Electrolyte polysulfide

Typical dimensions for the /5-alumina electrolyte tube are 380 mm long, with an outer diameter of 28 mm, and a wall thickness of 1.5 mm. A typical battery for automotive power might contain 980 of such cells (20 modules each of 49 cells) and have an open-circuit voltage of lOOV. Capacity exceeds. 50 kWh. The cells operate at an optimum temperature of 300-350°C (to ensure that the sodium polysulfides remain molten and that the /5-alumina solid electrolyte has an adequate Na" " ion conductivity). This means that the cells must be thermally insulated to reduce wasteful loss of heat atjd to maintain the electrodes molten even when not in operation. Such a system is about one-fifth of the weight of an equivalent lead-acid traction battery and has a similar life ( 1000 cycles). 

In the Na/S system the sulfur can react with sodium yielding various reaction products, i.e. sodium polysulfides with a composition ranging from Na2S to Na2S5. Because of the violent chemical reaction between sodium and sulfur, the two reactants have to be separated by a solid electrolyte which must be a sodium-ion conductor. / " -Alumina is used at present as the electrolyte material because of its high sodium-ion conductivity. 

The ammonium polysulfide, (NH4)2SX (with x=2 to 6) is produced in an electrochemical cell where aqueous ammonium sulfide, (NH4)2S, solution is supplied as electrolyte. The cell comprises an anode and a gas diffusion carbon cathode over which gaseous 02 is supplied in contact with the electrolyte.11 The cell operated continuously at pressures up to 60 bar. The applied potential, UWc> was 0.01 to 5 V. Pronounced electrochemical promotion behaviour was observed at Uwc values as low as 0.02 V with a current 1=0.5 A. 

Licht et al. [17] developed a method of numerical analysis to describe the above-quoted equilibria of the 11 participating species (including alkali metal cations) in aqueous polysulfide solution, upon simple input to the algorithm of the temperature and initial concentration of sulfur, alkali metal hydroxide, and alkali metal hydrosulfide in solution. The equilibria constants were evaluated by compensation of the polysulfide absorption spectrum for the effects of H8 absorption and by computer analysis of the resultant spectra. Results from these calculations were used to demonstrate that the electrolyte is unstable, and that gradual degradation of polysulfide-based PECs (in the long term) can be attributed to this factor (Chap. 5). 

Complex equilibria among multiple species are typical also of polyselenide and polytelluride solutions. Aqueous polyselenide solutions have been employed as electrolytes in high-elRciency PEC with photoanodes made of gallium arsenide and cadmium chalcogenides (Chap. 5). Unlike polysulfides, the fundamental equilibria constraining the distribution of polyselenide species have not been meticulously characterized [19]. 

Electrochemical preparation studies on low-dimensional structures of ternary or higher order compounds have appeared in the last few years. For example, whiskers of the quasi-ID copper(I) sulfide series KCuv-jcSa (0 < x < 0.34) were grown by employing electrochemical methods via anodic dissolution of copper electrodes, at 110 C in ethylenediamine solution of polysulfide K2Sn (n = 5, 6) electrolytes and, in some cases, CuCl [165]. 

The reaction occurring at the photoanode (CdS or CdSe) is the oxidation of sulfide or polysulfide, while at the cathode (Pt) some polysulfide species are reduced, so that the electrolyte undergoes no net chemical change. Here, the previous problem of poor stability in Fe(CN)g solutions could be minimized, at the expense of lowering the driving potential (and thus the conversion efficiency), by using fhe... 

After demonsfrafing fhe sfabilizafion of CdS- and CdSe-based PEC, using sulfide- or polysulfide-confaining elecfrolytes, Ellis el al. [51] proceeded lo show dial fhe photoanodic dissolution of single-crystal n-type CdTe, which was found to be unstable in a polysulfide electrolyte, could be completely quenched by adding Na2Te in the alkaline solution of NaOH. The photoelectrochemistry in their cell was considered to be consisting of the reactions... 

It has been illustrated that polycrystalline materials can be operated in regenerative electrolytic solar cells yielding substantial fractions of the respectable energy conversion efficiency obtained by using single crystals. Pressure-sintered electrodes of CdSe subsequently doped with Cd vapor have presented solar conversion efficiencies approaching 3/4 of those exhibited by single-crystal CdSe electrodes in alkaline polysulfide PEC [84]. 

An interesting idea has been to prepare the photosensitive electrode on site having the liquid play the dual role of a medium for anodic film growth on a metal electrode and a potential-determining redox electrolyte in the electrochemical solar cell. Such integration of the preparation process with PEC realization was demonstrated initially by Miller and Heller [86], who showed that photosensitive sulfide layers could be grown on bismuth and cadmium electrodes in solutions of sodium polysulfide and then used in situ as photoanodes driving the... 

Lyden et al. [92] used in situ electrical impedance measurements to investigate the role of disorder in polysulfide PEC with electrodeposited, polycrystalline CdSe photoanodes. Their results were consistent with disorder-dominated percolation conduction and independent of any CdS formed on the anode surface (as verified by measurements in sulfide-free electrolyte). The source of the observed frequency dispersion was located at the polycrystalline electrode/electrolyte interface. 

The photoelectrochemical behavior of ZnSe-coated CdSe thin Aims (both deposited by vacuum evaporation on Ti) in polysulflde solution has been described by Russak and Reichman [112] and was reported to be similar to MIS-type devices. Specifically, Auger depth profiling showed the ZnSe component of the (ZnSe)CdSe heterostructures to convert to ZnO after heat treatment in air, thus forming a (ZnO)CdSe structure, while the ZnO surface layer was further converted to a ZnS layer by cycling the electrode in polysulfide electrolyte. This electrochemically generated ZnS layer provided an enhanced open-circuit potential compared to CdSe alone. Efficiencies as high as 5.4% under simulated AM2 conditions were recorded for these electrodes. 

Russak MA, Reichman J (1982) Photoelectrochemical performance of ZnSe/CdSe thin film electrodes in aqueous polysulfide electrolyte. J Electrochem Soc 129 542-545... 

An alternative ambient temperature design based on sodium ion reaction refers to the domain of the so-calledbatteries. The polysulfide bromide cell (PSB) provides a reversible electrochemical reaction between two salt solution electrolytes (sodium bromide and sodium polysulfide), according to the scheme... 

Early tests [37] utilized a cell design similar to that of early MCFC experiments. The assembled cell, machined from graphite blocks, is shown as Fig. 24. The electrodes and current collectors were machined from graphite and dense carbon, respectively. The electrolyte was a mixture of 63% Na2S, 37% Li2S, believed to melt near 850 °C the melting point after several days of operation was below 700 °C, probably because of polysulfide formation. The electrolyte was immobilized in a matrix of MgO, the whole formed by hot-pressing a mixture of electrolyte and ceramic powders. 

The phase Na2Sx is sodium polysulfide, a material with a sulfur content of between 3 and 5. The anode reaction takes place at the liquid sodium - (3"-alununa interface. Here sodium atoms lose an electron and the Na+ ions formed enter the conduction planes in the electrolyte. The cathode reaction, which occurs at the interface between the (3"-alumina and the liquid sulfur forms sodium polysulfides. Despite the desirable properties of the cell, technical and economic considerations have acted so as to curtail large-scale commercial production. 

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