Electrochemical cells with solid electrodes

Among reliable systems for high-temperature measurements, gaseous (first of all, oxygen) electrodes based on solid electrolytes of the yttria-stabilized zirconia (YSZ) type can be recommended, which are always applied in the electrochemical cells with solid electrolytes. Gaseous electrodes based on Pt or carbon are also widely used in high-temperature melts, especially... 

Easily presented in Fig. 1, electrode reactions are much more difficult (see also Solid Electrolytes Cells, Electrochemical Cells with Solid Electrolytes in Equilibrium). The mass of oxygen transported through gas-dense solid electrolyte membranes (OT02) can be exactly determined by the measurement of electrical current (I) following due to the Faraday s law ... 

The fist of publications [1-50] covers the period from 1958 to 1990, i.e., up to the very last years of the united Soviet Union. It includes only a small part of the publications and stiU reflects the wide variety of research on solid electrolytes at the IE US AS/IE UD AS and the journals publishing these results. There were many theoretical and experimental studies on electrochemical cells with solid electrolytes [1, 4, 5, 21, 39] and an extensive research on the phase composition of oxide ionic conductors [2] and their electric properties [3, 6, 8, 10, 13, 14, 17, 18, 20, 30, 34, 48]. Many papers were related to practical applications like sohd electrolyte degradation [33, 47] or application limits related to the electronic conductivity of the solid electrolytes [5, 30, 40, 43]. There were many publications on the implementation of different electrodes and on the kinetics of electrode processes [23,27, 31, 35, 36, 45], on investigations of the electrode overvoltage [7, 12, 25, 28], on impedance spectroscopy of solid electrolytes [19, 27], and on isotope exchange research [15,16]. The double layer and electrocapUlarity of solid electrolytes were studied in detail [9, 11, 19, 32, 44]. Systematic studies were performed on the thermo-EMF of different solid electrolytes [22,24,29], the EMF of electrochemical cells with solid electrolytes [26, 39], and the thermodynamics of oxygen in molten copper [41]. Applied research was focused on electrochemical oxygen pumps... 

This chapter is mainly focused on the reference electrodes used for the developments of SOFCs and other electrochemical cells with solid oxide electrolytes. 

V.A. Sobyanin, V.I. Sobolev, V.D. Belyaev, O.A. Mar ina, A.K. Demin, and A.S. Lipilin, On the origin of the Non-Faradaic electrochemical modification of catalytic activity (NEMCA) phenomena. Oxygen isotope exchange on Pt electrode in cell with solid oxide electrolyte, Catal. Lett. 18, 153-164 (1993). 

This section deals with the electrodes in the electrochemical set-up, with special emphasis on the silicon electrode and its semiconducting character. An electrochemical cell with its complete electrical connections, as shown in Fig. 1.3 a and b, is similar to the well-known four-point probe used for applying a defined bias to a solid-state device. The two tines that supply the current are connected to... 

The desire to realise technological goals has spurred the discovery of many new solid electrolytes and intercalation compounds based on crystalline and amorphous inorganic solids. In addition an entirely new class of ionic conductors has been discovered by P. V. Wright (1973) and M. B. Armand, J. M. Chabagno and M. Duclot (1978). These polymer electrolytes can be fabricated as soft films of only a few microns, and their flexibility permits interfaces with solid electrodes to be formed which remain intact when the cells are charged and discharged. This makes possible the development of all-solid-state electrochemical devices. 

Liquid-solid mass transfer has also been studied, on a limited basis. Application to systems with catalytic surfaces or electrodes would benefit from such studies. The theoretical equations have been proposed based on film-flow theory (32) and surface-renewal theory (39). Using an electrochemical cell with rotating screen disks, liquid-solid mass transfer was shown to increase with rotor speed and increased spacing between disks but to decrease with the addition of more disks (39). Water flow over naphthalene pellets provided 4-6 times higher volumetric mass transfer coefficients compared to gravity flow and similar superficial liquid velocities (17). 

Fuel cell researchers deal primarily with interfaces between solid electrolyte materials and solid metallic electrodes. The characterization of electrochemical systems with solid-solid interfaces has become a major issue in the study of fuel cells. It is generally believed that the interface of a solid electrode and solid electrolyte is similar to the electrode/liquid electrolyte interface but more complicated [4],... 

A substrate (E) in a solid electrochemical cell with an external electric circuit connected from the source side by a counter electrode (C) and from the sink side by a working electrode (W),... 

