Abstract
The electrochemical CO(2) reduction reaction (CO2RR) to produce value-added products remains a developing technology for utilizing waste CO(2) streams. Most device-level CO2RR studies use pure CO(2) gas feeds; however, the effect of dilute CO(2) on the electrolyzer performance is an important consideration for large-scale electrolyzer operation, single-pass conversion, and real-world CO(2) source utilization. This work investigates the effect that the CO(2) concentration has on the performance of formic acid (HCOOH) producing tin oxide (SnO(2)) and bismuth oxide (Bi(2)O(3)) catalysts in an electrolyzer device setting. Surprisingly, SnO(2) demonstrated an approximately 20% increase in HCOOH selectivity (Faradaic efficiency) when the CO(2) concentration decreased from 100 to 20%. In contrast, Bi(2)O(3) consistently demonstrated high selectivity toward HCOOH across the same CO(2) concentration range. The effects of the CO(2) concentration on selectivity were further investigated with half-cell experiments and in situ Raman spectroscopy, which revealed dynamic changes in the cathodic overpotential and chemical state of the catalyst that depended on the CO(2) concentration. Density functional theory calculations showed how changes in the surface oxidation state of Sn, varying from fully oxidized SnO(2) to metallic Sn(0), affect the thermodynamic barriers of the three main observed products: HCOOH, CO, and H(2). Our results indicate that dilute CO(2) concentrations required larger cathodic overpotentials to sustain a fixed current density, which, in turn, pushed the Sn-based catalyst toward a more reduced surface that was favorable to HCOOH formation. On the other hand, the Bi-based catalyst remained in a metallic state at CO2RR-relevant potentials and demonstrated a consistent product selectivity regardless of CO(2) concentration. These findings highlight how varying the CO(2) inlet gas concentrations affects the chemical state of catalysts and the resulting performance metrics.