Temperature Effects in Electrode Kinetics

From a theoretical viewpoint, it must be said that such experiments are often unrewarding. The range of readily useable temperatures (in aqueous media) is not large and changes in rate constant are far smaller than changes 

for example, Modern Electrochemistry , Vol. 2, ed. J. O M. Bockris and A. K. N. Reddy, MacDonald, London 1970 Electrochemical Kinetics , ed. K. J. Vetter, Academic Press, London, 1967 Theory of Electrode Processes , ed. B. E. Conway, Ronald Press Co., New York, 1964. 

A final constraint on selection of electrode materials is frequently imposed by the problem of their corrosion, or oxidation. Thus accepting that a reduction such as that of ethene occurs over the potential range + 0.1 V to — 0.1 V, a number of metals such as Fe, Co, and Ni will not be stable, at least in more acidic solutions, but will corrode by means of the anodic dissolution reactions- 

---

A century has passed since Roszkowsky in 1894 indicated in Zeitschrift fur Physikalische Chemie that the electrode potential of the hydrogen evolution reaction, taken at constant current density, is affected by temperature. His paper was the first report on the effect of temperature on the kinetics of electrode reactions. During the next half century after the publication of Roszkowsky s paper, howeva, studies of temperature effects in electrode kinetics were scarce and involved mostly determinations of temperature coefficients of the potential or overpotential for a few electrode reactions. ... 

In 1929, on the basis of a purely formal analogy to the Arrhenius equation in chemical kinetics, Bowden introduced the energy of activation as a parameter for evaluation of the temperature effect in electrode kinetics. Mathematically, he proposed two alternative ways of defining the energy of activation. One was based on the temperature coefficient of the electrode potential taken at constant current density, and the other on the dependence of the logarithm of current density, taken at constant electrode potential, on the reciprocal of the temperature. Bowden was the first to emphasize that the energy of activation of electrode reactions is linearly dependent on electrode potential and also was the first to determine experimentally the energy of activation of, for example, the hydro-... 

In comparison with the perturbations to rate constants induced by the electrode potential, relatively little attention has been directed to experimental examinations of temperature effects in electrochemical kinetics. This is probably due, in part, to uncertainties in how to control the electrical variable while the temperature is altered. However, as noted in Sect. 3.4, in actuality there are no more ambiguities in interpreting electrochemical activation parameters than for the commonly encountered Arrhenius par-... 

Further measurements need to be made on the temperature and potential dependence of the rates of simple ionic redox reactions at electrodes with proper corrections for double-layer effects at various temperatures, so that the temperature dependence of (3 for an elementary electron transfer reaction, without chemisorption and coupled atom transfer, would become better known. This is an essential requirement for progress in understanding the true significance of the temperature effects on electrode-kinetic behavior reliable experiments will not, however, be easy to accomplish and will require parallel double-layer studies over a range of temperatures. 

What are ways oul of this extreme sensitivity to impurities in electrode kinetics for most electrochemical reactions One way to reduce the effects of trace impurities from the solution in electrode kinetics is to use a liquid electrode because such electrodes can be made to form drops, the lifetime of which is small, so that the impurities from the solution don t have time to adsorb on the drops before they break off from the elechxxle. The electrode material then has to be mercury (the only metal that is liquid at room temperatures), so this approach is limited because mercury is a poor catalyst and one wishes particularly to work with electrode materials that catalyze electrode reactions well.27... 

The great importance of the Tafel relation—because it is too widely observed to be applicable in electrode kinetics—does not seem to have been appreciated during the time (about 1960-1980) in which Gaussian concepts were frequently used to present a quantal approach to electrode kinetics. Supporting a theoretical view that does not yield what is in effect the first law of electrode kinetics is similar to supporting a theory of gas reactions that does not lead to the exponential dependence of rate on temperature. It represents a remarkable historical aberration in the field. Thus the... 

Once the SHE is taken as the electrical reference potential in evaluations of the effect of temperature in electrode kinetics, combinations of kinetic parameters pertinent to the standard equilibrium state of electrode reactions may be related to the corresponding equilibrium data. For instance, taking the difference of the natural logarithms of the preexponential current densities from Eqs. (57) for = assuming cq = cr = and Pc Pa- y one obtains... 

Finally, it may be concluded that the Agar-Temkin approach to the effect of temperature on the rate of electrode reactions is inadequate, in spite of the fact that the w-EAZOPs are determined by a correct procedure. Additional efforts are needed to find other theoretical descriptions or experimental innovations that would help to elucidate the phenomena of temperature dependence in accordance with its fundamental importance in electrode kinetics. 

