Multi-Atom PGM Based Catalyst for Highly Efficient Oxygen Reduction Reaction(ORR) and Hydrogen Oxidation Reaction (HOR) in Alkaline Environment

Abstract

Anion exchange membrane fuel cells (AEMFCs) have recently seen significant growth in interest as their achievable current density, peak power density, and longevity have been improved dramatically. Though these advances in performance have been important for demonstrating the feasibility of the technology, nearly all AEMFCs reported in the literature have required a relatively high loading of platinum group metal (PGM)-based catalysts at both the anode and cathode electrodes [1]. However, to take command of the low-temperature fuel cell market, AEMFCs cannot simply reach the same performance as incumbent proton exchange membrane fuel cells (PEMFCs), which have had decades of development and investment. AEMFCs must realize their most widely quoted advantage over PEMFCs and be produced at a much lower cost than PEMFCs. The most likely pathway to acceptably low cost will involve reducing the PGM loading in both electrodes. At the cathode, reasonable PGM-free catalysts exist, as will be shown in this work. At the anode; however, there are no practical contenders that exist to replace PGM-based catalysts. Hence, the most practical approach is to create transitional catalysts with ultra-low PGM content until future PGM-free catalysts can be realized. To reduce the platinum group metal (PGM) loading in anion exchange membrane fuel cell (AEMFC) electrodes, it is important to transition to catalysts with very low PGM content, and eventually to create catalysts that are completely PGM-free. One approach that can be used in both cases is to create atomically dispersed metals on a carbon support. In this work, four catalysts were prepared using a new, simple, scalable Controlled Surface Tension (CST) method: Pt/C, Pt/NC, PtRu/C, and PtRu/NC. CST is unique as it allows for a high density of very small multi-atom clusters, facilitated by altering the surface tension in the synthesis medium. The catalysts were physically characterized using a wide array of techniques, including high-resolution Cs aberration-corrected scanning transmission electron microscopy (STEM), extended X-ray absorption fine structure (EXAFS), and X-ray Absorption Near-Edge Structure (XANES). The catalysts were also tested for their oxygen reduction reaction and hydrogen oxidation reaction activity both ex-situ on a rotating ring-disk electrode and in-situ while integrated into the anode (PtRu) and cathode (Pt) of operating AEMFCs. With this new generation of low-PGM materials, it was possible to reduce the PGM loading by a factor of 14 while achieving comparable performance to commercial catalysts with a peak power density approaching 2 W/cm2. AEMFCs were also assembled with ultralow PGM loading (0.05 mgPGM/cm2), where PtRu/NC anodes were paired with Fe–N–C cathodes [2], which allowed for the demonstration of cells with a specific power of 25 W/mgPGM (40 W/mgPt).