(214e) The Effect of Membrane Thickness on Short- and Long-Term Performance of a Direct Methanol Fuel Cell

It is well known that the optimum proton-exchange membrane
material for a direct methanol fuel cell (DMFC) should have a high proton
conductivity and low methanol crossover. Such a combination of properties is
difficult to achieve and, in general, membranes with low methanol permeability
also exhibit sluggish proton conduction. Consequently, a major objective of DMFC
membrane development has been to maximize the conductance/permeability ratio. Alternatively,
attempts have been made to optimize both the MEA (membrane-electrode-assembly,
composed of the membrane and attached catalyst layers) structure and the
operational conditions of a DMFC in order to effectively utilize existing
membrane materials. Some investigators employ thin membranes to decrease the
ohmic resistance of an MEA and dilute methanol feed concentrations to lower
methanol crossover. Membrane thickness, in particular, has a direct bearing on
the primary processes that cause DMFC power losses, via ohmic losses (IR drop)
and methanol crossover flux (which causes cathode depolarization and poisoning
of the cathode catalyst by CO, a product of methanol oxidation). It is,
therefore, of prime importance to understand the interdependence of membrane
thickness and DMFC operating conditions (e.g., methanol feed concentration,
temperature and air/oxygen flow rate and pressure) on the short-term and
long-term performance of a direct methanol fuel cell.  In the present talk, the
effects of membrane thickness on the initial and long-term power output from a DMFC
will be presented (here, long-term is defined as a few days of fuel cell
operation). DMFC performance plots (voltage vs. current density) will be described/discussed
for different proton-exchange membranes (Nafion, sulfonated PEEK, and
sulfonated polyphosphazene) of varying thickness. In general, we have found
that the power generated using thin a membrane is initially high (due to lower
resistive losses) but falls rapidly (within a few hours) due to poisoning of
the cathode by CO, whereas the initial performance with a thicker membrane is
low (due to its high resistance) but is more stable during long-term operation because
there is less CO poisoning of the cathode (due to lower methanol crossover).