Reversible Performance Dynamics in PEM Water Electrolysis: An Electrochemical Study
Authors: Rex Alexander , Krenz Tobias , Trinke Patrick , Helmers Lennard , Bensmann Boris , Hanke-Rauschenbach Richard 
Published: December 2024

In Proton Exchange Membrane Water Electrolysis (PEMWE) systems, a notable phenomenon occurs following voltage transitions from low to high, such as after a shutdown and subsequent startup: the cell performance initially improves, higher currents are achieved, then gradually returns to baseline (s. Figure 1). This reversible performance behavior remains poorly understood despite documentation in the literature, see e.g., ref. [1, 2], and laboratory observations. However, distinguishing between reversible and irreversible performance losses is essential for accurate degradation assessments, emphasizing the need for detailed exploration of the underlying mechanisms.
The activity and stability of iridium-based electrocatalysts, crucial for the oxygen evolution reaction (OER) within PEMWE systems, depend on their iridium oxidation state, which varies with the applied voltage. While studies typically explore these oxidation state changes in aqueous model systems such as rotating disk electrodes (RDE) and scanning flow cells (SFC), the focus is often more on stability than on activity changes [2]. However, the current understanding of the catalysts' dynamic performance in PEMWE full-cell configurations remains incomplete.
In this study, a systematic experimental investigation of the reversible performance behavior of PEMWE cells was conducted. Tests were performed with single PEM water electrolysis cells with an active area of 4 cm², using a Nafion™ 115 catalyst-coated membrane. The anode and cathode loadings were 2.0 mgIr/cm² and 1.0 mgPt/cm², respectively. The voltage-driven experiments aimed to identify the voltage level that triggers performance recovery, evaluate the significance of this improvement depending on the low-voltage level, and assess how the duration of low-voltage phases affects recovery.
The results demonstrated that a recovery effect occurred once the low-voltage level dropped below 1.5 V, with further enhancements in recovery observed as the voltage continued to decrease. This effect was evident even with a brief holding time of 0.1 s at the lower voltage level. The reversible recovery is primarily driven by increased kinetics, likely influenced by voltage-dependent changes in iridium oxidation states. These observations are strongly supported by a robust alignment between the experimental data and a novel dynamic 0D model, which links iridium oxidation/reduction processes to performance metrics.
By investigating the dynamic behavior of iridium-based electrocatalysts in full-cell configurations, this research enriches the understanding of reversible performance phenomena in PEMWE cells. It provides an essential foundation for future studies to understand the mechanisms behind these reversible phenomena.
We gratefully acknowledge the financial support of the HyThroughGen project (BMBF 03HY108C) and the Deriel project (BMBF 03HY122A).

References

[1] M. Suermann et al., 2019, J. Electrochem. Soc. 166 F645
[2] J. Murawski et al., 2024, Johnson Matthey Technology Review 68.1

Figure 1