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In the present communication, an attempt is made to demonstrate (once again) some of the problems with the derivation of the “generalized Lippmann equation” considered to be valid by many researchers for solid electrodes and to address the problems in the framework of the Gibbs model of the interface by using only the basic principles of thermodynamics. By surveying the relevant literature, it has been shown that during the derivation of the equation, it was completely ignored that the Gibbs-Duhem equation (i.e., the electrocapillary equation) is a mathematical consequence which follows directly from the homogeneous degree one property of the corresponding thermodynamic potential function; consequently, the resulting expression cannot be correct. Some alternative approaches have also been considered. The adequacy of the open system and the partly closed system approach has been critically discussed, together with the possibility of introducing new thermodynamic potential functions.

On the basis of a survey on the relevant literature it can be stated that some views and approaches concerning the charged state of adsorbed species and the charge transfer processes occurring with them are far from being unambiguous even in some respect they contradict fundamental physical and physicochemical principles. The meaning of the electrosorption valency, the misleading formulation of the Gibbs adsorption equation, and the interpretation of redox processes occurring with adsorbed species, is discussed in detail. It has been concluded that although the electrosorption valency of an adsorbed species as usually defined is an extra-thermodynamic and self-contradictory concept, experimental determined formal partial charge numbers can be a useful tool for scientists investigating adsorption phenomena, since the observed deviation between its value and the charge number of the same species in the solution phase unequivocally indicates a non-simple mechanism of the adsorption process, which should be taken into account in theoretical interpretation of the experimental data. It has been emphasized that the evaluation of voltammetric curves obtained in the presence of adsorbed redox partners requires a cautious analysis of the accompanying chemical transformations. In the framework of a critical analysis it is demonstrated that probably one of the most important sources of the misinterpretations and misunderstandings is the inadequate approach to the concept of electrode charge. The possibility of a general and straightforward presentation of the Gibbs adsorption equation has also been discussed.

Poly(3,4-ethylenedioxypyrrole) films on gold substrate were prepared by electropolymerization of ethylenedioxypyrrole monomer (EDOP) under potentiodynamic conditions in aqueous sodium sulfate solutions. The aim of the work was to characterize the gold | poly(3,4-ethylenedioxypyrrole) electrodes in aqueous sulfuric acid solutions and to compare the results with those of other studies that investigated similar systems. However, the experimental results obtained for the PEDOP modified electrodes significantly deviated from the expectations based on studies with similar systems. According to the results, two distinct types of polymer films were formed depending on the storage history of the monomer solutions. By using “fresh” monomer solutions polymer films with nearly ideal capacitive behavior were obtained and almost rectangular-shaped cyclic voltammograms could be observed in a rather broad potential range. Impedance spectra revealed that the charge transfer resistance between the substrate and the film was low. According to the SEM images the polymer layer on the gold substrate was relatively smooth with some small and short cracks. In contrast, working with “old” monomer solution that was not properly stored, the shapes of the cyclic voltammograms recorded at the Au | PEDOP | 0.1 M sulfuric acid (aq) electrode exhibited two peaks, a reduction peak at about −0.2 V vs. SSCE and an oxidation peak close to 0.2 V vs. SSCE, and the charge transfer resistances were considerably higher than those estimated for the electrode prepared using “fresh” monomer solutions. SEM images showed that the surface of the polymer film was extremely rough, with several wrinkles, creases and large cracks. This behavior was quite unexpected, because the two types of samples were prepared in the same way and only the storage histories of the two commercial monomer solutions were different.

The present study describes the preparation and electrochemical characterization of poly(bisphenol A) (poly(BPA)) supported poly(3,4-ethylenedioxythiophene) (PEDOT) modified gold electrodes. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to characterize the electrochemistry of the Au | PEDOT | 0.1 mol/dm3 H2SO4(aq.) and Au | [poly(BPA)/PEDOT] | 0.1 mol/dm3 H2SO4(aq.) electrodes before and after overoxidation. Scanning electron microscopy (SEM) was used for the study of the structure/morphology of the polymer coatings. Similarly to overoxidized PEDOT modified electrodes, time evolution of the electrochemical properties of the poly(BPA)/PEDOT films could be observed after overoxidation. The four-dimensional (4D) analysis method (i.e., a post-experimental mathematical/analytical procedure to overcome the problems caused by non-stationarity) and complex nonlinear least-squares (CNLS) fitting has been used for the estimation of the impedance parameters corresponding to different time instants after overoxidation of the [poly(BPA)/PEDOT] film. The deviations of the impedance responses from the purely capacitive behavior predicted at low frequencies by the theoretical models could be well explained solely by the assumption of uneven film thickness. It has been found that the impedance model, which takes into account the film thickness distribution gives a good description of the impedance data, both before and after overoxidation. According to the results, after the electrochemical deposition of poly(BPA) on the PEDOT layer the resulting [poly(BPA)/PEDOT] modified electrode becomes more resistant against the negative effects of overoxidation.

