A cost-effective electrical energy storage system is needed to allow large-scale integration of intermittent renewable energy sources such as wind and solar into the electrical grid. The hydrogen-bromine reversible fuel cell system is a promising technology to meet this need because of its high round-trip conversion efficiency and potential low cost. Since the commonly used catalyst, platinum, is not stable in the corrosive HBr and Br₂ environment, an alternative catalyst that is more durable and equally active is needed. Rh_xS_y catalyst has been found to meet this requirement. ^1,2

An aqueous-based process to synthesize Rh_xS_y/C was developed by Allen et. al. , where XC72R is the carbon support. The catalyst synthesized by this process, however, has large particle sizes (12-40 nm). Other research groups ^4,5 have shown that smaller particle size can be achieved if surface with high affinity for precipitation of Rh cation is provided. Since rhodium cation is the source of rhodium, negatively charged carbon surface is preferred to achieve higher dispersion of catalyst nanoparticles. The measured mass-specific electrochemically active surface area (ECSA) of the catalysts in 1M nitrogen-saturated H₂SO₄ solution with HNO₃-pretreated carbon and untreated carbon substrates shown in Figure 1 supports this aspect. However, the subsequent H₂-Br₂ fuel cell test results given in Figure 2 show affinity issue with the catalyst with acid pretreated carbon substrate. It shows that while the negatively charged carbon surface helps to increase the catalyst dispersion, it results in poor Nafion ionomer coverage because of the electrostatic repulsion with the negatively charged Nafion ionomer ⁶. To improve the fuel cell performance of this high ECSA catalyst, a process to make the carbon support surface positively charge for high affinity to the Nafion ionomer is needed. This presentation will discuss the attempt to synthesize high ECSA Rh_xS_y catalyst on carbon substrate with high affinity for Nafion ionomer and the characteristics and performance of this catalyst.

Reference

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2. A. Ivanovskaya, N. Singh, R.-F. Liu, H. Kreutzer, J. Baltrusaitis, T. Van Nguyen, H. Metiu, and E. McFarland, Langmuir, 29, 480–492 (2013).
3. A.F. Gulla, R.J. Allen, US Patents, US20080202923 A1 (2008).
4. A. Eguizabal, L. Uson, V. Sebastian, J. L. Hueso, and M. P. Pina, RSC Advances, 5, 90691–90697 (2015).
5. M. S. Shafeeyan, W. M. A. W. Daud, A. Houshmand, and A. Shamiri, Journal of Analytical and Applied Pyrolysis, 89, 143–151 (2010).
6. F. Xu, H. Zhang, J. Ilavsky, L. Stanciu, D. Ho, M. J. Justice, H. I. Petrache, and J. Xie, Langmuir, 26, 19199–19208 (2010).

Acknowledgments

The work presented herein was funded by the National Science Foundation under Grant No. EFRI-1038234 and No. 1416874, as a sub-award from Proton Onsite.