Photoanode

PTh 333 - 335) evidently materials which are suitable for such applications and which stabilize the photoanodes considerably. 

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]. 

In their pioneering work on the formation of photoelectrochemically active metal sulfides by oxidation of the parent metal electrode. Miller and Heller [29] reported the anodic formation of polycrystalline Bi2S3 on a bismuth metal electrode in a sodium polysulfide cell, wherein this electrode was used in situ as photoanode. When a Bi metal electrode is anodized in aqueous sulfide solutions a surface film is formed by the reaction... 

Actually, G. C. Barker was the first to suggest the idea of stabrhzing the n-CdS photoanode by a sulfide solution - see discussion in Gerischer [44]. 

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... 

Added stability in PEC can be attained through the use of non-aqueous solvents. Noufi et al. [68] systematically evaluated various non-aqueous ferro-ferricyanide electrolytes (DMF, acetonitrile, PC, alcohols) for use in stabilizing n-CdSe photoanodes. Selection of the solvent was discussed in terms of inherent stability provided, the rate of the redox reaction, the tendency toward specific adsorption of the redox species, and the formal potential of the redox couple with respect to the flat band potential (attainable open-circuit voltage). On the basis of these data, the methanol/Fe(CN)6 system (transparent below 2.6 eV) was chosen as providing complete stabilization of CdSe. Results were presented for cells of the type... 

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... 

Fig. 5.4 Power characteristics of CdS/sodium polysulfide solar cells. Solution composition is 0.1 M NaiS + 0.01 M S+ 0.01 M NaOH. (i) Single-crystal photoanode ...

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. 

An early discovery about n-CuInSe2 and n-CuInS2 semiconductor crystals was that they are extremely stable when used as photoanodes in aqueous polysulfide... 

Noufi R, Tench D (1980) EUgh-efficiency GaAs photoanodes. J Electrochem Soc 127 188-190... 

Nakatani K, Matsudaiva S, Tsubomura H (1978) Photoanodic behavior of n-type cadmium sulfide in acetonitrile solutions containing iodide ion. J Electrochem Soc 125 406 09... 

Gronet CM, Lewis NS (1983) n-type GaAs photoanodes in acetonitrile Design of a 10.0% efficient photoelectrode. Appl Phys Lett 43 115-117... 

Bocarsly AB, Walton EG, Wrighton MS (1980) Use of chemically derivatized n-type sU-icon photoelectrodes in aqueous media. Photooxidation of iodide, hexacyanoiron(II), and hexaammineruthenium(II) at ferrocene-derivatized photoanodes. J Am Chem Soc 102 ... 

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