Liquid nonaqueous electrolytes electrodes

In comparison with aqueous electrolytes, liquid nonaqueous electrolytes offer larger liquid ranges, from below —150 °C [33] to above 300°C [34], voltage windows up to more than 5 V (see Section 17.4.1), a large range of acid-base properties, and often better solubility for many materials (electrolytes and nonelectrolytes), better compatibility with electrode materials, and increased chemical stability of the solution. Their drawbacks are lower conductivity, higher cost, flammability, and environmental problems. 

ITC) deposited on flexible, plastic substrates such as PMMA or polycarbonate was used as the conductive electrode substrate, with the active electrochromic electrodes producible as a roll which could be attached to window panes with common (e.g. cyanoacrylate) adhesives. This device again optionally used a counter electrode which was also electrochromic, with the difference that it could be not only a metal oxide such as WO3, but also, interestingly, an n-type CP, which of course displays electrochromism which is complementary to that of the more common p-type CPs. Thus, as cathode materials, the p-type CPs P(ANi) s, P(Py) s and poly(phenylene vinylene) were listed as usable, with virtually all the common dopants. As anode materials, WO3, M0O3, poly(isothianaphthene), and the -type CPs poly(alkoxy-thienylene vinylene) poly(p-phenylene), poly(phenyl quinoline) and poly(acetylene) were listed as usable. Liquid nonaqueous electrolytes based on common solvents such as DMSO and THF were used. No electrochromic data were however given in the patent or in subsequent publications. 

Water is involved in most of the photodecomposition reactions. Hence, nonaqueous electrolytes such as methanol, ethanol, N,N-d i methyl forma mide, acetonitrile, propylene carbonate, ethylene glycol, tetrahydrofuran, nitromethane, benzonitrile, and molten salts such as A1C13-butyl pyridium chloride are chosen. The efficiency of early cells prepared with nonaqueous solvents such as methanol and acetonitrile were low because of the high resistivity of the electrolyte, limited solubility of the redox species, and poor bulk and surface properties of the semiconductor. Recently, reasonably efficient and fairly stable cells have been prepared with nonaqueous electrolytes with a proper design of the electrolyte redox couple and by careful control of the material and surface properties [7], Results with single-crystal semiconductor electrodes can be obtained from table 2 in Ref. 15. Unfortunately, the efficiencies and stabilities achieved cannot justify the use of singlecrystal materials. Table 2 in Ref. 15 summarizes the results of liquid junction solar cells prepared with polycrystalline and thin-film semiconductors [15]. As can be seen the efficiencies are fair. Thin films provide several advantages over bulk materials. Despite these possibilities, the actual efficiencies of solid-state polycrystalline thin-film PV solar cells exceed those obtained with electrochemical PV cells [22,23]. 

Numerous other battery chemistries have evolved over time. The most prominent ones are assembled in Table 3.5.2. One possible categorization of battery technologies can be made according to the class of electrolyte they use. Here, we will distinguish between liquid aqueous, liquid nonaqueous, and solid electrolytes. To a certain degree, the phase state of the electrolyte determines the state of the electrodes. In general, it is advantageous to have a solid/liquid phase boundary between electrode and electrolyte because of much lower contact resistance in comparison to solid/solid contacts. Therefore, if the electrodes are solids, the electrolyte should be preferably liquid and vice versa. 

With minor modifications, the setup can also be used with a solid working electrode, or for nonaqueous electrolyte solutions. H-cells with solid plane parallel electrodes of the same area are frequently utilized for work in anhydrous media, also since they provide a uniform current distribution. A small distance between the electrodes, not only for this cell design, makes them suitable for work in media of low electrical conductivity. The cell design can be used for electrolysis in liquid ammonia, if a connection between the anode and cathode compartment above the solution level is ensured, to equilibrate the pressure in the system [iii]. 

