Single-electrode entropy

Kanamura K., Yoshioka S., Takehara Z., 1991. Dependence of entropy change of single electrodes on partial pressure in Solid Oxide Fuel Cells. Journal of the Electrochemical Society 138(7), 2165-2167. 

In these researches, a mainly purpose is to acquire EPHs of cell or half-cell reactions. The EPH could be considered as a basic issue of TEC. Before the identification of this problem there had been two puzzled questions, one is that the heat effects for a reversible reaction, Q can be calculated by the formula Q = TAS where AS is the entropy change of this reaction and T temperature in Kelvin. However, this formula that is valid for most reactions is not viable at least for a reversible single electrode reaction in aqueous solution. For a reversible single electrode reaction, the experimental value of the heat effect is not in agreement with that calculated on the current thermodynamic databank of ions, that is, with which, the product of the calculated entropy change and the temperature of the electrode reaction always differs from the experimental measurements [2]. For example, for the electrode reaction at the standard state ... 

On this scale, the entropy change for a single-electrode reaction, AS t will be characterized as ... 

In Eq. (30), Q is a product of temperature T and the entropy change derived from the current thermodynamic databank including the ion data which is constructed on the conventional scale, and can be named as "the traditional heat effect" while in Eq. (15), 77is the heat effect identified by the experiments, called as "the measured heat effect". The difference between them is z TAS (H+/H2). Consequently, the first problem mentioned above, why is this formula, Q = TAS, unsuitable for a reversible single electrode reaction, is answered. 

The most important conclusion of the above considerations is that measurements with non-isothermal cells may provide true single-electrode potentials (in this case, indeed potentials, not voltages ). The very important entropy of a single electrode Sf. can be determined this way. The corresponding relation is AEjAT=SJzF. 

Single-electrode potentials are important for some fundamental but unmeasurable quantities. The problem has been discussed in literature [2, 3]. Relative potential values (not absolute values which refer to an imaginary point in the universe ) can be calculated. Also single-electrode entropy values can be calculated by means of non-isothermal cells, but it is necessary to make use of some non-thermodynamic assumptions. 

By means of entropy calculations as mentioned above, fixup values of common reference electrodes have been calculated. On this basis, the non-isothermal temperature coefficient of some single-electrode potentials became available. Some results of such single-electrode potentials are given in Table 2.2. A comprehensive... 

The entropy of formation of the interface was calculated from the temperature coefficient of the interfacial tension.304 The entropy of formation has been found to increase with the nature of the electrolyte in the same sequence as the single cation entropy in DMSO.108, 09,329 The entropy of formation showed a maximum at negative charges. The difference in AS between the maximum and the value at ff=ocan be taken as a measure of the specific ordering of the solvent at the electrode/solution interface. Data 108,109304314 have shown that A(AS) decreases in the sequence NMF > DMSO > DMF > H90 > PC > MeOH. 

Climent V, Coles BA, Compton RG. 2002b. Coulostatic potential transients induced by laser heating of a Pt(lll) single-crystal electrode in aqueous acid solutions. Rate of hydrogen adsorption and potential of maximum entropy. J Phys Chem B 106 5988-5996. 

Climent V, Garcia-Araez N, Compton RG, Feliu JM. 2006. Effect of deposited bismuth on the potential of maximum entropy of Pt(lll) single-crystal electrodes. J Phys Chem B 110 21092-21100. 

Norskov and coworkers have determined the stability of reaction intermediates (mostly oxygen) on many single crystals and alloys of noble metals. Energy corrections due to solvent, zero-point energy, and entropy effects afforded experimentally relevant free energies for each mechanistic step. Their calculations treated the electrode potential with the same approach as Anderson, whereby each proton transfer was coupled with an energy shift of -et/ U being the potential difference between... 

In any practical electrochemical experiment the absolute, but unknown, metal/solution p.d. at the reference electrode must normally vary with temperature on account of the single interface reaction entropy change. This leads to the now well-known situation that measurements of or io as a function of temperature can never give the true or real heat of activation for the electrode process. This was first pointed out by Temkin who showed that... 

V. WATER REORIENTATION ON SINGLE-CRYSTAL ELECTRODES FROM NANOSECOND LASER-PULSED EXPERIMENTS. POTENTIAL OF MAXIMUM ENTROPY OF DOUBLE-LAYER FORMATION... 

In Fig. 2.3, the EP of Li cV205 I Li+ electrode and the electrode s temperature coefficient are overlapped in a single chart. Distinct dependence of entropy upon composition allows concluding that instead of the true plateaus on the equilibrium curve, a weak dependence upon composition takes place. A sharp peak at X = 0.5 most likely signals a phase transition at this composition. 

Among the papers dealing with potential measurements at isothermal half-cells at normal pressure, such of industrial interest are remarkable, e.g. of the chlorine electrode [41]. Very precise studies at thermocells with the hydrogen electrode, the silver-silver chloride electrode and the silver electrode [42] provided single ion entropies and activation entropies. 

Instead of continuous modulation, single thermal pulses have been imposed by laser beams. This can be seen as a continuation and an expansion of the temperature-jump technique which had been introduced to study kinetics of ionic processes [36]. The method has found application preferably with single-crystal electrodes [65-70]. Many fundamental quantities have been determined, among them the potential of zero charge (Fpzc) of Au(l 11) [65], the potential of maximum entropy [66, 70], the process of hydrogen adsorption at platinum surfaces [67, 68] and the entropy of double-layer formation [69]. This quantity also has been... 

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