It is a widespread opinion that the electrochemical behavior of gold in aqueous media has been thoroughly studied, and the interface stress changes of gold can be explained by monolayer oxide formation/removal and adsorption phenomena. However, there are some peculiarities to consider. For instance, a sudden change in the slope of the interface stress vs. electrode potential curve recorded during the negative-going scan of the cyclic voltammogram can be observed in the oxide reduction region of gold in sulfuric acid solutions (Fig. 1). The method of dual dynamic voltammetry, i.e. the simultaneous application of dynamic potential programs to the generator and the collector electrodes of a generator-collector system, is a promising new method for studying the mechanisms of electrochemical processes [1-3]. By applying this method to a gold/gold rotating ring-disk electrode (RRDE) it could be shown that the sharp change in the slope of the surface stress vs. potential curve recorded in the oxide reduction region of gold in sulfuric acid solutions [4] is accompanied by the formation of soluble, electrochemically reducible gold species which can be detected at the ring electrode. References: [1] S. Vesztergom, M. Ujvári, G.G. Láng, Electrochemistry Communications, 19, 1–4 (2012). [2] N. Kovács, M. Ujvári, G.G. Láng, P. Broekmann, S. Vesztergom, Instrumentation Science and Technology, 43, 633–648 (2015). [3] S. Vesztergom, N. Barankai, N. Kovács, M. Ujvári, P. Broekmann, H. Siegenthaler, G.G. Láng, Electrochemistry Communications, 68, 54–58 (2016). [4] G.G. Láng, C.A. Barbero, „Laser Techniques for the Study of Electrode Processes”, Springer-Verlag, Berlin, Heidelberg, 2012. _ Figure 1

In this review, results of recent studies on the electrochemical stability and degradation properties of poly(3,4-ethylenedioxythiophene) films are summarized, with particular emphasis on the structural changes induced by overoxidation and electrochemical degradation. The most important electrodeposition methods for the preparation of PEDOT films in surfactant free aqueous media have also been summarized, and several experimental techniques suitable for monitoring the degradation process have been discussed. Morphological changes in PEDOT films during overoxidation have been analyzed. Overoxidation mechanisms proposed in the literature have been surveyed.

Time dependence of the electrochemical impedance of an overoxidized glassy carbon|poly(3,4-ethylenedioxytiophene) (PEDOT)|0.1 mol·dm-3 sulfuric acid (aq.) elec­trode has been investigated. To follow the changes occurring at the film/substrate interface after the overoxidation procedure, successive impedance measurements were carried out. Although the system is intrinsically nonstationary, the charge transfer resis­tance (Rct) cor­res­ponding to different time instants could be determined by using the so-called 4-dimensional analysis method. The same post-experimental mathematic­cal/ana­lytical procedure could be used also for the estimation of the charge transfer resistance corresponding to the time instant just after overoxidation of the PEDOT film. The increase of the charge transfer resistance of the overoxidized system with respect to that of the pristine electrode suggests that during overoxidation the electrochemical activity of the film decreases and the charge transfer process at the metal/film interface beco­mes more hindered. After the overoxidation procedure, when the electrode potential was held in the “stability region” (at E = 0.4 V vs. SSCE in the present case) the Rct decre­ased continuously with experiment time to a value somewhat higher than that of the pristine electrode. By comparing the properties of the GC|PEDOT|0.1 M H2SO4 and the Au|PEDOT|0.1 M H2SO4 electrodes a possible mechanistic explanation for the observed behavior has been proposed. This is based on the assumption that in the case of the GC|PEDOT|0.1 M H2SO4 electrode two processes may occur simultaneously during the impedance measurements: (a) reduction of the oxidized surface of the GC substrate, including the reduction of the oxygen-containing surface functionalities and (b) read­sorption of the polymer chains (polymer chain ends) on the surface.

