Homogeneuosly sulfonated poly(styrene) (SPS) was prepared with various concentration of sulfonic acid groups in the base polymer. Membranes cast from these materials were investigated in relation to proton conductivity and methanol permeability in the temperature range from 20 • C to 60 • C. It was found that both these properties increase as the polymer is increasingly sulfonated, with abrupt jumps occurring at a concentration of sulfonic acid groups of about 15 mol%. The most extensively sulfonated membrane exhibited conductivity equal to that of Nafion. As a consequence, this membrane material is potentially an appealing alternative to the very expensive Nafion, for a number of electrochemical applications. For the membrane with the highest degree of sulfonation we measured a methanol permeability about 70% smaller than for Nafion. This characteristic is especially desirable in applications related to the direct methanol fuel cell (DMFC).

There is a growing interest in the use of low-temperature solid polymer electrolyte fuel cells (PEFCs) as modular power sources. These devices have the potential to deliver electrical energy from fuel oxidation with very high yeld. Moreover, the energy production process is not accompained by environment polluting emissions. These characteristics make PEFCs highly attractive especially for application in cars and other terrestrial vehicles. The H 2 /O 2 PEFC has experienced considerable progress during the last few years 1 and application of this technology to vehicular transportation now appears close to commercialization. 2 However, hydrogen storage in H 2 /O 2 PEFCs powered vehicles would entail problems related to pressurization and safety. One way to circumvent these problems would be producing hydrogen in situ from methanol reforming. Methanol would be a low cost liquid fuel with energy density comparable to that of petrol. Moreover, because methanol is liquid up to 65ЊC, it can be handled by the fuel supply infrastructure presently utilized for gasoline. Obviously, the reforming stage, in addition to being costly and cumbersome, would also reduce the overall efficiency of the system and its ability to respond to variations of load. Methanol exhibits conspicuous electrochemical activity. Recently, great attention has been devoted to PEFCs directly fed by liquid methanol. 3 However, at present, two major technical problems must be overcome before the direct methanol fuel cell (DMFC) can validly be proposed for electric vehicles application. One problem is slow methanol oxidation kinetics on the anode catalyst. The second is methanol diffusion from the anode to the cathode side, across the polymer electrolyte membrane. That causes loss of fuel, reduced cathode voltage, and excess thermal load in the cell. Poly(perfluorosulfonic acid) (Nafion ® ) membranes are commonly used as solid electrolytes in DMFCs, owing to good chemical and thermal resistance and ionic conductivity. However, it has been found that over 40% of the methanol can be wasted in DMFCs across such membranes. 4 The DMFC technology would greatly benefit both in terms of energy efficiency and output voltage from a drastic reduction of methanol crossover. Moreover, the high cost of Nafion and other perfluorosulfonated polymers is a key issue in large scale commercialization of PEFCs. As a result, a major research objective is to achieve or identify novel low cost proton conductive membranes with low methanol permeability. Use of poly(benzimidazoles) membranes doped with phosphoric acid has been claimed to reduce methanol crossover at high temperature. 5 An ionomer membrane composed of interpenetrating networks of poly(styrenesulfonic acid) and poly(vinylidene fluoride) was reported to exhibit methanol permeance about half of that of commercial Nafion 117 films. 6 Recently, it has been shown that when Nafion 117 is doped with cesium cations, its methanol permeability at room temperature reduces markedly; its proton conductivity decreases to a lesse...

No abstract

Poly(perfluorosulfonate acid) membranes were doped with cesium ions to several degrees. These, along with the H +form membrane, were investigated in relation to methanol permeability as well as hydrogen ion conductivity. While retaining considerable conductivity, the cesium-doped membranes are highly impermeable to methanol. We found that methanol permeability in the membrane reduced by over one order of magnitude, owing to the presence of cesium ions. These findings are discussed on the basis of alterations produced by cesium in the membrane microstructure. Also discussed is the potential implication of these results in the direct methanol fuel cell technology.

Composite Nafion membranes containing three natural zeolites (Mordenite, Chabazite and Clinoptilolite) were prepared by using a recast procedure for application in high temperature Direct Methanol Fuel Cells (DMFCs). The Nafion-zeolite membranes have shown good properties for high temperature DMFC application, due to their improved water retention characteristics. A maximum power density of 390 mW cm )2 was achieved at 140°C with the mordenite-based composite membranes in the presence of oxygen feed. The electrochemical behaviour of the composite membranes was interpreted in the light of surface properties and acidic characteristics of the fillers.