Operando Raman spectroscopy uncovers hydroxide and CO species enhance ethanol selectivity during pulsed CO₂ electroreduction

Pulsed CO₂ electroreduction (CO₂RR) has recently emerged as a facile way to in situ tune the product selectivity, in particular toward ethanol, without re-designing the catalytic system. However, in-depth mechanistic understanding requires comprehensive operando time-resolved studies to identify the kinetics and dynamics of the electrocatalytic interface. Here, we track the adsorbates and the catalyst state of pre-reduced Cu₂O nanocubes ( ~ 30 nm) during pulsed CO₂RR using sub-second time-resolved operando Raman spectroscopy. By screening a variety of product-steering pulse length conditions, we unravel the critical role of co-adsorbed OH and CO on the Cu surface next to the oxidative formation of Cu-O_ad or CuO_x/(OH)_y species, impacting the kinetics of CO adsorption and boosting the ethanol selectivity. However, a too low OH_ad coverage following the formation of bulk-like Cu₂O induces a significant increase in the C₁ selectivity, while a too high OH_ad coverage poisons the surface for C-C coupling. Thus, we unveil the importance of co-adsorbed OH on the alcohol formation under CO₂RR conditions and thereby, pave the way for improved catalyst design and operating conditions.