Zero-Gap Electrochemical CO₂ Reduction Cell Under Simulated Diurnal Conditions

We report on the investigation of zero-gap CO₂ electroreduction (CO₂R) cells operating under simulated diurnal conditions, to better understand the real-world dynamical operating characteristics of solar driven electrochemical cells, as compared to the constant current density and constant temperature operating conditions typically employed in a controlled lab environment. Various test modes, including cascade and pulse potential cycling, were employed to gain insights into the impact of diurnal cycling on integrated solar fuel devices. Notably, our findings revealed significant degradation of Cu gas diffusion electrodes (GDEs) during simulated day-night shifts, particularly when subjected to periods with an open-circuit voltage (OCV) operating configuration. Comparative analysis across different pulse cycling modes highlighted that Cu GDEs exhibited reduced selectivity only under OCV conditions with repeated cycles. Further investigation using X-ray tomography at the Advanced Light Source (ALS) on custom-designed membrane electrode assembly (MEA) cells elucidated the formation of salt precipitates during −150 mA/cm² operation, with rapid flooding occurring upon reaching an OCV operating point in Cu GDEs. Moreover, the severity of flooding correlated directly with the duration of the OCV condition. These insights provide a deeper understanding of potential degradation mechanisms in zero-gap CO₂R cells under diurnal conditions. Looking ahead, our research aims to explore CO₂ reduction under more complex operational scenarios and establish guidelines for the sustainable operation of solar-assisted electrochemical cells in diurnal settings.