Tomographic Reconstruction and Analysis of a Silver CO₂ Reduction Cathode

Abstract

electrochemical CO 2 RR enables direct power-to-X technology, that is the integration of electricity from (intermittent) renewable sources into the formation process of various valuable products. Technoeconomic analyses of CO 2 electrolyzers have identified current density and energy efficiency as the key benchmarks of this technology. [1][2][3][4][5] They further estimate that for adoption of this technology high current densities (hundreds of mA cm − ² or more) are required. The current density of the batch cells, which are ubiquitous in CO 2 RR catalyst research, is intrinsically limited. This points to flow cells with gas diffusion electrodes (GDEs) as a more promising solution. Endrődi et al. [6] and Weekes et al. [7] provide reviews of publications on flow cells using GDEs. Both highlight how flow cells and GDEs enable the aforementioned increase in current density and energy efficiency. Further recent publications demonstrate high current densities using GDEs. [8][9][10] The morphology of these electrodes is likely instrumental to their outstanding properties.Polymer electrolyte fuel cells (PEMFCs) and polymer electrolyte water electrolyzers (PEMWEs) serve as models for CO 2 electrolyzers, having achieved current densities in excess of amperes per square centimeter using GDE-based electrodes. [11][12][13][14] We suggest that if we wish to emulate the high current densities found in PEWMEs or PEMFCs, we need to understand the structure of CO 2 RR GDEs to subsequently improve it. Especially, we need to understand how the structure-property relations are similar and different from those found in PEMFCs and PEMWEs. This understanding in turn would enable rational design of catalyst layers (CLs), thus guiding the optimization of the manufacturing processes.To give an example: A point of discussion in some recent publications and perspectives has been the nature of the CO 2 delivery to the catalyst. Following the successful demonstration of GDEs application for CO 2 reduction, Cook et al. proposed a three-phase boundary mechanism according to which CO 2 reacts directly from the gas phase. [15] This model has been called into question. Burdyny and Smith [16] posit that the CL is fully flooded during operation; Weng et al. [17] propose that a thin layer of electrolyte covers the catalyst surface, in analogy to ionomer coverages often found in fuel cell models. This again points to the need for an improved understanding of electrode structure, especially its wetting behavior.Tomographic reconstruction has been well established as a valuable tool in the analysis of polymer electrolyte fuel cell (PEMFC) electrodes. While forays have been made into applying it to polymer electrolyte water electrolyzer (PEMWE) electrodes, CO 2 electrolyzer electrodes are still new ground. Here a tomographic analysis of an electrochemical CO 2 reduction gas diffusion electrode by means of focused ion beam scanning electron microscope tomography is presented. The reconstruction shows a porosity of 68%. While most of the porosity is on th...

Electrochemical Fixation of Carbon Dioxide in Molten Salts on Liquid Zinc Cathode to Zinc@Graphitic Carbon Spheres for Enhanced Energy Storage

“…The brighter areas in the backscatter electron image denote heavier elements while the darker regions point to lighter elements. [ 38 ] Therefore, the bright cores and the darker shells/flakes in the backscatter electron image in Figure 3e are attributed to the Zn and carbon, respectively, further depicting the fact that the core–shell Zn@C spheres are closely attached to the surface of carbon flakes.…”

Mass Transport in Catalytic Pores of GDE-Based CO2 Electroreduction Systems

“…McLaughlin et al 58 have recently reported surface area measurements for Ag GDE which are equivalent to a roughness factor of ∼ 50. f r will depend on the properties of the catalyst and the GDL/MPL layer as well as on the deposition technique and the CL thickness.…”

“…However, the GMPNP equations are numerically more stable than PNP and do not require additional numerical stabilization for convergence Bohra et al. 43 Lastly, based on the few attempts at measuring the pore structure properties of metal catalysts deposited on GDEs for eCO 2 R, 22,58 the pore radii are expected to lie between 100 to 1000 nm. We have assumed a radius of 5 nm for our simulations unless otherwise stated, primarily due to computational limitations since the finite element mesh grows with R 2 .…”

“…80 It is not expected for pores below 5 nm radius to have a significant population in the CL and therefore can be safely ignored for building models for the CL. 22,58…”

“…The pore structures in the actual CL deposited on the GDL are expected to be highly heterogeneous as reported in a very recent tomography study on Ag GDE cathode. 58 We assume the catalytic pore to be completely flooded with electrolyte such that the gas-electrolyte interface is located at the pore entry (the boundary between the MPL and CL, S 1 ). We believe that this is a reasonable assumption for a metal catalyst pore with small radii at a reducing potential and that a vast majority of current density in flow-cell systems can be attributed to the double-phase electrolyte-catalyst boundary (S 2 ) like in case of an H-cell.…”