NAMS 2002 Workshop - ICOM 2008

Plenary Lecture I
Monday July 14, 8:00 AM-9:00 AM, Hawai’i Ballroom
Fuel Cell Polymer Electrolyte (PEM) Derived from Disulfonated Random
and Block Poly(Arylene Ether) Copolymer System
Professor James E. McGrath, Virginia Tech, Blacksberg, VA, USA
Our research group has been engaged in the past few years in the synthesis of
biphenol based partially disulfonated poly(arylene ether sulfone) random
copolymers as potential PEMs.
This series of polymers has been named as BPSH-xx, where BP stands for
biphenol, S stands for sulfonated, H stands for acidified and xx represents the
degree of disulfonation. All of these sulfonated copolymers phase separate to
form nano scale hydrophilic and hydrophobic morphological domains. The
hydrophilic phase containing the sulfonic acid moieties causes the copolymer to
absorb water. Water confined in hydrophilic pores in concert with the sulfonic
acid groups serve the critical function of proton (ion) conduction and water
transport in these systems. Both Nafion and BPSH show high proton conductivity
at fully hydrated conditions. However proton transport is especially limited at low
hydration level for the BPSH random copolymer. It has been observed that the
diffusion coefficients of both water and protons change with the water content of
the pore. This change in proton and water transport mechanisms with hydration
level has been attributed to the solvation of the acid groups and the amount of
bound and bulk-like water within a pore. At low hydration levels most of the
water is tightly associated with sulfonic groups and has a low diffusion coefficient.
This results in an isolated domain morphology. Thus, although there may be
significant concentrations of protons, the transport is limited by the discontinuous
morphological structure.
Hence the challenge lies in how to modify the chemistry of the copolymers to
obtain significant protonic conductivity at low hydration levels. This has been
possible by altering the chemical structure to afford nanophase separated ion
containing block or segmented copolymers. Unlike the BPSH statistical or
random copolymers, where the sulfonic acid groups are randomly distributed
along the chain, the multi block copolymers feature an ordered sequence of
hydrophilic and hydrophobic segments. Connectivity is established between the
hydrophilic domains in these multi-block copolymers, they will not need as much
water, and hence will show much better protonic conductivity than the random
copolymers (with similar degree of sulfonation, or IEC) at partially hydrated
conditions. This is particularly valuable for H2/air systems and the self assembling
nanophase also has potential for direct methanol fuel cells (DMFC) for portable
power. The systhesis and characterization of these materials and their potential
applications will be described.