Degradation mechanisms of a proton exchange membrane water electrolyzer stack operating at high current densities

• A commercial proton exchange membrane water electrolysis (PEMWE) stack was operated at twice the nominal current for more than 2000h • Electrochemical and physical analysis revealed the dominant degradation phenomena • Increasing performance through membrane thinning was detected during operation resulting in higher cross-over. • Two different platinum group metal (PGM) loadings enabled the investigation of possible loading dependency of the phenomena On the path to an emission free energy economy, proton exchange membrane water electrolysis (PEMWE) is a promising technology for a sustainable production of green hydrogen at high current densities and thus high production rates. Long lifetime, increasing the current density and the reduction of platinum group metal loadings are major challenges for a widespread implementation of PEMWE. In this context, this work investigates the aging of a PEMWE stack operating at 4 A cm -2 , which is twice the nominal current density of commercial electrolyzers. Specifically, an 8-cells PEMWE stack using catalyst coated membranes (CCMs) with different platinum group metal (PGM) loading was operated for 2200 h. To understand degradation phenomena, physical ex-situ analyses, such as scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), were carried out. The same aging mechanism were observed in all cells, independent on their position in stack or the specific PGM loading of the membrane electrode assembly (CCM): (i) a decrease of ohmic resistance over time related to membrane thinning, (ii) a significant loss of ionomer at anodes, (iii) loss of noble metal from the electrodes leading to deposition of small Ir and Pt concentrations in the membrane, (iv) heterogeneous enrichment of Ti on the cathode side likely originating from the cathode-side of the Ti bipolar plates (BPPs). These results are in good agreement with the electrochemical performance loss. Thus, we were able to identify the degradation phenomena that dominate under high-current operation and their impact on performance. High current operation of proton exchange membrane water electrolyzer (PEMWE) stacks is a powerful approach to reduce hydrogen cost by increasing production rate without impacting the investments costs of the systems. Degradation phenomena were investigated for different platinum group metal (PGM) loadings during 2200h of operation at 4 A cm -2 .