Membrane‐Supported Nonvolatile Acidic Electrolytes Allow Higher Temperature Operation of Proton‐Exchange Membrane Fuel Cells

Abstract

The feasibility of using nonvolatile molten and solid acidic electrolyte impregnated ion-exchange membranes in higher temperature proton-exchange membrane fuel cells (PEMFC5) to alleviate their water dependence is investigated. Higher temperature PEMFC operation reduces CO poisoning as well as passivation of the Pt electrocatalyst by other condensable species. Further, higher temperature operation could eventually allow direct use of low-temperature reformable fuels such as methanol in the PEMFC. The methodology proposed here involves supporting an appropriate acidic solid, melt, or solution of low volatility within the pores of Nation® so as to enhance its protonic conductivity at higher temperatures and lower humidity levels. Preliminary experimental results reported here for a PEM fuel cell operating at temperatures of 110 to 120°C based on Nafion supported solutions of heteropolyacid indicate the feasibility of the technique.•• 0 densable species would be reduced, and it would also alleviate the water management problems in PEMFCs. This is the broad objective of this work.Before discussing the rationale behind the proposed methodology, it is useful to reflect briefly on the proton transport mechanism in conventional ion-exchange membranes. Aspects of proton transport in polymer ion-exchange membranes have been discussed by other authors based on transport and spectroscopic studies.713Yeager8 and Gierke and Hsu9 describe a structural three-zone pore model, namely, (i) a low dielectric constant region consisting of the hydrophobic fluorocarbon polymer matrix, (ii) a high dielectric constant inverted micellular region containing ion clusters including the sulfonate exchange sites, counterions, and sorbed water, and (iii) the interfacial region consisting mostly of the pendant side chains of the sulfonate groups and a small amount of water. In fully hydrated Nation®, these clusters are -4 nm in diameter, shrinking to -1.9 nm at low water concentrations.910 The interconnection of such spherical micelles within the membrane pores by channels creates a network for ion transport.'°F igure 1 shows the schematic diagram of a pore within an ionexchange membrane in which the interconnected cellular geometry of the ion-cluster network is approximated by straight-cylindrical geometry. The anionic groups (-SO) are tethered to the pore sur-

Approaches and technical challenges to high temperature operation of proton exchange membrane fuel cells

“…These gas streams will contain small amounts of carbon monoxide (CO) which poisons the platinum anode catalyst. High temperature operation of a PEMFC (above 1208C) has been investigated [1][2][3][4] to reduce the effects of CO adsorption onto the platinum electrocatalyst. In addition to increasing the tolerable CO concentration in the anode fuel, higher temperature operation can improve thermal management and heat utilization of the fuel cell stack, increase reaction rates at the anode and cathode, and potentially simplify fuel cell water management.…”

“…Hydrophilic, inorganic material may be incorporated into the perfluorinated ionomer membrane to increase the binding energy of water. The efficacy of these hydrophilic additives has been demonstrated in the case of heteropolyacids in Nafion [1]. Water molecules are strongly hydrogen bonded to the ions or dipoles in the inorganic material; additionally, the acid may increase proton conductivity by adding additional solvent and increasing proton density.…”

Impact of acid containing montmorillonite on the properties of Nafion® membranes

“…The DMFC technology relies on fuel cells equipped with a proton exchange membrane (PEMFC), that plays the role of the electrolyte. Nowadays, few membranes have been tested with some success, including perfluorosulfonate ionomer (PFSI), such as Nafion ® [1][2][3], polyaromatics, such as sulfonated polysulfone (PSU) [4][5][6], sulfonated polyetheretherketone (SPEEK) [7][8][9], sulfonated polyphenylene oxide (SPPO) [10][11] and polybenzimidazole (PBI) [12][13][14]. Except for PBI that needs doping by a strong acid for being conductive, all the other polymers combine high chemical stability and high ionic conductivity.…”

Development of Nafion/Tin Oxide Composite MEA for DMFC Applications

“…The enlarged stability domain of proton conductivity has been related to the higher stiffness of the composite membranes and thus to their enhanced dimensional stability [20]. The use of acidic fillers, such as heteropolyacids or sulfated metal oxides, has been also explored [21][22][23][24][25]. The acidic surface functionalities of inorganic fillers have been found to play a key role towards the improvement of the water retention properties of the composite membrane and thus of their transport properties.…”