(Invited) Pt-Based Aerogels As Enhanced-Durability Electrocatalysts for Polymer Electrolyte Fuel Cells

Abstract

Hydrogen-fed polymer electrolyte fuel cells (PEFCs) are excellently suited for the propulsion of environmentally benign electric vehicles, but their market penetration has been restricted by their excessive cost and insufficient service life.1 Both of these issues are closely related to the catalysts needed to speed up the kinetics of the H2-oxidation and O2-reduction reactions (HOR, ORR) taking place at the cell’s anode and cathode, respectively, which customarily consist of Pt-nanoparticles (often alloyed with a transition metal, M) supported on carbon black (i.e., Pt[M]/C). Specifically, PEFC startup/shutdown2 and gross fuel starvation3 can lead to potential excursions > 1 V vs. the reversible hydrogen electrode (RHE) that trigger the corrosion of the catalyst’s carbon support, causing a deterioration of the catalyst layer’s mass transport properties that negatively impact its performance at high current densities. On this basis, the durability of such Pt(M)/C catalysts could be enhanced by transitioning towards unsupported materials that elude the corrodible carbon support.4 Among these novel catalysts, aerogels consisting of tridimensionally interconnected nanoparticle ensembles can be synthesized in bimetallic alloy compositions like Pt3Ni,5 which additionally meets the ORR-activity target set by the US Department of Energy for automotive PEFCs.1 However, upon transfer of these catalysts to a PEFC cathode, the resulting aerogel catalysts layers (CLs) displayed a poor high current performance that was related to their low content in pores > 50 nm in width, which play a crucial role in the CL’s mass transport.6 To circumvent this drawback, a pore-inducing precursor (K2CO3) was included in the initial aerogel ink formulation and removed through acid washing after processing of the aerogel into a catalyst coated membrane – an approach that effectively led to a shift of the CL’s pore size distribution towards larger values and translated in a PEFC performance akin to state-of-the-art PtM/C. Most importantly, the resulting, optimized CLs displayed excellent durability when submitted to an accelerated stress test (AST) that mimics the rough potentials concomitant to PEFC startup and shutdown (cf. Figure 1).6,7 This outstanding corrosion resistance was subsequently shown to persis upon implementation of the same aerogel CLs in PEFC anodes, whereby no performance loss was observed following a gross H2-starvation AST.8 The latter is a particularly relevant result since, unlike in the case of startup/shutdown events (for which system engineering solutions can be envisaged), mitigating H2-starvation exclusively relies on the development of corrosion-resistant materials like the aerogels presented in this contribution.