Solid-Polymer Electrolytes

A solid polymer electrolyte may be defined as electrically conducting solution of a salt in a polymer. Solid polymer electrolytes are popular and practical applications in portable telecommunication devices, computers, and hybrid electric vehicles and for the possibility of using them in compact, lightweight, high-energy-density rechargeable lithium batteries. 

The definition of a solid polymer electrolyte should not be confused with that of a polymeric electrolyte [whose macromolecules contains ionic or ionizable groups, or both). Polyelectrolytes can be either synthetic or natural [12,16-18, 20-23]. 

For example, a solid polymer electrolyte is a solution of a lithium salt in a PEO matrix the ionic conductivity of such material is due to the mobility of lithium cations and their anions in an electric field. The objective of the electrolyte system is to provide mechanical integrity and ion-conducting properties. PEO is a semicrystalline polymer at room temperature and has an exceptional property to dissolve with high concentration of a wide variety of dopants. 

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Fig. 11. Solid polymer electrolyte (SPE) fuel cell (a) cell design and (b) power curve at 25°C.

Ishikawa, M., Morita, M., lhara, M. and Matsuda, Y., Electric double layer capacitor composed of activated carbon fiber cloth electrodes and solid polymer electrolytes containing alkylammonium salt, J. Electrochem. Soc., 1994, 141(7), 1730 1734. 

A possible solution to this problem is to use an electrolyte, such as a solid polymer electrolyte, which is less reactive with lithium metal [3]. Another simple solution is the lithium-ion cell. 

The most promising fuel cell for transportation purposes was initially developed in the 1960s and is called the proton-exchange membrane fuel cell (PEMFC). Compared with the PAFC, it has much greater power density state-of-the-art PEMFC stacks can produce in excess of 1 kWA. It is also potentially less expensive and, because it uses a thin solid polymer electrolyte sheet, it has relatively few sealing and corrosion issues and no problems associated tvith electrolyte dilution by the product water. 

The dendritic growth of lithium was suppressed on a lithium electrode surface modified by an ultrathin solid polymer electrolyte prepared from 1,1—difluoro-ethane by plasma polymerization [114]. 

A membrane ionomer, in particular a polyelectrolyte with an inert backbone such as Nation . They require a plasticizer (typically water) to achieve good conductivity levels and are associated primarily, in their protonconducting form, with solid polymer-electrolyte fuel cells. 

The majority of electrochemical cells to have been constructed are based on PEO, PAN, or PVdF [101]. Recently, the Yuasa Corporation have commercialized solid polymer electrolyte batteries, primarily for use in devices such as smart cards, ID cards, etc. To date, the batteries which have been manufactured and marketed are primary lithium batteries based on a plasticized polymer electrolyte, but a similar secondary battery is expected [120]. 

J F. M. Gray, Solid Polymer Electrolytes, Fundamentals and Technological Applications, VCH, New York, 1991. 

D. Tsiplakides, S. Neophytides, O. Enea, M.M. Jaksic, and C.G. Vayenas, Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA) of Pt Black Electrodes Deposited on Nafion 117 Solid Polymer Electrolyte, /. Electrochem. Soc. 144(6), 2072-2088 (1997). 

The oxidation of H2 at room temperature on Pt black electrodes deposited on Nafion 117 was the first electrochemical promotion study utilizing a solid polymer electrolyte.35... 

The experimental setup is shown in Figure 9.23. The Pt-black catalyst film also served as the working electrode in a Nafion 117 solid polymer electrolyte cell. The Pt-covered side of the Nafion 117 membrane was exposed to the flowing H2-02 mixture and the other side was in contact with a 0.1 M KOH aqueous solution with an immersed Pt counterelectrode. The Pt catalyst-working electrode potential, Urhe (=Uwr)> was measured with respect to a reversible reference H2 electrode (RHE) via a Luggin capillary in contact with the Pt-free side of the Nafion membrane. 

To keep the consumption of the valuable platinum low, thin foils have been glued to a graphite support [34] or thin-layers of platinum have been sputtered on to a glass base [59]. Platinum can be used in a particle electrode by plating silica gel with platinum [60], or in a solid polymer electrolyte where platinum is incorporated into a nafion ion exchange membrane [61]. 

We have already referred to the Mo/Ru/S Chevrel phases and related catalysts which have long been under investigation for their oxygen reduction properties. Reeve et al. [19] evaluated the methanol tolerance, along with oxygen reduction activity, of a range of transition metal sulfide electrocatalysts, in a liquid-feed solid-polymer-electrolyte DMFC. The catalysts were prepared in high surface area by direct synthesis onto various surface-functionalized carbon blacks. The intrinsic... 

State-of-the-art thin film Li" cells comprise carbon-based anodes (non-graphitic or graphite), solid polymer electrolytes (such as those formed by solvent-free membranes, for example, polyethylene oxide, PEO, and a lithium salt like LiPFe or LiCFsSOs), and metal oxide based cathodes, in particular mixed or doped oxides... 

Furan was dimethoxylated to give 2,5-dihydro-2,5-dimethoxyfuran, using electrogenerated bromine molecules generated from bromide salts in electrolyte solutions [71]. This reaction was characterized in classical electrochemical reactors such as pump cells, packed bipolar cells and solid polymer electrolyte cells. In the last type of reactor, no bromide salt or electrolyte was used rather, the furan was oxidized directly at the anode. H owever, high consumption of the order of 5-9 kWh kg (at 8-20 V cell voltage) was needed to reach a current efficiency of 75%. 