This special issue is the second part of the proceedings of the 6th Regional Symposium on Electrochemistry – South East Europe (RSE–SEE 6), held in Balatonkenese, Hungary, from 11−15 June 2017 (part 1 has been published in J. Electrochem. Sci. Eng. 8(1) (2018)). These are some of the best papers presented at the RSE-SEE 6 meeting and reflect the broad interests of the electrochemical community, including both new techniques and application areas. Since its start in 2008, the series of RSE-SEE symposia provides a stimulating international forum for electrochemistry researchers to share their current research results. Following this tradition, more than 110 participants from 24 countries presented papers in Balatonkenese and discussed recent progress in electrochemistry and electrochemical engineering. I would like to thank the authors for their contribution, the reviewers for their professional work and the jESE-Editorial Office for their assistance in the Proceedings publication. Also, I would like to acknowledge the financial and other support of the Hungarian Academy of Sciences, the International Society of Electrochemistry, the Furukawa Electric Group, Jászplasztik, Ametek, Zahner, Metrohm, PalmSens, Ivium, and Paks Nuclear Power Plant. The next 7th Regional Symposium on Electrochemistry – South East Europe will be organized jointly with the 8th Kurt Schwabe Symposium in Croatia in the late spring of 2019.

When studying electrochemical processes, one of the most widely used methods of determining the reaction pathway is an electrochemical assay of products using a rotating ring–disk electrode (RRDE). An RRDE tip consists of two electron conducting parts: a centrally located disk and a ring around it. When brought into contact with an electrolyte solution, the disk and the ring both form electrodes, the potentials of which can be independently controlled by a bi-potentiostat. When the tip is rotated, reactants from the solution arrive at the disk where they undergo an electrode reaction (oxidation or reduction) at a given rate, depending on the rotation speed. The formed products leave the disk surface and due to forced convection make their way towards the ring electrode where they can undergo another electrode reaction and can thus be detected. Normally, one applies potentiostatic control to at least one of the electrodes when carrying out an RRDE experiment; albeit the simultaneous potentiodynamic perturbation of the electrodes offers an increased applicability range. This paper presents the construction of a measuring system capable for the “bi-potentiodynamic” perturbation of two working electrodes, and demonstrates the use of such methods in case of a few chosen example systems.

The problems related to the electrochemical/electrocatalytic stability of perchlorate ions are reviewed in the light of recent experimental results. The electrocatalytic, catalytic, and electrochemical reduction processes are presented and the links between them are outlined. Some possible mechanisms of the complicated reduction processes are discussed. Various methods for the detection of reduction process are presented, e.g. voltammetry, impedance spectroscopy, and radiotracer methods. Environmental aspects and some methods for perchlorate removal and wastewater treatment are briefly summarized.

The carboxyl groups of atmospheric humic-like substances (HULIS) are of special interest because they influence the solubility in water, affect the water activity and surface tension of droplets in the air, and allow formation of chelates with biologically active elements. Experimentally determined abundances of the carboxyl group within HULIS by functional group analysis are consistent with our knowledge on the average molecular mass of HULIS if the number of dissociable carboxyl groups is assumed to be rather small. The best agreement between the average molecular mass derived from the existing abundance data and the average molecular mass published earlier occurs for assuming approximately one dissociable carboxyl group only. This implies that HULIS can not be regarded as polycarboxilic acid in diluted solutions. The average molecular mass of HULIS derived from our electrochemical measurements with the assumption of one dissociable carboxyl group or equivalently, one dissociable sulphate ester per molecule ranges from 250 to 310 Da. It was concluded that HULIS are a moderately strong/weak acid with a dissociation constant of about pK=3.4, which fits well into the interval represented by fulvic and humic acids. The mean number of dissociable hydrogen (i.e. of carboxyl groups and sulphate esters jointly) in HULIS molecules was refined to be between 1.1 and 1.4 in acidic solutions.

