Comparison of different cells for resistance determination of freely standing polymer membranes developed for direct methanol fuel cell (DMFC) applications

Cation conductive membranes, especially highly proton conductive membranes, are of interest not only for chlor-alkali electrolysis but for polymer electrolyte fuel cells as well. The very challenge for electrochemical characterization in this case is the low specific resistance of the polymer required for such applications, which in turn makes membrane resistance measurement a non-trivial problem. We investigate the different possibilities to characterize such membranes. For that purpose, a series of membranes made from sulfonated poly (phenylene oxide) (SPPO, degree of sulfonation (DS) from 7% up to 40%) was prepared. Commercially available membranes Nafion 117 as a reference are investigated, too. This contribution summarizes our new results concerning impedance spectroscopy measurements, using different measuring cells.

1. For fast screening, membrane resistance determination at phase angle φ = 0 is sufficient. From the difference between resistance with A ( R m + s ) and without MUT (RsA) nearly the same membrane resistance RmA is obtained as compared with fits covering a broad frequency spectrum. 2. Both cells are suited for resistance determination of proton conducting membranes, as well from φ = 0 as from model fits. The reproducibility amounts to 4% (gap cell) and 6% (“zero-gap” cell), respectively, for the determination from φ = 0. Results obtained for Nafion 117 are in agreement with literature data. 3. The model membrane Nafion 117 acts mainly as an ohmic resistor in series with the electrolyte resistance. Concerning SPPO membranes, a linear relationship between DS >12% and specific conductivity in H2SO4 is found. Adjusting the DS to 30%, membranes useful for DMFC application, obtained.