Hybrid Superlattice-Triggered Selective Proton Grotthuss Intercalation in δ-MnO2 for High-Performance Zinc-Ion Battery.

A great deal of attention has been paid on layered manganese dioxide (δ-MnO2 ) as promising cathode candidate for aqueous zinc-ion battery (ZIB) due to the excellent theoretical capacity, high working voltage and Zn2+ /H+ co-intercalation mechanism. However, caused by the insertion of Zn2+ , the strong coulomb interaction and sluggish diffusion kinetics have resulted in significant structure deformation, insufficient cycle stability and limited rate capability. And it is still far from satisfactory to accurately modulate H+ intercalation for superior electrochemical kinetics. Herein, the terrace-shape δ-MnO2 hybrid superlattice by polyvinylpyrrolidone (PVP) pre-intercalation (PVP-MnO2 ) was proposed with the state-of-the-art ZIBs performance. Local atomic structure characterization and theoretical calculations have been pioneering in confirming the hybrid superlattice-triggered synergy of electron entropy stimulation and selective H+ Grotthuss intercalation. Accordingly, PVP-MnO2 hybrid superlattice exhibits prominent specific capacity (317.2 mAh g-1 at 0.125 A g-1 ), significant rate performance (106.1 mAh g-1 at 12.5 A g-1 ), and remarkable cycle stability at high rate (≈100 % capacity retention after 20,000 cycles at 10 A g-1 ). Therefore, rational design of interlayer configuration paves the pathways to the development of MnO2 superlattice for advanced Zn-MnO2 batteries.

Improved Indicator Algorithms for Tracking a Hydrated Proton as A Local Structural Defect in Grotthuss Diffusion in Aqueous Solutions.

Keeping track of a hydrated proton in dynamics simulations is important and nontrivial. Here, we report two revised algorithms for the proton indicator, a pseudo-atom whose position approximates the location of an excess proton diffusing via the Grotthuss mechanism in aqueous solution. The new methods describe the delocalized proton as a structural defect. Encouragingly, in test simulations of a hydrated proton in bulk water, the new algorithms substantially outperform the original scheme by significantly reducing large displacements in the indicator positions upon donor switch, yielding smoother trajectories that effectively track the movement of the solvated proton.

Grotthuss Proton-Conductive Covalent Organic Frameworks for Efficient Proton Pseudocapacitors.

Herein, we describe the synthesis of two highly crystalline, robust, hydrophilic covalent organic frameworks (COFs) that display intrinsic proton conduction by the Grotthuss mechanism. The enriched redox-active azo groups in the COFs can undergo a proton-coupled electron transfer reaction for energy storage, making the COFs ideal candidates for pseudocapacitance electrode materials. After in situ hybridization with carbon nanotubes, the composite exhibited a high three-electrode specific capacitance of 440 F g-1 at the current density of 0.5 A g-1 , among the highest for COF-based supercapacitors, and can retain 90 % capacitance even after 10 000 charge-discharge cycles. This is the first example using Grotthuss proton-conductive organic materials to create pseudocapacitors that exhibited both high power density and energy density. The assembled asymmetric two-electrode supercapacitor showed a maximum energy density of 71 Wh kg-1 with a maximum power density of 42 kW kg-1 , surpassing that of all reported COF-based systems.

Proton Traffic Jam: Effect of Nanoconfinement and Acid Concentration on Proton Hopping Mechanism.

The properties of the water network in concentrated HCl acid pools in nanometer-sized reverse nonionic micelles were probed with TeraHertz absorption, dielectric relaxation spectroscopy, and reactive force field simulations capable of describing proton hopping mechanisms. We identify that only at a critical micelle size of W0 =9 do solvated proton complexes form in the water pool, accompanied by a change in mechanism from Grotthuss forward shuttling to one that favors local oscillatory hopping. This is due to a preference for H+ and Cl- ions to adsorb to the micelle interface, together with an acid concentration effect that causes a "traffic jam" in which the short-circuiting of the hydrogen-bonding motif of the hydronium ion decreases the forward hopping rate throughout the water interior even as the micelle size increases. These findings have implications for atmospheric chemistry, biochemical and biophysical environments, and energy materials, as transport of protons vital to these processes can be suppressed due to confinement, aggregation, and/or concentration.

