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Philippe H Hunenberger - One of the best experts on this subject based on the ideXlab platform.
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absolute proton hydration free energy surface potential of water and redox potential of the Hydrogen Electrode from first principles qm mm md free energy simulations of sodium and potassium hydration
Journal of Chemical Physics, 2018Co-Authors: Thomas S Hofer, Philippe H HunenbergerAbstract:The absolute intrinsic hydration free energy GH+,wat◦ of the proton, the surface electric potential jump χwat◦ upon entering bulk water, and the absolute redox potential VH+,wat◦ of the reference Hydrogen Electrode are cornerstone quantities for formulating single-ion thermodynamics on absolute scales. They can be easily calculated from each other but remain fundamentally elusive, i.e., they cannot be determined experimentally without invoking some extra-thermodynamic assumption (ETA). The Born model provides a natural framework to formulate such an assumption (Born ETA), as it automatically factors out the contribution of crossing the water surface from the hydration free energy. However, this model describes the short-range solvation inaccurately and relies on the choice of arbitrary ion-size parameters. In the present study, both shortcomings are alleviated by performing first-principle calculations of the hydration free energies of the sodium (Na+) and potassium (K+) ions. The calculations rely on thermodynamic integration based on quantum-mechanical molecular-mechanical (QM/MM) molecular dynamics (MD) simulations involving the ion and 2000 water molecules. The ion and its first hydration shell are described using a correlated ab initio method, namely resolution-of-identity second-order Moller-Plesset perturbation (RIMP2). The next hydration shells are described using the extended simple point charge water model (SPC/E). The hydration free energy is first calculated at the MM level and subsequently increased by a quantization term accounting for the transformation to a QM/MM description. It is also corrected for finite-size, approximate-electrostatics, and potential-summation errors, as well as standard-state definition. These computationally intensive simulations provide accurate first-principle estimates for GH+,wat◦, χwat◦, and VH+,wat◦, reported with statistical errors based on a confidence interval of 99%. The values obtained from the independent Na+ and K+ simulations are in excellent agreement. In particular, the difference between the two hydration free energies, which is not an elusive quantity, is 73.9 ± 5.4 kJ mol-1 (K+ minus Na+), to be compared with the experimental value of 71.7 ± 2.8 kJ mol-1. The calculated values of GH+,wat◦, χwat◦, and VH+,wat◦ (-1096.7 ± 6.1 kJ mol-1, 0.10 ± 0.10 V, and 4.32 ± 0.06 V, respectively, averaging over the two ions) are also in remarkable agreement with the values recommended by Reif and Hunenberger based on a thorough analysis of the experimental literature (-1100 ± 5 kJ mol-1, 0.13 ± 0.10 V, and 4.28 ± 0.13 V, respectively). The QM/MM MD simulations are also shown to provide an accurate description of the hydration structure, dynamics, and energetics.
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absolute proton hydration free energy surface potential of water and redox potential of the Hydrogen Electrode from first principles qm mm md free energy simulations of sodium and potassium hydration
Journal of Chemical Physics, 2018Co-Authors: Thomas S Hofer, Philippe H HunenbergerAbstract:The absolute intrinsic hydration free energy GH+,wat◦ of the proton, the surface electric potential jump χwat◦ upon entering bulk water, and the absolute redox potential VH+,wat◦ of the reference Hydrogen Electrode are cornerstone quantities for formulating single-ion thermodynamics on absolute scales. They can be easily calculated from each other but remain fundamentally elusive, i.e., they cannot be determined experimentally without invoking some extra-thermodynamic assumption (ETA). The Born model provides a natural framework to formulate such an assumption (Born ETA), as it automatically factors out the contribution of crossing the water surface from the hydration free energy. However, this model describes the short-range solvation inaccurately and relies on the choice of arbitrary ion-size parameters. In the present study, both shortcomings are alleviated by performing first-principle calculations of the hydration free energies of the sodium (Na+) and potassium (K+) ions. The calculations rely on the...
Kazuaki Yasuda - One of the best experts on this subject based on the ideXlab platform.
