Experimental Proof of Concept for an Innovative Evaporative Cooling Concept for Polymer Electrolyte Fuel Cells

Abstract

Polymer Electrolyte Fuel Cells (PEFCs) have a strong potential to enable emission-free, long-range and heavy-duty transportation, but high costs still limit this potential. Auxiliary systems used to tackle the critical water and heat management in a PEFC stack are often identified as some of the components which removal or simplification would result in a significant cost reduction1. Water management is needed to achieve a balance between providing the necessary gas humidification to have a high membrane ionic conductivity2; and avoiding the flooding of the Gas Diffusion Layer (GDL), which limits the transport of the gases to the catalyst layer and blocks the reaction. Water management usually requires costly external humidifiers. Heat management is also a common issue. While increased temperatures allow higher catalytic activity and lower sensitivity to impurities in the hydrogen stream, it also brings the danger of drying out the membrane, which would stop the reaction. Since the rate of heat released by the chemical reaction is of the same order of magnitude as the electrical power generated, the cell must be equipped with a cooling system to keep the temperature constant at an optimal value. Most PEFC systems rely on convective cooling by a flow of coolant in a dedicated space between two adjacent cells, which increases the volume of fuel cell stacks and lower their power density. Here we will present a proof-of-concept for an evaporative cooling design that would not only allow water and heat management without any external equipment, but would also have the potential to reduce considerably the stack’s volume, as the coolant is injected directly into the cell and requires no additional space. Through a process developed at PSI, we modify a commercial GDL into a succession of parallel zones of different wettability. While most of the surface is highly hydrophobic, about one third of the surface, under the form of parallel and equally-spaced lines, is hydrophilic enough to wick water at negative capillary pressures3. We place this patterned GDL on the anode side of a differential fuel cell, and we inject pure water into one of the anode flowfield’s channels, parallel to the gas channels (see Figure 1). The injected water is taken up into the hydrophilic lines, while the rest of the GDL remains a dry pathway to the catalyst layer for the reactant gases. The hydrophilic lines distribute the water across the entire surface of the cell and bring it in contact with the gas flow. The water evaporates, providing both the cooling power and the humidification needed for the cell, and the vapor is carried out of the cell by the remaining gases. In a previous study4, evaporation in a non-operating cell under representative conditions was investigated thanks to a test cell equipped with heat flux sensors, precise and fast temperature control and with the help of neutron imaging. Relations were established to determine the evaporation rates from the mass flow rates of gases, the temperatures and pressures and the geometry of the hydrophilic lines filled with water. Here we present the results of a detailed study to prove that our concept will provide the necessary cooling power and humidification to an operating fuel cell. We compare the performances of the same differential cell with and without evaporation cooling to determine how the evaporation is affecting the cell’s operation, and we thoroughly investigate mass and heat transport phenomena during operation to understand how water vapor transfer is modified by the chemical reaction compared to the case with evaporation only. Last, we discuss the possibility of extrapolating the results obtained on our test differential cell to a higher scale, and the potential of this concept for PEFC for automotive applications. References 1 Cano, Z. P. et al. Batteries and fuel cells for emerging electric vehicle markets. Nature Energy 3, 279-289, doi:10.1038/s41560-018-0108-1 (2018). 2 Barbir, F. PEM Fuel Cells. 295 (Elsevier Inc., 2013). 3 Forner-Cuenca, A. et al. Advanced Water Management in PEFCs: Diffusion Layers with Patterned Wettability: II. Measurement of Capillary Pressure Characteristic with Neutron and Synchrotron Imaging. J. Electrochem. Soc. 163, F1038-F1048, doi:http://doi.org/10.1149/2.0511609jes (2016). 4 Cochet, M. et al. Novel Concept for Evaporative Cooling of Fuel Cells: an Experimental Study Based on Neutron Imaging. Fuel Cells 18, 619-626, doi:doi:10.1002/fuce.201700232 (2018). Figure 1