A Molecular Simulation Study for Understanding the Effects of Two Water Channel Morphologies in Nafion with Various...

Fuel loss inside direct methanol fuel cells (DMFCs) having Nafion membranes with two distinct water channel morphologies (viz., lamellar water channel morphology (LWCM) and random water channel morphology (RWCM), both validated against the literature having comparable equivalent weights and membrane densities) has been quantified through a methanol adsorption (ρ_CH3OH) study using GCMC simulations. Subsequently, variations in nonideality of adsorbed methanol as a function of Nafion-water-methanol local mole fractions are estimated at DMFC operating conditions, which are governed by ρ_CH3OH inside different morphologies. For all temperatures with given hydration (λ), ρ_CH3OH values are significantly higher inside LWCM with a single ordered channel compared to RWCM with localized hydrophilic regions and inherent tortuosity. While ρ_CH3OH decreases with increasing λ inside LWCM, inside RWCM, ρ_CH3OH is constantly lower at λ = 3 and varies relatively for λ = 6 and 9, depending on the DMFC operating temperature. Pore volume and active site availability in hydrated Nafion result in a competitive arrangement of methanol with respect to the bound water and hydronium mixture, only at the LWCM centre and at localized hydrophilic sites of RWCM. These observations have been justified further by estimating the preferential solvation parameters. Interestingly, the molecular ordering of bound water inside all Nafion remained invariant upon methanol adsorption. However, enhanced molecular ordering of adsorbed methanol in comparison to its pure bulk phase also indicates the possibility of activity coefficients (γ_CH3OH) < 1 inside hydrated Nafion. Calculations of excess entropy substantiate ρ_CH3OH,LWCM > ρ_CH3OH,RWCM, at constant water compositions.