Modern Thermoelectrochemistry

Abstract

By definition, modern thermoelectrochemistry has the basic concept of temperature as an independent variable. The intention of this branch of science has been expressed by the following comprehensive definition (formulation by L. Dunsch in 2009 [1]):

Keywords

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4.1 Objectives

By definition, modern thermoelectrochemistry has the basic concept of temperature as an independent variable. The intention of this branch of science has been expressed by the following comprehensive definition (formulation by L. Dunsch in 2009 [1]):

Modern thermoelectrochemistry as a branch of electrochemistry is devoted to the influence of the temperature as an independent variable on all charge transfer reactions at condensed interphases.

Following this definition, experimental methods of modern thermoelectrochemistry should allow fast and arbitrary variation of electrode temperature. Also, they should provide measuring techniques to follow small temperature changes occurring at electrode surfaces. This way, the study of electrochemical transients at electrode surfaces as a result of temperature changes (thermal distortions) should be enabled, similar to the classic transient techniques like chronoamperometry where a transient current is recorded as result of a “voltage distortion”. Thermal distortions can be imposed in the form of a temperature jump, followed by investigating the relaxation of the electrode surface. Alternatively, “temperature modulation” can be applied, where electrochemical effects are studied as response on periodic temperature variations in the form of sinusoidal or rectangular thermal “waves”.

The techniques of modern thermoelectrochemistry should provide ways to impose temperature variations at the place where the interesting processes are occurring, i.e. at the surface of a working electrode and not necessarily at the entire cell volume. Consequences of these characteristics are:

• Main interest of modern thermoelectrochemistry is study of single electrode properties.

• Modern thermoelectrochemistry typically works with non-isothermal cells.

The term “modern” is appropriate mainly for such techniques which allow to impose fast temperature changes in the form of single pulses or periodic waves. Pure calorimetric methods without thermal excitation are placed somewhere between classic and modern thermoelectrochemistry. Modern techniques should display properties which otherwise would not be accessible.

4.2 Heated Electrodes

To impose a temperature jump, the electrolytic interface either could be heated or could be cooled. Existing contemporary state of the technology does not allow fast and arbitrary cooling; hence the dominant technique of modern electrochemistry is heating. We can study the transient from cold to hot by different methods. The opposite transient, from hot to cold, is available with some methods, but only if a jump from cold to hot is preceding and if the spontaneous cooling down of the interesting interface is running fast. With directly heated thin wire electrodes or with microwave heating, respectively, both transients can be studied this way.

There are two ways to make use of heated electrochemical interfaces. When steep heat pulses are imposed (the so-called pulse heating), thermal convection can be ignored within some tenths of a second, so as to work in stagnant solution with the result of attaining very high temperature. Even work in superheated state far above the boiling point is attainable. The alternative is permanent heating, which restricts working temperature to values below the boiling point. On the other hand, this variant generates a highly efficient stirring effect with a diffusion layer of constant thickness and with a stable working temperature. The resulting voltammograms are of ideal sigmoidal shape. The different modes of operation have consequences on the electrochemical behaviour, resulting from differences in streaming processes. Details will be discussed in more detail in Chap. 5.

4.2.1 Techniques of Heating

As mentioned above, heating in modern electrochemistry means heating the electrode/electrolyte interface. In order to heat this interesting place, different approaches are available. The electrode material can be heated either, e.g., by laser illumination, by resistive or by inductive heating. Otherwise, instead of the electrode itself, a spot of solution close to the electrode interface may be heated, e.g. by focused microwaves or by resistive electrolyte heating using high-frequency current in parallel to electrolysis current. In both modes of operation, only a very small part of the electrochemical cell is affected by temperature changes, so as to make the latter fast and leave bulk solution free from alterations.

A further classification results from the differentiation between indirect or direct heating. The former means that “heater” and active electrode area are not identical but separated by an insulating spacer or something similar. Such arrangements by nature are of somewhat higher thermal inertia compared with devices for direct heating, where no passive intermediate has to be heated up together with the active interface. Maximum heating rates can be achieved only by direct heating.

Among the electrochemical heating techniques discussed here, application of laser pulses or similar techniques working with focused light illumination have disappeared somewhat out of sight, maybe because younger techniques can be run easier and cheaper. The highest degree of perfection and of practicability so far has been achieved with focused microwaves and, even more, with resistive heating of thin metallic wires (“hot-wire electrochemistry”). The latter technology has found broad application during the last years. Thereby, its distinctive features and experimental details will be presented in an own chapter further below (Chap. 6).

