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

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. The

MEA will operate with reactants at a balance pressure of

30 PSIG at 70 ^oC. The NASA-JPL developed MEA is

designed 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 NASAJPL

MEA 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 MEA

at 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 30

PSIG 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.

References

[1] S. R. Narayanan, A. Kindler, A. Kisor, T. Valdez, R.

J. Roy, C. Eldridge, B. Murach, M. Hoberecht, J. Graf, J.

Electrochemical Society, 158 (11), B1348-B1357 (2011)

[2] S. R. Narayanan, T. I. Valdez, and S. Firdosy,

“Analysis of the Performance of Nafion-Based Hydrogen-

Oxygen Fuel Cells,” J. Electrochemical Society, 156(1),

B152-159 (2009)

[3] T. I. Valdez, J. Sakamoto, K. Billings, S. A. Firdosy,

F. Mansfeld, and S. R. Narayanan, Meet. Abstr,

Electrochem Soc. 801 360 (2008)