Highly Efficient Multi-Atom Pt and PtRu Catalysts for Anion Exchange Membrane Fuel Cells

Abstract

During the last decade, anion exchange membrane fuel cells (AEMFCs) have gained popularity due to theirpromise to provide low cost, high efficiency, high power density, and zero-emissions. Recent years have seena significant increase in the achievable peak power density and lifetime of AEMFCs, though that performancehas come with high loadings of Pt and PtRu catalysts at the cathode and anode, respectively1,2. However, to displace incumbent proton exchange membrane fuel cells (PEMFCs), AEMFCs must be able to offer muchlower cost3,4. Therefore, the U.S. Department of Energy (DOE) recently set some challenging activity targetsfor AEMFCs5; including a near-term target platinum group metal (PGM) loading of 0.2 mg/cm2 by 2023, 0.125mg/cm2 by 2024 and zero PGM by 2030.Recently, various efforts have sought to reduce the PGM loading in operating AEMFCs. Some efforts havefocused on developing completely PGM-free catalysts, such as Fe-N-C at the cathode6. However, the nearterm DOE targets can be met by reducing the loading of existing catalysts, which can be accomplished bymaximizing metal utilization7. One effective strategy is to improve the mass activity of Pt by increasing thenumber of active sites through catalyst size or structure control or improving the intrinsic activity of Pt throughthe manipulation of its electronic structure. One approach that can be used to reduce the PGM loading is tocreate atomically-dispersed catalysts. This can include single atoms or small multi-atom clusters 8,9.Therefore, in this work, Pt/C, Pt/NC, PtRu/C and PtRu/NC were fabricated using a simple, and scalable SwitchSolvent Synthesis (SWISS) method. This method synthesizes a high density of multi-atom catalysts. It is ableto do so by limiting the amount of water that is hydrating the synthesis precursors, which allows foragglomeration to be limited. The catalysts were physically characterized using a wide array of techniquesincluding x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy(TEM), and high-resolution Cs aberration-corrected scanning transmission electron microscopy (STEM). Thecatalysts were also tested for their oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR)activity in a rotating disk electrode setup. Their in-situ behavior was also investigated operating AEMFCs.With this new generation of low-PGM materials, it was possible to reduce the PGM loading by factor of 12while achieving comparable performance to commercial catalysts. Also, to assemble a cell with ultralow PGMloading, our previously developed Fe–N–C cathodes6 were paired with a low-loading PtRu/NC anodes (0.05mg PtRu per cm2, 0.08 mg Pt per cm2), which allowed for the demonstration of a specific power of 25 W per mg PGM (40 W per mg Pt).