4.2.1.1 Heating of Solution Fractions

Miniaturising heated isothermal cells with the ambition to achieve faster heating-up could be considered to be a first step from classical to modern thermoelectrochemistry. This approach obviously does make sense only if the heated solution can be exchanged fast. A practicable design is given with flow-through cells where a certain region of the flowing solution is exposed to electric heaters. An exceptional design has been presented [2], where concentric metal tubes inside a flow-through system formed working and counter electrodes, respectively. A central heater in the form of an internal cylinder was used to heat the flowing solution. Ferrocyanide/ferricyanide solution was electrolysed to study the hydrodynamic conditions. A microelectrode has been included into a tube-sized streaming arrangement which is heated by an external heater in the form of a coil [3]. Figure 4.1 gives an impression of the system. Obviously, it represents a tiny isothermal cell where working temperature can be changed rather fast.

Fig. 4.1

Top: Working scheme of a heated flow-through system with microelectrode included. Bottom: Design of the instrument (from [3], with permission)

Heated flow-through arrangements also were proposed for high-pressure, high-temperature electrochemistry [3–7]. In such cells, a preheating unit is heating up a certain region of the flowing solution stream before it is reaching the electrolysis compartment containing active electrodes. Devices of this type were used for spectroelectrochemical investigations with Raman spectroscopy at different temperatures [3, 5]. The method could be considered to be the first approach to a thermo-spectroelectrochemistry. Kinetic investigations with similar flow-through cells also were successful, e.g. for kinetics of the Fe²⁺/Fe³⁺ redox couple [6]. Determination of pH also has been reported [7]. A typical example for cell design is given in Fig. 4.2.

Fig. 4.2

High-temperature/high-pressure flow-through cell. From [6], with permission

Non-pressurised flow-through cells with outer heating unit have been used to study oxygen reduction [8].

A completely different approach to establish a heated solution region is followed by two techniques which make use of an ordinary electrolysis vessel that is converted to give a non-isothermal cell just by generating a “hot spot” in close vicinity to the electrode-solution interface. The electrode material itself is not heated up, but the active interface assumes increased temperature, probably including an extremely fine part of the metal surface. For proper function of electrochemical processes, indeed micrometre dimensions would be more than sufficient.

Two ways to generate a hot spot have been reported. One of them makes use of focused microwaves in front of a microelectrode; the second one causes ohmic heating of a solution spot in front of a microelectrode by means of high-power, high-frequency current flowing in parallel with the electrolysis current.

Microwave heating is a well-documented, well-developed technology of modern electrochemistry [9–24]. An overview has been given in two reviews [9, 10]. The method has been described first in 1998 by Compton and co-workers [11]. The electrochemical cell is placed inside a microwave oven. As a working electrode, a long wire lead ending in a thin noble metal wire forming at the front side of a microdisk is used. The connection cable of this microelectrode is exposed to the microwave field inside the oven and acts as a kind of antenna. The “antenna” receives the microwave oscillations which leave the metallic part at the solution side acting as a focused energy bundle at this place. As a result, a hot solution spot is formed. This spot is assuming the increased temperature extremely fast, and also the temperature will decrease extremely fast after finishing microwave energy supply. Streaming phenomena in the vicinity of the spot as well as many other characteristics have been described in detail [12, 14]. Extremely strong thermal convection has been described as some kind of “jet boiling” [14]. Local energy concentration allows to work with superheated water as a solvent, like in hot-wire electrochemistry. Unfortunately, there seem to exist some uncertainties with knowledge of the true local temperature. Partially, this may be a consequence that all the bulk solution is located inside the microwave oven and that an uncontrolled warming-up is occurring additionally. It seems reasonable to apply short heating pulses, but in contrast to hot-wire electrochemistry, the strong convection does not allow to make use of a stagnant solution layer. The method probably is not qualified so much for more fundamental studies in electrochemistry. This may be the reason that analytical applications are prevailing so far. Many examples of electrochemical stripping determinations have been reported [15–19]. A typical apparatus is depicted in Fig. 4.3. Characteristics of the method, for continuous as well as for pulsed heating, are sketched in Fig. 4.4. The behaviour with permanent as well as with pulsed heating is shown. The phenomena which are reason for the diagrams given are discussed in more detail in Chap. 5. They are very similar to those with heated wire electrodes.

