DMFC: Polarization curves, impedance spectra and how to make use of methanol crossover

We report non—stationary analytical models for DMFC performance. Separate models for DMFC anode and cathode are constructed and analytical expressions for the half—cell polarization curves are derived. The anode model takes into account methanol transport through the backing and catalyst layers, and methanol adsorption on the catalyst surface in the kinetics for methanol oxidation reaction (MOR). The cathode model includes the same mechanism of MOR on the cathode side, the Butler—Volmer equation for the oxygen reduction reaction and the diffusive oxygen transport in the backing and catalyst layers. Both models contain Ohm’s law for proton transport in the catalyst layer. The resulting half—cell polarization curves are discussed and compared to the experimental data. The cathode model is linearized and converted into frequency domain yielding a model for the cathode impedance. Comparison of calculated and experimental Nyquist spectra helps to understand the effect of methanol crossover on the cathode impedance. Finally, we discuss a three—electrode DMFC design, which enables to convert methanol crossover flux into a useful current. [1] A.A.Kulikovsky. An analytical model for DMFC anode (in preparation). [2] A.A.Kulikovsky. A model for DMFC cathode performance. J.ELectrochem.Soc., 159 (2012) F644. [3] A.A.Kulikovsky. A model for mixed potential in direct methanol fuel cell cathode and a novel cell design. Electrochim. Acta, 79 (2012) 52– 5