Photoelectrochemical devices can be used to convert CO2 into valuable chemical compounds, which can serve as energy storage and can be used to compensate for energy fluctuations from renewable energy sources. This promising strategy relies on the development of p-type photocathodes. Cu2O is a p-type semiconductor, which has a conduction band edge thermodynamically suitable for the reduction of CO2. Therefore, Cu2O could be a promising photocathode but commonly suffers from photocorrosion and chemical instability. For this purpose, commercial Cu2O powders have been investigated in detail using different techniques.
DRS measurement revealed that Cu2O absorbs light in the visible range, starting below 750 nm with a band gap value of 1.95 eV. On the other hand, the Mott-Schottky plot of Cu2O deposited on FTO reveals its p-type character with a value of 0.97 V vs. NHE being obtained for the valence band edge. The charge carrier dynamics have been studied by means of transient absorption spectroscopy. The transient absorption spectra of Cu2O in N2 atmosphere shows a peak at around 615 nm, which decreases in CO2 atmosphere and exhibits a faster decay than under N2 atmosphere.
Cyclo-voltammetry of Cu2O deposited on FTO measured in 0.5 M Na2SO4 in the dark and during illumination with a solar simulator exhibit two main peaks related to the oxidation of Cu+ to Cu2+ and the reduction of Cu2+ to Cu+. However, three main peaks are present with 0.1 M NaHCO3 used as electrolyte, related to the oxidation of Cu and Cu+ to Cu2+, the reduction of Cu2+ to Cu+, and the reduction of Cu+ to Cu. For both measurements, a decrease of the currents is noticed with an increasing number of cycles.
Following the PEC measurements, Raman spectra reveal a non-stability of the material as evinced by the appearance of Cu and Cu2+. Therefore, Cu2O electrodes have been prepared over self-prepared Cu-covered FTO substrates aiming to increase the stability. While the photocurrent of these prepared electrodes increases in the cathodic range, however, the increase of the dark current in the cathodic range is found to be significantly lower. The chrono-amperometric measurement of Cu2O deposited on Cu-covered FTO under chopped light conditions at an applied potential of -100 mV vs. NHE in 0.5 M NaHCO3 saturated with CO2 reveals that the electrode is able to generate a stable photocurrent.