Electrodes in Electrolytes

The following types of electrodes are considered in the following discussions of electrochemical systems  

Examples are the oxygen electrode, Equation (1), or the hydrogen electrode (i.e. the H+/H2 reaction), Equation (2). Electron transfer proceeds regularly with the reacting species adsorbed at an electrode surface. In many cases, electrocatalysis by the electrode metal plays an important role. Associated chemical reactions, such as protonation and dissociation, render the reaction mechanisms complex. This is true in particular for the oxygen electrode. 

An inert conductor, such as platinum, gold, or graphite, is immersed in an electrolyte containing a dissolved redox couple (e.g. Fe3+/Fe2+). Electron transfer proceeds between the electrode and the redox species  

At the equilibrium potential, a dynamic equilibrium state establishes, at which the rates of the forward and the back reactions are equal. 

Electrochemical cells may consist of two electrodes of the same type, but with different activities of the electroactive species in the electrolyte. Such systems will be discussed subsequently in the context of the Nemst equation. 

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Figure 52. Passivation of A1 substrate in LiBOB-based electrolytes Time-decaying current observed on an A1 electrode at various potentials containing 1.0 M LiBOB in EC/EMC. Inset the dependence of steady-state current density at t= 10 s) on applied potential as obtained on an A1 electrode in electrolytes based on various salts in the same mixed solvent. (Reproduced with permission from ref 155 (Eigure 1). Copyright 2002 The Electrochemical Society.)...

The Cd/CdS electrode was also prepared in solutions containing cadmium sulfate and sodium thiosulfate using the potentiodynamic method [183]. The electrochemical behavior of such electrodes in electrolytes containing sulfide ions was studied. 

Recently, the change in the interfacial free energy and surface stress has been determined for Au(lll) electrodes in electrolytes containing nonspecrfically adsorbed F anions. This behavior was compared to that of specifically adsorbed S04 anions [43]. It was found that the surface stress is more sensitive to the changes in electrode potential than the interfacial free energy. 

Carbon pitch is used for carbon electrodes in electrolytic reduction processes, such as aluminum reduction or the production of electro-steels in arc furnaces. Refractory pitch is used in the manufacture of refractory brick, usually burned magnesite or dolomite, the pores of which are filled with pitch by hot impregnation. Upon firing, the pitch in the brick is converted to carbon by carbonization. The remaining pitch coke within the refractory product retards penetration of molten metals and slags, thus prolonging the life of the brick furnace lining. Coke pitch is used in the production of foundry cores. 

Early work in our laboratory involved the examination of competitive photoanodic oxidation at a CdS electrode in electrolytes containing various reducing agents using the rotating ring-disk electrode (RRDE) technique.20 In the... 

FIGURE 18.19 Electrorefining of copper metal, (a) Alternating slabs of impure copper and pure copper serve as the electrodes in electrolytic cells for the refining of copper, (b) Copper is transferred through the CuS04 solution from the impure Cu anode to the pure Cu cathode. More easily oxidized impurities (Zn, Fe) remain in solution as cations, but noble metal impurities (Ag, Au, Pt) are not oxidized and collect as anode mud. 

With few exceptions the electrodes in electrolytic cells are arranged either vertically of horizontally. There is no exact rule as to which particular arrangement should be used as the position of the electrodes depends on the specific nature of the electrolytic process involved. Nevertheless, certain general rules may be laid down. 

A key feature of Li electrodes in electrolyte solutions, which is critical for their use as anodes in rechargeable batteries, is their cycling efficiency. In general, this term defines what percentage of Li deposited electrochemically remains electrically active and can thus be dissolved electrochemically. We can define Li cycling efficiency (LiCE.) per cycle as... 

Fig. 1. Apparatus for mapping of the potential across a corroding steel surface. A, monitoring instrumentation B, stationary reference electrode C, travelling reference electrode D, steel electrode in electrolyte.

Other methods of treating impure metals include magnetic separation and electrolytic refining, hi magnetic separation, the magnetic components of an ore are separated from the nonmagnetic residual material. In electrolytic refining, the metal is cast into plates that serve as electrodes in electrolytic tanks. The electric current causes the metal to dissolve, and the pure metal is deposited at the electrode of opposite polarity to the plates. 

For successful application of Faradaic electrochemical techniques, it is necessary to understand the fundamental processes that occur at the surface of the electrode in electrolyte solution. When a potential is applied between two electrodes in solution, a narrow interphase region, the electrical double-layer, develops at the surface of the electrodes. All oxidative or reductive electrochemical reactions between the electrode and the analyte occur in this interphase region between the electrode surface and bulk solution. The bulk solution will remain at electroneutrality because the potential drop between the electrodes only exists in the interphase region. Therefore, molecules in the bulk solution cannot feel the presence of the electrodes... 

