Date of Award

Spring 2022

Document Type

Open Access Dissertation

Department

Chemical Engineering

First Advisor

William E. Mustain

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 dramatically improved. 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. 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. All reasonable pathways to acceptably low cost involve reducing the PGM loading in both electrodes. At the cathode, highly performing 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 ultralow PGM content until future PGM-free catalysts can be realized.

Therefore, my dissertation has focused on understating the parameters that improve the performance and durability of low-PGM and PGM-free catalysts in AEMFCs. This has been achieved through two different approaches. First, low-PGM catalysts were synthesized using a new, simple, Controlled Surface Tension (CST) method. This allowed for the derived materials to have a high density of well-dispersed multi atom Pt and PtRu clusters. These materials were characterized using a wide array of techniques, including xray diffraction (XRD), transmission electron microscopy (TEM), high-resolution Cs aberration-corrected scanning transmission electron microscopy (STEM). They were also tested for their hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) activity and their in-situ behavior in operating AEMFCs. With this new generation of lowPGM materials, it was possible to reduce the PGM loading by a factor of 14 while achieving comparable performance to commercial catalysts. The second approach was to investigate the effect of structure on the behavior of Fe-N-C catalysts as the ORR cathode. It was demonstrated that single atoms are indeed the most active and important components. It is also shown that the physical properties of the carbon structure (porosity and graphitization) play a role in determining the operando ORR performance in AEMFCs. It will be shown that it is possible to create AEMFCs that meet both new and long-standing DOE targets, achieve a peak power density > 2.0 W/cm2 and a kinetic current at 0.9 V (iRfree) of 100 mA/cm2 . Finally, cells were assembled with ultralow PGM loading, where CST-PtRu/NC anodes were paired with Fe–N–C cathodes, which allowed for the demonstration of cells with a specific power of 25 W per mg PGM (40 W per mg Pt).

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