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

During the last decade, anion exchange membrane fuel cells (AEMFCs) have gained popularity due to their
 promise to provide low cost, high efficiency, high power density, and zero-emissions. Recent years have seen
 a significant increase in the achievable peak power density and lifetime of AEMFCs, though that performance
 has 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 much
 lower cost3,4. Therefore, the U.S. Department of Energy (DOE) recently set some challenging activity targets
 for AEMFCs5; including a near-term target platinum group metal (PGM) loading of 0.2 mg/cm2 by 2023, 0.125
 mg/cm2 by 2024 and zero PGM by 2030.
 Recently, various efforts have sought to reduce the PGM loading in operating AEMFCs. Some efforts have
 focused 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 by
 maximizing metal utilization7. One effective strategy is to improve the mass activity of Pt by increasing the
 number of active sites through catalyst size or structure control or improving the intrinsic activity of Pt through
 the manipulation of its electronic structure. One approach that can be used to reduce the PGM loading is to
 create 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 Switch
 Solvent Synthesis (SWISS) method. This method synthesizes a high density of multi-atom catalysts. It is able
 to do so by limiting the amount of water that is hydrating the synthesis precursors, which allows for
 agglomeration to be limited. The catalysts were physically characterized using a wide array of techniques
 including x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy
 (TEM), and high-resolution Cs aberration-corrected scanning transmission electron microscopy (STEM). The
 catalysts 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 12
 while achieving comparable performance to commercial catalysts. Also, to assemble a cell with ultralow PGM
 loading, our previously developed Fe–N–C cathodes6 were paired with a low-loading PtRu/NC anodes (0.05
 mg 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).