Infiltrated CsH2PO4-Carbon Nanotube Composites for High Performance Solid Acid Fuel Cathodes with Low Pt Loading

Abstract

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. Cathodes formed from these nanocomposite materials can exceed the activity of the state-of-the-art cathode (2.8 mgPt/cm2) at a fraction of the Pt content (0.7 mgPt/cm2) (see Figure 1a). Pt utilization is increased at lower Pt loadings, with Pt mass-specific activity greater than 1 A/mgPt at 0.5 V possible at 0.35 mgPt/cm2 (Figure 1b). Cells based on this architecture are stable for greater than 150 hours at 0.6 V and 250 °C. Acknowledgments This work is supported by ARPA-E via cooperative agreement 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). Figure 1