Protonic Defects Research Articles

Elucidating the Proton Transport Pathways in Liquid Imidazole with First-Principles Molecular Dynamics.

Imidazole is a promising anhydrous proton conductor with a high conductivity comparable to that of water at a similar temperature relative to its melting point. Previous theoretical studies of the mechanism of proton transport in imidazole have relied either on empirical models or on ab initio trajectories that have been too short to draw significant conclusions. Here, we present the results of multiple time-step ab initio molecular dynamics simulations of an excess proton in liquid imidazole reaching 1 ns in total simulation time. We find that the proton transport is dominated by structural diffusion, with the diffusion constant of the proton defect being ∼8 times higher than that of self-diffusion of the imidazole molecules. By using correlation function analysis, we decompose the mechanism for proton transport into a series of first-order processes and show that the proton transport mechanism occurs over three distinct time and length scales. Although the mechanism at intermediate times is dominated by hopping along pseudo-one-dimensional chains, at longer times the overall rate of diffusion is limited by the re-formation of these chains. These results provide a more complete picture of the traditional idealized Grotthuss structural diffusion mechanism.

Water-Assisted Proton Transport in Confined Nanochannels

Hydrated excess protons under hydrophobic confinement are a critical component of charge transport behavior and reactivity in nanoporous materials and biomolecular systems. Herein, excess proton confinement effects are computationally investigated for sub-2 nm hydrophobic nanopores by varying the diameters (d = 0.81, 0.95, 1.09, 1.22, 1.36, 1.63, and 1.90 nm), lengths (l ∼3 and ∼5 nm), curvature, and chirality of cylindrical carbon nanotube (CNT) nanopores. CNTs with a combination of different diameter segments are also explored. The spatial distribution of water molecules under confinement is diameter-dependent; however, proton solvation and transport are consistently found to occur in the water layer adjacent to the pore wall, showing an “amphiphilic” character of the hydrated excess proton hydronium-like structure. The proton transport free energy barrier also decreases significantly as the nanopore diameter increases and proton transport becomes almost barrierless in the d > 1 nm nanopores. Among the nanopores studied, the Zundel cation (H5O2+) is populated only in the d = 0.95 nm CNT (7,7) nanopore. The presence of the hydrated excess proton and K+ inside the CNT (7,7) nanopore induces a water density increase by 40 and 20%, respectively. The K+ transport through CNT nanopores is also consistently higher in the free energy barrier than proton transport. Interestingly, the evolution of excess protonic charge defect distribution reveals a “frozen” single water wire configuration in the d = 0.81 nm CNT (6,6) nanopore (or segment), through which hydrated excess protons can only shuttle via the Grotthuss mechanism. Vehicular diffusion becomes relevant to proton transport in the “flat” free energy regions and in the wider nanopores, where protons do not primarily shuttle in the axial direction.

Electronic Transport within Proton-Conducting Ceramics and Its Effect on Faradaic Efficiency of High-Temperature Water Electrolysis for Hydrogen Production

Proton conducting ceramics are used as electrolyte materials for protonic ceramic electrolysis cells to produce hydrogen through water electrolysis at high or intermediate temperatures. The widely applied barium-cerium-yttrium-zirconium oxides have been well studied as proton conductors for fuel cell applications, which present a promise in commercialization. In water electrolysis operating conditions electronic leakage within electrolyte can be observed, which reduces the Faradaic efficiency (FE) of hydrogen production. To address this electronic leakage problem, the intrinsic mechanism behind it should be understood. In this work, the defect chemistry in barium cerate and barium zirconate doped with yttrium is comparatively studied in terms of proton, electron and hole defect concentration/conductivity, and respective distribution profile within electrolyte, where electronic and protonic current density are calculated under electrolysis working conditions such as steam concentration, oxygen partial pressure, and over-potential. In addition, the conduction behaviors of three typical proton-conducting ceramics: BaCe0.7Zr0.1Y0.1Yb0.1O3-δ, BaCe0.4Zr0.4Y0.1Yb0.1O3-δ and BaZr0.8Y0.2O3-δ are experimentally investigated under different oxygen partial pressures and temperatures to validate with the prior defect equation derivation. The results indicated that high FE can be achieved under optimized conditions with reduced temperatures, implying numerous research opportunities in materials R&D of protonic ceramic electrochemical cells (PCECs) as well as the feasibility for widespread market adoption.

