For fast and reversible electrocatalytic reactions, like the hydrogen evolution on Pt in acidic media and in the presence of molecular H2, it is very challenging to obtain reliable kinetic information from the charge-potential relationship alone. To overcome this issue, we measure the time-resolved heat flux during hydrogen evolution on polycrystalline Pt as complementary information to chronopotentiometric measurements. The interpretation of the data is based on a comparison with a finite element simulation comprising a microkinetic model coupled to mass transport. Using this approach, we estimate the exchange current density to be around 10 – 15 mA cm−2, for both, the Tafel or the Heyrovský reaction being the rate limiting step. We also find that the coverage of the active hydrogen intermediate is low around the equilibrium potential, even in hydrogen saturated solution.

Hard carbon (HC) is commonly used as negative electrode material in sodium-ion batteries. Despite its extensive use, there is still a lack of comprehensive understanding of the sodiation mechanism. To obtain thermodynamic information on the storage processes, the reaction entropy of HC/Na metal cells in NaPF6/diglyme electrolyte solution is determined by measuring the temperature dependence of the equilibrium cell voltage at different states of charge (SoCs), commonly known as entropy profiling. In contrast to former studies, we changed the temperature of the cell stepwise within less than 1 min, which reduces the influence of baseline drift of the cell voltage. The cell reaction entropy varied between -10 and 6 J mol(-1)K(-1) for all SoCs, which indicates complete solvation/desolvation of the Na+ ions at the HC composite electrode. In addition, the fast temperature change led to characteristic features of the cell voltage response. These features were explained with a simple model including temperature gradients across the cell and different kinetics of the reactions at the HC and the Na metal electrode. From the cell voltage response, we inferred that the sodiation of HC is significantly slowed down with decreasing SoC.

Scanning probe methods were very successful for investigating ordered adlayer structures or static interface defects in electrochemical systems. However, information on more dynamic entities like fluctuating structures, diffusing species or solvent water is often difficult to obtain. In this contribution we will show how the determination of thermodynamic quantities like the free enthalpy of the reaction Δ R G , the reaction entropy Δ R S , or the reaction volume Δ R V may help to obtain independent, complementary information on the electrochemical processes. While Δ R G can be obtained directly from cyclic voltammetry, we address Δ R S by electrochemical microcalorimetry, where we measure the reversibly and irreversibly exchanged heat, evolved during the reaction. Δ R V is determined from pressure dependent measurements of the equilibrium potential of the respective surface electrochemical reaction. We will present results on three exemplary systems: i) On Δ R S obtained by electrochemical microcalorimetry for anion adsorption on Au(111), where the entropy variations with coverage point to substantial anion interactions at high coverages, while near the pzc the participation of solvent water is postulated. ii) On Na-insertion into Hard Carbon or Sodium-Vanadium-Phosphate as examples for common Na ion-battery electrodes, where time dependent measurements of the entropy point to consecutive reaction steps on different time scales. iii) We will also present our attempts to measure Δ R V of Cu bulk deposition and Cu UPD on Au(111) by pressure-dependent potential measurements, from which indications on strong electrostriction of solvent water in the UPD structure can be derived. Acknowledgements: We gratefully acknowledge contributions by the work groups of Christine Kranz and Timo Jacob. This work was partly funded by the German Research Foundation (DFG) under Project ID 390874152 (POLiS Cluster of Excellence).

Hard carbon (HC) is one of the most promising anode materials for sodium-ion batteries, but the different sodiation processes contributing to its reversible capacity are still under debate. In order to obtain thermodynamic information, we measured the heat exchanged at a single HC composite electrode during the (de)sodiation of HC in NaClO4/propylene carbonate and NaPF6/diglyme to determine the reaction entropy of the sodiation process of HC at different states of charge (SoCs). We found that it is dominated by the positive entropy contribution of the concomitant desolvation, leading to reaction entropies up to 85 J mol-1 K-1 for NaClO4/propylene carbonate and 250 J mol-1 K-1 for NaPF6/diglyme. Together with information on the electrochemical analysis of millisecond charging/discharging pulses, we postulate a capacitive adsorption process with incomplete desolvation at low SoC, while at medium to high SoC, a Faradaic insertion process with complete desolvation takes place.

