Electrochemical reactions definition

Corrosion is the deterioration of a substance or its properties because of a reaction with its environment. For our purposes, we can be a little more precise in this definition therefore, corrosion is a destructive attack of a metal by either chemical or electrochemical reaction with a given environment [183J. 

In a similar way, electrochemistry may provide an atomic level control over the deposit, using electric potential (rather than temperature) to restrict deposition of elements. A surface electrochemical reaction limited in this manner is merely underpotential deposition (UPD see Sect. 4.3 for a detailed discussion). In ECALE, thin films of chemical compounds are formed, an atomic layer at a time, by using UPD, in a cycle thus, the formation of a binary compound involves the oxidative UPD of one element and the reductive UPD of another. The potential for the former should be negative of that used for the latter in order for the deposit to remain stable while the other component elements are being deposited. Practically, this sequential deposition is implemented by using a dual bath system or a flow cell, so as to alternately expose an electrode surface to different electrolytes. When conditions are well defined, the electrolytic layers are prone to grow two dimensionally rather than three dimensionally. ECALE requires the definition of precise experimental conditions, such as potentials, reactants, concentration, pH, charge-time, which are strictly dependent on the particular compound one wants to form, and the substrate as well. The problems with this technique are that the electrode is required to be rinsed after each UPD deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems have been developed as an attempt to overcome these problems. 

Electrochemical reactions can be broken down into two groups outer-sphere electron-transfer reactions and inner-sphere electron transfer reactions. Outer-sphere reactions are reactions that only involve electron transfer. There is no adsorption and no breaking or forming of chemical bonds. Because of their simplicity, numerous studies have been performed, many entirely theoretical.18-25 By definition, though, electrode reactions are not outer-sphere reactions. However, if charge transfer is rate limiting for an electrode reaction, it typically takes a form similar to that of an outer-sphere reaction, which is described later in this section. 

Rate of Electrochemical Reaction in Terms of Current. In this part of the derivation we start with a definition of the rate of reaction and the definition of the electric current. The rate of the reduction reaction v, reaction (6.6) from left to right, is defined as the number of moles m of Ox reacting per second and per unit area of the electrode surface ... 

Charge transfer resistance, 1056 Charge transfer overpotential, 1231 Charge transfer, partial. 922. 954 Charges in solution, 882 chemical interactions, 830 Charging current. 1056 Charging time, 1120 Chemical catalysis, 1252 Chemical and electrochemical reactions, differences, 937 Chemical equilibrium, 1459 Chemical kinetics, 1122 Chemical potential, 937, 1058 definition, 830 determination, 832 of ideal gas, 936 interactions, 835 of organic adsorption. 975 and work function, 835... 

Clearly, techniques that provide definitive identification of intermediate or product species can be a valuable adjunct in the study of complicated electrochemical reaction sequences. Almost every imaginable analytical method has been used, and spectroscopic techniques have proven to be particularly valuable each particular method contributes a unique set of data for the experimentalist to interpret. Conversely, it should be recognized that electrochemistry has also aided spectroscopists by enabling them to prepare and study species that might otherwise be inaccessible. 

The pore/solid phase is further distinguished as transport and dead phase. The basic idea is that a pore phase unit cell surrounded by solid phase-only cells does not take part in species transport and hence in the electrochemical reaction and can, therefore, be treated as a dead pore and similarly for the electrolyte phase.25 The interface between the transport pore and the transport electrolyte phases is referred to as the electrochemically active area (ECA) and the ratio of ECA and the nominal CL cross-sectional area provides the ECA-ratio . It is be noted that in this chapter, ECA is normalized with the apparent electrode area and therefore differs from the definition in terms of the electrochemically active area per Pt loading reported elsewhere in the literature. 

