Proton Dynamics Research Articles

Understanding Li‐Ion Dynamics in Lithium Hydroxychloride (Li2OHCl) Solid State Electrolyte via Addressing the Role of Protons

AbstractLow‐melting‐point solid‐state electrolytes (SSE) are critically important for low‐cost manufacturing of all‐solid‐state batteries. Lithium hydroxychloride (Li2OHCl) is a promising material within the SSE domain due to its low melting point (mp < 300 °C), cheap ingredients (Li, H, O, and Cl), and rapid synthesis. Another unique feature of this compound is the presence of Li vacancies and rotating hydroxyl groups which promote Li‐ion diffusion, yet the role of the protons in the ion transport remains poorly understood. To examine lithium and proton dynamics, a set of solid‐state NMR experiments are conducted, such as magic‐angle spinning 7Li NMR, static 7Li and 1H NMR, and spin‐lattice T1(7Li)/T1(1H) relaxation experiments. It is determined that only Li+ contributes to long‐range ion transport, while H+ dynamics is constrained to an incomplete isotropic rotation of the OH group. The results uncover detailed mechanistic understanding of the ion transport in Li2OHCl. It is shown that two distinct phases of ionic motions appear at low and elevated temperatures, and that the rotation of the OH group controls Li+ and H+ dynamics in both phases. The model based on the NMR experiments is fully consistent with crystallographic information, ionic conductivity measurements, and Born–Oppenheimer molecular dynamic simulations.

Collective exchange processes reveal an active site proton cage in bacteriorhodopsin

Proton translocation across membranes is vital to all kingdoms of life. Mechanistically, it relies on characteristic proton flows and modifications of hydrogen bonding patterns, termed protonation dynamics, which can be directly observed by fast magic angle spinning (MAS) NMR. Here, we demonstrate that reversible proton displacement in the active site of bacteriorhodopsin already takes place in its equilibrated dark-state, providing new information on the underlying hydrogen exchange processes. In particular, MAS NMR reveals proton exchange at D85 and the retinal Schiff base, suggesting a tautomeric equilibrium and thus partial ionization of D85. We provide evidence for a proton cage and detect a preformed proton path between D85 and the proton shuttle R82. The protons at D96 and D85 exchange with water, in line with ab initio molecular dynamics simulations. We propose that retinal isomerization makes the observed proton exchange processes irreversible and delivers a proton towards the extracellular release site.

Proton dynamics in molecular solvent clusters as an indicator for hydrogen bond network strength in confined geometries.

Hydrogen bonding leads to the formation of strong, extended intermolecular networks in molecular liquids such as water. However, it is less well-known how robust the network is to environments in which surface formation or confinement effects become prominent, such as in clusters or droplets. Such systems provide a useful way to probe the robustness of the network, since the degree of confinement can be tuned by altering the cluster size, changing both the surface-to-volume ratio and the radius of curvature. To explore the formation of hydrogen bond networks in confined geometries, here we present O 1s Auger spectra of small and large clusters of water, methanol, and dimethyl ether, as well as their deuterated equivalents. The Auger spectra of the clusters and the corresponding macroscopic liquids are compared and evaluated for an isotope effect, which is due to proton dynamics within the lifetime of the core hole (proton-transfer-mediated charge-separation, PTM-CS), and can be linked to the formation of a hydrogen bond network in the system. An isotope effect is observed in water and methanol but not for dimethyl ether, which cannot donate a hydrogen bond at its oxygen site. The isotope effect, and therefore the strength of the hydrogen bond network, is more pronounced in water than in methanol. Its value depends on the average size of the cluster, indicating that confinement effects change proton dynamics in the core ionised excited state.

A Co(ii)-coordination polymer for ultrahigh superprotonic conduction: an atomistic insight through molecular simulations and QENS experiments

A superprotonic conducting coordination polymer PCM-1 was synthesized. Joint experimental/modeling study revealed water-assisted proton dynamics through the formation of a long-range H-bonded network mediated by the Teixeira jump-diffusion model.

