Photo-assisted electrochemical CO2 reduction at a boron-doped diamond cathode

We report on the coupling of photoelectrochemical water oxidation and electrochemical CO2 reduction to formic acid for a photoelectrochemical system capable of light and electrochemical energy conversion to chemical energy. Titanium oxide nanotubes (TiO2 NT) and boron-doped diamond (BDD) electrodes were used as a photoanode and a cathode, respectively. From CO2 electrochemical reduction at BDD, formic acid was obtained with a faradaic efficiency of about 86% and the system provided an overall energy conversion efficiency of 5.5%. From previous CO2 reduction at a dark electrolyzer based on a BDD cathode, the cell voltage was reduced from 2.7 V to 1.4 V with 52% saving of the electrical energy input. This resulted in an increase from 50% to nearly 80% of electrical-to-chemical energy conversion efficiency.