Mechanistic origin of the time-dependence of the open-circuit voltage of a galvanic cell involving a ternary or higher compound.

It has previously been predicted [H.-I. Yoo and M. Martin, Phys. Chem. Chem. Phys., 2010, 12, 14699] and observed [E. Kim, et al., Solid State Ionics, 2013, 235, 22] that the open-circuit voltage U of a galvanic cell, involving a ternary or higher compound with more than one kind of mobile ionic carrier, is path- and time-dependent upon imposition or removal of the mobile components' chemical potential differences, in contradistinction to the cell involving a binary compound. This has been attributed [H.-I. Yoo and M. Martin, Phys. Chem. Chem. Phys., 2010, 12 14699; J.-Y. Yoon, et al., Solid State Ionics, 2012, 213, 22] to the decoupled redistributions of multiple mobile components or multi-fold relaxation. We hereby experimentally demonstrate with SrTi0.982Al0.018O3-Δ, known to have an appreciable water solubility depending on temperature, that introduction of a secondary ionic carrier H+ in addition to the native O2- indeed renders the otherwise time-independent U time-dependent; and that this phenomenon may, thus, be employed to probe the presence of a secondary ionic carrier, e.g., H+ in addition to the primary O2- in BaTi0.982Al0.018O3-Δ whose water solubility is yet to be known. The temporal behavior of U of SrTi0.982Al0.018O3-Δ subjected to the two fixed chemical potential differences, ΔµO and ΔµH, is precisely delineated in terms of two-fold relaxation of H and O, yielding their chemical diffusivity values, and consequently, the ambiguity with the EMF-method to determine the ionic transference numbers of a multinary compound is cleared away.

On the kinetic decomposition voltage of ternary oxides.

In a previous article the authors reported the kinetic unmixing and decomposition of the ternary oxide NiTiO3 by an externally applied voltage. It was found [J. Chun, et al., J. Appl. Phys., 117, 2015, 124504] that while kinetic unmixing occurred for all magnitudes of the applied voltage, kinetic decomposition occurred only above a certain threshold voltage U. The experimentally determined value of U, however, did not coincide with the thermodynamic expectation according to the definition by Gibbs (). In this work the kinetic decomposition voltage U of a ternary oxide ABOν is theoretically derived and compared with the experimental results. It turns out that U depends on the mobility ratio of the cations and agrees with observed results for the system of NiTiO3 within the error bounds.

Oxygen thermomigration in acceptor-doped perovskite.

The recent proposal of possible oxygen-thermomigration as a plausible mechanism for unipolar resistive switching of oxide memristors is now widely employed in modelling or simulating their memristive function on the grounds of the conventional picture that the mobile component O is always thermophobic, with its reduced heat-of-transport being equal to its migrational enthalpy (qO* = ΔHm > 0). At 1000 °C, we measured the thermomigration of mobile-component O in a prototype memristive perovskite, CaTi0.90Sc0.10O2.95+δ, across its near-stoichiometric regime (δ ≈ 0), where oxygen vacancy concentration is essentially fixed by doping acceptor impurities (i.e., ). It has been found that the reduced heat-of-transport of mobile O (qO*) varies systematically in the range of -2 < qO*/eV ≤ +2, as the composition varies from hypo-(δ < 0) to hyper-stoichiometry (δ > 0), exhibiting a sign reversal or thermomigration direction change from thermophilic (qO* < 0) to thermophobic (qO* > 0). This sign-reversal is attributed to the change in the electronic ambipolar company of from electrons to holes crossing the stoichiometric composition δ = 0. The numerical data for qO* together with the measurement details are reported.

Comment on "How to interpret Onsager cross terms in mixed ionic electronic conductors" by I. Riess, Phys. Chem. Chem. Phys., 2014, 16, 22513.

Here we show that the Onsager cross terms for ion-electron interactions are not an artifact, but the necessity to phenomenologically and completely describe the mass/charge transport of a mixed ionic-electronic conductor in terms of mobile charged components which are the only experimentally operable species. The use of an appropriate comprehensive defect model may help to reduce the cross terms (which depend on the choice of formal charge of the mobile defects), but it cannot obviate them if long-range Coulombic interactions are in action among the defects.

