Small portable devices contain more and more functions and the lack of battery energy density becomes more serious year by year. Direct methanol fuel cells (DMFCs) are strong candidates for new portable power devices, such as future laptop computers or cellular phones, because of their potential advantages of low weight, high energy density compared to current portable power devices operating at low temperature, and simple system features.[1] Another potential use is for automobile power when used at high temperature, because methanol can be stored in its liquid phase with a high energy density, unlike gas-phase pressurized hydrogen. DMFCs use methanol as fuel by the following reactions:The stoichiometric methanol concentration required for complete reaction of all molecules to produce protons is around 64 wt % (in solution). To create DMFCs with high energy density, an extremely low-methanol-crossover, protonconductive membrane is required.To be able to compare the energy density between DMFCs and current Li-ion batteries, the weight or volume of the fuel cell itself must be low because fuel cells are open devices, unlike closed secondary batteries, and air can be used for the cathode reactant. DMFCs can potentially achieve 1500 Wh kg -1 at 0.5 V output; this value is 5 to 10 times larger than current Li-ion batteries.However, when methanol crossover occurs, unreacted methanol passes through the electrolyte membrane and reacts directly at the cathode catalyst. This causes a serious reduction of the DMFC voltage and results in catalyst poisoning and mixed potential loss at the cathode. Therefore, when commercial Nafion is used as the electrolyte membrane, a low concentration, typically less than 10 wt % methanol, is used to obtain a high DMFC performance while reducing the crossover.[2] Recently, some hydrocarbon electrolyte membranes have been synthesized to reduce the methanol-crossover effect.[3] Some membranes showed around 10 times lower methanol crossover than a Nafion membrane, and showed a high DMFC performance for higher methanol concentrations of around 30 wt %. However, the crossover reduction was not enough to achieve high energy densities. For example, a membrane we reported previously can reduce methanol crossover to less than one-tenth that of a Nafion membrane and showed a high DMFC performance at high methanol concentrations, because the cathode loss due to crossover could be reduced. However, still around half or one-third of the methanol fuel was transported through the membrane and reacted to CO 2 at the cathode catalyst without being converted to electricity. The energy density of the device was thus seriously reduced. It is necessary to achieve an extremely low-methanol-crossover electrolyte membrane. We have proposed a pore-filling electrolyte membrane concept for fuel-cell applications.[4] Such membranes are composed of two materials: a porous substrate having pore sizes on the sub-micrometer scale or less; and a polymer that fills the pores of the substrate, as shown in Figure 1. The poro...
We found that protons rapidly conduct through unfreezable and bound water in a pore-filling electrolyte membrane (PF-membrane), although many ions usually conduct through free water contained in polymer electrolytes. PF-membrane is a unique membrane that can suppress the swelling of filled sulfonated poly(arylene ether sulfone) (SPES) because of its rigid polyimide substrate. Based on low-temperature DSC measurements, this strong suppression of swelling resulting from the special structure of the polymer electrolyte results in unfreezable and bound water only; it does not contain any free water. Protons rapidly conduct through this structure. In addition, the activation energy of the proton conduction decreased from 16.3 to 9.1 kJ/mol in proportion to the increase in the ion exchange capacity (IEC) of the filled SPES, unlike the almost constant values of the SPES-cast membranes. This tendency of PF-membrane occurred because of the structure of the membrane, where the concentration of the sulfonic acid groups increased with increase in IEC, which became possible by squeezing free water using the swelling suppression of filled SPES. Without being constrained by the PF-membrane, this unique proton conduction through the structured water and highly concentrated sulfonic acid groups will help to develop future polymer electrolytes, particularly in the fuel cell field where protons need to conduct at various conditions such as temperatures below 0 degrees C, combined high temperature and low humidity, and the presence of fuels.
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