Electrode-electrolyte interface, chemical

The presented examples clearly demonstrate tliat a combination of several different teclmiques is urgently recommended for a complete characterization of tire chemical composition and tire atomic stmcture of electrode surfaces and a reliable interiDretation of tire related results. Stmcture sensitive metliods should be combined witli spectroscopic and electrochemical teclmiques. Besides in situ techniques such as SXS, XAS and STM or AFM, ex situ vacuum teclmiques have proven tlieir significance for tlie investigation of tlie electrode/electrolyte interface. 

Traditionally, the chemical stability of the electrode/electrolyte interface and its electronic properties have not been given as much consideration as structural aspects of solid electrolytes, in spite of the fact that the proper operation of a battery often depends more on the interface than on the solid electrolyte. Because of the high ionic conductivity in the electrolyte and the high electronic conductivity in the electrode, the voltage falls completely within a very narrow region at the electrolyte/electrode interface. 

The electrochemical oxidation of monomers such as pyrrole,2-5 thiophene,6-9 aniline,10-13 etc., or their derivatives, initiates a polymerization process at the electrode/electrolyte interface that promotes the formation of a polymeric film that adheres to the electrode. A similar homogeneous polymerization process can be initiated by chemical oxidation or chemical polymerization.14-21 Some monomers can be polymerized as well by electrochemical or chemical reduction. 

The activation overpotential Tiac,w is due to slow charge transfer reactions at the electrode-electrolyte interface and is related to current via the Butler-Volmer equation (4.7). A slow chemical reaction (e.g. adsorption, desorption, spillover) preceding or following the charge-transfer step can also contribute to the development of activation overpotential. 

Electrode potentials (as well as values of the EMF of galvanic cells) depend on the composition of the electrolyte and other phases of variable composition. The electrode potential corresponds to the Galvani potential of the electrode-electrolyte interface, up to a constant term f =(Po + const. Introducing the concendation dependence of the chemical potential p into Eq. (3.21), we find that... 

Figure 2.4 FT-IRRAS spectrum of the electrode/electrolyte interface for CO adsorbed qn a smooth platinum electrode in 0.5 M H2SOA at 0.4 V vs. NHE. From W.G. Golden, K. Kunimatsu and H. Seki, J. Phys. Chem., 88 (1984) 1275. Copyright 1984 American Chemical Society.

Despite the fact that electron transfer reactions at the electrode/electrolyte interface are of fundamental importance to many chemical processes, a quantitative understanding of the factors that influence the rate of these reactions is still lacking. Although the general theoretical framework was established many years ago by Marcus, Levich, Dogonadze, and oth-... 

GITT also provides very comprehensive information about the kinetic parameters of the electrode by analysis of the electrical current. The current 1, which is driven through the galvanic cell by an external current or voltage source, determines the number of electroactive species added to (or taken away from) the electrode and discharged at the electrode/ electrolyte interface. A chemical diffusion process occurs within the electrode and the current corresponds to the motion of mobile ionic species within the electrode just inside the phase boundary with the electrolyte (at x = 0)... 

Beyond any doubt, the electrode/electrolyte interfaces constitute the foundations for the state-of-the-art lithium ion chemistry and naturally have become the most active research topic during the past decade. However, the characterization of the key attributes of the corresponding surface chemistries proved rather difficult, and significant controversy has been generated. The elusive nature of these interfaces is believed to arise from the sensitivity of the major chemical compounds that originated from the decomposition of electrolyte components. 

Electrochemical Reaction/Transport. Electrochemical reactions occur at the electrode/electrolyte interface when gas is brought to the electrode surface using a small pump. Gas diffuses through the electrode structure to the electrode/electrolyte interface, where it is electrochemically reacted. Some parasitic chemical reactions can also occur on the electrocatalytic surface between the reactant gas and air. To achieve maximum response and reproducibility, the chemical combination must be minimized and controlled by proper selection of catalyst sensor potential and cell configuration. For CO, water is required to complete the anodic reaction at the sensing electrode according to the following reaction ... 

Thus, surface tension changes have been related to changes in the absolute potential differences across an electrode/electrolyte interface and to changes in the chemical potential of all the species, i.e., to changes in solution composition. Only one other quantity is missing, the surface excess. This can be easily introduced by recalling the definition of surface excess [Eq. (6.66)], i.e.,... 

One of the present authors has investigated the importance of the nature of the electrode/electrolyte interface for the yields and selectivities of some anodic electrosynthesis reactions. A series of four successive reviews reports on the gathered information and improved understanding of the chemical kinetics of reactive intermediates generated at the interface carbon elec-trode/nonaqueous solvent (208-212) and citations of detailed investigations therein. 

Since electrode reactions are heterogeneous chemical processes with transference of charge across the interface, for a net reaction to take place at commensurable rate reactants and products must be transported to and from the electrode surface to sustain a net flux and thus an electrical current across the electrode—electrolyte interface. 

Chemical reactions take place in the gas channels (typically the anode), and, as explained in Chapter 1, electrochemical reactions take place in the triple-phaseboundary (TPB), i.e. a reaction zone very close to the electrode-electrolyte interface. 

Here, however, unlike binaries or multinaries, the emf is inherently unstable273 274 as the same processes allowing for an excess emf to be measured are also allowing for sintering and grain growth (electrochemical Ostwald ripening275 273). In addition, there has to be paid attention to the fact that the emf refers to the excess chemical potential of silver directly at the electrode/electrolyte interface.273... 

Electrochemical energy storage and conversion systems described in this chapter comprise batteries and fuel cells [6-11], In both systems, the energy-supplying processes occur at the phase boundary of the electrode-electrolyte interface moreover, the electron and ion transports are separate [6,8], Figures 8.1 and 8.2 schematically illustrate the electron and ion conductions in both the electrodes and the electrolyte in Daniel and fuel cells. The production of electrical energy by the conversion of chemical energy by means of an oxidation reaction at the anode and a reduction reaction at the cathode is also described. 

Surface-active substances — are electroactive or elec-troinactive substances capable to concentrate at the interfacial region between two phases. Surface-active substances accumulate at the electrode-electrolyte - interface due to -> adsorption on the electrode surface (see -> electrode surface area) or due to other sorts of chemical interactions with the electrode material (see - chemisorption) [i]. Surface-active substances capable to accumulate at the interface between two immiscible electrolyte solutions are frequently termed surfactants. Their surface activity derives from the amphiphilic structure (see amphiphilic compounds) of their molecules possessing hydrophilic and lipophilic moieties [ii]. 

When we begin to investigate an electrochemical system, we normally know little about the processes or mechanisms within the system. Electrochemical impedance spectroscopy (EIS) can be a powerful approach to help us establish a hypothesis using equivalent circuit models. A data-fitted equivalent circuit model will suggest valuable chemical processes or mechanisms for the electrochemical system being studied. From Chapter 1, we know that a fuel cell is actually an electrochemical system involving electrode/electrolyte interfaces, electrode reactions, as well as mass transfer processes. Therefore, EIS can also be a powerful tool to diagnose fuel cell properties and performance. 

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