Distribution profile of water and suppression of methanol crossover in sulfonated polyimide electrolyte membrane for direct methanol fuel cells

We have developed novel cross-linked sulfonated polyimide (c-SPI) membrane as an electrolyte for direct methanol fuel cells (DMFCs). When the DMFC using the c-SPI membrane (thickness = 155 μm), Pt–Ru dispersed on carbon black (Pt–Ru/CB) anode and Pt/CB cathode with a Nafion® ionomer was operated at 80 °C and 0.1 A cm−2 with 1 M CH3OH and oxygen (oxidant), the methanol crossover rate, j(CH3OH), was suppressed to about 1/2 compared with that of the Nafion® 117 membrane (thickness = 180 μm) with the same electrodes. It was found for both cells that the j(CH3OH) was not so small as expected from the membrane thickness. In order to obtain a clue for the suppression of j(CH3OH), the distribution profiles of water (containing CH3OH) in thickness direction were investigated by measuring the specific resistances (ρ) between Pt probes inserted into the electrolyte membrane. Values of ρ at the anode side were low irrespective of the discharge current density, because such a part of the membrane was humidified thoroughly by liquid water (1 M CH3OH) allowing free penetration of CH3OH into the swollen polymer. In contrast, the values of ρ at the cathode side were high at the low current density due to drying of the membrane contacting with oxidant gas (O2 or air) in low humidity. We have succeeded to suppress the j(CH3OH) further (about 1/2 at 0.2 A cm−2) by using bilayer c-SPI, having a low ion exchanging (low swelling) barrier layer at the anode side without increasing the ohmic resistance, compared with that of the single c-SPI.