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High-temperature solid-oxide CO2 electrolysers enable high-efficiency conversion of electrical energy to valuable fuels and chemicals and as such facilitate a sustainable-energy technology. Conventional cermet-based fuel electrodes used in such solid-oxide cells (SOCs) like nickel–yttria-stabilized zirconia (Ni–YSZ) suffer from morphological degradation and destructive carbon deposition. In recent years, there has been an increasing interest in employing single-phase ceria-based fuel electrodes, which are known to exhibit excellent carbon deposition resistance. Under sufficiently reducing conditions, doped ceria (substituted with trivalent cations such as samarium or gadolinium to generate mobile oxygen vacancies) becomes a mixed ionic–electronic conductor, showing appreciable electronic conductivity. Here, we show for the first time stable high performance in CO2 electrolysis using a ceria-based SOC. The single full cell incorporating a 10 mol% gadolinium-doped ceria (GCO) fuel electrode delivers a current density as high as 1.51 A cm−2 at 800 °C during pure CO2 electrolysis, which is the best electrode performance reported to date among all-ceramic cathode materials. We demonstrate that the electrode performance in CO2 electrolysis is linked with the electronic conductivity, and hence, with the electronic charge carrier concentration in GCO. The results of the present work pave the way for development of robust, nickel-free SOCs for direct CO2 electrolysis. [ABSTRACT FROM AUTHOR]

Structural evolution, electrical conductivity, oxygen nonstoichiometry and oxygen transport properties of perovskite-type oxides La1−xCaxFeO3−δ (x = 0.05, 0.10, 0.15, 0.20, 0.30 and 0.40) are investigated. All investigated compositions exhibit, under ambient air, a phase transition from room-temperature orthorhombic (space group Pbnm) to rhombohedral (space group R3¯c) at elevated temperature. The transition temperature is found to decrease gradually from 900 °C for x = 0.05 to 625 °C for x = 0.40. Analysis of the data of oxygen nonstoichiometry obtained by thermogravimetry shows that under the given experimental conditions the Ca dopant is predominantly compensated by formation of electron holes rather than by oxygen vacancies. Maximum electrical conductivity under air is found for the composition with x = 0.30 (123 S cm−1 at 650 °C). Analysis of the temperature dependence of the mobility of the electron holes in terms of Emin–Holstein's theory indicates that small polaron theory fails for the compositions with high Ca contents x = 0.30 and x = 0.40. This is tentatively explained by the increased delocalization of charge carriers with increasing Ca dopant concentration. The oxygen transport properties of La1−xCaxFeO3−δ in the range 650–900 °C are evaluated using the electrical conductivity relaxation (ECR) technique. Combined with data of oxygen non-stoichiometry, the obtained results enable calculation of the oxygen vacancy diffusion coefficient and associated ionic conductivity. Both parameters increase with increasing Ca content in La1−xCaxFeO3−δ, while it is found that the effective migration barrier for oxygen diffusion decreases with decreasing oxygen vacancy formation enthalpy. [ABSTRACT FROM AUTHOR]

Layered Ruddlesden–Popper (RP) lanthanide nickelates, Lnn+1NinO3n+1 (Ln = La, Pr and Nd; n = 1, 2 and 3), are considered potential cathode materials in solid oxide fuel cells. In this study, the thermal evolution of the structure, oxygen nonstoichiometry, electrical conductivity and oxygen transport properties of La2NiO4+δ, Nd2NiO4+δ, La3Ni2O7−δ, La4Ni3O10−δ, Pr4Ni3O10−δ and Nd4Ni3O10−δ are investigated. Phase transitions involving a disruption of the cooperative tilting of the perovskite layers in the low-temperature structure thereby transforming it to a more symmetric structure are observed in several of the materials upon heating in air. Pr4Ni3O10−δ and Nd4Ni3O10−δ show no phase transition from room temperature up to 1000 °C. High density ceramics (>96%) are obtained after sintering at 1300 °C and (for n = 2 and n = 3 members) post-sintering annealing at reduced temperatures. Data for the electrical conductivity measurements on these specimens indicate itinerant behaviour of the charge carriers in the RP nickelates. The increase in p-type conductivity with the order n of the RP phase is interpreted as arising from the concomitant increase in the formal valence of Ni. The observations can be interpreted in terms of a simple energy band scheme, showing that electron holes are formed in the σx2−y2↑ band upon increasing the oxidation state of Ni. Electrical conductivity relaxation measurements reveal remarkable similarities between the surface exchange coefficients (kchem) of the different RP phases despite the differences in the order parameter n and the nature of the lanthanide ion. Calculation of the oxygen self-diffusion coefficients (Ds) from the experimental values of the chemical diffusion coefficients (Dchem), using the corresponding data of oxygen non-stoichiometry from thermogravimetry measurements, shows that these are strongly determined by the order parameter n. The value of Ds decreases almost one order of magnitude on going from the n = 1 members La2NiO4+δ and Nd2NiO4+δ to the n = 2 member La3Ni2O7−δ, and again one order of magnitude on going to the n = 3 members La4Ni3O10−δ, Pr4Ni3O10−δ and Nd4Ni3O10−δ. The results confirm that oxygen-ion transport in the investigated RP nickelates predominantly occurs via an interstitialcy mechanism within the rock-salt layer of the structures. [ABSTRACT FROM AUTHOR]

