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Functionalized mesoporous silica and carbon materials for fuel cell applications

Kim, Donghun
Degree Grantor:
University of California, Santa Barbara. Chemical Engineering
Degree Supervisor:
Bradley F. Chmelka
Place of Publication:
[Santa Barbara, Calif.]
University of California, Santa Barbara
Creation Date:
Issued Date:
Engineering, Chemical
Mesoporous material
Oxygen reduction
Fuel cell
Proton conducting membrane
Dissertations, Academic and Online resources
Ph.D.--University of California, Santa Barbara, 2014

Functionalized mesoporous silica and carbon materials have been developed as new approaches toward potentially overcoming the current limitations of polymer electrolyte fuel cell technologies. Functionalization of high-surface-area materials is a novel material synthesis method that can provide attractive macroscopic properties that are often difficult to achieve with single materials. Mesoporous silica membranes synthesized by using self-assembly of block copolymers were sequentially functionalized with aluminosilica and acid moieties to yield high proton conductivities, compared to perfluorosulfonic acid ionomers, the current industrial standard membrane, which provides insufficient proton conductivities under low humidity conditions, thus limiting the operating temperatures of polymer electrolyte fuel cells.

As pore connectivities of mesoporous silica membranes are highly dependent on mesostructural ordering, mesoporous silica membranes with transient structures between cubic and hexagonal structures were functionalized with aluminosilica and perfluorosulfonic acid moieties, which provide high hydrophilicities and proton concentrations, respectively. High acid loadings, high hydrophilicities, and the robust proton conducting channels of these membranes yield high proton conductivities, which are stable with humidity conditions and are therefore slightly higher (~3x10 -3 S/cm) than Nafion(TM) (~2x10-3 S/cm) under low humidity conditions (80°C and 20% relative humidity). Additionally, improving surface areas and mechanical properties of mesoporous silica membranes were sought to further increase proton conductivity.

Water-vapor treatments strengthen the silica frameworks, yielding micro- and macro-crack-free mesoporous silica membranes with increased surface areas (~1,000 m2/g), while incorporating organic materials blended with silica frameworks improves the mechanical properties, overcoming the brittleness of silica materials. Resultant membranes with high surface areas allow for a significantly increased amount of acid moieties (2.6 mmol/g) on the mesoporous silica membranes as well as improved proton conductivities (~5.8x10-2 S/cm at 80°C and 20% relative humidity).

Functionalized mesoporous carbon materials were prepared via a similar synthesis protocol to exhibit significantly different material properties, electrical conductivity and electrocatalytic activities, which show promise to develop an alternative to costly platinum-based cathode catalysts in polymer electrolyte fuel cells. Fe,N-doped mesoporous carbon catalysts were prepared from a widely available N-containing organic material, melamine, and iron nitrate salt pyrolyzed within the mesopores of silica templates. Resultant materials exhibit high surface areas of ~700 m2/g, surface N content of ~3 atom%, surface Fe content of ~0.2 atom%, and also high electrical conductivity of ~20 S/cm. Compared to activated-carbon-supported platinum catalysts, Fe,N-doped mesoporous carbon materials exhibit greater oxygen reduction activities under alkaline conditions, established by high half-wave potential of 0.87 V compared to Pt/C catalyst (0.85 V), and slightly lower activities under acidic conditions (half-wave potential of 0.72 V vs. 0.84 V for Pt/C catalyst). Favorable surface compositions have been established for Fe,N-doped mesoporous carbon catalysts by systematically varying surface N species to improve oxygen reduction activities under acidic conditions. Correlation between surface compositions and electrocatalytic activities reveals surface graphitic N and iron species are crucial to yield oxygen reduction activities. The catalysts also show high alcohol tolerance, which allows for improved peak power output from direct ethanol fuel cells, compared to activated-carbon-supported platinum catalysts.

Physical Description:
1 online resource (288 pages)
UCSB electronic theses and dissertations
Catalog System Number:
Inc.icon only.dark In Copyright
Copyright Holder:
Donghun Kim
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