Thermodynamics, underpotential

In order to examine the possible relationship between the bulk thermodynamics of binary transition metal-aluminum alloys and their tendency to form at underpotentials, the room-temperature free energies of several such alloys were calculated as a function of composition using the CALPHAD (CALculation of PHAse Diagrams) method [85]. The Gibbs energy of a particular phase, G, was calculated by using Eq. (14),... 

Swathirajan and Bruckenstein [113] have recently discussed the thermodynamics and kinetics of underpotential deposition phenomena on polycrystalline electrodes. Since work functions and surface structural features vary in different crystal faces, upd phenomena on single crystals have shown distinctive differences [109]. 

More recently, the use of a pyridinium mediator in an aqueous p-GaP photo-electrochemical system illuminated with 365 nm and 465 nm light has been reported [125], In this case, a near-100% faradaic efficiency was obtained for methanol production at underpotentials of 300-500 mV from the thermodynamic C02/methanol couple. Moreover, quantum efficiencies of up to 44% were obtained. The most important point here, however, was that this was the first report of C02 reduction in a photoelectrochemical system that required no input of external electrical energy, with the reduction of C02 being effected solely by incident fight energy. 

See also the main entry -+ underpotential deposition. Refs. [i] Haissinsky M (1933) J Chim Phys 30 27 [ii] Frumkin AN (1934) Zh Fiz Khimii 5 240 [iii] Kolb DM (1978) Physical and electrochemical properties of metal monolayers on metallic substrates. In Gerischer H, Tobias CW (eds) Advances in electrochemistry and electrochemical engineering, vol. 11. Wiley New York, p 125 [iv] Conway B (1984) Progr Surf Sci 16 1 [v] Ye S, Uosaki K (2003) Atomically controlled electrochemical deposition and dissolution of noble metals. In BardAJ, Stratmannn M, Gileadi E, Urbakh M (eds) Thermodynamics and electrified interfaces. Encyclopedia of electrochemistry, vol. 1. Wiley-VCH, p 471 [vi] Adzic R (2003) Electrocatalysis on surfaces modified by metal monolayers deposited at underpotentials. In Bard AJ, Stratmannn M, Gileadi E, Urbakh M (eds) Thermodynamics and electrified interfaces. Encyclopedia of electrochemistry, vol. 1. Wiley-VCH, p 561... 

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. 

So-called underpotential deposited species arise when an electrochemical reaction produces first, on a suitable substrate adsorbent metal, a two-dimensional array or in some cases two-dimensional domain structures (cf. Ref 100) at potentials lower than that for the thermodynamically reversible process of bulk crystal or gas formation of the same element. The latter often requires an overpotential for initial nucleation of the bulk phase. The thermodynamic condition for underpotential deposition is that the Gibbs energy for two-dimensional adatom chemisorption on an appropriate substrate must be more negative than that for the corresponding three-dimensional bulk-phase formation. Underpotential electrochemisorption processes commonly involve deposition of adatoms of metals, adatoms of H, and adspecies of OH and O. 

The formation of 2D Meads phases on a foreign substrate, S, in the underpotential range can be well described considering the substrate-electrolyte interface as an ideally polarizable electrode as shown in Section 8.2. In this case, only sorption processes of electrolyte constituents, but no Faradaic redox reactions or Me-S alloy formation processes are allowed to occur, The electrochemical double layer at the interface can be thermodynamically considered as a separate interphase [3.54, 3.212, 3.213]. This interphase comprises regions of the substrate and of the electrolyte with gradients of intensive system parameters such as chemical potentials of ions and electrons, electric potentials, etc., and contains all adsorbates and all surface energy. Furthermore, it is assumed that the chemical potential //Meads is a definite function of the Meads surface concentration, F, and the electrode potential, E, at constant temperature and pressure Meads (7", ). Such a model system can only be... 

Information about the influence of 2D UPD phases on thermodynamics and kinetics of subsequent 3D Me nucleation and growth can be obtained by UPD-OPD transition experiments. In general, the experiment has two stages. In the initial stage i, a 2D Me UPD phase is formed and eventually equilibrated at a selected underpotential AE. The final stage f of the system is characterized by an external potentiostatic pulse to t]f into the OPD range. There are two possibilities for pulse excitation techniques potentiostatic or galvanostatic conditions. 

The third part treats surface modification by underpotential deposition (UPD) of metals. Physical nature, thermodynamics, structural aspects, kinetics, as well as surface alloy formation are discussed. Experimental support is given based on classical electrochemical investigations as well as on some recent results from modern in situ surface analytical studies including atomic imaging by in situ STM and AFM. 

In electrochemistry, adsorbed hydrogen is denoted as either the underpotentially deposited hydrogen, Hypd, that is the H adlayer formed under thermodynamic equilibrium conditions where the coverage is changed reversibly with the potential applied or the over-potentially deposited hydrogen. Hqpd. as defined by Conway and co-workers [102] for the... 

A complete charge transfer makes it possible to introduce the term adatom, whereas the phenomenon of adatom formation, which occurs before the thermodynamic potential of the corresponding system is reached, was called underpotential deposition (upd) (see Chap. 6). 

It was described above how the metal ion is adsorbed on its own metal. Metal ions, however, can also be adsorbed on other metal substrates. This deposition is observed before bulk deposition and sometimes it looks as if a metal deposition occurs against the rules of thermodynamics. Therefore, this phenomenon is called underpotential deposition (UPD). Budevski, Staikov, and Lorenz gave a comprehensive treatment of UPD. In the following only a short description of the typical problems in this research field and its influence on surface treatment and phase formation will be described. 

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). 

Within the voltage limits set by the thermodynamic stability range of the electrolyte, foreign metal electrodes may sometimes be regarded as ideally polarizable or blocking. The metal electrodes must not react with the electrolyte, and for the moment adsorption and underpotential deposition will be neglected. From an electrochemical point of view, this is the simplest type of interface and has furnished much of the information we have about the electrified interface. 

Swathirajan S, Burckenstein S (1983) Thermodynamics and kinetics of underpotential deposition of metal monolayers on polyciystalline substrates. Electrochim Acta 28 865-877... 

For the alternated deposition of two elements, formation of a compound is a thermodynamically preferred result. The amount of the first element on the surface helps determine the amount of the next. For a binary compound like CdS, the elemental atomic layer coverages should be identical, beyond the first few cycles where the substrate may have an influence. The same compound should result over a range of underpotentials. The more of the first element deposited, the more of the second that will deposit, as the stoichiometry of the deposit controls the relative coverages. For CdS, the larger the underpotential used to deposit S, the less that will deposit. Consequently, in the subsequent step, less Cd will deposit, maintaining the CdS stoichiometry. CdS will still grow, though at a lower rate (nm/cycle). To form a conformal stoichiometric deposit, the applied potentials... 

Fundamental Thermodynamic Aspects of the Underpotential Deposition of Hydrogen, Semiconductors, and Metals... 

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