PEM Fuel Cell Operation at −20°C.

Abstract

An experimental procedure using isothermal galvanostatic operation was developed to quantify the charge ͑water͒ accumulation in proton exchange membrane ͑PEM͒ fuel cells at subfreezing conditions prior to voltage failure ͑i.e., zero cell voltage͒. The charge passed until voltage failure was compared to charge ͑water͒ storage estimates in the membrane phase and the cathode electrode void volume. Cryo-scanning electron microscope images of electrodes following voltage failure were used to assess ice filling of the cathode electrode void volume. At very low current densities, the membrane absorbs a maximum of Ϸ14 to Ϸ15 water molecules/per sulfonate group ͑ max Ϸ 14-15͒ and cathode electrode voids are completely ice filled. It is shown that the maximum charge storage of a membrane electrode assembly increases with electrode void volume and the difference between max and initial . With increasing current densities, decreasing fractions of the maximum charge storage can be utilized, which is shown to be related both to water transport resistances in the membrane phase and to reduced ice filling of the electrode void volume. Experimental results show that the charge storage utilization is mainly controlled by the current density and is less dependant on initial water content or electrode thickness.Proton exchange membrane ͑PEM͒ fuel cells are presently being investigated as an alternative power source for vehicles that require cold climate operation. At low temperatures, PEM fuel cell ͑PEMFC͒ performance suffers from the same irreversibilities that occur during operation at normal temperatures, namely, ohmic, kinetic, and mass transport losses. Our previous studies investigated the ohmic losses due to membrane conductivity 1 and oxygen reduction reaction ͑ORR͒ kinetic losses 2 at temperatures below 0°C. During operation at subfreezing temperatures, the amount of product water that can be produced prior to voltage failure ͑i.e., zero cell voltage͒ is essentially limited by the maximum water uptake of the membrane and the cathode electrode void volume, because the loss of water vapor into the reactant gases is negligible significantly below 0°C. This maximum water uptake can also be expressed in terms of a maximum charge storage ͑in coulombs͒. The latter is a more convenient measure because it corresponds to the maximum time integral of the current by which a frozen fuel cell must reach 0°C before voltage failure occurs as a result of reactant blockage by ice. This concept of maximum charge storage and its relation to mass transport of product water into the membrane as well as into the cathode electrode void volume is the focus of this study. Please note that the concept of charge storage in this study is synonymous with the quantity of water produced electrochemically; it is not to be confused with the platinum and double-layer charge capacity of the electrode, which is negligibly small 3 compared to the charge quantities discussed in this paper.During subfreezing operation, ice formation from product water in electrod...

Durability of Polymer Electrolyte Membrane Fuel Cells Operated at Subfreezing Temperatures

“…These results are consistent with those reported earlier where the initial membrane water content, λ, was 14 mol H 2 O / mol SO 3 − to 15 mol H 2 O / mol SO 3 − when operated under fully humidified conditions, and only drier membranes exhibited membrane hydration during the low temperature operations. 4 The increase in HFR can be attributed to either a loss in electronic conductivity of the CL (due to ice formation moving the carbon particles further apart) or an increase in the interfacial (CL/MEA or MPL/CL) resistances.…”

“…Controlled isothermal experiments of single cells have been conducted to infer the location of ice formation and to study the performance of fuel cells at subfreezing temperatures. [3][4][5][6][7][8][9][10][11] These results have all shown that ice formation in the CL leads to a loss in cell performance, but there are few reported results on the durability of fuel cell stacks that have been subjected to subfreezing temperatures. Moreover, there is wide variability in the results reported, with Knights et al reporting no degradation after 55 subfreezing starts 12 and Yan et al demonstrating significant damage within a few subfreezing starts on a 25 cm 2 fuel cell.…”

Mechanics-based model for non-affine swelling in perfluorosulfonic acid (PFSA) membranes

“…As discussed in the previous section, the maximum water content in previously predried membranes is similar whether the membrane is equilibrated in water vapor or liquid water [8,46,50,74]. In contrast to this, for membranes that are not predried (at elevated temperatures for a certain period of time), but subjected to standard treatment procedure, the water content is [10,38,45,46].…”

“…However, it has been reported that [8,46,50] the water content in predried 3 membranes is similar, whether the membrane is equilibrated in saturated water vapor or liquid water. Thus, we will here adopt the results from a number of experimental water uptake studies for predried PFSA membranes equilibrated in liquid water and/or water vapor from Zawodzinski et al [8,38], Hinatsu et al [45], Onishi et al [46], and Thompson et al [74]. According to these studies, the water content of the predried membrane saturated with water is in the range of l ¼ 12-20, in close agreement with the predictions of our cylindrical RVE model (Fig.…”

Cold start characteristics and freezing mechanism dependence on start-up temperature in a polymer electrolyte membrane fuel cell

“…Tajiri et al proposed a strict gas purge process before cold start operation, and estimated the ice distribution in the cathode catalyst layer at the end of cold starting at -30 °C at low and high current densities [5]. Recently, cryo-SEM observations were conducted to elucidate further details of the ice distribution in the catalyst layer at -25 °C or -20 °C; these observations were carried out with the examined components kept at very low temperature and in vacuum conditions to avoid thawing and covering by frost deposits [6][7][8][9]. At temperatures closer to zero, like -10 °C, it was reported that the produced water is present in a supercooled state, and that the freezing behavior in a PEFC can be visualized by infrared thermography and it was possible to capture the 2-dimensional propagation in the high temperature region caused by the release of heat due to freezing [10][11][12].…”