Effect of anion reorientation on proton mobility in the solid acids family CsHyXO4 (X = S, P, Se, y = 1, 2) from ab initio molecular dynamics simulations.

The high temperature phases of the solid acids CsHSeO4, CsHSO4 and CsH2PO4 show extraordinary high proton conductivities, while the low temperature phases do not conduct protons at all. We systematically investigate proton dynamics in the low and high temperature phases of these compounds by means of ab initio molecular dynamics simulations in order to develop a general picture of the proton transfer mechanism. For all of these compounds, proton conduction follows a Grotthuss mechanism via a combined proton transfer and subsequent structural reorientation of the environment. We demonstrate that the drastically reduced conductivity of the low temperature phases is caused by a highly ordered, rigid hydrogen bond network, while efficient long range proton transfer in the high temperature phases is enabled by the interplay of high proton transfer rates and frequent anion reorientation. Furthermore, we present a simple descriptor for the quantitative prediction of the diffusion coefficient within the solid acids family. As a side result, we show that the rate of the most elementary proton hopping reaction depends on the heavy-atom configuration of the nearest atoms in a ubiquitous manner, and is in turn almost independent from the global nature of the compound, i.e. whether it is organic or inorganic, ordered or disordered.

Thermal decomposition and structural dynamics in perovskite (C2H5NH3)2CdCl4 crystals

In this study, (C2H5NH3)2CdCl4 single crystals were grown, and their thermal properties were studied by the thermogravimetric analysis and differential scanning calorimetry. Phase transition temperature above room temperature was observed at 470 K. In addition, the cation dynamics in this crystal were investigated by 1H magic-angle spinning nuclear magnetic resonance (MAS NMR) and 13C cross-polarization (CP)/MAS NMR experiments. The Bloembergen–Purcell–Pound curves for 1H T1ρ and 13C T1ρ in C2H5NH3 cation exhibited minimum values, and these curves represent C2H5NH3 and C2H5 rotational motions. The activation energy for 1H in the C2H5NH3 cation is Ea = 22.63 kJ mol−1, whereas those for 13C in CH3 and CH2 are Ea = 18.05 and 19.14 kJ mol−1, respectively. Furthermore, the proton dynamics of C2H5NH3 undergo faster rotation than carbons. This implies that the molecular motion for 1H is enhanced at the C-end and N-end of an organic cation, whereas the molecular motion is not free at the main chain carbons of the organic cation.

Formation of Layered Proton-Conducting Zirconium and Titanium Organophosphonates by Topotactic Reaction: Physicochemical Properties, Proton Dynamics, and Atomic-Resolution Structure.

A series of new zirconium and titanium phosphates-organophosphonates, in which the organophosphonate moiety is functionalized with a sulfo group, was prepared by a topotactic reaction involving the gamma modification of zirconium or titanium hydrogen phosphate with 2-bis(phosphonomethyl)amino-ethan-1-sulfonic acid (H4TDP). H4TDP represents a new type of functionalizing agent, which can be easily prepared by a Moedritzer-Irani reaction from taurine (2-aminoethanesulfonic acid). The gamma modification of zirconium hydrogen phosphate (γ-ZrP) with H4TDP provides mixed phosphate-organophosphonate compounds with the formula Zr(PO4)(H2PO4)1-2x(H2TDP)x·yH2O, where x = 0.15, 0.34, 0.45, and is controlled by the γ-ZrP/H4TDP ratio in the starting mixture. On the contrary, by the topotactic gamma modification of titanium hydrogen phosphate (γ-TiP) with H4TDP, only one product with the formula Ti(PO4)(H2PO4)1-2z(H2TDP)z·yH2O, where z = 0.41 ± 0.01, was obtained regardless of the composition of the starting mixture. The synthesized compounds were characterized by elemental analysis, thermogravimetric analysis, energy-dispersive X-ray analysis, and infrared spectroscopy. The way the topotactic reaction proceeds and how the grafted organophosphonate groups are bonded to the layers of the host structure were suggested on the basis of the solid-state NMR data. It was found that the grafted moieties are spread evenly in the host layers among the hydrogen phosphate groups. The obtained solids are able to intercalate basic molecules, as was proved by the intercalation reactions of the zirconium series with butylamine. The amount of intercalated butylamine increases with increasing x. It is known that both host compounds, γ-ZrP and γ-TiP, are protonic conductors. It was found that the incorporation of H2TDP increases conductivity of the zirconium compound when x = 0.15, but further incorporation of H2TDP into the γ-ZrP host structure leads to a decrease of conductivity. This behavior is explained on the basis of the 1H MAS and the 1H-1H EXSY NMR data.