Urban-type PM2.5-fraction aerosol samples were collected and samples of pure atmospheric humic-like substances (HULIS) were isolated from them. Atmospheric concentrations of organic carbon (OC), water soluble organic carbon (WSOC) and HULIS were determined, and UV/Vis spectroscopic properties, solubility and conductivity of HULIS in aqueous samples were investigated. Atmospheric concentrations of OC and WSOC were 8.5 and 4.6 μg m−3, respectively. Hydrophilic WSOC accounted for 39% of WSOC, carbon in HULIS made up 47% of WSOC, and 14% of WSOC was retained on the separation column by irreversible adsorption. Overall average molecular mass and aromatic carbon abundance of HULIS were estimated from molar absorptivity to be 556 Da and 12%, respectively. Both results are substantially smaller than for standard reference fulvic acids, which imply different mechanisms for the formation processes of atmospheric HULIS and aquatic or terrestrial humic matter. HULIS were found to be water soluble as ionic unimers with a saturation concentration of 2–3 g l−1. Their solubility increased again with total HULIS concentration being above approximately 4 g l−1, which was most likely explained by the formation of HULIS aggregates. Solubility increased linearly from approximately 5 up to 20 g l−1 of dissolved HULIS concentration. The ionic dissolution was confirmed by electrochemical conductivity in the investigated concentration interval. Limiting molar conductivity was extrapolated and this was utilized to determine the apparent dissociation degree of HULIS for different concentrations. The dissociation degree was further applied to derive the concentration dependence of the van't Hoff factor of HULIS. The van't Hoff factor decreased monotonically with HULIS concentration; the decrease was substantial for dilute solutions and the relationship became weak for rather concentrated solutions.

Electrochemical impedance spectra obtained for poly( o -phenylenediamine) films on platinum in contact with 0.5 M H 2 SO 4 solution were analyzed on the basis of the brush model elaborated for conducting polymer films on electrodes. It is assumed that the polymer film consists of short and long polymer chains forming bundles in such a way that only a small part of the metal substrate is covered by the polymeric material. The analysis of the impedance spectra taken at several potentials in the region of hydrogen adsorption on platinum supports the validity of the model. The model has been tested also by varying the film thickness and the roughness of the platinum substrate.

The ability of simple inorganic reactions such as NaOH + CuCl 2 , NaOH + AgNO 3 , and CuCl 2 + K 3 [Fe-(CN) 6 ] to form various complex precipitation patterns has recently been discovered. When the first reagent penetrates into a hydrogel by diffusion and reacts with the second reagent homogenized in the gel, a precipitate is built up that has two kinds of bordering surfaces with different properties. Our experimental results suggest that one of these surfaces is permeable, while the other one is an ion-selective membrane that restrains the diffusion of the first reagent's reacting ion, preventing the reactions from proceeding ahead of the membrane. The presence of an intermediary compound, the precursor of the precipitate, is observed in the front of the permeable surface.

The problem of the reduction of perchlorate ions is discussed in the light of recent results bearing in mind the perchlorate environmental contamination challenge and recent problems concerning the electrochemical stability of ClO4− ions. The catalytic and electrocatalytic transformations are compared in the case of WC and Pt electrodes (catalysts). Various methods for the detection of the occurrence of the reduction process are presented (radiotracer technique, potential step method and voltammetry). It is shown that the interpretation of the results of studies of corrosion processes of Al, Zn, Cu, Ni and Fe carried out in the presence of ClO4− ions cannot be considered as adequate without taking into consideration the reduction of ClO4− ions and all previous work should be revisited on the basis of the new findings. Some elements of the mechanistic aspects of the complicated reduction processes are discussed.

Polarization curves of CdSe electrodeposited from aqueous ammonia solutions containing selenosulfate and the EDTA complex of cadmium ions follow Tafel lines for current densities smaller than cathodic and anodic reaction limited current densities. Reaction orders for electrodeposition are one with respect to selenosulfate and zero with respect to cadmium ions. At high flow velocities of the electrolyte, only thin layers of CdSe are obtained. Very small effective diffusion coefficients determined from diffusion limited currents indicate diffusion into the electrolyte of a reaction product formed in an equilibrium reaction. Three-dimensional nucleation on the thin layer occurs at low flow velocities. A mechanism modeling the nucleation and growth kinetics is proposed. The Gibbs energy of formation of CdSe is calculated using measurements of the equilibrium potential of CdSe.

