The electrochemical interface

The interface between the electrode and the electrolyte or electrochemical interface is the site where heterogeneous electrode reactions occur. The structure and electrical properties of this interfacial region are therefore relevant to electrode kinetics. 

Electrode kinetics can be regarded as a switch from electronic to ionic conductivity at the electrochemical interface [14]. The contact of two phases such as the electrode (in most cases an electronic conductor) and the electrolytic solution (ionic conductor) determines the structure and properties of the electrochemical interface. 

The electrochemical interface has a very large electrical capacity (— 10 s F cm-2) compared with the solid—gas or solid—vacuum interface due to the existence of an ionic space charge localized at a short distance from the electrode. 

The equilibrium condition between the two phases, containing charged particles, in contact with one another is given by 

The electrochemical potential can be separated into chemical and electrical contributions 

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Abruiia H D 1991 Electrochemical Interfaces Modern Techniques for In Situ Interface Characterization (New York VCH) Comprehensive introduction into in situ teclmiques for the investigation of the electrochemical interface. 

W. Schmickler, D. Henderson. New models for the electrochemical interface. Prog SurfSci 22 323-420, 1986. 

In solid-state systems it is often advantageous to have some of the electrolyte material mixed in with the reactant. There are two general advantages that result from doing this. One is that the contact area between the electrolyte phase and the electrode phase (the electrochemical interface) is greatly increased. The other is that the presence of the electrolyte material changes the thermal expansion characteristics of the electrode structure so as to be closer to that of the pure electrolyte. By doing so, the stresses that arise as the result of a difference in the expansion coefficients of the two adjacent phases that can use mechanical separation of the interface are reduced. 

The term polarography basically refers to a method, where the current flowing across the electrochemical interface is recorded as a function of the applied electrode potential, historically in most cases a mercury electrode is involved. Thus polarography might be called also voltammetry. This sometimes results in confusing terms like e.g. AC voltammetry, which is obviously equivalent to AC polarography (see following entry). (Data obtained with this method are labelled DCP.)... 

Yamakata, A., Uchida, T., Kubota, J. and Osawa, M. (2006) Laser-induced potential jump at the electrochemical interface probed by picosecond time-resolved surface-enhanced infrared absorption spectroscopy./. Phys. Chem. B, 110, 6423-6427. 

Specific aspects examined here include insights and conclusions derived from the most recently performed density functional theory (DFT) calculations, which have been based on a comprehensive model of the electrochemical interface, and the strong disagreements (which seem to defy all recent theoretical efforts) that remain regarding proper interpretation of experimental ORR results and proper identihcation of the ORR mechanism in a PEFC cathode employing Pt catalysts. 

Accordingly, Neurock and co-workers have developed models for the electrochemical interface that retain this concept of hexagonal stmcture over close-packed metal surfaces [FiUiol and Neurock, 2006 Taylor et al., 2006c]. With the use of a screening charge as described in Section 4.3, the sensitivity of the stmctural parameters of water with respect to the electrochemical environment were explored [Taylor et al., 2006a]. The predominant effect stems from the polar nature of the water molecule, in which the water molecules are observed to rotate as a function of the applied potential. 

Taylor CD, Wasileski SA, Filhol JS, Neurock M. 2006b. First principles reaction modeling of the electrochemical interface Consideration and calculation of a tunable surface potential fi om atomic and electronic structure. Phys Rev B, 73. 

Taylor CD, Kelly RG, Neurock M. 2007c. Theoretical analysis of the nature of hydrogen at the electrochemical interface between water and a Ni(lll) single-crystal electrode. J Electrochem Soc 154 F55-F64. 

Vidal F, Busson B, Tadjeddine A. 2005. Probing electronic and vibrational properties at the electrochemical interface using SFG spectroscopy Methanol electro-oxidation on Pt(llO). Chem Phys Lett 403 324-328. 

Tadjeddine A, Peremans A. 1996. Vibrational spectroscopy of the electrochemical interface by visible infia-ed sum-frequency generation. Surf Sci 368 377-383. 

IR absorbance was measured with a Fourier-transform IR spectrometer. The absorbance at wave number a is defined as (1 /TV) In [F(U0)/ F(U)], where N 10 is the number of useful reflections at the electrochemical interface, F(U) the light intensity at wave number a reaching the detector at potential U, and F(U0) the same but under reference conditions at potential U0. 

Specular reflection of electromagnetic radiation at the (electrochemical) interface is generally described by Fresnel equations. Supposing the most simple case that both the electrolyte and electrode are transparent and differ only in their refractive indexes, nx and n2, the reflectivity for normal incidence of the radiation equals ... 

Methods employing X-rays and y-radiation are used less often in electrochemistry. The possibility of using X-ray diffraction for in situ study of the electrode surface was first demonstrated in 1980. This technique has long been used widely as a method for the structural analysis of crystalline substances. Diffraction patterns that are characteristic for the electrochemical interface can be obtained by using special electrochemical cells and elec-... 

