Methodical Exploration on Silicotungstic Acid Tailored Hybrid Bio-polymeric Proton-Exchange Membrane Electrolytes: Correlating Positron Annihilation Characteristics with Electrochemical Selectivity for Direct Methanol Fuel Cells

A new class of proton exchange membrane based on chitosan (CS)-Pectin (PC) biopolymers are proposed as green electrolyte materials for direct methanol fuel cells (DMFCs). Caesium salt of silico-tungstic acid (STA) is incorporated into polymeric network to promote ionic conductance. In situ cross-linking with Glutaraldehyde (GA) is carried out to enhance membrane stability. These membranes are characterized by X-ray diffraction, thermo-gravimetric and Scanning electron microscopy studies. The presence of STA augmented thermo-mechanical stabilities and also caused a decrease in the fractional free volume (FFV) of membranes as evident from Positron Annihilation Lifetime Spectroscopy assessment. Contraction in FFV significantly suppresses methanol diffusion by generating more circuitous pathway, resulting in a decreased methanol crossover. Ion-exchange capacity, water sorption, proton conductivity and methanol permeability of these membranes are examined. These studies reveal that bio-polymeric hybrid membrane comprising optimal (10.wt%) STA provides a higher electrochemical selectivity suggesting their prospective applications in DMFCs. Maximum power density of 27 mW cm −2 is achieved for the membrane with optimised inorganic additive than 15 mW cm −2 obtained for pristine biopolymeric membrane.