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values of chitosan and Nafion 117 membrane were 1.49 x 104 and 1.71 x 104 Ss/cm3. The permeability of ethanol for Nafion was 5 times higher than other membranes.39 Meanwhile, the power density of a FC using a Nafion membrane was higher than that of other membranes.40 To improve the performance of hydrogen permeability and decreasing alcohol crossover, Sancho, et al was combining membrane Nafion- zeolite as a composite.23 Moreover, the main focus in DAFC is catalyst used to improve oxidation performance in anode side. Commonly used electrocatalyst are noble metal such as Pt, Pd, Au, which are expensive.20, 21 Consequently, some researchers are developing electrocatalyst prepared from metal alloy. Nowadays, catalyst Pd/MWCNT, PdSnNi/MWCNT (atomic ratio Pd:Sn:Ni ; 8:1:1) and Fe/C-PANI have been developed.41, 42 Method used to synthesis Pd/MWCNT and PdSnNi/ MWCNT catalyst is chemical reduction in glycol water mixture solvent.42, 43 Fe/CePANI is produced by direct pyrolysis, which uses Fe-doped polyaniline (PANI) precursors under N2 atmosphere. Yuan et alare also developing non-carbon-supported Pd-Ti nanoparticles as electrocatalyst in anode surface.44 This modification has aimed to get effective electrocatalyst supports because they combine high surface areas, good electronic conductivity, and strong metal–support interactions. Thus, the performance and life time of DAFC are more satisfied.45 Based on explanation above, there are three aspects to enhance DAFC performance such as anode material, catalyst, and membrane separator. Effect of modified membrane, zeolite membrane, catalyst in DAFCs performance will deeply expressed below. MEMBRANE IN DIRECT ALCOHOL FUEL CELLS The basic idea of FC is occurring two different reduction-oxidation reactions in both cathode and anode simultaneously. In a conventional FC using electrolyte, the reaction may occur in separate container for reduction and oxidation reactions. This limitation can be solved by applying a membrane to separate the cathode and anode. In a DAFC, the alcohol is oxidized at anode while the oxygen is reduced at cathode.46-48 Between the anode and cathode, a membrane exists (Fig. 3). The key component in FC is ion exchange membrane. Ion exchange membrane is a membrane with charge groups attached to a polymeric matrix. Based on the charge groups, ion exchange membrane can be classified as anion and cation exchange membrane. Furthermore, ion exchange membrane can also classified based on their structure, i.e. homogeneous and heterogeneous ion exchange membrane.49, 50 Ion exchange membrane has been widely used in various processes and applications, such as water treatment, wastewater treatment, production of high purity water, food processing, biotechnology, chemical synthesis, and energy conversion.51-61 The most desired properties of ion exchange membrane for those various applications are: high permselectivity, high conductivity, good mechanical, thermal, and chemical stability, and low production cost.54 Therefore, many efforts have been devoted to modify membrane in order to produce membrane with the desired characteristic. Modifications of ion exchange membrane have purposed to change surface properties such as surface charge, hydrophobic and hydrophilic balance, and roughness. All this time, Grebenyuk et al. have developed the AEM modification by molecular mass surfactants was successful to solve about organic fouling.62 Other than that, Japan researchers have been modified the ion exchange membrane surface by sulfonating agents to decrease fouling effect.63 Generally, commercial polymers such as Nafion, PVA (poly vinyl alcohol), SEBS (poly styrene-block poly (ethylene-ranbutylene)-block-polystyrene), CPP (chlorinated poly propylene), PEEK (poly ether ether ketone), PEI (poly ether imide), and PES (poly ether sulfone) are used as raw material to produce ion exchange membrane64 because their properties are complying of PEMFC properties. The basic function of a membrane in a DAFC is to retain the fuel in anodic area so the reaction can be occurred separately. Meanwhile, the reaction product, which could be the H+ or OH- must be able to travel through the membrane. Besides its selective-permeability, the membrane should also has good mechanical, thermal and chemical stability.65 In a real condition, the transport through membrane not only involves the ionic species, but also the alcohol itself. The alcohol crossover is a major hurdle in DAFC. The existence of alcohol in anode exposes itself to oxygen which initiates alcohol oxidation into carboxylic acid and/or combustion product. Both the acid and the intermediate products may reduce FC efficiency and poison the catalyst. Some strategies may be applied to overcome this problem including selection of the alcohol fuel, adjustment of temperature and membrane modification.47, 48, 65 Based on the ionic species passed through it, the membrane in DAFC can be categorized into two types: proton exchange membrane (PEM) and anion exchange membrane (AEM). The original DAFC works with PEM where the alcohol oxidized in anode releasing H+. The development of this type of membrane focuses on overcoming the 020030-4PDF Image | Zeolite modification for direct alcohol fuel cells DAFCs
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