Investigating the Durability of Unsupported Pt-Alloy Aerogel Cathode

Abstract

State-of-the-art polymer electrolyte fuel cells (PEFCs) require considerate amounts of carbon-supported platinum nanoparticle (Pt/C) catalysts (up to 0.4 mgPt/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/gPt) 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/gPt at 0.9 VRHE[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. [1] F.T. Wagner, B. Lakshmanan, M.F. Mathias, J. Phys. Chem. Lett. 1 (2010), 2204-2219. [2] H.H. Wang, Z.Y. Zhou, Q. Yuan, et al., Chem. Commun. 47 (2011), 3407-3409. [3] C. Hartnig, T. J. Schmidt, J. Pow. Sourc. 196 (2011), 5564-5572. [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.