Despite the advantages of CO 2 electrolyzers, efficiency losses due to mass and ionic transport across the membrane electrode assembly (MEA) are critical bottlenecks for commercial-scale implementation. In this study, more efficient electrolysis of CO 2 was achieved by increasing cation exchange membrane (CEM) hydration via the humidification of the CO 2 reactant inlet stream. A high current density of 755 mA/cm 2 was reached by humidifying the reactant CO 2 in a MEA electrolyzer cell featuring a CEM. The power density was reduced by up to 30% when the fully humidified reactant CO 2 was introduced while operating at a current density of 575 mA/cm 2 . We reduced the ohmic losses of the electrolyzer by fourfold at 575 mA/cm 2 by fully humidifying the reactant CO 2 . A semiempirical CEM water uptake model was developed and used to attribute the improved performance to 11% increases in membrane water uptake and ionic conductivity. Our CEM water uptake model showed that the increase in ohmic losses and the limitation of ionic transport were the result of significant dehydration at the central region of the CEM and the anode gas diffusion electrode−CEM interface region, which exhibited a 2.5% drop in water uptake.

Carbon dioxide (CO 2 ) reduction flow cells, coupled with renewable energy sources, are a promising means of curtailing anthropogenic CO 2 emissions by reducing CO 2 to generate useful carbon fuels. However, unstable mass transport overpotential due to gas evolution impedes high current density operation (>200 mA cm −2 ), preventing wide-scale commercialization. Here, we identify a real-time correlation between the electrolyte layer gas content and the cathode potential in an operating flow cell via concurrent galvanostatic operation and subsecond X-ray synchrotron imaging, whereby gas accumulation directly corresponds to increasing cathode overpotentials and gas removal corresponds to decreasing cathode overpotentials. Specifically, at 125 mA cm −2 , a 5% decrease in gas volume near the interface of the cathode gas diffusion electrode (GDE) and the electrolyte layer corresponds to a 12% decrease in the cathode overpotential. Moreover, gas saturation becomes more stable at high current densities (>175 mA cm −2 ) due to more frequent gas removal, consequently stabilizing cell performance. The findings from our work suggest that enhancing gas removal from the electrolyte layer minimizes cathode potential instability and enables current density operation greater than 200 mA cm −2 in alkaline flow cells for CO 2 reduction.

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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.