ABSTRACT An in situ scanning tunneling microscope has been used to study the dynamics of electrod... more ABSTRACT An in situ scanning tunneling microscope has been used to study the dynamics of electrode processes. The electrochemical oxidation of graphite and the dissolution of copper clusters from a polycrystalline gold surface are described as examples for such processes. It is also demonstrated that single platinum clusters exhibit a high mobility on graphite surfaces. The mobility is reduced when the platinum forms a network on the graphite surface. In addition, the use of the scanning tunneling microscope as a tool for nanomodifications on graphite surfaces is demonstrated. Holes and copper clusters of nanometer size were generated under in situ conditions using the tunneling tip.
Journal of The Electrochemical Society, Nov 1, 2021
Lithium-ion batteries are prevalent in everyday usage and ensuring they are used efficiently is o... more Lithium-ion batteries are prevalent in everyday usage and ensuring they are used efficiently is of paramount concern. The degradation of lithium-ion batteries under cycling can affect various components in the cells. Understanding the degradation characteristics of the cell can aid in ensuring longer lifetime through optimized usage. Long-term automated EIS data collection has been used as a non-destructive tool to track the impedance changes within the cells over time due to repeated cycling. Separating the different degradation factors contributed by the graphite and NMC electrodes in a non-invasive way is still a challenge. In this work, EIS measurements of half-cells dissembled from full cells are shown to provide further insight into cell degradation characteristics. By using this methodology to differentiate the degradation at different electrodes, new materials can be easily screened for their viability in next generation batteries. The methodology also helps estimate the degradation changes with cycling and provides useful information for further understanding the degradation mechanism.
Berichte der Bunsengesellschaft für physikalische Chemie, Sep 1, 1991
Hydrates of the alkaline hydroxides LiOH, NaOH, KOH and CsOH in their liquid and solid states wer... more Hydrates of the alkaline hydroxides LiOH, NaOH, KOH and CsOH in their liquid and solid states were investigated using impedance spectroscopy. KOH · 4H2O was studied using deuteron NMR. From the impedance measurements the electrical conductivity of the hydrate was evaluated. It is an important result of these measurements that the lithium and sodium hydroxide hydrates exhibit a poor conductivity in the solid state; the activation energies of conductivity exceed 3 eV. The potassium and cesium hydroxide hydrates, however, are considerably more conductive in the solid state, especially at low temperatures with activation energies around 0.7 eV. Linewidths and T1 and T2 relaxation times were determined as a function of temperature in the liquid and solid hydrate. The NMR measurements indicate that the protons (deuterons) are the mobile species. The differences in conductivity may be due to a size effect of the cation in that large cations with a single charge favor an extended water network which maybe of clathrate‐like structure.
Berichte der Bunsengesellschaft für physikalische Chemie, Apr 1, 1982
The capacity and photocurrent behaviour of passive titanium electrodes has been investigated in d... more The capacity and photocurrent behaviour of passive titanium electrodes has been investigated in dependence on the film thickness and the electrode potential, the photocurrents additionally on the wavelength. From capacity measurements a high donor concentration of 1020 cm−3 and a dielectric constant of D = 9 can be concluded. Compared to bulk TiO2 electrodes, the onset of the photocurrent is shifted to higher photon energies. The band gap is estimated to be Eg = 3.4 eV. At constant electrode potential ε the photocurrent decreases with increasing film thickness though the absorption of the film should be higher. This indicates the strong influence of the field strength in the film on the photocurrent which increases with thickness at constant field.
