Date of Award

12-2009

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemistry

Advisor

Creager, Stephen E

Committee Member

Creager , Stephen E

Committee Member

DesMarteau , Darryl D

Committee Member

Smith , Dennis W

Committee Member

Chumanov , George

Abstract

Mesoporous carbon materials of carbon xerogel (CX) and silica-templated carbon (MC) were synthesized and explored as catalyst supports alternative to the most-commonly-used carbon black (CB) support, for polymer-electrolyte-membrane fuel cell (PEMFC) application. Pt catalyst was loaded on these carbons and electrodes were fabricated from them. These Pt-loaded carbon supports were characterized with XRD, TEM, ex-situ and in-situ cyclic voltammetry, etc. The fabricated electrodes were evaluated in single-cell testing in comparison with commercial CB-supported Pt catalyst fabricated electrodes. The experimental results showed that CX-supported Pt catalyst had close or better performance than that of CB-supported Pt, possibly due to CX's 3-D porous structure, but MC had inferior performance to that of CB. MC's high specific surface area, large pore size, high pore volume structure advantages did not transfer to a higher cell performance as expected. The reasons for MC support's poor performance were discussed.
Monofunctional fluorosulfonimide electrolyte (-C6H4SO2N(H)SO2CF3) was electrochemically grafted via its parent diazonium zwitterion onto planar glassy carbon electrode and the properties of the grafted layer on the electrode were investigated with electrochemical probes, XPS and chemical methods. The same monofunctional fluorosulfonimide electrolyte was also chemically grafted onto mesoporous CX and CB supports, a polymer electrolyte of sulfonated poly(arylene ether sulfone) was grafted onto CB support via the step-growth polymerization method. These monofunctional or polymeric electrolyte grafted mesoporous carbons were applied in PEMFC electrodes in the hope to increase three-phase zone and stability of the electrodes via covalently bonding of electrolyte onto electrodes. Single-cell testing results of MEAs made from these Pt-loaded, the sulfonimide-grafted CX or the polysulfone-electrolyte-grafted CB supports showed unexpectedly lower performance than that of un-graft commercial Pt-loaded CB support. The reasons for the poor performance were explored.
In addition, sulfonimide polymers prepared by blending two different equivalent weight (EW) plain sulfonimide polymers or crosslinking a low EW polymer were evaluated as membrane materials for PEMFCs in comparison with Nafion membranes. The results showed even blending the same sulfonimide polymer with different EWs might improve membrane performance, and cross-linking of low sulfonimide also improved the membrane.
From current work, it is worth to mention, PEMFC is a complicated and delicate system, whose performance is a combination of many different, even conflicting parameters of the Pt catalyst, the catalyst support, the electrolyte in the electrode, the membrane, the gas diffusion layer, and others. For fair comparison of cell performance in PEMFCs, well-designed, well-controlled experiments and methods are needed.

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