The aqueous proton displays an anomalously large diffusion coefficient that is up to 7 times that of similarly sized cations. There is general consensus that the proton achieves its high diffusion ...through the Grotthuss mechanism, whereby protons hop from one molecule to the next. A main assumption concerning the extraction of the timescale of the Grotthuss mechanism from experimental results has been that, on average, there is an equal probability for the proton to hop to any of its neighboring water molecules. Herein, we present
simulations that show this assumption is not generally valid. Specifically, we observe that there is an increased probability for the proton to revert back to its previous location. These correlations indicate that the interpretation of the experimental results need to be re-examined and suggest that the timescale of the Grotthuss mechanism is significantly shorter than was previously thought.
ABSTRACT
Proton transport is essential in many areas of chemistry and biology and is especially important in the fields of proton exchange membrane fuel cells and biocompatible, protonic ...semiconductors. These devices make use of membranes to control the flow of protons for either the generation of energy or to more closely couple electronics and biology. In the present study, we make use of ab initio molecular dynamics simulations, including the effect of applied electric fields, to gain atomistic insight into the intrinsic conductivity of chitosan‐based polymers and demonstrate that chitosan does not act as a significant source of friction for the transport of protons while increasing the number of free ions. Published 2017.† J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1103–1109
Ab initio molecular dynamics simulations that include the effects of applied electric fields demonstrate that chitosan membranes can support high proton mobilities. The simulations also provide details on how acidic functional groups on the polymer enhance proton conduction while basic functional groups enhance hydroxide conduction.
We review the basics and the evolution of a powerful and widely applicable general approach to the systematic reduction of computational burden in many-electron calculations. Variational fitting of ...electron densities (either total or partial) has the great advantage, for quantum mechanical calculations, that it respects the stationarity property, which is at the heart of the success of the basis set expansion methods ubiquitous in computational chemistry and materials physics. The key point is easy. In a finite system, independent of whether the fitted charge distribution is constrained to contain the proper amount of charge, variational fitting guarantees that the quantum mechanical total energy retains the stationarity property. Thus, many-electron quantum mechanics with variational fitting of an electronic density in an incomplete density-fitting basis set behaves similarly as the exact quantum mechanical energy does when evaluated with an incomplete basis set to fit wavefunctions or spin-orbitals. Periodically bounded systems are a bit more subtle but the essential stationarity is preserved. This preservation of an exact property is quite distinct from truncation of the resolution of the identity in a basis. Variational fitting has proven to have benefits far beyond the original objective of making a Gaussian-orbital basis calculation of an early density functional computationally feasible. We survey many of those developments briefly, with guidance to the pertinent literature and a few remarks about the connections with Quantum Theory Project.
The stability of the fast ion conducting cubic phase of zirconia has been investigated using atomistic molecular dynamics to determine the effect of clustering of dopant ions for zirconia containing ...11mol% of yttria or scandia stabilizing additive. Large clusters of dopant ions entrap vacancies within the dopant cluster, which increases the average oxygen coordination of the zirconium ions. Not only does this reduce the number of vacancies available for ionic conduction, but it also destabilizes the cubic phase, causing a significant reduction in ionic conductivity as a whole. Using scandia as a stabilizing additive provides better conductivity than yttria, despite the fact that yttria causes a lower coordination for zirconium ions. Cationic migration in the cubic phase occurs too slowly to be captured in molecular dynamics simulations; instead, energy calculations are made, which predict that yttrium dopants exhibit stronger mutually repulsive interactions than scandium ions, which impede the energetically favorable phase segregation process. As a result, aging of yttria stabilized zirconia will proceed at a slower rate than scandia stabilized zirconia.
•Studied effect of dopant stratification on scandia- and yttria-stabilized zirconia.•Energy barriers are present in the transition path to stratification.•Nanometer scale precipitates of non-cubic phases can co-exist with the cubic phase.•Energy calculations indicate a preference for a stratified dopant arrangement.
K
Sb
and K
Sb
Zintl ion precursors react with Pd(PPh
)
in ethylenediamine/toluene/PBu
solutions to give crystals of Sb@Pd
@Sb
/PBu
salts, where n = 3, 4. The clusters are structurally identical in ...the two charge states, with nearly perfect I
point symmetry, and can be viewed as an Sb@Pd
icosahedron centered inside of an Sb
dodecahedron. The metric parameters suggest very weak Sb-Sb and Pd-Pd interactions with strong radial Sb-Pd bonds between the Sb
and Pd
shells. All-electron DFT analysis shows the 3- ion to be diamagnetic with I
symmetry and a 1.33 eV HOMO-LUMO gap, whereas the 4- ion undergoes a Jahn-Teller distortion to an S = 1/2 D
structure with a small 0.1 eV gap. The distortion is predicted to be small and is not discernible by crystallography. Laser desorption-ionization time-of-flight mass spectrometry (LDI-TOF MS) studies of the crystalline samples show intense parent Sb@Pd
@Sb
ions (negative ion mode) and Sb@Pd
@Sb
(positive ion mode) along with series of Sb@Pd
@Sb
ions. Ni(cyclooctadiene)
reacts with K
Sb
in en/tol/Bu
PBr solvent mixtures to give black precipitates of Sb@Ni
@Sb
salts that give similar Sb@Ni
@Sb
parent ions and Sb@Ni
@Sb
degradation series in the respective LDI-TOF MS studies. The solid-state and gas-phase studies of the icosahedral Sb@M
@Sb
ions show that the clusters can exist in the -4, -3, -1, +1 (M = Pd) and +1, -1 (M = Ni) oxidation states. These multiple-charge-state clusters are reminiscent of redox-active fullerenes (e.g., C
, where n = +1, 0, -1, -2, -3, -4, -5, -6).
