@article{63675, abstract = {{Cobalt spinel (Co3O4) catalysts are widely studied in scope of the electrocatalytic oxygen evolution reaction (OER), yet the role of interfacial structural transformation under anodic bias remains under debate. Here, we employ an operando approach, combining a fast electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D), electrochemical impedance spectroscopy (EIS), and Raman spectroscopy to investigate interfacial transformations of Co3O4 nanoparticle electrodes in alkaline electrolyte. We identify two distinct regimes during the anodic sweep prior to the macroscopic OER onset. At lower potentials, the catalyst interface remains mechanically rigid while reversibly associating several OH−/H2O species per oxidized cobalt site. At higher potentials, pronounced softening of the interface occurs alongside further uptake of electrolyte species. This indicates amorphization and a ‘swelling process’ beyond simple adsorption. Notably, an electrochemical conditioning treatment can suppress mass and compliance hysteresis without affecting OER activity, suggesting that most incorporated electrolyte species do not participate in the OER. EIS further reveals that OER intermediates form well below the apparent OER onset potential. These results advance our mechanistic understanding of interfacial transformations in cobalt-based OER catalysts and establish EQCM-D as a sensitive operando technique for probing electrocatalyst transformations.}}, author = {{Leppin, Christian and Placke‐Yan, Carsten and Bendt, Georg and Hernandez, Sheila and Tschulik, Kristina and Schulz, Stephan and Linnemann, Julia}}, issn = {{1867-3880}}, journal = {{ChemCatChem}}, keywords = {{electrocatalysis, Co3O4, EQCM-D, OER}}, number = {{2}}, publisher = {{Wiley}}, title = {{{Interfacial Softening and Electrolyte Uptake in Co3O4 OER Catalysts: Insight from Operando Spectroscopy and Fast EQCM‐D}}}, doi = {{10.1002/cctc.202501104}}, volume = {{18}}, year = {{2026}}, } @article{64182, abstract = {{Overcoming the slow kinetics of the oxygen evolution reaction at the anode is a key challenge for the production of hydrogen via electrolysis. This reaction operates at very positive potentials, where the electrocatalyst is exposed to highly oxidative conditions and prone to potential-dependent transformation of the near-surface region. While substantial evidence for such surface restructuring exists, its extent and relevance for the catalyst’s activity are unclear. We address this topic for the case of Co3O4, one of the best-known electrocatalysts exhibiting surface restructuring, by studies of epitaxial (111)-ordered electrodeposited films with combined operando X-ray surface diffraction and absorption spectroscopy, electrochemical impedance spectroscopy, and electrochemical measurements on rotating disk electrodes. Comparison of the as-prepared and annealed state of the same samples, which both are stable even under long-term oxygen evolution conditions, provides clear insight into the role of surface defects. Our results show that defect-free annealed Co3O4(111) surfaces are structurally stable over a wide potential range and hydroxylate via adsorption at surface oxygen and Co sites. Potential-induced surface restructuring of the Co3O4 lattice occurs only in the presence of surface defects, leading to the formation of the well-known nanometer-thick oxyhydroxide skin layer. The presence of this skin layer promotes oxygen evolution at low overpotentials but results in higher Tafel slopes. As a result, highly ordered Co3O4(111) surfaces are more active at high current densities than defective Co3O4 surfaces that undergo surface restructuring. These results highlight that strategies for catalyst surface defect engineering need to be application-oriented.}}, author = {{Scharf, Carl Hendric and Chandraraj, Alex and Dyk, Konrad and Stebner, Felix and Lepin, Sören and Tian, Jing and El Bergmi Byaz, Laila and Stettner, Jochim and Leppin, Christian and Kotova, Anastasiia and Reinke, Sebastian and Linnemann, Julia and Maroun, Fouad and Magnussen, Olaf M.