Unsupported Pt-Ni Aerogels As O2-Reduction Catalysts for PEFCs

Abstract

State-of-the-art polymer electrolyte fuel cells (PEFCs) require large amounts of carbon-supported platinum nanoparticle (Pt/C) catalysts (~ 0.4 mgPt/cm2 geometric) to account for the large overpotential of the oxygen reduction reaction (ORR).1 These excessive Pt-loadings that impede widespread commercialization of PEFCs can be mitigated by increasing the catalysts’ ORR activity, e.g. by alloying platinum with other metals like Ni, Cu and Co, to form materials which show up to one order of magnitude higher mass-specific activity than commercial Pt/C catalysts.2 On the other hand, state-of-the art carbon-supported materials suffer from significant carbon and Pt corrosion during the normal operation of PEFCs, gradually compromising their performance. To partially overcome these stability issues, a lot of research effort is dedicated to the development of unsupported ORR catalysts. Among these materials, bimetallic alloy aerogels consisting of nanoparticles interconnected to nanochains3 present an interesting option, since their extended 3D structure should facilitate transfer to actual PEFC cathodes. With this motivation in mind, we have modified a previously published synthesis3 in aqueous environment to prepare ‘clean’ bimetallic Pt-Ni aerogels with different stoichiometries, among which Pt3Ni and Pt1.5Ni were characterized in depth and tested for ORR activity. The alloy formation and extended 3D nanochain structure were confirmed by X-ray diffraction and transmission electron miscroscopy, respectively. Moreover, X-ray photoelectron and X-ray absorption spectroscopy indicated the existence of a separate Ni-(hydr)oxide phase in the Pt1.5Ni aerogel that was not present in the Pt3Ni sample. This hypothesis was further verified by complementing electrochemical measurements in alkaline electrolyte. Despite this difference in composition, both Pt-Ni aerogels showed comparable ORR activities in rotating disk electrode measurements in 0.1 M HClO4 electrolyte, reaching the activity target of 440 A/gPt at 0.9 VRHE set by the U.S. Department of Energy4 for automotive PEFCs. However, accelerated stress tests5 using the above mentioned RDE setup did not reveal significant durability improvements with respect to a commercial Pt/C benchmark catalyst. Consequently, these results were compared to those obtained in a differential PEFC setup to better understand the degradation behavior in an environment closer to real application. In summary, this contribution reports the detailed investigation of surface and bulk properties of bimetallic Pt-Ni aerogels alongside with performance and stability assessments in aqueous electrolyte and a differential PEFC setup. Acknowledgement Funding from the Swiss National Science Foundation (20001E_151122/1), the German Research Foundation (EY 16/18-1) and the European Research Council (ERC AdG 2013 AEROCAT) is greatly acknowledged.