Three-center integrals over Gaussian orbital and Kohn–Sham (KS) basis sets are reviewed. An orbital basis function carries angular momentum about its atomic center. That angular momentum is created ...by solid harmonic differentiation with respect to the center of an s-type basis function. That differentiation can be brought outside any purely s-type integral, even nonlocal pseudopotential integrals. Thus the angular factors associated with angular momentum and differentiation with respect to atom position can be pulled outside loops over orbital and KS Gaussian exponents.
The fast oxygen-conducting cubic phase of zirconia is commonly stabilized by the substitution of group III cations such as yttrium and scandium, which creates vacancies in the oxygen sublattice. ...While scandia stabilized zirconia can achieve higher conductivity in freshly prepared specimens, it has previously been shown that yttria provides better stabilization characteristics than scandia, especially after extended operation at high temperatures. Density functional simulations of yttria and scandia zirconia show that oxygen ions neighboring vacancies can move into interstitial locations, effectively causing partial occupancy of neighboring anion sites, particularly in specimens containing scandia. Such partial occupancies can stabilize the substance in an ordered arrangement, causing age-related degradation over time, and can explain the diffuse scattering seen in X-ray diffraction studies of aged electrolytes. This tendency is reduced by increasing the yttria content of the mixture. Comparisons against rigid ion models show that the bonding is primarily ionic, but when cations become lower coordinated, the strength of the ionic bond increases, which can pull oxygen ions away from their ideal cubic lattice sites into interstitial locations that are shown to be stable at low temperature, a phenomenon which is more common in proximity to scandium ions than near yttrium ions.
► Zirconia stabilized by yttria, scandia and a yttria-scandia blend were analyzed. ► Oxygen ions displaced into interstitial locations between adjacent anion sites. ► Partial occupancy of anion sites may increase correlation of ion migration.
K5Sb4 and K3Sb7 Zintl ion precursors react with Pd(PPh3)4 in ethylenediamine/toluene/PBu4+ solutions to give crystals of Sb@Pd12@Sb20n-/PBu4+ salts, where n = 3, 4. The clusters are structurally ...identical in the two charge states, with nearly perfect Ih point symmetry, and can be viewed as an Sb@Pd12 icosahedron centered inside of an Sb20 dodecahedron. The metric parameters suggest very weak Sb-Sb and Pd-Pd interactions with strong radial Sb-Pd bonds between the Sb20 and Pd12 shells. All-electron DFT analysis shows the 3- ion to be diamagnetic with Ih symmetry and a 1.33 eV HOMO-LUMO gap, whereas the 4- ion undergoes a Jahn-Teller distortion to an S = 1/2 D3d structure with a small 0.1 eV gap. The distortion is predicted to be small and is not discernible by crystallography. Laser desorption-ionization time-of-flight mass spectrometry (LDI-TOF MS) studies of the crystalline samples show intense parent Sb@Pd12@Sb20- ions (negative ion mode) and Sb@Pd12@Sb20+ (positive ion mode) along with series of Sb@Pd12-y@Sb20-x-/+ ions. Ni(cyclooctadiene)2 reacts with K3Sb7 in en/tol/Bu4PBr solvent mixtures to give black precipitates of Sb@Ni12@Sb20n- salts that give similar Sb@Ni12@Sb20-/+ parent ions and Sb@Ni12-y@Sb20-x-/+ degradation series in the respective LDI-TOF MS studies. The solid-state and gas-phase studies of the icosahedral Sb@M12@Sb20n-/n+ ions show that the clusters can exist in the -4, -3, -1, +1 (M = Pd) and +1, -1 (M = Ni) oxidation states. These multiple-charge-state clusters are reminiscent of redox-active fullerenes (e.g., C60n, where n = +1, 0, -1, -2, -3, -4, -5, -6).
A hydrogen-powered solid oxide fuel cell (SOFC), with a Pt cathode and a Ni anode, is modeled with a kinetic Monte Carlo (KMC) simulation technique. A series of reversible elementary steps are ...adopted from experiments and theories for simulating the oxygen reduction reaction near the cathode–electrolyte interface and the hydrogen-oxidation mechanism near the anode–electrolyte interface. By studying the change in the ionic current density, the sensitivity of the kinetic parameters is analyzed, and the influence of various operating conditions and different material properties are also explored. The results show that the dominant elementary process is the oxygen incorporation into the yttria-stabilized zirconia (YSZ) electrolyte at the cathode. Increasing the applied bias voltage, operating temperature, and relative permittivity of the YSZ, but reducing the thickness of the YSZ enhance the ionic current density and improve the efficiency of the SOFC.
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