Hydrogen Crossover in Anion Exchange Membrane Fuel Cells

Abstract

In recent years anion exchange membrane fuel cells (AEMFC) have started to draw more attention by potentially allowing catalysts based on more abundant materials, and by showing high performance [1]. One often overlooked parameter in these systems is membrane permeability of hydrogen, often referred to as hydrogen crossover, which directly affects fuel cell stability and efficiency. This undesirable effect creates mixed potential due to the permeated hydrogen reacting with oxygen on the cathode, and not generating useful electric energy, only water and heat [2]. The generated heat creates hotspots of temperatures enough to cause membrane defects or even destruction which further accelerates the degrading process [3].During fuel cell operation relative humidity (RH) and temperature varies in the system, therefore in this study hydrogen crossover is measured at different temperatures (between 50 and 70 ⁰C) and RH (between 30 and 100 %), on Aemion anion exchange membranes (AEMs) with and without reinforcement and with a variety of membrane thicknesses (25 and 50 μm in Figure 1. a). The hydrogen crossover is measured using a mass spectrometer (MS) at the inert gas exhaust. To avoid effects of other components pure membranes and gas diffusion layers (GDLs) were utilized without electrodes.It is shown that AEMs increase in permeability with temperature for all tested while the effect of RH depends on if they are reinforced or not (Figure 1. a). The non-reinforced membranes show a constant decrease in hydrogen crossover with increasing RH, while the reinforced show constant hydrogen crossover until full humidification where it sharply decreases. While the observed trends are different, the quantitative values between reinforced and non-reinforced membranes were relatively similar: at dry conditions, RHs lower than 50 %, crossover is 10 to 30 % lower for reinforced membranes, around 10 % higher at 70 % RH, and at full humidification a non-significant difference compared with non-reinforced (Figure 1. a). It is shown that an increase in the membrane thickness is not proportional to the decrease in hydrogen crossover, suggesting that interface resistance is not negligible. The calculated interface resistance is higher than the bulk resistance for both reinforced and non-reinforced membranes at all RHs (Figure 1. b). However, the bulk resistance is higher for reinforced membranes than for non-reinforced, while interface resistances are relatively similar. Interestingly, the ratio between the interface and the bulk resistance remains constant at all RHs. However, for the membranes without reinforcement the bulk resistance is 20 % of total crossover resistance, while for reinforced it is two times higher, 40 %. Scanning electron microscope (SEM) cross-section analysis shows that the reinforcement is a relatively thin layer of a different polymer in the membrane, which suggests that even a very thin reinforcement could affect membrane permeability properties. Understanding the effects of the reinforcement and their influence on fuel cells will allow for more durable and safer AEMFCs.Figure 1. H2 crossover measured using a MS at the inert gas side. a) At different temperatures, relative humidities and membrane thicknesses. b) H2 crossover resistance at the interface and in the bulk for membranes with and without reinforcement. Measurements are done at 70 °C and at flows 20 ml/min of both hydrogen and the inert gas (Ar).[1] D. R. Dekel, “Review of cell performance in anion exchange membrane fuel cells,” J. Power Sources, vol. 375, pp. 158–169, 2018, doi: 10.1016/j.jpowsour.2017.07.117.[2] S. S. Kocha, J. D. Yang, and J. S. Yi, “Characterization of Gas Crossover and Its Implications in PEM Fuel Cells,” 2006, doi: 10.1002/aic.10780.[3] Q. Tang, B. Li, D. Yang, P. Ming, C. Zhang, and Y. Wang, “Review of hydrogen crossover through the polymer electrolyte membrane,” Int. J. Hydrogen Energy, vol. 46, no. 42, pp. 22040–22061, 2021, doi: 10.1016/j.ijhydene.2021.04.050. Figure 1