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Electrochemical microcalorimetry at single electrodes
Accepted Manuscript
Electrochemical Microcalorimetry at single electrodes Rolf Schuster PII: DOI: Reference:
S2451-9103(16)30049-7 10.1016/j.coelec.2017.01.007 COELEC 21
To appear in:
Current Opinion in Electrochemistry
Received date: Accepted date:
22 December 2016 18 January 2017
Please cite this article as: Rolf Schuster , Electrochemical Microcalorimetry at single electrodes, Current Opinion in Electrochemistry (2017), doi: 10.1016/j.coelec.2017.01.007
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ACCEPTED MANUSCRIPT Highlights
The heat effects at single electrodes can be measured with minute conversions.
Direct measurement of the reaction entropy, including all side reactions.
The reaction entropy is measured as a function of the potential.
The reaction entropy provides information complementary to a cyclic
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voltammogram.
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ACCEPTED MANUSCRIPT Electrochemical Microcalorimetry at single electrodes
Rolf Schuster Karlsruhe Institute of Technology, Institute of Physical Chemistry, 76131 Karlsruhe, Germany
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Abstract
The heat, which is reversibly exchanged at a single electrode, directly correlates with the entropy change during the electrochemical reaction. With recent experimental improvements also surface electrochemical reactions, i.e., reactions with only
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submonolayer conversion became accessible for heat measurements. Since the reaction entropy includes all side reactions, e.g., also charge-neutral coadsorption or ordering processes of the solvent, it provides complementary information to the current-potential relation, as for example measured by conventional cyclic voltammetry. Here we will briefly review the theoretical background and recent
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developments and compare the method to other approaches for the measurement of
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the entropy of electrochemical systems.
email: [email protected] 2
ACCEPTED MANUSCRIPT Introduction The central objectives of many electrochemical studies are specified by two questions: What are the species, which are involved in the electrochemical process? And what are the reaction steps and side reactions? Species may not directly be involved in the reaction but rather act as spectators, as recently demonstrated e.g. for alkali cations during the oxygen reduction reaction on Pt(111) in alkaline media [1, 2].
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Reaction steps may also include charge neutral side reactions like coadsorption processes during the underpotential deposition (UPD) of Cu on Au(111) in sulfate solutions [3, 4] or potential-dependent ordering processes of the solvent molecules on surfaces [5]. Even today definite answers to the above questions are difficult to obtain and e.g. the potential-dependence of the surface excesses of cations or
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anions in the double layer of a simple metal electrode is only known for few systems, e.g. mercury in NaF and NaCl solutions [6], Au(111) in sulfate and halogenide solutions [7] or Cu UPD on Au(111) [3].
Various methods exist for the study of electrochemical systems. With the advent of surface science powerful tools were developed, which were proven to be very
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successful, e.g., for the investigation of the structure of adlayers and the identification of adsorbed species (see e.g. references [1, 2, 4, 5] above). Traditionally very often
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thermodynamic or electric observables are employed for the characterization of the electrochemical system (Fig. 1). The most prominent examples include cyclic
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voltammetry and impedance spectroscopy, where the response of the current on variations of the potential drop across the electrochemical interface is studied. From
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the thermodynamic point of view those methods exploit the potential dependent flow of charge and thus the electrical work involved in the electrochemical reaction.
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Another thermodynamic quantity which provides very valuable information on the interfacial properties is the surface tension of the electrode surface. By employing the electrocapillary equation, surface excesses of the species in the electrochemical interface could be determined as mentioned above [3, 6, 7]. Historically also the heat, exchanged at an electrode during an electrochemical reaction was considered an important thermodynamic quantity. The experimental observation of heat effects at single electrodes dates back to 1879 when Bouty observed that the electrochemical deposition of Cu led to cooling of the electrode whereas the dissolution led to warming [8]. Bouty correctly realized that the observed 3
ACCEPTED MANUSCRIPT phenomenon constitutes the electrochemical analogue to the Peltier effect at junctions of solid conductors. As pointed out in the following, the reversibly exchanged heat, the so-called Peltier heat at a single electrode reflects the entropy change at the electrochemical interface upon the electrochemical reaction. Until recently, such measurements required relatively large electrochemical conversions, typically of the order of hundred to several thousand monolayers (referenced to the number of surface atoms of a typical electrode). Electrochemical calorimetry was
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hence applied mainly for the study of bulk electrochemical reactions, like bulk Ag or Cu deposition or the [Fe(CN)6]3+/4+ electron transfer reaction [9, 10]. Such
experiments were very valuable, e.g. for the investigation of the heats of transport of various ions [9, 11, 12] or for estimates of the absolute entropy of the hydrogen
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electrode [13-15].