Catalytic effects of surface groups on porous carbon electrodes are claimed in literature for the... more Catalytic effects of surface groups on porous carbon electrodes are claimed in literature for the redox reactions V(II)/V(III) and V(IV)/V(V). The literature is critically analysed also in relation to work of this group. A method on how to overcome the problem of assessing the electrochemically active surface area on porous electrodes is presented [1]. Applying this method, no catalytic effects for above reactions can be substantiated. It is further pointed out that the parameters electrochemical potential and temperature need to be used to assess electrocatalysis. The main control parameter determining the rate of an electrochemical reaction is the Gibbs free enthalpy of activation, ΔG≠ , in contrast to ΔH≠ for e.g. a heterogeneous surface reaction. This would suggest to clearly define electrocatalysis as the “lowering the activation barrier, ΔG≠ , in an electrochemical reaction” where, in fact, the lowering of ΔG≠ can result from a change in ΔH≠ and/or ΔS≠ (∂ ΔG≠ /∂η=∂ ΔH≠ /∂η−T∂ΔS≠/∂η , as in Figure1). Such a treatment of data of temperature dependent electrochemical reactions has been described in literatures [2,3]. An analysis according to the above-discussed procedures would pin down which quantity is responsible for the lowering of ΔG ≠ that may allow for a better interpretation of the electrocatalytic effect. Using the example of the redox reactions V(II)/V(III) and V(IV)/V(V) the literature was analysed regarding possible catalytic effects of functional groups on carbon electrodes, as in Figure2. The conclusion was that no electrocatalysis of either of these reactions can be confirmed. Observed enhancements of currents can rather be attributed to a change in the effective surface area of the, usually, porous electrode. A method is suggested how such effects can be distinguished from electrocatalysis by using electrochemical impedance spectroscopy (EIS). Figure2 gives an example how EIS data are used for evaluation. In more general terms, the criteria for electrocatalysis were described. Experimentally, an investigation should contain both, measurements of the potential dependence and the temperature dependence since the main parameter in electrocatalysis is the lowering of ΔG≠ . Only then, the respective influence of ΔH≠ and ΔS≠ can be assessed. Acknowledgments This work was supported in part by Siemens AG through a research grant and by EPSRC through funding for the North East Centre of Energy Materials (NECEM) (EP/R021503/10). References: [1] U. Stimming, J. Wang, and A. Bund, ChemPhysChem, 2019, 20, 1-7. [2] B. E. Conway, In Modern Electrochemistry, Vol. 16. New York: Plenum Press, 1987. [3] Z. Borkowska, M. Cappadonia, and U. Stimming, Electrochim. Acta, 1992, 37(3), 565-568. [4] J. Friedl, C. M. Bauer, A. Rinaldi, and U. Stimming, Carbon, 2013, 63, 228-239. Figure 1
is carried out at a Pt electrode at temp. in the range 130‐300 K. The exchange current density is... more is carried out at a Pt electrode at temp. in the range 130‐300 K. The exchange current density is found to follow an Arrhenius law in the liquid and in the solid state.
ABSTRACT In this work, two types of electroactive proteins, namely azurin and ferrocene-labeled p... more ABSTRACT In this work, two types of electroactive proteins, namely azurin and ferrocene-labeled papain, were adsorbed on differently oxidized diamond and investigated by cyclic voltammetry. A comparison was made with oxidized highly oriented pyrolytic graphite (HOPG). A direct electron transfer to the biomolecules was confirmed for all oxidized carbon electrodes. A strong influence of the oxygen termination process of diamond on the charge transfer through the interface has been observed. This effect has been attributed to different defects and electronic states at the interface, as confirmed by capacitance-voltage measurements in electrolyte, electrical characterisation and X-ray photoemission spectroscopy (XPS). Wet chemical oxidized diamond was proved to be the most effective electrode material for biomolecule anchoring with an electron transfer rate higher by factor of three than that of HOPG.
In this work, the authors explore the sodium salt of the 14‐vanado(V)phosphate, Na7[H2PV14O42], a... more In this work, the authors explore the sodium salt of the 14‐vanado(V)phosphate, Na7[H2PV14O42], as a potential anode material for sodium‐ion batteries (NIBs). The multi‐electron redox activity of the polyoxovanadate [H2PV14O42]7‐leads to high capacity. This polyanion is synthesized by a simple aqueous solution procedure and isolate as a sodium salt with different numbers of crystal waters, Na7[H2PV14O42]·nH2O (n = 15–24). Na7[H2PV14O42] as anode in NIBs exhibits a high and reversible capacity of 322 mA h g−1 at 25 mA g−1 with a high cycling stability (with capacity retention of 87% after 120 cycles). Some of the V5+ ions in [H2PV14O42]7‐ can be reduced to V3+ after being discharged to 0.01 V versus Na/Na+, resulting in an average oxidation state of V3.7+, as based on ex situ X‐ray photoelectron spectroscopy and in situ synchrotron X‐ray absorption near edge structure studies. The crystalline material becomes amorphous during the charge/discharge processes, which can be observed by in situ synchrotron X‐ray diffraction, indicating that functionality does not require crystallinity. The authors propose that the charge storage mechanism of Na7[H2PV14O42] anodes mainly involves redox reactions of V accompanied by insertion/extraction of Na ions in‐between polyoxo‐14‐vanadate ions and adsorption/desorption of Na ions on the surface of the vanadate material.
ABSTRACT BaZr0.9Y0.1O3-δ has been synthesised by three different methods: the solid-state reactio... more ABSTRACT BaZr0.9Y0.1O3-δ has been synthesised by three different methods: the solid-state reaction, the spray pyrolysis and the spray drying. Significantly different apparent lattice parameters (between 0.4192 nm for the sample prepared by the solid-state reaction method and sintered at 1,500 °C and 0.4206 nm for the sample prepared by the solid-state reaction method and sintered at 1,720 °C) are observed after calcination and sintering, depending on the synthesis method and the sintering temperature. The bulk conductivity values also vary over several orders of magnitude (–7.2< log σb
ABSTRACT An in situ scanning tunneling microscope has been used to study the dynamics of electrod... more ABSTRACT An in situ scanning tunneling microscope has been used to study the dynamics of electrode processes. The electrochemical oxidation of graphite and the dissolution of copper clusters from a polycrystalline gold surface are described as examples for such processes. It is also demonstrated that single platinum clusters exhibit a high mobility on graphite surfaces. The mobility is reduced when the platinum forms a network on the graphite surface. In addition, the use of the scanning tunneling microscope as a tool for nanomodifications on graphite surfaces is demonstrated. Holes and copper clusters of nanometer size were generated under in situ conditions using the tunneling tip.