Isothermal transport properties and majority-type defects of BaCo(0.70)Fe(0.22)Nb(0.08)O(3-δ).

(Ba,Sr)(Co,Fe)O3-δ based mixed conducting oxides, e.g. (Ba0.5Sr0.5)(Co1-xFex)O3-δ and Ba(Co0.7Fe0.3-xNbx)O3-δ, are promising candidates for oxygen permeable membranes and SOFC cathodes due to their excellent ambipolar conductivities. Despite these excellent properties, however, their mass/charge transport properties have not been fully characterized and hence, their defect structure has not been clearly elucidated. Until now, the majority types of ionic and electronic defects have been regarded as oxygen vacancies and localized holes. Holes, whether localized or not, are acceptable as majority electronic carriers on the basis of the as-measured total conductivity, which is essentially electronic, and electronic thermopower. On the other hand, the proposal of oxygen vacancies as majority ionic carriers lacks solid evidence. In this work, we document all the isothermal transport properties of Ba(Co0.70Fe0.22Nb0.08)O3-δ in terms of a 2 × 2 Onsager transport coefficient matrix and its steady-state electronic thermopower against oxygen activity at elevated temperatures, and determine the valences of Co and Fe via soft X-ray absorption spectroscopy. It turns out that the ionic and electronic defects in majority should be oxygen interstitials and at least two kinds of holes, one free and the other trapped. Furthermore, the lattice molecule should be Ba(Co0.7Fe0.3-xNbx)O2+δ, not Ba(Co0.7Fe0.3-xNbx)O3-δ, to be consistent with all the results observed.

Isothermal Onsager matrices and acceptor size effect on mass/charge transport properties of La(1.9)A(0.1)NiO(3.95+δ) (A = Ca, Sr).

Isothermal Onsager transport coefficient matrices have been established experimentally for the systems of La1.9A0.1NiO3.95+δ doped with different-sized acceptors A = Ca(2+) and Sr(2+), in the range of oxygen activity -6 < log aO2 < 0 at 800°, 900° and 1000 °C, respectively. The oxygen self-diffusivity, oxygen defect (interstitial) diffusivity, hole mobility, and partial conductivities in the reversible electrode condition are thereby evaluated against defect concentration, and compared with those of the host La2NiO4+δ. It has been found that acceptor-doping suppresses the oxygen defect diffusivity by ca. an order of magnitude compared with the undoped host by increasing the migrational enthalpy by 0.3 eV or so. Hole mobility is in the range of 0.15 to 0.20 cm(2) V(-1) s(-1) for both the undoped and Ca-doped specimens, and 0.21-0.25 cm(2) V(-1) s(-1) for the Sr-doped specimens, with their temperature dependence indicating band conduction. The ionic charge-of-transport, corresponding phenomenologically to the number of holes dragged by an oxygen interstitial upon its transfer, appears to increase with increasing defect concentration and decreasing temperature in the range of 0 to 0.5. The ionic and electronic mobilities depending on the types of dopants are discussed in terms of dopant size.

A new high-energy cathode for a Na-ion battery with ultrahigh stability.

Large-scale electric energy storage is a key enabler for the use of renewable energy. Recently, the room-temperature Na-ion battery has been rehighlighted as an alternative low-cost technology for this application. However, significant challenges such as energy density and long-term stability must be addressed. Herein, we introduce a novel cathode material, Na1.5VPO4.8F0.7, for Na-ion batteries. This new material provides an energy density of ~600 Wh kg(-1), the highest value among cathodes, originating from both the multielectron redox reaction (1.2 e(-) per formula unit) and the high potential (~3.8 V vs Na(+)/Na) of the tailored vanadium redox couple (V(3.8+)/V(5+)). Furthermore, an outstanding cycle life (~95% capacity retention for 100 cycles and ~84% for extended 500 cycles) could be achieved, which we attribute to the small volume change (2.9%) upon cycling, the smallest volume change among known Na intercalation cathodes. The open crystal framework with two-dimensional Na diffusional pathways leads to low activation barriers for Na diffusion, enabling excellent rate capability. We believe that this new material can bring the low-cost room-temperature Na-ion battery a step closer to a sustainable large-scale energy storage system.

