Polymer electrolytes based on natural polymers

Abstract The development of new materials that can be apphed as solid electrolytes has led to the creation of modern systems of energy generation and storage. Among different poly(ethylene oxide)-based electrolytes, natural polymers, snch as hydroxyethylcellnlose, hydroxypropylcellulose or carboxymethylcellulose, starch and chitosan or proteins like gelatin, are also proposed. After salt or acid addition, the transparent membranes show ionic conductivity values of 10 S/cm and can be apphed with good stability results to electrochromic devices. 

The present review shows the principal results of ionic condnctivity characterization of polymer electrolytes based on natnral polymers with emphasis on sohd state nnclear magnetic resonance (NMR) measurements. 

Key words polymer electrolytes, natural polymers, ionic condnctivity, RMN. 

Natural polymer-based plastics can be also obtained through plasticiza-tioni5.i6.2o,2i Qj. g]-afting22 processes. The polymer electrolytes obtained by the plasticization process with low molecular weight substances increase the values of the ionic conductivity of the formed blends. TVlthough the mechanism is not yet well known, there is an interaction of ions with mobile specific sites along the polymer chain. Also the fact that a plasticizer separates the polymer chain enhances the assumptions about their important role in the increase of the ionic conductivity values.  

2 Grafted natural polymer-based solid polymer electrolytes 

---

Polymer electrolytes based on natural polymer matrix... 

Rawlicka, A., D.C. Dragunski, K.V. Guimaraes, and C.O. Avellaneda. 2004. Electrochromic devices with solid electrolytes based on natural polymers. Mol Cryst Liq Crysf 416 105-112. 

Figure 3.8 displays the Arrhenius plots of the Li spin-lattice relaxation rates (T ) for three electrolytes based on natural polymers complexed with lithium perchlorate, aU measured at the Larmor frequency of 155 MHz. The... 

Ending this review on electrolytes based on natural polymers we can cite again Michel Armand, who 15 years ago said The research on polymer electrolytes is certainly not on a slowing course, with yet unsettled scientific problems like dissociation and transport numbers which still call for novel ideas, techniques and, surely, patient work . This remark is still valid, even with much more knowledge. 

Not long ago,this group first described microelectrochemical devices, which are based on microfabricated arrays of electrodes, connected by electroactive materials. Because the active components of these devices are chemical in nature, many of these devices are chemically sensitive,and comprise a potentially useful class of chemical sensors. Devices showing sensitivity to pH, 02r 2 f and Na" have been demonstrated. These devices are, typically, operated in fluid solution electrolytes. If this class of devices is to be useful as gas sensors, systems which are not dependent on liquid electrolytes need to be developed. We have recently reported solid state microelectrochemical transistors, which replace conventional liquid electrolytes with polymer electrolytes based on polyethyleneoxide (PEG) and polyvinylalcohol (PVA). In this report, we discuss additional progress toward solid-state devices by employing a new polymer ion conductor based on the polyphosphazene comb-polymer, MEEP (shown below). By taking advantage of polymer ion conductors we have developed microelectrochemical devices, where all of the components of the device are confined to a chip. 

Some experimental results obtained from NMR and EPR spectroscopies in a series of polymer electrolytes based on cellulose derivatives, chitosan, starch, agar and other natural polymers are now presented. 

Single-ion conductors can be obtained by the intercalation of PEO on clay due to the presence of cation charge at the silicate surface. The conductivity values of electrolytes based on POEM with the addition of 2 and 5 wt% clay were found to be around 4 x 10 S/cm at 70 °CF The conductivity obtained can be anisotropic. Molecular dynamic simulation has shown that the Li" ions are solvated preferentially by the silicate oxygen atom rather than PEO. The conductivity is too low for practical applications, even with a cationic transference number equal to one. In order to increase conductivity, but with a cationic transference number different from one, lithium salts were added to PEO/clay nanocomposites. At room temperature, the nanocomposite electrolyte exhibited higher ionic conductivity than unfilled polymer due to the larger content of the PEO amorphous phase. The improvement in conductivity depends on the nature of the clay. Fan et al. have shown that 250-Li-MMT, i.e. Li-MMT heated to 250°C, was more effective in enhancing the conductivity of (PE0)i6LiC104 than Org-MMT, dodecylamine modified Li-MMT, and Li-MMT, since 250-Li-MMT forms an exfoliated structure in the PEO matrix. 

---