Overpotentials

The Nernst potential (Vn) of Eq. 13 accounts for changes in the activity of the reaction and for nonstandard conditions, 

The partial pressure of water is determined with the empirical formula from Ref. 21 and is shown in Eq. 14. This relationship enables the partial pressure of water to be calculated from experimental data as the temperature of the DI water into the stack anode varies, 

The partial pressures of hydrogen and oxygen are determined using measurements from the stack cathode and anode and Eqs. 15 and 16, 

Water electrolysis is an electrochemical reaction where water is split into hydrogen and oxygen in the presence of a catalyst and applied electric field. As current density increases the cell losses due to membrane, electrode, and interfacial resistances dominate and are referred to as ohmic overpotential. 

At equilibrium (i.e., no current) there exist dynamic currents, measured in amps, at each electrode and are a fundamental characteristic of electrode behavior. The anode and cathode exchange current densities can be defined as the rate of oxidation and reduction respectively. The exchange current density is a measure of the electrode s ability to transfer electrons and occurs equally in both directions resulting in no net change in composition of the electrode.22 A large exchange current density represents an electrode with fast kinetics where there is a lot of simultaneous electron transfer. A small exchange current density has slow kinetics and the electron transfer rate is less. 

The higher the exchange current density the easier it is for reaction to continue when current is supplied to the stack. The cathode exchange current density is thus not the limiting parameter of the activation overpotential term and is often ignored. The current density ( ) normalizes the stack current (7) to the active area of the cell. 

Ohmic losses occur because of resistance to the flow of ions in the solid electrolyte and resistance to flow of electrons through the electrode materials. Because the ionic flow in the electrolyte obeys Ohm s law, the ohmic losses can be expressed by Ohm s law. The ohmic overpotential, r o of Eq. 19 is a function of the stack current density ( ), membrane thickness (cp), and the conductivity of the stack (a), 

The cathode exchange current density is typically four orders of magnitude greater than the anode exchange current density and supported by Choi and Beming in Ref. 24 and 25. The anode side is therefore limiting the reaction and dominates the activation overpotential. 

The formation of hypochlorous acid is thermodynamically highly favored, for example, at 20 °C the equilibrium constant is 1.2 x 10 (Hamann and Vielstich, 2005). This leads to a high concentration of hypochlorite ions at the anode and in a subsequent reaction oxygen is formed, which contaminates the sodium hydroxide solution with chlorate ions and chloride  

It is essential for all technical chlor-alkali electrolysis processes - as subsequently discussed in Section 6.19.2.2 - that the transport of hydroxide ions formed at the cathode into the anode compartment is excluded (membrane process) or at least largely suppressed (diaphragm process). In the mercury cell process, OH ions are not formed in the entire process. 

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Once current passes tluough the interface, the Galvani potential difference will differ from that expected from the Nemst equation above the magnitude of the difference is temied the overpotential, which is defined heiiristically as... 

At low currents, the rate of change of die electrode potential with current is associated with the limiting rate of electron transfer across the phase boundary between the electronically conducting electrode and the ionically conducting solution, and is temied the electron transfer overpotential. The electron transfer rate at a given overpotential has been found to depend on the nature of the species participating in the reaction, and the properties of the electrolyte and the electrode itself (such as, for example, the chemical nature of the metal). 

At higher current densities, the primary electron transfer rate is usually no longer limiting instead, limitations arise tluough the slow transport of reactants from the solution to the electrode surface or, conversely, the slow transport of the product away from the electrode (diffusion overpotential) or tluough the inability of chemical reactions coupled to the electron transfer step to keep pace (reaction overpotential). 

Iimnediately after the imposition of a large negative overpotential in a solution containing oxidized species,... 

The overpotential is defined as the difference between the actual potential of an electrode at a given current density and the reversible electrode potential for the reaction. 

The overpotential required for the evolution of O2 from dilute solutions of HCIO4, platinum electrodes is approximately 0.5 V. 

If the initial concentration of Cu + is 1.00 X 10 M, for example, then the cathode s potential must be more negative than -1-0.105 V versus the SHE (-0.139 V versus the SCE) to achieve a quantitative reduction of Cu + to Cu. Note that at this potential H3O+ is not reduced to H2, maintaining a 100% current efficiency. Many of the published procedures for the controlled-potential coulometric analysis of Cu + call for potentials that are more negative than that shown for the reduction of H3O+ in Figure 11.21. Such potentials can be used, however, because the slow kinetics for reducing H3O+ results in a significant overpotential that shifts the potential of the H3O+/H2 redox couple to more negative potentials. 

Overvoltages for various types of chlor—alkali cells are given in Table 8. A typical example of the overvoltage effect is in the operation of a mercury cell where Hg is used as the cathode material. The overpotential of the H2 evolution reaction on Hg is high hence it is possible to form sodium amalgam without H2 generation, thereby eliminating the need for a separator in the cell. 

The mercury cell operates efficiently because of the higher overpotential of hydrogen on mercury to achieve the preferential formation of sodium amalgam. Certain trace elements, such as vanadium, can lower the hydrogen overpotential, however, resulting in the release of hydrogen in potentially dangerous amounts. 

One factor contributing to the inefficiency of a fuel ceU is poor performance of the positive electrode. This accounts for overpotentials of 300—400 mV in low temperature fuel ceUs. An electrocatalyst that is capable of oxygen reduction at lower overpotentials would benefit the overall efficiency of the fuel ceU. Despite extensive efforts expended on electrocatalysis studies of oxygen reduction in fuel ceU electrolytes, platinum-based metals are stiU the best electrocatalysts for low temperature fuel ceUs. 

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