Challenges of Pt-Ni Aerogel PEMFC-Cathode Catalyst Layers Under Automotive-Relevant Conditions

Abstract

Proton exchange membrane fuel cells (PEMFCs) constitute an emerging technology for the decarbonization of the transportation sector. However, in order for the price of fuel cell electric vehicles (FCEVs) to be comparable to that of internal combustion engine cars, the manufacturing costs of PEMFCs, must be significantly reduced. These costs are mainly related to the Pt-based catalysts used in PEMFCs’ anode and cathode electrodes and, as a result, extended FCEV-commercialization passes by decreasing these Pt loading to a value of 0.06-0.11 mgPt cm-2. Moreover, the automotive PEMFCs suffer from an insufficient durability that is partially due to the corrosion of the carbon support used in state-of-the-art Pt-catalysts, which is in turn induced by the high cathode potentials (> 1.5 V vs. the reversible hydrogen electrode) reached upon PEMFC start-up and/or shutdown.To tackle both these needs, in our previous work we introduced a novel catalyst layer (CL) based on a carbon-free bimetallic Pt-Ni alloy nanochain network (referred to as “aerogel”) which featured an excellent performance and start-up/shutdown durability upon PEMFC operation [1]. In a more recent work, the synthesis of this Pt-Ni aerogel was up-scaled (as to yield ≈ 120 mg of material per batch) and catalyst-coated membrane (CCM) processing methods were adapted and optimized using an automated spray-coating machine that yielded homogenous and high-performing aerogel CLs with a Pt-loading of ≈ 0.3 mgPt cm-2.The decrease of the above loading to the needed values of ≤ 0.1 mgPt cm-2 discussed above is expected to pose a challenge in terms of high current density operation due to limitation in the transport of reactants (oxygen and protons) to such a reduced catalyst area[2]. Part of this performance loss includes the transport of O2 through the ionomer film that generally covers the Pt nanoparticles in the cathode CL. This O2 transport resistance can be quantified via O2 limiting current measurements through which the non-Fickian, Knudsen diffusion through the CL can be quantified[3]. Thus, in this work we present our results in the manufacture and PEMFC-testing of low loading Pt-Ni aerogel CLs, which we tackled with and without the use of an ionomer and benchmarked against equivalently -loaded CLs prepared with a commercial Pt/C catalyst. Our results unveil a relation between ionomer presence/absence, non-Fickian oxygen transport resistance and roughness factor that is comparable to that reported by Kongkanand and Mathias for unsupported, Pt-based nanostructured thin films[4].In addition to this, aerogel electrodes feature a reduced thickness (≈ 5 μm/(mgPt∙cm− 2) that is likely to result in water flooding and ice accumulation at low temperatures and during cold-starts (< 0˚C), respectively. Under the latter, extreme conditions, the water produced by the reduction of oxygen can form ice, which is expected to accumulate within the cathode pore volume and lead to a high oxygen transport resistance[5]. Moreover, the lack of hydrophobicity and of storage capacity (i.e. void volume) within the catalyst layer is susceptible to cause water flooding and/or ice formation, ultimately leading to cell failure[6]. Thus, this contribution will also showcase our results under such low temperature and cold start conditions (in the latter case, at − 10 °C), and do so while considering the combinations of low vs. high aerogel loadings and presence vs. absence of ionomer discussed above. In doing so, this work will provide a holistic picture of the mass-transport effects determining the performance of these aerogel CLs under automotive-relevant conditions.