Solids, electrode kinetics

W. Gopel, and H.-D. Wiemhofer, Electrode kinetics and interface analysis of solid electrolytes for fuel cells and sensors, Ber. Buns. Phys. Chem. 94, 981-987 (1990). 

Solid alkaline membrane fuel cells (SAMECs) can be a good alternative to PEMFCs. The activation of the oxidation of alcohols and reduction of oxygen occurring in fuel cells is easier in alkaline media than in acid media [Wang et al., 2003 Yang, 2004]. Therefore, less Pt or even non-noble metals can be used owing to the improved electrode kinetics. Eor example, Ag/C catalytic powder can be used as an efficient cathode material [Demarconnay et al., 2004 Lamy et al., 2006]. It has also... 

Early studies of ET dynamics at externally biased interfaces were based on conventional cyclic voltammetry employing four-electrode potentiostats [62,67 70,79]. The formal pseudo-first-order electron-transfer rate constants [ket(cms )] were measured on the basis of the Nicholson method [99] and convolution potential sweep voltammetry [79,100] in the presence of an excess of one of the reactant species. The constant composition approximation allows expression of the ET rate constant with the same units as in heterogeneous reaction on solid electrodes. However, any comparison with the expression described in Section II.B requires the transformation to bimolecular units, i.e., M cms . Values of of the order of 1-2 x lO cms (0.05 to O.IM cms ) were reported for Fe(CN)g in the aqueous phase and the redox species Lu(PC)2, Sn(PC)2, TCNQ, and RuTPP(Py)2 in DCE [62,70]. Despite the fact that large potential perturbations across the interface introduce interferences in kinetic analysis [101], these early estimations allowed some preliminary comparisons to established ET models in heterogeneous media. 

Adams, R. N., Electrochemistry at Solid Electrodes, M. Dekker, New York, 1969. Albery, W. J., Electrode Kinetics, Oxford University Press, 1975. 

Sections 5.6.2 and 5.6.3 dealt with the deposition of metals from complexes these processes follow the simple laws dealt with in Sections 5.2 and 5.3, particularly if they take place at mercury electrodes. The deposition of metals at solid electrodes (electrocrystallization) and their oxidation is connected with the kinetics of transformation of the solid phase, which has a specific character. A total of five different cases can be distinguished in these processes ... 

Jiang SP and Badwal SPS. An electrode kinetics study of H2 oxidation on Ni/Y203-Zr02 cermet electrode of the solid oxide fuel cell. Solid State Ionic 1999 123 209-224. 

Radiometric methods are unique for their ability to provide directly the surface concentration of the adsorbate. A method for in situ study of electrochemical reactions on solid electrodes was invented by Joliot. ° He used a thin gold foil as an electrode which at the same time served as the window of the radiation counter. Johot determined the kinetics and the effect of tartaric acid on polonium electrodeposition on gold. The method was later further developed and improved (e.g.. Refs. 102,103). 

In the early days of electrode kinetics studies (1928-1950), the results of the measurements of rates of electrode reactions on solid electrodes obtained in one laboratory were usually irreproducible and disagreed by large percentages with those obtained in another laboratory. It was found (Bockris and Conway, 1949) that for electrode reactions involving intermediate radicals adsorbed on the electrode, as little as 10-10 mol liter-1 of impurity in the solution affected the measured rate of the electrode reaction (Fig. 7.30). There are two mechanisms by which impurities (usually traces of organics from the environment) can interfere with a satisfactory measurement. 

A. M. Azzam, J. O M. Bockris, B. E. Conway, and H. Rosenberg, Trans. Faraday Soc. 46 918 (1950). Technique of steady-state electrode kinetics on solid electrodes involving intermediates. 

The symmetry factor P is obviously a central entity in electrodics and a fundamental quantity in the theoretical treatment of charge transfer at surfaces, particularly in relating electrode kinetics to solid-state physics. 

Several factors have contributed to this goal in the recent past development of electrochemical techniques for the study of complex reactions at solid electrodes, use of physical methods such as ESCA, Auger, LEED, etc. for the study of surfaces in the ultrahigh vacuum (UHV) environment and in situ techniques under the same conditions as the electrode reaction. Ellipsometry, electroreflectance, Mossbauer, enhanced Raman, infrared, electron spin resonance (ESR) spectroscopies and measurement of surface resistance and local changes of pH at surfaces were incorporated to the study of electrode kinetics. 

The potential response of the RDE to current steps has been treated analytically [3, 237, 251] and accurately by Hale using numerical integration [252] this enables the elucidation of kinetic parameters [185, 253]. A current density—transition time relationship at the RDE has been established which accounts for observed differences from the Sand equation [eqn. (218)] and which has been applied to EC reactions [254]. Other hydrodynamic solid electrodes have not been considered in detail, although reversible reactions at channel electrodes have been discussed [255, 256]. 

Following the introduction of basic kinetic concepts, some common kinetic situations will be discussed. These will be referred to repeatedly in later chapters and include 1) diffusion, particularly chemical diffusion in different solids (metals, semiconductors, mixed conductors, ionic crystals), 2) electrical conduction in solids (giving special attention to inhomogeneous systems), 3) matter transport across phase boundaries, in particular in electrochemical systems (solid electrode/solicl electrolyte), and 4) relaxation of structure elements. 

