Dependence of the cell resistance on external potential in DMFC system

The polarization properties of a portable direct methanol fuel cell (DMFC) subjected to an external potential are experimentally investigated. In DMFC, polymer electrolyte membrane plays important role as a proton conductor. The resistance of the membrane, which reflects ion-transport and mobility of ions through the membrane, is one of the fundamental factors that determine electric activity of the fuel cell. We applied an external potential in the reverse and forward direction to the cell potential of the DMFC system. From the dependencies of the effective membrane resistance on the external potential, it was found that there are several thresholds in the external potential at which the main reactions of the cell are switched. In the case of the reverse external potential, electrolysis of water and methanol induced by the accumulated electrons and protons reduce the cell resistance. For the forward external potential, on the other hand, the resistance becomes high due to the polarization inside the membrane. The resistance decreases as electric decomposition of water and methanol proceeds.