Experimental evidence of tunable space-charge-layer-induced electrical properties of nanocrystalline ceria thin films.

Fully dense nanocrystalline ceria films were successfully deposited on a MgO single crystal by pulsed laser deposition (PLD). The electrical conductivity of the nanocrystalline thin film was 20 times higher than that of the bulk sample. The activation energy of bulk ceria was 2.3 eV, whereas the activation energy of the nanocrystalline sample was only 1.2 eV. After post-annealing at 1273 K in which the grain size of the nanocrystalline thin film increased to ~400 nm, the electrical conductivity and activation energy of the film were changed similar to those of bulk. These unique electrical properties of the nano-crystalline thin-film can be attributed to the grain size effect, or more specifically, to the space charge layer (SCL) effect. Furthermore, the electrical conductivity of the nanocrystalline thin film became similar to that of the bulk in an extremely reducing atmosphere because of the unusual dependence of the SCL effect on the oxygen partial pressure.

Defect structure and Fermi-level pinning of BaTiO3 co-doped with a variable-valence acceptor (Mn) and a fixed-valence donor (Y).

Defect structures of BaTiO(3) and the like co-doped with variable-valence acceptors and donors are not clear particularly in transition from acceptor domination to donor domination with increasing oxygen activity. We have, thus, examined the electrical conductivity and thermoelectric power of BaTiO(3) co-doped with a variable-valence acceptor Mn(Mn(Ti)'', Mn(Ti)') and a fixed-valence donor Y(Y(ba)·) in different co-doping ratios (m(d)/m(a)) as functions of oxygen activity in the range of -20 < log a(O(2))≤ 0 at elevated temperatures of 900-1100 °C. Their systematic variations with m(d)/m(a) and log a(O(2)) are reported, and thereby defect structures of the co-doped BaTiO(3) depending on m(d)/m(a) are determined. It is found that for the co-doping ratio 1 < m(d)/m(a) < 2, the Fermi level is pinned at a few kT's around the deep level of Mn(Ti)'' across the otherwise p-type semiconducting log a(O(2))-region of Mn-singly doped BaTiO(3), and attributed to deep acceptor-shallow donor mutual compensation 2[Mn(Ti)''] + [Mn(Ti)'] ≈ [Y(ba)·], thus turning otherwise p-type semiconducting BaTiO(3) semi-insulating.

Oxidation kinetics of nitrogen doped TiO(2-δ) thin films.

The oxidation kinetics of nitrogen doped, oxygen deficient titanium dioxide thin films has been studied in atmospheres of pure oxygen or nitrogen at 500 °C, 550 °C, and 600 °C, respectively, by means of in situ optical spectroscopy. The thin films show high electronic absorbance in the visible and NIR region, accompanied by a red shift of the absorption edge of about 0.4 eV, e.g., from about 2.9 to 2.5 eV at 600 °C. The time dependent decrease of absorbance due to oxidation is found to follow a parabolic rate law. An activation energy of about 1.96 eV can be obtained from the temperature dependence of the parabolic oxidation rate constant. In the framework of a microscopic oxidation model, this energy barrier is attributed to the diffusion of titanium interstitials in the re-oxidized part of the thin films as a rate-determining process. In addition, an attempt is made to evaluate the kinetics of nitrogen release from the time dependent blue shift of the absorption edge during re-oxidation.

Compilation of all the isothermal mass/charge transport properties of the mixed conducting La2NiO(4+δ) at elevated temperatures.

It is known [H.-S. Kim and H.-I. Yoo, Phys. Chem. Chem. Phys., DOI:10.1039/c0cp00722f] that all the isothermal mass and charge transport properties of a mixed ionic electronic conductor compound can be universally represented by a 2 × 2 Onsager transport coefficient matrix. Furthermore, the three independent coefficients of the matrix can be determined from a simple relaxation experiment under the ion-blocking condition in association with the equation of state with respect to the nonstoichiometry or thermodynamic factor. By using this method, we compile the transport matrices at 800 °C, 900 °C and 1000 °C, respectively, on the system of La(2)NiO(4+δ) across its entire stability range, and calculate thereby its transport properties to compare with the literature values available. The interference effect between mobile oxygen ions and holes upon their transfer is discussed.

