PVA-PSSA Membrane with Interpenetrating Networks and its Methanol Crossover Mitigating Effect in DMFCs

Abstract

A membrane with interpenetrating networks between poly͑vinyl alcohol͒ ͑PVA͒ and poly͑styrene sulfonic acid͒ ͑PSSA͒ coupled with a high proton conductivity is realized and evaluated as a proton exchange membrane electrolyte for a direct methanol fuel cell ͑DMFC͒. Its reduced methanol permeability and improved performance in DMFCs suggest the new blend as an alternative membrane to Nafion membranes. The membrane has been characterized by powder X-ray diffraction, scanning electron microscopy, time-modulated differential scanning calorimetry, and thermogravimetric analysis in conjunction with its mechanical strength. The maximum proton conductivity of 3.3 ϫ 10 −2 S/cm for the PVA-PSSA blend membrane is observed at 373 K. From nuclear magnetic resonance imaging and volume localized spectroscopy experiments, the PVA-PSSA membrane has been found to exhibit a promising methanol impermeability, in DMFCs. On evaluating its utility in a DMFC, it has been found that a peak power density of 90 mW/cm 2 at a load current density of 320 mA/cm 2 is achieved with the PVA-PSSA membrane compared to a peak power density of 75 mW/cm 2 at a load current density of 250 mA/cm 2 achievable for a DMFC employing Nafion membrane electrolyte while operating under identical conditions; this is attributed primarily to the methanol crossover mitigating property of the PVA-PSSA membrane. Direct methanol fuel cells ͑DMFCs͒ using a proton exchange membrane have been identified as one of the most promising candidates for portable power applications.1,2 Unlike hydrogen-air polymer electrolyte fuel cells, DMFCs do not require a fuel reformer or a high-volume hydrogen storage system. The membrane electrolyte employed with the DMFC, besides exhibiting a good proton conductivity, should act as a physical separator to prevent fuel crossover from the anode to the cathode. At present, Nafion a perfluorosulfonated membrane with a hydrophobic fluorocarbon backbone and hydrophilic sulfonic acid pendant side chains, happens to be the only commercially available and widely used membrane electrolyte in the DMFC. It has been documented that proton conduction in Nafion occurs through the ionic channels formed by micro-or nanophase separation between the hydrophilic proton exchange sites and the hydrophobic domains.3 However, the methanol crossover from anode to cathode across the Nafion membrane brings about a mixed potential at the cathode causing both the loss of fuel and cell polarization impeding their commercial realization. [4][5][6] It has been reported that even over 40% of methanol could be lost in a DMFC due to crossover across the membrane.7 Methanol crossover across the Nafion membrane can be kept to a minimum by controlling the methanol-feed concentration. Alternatively, membranes that are relatively impermeable to methanol have been employed for this purpose. [8][9][10][11][12] Membranes with a lower methanol permeability allow a higher methanol-feed concentration, enhancing the performance of the DMFC. To optimize fuel cell performance, it is neces...

Water Soluble Polymers as Proton Exchange Membranes for Fuel Cells

“…It was found that the proton conductivity of the PVA/PSSA or PVA/PSSA-PMA semi-IPN membranes was strongly dependent on the content of the linear acid polymer. Maximum proton conductivity (~10 Semi-IPNs combining linear PSSA and PVA-GA polymer networks, showing high proton conductivities and high water retention capabilities for PEM applications in fuel cells, have also been prepared [60,68]. The maximum proton conductivity was observed for a PVA/PSSA semi-IPN membrane with about 35 wt% PSSA (PVA/PSSA 35 wt%) at all humidity values (0-100% RH).…”

“…However, the excessive swelling of PVA-PWA composite membranes limits their mechanical strength. For PEM applications, PVA is commonly incorporated into the PEM as a crosslinked partner via aldol condensation [60][61][62][63][64][65][66][67][68][69][70] or esterification [49,[71][72][73][74][75][76] to form a 3D network structure. Moreover, the degree of crosslinking of the PVA-based membranes has been shown to be easy to control via successive chemical treatments (aldol condensation or esterification).…”

“…The numerous crosslinked membranes that have been designed with a view to meeting these requirements include common chemical networks (a polymer comprising only one network without a linear polymer entrapped within the network polymer) [121][122][123][124], interpenetrating polymer networks (IPNs) [68,125] and semi-IPNs [126][127][128]: an illustrated common polymer network and the associated IUPAC polymer network definition are shown in Figure 6 [129]. A possible drawback to the use of cross-linked polymers is that the crosslinking might hinder the mobility of ions and hence lower the final conductivity.…”

“…Based on the advantages of PVA-based crosslinked membranes, semi-IPNs combining PVA network and linear PSSA or poly (styrene sulfonic acid-co-maleic acid) (PSSA-PMA) copolymers have been produced according to an impregnation [62] or an in situ [60,[66][67][68] process to form semi-IPN membranes (Table 5; 1-3). To form a PVA network, the PVA is most commonly combined with a dialdehyde, such as glutaraldehyde (GA), and a dicarboxylic acid, such as sulfosuccinic acid (SSA), using an aldol condensation followed by esterification to form the 3D network structure.…”

“…PSSA homopolymer, or as copolymer, has often been combined with a PVA network to form a semi-IPN architecture through an impregnation process [60,62] or by in situ synthesis [66][67][68]. This strategy is similar to the PAMPS polymer system that involves the use of intra-and inter-molecular hydrogen bonds to form a dense composite network structure, resulting in good film forming properties, good resistance to chemical attack, and also low permeability to alcohols on crosslinked PVA-based composite membranes.…”