Nafion methanol permeability

Although generally the Nafion s proton conductivities are higher than those of the S-PEEKs, the Nafion methanol permeability increases with increasing temperature and the P factor decreases making the S-PEEKs more attractive than Nafion for DMFC applications. 

When these structures are compared, it can be observed that in wet systems, proton transport as well as fuel crossover in Nafion are high, and in the case of PAEK, these incidents are low. A comparative study was reported by Xue and Yin [2] on methanol permeability and proton conductivity of SPEEK with DS between 59% and 93% with Nafion. Methanol permeability strongly depends on the DS and increased in that range from 27 x 10" to 154 x 10" cm s". The lowest permeability was 6.5 times lower than for Nafion, whereas this factor for proton conductivity was just 2.4. These results were obtained at 22°C. The fuel crossover of PAEKs membranes is relatively low, but membrane modifications should lead to further decrease in fuel crossover while maintaining or improving the proton conductivity. Obviously, the stability features (e.g., swelling and mechanical) should be optimal to obtain good performance in fuel cell tests [1,2]. 

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). 

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].

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. 

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. 

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. 

To serve as a preliminary membrane screening method, the methanol permeability was measured for a number of Nafion -type samples by the... 

Composite membranes prepared by casting Nafion 15 on films made from PPE and phosphomolybdic acid show a lower methanol permeability in comparison to pure Nafion 15. The composite membranes have a potential application as electrol5d es in direct methanol fuel cells. 

It can be observed that near room temperature the permeability data spreads over almost one order of magnitude, as a reflect of the different membrane treatment. In the case of expanded Nafion membranes (pretreated by boiling in H2O2, water and acid) [188, 280, 296, 299] the permeabilities were lower than those of the membranes hydrated in water at 20-80 °C [284,289, 290, 293], with the exception of the data by Wu et al. [116, 288] that being expanded membrane show one of the highest methanol permeabilities. 

Paradoxically, the efforts to reduce the methanol permeabilities of Nalion with inorganic or organic fillers in most cases yield composite membranes with permeabilities similar to that obtained by optimizing the cast procedure of pure Nafion [302]. Nevertheless, the reduction of methanol permeability by itself is not a criterion for improving DMFC performance because it is usually associated to a reduction of the proton conductivity. We will analyze this property in Sect. 6.5.5 as a previous step to discuss the behavior of the proton-conducting membranes in terms of alcohol selectivity defined by Eq. 6.2. 

As discussed in Sect. 6.4.3 in the case of methanol permeability through Nafion composite membranes, proton conductivity along is not a criteria for an optimal DMFC membrane. Therefore, in the next Section, membrane selectivity to methanol and other alcohols will be critically reviewed. 

In Fig. 6.22 are plotted the relative conductivity vs. relative methanol permeability for Nafion/inorganic composites. The dashed lines indicate relative selectivity values of 1 and 10, as illustrative boundaries. 

It can be observed that most of the studied inorganic compounds exhibit and Pj close to unity, that is, the filler does not affect significantly the conductivity neither the methanol permeability as compare to the pure Nafion membrane. Few composites lie on the undesirable Cl, C2 and D region (fit < 1), and in the B1 octant, corresponding to membranes with higher conductivity and higher permeability than pure Nafion. 

Other Nafion-based membranes described in Sect. 6.3.4, like organicfinorganic ternary composites with Nafion, have high methanol relative selectivity, such as the Nafion/PEG/Si02 (fir = 10-20) [155] but DMFC test has not been reported for this composite. A trilayer membrane composed of one central methanol-barrier layer of PVdF and two Nafion layers was prepared [164]. Although the membranes could reduce methanol permeability between one and more than two orders of magnitude, depending on the layer thickness, proton conductivity is also reduced by a similar factor, resulting in a moderate fir value. 

