Electrosorption thermodynamics

Refs. [i] Gileadi E (1967) Electrosorption. Plenum Press, New York [ii] Bockris TOM, Srinivasan S (1969) Fuel cells their electrochemistry. McGraw-Hill, New York, p 96 [iii] Hordnyi G (2002) State of art present knowledge and understanding. In Bard A], Stratmann M, GileadiE, Ur-bakh M (eds) Thermodynamics and electrified interfaces. Encyclopedia of Electrochemistry, vol. 1. Wiley-VCH, Weinheim, p 349... 

In this review, we will consider the adsorption of a single species coadsorption phenomena will not be considered, since it is generally impossible to divide the flow of charge among several species. We will present the thermodynamics on which the concept of the electrosorption valency is based, discuss methods by which it can be measured, and explain its relation to the dipole moment and to partial charge transfer. The latter can be explained within an extension of the Anderson-Newns model for adsorption, which is useful for a semi-quantitative treatment of electrochemical adsorption. Our review of concepts and methods will be concluded by a survey of experimental data on thiol monolayers, which nowadays are adsorbates of particular interest. 

Another useful thermodynamic relationship that allows the potential dependence of l to be determined from the differential capacity Co, of the interphase at constant rs is readily obtained from the very definition of l. Choosing Ez as the reference potential and denoting by lz the electrosorption valency at Ez, the l value at any other applied potential E is given by ... 

The most common procedure for estimating X consists in measuring the thermodynamically significant electrosorption valency l and in evaluating its extrathermodynamic contributions k vkw and g of Eqs. (26) and (27) in order to extract the X value. A more direct extrathermodynamic procedure that relies exclusively on the Gouy-Chapman theory can be applied to the important class of self-assembled thiol monolayers anchored to a metal surface. 

Chapter 3, by Rolando Guidelli, deals with another aspect of major fundamental interest, the process of electrosorption at electrodes, a topic central to electrochemical surface science Electrosorption Valency and Partial Charge Transfer. Thermodynamic examination of electrochemical adsorption of anions and atomic species, e.g. as in underpotential deposition of H and metal adatoms at noble metals, enables details of the state of polarity of electrosorbed species at metal interfaces to be deduced. The bases and results of studies in this field are treated in depth in this chapter and important relations to surface -potential changes at metals, studied in the gas-phase under high-vacuum conditions, will be recognized. Results obtained in this field of research have significant relevance to behavior of species involved in electrocatalysis, e.g. in fuel-cells, as treated in chapter 4, and in electrodeposition of metals. 

As a result, the entropy of electrosorption is usually positive. Remembering the well known thermodynamic relationship... 

Several important features of electrosorption follow from this simple equation. First it becomes clear that the thermodynamics of electrosorption depends not only on the properties of the organic molecule and its interactions with the surface, but also on the properties of water. In other words, the free energy of electrosorption is the difference between the free energy of adsorption of RH and that of n water molecules ... 

No more than the slopes do they contain the electrosorption Gibbs energy. The surface pressure is experimentally accessible by Integration of the Gibbs equation dy = -Xjr d/i. The fact that some components are dubbed "charge determining" does not matter provided the sum Is taken over electroneutral components. This Is our choice, not that of thermodynamics which Is model-free. In practice this means that the r.h.s. of. for instance (3.12.4b( Is Integrated from the pristine surface at the p.z.c. to the final condition, determined by the final values of a°. cr and r. The surface pressure is a function of state, therefore... 

In contrast to this thermodynamic interpretation of /, an attempt was made by Vetter and Schultze [3.226, 3.227] to obtain information on the location and the partial charge of (IP) in the compact double layer from the electrosorption valency. 

Extended thermodynamic [3.99] as well as kinetic [3.100] investigations have been carried out in this system (cf. Section 3.5) characterized by a significant positive Me-S lattice misfit (do.Pb = 0.3500 nm, rfo,Au = 0.2884 nm), which is equal to the misfit in the system AuQikl). Cyclic voltammograms and IXE) isotherms of the system Ag(/tA0/Pb, H, C104, measured with the TIL technique [3.99], are presented in Figs. 3.35 and 3.36, respectively. The electrosorption valency was found to he y= z = 2 in the entire UPD range (Fig. 3.37). Therefore, cosorption or competitive adsorption processes of anions can be excluded. 

It seems reasonable that these crihcal potentials may be explained on a thermodynamical basis, fp has been interpreted by electrosorption of the aggressive anions at the bare metal surface of a pit to a minimum surface concentration, in order to inhibit its repassivahon [3, 7, 9]. Similarly,... 

The thermodynamic interpretation is based on the derivative of the Lippmann equation defining the electrosorption valency y ... 

Strictly thermodynamically, the electrosorption valency only shows that no other co-adsorption of other ions occurs. However, there may be a compact adsorption layer. STM gives images for the two surfaces (Figure 4.32). For Pb on Ag(l 11) the image was interpreted to be a so-called filled honeycomb 3(2X2) structure and a compressed hep layer. For Pb on Ag(lOO) a c(2X2) structure was observed. Otherwise the adsorbed mass was similar to the Ag(l 11) substrate which lead to the model of a bi-layer. 

Differential values of Y can be also useful for interpretation of electrosorption valency in terms of sulfate vesus bi-suUate adsorption, as one of mostly untapped resources of platinum electrochemistry. However the main progress is expected from combination of precise electrochemistry, thermodynamic analysis, and independent physical information supported by an appropriate theoretical basis, with necessary links provided by computational community. The final Section contains some brief notes in this respect. 

The major difference between gas phase adsorption and electrosorption is that in the former case adsorption occurs on a bare surface, while in the latter case the metal substrate is solvated, i.e., covered with an adsorbed layer of solvent molecules. Thus it is evident that adsorption on electrodes is a replacement reaction. The observed standard free energy, enthalpy and entropy of adsorption can therefore only be related to the type of interaction between the adsorbate and the electrode if the corresponding thermodynamic quantities for the solvent are known and the number of solvent molecules replaced by each adsorbed organic molecule can be estimated. 

The same relationships also apply to the enthalpy and the entropy of electrosorption. The enthalpy of electro sorption turns out to be less (in absolute value) than the enthalpy of chemisorption of the same molecule on the same surface from the gas phase. On the other hand, the entropy of chemisorption from the gas phase is, as a rule, negative, since the molecule RH is transferred from the gas phase to the surface, losing in the process three degrees of freedom of translation. This is also true for electrosorption, but in this case n molecules of water are transferred from the surface to the solution, leading to a net increase of 3 (n— 1) degrees of freedom. As a result, the entropy of electrosorption is usually positive. Remembering the well known thermodynamic relationship... 

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