Bimetallic Pt-Ni Aerogels for Electrocatalysis of the Oxygen Reduction Reaction
Authors: Henning Sebastian , Herranz Juan , Kühn Laura , Herrmann Anne-Kristin , Liu Wei , Eychmüller Alexander , Schmidt Thomas J. 
Published: February 2020

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] One approach to reduce this excessive Pt-loading relies on 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 specific activity than commercial Pt/C catalysts.[2] On the other hand, these carbon-supported materials suffer from significant carbon- and Pt-corrosion during the normal operation of PEFCs, gradually compromising their reliability and profitability.[3] To partially overcome these stability issues, unsupported mono- and bimetallic electrocatalysts with very high surface areas (~ 100 m2
metal/gmetal) can be synthesized using a nanoparticle gel formation and destabilization process.[4,5] In the case of PtPd-alloy compositions, this new class of materials, often referred to as aerogels, showed an up to 7-fold improvement in ORR mass activity vs. Pt/C, along with greater stability upon potential cycling.[6] However, we are not aware of any studies having investigated the ORR activity and stability of bimetallic aerogels consisting of the combination of a noble and a non-noble metal, even if such materials could lead to a further reduction in noble metal content and costs, and thus are of great interest.
With this motivation in mind, we have studied both the ORR activity and potential stability of PtxNi aerogels (with x=1,3) consisting of an interconnected nanoparticle network, as exemplified by the transmission electron micrograph of the Pt3Ni aerogel displayed in Fig. 1. Derivatives of this material were subsequently obtained by potential-induced and chemically-driven leaching of the non-noble metal, along with mild heat-treatment to induce surface reconstruction.[2,7] The resulting materials were electrochemically characterized using the thin-film RDE technique [8] to quantify the catalyst’s surface area and specific activity in 0.1 M HClO4electrolyte using a three electrode setup and a custom-made glass cell. Additionally, extended potential cycling was performed to probe the stability of the as-synthesized, acid-leached and heat-treated aerogels. Ultimately, the obtained results were compared to those gathered on monometallic Pt aerogel and commercial Pt/C catalysts.
In summary, this contribution will report on the ORR activity and stability of new bimetallic noble metal/non-noble metal aerogels based on platinum and nickel. These results will provide valuable insights for the synthesis of further aerogel compounds with various compositions.

Figure 1. Transmission electron micrographs of a Pt3Ni aerogel catalyst, showing the individually connected nanoparticles with an averaged diameter of ~ 5.0 nm at low and high magnification (inset).

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.

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Figure 1