Electrochemical systems, external

Electrochemical noise is the name given to spontaneous fluctuations of parameters in an electrochemical system. Difierent types of such noise are encountered (1) fluctuations of an electrode s potential at zero external current (2) fluctuations of electrode potential when the system is galvanostatically controlled (a current of constant density passes through the electrode), (3) fluctuations around a theoretical zero value of the current flowing between two perfectly identical electrodes, (4) fluctuations of the imposed current when the system is potentiostatically controlled, and (5) fluctuations of the potential difference AE between unlike electrodes, and similar phenomena. [Pg.626]

The potential that develops in an electrochemical system such as a fuel cell can also act to significantly influence the energies, kinetics, pathways, and reaction mechanisms. The double-reference potential DFT method [Cao et al., 2005] described earlier was used to follow the influence of an external surface potential on the reaction... [Pg.115]

In this chapter, we will give a general description of electrochemical interfaces representing thermodynamically closed systems constrained by the presence of a hnite voltage between electrode and electrolyte, which will then be taken as the basis for extending the ab initio atomistic thermodynamics approach [Kaxiras et ah, 1987 Scheffler and Dabrowski, 1988 Qian et al., 1988 Reuter and Scheffler, 2002] to electrochemical systems. This will enable us to qualitatively and quantitatively investigate and predict the structures and stabilities of full electrochemical systems or single electrode/electrolyte interfaces as a function of temperature, activi-ties/pressures, and external electrode potential. [Pg.131]

Fuel cells are electrochemical systems that convert the energy of a fuel directly into electric power. The design of a fuel cell is based on the key components an anode, to which the fuel is supplied a cathode, to which the oxidant is supplied and an electrolyte, which permits the flow of ions (but no electrons and reactants) from anode to cathode. The net chemical reaction is exactly the same as if the fuel was burned, but by spatially separating the reactants, the fuel cell intercepts the stream of electrons that spontaneously flow from the reducer (fuel) to the oxidant (oxygen) and diverts it for use in an external circuit. [Pg.298]

One electrochemical system that can be used to measure the surface tension of the mercuiy/solution interface is shown in Fig. 6.50. The essential parts are (1) a mercuiy/solution polarizable interface, (2) a nonpolarizable interface, (3) an external source of variable potential difference V, and (4) an arrangement to measure the surface tension of the mercuiy in contact with the solution.39... [Pg.131]

A qualitative understanding of the change of cell potential with current in the case of driven electrochemical systems (substance producers) can be developed on similar lines. Here, an external current source has to oppose the spontaneous current flow from the cell. That means that it has to promote a net electronation reaction at the electrode that would tend to run spontaneously as a sink and a net deelectronation at the electrode that would tend to be a source. [Pg.647]

The net electrical current measured in the external circuit is due to both oxidation and reduction processes taking part in the whole electrochemical system of two electrodes. In order to study electrcchemical reactions in a controlled way, the processes taking place at only one electrode need be considered and the experiment must be designed for that purpose, making negligible the electrokinetics at the counterelectrode. [Pg.7]

Electrochemical systems for the reduction of C02 require the application of an external bias or current to supply the electrons to reduce C02. Instead of a sacri-... [Pg.296]

More recently, the use of a pyridinium mediator in an aqueous p-GaP photo-electrochemical system illuminated with 365 nm and 465 nm light has been reported [125], In this case, a near-100% faradaic efficiency was obtained for methanol production at underpotentials of 300-500 mV from the thermodynamic C02/methanol couple. Moreover, quantum efficiencies of up to 44% were obtained. The most important point here, however, was that this was the first report of C02 reduction in a photoelectrochemical system that required no input of external electrical energy, with the reduction of C02 being effected solely by incident fight energy. [Pg.309]

The impedance for the study of materials and electrochemical processes is of major importance. In principle, each property or external parameter that has an influence on the electrical conductivity of an electrochemical system can be studied by measurement of the impedance. The measured data can provide information for a pure phase, such as electrical conductivity, dielectrical constant or mobility of equilibrium concentration of charge carriers. In addition, parameters related to properties of the interface of a system can be studied in this way heterogeneous electron-transfer constants between ion and electron conductors, or capacity of the electrical double layer. In particular, measurement of the impedance is useful in those systems that cannot be studied with DC methods, e.g. because of the presence of a poor conductive surface coating. [Pg.50]

The most convenient means of making time-resolved SH measurements on metallic surfaces is to use a cw laser as a continuous monitor of the surface during a transient event. Unfortunately, in the absence of optical enhancements, the signal levels are so low for most electrochemical systems that this route is unattractive. A more viable alternative is to use a cw mode-locked laser which offers the necessary high peak powers and the high repetition rate. The experimental time resolution is typically 12 nsec, which is the time between pulses. A Q-switched Nd YAG provides 30 to 100 msec resolution unless the repetition rate is externally controlled. The electrochemical experiments done to date have involved the application of a fast potential step with the surface response to this perturbation followed by SHG [54, 55,116, 117]. Since the optical technique is instantaneous in nature, one has the potential to obtain a clearer picture than that obtained by the current transient. The experiments have also been applied to multistep processes which are difficult to understand by simple current analysis [54, 117]. [Pg.157]

