Dynamic vs. Stationary Analysis of Electrochemical Carbon Dioxide Reduction: Profound Differences in Local States

Abstract

Electrochemical CO2 reduction is crucial for mitigating emissions by converting them into valuable chemicals. Stationary methods suffer from drawbacks like gas bubble distortion and long measurement times. However, dynamic cyclovoltammetry in rotating disc electrode setups is employed to infer performance. This study uncovers limitations when applying this approach to CO2 reduction in aqueous electrolyte. Here, we present a model-based analysis considering electrochemical reactions, species and charge transport, and chemical carbonation. Experimental and simulated potential cycles demonstrate scan rate dependence, significantly deviating from stationary curves at low rotation rates (50 rpm). Such low rotation rates mimic real diffusion layer thicknesses in practical cell systems, thus a transport impact can be expected also on cell level. This behavior arises from slow transport and carbonation, causing time-dependent CO2 depletion and electrolyte buffering. Dynamic investigation reveals strong species transport effects. Furthermore, dynamic operation enhances Faradaic efficiency due to a shift in the carbonate reaction system, favoring electrochemical CO2 consumption over chemical CO2 consumption. By clarifying dynamic vs. stationary operation, this research contributes to understanding electrochemical CO2 reduction processes, how to determine transport limitations via dynamic measurements, and provides guidelines for more accurate performance assessment.