Abstract
Polymer electrolyte fuel cells (PEFCs) are a promising and sustainable alternative to the internal combustion engine for automotive applications. DOE targets [1] require the fuel cells to achieve unassisted start at -40oC and achieve 50% rated power in 30 seconds when starting from -20oC. An understanding of the effect of the cold conditions on the electrochemical and transport processes in the PEFC are crucial to achieve the DOE targets. Wan et al. [2] provides a good overview of the modeling and experimental studies highlighting the impact of freeze start-ups on the membrane, catalyst layer (CL) and gas diffusion layer (GDL). However, only few studies [3-5] in literature have investigated non-isothermal freeze starts in fuel cells. Recently, Naganuma et al. [6] reported the transient behavior of the Toyota FC vehicle for a rapid warm up from -20oC. Understanding the transient behavior of the PEFC during the warm up can aid in designing efficient and reliable warm up strategies for PEFC vehicles.
Advances in the field of X-ray computed tomography (XCT) have enabled sub-second imaging [7] of the transport processes in an operando PEFC GDL. This makes XCT an excellent tool to analyze the transient behavior in the GDLs during the non-isothermal freeze start. In this work, non-isothermal freeze starts from -30oC are performed to investigate: a) effect of cell drying and heat up rate on the freeze start; and b) effect of micro-porous layer on the freeze start. XCT was performed at the TOMCAT beamline at the Swiss Light Source to characterize the supercooled/frozen water phase in the GDL during the freeze starts.
Electrochemical results indicate that pre-drying the cell results in a higher cell power during the freeze start-up. The XCT data show that in the presence of an MPL no supercooled/frozen water was observed in the GDL when the cell is dried before freezing. For the freeze starts performed without drying, frozen/supercooled water clusters are observed in the GDL which evaporate as the cell temperature increases above 0oC. However, no new water clusters were observed during the cell operation. This suggests that the water produced during the freeze start remained in the CL and membrane. Imaging of isothermal freeze starts at temperatures ranging from -30 to 0oC will give further insight into the observed electrochemical transients during the freeze start.
References:
[1] Patterson T., O' Neill J., Perry M., Epping K., Gruber K, Podolski W., PEM Fuel Cell Freeze Durability and Cold Start Project, DOE Annual Progress Report, 2007.
[2] Wan, Z., Chang, H., Shu, S., Wang, Y., Tang, H., A review on cold start of proton exchange membrane fuel cells, Energies, 2014.
[3] Schießwohl E., von Unwerth, T., Seyfried, F., Brüggemann, D., Experimental investigation of parameters influencing the freeze start ability of a fuel cell system, Journal of Power Sources, 2009.
[4] Oszcipok, M., Zedda, M., Riemann, D., & Geckeler, D., Low temperature operation and influence parameters on the cold start ability of portable PEFCs, Journal of Power Sources, 2006.
[5] Alink, R., Gerteisen, D., and Oszcipok, M., Degradation effects in polymer electrolyte membrane fuel cell stacks by sub-zero operation—An in situ and ex situ analysis, Journal of Power Sources, 2008.
[6] Naganuma, Y., Manabe, K., Imanishi, H. and Nonobe, Y., Development of System Control for Rapid Warmup Operation of Fuel Cell, SAE Int. J. Alt. Power., 2012.
[7] Xu, H., Bührer, M., Marone, F., Schmidt, T.J., Büchi, F.N. and Eller, J., Influence of Pore Size Distribution on Operando Gas Diffusion Layer Liquid Saturation. In Meeting Abstracts, The Electrochemical Society, 2019.