There are two aspects to reference redox systems. One point is the possibility of compiling electrode potentials in a variety of solvents and solvent mixtures, which are not affected by unknown liquid junction potentials. Unfortunately very frequently aqueous reference electrodes are employed in electrochemical studies in nonaqueous electrolytes. Such data, however, include an unknown, irreproducible phase boundary potential. Electrode potentials of a redox couple measured in the same electrolyte together with the reference redox system constitute reproducible, thermodynamic data. In order to stop the proliferation of—in the view of the respective authors— better and better reference redox systems, the lUPAC recommended that either ferrocenium ion/ferrocene or bw(biphenyl)chromium(l)/te(biphenyl)chromium(0) be used as a reference redox system [5]. 

Furthermore, many investigations of nonaqueous electrolytes have even been performed with a saturated calomel electrode despite obvious problems such as contamination by water. In addition, unknown liquid junction potentials and insufficient knowledge of electrode reactions must be taken into account in addition to differing experimental conditions for the interpretation of such data. 

Film-forming chemical reactions and the chemical composition of the film formed on lithium in nonaqueous aprotic liquid electrolytes are reviewed by Dominey [7], SEI formation on carbon and graphite anodes in liquid electrolytes has been reviewed by Dahn et al. [8], In addition to the evolution of new systems, new techniques have recently been adapted to the study of the electrode surface and the chemical and physical properties of the SEI. The most important of these are X-ray photoelectron spectroscopy (XPS), SEM, X-ray diffraction (XRD), Raman spectroscopy, scanning tunneling microscopy (STM), energy-dispersive X-ray spectroscopy (EDS), FTIR, NMR, EPR, calorimetry, DSC, TGA, use of quartz-crystal microbalance (QCMB) and atomic force microscopy (AFM). 

Electrolytes are ubiquitous and indispensable in all electrochemical devices, and their basic function is independent of the much diversified chemistries and applications of these devices. In this sense, the role of electrolytes in electrolytic cells, capacitors, fuel cells, or batteries would remain the same to serve as the medium for the transfer of charges, which are in the form of ions, between a pair of electrodes. The vast majority of the electrolytes are electrolytic solution-types that consist of salts (also called electrolyte solutes ) dissolved in solvents, either water (aqueous) or organic molecules (nonaqueous), and are in a liquid state in the service-temperature range. [Although nonaqueous has been used overwhelmingly in the literature, aprotic would be a more precise term. Either anhydrous ammonia or ethanol qualifies as a nonaqueous solvent but is unstable with lithium because of the active protons. Nevertheless, this review will conform to the convention and use nonaqueous in place of aprotic .]... 

For most potentiometric measurements either the saturated calomel reference electrode or the silver/silver chloride reference electrode are used. These electrodes can be made compact, are easily produced, and provide reference potentials that do not vary more than a few millivolts. The discussion in Chapter 5 outlines their characteristics, preparation, and temperature coefficients. The silver/silver chloride electrode also finds application in nonaqueous titrations, although some solvents cause the silver chloride film to become soluble. Some have utilized reference electrodes in nonaqueous solvents that are based on zinc or silver couples. From our own experience, aqueous reference electrodes are as convenient for nonaqueous systems as are any of the prototypes that have been developed to date. When there is a need to rigorously exclude water, double-salt bridges (aqueous/nonaqueous) are a convenient solution. This is true even though they involve a liquid junction between the aqueous electrolyte system and the nonaqueous solvent system of the sample solution. The use of conventional reference electrodes does cause some difficulties if the electrolyte of the reference electrode is insoluble in the sample solution. Hence the use of a calomel electrode saturated with potassium chloride in conjunction with a sample solution that contains perchlorate ion can cause erratic measurements due to the precipitation of potassium perchlorate at the junction. Such difficulties normally can be eliminated by using a double junction that inserts another inert electrolyte solution between the reference electrode and the sample solution (e.g., a sodium chloride solution). 

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