Photoelectrochemical reduction of carbon dioxide in aqueous solutions on p-GaP electrodes: An ac impedance study with phase-sensitive detection

The mechanism of the photoelectrochemical reduction of water and CO2 on p-GaP electrodes in aqueous solutions was studied using photocurrent spectroscopy with chopped light excitation, revealing both a photo-anodic sub-bandgap response and a photo-cathodic current due to the bandgap response. A.c. power curves were recorded for p-GaP in pH 7 phosphate buffer electrolyte using modulated irradiation and a lock-in amplifier. At a light modulation frequency of 150 Hz, CO2 bubbling effected a 15% increase in the photo-cathodic current, relative to that in an Ar flushed solution. This effect disappeared at a light modulation frequency of 1560 Hz. Ruthenium pretreatment of p-GaP resulted in a doubling of both the cathodic photocurrent and the photovoltage. Pretreatment of p-GaP by dipping in a solution of CuSO4 caused a marked enhancement in both the dark current and photocurrent at reverse bias. In preparative reduction of aqueous bicarbonate, using light chopped at 22 Hz and -1.08 V between the p-GaP and a stainless steel counter electrode separated by a cation selective membrane, the production of formic acid was determined. The efficiency of conversion of the incident light energy to the Gibbs energy of formation of the formic acid produced amounted to 0.3%, both with bare and with Cu treated p-GaP.