FIB/SEM Nanotomography of a CO2 Electrolyzer Cathode Catalyst Layer

Abstract

Silver catalysts have demonstrated selective reduction of CO2 to CO. CO produced by this reaction could replace fossil fuel derived CO as a feedstock for fuels and value added chemicals. Furthermore, gas diffusion electrodes based on silver catalyst have additionally achieved high current density. Thus, the foundation for development of this promising defossilization strategy has been laid. In order to implement this strategy it is necessary to optimize catalyst layers to improve their durability and efficiency. At the same time, current density and selectivity need to be maintained. Detailed understanding of the electrodes is necessary to achieve these goals. In particular, the wetting parameters and the delicate balance of molecular and ionic transport needs to be investigated. We posit that in analogy to fuel cells, resolving the catalyst layers is essential.Tomographic reconstructions have successfully resolved the catalyst layers of fuel cells and water electrolyzers down to the nanoscale. So far, CO2 reduction gas diffusion electrodes have been imaged using X-Ray microtomography. However, the applied X-ray tomography was insufficient in resolving the nano-porous structure of these catalyst layers. We have reconstructed a porous silver catalyst layer using focused ion beam scanning electron microscope (FIB/SEM) tomography. It is thus possible to resolve the pores down to the nanoscale. Using the geometric representation, obtained from the reconstruction, we calculated geometric structure and mass transport parameters. Finally, we modeled the effect of (partial) flooding of the catalyst layer. We present the tomography alongside results for the porosities, diffusivities and conductivities of the catalyst layer.