Any surface (typically a piece of metal) on which an electrochemical reaction takes place will produce an electrochemical potential when in contact with an electrolyte (typically water containing dissolved ions). The unit of the electrochemical potential is volt (TV = 1JC1 s 1 in SI units).The metal, or strictly speaking the metal-electrolyte interface, is called an electrode and the electrochemical reaction taking place is called the electrode reaction. The electrochemical potential of a metal in a solution, or the electrode potential, cannot be determined absolutely. It is referred to as a potential relative to a fixed and known electrode potential set up by a reference electrode in the same electrolyte. In other words, an electrode potential is the potential of an electrode measured against a reference electrode. The standard hydrogen electrode (SHE) is universally adopted as the primary standard reference electrode with which all other electrodes are compared. By definition, the SHE potential is OV, i.e. the zero-point on the electrochemical potential scale. Electrode potentials may be more positive or more negative than the SHE. 

Spectroscopy is also extensively applied to determination of reaction mechanisms and transient intermediates in homogeneous systems (34-37) and at interfaces (38). Spectroscopic theory and methods are integral to the very definition of photochemical reactions, i.e. chemical reactions occurring via molecular excited states (39-42). Photochemical reactions are different in rate, product yield and distribution from thermally induced reactions, even in solution. Surface mediated photochemistry (43) represents a potential resource for the direction of reactions which is multifaceted and barely tapped. One such facet, that of solar-excited electrochemical reactions, has been extensively, but by no means, exhaustively studied under the rubric photoelectrochemistry (PEC) (44-48). 

The diffusion and chemical reaction rates depend only on the concentration of gaseous reactants at the working electrode surface, and by definition, are independent of electrode potential. When concentration overpotentiai dominates the total overpotentiai at the working electrode, a limiting current exists. This limiting current is the maximum current obtained when the electrochemical reaction is completely mass-transfer controlled. ... 

The physical meaning of i, which is called the exchange current density, should be clear from its definition in Eq. 34E. It represents the rate at which the electrochemical reaction proceeds back and forth at equilibrium when the net reaction rate, observed as a current flowing through the external circuit, is zero. It is similar to the exchange rate discussed earlier in connection with Eq. 5E. We also note that i... 

Although catalysis in electrochemical reactions was probably first specifically recognized by Frumkin at a conference in Leningrad in 1939, a first and perceptive definition of electrocatalysis seems to have been by Busing and Kauzmann in 1952 (72) in terms of the ability of various electrode surfaces to promote the velocity of the rate-determining step of the reaction. In this respect, their definition preceded the common use of this term in North America in the 1960s by some years, when it was applied to the activities of fuel-cell electrodes by Liebhafsky (7i). 

There are some recent reviews on the topic of organic electrochemical processes in industry [13,64-69]. Most of these reviews list several dozens of electrochemical reactions that are reputed to have reached commercial status, at least for a period of time. Many cases are not definitely confirmed some have been operated commercially for some years but are believed to be obsolete today (see Table 1). D. Degner has compiled the relevant patent literature for industrially important reactions that have been studied between the early 1970s and the late 1080s [70]. Table 1 presents many of these examples, but its message often is only These are electrochemical reactions that have or had the opportunity to achieve costs equal to those of alternatives. 

In the following, similarities and differences in the fundamental description of chemical and electrochemical reactions are considered with a definition of symbols and signs [51,52]. 

Other authors [1] define the theoretical capacity in a different way. They define it as the total quantity of electricity involved in the electrochemical reactions, and it is usually given in ampere hour per gram. According to their definition the capacity is given by... 

Finally water removal capacity of reactant streams, especially air stream, withdraws definitively a part of the water from the cell module. These quantities could become very significant at high temperature, exceeding the water produced by electrochemical reaction. This happens because evaporation rate and saturated vapor pressure of water increase with temperature in a non-linear relationship [1],... 

The over potential plays a central role in electrochemistry as it controls the electrochemical reactions. By convention it is generally measured as a positive value for reactions where electrons are transferred to the electrode. The associated current is also counted positively. In this case the electrode is called an anode. If electrons are transferred from, the electrode to the ions of the electrolyte, the over potential and the associated current are measured as negative values. The electrode is termed cathode. Using the definition of the over potential, the terminal voltage for an electrochemical cell is given by (see Fig. 3.2(b)) ... 

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