Equation (4) is thus a time-dependent boundary condition to Eqs. (6, 7), which, supplemented by the remaining boundary conditions (which also involve external constraints resulting from the operation mode of the experiment, s.b.) and possibly by the incorporation of convection, form the most basic Ansatz for modeling patterns of the reaction-transport type in electrochemical systems. However, so far, there are no studies on electrochemical pattern formation that are based on this generally applicable set of equations. Rather, one assumption was made throughout that proved to capture the essential features of pattern formation in electrochemistry and greatly simplifies the problem it is assumed that the potential distribution in the electrolyte can be calculated by Laplace s equation, i.e. Poisson s equation (6) becomes ... [Pg.97]

Usually, that is except under open circuit conditions, the working electrode is embedded in an electric circuit, which imposes a constraint on WE(f) and thus defines its value with respect to some potential scale (in our case Eq. (14b)). Moreover, WE will in general evolve in time and thus the external constraint directly influences the dynamics of the system. The two most important operation modes of electrochemical systems are the potentiostatic and the galvanostatic operation. [Pg.107]

Instead of attempting a general discussion of the three conditions characterizing a particular diffusion problem, it is best to treat a typical electrochemical diffusion problem. Consider that in an electrochemical system a constant current is switched on at a time arbitrarily designated t=0 (Fig. 4.17). The current is due to charge-transfer reactions at the electrode-solution interfaces, and these reactions consume a species. Since the concentration of this species at the interface falls below the bulk concentration, a concentration gradient for the species is set up and it diffuses toward the interface. Thus, the externally controlled current sets up a diffusion flux within the solution. [Pg.387]

It is a great advantage of electrochemical system that the other parameter that produces a shift in the density of states of the reactant is the potential applied externally to the interface. The driving force of interfacial reactions can be varied with the elec-... [Pg.47]

In Secs. 5-10 we present a series of selected examples of the use of the external reflectance technique to investigate some electrochemical systems of interest. Results from the electrochemical literature on the adsorption of hydrogen, carbon monoxide and alcohols are discussed and compared with the data from UHV measurements (Secs. 5-7). [Pg.145]

Whether a mineral undergoes oxidation or reduction is determined by the rest potential if there is only one pair of redox process occurring on a mineral surface (the rest potential is the potential an electrochemical system will naturally approach if no external voltage is applied). The redox behavior will be determined by the mixed potential if there... [Pg.46]

Faraday s law relates the charge transferred by ions in the electrolyte and electrons in the external circuit, to the moles of chemical species reacted (Newman and Thomas-Alvea, Electrochemical Systems, 3d ed., Wiley Interscience, 2004) ... [Pg.32]

The potential in standard conditions ( °) of other electrochemical pairs can be obtained with respect to Eq. 3.4, permitting the compilation of a list of semireaction potentials (electrochemical series ). In this list, all the semi-reactions are written in such a way to evaluate the tendency of the oxidized forms to accept electrons and become reduced forms (positive potentials correspond to spontaneous reductions) [2]. These potentials can be correlated to thermodynamic quantities if the electrochemical system behaves in a reversible way from a thermodynamic point of view, i.e., when the electrochemical system is connected against an external cell with the same potential, no chemical reaction occurs, while any inhnitesimal variation of the external potential either to produce or to absorb current is exactly inverted when the opposite variation is applied (reversible or equilibrium potentials, Eeq)- When the equilibrium of the semi-reaction considered is established rapidly, its potential against the reference can be experimentally determined. [Pg.73]

As mentioned in the introduction, the electrical nature of a majority of electrochemical oscillators turns out to be decisive for the occurrence of dynamic instahilities. Hence any description of dynamic behavior has to take into consideration all elements of the electric circuit. A useful starting point for investigating the dynamic behavior of electrochemical systems is the equivalent circuit of an electrochemical cell as reproduced in Fig. 1. The parallel connection between the capacitor and the faradaic impedance accounts for the two current pathways through the electrode/electrolyte interface the faradaic and the capacitive routes. The ohmic resistor in series with this interface circuit comprises the electrolyte resistance between working and reference electrodes and possible additional ohmic resistors in the external circuit. The voltage drops across the interface and the series resistance are kept constant, which is generally achieved by means of a potentiostat. [Pg.6]

Finally, before discussing oscillatory behavior, it is worth noting that a circuit equivalent to that shown in Fig. 1 also arises in semiconductor physics where a semiconductor device takes on the role of the faradaic impedance and the other elements of the circuit are electronic elements. Thus interesting parallels can be drawn between the dynamics of electrochemical and semiconductor systems. Furthermore, stability criteria derived for the latter can be directly applied to electrochemical systems. This is especially interesting for the interaction of S- or Z- shaped current-potential curves with the external circuit, which are not considered here owing to the presence of chemical instabilities. [Pg.11]

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