Microscopic Insights on the Degradation of Electrocatalysts in PEM Electrolyzers at Low Catalyst Loading

Abstract

Water electrolysis has the potential to become a key element in coupling the electricity, mobility, heating and chemical sector via power-to-liquids or power-to-gas in a future sustainable energy system [1]. While proton exchange membrane (PEM) water electrolysis offer several advantages over traditional electrolysis technology, catalyst durability at low catalyst loading remains a barrier for industrial commercialization [2].Fundamental understanding of degradation mechanisms is crucial to elucidate the key parameters for high performance and durability of low-catalyst-loading electrodes. In this work, electron microscopy methods, including transmission electron microscopy/scanning transmission electron microscopy (TEM/STEM) and X-ray energy dispersive spectroscopy (XEDS) are developed to quantify the degradation of iridium- and platinum-based electrocatalysts in membrane electrode assemblies (MEAs).This work aims for a comprehensive understanding of the degradation of PEM water electrolyzer with a low catalyst loading of 0.3 mg cm−2 Pt on the cathode and 0.08 mg cm−2 Ir on the anode. A long-term electrolysis operation of ~4500 h at 1.8 A cm−2 and 2.76 MPa differential pressure was performed. Degradation studies on such low catalyst loading and at high operating current are still lacking in this field. For the first time, the mechanism of cathode degradation is proposed using established physical models from two aspects: (1) direct Pt dissolution from nanoparticles due to Gibbs-Thomson relation [3] and (2) transient Pt dissolution due to rapid Pt oxide reduction [4] during start/stop of operation. Furthermore, iridium dissolution and subsequent re-deposition on the cathode constitute a major portion (42%) of the anode catalyst loss [5]. As Ir dissolves and migrates through the membrane, Pt-Ir precipitates are formed in the membrane [5] . The overall picture of degradation mechanisms and the distribution of platinum and iridium across the MEA are provided in Figure 1 [5]. In summary, quantitative analysis from microscopic imaging (TEM/STEM) and chemical analysis (XEDS) is an effective tool to characterize catalyst morphology, dissolution, and re-distribution in degraded MEAs. Quantitative results are helpful defining the target of research to improve the performance and durability of PEM electrolyzers.Figure 1. Schematic diagram depicting the mechanism leading to the formation of Pt-Ir precipitates and Ir band in the membrane, adapted from [5]. The distribution of Pt and Ir at different regions of the MEA is shown as weight percent. Irn+ is used to universally represent ionic Ir species.