Hydroxide Exchange Membrane Fuel Cells (HEMFC) are continually being researched as a potentially more cost-effective alternative to Proton Exchange Membrane Fuel Cells (PEMFC). In the high pH environment in HEMFCs, platinum group metal free catalysts have shown promising results, and bipolar plates can be manufactured using more cost-effective material1. These aspects can significantly reduce the overall cost; however, improvements in HEMFCs performance and durability are needed for commercialization.A key challenge in the design of PEM and HEM fuel cells is optimizing water and thermal balance to achieve maximum performance and durability. Water and thermal management are extremely important to understand and optimize in HEMFCs due to their influence on kinetic, ohmic, and transport overpotentials2. Significant efforts in water management have been made in PEMFCs in order to achieve higher performance and durability, specifically using a microporous layer (MPL) to optimize oxygen transport in the cathode where water and heat are generated3. We can use knowledge gained from PEMFC in HEMFCs; however, our water generation and movement are flipped, as shown in Figure 1. In PEMFCs the cathode generates 2 water molecules per 4 electrons, and in HEMFCs the anode produces 4 water molecules, and the cathode consumes 2 water molecules per 4 electrons. The water consumption in the cathode along with the heat generation increases the risk of dry-out. On the other side, the anode produces water and could possibly lead to flooding. Furthermore, the goal in HEMFCs, in order to achieve better performance and durability, is to use the water produced in the anode to help hydrate the membrane and cathode ionomer to minimize dry-out without causing flooding losses4.Our current investigation is to manipulate the water and thermal management through the gas diffusion layers (GDLs). An MPL can be added to influence liquid water within the cell, specifically on the anode. Figure 2 (a) shows four identical cells with different GDL combinations, 29BC is with an MPL and 25BA is without an MPL. At high temperature and flowrates, the capillary pressure from liquid water in the anode catalyst layer does not exceed that needed to enter the MPL and, therefore, hydrates the membrane. Other benefits from the MPL includes improved contact between the GDL and catalyst layer and higher electronic conductivity. Another aspect of GDLs that can manipulate water management is through altering the thermal conductivity. Figure 2 (b) shows three identical cells with Sigracet (SGL) and Toray GDLs, where Toray thermal conductivity is 4 times higher than SGL. Higher thermal conductivity decreases temperature gradients within the MEA and can decrease risk of dry out. But lower temperature gradients could lead to less liquid water vaporization within the catalyst layer and increases risk of flooding. We report the MPL is beneficial between the anode catalyst layer and GDL to promote back diffusion, and the temperature gradients effected by the thermal conductivity have a significant impact on water content in the cathode. Setzler, B. P., Zhuang, Z., Wittkopf, J. A. & Yan, Y. Activity targets for nanostructured platinum-group-metal-free catalysts in hydroxide exchange membrane fuel cells. Nat. Nanotechnol. 11, 1020–1025 (2016).Weber, A. Z. & Newman, J. Coupled Thermal and Water Management in Polymer Electrolyte Fuel Cells. J. Electrochem. Soc. 153, A2205 (2006).Weber, A. Z. & Newman, J. Effects of Microporous Layers in Polymer Electrolyte Fuel Cells. J. Electrochem. Soc. 152, A677 (2005).Omasta, T. J. et al. Beyond catalysis and membranes: Visualizing and solving the challenge of electrode water accumulation and flooding in AEMFCs. Energy Environ. Sci. 11, 551–558 (2018). Figure 1
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Oct 29, 2021
Lei Zang + 5
Open Access