Electrochemical cell slow electron transfer

For a redox reaction in an electrochemical cell the decrease in free enthalpy (- AG) is in accordance with the energy delivered by the transfer of electrons through an external circuit if this takes place in a reversible way, i.e., at a rate slow enough to allow complete attainment of equilibrium, the conversion of 1 gram mole will deliver an electrical energy of - AG = z FE. In total cell reaction mred, + n ox2 m ox, + nred2, where m81 = nS2 electrons are transfered (<5, and S2 represent the respective valence differences of the two redox systems), we have... 

Electrochemical reactions in fuel cells occurring on an electrode surface involve several steps. The electroactive species need to reach the electrode surface and adsorb on it, and then the electron transfer occurs at the electrode/electrolyte interface. The first step is mass transfer, and the second and third steps are electrode kinetics. If the mass transfer is fast, and the absorption and charge transfer are slow, the total reaction rate is determined by the electrochemical reaction kinetics. However, in the case of slow mass transfer and fast electrochemical kinetics, the mass transfer limits the whole reaction speed. In other words, the reactant that can reach the electrode surface will be consumed immediately, and the problem will be insufficient reactant on the electrode surface. 

An ionically conductive polymer is applied to the lithium battery, the fuel cell, etc. These polymer electrolytes have a fairly high ionic conductivity. However, general electrochemical measurement could not be performed in such polymer electrolytes. Electrochemical measmement in such polymer electrolytes has been possible only by special electrode systems described earlier. The reason is that the diffusion of ions or redox molecules and the rate of electron transfer are slow in these polymer electrolytes. 

This chapter is concerned with measurements of kinetic parameters of heterogeneous electron transfer (ET) processes (i.e., standard heterogeneous rate constant k° and transfer coefficient a) and homogeneous rate constants of coupled chemical reactions. A typical electrochemical process comprises at least three consecutive steps diffusion of the reactant to the electrode surface, heterogeneous ET, and diffusion of the product into the bulk solution. The overall kinetics of such a multi-step process is determined by its slow step whose rate can be measured experimentally. The principles of such measurements can be seen from the simplified equivalence circuit of an electrochemical cell (Figure 15.1). 

In summarizing, one can say that the conversion efficiencies of electrochemical solar cells with semiconductor electrodes are very similar to those for solid state devices. Additional problems arise by the possibility that the electron transfer reactions at the interfaces can be slow. This disadvantage may however be compensated by the larger flexibility in the adjustment of the redox potentials of the electrolytes to the properties of the semiconductors and by the very simple formation of the heterojunction at which the unfavorable effects of interfacial electronic states are less pronounced. The most serious problem of such cells remains the photodecomposition which has to be overcome before such devices can reach practical importance. 

The electrochemical kinetics study involves the study of electrochemical reaction rates and the key factors that determine whether a reaction will be fast or slow, and how the reaction rate may be changed. The study of kinetics is important in the design and operation of a fuel cell. The rate of electron transfer at the electrodes or the current produced by the fuel cell depends on the rate of electrochemical reaction. The key factors that affect the electron transfer are ionic and electronic resistances in electrolyte and in electrodes and the rate of mass transport through the electrodes. In order to understand how these factors affect reaction rates, phenomena at molecular level during a chemical reaction need to be imderstood. The processes at the electrode and electrolyte interface are illustrated in Figure 5.1. 

Because the metal dissolution is an anodic process, for example, Fe(s) Fe +(aq) + 2e , the current of the process is assumed to be positive. When potential increases from Mez+zMe lo f (passivation or Flade potential), the current is increasing exponentially due to the electron transfer reaction, for example, Fe(s) -> Fe +(aq) + 2e", and can be described using Tafel s equation. At a E the formation of an oxide layer (passive film) starts. When the metal surface is covered by a metal oxide passive film (an insulator or a semiconductor), the resistivity is sharply increasing, and the current density drops down to the rest current density, 7r. This low current corresponds to a slow growth of the oxide layer, and possible dissolution of the metal oxide into solution. In the region of transpassivation, another electrochemical reaction can take place, for example, H20(l) (l/2)02(g) + 2H+(aq) + 2e, or the passive film can be broken down due to a chemical interaction with environment and mechanical instability. Clearly, a three-electrode cell and a potentiostat should be used to obtain the current density-potential curve shown in Figure 9.3. 

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