Microscopic Study of Proton Kinetic Energy Anomaly for Nanoconfined Water.

The reported anomalies of the proton mean kinetic energy, Ke(H), in nanoconfined water, as measured by deep inelastic neutron scattering (DINS), constitute a longstanding problem related to proton dynamics in hydrogen-bonded systems. A considerable number of theoretical attempts to explain these anomalies have failed. The mean vibrational density of states (VDOS) of protons in water nanoconfined inside single wall carbon nanotubes (SWCNTs) is calculated as a function of temperature and SWCNT diameter, DCNT, by classical molecular dynamics (MD) simulation using the TIP4P-2005f water model. The calculated VDOS are utilized for deducing the mean kinetic energy of the water protons, Ke(H), by treating each phonon state as a harmonic oscillator. The calculation depicts a strong confinement effect as reflected in the drop of the value of Ke(H) at 5 K for DCNT < ∼12 Å, while absent for larger diameters. The results also reveal very significant blue and red shifts of the stretching and bending modes, respectively, compared to those in bulk ice, in agreement with experiment.

Induced Proton Dynamics on Semiconductor Surfaces for Sensing Tight Junction Formation Enhanced by an Extracellular Matrix and Drug.

In the fields of tissue engineering and drug discovery, confirming the formation and maturation of epithelial cell tight junctions (TJs), which are necessary for blocking pathogenic invasion and absorption of nutrients and ions, at a high spatiotemporal resolution is essential. We previously developed a system of monitoring pH perturbation induced by weak acid exposure to cells cultured on an ion-sensitive field-effect transistor that enables a sensitive and specific detection of biomembrane injuries and TJ breakdowns caused by external stimuli such as nanomaterials and cytotoxins. In this study, we monitor time-lapse changes in the paracellular diffusion of growing epithelial cell monolayers using the pH perturbation assay as well as conventional permeability and trans-epithelial electrical resistance assays. The effects of the extracellular matrix and a TJ potentiator (KN-93) on epithelial TJ formation are evaluated. TJ formations were promoted on the substrate coated with Matrigel more than on the one coated with poly(l-lysine). KN-93 accelerated TJ formations in a dose-dependent manner. The pH perturbation assay denoted a longer incubation time for the completion of TJ formation compared with the conventional assays under the same conditions. Importantly, the pH perturbation assay is able to rigorously evaluate TJ formation, as the assay uses protons as the smallest indicator for detecting paracellular gaps, and the pH perturbation is specific to TJ alterations. These features for in vitro TJ evaluation using proton dynamics are advantageous for applications in tissue engineering and drug development.

Vibrational modes and quantum zero-point energy of hydrogen in ZrH0.0155 and ZrH2

We report on an inelastic neutron scattering study of the proton dynamics in ZrH0.0155 and ε-ZrH2. In particular, we present measurements of the incoherent dynamic structure factor, generalized vibrational density of states, and proton momentum distribution of these two materials. Our results are generally consistent with theoretical predictions of Elsässer et al. [Mat. Res. Soc. Symp. Proc. 453 221–226 (1997)]. They argued that the effective Born-Oppenheimer potential experienced by the hydrogen atoms in ε-ZrH2 is nearly isotropic and harmonic at energies below 0.3 eV, but becomes anisotropic and anharmonic for higher energies. At low temperatures, the proton momentum distribution is dominated by the quantum-mechanical ground state of the protons. We find that it assumes a Gaussian shape, consistent with the concept that the potential surface is approximately harmonic for small displacements of the hydrogen atoms. However, the anharmonicity of the potential becomes readily apparent in the excited states of the hydrogen atoms, as the harmonic approximation breaks down in the description of the multiphonon bands.

