Status of Regenerative Fuel Cell Membrane Electrode Assembly Development for Space-Based Energy Storage

Abstract

Introduction The National Aeronautics and Space Administration (NASA) has led a thrust for the development of advanced regenerative fuel cell systems (RFCs) to be used as energy storage for space-based robotics, mobility systems, and human habitats. The goal of the program has been to develop an RFC system capable of a round trip efficiency of 64%. The focus at the Jet Propulsion Laboratory (JPL) in RFC research has been in developing proton exchange membrane electrode assemblies (MEAs) for both the fuel cell and the electrolyzer subsystems [1-3]. In a RFC system, energy storage is achieved via the electrolysis of water to hydrogen and oxygen gases during the charge phase. Consumption of the hydrogen and oxygen gases then occurs during the discharge phase, with the subsequent generation of water. For space applications, the energy for the electrolysis of water will be supplied via solar or nuclear power. The power delivered during the discharge of the RFC system can be used by robots, mobility systems, and human habitats operating on the moon, near-Earth asteroids or Mars. Such a system could also be used for load balancing in both space and terrestrial applications. This paper will discuss fuel cell MEAs that have been developed for a future NASA regenerative fuel cell system. RFC system trades and NASA mission concepts that will feature RFC-based energy storage systems will also be discussed. Results and Discussion The current-voltage polarization of two MEAs are shown in Figure 1. To achieve high voltage efficiencies, the MEAs will be expected to operate at current densities in the range of 200 to 600 mA/cm2. TheMEA will operate with reactants at a balance pressure of 30 PSIG at 70 oC. The NASA-JPL developed MEA isdesigned to operate in a non-flow-through stack. The MEA current-voltage polarization reported are for MEAs that operate with a reactant feed rate of approximately 3x stoic. As shown in Figure 1, the polarization for the NASA-JPL MEA is reported to be 0.92, 0.86 and 0.81 Volts at an applied current density of 200, 600 and 1000 mA/cm2, respectively. The polarization of the NASAJPLMEA is approximately 80 mV greater than the commercially available MEA at a current density of 200 mA/cm2. The voltage efficiency for the JPL-NASA MEAat 200 and 600 mA/cm2 is approximately 75 and 70%,respectively. To achieve the desired RFC system efficiency, the fuel cell MEA will need to operate at approximately 73% voltage efficiency. It is envisioned that MEA operation at current densities above 1 A/cm2 would be for short periods (< 1 hour) for NASA applications. The short-term durability studies for the NASAJPL MEA is shown as Figure 2. The MEA operated at an applied current density of 200 mA/cm2, 70 oC, and 30PSIG balanced reactants during the durability studies. The initial voltage for the MEA is reported to be greater than 0.92 Volts, the voltage drops to 0.91 Volts after 13 hours of operation. The initial voltage is recoverable after current cycling as shown at 243 hours of operation. The cell degradation is reported to be approximately 7 μVolts after more than 1000 hours of operation. The degradation is, in part, attributed to test hardware. The minimum MEA voltage efficiency is approximately 73% for the duration of the test. Future work will investigate the integration of the NASA-JPL MEA in NASA RFC hardware. Acknowledgements The work presented here was carried out at the Jet Propulsion Laboratory, California Institute of Technology for the National Aeronautics and Space Administration.