Sodium-ion batteries are among the most promising alternatives to lithium-ion batteries. Hard carbon (HC) electrodes have been recognized as suitable active anode material for mono-valent ion batteries. Here, we present a simple and cost-effective spray-coating process to prepare HC composite electrodes on copper current collectors with different binder (sodium carboxymethyl cellulose, CMC) content and different HC particle sizes. The spray-coated electrodes were evaluated and tested in 1 M sodium perchlorate (NaClO4) in propylene carbonate (PC) in dependence of the CMC content with and without fluoroethylene carbonate (FEC) as additive, and the performance was also compared to doctor bladed HC electrodes. Spray-coated anodes in Na half-cells revealed improved capacity during the first cycles compared with doctor bladed anodes with similar thicknesses. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) studies were performed, which revealed a significant increase of inorganic fluoro-compounds in the formed solid electrolyte interphase (SEI) when FEC was present as additive. In addition, first single electrode microcalorimetry studies on spray-coated thin HC composite electrodes yielded an entropy of the sodiation process of 80 J mol-1 K-1 at high state of charge (SoC), comparable to that of bulk Na deposition. Spray coating: The study presents a cost-effective, spray-coating process for HC composite electrodes on copper current collectors using different binder content and particle sizes with controlled film thicknesses and mass loading. The spray-coated anodes revealed excellent performance.image

Investigations on electrochemical kinetics usually rely on the measurement of current or potential as a function of time. Charge-neutral process steps or side reactions are naturally disguised in the electrical signals and have only indirect impact. However, all processes will contribute to heat evolution. In this work, heat absorption/liberation is measured as a function of time for pulsed Na deposition/dissolution on a Na-electrode in a 1 m NaPF6/diglyme solution, in addition to the standard electrochemical signals. While potential and current transients both exhibited sharp rectangular shapes, indicating instantaneous electrochemical Na deposition or dissolution on the time scale of the pulse (10 ms), heat absorption or liberation continued up to about 0.5 s after the pulse. Since heat evolution is to large extent reversible, this corresponded to entropy changes in the absence of external electric current flow, pointing to a reversible, charge-neutral chemical process accompanying Na deposition or dissolution. From the observed entropy changes, it is suggested that upon Na deposition solvated Na+ ions are instantaneously transferred into the outer layers of the solid electrolyte interphase, followed by slow desolvation. Charge-neutral processes in electrochemical systems do not contribute to the cell current. However, they participate in the heat evolution at the electrode during the reaction. By time-dependent heat measurements upon Na deposition at a Na metal electrode slow charge-neutral reaction steps are detected. This possibly points to incomplete desolvation of Na+ ions upon incorporation into the solid electrolyte interface.image

The anode/electrolyte interface behavior, and by extension, the overall cell performance of sodium-ion batteries is determined by a complex interaction of processes that occur at all components of the electrochemical cell across a wide range of size- and timescales. Single-scale studies may provide incomplete insights, as they cannot capture the full picture of this complex and intertwined behavior. Broad, multiscale studies are essential to elucidate these processes. Within this perspectives article, several analytical and theoretical techniques are introduced, and described how they can be combined to provide a more complete and comprehensive understanding of sodium-ion battery (SIB) performance throughout its lifetime, with a special focus on the interfaces of hard carbon anodes. These methods target various length- and time scales, ranging from micro to nano, from cell level to atomistic structures, and account for a broad spectrum of physical and (electro)chemical characteristics. Specifically, how mass spectrometric, microscopic, spectroscopic, electrochemical, thermodynamic, and physical methods can be employed to obtain the various types of information required to understand battery behavior will be explored. Ways are then discussed how these methods can be coupled together in order to elucidate the multiscale phenomena at the anode interface and develop a holistic understanding of their relationship to overall sodium-ion battery function. Here, several analytical methods across multiple time and length scales are discussed, covering a wide range of physical and (electro)chemical properties. To fully grasp the complexity of sodium-ion battery anodes, integrated studies on the same battery system, ranging from the cellular level to the atomic level, are required.image

A common way to benchmark catalysts is to perform a Tafel analysis on the current-overpotential relationship obtained by potential sweep methods and extract the exchange-current density. However, especially for fast reactions this analysis underestimates the reaction rate. In this contribution, we introduce an alternative way to determine the activation overpotential from the irreversible part of the heat exchanged during the electrode reaction, which was measured with electrochemical microcalorimetry (ECM). In addition, the reversible part of the exchanged heat allows access to the reaction entropy. We evaluate the proposed procedure using the fast hydrogen evolution at platinum in acidic solution as model reaction.