Local Coordination of Protons in In- and Sc-Doped BaZrO3

Acceptor-doped barium zirconate-based proton conductors are currently receiving considerable attention because of their promise as electrolytes in future electrochemical devices, such as solid oxide fuel cells, but the defect chemistry, especially in regard to the local coordination environment and dynamics of protons in these materials, is unclear. Here, we investigate the local coordination environments and vibrational dynamics of protons in samples of the proton conducting material BaZr1-xMxO3Hx with M = In and Sc, and x = 0.1 and 0.5, using inelastic neutron scattering (INS), infrared (IR), and Raman spectroscopy together with ab initio molecular dynamics (AIMD) simulations. The local coordination of protons is shown to exhibit a rather peculiar dependence on the type and concentration of dopant atoms, as they are found to be similar for BaZr1-xScxO3Hx with x = 0.1 and 0.5 and BaZr1-xInxO3Hx with x = 0.1, whereas for BaZr1-xInxO3Hx with x = 0.5 additional proton sites seem to be present. It is argued that these additional proton sites are characterized by local structural arrangements reminiscent of the fully In-substituted material BaInO3H. The presence of these local structural arrangements points toward different local proton mobilities between BaZr1-xInxO3Hx with x = 0.5 and the other three materials and a higher rate of proton transfer events in brownrnillerite-type local structures.

Charge delocalization effects on Nafion structure and water /proton dynamics in hydrated environments

In this work, using molecular dynamics simulations, we examine the effect of atomic charge delocalization on the pendant side chain of Nafion membrane on the structural and dynamical properties in various hydrated environments. The sulfur-sulfur radial distribution functions suggest that the sulfonate groups of the pendant side chain have closer geometric proximity with an increase in charge delocalization. However, the interactions of the sulfonate groups with water molecules/hydronium ions show a slight change with the charge delocalization. The average water cluster size decreases significantly with charge delocalization, though the diffusion coefficients of water molecules (at medium and higher water concentration) increase initially and then decreases slightly with excessive charge delocalization. The diffusion coefficients of hydronium ions do not follow any particular trend with charge delocalization. A complex interplay between sulfur-sulfur, sulfur-water/hydronium interactions, and water cluster distribution plays an essential role in the magnitude of the diffusion coefficient of water molecules and hydronium ions.

ViDA: a Vlasov–DArwin solver for plasma physics at electron scales

We present a Vlasov–DArwin numerical code (ViDA) specifically designed to address plasma physics problems, where small-scale high accuracy is requested even during the nonlinear regime to guarantee a clean description of the plasma dynamics at fine spatial scales. The algorithm provides a low-noise description of proton and electron kinetic dynamics, by splitting in time the multi-advection Vlasov equation in phase space. Maxwell equations for the electric and magnetic fields are reorganized according to the Darwin approximation to remove light waves. Several numerical tests show that ViDA successfully reproduces the propagation of linear and nonlinear waves and captures the physics of magnetic reconnection. We also discuss preliminary tests of the parallelization algorithm efficiency, performed at CINECA on the Marconi-KNL cluster. ViDA will allow the running of Eulerian simulations of a non-relativistic fully kinetic collisionless plasma and it is expected to provide relevant insights into important problems of plasma astrophysics such as, for instance, the development of the turbulent cascade at electron scales and the structure and dynamics of electron-scale magnetic reconnection, such as the electron diffusion region.

