Bulk Oxygen Research Articles

Surface and Bulk Oxygen Kinetics of BaCo0.4Fe0.4Zr0.2-XYXO3-δ Triple Conducting Electrode Materials.

Triple ionic-electronic conductors have received much attention as electrode materials. In this work, the bulk characteristics of oxygen diffusion and surface exchange were determined for the triple-conducting BaCo0.4Fe0.4Zr0.2−XYXO3−δ suite of samples. Y substitution increased the overall size of the lattice due to dopant ionic radius and the concomitant formation of oxygen vacancies. Oxygen permeation measurements exhibited a three-fold decrease in oxygen permeation flux with increasing Y substitution. The DC total conductivity exhibited a similar decrease with increasing Y substitution. These relatively small changes are coupled with an order of magnitude increase in surface exchange rates from Zr-doped to Y-doped samples as observed by conductivity relaxation experiments. The results indicate that Y-doping inhibits bulk O2− conduction while improving the oxygen reduction surface reaction, suggesting better electrode performance for proton-conducting systems with greater Y substitution.

High-performance and CO2‑resistant cathode toward electrocatalytic oxygen reduction for solid oxide fuel cells: Doped ceria and SrCo0.7Nb0.1Ni0.2O3-δ composite

High oxygen reduction reaction (ORR) activity and sufficient stability are main challenges for high performance cathodes for solid oxide fuel cells (SOFCs) operated at medium temperature (550-700°C). In this study, ionic conducting Sm0.2Ce0.8O2-δ (SDC) are incorporated into perovskite SrCo0.7Nb0.1Ni0.2O3-δ (SCNN) cathode to synergistically improve its ORR activity as well as CO2 tolerance. A series of composite oxide, SCNN-xSDC (x=0, 20, 30, 40 and 50 wt %), were systematically evaluated as candidate cathodes for intermediate temperature SOFCs. Among all cathodes, SCNN-40SDC has the lowest polarization resistance (0.019 Ω cm2), only ∼30% of SCNN (0.066 Ω cm2) at 700°C. Introduction appropriate SDC boosts ORR activity of the composite cathodes, which is owing to enhanced oxygen surface exchange process, as evidenced by higher oxygen surface exchange coefficient (Kchem) and bulk diffusion coefficient (Dchem). Furthermore, the composite cathode shows much enhanced stability under CO2 poisoning, which was ascribed to the suppression of Sr segregation from SrCo0.7Nb0.1Ni0.2O3-δ and alleviated formation of carbonate in condition of SDC addition. Introducing ionic conductor Sm0.2Ce0.8O2-δ to construct a dual-phase cathode is a promising way to improve stability and ORR activity of perovskite oxides for SOFCs.

Facile Removal of Bulk Oxygen Vacancy Defects in Metal Oxides Driven by Hydrogen-Dopant Evaporation.

Oxygen vacancy is a common defect in metal oxides that causes appreciable damage to material properties and performance. Removing bulk defects of oxygen vacancy (VO) typically needs harsh conditions such as high-temperature annealing. Supported by first-principles simulations, we propose an effective strategy of removing VO bulk defects in metal oxides by evaporating hydrogen dopants. The hydrogen dopants not only lower the migration barrier of VO but also push VO away due to their repulsive interaction. The coevaporation mechanism was supported by a neural networks potential-based molecular dynamics simulation, which shows that the migration of hydrogen dopants from inside to surface at 400 K promotes the migration of VO as well. Our proof-of-concept study suggests an alternative and efficient way of modulating oxygen vacancies in metal oxides via reversible hydrogen doping.

A rational design of FeNi alloy nanoparticles and carbonate-decorated perovskite as a highly active and coke-resistant anode for solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are a kind of clean and efficient device to convert chemical energy in fuels into electricity. However, since anodes with high catalytic activity and carbon tolerance are still underdeveloped, the consequent serious performance degradation of the cells under operational conditions significantly confines their commercial applications. Here we propose a new strategy to remove carbon deposition by in-situ formation of alkali metal carbonate on the anode surface. A multi-phase composite anode, which is composed of an orthorhombic single perovskite main phase, a Ruddlesden-Popper (RP) layered perovskite second phase, and an in-situ exsolved FeNi alloy minor phase, is developed by one-step reduction of La0.65Li0.05Sr0.3Fe0.8Ni0.2O3-δ (LLSFN0.05) at a high temperature. The deficiencies of the RP phase and A-site caused by Li dopant would increase oxygen bulk diffusion, and FeNi nanoparticles would boost the catalytic activity. Moreover, when dealing with carbon fuel, lithium carbonate can be synthesized on the anode surface, serving as a good oxygen ion conductor and an efficient catalyst for coke removal by gasification. A single cell with our reduced LLSFN0.05 anode exhibited maximum power densities of 596, 467, and 424 mW cm−2 at 750 ℃ with H2, CO, and wet C2H6 as the fuel, respectively. In addition, the cells could have a long-term stable operation for over 80 h using CO as the fuel at 200 mA cm−2. This study provides a new material design strategy to develop a highly active and coke-resistant anode.