Fig. 4.3

Arrangement for microwave-activated voltammetry. From [15], with permission

Fig. 4.4

Electrochemical characteristics of microwave-heated solution spot. From [15], with permission

Examples of stripping analyses with microwave-enhanced voltammetry are trace determination of cadmium [15–17], of lead (deposited as lead dioxide and as elemental lead) [16, 18] and of palladium [19].

Ohmic heating of electrolyte solution as a consequence of electrolysis current is well known and has been utilised casually, e.g. for cooking food [25, 26]. Often, this side effect is undesirable and measures to avoid it are discussed, e.g. when batteries or electrochemical capacitors are charged/discharged [27, 28], and also in technical application of electro-osmosis [29]. The phenomena connected with ohmic heating in traditional cells are investigated [30–32]. Also, it was well known that thermoelectric convection as a result of ohmic heating occurred in studies with asymmetric high-frequency electric fields in electrolyte solutions, which intended to study dielectrophoretic effects [33–35].

Use of ohmic electrolyte heating as a scientific tool was an idea of the later Nobel Prize winner Manfred Eigen. His famous work of kinetic investigations by means of temperature jump started 1954 with heating experiments in electrolyte solutions [36]. Eigen was not interested in chemical processes at the electrode surface, but aimed at kinetic phenomena of ionic reactions occurring in homogeneous solution. Only recently, ohmic solution heating has been rediscovered and efforts have been made to use it intentionally as a tool of modern thermoelectrochemistry. Baranski and co-workers established a method which can be run with low effort [37–40]. The working principle is to send a strong high-power, high-frequency current through a microelectrode disk in admixture with all signal currents. This way, a hot solution spot close to the electrode/electrolyte interface is created similar to spots made by focused microwaves. The working frequency has to be chosen very high, up to gigahertz range. This is presumption to efficiently separate the high-power heating current from low-power electrochemical signals. The electronic equipment, nevertheless, is rather simple and easy to establish with low cost (see Fig. 4.5). The results are impressing. Voltammograms and other electrochemical functions are similar to results of microwave voltammetry and to voltammetry with hot-wire electrochemistry (Fig. 4.6). Pulsed as well as continuous heating has been reported. Dielectrophoretic phenomena caused by the heating AC as well as other side effects have been discussed. Meanwhile, a well-founded theoretical basis of the method has been established [39]; however, no practical application has been reported so far. The method has been refined in direction of a valuable diagnostic tool to study fundamental phenomena like dielectric relaxation of water at ultrahigh frequencies and of faradaic rectification effects (the latter as a method for indirect estimation of electrode impedance). With this orientation, less powerful AC current has been imposed. In such studies, not heating of solution was desired, but a variety of other effects caused by overlaid high-frequency current [41].

Fig. 4.5

Electronic set-up for generation of a hot solution spot by high-frequency heating current in parallel to electrolysis current. From [37], with permission

Fig. 4.6

Left: Changes in cyclic voltammograms caused by a superimposed sinusoidal waveform at frequency 200 MHz. AC amplitudes in V _rms: (a) 0, (b) 1.62 and (c) 2.3. Right: Changes in steady-state voltammograms caused by a superimposed sinusoidal waveform at frequency 150 MHz. AC amplitudes in V _rms: (a) 0, (b) 0.79, (c) 1.01, (d) 1.13, (e) 1.28,(f) 1.44, (g) 1.62, (h) 1.82, (i) 2.05 and (j) 2.31. Solution: 0.01 M Ru(NH₃)₆ ³⁺, 2.5 M (NH₄)₂SO₄ and 1 M NH₃. Working electrode: Au disk, 12.5 μm in radius; sweep rate, 0.02 V/s. From [37], with permission

4.2.1.2 Heating of the Electrode Body

At present, altogether four ways are existing to heat an electrode body in situ:

• Heating a disk-shaped electrode by illumination with laser or focused light beams

• Electric heating by an external heater (indirect electric heating)

• Electric heating (ohmic or Joule heating) of the electrode body by an imposed

  heating current (direct electric heating)

• Inductive heating of the electrode body by an external high-frequency field

4.2.1.2.1 Heating by Laser or by Focused Tungsten Light

Apart from two single papers dealing with an early variant of direct electric heating [42, 43], the oldest way to heat up electrodes in situ has been laser illumination. Indeed, modern thermoelectrochemistry starts with laser-activated methods. Meanwhile, a large number of papers have been published [44–112], although during last years the output has decreased.