By employing high surface area platinized Pt electrodes, the heat effects upon a surface adsorption processes, e.g. hydrogen adsorption on Pt became experimentally accessible [16, 17]. Later Hai and Scherson applied the mirage effect for the sensitive temperature measurement of the heat evolution upon the
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electrochemical surface oxidation of a Au film [18]. Recently our group combined sensitive pyroelectric temperature detection by a thin electrode-sensor assembly with
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pulsed electrochemistry. With these improvements the quantitative determination of the Peltier heat of electrochemical reactions with conversions down to about 0.01 ML
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became possible, as demonstrated e.g. for the study of the different steps of Ag and Cu UPD [19, 20]. Also the study of non-Faradaic processes like double layer
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charging comes into reach [21]. Due to improved sensitivity, electrochemical microcalorimetry at single electrodes
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now provides access to the reaction entropy of electrochemically stimulated surface processes. The measured entropy includes contributions, which do not necessarily show up in the cell current and may stem, e.g., from charge-neutral side reactions or ordering processes of the solvent. Hence, information on the entropy is complementary e.g., to the current-potential relation, as determined by standard electrochemical methods. This may help to identify reaction steps and side reactions or to determine the surface coverages of the involved species as recently demonstrated in [20].
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ACCEPTED MANUSCRIPT It should be noted that this brief review is limited to processes at single electrodes. Calorimetric measurements of complete cells as well as measurements far from equilibrium, e.g., in battery or fuel cell systems are not considered (see e.g. references in [22-26] [27] for further information).
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What do we measure by electrochemical calorimetry? Electrochemical processes at a single electrode involve transport of ions and
electrons into and out of the electrochemical interface. Heat effects have to be
ascribed to an electrochemical Peltier effect and even at infinitesimally small current flow, i.e., close to thermal equilibrium, the reversible heat effects qrev are determined
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by both, changes of the interfacial entropy due to the reaction entropy of the overall electrochemical process RS as well as by contributions due to ion and electron transport TransS. With the usual sign convention, where heat uptake by the electrochemical system is counted positive, the infinitesimal reversible heat effects
qrev T R S d TTransS d ,
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qrev are given by:
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where denotes the reaction variable, which states the progress of the reaction and T is the temperature. The contributions from transport were first realized in the late
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1920ies by Lange and coworkers and by Wagner [11, 12, 28, 29], after Eastman introduced the concept of the so called Eastman entropy of transport [30]. A seminal
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review on thermoelectrochemical phenomena was presented by Agar [31]. Application oriented summaries can be found e.g. in [9, 32]. The entropic contribution
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due to transport can be calculated from the Eastman entropies of transport, which are known for several systems [31]. Furthermore, the transport contribution will be constant if the electrolyte composition does not change or if highly concentrated supporting electrolyte is used. In addition to the reversible heat effects, which are termed „Peltier heat‟, in a real experiment with finite electrochemical currents, there exist irreversible contributions, which always generate heat. They can be separated into heat generation due to the deviation from thermal equilibrium, which in the electrochemical language means heat due to „overpotential‟, and heat generation due to the current flow through the 5
ACCEPTED MANUSCRIPT electrolyte with finite conductance. The latter is usually termed Joule heat. As pointed out in [33], the heat due to overpotential is generated at the electrochemical interface whereas Joule heat is produced along the whole current path through the electrolyte. Usually the reversible contributions can be separated from the irreversible ones, e.g. by changing the direction of the electrochemical reaction, which will cause a sign reversal of the reversible heat effects, whereas the irreversible part will keep its sign
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[9, 34] , or by extrapolation of measurements at varying overpotential [33, 35].
