Determination of proton conductivity in anode catalyst layers of the direct methanol fuel cell (DMFC)

A new method for the determination of the specific proton conductivity in PEMFC and direct methanol fuel cell (DMFC) electrodes is proposed. The proton conductivity of DMFC anode catalyst layers was determined as a function of the Nafion® content (5–16 wt%, 6–12 vol.%). The catalyst layers consisted of Pt/Ru black and Nafion® polymer. Following the theory published in [J. Electroanal. Chem. 475 (1999) 107], proton conductivities were calculated from the slope of the high frequency linear part of impedance spectra and the pseudo-double layer capacitance obtained from cyclic voltammograms. It turns out that the values of the proton conductivity are of the order of mS/cm, which is more than one order of magnitude lower than the bulk conductivity of Nafion® under these conditions. This means that the microstructure and tortuosity of the Nafion® phase in the catalyst layer significantly influences the proton conductivity. Compared to the influence of the mass transport and the active catalyst surface, the proton conductivity is a less important factor for the performance of DMFC anodes with unsupported Pt/Ru-catalyst in the range of Nafion® fraction investigated.