Developing Microporous Transport Layers for Polymer Electrolyte Membrane (PEM) Water Electrolyzer Anodes

Abstract

Polymer electrolyte membrane water electrolysis (PEMWE) is a promising technology for the production of hydrogen from fluctuating renewable energies [1]. Due to the scarcity of iridium that is commonly used as anode catalysts for the oxygen evolution reaction (OER), low anode loadings have become increasingly important [2]. Some low iridium loaded electrodes come with the drawback of a low electrical conductivity, which can be mitigated if the adjacent porous transport layer (PTL) has a similarly fine pore structure as that of the carbon black based microporous layers (MPL) used in PEM fuel cells [3]. First developments of titanium MPLs for PEMWE were based on vacuum plasma spraying [4, 5]. In recent work, titanium MPLs have also been fabricated by powder sintering, however these MPLs are very thick (200-300 µm) [6] and thus are almost as thick as the complete PTL used in other work.In this work, we present a method to produce microporous layers for PEMWE anodes by a titanium powder sintering process. For this, we coat a titanium slurry on top of a commercial powder-sintered PTL substrate. The successive sintering step solidifies the MPL coating and forms a single component from the two layers. The effects of the sintering temperature on the surface morphology are investigated by scanning electron microscopy (SEM) top-view and cross-sectional imaging (see Figure 1). An analysis of the pore structure of the developed MPL/PTL composite through mercury intrusion porosimetry (MIP) reveals pore-sizes of approximately one order of magnitude below those of the PTL substrate. Electrochemical characterization is carried out in 5 cm2 PEMWE single-cells with low-iridium loadings (0.2 mg/cm2), measuring polarization curves up to 6 A/cm2 and performing electrochemical impedance spectroscopy (EIS) measurements. Our study demonstrates the benefits of MPLs for highly efficient PEM water electrolyzers with low-iridium loadings, as a 15% lowered HFR and a slightly improved iR-free cell voltage can be observed for our MPL compared to the pristine PTL substrate without MPL. Further analysis will be conducted to disentangle the contribution of the MPL (surface) morphology and of its surface properties to the observed performance benefits.