Experimental challenges and solutions
The measurement of heat effects at single electrodes dates back to the experiments
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of Bouty. Early measurements are summarized in [36, 37]. More recent experiments are reviewed e.g. in [10, 32, 38]. Essentially two approaches were applied. Either the temperature difference evolving between a single electrode and its surrounding was directly measured similar to the early experiments of Bouty [9, 10, 18, 34, 35, 39-54] or the heat effects of a complete half-cell, i.e. the electrode and the surrounding
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electrolyte, were investigated by calorimetry [37, 55, 56].
Until recently in most of the experiments bulk electrochemical reactions were studied.
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The measurement of the heat of surface electrochemical reactions with small conversions is hampered by the large heat capacity of a typical electrode and the
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surrounding electrolyte. The sensitivity can be increased by the use of an electrodesensor assembly with a small heat capacitance. This concept was introduced by the
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groups of King and Campbell for the study of heats of adsorption at surfaces in UHV [57-59]. They employed thin, free-standing samples, only a few micrometer thick.
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Campbell‟s group introduced a pyroelectric PVDF ribbon, contacting the backside of the sample for temperature measurement. A similar approach was used before for the measurement of heat effects upon nerve excitation [60]. The dissipation of heat into the electrolyte can be minimized by keeping the reaction time short. When the electrochemical reactions are conducted for only 10 ms, the evolved heat will heat up a water layer in front of the electrode which is only a few micrometer thick [33]. Our group recently combined both approaches [21, 61]. We use a thin (ca. 50 to 100 µm) electrode, which is mounted on top of a freely suspended 25 µm thick LiTaO3 pyroelectric sensor, both pressed together by the air pressure. On top of the 6
ACCEPTED MANUSCRIPT electrode-sensor assembly an O-ring sealed electrochemical cell is mounted, which leaves an active electrode area of about 0.2 cm2. The electrochemical reactions are conducted for only 10 ms, which is fast enough to avoid heat flow into the electrolyte and long enough to allow for thermal equilibration of the thin electrode-sensor assembly. This allows us to measure heat effects, e.g. for Cu UPD on (111)-textured Au films from sulfate containing electrolytes. Fig. 2 shows an example for the temperature decrease upon 10 ms long underpotential deposition of Cu. The charge,
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which flowed in the outer cell circuit amounted to 4 µC/cm2, which corresponds to about 1% of a complete Cu UPD layer. Absolute heat values were obtained by
calibration of the calorimeter e.g. with the [Fe(CN)6]3+/4+ electron transfer reaction [61], for which the Peltier heat is known from literature. The general applicability of the method is currently hampered by two constraints: i) Preparation of well-oriented
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surfaces is difficult for thin electrodes. We therefore often work on (111)-textured Au films, which are easy to prepare. ii) The electrochemical reactions should behave reversibly on a timescale of 10 ms, which excludes slow electrochemical reactions. So far, further applications included the determination of the entropy of water in swelling ferrocyanide-loaded polyelectrolytes [62] or the entropy change upon the
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electrochemically driven surface aggregation of dodecyl sulfate [63]. Also the
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deposition [64, 65].
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solvation of Li ions in battery electrolytes was studied by calorimetry of Li bulk
Alternative methods for the determination of the reaction entropy of electrochemical
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reactions
One alternative for the determination of entropies of electrochemical systems is
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provided by measuring the temperature dependence of the cell potential. A variation, which is applicable to surface electrochemical systems with continuously changing state of the surface, employs the temperature dependence of the cyclic voltammogram, e.g. in isothermal cells. This method was first applied for the determination of the entropy change upon hydrogen adsorption on polycrystalline Pt by Breiter and Böld [66, 67] and later refined by Conway et al. [68]. Later several groups studied the hydrogen and OH adsorption on single crystalline Pt surfaces with an improved treatment based on a generalized adsorption isotherm [69-77]. GarciaAraez et al. introduced an interpretation of such data on the basis of the 7
ACCEPTED MANUSCRIPT electrocapillary equation [76-78]. As pointed out by Harrison et al. [79] on the basis of previous work, e.g. [80, 81] the entropy of the formation of the interface can be obtained from the temperature effects on the charge-density or capacitance data. This procedure was modified by Garcia-Araez et al. to include charge-transfer processes at the interface. Temperature effects on the capacitance of solid electrodes were already previously employed for the investigation of double-layer properties at Au or Ag [82-84]. A related approach to measure the entropy of
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formation of the double layer is provided by studying the change of the open cell potential of an electrode upon a (laser-induced) temperature jump [85-89].
