Selectivity Control of Cu Nanocrystals in a Gas-Fed Flow Cell through CO₂ Pulsed Electroreduction

In this study, we have taken advantage of a pulsed CO₂ electroreduction reaction (CO₂RR) approach to tune the product distribution at industrially relevant current densities in a gas-fed flow cell. We compared the CO₂RR selectivity of Cu catalysts subjected to either potentiostatic conditions (fixed applied potential of -0.7 V_RHE) or pulsed electrolysis conditions (1 s pulses at oxidative potentials ranging from E_an = 0.6 to 1.5 V_RHE, followed by 1 s pulses at -0.7 V_RHE) and identified the main parameters responsible for the enhanced product selectivity observed in the latter case. Herein, two distinct regimes were observed: (i) for E_an = 0.9 V_RHE we obtained 10% enhanced C₂ product selectivity (FE_C2H4 = 43.6% and FE_C2H5OH = 19.8%) in comparison to the potentiostatic CO₂RR at -0.7 V_RHE (FE_C2H4 = 40.9% and FE_C2H5OH = 11%), (ii) while for E_an = 1.2 V_RHE, high CH₄ selectivity (FE_CH4 = 48.3% vs 0.1% at constant -0.7 V_RHE) was observed. Operando spectroscopy (XAS, SERS) and ex situ microscopy (SEM and TEM) measurements revealed that these differences in catalyst selectivity can be ascribed to structural modifications and local pH effects. The morphological reconstruction of the catalyst observed after pulsed electrolysis with E_an = 0.9 V_RHE, including the presence of highly defective interfaces and grain boundaries, was found to play a key role in the enhancement of the C₂ product formation. In turn, pulsed electrolysis with E_an = 1.2 V_RHE caused the consumption of OH^- species near the catalyst surface, leading to an OH-poor environment favorable for CH₄ production.