Azole structures influence fuel cell performance of phosphoric acid-doped poly(phenylene oxide) with azoles on side chains

We synthesized three types of polymer electrolyte membranes (PEMs) by functionalizing poly(phenylene oxide) with triazole, benzimidazole and imidazole groups, respectively on the side chains. The effect of azole structures on the fuel cell performance was investigated by analyzing and comparing the PEM properties at the sufficient PA doping level. We reveal for the first time that PEM performance is influenced by azole structures (pKa and steric hindrance). The higher pKa of imidazole was advantageous to the formation of protonic defects, enhancing the proton conduction. However, the steric hindrance of bulky benzimidazole interrupted acid-base interaction and the formation of protonic defects. The imidazole-containing membrane (XIMPPO) showed the anhydrous proton conductivity 130 mS/cm at 150 °C and the triazole-containing membrane (XTAPPO) and the benzimidazole-containing membrane (XBIPPO) showed 114 mS/cm and 102 mS/cm, respectively. Imidazole was also the most effective for the fuel cell performance, showing the peak power density of 0.20 W/cm2 and XTAPPO and XBIPPO showed 0.16 W/cm2 and 0.17 W/cm2, respectively.

Computational investigation of Zn-doped and undoped SrEu2Fe2O7 as potential mixed electron and proton conductors.

Understanding the electrode properties at the atomistic level is of great benefit to the evaluation of electrode performance and design of better electrode materials in solid oxide fuel cells. In this work, density functional theory (DFT) calculations are employed to investigate the formation and conducting behaviors of oxygen vacancies and proton defects in Ruddlesden–Popper oxide SrEu2Fe2O7 (SEFO), which has been experimentally characterized as a promising cathode. The calculation results suggest both oxygen vacancies and proton defects can be formed in SEFO, and especially, the formation of these defects is largely dependent on oxygen sites in the special crystal structure with alternative stacking of rock-salt layers and double-layered perovskite slabs. The oxygen vacancies within the perovskite slabs have very low formation energies, but demonstrate high energy barriers for migration and low hydration properties; while in the case of those in the rock salt layers, it’s contrary. Interestingly, protons have similar migration abilities in the perovskite slabs and rock salt layers. And therefore, increasing the vacancy concentration of the rock salt layer is beneficial to increase the concentration of proton defects and to improve the proton conductivity. DFT calculations also indicate that substituting Zn for Fe in SEFO can largely depress the oxygen vacancy formation energy, which helps to increase the concentration of both defects. Importantly, the energy barriers for migration of both oxygen ions and protons are barely enhanced, implying a negligible trapping effect of the Zn dopant.

The effect of grain size on the hydration of BaZr0.9Y0.1O3-δ proton conductor studied by ambient pressure X-ray photoelectron spectroscopy.

Three BaZr0.9Y0.1O3-δ (BZY10) pellets were prepared using different sintering processes, resulting in samples with different grain sizes, from 0.3 to 5 microns. Ambient pressure X-ray photoelectron spectra were recorded in argon, steam and oxygen atmospheres (100 mTorr) in the 300-500 °C temperature range. Deconvolution of O 1s peaks reveals 4 distinct contributions: sub-surface lattice oxide, termination layer oxides, OH- and gas-phase steam in wet environments. The OH- contribution of the O 1s peak includes sub-surface incorporation of protonic defects in the lattice related to hydration as well as surface hydroxylation and molecular water adsorption. The OH- concentration increases with grain size and with decreasing the analysis depth. These results suggest that grain boundaries associated with the larger grains adsorbed water more effectively. Thus, larger grains, which increase proton conductivity in BZY10, may also enhance catalytic activity for carbonaceous fuel oxidation by facilitating increased hydration and surface carbon removal.

