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

Abstract

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.

Figure 1

(a) Cyclic voltammogram of the Cu NCs obtained at a scan rate of 1 mV/s in the 1 M KOH electrolyte. (b) FE of Cu NCs under potentiostatic conditions at −0.7 V vs RHE in the flow cell.

Figure 2

Current density (top) and FE (bottom bar graph) at a potentiostatic −0.7 V vs RHE and under pulsed electrolysis conditions with the different E_an values indicated and the same E_ca = −0.7 V vs RHE cathodic potential in all cases. The activity and selectivity data reported are an average of at least three different measurements on analogously prepared fresh independent samples, and the error is estimated as the standard deviation.

Figure 3

Current density (top) and FE (bottom bar graph) of the pulsed electrolysis with E_an = 0.9 and 1.2 V and the cathodic pulse E_ca = −0.7 V. The arrows indicate that the same samples pretreated using pulsed electrolysis were subsequently measured at a constant potential of −0.7 V vs RHE. The activity and selectivity data reported are an average of at least three different measurements on analogously prepared fresh independent samples, and the error is estimated as the standard deviation.

Figure 4

(a–d) SEM and (e–h) TEM images of Cu NCs samples (a, e) before and (b,f) after potentiostatic electrolysis and pulsed CO₂RR conditions with (c, g) E_an = 0.9 V and (d, h) 1.2 V. Scale bars: (a–d) 100 nm; (e–h) 20 nm.

Figure 5

Time-dependent Cu K-edge (a) XANES and (b) Fourier-transformed (FT) EXAFS spectra showing the reduction of Cu NCs under the pulsed CO₂RR with 1 s pulses and E_an = 0.9 V. (c) Results of a linear combination fitting of XANES spectra for Cu NCs under the potentiostatic CO₂RR and pulsed reaction conditions with 1 s pulses and E_an = 0.9 and 1.2 V. Spectra for metallic Cu, Cu₂O, CuO, and Cu(OH)₂ were used as references for the LCA fitting. The Cu(II) concentration reported is the sum of CuO and Cu(OH)₂ contributions. Weights of the other components are shown in Figure S11.

Figure 6

Periodic oxidation and reduction of pre-reduced Cu NCs under 60 s pulses with E_ca = −0.7 V and E_an = 0.9 and 1.2 V. (a) Time dependence of the Cu(0), Cu(I), and Cu(II) concentrations, as obtained from LCA-XANES. XANES spectra corresponding to metallic Cu, Cu₂O, CuO, and Cu(OH)₂ were used as references. Weights corresponding to CuO and Cu(OH)₂ are shown separately (green and purple circles, respectively). The sequence of applied potential pulses is also shown in (a). (b, c) Enlarged regions of (a), corresponding to the first three pulses with (b) E_an = 0.9 V and (c) E_an = 1.2 V.

Figure 7

(a) Operando surface-enhanced Raman spectra under OCP, potentiostatic operation at −0.7 V, and pulsed conditions with different E_an values. Dashed lines represent the Raman bands of Cu₂O (black), Cu–OH (red), and Cu–CO (blue).