Influence of the Ionomer Content in Low-Loaded Anode Electrodes on Interfacial Resistances in Proton Exchange Membrane Water Electrolyzers

Abstract

Proton exchange membrane water electrolysis (PEM-WE) is a rapidly developing and promising technology for the production of green hydrogen from renewable energy [1]. For its large-scale application it is inevitable to substantially reduce the currently used high iridium loadings (on the order of 1 mgIr/cm2) required for the oxygen evolution reaction (OER) in PEM-WE anodes [2]. Furthermore, the interface between the anode catalyst layer and the porous transport layer (PTL) is of critical importance to reduce interfacial resistances and avoid additional performance losses [3]. For example, coating the titanium PTL at the anode with ultra-thin layers of iridium or platinum mitigates the growth of a TiOx passivation layer and improves the long-term durability of PEM water electrolyzers [4]. With regards to the electrode, Bernt et al. [5] and Böhm et al. [6] have shown for a TiO2-supported OER catalyst (based on a low-iridium-packing-density catalyst concept) that a reduced electrical conductivity of the catalyst material has a detrimental effect on the PEM-WE cell performance: severe interfacial contact resistances between the electrode and the PTL have been observed for anodes based on OER catalysts with a low electrical conductivity in combination with an uncoated titanium PTL, which can be mitigated by improving catalyst conductivity or by using a platinum-coated PTL.Since the ionomer in electrodes not only influences proton and oxygen/water transport, but also acts as a binder that can affect the electrical resistance of the electrode, the work presented herein aims for a better understanding of the influence of the ionomer content in low-loaded PEM-WE anodes on cell performance and on the interfacial resistances between the anode electrode and the PTL. A TiO2-supported IrOx catalyst with 33 wt.% iridium and low electrical conductivity (approximately one order of magnitude lower compared to a commercial TiO2-supported catalyst with 75 wt.% iridium) is examined in 5 cm² single cells with titanium PTLs on the anode side. The ionomer content within the anode catalyst layer is varied between 6 and 38 wt.% at iridium loadings of ~0.3 mgIr/cm². The potentiostatic testing protocol includes electrochemical impedance spectroscopy (EIS) measurements and yields stable current densities at low to medium ionomer contents. In contrast, a rapid and severe decrease in cell performance is observed at high ionomer contents, and the EIS-spectra reveal a dynamic growth of interfacial contact resistances (see figure 1).Besides presenting a detailed performance and EIS analysis, we will investigate the effect of a protective platinum coating covering the titanium PTL on the development of contact resistances for the different ionomer contents. Further operando and ex situ analyses will be performed to differentiate between degradation effects of the PTL and the electrodes and to gain a better understanding of the occurring interfacial resistances.