In order to explain the impedance spectra of polymer film electrodes a modification of the porous model is suggested based on experimental observations. In this ‘brush’ model it is assumed that a polymer film consists of short and long polymer chains forming bundles in such a way that only a small part of the metal substrate is covered by the polymeric material. The oxide layer formation on the gold substrate attests the reality of this physical model. The model has been tested by the analysis of the impedance spectra of Au/poly( o -phenylenediamine) electrodes in contact with 1 mol dm −3 perchloric acid. The spectra were taken at different potentials and the film thicknesses as well as the roughness of the gold substrate were systematically varied.

Measuring the bending of a plate to determine the stress in thin films coated on one side of a substrate is a common technique. The surface stress changes can be estimated by using an appropriate form of Stoney's equation, if the thickness of the film is sufficiently less than that of the substrate. The changes in the bending of the substrate are usually calculated from the displacement of the reflected laser beam measured, e.g., with a position sensitive photo detector (PSD). Since the displacement of the light spot on the PSD is measured in air outside the liquid phase (where the mirroring surface is located), the measured values should be corrected according to Snell's law of refraction. A relation between the reciprocal of curvature radius of the substrate and the displacement of the laser beam on PSD was derived by taking into account the bending of the laser beam due to refraction at the optical window. It is shown that the neglect of the refraction at the optical window may cause an error of 25–30% in the estimation of the stress changes.

The results presented on simultaneous oscillations of the potential and deflection of a platinum cantilever electrode during the galvanostatic oxidation of formic acid in acid media are the first experimental evidence that the oscillation arising in the course of galvanostatic oxidation of simple organic compounds is associated with the change of surface stress of the electrode. The results of the bending beam experiments may open up new vistas in the quantitative description of the surface energetics of heterogeneous oscillation phenomena.

Equilibrium thermodynamics rests on the assumption that fundamental equations exist containing all thermodynamic information about a given system. Fundamental relations describing the thermodynamic properties of homogeneous phases are homogeneous functions of the first degree in the extensive variables. Homogeneous functions obey Euler’s theorem. This is also true for the surface thermodynamics of stressed solids. The derivation of a ‘generalised Lippmann equation’ for stressed solids first proposed by Couchman et al. and adopted by IUPAC violates the mathematical requirements for a thermodynamic equation and thus cannot be valid.

A new model for the impedance of polymer film electrodes has been developed in that three mobile charge carriers are considered. It has been observed especially in the case of organic conductive polymer that beside the electron diffusion and counterion transport very often hydrogen ions also participate in the charge transport and charge transfer processes in the course of redox transformation of these polymers, i.e. a model describing this case is essential. It has been demonstrated that by the help of reasonable assumptions some of the mathematical problems can be overcome and an analytical solution can be obtained for the complex impedance in the case of low amplitude variation of the electrode polarization. The equations derived for the charge transport and charge transfer impedances are tested by CNLS fitting data obtained for an Au∣poly( o -phenylenediamine) electrode.

The energetics of electrified interfaces exhibiting constant phase element (CPE) behaviour is discussed. In the double layer region energy can be dissipated in various ways other than by charge transfer. Therefore, contrary to the statement in Zoltowski, J. Electroanal. Chem. 443 (1998) 149, an electrode with a double layer of CPE character can be an ideally polarisable electrode.

Electrochemical impedance spectra obtained for poly(o-phenylenediamine) films in contact with solutions of different compositions were analyzed on the basis of selected models. The Kramers-Kronig transformation has been used for validation of the impedance data. Complex non-linear least squares fitting and computer simulation have been used to derive reliable quantities characterising the electrochemical behaviour of these systems. It has been shown that the charge transport diffusion coefficient strongly decreases with increasing potentials. The potential dependence of low-frequency capacitance attests to strong interactions within the film that depends upon the nature of anions. The participation of protons in the redox reactions has also been demonstrated.

Changes of the specific surface energy of sputtered gold films on an oscillating quartz plate were measured by Koesters laser interferometry as functions of the electrode potential and of the concentration of sulphate in aqueous solutions of different pH simultaneously with changes of mass and current density. Impedance spectra were obtained under the same conditions. Equilibrium with respect to adsorbed mass was attained much faster than equilibrium with respect to electrode deformation. The potential of zero charge was independent of pH in the concentration range investigated. Capacitances derived from specific surface energies agreed well with those obtained from impedance spectra. The time dependence of the surface energy corresponds to a frequency dependence of the double-layer capacitance. The frequency dependence of the double-layer capacitance was characterized by a constant-phase element (CPE) with impedance ZCPE = (1B)(jω)−α. The parameters B and α were independent of frequency, but depended on the electrode potential. It was concluded that the CPE behaviour is probably due to energy dissipation in the solid during potential perturbation.

