Electrochemical systems, thermodynamics electrochemistry

R. Parsons, Thermodynamic methods for the study of interfacial regions in electrochemical systems, in Comprehensive Treatise of Electrochemistry. Vol. 1, J. O M. Bockris, B. E. Conway and E. Yeager, editors. Plenum Press, New York, 1980, pp. 1-44. 

Another well-established heterogeneous interface is that between the electrode surface and the electrolyte in electrochemistry, where there are regimes of various degrees of order, characterized by differing mass transport phenomena and involving different kinetic and thermodynamic requirements. Adsorption and surface phenomena are important and in general it has been recognized for some time that vibration of an electrochemical system can produce a variety of effects. 

As intensive studies on the ECPs have been carried out for almost 30 years, a vast knowledge of the methods of preparation and the physico-chemical properties of these materials has accumulated [5-17]. The electrochemistry ofthe ECPs has been systematically and repeatedly reviewed, covering many different and important topics such as electrosynthesis, the elucidation of mechanisms and kinetics of the doping processes in ECPs, the establishment and utilization of structure-property relationships, as well as a great variety of their applications as novel electrochemical systems, and so forth [18-23]. In this chapter, a classification is proposed for electroactive polymers and ion-insertion inorganic hosts, emphasizing the unique feature of ECPs as mixed electronic-ionic conductors. The analysis of thermodynamic and kinetic properties of ECP electrodes presented here is based on a combined consideration of the potential-dependent differential capacitance of the electrode, chemical diffusion coefficients, and the partial conductivities of related electronic and ionic charge carriers. 

By extension, the term electrochemistry stretches to include systems which have no controlled exchange of electrical energy with the exterior. The overall electric current is zero the electrochemical system Is at open circuit. The term combines two very different situations. The first applies to any system in thermodynamic equilibrium, that is in which no transformation of matter occurs. This is the case with many potentiometric sensors. The second situation covers systems that are likely to react spontaneously, namely with a transformation of matter and the internal exchange of electric energy, such as in corrosion. The concepts of electrochemistry are the suitable tools to describe such systems. 

Aimed at master s degree or PhD students as well as researchers and specialist engineers, this work focuses on electrochemical systems using electrolytes in solid phases (ionic crystals, ceramics, different types of glass and polymers). The fundamental concepts of electrochemistry are laid out (the thermodynamics of point defects and amorphous phases, transport mechanisms, mixed conduction, and gas electrode reactions) alongside the specific research methods used. Several applications are also described. 

This work presents electrochemistry from a macroscopic viewpoint, and is divided into 4 parts the thermodynamics of electrochemical cells, electrochemical kinetics, transport processes, and finally current distribution and mass transfer in electrochemical systems (including porous electrodes and semiconducting electrodes). Problems to solve are presented at the end of each chapter, without the answers. 

The first chapter focuses on the basic notions that need to be mastered before being able to go on and tackle the following chapters. The reader is reminded of the basic concepts, all defined in precise detail, as well as being introduced to certain experimental aspects. This chapter is therefore meant more or less for beginners in electrochemistry. The common electrochemical systems are described in the second chapter, which introduces the elementary laws so that they can be applied immediately by the reader. This chapter does not therefore provide any in-depth demonstrations. However, it is the last two chapters and the appendices that go into greater depth to tackle the key notions in a thorough and often original way. The third chapter focuses on aspects related to thermodynamic equilibrium, and the fourth chapter deals with electrochemical devices with a current flow, and which are therefore not in equilibrium. 

We started from the soUd thermodynamic background of equUihrium, then went through and finally came to unsteady field of nmi-linear dynamics of electrochemical systems. The results in this field are scarce apart from the above, only a few works can be mentioned related to aqueous electrochemistry. This wild land is still waiting for explorers. 

Thermodynamics has countless applications other than the expansion of gases to run steam engines. In fact, probably within your reach right now are a laptop computer, IVIP3 player, cell phone, and/or wristwatch, which represent one type of application. Metal-plated jewelry and silverware represent the other. These are a few of the objects you use every day that rely on a major field in applied thermodynamics— electrochemistry, the study of the relationship between chemical change and electrical work. We typically study this relationship with electrochemical cells, systems that incorporate a redox reaction to produce or utilize electrical energy. In this chapter, we examine the essential features of the two types of electrochemical cells as well as the quantitative relationship between free energy and electrical work. 

