Impact of Interposer Microstructure on Ionic Transport in Liquid-Phase Bicarbonate Electrolysis

The electrochemical reduction of CO₂ (CO₂RR) is a potentially scalable approach for converting captured carbon dioxide into value-added products. Conventional gas-phase electrolysis systems can suffer from carbonate crossover, which limits the efficiency of the system. Liquid-phase (bi)carbonate electrolysis using bipolar membrane electrode assemblies (BPM-MEA) has emerged as a promising alternative. The interposer layer, a porous mass-transport material between the BPM and the catalyst, is an essential component of the MEA, as it allows evolved CO₂ to reach the catalyst surface for reaction. In the absence of this layer, evolved CO₂ generated by the pH swing process at the BPM can be converted back into (bi)carbonate (CO₂ recapture) due to the high bulk pH. Thus, clear design guidelines are needed to maximize CO₂ conversion, minimize CO₂ recapture in the catholyte, and improve energy efficiency. Here, the transport properties of the interposer are systematically characterized by X-ray tomography and symmetric-cell impedance spectroscopy to quantify porosity, tortuosity, and the resulting MacMullin number. We then examine the correlation between these material properties and the electrolyzer performance. We focus on characterizing two commercial porous membrane filters, mixed cellulose ester (MCE) and poly(ether sulfone) (PES).