The electrochemical CO2 reduction reaction (CO2RR) to produce value-added products remains a developing technology for utilizing waste CO2 streams. Most device-level CO2RR studies use pure CO2 gas feeds; however, the effect of dilute CO2 on the electrolyzer performance is an important consideration for large-scale electrolyzer operation, single-pass conversion, and real-world CO2 source utilization. This work investigates the effect that the CO2 concentration has on the performance of formic acid (HCOOH) producing tin oxide (SnO2) and bismuth oxide (Bi2O3) catalysts in an electrolyzer device setting. Surprisingly, SnO2 demonstrated an approximately 20% increase in HCOOH selectivity (Faradaic efficiency) when the CO2 concentration decreased from 100 to 20%. In contrast, Bi2O3 consistently demonstrated high selectivity toward HCOOH across the same CO2 concentration range. The effects of the CO2 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 CO2 concentration. Density functional theory calculations showed how changes in the surface oxidation state of Sn, varying from fully oxidized SnO2 to metallic Sn(0), affect the thermodynamic barriers of the three main observed products: HCOOH, CO, and H2. Our results indicate that dilute CO2 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 CO2 concentration. These findings highlight how varying the CO2 inlet gas concentrations affects the chemical state of catalysts and the resulting performance metrics.
Ellis et al. (Fri,) studied this question.