Alcohol permeability

Table VIII. Water and Alcohol Permeability of AN/S Copolymers...

Adhesion of end-sealing compounds Alcohol permeability of acrylonitrile-styrene copolymers. .. Alcoholic beverages stored in... 

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. 

Three approaches to reach those purpose have been reviewed and discussed in this Chapter the improvement of Nafion with fillers that reduce alcohol permeability, the synthesis of new polymers and blends with better alcohol selectivity, and the development of AEM with good conductivities for use in alkaline fuel cells where the alcohol crossover is not important. 

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. 

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