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characteristics of a platinum black catalyst layer with regard to platinum dissolution phenomena in a membrane Electrode assembly
Journal of The Electrochemical Society, 2006Co-Authors: Kazuaki Yasuda, Tsutomu Ioroi, Akira Taniguchi, Tomoki Akita, Zyun SiromaAbstract:The nature of platinum dissolution and precipitation in a polymer electrolyte membrane of a membrane Electrode assembly (MEA) for a proton-exchange membrane fuel cell (PEMFC) was studied using a potential holding experiment at 1.0 V vs a reversible Hydrogen Electrode and high-resolution transmission electron microscopy. The electrochemically active surface area decreased depending on the holding time, and platinum deposition was observed in the polymer electrolyte membrane near a cathode catalyst layer. However, platinum dissolution and deposition out of the catalyst layer were greatly reduced when a platinum black Electrode was used. In the experiment using a double-layered catalyst layer, platinum redeposited not on the carbon black surface but rather on the platinum black surface.
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compact dynamic Hydrogen Electrode unit as a reference Electrode for pemfcs
Journal of Power Sources, 2006Co-Authors: Zyun Siroma, Ryou Kakitsubo, Naoko Fujiwara, Shinichi Yamazaki, Tsutomu Ioroi, Kazuaki YasudaAbstract:Abstract A compact and easy-handling design for the dynamic Hydrogen Electrode (DHE) was proposed. Before mounting in the cell, a unitized set was made by the hot-pressing of two 2 mm × 5 mm ionomer membranes, between which two PTFE-coated platinum wires were sandwiched. Before the hot-pressing, a small amount of paste composed of platinum black powder and an ionomer solution was dropped at the end of each wires. Prior to use in a PEMFC, a proper current value for the operation was found in an aqueous acid solution. Mounting of the DHE on a membrane Electrode assembly (MEA) was done by local hot-pressing. The space for the mounting was only 2 mm in width between the gasket and edge of the gas diffusion Electrode. The potential of the DHE fluctuated, but it was in the range of about 5 mV. A PEMFC with both the DHE and conventional RHE was operated, and the I – V performance of the PEMFC was measured. The potential difference between the conventional RHE and DHE was about 5 mV, which is due to the overpotential of the Hydrogen evolution reaction at the DHE. Considering this difference, this DHE is available to monitor single Electrode potentials.
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compact dynamic Hydrogen Electrode unit as a reference Electrode for pemfcs
Journal of Power Sources, 2006Co-Authors: Zyun Siroma, Ryou Kakitsubo, Naoko Fujiwara, Shinichi Yamazaki, Tsutomu Ioroi, Kazuaki YasudaAbstract:Abstract A compact and easy-handling design for the dynamic Hydrogen Electrode (DHE) was proposed. Before mounting in the cell, a unitized set was made by the hot-pressing of two 2 mm × 5 mm ionomer membranes, between which two PTFE-coated platinum wires were sandwiched. Before the hot-pressing, a small amount of paste composed of platinum black powder and an ionomer solution was dropped at the end of each wires. Prior to use in a PEMFC, a proper current value for the operation was found in an aqueous acid solution. Mounting of the DHE on a membrane Electrode assembly (MEA) was done by local hot-pressing. The space for the mounting was only 2 mm in width between the gasket and edge of the gas diffusion Electrode. The potential of the DHE fluctuated, but it was in the range of about 5 mV. A PEMFC with both the DHE and conventional RHE was operated, and the I – V performance of the PEMFC was measured. The potential difference between the conventional RHE and DHE was about 5 mV, which is due to the overpotential of the Hydrogen evolution reaction at the DHE. Considering this difference, this DHE is available to monitor single Electrode potentials.
Thomas S Hofer - One of the best experts on this subject based on the ideXlab platform.
