Electrocatalytic CO₂ reduction to produce valuable fuels and chemicals from waste CO₂ suffers from high overpotentials and low product selectivity in part due to competition with water reduction and due to the complex kinetics of the reaction and the myriad of products typically observed. In recent years, chirality induced spin selectivity (CISS) has gained traction as a means of controlling product selectivity in electrocatalysis especially for controlling the oxygen evolution reaction. In this paper, we report direct electrochemical, spectroscopic and mechanistic evidence that CISS is also operative in electrocatalysis of CO₂. We demonstrate a method to create highly chiral nanostructured electrodes made from copper and show that nanoscale topological chirality induces an increase in product selectivity towards CO₂reduction products over hydrogen production which is the main competing reaction. We elucidate the mechanism in which chirality controls spin selectivity and reaction kinetics using (spectro)electrochemistry and ultrafast spectroscopy. It is shown that the chiral Cu electrodes exhibit a lower onset potential for CO₂RR compared to their achiral counterparts and that the hydrogen evolution reaction is strongly suppressed. We also show that formate is only produced on the chiral electrodes and that the yield of CO formation has increased above that on achiral electrodes. We further study the mechanism through which spin polarization occurs through time-resolved Kerr ellipticity (TRKE). TRKE indicates that carriers traveling through the chiral and helical structure become spin polarized, which in turn creates an accumulation of similar electron spins at the electrode surface. This spin polarization reduces hydrogen formation due to the Pauli exclusion principle for two-electron bond formation between two reduced hydrogen atoms and thereby promotes CO₂ reduction to CO and formate. These measurements provide strong direct evidence that the precise chiral nano and micro structure of the electrocatalyst causes the spin of conduction electrons that travel through it to polarize. These findings show that chiral catalysts can indeed control selectivity during the CO₂RR reaction and potentially indicate that also other valuable reduction reactions such as nitrogen to ammonia or CO to methane or ethylene might benefit from the CISS effect since in these reactions, hydrogen evolution is also an undesired side reaction.

Md Azimul Haque

Appearances