Alternatively also nonisothermal electrochemical cells may be studied, e.g. to
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measure the thermopower of the electrochemical system, as discussed e.g. in [31].
Conclusions
With recent improvements, calorimetry becomes applicable also for the study of electrochemical surface reactions. From such measurements the reaction entropy
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including all side reactions can be determined as a function of the electrode potential, which provides complementary information to the current-potential characteristics, as
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obtained for example by cyclic voltammetry. Electrochemical microcalorimetry provides an attractive alternative to existing methods for the determination of the
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entropy of electrochemical systems, albeit further instrumental amendments, particularly with respect to its applicability to single crystalline electrodes, are
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necessary.
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Acknowledgements
I am particularly grateful to Kai Etzel, Katrin Bickel, Stefan Frittmann, Dieter Waltz and the colleagues from the machine shop for their contributions during the development of our microcalorimeter. This work was supported by the Deutsche Forschungsgemeinschaft (SCHU 958/7).
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[81] Hills GJ, Payne R. Temperature and Pressure Dependence of the Double Layer Capacity at the Mercury-Solution Interface. Trans Faraday Soc. 1965;61:326-49. [82] Hamelin A, Stoicoviciu L, Silva F. The temperature dependence of the doublelayer properties of gold faces in perchloric acid solutions: Part I. The (210) gold face. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry. 1987;229:107-24. [83] Hamelin A, Stoicoviciu L, Silva F. The temperature dependence of the doublelayer properties of gold faces in perchloric acid solutions: Part II. The (110) gold face. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry. 1987;236:283-94. [84] Silva F, Sottomayor MJ, Martins A. Temperature coefficient of the potential of zero charge and entropies of formation for the interface of stepped faces of gold in 14
ACCEPTED MANUSCRIPT contact with aqueous perchloric acid solutions. J Chem Soc, Faraday Trans. 1996;92:3693-9. [85] Benderskii VA, Velichko GJ. Temperature jump in electric double-layer study, Part I. Method of measurement. J Electroanal Chem. 1982;140:1-22. [86] Climent V, Coles BA, Compton RG. Laser Induced Current Transients applied to a Au(111) Single Crystal Electrode. A General Method for the Measurement of Potentials of Zero Charge of Solid Electrodes. J Phys Chem B. 2001;105:1066973.
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[87] Climent V, Coles BA, Compton RG. Laser-Induced Potential Transients on a Au(111) Single-Crystal Electrode. Determination of the Potential of Maximum Entropy of Double Layer Formation. J Phys Chem B. 2002;106:5258-65. [88] Climent V, Garcia-Araez N, Compton RG, Feliu JM. Effect of Deposited Bismuth on the Potential of Maximum Entropy of Pt(111) Single-Crystal Electrodes. The Journal of Physical Chemistry B. 2006;110:21092-100.
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[89] Garcia-Aráez N, Climent V, Feliu JM. Evidence of Water Reorientation on Model Electrocatalytic Surfaces from Nanosecond-Laser-Pulsed Experiments. J Am Chem Soc. 2008;130:3824-33.
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ACCEPTED MANUSCRIPT Figure 2. (A) Potential E, current-density j, and temperature T transients during Cu underpotential deposition onto a (111)-textured Au film in 10 mM CuSO4 / 0.1 M H2SO4 by a 10 ms long current pulse. The electrochemical conversion amounts to about 1% of a complete Cu UPD layer. During the deposition the temperature of the electrode decreased about linearly with time, signaling the cooling of the electrode due to the electrochemical Peltier effect. After the current pulse the temperature slowly increased towards its starting value before the pulse, due to thermal
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equilibration with the surrounding. The temperature decrease at the end of the 10 ms pulse is of the order of 10 µK. (B) Cyclic voltammogram and potential-dependent molar Peltier heat of Cu UPD on Au(111). The Peltier heat data was obtained by cycling through the Cu UPD region by sequences of positive (red, upward triangles) and negative (blue, downward triangles) current pulses (see (A) for an exemplary
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Electrochemical Microcalorimetry at single electrodes Rolf Schuster PII: DOI: Reference:
S2451-9103(16)30049-7 10.1016/j.coelec.2017.01.007 COELEC 21
To appear in:
Current Opinion in Electrochemistry
Received date: Accepted date:
22 December 2016 18 January 2017
Please cite this article as: Rolf Schuster , Electrochemical Microcalorimetry at single electrodes, Current Opinion in Electrochemistry (2017), doi: 10.1016/j.coelec.2017.01.007
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Highlights
The heat effects at single electrodes can be measured with minute conversions.