Symmetry-related proton transfer pathways in respiratory complex I.

Complex I functions as the initial electron acceptor in aerobic respiratory chains of most organisms. This gigantic redox-driven enzyme employs the energy from quinone reduction to pump protons across its complete approximately 200-Å membrane domain, thermodynamically driving synthesis of ATP. Despite recently resolved structures from several species, the molecular mechanism by which complex I catalyzes this long-range proton-coupled electron transfer process, however, still remains unclear. We perform here large-scale classical and quantum molecular simulations to study the function of the proton pump in complex I from Thermus thermophilus The simulations suggest that proton channels are established at symmetry-related locations in four subunits of the membrane domain. The channels open up by formation of quasi one-dimensional water chains that are sensitive to the protonation states of buried residues at structurally conserved broken helix elements. Our combined data provide mechanistic insight into long-range coupling effects and predictions for site-directed mutagenesis experiments.

Electrostatics, hydration, and proton transfer dynamics in the membrane domain of respiratory complex I.

Complex I serves as the primary electron entry point into the mitochondrial and bacterial respiratory chains. It catalyzes the reduction of quinones by electron transfer from NADH, and couples this exergonic reaction to the translocation of protons against an electrochemical proton gradient. The membrane domain of the enzyme extends ∼180 Šfrom the site of quinone reduction to the most distant proton pathway. To elucidate possible mechanisms of the long-range proton-coupled electron transfer process, we perform large-scale atomistic molecular dynamics simulations of the membrane domain of complex I from Escherichia coli. We observe spontaneous hydration of a putative proton entry channel at the NuoN/K interface, which is sensitive to the protonation state of buried glutamic acid residues. In hybrid quantum mechanics/classical mechanics simulations, we find that the observed water wires support rapid proton transfer from the protein surface to the center of the membrane domain. To explore the functional relevance of the pseudosymmetric inverted-repeat structures of the antiporter-like subunits NuoL/M/N, we constructed a symmetry-related structure of a possible alternate-access state. In molecular dynamics simulations, we find the resulting structural changes to be metastable and reversible at the protein backbone level. However, the increased hydration induced by the conformational change persists, with water molecules establishing enhanced lateral connectivity and pathways for proton transfer between conserved ionizable residues along the center of the membrane domain. Overall, the observed water-gated transitions establish conduits for the unidirectional proton translocation processes, and provide a possible coupling mechanism for the energy transduction in complex I.

A GH115 α-glucuronidase structure reveals dimerization-mediated substrate binding and a proton wire potentially important for catalysis.

Xylan is a major constituent of plant cell walls and is a potential source of biomaterials, and the derived oligosaccharides have been shown to have prebiotic effects. Xylans can be highly substituted with different sugar moieties, which pose steric hindrance to the xylanases that catalyse the hydrolysis of the xylan backbone. One such substituent is α-D-glucuronic acid, which is linked to the O2' position of the ß-1,4 D-xylopyranoses composing the main chain of xylans. The xylan-specific α-glucuronidases from glycoside hydrolase family 115 (GH115) specifically catalyse the removal of α-D-glucuronic acid (GlcA) or methylated GlcA (MeGlcA). Here, the molecular basis by which the bacterial GH115 member wtsAgu115A interacts with the main chain of xylan and the indirect involvement of divalent ions in the formation of the Michaelis-Menten complex are described. A crystal structure at 2.65 Šresolution of wtsAgu115A originating from a metagenome from an anaerobic digester fed with wastewater treatment sludge was determined in complex with xylohexaose, and Asp303 was identified as the likely general acid. The residue acting as the general base could not be identified. However, a proton wire connecting the active site to the metal site was observed and hence a previous hypothesis suggesting a Grotthuss-like mechanism cannot be rejected. Only a single molecule was found in the asymmetric unit. However, wtsAgu115A forms a dimer with a symmetry-related molecule in the crystal lattice. The xylohexaose moieties of the xylohexaose are recognized by residues from both protomers, thus creating a xylohexaose recognition site at the dimer interface. The dimer was confirmed by analytical size-exclusion chromatography in solution. Kinetic analysis with aldouronic acids resulted in a Hill coefficient of greater than 2, suggesting cooperativity between the two binding sites. Three Ca2+ ions were identified in the wtsAgu115A structures. One Ca2+ ion is of particular interest as it is coordinated by the residues of the loops that also interact with the substrate. Activity studies showed that the presence of Mg2+ or Mn2+ resulted in a higher activity towards aldouronic acids, while the less restrictive coordination geometry of Ca2+ resulted in a decrease in activity.

