Membrane alcohol permeability

Although membranes could be used for the entire alcohol purification process, a great many stages would be needed, and it is cheaper to use distillation for most of the separation. If a membrane selectively permeable to alcohol could be developed, it might be feasible to separate alcohol from dilute solutions entirely by membranes. 

For the 50/50 mixture, the flux of toluene is 65% of the flux with pure toluene, but the alcohol flux is only 18% of that for pure alcohol. The membrane absorbs several percent toluene, and toluene molecules in th< free volume of the polymer phase may be responsible for the reduction in alcohol permeability. 

The decrease of alcohol permeability and, consequently, of alcohol crossover, even if accompanied by a reduction of the proton conductivity, open the possibility to new strategies of MEAs preparation by choosing the optimal membrane thickness and alcohol concentration, among several parameters, in order to increase DAFC performance. Other beneficial effect of incorporating fillers in Nafion-based membranes, is the chance of increasing the operation temperature of the fuel cell, due to the retention of water, avoiding the dramatic drop of proton conductivity taking place in Nafion above 100 °C. 

In the search for PEMs with lower alcohol permeability than Nafion and other perfuorinated membranes, without degradation of the proton conductivity, a number of new polymeric membranes were synthetized and characterized, such as sulfonated polyimides, poly(arylene ether)s, polysulfones, poly(vinyl alcohol), polystyrenes, and acid-doped polybenzimidazoles. A comprehensive discussion of the properties of these alternative membranes is given in Chap. 6, along with those of Nafion and Nafion composites. 

This chapter aims to cover most of the membranes that have been characterized in relation to alcohol permeability (crossover) or have been tested in single DAFC. 

Fig. 6.2 Selectivity plot relative proton conductivity versus relative alcohol permeability, showing regions with membrane properties better, equal, and worse than Nafion...

Membrane conductivity and alcohol permeability depend on temperature and degree of hydration of the membrane. Therefore, it is useful to define the relative selectivity, as the ratio between the selectivity of the membrane to the selectivity of Nafion membrane measured to under similar experimental conditions. 

Thus, in a plot of the membrane relative electrical conductivity vs. relative alcohol permeability, the best candidates as membranes for DAFC are those lying in the upper left-hand comer, as shown in the plot of Fig. 6.2. 

As mentioned above, Naflon (DuPont) membranes are currently used for hydrogen feed PEM fuel cells due to their excellent chemical stability and high proton conductivity. It is also the most common PEM in DAEC, in spite that alcohol permeability leads to loss of efficiency. In this section we summarized all the properties of Naflon membranes that are relevant to its use in DAEC in order to them with those of alternative membranes. 

Thus, the uptake of alcohol by Naflon membranes is fundamental to assess their behavior in DAEC because alcohol permeability is proportional to the alcohol content and, consequently, a membrane with low alcohol solubility is preferred. 

The most common method to measure alcohol permeability through membranes is the diffusion cell method under non-stationary conditions. In this method the membrane separates two reservoirs the receptor reservoir containing pure water, and the donor reservoir containing the alcohol solution of known concentration. Usually the alcohol solution in the donor reservoir is refreshed during the experiment to maintain its concentration, Cj, constant along the time. The non-stationary alcohol concentration in the receptor reservoir, c, is followed as a function of time by in situ or ex situ sensors. By integrating Eq. 6.4 the time dependence of is given by... 

Other method used to determine alcohol permeability is the pervaporation method, where the membrane is embedded in a cell and one is continuously fed, by a pump, with a alcohol solution, and the other side is purged with a continuous flow of an inert gas with a fixed flow rate for carrying the permeate to a gas chromatograph [33, 34]. fii this case the state of solvation of the membrane on the gas side is not well defined. 

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. 

Shroti N, Barbora L, Verma A (2011) Neodymium triflate modified Nafion composite membrane for reduced alcohol permeability in direct alcohol fuel cell. Int J Hydrogen Energ 36 14907-14913... 

Membrane. Perfluorosulfonic acid (PFSA) is the most commonly used membrane material [4], PFSA membranes are relatively strong and stable in both oxidative and reductive environments, since the structure of PFSA is based on a PTFE backbone. The conductivity of a well-humidified PFSA membrane can be as high as 0.2s cm. As is well known, fuel cell operation at elevated temperatures can increase the rates of reaction, reduce problems related to catalyst poisoning, reduce the use of expensive catalysts, and minimize problems due to electrode flooding. Unfortunately, a PFSA membrane must be kept hydrated to retain its proton conductivity. Moreover, a PFSA membrane is alcohol permeable if it is used in DAFCs. Because of the disadvantages of PFSA membranes, many alternatives have been proposed [106]. Five categories of membranes are classified (1) perfluorinated, (2) partially fluorinated, (3) non-fluorinated, (4) non-fluorinated composite, and (5) others. 

First, we consider the experimental aspects of osmometry. The semiperme-able membrane is the basis for an osmotic pressure experiment and is probably its most troublesome feature in practice. The membrane material must display the required selectivity in permeability-passing solvent and retaining solute-but a membrane that works for one system may not work for another. A wide variety of materials have been used as membranes, with cellophane, poly (vinyl alcohol), polyurethanes, and various animal membranes as typical examples. The membrane must be thin enough for the solvent to pass at a reasonable rate, yet sturdy enough to withstand the pressure difference which can be... 

Reverse Osmosis. A reverse osmosis (RO) process has been developed to remove alcohol from distilled spirits without affecting the sensory properties (14). It consists of passing barrel-strength whiskey through a permeable membrane at high pressure, causing the alcohol to permeate the membrane and concentrating the flavor components in the retentate. 

An extraordinary way of stabilizing RUO2-coated CdS colloids for H2 generation was chosen by Fendler and co-workers The colloidal particles were generated in situ in surfactant vesicles of dioctadecyldimethylammonium chloride and dihexa-decyl phosphate. Thiophenol as a membrane permeable electron donor acted as a sacrificial additive. Later, a surface active re-usable electron donor (n-C,gH3,)2N — (CHj)—CH2—CHj—SH, Br was incorporated into the vesicles. Its R—SS—R oxidation product could be chemically reduced by NaBH to regenerate the active electron donor. The H2 yields in these systems were only 0.5 %. However, yields up to 10% were later reported for a system in which CdS was incorporated into a polymerizable styrene moiety, (n-C,jH3jC02(CH2)2) N (CH3) (CH2CgH4CH=CH2>, CP, and benzyl alcohol was used as the electron donor. 

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). 

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