Special attention deserves Nafion layered membrane prepared by the LBL self assembly of polyelectrolytes [25, 167-171]. A high selectivity membrane was prepared by Tang et al. [25] by self-assembling multi-layer Pd nanoparticles onto Nafion, using poly(diallyl dimethylammonium chloride) (PDAC) for charging the Pd particles. A Nafion 112 membrane was immersed in a Pd/PDDA dispersion and then in a Nafion dispersion. The process was repeated five times to obtain a multilayer self-assembly Nafion composite that shows a small decrease in conductivity (from 112 mS.cm for Nafion 112 to 81 mS.cm for the composite). However, the reported methanol permeability was reduced by a factor 0.0085 (out of scale in Fig. 6.22), leading to k 85. This composite membrane, whose strucmre is depicted in Fig. 6.24, was not tested in a DMFC. 

Woo et al. [452] synthesized sPI membranes for DMFC using BDSA, ODA and 3,3, 4,4 -benzophenonetetracarboxylic dianhydride (BTDA), with different sulfonation levels controlled by the BDSA/ODA molar ratio. The proton conductivity was moderate (1.7-41 mS.cm ), but the methanol permeability is much lower than Nafion 117. 

All the sPI composite membranes are located in flie A quadrant, that is have higher proton conductivities and lower methanol permeability than Nafion. The sPl composite with PTA [460] has a poor DMFC performance, but sPBl layered with PEGDMA reached MPD of 120 mW.cm at 60 °C [462]. The behavior of the sPI/Nafion composite is exceptional, exhibiting =79 and a MPD of 130 mW. cm at 70 °C with 5 M methanol. 

PEGDMA-SSA membranes exhibit proton conductivities similar to Nafion and methanol permeabilities slightly lower than Nafion, leading to relative selectivities in the range 1.5-2.0, while PDEGDMA-SSA membranes have <1 [482]. HNBR/... 

Park YS, Yamazaki Y (2005) Low methanol permeable and high proton-conducting Nafion/ calcium phosphate composite membrane for DMFC. Solid State Ion 176 1079-1089... 

Casciola M, Bagnasco G, Domiadio A, Micoli L, Pica M, Sganappa M, Turco M (2009) Conductivity and methanol permeability of Nafion-zirconium phosphate composite membranes containing high aspect ratio filler particles. Fuel Cells 9 394-400... 

Arbizzani C, Donadio A, Pica M, Sganappa M, Vatzi A, Casciola M, Mastragostino M (2010) Methanol permeability and performance of Nafion-zirconum phosphate composite membranes in active and passive direct methanol fuel cells. J Power Sources 195 7751-7756... 

Huang QM, Zhang QL, Huang HL, Li WS, Huang YJ, Luo JL (2008) Methanol permeability and proton conductivity of Nafion membranes modified electrochemically with polyaniline. J Power Sources 184 338-343... 

Wang CH, Chen CHC, Du HY, Chen CP, Hwang JY, Chen LC, Shih HC, Stejskal J, Chen KH (2009) Low methanol-permeable polyaniline/Nafion composite membrane for direct methanol fuel cells. J Power Sources 190 279-284... 

Tsai JC, Cheng HP, Kuo JF, Huang YH, Chen CY (2009) Blended Nafion/SPEEK direct methanol fuel cell membranes for reduced methanol permeability. J Power Sources 189 958-965... 

Xue S, Yin G (2006) Methanol permeability in sulfonated poly (etheretherketone) membranes A comparison with Nafion membranes. Eur Polym J 42 776-785... 

It has been demonstrated that the majority of methanol molecules are bound onto the backbone of sulfonated poly(phenylene ether ether sulfone). In contrast, almost all of the methanol molecules in Nafion are in a free state. Thus, a larger amount of bound methanol molecules and a stronger interaction between methanol molecules in sulfonated poly-(phenylene ether ether sulfone) are seen. Therefore, their low methanol permeability is observed [102]. 

The reason for the interest in these materials is that the membranes based on poly(ether ketone)s show a good chemical and mechanical stability, high proton conductivity, a reduced methanol permeability, and a lower cost with respect to a Nafion membrane [64,65]. 

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