Electrode-electrolyte interface, kinetic theory

By tradition, electrochemistry has been considered a branch of physical chemistry devoted to macroscopic models and theories. We measure macroscopic currents, electrodic potentials, consumed charges, conductivities, admittance, etc. All of these take place on a macroscopic scale and are the result of multiple molecular, atomic, or ionic events taking place at the electrode/electrolyte interface. Great efforts are being made by electrochemists to show that in a century where the most brilliant star of physical chemistry has been quantum chemistry, electrodes can be studied at an atomic level and elemental electron transfers measured.1 The problem is that elemental electrochemical steps and their kinetics and structural consequences cannot be extrapolated to macroscopic and industrial events without including the structure of the surface electrode. 

Electrochemical impedance spectroscopy leads to information on surface states and representative circuits of electrode/electrolyte interfaces. Here, the measurement technique involves potential modulation and the detection of phase shifts with respect to the generated current. The driving force in a microwave measurement is the microwave power, which is proportional to E2 (E = electrical microwave field). Therefore, for a microwave impedance measurement, the microwave power P has to be modulated to observe a phase shift with respect to the flux, the transmitted or reflected microwave power APIP. Phase-sensitive microwave conductivity (impedance) measurements, again provided that a reliable theory is available for combining them with an electrochemical impedance measurement, should lead to information on the kinetics of surface states and defects and the polarizability of surface states, and may lead to more reliable information on real representative circuits of electrodes. We suspect that representative electrical circuits for electrode/electrolyte interfaces may become directly determinable by combining phase-sensitive electrical and microwave conductivity measurements. However, up to now, in this early stage of development of microwave electrochemistry, only comparatively simple measurements can be evaluated. 

Activation polarization arises from kinetics hindrances of the charge-transfer reaction taking place at the electrode/electrolyte interface. This type of kinetics is best understood using the absolute reaction rate theory or the transition state theory. In these treatments, the path followed by the reaction proceeds by a route involving an activated complex, where the rate-limiting step is the dissociation of the activated complex. The rate, current flow, i (/ = HA and lo = lolA, where A is the electrode surface area), of a charge-transfer-controlled battery reaction can be given by the Butler—Volmer equation as... 

Here we have sought to show how currently accessible simulation techniques can be brought to bear on qualitative and quantitative issues in the study of electrode-electrolyte interface. In our view, which we hope is exemplified by the cases described in detail, the firmest and most enlightening conclusions can be drawn when there are very tight links between theory, simulation, and experiment. One hears it said, for example, that there is no need for more theory in electrochemistry because we have the theory. But in the examples cited, we have seen that studies in which careful attention is paid to making simulations quantitatively realistic, qualitative conclusions can emerge which are not part of the currently accepted theoretical picture. This occurred in our studies of the fields near electrodes and also in our discovery of the importance of approach free energy in the kinetic barrier for the cuprous-cupric electron transfer. 

The activation polarization takes place from kinetics impediments of the charge-transfer reaction occurring at the electrode/electrolyte interface this form of kinetics is better understood applying the transition state theory. 

Although double layers are a general interfacial phenomena, we shall first consider only electrode-electrolyte interfaces because of their importance in electrode kinetics and because the theory leads on to a general treatment of all such phenomena. 

It can be expected from the nature of silicon/electrolyte interfaces described in the previous sections that the surface states on silicon electrodes may have different physical and chemical characteristics such as type, quantity, distribution, transfer kinetics, and so on, depending on the surface condition. Table 2.12 shows examples of measurements of surface states reported in the literature. Thus, while the energy levels in bulk silicon and electrolyte can be described by a general theory, those of surface states can only be dealt with by specific theories applicable to the specific situations. 

Cathodic protection (CP) is defined as the reduction or elimination of corrosion by making the metal a cathode by means of impressed current or sacrificial anode (usually magnesimn, aluminum, or zinc) [11]. This method uses cathodic polarization to control electrode kinetics occurring on the metal-electrolyte interface. The principle of cathodic protection can be explained by the Wagner-Traud mixed potential theory [12]. 

Fig. 9.1 Dr. Alexander Borisovich Ershler (1935-1989) with his group. From right Dr. chem. A.B. Ershler, Ph.D. phys. Eduard M. Podgaetskii, Engr. Tatyana S. Orekhova, Ph.D. chem. Ida M. Levinson another member of the group, postgraduate student Vladimir Kurmaz, was drafted at this time into the Soviet army. Areas of research adsorption theory of neutral organic compounds and its influence on kinetics of electrode reactions, electrode reactions of organomercury compounds, development of new electrochemical methods (high-speed pulse chronopotentiometry, electroreflection, and optical transitions at the metal-electrolyte interface, etc.). ELAN, 1975...

Electrode kinetics, to be considered in the next chapter, are profoundly influenced by the structure of the double layer at an electrode-solution interface and it is with such systems that we shall be primarily concerned. However, double layer theory, as developed for electrode-electrolyte solution interfaces, leads on to the proper interpretation of electrokinetic phenomena, an understanding of the factors affecting colloid stability, and to the elucidation of cell membrane and ion-exchange processes. 

The mercury/electrolyte interface played a major role in the early studies of the structure of metal/solution interfaces, and electrode kinetics in general. The surface of the liquid metal is highly reproducible and the low catalytic activity of Hg towards hydrogen evolution provided a rather wide range of potentials where the thermodynamic properties of the interface coidd be determined experimentally, allowing theories to be verified or discarded. However, mercury is of little industrial interest, and its use has been all but eliminated in recent decades because of its high toxicity and devastating influence on the environment. 

Abstract Recent advances in molecular modeling provide significant insight into electrolyte electrochemical and transport properties. The first part of the chapter discusses applications of quantum chemistry methods to determine electrolyte oxidative stability and oxidation-induced decomposition reactions. A link between the oxidation stability of model electrolyte clusters and the kinetics of oxidation reactions is established and compared with the results of linear sweep voltammetry measurements. The second part of the chapter focuses on applying molecular dynamics (MD) simulations and density functional theory to predict the structural and transport properties of liquid electrolytes and solid elecfiolyte interphase (SEI) model compounds the free energy profiles for Uthium desolvation from electrolytes and the behavior of electrolytes at charged electrodes and the electrolyte-SEl interface. 

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