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absolute proton hydration free energy surface potential of water and redox potential of the Hydrogen Electrode from first principles qm mm md free energy simulations of sodium and potassium hydration
Journal of Chemical Physics, 2018Co-Authors: Thomas S Hofer, Philippe H HunenbergerAbstract:The absolute intrinsic hydration free energy GH+,wat◦ of the proton, the surface electric potential jump χwat◦ upon entering bulk water, and the absolute redox potential VH+,wat◦ of the reference Hydrogen Electrode are cornerstone quantities for formulating single-ion thermodynamics on absolute scales. They can be easily calculated from each other but remain fundamentally elusive, i.e., they cannot be determined experimentally without invoking some extra-thermodynamic assumption (ETA). The Born model provides a natural framework to formulate such an assumption (Born ETA), as it automatically factors out the contribution of crossing the water surface from the hydration free energy. However, this model describes the short-range solvation inaccurately and relies on the choice of arbitrary ion-size parameters. In the present study, both shortcomings are alleviated by performing first-principle calculations of the hydration free energies of the sodium (Na+) and potassium (K+) ions. The calculations rely on thermodynamic integration based on quantum-mechanical molecular-mechanical (QM/MM) molecular dynamics (MD) simulations involving the ion and 2000 water molecules. The ion and its first hydration shell are described using a correlated ab initio method, namely resolution-of-identity second-order Moller-Plesset perturbation (RIMP2). The next hydration shells are described using the extended simple point charge water model (SPC/E). The hydration free energy is first calculated at the MM level and subsequently increased by a quantization term accounting for the transformation to a QM/MM description. It is also corrected for finite-size, approximate-electrostatics, and potential-summation errors, as well as standard-state definition. These computationally intensive simulations provide accurate first-principle estimates for GH+,wat◦, χwat◦, and VH+,wat◦, reported with statistical errors based on a confidence interval of 99%. The values obtained from the independent Na+ and K+ simulations are in excellent agreement. In particular, the difference between the two hydration free energies, which is not an elusive quantity, is 73.9 ± 5.4 kJ mol-1 (K+ minus Na+), to be compared with the experimental value of 71.7 ± 2.8 kJ mol-1. The calculated values of GH+,wat◦, χwat◦, and VH+,wat◦ (-1096.7 ± 6.1 kJ mol-1, 0.10 ± 0.10 V, and 4.32 ± 0.06 V, respectively, averaging over the two ions) are also in remarkable agreement with the values recommended by Reif and Hunenberger based on a thorough analysis of the experimental literature (-1100 ± 5 kJ mol-1, 0.13 ± 0.10 V, and 4.28 ± 0.13 V, respectively). The QM/MM MD simulations are also shown to provide an accurate description of the hydration structure, dynamics, and energetics.
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absolute proton hydration free energy surface potential of water and redox potential of the Hydrogen Electrode from first principles qm mm md free energy simulations of sodium and potassium hydration
Journal of Chemical Physics, 2018Co-Authors: Thomas S Hofer, Philippe H HunenbergerAbstract:The absolute intrinsic hydration free energy GH+,wat◦ of the proton, the surface electric potential jump χwat◦ upon entering bulk water, and the absolute redox potential VH+,wat◦ of the reference Hydrogen Electrode are cornerstone quantities for formulating single-ion thermodynamics on absolute scales. They can be easily calculated from each other but remain fundamentally elusive, i.e., they cannot be determined experimentally without invoking some extra-thermodynamic assumption (ETA). The Born model provides a natural framework to formulate such an assumption (Born ETA), as it automatically factors out the contribution of crossing the water surface from the hydration free energy. However, this model describes the short-range solvation inaccurately and relies on the choice of arbitrary ion-size parameters. In the present study, both shortcomings are alleviated by performing first-principle calculations of the hydration free energies of the sodium (Na+) and potassium (K+) ions. The calculations rely on the...
Evan R Williams - One of the best experts on this subject based on the ideXlab platform.
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directly relating gas phase cluster measurements to solution phase hydrolysis the absolute standard Hydrogen Electrode potential and the absolute proton solvation energy
Chemistry: A European Journal, 2009Co-Authors: William A Donald, Ryan D Leib, Jeremy T Obrien, Evan R WilliamsAbstract:Hydrated ion nanocalorimetry is used to measure reduction energies and H atom affinities of gaseous hydrated ions by determining the energy deposited into these nanodrops from the number of water molecules lost upon reduction by thermally generated electrons (see figure). Solution-phase, half-cell potentials are measured relative to other half-cell potentials, resulting in a thermochemical ladder that is anchored to the standard Hydrogen Electrode (SHE), which is assigned an arbitrary value of 0 V. A new method for measuring the absolute SHE potential is demonstrated in which gaseous nanodrops containing divalent alkaline-earth or transition-metal ions are reduced by thermally generated electrons. Energies for the reactions 1) M(H2O)242+(g)+e−(g)→M(H2O)24+(g) and 2) M(H2O)242+(g)+e−(g)→MOH(H2O)23+(g)+H(g) and the Hydrogen atom affinities of MOH(H2O)23+(g) are obtained from the number of water molecules lost through each pathway. From these measurements on clusters containing nine different metal ions and known thermochemical values that include solution hydrolysis energies, an average absolute SHE potential of +4.29 V vs. e−(g) (standard deviation of 0.02 V) and a real proton solvation free energy of −265 kcal mol−1 are obtained. With this method, the absolute SHE potential can be obtained from a one-electron reduction of nanodrops containing divalent ions that are not observed to undergo one-electron reduction in aqueous solution.
