Rolf Schuster

A microcalorimeter for measuring heat effects of electrochemical reactions with submonolayer conversions

We present a microcalorimeter for measuring heat effects during electrochemical reactions with conversions down to a few percent of a monolayer, referenced to the electrodes surface atoms. The design uses a thin pyroelectric polymer foil for temperature measurement at the backside of a thin electrode, similar to the concepts pioneered by the groups of D. A. King and Ch. T. Campbell for UHV adsorption microcalorimetry. To establish intimate thermal contact between electrode and sensor and utmost sensitivity, the free standing sensor and electrode foils are pressed together by air pressure, acting on the electrochemical cell. Pyroelectric temperature sensing is combined with pulsed electrochemistry, where the electrochemical heat is released on a time scale of about 10 ms, which is long enough for thermal equalization of the electrode-sensor assembly but short enough to avoid significant heat loss into electrolyte and cell compartment. As examples heat effects upon Ag deposition and dissolution as well as the electron transfer reaction of [Fe(CN)(6)](4-)/[Fe(CN)(6)](3-) are presented. The latter reaction was also employed for the calibration of the calorimeter.

Heat Effects upon Electrochemical Copper Deposition on Polycrystalline Gold

Heat effects upon Cu deposition on polycrystalline Au surfaces from sulphuric acid electrolytes were calorimetrically measured. By combination of pyroelectric temperature detection at the backside of a thin electrode foil with pulsed electrochemistry, sensitivities to electrochemical conversions of a few percent of a Cu monolayer (ML), corresponding to about 1 microJ cm(-2) were achieved. We compared the heat evolution upon Cu under potential deposition (UPD), Cu deposition onto a fully developed Cu UPD layer and bulk Cu deposition onto a 300 ML thick Cu layer on Au. The heat effects were measured dependent on the amplitudes of the applied potential steps, that is, the driving forces of the respective reactions. From the differences of the heat effects among the Cu deposition processes, we deduced implications on the reaction mechanisms. For Cu UPD, the heat effects were explicable by the deposition of 1.3 Cu atoms per two electrons flowing to the electrode accompanied by sulphate coadsorption, similar to Cu UPD from sulphuric acid solutions on Au(111). Upon Cu deposition on a Cu UPD layer the heat effects signal considerable anion coadsorption up to the deposition of about 0.5 ML of Cu. At higher conversions the deposition mechanism gradually changes towards bulk Cu deposition, accompanied by reduction of the sulphate coverage.

Microcalorimetric Measurements of the Solvent Contribution to the Entropy Changes upon Electrochemical Lithium Bulk Deposition

