Thermodynamics electrochemical cells

This observation led the pioneers of electrochemical thermodynamics to construct a series of cells, each with the H+ H2 couple as one half-cell. The emf of each was measured. Unfortunately, there were always more couples than measurements, so they could never determine values for either Cu +Cu or h+,h2 (nor, indeed, for any electrode potential), so they commented on their relative magnitudes, and compiled a form of ranking order. 

Electrochemical energy conversion devices are pervasive in our daily lives. Batteries, fuel cells and supercapacitors belong to the same family of energy conversion devices. They are all based on the fundamentals of electrochemical thermodynamics and kinetics. All three are needed to service the wide energy requirements of various devices and systems. Neither... 

Tower, Stephen. All About Electrochemistry. Available online. URL http //www.cheml.com/acad/webtext/elchem/. Accessed May 28, 2009. Part of a virtual chemistry textbook, this excellent resource explains the basics of electrochemistry, which is important in understanding how fuel cells work. Discussions include galvanic cells and electrodes, cell potentials and thermodynamics, the Nernst equation and its applications, batteries and fuel cells, electrochemical corrosion, and electrolytic cells and electrolysis. 

The sensing mechanism was studied in detail by Hotzel and Weppner from a thermodynamic point of view. Under open circuit conditions, the equilibrium half cell electrochemical reaction at the reference electrode is... 

The potential profiles in this PEVD system are illustrated in Figure 17. Although there is no driving force due to a difference in the chemical potential of sodium in the current PEVD system, the applied dc potential provides the thermodynamic driving force for the overall cell reaction (62). Consequently, electrical energy is transferred in this particular PEVD system to move Na COj from the anode to the cathode of the solid electrochemical cell by two half-cell electrochemical reactions. In short, this PEVD process can be used to deposit Na CO at the working electrode of a potentiometric CO sensor. 

This chapter explains the fundamental principles and applications of electrochemical cells, the thermodynamics OF electrochemical reactions, and the cause and prevention of corrosion BY ELECTROCHEMICAL MEANS. SOME SIMPLE ELECTROLYTIC processes and the QUANTITATIVE ASPECTS OF ELECTROLYSIS ARE ALSO discussed. 

Our interest in this chapter is with the mass and energy balances for chemical reactors, and in electrochemical cells. We consider first the mass and energy balances for tank and tubular reactors, and then for a general black-box chemical reactor, since these balance equations are an important application of the thermodynamic equations for reacting mixtures and the starting point for practical reactor design and analysis. Finally, we consider equilibrium and the energy balance for electrochemical systems such as batteries and fuel-cells, and the use of electrochemical cells for thermodynamic measurements. 

Corresponding cell values of AG° for the formation of many ternary compounds have been obtained. Besides doped zir-conia, also some iodides, Agl, Cul have been used as cation conductors in electrochemical cells for thermodynamic and kinetic studies. The Gibbs energy of a sulfide formation can be determined [21], for example, by the cell... 

J. P. Meyers and J. Newman, Simulation of the Direct Methanol Fuel Cell -1. Thermodynamic Framework for a Multicomponent Membrane, Journal of the Electrochemical Society, 149, A710 (2002). 

Meyers JP, Newman J (2002) Simulation of the direct methanol fuel cell I. Thermodynamic framework for a multicomponent membrane. J Electrochem Soc 149 A710-A717... 

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. 

Electrochemical cells are of course of great practical importance in the form of batteries and fuel cells. In thermodynamics, the relationship between cell potential and Gibbs energy is often used the other way around. That is, cell potentials are one of the most accurate and useful sources of information about Gibbs energies of reactions and dissolved substances. Not all Gibbs energy data come from calorimetry. Most data for / cu2+ ... 

Gopalan S and Wkar A V (1993), Thermodynamic stabilities of SrCe03 and BaCe03 using a molten salt method and galvanic cells ,/ Electrochem Soc, 140,1060-1065. 

Spacil HS, Tedmon CS Jr (1969) Electrochemical dissociation of water vapor in solid oxide electrolyte cells. I. Thermodynamics and cell characteristics. J Electrochem Soc 116 1618-1626... 

To better understand how a PEM fuel cell works, it is necessary to grasp the fundamentals of PEM fuel cells, including their cell structure and the thermodynamics and kinetics of fuel cell electrochemical reactions. In the following sections of this chapter, the fundamentals of H2/air PEM fuel cells will be discussed in detail. 

Wagner, C. (1957). Galvanic Cells with Solid Electrolytes Involving Ionic and Electronic Conduction, In Proceedings of the International Committee Electrochemical Thermodynamics and Kinetics, Chap. 8.5, 7th C.I.T.C.E. Meeting, 1955, Butterworths, London, pp. 361-377... 

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. 

Based on the fundamental principles of electrochemical engineering (electrochemical thermodynamics and kinetics), the fuel cell polarization curves can easily be understood. 

Owing to the size of the subject, it has proved necessary in the following chapters, to be selective in the choice of material presented. Earlier chapters are concerned with ionics and its applications. Here are considered ion interactions in solution, acid-base equilibria, transport phenomena, and the concept of reversible electrode potential. This last named leads to the development of reversible cells and their exploitation. Here one is dealing with electrochemical thermodynamics - with the rapid attainment of equilibrium between species at an electrode surface and charged species in solution. 

Using the method presented in Appendix E, construct a concept map showing the relationship between electrochemical cells and thermodynamic properties. 

The potential of the reaction is given as = (cathodic — anodic reaction) = 0.337 — (—0.440) = +0.777 V. The positive value of the standard cell potential indicates that the reaction is spontaneous as written (see Electrochemical processing). In other words, at thermodynamic equihbrium the concentration of copper ion in the solution is very small. The standard cell potentials are, of course, only guides to be used in practice, as rarely are conditions sufftciendy controlled to be called standard. Other factors may alter the driving force of the reaction, eg, cementation using aluminum metal is usually quite anomalous. Aluminum tends to form a relatively inert oxide coating that can reduce actual cell potential. 

Whenever energy is transformed from one form to another, an iaefficiency of conversion occurs. Electrochemical reactions having efficiencies of 90% or greater are common. In contrast, Carnot heat engine conversions operate at about 40% efficiency. The operation of practical cells always results ia less than theoretical thermodynamic prediction for release of useful energy because of irreversible (polarization) losses of the electrode reactions. The overall electrochemical efficiency is, therefore, defined by ... 

The industrial economy depends heavily on electrochemical processes. Electrochemical systems have inherent advantages such as ambient temperature operation, easily controlled reaction rates, and minimal environmental impact (qv). Electrosynthesis is used in a number of commercial processes. Batteries and fuel cells, used for the interconversion and storage of energy, are not limited by the Carnot efficiency of thermal devices. Corrosion, another electrochemical process, is estimated to cost hundreds of millions of dollars aimuaUy in the United States alone (see Corrosion and CORROSION control). Electrochemical systems can be described using the fundamental principles of thermodynamics, kinetics, and transport phenomena. 

Determining the cell potential requites knowledge of the thermodynamic and transport properties of the system. The analysis of the thermodynamics of electrochemical systems is analogous to that of neutral systems. Eor ionic species, however, the electrochemical potential replaces the chemical potential (1). 

Evans considers that corrosion may be regarded as a branch of chemical thermodynamics or kinetics, as the outcome of electron affinities of metals and non-metals, as short-circuited electrochemical cells, or as the demolition of the crystal structure of a metal. 

A system is any part of external reality that can be subjected to thermodynamic treatment the material with which the system is in contact forms the surroundings, e.g. an electrochemical cell could be the system and the external atmosphere the surroundings. 

---