Structural evolution, electrical conductivity, oxygen nonstoichiometry and oxygen transport properties of perovskite-type oxides La0.6A0.4FeO3−δ (A = Ca, Sr, and Ba) were investigated. La0.6Ca0.4FeO3−δ (LCF64) and La0.6Sr0.4FeO3−δ (LSF64) show a phase transformation in air at elevated temperature, i.e., from orthorhombic (Pnma) to rhombohedral (R3¯c) and from rhombohedral to cubic (Pm3¯m), respectively, while La0.6Ba0.4FeO3−δ (LBF64) remains cubic over the entire temperature range from room temperature to 1000 °C. The different phase behaviour of the solids is interpreted to reflect the decreased tendency for octahedral tilting with increasing alkaline-earth-metal dopant ion radius. The electrical conductivity of LSF64 is 191 S cm−1 in air at 800 °C, decreasing to a value of 114 S cm−1 at a pO2 of 0.01 atm, and found over this pO2 range roughly twice as high as those of LCF64 and LBF64. Failure to describe the data of electrical conductivity using Holstein's small polaron theory is briefly discussed. Chemical diffusion coefficients and surface exchange coefficients of the materials in the range 650–900 °C were extracted from data of electrical conductivity relaxation. Data of oxygen nonstoichiometry was used to calculate the vacancy diffusion coefficients from the measured chemical diffusion coefficients. The calculated migration enthalpies are found to decrease in the order LCF64 (1.08 ± 0.04 eV) > LSF64 (0.95 ± 0.01 eV) > LBF64 (0.81 ± 0.01 eV). The estimated ionic conductivities of the materials, at 900 °C, are within a factor of 1.4. [ABSTRACT FROM AUTHOR]

High-temperature solid oxide cells afford chemical storage of renewable electricity. In particular, the electrochemical conversion of the greenhouse gas CO2 is attracting increasing interest to facilitate a sustainable energy technology. In this work, the effectiveness of perovskite-structured Sr2Fe1.4Mn0.1Mo0.5O6−δ (SFMM0.1) for use as cathode material for CO2 electrolysis has been investigated. Both parent Sr2Fe1.5Mo0.5O6−δ (SFM) and SFMM0.1 are found to be redox stable in air and 5% H2/Ar at 850 °C. Electrical conductivity relaxation experiments and first-principle calculations reveal that oxygen transport, CO2 adsorption and reduction kinetics are enhanced upon doping of SFM with Mn. The faster CO2 reduction kinetics observed for SFMM0.1 relative to SFM is reflected in a lower polarization resistance when both materials are used as single-phase electrodes in symmetrical cells. The polarization resistance in 50% CO/CO2 at 800 °C decreases from 1.15 Ω cm2 for SFM to 0.60 Ω cm2 for SFMM0.1. Under similar conditions, the polarization resistance decreases further to 0.50 Ω cm2 for a symmetrical cell with dual-phase SFMM0.1-SDC (samaria-doped ceria) electrodes. Unprecedented performance is demonstrated when SFMM0.1-SDC is integrated as the cathode in a solid oxide cell for electrolysis of pure CO2, achieving a current density of 1.35 A cm2 at 800 °C at 1.5 V. [ABSTRACT FROM AUTHOR]