For convenience and simplicity, some assumptions are made. The solid electrolyte (E) is assumed to be an exclusive ionic conductor of mono-valent cation (A+). Two porous electroiuc conducting electrodes (C) and (W) are attached to the solid electrolyte (E) from the source and sink side, respectively. An external electric circuit with a dc source is coimected to the solid electrochemical cell via both electrodes. 

One of the first key steps in understanding the origin of electrochemical promotion was the realization that solid electrolyte cells with metal electrodes are both work function probes and work function controllers for the gas-exposed surfaces of their electrodes (Figure 18) ... 

Consider an electrochemical cell with a solid host MO2 as one electrode, an alkali metal A as the other electrode, and an electrolyte in which the monovalent cation A + is dissolved. The intercalation/deintercalation between the host MO2 and the guest ion A is given by Equation (5.1). On the other hand, the redox reaction between A and A may be written as... 

Hamamoto, K, Fujishrro, T. and Awano, M. (2008) Gas sensing property of the electrochemical cell with a multilayer catalytic electrode. Solid State Ionics, 179, 1648-51. 

The most important advantage of photoelectrochemical cells with semiconductor electrodes, as compared to, for example, solid-state semiconductor solar cells, is a relatively low sensitivity of their characteristics to the crystalline perfection of the semiconductor and the degree of its purification. Polycrystalline semiconductor electrodes in electrochemical solar cells exhibit both high absolute and high relative (as compared to single-crystal electrodes) conversion efficiency. This opens, at least in principle, the way of... 

M.V. PerfiTev and G.I. Fadeev, Determination of Limitedly Low Temperature of Reversible Work of Electrodesin Cells with Solid Oxide Electrodes, Coll. Inst. Electrochem. Ural Sci. Cent. Acad. Sci. USSR N25 (1977) 102-106. 

It is worth noting that the remarkable effect described for the carbon support porosity on the metal utilization factor and hence on the specific electrocat-alytic activity in methanol electrooxidation was only observed when the catalysts were incorporated in ME As and measured in a single cell. The measurements performed for thin catalytic layers in a conventional electrochemical cell with liquid electrolyte provided similar specific catalytic activities for Pt-Ru/C samples with similar metal dispersions but different BET surface areas of carbon supports [223]. The conclusions drawn from measurements performed in liquid electrolytes are thus not always directly transferable to PEM fuel cells, where catalytic particles are in contact with a solid electrolyte. Discrepancies between the measurements performed with liquid and solid electrolytes may arise from (1) different utilization factors (higher utilization factors are usually expected in the former case), (2) different solubilities and diffusion coefficients, and (3) different electrode structures. Thus, to access the influence of carbon support porosity... 

Figure 8. Reactor assembly and fiimace used with the single-pellet configuration. 1 glass tube volume SO-i 00 mL. 2 electrochemical cell. WE working electrode CE counter electrode RE reference electrode SE solid electrolyte. Reprinted from / Electroanal. Chem., G. F6ti, V. Stankovid, I. Boizonella, and Ch. Comninellis, Transient Behavior of Electrochemical Promotion of as-Phase Catalytic Reactions, (2002) in press, with permission from Elsevier Science.

Fig. 15.4. Plot of log Ph2 versus EMF of an electrochemical cell with a NASICON solid electrolyte and Pt electrodes. The slope of the line indicates near Nernstian behaviour (r=25 2°C).

As the result, the basic electrochemical relationships binding the rate of the electrochemical reaction with the electrode potential are insufficient for the complete description of the cell processes. Instead, rather sophisticated macro-kinetic models have been developed. On the other hand, all models require quantitative data on several kinetic parameters. In the simplest case, it is enough to know the rate constant of the charge transfer step, or exchange current (EC), and diffusion coefficients (DC) in solid phase and electrolyte solution, assuming that both EC and DC depend on concentration. 

In summarizing, one can say that the conversion efficiencies of electrochemical solar cells with semiconductor electrodes are very similar to those for solid state devices. Additional problems arise by the possibility that the electron transfer reactions at the interfaces can be slow. This disadvantage may however be compensated by the larger flexibility in the adjustment of the redox potentials of the electrolytes to the properties of the semiconductors and by the very simple formation of the heterojunction at which the unfavorable effects of interfacial electronic states are less pronounced. The most serious problem of such cells remains the photodecomposition which has to be overcome before such devices can reach practical importance. 

Reference Electrodes for Electrode Potential Measurements in Fuel Cells, Gas Electrolyzers, Gas Pumps, and Electrochemical Converters with Solid-Electrolyte Membranes... 

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