Elsevier

Journal of Alloys and Compounds

Volumes 404–406, 8 December 2005, Pages 653-656
Journal of Alloys and Compounds

100–200 ° C polymer fuel cells for use with NaAlH4

https://doi.org/10.1016/j.jallcom.2005.02.091Get rights and content

Abstract

The complex hydride NaAlH4 is attracting much attention right now because of high hydrogen storage capacity and a practical desorption temperature that can be brought down below 150 ° C. However, this is still far from the working temperature of conventional perfluorosulfonic acid based polymer fuel cells (e.g. Nafion cells). A hydrogen desorption temperature below 80 ° C is mandatory if the excess heat of such a fuel cell shall be used for hydrogen desorption from a NaAlH4 storage system. Our approach is to increase the working temperature of the polymer fuel cell. The present paper reports our recent work on polymer fuel cells operating at temperatures up to 200 ° C. The key component to make this possible is an electrolyte membrane of polybenzimidazole (PBI) doped with phosphoric acid. Cells are operated at temperatures up to 200 ° C without any humidification. Continuous service for more than 6 months at 150 ° C is demonstrated. The temperature and amount of the excess heat is more than enough to run a metal hydride tank based on NaAlH4.

Introduction

Metal hydride systems for use at near ambient temperature and pressure generally suffer from a low gravimetric hydrogen storage capacity. Hydride systems with much higher capacities are known, but only with higher working temperatures like MgH2 with 7.6 wt.% at 300 ° C. The working temperature of the metal hydride is an important issue. Not only must the hydride be heated to the working temperature, but a significant amount of heat (the desorption enthalpy) must also be supplied at this temperature. This heat can be produced by burning part of the stored hydrogen, but in case of MgH2 at least 31% of the stored hydrogen will then be wasted just to provide heat for hydrogen desorption. When fuelling a fuel cell, a more effective way is to make use of the heat that is always produced by the fuel cell along with the electricity. Therefore, the working temperature of the hydride should not be higher than the working temperature of the fuel cell. Conventional proton exchange membrane fuel cells (PEMFC) have a working temperature of about 80 ° C, and consequently, a compatible metal hydride system must have an equilibrium pressure of at least 1 bar and sufficient desorption kinetics, at temperatures significantly below 80 ° C. During recent years several groups have reported reversible hydrogen storage capacities in the range of 3–5 wt.% and release temperatures in the range of 100–200 ° C for NaAlH4[1], [2], [3]. A further temperature reduction down below 80 °C has not proven easy. Why not increase the temperature of the PEMFC instead?

The electrolyte of a normal PEMFC is a perfluorinated sulfonic acid polymer membrane (e.g. Nafion), which is only proton conductive with high water content. This is the reason why the working temperature cannot easily be raised above 100 ° C. For a high-temperature system, another membrane material is needed. In 1995, Wainright and co-workers [4] proposed phosphoric acid doped polybenzimidazole (PBI) as a high-temperature proton conductor, and our group has followed that line [5], [6]. Other high-temperature membranes under development have been reviewed recently [7]. The driving force for the development of high-temperature PEM fuel cells has to a large extend been to increased the tolerance to carbon monoxide (over three orders of magnitude at 200 °C [8]) and consequently the possibility of powering the fuel cell directly from a reformer/shift reactor without purification [9]. However, in this paper the focus is on the possible interplay with complex hydrides like NaAlH4. The high operating temperature offers an excellent opportunity for improving the energy efficiency by using the excess heat for hydrogen desorption.

Section snippets

Experimental

The PBI used in this work was poly-2,2-m-(phenylene)-5,5-bibenzimidazole (see Fig. 1) obtained from Celanese or later synthesized [10]. After casting to a thickness of about 50 μm, the membrane was doped with phosphoric acid to a level of 5–6 H3PO4 molecules per repeat unit of PBI. The catalyst applied was platinum on carbon, about 0.5 mg/cm2 for all electrodes. The experimental details are given elsewhere [6], [10].

Results

The ionic conductivity of acid doped PBI was measured earlier [10] and the value is plotted as a function of temperature and relative humidity in Fig. 2. It can be seen that the conductivity at 200 °C and only 5% relative humidity is similar to that of Nafion at 80 ° C and 90% relative humidity. Even at very low relative humidity ,the conductivity of doped PBI is still significant. In contrast, the conductivity of Nafion depends on a very high relative humidity and that is the reason why Nafion

Discussion

At standard temperature and pressure hydrogen fuel cells have a maximum electrical efficiency of 95% based on the lower heating value or 83% based on the higher heating value. However, the practical electrical efficiency depends on the cell load and is only about 50% or less. The remaining 50% or more is converted into heat. At 200 ° C, only the lower heating value of 244 kJ/mole H2 is relevant. Fifty percent of that is 122 kJ/mole H2.

Hydrogen desorption from NaAlH4occurs in two steps.

Conclusion

The problem of temperature mismatch between conventional Nafion based polymer fuel cells and the high capacity metal hydride systems like NaAlH4 can be overcome with the use of a high-temperature polymer, in this case polybenzimidazole (PBI). It is shown that polymer fuel cells based on PBI can be operated at temperatures up to 200 ° C. Long term experiments have shown a lifetime of more than 6 months at 150 ° C. In contrast to Nafion cells PBI cells need no humidification of the gases. This

Acknowledgements

The authors wish to thank the following for the financial support of the present work:

The Nordic Energy Research Programme, the Nordic Industrial Fond, Danish Technical Research Council, Danish Power Systems ApS, the European Commission in the framework of the Non-Nuclear Energy Programme JOULE III and the Fifth Framework Programme.

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