The formation of an appropriate solid electrolyte interphase (SEI) at the anode of a sodium battery is crucially dependent on the electrochemical stability of solvent and electrolyte at the redox potential of Na/Na+ in the respective system. In order to determine entropic contributions to the relative stability of the electrolyte solution, we measure the reaction entropy of Na metal deposition for diglyme (DG) and propylene carbonate (PC) based electrolyte solutions by electrochemical microcalorimetry at single electrodes. We found a large positive reaction entropy for Na+ deposition in DG of Delta S-R(DG) approximate to 234 Jmol(-1)K(-1) (c.f.: Delta(R) S(PC) approximate to 83 Jmol(-1)K(-1)), which signals substantial entropic destabilization of Na+ in DG by about 0.73 eV, thus increasing the stability of solvent and electrolyte relative to Na+ reduction. We attribute this strong entropic destabilization to a highly negative solvation entropy of Na+, due to the low dielectric constant and high freezing entropy of DG.

We measured the entropy of hydrogen adsorbed on Pt NPs with 3 different shapes. The entropy depends on the local geometry of the particles and matches the entropy of hydrogen adsorbed on single crystals with the corresponding orientation.

Iron‐doped tin oxide (Sn0.9Fe0.1O2), and specifically carbon‐coated Sn0.9Fe0.1O2 (Sn0.9Fe0.1O2‐C) provides high reversible capacity and a reasonably low de‐/lithiation potential owing to the combined conversion and alloying mechanism. The initial (quasi‐)amorphization during the first lithiation, however, renders an in‐depth understanding of the reaction mechanism challenging. Herein, a comprehensive investigation via a set of highly complementary characterization techniques is reported, including operando X‐ray diffraction, ex situ 119Sn and 57Fe Mössbauer spectroscopy, ex situ 7Li NMR spectroscopy, operando isothermal microcalorimetry (IMC) of Li‖Sn0.9Fe0.1O2‐C coin cells, and electrochemical microcalorimetry of single Sn0.9Fe0.1O2‐C electrodes. The combination of these advanced techniques allows for detailed insights into the lithiation and delithiation mechanism and the potential determining processes, despite the (quasi‐) amorphous nature of the active material after the initial lithiation.

To explore a minimal feature size of <100 nm with electrochemical additive manufacturing, we use a strategy originally applied to microscale electrochemical machining for the nanoscale deposition of Co on Au. The concept’s essence is the localization of electrochemical reactions below a probe during polarization with ns-long voltage pulses. As shown, a confinement that exceeds that predicted by a simple model based on the time constant for one-dimensional double layer charging enables a feature size of <50 nm for 2D patterning. We further indirectly verify the potential for out-of-plane deposition by tracking growth curves of high-aspect-ratio deposits. Importantly, we report a lack of anodic stability of Au tips used for patterning. As an inert probe is the prerequisite for controlled structuring, we experimentally verify an increased resistance of Pt probes against degradation. Consequently, the developed setup and processes show a path towards reproducible direct 2D and 3D patterning of metals at the nanoscale.

Electrochemical microcalorimetry measures the heat, which is evolved during an electrochemical process. With our experimental setup tiny electrochemical conversions down to a few percent of a monolayer are sufficient for the determination of the heat, which is evolved at a single electrode. This allows to stay close to equilibrium conditions and thus to determine the reversible contribution to the heat, which is directly proportional to the entropy changes during the electrochemical reaction, including all side reactions like codeposition processes, polarization of the double layer etc.. In this contribution we will present examples, how from the knowledge of the reaction entropy conclusions can be drawn i) on the solvation of Na ions for Na deposition from a diglyme electrolyte, ii) on the reactions steps for the reduction of Ag from Ag-cyanide complexes and iii) the deposition process of Cu2+ from aqueous solutions. We also will present first results of our attempts to measure the reaction volume of Cu bulk deposition and Cu UPD on Au(111) by pressure dependent potential measurements.

The front cover artwork is provided by Dr. Marco Schönig from Prof. Rolf Schuster's group at the Karlsruhe Institute of Technology. The image shows a gold surface under electrochemical control covered by two different, unspecified adsorbed species. In the inset, we present the result of our heat of reaction measurements, which showed that for Au(111) in sulfuric acid solutions the adsorbing anionic species is sulfate. Read the full text of the Research Article at 10.1002/cphc.202200227.