Influence of hydration and dopant ionic radius on the elastic properties of BaZrO3

The influence of point defects (acceptor dopants, oxygen vacancies, protonic defects) on the macroscopic elastic properties of acceptor-doped BaZrO3 ceramics is investigated. Ultrasonic pulsed echo time of flight measurements are used to study the impact of dopant size, concentration and degree of hydration. Ceramics of BaXxZr1-xO3-x/2+δH2δ with X = Y, Sc and 0.05 ≤ x ≤ 0.2 were prepared by solid state reactive sintering with NiO sintering aid, added to achieve mechanical robustness sufficient for high-degree (58–71%) hydration without disintegration. Introduction of the dopants causes linear decrease in the Young's and shear moduli. By comparing the rate of decrease upon doping with Y3+ and Sc3+, the contribution of the lattice expansion was separated from the contribution of vacancy formation. Dissociative water incorporation also decreases the elastic moduli, however, for the case of Sc-doping the effect of hydration on the elastic moduli is much larger. All experimental data agree well with predictions by ab initio calculations.

Impact of Proton and Electron Irradiation-Induced Defects on the Dark Current of GaAs Solar Cells

The same amount of non-ionizing energy is deposited in GaAs solar cells through 1 MeV proton and electron radiation at specific fluence values. The defects created are detected and characterized via temperature-dependent dark I–V analysis, and the energy levels are correlated to trap states observed via admittance spectroscopy. A remarkable difference is observed between the defect energy levels introduced in the proton and electron cases: in the former, the recombination centers lie around the mid-gap position, while in the latter they are spread over a wider energy range in the band-gap. This induces a profound difference in the degradation of the recombination current in the space-charge region. On the other hand, the degradation of the diffusion current in the neutral regions is found to be determined by the recombination velocities at the back and front hetero-interfaces of the solar cell. They depend only on the displacement damage dose and are independent of the particle type.

Hiding in the Crowd: Spectral Signatures of Overcoordinated Hydrogen-Bond Environments.

Molecules with an excess number of hydrogen-bonding partners play a crucial role in fundamental chemical processes, ranging from anomalous diffusion in supercooled water to transport of aqueous proton defects and ordering of water around hydrophobic solutes. Here we show that overcoordinated hydrogen-bond environments can be identified in both the ambient and supercooled regimes of liquid water by combining experimental Raman multivariate curve resolution measurements and machine learning accelerated quantum simulations. In particular, we find that OH groups appearing in spectral regions usually associated with non-hydrogen-bonded species actually correspond to hydrogen bonds formed in overcoordinated environments. We further show that only these species exhibit a turnover in population as a function of temperature, which is robust and persists under both constant pressure and density conditions. This work thus provides a new tool to identify, interpret, and elucidate the spectral signatures of crowded hydrogen-bond networks.

Interaction of O2, H2O and H2 with proton-conducting oxides based on lanthanum scandates

An interaction of components from the gas phase, containing gaseous oxygen, hydrogen and water, with the La1–xSrxScO3–α oxides in the temperature range of 300–950 °С and partial pressure of 6.1–24.3 kPa 8.1–50.7 kPa has been studied using a high temperature thermogravimetric analysis. The effects of partial pressure of the gaseous oxygen, water and hydrogen on the apparent uptake level of protons with oxides has been found. The studies of incorporation processes of hydrogen from molecular hydrogen atmosphere into the structure of proton-conducting oxides based on strontium-doped lanthanum scandates were performed in the temperature range of 300–800 °C and hydrogen pressure of 0.2 kPa by means of the isotope exchange method with the equilibration of isotope composition in the gas phase. The protons and deuterons concentrations were determined for the La0.91Sr0.09ScO3–α oxide. The existence of proton defects in the structure of studied oxides after the exposure in H2O and H2-containing atmospheres was revealed using the 1Н NMR. The role of oxygen vacancies in the proton incorporation processes is considered in the present work.

