Oxide Acidity Modulates Structural Transformations in Hydrogen Titanates during Electrochemical Li-Ion Insertion

Hydrogen titanates (HTOs) form a diverse group of metastable, layered titanium oxides with an interlayer containing both water molecules and structural protons. We investigated how the chemistry of this interlayer environment influenced electrochemical Li+-insertion in a series of HTOs, H2TiyO2y+1·nH2O (y = 3, 4, and 5). We correlated the electrochemical response with the physical and chemical properties of HTOs using operando X-ray diffraction, in situ differential electrochemical mass spectroscopy, solid-state proton nuclear magnetic resonance, and quasi-elastic neutron scattering. We found that the potential for the first reduction reaction trended with the relative acidity of the structural protons. This mechanism was supported with first-principles density functional theory (DFT) calculations. We propose that the electrochemical reaction involves reduction of the structural protons to yield hydrogen gas and formation of a lithiated hydrogen titanate (H2-xLixTiyO2y+1). The hydrogen gas is confined within the HTO lattice until the titanate structure expands upon subsequent oxidation. Our work has implications for the electrochemical behavior of insertion hosts containing hydrogen and structural water molecules, where hydrogen evolution is expected at potentials below the hydrogen reduction potential and in the absence of electrolyte proton donors. This behavior is an example of electrochemical electron transfer to a nonmetal element in a metal oxide host, in analogy to anion redox.

Participation of electrochemically inserted protons in the hydrogen evolution reaction on tungsten oxides

Understanding the mechanisms by which electrodes undergo the hydrogen evolution reaction (HER) is necessary to design better materials for aqueous energy storage and conversion. Here, we investigate the HER mechanism on tungsten oxide electrodes, which are stable in acidic electrolytes and can undergo proton-insertion coupled electron transfer concomitant with the HER. Electrochemical characterization showed that anhydrous and hydrated tungsten oxides undergo changes in HER activity coincident with changes in proton composition, with activity in the order HxWO3·H2O > HxWO3 > HxWO3·2H2O. We used operando X-ray diffraction and density functional theory to understand the structural and electronic changes in the materials at high states of proton insertion, when the oxides are most active towards the HER. H0.69WO3·H2O and H0.65WO3 have similar proton composition, structural symmetry, and electronic properties at the onset of the HER, yet exhibit different activity. We hypothesize that the electrochemically inserted protons can diffuse in hydrogen bronzes and participate in the HER. This would render the oxide volume, and not just the surface, as a proton and electron reservoir at high overpotentials. HER activity is highest in HxWO3·H2O, which optimizes both the degree of proton insertion and solid-state proton transport kinetics. Our results highlight the interplay between the HER and proton insertion-coupled electron transfer on transition metal oxides, many of which are non-blocking electrodes towards protons.

Comparative Study of Proton Exchange in Tri- and Hexatitanates: Correlations between Stability and Electronic Properties

A hydrothermal method is considered to be convenient and is extensively used in preparing titanate architectures, but the intermediate and final products are complicated and variable. To date, it is accepted that intermediates are tri- and hexatitanates. Here, atomic structures, energetics, and correlations between stability and electronic properties of proton exchange in tri- and hexatitanates, i.e., Na_2-xH_xTi₃O₇ and Na_2-xH_xTi₆O₁₃, are investigated by first-principles calculations. We found that the bond length of Na-O bonds plays a significant role in determining the activity of tunnel oxygen atoms, while the proton substitution sites are closely related to the activity of tunnel O atom in titanates. As H⁺ concentration increases, the formation energy of Na_2-xH_xTi₃O₇ and Na_2-xH_xTi₆O₁₃ decreases first and then increases, suggesting that completely protonated titanates, i.e., H₂Ti₃O₇ and H₂Ti₆O₁₃, are unstable. However, we found that H⁺ substitution would take place even in an alkaline solution both for Na₂Ti₃O₇ and Na₂Ti₆O₁₃. With a decrease in the pH, the process of H⁺ exchange becomes more energetically favorable. Compared to Na₂Ti₃O₇, Na⁺ ions are more easily exchanged by H⁺ ions in Na₂Ti₆O₁₃ at the same pH value. We found that there is a strong correlation between stability and electronic properties during the Na⁺-H⁺ exchange process. Finally, hydrogen bonds are observed in H₂Ti₃O₇ and Na_2-xH_xTi₆O₁₃ complexes, which make them more stable than Na_2-xH_xTi₃O₇ complexes without H-bonds. All of these findings provide insight into understanding the geometry of possible intermediates in the preparation of titanates and suitable conditions for the synthesis of titanates.

