Observing Oxygen Transport in Polymer Electrolyte Water Electrolysis Via Operando X-Ray Tomographic Microscopy

Abstract

The power-to-gas process using polymer electrolyte water electrolysis (PEWE) to produce hydrogen, is increasingly attracting interest as a mean to store the surplus electricity generated from intermittent renewable sources. However, high capital and operational costs limit the widespread implementation of the PEWE technology in the current energy system. The operational costs are dominated by electricity prices and therefore improving PEWE efficiency is crucial. In PEWE cells, water is supplied at the anode side and it is distributed to the active electrode (catalyst layer, CL) through a porous transport layer (PTL). During operation, oxygen is produced at the anodic CL and it is discharged at the flow channel. However, the undesired accumulation of oxygen in the anodic porous transport layer (PTL) may interfere with the transport of water to the active sites, contributing to efficiency losses, especially for high current density operations.The evolution and transport of oxygen in PEWE leads to a complex two-phase flow regime that is governed by a complex interplay of viscous and capillary forces in the PTL microstructure. To date, the precise relationship between current density, PTL morphology (pore dimensions, shape) and oxygen pathways is largely unknown. Furthermore, the overall influence of the water/gas transport on the mass transport losses in PEWE is still largely debated.In the past decade, many studies have been conducted to directly observe the oxygen transport within operating PEWE cells. Neutron radiography has been employed to compute the average water-gas saturation in PTLs for different current densities, temperature, or operating pressures [1, 2]. Furthermore, X-ray radiography was used to examine bubble formation and growth in the flow channel [3]. However, radiographic studies can only provide average information on the gas-water distribution in PTLs and a spatially-resolved description of the oxygen pathways at the microscale is crucially needed.In this work, we present the first operando X-ray tomographic microscopy (XTM) experiment where oxygen pathways inside a representative PTL material are imaged in three dimensions. The shape, dimension, and connectivity of oxygen pathways in the PTL microstructure are described for current densities ranging from 0.1 to 2.5 Acm-2. Furthermore, a precise measure of oxygen saturation in the PTL porosity is discussed in terms of current density and distance from the catalyst layer. Finally, the differences in oxygen saturation between regions under the channel and rib are also presented. We believe that these results will represent a step forward in the understanding of two-phase flow in PEWE cells, paving the way for a more rational design of better performing PTL materials.[1] Zlobinski, M., et al., 2020. Transient and steady state two-phase flow in anodic porous transport layer of proton exchange membrane water electrolyzer. Journal of the Electrochemical Society[2] Selamet, O.F., et al., 2013. Two-phase flow in a proton exchange membrane electrolyzer visualized in situ by simultaneous neutron radiography and optical imaging. International Journal of Hydrogen Energy 38, 5823–5835.[3] Hoeh, M.A., et al., 2015. In operando synchrotron X-ray radiography studies of polymer electrolyte membrane water electrolyzers. Electrochemistry Communications 55, 55–59 Figure 1