Effect of Low and Sub-Freezing Temperature on the PEFC Performance of Unsupported Pt-Ni Aerogel Cathode Catalyst Layers

Abstract

Polymer electrolyte fuel cells (PEFCs) have received considerable attention on account of their ability to use green hydrogen as an environmentally benign fuel. For fuel cells to replace entirely combustion engines in automotive applications, though, their durability must be enhanced and the manufacturing costs must be reduced. The latter can be accomplished by alloying platinum with other metals such as Ni, as to increase its oxygen reduction reaction (ORR)-activity and decrease the required loading of this expensive metal. Complementary, novel catalysts are needed that can sustain the corrosion of the carbon support used in the state-of-art platinum nanoparticle (Pt/C) ORR- catalysts, which is induced by the high potentials ( > 1.5 V) reached upon PEFC start-up and/or shut downs.To meet the demands mentioned above, in our previous work we presented a novel cathode catalyst layer (CL) based on an un-supported, bimetallic Pt-Ni nanochain network referred to as aerogel. This material 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]. Such CLs had been prepared by hand-spraying, which is highly sensitive to the handler and prone to irreproducibility. Thus, in the first part of this contribution we will show how a careful optimization of an automated spraying procedure allowed us to produce aerogel catalyst coated membranes (CCMs) that display a PEFC performance (at 80˚C, 100 % RH and 1.5 barabs) similar to our previous best results in Ref. 1.On top of this, in automotive applications PEFCs are exposed to a wide range of temperatures (including < 0°C) [2], dry conditions, and fast transients to high current densities. Such operative modes can imply problems 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, the water in the fuel cell could freeze when the temperature drops below 0˚C resulting in ice formation, and thus in a blockage of the pathways through the pores in the catalyst layer, thereby preventing the supply of reactants and leading to a cell failure. Thus, the second part of this contribution will feature our results with Pt3Ni CLs exposed to PEFC tests at ≤ 60 °C including sub-freezing temperature and start-up. Specifically, our PEFC tests include electrochemical impedance spectroscopy (EIS) measurements from which we estimate the effect of the temperature on the proton transport resistance through the aerogel CL, thus allowing us to quantify the mass transport losses along this broad range of conditions [3]. Based on these results, specific temperature and RH conditions were selected to conduct additional current up-transient failure tests [4, 5].In summary, this contribution will portray our efforts at demonstrating the applicability of aerogel CLs in PEFC cathodes operated under application-relevant conditions.