The experimental determination of a potential change across a solid/solid interface is a most difficult task since it means that potential probes have to be placed very near the interface. Electrochemists face a similar problem when they study electrode kinetics, but the handling of fluids in this respect is much easier. Nevertheless, we will exploit their concepts and methods to some extent in what follows. 

Electrode Kinetics Principles and Methodology Electrode Kinetics Reactions Reactions at the Liquid-Solid Interface... 

When one considers a distance scale much smaller than 1 pm, surface roughness also is an issue to observed electrode behavior. The ratio of the microscopic surface area to the projected electrode area is usually designated the roughness factor, and can vary from 1.0 to 5 or so for typical solid electrodes, or much higher for porous electrodes. Capacitance, surface faradaic reactions, adsorption, and electrode kinetics all depend on microscopic area. For example, double-layer capacitance increases with roughness such that the apparent capacitance (C°bs) is larger than the value for a perfectly flat electrode (Cflat) as shown in Equation 10.1 ... 

The most popular electroanalytical technique used at solid electrodes is Cyclic Voltammetry (CV). In this technique, the applied potential is linearly cycled between two potentials, one below the standard potential of the species of interest and one above it (Fig. 7.12). In one half of the cycle the oxidized form of the species is reduced in the other half, it is reoxidized to its original form. The resulting current-voltage relationship (cyclic voltammogram) has a characteristic shape that depends on the kinetics of the electrochemical process, on the coupled chemical reactions, and on diffusion. The one shown in Fig. 7.12 corresponds to the reversible reduction of a soluble redox couple taking place at an electrode modified with a thick porous layer (Hurrell and Abruna, 1988). The peak current ip is directly proportional to the concentration of the electroactive species C (mM), to the volume V (pL) of the accumulation layer, and to the sweep rate v (mVs 1). 

In Fig. 3.14a, the dimensionless limiting current 7j ne(t)/7j ne(tp) (where lp is the total duration of the potential step) at a planar electrode is plotted versus 1 / ft under the Butler-Volmer (solid line) and Marcus-Hush (dashed lines) treatments for a fully irreversible process with k° = 10 4 cm s 1, where the differences between both models are more apparent according to the above discussion. Regarding the BV model, a unique curve is predicted independently of the electrode kinetics with a slope unity and a null intercept. With respect to the MH model, for typical values of the reorganization energy (X = 0.5 — 1 eV, A 20 — 40 [4]), the variation of the limiting current with time compares well with that predicted by Butler-Volmer kinetics. On the other hand, for small X values (A < 20) and short times, differences between the BV and MH results are observed such that the current expected with the MH model is smaller. In addition, a nonlinear dependence of 7 1 e(fp) with 1 / /l i s predicted, and any attempt at linearization would result in poor correlation coefficient and a slope smaller than unity and non-null intercept. 

Fig. 7.12 Comparison between double (DDPV, dotted lines) and multipulse (DMPV, solid lines) techniques for different electrode kinetics ...

During the past four decades the dynamics and mechanisms of electron-transfer processes have been studied via the application of transition-state theory to the kinetics for electrochemical processes. As a result, both the kinetics of the electron-transfer processes (from solid electrode to the solution species) as well as of pre- and post-electron-transfer homogeneous processes can be characterized quantitatively. 

Another limitation of solid electrodes has been their complex diffusion-current response relative to time with slow-sweep voltammetry. The development of a capillary hanging-mercury-drop electrode (HMDE) by Kemula and Kublik,4,5 together with modem electronic instrumentation, allowed the principles of voltage-sweep voltammetry and cyclic voltammetry to be established. The success has been such that this has become one of the most important research tools for electrochemists concerned with the kinetics and mechanisms of electrochemical processes. These important contributions by Nicholson and Shain6 7 rely, as have all electrochemical kinetic developments, on the pioneering work by Eyring et al.8... 

Fig. 13.27. Potential vs. current density plots for state-of-the-art fuel cells, o, proton exchange membrane fuel cell , solid oxide fuel cell , pressurized phosphonic acid fuel cell (PAFC) a, direct methanol fuel cell, direct methanol PAFC , alkaline fuel cell. (Reprinted from M. A. Parthasarathy, S. Srinivasan, and A. J. Appleby, Electrode Kinetics of Oxygen Reduction at Carbon-Supported and Un-supported Platinum Microcrystal-lite/Nafion Interfaces, J. Electroanalytical Chem. 339 101-121, copyright 1992, p. 103, Fig. 1, with permission from Elsevier Science.)...

Fleig reviews fundamental aspects of solid state ionics, and illustrates many similarities between the field of solid state electrochemistry and liquid electrochemistry. These include the consideration of mass and charge transport, electrochemical reactions at electrode/solid interfaces, and impedance spectroscopy. Recent advances in microelectrodes based on solid state ionics are reviewed, along with their application to measuring inhomogeneous bulk conductivities, grain boundary properties, and electrode kinetics of reactions on anion conductors. 

An alternative, well-studied approach to develop Na metal anode rechargeable cells is to isolate the Na electrode from the cathode via a ceramic Na ion conductor, i.e., a solid electrolyte. Such batteries are operated above the melting point of Na, thereby enhancing electrode kinetics and solid state diffusion of Na ions through the special alumina ceramic. The most commonly employed material is beta double prime alumina, which has channels permitting the facile diffusion of Na ions. 

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