The Grotthuss mechanism for bifunctional proton transfer in poly(benzimidazole).

Poly(benzimidazole) (PBI) has received considerable attention as an effective high-temperature polymer electrolyte membrane for fuel cells. In this work, the Grotthuss mechanism for bifunctional proton transfer in PBI membranes was studied using density functional theory and transition state theory. This study focused on the reaction paths and kinetics for bifunctional proton transfer scenarios in neutral ([PBI]2), single (H+[PBI]2) and double-protonated (H2+[PBI]2) dimers. The theoretical results showed that the energy barriers and strength for H-bonds are sensitive to the local dielectric environment. For [PBI]2 with ε = 1, the uphill potential energy curve is attributed to extraordinarily strong ion-pair H-bonds in the transition structure, regarded as a 'dipolar energy trap'. For ε = 23, the ion-pair charges are partially neutralized, leading to a reduction in the electrostatic attraction in the transition structure. The dipolar energy trap appears to prohibit interconversion between the precursor, transition and proton-transferred structures, which rules out the possibility for [PBI]2 to be involved in the Grotthuss mechanism. For H+[PBI]2 and H2+[PBI]2 with ε = 1, the interconversion involves a low energy barrier, and the increase in the energy barrier for ε = 23 can be attributed to an increase in the strength of the protonated H-bonds in the transition structure: the local dielectric environment enhances the donor-acceptor interaction of the protonated H-bonds. Analysis of the rate constants confirmed that the quantum effect is not negligible for the N-H+ … N H-bond especially at low temperatures. Agreement between the theoretical and experimental data leads to the conclusion that the concerted bifunctional proton transfer in H2+[PBI]2 in a high local dielectric environment is 'the rate-determining scenario'. Therefore, a low local dielectric environment can be one of the required conditions for effective proton conduction in acid-doped PBI membranes. These theoretical results provide insights into the Grotthuss mechanism, which can be used as guidelines for understanding the fundamentals of proton transfers in other bifunctional H-bond systems.

Substitutional Doping of MoS2 for Superior Gas-Sensing Applications: A Proof of Concept.

Two-dimensional layered materials (like MoS2 and WS2) those are being used as sensing layers in chemoresistive gas sensors suffer from poor sensitivity and selectivity. Mere surface functionalization (decorating of material surface) with metal nanoparticles (NPs) might not improve the sensor performance significantly. In this respect, doping of the layered material can play a significant role. Here, we report a simple yet effective substitutional doping technique to dope MoS2 with noble metals. Through various material characterization techniques like X-ray diffraction, scanning tunneling spectroscopy images, and selected area electron diffraction pattern, we were able to put forward the difference between surface decoration and substitutional doping by Au at S-vacancy sites of MoS2. Lattice strain was found to exist in the Au-doped MoS2 samples, while being absent in the Au NP-decorated samples. Surface chemistry studies performed using X-ray photoelectron spectroscopy showed a shift of Mo 3d peaks to lower binding energies, thus realizing p-type doping due to Au. The blue shift of the peaks as observed in Raman spectroscopy further confirmed the p-type doping. We found that gold-doped MoS2 was more sensitive and selective toward ammonia (with a response of 150% for 500 ppm of ammonia at 90 °C) as compared to gold NP-decorated MoS2. The advantages of substitutional doping and the gas-sensing mechanism were also explained by the density functional theory study. From the first principles study, it was found that the adsorption of Au atoms on the S-vacancy site of a monolayer of the MoS2 sheet was thermodynamically favorable with the adsorption energy of 2.39 eV. We also successfully doped MoS2 with Pt using the same technique. It was found that Pt-doped MoS2 gives huge response toward humidity (60,000% at 80% relative humidity). Thus, various noble metal doping of MoS2 selectively improved the sensing response toward specific analytes. From this work, we believe that this method could also be useful to dope other layered nanomaterials to design gas sensors with improved selectivity.