Direct measurement of the reaction entropy, including all side reactions.
The reaction entropy is measured as a function of the potential.
The reaction entropy provides information complementary to a cyclic
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voltammogram.
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ACCEPTED MANUSCRIPT Electrochemical Microcalorimetry at single electrodes
Rolf Schuster Karlsruhe Institute of Technology, Institute of Physical Chemistry, 76131 Karlsruhe, Germany
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Abstract
The heat, which is reversibly exchanged at a single electrode, directly correlates with the entropy change during the electrochemical reaction. With recent experimental improvements also surface electrochemical reactions, i.e., reactions with only
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submonolayer conversion became accessible for heat measurements. Since the reaction entropy includes all side reactions, e.g., also charge-neutral coadsorption or ordering processes of the solvent, it provides complementary information to the current-potential relation, as for example measured by conventional cyclic voltammetry. Here we will briefly review the theoretical background and recent
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developments and compare the method to other approaches for the measurement of
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the entropy of electrochemical systems.
email: [email protected] 2
ACCEPTED MANUSCRIPT Introduction The central objectives of many electrochemical studies are specified by two questions: What are the species, which are involved in the electrochemical process? And what are the reaction steps and side reactions? Species may not directly be involved in the reaction but rather act as spectators, as recently demonstrated e.g. for alkali cations during the oxygen reduction reaction on Pt(111) in alkaline media [1, 2].
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Reaction steps may also include charge neutral side reactions like coadsorption processes during the underpotential deposition (UPD) of Cu on Au(111) in sulfate solutions [3, 4] or potential-dependent ordering processes of the solvent molecules on surfaces [5]. Even today definite answers to the above questions are difficult to obtain and e.g. the potential-dependence of the surface excesses of cations or
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anions in the double layer of a simple metal electrode is only known for few systems, e.g. mercury in NaF and NaCl solutions [6], Au(111) in sulfate and halogenide solutions [7] or Cu UPD on Au(111) [3].
Various methods exist for the study of electrochemical systems. With the advent of surface science powerful tools were developed, which were proven to be very
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successful, e.g., for the investigation of the structure of adlayers and the identification of adsorbed species (see e.g. references [1, 2, 4, 5] above). Traditionally very often
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thermodynamic or electric observables are employed for the characterization of the electrochemical system (Fig. 1). The most prominent examples include cyclic
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voltammetry and impedance spectroscopy, where the response of the current on variations of the potential drop across the electrochemical interface is studied. From
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the thermodynamic point of view those methods exploit the potential dependent flow of charge and thus the electrical work involved in the electrochemical reaction.
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Another thermodynamic quantity which provides very valuable information on the interfacial properties is the surface tension of the electrode surface. By employing the electrocapillary equation, surface excesses of the species in the electrochemical interface could be determined as mentioned above [3, 6, 7]. Historically also the heat, exchanged at an electrode during an electrochemical reaction was considered an important thermodynamic quantity. The experimental observation of heat effects at single electrodes dates back to 1879 when Bouty observed that the electrochemical deposition of Cu led to cooling of the electrode whereas the dissolution led to warming [8]. Bouty correctly realized that the observed 3
ACCEPTED MANUSCRIPT phenomenon constitutes the electrochemical analogue to the Peltier effect at junctions of solid conductors. As pointed out in the following, the reversibly exchanged heat, the so-called Peltier heat at a single electrode reflects the entropy change at the electrochemical interface upon the electrochemical reaction. Until recently, such measurements required relatively large electrochemical conversions, typically of the order of hundred to several thousand monolayers (referenced to the number of surface atoms of a typical electrode). Electrochemical calorimetry was
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hence applied mainly for the study of bulk electrochemical reactions, like bulk Ag or Cu deposition or the [Fe(CN)6]3+/4+ electron transfer reaction [9, 10]. Such
experiments were very valuable, e.g. for the investigation of the heats of transport of various ions [9, 11, 12] or for estimates of the absolute entropy of the hydrogen
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electrode [13-15].
