Charge Transport Barriers Limit the Performance of Amorphous Ir-Oxide Oer Electrocatalysts

Abstract

Significant research efforts are dedicated to reducing the iridium loading in proton-exchange membrane water electrolyzers. Recently, the focus shifted toward better understanding the interplay between the titanium porous transport layers (PTLs) and the Ir-oxide-based catalyst layers, where significant performance losses were observed for low-loaded anodes based on intrinsically highly active but poorly conductive amorphous IrOx. In the presented study, a gas-diffusion electrode half-cell setup is used to promote an understanding of the underlying phenomena leading to this low performance. The influence of the PTL platinization on the performance of realistic porous transport electrodes (PTEs) for the oxygen evolution reaction (OER) is investigated by gradually increasing the platinum layer thickness for PTEs based on amorphous and rutile Ir-oxide. Electrochemical measurements show a beneficial influence of platinization on the activity for amorphous, but not for rutile Ir-oxide. Impedance analysis corroborates the formation of a Schottky-type interface between the PTL and the amorphous IrOx catalyst layers, depending on the PTL’s platinum layer thickness. We presume that this heterogeneous Schottky-type interface induces an additional voltage drop and influences the utilization of the catalyst layer, leading to increased overpotentials. The measurements were complemented by inductively coupled plasma mass spectrometry showing constant integral amounts of Ir dissolving from the catalyst layers during the OER, independent of the platinization. Transferring the obtained knowledge to single cells, the whole composite anode, including e.g. the interplay between PTL and catalyst layer, must be optimized in conjunction to achieve optimum OER performance and long-term stability.

Keywords: Proton Exchange Membrane Water Electrolysis, Gas Diffusion Electrode, Oxygen evolution reaction, Schottky Junction, Impedance Spectroscopy