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Gurudayal Gurudayal1 Joel Ager1

1, Lawrence Berkeley National Laboratory, Berkeley, California, United States

Silicon based photocathodes with various co-catalysts have been studied extensively for solar fuels production, especially for hydrogen evolution reaction of photoelectrochemical (PEC) water splitting.1 In contrast, Si photocathodes which perform CO2 reduction (CO2R) are less well investigated.2 Moreover, in most prior studies, Si photocathodes produce two electron reduction products such as carbon monoxide and formate.
A strategy to produce C2 and C3 products such as ethylene, ethanol, and propanol will described. The choice of the CO2 reduction catalyst is crucial as Si is not able to selectively convert CO2 in to C2-C3 products by itself. Also, the light absorption properties and effects on surface recombination need to be considered, as well as the selectivity for CO2R over hydrogen evolution.
To permit the use of absorbing metallic catalysts, we employed a back illumination geometry using a both side-textured n-type Si absorber and a p++ implanted hole back contact (illumination side) and n++ electron front contact (electrolyte side). Front surface passivation, electron collection, and stability under aqueous PEC CO2R conditions are achieved via atomic layer deposition of TiO2. A CuAg nanococtus catalyst was deposited in two-step methods; i) a 100 nm layer of Ag was deposited via ebeam and ii) copper was electrodeposited at high current to generate nanocactus morphology.
Under 1-sun back illumination, the onset potential for production of hydrocarbon products is shifts to 0.5-0.6 V vs. RHE, which represents a cathodic shift of ~500 mV due to the Si PV. The device has excellent stability (6 hrs in 0.1 M CsHCO3) and over 60% faradic efficiency for hydrocarbons and oxygenates. This nanocactus bimetallic catalyst grown on planar and textured silicon and compared for CO2 reduction. Textured silicon shows higher photocurrent than planar silicon at a fixed potential due to high surface area but there was no change observed in products distribution.

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