An experimental study on oxygen isotope exchange reaction between CAI melt and low-pressure water vapor under simulated Solar nebular conditions

Calcium-aluminum-rich inclusions (CAIs) are known as the oldest high-temperature mineral assemblages of the Solar System. The CAIs record thermal events that occurred during the earliest epochs of the Solar System formation in the form of heterogeneous oxygen isotopic distributions between and within their constituent minerals. Here, we explored the kinetics of oxygen isotope exchange during partial melting events of CAIs by conducting oxygen isotope exchange experiments between type B CAI-like silicate melt and 18O-enriched water vapor (PH2O = 5 × 10−2 Pa) at 1420 °C. We found that the oxygen isotope exchange between CAI melt and water vapor proceeds at competing rates with surface isotope exchange and self-diffusion of oxygen in the melt under the experimental conditions. The 18O concentration profiles were well fitted with the three-dimensional spherical diffusion model with a time-dependent surface concentration. We determined the self-diffusion coefficient of oxygen to be ∼1.62 × 10−11 m2 s−1, and the oxygen isotope exchange efficiency on the melt surface was found to be ∼0.28 in colliding water molecules. These kinetic parameters suggest that oxygen isotope exchange rate between cm-sized CAI melt droplets and water vapor is dominantly controlled by the supply of water molecules to the melt surface at PH2O < ∼10−2 Pa and by self-diffusion of oxygen in the melt at PH2O > ∼1 Pa at temperatures above the melilite liquidus (1420–1540 °C). To form type B CAIs containing 16O-poor melilite by oxygen isotope exchange between CAI melt and disk water vapor, the CAIs should have been heated for at least a few days at PH2O > 10−2 Pa above temperatures of the melilite liquidus in the protosolar disk. The larger timescale of oxygen isotopic equilibrium between CAI melt and H2O compared to that between H2O and CO in the gas phase suggests that the bulk oxygen isotopic compositions of ambient gas at ∼1400 °C in the type B CAI-forming region is preserved in the oxygen isotopic compositions of type B CAI melilite. Based on the observed oxygen isotopic composition, we suggest that a typical type B1 CAI (TS34) from Allende was cooled at a rate of ∼0.1–0.5 K h−1 during fassaite crystallization.

Building the Stable Oxygen Framework in High‐Ni Layered Oxide Cathode for High‐Energy‐Density Li‐Ion Batteries

High‐Ni layered oxide cathodes hold a great promise for fabricating high‐energy lithium‐ion batteries. However, the oxygen evolution during cycling is a crucial factor in the structure deterioration, potential change, and capacity decay of cathodes, limiting the commercial application of high‐Ni (Ni &gt; 0.9) layered oxides in batteries. Herein, we demonstrate a feasible approach to enhance the stability of oxygen framework, through the surface oxygen immobilization with yttrium and bulk oxygen stabilization with aluminum in high‐Ni layered oxides. As expected, benefiting from the oxygen‐stabilized framework, the bulk structure deterioration, and interfacial parasitic reaction are mitigated obviously during battery operation, along with the improved thermal stability of cathode. Correspondingly, the as‐prepared high‐Ni oxide delivers high reversible capacity, impressive cycle ability, and low potential polarization upon cycling. Such significant improvement on the electrochemical performance is primarily attributed to the strong oxygen affinities of both yttrium at the surface layer and aluminum in the bulk, which synergistically stabilizes the oxygen framework of high‐Ni oxide via raising the energy barrier for oxygen evolution. Therefore, building the stable oxygen framework is critical for enhancing the energy density output, cycle operation, and thermal stability of high‐Ni oxide cathodes.