Fhosphoric acid does not have all the properties of an ideal fuel cell electrolyte. Because it is chemically stable, relatively nonvolatile at temperatures above 200 C, and rejects carbon dioxide, it is useful in electric utility fuel cell power plants that use fuel cell waste heat to raise steam for reforming natural gas and liquid fuels. Although phosphoric acid is the only common acid combining the above properties, it does exhibit a deleterious effect on air electrode kinetics when compared with other electrolytes ( ) including such materials as sulfuric and perchloric acids, whose chemical instability at T > 120 C render them unsuitable for utility fuel cell use. In the second part of this paper, we will review progress towards the development of new acid electrolytes for fuel cells. 

Temperature effect on the electrodeposition of zinc on the static mercury drop electrode (SMDE) and glassy carbon (GG) electrode was studied in acetate solutions [44]. From the obtained kinetic parameters, the activation energies of Zn(II)/Zn(Hg) process were determined. 

The double-layer effect in the electrode kinetics of the amalgam formation reactions was discussed [67]. The dependences on the potential of two reduction (EE) mechanisms of divalent cations at mercury electrode, and ion transfer-adsorption (lA) were compared. It was suggested that a study of temperature dependence of the course of these reactions would be helpful to differentiate these two mechanisms. 

Although a couple of outstanding original publications on electrode kinetics appeared as early as 1928, the bulk of the experimental work in this field was carried out in the second half of the twentieth century. Experimental work with mercury as the electrode was found to be relatively easy. For one thing, because mercury is a liquid at room temperature, there were no crystal planes of differing reactivity to worry about, and mercury drops can easily be renewed, so impurity adsorption with its anticatalytic effects is not a problem. 

In particular, Yeager and co-workers (42) have described a = a 4- j3 T for the reduction of oxygen on platinum in concentrated phosphoric acid in the temperature range 25—250°C with cathodic Tafel slopes nearly independent of temperature (a — 0.08 and j3 = 0.0012/K). The effect of electrode potential on the reaction kinetics is mainly through the entropy of activation [43]. 

The electrolysis measurements were conducted at three flow rates i) anolyte 3.4 ml/min and catholyte 4.4 ml/min ii) anolyte 11.8 ml/min and catholyte 11 ml/min) in) anolyte 22 ml/min and catholyte 27 ml/min). The tests were run at ambient temperature and pressure. Linear sweep voltammetry data obtained for the AHA and Nafion 115 membranes indicated very little effect of the flow rate on the electrode kinetics as long as the mass transport limitation is not reached. Apparently, the higher flow rates of reactants passing through the electrodes do not speed up the electrochemical conversion rates in the electrolyser used in this study. 

Temperature variations. Essentially all kinetic phenomena are temperature dependent ion diffusion (in both electrolyte and active materials), electron transfer, desolvation, adsorption, etc. Additionally, thermodynamic equilibrium constants are temperature dependent, so any temperature variations within the cell will produce uneven plating and stripping, electrode shape change effects and uneven utilization again leading to compromised performance. 

Experimental systems used for electrochemical measurements should be selected to take maximum advantage of well-imderstood phenomena such as mass transfer so as to focus attention on the less-understood phenomena such as electrode kinetics. For example, the study of electrochemical reactions in stagnant environments should be avoided because concentration and temperature gradients give rise to natural convection, which has an effect on mass transfer that is difficult to characterize. It is better to engage in such experimental investigations in systems for which mass transfer is well defined. To simplify interpretation of the impedance data, the electrode should be uniformly accessible to mass transfer. 

To function as an effective polyelectrolyte, the polymer should have a low glass transition temperature to allow the freedom of molecular movement necessary for ion transport. While the advantages of PEO-alkali metal complexes are their good electrochemical stability as well as faster kinetics at the ion transfer between electrode and electrolyte, the disadvantage is the low conductivity of the polymeric electrolyte at normal operating temperatures. Thus, in order to obtain high-power densities, it is necessary to work... 

Particularly, temperatures above ambient are used because of the beneficial effect on the kinetics of all steps in an electrode process. The diffusion coefficient, the exchange current density and the rate of chemical reactions are all increased. 

The temperature dependence of the catalyst activity of an iron fluoro-porphyrin-coated graphite electrode was studied by RDE coupled with the surface cyclic voltammetry. The purpose was to investigate the surface adsorption and reaction, O2 reduction catalysis kinetics, and especially the temperature effect on the catalyst activity. Figure 7.11(A) shows the surface CVs of 5,10,15,20-Tetrakis(pentafluorophenyl)-21H,23H-porphine iron (III) chloride (abbreviated as Fe TPFPP)-coated graphite electrode, recorded in a pH 1.0 Ar-saturated solution at different potential scan rates. The 1-electron reversible redox peak of approximately 0.35 V can be seen, which has a peak current increased linearly with increasing the potential scan rate, indicating the electrochemical behavior of this peak follows the feature of a reversible redox reaction of an adsorbed species on the electrode surface. 

The electrode kinetics and effects on the ES electrochemical performance have also been studied. At an extremely low kinetic rate, capacitance loss accompanied by an increase in series resistance could occur, leading to aging. This aging is accelerated at both high temperatures and voltages. 

---