Catalyst layers of direct methanol fuel cells (DMFCs) are modified by in situ electropolymerization of m-aminobenzenesulfonic acid. By using electrochemical impedance spectroscopy and porosimetry, this modification is found to add polymer electrolyte into primary pores (<10 nm), where ionic resistance is high for lack of polymer electrolyte (i.e., Nafion), and the additional electrolyte successfully decreases the ionic resistance by 10-15% compared to the plain carbon surface with a slight ion-conductivity (>40 kΩ cm). In view of methanol oxidation characteristics, this modification decreases the resistance by ca. 25% (from 5.1 Ω cm2 to 3.7 Ω cm2) at 0.6 V vs. DHE, resulting in the increase in the cell voltage of DMFC test by ca. 20 mV. The clear relation between the performance and the microstructures is concluded to be helpful to understand the performance of fuel cell electrodes in detail.
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