On the path-dependence of the open-cell voltage of a galvanic cell involving a ternary or multinary compound with multiple mobile ionic species under multiple chemical potential gradients.

It is well known that the open-cell voltage (U) of a galvanic cell involving a binary compound, or a multinary compound with a single kind of mobile ionic species, is a state property under a gradient of chemical potential of the mobile component. It is not so transparent, however, whether U is still a state property when involving a ternary or multinary compound with two or more kinds of mobile ions under multiple chemical potential gradients of those mobile components. We clarify this issue with a multinary oxide that conducts oxide ions, protons and electron holes and is exposed to the chemical potential gradients of both water and oxygen. We show that U is path- and history-dependent, and manifests itself along the diffusion paths of the two mobile components H and O under given boundary conditions.

Complete representation of isothermal mass and charge transport properties of mixed ionic-electronic conductor La(2)NiO(4+δ).

A variety of electrochemical applications of mixed ionic-electronic conductor compounds hinge on their variety of mass and charge transport properties, e.g., ionic conductivity, electronic conductivity, conductivity with suppressed transfer of either electrons or ions, chemical diffusivity, and permeability. All these transport properties of a compound may be represented completely by an Onsager transport coefficient matrix. Here we introduce a simple electrochemical method to measure all the Onsager coefficients for a mixed conductor oxide with known nonstoichiometry (δ), apply it to construct the Onsager matrix for the system of La(2)NiO(4+δ), and derive thereby its transport properties to compare with the literature values available.

Defect-chemical analysis of the nonstoichiometry, conductivity and thermopower of La(2)NiO(4+delta).

La(2)NiO(4+delta) is an oxygen excess compound (delta > 0) with oxygen interstitials (O(i) and holes (h*) in majority, which deviates positively from the ideal-dilute-solution behavior of defects. It was earlier attempted to interpret this positive deviation by taking into account the activity coefficients of both O(i) and h*. In this work, we examined the nonstoichiometry, electrical conductivity, and thermopower against oxygen activity in the entire stability range of the oxide at 800 degrees , 900 degrees and 1000 degrees C, respectively. It has been found that the positive deviation is ascribed essentially to the hole degeneracy and quantitatively described by using the Joyce-Dixon approximation of the Fermi-Dirac integral. Consequently evaluated are the effective density of states of the valence band (20.3

Experimental determination of the Onsager coefficients of transport for Ce(0.8)Pr(0.2)O(2-delta).

For a mixed oxide-ion and electron conducting oxide, with oxygen vacancies (V(O)) and electrons (e') or holes (h ) as charge carriers, a flux of (V(O)) (J(i)) can in principle be driven, not only directly by its own electrochemical potential gradient (inverted Delta eta(i)), but also indirectly by that of electrons (inverted Delta eta(e)), and vice versa for the flux of electrons (J(e)). It is common practice to assume that electrons and mobile ions migrate independently, despite the lack of experimental evidence in support of this. Here, all the Onsager coefficients, including the cross coefficients, have been measured for Ce(0.8)Pr(0.2)O(2-delta) within the a(O(2)) range 10(-21)-1 at 800 degrees C, using local ionic and electronic probes in a four-probe configuration. The cross coefficients of transport were found to be negligible in comparison to the direct coefficients in the a(O(2)) range 10(-21)-10(-4), but of the same order of magnitude as the direct coefficients for high a(O(2)) values (10(-2)-1). This is in contrast to the commonly used assumption that the two types of carriers migrate independently, i.e. that L(ie) = 0.

Hydration of Proton-conducting BaCe0.9Y0.1O3-δ by Decoupled Mass Transport.

