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 CO$_2$ electroreduction reaction (CO$_2$RR) approach to tune the product distribution at industrially relevant current densities in a gas-fed flow cell. We compared the CO$_2$RR 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 VRHE) 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 C$_2$ product selectivity (FE$_{C_2H_4}$ = 43.6% and FE$_{C_2H_5OH}$ = 19.8%) in comparison to the potentiostatic CO$_2$RR at −0.7 V$_{RHE}$ (FE$_{C_2H_4}$ = 40.9% and FE$_{C_2H_5OH}$ = 11%), (ii) while for $E_{an}$ = 1.2 V$_{RHE}$, high CH$_4$ selectivity (FE$_{CH_4}$ = 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 C$_2$ 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 CH$_4$ production.