We measure the entropy of formation of the interface upon anion adsorption (Cl-, Br- I- and SO42-) on Au(111) as an important indicator for the structure, order and composition of the interface. The entropy of formation of the interface exhibits a rather universal behaviour for all anions with a steep decrease upon initial adsorption followed by a shallow minimum at intermediate anion coverages and a strong increase close to the completion of the adsorbate adlayer. The strong variation of the entropy signals significant entropic contributions to the free enthalpy of the adsorption process and thus the stability of the adsorbed phase. At low anion coverages, close to the potential of zero charge, we attribute the entropy variations to the rearrangement of the interfacial water structure. At intermediate and high anion coverages, a comparison with the results of a lattice-gas model, considering pairwise repulsive interactions within the quasi-chemical approximation, shows that the entropy changes upon anion adsorption can be explained by the configurational entropy of the adsorbed phase. Thus, entropic contributions from both the solvent and the adsorbate are important for the stability of surface phases, particularly for disordered systems.

We investigated the entropy of the electrical double layer (EDL) formation at a (111)-textured Au film in the ionic liquid 1-butyl-1-methylpyrrolidinium bis[(trifluoromethypsulfonyl]imide by electrochemical microcalorimetry. We found reversible heat exchange upon EDL charging in the potential range of -0.6 to 1 V versus Fc/Fc(+). Dependent on the potential, the heat evolution continued for 10 to 200 ms after the current flow, indicating slow reorganization of the EDL on this time scale. The partial molar entropy of the formation of the EDL Delta S-R(form)., was derived from the reversibly exchanged heat upon EDL charging. It varied about linearly with the potential from 27 J K-1 moL(-1) at -0.6 V to -79 J K-1 mol(-1) at 1 V versus Fc/Fc(+). Delta S-R(form) crosses zero close to the potential of zero charge of the system. The entropy values are much higher than those arising from purely configurational entropy of a 2D lattice gas, thus pointing to strong, potential dependent immobilization of the constituents of the EDL, where the immobilization is correlated with the strength of the electric field in the EDL. In addition, from our entropy values, we derive an apparent contradiction with experimentally determined temperature coefficients of the EDL capacitance, which implies a strongly varying activity of the ions participating in charging of the EDL with temperature.

Thermogalvanic cells may be viewed as electrochemical thermocouples. Here, we demonstrate their potential for sensitive and fast sensing of the surface temperature of a thin Au sheet, in contact with a 0.08 M equimolar ferri-/ferrocyanide solution in 26% KOH. We obtained a sensitivity of about 10 μK, because of the large Peltier coefficient of the electrochemical reaction at the electrode-electrolyte junction. The resolution is limited by the thermal noise of the electrochemical system and could be quantitatively explained by the thermal noise of a Randles equivalent circuit of the cell. Since no thermal contact resistances to bulky temperature sensors are involved, we achieved a response time smaller than 300 μs, which may be potentially lowered to a few microseconds. By using the thermogalvanic cell as a sensor for electrochemical microcalorimetry, we calibrated the system, in full agreement with a simulation of its thermal behavior.

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How particles can move on a catalyst surface that, under the conditions of an industrial process, is highly covered by adsorbates and where most adsorption sites are occupied has remained an open question. We have studied the diffusion of O atoms on a fully CO-covered Ru(0001) surface by means of high-speed/variable-temperature scanning tunneling microscopy combined with density functional theory calculations. Atomically resolved trajectories show a surprisingly fast diffusion of the O atoms, almost as fast as on the clean surface. This finding can be explained by a "door-opening" mechanism in which local density fluctuations in the CO layer intermittently create diffusion pathways on which the O atoms can move with low activation energy.

The potential-induced sulfate adsorption on (111)-textured Au films in 0.1 M sulfuric acid was studied by surface plasmon resonance (SPR) measurements. Full SPR curves were recorded with millisecond time resolution allowing for simultaneous measurement of SPR and cyclic voltammetry data. We could quantitatively account for the observed SPR shift within a five-phase model, which includes the effects of electron depletion in the topmost electrode layer upon positive polarization. From the refractive index change in the modeled sulfate layer and the sulfate surface excess, as known from the literature, we derived an effective polarizability of alpha(eff) = 3.9 angstrom(3) per sulfate species entering the adlayer from the bulk of the solution. Since this value comes close to that of sulfate ions in solution, no significant side processes like exchange of water between the adlayer and the solution are expected. This directly implies compression of the adlayer species upon positive polarization of the electrode and hence a negative reaction volume for the sulfate layer formation process, signaling strong electrostriction of water in the adlayer.