Chapter 1 deals with the definitions of some general and specific terms concerning the electrochemistry of solid–liquid interfaces. The definitions of the terms “electrode” and “solid electrode” as well as selected concepts in electrochemistry and surface science are discussed. It has been stressed that a “solid electrode” is a multicomponent, multiphase electrochemical system, containing at least one solid phase, which means that it cannot be treated as a pure solid in vacuum. It has been concluded that in case of a “solid electrode,” the experimentally determinable quantity is the change of the intensive parameter conjugate to the interface area. This quantity is intimately related to the properties of the solid-liquid interface and not just to the properties of the surface of the solid phase; therefore, it seems reasonable to refer to it as the “interface stress change” or “interfacial stress change.”

The importance of probe beam deflection (PBD) techniques in the electrochemical field of study is evaluated. A review of the historical development of PBD techniques, and of the closely related photothermal deflection spectroscopy, is then performed.

In this chapter, selected results of bending beam (bending plate) experiments with optical detection are presented as illustrative examples. In order to demonstrate the capabilities and limitations of stress measurements in electrochemical systems, relevant results obtained using the bending beam method and Kosters laser interferometry are discussed (e.g., platinum and gold in contact with electrolyte solutions, cyclic voltammetry and potential pulse experiments, stress changes in thin films and passive layers on metals, insertion of species into materials, monitoring of the electrochemical degradation of polymer films, simultaneous oscillations of electrode potential, surface mass and interfacial stress in the course of galvanostatic oxidation of formic acid on platinum, etc.).

In Chap. 6, advantages and limitations of some nonoptical techniques (e.g., scanning tunneling microscopy) used for the experimental determination of changes in cantilever deflection or for the measurement of the deformation of thin plates/disks in electrochemical systems are discussed. Typical experimental arrangements have been presented. Possible sources of errors (interaction forces between surfaces, clamping of probes, contact angles, design of the electrochemical cells) have been analyzed.

The theoretical framework of probe beam deflection (PBD) techniques is described. First, the optical principles underlying the measurement are discussed. Then, the analytical solutions are presented for electrochemical systems subjected to different potentials and current perturbations. Among them are potential pulses (chronodeflectometry), current pulse, and sinusoidal perturbations. The behavior of continuous and discontinuous processes is discussed. The possibility to study multiflux processes by chronodeflectometry is explored. New techniques, such as normal pulse voltadeflectometry (NPVD) and differential pulse voltadeflectometry (DPVD), are proposed. Then, different approaches used to simulate or process the probe beam deflection data measured along cyclic voltammograms are discussed. Those include digital simulation, Laplace transform, and convolution. Finally, a typical experimental setup for PBD is described.

Several probe beam deflection (PBD) studies of the ion exchange, occurring during electrochemical processes of surface-confined redox species, are described. Among them are the formation and reduction of oxide surface layers on noble metal electrodes, the double layer charging and specific ion adsorption on carbon and metal surfaces, the redox reactions of carbon surface groups, the charge compensation processes in electroactive oxides, redox and conductive polymer films, the irreversible oxidation of adsorbed species (e.g., CO), and the charge compensation processes in redox self-assembled multilayers. The systems are usually explored qualitatively using cyclic voltadeflectometry and the quantitatively analyzed using chronodeflectometry and/or diffusion deconvolution. The combination of the PBD measurements with other techniques (e.g., electrochemical quartz crystal microbalance) has proved to be useful to a more detailed insight on the ion exchange mechanism. The review of the published data suggests that surface-confined electrochemical systems are ideally suited to probe beam deflection measurements.

This chapter gives a brief overview of the mathematical background, some important mathematical concepts, and various functional relationships underlying the thermodynamic theory of interfaces (theory of homogeneous functions and partly homogeneous functions, Euler’s theorem and the Gibbs–Duhem equation, and Legendre transformation). The extensive or intensive nature of thermodynamic variables is emphasized, and the relationship to the theory of homogeneous functions is presented. The mathematical principles are explained and illustrated with simple examples.