Conduction and relaxation phenomena in barium zirconate ceramic in wet N2 environment

Proton conducting barium zirconate perovskite was prepared by conventional mixed oxide reaction route. The x-ray diffraction patterns proved the single phase cubic perovskite structure of the synthesized ceramic. SEM analysis of the ceramic pellets sintered at 1600 °C reveals the formation of dense microstructure with average grain size of 5–6 μm. Complex impedance spectroscopy study was performed over a wide range of frequency (10 Hz-5MHz) under wet N2 (pH2O = 0.031 atm) and dry N2 environment respectively varying the temperature from 300 °C to 600 °C. Two types of relaxation phenomena were evident from the impedance spectroscopic analysis, indicating the effect of grain and grain boundary to the overall resistance of the compositions. The conductivity due to grain and grain boundary contributions measured for the sample under wet N2 environment was found fairly one order higher in magnitude than that measured in dry N2 atmosphere respectively. The activation energy for conduction under wet N2 condition is found lower than that measured in dry conditions. The comparison of conductivity under two different environments reveals that additional charge species take part in conduction process under wet condition. These are attributed to the protonic defects which are incorporated into barium zirconate through extrinsic oxygen vacancies generated due to sintering at higher temperature.

Interaction of H2O, O2 and H2 with Proton Conducting Oxides Based on Lanthanum Scandates

The research uses the method of high-temperature thermogravimetric analysis to study the processes of interaction of the gas phase in the temperature range 300–950 °C in the partial pressure ranges of oxygen 8.1–50.7 kPa, water 6.1–24.3 kPa and hydrogen 4.1 kPa with La1–xSrxScO3–α oxides (x = 0; 0.04; 0.09). In the case of an increase in the partial pressure of water vapor at a constant partial pressure of oxygen (or hydrogen) in the gas phase, the apparent level of saturation of protons is shown to increase. An increase in the apparent level of saturation of protons of the sample also occurs with an increase in the partial pressure of oxygen at a constant partial pressure of water vapor in the gas phase. The paper discusses the causes of the observed processes. The research uses the hydrogen isotope exchange method with the equilibration of the isotope composition of the gas phase to study the incorporation of hydrogen into the structure of proton-conducting oxides based on strontium-doped lanthanum scandates. The concentrations of protons and deuterons were determined in the temperature range of 300–800 °C and a hydrogen pressure of 0.2 kPa for La0.91Sr0.09ScO3–α oxide. The paper discusses the role of oxygen vacancies in the process of incorporation of protons and deuterons from the atmosphere of molecular hydrogen into the structure of the proton conducting oxides La1–xSrxScO3–α (x = 0; 0.04; 0.09). The proton magnetic resonance method was used to study the local structure in the temperature range 23–110 °C at a rotation speed of 10 kHz (MAS) for La0.96Sr0.04ScO3–α oxide after thermogravimetric measurements in an atmosphere containing water vapor, and after exposures in molecular hydrogen atmosphere. The existence of proton defects incorporated into the volume of the investigated proton oxide from both the atmosphere containing water and the atmosphere containing molecular hydrogen is unambiguously shown. The paper considers the effect of the contributions of the volume and surface of La0.96Sr0.04ScO3–α oxide on the shape of the proton magnetic resonance spectra.

High-temperature properties of titanium-substituted yttrium niobate

Abstract

Review of proton- and oxide-ion-conducting perovskite materials for SOFC applications

In this review, the essential use of perovskites and their structural properties associated with proton and oxide-ion conductivity are explained specifically for applications in solid oxide fuel cells (SOFCs), electrolyser cells and so on. The number of oxygen vacancies and their distribution in the crystal structure play a crucial role in perovskites, which exhibit good protonic and oxide-ion conductivity. Atmospheric stability at the operating temperature is another important factor that decides the long-term usage of the materials. Here the properties are summarised in terms of stability, proton defects, ordering and cation substitution. A basic understanding of all of these properties will give a direction to synthesis of compounds with desired proton and oxide-ion conductivity for applications in renewable and sustainable energy devices. A certain group of perovskite oxides, when prepared as nanostructures, is specifically used in fuel cell electrodes for improved performance. A few trends and highlights of current scientific advances in nanoperovskites for energy applications are discussed. The high stability of the proton conductor Ba3Ca1⋅18Nb1⋅82O8⋅73and substitution of new dopants in this compound for SOFC devices are highlighted in this review.