The resistor R and capacitor C shown in Table 16.1 can take on different meanings for different electrochemical systems. The resistance may, for example, be associated with the charge-transfer resistance of an electrochemical reaction, with the resistance of an oxide or porous layer, or with the electronic resistance of a semiconductor. The capacitor C may be associated with the double layer for an electrode in electrolyte, with surface capacitance of a film, or with the space-charge region of a semiconductor. The resistor Re may be associated with the Ohmic resistance of the electrolyte or with the frequency-independent resistance of a solid. 

In what follows, heterogeneous transformations are understood as chemical or physical-chemical transformations that take place on some surfaces, for example, on interfaces or surfaces possessing catalytic properties. This wide understanding of the term heterogeneous transformation includes surface catalytic reactions, adsorption and desorption on solid and fluid surfaces, dissolving of crystals in fluid, electrochemical reactions on the surface of an electrode in electrolyte, sublimation and condensation, sedimentation of aerosols and colloids, etc. Chemical transformations taking place in the bulk of fluid will be called homogeneous transformations or volume chemical reactions. 

Let us consider only the cathode (negative electrode in electrolytic cells) at this time. What happens if we make the cathode slightly negative Electrons are forced into the cathode by the external battery. The negative charge attracts the positive ions (Mn, Sn ) in the solution. If the metal ions pick up these electrons, they will be reduced to the... 

Fletcher S 1993 Contribution to the theory of conducting-polymer electrode in electrolyte solutions J. Chem. Soc. Faraday Trans. 89 311-20 Kanatzidis M G 1990 Conductive Polymers Chem. Eng. News 68(49) 36-54 Bard A J and Faulkner L R 1980 Electrochemical Methods Fundamentals and Applications (New York Wiley)... 

In-situ Electrochemical AFM Study of Semiconductor Electrodes in Electrolyte Solutions... 

Summary. Atomically resolved surface structures were observed at the van der Waals face of InSe and the (100) face of p- and n-GaAs electrodes in electrolyte solutions imder potential control by electrochemical atomic force microscope (AFM). The surface structure change of these electrodes during electrochemical reactions was followed. 

Meniscus height Sediment layer height Spacing between parallel flat plates, charged flat plates, and flat electrodes in electrolytic cell Specific enthalpy... 

Voltammetry of Redox Solutes. We have studied the voltammetric response of the cuprate ceramic electrodes in electrolyte solutions... 

Uses Reduction of iron ore in blast furnaces as source of synthesis gas refractory furnace linings in electrorefining of aluminum and other high-temp, service electrodes in electrolytic reduction of AI2O3 to aluminum filter medium fuel electrothermal prod, of phosphorus, silicon carbide, and calcium carbide plastics additive providing wear reduction in PTFE, creep resist., chemical inertia and stability, thermal conductivity and stability... 

Electrolytes provide a new example of electrical conduction in which chemical changes (chemical reactions) are occurring at the two electrodes. In electrolytes, the electric charge carriers are not discrete electrons but positive and negative ions (cations and anions). 

Galvanic cell Arrangement for the conversion of chemical energy to electrical energy using electrodes in electrolytes. 

Braun F (1898) On evolution of light at some electrodes in electrolyte solutions. Ann Physik Chem WF 65 361... 

S. Fletcher [1992] An Electrical Model Circuit that Reproduces the Behaviour of Conducting Polymer Electrodes in Electrolyte Solutions, J. Electroanal. Chem. 337, 127-145. 

R. de Levie [1963] Porons Electrodes in Electrolyte Solntions, I, Electrochim. 

On Porons Electrodes in Electrolyte Solutions, n, Electrochim. Acta 9, 1231-1245. 

Figure 7 shows the cyclic voltammetry (CV) results for the catalyst electrodes in electrolytes of 1.0 M methanol -n 0.5 M sulfuric acid. In all of the curves, at the forward scan, the current began to rise and showed an anodic peak I at a potential of about 540mV. The curves for Pt/BCB and Pt/NCB began to rise at about 300 mV, which could be considered the onset potential, 100 mV lower than that of Pt/VCB. This implies that Pt/BCB and Pt/NCB show easier and faster electrochemical reactions due to the reduction of the overpotential in... 

Aurbach D et al (1996) A comparative study of synthetic graphite and Li electrodes in electrolyte solutions based on ethylene carbonate dimethyl carbonate mixtures. J Electrochem... 

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