Calcium manganite-based perovskite-type oxides hold promise for application in chemical looping combustion processes and oxygen transport membranes. In this study, we have investigated the structure, electrical conductivity and oxygen transport properties of perovskite-type oxides CaMn1−x−yTixFeyO3−δ. Distinct from previous work, data of high-temperature X-ray diffraction (HT-XRD) in the temperature range 600–1000 °C (with intervals of 25 °C) demonstrates that CaMnO3−δ (CM) transforms from orthorhombic to a mixture of orthorhombic and tetragonal phases between 875 °C and 900 °C. Rietveld refinements show the formation of a pure tetragonal phase at 975 °C and of a pure cubic phase at 1000 °C. Partial substitution of manganese by iron and/or titanium to yield CaMn0.875Ti0.125O3−δ (CMT), CaMn0.85Fe0.15O3−δ (CMF) or CaMn0.725Ti0.125Fe0.15O3−δ (CMTF) leads to different phase behaviours. While CMT remains orthorhombic up to the highest temperature covered by the HT-XRD experiments, CMF and CMTF undergo an orthorhombic → tetragonal → cubic sequence of phase transitions. Electrical conductivity relaxation measurements are conducted to determine the chemical diffusion coefficient (Dchem) and the surface exchange coefficient (kchem) of the materials. The results demonstrate that oxygen transport is hindered in the tetragonal phase, when occurring, which is attributed to a possible ordering of oxygen vacancies. The small polaron electrical conductivity of CM in the cited temperature range is lowered upon partial manganese substitution, by about 10% for CMF and up to half an order of magnitude for CMT and CMTF. [ABSTRACT FROM AUTHOR]

The influence of partial substitution of manganese by nickel or magnesium in Na0.44MnO2on cathode performance in sodium ion batteries has been investigated. Partial substitution changes the structure of parent Na0.44MnO2 from tunnel into layered P2-type, or a mixture of P2- and P3-type materials. Substitution smoothes the charge/discharge curves and may significantly improve capacity, albeit with a lower capacity retention relative to pristine Na0.44MnO2. In particular, high discharge capacities are found in the voltage range 2.0–4.2 V at 0.1 C rate for Na0.44Mn0.89Ni0.11O2 (193 mAh g−1) and Na0.44Mn0.89Mg0.11O2 (188 mAh g−1), with a capacity retention of 74% and 81%, respectively, after 70 cycles. [ABSTRACT FROM AUTHOR]

Electrical conductivity relaxation (ECR) and oxygen permeation measurements were conducted, at 750 °C, to assess the long-term oxygen transport characteristics of the mixed ionic–electronic conducting Pr2NiO4+δ with a K2NiF4 structure. The results show that the apparent values for the oxygen diffusion and surface exchange coefficients extracted from the data and the associated oxygen flux increase over 120 h by 1–2 orders of magnitude. The results of post-mortem X-ray diffraction analysis of the samples show partial to virtually complete decomposition of Pr2NiO4+δ under the conditions of the experiments to Pr4Ni3O10+δ, PrNiO3−δ, Pr6O11, and traces of NiO. Pulse 18O–16O isotopic exchange (PIE) measurements confirmed fast surface exchange kinetics of the higher-order Ruddlesden–Popper phase Pr4Ni3O10+δ and Pr6O11 formed upon decomposition. Additional factors related to the microstructure, however, need to be considered to explain the observations. [ABSTRACT FROM AUTHOR]

Fe2NiO4–Ce0.8Tb0.2O2−δ (NFO–CTO) composite membranes are of interest to separate oxygen from air. In this study, we investigate the influence of the catalytic activation of NFO–CTO membranes on the oxygen permeation rate. Specifically, the effect of activating porous NFO–CTO layers –coated on both sides of the dense NFO–CTO membrane – with Pr6O11 nanoparticles is studied. Measurements in the temperature range 850–700 °C revealed a 2–4 fold increase in the oxygen flux after coating a 30 μm-thick porous NFO–CTO layer on both membrane sides, and a 6–12 fold increase relative to the bare membrane after activating the porous layers coated on both sides of the membrane with Pr6O11 nanoparticles. No degradation of the oxygen fluxes was found in CO2-containing atmospheres. Pulse isotopic exchange measurements confirmed an increase in the oxygen surface exchange rate of more than one order of magnitude after dispersion of Pr6O11 nanoparticles on the surface of NFO–CTO composite powders. Electrochemical impedance spectroscopy measurements on symmetrical cells, using Gd-doped ceria (CGO) as the electrolyte and Pr6O11-activated NFO–CTO electrodes, showed a 10-fold decrease in the polarization resistance compared to non-infiltrated electrodes in air. Modification of porous layers by activation with Pr6O11 nanoparticles is considered a viable route to enhance the oxygen fluxes across composite membranes. [ABSTRACT FROM AUTHOR]