Reactivity of solid BaCe0.9Y0.1O3-δ towards melted WO3

High-temperature impregnation of solid single-phase Y-doped BaCeO3 with melted WO3 was performed towards the evaluation of the possibility of formation of chemically stable material with relatively high electrical conductivity in this system. Impregnation was performed at 1470 °C as a function of time. Due to the relatively high reactivity in solid BaCe0.9Y0.1O3-δ - liquid WO3 system the reactive and the inner layers were distinguished and analysed. The obtained results clearly indicate the decomposition of Y-doped BaCeO3 in contact with melted WO3. X-ray diffraction method together with SEM observation, EDS analysis and EIS measurements results were applied to discuss the mechanism of this multi-stage process and the influence of the reaction time on materials structure and microstructure. As it was observed, partial incorporation of tungsten into the barium cerate structure can process not only on the surface of the materials but also in the inner layer of the synthesised materials. This donor-type doping of the perovskite structure finds reflection in both, materials electrical properties and chemical stability towards CO2/H2O. The decrease of proton defects concentration was observed together with the significant increase of the materials chemical stability in comparison to un-modified single-phase BaCe0.9Y0.1O3-δ. The calculated values of activation energy of conductivity confirm the change in the conduction mechanism. Also, the presence of an additional phase in the intergranular voids of doped BaCeO3 was postulated.

(Keynote) Nuclear and Electronic Quantum Effects in the Chemical Dynamics of Proton Defects in Hydrogen-Bonded Liquids

Proton defects in hydrogen bonded liquids exhibit a variety of complex structural and dynamical features ranging from rapid diffusion to broad spectral features and perturbed reactivity at interfaces. Accurately describing the chemical dynamics of proton defects requires the ability to simultaneously treat the quantum mechanical nature of the electrons, to describe the constant breaking and forming of chemical bonds, and the nuclei, to include the large zero point energy of the light hydrogen atoms. In this talk I will discuss our recent developments that allow for the efficient quantum mechanical treatment of the electrons and nuclei which now enable us to perform such simulations 100’s of times faster than was previously possible. I will then show how these advances can be used to elucidate transport mechanisms and spectroscopic features of these defects in aqueous and non-aqueous hydrogen bonded liquids in bulk and interfacial environments.

Investigation of proton conductivity in Sc and Yb co-doped barium zirconate ceramics

The oxygen deficient BaZr0.85Sc0.15−xYbxO3-δ (x = 0, 0.05, 0.10) ceramics were synthesized via conventional solid state reaction route. The Rietveld analysis of the x-ray diffraction profile proved that all the prepared compositions presents single phase cubic perovskite symmetry and space group. The actual oxygen occupancy of the materials has been derived from the Rietveld study. Thermo-gravimetric study of the pre-hydrated samples revealed a considerable mass loss, indicating more than 90% of the oxygen vacancies have been successfully filled by protonic defects upon hydration. FESEM images of the fractured surfaces of sintered ceramics showed dense microstructures with sub-micron sized grains and well resolved grain boundaries. The Nyquist plots differentiated the bulk, grain boundary and electrode response at lower temperatures (≤300 °C). The bulk conductivity of the Sc and Yb co-doped barium zirconate was higher compared to that of the respective Sc-doped perovskite oxide. The activation energy of bulk conductivity and total conductivity decreased with the increase in Yb content. The total conductivity of (4.01 × 10−3 Scm−1) at 600 °C has been achieved for BaZr0.85Sc0.10Yb0.05O3, suggesting the composition suitable for SOFC applications.

