Lifetime and degradation of high temperature PEM membrane electrode assemblies

https://doi.org/10.1016/j.ijhydene.2015.07.152Get rights and content

Abstract

This work will provide details on some of the high temperature polymer electrolyte membrane (HTPEM) membrane-electrode-assembly (MEA) performance targets most recently achieved by Danish Power Systems. These include (i) MEA performances of >0.67 V at 0.2 A cm−2 using dry H2/Air, (ii) MEA lifetime of 17.000 h at 0.24 A cm−2 using dry H2/Air with an average degradation rate of 9 μV h−1, and (iii) an integrated 5 kW stack/reformer system using methanol reformate as fuel. Post mortem SEM, TEM, micro-tomography and XRD showed membrane thinning and catalyst particle growth that is typical for PEM fuel cells. Platinum particles grew from an initial 2–3 nm to 6–8 nm at the cathode and 4–5 nm at the anode, while the membrane showed thinning from an undoped 40 μm–18 μm in some areas after testing. Studies using reformate have also led to promising initial results, while the rate of degradation for an MEA supplied with wet H2 (30 mol%)/Air for 2000 h was found to be very similar to the rate when supplied with dry H2. In addition to reaching these performance benchmarks, a reduction in the standard deviation for MEA cell voltage at 0.2 A cm−2 to <1% has been achieved through efforts aimed at improving the uniformity of the membrane and catalyst layer thicknesses.

Introduction

Polymer electrolyte membrane (PEM) fuel cells have long since been identified as a promising technology in meeting some of the challenges faced with shifting towards a clean and renewable energy economy. High temperature PEM (HTPEM) fuel cells provide an attractive alternative to low temperature PEM fuel cells with respect to fuel flexibility, simplified cooling system, balance of plant and higher value waste heat without reactant humidification [1]. Furthermore, the higher operating temperature range of 140–180 °C dramatically increases the tolerance of the noble metal catalysts to impurities in the reactants at both the anode and cathode.

To date, most of the success in the field of HTPEM fuel cell development has been realized through the implementation of a phosphoric acid-doped polybenzimidazole (PBI) membrane electrolyte [2]. One of the most useful PBI-based PEMs for high temperature applications are those derived from 3,3′,4,4′-tetraaminobiphenyl and isophthalic acid. The use of such a material allows for operating temperatures in the range of 140–180 °C.

Losses in MEA performance associated with fuel cell operation typically occur via processes related to degradation of the catalyst layer [3], the re-distribution of phosphoric acid within the PBI membrane and catalyst layers, and thinning of the membrane itself [4], [5]. We have previously presented work regarding the effect of m-PBI molecular weight on membrane oxidative stability and MEA performance [6], [7]. It is believed that Ostwald ripening [8] and coalescence [9] are the two main mechanisms through which degradation of the catalyst layer occurs, thereby reducing the electrochemical surface area of the carbon-supported platinum electrodes.

As one of the few commercial suppliers of PBI membranes and MEAs for HTPEM fuel cells, we hereby seek to provide an update regarding some of the continuing challenges related to the performance, durability, and commercialization of HTPEM membrane electrode assemblies (MEAs) and our approach to overcoming these obstacles.

Section snippets

Materials

m-PBI was synthesized in polyphosphoric acid (PPA) at 200–230 °C under an argon atmosphere and prepared by Danish Power Systems (DPS) [2]. m-PBI membranes (Dapozol® membranes) were manufactured by DPS using previously described methods [10].

Fuel cell testing

Anodes and cathodes were prepared in an identical fashion by spraying a platinized carbon black (Pt/C) dispersion onto a carbon-based gas diffusion layer, as previously described [10]. The Pt loading was measured gravimetrically for each electrode after

MEA performance and durability

Fig. 1a shows the cell performance of standard DPS MEAs operated under a constant current of 0.2 A cm−2. Polarization curves were recorded intermittently after 5, 24 and 48 h of operation, giving rise to the transients that are seen for each MEA. To date, we are able to achieve performances consistently >0.67 V when operated continuously at 0.2 A cm−2. However, reducing the high Pt loading of 1.5 mg cm−2 at both the cathode and anode of these MEAs remains a high priority for further commercial

Conclusion

The measurements presented here have demonstrated the feasibility of HTPEM fuel cell technology towards a number of applications – particularly with the ability to operate using reformate. The performance and durability of DPS MEAs has been shown to fulfil several of the requirements for commercialization. However, the high platinum loading remains a significant obstacle that should be addressed in the next phase of MEA development.

Acknowledgments

Funding of this work is gratefully acknowledged from the Danish ForskEL program, the Danish Advanced Technology Foundation, Danish National Research Foundation and Danish Energy Agency. We further thank the European Commission as some of this work was supported by the Seventh Framework Programme through the project CISTEM (Grant Agreement Number 325262, 01.06.2013-31.05.2016). SEM imaging was carried out by Larisa Seerup at the Technical University of Denmark.

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