Deciphering Anion Exchange Membrane Water Electrolysis: A Distribution of Relaxation Times Approach
Authors: Ranz Matthias , Grabner Bianca , Schweighofer Bernhard , Wegleiter Hannes , Trattner Alexander 
Published: December 2024

Anion exchange membrane water electrolysis (AEM-WE) stands out as a promising method for hydrogen production from renewable energy sources. Unlike proton exchange membrane water electrolysis (PEM-WE), AEM-WE offers the advantage of nonprecious metal catalysts due to mild alkaline conditions, while still enabling a compact cell design and operation under differential pressure. Remarkable performances of AEM-electrolysis cells have been demonstrated in literature, achieving current densities of up to 7.68 A·cm−2 [1], surpassing the record current density of PEM-WE at 6 A·cm−2 [2], both measured at 80 °C and 2 V. However, challenges related to efficiency and stability persist and demand effective solutions to unlock the full potential of AEM-WE. In order to achieve this, cell components, particularly the membrane, catalysts, and electrode structure have to be improved and advanced measurement techniques have to be applied.

Different measurement tools serve to characterize, separate, and quantify the dynamic processes inherent in AEM-WE. Electrochemical impedance spectroscopy (EIS) serves as a valuable non-destructive, in-situ diagnostic tool for electrochemical cells, enabling the differentiation of various phenomena based on their respective relaxation times. Moreover, EIS enables the quantification of loss mechanisms thereby facilitating a targeted optimization of electrolysis components and the identification of weaknesses limiting long-time stability.

Despite its utility, the broad use of EIS is hindered by its complex interpretation. The widely applied approach of fitting equivalent circuit models (ECMs) aims to extract quantitative data from the semi-circular shapes observed in Nyquist plots. However, the assumption of a physico-chemical meaningful ECM is challenging without prior knowledge of the number, size, and time constants of appearing loss mechanisms. Furthermore, the potential for multiple ECMs to fit one and the same experimental result introduces ambiguity, leading to potential misinterpretations of data.

An alternative approach to analyse EIS spectra involves converting the measured data into the time (τ) domain and characterize the different mechanisms based on their characteristic relaxation times. The distribution of relaxation times (DRT) analysis has already been applied across diverse electrochemical systems, spanning from solid oxide fuel cells (SO-FC) and PEM fuel cells (PEM-FC) to PEM water electrolysis and AEM fuel cells (AEM-FC). However, to the best of the authors’ knowledge, DRT analysis of AEM-WE is still absent in literature.

In this study we present a thorough investigation of an AEM-WE single-cell, employing a combination of electrochemical impedance spectroscopy (EIS) and the equivalent circuit model (ECM). Notably, we demonstrate the distribution of relaxation times (DRT) analysis at AEM-WE cells for the first time, utilizing a reversible hydrogen electrode (RHE) as a reference. Through half-cell EIS measurements and subsequent DRT spectra analysis, anodic and cathodic half-cell reactions are clearly identified and quantified. The DRT analysis differentiates five loss mechanisms within the AEM-WE system, encompassing the hydrogen evolution reaction, the oxygen evolution reaction, and ionic transport losses within the catalyst layers. By systematically varying operating parameters, we successfully attribute DRT peaks to their respective physicochemical origins. These findings provide valuable insights into the electrochemical processes within the AEM-WE single-cell, significantly advancing our understanding of underlying mechanisms.

In fig. 1, we investigate the degradation of AEM-WE single cells through long-term experiments. Periodic EIS measurements enable an in-operando analysis of catalyst degradation. The impedance data is analysed using ECM and DRT. Ohmic contributions resulting from electric and ionic charge transport are distinguished from polarisation resistances arising from electrochemical reactions. A catalyst degradation is observable in fig 1b & c, where the low frequency resistance (LFR) in the Nyquist plot and the DRT peak area increases over time. However, fig. 1b also reveals the reduction of the high frequency resistance (HFR) resulting in an overall performance increase and voltage decrease.

[1] N. Chen, S. Y. Paek, J. Y. Lee, J. H. Park, S. Y. Lee, Y. M. Lee, High-performance anion exchange membrane water electrolyzers with a current density of 7.68 a cm −2 and a durability of 1000 hours, Energy & Environmental Science 14 (12) (2021) 6338–6348. doi:10.1039/D1EE02642A.

[2] M. Braig, R. Zeis, Distribution of relaxation times analysis of electrochemical hydrogen pump impedance spectra, Journal of Power Sources 576 (2023) 233203. doi:10.1016/j.jpowsour.2023.233203.

Figure 1