Porphyrin-like Iron Single Atom Catalysts on N-doped Carbons for the Oxygen Reduction Reaction

Abstract

The oxygen reduction reaction (ORR) plays an important role in a number of modern energy conversion and storage devices, including polymer electrolyte membrane fuel cells (PEMFCs) and metal-air batteries. Platinum (Pt)-based catalysts are currently the benchmark catalysts for ORR, due to their high ORR activities and stability. However, the scarcity and high cost of platinum are obstacles to the uptake of fuel cell technology. In recent years, M-N-C electrocatalysts comprising metal single atom catalysts (MNx SACs) supported in N-doped carbons, have emerged as promising low-cost catalysts for ORR. However, the activity and stability of M-N-C electrocatalysts for ORR still need to be significantly improved before their large scale implementation in practical applications. This PhD project targeted the discovery of high performance Fe-N-C electrocatalysts for ORR. In Chapter 1, we introduced and summarized the synthetic approaches towards M-N-C materials, with emphasis placed on controlling the types and site density (SD) of metal SAC active sites on carbon-based supports for high-performance ORR. In Chapter 2, some important methodologies used in this PhD research project to characterize the structure and ORR activity of M-N-C materials are introduced. Chapter 3 examined the evolution of Zn single atom catalyst sites during the pyrolysis of Zeolitic imidazolate framework 8 (ZIF-8) at temperatures ranging from 500 to 900 ºC. The obtained products are denoted as Zn-N-C-T (T = 600, 700, 750, 800, 900 ºC). ZIF-8 is a metal organic framework (MOF) consisting of Zn2+ nodes tetrahedrally coordinated by four nitrogen atoms from 2-methylimidazole linkers, and a commonly used precursor in the synthesis of M-N-C electrocatalysts for ORR. ZIF-8 was observed to be thermally stable up to 500 ºC. With increasing pyrolysis temperature from 500 to 900 ºC, ZIF-8 decomposition into Zn-N-C catalysts occurred. Some of the Zn from the ZIF-8 precursor was lost by sublimation with sample heating at these temperatures. Zn L-edge and Zn K-edge XAS revealed that the Zn sites in all Zn-N-C-T (T = 600, 700, 750, 800, 900) samples were in the form of porphyrin-like ZnN4 sites. Interestingly, the Zn atoms were above the N4 plane in Zn-N-C-600, gradually moving closer to the N4 coordination plane as the pyrolysis temperature was increased up to 900 ºC. The Zn-N-C-800 sample showed excellence activity for the peroxidase-like decomposition of H2O2, highlighting the importance of the exact ZnN4 site geometry for efficient catalysis. On pyrolyzing microporous ZIF-8 (1498 m2 g-1) to obtain N-doped carbon supports for ORR, most of the micropores collapse yielding lower surface area products (e.g.the Zn-N-C-T catalysts above had surface areas of only 9-596 m2 g-1. For Fe-N-CORR electrocatalysts, achieving a high site density (SD) of FeNx sites is important formaximizing ORR activity. Achieving a high SD of FeNx sites, whilst preventing undesirable Fe nanoparticle formation, typically requires the use of a high surface area, microporous N-doped carbon support. This motivated the search for a new synthetic approach for fabricating N-doped carbons of very high surface area from ZIF-8. In Chapter 4, a simple molten NaCl-assisted method towards Fe/NC-NaCl electrocatalysts with high surface area and abundance of surface micropores is reported, which in turn enabled Fe-N-C catalysts with highly accessible FeN4 sites and high site densities to be synthesized. By pyrolyzing microporous ZIF-8 together with NaCl (melting point 802 oC) under an N2 atmosphere, then post-synthetically dissolving the NaCl in water, an N-doped carbon product (denoted herein as NC-NaCl) was obtained with a dodecahedron morphology and abundance of micropores and mesopores. After the adsorption of Fe3+ ions and a second pyrolysis step, the Fe/NC-NaCl electrocatalyst produced a remarkable BET surface area of 1911 m2 g-1, which ensured high dispersion and high density of accessible FeN4 sites (26.3×1019 sites g-1). The developed Fe/NC-NaCl electrocatalyst exhibited outstanding ORR performance with a high half-wave potential of 0.832 V (vs. RHE) in 0.1 M HClO4. When used as the ORR cathode catalyst in 1.0 bar H2-O2 fuel cell, Fe/NC-NaCl offered a high peak power density of 0.89 W cm-2, ranking it one of the most active M-N-C materials reported to date. In Chapter 5, we studied the direct pyrolysis of a Fe-containing ZIF-8 as a route towards Fe-N-C catalysts rich in FeNx sites. Experiments were performed in the presence or absence of molten NaCl during the pyrolysis step, yielding Fe-ZIF+NaCl and Fe-ZIF catalysts, respectively. Results show that molten NaCl strategy produced an N-doped carbon support richer in micropores and small mesopores (3.9 Å), which created tailored FeN4 sites with higher TOF and site density. Owing to the presence of the abundant mesopores in Fe-ZIF+NaCl, containing catalytically accessible FeN4 sites, the catalyst exhibited outstanding ORR performance with a high half-wave potential of 0.90 V (vs. RHE) in 0.1 M KOH. A zinc-air battery driven by the Fe-ZIF+NaCl electrocatalyst delivered a high peak power density of 225 W cm-2. The support porosity, FeN4 site density, tailored FeN4 coordination thus all influence the activity of Fe-N-C electrocatalysts for ORR. Results of this thesis guide the development of improved M-N-C materials for oxygen electrocatalysis.