Of polycrystalline electrolytes

Nucleation does not occur simultaneously over the entire cathode surface, but it is a process extended in time so that crystals generated earlier may be considerably larger than those generated later. This causes periodicity in the surface stracture of polycrystalline electrolytic deposits, as well as coarseness of the obtained thin metal film even when they are formed on ideally smooth substrate. Hence, the larger the nucleation rate, the more homogeneous the crystal grain size distribution is obtained leading to formation of smoother deposit. Obviously, periodicity in the surface structure is a more complicated problem, as shown by Kovarskii et al. [60-62], but for the purpose of this analysis, the above conclusion is sufficient. 

Kovarskii NY, Lisov AV (1984) Periodicities in the surface structure of polycrystalline electrolytic deposits. Elektrokhimiya 20 221-225 (in Russian)... 

In any case, it is perceived from the above discussion that the problem of longterm chemical stability of polycrystalline semiconductor liquid junction solar cells is far from being solved. Still, as already pointed out in the early research, any practical photovoltaic and PEC device would have to be based on polycrystalline photoelectrodes. Novel approaches mostly involving specially designed PEC systems with alternative solid or gel electrolytes and, most importantly, hybrid/sensitized electrodes with properties dictated by nanophase structuring - to be discussed at the end of this chapter - promise new advances in the field. 

Zinc sulfide, with its wide band gap of 3.66 eV, has been considered as an excellent electroluminescent (EL) material. The electroluminescence of ZnS has been used as a probe for unraveling the energetics at the ZnS/electrolyte interface and for possible application to display devices. Fan and Bard [127] examined the effect of temperature on EL of Al-doped self-activated ZnS single crystals in a persulfate-butyronitrile solution, as well as the time-resolved photoluminescence (PL) of the compound. Further [128], they investigated the PL and EL from single-crystal Mn-doped ZnS (ZnS Mn) centered at 580 nm. The PL was quenched by surface modification with U-treated poly(vinylferrocene). The effect of pH and temperature on the EL of ZnS Mn in aqueous and butyronitrile solutions upon reduction of per-oxydisulfate ion was also studied. EL of polycrystalline chemical vapor deposited (CVD) ZnS doped with Al, Cu-Al, and Mn was also observed with peaks at 430, 475, and 565 nm, respectively. High EL efficiency, comparable to that of singlecrystal ZnS, was found for the doped CVD polycrystalline ZnS. In all cases, the EL efficiency was about 0.2-0.3%. 

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]. 

The catalytic activity and selectivity of polycrystalline silver catalysts used for ethylene2epoxidation can be affected significantly by electrochemical 0 pumping. This new phenomenon was studied in the solid electrolyte cell... 

CdSe CdSe was deposited on different substrates. The two-step method of the electrosynthesis of CdSe films, based on the initial chemical modification of polycrystalline gold surface with selenium overlayer was described [157]. In the second step, this overlayer was cathodically stripped as a Se in a Se(IV)-free electrolyte medium that was dosed with the requisite amounts of Cd(II) ions. 

Some attention has also been paid to the simultaneous adsorption of sulfate anions and organic compounds. Futamata [44] has detected coadsorption of water molecules and sulfate species with uracil on polycrystalline gold electrode, applying attenuated total reflection-infrared spectroscopy. The adsorbed sulfate species appeared either as S04 or HS04, depending on the pH of the electrolyte solution. Skoluda... 

Compounds of the I—VII group in the periodic table are known to exhibit good ionic conductivity and have attracted much attention as possible candidates for solid electrolytes. A typical family of compounds is Lil, CuCl, CuBr, and Agl. Historically, polycrystalline solid electrolytes were noticed to show significantly higher ionic conductivity than bulk crystals, since a half century ago. Furthermore, a large increase in conductivity was reported for the system of the mixture of a solid electrolyte such as CuCl (1) and Agl (2) with submicrometer particles of several sorts of insulating materials. In this case, the size of the metal halide itself was on the order of a micrometer or larger. It was also reported that the enhanced conductivity was approximately proportional to the inverse of the size of the electrolyte substances (2). Hence it is natural to make an effort to obtain fine particles of metal halides in order to get better conductivity. 

