Effects of cell design and operating parameters on oscillatory instability in an anion-exchange membrane CO2 electrolyzer

Periodic oscillations in cell voltage and CO2 reduction product selectivity are commonly observed in laboratory studies using membrane electrode assembly (MEA) CO2 electrolyzers with anion-exchange membranes (AEM). These oscillations hinder mechanistic studies and progress toward industrial deployment of CO2 electrolyzers. To understand how to mitigate these oscillations in laboratory experiments, we investigated the effects of three electrolyzer design parameters on system stability: (i) membrane type and thickness, (ii) gas diffusion layer (GDL) properties, and (iii) cell compression, characterized as a normalized thickness difference (NTD) between the GDL and gasket and assembly torque. In our system, we found pairing a higher ionic conductivity AEM with a GDL with a cracked microporous layer (MPL) at a medium cell compression (NTD: 40%, torque: 2 Nm), led to the most stable CO2 to CO electrolysis with pseudo-steady-state CO Faradaic efficiency of around 85% achieved at 100 mA cm–2 for over 50 h. In contrast, lower conductivity AEMs showed earlier and more frequent oscillations related to water flooding, enhanced K+ crossover, and eventual short-circuiting. A lower compression assembly (NTD: 0%, torque: 2 Nm) exhibited a higher initial CO selectivity (>95% for 50 h) but developed more frequent oscillations associated with salt accumulation in the cathode flow channel. Our hypotheses are further supported by tests of an ”over-compressed” cell (NTD: 40%, torque: 4 Nm), which exhibited poor water drainage, rapid salt accumulation in the cathode gas channel, and an increased risk of electrode short-circuiting. These observations lead to practical AEM electrolyzer design rules: pairing high-conductive AEMs with crack-rich MPLs and optimizing cell compression to balance membrane-electrode interfacial contact and water drainage to maintain selective and stable CO2 electrolysis.