Nafion composite membranes methanol selectivity

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. 

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. 

In Sect. 6.1 we defined the alcohol selectivity coefficient, / , of a membrane Eq. 6.2, which accounts for the ratio between the proton and alcohol transport through it, and the relative selectivity, / r, Eq. 6.3 which compare the alcohol selectivity of the membrane to Nafion under the same experimental conditions. Here we will analyze the abundant data on the methanol selectivity in inorganic and organic Nafion composites in comparison with the selectivity of pure Nation membranes. In order to perform this analysis we will resort to the general selectivity plot described in Fig. 6.2. 

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. 

Watanabe and coworkers [5, 454, 455] studied the DMFC performance of sPI and crosslinked sPI membranes obtained by polycondensation of NTDA, bis (3-sulfo propoxy)benzidine (BSPB), and alkaline diamine [5]. Other studies on polyimide membranes focused on methanol selectivity [456], DMFC performance [457], and the effect of crosslinking on the methanol selectivity [458]. Hybrid sPI membranes were prepared, including composites with PAMPS [459], PTA [460], Nafion infiltrated sPI membranes [461], sPI membranes coated with crosslinkable poly(ethylene glycol) dimethylacrilate (PEGDMA) [462], and mesoporous silica [463, 464],... 

Composite membranes with a poly(benzimida-zole) content of 2.5-7.5% show a better selectivity than Nafion 117 taking into consideration the methanol swelling ratio and the proton conductivity comprehensively. 

Supercritical CO2 activation of a Naflon membrane prior to zeolite deposition was used to modify its structure. The resultant Nafion-zeolite composite membranes showed a dramatic decrease in methanol permeability (if the colloidal rather than the suspended Fe-silicalite-1 particles were used for the deposition) and a 19-fold higher selectivity compared with either the composite membranes prepared without previous supercritical treatment or the pure commercial Nafion-115 membrane. The method of the in situ synthesis of a zeolite inside the membrane pores was found to be very effective for preparing the composites, giving a sixfold higher selectivity for the composite manbrane compared with the pure Nation membranes (Gribov et al. 2007). 

Tricoli and Nanetti [17] prepared a novel zeolite-Naflon composite manbrane by embedding zeolite fillers in Nafion. The zeolites used in this study were chabazite and clinoptilolite. The presence of zeolites in the membranes caused notable changes in conductivity, methanol permeability, and selectivity with respect to pure Nafion. In another interesting study, Holmberg et al. synthesized and characterized zeolite-Y nanocrystals for Nafion-zeolite-Y composite proton exchange membranes. The composite membranes were found to be more hydrophilic and proton conductive than the base-unmodified membranes at high temperatures [18]. 

Such reaction will generate more delocalized protons and increase proton s mobility. Doping with phosphosilicate gels brings about an increase in not only membrane conductivity but also the selectivity parameter, namely, the ratio of conductivity to methanol permeability. Compared with Nafion 112, the composite membrane has lower methanol permeability and comparable conductivity, but appreciably higher selectivity [423]. 

---