}}, issn = {{2155-5435}}, journal = {{ACS Catalysis}}, keywords = {{electrocatalysis, oxygen evolution reaction, cobalt spinel, operando characterization}}, publisher = {{American Chemical Society (ACS)}}, title = {{{Role of Defects in Reversible Surface Restructuring and Activity of Co3O4 Oxygen Evolution Electrocatalysts}}}, doi = {{10.1021/acscatal.5c08785}}, year = {{2026}}, } @article{66488, abstract = {{Alkaline oxygen evolution on Co3O4 involves more than adsorption and electron transfer at a fixed surface. The article draws on operando electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D), surface X-ray diffraction (SXRD), Raman, UV/Vis and impedance spectroscopy. On this basis, it discusses pseudocapacitive oxidation of cobalt ion sites, electrolyte uptake and near-surface transformation, and asks which of these changes are kinetically coupled to electrocatalysis.}}, author = {{Linnemann, Julia and Leppin, Christian}}, journal = {{Bunsen-Magazin}}, keywords = {{electrocatalysis, oxygen evolution reaction, cobalt spinel, operando characterization, spectroelectrochemistry}}, location = {{Dresden}}, number = {{4}}, pages = {{81--84}}, publisher = {{Deutsche Bunsen-Gesellschaft für physikalische Chemie e.V.}}, title = {{{The catalyst that stores charge first}}}, doi = {{10.26125/6G4P-8386}}, year = {{2026}}, } @article{61982, abstract = {{Doped Co3O4 nanoparticles are investigated via spectro-electrochemistry in the (pre-) oxygen evolution reaction (OER) regime by tracing the absorption signal of the Co3+ d–d transition under applied bias for getting insight into the catalysts activation and the formation of catalytically active phases. In the low potential regime up to 1.37 VRHE, a rise in the optical absorption signal of the [Co3+]oct d–d transition is observed and attributed to a structural change from [Co2+]tet to [Co3+]oct due to an electrochemically induced surface restructuring with water. For applied potentials higher than 1.37 VRHE an overall offset of the absorption spectra in the UV–vis range, equivalent to a darkening of the materials is detected. This is attributed to the formation of a CoOx(OH)y skin layer as supported by high-energy X-ray diffraction (HE-XRD) measurements. We found that the kinetics of the Co3+ states are heavily influenced by the type of dopant with V-doped Co3O4 exhibiting stable Co3+ states (>20 min) while the Mn-doped Co3O4 Co3+ states reduce within 36 s under reductive bias. We conclude that doping Co3O4 with transition metals affects the formation and potential-dependent thickness of the CoOx(OH)y skin layer as the catalytically active phase and the formation of long-time stable surface Co3+ states after activation in the first OER cycle.}}, author = {{Kampermann, L. and Klein, J. and Wagner, T. and Kotova, A. and Placke-Yan, C. and Yasar, A. and Jacobse, L. and Lasagna, S. and Leppin, Christian and Schulz, S. and Linnemann, Julia and Bergmann, A. and Roldan Cuenya, B. and Bacher, G.}}, issn = {{2155-5435}}, journal = {{ACS Catalysis}}, keywords = {{electrocatalysis, oxygen evolution reaction, cobalt spinel, operando characterization, spectroelectrochemistry}}, number = {{21}}, pages = {{18391--18403}}, publisher = {{American Chemical Society (ACS)}}, title = {{{Operando Analysis of the Pre-OER Activation of Metal-Doped Co3O4 Nanoparticle Catalysts}}}, doi = {{10.1021/acscatal.5c03900}}, volume = {{15}}, year = {{2025}}, } @article{63223, abstract = {{AbstractThe quartz crystal microbalance with dissipation monitoring (QCM‐D) is routinely used to investigate structured samples. Here, a simulation technique is described, that predicts the shifts of frequency and half bandwidth, Δfn and ΔΓn, of a quartz resonator operating on different overtone orders, n, induced by structured samples in contact with the resonator surface in liquid. The technique, abbreviated as FreqD‐LBM, solves the Stokes equation in the frequency domain. The solution provides the complex amplitude of the area‐averaged tangential stress at the resonator surface, from which Δfn and ΔΓn are derived. Because the dynamical variables are complex amplitudes, the viscosity can be complex, as well. The technique naturally covers viscoelasticity. Limitations are linked to the grid resolution and to problems at large viscosity. Validation steps include viscoelastic films, rough surfaces, an oscillating cylinder in a viscous medium, and a free‐floating sphere above the resonator. Application examples are soft adsorbed particles, stiff adsorbed particles, and a large, immobile spherical cap above the resonator, which allows to study the high‐frequency properties of the material in the gap. FreqDLBM runs on an office PC and does not require expert knowledge of numerical techniques. It is accessible to an experimentalist.