(29f) Polymeric Membranes for Fuel Cells: Overview and Recent Developments

Polymeric membranes play a critical role during the
generation of electricity in hydrogen/air and direct methanol proton-exchange
membrane (PEM) fuel cells. The membrane in such devices performs three roles: It
physically separates the positive and negative electrodes, prevents mixing of
the fuel and oxidant, and provides a conduit for ion (proton) movement between
the electrodes.  For hydrogen/air proton-exchange membrane fuel cells, the
membrane must exhibit low gas permeability and high proton conductivity.  For a
direct liquid methanol PEM fuel cell, the ion-exchange membrane must conduct
protons and be a good methanol barrier.  DuPont's Nafion^TM (a perfluoro
sulfonic acid polymer) is the membrane of choice for H₂/air PEM fuel
cells that operate at a temperature of about 80^oC. For a reformate
fuel, it is desirable to operate the fuel cell at a temperature greater than
100^oC (to reduce the effects of CO poisoning). Unfortunately, Nafion
looses water and the conductivity is low at such high temperatures, unless the
water activity in the fuel gas feed is near unity.  Nafion has also been used
in direct methanol fuel cells, but high methanol crossover leads to unwanted
fuel losses, poisoning of the air cathode by CO (which is produced when
methanol permeates through the membrane and is chemically oxidized at the
cathode), and low power output (due to cathode depolarization by methanol). There
is considerable research and development around the world to develop new
membrane materials for PEM fuel cells with tailored physical and transport
properties. Current membrane fuel cell work is focused on:  (i) proton
conductors that operate at a temperature of 100-120^oC under low
humidity conditions (25% relative humidity) for hydrogen/air fuel cells, (ii) polymeric
membranes that conduct protons in the dry state during H₂/air fuel
cell operation at high temperatures (160-200^oC), and (iii) proton
conducting ion-exchange membranes with low methanol permeability for operating temperatures
in the range of 60-140^oC. This talk will present and critique fuel
cell membrane research efforts in each of these areas.