Low Temperature PEFC Performance of Unsupported Pt-Ni Aerogel Cathode Catalyst Layers

Abstract

The commercialization of polymer electrolyte fuel cells (PEFCs) requires Pt-based, O2-reduction reaction (ORR) catalysts with improved stability and activity, in order to be able to reach cathode Pt-loadings << 0.4 mgPt·cm-2. This can be done by alloying platinum with other metals such as Ni (as to increase its ORR-activity), while complementary developing novel catalysts that can endure the oxidizing conditions concomitant to PEFC startup/shutdown, which cause the corrosion (and progressive failure) of the carbon-supported Pt-catalysts (Pt/C) used in state-of-the-art PEFCs. Aiming to tackle both requirements, in our previous work we showed that an optimized cathode catalyst layer (CL) based on an unsupported, bimetallic Pt-Ni nanochain network (aerogel) displayed a PEFC ORR-activity ≈2.5-fold larger than a commercial Pt/C at 80 °C and 100 % relative humidity (RH), while preserving 90 % of this initial performance after an accelerated stress test that mimicked PEFC startup/shutdown (vs. only 40 % for Pt/C) [1]. However, those studies did not include PEFC measurements at lower temperatures and relative humidity (RH) conditions which are relevant for PEFC powered vehicles, and that have been shown to be problematic for other unsupported Pt-based catalysts [2].To tackle this needs, we first had to re-assess the procedures used for the preparation of aerogel CLs. More precisely, in our previous work [1] aerogel-based catalyst-coated membranes (CCMs) were prepared by hand spraying, which is highly dependent on the handler and can therefore lead to irreproducibility. To overcome this issue, we have used an automated spray-coating machine that on the other hand requires a larger amount of material for operation, thus implying the need for an up-scale of the aerogel synthesis up to 125 mg batches (≈ 5-fold larger than those prepared for our previous work). Following a careful optimization of this spraying procedure, we managed to prepare an up-scaled, automatically sprayed aerogel CCM that displayed a PEFC performance similar to our previous best results in Ref. 1. This Pt3Ni CL was subsequently submitted to additional PEFC tests at temperatures ≤ 60 °C and under RH conditions between 100 and 20 %, whereby the latter can imply issues in the evacuation of product liquid water caused by the limited water storage capacity (i.e., void volume) of the pores in such ultra-thin CLs (≈ 1.5 μm). Specifically, our PEFC tests included electrochemical impedance spectroscopy (EIS) measurements from which we estimated the effect of temperature and RH on the proton transport resistance through the catalyst layer, thus allowing us to quantify the mass transport losses along this broad range of temperature and RH [3]. Based on these results, specific temperature and RH conditions were selected to conduct additional current up-transient failure tests [2, 4].