Electrochemical potential and temperature

The penetration of stress corrosion cracking as a function of time depends on the alloy composition, structure, pH, environmental species present, stress, electrochemical potential and temperature. 

From a materials science point of view, PEMFC components are exposed to a very aggressive environment where strong oxidizing and reducing agents are present together with water, acids, and radicals while subjected to elevated electrochemical potentials and temperatures. As a result, the durability of PEMFCs is recognized as the most critical issue... 

Dam VAT, de Bmijn FA. 2007. The stability of PEMFC electrodes—Platinum dissolution vs. potential and temperature investigated by quartz crystal microbalance. J Electrochem Soc 154 B494-B499. 

Besides being of obvious synthetic utility, these results remind us that changes in experimental parameters, particularly solvent, electrolyte, potential, and temperature, may cause substantial changes in the composition of the double layer and therefore may provoke changes in the course of electrochemical processes occurring at the electrode surface. 

Electrochemical methods are very useful in structural studies but are barely applicable for preparative aims. The cause is the limited stability of cation radicals. It is difficult to do low-temperature preparative electrolysis, and the main problem is to dispose of the large amount of heat generated during the electrode work. That is, not much current can be passed through an ordinary-sized electrode without generating too much heat. When potential and temperature control are necessary, only small quantities of a material can be obtained in a reasonable period of time. When potential and temperature control are not necessary, as in Kolbe electrolysis, anodic oxidation is indeed useful as a preparative method. 

This phenomenon has been studied by different combined electrochemical techniques such as -> spectroelec-trochemistry, radioactive -> tracer method, -> electrochemical quartz crystal microbalance, conductivity etc. by varying the experimental parameters, e.g., film thickness, the composition and concentration of the electrolyte solutions, the wait-time at different waiting potentials, and temperature [iii-x]. Several interpretations have been developed beside the ESCR model. The linear dependence of the anodic peak potential on the logarithm of the time of cathodic electrolysis (wait-time) -when the polymer in its reduced state is an insulator -has been interpreted by using the concept of electric percolation [ix]. Other effects have also been taken into account such as incomplete reduction [vii], slow sorp-tion/desorption of ions and solvent molecules [iii-vi], variation of the equilibrium constants of -+polarons and - bipolarons [viii], dimerization [xi], heterogeneous effects [xii], etc. 

The phenomenological equations (6.9.1) have thus been reexpressed in (6.9.9) solely in terms of the measurable transport coefficients a, k, and o. The Seebeck coefficient may be interpreted as the entropy carried per electronic charge. Equation (6.9.9a) represents a further generalization of Ohm s Law, showing how the current density behaves in the presence of a temperature gradient see also Exercise 6.9.3. Equation (6.9.9b) specifies the entropy flux under the joint action of a gradient in electrochemical potential and in temperature this represents a generalization of Fourier s Law. 

The activity, stability, and tolerance of supported platinum-based anode and cathode electrocatalysts in PEM fuel cells clearly depend on a large number of parameters including particle-size distribution, morphology, composition, operating potential, and temperature. Combining what is known of the surface chemical reactivity of reactants, products, and intermediates at well-characterized surfaces with studies correlating electrochemical behavior of simple and modified platinum and platinum alloy surfaces can lead to a better understanding of the electrocatalysis. Steps, defects, and alloyed components clearly influence reactivity at both gas-solid and gas-liquid interfaces and will understandably influence the electrocatalytic activity. 

Construction of Apparatus. The schematic of the apparatus for supercritical corrosion studies is shown in Figure 1. The important components include a type 396-89 Simplex Minipump which can accurately meter (between 46 and 460 ml/hr) a wide variety of solvents at pressures up to 6000 psi (about 400 atm) an EG G Model 362 Scanning Potentiostat the electrochemical cell an IBM PC computer with interface hardware for electrochemical potential and current, temperature, and pressure measurement and control and a 316 stainless steel reactor, which holds the supercritical fluid for the measurements. The alloy was selected for excellent corrosion resistance properties and relatively low cost when compared with other exotic alloys such as Hastelloy C. 

For references on electrochemical reaction kinetics and mechanism, see, e.g., Newman and Thomas-Alvea, Electrochemical Systems, 3d ed., Wiley Interscience, 2004 Bard and Faulkner, Electrochemical Methods Fundamentals and Applications, 2d ed., Wiley, 2001 Bethune and Swendeman, Table of Electrode Potentials and Temperature Coefficients, Encyclopedia of Electrochemistry, Van Nostrand Reinhold, New York 1964, pp. 414-424 and Bethune and Swendeman, Standard Aqueous Electrode Potentials and Temperature Coefficients, C. A. Hampel Publisher, 1964. 

W. M. Latimer, The Oxidation States of the Elements and their Potentials in Aqueous Solutions, 2nd ed., Prentice-Hall, Englewood Cliffs, NJ, 1952. Although a useful source in its time, this book is now outdated. Unfortunately, no comprehensive compilation of electrochemical thermodynamic data has entirely replaced it. However, useful data sources include A. 1. de Be-thune, N. A. S. Loud, Standard Aqueous Electrode Potentials and Temperature Coefficients at 2S°C, C. A. Hampel, Skokie, IL, 1964 A. J. Bard, ed.. Encyclopedia of Electrochemistry of the Elements, Vols. 1-10, Marcel Dekker, New York, 1973-1976. 

Polypyrrole and many of its derivatives can be synthesized via simple chemical or electrochemical methods [120]. Photochemically initiated and enzyme-catalyzed polymerization routes have also been described but less developed. Different synthesis routes produce polypyrrole with different forms chemical oxidations generally produce powders, while electrochemical synthesis leads to films deposited on the working electrode and enzymatic polymerization gives aqueous dispersions [Liu. Y. C, 2002, Tadros. T. H, 2005 and Wallace. G. G, 2003]. As mentioned above the electrochemical polymerization method is utilized extensively for production of electro active/conductive films. The film properties can be easily controlled by simply varying the electrolysis conditions such as electrode potential, current density, solvent, and electrolyte. It also enables control of thickness of the polymers. Electrochemical synthesis of polymers is a complex process and various factors such as the nature and concentration of monomer/electrolyte, cell conditions, the solvent, electrode, applied potential and temperature, pH affects the yield and the quality of the film... 

Electrochemical critical temperature (ECT) testing has been evolving for more than two decades [48,50,64,73-75,79, 91-94,101] The programmed changes in applied potential and temperature that constitute the ECT method Eire schematically illustrated in Fig. 4. (Note that the corresponding changes in current density are not illustrated in Fig. 4.)... 

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