Intermediate temperature fuel cells, operating between 150-300 °C offer several advantages over traditional low temperature systems including faster kinetics, increased tolerance to catalyst poisons such as CO, and better fuel flexibility. Fuel cells based on the inorganic proton conductor CsH₂PO₄ (CDP)¹ have the potential to be efficient devices for intermediate-temperature (230-260 °C) distributed generation applications². Realizing this potential is contingent on the development of next-generation electrodes with significantly higher activity and lower precious metal content than the state of the art.

Current SAFC electrodes are based on a porous framework of the CDP electrolyte functionalized with an interconnected film of platinum nanoparticles that serves as both the ORR catalyst and the electronic conductor³. The activity of these electrodes scales with the surface area of the electrolyte in the electrode², but the requirement of a percolating Pt network also unfavorably scales the precious metal content of these high surface area electrodes.

We have addressed this problem by developing a new electrode concept based on a multi-step, multiphase infiltration strategy. The CDP electrolyte is first infiltrated in the liquid phase into chemically-treated multi-walled carbon nanotube (MWNT) bundles, followed by vapor-phase Pt deposition on the nanocomposite structure. In addition to exerting a structural templating influence on the solid electrolyte, the role of electronic conduction is assumed by the MWNTs, allowing for much greater tri-phase boundary densities to be obtained at lower Pt content. We further examined the effect of electrode thickness and have observed that the majority of the electrochemical activity is within the first micron of the catalyst layer.

Cathodes formed from these nanocomposite materials can exceed the activity of the state-of-the-art cathode (2.8 mg_Pt/cm²) at a fraction of the Pt content (0.18 mg_Pt/cm²). Pt utilization is increased at lower Pt loadings, with Pt mass-specific activity greater than 3 A/mg_Pt at 0.5 V. Cells based on this architecture are stable for greater than 300 hours at 0.6 V and 250 °C.

In this presentation we will highlight our progress over the last few years and comment on efforts to commercialize this technology. A critical review of the unique features of the electrode architecture will be presented along with some commentary on recent observations that suggest that single atom Pt catalysts exist on the nanocomposite electrode that may play a role in the enhanced Pt activity.

Acknowledgements

This work is supported by ARPA-E under Cooperative Agreement Number DE-AR0000499.

References

¹ S.M. Haile, C.R.I. Chisholm, K. Sasaki, D.A. Boysen, and T. Uda, Faraday Discuss. 134, 17 (2007).

² C. R. I. Chisholm, D. A. Boysen, A. B. Papandrew, S. K. Zecevic, S. Cha, K. A. Sasaki, Á. Varga, K. P. Giapis, and S.M. Haile, Electrochem. Soc. Interface 18, 53–59 (2009).

³ A. B. Papandrew, C. R.I. Chisholm, R. A. Elgammal, M. M. Özer, and S. K. Zecevic, Chem. Mater. 23, 1659–1667 (2011).