Bismuth has been doped into mixed ionic–electronic conducting La1.75Sr0.25NiO4+δ (LSN) with the 2D K2NiF4-type structure to evaluate its influence on various properties of the host material, which include its potential use as a SOFC cathode. X-ray powder diffraction indicates that LSN retains its tetragonal structure after doping with 5 mol% bismuth to form La1.65Bi0.1Sr0.25NiO4+δ (LSN–Bi). Bismuth doping profoundly lowers (by ∼150 °C) the sintering temperature of LSN. Both LSN and LSN–Bi show excellent compatibility with electrolytes yttria-stabilized zirconia (YSZ) and samaria-doped ceria (SDC) in terms of thermal expansion and chemical reactivity (＜900 °C). The electrical conductivity of both materials is metallic like and reaches values of 99.3 S cm−1 and 100.3 S cm−1 at 550 °C for LSN and LSN–Bi, respectively. The data from electrical conductivity relaxation (ECR) measurements demonstrate that the substitution of lanthanum by bismuth enhances the chemical diffusion coefficient (Dchem) and surface exchange coefficient (kchem) by factors of 2–3. The faster kinetics of oxygen transport exhibited by LSN–Bi relative to parent LSN is reflected by a lower polarization resistance of the former when the electrode performance of both materials is compared in symmetric cells. The corresponding values at 700 °C are 4.2 Ω cm2 and 0.61 Ω cm2 for LSN and LSN–Bi, respectively. High peak power densities are achieved (328 mW cm−2 and 131 mW cm−2 at 700 and 600 °C, respectively), when LSN–Bi is incorporated as the cathode in a fuel cell operated with humidified hydrogen as the fuel and air as the cathode gas. The material is considered a promising candidate for further study. [ABSTRACT FROM AUTHOR]

The oxygen surface exchange kinetics of mixed-conducting perovskite La0.58Sr0.4Co0.2Fe0.8O3−δ (LSCF) ceramics coated with a porous nano-particulate layer of either gadolinea (Gd2O3), ceria (CeO2) or 20 mol% Gd-doped ceria (GCO) was determined by electrical conductivity relaxation (ECR). The measurements were performed in the temperature range 700–900 °C, following pO2-step changes between 0.2 and 0.4 atm. The apparent value of the surface exchange coefficient, kchem, is found to vary with the loading amount and ionic conductivity of the coated phase whilst, as expected, the chemical diffusion coefficient Dchem remains invariant with the applied coating. Partial coverage of the LSCF surface with non-ionic conductive Gd2O3 or CeO2 lowers the value of kchem relative to that observed for bare LSCF, which is attributed to surface blocking effects. In contrast, partial coverage of LSCF with GCO electrolyte particles enhances the apparent value of kchem up to a factor of ∼6 compared to bare LSCF. The data of pulse isotope exchange (PIE) measurements show that the surface exchange reaction on bare LSCF is predominantly limited by dissociative adsorption of O2. Different mechanisms for the improved oxygen surface exchange kinetics after partially covering the LSCF surface with GCO are discussed. [ABSTRACT FROM AUTHOR]

In this study, the influence of long-term annealing at intermediate temperatures on oxygen transport of Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and 3 mol% Zr-doped BSCF (BSCF-Z3) ceramics with different grain sizes was studied by means of in situ electrical conductivity relaxation (ECR) measurements. Ceramics with different grain sizes in the range of 3–80 μm were obtained by varying the temperature and dwell time during sintering. For both compositions, the apparent values of the chemical diffusion coefficient Dchem and surface exchange coefficient kchem extracted from the data of ECR measurements are found to decrease with the time of annealing. The strongly correlated decreases in Dchem and kchem, being greater in magnitude at a lower grain size and temperature, are observed significantly more pronounced for BSCF than for BSCF-Z3. The results from microstructural analysis provide a clear rationale for the observations from ECR. High-resolution transmission electron microscopy (TEM) images recorded before and after annealing under pure oxygen at 700 °C for 13 d show excessive formation of hexagonal and plate-like lamellar precipitates at the grain boundaries and in the interior of grains of BSCF ceramics during the annealing process, whilst secondary phase formation is restricted solely to the grain boundary regions in BSCF-Z3. The possible importance of grain boundaries in determining the oxygen surface exchange kinetics is emphasized. [ABSTRACT FROM AUTHOR]