Sodium /3-aluminas (often known as simply /3-aluminas ) are examples of solid electrolytes, i,e. compounds which permit fast ionic motion (here of sodium ions) within a solid lattice, While /3-aluminas conduct reasonably well at room temperature ( 1 S/m for polycrystalline material), they are generally used at temperatures over 300°C where their conductance is greater than 10 S/m. ( Ambient solid electrolytes and batteries based on these will be considered in the next chapter.)... 

Na6Al32VAi051, respectively. This suggests that compared to //-alumina the fi" modification contains an aluminium vacancy compensated by three extra sodium ions in the conduction plane. The consequent higher conductivity of the ft" modification makes it favoured for battery electrolytes. The conductivity of polycrystalline //-alumina at 350 °C (the temperature appropriate to battery operation) is about 5Sm-1 and for polycrystalline //"-alumina about 50 8 m-1. 

The oxidation of propylene oxide on porous polycrystalline Ag films supported on stabilized zirconia was studied in a CSTR at temperatures between 240 and 400°C and atmospheric total pressure. The technique of solid electrolyte potentiometry (SEP) was used to monitor the chemical potential of oxygen adsorbed on the catalyst surface. The steady state kinetic and potentiometric results are consistent with a Langmuir-Hinshelwood mechanism. However over a wide range of temperature and gaseous composition both the reaction rate and the surface oxygen activity were found to exhibit self-sustained isothermal oscillations. The limit cycles can be understood assuming that adsorbed propylene oxide undergoes both oxidation to CO2 and H2O as well as conversion to an adsorbed polymeric residue. A dynamic model based on the above assumption explains qualitatively the experimental observations. 

The last decade has seen an explosion of activity in the field as electrochemists have wrestled with unfamiliar, and often intractable, problems generated by the very wide range of materials investigated, difficulties often compounded by the use of polycrystalline samples whose bulk and surface properties have proved resistant to control. In addition to the elemental semiconductors and the III/V materials, a huge range of n- and p-type oxides, sulphides, selenides, and tellurides have been described and surface and bulk modifications carried out in the hope of enhancing photoelectrochemical efficiency. New theoretical and experimental tools have developed apace and our fundamental understanding of the semiconductor-electrolyte interface has deepened substantially. 

With the conductivity of an aqueous electrolyte (e.g., IN KCl) serving as a reference, comparable conductivities can be achieved in solid electrolytes under certain conditions. Some of the best solid ionic conductors, commonly referred to as superionic conductors , have resistivities comparable to those of aqueous electrolytes at room temperature (e.g., RbAg4l5 and single crystal MgO-stabilized 6"-alumina). However, they are either in the form of single crystals, which is impractical for most applications, or composed of very expensive and relatively unstable materials. Resistivities comparable to those of aqueous electrolytes can be achieved in solid electrolytes at higher temperatures in both superionic conductors like 6"-alumina (i.e., 300°C) and normal ionic conductors such as stabilized zirconia (800-1000°C), stabilized cerium oxide (>800 C), and stabilized bismuth oxide (>600°C). Sodium ion conducting glasses are much less conductive than polycrystalline 8 -alumina. 

The data presented in Table 1 permit several conclusion to be drawn on the potential use of these materials as electrolytes in electrochemical devices with practical values for the area-specific resistance. Only the j0"-alumina and NASICON electrolytes possess sufficient conductivities for use as membranes with thicknesses of 1 mm or greater. The sodium ion conductivities of polycrystalline NASICON and /l"-alumina are comparable. The glassy electrolytes must be used in the form of thin films ( < 50 pm and possibly under 10-15 pm) or as capillary tubes with very thin walls (10-50 pm). 

---