School of Energy and Chemical Engineering (Chemical Engineering)ope

Woo, Jinwoo

School of Energy and Chemical Engineering (Chemical Engineering)Owing to the rapid depletion of fossil fuels and the subsequent increase in environmental pollution, the search for an alternative energy source has gained interest worldwide. Among the several alternative sources of energy, the utilization of renewable energy, such as hydrogen for fuel cells, has drawn significant attention. A polymer electrolyte fuel cell (PEFC) is considered as one of the most promising energy conversion devices because of its high efficiency and zero carbon emission, and thus, constitutes a key element of the hydrogen economy. Although PEFCs offer many advantages, there are still several challenges hindering their large-scale implementation and commercialization, including the oxygen reduction reaction (ORR) at the cathode. The ORR involves multiple proton-coupled electron transfers and the cleavage of a strong O=O double bond, rendering it energetically demanding. To circumvent the sluggish kinetics of the ORR, a large amount of a Pt-based catalyst is typically used at the cathode of PEFCs. However, the high cost of Pt combined with its scarcity has hindered the widespread deployment of PEFCs. In this regard, the development of active nonprecious metal-based catalysts (NPMCs) is of major significance, and iron- and nitrogen-doped carbon (Fe???N/C) catalysts have been of particular interest among them owing to their excellent ORR activity.

In this dissertation, we present rationally designed synthetic strategies for atomically dispersed Fe???N/C catalysts and clarify their active site structures. In particular, we discuss the effects of the active site structures and structural properties of the Fe???N/C catalysts on the ORR, an important reaction that critically affects fuel cell performance.

In Chapter 2, we show the relationship between the textural properties of mesoporous silica-templated Fe???N/C catalysts and their ORR activity. Iron-doped mesoporous porphyrinic carbons (Fe-MPCs) with varying textural properties were synthesized by nanocasting using mesoporous silica as the template and Fe porphyrin as the precursor. The resulting Fe-MPC catalysts possessed different pore sizes, pore connectivities, and particle sizes, and were used as model catalysts for investigating the impacts of textural properties on the ORR activity. In general, the Fe-MPC catalysts presented excellent ORR activity owing to their high surface area and high density of the active Fe???Nx sites. Correlations between the textual properties of the Fe-MPC catalysts and their ORR activity revealed that reducing the particle size, increasing the pore size, and three-dimensional connectivity could be beneficial for enhancing the ORR activity.

In Chapter 3, we present a silica-coating-mediated synthetic strategy for the preparation of Fe???N/C catalysts enriched with active Fe???Nx sites while mitigating the formation of lesser active Fe and Fe3C species. The silica-coating-mediated method was applicable to various types of Fe and N precursors, including iron porphyrin, iron acetate/1,10-phenanthroline, and iron chloride/polyaniline. This strategy was also effective for preparing Fe???N/C catalysts with various carbon supports and wide ranges of Fe content and pyrolysis temperature. We extended our strategy to synthesize S- or P-doped Fe???N/C catalysts with suppressed formation of the inactive FeS and Fe2P species. Consequently, the Fe???N/C catalysts prepared with silica coating exhibited improved ORR activity up to a factor of 11 compared to the uncoated counterparts. Significantly, in anion exchange membrane fuel cell (AEMFC) tests, an S-doped Fe???N/C-based cathode showed the highest performance among reported AMEFCs with NPMC-based cathodes. Furthermore, the S-doped Fe???N/C-based cathode demonstrated promising volumetric current density in an acidic proton-exchange membrane fuel cell. The results revealed that the silica-coating-mediated strategy is generally effective for preparing atomically dispersed catalytic entities.

In Chapter 4, we use X-ray absorption and 57Fe M??ssbauer spectroscopies to determine the evolution of the structure of Fe-based species during the silica-coating-mediated synthesis. By conducting X-ray absorption near-edge structure and 57Fe M??ssbauer spectroscopy analyses, the formation of Fe???Si species after silica coating was identified. Peak parameter analyses of the 57Fe M??ssbauer spectroscopy data suggested that the density of the active Fe???Nx species in the Fe???N/C catalyst prepared with silica coating was twice that in the Fe???N/C catalyst without silica coating. Accordingly, the Fe???N/C catalyst with silica coating exhibited a kinetic current density for the ORR twice that without the silica coating.

In Chapter 5, we present a solid-state synthesis route for Fe???N/C catalysts that allows a systematic control of the graphitic layer number. In the solid-state impregnation of an iron???phenanthroline complex within the pores of a mesoporous silica, an Fe???N-containing carbon layer was uniformly generated from the pore walls, allowing tuning the framework structure from tube- to rod-like structures. The tube-type Fe???N/C catalyst exhibited a larger surface area and CO adsorption than the rod-type catalyst. Unexpectedly, the ORR activity of the rod-type catalyst was much greater than that of the tube-type catalyst by ca. 31 times in terms of the turnover frequency. In the rod-type catalyst with 20 carbon layers, O2 is expected to be adsorbed and dissociated between two metal centers, whereas in the tube-type catalyst with 3???4 carbon layers, O2 is expected to be mostly adsorbed on the on-top site with one metal center. This difference in the O2 binding mode in the two catalysts appears to be responsible for their activity difference.ope

Fe???N/C Oxygen Reduction Reaction Catalysts with Controlled Structures for Understanding Catalyst Structure???Activity Relationship

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