Exploring Degradation in PEM Water Electrolysis Cells Due to Metal Migration: Insights from X-Ray Fluorescence Analysis

Abstract

Green hydrogen production through proton exchange membrane (PEM) water electrolysis offers a viable renewable energy storage and transport solution. The Hydrogen Evolution Reaction (HER) and the Oxygen Evolution Reaction (OER) are central to this technology, with the latter’s slow kinetics posing a significant bottleneck. Despite research efforts, iridium (Ir) is the only element which currently meets the stability and activity needs of OER, challenging the search for alternatives and shifting focus towards enhancing catalyst stability [1]. Recent studies have identified metal migration, particularly of Ir, as a critical factor in the degradation of PEM water electrolyzers [2]. This finding has highlighted the importance of understanding metal migration mechanisms and the need to investigate the contribution or interaction of less studied elements such as platinum (Pt), titanium (Ti), and iron (Fe) to the degradation process.This study investigates the migration of both catalytically active (Ir, Pt) and inactive (e.g., Ti, Fe, Ca) metals within PEM water electrolysis membranes, using ex-situ 2D X-ray fluorescence (XRF) mapping (s. Fig. 1). Tests were conducted on PEM water electrolysis single cells (active area: 4 cm2) with a NafionTM 115 catalyst-coated membrane, with anode and cathode loadings of 2.0 mgIr/cm2 and 1.0 mgPt/cm2, respectively. The study entailed varying operating times (10 h, 100 h, 500 h, 1500 h) under constant load, different load profiles (e.g., open-circuit voltage (OCV) and accelerates stress test (AST) phases) for 500 h, and the introduction of specific impurities like iron (Fe) and calcium (Ca) through the anodic feed water. Complementary ICP-MS analysis of the anode and cathode waters was also performed. Selected segments of the catalyst-coated membranes (CCMs) were investigated using synchrotron radiation-based XRF at microXAS beamline (X05LA) of the Swiss Light Source (Paul Scherrer Institute, Villigen, Switzerland) at 11.0, 11.4, and 11.7 keV (below and above the Ir L3- and Pt L3-edges, for IrO2, see e.g. ref. [3]). The beam was focused using a Kirkpatrick-Baez (KB) mirror system. The spatial resolution of the measurements was approx. 1 μm, allowing selective identification and relative concentration of elements to be mapped within the entire cell cross-section (electrodes and polymer layer).Through our XRF analysis, it is possible to detect migration of the catalytically active materials (Ir, Pt) through the Nafion membrane depending on the degradation time. Interestingly, while impurity tests with Ca led to its accumulation in the membrane, Fe impurities introduced similarly did not show significant accumulation. These results, demonstrating the intricate details of metal migration and their varying behaviors under different operational conditions, underscore the value of high-resolution XRF mapping with synchrotron radiation in providing critical insights. This understanding is vital to unraveling the complex degradation mechanisms of PEM water electrolyzers. Further experiments may in principle allow for precise quantification of metal migration, with measurement of appropriate elemental standards. Figure 1. Schematic overview of the study: Catalyst-coated membranes (CCMs) at various stages of degradation are investigated using synchrotron radiation-based X-ray fluorescence (XRF) mapping. This method provides a 2D elemental distribution map, as demonstrated with Iridium (Ir) in this example. Analysis of the cross-sectional distribution of the element within the membrane allows for inferences regarding its migration. We thank the SLS for providing beam-time through an accepted proposal (20221749) and appreciate the micro-XAS beamline team’s help during measurements and data processing.