State-of-the-art polymer electrolyte fuel cells (PEFCs) require considerate amounts of carbon-supported platinum nanoparticle (Pt/C) catalysts (up to 0.4 mg_Pt/cm²_MEA) to catalyse the cathodic oxygen reduction reaction (ORR) [1]. To reduce overall PEFC costs, these excessive Pt-loadings must be reduced by enhancing the catalysts’ ORR activity and stability; e.g. by alloying platinum with other metals like Ni, Cu, Co [2] and by replacing or completely removing the carbon support that suffers from significant corrosion during the normal operation of PEFCs [3].

To overcome these activity and stability challenges, unsupported bimetallic Pt-alloy aerogels consisting of a 3D nanochain network (~ 30 m²_ECSA/g_Pt) were synthesized (see Figure 1 for a TEM image of Pt₃Ni aerogel) [4, 5]. Both Pt-Ni and Pt-Cu aerogels meet the US Department of Energy ORR activity target for 2017 of 440 A/g_Pt at 0.9 V_RHE[6] when tested in liquid half cells. Considering that this outstanding performance needs to be demonstrated in an actual PEFC, Pt‑Ni and Pt-Cu aerogels were processed into membrane electrode assemblies (MEAs) and characterized in a differential PEFC [7].

In this contribution, we present the MEA optimization process for unsupported Pt‑alloy aerogel ORR catalysts and compare cell performance and durability (start-stop cycles, load cycles) to a conventional Pt/C benchmark. To explain the outstanding stability of Pt-alloy aerogel catalysts for start-stop cycling (see Figure 1), catalyst layers were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX) at beginning-of-life and end-of-life.

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[4] S. Henning, L. Kühn, J. Herranz, et al., J. Electrochem. Soc. 163 (2016), F998-F1003.

[5] S. Henning, L. Kühn, J. Herranz, et al., Electrochim. Acta 233 (2017), 210-217.

[6] O. Gröger, H.A. Gasteiger, J.P. Suchsland, J. Electrochem. Soc. 162 (2015), A2605-A2622.

[7] P. Oberholzer, P. Boillat, R. Siegrist, et al., Electrochem. Commun. 20 (2012), 67-70.