Controlling Mass Transport in Direct Carbon Dioxide Zero-Gap Electrolyzers via Cell Compression

Abstract

The development of high-performance CO2 electrolyzers is crucial for accelerating the sustainable production of fuels and chemicals integrated with renewable energy sources. Here, we introduce a methodology to actively control mass transport inside a realistic zero-gap membrane electrode assembly of a CO2 electrolyzer by varying the gasket thickness, which consequently changes the cell compression. This allows control over the thickness and porosity of the gas diffusion electrodes, influencing the overall electrolyzer performance, as demonstrated using Ag-deposited electrodes. At low operating voltages (<2.9 V), both high- and low-compression electrolyzers exhibit similar faradaic efficiencies and partial current densities for CO formation. However, at high voltages, the low-compression electrolyzer with high electrode porosity demonstrates superior CO selectivity and activity with suppressed H2 formation. These experimental results are validated by the computational membrane electrode assembly (MEA) model developed by using the measured in situ electrode thicknesses and electrode porosities. Additionally, liquid electrolyte saturation at the catalyst layer is found to play a dominant role in determining the mass transport, resulting in a decreased electrolyzer performance with low electrode porosity. The systematic investigation in this study improves the understanding of the transport dynamics in MEA-based devices and provides insights into optimizing device design parameters for industry-relevant CO2 electrolysis.

Modeling Planar Electrodes and Zero‐Gap Membrane Electrode Assemblies for CO2 Electrolysis

“…The microporous layer and diffusion media material in this study have similar hydrophobicity as those from Zenyuk et al However, the silver catalyst used in this study may be more hydrophobic than the catalyst layer from Zenyuk et al, which consists of platinum nanoparticles on carbon support with Nafion. [40] Further model development is needed to fully capture the water balance and multiphase equilibrium and its impacts on mass transport. [5,6,46] At the anode diffusion media/electrolyte channel boundary, the concentrations are assumed to be at equilibrium (see Table S2) and the solid potential is set to the specified applied cell potential.…”

“…At low voltages (< 2.8 V) electrolyzers at both low compression (0.010" gasket thickness) and high compression (0.008" gasket thickness) perform similarly in both partial current density and Faradaic efficiency towards CO production. [40] However, at high voltages the low compression electrolyzer demonstrates higher CO selectivity. Compression test results show that the thickness of the catalyst layers, membrane, and diffusion media decreases with increasing compression, which is confirmed with cross-sectional SEM images.…”

“…Then we developed a full-cell 1D model for an MEA electrolyzer, which accounts for mass transport limitations including flooding in the cathode and crossover of CO 2 to the anode. A related and simplified half-cell MEA model was presented in Lee et al, [40] and in this work, we expand on that model by comprehensively exploring the parameter space and resolving the catalyst layers and anode. The model was then validated against experimental data from Lee et al and used to analyze the effects of flooding and bicarbonate formation in the cell.…”