Laser beams can be directed to an electrode disk either from the rear (beam does not cross the solution) or from the electrolyte side. The former case obviously will generate exclusively thermal effects, whereas the latter may bring about photoelectrochemical as well as thermoelectrochemical phenomena. Only a few experiments may be assigned clearly to photoelectrochemistry, as with studies of photoelectron emission [46–48]. Often, a clear distinction is difficult. In 1985, Konovalov and Raitsimring differentiated the contributions of both effects in their experiments with short time laser pulses [49]. By far the most laser applications aimed at thermal effects on electrochemical processes.

Laser illumination of electrodes as a thermoelectrochemical method dates back to 1975 [50], when Barker and Gardner applied pulsed diode laser light to implement thermal modulation as a new method. That time, many authors experimented with different modulation techniques in order to separate useful signal from noise. A scheme of the instrument of Barker and Gardner is given in Fig. 4.7. When periodic heat pulses in the form of a square wave function are imposed at the electrode interface, thermal changes of the electrode processes result in periodic oscillations of electrochemical quantities which can be separated by electronic means. It is not a problem to do this separation in a phase-selective manner. This way, the (aperiodic) electrochemical noise is separated efficiently from signals which clearly can be ascribed to pure electrode processes.

Fig. 4.7

Thermal laser modulation of Hg electrode. (M) Modulator for diode laser DL, (C) electrolysis cell, (A) f _m amplifier, (SD) synchronous detector for signals of frequency f _m , (P) polarising voltage, (X-Y) pen recorder. From [50], with permission

After the pioneering work of Barker and Gardner, the interest in thermal modulation weakened. Beginning in the eighties of the last century, the method became revitalised in conjunction with growing interest in electrochemical kinetics [51–64]. In 1983, Miller re-established a thermal laser modulation technique [51] which was applied to rotating-disk electrodes during the following years [55]. The scheme of an improved arrangement is given in Fig. 4.8. Electrodes were illuminated from the rear side by laser pulses with frequencies of 1 till 30 Hz. This way, pure thermal effects were acting. Since rotating disk electrodes are hydrodynamic systems, the overlapping of mechanical streaming phenomena with thermal convection results in highly complex streaming conditions [52, 53]. The authors considered the influence of thermodiffusion (Soret effect) [54]. They presented a comprehensive theoretical treatment of reversible redox couple behaviour [55]. Many of their insights have been verified later by hot-wire electrochemistry and also partially by laser-activated voltammetry.

Fig. 4.8

Thermal modulation of an RDE (from [53], with permission)

Thermal modulation turned out to be a valuable tool of modern thermoelectrochemistry. Instead of lasers, focused light of a tungsten lamp has been applied to determine the activation energy of electrochemical processes [56, 57], as well as thermal effects on the limiting current of reversible processes at platinum electrodes [58].

An outstanding example for laser application was investigation of the interface between two immiscible liquids. The scheme of an arrangement is shown in Fig. 4.9. For such systems, laser heating seems to be the only existing way to perform thermoelectrochemical experiments. Ion-transfer entropy across the interface has been determined successfully [59]. For theoretical interpretation of the oscillations caused by thermal modulation, Olivier et al. introduced the term “thermoelectrochemical impedance” [60]. A comprehensive treatment of this term has been given by Rotenberg [61, 62]. The activation energy of the diffusion process has been determined [61], and the studies have been extended to the interface conducting polymer/electrolyte solution [63]. By means of an oscillating IR diode, Aaboubi et al. studied the complex overlapping phenomena at the limiting current region of a vertically oriented electrode and proposed a special transfer function [64].

Fig. 4.9

Laser-activated temperature-modulated voltammetry at an interface between immiscible liquids. From [59], with permission

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 (E _pzc) of Au(111) [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 determined for polycrystalline electrodes [71–73]. The implications of the Soret effect at such electrodes have been studied by means of the temperature-jump technique [74–76]. An interesting approach was to apply the technique for determination of solid-state properties in conducting polymers. The temperature jump is generating an electrochemical thermocouple effect which contains information about the nature of carriers (either electrons or holes [77].