Structure of the amorphous titania precursor phase of N-doped photocatalysts

Amorphous titania samples prepared by ammonia solution neutralization of titanyl sulphate have been characterized by chemical and thermal analyses, and with reciprocal-space and real-space fitting of wide-angle synchrotron X-ray scattering data. A model that fits both the chemical and structural data comprises small segments of lepidocrocite-type layer that are offset by corner-sharing as in the monoclinic titanic acids H2Ti n O2n+1·mH2O. The amorphous phase composition that best fits the combined chemical and scattering data is [(NH4)3H21Ti20O52]·14H2O, where the formula within the brackets is the cluster composition and the H2O outside the brackets is physically adsorbed. The NH4 + cations are an integral part of the clusters and are bonded to layer anions at the corners of the offset layers, as occurs in the alkali metal stepped-layer titanates. The stepped-layer model is shown to give a consistent mechanism for the reaction of aqueous ammonia with solid hydrated titanyl sulphate, in which the amorphous product retains the exact size and shape of the reacting titanyl sulphate crystals.

Theoretical Description, Synthesis, and Structural Characterization of β-Na0.33V2O5 and Its Fluorinated Derivative β-Na0.33V2O4.67F0.33: Influence of Oxygen Substitution by Fluorine on the Electrochemical Properties

The structure of β-Na_0.33V₂O_4.67F_0.33 has been investigated by both theoretical and experimental methods. It exhibits the same structure as that of the parent bronze β-Na_0.33V₂O₅. The partial substitution of oxygen by fluorine has little effect on the average structure and cell parameters, but the sodium environment changes significantly. Using DFT calculations, we determined the most stable positions of fluorine atoms in the unit cell. It was found that the partial replacement of oxide by fluoride takes mainly place in the coordination sphere of Na producing a shortening of the Na-anion bond lengths. We also analyzed the electronic properties based on density of states and Bader charge distribution. The crystallochemical situation of sodium ions in β-Na_0.33V₂O_4.67F_0.33 oxyfluoride, detected by both experimental and computational methods, affects its mobility with respect to the parent oxide. The higher ionicity in the Na coordination sphere of β-Na_0.33V₂O_4.67F_0.33 is related to a sodium ion diffusion coefficient, D_Na+, that is 1 order of magnitude lower (1.24 × 10^-13 cm² s^-1) than in the case of β-Na_0.33V₂O₅ (1.13 × 10^-12 cm² s^-1). Electrochemical sodium insertion/deinsertion properties of the oxyfluoride have been also investigated and are compared to the oxide. Insertion/deinsertion equilibrium potential for the same formal oxidation state of vanadium increases due to fluorination (for instance reduction of V^+4.3 occurs at 1.5 V in the oxide and at 1.75 V in the oxyfluoride). However, the capacity of Na_0.33V₂O_4.67F_0.33 at constant current is lower than in the case of β-Na_0.33V₂O₅ due to a less adequate morphology, a lower D_Na+, and a lower oxidation state of vanadium owing to the aliovalent O/F substitution.

Structural design of alkali-metal titanates: electrochemical growth of KxTi8O16, Na2+xTi6O13, and Li2+xTi3O7 single crystals with one-dimensional tunnel structures

Crystal growth of alkali-metal titanates was achieved by employing constant-voltage electrolysis of molten TiO₂/A₂MoO₄ (A = Li, Na, and K).

