Demystifying Z-behavior of hydrogen in electrochemical CO2 reduction

Abstract

The Hydrogen Evolution Reaction (HER) occurs concurrently with the electrochemical CO${}_2$ Reduction Reaction (eCO2RR). Several studies on HER partial currents using silver-based catalysts in bicarbonate electrolytes have observed a peculiar phenomenon: HER currents initially increase, then decrease, and finally increase again with overpotential, a pattern we refer to as the “Z-shape behavior.” Although widely documented, the underlying mechanism responsible for this Z-shaped behavior in H${}_2$ partial currents remains unclear. Understanding this phenomenon is crucial, as it defines the potential region with high faradaic efficiency for CO production. Therefore, gaining insights into this mechanism and identifying the influencing parameters could enhance the efficiency of CO${}_2$ electrolysis and have practical applications. In this study, we provide a model-based analysis of experiments to elucidate the interplay of various processes contributing to this behavior. Our model integrates micro-kinetic and continuum approaches and considers two distinct kinetic mechanisms for HER proposed in the literature. We offer explanations for the Z-shaped behavior for both mechanisms, highlighting their differences and implications for model predictions. Additionally, we conduct a sensitivity analysis on various parameters related to kinetic and transport phenomena, aiding in the interpretation of experimental partial currents obtained with different electrolyte concentrations.