On the Nature of Mixed Potential in a DMFC

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Abstract
A 1D+1D1D+1D model of a direct methanol fuel cell (DMFC) is developed. Analytical solution for the case of equal oxygen and methanol stoichiometries λ is obtained. The model predicts a new effect: formation of a narrow zone of local current (bridge), which “short-circuits” cell electrodes at vanishingly small current in the load. The bridge arises only under nonzero flux of methanol through the membrane. This effect explains dramatic decrease of DMFC open-circuit voltage known as mixed potential. The expression for voltage loss at open-circuit E0E0 is derived; E0E0 increases with the rate of crossover and tends to zero with the growth of λ. Numerical estimate for typical cell gives E0≃500-700mVE0≃500-700mV , which agrees with the experimental data. The model suggests a method for measuring mean crossover flux through the membrane and indicates conditions for cell operation with zero crossover, regardless of the transport properties of the membrane