Functionalized fullerene embedded in Nafion matrix: A modified composite membrane electrolyte for direct methanol fuel cells

Methanol permeability is a major concern for Nafion((R)) membranes used as polymer electrolyte in direct methanol fuel cells (DMFCs). In the present study, composite membranes are formed by incorporation of functionalized fullerene (FF) in Nafion((R)) ionomer for its use as electrolytes in direct methanol fuel cells at various methanol concentrations. Fullerene was functionalized by 4-benzene diazonium sulfonic acid precursor formed via diazotization reaction of sulfanilic acid. Surface functionalization and sulfonation of fullerene is confirmed by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and elemental analysis. Structural changes in fullerene are determined by transmission electron microscopy (TEM) analysis. The composite membranes of Nafion((R))-FF are prepared by solvent casting technique and characterized for their physico-chemical properties in terms of water uptake, proton conductivity and methanol permeability. The morphological changes are observed by field emission scanning electron microscopy (FE-SEM) suggesting the FF distribution in Nafion((R)) and atomic force microscopy (AFM) explaining the presence of FF in the ionic clusters of Nafion((R)) thereby increasing the surface roughness. Nafion((R))-FF composite membranes show improved proton conductivity due to the presence of surface functional-SO3H groups. Composite membranes exhibit better electrochemical selectivity and as a result enhanced DMFC power output in comparison to recast Nafion((R)). In addition, methanol permeability and DMFC polarization for the optimized composite membrane are carried out at different methanol concentration. The peak power density of 146 mW cm(-2) in DMFC is obtained for Nafion((R))-FF (1 wt.%) at 2 M methanol which is higher than the performance observed for Nafion-117. Open circuit voltage change is minimal with respect to time for Nafion((R))-FF (1 wt.%) confirming its better stability in comparison with recast Nafion((R)) and on par with Nafion-117. (C) 2016 Elsevier B.V. All rights reserved.