Direct methanol fuel cell, membrane stability

Polyphosphazene-based PEMs are potentially attractive materials for both hydrogen/air and direct methanol fuel cells because of their reported chemical and thermal stability and due to the ease of chemically attaching various side chains for ion exchange sites and polymer cross-linking onto the — P=N— polymer backbone. Polyphosphazenes were explored originally for use as elastomers and later as solvent-free solid polymer electrolytes in lithium batteries, and subsequently for proton exchange membranes. 

P. Zelenay, S. C. Thomas, S. Gottesfeld, Direct Methanol Fuel Cells Recent Progress in Fuel Efficiency, Cell Performance and Performance Stability, in Proton Conducting Membrane Fuel Cells (Second International Symposium) (Eds. S. Gottesfeld, T. F. Fuller, G. Halpert), Proc. Vol. 98-27 Electrochemical Society, Pennington, 1999, pp. 300-315. 

Blends of a PAI and poly(aryl ether ketone) exhibit improved solvent resistance and hydrolytic stability. Blends of sulfonated poly(ether ether ketone) and PAI have been tested as membrane materials for direct methanol fuel cells. Miscible blends can be obtained. Blends of poly(urethane)s (PU)s and PAI, as the minor component have been reported for membrane applications. The resulting membranes are immiscible. Phase separation occurs when the amount of PU decreases. 

Very recently, Wu et al. reported the preparation of PBI composite membranes containing small amounts (lower than 1 %) of functimialized multi-walled CNTs (MWCNTs). The membranes were doped with potassium hydroxide and applied in an alkaline direct methanol fuel cell [77]. The main advantages obtained by the incorporation of the CNT fillers were the reduction of the methanol permeability, a higher thermal stability, and the improvement of the irniic conductivity. The maximum power density (104.7 mW cm ) was achieved for the composite PBI membrane with the lowest CNT cmitent, 0.05 %, miming at 90 °C and with 2 M methanol in 6 M KOH. This peak power density was more than three times higher than for a KOH-doped pristine PBI [78]. Nonetheless, the amount of Pt catalyst on each electrode was veiy high, around 5 mg Pt cm , which could be the actual reason for the better performance achieved by the composite PBI-based alkaline direct methanol fuel cell (DMFC). 

This chapter is a review focussed on the development of ionomers based on aromatic polysulfones for their application as Polymer Electrolyte Membrane (PEM) in Proton Exchange Membrane Fuel Cells (PEMFC) or in Direct Methanol Fuel Cells (DMFC). Different types of synthesis routes have been discussed in this chapter in order to obtain ionomers based on polysulfones with variation in structural designs. Special attention is given to the impact of the structural design of the ionomer on various properties such as membrane morphology, thermo-mechanical stability and protonic conductivity of the membranes for their utilization as PEMs. 

These preliminary results are a first demonstration that the shape-selected particles concept may work in a realistic fuel cell environment. Future research will focus on degradation and stability tests of the novel materials as well as their application in other fuel cell types, as for instance direct methanol fuel cells and high-temperature pol)uner electrolyte membrane fuel cells. Moreover, the effect of the surfactant requires special attention, as the surfactant molecules may also influence the electrocatalysis by a ligand effect or an ensemble effect directing the adsorption of reactants to specific surface sites. 

Silva et al. [63] developed a mixture of sulfonated-poly(etheretherketone) (s-PEEK) with a 42 or 68% degree of sulfonation, m-PBI, and zirconium phosphate (ZrP) for use in a direct methanol fuel cell. This blend was originally designed to increase the chemical and thermal stability of the s-PEEK. The proton conductivity and DMFC performance were also tested. Although the membrane swelling and methanol permeability decreased, the membrane conductivity also decreased. In general terms, however, the addition of the m-PBI and ZrP imparted chemical stability and increased DMFC efficiency at temperatures up to 130 °C. 

The most widely studied fuel-ceU membrane is DuPont s Nafion , a copolymer of tetrafluoroethylene and perfluoro(4-methyl-3,6-dioxa-7-octene-l-sulfonic acid). Nafion is the membrane material of choice for most proton-exchange membrane fuel cells that operate at a temperature <80 °C. While Nafion offers high conductivity combined with exceptional chemical and mechanical stability [3], it suffers from several critical drawbacks. When used in a direct methanol fuel cell, Nafion shows significant methanol leakage (crossover from the anode to the cathode) with the resultant reduction in fuel-ceU performance. To overcome this shortcoming the methanol concentration in the anode feed is usuaUy reduced to 0.5-2.0 M, which necessitates... 

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