2. I MPACT OF T EXTURAL P ROPERTIES OF M ESOPOROUS P ORPHYRINIC C ARBON
3.1. I NTRODUCTION
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P ROMOTING O XYGEN R EDUCTION R EACTION
A CTIVITY OF F E −N/C E LECTROCATALYSTS BY
S ILICA -C OATING -M EDIATED S YNTHESIS FOR A NION
E XCHANGE M EMBRANE F UEL C ELLS
This chapter includes the published contents:
Woo, J.; Yang, S. Y.; Sa, Y. J.; Choi, W.-Y.; Lee, M.-H.; Lee, H.-W.; Shin, T. J.; Kim, T.-Y.; Joo, S.
H. Chem. Mater. 2018, 30, 6684–6701. DOI: 10.1021/acs.chemmater.8b02117. Reproduced with permission. Copyright © 2018 American Chemical Society.
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linear correlation between the ORR activity and the number of Fe atoms coordinated to N atoms in the carbon matrix (denoted as Fe–Nx sites) using 57Fe Mӧssbauer spectroscopy.44 Mukerjee et al. showed the adsorption/desorption of oxygenated species on Fe–Nx sites using electrochemical in-situ X-ray absorption spectroscopy (XAS), indicating the direct participation of such sites in the ORR.46 Jaouen and coworkers combined experimental and simulated X-ray absorption near edge structure (XANES) data, and they identified the FeN4C12 moiety as a highly active structure for the ORR.47 Furthermore, atomic-resolution transmission electron microscopy study of Fe–N/C catalysts was used to directly observe molecularly dispersed Fe–Nx sites.48,49 These studies suggest that the active sites in Fe–N/C catalysts are composed of Fe–N moieties dispersed on a carbon support.
Design of Fe–N/C catalysts with abundant active sites has been actively pursued on the basis of understanding of the active sites in Fe–N/C catalysts. General preparation methods for Fe–N/C catalysts involve high-temperature (600–1100 °C) pyrolysis of a mixture of Fe, N, and C precursors, as the pyrolysis step has proven to be essential for endowing conductivity and structural integrity in Fe–N/C catalysts.54,55 However, a significant portion of Fe atoms aggregate into less active, large Fe- based particles during pyrolysis, thus decreasing the mass activity.56,57 Furthermore, the Fe-based particles are often encapsulated by graphitic carbon shells generated in situ via a Fe particle-catalyzed Fischer-Tropsch reaction, rendering them resistant against acid etching.58,59 Hence, the development of synthetic routes towards Fe–N/C catalysts with predominant Fe–Nx sites has remained a significant challenge. Earlier efforts in this direction relied on repetitive heat/acid treatments39,60 or corrosive activation under NH3 gas flow at high temperatures60,61 to remove Fe-based particles. Kramm et al.
recently demonstrated that heat-treatment in diluted H2 gas can effectively etch Fe-based particles from the as-prepared Fe–N/C catalysts, thereby boosting the ORR activity.45 The addition of heteroatoms such as sulfur in the precursor mixture was also suggested to increase the mass activity of Fe–N/C catalysts via formation of acid-leachable FeSx species instead of insoluble Fe3C.57 Another notable method is based on thermal conversion of metal-organic frameworks (MOFs); Li and coworkers prepared MOF with a controlled number of Fe sites, which, upon thermal pyrolysis, could yield Fe–N/C catalysts with abundant Fe–Nx sites.62
We recently developed the “silica-protective-layer-assisted” strategy that could preferentially generate catalytically active Fe–Nx sites.42 In this method, a silica layer was over-coated on an iron porphyrin adsorbed carbon support, and the silica layer was etched after high-temperature pyrolysis to yield a Fe–N/C catalyst. The silica layer was suggested to preserve Fe–N4 sites in the iron porphyrin precursor through axial direction bonding between Fe and O, thus preventing the aggregation of Fe particles during pyrolysis. The resulting Fe–N/C catalyst contained a higher density of Fe–Nx sites compared to the catalyst prepared without the silica coating step, and hence showed better ORR activity in both alkaline and acidic media. In that work, the silica coating strategy was demonstrated using iron porphyrin as the precursor for Fe and N species. Although metallomacrocyclic compounds
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containing Fe–N4 centers have been used as versatile precursors for preparing Fe–N/C catalysts due in part to their inherent Fe–N bonding, these compounds are commonly expensive, which may offset their advantages. Hence, the preparation of Fe–N/C catalysts from low-cost precursors is important.
Recently, anion exchange membrane fuel cells (AEMFCs) have garnered increasing attention in the fuel cell community.63–66 AEMFCs have a number of advantages over proton exchange membrane fuel cells (PEMFCs), including the promising applicability of NPMCs for use as electrodes, a wide range of available materials for cell and stack, and the use of less expensive membranes. However, the cell performance of AEMFCs has long been inferior to that of PEMFCs, primarily due to the considerably lower conductivity of OH– compared to H+. Recent emergence of highly conductive anion exchange membranes has enabled remarkable enhancement in the performance of AEMFCs.65 However, AEMFCs based on NMPCs still exhibit much lower current and power densities than those of AEMFCs employing Pt-based catalysts. Given the use of NMPCs can significantly lower the total cost of an AEMFC stack, the development of high-performance NMPC-based AEMFCs is highly desirable.
In the present work, we demonstrate the general applicability of the silica coating-mediated synthetic strategy using a variety of precursors (Fe porphyrin, Fe(II) acetate/1,10-phenanthroline, and Fe(III) chloride/polyaniline) in the formation of highly active Fe–N/C catalysts with abundant Fe–Nx
sites for the ORR. We also investigated the efficacy of the silica coating-mediated synthesis for three commercial carbon supports (carbon nanotubes, Ketjen black, and Vulcan carbon) with different textural properties. Furthermore, the impacts of Fe contents in the precursor mixture and pyrolysis temperatures on the ORR activity of Fe–N/C catalysts were explored. It was found that the silica coating was effective in producing Fe−N/C catalysts nearly free of less active Fe-based particles, as revealed by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and XAS examinations. The Fe−N/C catalysts generated with the silica coating step showed enhanced ORR activity compared to catalysts without the silica coating, regardless of precursor types, carbon support types, and pyrolysis temperatures. This result demonstrates the generality of this strategy in boosting the ORR activity. We found that this synthetic method can be further extended to S- and P-doped Fe−N/C catalysts. Significantly, the S-doped Fe−N/C exhibited excellent ORR activity with half-wave potential at 0.91 V (vs. reversible hydrogen electrode, RHE).
Furthermore, in AEMFC single cell tests, a membrane electrode assembly (MEA) employing S-doped Fe−N/C cathode exhibited very high performance, with its current density (977 mA cm−2 @ 0.6 V) being the highest value among those of NMPC-based AEMFC MEAs. In addition, the S-doped Fe−N/C-based MEA also exhibited promising single cell performance for a PEMFC.
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