}}, author = {{Johannsmann, Diethelm and Häusner, Paul and Langhoff, Arne and Leppin, Christian and Reviakine, Ilya and Vanoppen, Viktor}}, issn = {{2513-0390}}, journal = {{Advanced Theory and Simulations}}, number = {{7}}, publisher = {{Wiley}}, title = {{{The Frequency‐Domain Lattice Boltzmann Method (FreqD‐LBM): A Versatile Tool to Predict the QCM Response Induced by Structured Samples}}}, doi = {{10.1002/adts.202401373}}, volume = {{8}}, year = {{2025}}, } @article{63222, abstract = {{The solid electrolyte interphase (SEI) on the anode of lithium-ion batteries (LIBs) has been studied thoroughly due to its crucial importance to the battery’s long-term performance. At the same time, most studies of the SEI apply ex situ characterization methods, which may introduce artifacts or misinterpretations as they do not investigate the SEI in its unaltered state immersed in liquid battery electrolyte. Thus, in this work, we focus on using the non-destructive combination of electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) and impedance spectroscopy (EIS) in the same electrochemical cell. EQCM-D can not only probe the solidified products of the SEI but also allows for the monitoring of viscoelastic layers and viscosity changes of the electrolyte at the interphase during the SEI formation. EIS complements those results by providing electrochemical properties of the formed interphase. Our results highlight substantial differences in the physical and electrochemical properties between the SEI formed on copper and on amorphous carbon and show how formation parameters and the additive vinylene carbonate (VC) influence their growth. The EQCM-D results show consistently that much thicker SEIs are formed on carbon substrates in comparison to copper substrates.}}, author = {{Stich, Michael and Leppin, Christian and Krauss, Falk Thorsten and Valdes Landa, Jesus Eduardo and Pantenburg, Isabel and Roling, Bernhard and Bund, Andreas}}, issn = {{2313-0105}}, journal = {{Batteries}}, number = {{7}}, publisher = {{MDPI AG}}, title = {{{Comparing the SEI Formation on Copper and Amorphous Carbon: A Study with Combined Operando Methods}}}, doi = {{10.3390/batteries11070273}}, volume = {{11}}, year = {{2025}}, } @article{63224, abstract = {{By monitoring the solidification of droplets of plant latices with a fast quartz crystal microbalance with dissipation monitoring (QCM-D), droplets from Campanula glomerata were found to solidify much faster than droplets from Euphorbia characias and also faster than droplets from all technical latices tested. A similar conclusion was drawn from optical videos, where the plants were injured and the milky fluid was stretched (sometimes forming fibers) after the cut. Rapid solidification cannot be explained with physical drying because physical drying is transport-limited and therefore is inherently slow. It can, however, be explained with coagulation being triggered by a sudden decrease in hydrostatic pressure. A mechanism based on a pressure drop is corroborated by optical videos of both plants being injured under water. While the liquid exuded by E. characias keeps streaming away, the liquid exuded by C. glomerata quickly forms a plug even under water. Presumably, the pressure drop causes an influx of serum into the laticifers. The serum, in turn, triggers a transition from a liquid–liquid phase separated state (an LLPS state) of a resin and hardener to a single-phase state. QCM measurements, optical videos, and cryo-SEM images suggest that LLPS plays a role in the solidification of C. glomerata.