Constructing Long-Range Transfer Pathways with Ordered Acid-Base Pairs for Highly Enhanced Proton Conduction.

Acid-base pairs hold great superiority in creating proton defects and facilitating proton transfer with less or no water. However, the existing acid-base complexes fail in assembling into ordered acid-base pairs and thus cannot always take full advantage of the acid-base synergetic effect. Herein, polymer quantum dots with inherent ordered acid-base pairs are utilized and anchored on dopamine-coated graphene oxide, thus forming into long-range conducting pathways. The resultant building blocks ( nPGO) are integrated in a sulfonated poly(ether ether ketone) matrix to fabricate composite membranes. The constructed long-range transfer highways with ordered acid-base pairs impart to the composite membrane significantly enhanced proton conduction ability. Under the hydrated state, the composite membrane attains 91% increase over the control membrane in conductivity, and the single-cell fuel based on the membrane achieves 71% promotion in maximum power density. Under anhydrous conditions, more striking augment in conduction is observed for the composite membrane, reaching 7.14 mS cm-1, almost 10 times of the control membrane value (0.78 mS cm-1). Remarkably, such anhydrous proton conduction performance is even comparable to that of the composite membrane impregnated with ionic liquids, which is hard to realize with conventional fillers. Collectively, these results endow composite membranes great potential for applications in hydrogen-based fuel cells, sensors, and catalysis.

Structural manifestation of partial proton ordering and defect mobility in ice Ih.

Water ice is one of the most fundamental and well-studied molecular solids, which continues to provide surprising and useful insights into its emergent complexity as ever more precise experimental techniques are applied. Using one of the highest resolution neutron powder diffraction instruments in the world, I report a small but systematic distribution in the c/a axial ratios of D2O ice Ih below 160 K that depends upon both the preparation method and the thermal history of the sample. The general decrease in c/a on cooling is interpreted as a consequence of short-range partial ordering in the hydrogen-atom substructure: the synthesis-dependent variation then follows from there being a spectrum of relaxation times between samples formed by slow natural freezing, samples formed by very rapid quench-freezing and samples formed with a small amount of alkali hydroxide dopant, probably due to differences in the relative abundance of orientational versus ionic point defects.

Structures, Phase Fields, and Mixed Protonic-Electronic Conductivity of Ba-Deficient, Pr-Substituted BaZr0.7Ce0.2Y0.1O3-δ.

The BaZr0.7Ce0.2Y0.1O3-δ-BaPrO3-δ perovskite system, of interest for high-temperature electrochemical applications involving mixed protonic-electronic conductivity, forms a solid-solution with a wide interval of Ba substoichiometry in the range Ba(Ce0.2Zr0.7)1- xPr xY0.1O3-δ, 0 ≤ x ≤ 1. Structural phase transitions mapped as a function of temperature and composition by high-resolution neutron powder diffraction and synchrotron X-ray diffraction reveal higher symmetry for lower Pr content and higher temperatures, with the largest stability field observed for rhombohedral symmetry (space group, R3̅ c). Rietveld refinement, supported by magnetic-susceptibility measurements, indicates that partitioning of the B-site cations over the A and B perovskite sites compensates Ba substoichiometry in preference to A-site vacancy formation and that multiple cations are distributed over both sites. Electron-hole transport dominates electrical conductivity in both wet and dry oxidizing conditions, with total conductivity reaching a value of ∼0.5 S cm-1 for the x = 1 end-member in dry air at 1173 K. Higher electrical conductivity and the displacement of oxygen loss to higher temperatures with increasing Pr content both reflect the role of Pr in promoting hole formation at the expense of oxygen vacancies. In more reducing conditions (N2) and at low Pr contents, conductivity is higher in humidified atmospheres (∼0.023 atm pH2O) indicating a protonic contribution to transport, whereas the greater electron-hole conductivity with increasing Pr content results in lower conductivity in humidified N2 due to the creation of protonic defects and the consumption of holes.