Oxygen vacancies-enriched Mn3O4 enabling high-performance rechargeable aqueous zinc-ion battery

The development of high-energy cathode for rechargeable aqueous zinc-ion batteries (ZIBs) is highly attractive. However, the disproportionation effect of Mn2+ seriously affects the capacity retention of ZIBs during cycling. Defect engineering provides efficient methods to enhance conductivity and structural stability of active materials. Here, a novel in situ generated bulk oxygen deficient Mn3O4 nanoframes cathode for rechargeable aqueous ZIBs is reported, with high capacity and good electrochemical stability. The oxygen-deficient Mn3O4 spheres display an excellent gravimetric capacity of 325.4 mAh g−1 and a high energy density of 423 Wh kg−1 at a power density of 2257.2 W kg−1. Ex situ X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) characterization demonstrate the initial Mn3O4 is converted to ramsdellite MnO2 for insertion and extraction of H+ and Zn2+. Theoretical modeling reveal that numerous edge sites and oxygen vacancies act as preferential intercalation sites for the zinc ions, leading to a much greater capacity than that of defect-free Mn3O4. These results highlight the potentials of defect engineering as a strategy of improving the electrochemical performance of Mn3O4 in aqueous rechargeable batteries.

Boosting the Catalytic Performance of CeO2 in Toluene Combustion via the Ce-Ce Homogeneous Interface.

Catalytic combustion is an advanced technology to eliminate industrial volatile organic compounds such as toluene. In order to replace the expensive noble metal catalysts and avoid the aggregation phenomenon occurring in traditional heterogeneous interfaces, designing homogeneous interfaces can become an emerging methodology to enhance the catalytic combustion performance of metal oxide catalysts. A mesocrystalline CeO2 catalyst with abundant Ce-Ce homogeneous interfaces is synthesized via a self-flaming method which exhibits boosted catalytic performance for toluene combustion compared with traditional CeO2, leading to a ∼40 °C lower T90. The abundant Ce-Ce homogeneous interfaces formed by both highly ordered stacking and small grain size endow the CeO2 mesocrystal with superior redox property and oxygen storage capacity via forming various oxygen vacancies. Surface and bulk oxygen vacancies generate and activate crucial oxygen species, while interfacial oxygen vacancies further promote the reaction behavior of oxygen species (i.e., activation, regeneration, and migration), causing the splitting of redox property toward lower temperature. These properties facilitate aromatic ring decomposition, the important rate-determining step, thus contributing to toluene catalytic degradation to CO2. This work may shed insights into the catalytic effects of homogeneous interfaces in pollutant removal and provide a strategy of interfacial defect engineering for catalyst development.

First-principles-based microkinetic rate equation theory for oxygen carrier reduction in chemical looping

The reduction kinetics of oxygen carriers with reacting gases are typically modeled using shrinking-core-based models, in which, the elemental surface reaction mechanisms are simplified as only one global reaction, and the kinetic parameters are obtained by fitting with experimental data. In this study, a theoretical scheme is proposed to couple the elemental surface reaction mechanisms with the bulk diffusion of lattice oxygen, and a first-principle-based microkinetic rate equation theory is developed to model the reduction kinetics of oxygen carrier in chemical looping. The elemental surface reaction mechanisms are computed using quantum chemistry and are integrated into the mean-field microkinetic model, in which, oxygen bulk diffusion is also included. The elemental steps of gas adsorption, surface reaction, and surface species desorption are considered in the theory. The kinetic parameters used in the model are directly calculated by using transition state theory and density functional theory instead of fitting with experimental data. The developed theory is applied to predict the reduction kinetics of a CaMn0.775Ti0.125Mg0.1O2.9-δ perovskite oxygen carrier and validated to accurately predict the reduction kinetics by H2 and CO under a wide range of temperatures and gas concentrations. The effect of surface reaction and bulk diffusion on the overall reaction rate is analyzed and the rate-controlling step is discussed. When the bulk diffusion is considerably rapid, the interior oxygen concentration in the grain remains uniform throughout the reduction process and can be considered as a function of time only. Lumped parameter analysis is used to simplify the bulk diffusion of lattice oxygen without sacrificing accuracy. The developed microkinetic rate equation theory couples the surface reaction mechanism with the bulk diffusion, bridges the gap between atom-scale first-principles quantum chemistry calculations with particle-scale heterogeneous kinetics, and provides a theoretical framework for oxygen carrier reduction/oxidation kinetics, gas conversion mechanisms, and product selectivity in chemical looping.