Protein Motifs for Proton Transfers That Build the Transmembrane Proton Gradient.

Biological membranes are barriers to polar molecules, so membrane embedded proteins control the transfers between cellular compartments. Protein controlled transport moves substrates and activates cellular signaling cascades. In addition, the electrochemical gradient across mitochondrial, bacterial and chloroplast membranes, is a key source of stored cellular energy. This is generated by electron, proton and ion transfers through proteins. The gradient is used to fuel ATP synthesis and to drive active transport. Here the mechanisms by which protons move into the buried active sites of Photosystem II (PSII), bacterial RCs (bRCs) and through the proton pumps, Bacteriorhodopsin (bR), Complex I and Cytochrome c oxidase (CcO), are reviewed. These proteins all use water filled proton transfer paths. The proton pumps, that move protons uphill from low to high concentration compartments, also utilize Proton Loading Sites (PLS), that transiently load and unload protons and gates, which block backflow of protons. PLS and gates should be synchronized so PLS proton affinity is high when the gate opens to the side with few protons and low when the path is open to the high concentration side. Proton transfer paths in the proteins we describe have different design features. Linear paths are seen with a unique entry and exit and a relatively straight path between them. Alternatively, paths can be complex with a tangle of possible routes. Likewise, PLS can be a single residue that changes protonation state or a cluster of residues with multiple charge and tautomer states.

Bidirectional Superionic Conduction in Surface-Engineered 2D Hexagonal Boron Nitrides.

We designed functionalized hexagonal boron nitride (FhBN) nanoflakes with high proton conductivity in both in- and through-plane directions as next generation polymer electrolyte membranes (PEMs) for energy storage and conversion systems. The synthesis and functionalization of hBN nanoflakes with sulfonic acid (SA) groups are obtained by one-step and in situ liquid-phase exfoliation with excellent dispersibility and stability over a period of three months. The physico/chemical properties of FhBN nanoflakes were investigated by different spectroscopic and microscopic characterization, confirming chemical interactions between hBN lattice and SA groups. High concentrations (65 and 75 wt %) of FhBN nanoflakes composed with Nafion solution formed stable FhBN-Nafion nanocomposite PEMs, offering extra proton conduction sites, doubling ion-exchange capacity, and reducing the swelling ratio compared to those of Nafion. Our results demonstrate that both the in-plane and through-plane proton conductivities of FhBN-Nafion PEMs significantly improve under various conditions comparative to that of Nafion. The maximum values of both in- and through-plane conductivities for FhBN75%-Nafion PEM at 80% of humidity and 80 °C are 0.41 and 0.1 S·cm-1, respectively, which are 7 and 14 times higher than those of Nafion. The bidirectional superionic transport in highly concentrated FhBN PEMs is responsible for outstanding properties, useful for electrochemical energy devices.

Direct Measurement of Water-Assisted Ion Desorption and Solvation on Isolated Carbon Nanotubes.

