Photoelectrochemical CO2 reduction to adjustable syngas on grain-boundary-mediated a-Si/TiO2/Au photocathodes with low onset potentials

Abstract

A photoelectrochemical (PEC) CO₂ reduction reaction (CRR) in an aqueous medium is among the most promising methods to produce renewable fuels with the sun and water as the energy and electron source. But a high negative bias remains a prerequisite to achieve an appreciable conversion at present due to the large overpotential of CO₂ reduction. This communication describes a PEC CRR with low onset potential on an amorphous silicon (a-Si) photocathode decorated with grain-boundary-rich gold catalysts. Syngas, with high throughput and desirable CO/H₂ ratios such as 1 : 2 and 1 : 1, was produced in a wide PEC range from the equilibrium potential E⁰(CO₂/CO) of −0.1 V vs. the reversible hydrogen electrode (V_RHE) to as positive as 0.4 V_RHE, leading to a half-cell conversion efficiency of 0.42%. This performance represents the highest half-cell efficiency and lowest onset potential for CO₂ reduction ever reported among aqueous PEC cathodes under 1 sun illumination. The a-Si/TiO₂/Au photocathodes exhibit excellent stability in a wide potential range, with a high CO mass activity up to 180 ampere per gram of gold at −0.1 V_RHE. Controlling the grain boundary provides powerful potential to adjust the product selectivity in PEC CO₂ reduction.