Multifunctional oxygen vacancies in WO3–x for catalytic alkylation of C–H by alcohols under red-light

Surface reaction kinetics and light absorption properties of a photocatalyst are essential demands for efficiently solar to chemical energy converting. In this study, plasmonic WO3–x was firstly applied to photocatalytic alkylation of arenes under red light irradiation. The oxygen vacancies, both on the surface and in the bulk of WO3–x, allow abundant free electrons to increase carrier densities and support its LSPR using low energy photons. The surface oxygen vacancies have more functions: they not only release surface tungsten sites which ensure the chemisorption of alcohols due to the coordianation ability but also promote the activation of alcohols via an efficient transport of the holes on the neighbouring O sites to chemisorption alcohol species. In brief, the bulk oxygen vacancies provide abundant charges and the surface vacancies promote the bond adsorption and activation abilities, which ensure the high efficiency of photocatalytic alkylation of C–H.

Local Structure and Redox Properties of Amorphous CeO2-TiO2 Prepared Using the H2O2-Modified Sol-Gel Method.

Amorphous CeO2-TiO2 nanoparticles synthesized by the H2O2-modified sol-gel method were investigated in terms of the Ce-O-Ce and Ti-O-Ti linkage, local structure, and redox properties. The decrease in the crystallinity of CeO2-TiO2 by H2O2 addition was confirmed. The metal–oxygen linkage analysis showed the difference in size of the metal–oxygen network between crystalline CeO2-TiO2 and amorphous CeO2-TiO2 due to the O22− formed by H2O2. The local structure of CeO2-TiO2 was analyzed with an extended X-ray absorption fine structure (EXAFS), and the oscillation changes in the k space revealed the disordering of CeO2-TiO2. The decrease in Ce-O bond length and the Ce-O peak broadening was attributed to O22− interfering with the formation of the extended metal–oxygen network. The temperature-programmed reduction of the H2 profile of amorphous CeO2-TiO2 exhibited the disappearance of the bulk oxygen reduction peak and a low-temperature shift of the surface oxygen reduction peak. The H2 consumption increased compared to crystalline CeO2-TiO2, which indicated the improvement of redox properties by amorphization.

Cu-Ce0.8Sm0.2O2-δ anode for electrochemical oxidation of methanol in solid oxide fuel cell: Improved activity by La and Nd doping

CuCe0.8La0.1Sm0.1O2-δ and CuCe0.8Nd0.1Sm0.1O2-δ are studied as anode materials for solid oxide fuel cells with methanol as fuel. The oxygen surface exchange and bulk diffusion coefficients of Ce0.8Sm0.2O2-δ both increase with La and Nd doping. The CH3OH temperature-programmed surface reaction results show that the addition of La and Nd accelerates the chemical oxidation of CH3OH. Furthermore, compared with CuCe0.8Sm0.2O2-δ, the anodes with La and Nd show higher resistance to coking in CH3OH atmosphere. The Cu-based cermet anode exhibits a low catalytic activity for the electrochemical oxidation of H2, and a single cell supported by a Ce0.8Sm0.2O2-δ‑carbonate composite electrolyte with CuCe0.8Sm0.2O2-δ anode exhibits a maximum power density of 160 mW cm−2 at 650 °C using dry hydrogen as fuel. However, the maximum power density reaches 550 mW cm−2 when CH3OH is used as fuel, and further increases to 730 and 830 mW cm−2 with the addition of La and Nd in the anode, respectively. The results indicate that with the promotion of the oxygen activity, the Cu-based cermet is a promising anode material for solid oxide fuel cells using CH3OH as fuel.

Controllable synthesis of argentum decorated CuOx@CeO2 catalyst and its highly efficient performance for soot oxidation