Membrane-type fuel cells. The electrolyte is a polymeric ion-exchange membrane the working temperatures are 60 to 100°C. Such systems were first used in Gemini spaceships. These fuel cells subsequently saw a rather broad development and are known as (solid) polymer electrolyte or proton-exchange membrane fuel cells (PEMFCs). 

Fuel cells (hydrogen-oxygen, hydrogen-air, methanol-air) and industrial electrolysis (water, chlor-alkali) using ion-exchange membranes are the most demanding applications for the membranes. In these apphcations, the membranes have often been designated as SPE, which can be read as solid polymer electrolyte or solid... 

Incorporation into a Polymer Layer In recent years a new electrode type is investigated which represents a layer of conducting polymer (such as polyaniline) into which a metal catalyst is incorporated by chemical or electrochemical deposition. In some cases the specific catalytic activity of the platinum crystallites incorporated into the polymer layer was found to be higher than that of ordinary dispersed platinum, probably because of special structural features of the platinum crystallites produced within the polymer matrix. A variant of this approach is that of incorporating the disperse catalyst directly into the surface layer of a solid polymer electrolyte. 

Neergat N, Sbukla AK, Gandhi KS. 2001. Platinum-based alloys as oxygen-reduction catalysts for solid-polymer-electrolyte direct methanol fuel cells. J Appl Electrochem 31 373-378. 

Thamizhmani G, Capuano GA. 1994. Improved electrocatal)Tic oxygen reduction performance of platinum ternary alloy-oxide in solid-polymer-electrolyte fuel cells. J Electrochem Soc 141 968-975. 

Zecevic SK, Wainright JS, Litt MH, Gojkovic SL, Savinell RF. 1997. Kinetics of O2 reduction on a Pt electrode covered with a thin film of solid polymer electrolyte. J Electrochem Soc 144 2973-2982. 

Arico AS, Creti P, Antonucci PL, Antonucci V. 1998. Comparison of ethanol and methanol oxidation in a liquid-feed solid polymer electrolyte fuel cell at high temperature. Electrochem Sol Lett 1 66-68. 

Paulus UA, Veziridis V, Schnyder B, Kuhnke M, Scherer GG, Wokaun A. 2003. Fundamental investigation of catalyst utilization at the electrode/solid polymer electrolyte interface. Part I. Development of a model system. J Electroanal Chem 541 77-91. 

Stonehart P. 1994. The role of electrocatalysis in solid polymer electrolyte fuel cells. In Drake JAG, editor. Electrochemistry and Clean Energy. Cambridge The Royal Society of Chemistry. 

ELAT Solid Polymer Electrolyte Electrode 20% Pt/C with 0.4 mg/cm2 Pt loading E-Tek, Inc., 1 Mountain Rd, Framingham Industrial Park, Framingham, MA 01701 508-879-0733... 

Materials similar to Nation containing immobilized —COO- or —NR3+ groups on a perfluorinated skeleton were also synthesized. These are available in the form of solid membranes or solutions in organic solvents the former can readily be used as solid electrolytes in the so called solid polymer electrolyte (SPE) cells, the latter are suitable for preparing ion-exchange polymeric films on electrodes simply by evaporating the polymer solution in a suitable solvent. 

Skotheim et al. [286, 357, 362] have performed in situ electrochemistry and XPS measurements using a solid polymer electrolyte (based on poly (ethylene oxide) (PEO) [363]), which provides a large window of electrochemical stability and overcomes many of the problems associated with UHV electrochemistrty. The use of PEO as an electrolyte has also been investigated by Prosperi et al. [364] who found slow diffusion of the dopant at room temperature as would be expected, and Watanabe et al. have also produced polypyrrole/solid polymer electrolyte composites [365], The electrochemistry of chemically prepared polypyrrole powders has also been investigated using carbon paste electrodes [356, 366] with similar results to those found for electrochemically-prepared material. 

What can be learnt from XPS about electrochemical processes will be demonstrated and discussed in the main part of this chapter by means of specific examples. Thereby a survey of new XPS and UPS results on relevant electrode materials will be given. Those electrode materials, which have some potential for a technical application, are understood as practical and will be discussed with respect to the relevant electrochemical process. The choice of electrode materials discussed is of course limited. Emphasis will be put on those materials which are relevant for technical solid polymer electrolyte cells being developed in the author s laboratory. 

In the following chapter examples of XPS investigations of practical electrode materials will be presented. Most of these examples originate from research on advanced solid polymer electrolyte cells performed in the author s laboratory concerning the performance of Ru/Ir mixed oxide anode and cathode catalysts for 02 and H2 evolution. In addition the application of XPS investigations in other important fields of electrochemistry like metal underpotential deposition on Pt and oxide formation on noble metals will be discussed. 

For Cl2 or 02 evolution the stability of ruthenium based electrodes is not sufficient on a technical scale. Therefore the possibility of stabilizing the ruthenium oxide without losing too much of its outstanding catalytic performance was investigated by many groups. For the Cl2 process, mixed oxides with valve metals like Ti or Ta were found to exhibit enhanced stability (see Section 3.1), while in the case of the 02 evolution process in solid polymer electrolyte cells for H2 production a mixed Ru/Ir oxide proved to be the best candidate [68, 80]. 

Significant advances have been made in this decade in electrochemical H2 separation, mostly through the use of solid polymer electrolytes. Since the overpotentials for H2 reduction and oxidation are extremely low at properly constructed gas diffusion electrodes, very high current densities are achievable at low total polarization. Sedlak [13] plated thin layer of Pt directly on Nafion proton conductors 0.1-0.2cm in thickness, and obtained nearly 1200 mA/cm2 at less than 0.3 V. The... 

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