New Characterizations for PEFC Under Sub-Zero Temperature

Abstract

The operation of fuel cells at sub-zero temperature results in direct deposition of ice or production of supercooled water that freezes upon interacting with nucleation seeds. Ice formation can lead to irreversible mechanical damages in membrane electrode assembly (MEA), particularly at interfaces, which is detrimental to the performance and safety of fuel cells. The mitigation strategies are focused on temperature elevation, including assisted heating, catalytic heating, and starvation (low potential) heating; however, these are associated with parasitic loss that reduces overall efficiency of the system. An alternative method is to develop porous media that are capable of ensuring oxygen transport pathways in the presence of ice or retaining supercooled water without phase transition until the cell reaches above 0ºC.The development of ice-resistant porous media for sub-zero operation is hindered by limitations on material characterization and performance evaluation techniques. The most common characterization technique is differential scanning calorimetry (DSC), by which water-saturated porous media is subjected to rapid temperature cycles to measure melting and freezing peaks. The DSC is a well-established and convenient technique that is suitable for statistical analysis; however, the sample size is limited to the mm-scale, and water must be injected in hydrophobic media prior to the DSC measurement where water saturation level and morphology are unpredictable. In situ testing of novel materials are performed on single cell hardware, in which the temperature is controlled by circulating a coolant through endplates or cooling channels that are placed several mm to cm away from an MEA. Such pseudo-active cooling system results in a significant difference (and delay) between the coolant and cell temperatures, influenced by heat production due to cell operation as well as freezing/melting. On the other hand, free heating operation starting from sub-zero is not desirable, since product heat is dissipated to current collectors and endplates that have much greater mass per active area in comparison to stack. As a result, most of sub-zero tests are performed on isothermal condition.Our previous works [1,2] introduced a unique single fuel cell hardware featuring temperature control and heat flux measurement (TCHM) system. Thermoelectric coolers and tangential gradient heat flux sensors enable a precise control of temperature in the MEA and detection of freezing and melting events during the cell operation. We have advanced the TCHM controlling software and integrated them on the custom fuel cell test bench. In this work, two novel techniques are introduced for sub-zero application for fuel cell porous media: (1) advanced calorimetry and (2) non-isothermal cold start.The advantages of our advanced calorimetry include testing of a large sample area up to 12cm2 and control of water quantity in a gas diffusion layer (GDL). Through neutron imaging, we validated capillary water injection method in the GDL and reduced saturation level with drying gas purging. Using techniques, we analyzed the effect of GDL structures and hydrophobicity on repeated freezing-thaw cycle to assist design of ice-resistant material. Additionally, we successfully implemented non-isothermal cold-start at -20 ºC in addition to isothermal condition, as seen in Figure 1. After the calibration of the TCHM units, the heating load was emulated to match that of the stack, corresponding to the heating rate between 10-15 K/min.The methods introduced in this study are expected to make a significant contribution towards the development of GDLs and catalyst layers that suppress ice formation and related mechanical damages. In particular, the non-isothermal cold-start provides a flexible, precise temperature control in the MEA, which can bridge the gap between the single cell and stack testing. Reference [1] J. Lee, E.R. Carreon-Ruiz, M. Siegwart, P. Boillat, Segmentation for Preventing Ice Propagation in Operating Fuel Cells, Meet. Abstr. MA2021-02 (2021) 1066. https://doi.org/10.1149/MA2021-02361066mtgabs.[2] M. Siegwart, F. Huang, M. Cochet, T.J. Schmidt, J. Zhang, P. Boillat, Spatially Resolved Analysis of Freezing during Isothermal PEFC Cold Starts with Time-of-Flight Neutron Imaging, Journal of The Electrochemical Society. 167 (2020) 064510. https://doi.org/10.1149/1945-7111/ab7d91. Figure 1