Cyclic two-step electrolysis for stable electrochemical conversion of carbon dioxide to formate.

Pd metal and Pd-based alloys are ideal catalysts that allow for the electrochemical conversion of CO2 to HCOO- at almost zero-overpotential with high selectivity, but catalyst degradation caused by concurrent CO poisoning limits their practical implementation. Here, we demonstrate that cyclic two-step electrolysis, by applying the reduction and oxidation potentials alternately, achieves 100% current density stability and 97.8% selectivity toward HCOO- production for at least 45 h. The key idea for achieving the reliability is based on the selective removal of CO by controlling the parameters during the oxidation step, which utilizes the different reversibility of HCOO- and CO production reactions. Furthermore, it is found that potentiostatic electrolysis causes CO adsorption and subsequent dehydridation, which in turn lowers HCOO- selectivity. Our work provides a system-level strategy for solving the poisoning issue that is inevitable in many electrocatalytic reactions.