Enhancing Proton Exchange Membrane Fuel Cell Durability By Using Porous Carbon-Supported Cathode Catalysts with Various Pt-Particle Sizes

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Nowadays, proton exchange membrane fuel cells (PEMFCs) are envisioned for heavy-duty vehicles (HDVs) such as trucks and buses, due to their high power density, low emissions, and quiet operation.1 However, to be viable for HDVs, PEMFCs require cathode catalyst materials that are highly stable and durable in order to maintain performance over at least 30,000 hours of operation. One well-known challenge is that voltage cycling of the cathode leads to significant losses in the electrochemically active surface area (ECSA) due to Pt-dissolution and subsequent growth via Ostwald ripening or particle coalescence, as well as Pt loss into the ionomer/membrane phase.2 While larger Pt particles are less prone to dissolution due to their lower surface free energy,3 depositing Pt particles within the internal pores of primary porous carbon particles has been shown to effectively mitigate particle coalescence.4 Therefore, combining larger particles with locating them internally of the carbon pores shows promise for significantly reducing Pt surface area loss.In this study, voltage cycling-based accelerated stress tests (ASTs) were performed using 5 cm2 membrane electrode assemblies (MEAs) with 0.1 mgPt cmMEA -2 loaded Pt-based cathode catalysts supported on porous Ketjenblack carbon (TEC10E20E, TKK, referred to as “pristine” Pt/KB). To increase the Pt-particle size, the catalyst was subjected to a heat-treatment under an reductive atmosphere (5% H2/Ar mixture) at 900 °C for 1 h (referred to as “HT-900°C/1h”), while a subsequent treatment under oxidative atmosphere (under air/Ar mixture) at 250 °C for 12 h (referred to as “ox-HT-900°C/1h”) was also included to enhance catalyst accessibility by inducing a Pt-catalyzed local carbon support oxidation of the bottleneck pore entrances, as described by Lazaridis et al.5 A full MEA characterization was carried out, encompassing determination of the ECSA by CO-stripping, of the H2/air and H2/O2 performance, and of the O2 and H+ transport resistances in the cathode by limiting current measurements and electrochemical impedance spectroscopy, respectively.We found that the HT-900°C/1h cathode exhibited significantly improved ECSA retention compared to the pristine Pt/KB, primarily due to its larger Pt particle size (~5 vs. ~2.5 nm), which reduces Pt dissolution. However, it was also observed that these catalysts had a much lower beginning-of-life (BoL) H2/air performance, as evidenced by comparing the rightmost data points in Fig. 1 for the pristine (gray squares) and the HT-900°C/1h (blue triangles) catalysts. These performance losses are attributed to the reduced roughness factor (rf), i.e., the lower available Pt surface area per electrode area, which was demonstrated to affect various voltage loss terms.6 Especially at higher current densities, an increased impact of the local oxygen transport resistance is observed, due to its inverse proportionality to the cathode rf. To address these challenges, it was demonstrated that enhancing catalyst accessibility substantially improves H2/air performance at higher current densities, so that the ox-HT-900°C/1h catalyst surpasses the initial performance of the pristine Pt/KB (compare rightmost green circles). Thus, combining heat treatment of a Pt/KB catalyst under both reductive and oxidative atmosphere enhances cathode catalyst durability and concomitantly high current density H2/air performance.