Journal of The Electrochemical Society, Nov 1, 2021
Lithium-ion batteries are prevalent in everyday usage and ensuring they are used efficiently is o... more Lithium-ion batteries are prevalent in everyday usage and ensuring they are used efficiently is of paramount concern. The degradation of lithium-ion batteries under cycling can affect various components in the cells. Understanding the degradation characteristics of the cell can aid in ensuring longer lifetime through optimized usage. Long-term automated EIS data collection has been used as a non-destructive tool to track the impedance changes within the cells over time due to repeated cycling. Separating the different degradation factors contributed by the graphite and NMC electrodes in a non-invasive way is still a challenge. In this work, EIS measurements of half-cells dissembled from full cells are shown to provide further insight into cell degradation characteristics. By using this methodology to differentiate the degradation at different electrodes, new materials can be easily screened for their viability in next generation batteries. The methodology also helps estimate the degradation changes with cycling and provides useful information for further understanding the degradation mechanism.
Berichte der Bunsengesellschaft für physikalische Chemie, Sep 1, 1991
Hydrates of the alkaline hydroxides LiOH, NaOH, KOH and CsOH in their liquid and solid states wer... more Hydrates of the alkaline hydroxides LiOH, NaOH, KOH and CsOH in their liquid and solid states were investigated using impedance spectroscopy. KOH · 4H2O was studied using deuteron NMR. From the impedance measurements the electrical conductivity of the hydrate was evaluated. It is an important result of these measurements that the lithium and sodium hydroxide hydrates exhibit a poor conductivity in the solid state; the activation energies of conductivity exceed 3 eV. The potassium and cesium hydroxide hydrates, however, are considerably more conductive in the solid state, especially at low temperatures with activation energies around 0.7 eV. Linewidths and T1 and T2 relaxation times were determined as a function of temperature in the liquid and solid hydrate. The NMR measurements indicate that the protons (deuterons) are the mobile species. The differences in conductivity may be due to a size effect of the cation in that large cations with a single charge favor an extended water network which maybe of clathrate‐like structure.
Berichte der Bunsengesellschaft für physikalische Chemie, Apr 1, 1982
The capacity and photocurrent behaviour of passive titanium electrodes has been investigated in d... more The capacity and photocurrent behaviour of passive titanium electrodes has been investigated in dependence on the film thickness and the electrode potential, the photocurrents additionally on the wavelength. From capacity measurements a high donor concentration of 1020 cm−3 and a dielectric constant of D = 9 can be concluded. Compared to bulk TiO2 electrodes, the onset of the photocurrent is shifted to higher photon energies. The band gap is estimated to be Eg = 3.4 eV. At constant electrode potential ε the photocurrent decreases with increasing film thickness though the absorption of the film should be higher. This indicates the strong influence of the field strength in the film on the photocurrent which increases with thickness at constant field.
