Polymeric Ionic Conductors

2 Polymeric ionic conductors. One of the most unexpected developments in recent decades in the whole domain of electrochemistry has been the invention of and gradual improvements in ionically conducting polymeric membranes, to the 

Du Pont in America developed its own ionically conducting membrane, mainly for large-scale electrolysis of sodium chloride to manufacture chlorine, Nafion , (the US Navy also used it on board submarines to generate oxygen by electrolysis of water), while Dow Chemical, also in America, developed its own even more efficient version in the 1980s, while another version will be described below in connection with fuel cells. Meanwhile, Fenton et al. (1973) discovered the first of a 

---

On this basis, five classes of different polyphosphazenes are considered as outstanding examples of this type of macromolecules, in which skeletal and substituent features overlap to the highest extent. The reported materials are elastomers, flame retardants and self-extinguishing macromolecules, polymeric ionic conductors, biomaterials, and photosensitive polymeric compounds all of them based on the polyphosphazene structure. 

The versatility of water-soluble polyphosphazenes is in the variations in the structures that can be prepared. Structures with a low glass-transition temperature backbone can be modified with a variety of versatile side units. These may find use in solid polymeric ionic conductors, as a means to entrap and immobilize enzymes with retention of enzymic activity, and in biological functions as hydrogels with the capability of exhibiting biocompatibility and... 

Today, the term solid electrolyte or fast ionic conductor or, sometimes, superionic conductor is used to describe solid materials whose conductivity is wholly due to ionic displacement. Mixed conductors exhibit both ionic and electronic conductivity. Solid electrolytes range from hard, refractory materials, such as 8 mol% Y2C>3-stabilized Zr02(YSZ) or sodium fT-AbCb (NaAluOn), to soft proton-exchange polymeric membranes such as Du Pont s Nafion and include compounds that are stoichiometric (Agl), non-stoichiometric (sodium J3"-A12C>3) or doped (YSZ). The preparation, properties, and some applications of solid electrolytes have been discussed in a number of books2 5 and reviews.6,7 The main commercial application of solid electrolytes is in gas sensors.8,9 Another emerging application is in solid oxide fuel cells.4,5,1, n... 

Membranes from Polymeric Solid Ionic Conductors... 

Phosphoric-acid fuel cell (PAFC) — In PAFCs the -> electrolyte consists of concentrated phosphoric acid (85-100%) retained in a silicon carbide matrix while the -> porous electrodes contain a mixture of Pt electrocatalyst (or its alloys) (-> electrocatalysis) supported on -> carbon black and a polymeric binder forming an integral structure. A porous carbon paper substrate serves as a structural support for the electrocatalyst layer and as the current collector. The operating temperature is maintained between 150 to 220 °C. At lower temperatures, phosphoric acid tends to be a poor ionic conductor and poisoning of the electrocatalyst at the anode by CO becomes severe. 

The polymer films or membranes used as ionic conductors are often made of composite polymeric materials made up of inert and ionic polymers. In such a material the inert polymer ensures the necessary elasticity and mechanical stability of the membranes, whereas the ionic component is responsible for the electrochemical properties. 

It should be noted that, in addition to their use as electronic conductors, polymers can also function as ionic conductors. Materials such as poly(ethylene oxide) and certain oligoethyleneoxy-substituted polyphosphazenes and polysiloxanes, which conduct Li" ions, are used in this regard as polymeric electrolytes for battery applications (9]. 

However, as already pointed out, a widespread application of laminated ECDs calls for the use of polymer electrolytes having an appreciable conductivity at ambient temperature. A possible choice in this respect are sulphonic acid polymers, which are good ionic conductors, do not dissolve WO3 and can be conveniently fabricated in the thin-layer form. The most classical example is polymerized 2-methylpropane sulphonic acid, commonly called poly-AMPS . 

Chemistry and structural chemistry of important oligomeric and polymeric chalcogen compounds of main group elements novel sulphide ionic conductors development of boron-sulphur and boron-selenium... 

The last but not the least approach is addition of ceramic fillers. There is a variety of ceramic materials that offer relatively high ionic conductivities at room temperature (Goodenough et al. 1976 Hong 1976 Hooper 1977 Kafalas and Hong 1978 Sebastian and Gopalakrishnan 2003), however, applying them directly as electrolytes in commercial cells for mobile applications is not possible because they are brittle, difficult to process, and they cannot provide contact with the entire surface of porous electrodes made of powders as easily as polymers and liquids can do. Thus the first approach was using the polymeric electrolytes as conductive binders for the ceramic ionic conductors added to the polymer in a form of powder. Then inert materials were used. This will be described in detail in the next section. 

---