Polymer Electrolyte Water Electrolysis: Correlating Porous Transport Layer Structural Properties and Performance: Part I. Tomographic Analysis of Morphology and Topology

By Tobias Schuler, Ruben De Bruycker, Thomas J. Schmidt, Felix N. Büchi. Journal of the Electrochemical Society, 2019

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In the first paper of this series, the bulk, surface and transport properties of porous transport layer materials (PTL) for polymer electrolyte water electrolysis cells (PEWE) are characterized. A systematic PTL matrix of Ti fiber materials with 3 fiber diameters and 2 nominal porosities as well as a state of the art sintered powder material are investigated to get a better understanding of the governing parameters in electrolysis cells. X-ray tomographic microscopy analysis of ex situ PTL structures and post operando membrane electrode assemblies is performed. On the tomographic structures, bulk (porosity, pore/solid size distributions, fiber orientation), mass transport (diffusivity, permeability, conductivities) and surface parameters (roughness, membrane deformation, PTL surface area and interfacial contact area) are determined. The second paper of this series will correlate the structural parameters with in-depth electrochemical analysis. The new insights into the effect of PTL properties on PEWE performance allow to isolate governing key parameters. From know-how obtained on the fiber based materials fundamental design guidelines for optimization of PTL structures are deduced. X-ray tomographic microscopy.— The significant difference of X-ray mass attenuation coefficient between Ti and air of 1.95 cm 2 /g (contrast @ 40 keV, monochromatic) 30 makes XTM a well suited tool for structural analysis. Surface and bulk properties were derived from dry structure imaging using a CT-scanner phoenix nanotom m (Gen-eral Electric, Germany). Samples with dimensions of 20 × 2.0 × 1.0 mm 3 were mounted vertically for imaging. A 0.25 mm copper filter was used to suppress beam hardening effects. Acquisition pa- rameters were set to 60 kV and 310 μ A. A voxel cube edge length of 2 μ m was chosen for all PTL samples resulting in a minimum of 5 to 6 voxels/feature size. 1500 projections over 360° were acquired with an exposure time of 1500 ms, resulting in an acquisition time per sample of approximately 2 h. To characterize the membrane deformation on the surface of the PTL, 20 × 20 mm 2 electrochemically operated CCM/PTL sandwiches were imaged directly after cell disassembling. To ensure a reliable analysis of plastic membrane deformation, exclusively CCM/PTL sandwiches adhering to each other after disassembly were analyzed. The high attenuation coefficients of the precious metal catalysts in the anodic and cathodic electrodes made catalyst layer deformation easily observable. For this post operando imaging, local tomography was conducted with a voxel size of 3 μ m resulting in 3 to 4 voxels for the catalyst layer thickness. Prior to imaging, the sam- ples were hydrated with deionized water and sealed with parafilm to prevent membrane shrinking during image acquisition.

Subfields: Automotive Engineering, Electrical and Electronic Engineering, Energy Engineering and Power Technology

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