}}, author = {{Langhoff, Arne and Peschel, Astrid and Leppin, Christian and Kruppert, Sebastian and Speck, Thomas and Johannsmann, Diethelm}}, issn = {{2223-7747}}, journal = {{Plants}}, number = {{5}}, publisher = {{MDPI AG}}, title = {{{Rapid Solidification of Plant Latices from Campanula glomerata Driven by a Sudden Decrease in Hydrostatic Pressure}}}, doi = {{10.3390/plants14050798}}, volume = {{14}}, year = {{2025}}, } @article{63225, abstract = {{Various polycations and polyanions were sequentially adsorbed onto the gold electrode of a quartz crystal microbalance with dissipation monitoring. The study focused on determining the adsorption kinetics, viscoelastic properties, and electroresponsivity of polyelectrolyte layers. For the first time, it was demonstrated that the structure (compact or expanded) of the layers can be determined by electroresponsivity. Viscoelastic modeling alone did not provide a conclusive answer as to whether the layers were compact or expanded. The study was further enriched by streaming potential and contact angle measurements, where polyelectrolyte multilayers were formed on mica. It was found that successive adsorption of layers led to periodic inversion of the zeta potential. Systematic differences were observed between the different top layers, which were explained by intermixing between layers. The presence or absence of interpenetration, as determined by the measurements of streaming potential and contact angles, correlated well with electroresponsivity.}}, author = {{Leppin, Christian and Pomorska, Agata and Morga, Maria and Pomastowski, Pawel and Fijałkowski, Piotr and Michna, Aneta and Johannsmann, Diethelm}}, issn = {{1525-7797}}, journal = {{Biomacromolecules}}, number = {{2}}, pages = {{914--928}}, publisher = {{American Chemical Society (ACS)}}, title = {{{Swelling Degree of Polyelectrolyte Layers Determined by an Electrochemical Quartz Crystal Microbalance}}}, doi = {{10.1021/acs.biomac.4c01205}}, volume = {{26}}, year = {{2025}}, } @article{63226, abstract = {{Nanobubbles in water splitting are recognized by the EQCM-D. They are ubiquitous. Lifetimes are in the range of seconds.}}, author = {{Leppin, Christian and Langhoff, Arne and Johannsmann, Diethelm}}, issn = {{1463-9076}}, journal = {{Physical Chemistry Chemical Physics}}, number = {{37}}, pages = {{19733--19747}}, publisher = {{Royal Society of Chemistry (RSC)}}, title = {{{A fast electrochemical quartz crystal microbalance (EQCM) evidences the presence of nanobubbles in alkaline water splitting}}}, doi = {{10.1039/d5cp02691a}}, volume = {{27}}, year = {{2025}}, } @article{63227, abstract = {{Using a precise electrochemical quartz crystal microbalance (EQCM), it was shown that electrogravimetry can be carried out with microelectrode arrays (MEAs). Significant differences between the potential dependent adsorption of a redox-active molecule and electroplating were presented.}}, author = {{Biermann, Michael and Leppin, Christian and Langhoff, Arne and Ziemer, Thorben and Rembe, Christian and Johannsmann, Diethelm}}, issn = {{0003-2654}}, journal = {{The Analyst}}, number = {{7}}, pages = {{2138--2146}}, publisher = {{Royal Society of Chemistry (RSC)}}, title = {{{An electrochemical quartz crystal microbalance (EQCM) based on microelectrode arrays allows to distinguish between adsorption and electrodeposition}}}, doi = {{10.1039/d3an02210b}}, volume = {{149}}, year = {{2024}}, } @article{63231, abstract = {{ A QCM-D probes the temperature- and concentration-dependent complex high-frequency viscosity and provides information on protein-protein interactions in solutions of monoclonal antibodies.}}, author = {{Rott, Emily and Leppin, Christian and Diederichs, Tim and Garidel, Patrick and Johannsmann, Diethelm}}, issn = {{0003-2654}}, journal = {{The Analyst}}, number = {{8}}, pages = {{1887--1897}}, publisher = {{Royal Society of Chemistry (RSC)}}, title = {{{Protein–protein interactions in solutions of monoclonal antibodies probed by the dependence of the high-frequency viscosity on temperature and concentration}}}, doi = {{10.1039/d3an00076a}}, volume = {{148}}, year = {{2023}}, } @article{63228, abstract = {{AbstractA simulation based on the frequency‐domain lattice Boltzmann method (FreqD‐LBM) is employed to predict the shifts of resonance frequency, Δf, and half bandwidth, ΔΓ, of a quartz crystal microbalance with dissipation monitoring (QCM‐D) induced by the adsorption of