Methanol permeability

These main objectives can be reached only by modifying the structures and compositions of primarily the anode (methanol electrode) and secondarily the cathode (oxygen electrode) as discussed in Sections 111 and IV, respectively. In addition. Section IV discusses the conception of new proton exchange membranes with lower methanol permeability in order to improve the cathode characteristics. Section V deals with the progress in the development of DMFCs, while in Section VI the authors attempt to make a prognosis on the status of DMFC R D and its potential applications. 

The development of highly efficient methanol fnel cells depends on a nnmber of scientific aspects (1) the development of more highly active catalysts for methanol oxidation at temperatnres not over 60 to 70°C (desirable in cells without ruthenium, which is in short supply) (2) the development of selective catalysts for the oxygen electrode (i.e., of catalysts insensitive to the presence of methanol) and (3) the development of new membrane materials having a lower methanol permeability. 

The effect of annealing temperatures (65 - 250 °C) and blend composition of Nafion 117, solution-cast Nafion , poly(vinyl alcohol) (PVA) and Nafion /PVAblend membranes for application to the direct methanol fuel cell is reported in [148], These authors have found that a Nafion /PVAblend membrane at 5 wt% PVA (annealed at 230 °C) show a similar proton conductivity of that found to Nafion 117, but with a three times lower methanol permeability compared to Nafion 117. They also found that for Nafion /PVA (50 wt% PVA) blend membranes, the methanol permeability decreases by approximately one order of magnitude, whilst the proton conductivity remained relatively constant, with increasing annealing temperature. The Nafion /PVA blend membrane at 5 wt% PVA and 230 °C annealing temperature had a similar proton conductivity, but three times lower methanol permeability compared to unannealed Nafion 117 (benchmark in PEM fuel cells). 

Kauranen, P. S. and Skou, E. 1996. Methanol permeability in perfluorosul-fonate proton exchange membranes at elevated temperatures. Journal of Applied Electrochemistry 26 909-917. 

Kim, J., Kim, B. and Jung, B. 2002. Proton conductivities and methanol permeabilities of membranes made from partially sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene copolymers. Journal of Membrane Science 207 129-137. 

Yin, Y, Fang, J., Cui, Y, Tanaka, K., Kita, H. and Okamoto, K. 2003. Synthesis, proton conductivity and methanol permeability of a novel sulfonated polyim-ide from 3-(2, 4 -diaminophenoxy)propane sulfonic acid. Polymer 44 4509-4518. 

Gasa, J. V., Weiss, R. A. and Shaw, M. T. 2006. Influence of blend miscibility on the proton conductivity and methanol permeability of polymer electrolyte blends. Journal of Polymer Science Part B 44 2253-2266. 

DMFC low methanol permeability membrane, high performance catalyst. 

Research Focus Development of bipolar polymer electrolytes, which are thermally and hydrolytically stable, flexible, and exhibit low methanol permeability and high proton conductivity. 

Figure 19. Methanol permeability as a function of temperature and diffusiv-ity data evaluated using the atomistic simulation method.138 Reprinted from Journal of Electrochemical Society, X. Zhou, Z. Chen, F. Delgado, D. Brenner, R. Srivastava, J. Electrochem. Soc. 154, B82 (2007)- Reproduced by permission of the Electrochemical Society.

In addition to the slow methanol oxidation kinetics, methanol that crosses over from the anode to the cathode side through the membrane can react with 02 at the cathode catalyst, leading to a mixed potential at the cathode side and thereby reducing cell performance. To solve this problem, methanol-tolerant catalysts as well as membranes with low methanol permeability have been investigated. However, these materials are still in the research stages and commercial applications have not been developed. 

Hung [3] incorporated benzimidazole derivatives, (II), into sulfonated poly-ethersulfones as a method for preventing filler leaching, improving mechanical properties, and decreasing methanol permeability. 

Sulfonated polyaromatics (partly fluorinated in this specific case), tested for possible operation at "Feel 1 > 100°C and/or lower methanol permeability... 

Replace the PFSA membrane by another polymeric membrane of lower methanol permeability (while maintaining good cell performance)... 