Lithium Hexastannate: A Potential Material for Energy Storage

The prediction of a new lithium compound, Li2Sn6O13, is made from a combined first‐principles and classical force‐field approach. The electronic, structural, and mechanical properties of monoclinic Li2SnO3, Li2Ti6O13, and Li2Sn6O13 are explored. The calculated results for the equilibrium lattice parameters are in agreement with the available experimental data. The thermodynamic stabilities of Li2Ti6O13 and Li2Sn6O13 are evaluated. Both compounds are demonstrated to be thermodynamically stable with standard molar formation enthalpies of −5553 and −6740 kJ mol−1, respectively. Reaction energies for delithiation of 6.41 and 6.90 eV atom−1 are also determined for Li2Sn6O13 and Li2Ti6O13, respectively. The predicted voltage of Li insertion/extraction process per Li+/Li is 1.6 V for Li2Sn6O13, comparable to its isostructural counterpart Li2Ti6O13. Electronic band structure calculations indicate the insulating character of Li2SnO3 with an indirect band gap of 4.4 eV, whereas both Li2Ti6O13 and Li2Sn6O13 appear to be semiconductor compounds with band gaps of 3.1 and 3.0 eV, respectivley. The energy barriers for Li+ migration amount to ≈0.5 eV for both materials. Elastic stiffness coefficients and bulk, shear and Young's moduli were also calculated. The Li2Sn6O13 derivative is mechanically stable and can be predicted to be a brittle compound that is more resistant to volume change than Li2Ti6O13. If the Li2Sn6O13 compound could experimentally be prepared by using ion exchange, it could potentially be an efficient material for anodes in lithium‐ion batteries.

Formation of Amorphous H3Zr2Si2PO12 by Electrochemical Substitution of Sodium Ions in Na3Zr2Si2PO12 with Protons

The sodium ions in Na₃Zr₂Si₂PO₁₂ (NASICON) were substituted with protons using an electrochemical alkali-proton substitution (APS) technique at 400 °C under a 5% H₂/95% N₂ atmosphere. The sodium ions in NASICON were successfully substituted with protons to a depth of <400 μm from the anode. Completely protonated NASICON, i.e., H₃Zr₂Si₂PO₁₂, was obtained to a depth <40 μm from the anode, although complete protonation of NASICON cannot be achieved by ion exchange in aqueous acid. H₃Zr₂Si₂PO₁₂ was amorphous, whereas the partially protonated NASICON was crystalline, and its unit cell volume decreased with an increase in the extent of substitution. Amorphous H₃Zr₂Si₂PO₁₂ was prepared by pressure-induced amorphization of the NASICON framework, in which an internal pressure of ∼3.5 GPa was induced by the substitution of large sodium ions with small protons during APS at 400 °C.

Applications of Novel Carbon/AlPO4 Hybrid-Coated H2Ti12O25 as a High-Performance Anode for Cylindrical Hybrid Supercapacitors

The hybrid supercapacitor using carbon/AlPO₄ hybrid-coated H₂Ti₁₂O₂₅/activated carbon is fabricated as a cylindrical cell and investigated against electrochemical performances. The hybrid coating shows that the conductivity for the electron and Li ion is superior and it prevented active material from HF attack. Consequently, carbon/AlPO₄ hybrid-coated H₂Ti₁₂O₂₅ shows enhanced rate capability and long-term cycle life. Also, the hybrid coating inhibits swelling phenomenon caused by gas generated as decomposition reaction of electrolyte. Therefore, the hybrid supercapacitor using carbon/AlPO₄ hybrid-coated H₂Ti₁₂O₂₅/activated carbon can be applied to an energy storage system that requires a long-term life.

Structure of H2Ti3O7and its evolution during sodium insertion as anode for Na ion batteries

Three protons in H₂Ti₃O₇shown by neutron powder diffraction and¹H-ssNMR.