We have investigated the change in mean residence time of gaseous ions adsorbed on the surface of suspended carbon nanotube field-effect transistors (CNT-FETs) with and without native surface water layers that exists in atmospheric conditions. Devices were characterized electrically before and after dehydration by thermal, dry gas, and vacuum desiccation and in each scenario were found to have substantially higher mean ion residence times. It is proposed that water molecules native to the CNT surface in ambient conditions provide a reduction pathway for incoming gaseous ions, yielding hydronium ions (H3O+). This is supported by the appearance of frequent clustered readsorption events in the presence of surface water, caused by the rapid hopping of H+ between the device surface and the lowest water layer, which are not present in data collected from desiccated devices. After desiccation of the device, a thermal trial was conducted to determine the adsorption energy of N2+ ions on the CNT surface. This work has profound implications for our understanding of wetting in one-dimensional systems and the chemistry of ion chemisorption and solvation on the surfaces of materials in general.

Hydrous Nickel-Iron Turnbull's Blue as a High-Rate and Low-Temperature Proton Electrode.

Proton batteries are emerging as a promising solution for energy storage; however, their development has been hindered by the lack of suitable cathode materials. Herein, a hydrous Turnbull's blue analogue (TBA) of Ni[Fe(CN)6]2/3·4H2O has been investigated as a viable proton cathode. Particularly, it shows an extremely high rate performance up to 6000 C (390 A g-1) at room temperature and delivers good capacity values at a low temperature of -40 °C in an aqueous electrolyte. The excellent rate capability is also amenable to high mass loadings of 10 mg cm-2. Such fast and low-temperature rate behavior likely stems from the fast proton conduction that is afforded by the Grotthuss mechanism inside the TBA structure. Furthermore, advanced characterization, including in operando synchrotron X-ray diffraction (XRD), and X-ray absorption near-edge structure (XANES) were employed to understand the changes of crystal structures and the oxidation-states of metal elements of the electrodes.

A hypothesis concerning callose.

Background Concept: Certain proteins and the glucose monomer have spacing of their carbonyl oxygen atoms that match the spacing of the oxygen atoms of hexagonal ice. This opens the possibility that a sequence of linked glucose residues may have a sequence of equally spaced carbonyl oxygen atoms. Hypothesis: Callose In plants is a duality consisting of the callose itself and a layer of ordered water whose oxygen atoms are hydrogen bonded to the carbonyl oxygen atoms in the callose. The atomic basis of the hypothesis is that the 1-3 linkage between glucose residues in callose results in equally spaced carbonyl oxygen atoms within and between residues. Properties of Callose/Ordered Water: The physical properties of the duality are the properties of callose itself: 1) it is immobile, 2) it can be created and dissolved, 3) it can exist at a submicrometer to micrometers space scale. The electrical properties of ordered water in a botanical platform are not known at the present time. They can be derived only from limited data in non biological platforms and inferences from the electrical properties of ice. These properties are 1) proton movement is governed by the Grotthuss mechanism, 2) there is insignificant movement of non-protonic ions and larger molecules through the ordered water, 3) proton movement is isotropic. Proposed Functionality of Callose/Ordered Water: Known locations of callose were examined theoretically to determine the functionality of a callose/ordered water duality. These locations were sieve plate pores, plasmodesmata and pollen tubes, stomatal guard cells, companion cell/sieve tube complex and micro and megasporocytes. Protonic Circuits: In a botanical context, protonic circuits at a single cell and supracellular level take the form of a proton microloop wherein callose/ordered water is one component in the loop. These circuits use both the enhanced proton mobility and the ion blocking ability of ordered water.

Phospholipids and glycolipids mediate proton containment and circulation along the surface of energy-transducing membranes.