Mass relaxation profile of a perovskite-type oxide, BaCe0.9Y0.1O3-δ, was studied to understand decoupled diffusion of oxygen and hydrogen species during hydration/dehydration. The mass relaxation measurements are performed by thermogravimetric analysis (TGA) under various humidity conditions (Dry, -3.0 ≤ log(pH2O/atm) ≤ -1.6) at a constant oxygen partial pressure (log(pO2/atm) = -1.00 ± 0.01). The decoupled ions participated in hydration/dehydration reactions were proven to be at different ratios from the result introduced by the 8R m function. The enthalpy and entropy of non-stoichiometric hydration reaction, which considers each ratio of charge-carrier species, were -144.7 ± 3.7 kJ/mol and -147.8 ± 3.2 J/mol · K, respectively.

Thermoelectric behavior of a mixed ionic electronic conductor, Ce(1-x)GdxO(2-x/2-delta).

The thermoelectric power of a mixed ionic electronic conductor oxide Ce(1-x)Gd(x)O(2-x/2-delta) (x=0.1) was measured as a function of oxygen activity in the range of -20

Electrical conductivity-defect structure correlation of variable-valence and fixed-valence acceptor-doped BaTiO(3) in quenched state.

Insulation resistance degradation of dielectric BaTiO(3) is expected to be closely correlated to its defect structure frozen in from elevated processing temperatures. For BaTiO(3), respectively doped with variable-valence (Mn(Ti)) and fixed-valence acceptors (Al(Ti)), their defect structures were frozen in by quenching at different equilibrium oxygen activities in the range of -18 < log a(O(2))< or = 0 at 1000 and 900 degrees C, respectively, and their electrical conductivities were measured against temperature in the range of 200 < or =T/K < or = 494 by impedance spectroscopy. Frozen-in defect structures were calculated and compared with the conductivity as measured in the quenched state. A close correlation has been confirmed between the bulk conductivity as measured in the quenched state and the frozen-in defect structure as calculated. The effects of variable- and fixed-valence acceptor impurities on the defect structure and electrical conductivity in the quenched state are highlighted in the light of hole trapping, and the charge transport behavior in the quenched state is discussed.

Electrical conductivity and oxygen nonstoichiometry of acceptor (Ga)-doped titania.

Electrical conductivity and oxygen nonstoichiometry (delta) have been measured, respectively, by a dc 4-probe technique and coulometric titrometry on the system of polycrystalline Ti0.99Ga0.01O1.995-delta against oxygen activity in the range of -20 < log aO2 < or = 0 at different temperatures in the range of 1073 < or =T/K < or = 1373. It is found that isothermal conductivity varies as sigma alpha aO2m with m approximately -1/4, -1/5, -1/4, +1/4, in order of increasing aO2, finally exhibiting an n-type (m = -1/4) to p-type (m = +1/4) transition crossing the stoichiometric composition (delta = 0), in perfect agreement with the oxygen nonstoichiometry variation. The electrical conductivity and oxygen nonstoichiometry isotherms are combined to evaluate the ionic partial conductivity and transference number, the intrinsic electronic excitation equilibrium constant, and eventually both mobilities of electrons and holes without employing any assumption at all. The partial molar enthalpy and entropy of the component oxygen are also evaluated as functions of nonstoichiometry.

Hydration and oxidation kinetics of a proton conductor oxide, SrCe(0.95)Yb(0.05)O(2.975).

The oxidation and hydration kinetics of a proton conductor oxide, SrCe(0.95)Yb(0.05)O(2.975), were examined via conductivity relaxation upon a sudden change of oxygen activity in a fixed water-activity atmosphere, and vice versa, in the ranges of -4.0 < log a(O(2)) < or = 0.01 and -5.0 < log a(H(2)O) < -2.0 at 800 degrees C. It was found that under an oxygen-activity gradient in a fixed water-vapor-activity atmosphere, the conductivity relaxation with time is monotonic with a single relaxation time (as usual), yielding a chemical diffusivity that is unequivocally that of the component oxygen. In a water-activity gradient in a fixed oxygen activity atmosphere, on the other hand, the conductivity relaxation appears quite unusual, exhibiting an extremum after an initial transient. The conductivity relaxation upon hydration or oxidation, in general, is quantitatively analyzed in terms of two apparent chemical diffusivities for component oxygen and hydrogen, respectively. The inner workings of hydration is discussed, and the as-evaluated chemical diffusivities are reported and compared with the conventional chemical diffusivity of water.