PBI-Type Polymers and Acidic Proton Conducting Ionic Liquids – Conductivity and Molecular Interactions

The operation of polymer electrolyte fuel cells (PEFC) at elevated temperatures above 100°C would allow a much simpler system setup: i) no feed gas humidification, ii) a more efficient cooling system (easier water and heat management), iii) the possibility of reco­vering high-grade waste heat, and iv) a higher tolerance against feed gas impurities. Cur­rently, (high temperature) HT-PEFC, based on phosphoric acid doped polybenzimidazole (PBI) membranes, cannot compete with the performance characteristics of NAFION-based low temperature-PEFCs. The presence of H3PO4 causes a slow cathodic oxygen reduction reaction kinetics (ORR). Thus, there is a necessity for new non-aqueous proton conducting electrolytes operational for the temperature range between 100–120 °C. Proton conducting ionic liquids (PILs) with acidic cations are promising candidates for the use as non-aqueous electrolytes at operation temperatures above 100 °C. In this con­tribution, an experimental study on the interaction of PBI based proton exchange mem­brane (PEM) with highly acidic betaine-type zwitterion PILs is presented. 2-Sulfo­ethyl­methylammonum triflate ([2-Sema][TfO]) exhibits a ~3 times higher ORR current densities on Pt compared to H3PO4. There is a (slow) uptake of [2-Sema][TfO] by PBI due to a swelling process, up a weight increase of 135%. The composition of doped membrane was determined by NMR. The doping process was monitored by Raman spectroscopy, proving the protonation of the imidazole groups in the polymer chains. The thermal sta­bility is measured by TGA. NMR analysis and quasielastic neutron scattering (QENS) has been applied to elucidate the proton dynamics and the molecular interactions between PBI, PIL and residual water, which is present during fuel cell operation. Residual water concentrations up to 6 wt% are investigated. Considering the change of viscosity, the total conductivity in [2-Sema][TfO] depends highly on the H2O concentration. A high acidic PIL is able to protonate H2O. Thus, compared to low and medium acidic PILs, the conductivity of high acidic PIL shows a (quasi-)exponential increase. The self-diffusion coefficients are measured by DOSY 1H-NMR, revealing that the diffusion of the active (acidic) proton is faster than the diffusion of the PIL cation. The results indicate a cooperative mechanism involving a fast proton exchange between PIL cations, H2O and H3O+. Beyond the time scale of the NMR mea­surement, two Lorentzian components can be observed by QENS in [2-Sema][TfO]/H2O. Besides the diffusion of PIL cations and the H2O/cation proton exchange, an additional faster dynamic process of the active protons is present. Therefore, an intra-molecular proton dynamic is inferred in cation. Compared to the neat PIL, the proton conduction in the doped PBI membrane is re­stric­ted due to the constraining network of the polymer chains. To optimise the conductivity but also the uptake of the PIL into the polymer, the use of solution casting methods have been studied for these materials.

Alternate way of soliton solutions in hydrogen-bonded chain

The transport of energy and charge along one-dimensional chains of hydrogen bonds is an extremely important problem in bioenergetic and solid-state situations. We present an alternate way to investigate the proton dynamics in a symmetric double-well potential created by the potentials of two heavy ions in a long periodic chains of hydrogen bonds forming the channels for proton transport. We formulate the Hamiltonian by incorporating quartic anharmonic proton–proton interaction. We invoke the modified extended tangent hyperbolic method (METF) embedded with symbolic computation to solve the associated two coupled equations of motion. We propose a series of new forms of the potential relief of hydrogen bonds. We have shown that the symmetric double-well potential in the presence of anharmonic quartic proton–proton interaction gives rise to the anti-kink, anti-soliton, and soliton solutions for the equations of motion. Thereby the proton motion in the form of coherent energy profiles such as kink and solitons may provide the mechanism of bioenergy transport in biological systems.

Exact Potential Energy Surface for Molecules in Cavities.

We find and analyze the exact time-dependent potential energy surface driving the proton motion for a model of cavity-induced suppression of proton-coupled electron transfer. We show how, in contrast to the polaritonic surfaces, its features directly correlate to the proton dynamics and we discuss cavity modifications of its structure responsible for the suppression. The results highlight the interplay between nonadiabatic effects from coupling to photons and coupling to electrons and suggest caution is needed when applying traditional dynamics methods based on polaritonic surfaces.

