Date of Award


Document Type

Open Access Dissertation


Chemistry and Biochemistry



First Advisor

Brian C Benicewicz


The central theme of my research lies in the investigation of novel polybenzimidazole (PBI)-based materials for different energy related applications ranging from proton exchange membrane fuel cells (PEMFC) to high temperature gas separation. With the aid of a deeper understanding of the structure-property relationships in this class of materials, a better control on PBI chemistry - from monomer structure to polymer morphology to membrane/film processing method was able to be performed in order to achieve greater performance in targeted applications.

In Chapter 1, the overall background of two energy related applications - fuel cells and gas separation was first introduced as well as their recent developments based on polymeric materials. Next, the history of PBI materials and the role they are playing in these two main areas were briefly discussed. Major research objectives of my doctoral study were described in the end.

The first section of the dissertation, on the synthesis and characterization of novel PBI materials for fuel cell uses was provided in Chapter 2 and Chapter 3. In Chapter 2, the synthesis and characterization of phenylindane-containing PBI for high-temperature polymer electrolyte membrane fuel cells was described. The introduction of a bulky, rigid, and bent phenylindane moiety into the PBI background help the PBI achieve greater solubility in organic solvents, which has been a challenging topic in the PBI industry, and also better proton conductivity and fuel cell performance. Chapter 3 described the synthesis and characterization of a new fluorine-containing PBI for high-temperature polymer electrolyte membrane fuel cells. In this chapter, a new synthetic route of a fluorine-containing monomer (2,2'-bis(trifluoromethyl)-4,4'-biphenyldicarboxylic acid) was introduced. The PBI based on this new fluorine-containing monomer exhibited better organo-solubility and also better oxidative stability. These two new PBIs broadened our knowledge in PBI chemistry and provided new potential candidates for fuel cell related applications.

The second part of the dissertation is the understanding the structure-property relationships in PBI films for high temperature gas separation (Chapter 4 and Chapter 5). In Chapter 4, the influence of PBI main chain structures on H2/CO2 separation at elevated temperatures was studied and discussed. Four PBI derivatives with different main chain structures were designed to exhibit highly localized mobility at high temperatures, contain rigid and bent configurations that frustrated close chain packing, or possess bulky side groups. These PBIs were found to exhibit much improved H2 permeability (up to 997.2 barrer) compared with base m-PBI (76.81 barrer) at 250 °C and 50 psia. Chapter 5 introduced random PBI based copolymers containing hexafluoroisopropylidene functional groups for gas separations at elevated temperatures. It was found that by using a random copolymerization method, a relative control can be realized on the free volume cavity size and concentration within the polymers and also on materials corresponding H2/CO2 separation performance (gas permeability & selectivity).


© 2014, Xin Li

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