Catalytic effects of surface groups on porous carbon electrodes are claimed in literature for the... more Catalytic effects of surface groups on porous carbon electrodes are claimed in literature for the redox reactions V(II)/V(III) and V(IV)/V(V). The literature is critically analysed also in relation to work of this group. A method on how to overcome the problem of assessing the electrochemically active surface area on porous electrodes is presented [1]. Applying this method, no catalytic effects for above reactions can be substantiated. It is further pointed out that the parameters electrochemical potential and temperature need to be used to assess electrocatalysis. The main control parameter determining the rate of an electrochemical reaction is the Gibbs free enthalpy of activation, ΔG≠ , in contrast to ΔH≠ for e.g. a heterogeneous surface reaction. This would suggest to clearly define electrocatalysis as the “lowering the activation barrier, ΔG≠ , in an electrochemical reaction” where, in fact, the lowering of ΔG≠ can result from a change in ΔH≠ and/or ΔS≠ (∂ ΔG≠ /∂η=∂ ΔH≠ /∂η−T∂ΔS≠/∂η , as in Figure1). Such a treatment of data of temperature dependent electrochemical reactions has been described in literatures [2,3]. An analysis according to the above-discussed procedures would pin down which quantity is responsible for the lowering of ΔG ≠ that may allow for a better interpretation of the electrocatalytic effect. Using the example of the redox reactions V(II)/V(III) and V(IV)/V(V) the literature was analysed regarding possible catalytic effects of functional groups on carbon electrodes, as in Figure2. The conclusion was that no electrocatalysis of either of these reactions can be confirmed. Observed enhancements of currents can rather be attributed to a change in the effective surface area of the, usually, porous electrode. A method is suggested how such effects can be distinguished from electrocatalysis by using electrochemical impedance spectroscopy (EIS). Figure2 gives an example how EIS data are used for evaluation. In more general terms, the criteria for electrocatalysis were described. Experimentally, an investigation should contain both, measurements of the potential dependence and the temperature dependence since the main parameter in electrocatalysis is the lowering of ΔG≠ . Only then, the respective influence of ΔH≠ and ΔS≠ can be assessed. Acknowledgments This work was supported in part by Siemens AG through a research grant and by EPSRC through funding for the North East Centre of Energy Materials (NECEM) (EP/R021503/10). References: [1] U. Stimming, J. Wang, and A. Bund, ChemPhysChem, 2019, 20, 1-7. [2] B. E. Conway, In Modern Electrochemistry, Vol. 16. New York: Plenum Press, 1987. [3] Z. Borkowska, M. Cappadonia, and U. Stimming, Electrochim. Acta, 1992, 37(3), 565-568. [4] J. Friedl, C. M. Bauer, A. Rinaldi, and U. Stimming, Carbon, 2013, 63, 228-239. Figure 1
is carried out at a Pt electrode at temp. in the range 130‐300 K. The exchange current density is... more is carried out at a Pt electrode at temp. in the range 130‐300 K. The exchange current density is found to follow an Arrhenius law in the liquid and in the solid state.
ABSTRACT In this work, two types of electroactive proteins, namely azurin and ferrocene-labeled p... more ABSTRACT In this work, two types of electroactive proteins, namely azurin and ferrocene-labeled papain, were adsorbed on differently oxidized diamond and investigated by cyclic voltammetry. A comparison was made with oxidized highly oriented pyrolytic graphite (HOPG). A direct electron transfer to the biomolecules was confirmed for all oxidized carbon electrodes. A strong influence of the oxygen termination process of diamond on the charge transfer through the interface has been observed. This effect has been attributed to different defects and electronic states at the interface, as confirmed by capacitance-voltage measurements in electrolyte, electrical characterisation and X-ray photoemission spectroscopy (XPS). Wet chemical oxidized diamond was proved to be the most effective electrode material for biomolecule anchoring with an electron transfer rate higher by factor of three than that of HOPG.
In this work, the authors explore the sodium salt of the 14‐vanado(V)phosphate, Na7[H2PV14O42], a... more In this work, the authors explore the sodium salt of the 14‐vanado(V)phosphate, Na7[H2PV14O42], as a potential anode material for sodium‐ion batteries (NIBs). The multi‐electron redox activity of the polyoxovanadate [H2PV14O42]7‐leads to high capacity. This polyanion is synthesized by a simple aqueous solution procedure and isolate as a sodium salt with different numbers of crystal waters, Na7[H2PV14O42]·nH2O (n = 15–24). Na7[H2PV14O42] as anode in NIBs exhibits a high and reversible capacity of 322 mA h g−1 at 25 mA g−1 with a high cycling stability (with capacity retention of 87% after 120 cycles). Some of the V5+ ions in [H2PV14O42]7‐ can be reduced to V3+ after being discharged to 0.01 V versus Na/Na+, resulting in an average oxidation state of V3.7+, as based on ex situ X‐ray photoelectron spectroscopy and in situ synchrotron X‐ray absorption near edge structure studies. The crystalline material becomes amorphous during the charge/discharge processes, which can be observed by in situ synchrotron X‐ray diffraction, indicating that functionality does not require crystallinity. The authors propose that the charge storage mechanism of Na7[H2PV14O42] anodes mainly involves redox reactions of V accompanied by insertion/extraction of Na ions in‐between polyoxo‐14‐vanadate ions and adsorption/desorption of Na ions on the surface of the vanadate material.
ABSTRACT BaZr0.9Y0.1O3-δ has been synthesised by three different methods: the solid-state reactio... more ABSTRACT BaZr0.9Y0.1O3-δ has been synthesised by three different methods: the solid-state reaction, the spray pyrolysis and the spray drying. Significantly different apparent lattice parameters (between 0.4192 nm for the sample prepared by the solid-state reaction method and sintered at 1,500 °C and 0.4206 nm for the sample prepared by the solid-state reaction method and sintered at 1,720 °C) are observed after calcination and sintering, depending on the synthesis method and the sintering temperature. The bulk conductivity values also vary over several orders of magnitude (–7.2< log σb
Uploads
Papers by Ulrich Stimming