Thermodynamic reversible potential

Observance of a mixed potential of about 1.0 V (instead of the equilibrium thermodynamic reversible potential Ec= 1.23 V vs. SHE) due to the formation of surface oxides at the platinum electrode, according to different electrode reactions ... 

This is the maximum amount of useful work that can be derived from the system on driving the reaction in the opposite direction. Thus, Vrev corresponds to the reversible work and is consequently called the thermodynamic reversible potential. At 25°C and 1 bar, the AG for the water-splitting reaction is 237.178 kj/mol [10]. Therefore,... 

This is, in fact, the way electrode potentials are measured in practice. A cell is made up of the electrode of interest (the working electrode, e.g., Cu in Fig. 7.14) and a reference electrode made of Pt over which is bubbled Hj- No current passes through the reference electrode, which is therefore at its thermodynamically reversible potential. A counter-electrode (not shown in Fig. 7.14) is coupled through a power source... 

Note that some electrochemical cells use, instead of conventional reference electrodes, indicator electrodes. These are electrodes that are not thermodynamically reversible but which may hold then-potential constant 1 mV for some minutes—enough to make some nonsteady-state measurements (see Chapter 8). Such electrodes can simply be wires of inert materials, e.g.. smooth platinum without the conditions necessary to make it a standard electrode exhibiting a thermodynamically reversible potential. However, many different electrode materials may serve m this relatively undemanding role. 

Going downhill, i.e., discharging the cell, is subject to similar thinking but of course reversed. The potential available at the terminals of a battery in discharge will be the thermodynamically reversible potential but now diminished by the sum of the overpotential and the IR drop. 

The Flade potential, " the limiting potential at which film formation occurs, is discussed below it appears to correspond to the thermodynamic reversible potential of the stoi -chiometric oxide in the case of several metals such as silver,... 

Electrocatalysis is manifested when it is found that the electrochemical rate constant, for an electrode process, standardized with respect to some reference potential (often the thermodynamic reversible potential for the same process) depends on the chemical nature of the electrode metal, the physical state of the electrode surface, the crystal orientation of single-crystal surfaces, or, for example, alloying effects. Also, the reaction mechanism and selectivity 4) may be found to be dependent on the above factors in special cases, for a given reactant, even the reaction pathway [4), for instance, in electrochemical reduction of ketones or alkyl halides, or electrochemical oxidation of aliphatic acids (the Kolbe and Hofer-Moest reactions), may depend on those factors. 

Changes of A from one metal to another, for a given process (e.g. the HER), provide the principal basis for dependence of the kinetics of the electrode process on the metal and are recognized as the origin of electrocatalysis associated with a reaction in which the first step is electron transfer, with formation of an adsorbed intermediate. In the case of the HER, this effect is manifested in a dependence of the logarithm of the exchange current density, I o (i.e., the reversible rate of the process, expressed as A cm , at the thermodynamic reversible potential of the reaction) on metal properties such as 0 (Fig. 2) (14-16, 20). However, as was noted earlier, for reasons peculiar to electrochemistry, reaction rate constants cannot depend on under the necessary condition that currents must be experimentally measured at controlled potentials (referred to the potential of some reference... 

For the above reaction, a shorter extrapolation to = 0, to determine k°, is somewhat better statistically, with overlap only of the lower confidence interval for Pd and of the upper one for Pt. In addition, the value of k° can always be defined for a reaction path, while estimation of I o requires knowledge of the thermodynamic reversible potential. The latter is not always... 

In order to understand the variation of the electrochemical reaction rate (measured in current density) with the potential of the electrode, it is best to start by thinking of the situation at equilibrium. As explained above there is a certain potential (referred to as the thermodynamic reversible potential) at which the two reaction rates, the anodic and the cathodic, are equal in magnitude and opposite in direction. 

Nakamura and coworkers studied the pH effect (13 > pH > 4) for water oxidation at S-MnOa modified electrode. It was suggested that the decreased activity under neutral or acidic conditions was due to the instability of manganese(m) at pH < 9 that disproportionates to manganese(n) and manganese(iv). The presence of manganese(iii) on the 8-Mn02 surface was concluded to be essential to reduce the overpotential for water oxidation and resulted in the ojgrgen production at an onset potential close to the thermodynamic reversible potential of the four-electron oxidation of water, E(02/H20) = 0.76 V, at pH... 

The four-electron reduction of oxygen [reaction (I)] is very irreversible and therefore experimental verification of the thermodynamic reversible potential of this reaction is very difficult. The exchange current densities for reactions (I) and (II) are typically 10" -10" A/cm of real surface area for Pt and other noble metals at room temperatures. Any other side reaction, even if slow and otherwise difficult to detect, may compete with reaction (I) or (II) in establishing the rest potential. Indeed, unless special experimental procedures are used, the thermodynamic potential cannot be obtained at ambient temperature in aqueous electrolytes. Even on the most active platinum electrode in pure acid or alkaline aqueous solution under ordinary conditions, the rest potential in the presence of oxygen at 1 atm and ambient temperature usually does not exceed 1.1 V vs. the NHE and most often has a value close to 1.0 V. In early work on O2 electrochemistry, before reliable thermodynamic data were available, the potential 1.08 V vs. RHE was considered as the reversible value for reactions (I) and (II). 

The development of the ultrasensitive potential sweep technique, capable of detecting submonolayer amounts of substance on electrode surfaces and its application to metal deposition studies, resulted in detailed investigations of the phenomenon of deposition of metals on foreign substrates at potentials more positive than the thermodynamic reversible potential for the given conditions.This phenomenon has been termed underpotential deposition (UPD). 

Rhodium, Rh, like other noble metals, forms surface oxides upon anodic polarization even in the region of water stability, thus below the thermodynamic reversible potential of the oxygen evolution reaction, = 1.23 V, SHE 1-20). In aqueous H2SO4 solution, the oxide growth on Rh commences at 0.55 V, RHE (reversible hydrogen electrode), and up to ca. 1.40 V, RHE, a complete monolayer (ML) of Rh(OH)3 is formed as revealed by coupled CV and XPS measurements (27). 

I/q, the thermodynamic reversible potential, is the potential that can be measured... 

The positive electrode does not accept charge, and convert nickel hydroxide to nickel oxyhydroxide, at the thermodynamically reversible potential. In fact, with a low enough charge rate, gassing according to the equation below occurs ... 

A good reference electrode is always a reversible electrode. The inverse is not necessarily true. Not every reversible electrode is suitable as a reference electrode. For example, the correct thermodynamic reversible potential of a metal/metal-ion electrode may be hard to reproduce, because of impurities in the metal or complexing agents in the solution, even when the interface is highly nonpolarizable. 

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