Electropolymerized porphyrin films as methanol barriers in direct methanol fuel cells

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Abstract

Methanol crossover from the anode side to the cathode site across the membrane in direct methanol fuel cells causes loss of fuel and catalyst poisoning. Many reports deal with the modification of the commonly used Nafion® membrane or the development of new polymer membranes exhibiting reduced methanol permeability. The present study reports on the use of tetra(orthoaminophenyl)porphyrin ((o-NH2)TPP) as the methanol barrier when electropolymerized on a direct methanol fuel cell cathode. Using the electrochemical quartz crystal microbalance and atomic force microscopy techniques, the (poly(o-NH2)TPP) films which efficiently reduced methanol permeability were shown to be non-porous, 0.55 μm thick and characterized by a high density (0.94 g/cm3) and high porphyrin site concentration (1.4 M). For a glassy carbon electrode electrodeposited with Pt (Pt/GC), the oxygen reduction current density in 1 M H2SO 4 at +0.1 V decreases from 0.42 to 0.23 mA/cm2 in the presence of 0.5 M methanol while a similar electrode coated with the electropolymerized porphyrin shows almost no decrease after methanol addition (0.58 and 0.56 mA/cm2, respectively). For Pt/C electrodes with a surface morphology coarser than that of Pt/GC, the surface was first coated with electropolymerized aniline-2-sulfonic acid (PAS) and then with the electropolymerized porphyrin. The ratio of oxygen reduction currents after and before adding 0.25 M methanol was approximately 90% in this case. Experiments in a two-electrode configuration showed that it is possible to operate a DMFC with low current output without using an electrolytic membrane separating the anode from the cathode, provided that the cathode is coated with a bilayer of PAS/poly(o-NH2)TPP.

Original languageEnglish
Pages (from-to)265-272
Number of pages8
JournalJournal of Electroanalytical Chemistry
Volume571
Issue number2
DOIs
StatePublished - 1 Oct 2004

Keywords

  • Aniline sulfonic acid
  • Crossover
  • Electropolymerized films
  • Fuel cells
  • Methanol
  • Porphyrins

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