In this paper, CuOx@Ag/CeO2 catalysts were synthesized by simple wet-chemical method and equal volume impregnation method. The obtained catalysts were subjected to soot temperature programmed oxidation (soot-TPO) activity tests and were further characterized by various techniques such as X-ray diffraction (XRD), transmission electron microscopy/high-resolution transmission electron microscopy (TEM/HR-TEM), N2 physisorption, X-ray photoelectron spectroscopy (XPS) and H2-temperature programmed reduction (H2-TPR). The results show that CuOx@Ag/CeO2 synthesized presents well controlled core-shell structures, with nano-cube like Cu2O as the core and Ag decorated polycrystalline CeO2 grafting layers as the shell. Such core-shell structured CuOx@Ag/CeO2 can successfully construct a secondary oxygen delivery channel (CuOx → CeO2 → Ag) to effectively transfer bulk oxygen of the catalyst to the soot, resulting in its excellent soot oxidation activity compared to CuOx@CeO2. The potential benefiting effect by Ag introduction over Cu@Ag/Ce can be concluded as: (i) pumping lattice oxygen and accelerating gaseous O2 dissociation to generate significantly increased active surface oxygen content; (ii) modulating a moderate surface oxygen vacancies concentration to maintain more highly active O2– species.

Enhanced photoelectrochemical water oxidation in Hematite: Accelerated charge separation with Co doping

Hematite (α-Fe2O3) is one of the most promising candidates for a photoanode for photoelectrochemical water splitting. However, it has low efficiency because of poor conductivity in the bulk and sluggish oxygen evolution (OER) kinetics on the surface. In this study, a Co-doped α-Fe2O3 photoanode was prepared by the hydrothermal method. It had increased charge separation efficiency and accelerated surface reaction kinetics simultaneously. At the optimal Co content, the photocurrent density of the Co-doped α-Fe2O3 increased by 23 times to 0.54 mA/cm2 at 1.23 VRHE in 1 M NaOH electrolyte; this increase was attributed to the improvement in carrier density and charge transfer behavior. Moreover, the cathodic shift of onset potential for Co-doped α-Fe2O3 reached 290 mV owing to the accelerated OER kinetics. This work reveals the key roles of Co doping and provides a suitable photoanode for solar hydrogen technology.

Spatial evolution characteristics of active components of copper-iron based oxygen carrier in chemical looping combustion

Reactivity of oxygen carriers (OCs) is a key issue in chemical looping technology. Exploring spatial evolution characteristics of active components and lattice oxygen migration during reduction reaction is of great importance in improving the reactivity of OCs. In this study, thermogravimetric analyzer (TGA) is used to control the reduction degree of OCs. Oxygen migration to the surface of CuFe2O4 (100) and CuO (111) is studied based on density functional theory (DFT). X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are used to investigate phase change and valence evolution of active components of the oxygen carriers. The results show that the ratios of lattice oxygen, chemical adsorbed oxygen and physical adsorbed oxygen at three points are very close in OCfresh. As the reduction reaction progresses, lattice oxygen is gradually consumed and the content of Cu0 and Fe2+ is increased. The closer to the edge point, the more lattice oxygen is consumed and the larger ratio of Cu0 and Fe2+. Moreover, there is no Fe0 in sample OC6.24% or OC9.19%, which is caused by the rapid migration and replenishment of lattice oxygen. The calculation results show that bulk oxygen migration to the surfaces of the OCs is exothermic and has a certain energy barrier.

Confined Ir single sites with triggered lattice oxygen redox: Toward boosted and sustained water oxidation catalysis

Confined Ir single sites with triggered lattice oxygen redox: Toward boosted and sustained water oxidation catalysis

Understanding the Role of Sr as a Dopant in a Double Perovskite Material for Solid Oxide Fuel Cells

LnBaCo2O5+d type of layered double perovskite materials has received much attention as solid oxide fuel cell cathode materials owing to their high oxygen ion concentration, high electronic conductivity, and catalytic activity towards oxygen reduction. In the present work, Nd-based double perovskites NdBa1-xSrxCo2O5+d (NBSCO, x= 0, 0.25 and 0.50) have been studied computationally.Plane-wave density functional theory-based calculations using VASP were performed to examine the electrochemical properties of NdBa1-xSrxCo2O5+d double perovskite material. Given the application of NBSCO material for SOFC cathodes, the bulk oxygen vacancy formation energies (Eov) have been calculated computationally for oxygen vacancy created in all possible planes (BaSr/O, Nd/O, and Co/O) and surface energies (γ) of the structure with different terminations (BaSr/Co, Nd/Co, Co/BaSr and Co/Nd) along (001) direction. Fig 1. and Fig. 2 shows the structure with oxygen vacancy created in various planes and with different terminal surfaces. Computed oxygen vacancy formation energies and surface energies in NBSCO are shown in table 1 and table 2. Nd/O plane observed to have the lowest oxygen vacancy formation energy among all the three possible planes in (001) direction of bulk NBSCO. This shows Nd/O plane to have high oxygen vacancy concentration. For x=0, 0.25, and 0.50, BaSr/O plane has the highest oxygen vacancy formation energies showing difficulty in oxygen vacancy creation in BaSr/O plane as compared to other planes. This suggested less oxygen anion diffusivity in the BaSr/O plane. However, doping one fourth and half of the Ba with Sr resulted in an improved bulk oxygen vacancy characteristic of the BaSr/O plane. Different energetics of different terminal surfaces shows the importance of terminations. The results of first-principles calculations for surface energies were analyzed and compared for different terminal surfaces. For NBCO material, low surface energies have been observed for Ba/Co termination. Due to lower surface energies, Ba ions have tendencies to segregate towards the surface. This is in accordance with the DFT based calculations of surface energies and molecular dynamics simulations for diffusivity and root mean square deviation values of other Ba containing LnBaCo2O5+d layered perovskites [1-5]. Doping the material with Sr has also shown a similar trend in surface energies of BaSr/Co terminal surface.