A significant number of efforts to prepare membranes of lower methanol permeability for use in DMFCs have been reported. The general types of new DMFC membrane preparations tried to date include (1) replacing poly(PFSA) by sufonated polyaromatic polymers [105],... 

Fig. 53 Comparison of DMFC polarizations curves at 80 °C with anode feed of 0.5 M methanol solution and the air cathode operating at zero backpressure for a cell with a Nafion 115 membrane and a cell with a polyaromatic membrane of lower methanol permeability [105].

Kauranen, P.S. Skou, E. Methanol permeability in perfluorosulfonate proton exchange membranes at elevated temperatures. J. Appl. Electrochem. 1996, 26, 909-915. 

The methanol permeability of the nanocomposite membranes was shown to decrease on addition of the sulfonated titanate. Functionalized montmorillonite (MMT) was also employed to improve PFSA [58, 59] these composite membranes provide a low methanol crossover, without sacrifidng proton conductivity due to the introduction of sulfonic acid groups at the MMT surface, followed by blending with the Nafion ionomer. 

Despite perfluorinated polymer electrolytes (Nafion, Flemion, Aciplex) having been used extensively in PEMFCs, their poor thermal mechanical stability (low Tg), high methanol permeability, and extremely high production costs have led to limitations of their large-scale application. Therefore, a variety of hydrocarbon polymer electrolytes have been developed during the past decade, with the greatest emphasis placed on the costs, conductivity, and durability of these materials. At the same time, the poor mechanical stability and inadequate durability of the hydrocarbon polymer membranes were identified as the main barriers to their practical application. 

Table 4.1 Proton conductivity and methanol permeability of the composite membrane with three-dimensionally ordered macroporous silica matrix. Measurement temperature 30°C, Methanol concentration 10 mol dm ...

The membrane shown in Fig. 4.10 was prepared using this three-dimensionally ordered macroporous polyimide obtained according to the above process with AMPS polymer. The proton conductivity and methanol permeability of the composite membrane are summarized in Table 4.2. The proton conductivity of the composite membrane was higher than that of Nafion and the methanol permeability of the composite membrane was slightly lower than that of Nafion . Both tendencies are good for membrane for direct methanol fuel cell. In this way, three-dimensionally ordered macroporous materials are suitable for matrix of soft proton conductive polymer with higher proton conductivity. 

N. Miyake, J.S. Wainright and R.F. Savinell, Evaluation of a sol-gel derived Nafion/ silica hybrid membrane for polymer electrolyte membrane fuel cell applications. II. Methanol uptake and methanol permeability, J. Electrochem. Soc., 2001, 148, A905-A909. 

While Nafion , a perfluorinated polymer developed by DuPont, is the most commonly used proton conductive polymer electrolyte membrane it is an insufficient solution in a number of areas. It has high cationic transport (approximately 9.56 5/cm) [8] but also has high levels of methanol fuel crossover, slow anode kinetics and very high cost [12]. Fuel cell membrane performance can be estimated from the ratio of proton conductivity (a) to methanol permeability (P). The higher the value of a/P, the better the membrane performance would be [13]. Chitosan has been shown to have a much lower methanol permeability than Nafion [14], and as such, a great deal of attention focused on developing chitosan membranes with high levels of ionic conduction and low methanol permeability as delineated in Table 3.1. 

Modification Conductivity Type Measured Conductivity (S/cm) Methanol Permeability (cm2/s) Fuel Cell Membrane Efficiency... 

To further elucidate the nature of methanol permeability through ion exchange membranes it is helpful to consider the theoretical basis for the understanding of molecular permeation. The equilibrium condition for a compound which is distributed between two phases will have an identical chemical potential in the two phases ... 

The methanol permeability in perfluorosulfonate proton exchange membranes at elevated temperature has been also investigated by other electrochemical techniques. One technique involves using carbon supported Pt electrodes placed to both sides of the membrane to serve as concentration sensors. By adding methanol to one or both sides of the membrane, one can calculate the methanol permeability from the time responses of anodic peak currents on the two working electrodes. Experiments have been performed on a Nafion -117 membrane in 2.0 M H2SO4 at 60 and 70 C. 

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