A large number of papers dealing with laser-induced temperature jumps were addressed to heterogeneous rate constants in electrochemical kinetics [78–83]. Heterogeneous rate processes have been studied [78], as well as double-layer formation at glassy carbon electrodes [79]. Superfast electrode reactions [80] and short-lived intermediates at electrode surfaces [81, 82] were the subject of investigations. Anodic silver oxidation in the presence of different anions has been studied [83, 84].

Adsorption phenomena were followed by temperature-jump techniques [85–87], and layers of surface-attached species have been analysed [88–91]. Surface modification by gold nanoparticles [89] and by self-assembled monolayers at gold surfaces [90, 91] has been the subject of investigations.

Electrochemical methods of analysis (electroanalysis) have made progress by laser-assisted techniques [44, 92–94]. They were useful to detect ascorbic acid at a carbon electrode in flow injection [44]. Capabilities of pulsed laser beam illumination of gold and platinum disk electrodes were tested with the well-known redox couples toluidine blue, iodide, ferricyanide, ruthenium hexammine and ferrocene (see Fig. 4.10) [92]. Laser-activated voltammetry proved useful for selective removal of impurities from glassy carbon- and boron-doped diamond surfaces [93]. Several effects of pulsed lasers at electrode surfaces were studied to find optimum conditions for electroanalytical chemistry [94].

Fig. 4.10

Linear sweep voltammograms (5 mV s⁻¹) at a 1 mm diameter platinum disk electrode subjected to increasing laser intensity (left: 0–1.2 W cm⁻²; right: 0–0.8 W cm⁻²). Left: Ru(NH₃)₆Cl₃ in 0.1 M KCl; right: ferrocene/0.1 M TBAH in acetonitrile. From [92], with permission

Industrial electroplating processes have been improved by means of laser irradiation [95–102]. Enhancement of plating has been discussed as resulting from acceleration of charge transfer rate, potential changes and thermal stirring effects [95, 96]. Copper plating on different substrates was found to be enhanced by laser application [97, 98]. Laser pulses which strongly interacted with hydrodynamic conditions had a remarkable effect on nucleation and growth of zinc electrodeposits [99, 100]. By means of focused laser beams, maskless surface patterns were generated [101]. Nickel electrodeposition has been discussed [102]. Laser treatment of electrodes has been studied with further industrial electrochemical processes like hydrogen evolution on nickel electrodes [103] and etching of manganese-zinc ferrites in KOH induced by a focused laser beam [104].

An important task of laser activation was cleaning of electrode surfaces in situ [45]. By means of high-power laser beams, organic residues, adsorptive layers and other contaminations have been removed successfully [105–109]. Such treatment procedures proved useful mainly for carbonaceous surfaces [105–108], but also metallic electrodes have been treated by strong lasers, even till plastic deformation occurred [109].

4.2.1.2.2 Indirect Electric Heating

Electric heating of electrodes by means of an external heater (indirect electric heating) highly simplifies apparatus in comparison to laser heating. It is advantageous that there is no mutual interference between heating and measuring circuits. Hence, heating can be done by direct current. On the other hand, there is introduced an additional barrier between heater and active electrode surface with the consequence of higher thermal inertia. The insulating layer should be made of material with high thermal conductivity. Ceramic materials are preferred. Harima and Aoyagui in 1976 proposed an arrangement consisting of a thin aluminium foil as the heater in close contact with a thin mylar foil carrying a gold film as active electrode [110, 111]. The authors presented a theoretical treatment of transient processes following a rapid temperature perturbation [110], but an experimental application has not been published. Indirect heating of large electrodes was used to study slow processes [112]. An indirectly heated iron disk electrode was used to study thermal calcium carbonate scaling by means of electrochemical impedance studies [113].

Later, indirect electrode heating was rediscovered when a technology of microelectronics has proved useful, namely the Low-Temperature Cofired Ceramics (LTCC) [114–119] (Fig. 4.11). In this technique, stacks of thin ceramic plates containing screen-printed patterns are interconnected by vertical holes filled with conducting material (so-called VIAs). Indirectly heated electrodes have been designed with platinum heaters and gold electrodes at the surface. On this basis, a variety of biosensors, among them such with different enzyme layers, have been designed and tested successfully [116–119]. Alternative indirect electrode heating has been proposed based on polysilicon layers [120] and on CMOS structures [121]. With indirectly heated LTCC sensors, rather fast temperature change can be achieved. This gave rise to do experiments with the new thermoelectrochemical technique TPV (temperature pulse voltammetry, see later below), which had been developed originally for hot-wire techniques. Some interesting results for analysis of reactants with sluggish kinetics were obtained, e.g. for nitrogen oxide [114].