rigid spheres to the resonator surface. The comparison with the experimental values of Δf and ΔΓ allows to estimate the stiffness of the contacts between the spheres and the resonator surface. The contact stiffness is of interest in contact mechanics, but also in sensing because it depends on the properties of thin films situated between the resonator surface and the sphere. The simulation differs from previous implementations of FreqD‐LBM insofar, as the material inside the particles is not included in the FreqD‐LBM algorithm. Rather, the particle surface is configured to be an oscillating boundary. The amplitude of the particles' motions (displacement and rotation) is governed by the force balance at the surface of the particle. Because the contact stiffness enters this balance, it can be derived from experimental values of Δf and ΔΓ. The simulation reproduces experiments by the Krakow group. For sufficiently small spheres, a contact stiffness can be derived from the comparison of the simulation with the experiment.}}, author = {{Johannsmann, Diethelm and Leppin, Christian and Langhoff, Arne}}, issn = {{2513-0390}}, journal = {{Advanced Theory and Simulations}}, number = {{11}}, publisher = {{Wiley}}, title = {{{Stiffness of Contacts between Adsorbed Particles and the Surface of a QCM‐D Inferred from the Adsorption Kinetics and a Frequency‐Domain Lattice Boltzmann Simulation}}}, doi = {{10.1002/adts.202300190}}, volume = {{6}}, year = {{2023}}, } @article{63230, abstract = {{Quartz crystal microbalance with dissipation monitoring (QCM-D) is a well-established technique for studying soft films. It can provide gravimetric as well as nongravimetric information about a film, such as its thickness and mechanical properties. The interpretation of sets of overtone-normalized frequency shifts, ∆f/n, and overtone-normalized shifts in half-bandwidth, ΔΓ/n, provided by QCM-D relies on a model that, in general, contains five independent parameters that are needed to describe film thickness and frequency-dependent viscoelastic properties. Here, we examine how noise inherent in experimental data affects the determination of these parameters. There are certain conditions where noise prevents the reliable determination of film thickness and the loss tangent. On the other hand, we show that there are conditions where it is possible to determine all five parameters. We relate these conditions to the mathematical properties of the model in terms of simple conceptual diagrams that can help users understand the model’s behavior. Finally, we present new open source software for QCM-D data analysis written in Python, PyQTM.}}, author = {{Johannsmann, Diethelm and Langhoff, Arne and Leppin, Christian and Reviakine, Ilya and Maan, Anna M. C.}}, issn = {{1424-8220}}, journal = {{Sensors}}, number = {{3}}, publisher = {{MDPI AG}}, title = {{{Effect of Noise on Determining Ultrathin-Film Parameters from QCM-D Data with the Viscoelastic Model}}}, doi = {{10.3390/s23031348}}, volume = {{23}}, year = {{2023}}, } @article{63229, author = {{Johannsmann, Diethelm and Petri, Judith and Leppin, Christian and Langhoff, Arne and Ibrahim, Hozan}}, issn = {{2211-3797}}, journal = {{Results in Physics}}, publisher = {{Elsevier BV}}, title = {{{Particle fouling at hot reactor walls monitored In situ with a QCM-D and modeled with the frequency-domain lattice Boltzmann method}}}, doi = {{10.1016/j.rinp.2023.106219}}, volume = {{45}}, year = {{2023}}, } @article{63234, author = {{Wiegmann, Jens and Leppin, Christian and Langhoff, Arne and Schwaderer, Jan and Beuermann, Sabine and Johannsmann, Diethelm and Weber, Alfred P.