Research On Subfreezing Characteristics Of CCM For Proton Exchange Membrane Fuel Cells

Catalyst Coated Membrane (CCM) is one of the key components for proton exchange membrane fuel cells (PEMFCs) which are composed of proton exchange membrane (PEM) and a catalyst layer coated on both sides. The performance of CCM directly determines the performance of a fuel cell, as well as playing an important role in cost reduction, power density improvement and quickening the commercialization of fuel cells. The subfreezing environmental adaptability and freeze durability are a key issue in the commercialization of proton exchange membrane fuel cell, and so do they as a challenging frontier of fuel cell research. This paper was designed to perform research and evaluation on subzero characteristics of homemade CCM through experiment.PEM is a central functional compartment for PEMFCs and is also the main lieu for water storage inside a fuel cell. So what is the state of water in a subzero temperature requires to answer first of all. Whereas hereupon, this paper combined differential scanning calorimetry with humidity fixed point by saturated saline solution to investigate subzero water state in the membrane. At present, subfreezing behavior and freeze durability have become a hotspot in PEMFC research. Cost reduction and durability improvement have brought fuel cell to the edge of industrialization. Meanwhile, cold start issue has been given prominence. Freeze start, subfreezing performance and the impact of subzero temperature on fuel cell durability have attracted great interest from more and more investigators. Thus, subfreezing characteristics for fuel cell has become important part of cold start ability and freeze durability on the road to commercialization.Low temperature failure behavior of fuel cell components regarding membrane and CCM was investigated by contrastive experiment under freeze/thaw experiment, by means of which, related mechanism was discussed initially. Subfreezing behavior and performance decay of CCM were investigated under freeze/thaw operation on the basis of various purging scenarios. Experiment was setup along with electrochemistry workstation to investigate performance of CCM before and after freeze/thaw operation. By means of polarization curves, cyclic voltammetry, electrochemical impedance spectra and hydrogen crossover test, the electrochemical performance characterization was completed. Surface appearance of components was characterized by SEM and Image Analysis System.By means of above research methods, related results and main conclusions are asfollows:(1) Frozen behaviors of water inside membrane were discussed under mol water content. Nonfreezable water contentλ₂ was always between 4 and 5. The cause for frozen temperature change was ascribed to freezable water contentλ₁. As water content increased, the frozen temperatures for water inside membrane shifted to much higher values. In temperature range from -25℃to 0℃, freezable water content inside membrane and frozen temperatures follow linear relationship. The data of frozen temperatures with same water content indicates that composite membrane bears more excellent frozen nature.(2) Local swelling and expansion, stress concentration along with penetration from ice lens or ice crystals might be the root of subfreezing membrane failure. Hydrogen crossover results indicated that there's little change in membrane microstructure, which would less likely lead to fuel cell performance degradation in subfreezing temperature.(3) Reasonable dehumidification scenario is important in retaining integral long-term performance under subzero temperature. The results indicated here that both "cold purge" and "hot purge" were effective in preventing frostbite if proper parameters had been chosen. From the point of activation necessity and performance reproducibility, cold purge excels hot purge.(4) By means of single cell performance testing and Cyclic Voltammograms testing, the potential cause of subzero performance decay was ascribed to: decrease in electrochemical active area for low current densities; mass transport characteristics in gas diffusion layer (GDL), and interfacial coupling (electric) characteristics between catalyst coated membrane (CCM) and GDL. In situ ac impedance validated the discrepancy in high current region and low current region. As for subzero performance degradation, polarization resistance plays an important role in low current density region; mass diffusion resistance and contact resistance at CCM/GDL interfaces plays an important role in high current density region.The results from this paper indicated that homemade CCM bears not only excellent performance and environmental adaptability but also sound freeze durability. What's more, proper purging scenarios would benefit subzero storage and maintain performance integrity.