Electrochemical processes are always heterogeneous and confined to the electrochemical interface between a solid electrode and a liquid electrolyte (in this chapter always aqueous). The knowledge of the actual composition of the electrode surface, of its electronic and geometric structure, is of particular importance when interpreting electrochemical experiments. This information cannot be obtained by classical electrochemical techniques. Monitoring the surface composition before, during and after electrochemical reactions will support the mechanism derived for the process. This is of course true for any surface sensitive spectroscopy. Each technique, however, has its own spectrum of information and only a combination of different surface spectroscopies and electrochemical experiments will come up with an almost complete picture of the electrochemical interface. XPS is just one of these techniques. 

Electrochemical reactions are driven by the potential difference at the solid liquid interface, which is established by the electrochemical double layer composed, in a simple case, of water and two types of counter ions. Thus, provided the electrochemical interface is preserved upon emersion and transfer, one always has to deal with a complex coadsorption experiment. In contrast to the solid/vacuum interface, where for instance metal adsorption can be studied by evaporating a metal onto the surface, electrochemical metal deposition is always a coadsorption of metal ions, counter ions, and probably water dipols, which together cause the potential difference at the surface. This complex situation has to be taken into account when interpreting XPS data of emersed electrode surfaces in terms of chemical shifts or binding energies. 

These measurements have verified that the work function of an electrode, emersed with the double layer intact, depends only on the electrode potential and not on the electrode material or the state of the electrode (oxidized or covered with submonolayer amounts of a metal) [20]. Work function measurements on emersed electrodes do not serve the same purpose as in surface science investigations of the solid vacuum interface. At the electrochemical interface, any change of the work function by adsorption is compensated by a rearrangement of the electrochemical double layer in order to keep the applied potential i.e. overall work function, constant. Work function measurements, however, could well be used as a probe for the quality of the emersion process. Provided the accuracy of the measurement is good enough, a combination of electrochemical and UPS measurements may lead to a determination of the components of equation (4). 

More than a decade ago, Hamond and Winograd used XPS for the study of UPD Ag and Cu on polycrystalline platinum electrodes [11,12]. This study revealed a clear correlation between the amount of UPD metal on the electrode surface after emersion and in the electrolyte under controlled potential before emersion. Thereby, it was demonstrated that ex situ measurements on electrode surfaces provide relevant information about the electrochemical interface, (see Section 2.7). In view of the importance of UPD for electrocatalysis and metal deposition [132,133], knowledge of the oxidation state of the adatom in terms of chemical shifts, of the influence of the adatom on local work functions and knowledge of the distribution of electronic states in the valence band is highly desirable. The results of XPS and UPS studies on UPD metal layers will be discussed in the following chapter. Finally the poisoning effect of UPD on the H2 evolution reaction will be briefly mentioned. 

While the above XPS results give the impression, that the electrochemical interface and the metal vacuum interface behave similarly, fundamental differences become evident when work function changes during metal deposition are considered. During metal deposition at the metal vacuum interface the work function of the sample surface usually shifts from that of the bare substrate to that of the bulk deposit. In the case of Cu deposition onto Pt(l 11) a work function reduction from 5.5 eV to 4.3 eV is observed during deposition of one monolayer of copper [96], Although a reduction of work function with UPD metal coverage is also observed at the electrochemical interface, the absolute values are totally different. For Ag deposition on Pt (see Fig. 31)... 

The various examples discussed above and several others, not mentioned in this necessarily incomplete chapter, demonstrate that XPS and also UPS assist and improve our understanding of the electrode/electrolyte interface and of electrochemical reactions. XPS, UPS and other ex situ techniques will continue to play a key role in providing information about the structure and composition of the electrochemical interface on a microscopic scale. 

Since the electrochemical interface is usually the interface between a metal and an electrolyte, all properties of the interface may be expected to involve contributions of the metal and of the electrolyte. However, most theories of the electrochemical interface are theories of the electrolyte phase, with no reference to the contributions of the metal. Here, we discuss more recent theoretical work which attempts to redress this inequity. As we shall see, it is not, in general, possible to separate, experimentally, the metal contribution from the electrolyte contribution. 

It is, of course, usual in discussing the electrochemical interface to use a dielectric constant, which is the ratio of the electric displacement to the electric field. By Fourier transforming the dielectric function e(k), one would obtain an effective dielectric constant, which would, however, depend on position. In fact,48 the screening... 

With the proper definitions of ex and k0, this equation is applicable to the metal as well as to the electrolyte in the electrochemical interface.24 Kornyshev et al109 used this approach to calculate the capacitance of the metal-electrolyte interface. In applying Eq. (45) to the electrolyte phase, ex is the dielectric function of the solvent, x extends from 0 to oo, and x extends from L, the distance of closest approach of an ion to the metal (whose surface is at x = 0), to oo, so that kq is replaced by kIo(x — L). Here k0 is the inverse Debye length for an electrolyte with dielectric constant of unity, since the dielectric constant is being taken into account on the left side of Eq. (45). For the metal phase (x < 0) one takes ex as the dielectric function of the metal and limits the integration over x ... 

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