Effect of Elevated PEM Fuel Cell Operating Temperature (120°C and 140°C) and Membrane Thickness on Proton Conductivity for Combat Vehicle Use

Abstract

Energy required from ground vehicles in the United States (U.S.) Army is increasing due to the need for increased capabilities and mission duties required for an ever changing combat environment. These additional capabilities include silent watch (long term mounted surveillance), advanced radios/jamming devices/sensors, directed energy weapons, exportable power supporting stationary applications, and vehicle-to-grid connectivity. Increasing the combat vehicle energy output, while still fitting into the limited space inside each vehicle, requires an energy source that is compact, power dense and energy efficient. PEM Fuel Cells (PEMFCs) meet those requirements for primary and/or auxiliary power generators placed in vehicles for short or long-term mission roles. While PEMFCs provide a potential solution for the U.S. Army’s vehicle energy requirements, there still exists a number of critical areas which need to be resolved before integration into combat vehicles is realized. These issues include: 1. Stack degradation from thermal-cycling and stack sealing loss, 2. Electrocatalyst degradation and 3. Cell membrane thermal degradation. In addition, specifically for the U.S. Army, vehicles significantly restrict heat rejection (due to low air flow from ballistic grills) to the PEMFC system in addition to vehicles operating in locations with elevated ambient temperatures, which can increase the stack operating temperature up to 140°C. Since cooling requirements for PEMFCs are typically engineered for operating temperatures closer to 65°C, these elevated stack temperatures could result in thermal degradation of the cell membrane.Since the cell membrane is a vital component for stack operation, membrane material formulations have been developed with increased chemical and thermal degradation resistances. One such membrane material is Nafion, which starts to thermally decompose around 300°C. Even when the operating temperature is lower than the decomposition temperature, a small percentage of the Nafion membrane potentially could degrade as temperatures reach between 120°C and 140°C which could change the performance of the PEMFC through changes in proton transport resistance. Another possible method of reducing membrane thermal degradation would be engineering cells with thicker membranes, where the added mass could withstand the increased temperature for longer periods of time. These potential changes to the Nafion membrane at elevated temperatures (120°C and 140°C) using different membrane thicknesses, when compared to stack operation at 65°C, were investigated in this study.Pristine Nafion membranes (Nafion 115, 117 and 1110) were used in the following analysis and were heated at 65°C, 120°C and 140°C, for 2, 8 and 24hrs at each temperature. The membranes all had similar polymer formulations with thicknesses of 127, 183 and 254μm, respectively. All three polymers were heated for 2, 8 and 24hrs in 16MΩ water at 65°C, 120°C and 140°C and were characterized to determine the impact of elevated temperatures and material thickness on proton conductivity and structural changes to each material. Figure 1a shows the calculated proton conductivities, from raw EIS data, at 65°C, 120°C and 140°C for all three membranes heated for 2, 8 and 24hrs. Results from heating at 120°C showed all three membranes had increased conductivities between 14% and 30% after 24hrs, compared to the 65°C results, and were statistically similar to each other. While large variations in the conductivities occurred, the majority of the 120°C measurement standard deviations did not overlap with the 65°C results. Results from heating at 140°C showed the three membranes had increased their conductivities by different amounts, depending on membrane thickness. Nafion 115 had been statistically increased by 30% compared to 115 heated at 65°C for 24hrs, while the 117 and 1110 had returned to a statistically similar value as the 65°C 24hr results.Figures 1b,c,d show internal structural changes to the vibrational modes for each membrane material using FTIR characterization, which support the changes in proton conductivities reported in Figure 1a. All three membranes, after 24hrs at 65°C, showed near identical scans. All three membrane materials, after heating at 120°C and 140°C, showed changes to the vibrational modes observed for the 65°C results and produced additional vibrations modes not originally present. These conductivity and FTIR results show heating Nafion, even at elevated temperatures, was sufficient to dynamically change its operating performance over time. Altering the membrane thickness did not mitigate performance changes over time however, with even the thicker membranes being less stable with time. Based on these results, an alternative approach or material should be used if operating the PEM fuel cell at or above 100°C. Figure 1