By employing high surface area platinized Pt electrodes, the heat effects upon a surface adsorption processes, e.g. hydrogen adsorption on Pt became experimentally accessible [16, 17]. Later Hai and Scherson applied the mirage effect for the sensitive temperature measurement of the heat evolution upon the
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electrochemical surface oxidation of a Au film [18]. Recently our group combined sensitive pyroelectric temperature detection by a thin electrode-sensor assembly with
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pulsed electrochemistry. With these improvements the quantitative determination of the Peltier heat of electrochemical reactions with conversions down to about 0.01 ML
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became possible, as demonstrated e.g. for the study of the different steps of Ag and Cu UPD [19, 20]. Also the study of non-Faradaic processes like double layer
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charging comes into reach [21]. Due to improved sensitivity, electrochemical microcalorimetry at single electrodes
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now provides access to the reaction entropy of electrochemically stimulated surface processes. The measured entropy includes contributions, which do not necessarily show up in the cell current and may stem, e.g., from charge-neutral side reactions or ordering processes of the solvent. Hence, information on the entropy is complementary e.g., to the current-potential relation, as determined by standard electrochemical methods. This may help to identify reaction steps and side reactions or to determine the surface coverages of the involved species as recently demonstrated in [20].
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ACCEPTED MANUSCRIPT It should be noted that this brief review is limited to processes at single electrodes. Calorimetric measurements of complete cells as well as measurements far from equilibrium, e.g., in battery or fuel cell systems are not considered (see e.g. references in [22-26] [27] for further information).
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What do we measure by electrochemical calorimetry? Electrochemical processes at a single electrode involve transport of ions and
electrons into and out of the electrochemical interface. Heat effects have to be
ascribed to an electrochemical Peltier effect and even at infinitesimally small current flow, i.e., close to thermal equilibrium, the reversible heat effects qrev are determined
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by both, changes of the interfacial entropy due to the reaction entropy of the overall electrochemical process RS as well as by contributions due to ion and electron transport TransS. With the usual sign convention, where heat uptake by the electrochemical system is counted positive, the infinitesimal reversible heat effects
qrev T R S d TTransS d ,
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qrev are given by:
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where denotes the reaction variable, which states the progress of the reaction and T is the temperature. The contributions from transport were first realized in the late
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1920ies by Lange and coworkers and by Wagner [11, 12, 28, 29], after Eastman introduced the concept of the so called Eastman entropy of transport [30]. A seminal
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review on thermoelectrochemical phenomena was presented by Agar [31]. Application oriented summaries can be found e.g. in [9, 32]. The entropic contribution
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due to transport can be calculated from the Eastman entropies of transport, which are known for several systems [31]. Furthermore, the transport contribution will be constant if the electrolyte composition does not change or if highly concentrated supporting electrolyte is used. In addition to the reversible heat effects, which are termed „Peltier heat‟, in a real experiment with finite electrochemical currents, there exist irreversible contributions, which always generate heat. They can be separated into heat generation due to the deviation from thermal equilibrium, which in the electrochemical language means heat due to „overpotential‟, and heat generation due to the current flow through the 5
ACCEPTED MANUSCRIPT electrolyte with finite conductance. The latter is usually termed Joule heat. As pointed out in [33], the heat due to overpotential is generated at the electrochemical interface whereas Joule heat is produced along the whole current path through the electrolyte. Usually the reversible contributions can be separated from the irreversible ones, e.g. by changing the direction of the electrochemical reaction, which will cause a sign reversal of the reversible heat effects, whereas the irreversible part will keep its sign
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[9, 34] , or by extrapolation of measurements at varying overpotential [33, 35].
Experimental challenges and solutions
The measurement of heat effects at single electrodes dates back to the experiments
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of Bouty. Early measurements are summarized in [36, 37]. More recent experiments are reviewed e.g. in [10, 32, 38]. Essentially two approaches were applied. Either the temperature difference evolving between a single electrode and its surrounding was directly measured similar to the early experiments of Bouty [9, 10, 18, 34, 35, 39-54] or the heat effects of a complete half-cell, i.e. the electrode and the surrounding
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electrolyte, were investigated by calorimetry [37, 55, 56].