Research on the deposition of lithium from organic electrolytes has been pursued very thoroughly since the development of the lithium-ion battery. Although graphite has replaced lithium metal as anode material owing to safety and performance issues, the deposition of lithium is still a major concern. During the charging of the battery, lithium ions are intercalated into the graphite lattice. There are however various factors that may lead to the deposition of lithium on the electrode instead of intercalation. Low temperature and high current density are among the main reasons for lithium plating. Dendritic growth of lithium in turn can lead to a short-circuit fault of the cell and a thermal runaway of the battery. Electrochemical microcalorimetry can help to shed some light on the thermodynamic aspects of lithium deposition. Because electrical work is performed during this reaction, the exchanged heat is not linked to the enthalpy of the reaction but to the entropy, as stated already by Onsager and discussed in detail, for example, by Agar. We measured the temperature change on the back side of a thin electrode during short periods of bulk deposition of lithium. This allows us to quantitatively determine the reversible heat change accompanying the reaction. The heat, usually referred to as Peltier heat, corresponds to the entropy of reaction, which includes contributions from all species involved in the half-cell reaction. We studied the lithium bulk deposition in a typical battery electrolyte, LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC). In this electrolyte the entropy of Li bulk deposition is dominated by entropy changes from solvation of Li. Within a simple model we derived the coordination number of Li in the EC/DMC mixture. First quantitative measurements of the Peltier heat of single electrode reactions date back to the beginning of the 20th century (see for example Ref. [4] for a compilation of early data). Most of those studies concentrated on bulk metal deposition, for example, of Ag, Cu, Cd, and Zn from aqueous solutions. With experimental improvements, reliable data also became available for electron-transfer reactions, for example, for [Fe(CN)6] 4 /3 . Also the influence of cations and anions and their transport processes on the heat changes was studied in detail (see for example Refs. [5a,6]). Battery-related systems came into focus with the work of Conway and coworkers, who calorimetrically studied Zn and bromine electrodes to gain understanding of the thermal behavior of battery charge and discharge processes. Most research on reversible heat changes of battery related single electrode reactions concentrated on the hydrogen evolution reaction (see for example Ref. [8] and references therein). Very little work has been performed on Li-ion batteries, mostly considering complete cells. Maeda studied the heat change of a single graphite electrode upon intercalation of ClO4 , BF4 , K, and Li. Unfortunately the sensitivity of the calorimetric setup was insufficient to quantitatively measure the Peltier heat of the Li intercalation process. It should be noted that all of the above cited studies required rather high electrochemical conversions of the order of several 10 to 10000 monolayers, referenced to the electrode surface, to obtain reliable heat measurements. Only recently reactions with submonolayer conversions, such as oxide formation on a Au surface, hydrogen adsorption on a platinized Pt foil, or Cu underpotential deposition on a Au surface, became accessible. Herein we apply our microcalorimetric approach to the study of Li deposition. As heat changes upon deposition or dissolution down to a few percent of a monolayer of Li can be reliably determined with this method, we were able to directly measure the reversible entropy change of the Li electrode. Prior to the calorimetric experiments, we deposited a thick Li layer (ca. 1800 monolayers) on a Ni working electrode. Subsequently, pulsed Li deposition and dissolution was conducted on that surface. Figure 1 shows the potential, current, and temperature transients during a typical experiment. At 10 ms, a potential pulse with amplitude 50 mV starting at the Li/Li equilibrium potential, that is, with 50 mV overpotential, was applied for 10 ms. At the end of the pulse, at t = 20 ms, the external cell current was interrupted by an electronically operated switch. The potential then relaxed towards the Li/Li equilibrium potential. During the negative potential pulse we measured a negative current, which corresponds to the deposition of lithium. The corresponding charge can be retrieved by integration of the current transient and amounts to 6.3 mC in the shown experiment. This is 20% of a monolayer of Li assuming a surface density of 10 atomscm 2 for Li. The temperature signal started to drop immediately with the beginning of the pulse, that is, with the beginning of Li deposition. After the end of the pulse the [*] M. J. Schmid, K. R. Bickel, Prof. R. Schuster Institut f r Physikalische Chemie Karlsruhe Institute of Technology Fritz-Haber-Weg 2, 76131 Karlsruhe (Germany) E-mail: rolf.schuster@kit.edu

Cooperative Behaviour of Pt Microelectrodes during CO Bulk Electrooxidation

Cooperative behaviour of an array of microelectrodes: The interplay of bistable reaction kinetics with global coupling results in a spontaneous sequential activation of electrodes when the applied current is increased.

Electrochemical Behaviour of Iron in a Third‐Generation Ionic Liquid: Cyclic Voltammetry and Micromachining Investigations

The electrochemical behaviour of Fe in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim](+)Ntf2(-)) and mixtures with Cl(-) is studied with the aim of investigating the applicability of ionic liquids (IL) for the electrochemical machining of iron. Whereas in pure IL iron could not be significantly dissolved, the addition of Cl(-) enables the active dissolution with anodic current densities up to several mA cm(-2). Although several anodic peaks appear in the cyclic voltammograms (CV), the distinct assignment of those electrochemical processes remain difficult. In particular no proof for the formation of FeCl(x) (2-x) complexes during Fe dissolution are deduced from the CV, although such complexes are shown to be stable in the employed electrolyte. In addition, we present electrochemical drilling experiments with short potential pulses, which demonstrate that electrochemical machining of Fe is, in principle, possible in IL based electrolytes, even though hampered by slow machining speed.

