Observations of Mass Transport Losses in Two-Phase CO₂ electroreduction

Abstract

The need for capturing CO2 has become urgent with a record-breaking concentration of CO2 in our atmosphere (average of 407.4 ppm in 2018 [1]). CO2 electrolyzers are a promising technology capable of achieving a net-negative CO2 cycle when coupled with renewable energy sources. Aqueous electrolyte-based alkaline CO2 electrolyzers have recently been demonstrated to exhibit high selectivity for CO2 reduction to CO [2]. While the selectivity and enhanced performance of the aqueous alkaline environment are desired, the mass transport limitations introduced with this design have largely been overlooked. In this study, we investigated the transport mechanisms near the electrolyte-catalyst interface over a range of current densities via sub-second in-operando X-ray synchrotron radiography. For the first time, we report fluctuations in cell overpotential driven by the dynamic accumulation and removal of gas at the electrolyte-catalyst interface region. Moreover, this fluctuation in potential due to the gas accumulation and removal in the liquid electrolyte layer was also observed to occur at a periodic rate that became increasingly frequent with increasing current density. The transient gas behavior observed from this study must be accounted for to further enhance the performance of alkaline CO2 electrolyzers with a liquid electrolyte. References: [1] Lindsey, R. “Climate Change: Atmospheric Carbon Dioxide”, NOAA Climate.gov, 2019. [2] A. Martín, G. Larrazábal and J. Pérez-Ramírez, "Towards sustainable fuels and chemicals through the electrochemical reduction of CO2: lessons from water electrolysis", Green Chemistry, vol. 17, no. 12, pp. 5114-5130, 2015.