Insights into the Self-Inhibiting Photoreduction of Cu₂O in Bicarbonate Electrolytes

Cuprous oxide (Cu₂O) has long been regarded as the quintessential photoelectrochemical cathode due to its narrow band gap, native p-type behavior, and low cost. However, it is not stable in aqueous electrolytes upon irradiation, limiting its potential application for producing hydrogen or converting wastewater or CO₂ by using direct sunlight. Although Cu₂O's instability is well-documented, the mechanism and factors governing its photocurrent decay over time are not. In this work, electrodeposited Cu₂O layers with increasing thickness were synthesized to investigate their transient structure-photocurrent density relationships in bicarbonate electrolytes. The photocurrent decay for all the thicknesses could be modeled by an exponential function with an initial photocurrent j _ph,0 and a decay rate constant c. It was found that j _ph,0 follows the Gärtner equation, while the contraction of the space charge layer and the light-dampening effect of the metallic Cu overlayer were evaluated to explain c. Finally, the use of a fast reduction kinetics species (NO₃ ^-) and the comparison with another electrolyte provided critical insights into the effect of KHCO₃ in the self-reduction of Cu₂O.