Bimetallic Aerogels: Nanostructured Oxygen Reduction Reaction Electrocatalysts

The kinetics of the oxygen reduction reaction (ORR) is known to be sluggish as compared to fast kinetics of the hydrogen oxidation in both acidic an alkaline environment, leading to the fact that the ORR in polymer electrolyte fuel cell (PEFC) cathodes are typically considered to significantly contribute to losses to PEFC voltage efficiencies.^[1] Therefore, the catalyst loading, predominantly Pt-based systems for PEFCs, on technical cathodes need to be relatively high, e.g., around 0.4 mg_Ptcm^‑2 which in turn strongly contributes to the high relative cost of membrane electrode assemblies within the fuel cell stack. In addition, state-of-the-art cathode catalysts are supported on high surface area carbons in order to improve Pt dispersion and maximize the specific surface areas. The carbon supports, however, are known to be prone to corrosion under certain PEFC operation conditions, such as excursions to high potentials and longer times at cell open circuit. ^[1]

A widely known strategy to improve the Pt ORR kinetics is alloying Pt with transition metals, e.g., Ni, Co, Cu etc., typically leading to kinetic improvements between a factor of two to 10.^[2] In order to address the inherent thermodynamic instability of carbon supports under PEFC cathode operating conditions, there are several strategies followed: (i) replacing carbon by high surface area transition metal oxides (e.g., doped SnO2)^[3], or (ii) using unsupported purely metal containing catalysts as e.g. the 3M nanostructured thin film, NSTF, systems.^[4]

Our approach to both increase ORR activity and stability follows the route of preparing nanostructured, high surface area aerogels.^[5] We recently could demonstrate the feasibility of this approach with PtPd aerogels which showed a 7-fold improved activity and significantly increased stability during potential cycling as compared to Pt/C. ^[6]

In this study, we extended this approach and, for the first time, prepared bimetallic aerogels consisting of Pt and a non-noble transition metal, such as Ni and Co. The systems are purely metallic (no support) and provide a specific surface area similar than carbon supported Pt catalysts, however, in principle can be considered as an extended surface catalyst since basically all the chain-type bimetallic structures are connected in a fractal way. This is one of the properties which leads to significantly improved mass and surface specific activities as compared to supported catalyst systems.

Acknowledgement

Funding from the Swiss National Science Foundation for financial support (contract number 20001E_151122/1) and the Deutsche Forschungsgemeinschaft (contract number EY 16/18-1) is greatly acknowledged.

References

[1]          A. Rabis, P. Rodriguez, T. J. Schmidt, ACS Catal. 2012, 2, 864-890.

[2]          T. J. Schmidt, ECS Transactions 2012, 45, 3-14.

[3]          A. Rabis, E. Fabbri, A. Foelske, M. Horisberger, R. Kötz, T. J. Schmidt, ECS Transactions 2013, 50, 9-17.

[4]          M. K. Debe, Nature 2012, 486, 43-51.

[5]          W. Liu, A.-K. Herrmann, N. C. Bigall, P. Rodriguez, D. Wen, M. Oezaslan, T. J. Schmidt, N. Gaponik, A. Eychmüller, Accounts of Chemical Research 2015, 48, 154-162.

[6]          W. Liu, P. Rodriguez, L. Borchardt, A. Foelske, J. Yuan, A.-K. Herrmann, D. Geiger, Z. Zheng, S. Kaskel, N. Gaponik, R. Kötz, T. J. Schmidt, A. Eychmüller, Angew. Chem. Int. Ed. 2013, 52, 9849-9852.