We close this section with a reminder of a fnndamental issue in electrochemistry Not all the quantities in Equations 13.8 throngh 13.13 are accessible to measurement by electrochemical or thermodynamic methods. Only the electrochemical potential ( i ), the work function (W ) or equivalently the real potential (a ) and the Volta potential ( / ) are. Equations 13.9, 13.11, and 13.13 are therefore formal resolutions. It is not possible to assign actual values to the separate terms, the chemical potential ( t ), the Galvani potential (cp ), nor the surface potential (x ), without making extrathermodynamic assumptions. These quantities must therefore be considered unphysical, at least from the point of view of thermodynamics. This statement, which is called the Gibbs-Guggenheim Principle in [42], is often met with disbelief from theoretical and computational chemists, particularly in the case of the chemical potential (Equation 13.10). The standard chemical potential is essentially the (absolute) solvation free energy AjG of species i. One would hope that a molecular simulation contains all information needed to compute AjG . Indeed, there seems to be a way around this thermodynamic verdict for computation and also mass spectroscopic. This continues to be, however, hazardous territory, particularly for DFT calculations in periodic systems. ... 

The main objective of this chapter is to introduce students to one of the most important subjects of the book, equilibrium electrochemistry, which is mainly based on equilibrium thermodynamics. Equilibrium electrochemistry is usually the first and required step in analyzing any electrochemical system. How to estimate the equilibrium potential of a half-reaction and the electric potential difference of an electrochemical cell are described in this chapter. One of the most fundamental equations of electrochemical science and engineering, the Nemst equation, is introduced and anployed for composing the potential-pH (Pourbaix) diagrams. Temperature dependence of the electrode potential and the cell potential difference is also described. 

Equilibrium electrochemistry allows us to calculate the standard values of open circuit potential (OCP) of a fuel cell and the decomposition potential (DP = -OCP) of an electrolytic cell if thermodynamic properties required for such calculations are available. The equilibrium electrochemical calculations should be done first before any other calculations or even experimental measurements to see any thermodynamic constrains of the electrochemical system. As an example. Figure 4.3 shows results of such calculations for three fuel cell reactions over a wide tanperature range from ambient up to 900°C. 

When electrochemical systems follow "Nemstian behavior", i.e. reversible thermodynamics and kinetics described by the Nemst equation of electrochemistry, CVs have well established characteristics for such properties as anodic/cathodic peak separation, peak half width, and scan rate dependence. 

Chemistry. There are many parts of mainline chemistry that originated in electrochemistry. The third law of thermodynamics grew out of observations on the temperature variations of the potential of electrochemical reactions occurring in cells. The concepts of pH and dissociation constant were formerly studied as part of the electrochemistry of solutions. Ionic reaction kinetics in solution is expressed in terms of the electrochemical theory developed to explain the activity of ions in solution. Electrolysis, metal deposition, syntheses at electrodes, plus half of the modem methods of analysis in solution depend on electrochemical phenomena. Many biomolecules in living systems exist in the colloidal state, and the stability of colloids is dependent on the electrochemistry at their contact with the surrounding solution. 

In all of these systems, certain aspects of the reactions can be uniquely related to the properties of a surface. Surface properties may include those representative of the bulk material, ones unique to the interface because of the abrupt change in density of the material, or properties arising from the two-dimensional nature of the surface. In this article, the structural, thermodynamic, electrical, optical, and dynamic properties of solid surfaces are discussed in instances where properties are different from those of the bulk material. Predominantly, this discussion focuses on metal surfaces and their interaction with gas-phase atoms and molecules. The majority of fundamental knowledge of molecular-level surface properties has been derived from such low surface area systems. The solid-gas interface of high surface area materials has received much attention in the context of separation science, however, will not be discussed in detail here. The solid-liquid interface has primarily been treated from an electrochemical perspective and is discussed elsewhere see Electrochemistry Applications in Inorganic Chemistry). The surface properties of liquids (liquid-gas interface) are largely unexplored on the molecular level experimental techniques for their study have begun only recently to be developed. The information presented here is a summary of concepts a more complete description can be found in one of several texts which discuss surface properties in more detail. ... 

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