin layers were deposited on TiO2-CdS photoelectrodes by successive ionic layer absorption and reaction (SILAR) and sol-gel methods, respectively. The photocurrent stability of CdS with ZnS and SiO2 improved 4 and 3 times respectively compared to bare CdS. Transient absorption results showed slower bleach and absorption decay due to reduced trap-mediated recombination when ZnS or SiO2 was present on CdS photoelectrodes. Higher charge transfer resistance in the presence of ZnS or SiO2 was indicated from electrochemical impedance measurements. Tafel plots showed that the photocorrosion current decreased 55% for ZnS-coated CdS and 63% for SiO2-coated CdS compared to bare CdS. Charge kinetics and photoresponses of TiO2-CdS photoelectrodes were dependent on the CdS nanoparticle size. The non-radiative recombination and electron transfer rate reduced with increased CdS nanoparticle size. The photocurrent density increased 4 times when SILAR cycles of CdS increased from 3 to 9, and then decreased with increased SILAR cycles. The stability of photocurrent improved with increased CdS nanoparticle size. The effect of solvent on charge kinetics in CdS quantum dots (QDs) were investigated. Inorganic Na2SO4 aqueous solution and organic DMSO were employed to dissolve urea. The transient absorption spectroscopy measurements showed that, the lifetime of electrons and holes in CdS was shortest in vacuum, followed by in inorganic Na2SO4 and longest in organic DMSO. Fluorescence and transient absorption spectroscopy measurement results indicated that urea could passivate CdS surface states and donate electrons to CdS. Polarity and viscosity of the electrolyte affected charge kinetics. Hole transfer rate constant from CdS to urea in Na2SO4-urea solution was 10 times of that in DMSO-urea solution.