Fig. 4.11

LTCC sensor with indirect heating of a Pt electrode. “VIA” means a conducting hole through a thin ceramic plate. From [114], with permission

4.2.1.2.3 Direct Electric Heating: “Hot-Wire Electrochemistry” and “Hot-Layer Electrochemistry”

Joule heating of thin metallic wire electrodes started with some experiments where line frequency was utilised for heating [42, 43]. An important step was the work of Gabrielli and co-workers [122, 123] which heated a 100 μm platinum wire by an alternating current with a frequency of 250 kHz. The resulting temperature jump of some Kelvin was intended for kinetic studies. The method was useful only for slight temperature changes, since otherwise a strong AC distortion of the electrochemical signal would occur caused by the high-frequency iR drop along the wire. Radio frequency heating of wire electrodes became meaningful when a way had been found to avoid completely this distortion [124]. The principle, sketched in Fig. 4.12, is based on the idea to keep away any AC voltage from the potentiostat working electrode input. It can be assumed that in the frequency region of 100 kHz and more, no substantial faradaic processes will occur. If the working electrode input would be connected “asymmetrically” (like a) in Fig. 4.12), along the wire length, with maximum at the distant wire end, heating current would develop an iR voltage which would be “seen” by the potentiostat input with the result of strong distortion. The first way to compensate for this influence was a symmetric arrangement (like b) in Fig. 4.12) where the working electrode wire is connected to the potentiostat input at the centre between two equal halves of its length. As shown in the figure, now two AC voltage values with opposite sign are “seen” by the working electrode input. This way, the distortion is avoided. Even more elegant is c) in Fig. 4.12, where the electrode wire is not divided, but compensation is made by a bridge arrangement [125]. In this case, protection of potentiostat input is improved further by means of two inductive elements as branches of the bridge. Although direct ohmic heating of thin wires (without isolation between heating and measuring circuits) needs all the precautions described, it is a very powerful method of modern thermoelectrochemistry due to its very short heating-up and cooling-down periods and its inexpensive instrumentation. Experimental details of the technology together with application examples will be given in Chap. 6.

Fig. 4.12

Distortion of potentiostat measuring circuit by AC heating current and its compensation. (a) Uncompensated heating (asymmetric arrangement). (b) Compensated AC heating (symmetric arrangement). (c) Compensated AC heating (bridge arrangement). AC voltage amplitude caused by iR drop of heating current “as seen by the potentiostat” indicated schematically. From [1], with permission

Advantages of direct electrode heating are demonstrated best with heated microwires due to their extremely low heat capacity, the resulting short heating-up period and consequently the chance to do hot-wire electrochemistry above the boiling point without autoclaving. Anyway, when the principle described above had been proven successful for AC distortion compensation, the techniques of direct AC heating have been used not only with thin wires but also with macro structures. Metallic bands made by thin film techniques as well as by screen printing have been tested, also screen-printed carbon electrodes. A special case were heated ITO (indium tin oxide) structures which proved advantageous for electrochemiluminescent studies described further below. Heated carbon paste electrodes have been used also broadly. Paste electrodes can be modified easily by the addition of diverse agents. Examples were pastes made of multi-wall carbon nanotubes modified by agents like ruthenium bipyridyl or by special enzymes. Pastes with ionic liquids as binders also have been used. Application examples of such heated macro structures will be given in the Chap. 6, which is dedicated to the experimental work with electrically heated electrodes.

4.2.1.2.4 Inductive Heating

As the last method of electrode heating, the inductive heating technique has been mentioned above. Here, eddy currents are induced in metallic electrodes by a strong alternating magnetic field of radio frequency, similar to the action of inductive hot plates on a saucepan. Platinum macroelectrodes have been heated in electrochemical flow-stream cells [126–128], as shown in Fig. 4.13. The method has been tested for analyses of reversible redox couples [126, 127] and of organic redox active compounds [128]. A special advantage is that no galvanic connection exists between heating and measuring circuits.

Fig. 4.13

RF-heated electrochemical channel flow system. From [126], with permission