PVA–SSA–HPA Mixed-Matrix-Membrane Electrolytes for DMFCs

Abstract

Stabilized forms of heteropolyacids (HPAs), namely phosphomolybdic acid (PMA), phosphotungstic acid (PTA), and silicotungstic acid (STA), are incorporated into poly(vinyl alcohol) (PVA) cross-linked with sulfosuccinic acid (SSA) to form mixed-matrix membranes for application in direct methanol fuel cells (DMFCs). Bridging SSA between PVA molecules not only strengthens the network but also facilitates proton conduction in HPAs. The mixed-matrix membranes are characterized for their mechanical stability, sorption capability, ion-exchange capacity, and wetting in conjunction with their proton conductivity, methanol permeability, and DMFC performance. Methanol-release kinetics is studied ex situ by volume-localized NMR spectroscopy (employing "point-resolved spectroscopy") with the results clearly demonstrating that the incorporation of certain inorganic fillers in PVA–SSA viz., STA and PTA, retards the methanol-release kinetics under osmotic drag compared to Nafion, although PVA–SSA itself exhibits a still lower methanol permeability. The methanol crossover rate for PVA–SSA–HPA-bridged-mixed-matrix membranes decreases dramatically with increasing current density rendering higher DMFC performance in relation to a DMFC using a pristine PVA–SSA membrane. A peak power density of at a load current density of is achieved for the DMFC using a PVA–SSA–STA-bridged-mixed-matrix-membrane electrolyte.