Understanding the Activity and Stability of Single-Atom Catalysts for the ORR in Anion Exchange Membrane Fuel Cell(AEMFCs)

Abstract

To be considered the ultimate power source for transportation systems and portable devices, anion exchange membrane fuel cells (AEMFCs) need to overcome significant cost barriers1,2. For this, the U.S. Department of Energy (DOE) recently has set some challenging activity targets for AEMFCs; this includes the near-term target PGM loading is 0.2 mg/cm2 for 2021–2023, and by 2024 the total loading should be limited to ⩽ 0.125 mg/cm2.This final milestone is the first step in enabling a shift to completely platinum group metal (PGM)-free membrane electrode assemblies (MEAs) by 20303. Unfortunately, today’s high performing4 and long-life5,6 MEAs all rely on a high loading of PGM in both the anode and the cathode, resulting in a total PGM loading of ~1 mgPGM/cm2.To reduce the PGM loading in AEMFCs, it is important to transition to very low PGM electrodes, and eventually to catalysts that are PGM-free but do not contain bulk transition metals that passivate or dissolve in the electrolyte. One approach that can be used in both cases is to create atomically-dispersed ORR catalysts. On the PGM side, single atom catalysts (or even dual atom clusters) have the potential to drastically lower the PGM loading and cost while maximizing utilization7. The rational design of electrocatalyst, either through modification of commercial catalysts or entirely new structures, is very important to reach low loading high activity electrocatalusts for fuel cells.8–10. On the non-PGM side, the most common catalyst moiety has been single atoms of Fe in N4-funcitonal groups in carbon, or Fe-N-C. Though there have been Fe-N-C catlaysts that have shown good activity and durability in ex-situ tests, these materials are yet to be transitioned successfully to MEAs that can compete with their PGM-containing counterparts.In this study, two approaches will be discussed. The first synthesizes a high densities of atomically dispersed single-atom Pt catalysts, which were prepared in-house by a simple, and scalable synthesis method of chelate fixation (CheFi). These materials have been physically characterized using a wide array of techniques including x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution Cs aberration-corrected scanning transmission electron microscopy (STEM). The catalysts were also tested for their ORR activity and their in-situ behavior in operating AEMFCs. In addition to their performance data, these catalysts yielded interesting mechanistic insight that will be discussed. The second approach was to investigate the effect of structure on the behavior of Fe-N-C catalysts. In this part of the work, it was demonstrated that single atoms are indeed the most active and important componenes. It is also shown that the physical properties of the carbon structure (porosity and graphitization) play a role in determining not only the ORR activity, but also the ability to translate that activity to highly performing AEMFCs. In fact, it will be shown that it is possible to create AEMFCs with a PGM-free cathode that are able to achieve a peak power density > 2.0 W/cm2 and a kinetic current at 0.9 V (iR-free) of 100 mA/cm2. These cells are also able to out-perform and out-live their PEMFC counterparts.