Microfluidic and polymer electrolyte membrane (PEM) reactor designs with different configurations were used for studying electrochemical CO₂ reduction at room temperature, each having particular advantages. PEM based reactors with zero-gap configuration containing a membrane electrode assembly (MEA) might offer several advantages over other device architectures such as having lower ohmic drops at high current density, higher volumetric energy density, making them more suitable to scale-up.^1,2 However, PEM based and microfluidic reactors with flowing liquid catholyte are the most numerous configuration in electrochemical CO₂ reduction so far.^3,4 The studies in gas-fed electrochemical cells with flowing liquid catholyte provided crucial information on the effect of process conditions and material parameters to the selectivity and activity of the electrocatalytic process. Although this configuration might suffer from huge ohmic loses at high current density and require more technical control for scaling up ,from both a fundamental and applied perspective, it is important understand the effect of mass transport and process conditions on the performance and catalyst screening in these systems.

We will present results of a 2-D transport model for a gas diffusion electrode performing CO₂ reduction to CO with a flowing catholyte, including the concentration gradients along the flow cell, spatial distribution of the current density and local pH in the catalyst layer. The model predicts that both the concentration of CO₂ and the buffer electrolyte gradually diminish along the channel for a parallel flow of gas and electrolyte as a result of electrochemical conversion and non-electrochemical consumption. The effect of concentration gradients along the flow channel on the current density distribution becomes prominent at high conversions (e.g. current density) when compared to the ohmic drops across the electrochemical cell (Figure 1a), and a strong variation of the electrochemical performance is observed along the flow path (Figure 1b). In addition, the contribution of concentration overpotentials to overall potential losses dramatically changes with the CO₂ gas inlet feed flow rate, which results in differences in outlet concentrations at high conversions. Fundamental and practical implications of our findings to electrochemical CO₂ reduction are discussed particularly at high single-pass conversions.