Gas diffusion electrodes (GDEs) improve the performance of CO₂ reduction by enhancing the gaseous species transport. GDEs also increase the accessibility of the catalyst and the reaction surface area. A membrane electrode assembly (MEA) electrolyzer is advantageous due to its ease of assembly and low ohmic loss. However, the impact of operational conditions on cell performance and the local variations of mass and charge transfer in the GDE MEA electrolyzer is not well understood, and studies typically focus on individual components rather the complete cell. This study aims to investigate CO₂ electrolysis in a GDE MEA configuration using a porous Ni-based single-site catalyst through a combined computational and experimental approach.

The approach followed here was: i) we developed a model based on the assembly structure and geometry of the MEA cell to represent the experimental setup; ii) we extracted morphological information of the catalyst layer and the diffusion layer by pore-level investigations utilizing the exact mesostructured obtained from tomography, as well as kinetics parameters extracted by a boundary layer model that was fit to the H-cell experiments with the same catalytic material as the one used in the MEA cell; iii) we fed these parameters into the MEA cell model that accounts for the three relevant phases (gas phase for reactant/product transport, membrane phase for ion transport, and solid phase for electron transport) and predicted the cell performance; and iv) we compared the predicted results with the experimental data obtained with the MEA cell to validate our model.

The computational model of the GDE MEA cell was able to predict the behavior of the experiments with less than 15% error. The computed potential was about 0.2 V to 0.4 V lower than the potential in the experiment at the same current, mainly attributed to contact resistances in the system. The model provides local distributions of gaseous species and current density at the catalyst layer and diffusion layer interface. At 15 sccm flow rate, the total current density decreased along the gas flow direction and reached 100 mA/cm² at 50% downstream of the channel. CO₂ concentration depleted after 50% of the channel length. To operate at more negative cell potentials, it was necessary to increase the flow rate or reduce the gas channel length to avoid the competing hydrogen evolution reaction becoming the dominant reaction. For a cell running at 15 sccm, the conversion was approaching 99% at a cell potential of -2.7 V. This was also near the point (-2.6 V) where maximum Faradaic efficiency was reached. Beyond this potential, the additional charge will not contribute to CO evolution but rather hydrogen evolution, because of the depletion of CO₂.

This work offers a practical methodology to provide first insights into the performance-significant design and operational choices of GDE-MEA CO₂ reduction devices. Differences between the model and the experiment are attributed to: i) Competitive adsorption between reactant CO₂ and H₂O on catalytic sites in the experiment; ii) bypass of flow along the GDE, especially at high flow rate, and iii) contact resistance induced potential drops in the cell in between each component (cathode, membrane, anode, current collector, etc). Future work will account for these factors.