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cover picture directly relating gas phase cluster measurements to solution phase hydrolysis the absolute standard Hydrogen Electrode potential and the absolute proton solvation energy chem eur j 24 2009
Chemistry: A European Journal, 2009Co-Authors: William A Donald, Ryan D Leib, Jeremy T Obrien, Evan R WilliamsAbstract:A new method for measuring the absolute standard Hydrogen Electrode (SHE) potential is demonstrated by E. R. Williams et al. on page 5926 ff. Gaseous clusters containing one of nine different divalent metal ions and 24 water molecules (red and gray spheres) are reduced by thermally generated electrons. From reaction energies measured for nine different metal-ion-containing clusters, and other thermochemical data, an average absolute value for the SHE potential of +4.29±0.02 V is obtained. The cover image was created by Mariam ElNaggar.
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absolute standard Hydrogen Electrode potential measured by reduction of aqueous nanodrops in the gas phase
Journal of the American Chemical Society, 2008Co-Authors: William A Donald, Ryan D Leib, Jeremy T Obrien, Matthew F Bush, Evan R WilliamsAbstract:In solution, half-cell potentials are measured relative to those of other half cells, thereby establishing a ladder of thermochemical values that are referenced to the standard Hydrogen Electrode (SHE), which is arbitrarily assigned a value of exactly 0 V. Although there has been considerable interest in, and efforts toward, establishing an absolute electrochemical half-cell potential in solution, there is no general consensus regarding the best approach to obtain this value. Here, ion-electron recombination energies resulting from electron capture by gas-phase nanodrops containing individual [M(NH3)6]3+, M = Ru, Co, Os, Cr, and Ir, and Cu2+ ions are obtained from the number of water molecules that are lost from the reduced precursors. These experimental data combined with nanodrop solvation energies estimated from Born theory and solution-phase entropies estimated from limited experimental data provide absolute reduction energies for these redox couples in bulk aqueous solution. A key advantage of this a...
Mogens Bjerg Mogensen - One of the best experts on this subject based on the ideXlab platform.
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solid oxide electrolysis cells degradation at high current densities
Journal of The Electrochemical Society, 2010Co-Authors: Ruth Knibbe, Sune Dalgaard Ebbesen, Anne Hauch, Marie Lund Traulsen, Mogens Bjerg MogensenAbstract:The degradation of Ni/yttria-stabilized zirconia (YSZ)-based solid oxide electrolysis cells operated at high current densities was studied. The degradation was examined at 850 degrees C, at current densities of -1.0, -1.5, and -2.0 A/cm(2), with a 50:50 (H(2)O:H(2)) gas supplied to the Ni/YSZ Hydrogen Electrode and oxygen supplied to the lanthanum, strontium manganite (LSM)/YSZ oxygen Electrode. Electrode polarization resistance degradation is not directly related to the applied current density but rather a consequence of adsorbed impurities in the Ni/YSZ Hydrogen Electrode. However, the ohmic resistance degradation increases with applied current density. The ohmic resistance degradation is attributed to oxygen formation in the YSZ electrolyte grain boundaries near the oxygen Electrode/electrolyte interface. (C) 2010 The Electrochemical Society. [DOI:10.1149/1.3447752] All rights reserved.
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solid oxide electrolysis cells microstructure and degradation of the ni yttria stabilized zirconia Electrode
Journal of The Electrochemical Society, 2008Co-Authors: Anne Hauch, Sune Dalgaard Ebbesen, Soren Hojgaard Jensen, Mogens Bjerg MogensenAbstract:Solid oxide fuel cells produced at Riso DTU have been tested as solid oxide electrolysis cells for steam electrolysis by applying an external voltage. Varying the sealing on the Hydrogen Electrode side of the setup verifies that the previously reported passivation over the first few hundred hours of electrolysis testing was an effect of the applied glass sealing. Degradation of the cells during long-term galvanostatic electrolysis testing [850°C, -1/2 A/cm 2 , p(H 2 O)/p(H 2 ) = 0.5/0.5] was analyzed by impedance spectroscopy and the degradation was found mainly to be caused by increasing polarization resistance associated with the Hydrogen Electrode. A cell voltage degradation of 2%/1000 h was obtained. Postmortem analysis of cells tested at these conditions showed that the Electrode microstructure could withstand at least 1300 h of electrolysis testing, however, impurities were found in the Hydrogen Electrode of tested solid oxide electrolysis cells. Electrolysis testing at high current density, high temperature, and a high partial pressure of steam [-2 A/cm 2 , 950°C, p(H 2 O) = 0.9 atm] was observed to lead to significant microstructural changes at the Hydrogen Electrode-electrolyte interface.