}}, issn = {{0921-8831}}, journal = {{Advanced Powder Technology}}, number = {{3}}, publisher = {{Elsevier BV}}, title = {{{Influence of the solvent evaporation rate on the β-Phase content of electrosprayed PVDF particles and films studied by a fast Multi-Overtone QCM}}}, doi = {{10.1016/j.apt.2022.103452}}, volume = {{33}}, year = {{2022}}, } @article{63233, author = {{Leppin, Christian and Langhoff, Arne and Johannsmann, Diethelm}}, issn = {{0003-2700}}, journal = {{Analytical Chemistry}}, number = {{28}}, pages = {{10227--10233}}, publisher = {{American Chemical Society (ACS)}}, title = {{{Square-Wave Electrogravimetry Combined with Voltammetry Reveals Reversible Submonolayer Adsorption of Redox-Active Ions}}}, doi = {{10.1021/acs.analchem.2c01763}}, volume = {{94}}, year = {{2022}}, } @article{63236, abstract = {{The response of the quartz crystal microbalance (QCM, also: QCM-D for “QCM with Dissipation monitoring”) to loading with a diverse set of samples is reviewed in a consistent frame. After a brief introduction to the advanced QCMs, the governing equation (the small-load approximation) is derived. Planar films and adsorbates are modeled based on the acoustic multilayer formalism. In liquid environments, viscoelastic spectroscopy and high-frequency rheology are possible, even on layers with a thickness in the monolayer range. For particulate samples, the contact stiffness can be derived. Because the stress at the contact is large, the force is not always proportional to the displacement. Nonlinear effects are observed, leading to a dependence of the resonance frequency and the resonance bandwidth on the amplitude of oscillation. Partial slip, in particular, can be studied in detail. Advanced topics include structured samples and the extension of the small-load approximation to its tensorial version.}}, author = {{Johannsmann, Diethelm and Langhoff, Arne and Leppin, Christian}}, issn = {{1424-8220}}, journal = {{Sensors}}, number = {{10}}, publisher = {{MDPI AG}}, title = {{{Studying Soft Interfaces with Shear Waves: Principles and Applications of the Quartz Crystal Microbalance (QCM)}}}, doi = {{10.3390/s21103490}}, volume = {{21}}, year = {{2021}}, } @article{63235, abstract = {{AbstractA fast electrochemical quartz crystal microbalance with dissipation monitoring (EQCM−D) was applied to copper electrodeposition and subsequent stripping. Accumulation brings the frequency noise down to the mHz range, corresponding to 0.1 % of a monolayer. With this precision, the apparent mass transfer rate as determined from the time‐derivative of the frequency shift can be directly compared to the current. Small but systematic deviations between the two can be attributed to nanoscale roughness. In the voltage range of underpotential deposition (UPD), the apparent mass transfer rate shows peaks and shoulders. The plating additive benzotriazole (BTA) leaves the magnitude of electrogravimetric signals unchanged, but shifts the UPD onset potential. The additive thiourea (TU) promotes UPD and strongly increases the bandwidth.}}, author = {{Leppin, Christian and Langhoff, Arne and Höfft, Oliver and Johannsmann, Diethelm}}, issn = {{1040-0397}}, journal = {{Electroanalysis}}, number = {{12}}, pages = {{2529--2538}}, publisher = {{Wiley}}, title = {{{A Modulation QCM Applied to Copper Electrodeposition and Stripping}}}, doi = {{10.1002/elan.202100471}}, volume = {{33}}, year = {{2021}}, } @article{63237, abstract = {{Using a fast electrochemical quartz crystal microbalance (EQCM), zwitterionic electrolytes were studied with regard to changes of resonance frequency and resonance bandwidth after the electrode potential was switched.}}, author = {{Leppin, Christian and Langhoff, Arne and Poggemann, Hanna-Friederike and Gödde, Alexander Simon and Johannsmann, Diethelm}}, issn = {{0003-2654}}, journal = {{The Analyst}}, number = {{19}}, pages = {{6005--6013}}, publisher = {{Royal Society of Chemistry (RSC)}}, title = {{{Fast and slow EQCM response of zwitterionic weak electrolytes to changes in the electrode potential: a pH-mediated mechanism}}}, doi = {{10.1039/d1an01306h}}, volume = {{146}}, year = {{2021}}, } @article{63238, abstract = {{

A fast EQCM measures the kinetics of the viscosity changes inside the double layer following voltage jumps.

}}, author = {{Leppin, Christian and Peschel, Astrid and Meyer, Frederick Sebastian and Langhoff, Arne and Johannsmann, Diethelm}}, issn = {{0003-2654}}, journal = {{The Analyst}}, number = {{7}}, pages = {{2160--2171}}, publisher = {{Royal Society of Chemistry (RSC)}}, title = {{{Kinetics of viscoelasticity in the electric double layer following steps in the electrode potential studied by a fast electrochemical quartz crystal microbalance (EQCM)}}}, doi = {{10.1039/d0an01965h}}, volume = {{146}}, year = {{2021}}, }