How fuel permeation affects the hydrogen bond network in bio-polymericmembranes

Chitosan biopolymers have been recently employed in various forms as an alternative protonconductingmembrane to Nafion due to its considerably lower methanol crossover properties orhigher selectivity (i.e., the ratio of proton conductivity to methanol permeability) [1-3]. The abilityof water molecules to transport the proton from anode to cathode compartments relies upon thenetwork of hydrogen bonds emerged among them, which is influenced by the amount of methanoluptake inside the DMFC membrane [4]. In the present research, the effects associated with themethanol crossover on the water hydrogen bonding characteristics across the water/methanolsolvated chitosan membrane were examined by means of molecular dynamics (MD) simulationapproaches. To this purpose, a range of methanol concentrations (including 0, 10, 20, 40, 60, and80 wt%) were taken into account. Intermolecular hydrogen bonding interactions of water withchitosan functional fragments, namely, amine (-NH2), hydroxyl (-OH) as well as hydroxymethyl (-CH2OH) were studied using radial distribution function (RDF), coordination number (CN), andtime evolution of the number of hydrogen bonds. The obtained simulation results for RDFs andCNs (the insets in Figure 1) of water around each function group suggested that water interactionswith chitosan chains weakened against an increase in methanol concentration. Such an observationwas further confirmed by the reduced number of hydrogen bonds in increasingly methanol solvatedchitosan membrane. These findings propose that an increase in methanol sorption (and itssubsequent crossover) could significantly diminish the proton conductivity feature of DMFCmembranes fabricated from chitosan biopolymers.

Keywords:

Fuel permeation , Bio-polymeric membrane , Direct methanol fuel cell , Modeling