In this chapter, optical methods for the experimental determination of interfacial stress changes in electrochemical systems are discussed. Typical experimental arrangements are presented (for “single beam” and the “multibeam” systems). The essential components of two frequently used methods—the “bending beam method” and “Kosters interferometry”—are lasers. The main difference between the two above techniques is the detection system: position sensing with photodiodes (“position-sensitive detectors,” PSD) in case of the bending beam method and automated detection of light intensity changes in case of the laser interferometer. Basic principles of these methods have been discussed. Possible sources of errors have been analyzed (effect of refraction, error due to nonnormal incidence or due to the lateral shift of the reflected laser beam at the optical window, errors related to the structure of the cantilever probe, etc.).

In this chapter, existing thermodynamic theories for solid–liquid interfaces are critically reviewed. The definition of the “generalized surface intensive parameter” is discussed. Some critical remarks have been made concerning the approach generally used in the literature. A brief review of the derivation of the “generalized Lippmann equation” held to be valid generally for solid electrodes is given. Special attention has been paid to the mathematical consequences of the homogeneous property of the fundamental equation. Inconsistencies in the derivation are cited. Problems related to the so-called “surface Gibbs−Duhem equation” are discussed. Some remarks concerning a recent theoretical debate on the derivation of the Shuttleworth equation are also included.

Different experimental techniques, which have been used to monitor the ion transport in electrochemical systems, are briefly reviewed. Among them are electrochemical techniques (scanning electrochemical microscopy, ring-disk rotating electrode, ion sensors), spectroelectrochemical techniques (in situ UV–visible spectroscopy with indicator dyes, infrared, and Raman), and various others (radiotracer detection, surface plasmon resonance, interferometry, electrochemical quartz crystal microbalance, etc.). The strengths and weakness of each technique is analyzed. In those cases where the same or similar systems have been also studied using PBD, the results are compared.

In Chap. 2, several aspects of the thermodynamic theory and their physical interpretation are discussed, and a simple and concise treatment of electrified interfaces within the framework of classical thermodynamics is presented. Basic concepts such as “surface,” “interface,” “interphase,” “interfacial or interface region,” “dividing surface,” and “adsorption” are reviewed; Gibbs and Guggenheim models in interfacial thermodynamics are discussed. The derivation of the electrocapillary equation, the Lippmann equation, and the Gibbs adsorption equation for an ideally polarizable electrode is presented. An example for the application of the electrocapillary equation is provided. An equation has been derived for ideally polarizable electrodes for the case in which there is no complete equilibrium between the bulk of a solid and the interface.

In Chap. 4, methods for the experimental determination of (interfacial) stress changes in electrochemical systems are presented. The piezoelectric method, the extensometer method and its variants (e.g., optical fiber interferometry with Mach–Zehnder interferometer), the “bending beam” (“bending cantilever,” “laser beam deflection,” “wafer curvature”) method and related techniques, and the method based on the measurement of contact angle are discussed in detail. Special attention has been paid to problems related to the use of the electrochemical bending beam and bending plate/disc methods. The theories of the methods are summarized, and typical experimental arrangements are presented. The kind and quality of information that can be achieved using these methods are discussed.

The theory of photothermal deflection spectroscopy (PDS) applied in situ to electrochemical systems is outlined. The predicted response to different forms of light excitation is described. Then, the experimental parameters relevant to the measurement of photothermal deflection signal in an electrochemical environment are discussed, along with the description of an actual photothermal deflection setup. Finally, experimental results of in situ electrochemical PDS and single wavelength photothermal deflection measurements are described. The techniques are compared with conventional optical absorption techniques such as transmission and reflectance spectroscopy.

Electrochemical impedance spectra obtained for poly(vinylferrocene) films are analyzed on the basis of selected theoretical models for the impedance of polymer-modified electrodes. The Kramers-Kronig transformation has been used for validation of the impedance data. Complex non-linear least squares fitting and computer simulation have been used to derive reliable quantities characterizing the electrochemical behaviour of these systems. It is demonstrated that the goodness of fit should be carefully checked in order to estimate parameters with adequate precision. It is emphasized that a relatively good fit concerning an impedance spectrum is a necessary, but not sufficient condition for the suitability of a model. Additional experimental test is required, e.g. measurements at a range of potentials, temperatures and film thicknesses.