Impact of Intermittent Operation on the Lifetime and Performance of a PEM Water Electrolyzer

In times of an increasing demand for energy production through renewable but fluctuating energy sources, such as wind or solar energy, hydrogen as an energy carrier becomes more and more important. Proton-exchange membrane water electrolysis (PEM-WE) is a suitable and already quite advanced technique for sustainable production of hydrogen. 1 However, coupling a PEM-WE with intermittent renewable energy sources will induce frequent current interrupts of the PEM-WE system. These events can potentially lead to rapid degradation of the membrane electrode assemblies (MEAs) and hence, a thorough understanding of the underlying mechanisms is crucial to assess the stability and lifetime of a PEM-WE and to choose appropriate operating conditions. In this work, we present a test protocol involving operation at high (3 Acm -2 geo ) and low (0.1 Acm -2 geo ) current density, alternating with current interrupts during which the system remains at the open circuit voltage (OCV). Previous studies in our lab revealed that the permeation of hydrogen through the membrane into the anode compartment during extended OCV periods can cause the reduction of IrO x 2 , the most commonly used anode catalyst for the oxygen evolution reaction (OER) owing to its decent activity and high stability. During a subsequent start-up of the PEM-WE, metallic Ir is oxidized to a hydrous Ir-oxide. The transformation of the catalyst surface was probed by cyclic voltammetry (CV) during the degradation test. While the initial CV (Fig. 1, black curve) typical for crystalline IrO x is essentially featureless, CVs recorded after ten current-interrupt cycles revealed the formation of hydrogen under-potential-deposition (H-UPD) features (region 1, blue curve), which are characteristic for metallic Ir electrodes. 3 The redox-features evolving at ≈0.8 V are characteristic of an amorphous, hydrous Ir-oxide (region 2). 4 The appearance of these hydrous Ir-oxide features indicates a change in hydration state as well as in surface chemistry, which is known to affect both the OER activity and the stability of IrO x . 5 Amorphous hydrous Ir-oxide exhibits higher OER activity but lower stability compared to crystalline thermally grown IrO x. Interestingly, the polarization curve recorded directly after IrO x reduction during an OCV period shows a lower cell voltage (i.e., improved OER activity), thus supporting the formation of a hydrous Ir-oxide. However, since this hydrous oxide is less stable, a rapid decay of