Until recently in most of the experiments bulk electrochemical reactions were studied.
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The measurement of the heat of surface electrochemical reactions with small conversions is hampered by the large heat capacity of a typical electrode and the
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surrounding electrolyte. The sensitivity can be increased by the use of an electrodesensor assembly with a small heat capacitance. This concept was introduced by the
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groups of King and Campbell for the study of heats of adsorption at surfaces in UHV [57-59]. They employed thin, free-standing samples, only a few micrometer thick.
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Campbell‟s group introduced a pyroelectric PVDF ribbon, contacting the backside of the sample for temperature measurement. A similar approach was used before for the measurement of heat effects upon nerve excitation [60]. The dissipation of heat into the electrolyte can be minimized by keeping the reaction time short. When the electrochemical reactions are conducted for only 10 ms, the evolved heat will heat up a water layer in front of the electrode which is only a few micrometer thick [33]. Our group recently combined both approaches [21, 61]. We use a thin (ca. 50 to 100 µm) electrode, which is mounted on top of a freely suspended 25 µm thick LiTaO3 pyroelectric sensor, both pressed together by the air pressure. On top of the 6
ACCEPTED MANUSCRIPT electrode-sensor assembly an O-ring sealed electrochemical cell is mounted, which leaves an active electrode area of about 0.2 cm2. The electrochemical reactions are conducted for only 10 ms, which is fast enough to avoid heat flow into the electrolyte and long enough to allow for thermal equilibration of the thin electrode-sensor assembly. This allows us to measure heat effects, e.g. for Cu UPD on (111)-textured Au films from sulfate containing electrolytes. Fig. 2 shows an example for the temperature decrease upon 10 ms long underpotential deposition of Cu. The charge,
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which flowed in the outer cell circuit amounted to 4 µC/cm2, which corresponds to about 1% of a complete Cu UPD layer. Absolute heat values were obtained by
calibration of the calorimeter e.g. with the [Fe(CN)6]3+/4+ electron transfer reaction [61], for which the Peltier heat is known from literature. The general applicability of the method is currently hampered by two constraints: i) Preparation of well-oriented
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surfaces is difficult for thin electrodes. We therefore often work on (111)-textured Au films, which are easy to prepare. ii) The electrochemical reactions should behave reversibly on a timescale of 10 ms, which excludes slow electrochemical reactions. So far, further applications included the determination of the entropy of water in swelling ferrocyanide-loaded polyelectrolytes [62] or the entropy change upon the
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electrochemically driven surface aggregation of dodecyl sulfate [63]. Also the
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deposition [64, 65].
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solvation of Li ions in battery electrolytes was studied by calorimetry of Li bulk
Alternative methods for the determination of the reaction entropy of electrochemical
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reactions
One alternative for the determination of entropies of electrochemical systems is
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provided by measuring the temperature dependence of the cell potential. A variation, which is applicable to surface electrochemical systems with continuously changing state of the surface, employs the temperature dependence of the cyclic voltammogram, e.g. in isothermal cells. This method was first applied for the determination of the entropy change upon hydrogen adsorption on polycrystalline Pt by Breiter and Böld [66, 67] and later refined by Conway et al. [68]. Later several groups studied the hydrogen and OH adsorption on single crystalline Pt surfaces with an improved treatment based on a generalized adsorption isotherm [69-77]. GarciaAraez et al. introduced an interpretation of such data on the basis of the 7
ACCEPTED MANUSCRIPT electrocapillary equation [76-78]. As pointed out by Harrison et al. [79] on the basis of previous work, e.g. [80, 81] the entropy of the formation of the interface can be obtained from the temperature effects on the charge-density or capacitance data. This procedure was modified by Garcia-Araez et al. to include charge-transfer processes at the interface. Temperature effects on the capacitance of solid electrodes were already previously employed for the investigation of double-layer properties at Au or Ag [82-84]. A related approach to measure the entropy of
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formation of the double layer is provided by studying the change of the open cell potential of an electrode upon a (laser-induced) temperature jump [85-89].
Alternatively also nonisothermal electrochemical cells may be studied, e.g. to
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measure the thermopower of the electrochemical system, as discussed e.g. in [31].