Identification of Non-Faradaic Processes by Measurement of the Electrochemical Peltier Heat during the Silver Underpotential Deposition on Au(111)

We measured the heat which is reversibly exchanged during the course of an electrochemical surface reaction, i.e., the deposition/dissolution of the first two monolayers of Ag on a Au(111) surface in (bi)sulfate and perchlorate containing electrolytes. The reversibly exchanged heat corresponds to the Peltier heat of the reaction and is linearly related to its entropy change, including also non-Faradaic side processes. Hence, the measurement of the Peltier heat provides thermodynamic information on the electrochemical processes which is complementary to the current-potential relations usually obtained by conventional electrochemical methods. From the variation of the molar Peltier heat during the various stages of the deposition reaction we inferred that co-adsorption processes of anions and Ag do not play a prominent role, while we find strong indications for a charge neutral substitution reaction of adsorbed anions by hydroxide, which would not show up in cyclic voltammetry.

An improved sensor for electrochemical microcalorimetry, based on lithiumtantalate

We have developed a pyroelectric sensor for electrochemical microcalorimetry, based on LiTaO3, which provides unprecedented sensitivity for the detection of electrochemically induced heat effects. Deterioration of the heat signal by electrostriction effects on the electrode surface is suppressed by a multilayered construction, where an intermediate sapphire sheet dampens mechanical deformations. Thus, well textured thin metal films become viable candidates as electrodes. We demonstrate the sensor performance for Cu underpotential deposition on (111)-textured Au films on sapphire. The sensor signal compares well with a purely thermal signal induced by heating with laser pulses. The high sensitivity of the sensor is demonstrated by measuring heat effects upon double layer charging in perchloric acid, i.e., in the absence of electrochemical charge- or ion-transfer reactions.

Plasmonic nanospheres with a handle—Local electrochemical deposition of Au or Ag at the apex of optically inactive W- or C-tips

We developed an electrochemical method for the local deposition of spherical particles of plasmonic metals like Au or Ag at the apex of conductive tips. The electrochemical metal deposition was confined to the tip apex by the application of short potential pulses between the tip and a sacrificial electrode in close proximity. The diameter of the deposits could be varied between several 10 nm to about 1 μm. Cathodoluminescence maps of the decorated tips showed enhanced luminescence at the Au or Ag nanospheres. Local cathodoluminescence spectra proved excitation of surface plasmons characteristic for metallic Ag or Au.

Electrochemical Machining of Gold Microstructures in LiCl/Dimethyl Sulfoxide

LiCl/dimethyl sulfoxide (DMSO) electrolytes were applied for the electrochemical micromachining of Au. Upon the application of short potential pulses in the nanosecond range to a small carbon-fiber electrode, three-dimensional microstructures with high aspect ratios were fabricated. We achieved machining resolutions down to about 100 nm. In order to find appropriate machining parameters, that is, tool and workpiece rest potentials, the electrochemical behavior of Au in LiCl/DMSO solutions with and without addition of water was studied by cyclic voltammetry. In waterless electrolyte Au dissolves predominantly as Au(I), whereas upon the addition of water the formation of Au(III) becomes increasingly important. Because of the low conductivity of LiCl/DMSO compared with aqueous electrolytes, high machining precision is obtained with moderately short pulses. Furthermore, the redeposition of dissolved Au can be effectively avoided, since Au dissolution in LiCl/DMSO is highly irreversible. Both observations render LiCl/DMSO an appropriate electrolyte for the routine electrochemical micromachining of Au.

Oscillations in an array of bistable microelectrodes coupled through a globally conserved quantity

The dynamical behavior of an array of microelectrodes is investigated under controlled current conditions during CO electrooxidation, a bistable electrochemical reaction with an S-shaped negative differential resistance (S-NDR) current-potential curve. Under these conditions, the total current constitutes a globally conserved quantity, thus coupling all microelectrodes globally. Upon increasing the total current, the microelectrodes activate one by one, with a single microelectrode being on its intermediate S-NDR current branch and the other ones being either on their passive or their active branches. When a few coupled microelectrodes are activated, the electrochemical system exhibits spontaneous potential oscillations. Mathematical analysis shows that oscillations arise already in a two group approximation of the dynamics, the two groups consisting of 1 electrode and n - 1 electrodes with n ≥ 3, respectively, with each group being described by a single evolution equation. In this minimal representation, oscillations occur when the single electrode is on the intermediate branch and the larger group is on the active branch.