Exploring the Effects of Recombination Interlayers on Hydrogen and Oxygen Crossover in PEM Water Electrolysis: Insights from 1D Model-Based Investigations
Authors: Brundiers Steffen , Trinke Patrick , Bensmann Boris , Hanke-Rauschenbach Richard 
Published: December 2024

Proton exchange membrane (PEM) water electrolysis presents a promising solution for renewable energy storage and transportation through the production of green hydrogen. A major problem during the operation of PEM electrolysis, regarding both efficiency and safety, is the gas crossover. Small amounts of the produced hydrogen and oxygen gases permeate through the cell membrane to the opposite electrode side. This is problematic because the hydrogen that permeates to the anode is no longer available as product gas, resulting in Faradaic efficiencies less than unity [1]. Furthermore, hydrogen in oxygen mixtures at the anode with hydrogen contents above 4 vol.% are explosive and pose a critical safety issue [2,3]. Hence, several mitigation strategies like thicker membranes, external recombiners, or recombination interlayers (ILs) inside of the membrane have been developed to reduce gas crossover [4].
In this study, we focus on ILs that are used to recombine permeating hydrogen with oxygen to water at catalyst particles (e.g., Pt) inside of the membrane, thus lowering the permeating fluxes. The advantages of ILs are that they can be easily integrated into standard cells, no external devices are needed, and previous experimental work has shown that the hydrogen in oxygen content can be lowered substantially. Until today, there are only experimental investigations regarding this topic, which fail to resolve local effects and only show the integral behavior of a PEM electrolysis cell with an IL. In this contribution, we propose the first model-based approach to investigate local potential, temperature, and concentration profiles during the operation of PEM electrolysis cells with a recombination IL. This further facilitates the understanding of the underlying local mechanisms through numerical investigations and enables further optimization of this mitigation strategy. The proposed model is a 1D through-plane stationary model that takes into account the transport of protons, electrons, heat, and gases dissolved in the water. The domain is represented as a series of seven adjacent layers from the anode to the cathode as shown in Figure 1a. A unique feature of the model is the simulation of the local profiles of the gases dissolved in the water of the membrane. These local concentrations allow us to model the reactions taking place in the IL. The proposed model is fully operational and validated with experimental data for different pressure conditions.
The model results show that the IL has a major influence on the local concentrations and crossover fluxes (see Figure 1b). When using an IL, the hydrogen crossover flux entering the anode can be reduced compared to operations without an IL. However, the hydrogen crossover flux leaving the cathode, which is the relevant measure for calculating the Faraday efficiency, is increased when using an IL. This is due to a larger concentration gradient from the cathode to the IL compared to the concentration gradient without an IL. Analogous observations can be made for the oxygen crossover. In addition to these investigations, a series of unresolved issues regarding ILs are tackled with the proposed model. These are namely i) the influence of ILs on the operational limits of a PEM electrolysis cell under different pressure configurations, ii) the influence of recombination heat on the cell’s local heat distribution, and lastly iii) the influence of ILs on the Faraday efficiency of the cell. With regard to the latter investigation, the results show that despite having a major influence on the crossover fluxes, the Faraday efficiency stays constant when using ILs. This can be explained by two counter-acting loss processes, which are identified by the proposed model. These numerical results are consistent with experimental observations. Currently, additional experimental work to further elucidate the reaction kinetics inside of the IL and the relevant transport effects to update the model is underway.
The authors gratefully acknowledge funding by BMBF in the framework of project evolver, FKZ 03SF0765.

References:

[1] M. Schalenbach, M. Carmo, D. L. Fritz, J. Mergel, and D. Stolten, Int. J. Hydrog. Energy, 38, 14921–14933 (2013).
[2] H. Ito, N. Miyazaki, M. Ishida, and A. Nakano, Int. J. Hydrog. Energy, 41, 20439–20446 (2016).
[3] S. A. Grigoriev et al., Int. J. Hydrog. Energy, 34, 5986–5991 (2009).
[4] P. Trinke, P. Haug, J. Brauns, B. Bensmann, R. Hanke-Rauschenbach, and T. Turek, J. Electrochem. Soc., 165, F502 (2018).

Figure 1