Conclusions
With recent improvements, calorimetry becomes applicable also for the study of electrochemical surface reactions. From such measurements the reaction entropy
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including all side reactions can be determined as a function of the electrode potential, which provides complementary information to the current-potential characteristics, as
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obtained for example by cyclic voltammetry. Electrochemical microcalorimetry provides an attractive alternative to existing methods for the determination of the
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entropy of electrochemical systems, albeit further instrumental amendments, particularly with respect to its applicability to single crystalline electrodes, are
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necessary.
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Acknowledgements
I am particularly grateful to Kai Etzel, Katrin Bickel, Stefan Frittmann, Dieter Waltz and the colleagues from the machine shop for their contributions during the development of our microcalorimeter. This work was supported by the Deutsche Forschungsgemeinschaft (SCHU 958/7).
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[65] Schmid MJ, Xu J, Lindner J, Novák P, Schuster R. Concentration Effects on the Entropy of Electrochemical Lithium Deposition: Implications for Li+ Solvation. The Journal of Physical Chemistry B. 2015;119:13385-90.
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[67] Breiter M. Über die Art der Wasserstoffadsorption an Platinmetallelektroden. Electrochim Act. 1962;7:25-38.
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[68] Conway BE, Angerstein-Kozlowska H, Sharp WBA. Temperature and Pressure Effects on Surface Processes at Noble Metal Electrodes. Part 1.-Entropy of Chemisorption of H at Pt Surfaces. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases. 1978;74:1373-89. [69] Jerkiewicz G, Zolfaghari A. Determination of the Energy of the Metal−Underpotential-Deposited Hydrogen Bond for Rhodium Electrodes. The Journal of Physical Chemistry. 1996;100:8454-61. [70] Zolfaghari A, Chayer M, Jerkiewicz G. Energetics of the Underpotential Deposition of Hydrogen on Platinum Electrodes. J Electrochem Soc. 1997;144:3034-41. [71] Jerkiewicz G. Hydrogen sorption ATIN electrodes. Prog Surf Sci. 1998;57:13786. 13
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ACCEPTED MANUSCRIPT contact with aqueous perchloric acid solutions. J Chem Soc, Faraday Trans. 1996;92:3693-9. [85] Benderskii VA, Velichko GJ. Temperature jump in electric double-layer study, Part I. Method of measurement. J Electroanal Chem. 1982;140:1-22. [86] Climent V, Coles BA, Compton RG. Laser Induced Current Transients applied to a Au(111) Single Crystal Electrode. A General Method for the Measurement of Potentials of Zero Charge of Solid Electrodes. J Phys Chem B. 2001;105:1066973.
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[89] Garcia-Aráez N, Climent V, Feliu JM. Evidence of Water Reorientation on Model Electrocatalytic Surfaces from Nanosecond-Laser-Pulsed Experiments. J Am Chem Soc. 2008;130:3824-33.
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electrochemical system
wel
q
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cyclic voltammetry, d calorimetry impedance spectroscopy, chronoamperometry, ... Gibbs adsorption equation, electrocapillary Figure 1. Thermodynamic quantities (electric work wel, surface tension , heat q) and
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corresponding methods for the investigation of electrode processes.
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A
0 -20 -40 -60 0
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t [ms]
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T [a.u.] j [µA/cm²] E [V]
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E [V] vs. Cu | Cu
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ACCEPTED MANUSCRIPT Figure 2. (A) Potential E, current-density j, and temperature T transients during Cu underpotential deposition onto a (111)-textured Au film in 10 mM CuSO4 / 0.1 M H2SO4 by a 10 ms long current pulse. The electrochemical conversion amounts to about 1% of a complete Cu UPD layer. During the deposition the temperature of the electrode decreased about linearly with time, signaling the cooling of the electrode due to the electrochemical Peltier effect. After the current pulse the temperature slowly increased towards its starting value before the pulse, due to thermal
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equilibration with the surrounding. The temperature decrease at the end of the 10 ms pulse is of the order of 10 µK. (B) Cyclic voltammogram and potential-dependent molar Peltier heat of Cu UPD on Au(111). The Peltier heat data was obtained by cycling through the Cu UPD region by sequences of positive (red, upward triangles) and negative (blue, downward triangles) current pulses (see (A) for an exemplary
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single pulse) [20].
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electrode temperature
TOC-graphics
qrev RS 20
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time [ms]
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