Alkaline cationic polymer electrolyte

Alkaline-based polymer electrolyte membranes are relative newcomers on the scene. It has been widely believed that the quaternary ammonium hydroxide functional group (RN4 , OH ), which is the one used in most anion exchange membranes, is self-destructive, because the OH- ion is likely to attack the RN4 cation. In addition, the specific conductivity of an OH conducting ionomer was suspected of being at least a factor 3-A lower than that of the H" conducting... 

PEO is found to be an ideal solvent for alkali-metal, alkaline-earth metal, transition-metal, lanthanide, and rare-earth metal cations. Its solvating properties parallel those of water, since water and ethers have very similar donicites and polarizabilities. Unlike water, ethers are unable to solvate the anion, which consequently plays an important role in polyether polymer-electrolyte formation. 

Long-term stability and contamination is a problem with both alkaline and polymer membrane electrolysis electrolytes. Alkaline electrolytes adsorb carbon dioxide readily and form carbonates. Polymer membrane systems must use very pure de-ionized water or they will accumulate cations that displace protons and increase the cell resistance over time. 

A particular approach adopted by General Electric In U.S.A. is the solid polymer electrolyte (SPE) cell in which the porous cloth-type separator is replaced by a polymeric ion exchange membrane which is conductive to cations (Figure 5). The particular membrane employed, NAFION, is a perfluorsulphonlc acid pol3nner which is extremely stable in both acid alkaline solution. Appropriate electrocatalysts are coated on each face of the polymer sheet and these are contacted by a metal mesh current collector. Further research is aimed at reducing the cost and improving the electrical efficiency of the system to make it competitive with conventional electrolyzers. 

Varcoe JR, Slade RCT, Yee ELH, Poynton SD, Driscoll DJ, Apperley DC (2007) Poly(ethylene-co-tetrafluoroethylene)-derived radiation-grafted anion-exchange membrane with properties specifically tailored for application in metal-cation-free alkaline polymer electrolyte fuel cells. Chem Mater 19(10) 2686-2693. doi 10.1021/cm062407u... 

Direct methanol fuel cells are a class of polymer electrolyte membrane (PEM) fuel cells that typically employ a cation exchange membrane to separate the anode and cathode compartments. To illustrate the basic principles of DMFC operations, we shall take a typical, liquid-feed cell with a cation exchange membrane (alkaline exchange membranes are an alternative, and these are discussed later in this chapter). This is depicted in Figure 5.1. 

From the electrochromic mechanism described above, it can be concluded that the polymer electrolyte must be employed in the ion conducting layer. However, what type of polymer electrolyte is adopted depends on which ion is used in an electrochromic device. Theoretically, all cations can be used in electrochromic devices. Actually, only a few kinds of cations, such as H+, Li", Na", OH, F" and the like, are appropriate. As a result, when H" is used, the ion conducting layer is based on PFSA and so on. If LF is used, the ion conducting layer is similar to that used in a lithium ion battery. And for OH , it is an alkaline polymer electrolyte. 

Some vinyl fluoride-based polymers with side chains of perfluorosulfonic acid (the Nation family) are important ion-exchange membrane materials used in practice for electrolysis of NaCl and in certain fuel cells. They show a proton conductivity of 0.01 S cm- at room temperature. However, such fast ionic transport occurs only when they are swollen with water. It is therefore not appropriate to call them solid electrolytes in the tme sense of the word. It was in 1970 that anionic conductivity, though not high, was reported for crown ether complexes such as dibenzo-18-crown-6 KSCN, in which cations are trapped by the ligand. " A few years later much higher cationic (instead of anionic) conduction was found in complexes of a chain-like polyether such as PEO or PPO with alkaline salts here, PEO stands for poly(ethyleneoxide), (CHjCHj-O), and PPO for poly(propyleneoxide)."2>"3 These were the flrst examples of tme polymer solid electrolytes and were followed by a great number of studies. Polymeric electrolytes are advantageous in practice because they are easily processed and formed into flexible Aims. 

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