Proton bioenergetics provides the energy for growth and survival of most organisms in the biosphere ranging from unicellular marine phytoplankton to humans. Chloroplasts harvest light and generate a proton electrochemical gradient (proton motive force) that drives the production of ATP needed for carbon dioxide fixation and plant growth. Mitochondria, bacteria and archaea generate proton motive force to energize growth and other physiologies. Energy transducing membranes are at the heart of proton bioenergetics and are responsible for catalyzing the conversion of energy held in high-energy electrons→electron transport chain→proton motive force→ATP. Whereas the electron transport chain is understood in great detail there are major gaps in understanding mechanisms of proton transfer or circulation during proton bioenergetics. This paper is built on the proposition that phospho- and glyco-glycerolipids form proton transport circuitry at the membrane's surface. By this proposition, an emergent membrane property, termed the hyducton, confines active/unbound protons or hydronium ions to a region of low volume close to the membrane surface. In turn, a von Grotthuß mechanism rapidly moves proton substrate in accordance with nano-electrochemical poles on the membrane surface created by powerful proton pumps such as ATP synthase.

Proton Transport in Imidazoles: Unraveling the Role of Supramolecular Structure.

The impact of supramolecular hydrogen bonded networks on dynamics and charge transport in 2-ethyl-4-methylimidazole (2E4MIm), a model proton-conducting system, is investigated by broadband dielectric spectroscopy, depolarized dynamic light scattering, viscometry, and calorimetry. It is observed that the slow, Debye-like relaxation reflecting the supramolecular structure in neat 2E4MIm is eliminated upon the addition of minute amounts of levulinic acid. This is attributed to the dissociation of imidazole molecules and the breaking down of hydrogen-bonded chains, which leads to a 10-fold enhancement of ionic conductivity.

Mechanisms for mechanical trapping of geologically sequestered carbon dioxide.

Carbon dioxide (CO2) sequestration in subsurface reservoirs is important for limiting atmospheric CO2 concentrations. However, a complete physical picture able to predict the structure developing within the porous medium is lacking. We investigate theoretically reactive transport in the long-time evolution of carbon in the brine-rock environment. As CO2 is injected into a brine-rock environment, a carbonate-rich region is created amid brine. Within the carbonate-rich region minerals dissolve and migrate from regions of high-to-low concentration, along with other dissolved carbonate species. This causes mineral precipitation at the interface between the two regions. We argue that precipitation in a small layer reduces diffusivity, and eventually causes mechanical trapping of the CO2. Consequently, only a small fraction of the CO2 is converted to solid mineral; the remainder either dissolves in water or is trapped in its original form. We also study the case of a pure CO2 bubble surrounded by brine and suggest a mechanism that may lead to a carbonate-encrusted bubble owing to structural diffusion.

Lewis-inspired representation of dissociable water in clusters and Grotthuss chains.

Proton transfer to and from water is critical to the function of water in many settings. However, it has been challenging to model. Here, we present proof-of-principle for an efficient yet robust model based on Lewis-inspired submolecular particles with interactions that deviate from Coulombic at short distances to take quantum effects into account. This "LEWIS" model provides excellent correspondence with experimental structures for water molecules and water clusters in their neutral, protonated and deprotonated forms; reasonable values for the proton affinities of water and hydroxide; a good value for the strength of the hydrogen bond in the water dimer; the correct order of magnitude for the stretch and bend force constants of water; and the expected time course for Grotthuss transport in water chains.

Hydroxide Hydrogen Bonding: Probing the Solvation Structure through Ultrafast Time Domain Raman Spectroscopy.

The mechanism of charge transport in aqueous media is critical in molecular, materials, and life sciences. The structure of the solvated hydroxide ion has been an area of some controversy. Polarization-resolved ultrafast time domain polarizability relaxation is used here to resolve the terahertz frequency Raman spectrum of hydroxide solutions. The measurements reveal the totally symmetric hydrogen-bond stretching (HO(-)···HOH) mode of the solvated hydroxide, permitting an experimental measurement of the bond force constant. The observed polarized Raman spectra are compared with those obtained from DFT calculations performed on HO(-)(H2O)n clusters. Good agreement between the observed frequency and the polarization dependence is found for the n = 3 or 4 clusters, particularly for those in which the solvating water molecules adopt a planar structure. The frequency of the symmetric stretch increases with concentration, consistent with an effect of ionic strength on either the H-bond or the structure of the cluster.