Proton conductivity and proton dynamics in nanocrystalline cellulose functionalized with imidazole

In the present study, we report the synthesis, electrical and dynamic properties of a new generation bio-based nanocomposite, that is a proton-exchange membrane based on nanocrystalline cellulose (CNC) and imidazole (Im). CNC serves as supporting material and imidazole acts as a proton donor and proton acceptor at the same time. The nanocomposite (1.3 CNC-Im) was synthesized as a film and shows proton conductivity equal to 4.0 × 10−1 S/m at 160 °C in anhydrous conditions. Analysis of impedance measurements and NMR spectra provided some insight into the macroscopic and microscopic processes involved in proton transport in 1.3 CNC-Im. Local processes such as reorientation of imidazole rings and breaking of hydrogen bonds are identified and their activation energies are calculated. The energies of the macroscopic and microscopic proton transport in CNC-Im film are correlated. The percolation model used confirmed the percolation nature of conductivity in cellulose composites with imidazole.

Collision dynamics of proton to water dimer at 250 eV

Applying a real-space, real-time implementation of time-dependent density functional theory coupled to molecular dynamics (TDDFT-MD) non-adiabatically, we study the ionization and fragmentation of water dimer in collision with a proton at 250 eV. Four different incident orientations with various impact parameters are employed to account for orientation effects. The reaction channels, electronic density evolution, scattering pattern and energy loss of proton are obtained. We find that proton is scattered away for all impact parameters and the head-on collision effects the energy loss of proton dominantly as well as the scattering angle. The locations of peaks of the scattering angles are similar to those corresponding to the energy loss. The single-electron capture, the double-electron capture as well as the total electron capture cross-sections are obtained. We find that the single-electron capture cross-section contributes most to the total electron capture cross-section and the calculated total electron capture cross-section is in reasonable agreement with experimental and other theoretical results with respect to water gas and liquid water.

The Role of Sodium Hydrogen Exchanger 1 in Dysregulation of Proton Dynamics and Reprogramming of Cancer Metabolism as a Sequela

Cancer cells have an unusual regulation of hydrogen ion dynamics that are driven by poor vascularity perfusion, regional hypoxia, and increased glycolysis. All these forces synergize/orchestrate together to create extracellular acidity and intracellular alkalinity. Precisely, they lead to extracellular pH (pHe) values as low as 6.2 and intracellular pH values as high as 8. This unique pH gradient (∆pHi to ∆pHe) across the cell membrane increases as the tumor progresses, and is markedly displaced from the electrochemical equilibrium of protons. These unusual pH dynamics influence cancer cell biology, including proliferation, metastasis, and metabolic adaptation. Warburg metabolism with increased glycolysis, even in the presence of Oxygen with the subsequent reduction in Krebs’ cycle, is a common feature of most cancers. This metabolic reprogramming confers evolutionary advantages to cancer cells by enhancing their resistance to hypoxia, to chemotherapy or radiotherapy, allowing rapid production of biological building blocks that support cellular proliferation, and shielding against damaging mitochondrial free radicals. In this article, we highlight the interconnected roles of dysregulated pH dynamics in cancer initiation, progression, adaptation, and in determining the programming and re-programming of tumor cell metabolism.

Investigation of the Effect of a Longitudinal Magnetic Field Component on the Dynamics of Protons inside a Plasmoid

Kinetic effects of the dynamics of protons in plasmoids with a non-zero longitudinal (By) magnetic field component in a current sheet (CS) of a geomagnetic tail are considered. The results of modeling proton dynamics and a description of the mechanism of emergence of “north-south” density asymmetry are presented. The mechanism that is possibly responsible for maintaining the longitudinal magnetic field component is described. The obtained parameters are evaluated and the results are compared with observations of the Cluster mission.