Impregnation of microporous SDC scaffold as stable solid oxide cell BSCF-based air electrode

Barium strontium cobaltite-ferrite (Ba1-xSrxCoyFe1-yO3-δ, BSCF) is a widely studied mixed ionic-electronic conductor material for air electrode in solid oxide cells (SOC). Despite having excellent features, due to fast oxygen surface exchange and oxygen bulk diffusion, it lacks long-term stability. Electrode/electrolyte thermal expansion coefficient (TEC) mismatch and structural instability at temperature lower than 900 °C are responsible for the increase of electrode polarization which becomes a crucial issue for the long-term stability.In this work, SOC stability was studied by adding a thin porous samarium-doped ceria (SDC) backbone on top of the dense SDC electrolyte. The porous SDC backbone was then infiltrated by precursor nitrates to obtain a Ba0.5Sr0.5Co0.8Fe3-δ composition. The SEM investigation showed a nano-sized BSCF-based layer covering the backbone structure. In addition, symmetrical cells were studied in the 400–700 °C temperature range and under anodic and cathodic polarization showing unexpected behavior associated to the electrode microstructure. The modified electrode synergistically enhanced ORR and OER by showing no oxygen vacancies clustering which induces a higher polarization resistance. Ageing procedure was performed for over 120 h at 600 °C under switched current load of ±0.2 A cm−2. The prepared system showed high stability coupled with remarkable electrocatalytic performance and good mechanical properties.

Enhanced Electrochemical Performance of the Fe-Based Layered Perovskite Oxygen Electrode for Reversible Solid Oxide Cells

Reversible solid oxide cells (RSOCs) present a conceivable potential for addressing energy storage and conversion issues through realizing efficient cycles between fuels and electricity based on the reversible operation of the fuel cell (FC) mode and electrolysis cell (EC) mode. Reliable electrode materials with high electrochemical catalytic activity and sufficient durability are imperatively desired to stretch the talents of RSOCs. Herein, oxygen vacancy engineering is successfully implemented on the Fe-based layered perovskite by introducing Zr4+, which is demonstrated to greatly improve the pristine intrinsic performance, and a novel efficient and durable oxygen electrode material is synthesized. The substitution of Zr at the Fe site of PrBaFe2O5+δ (PBF) enables enlarging the lattice free volume and generating more oxygen vacancies. Simultaneously, the target material delivers more rapid oxygen surface exchange coefficients and bulk diffusion coefficients. The performance of both the FC mode and EC mode is greatly enhanced, exhibiting an FC peak power density (PPD) of 1.26 W cm-2 and an electrolysis current density of 2.21 A cm-2 of single button cells at 700 °C, respectively. The reversible operation is carried out for 70 h under representative conditions, that is, in air and 50% H2O + 50% H2 fuel. Eventually, the optimized material (PBFZr), mixed with Gd0.1Ce0.9O2, is applied as the composite oxygen electrode for the reversible tubular cell and presents excellent performance, achieving 4W and 5.8 A at 750 °C and the corresponding PPDs of 140 and 200 mW cm-2 at 700 and 750 °C, respectively. The enhanced performance verifies that PBFZr is a promising oxygen electrode material for the tubular RSOCs.

Universal alignment of surface and bulk oxygen levels in semiconductors

Oxygen and hydrogen are the two most important impurities in semiconductors because of their ubiquitous presence in growth and device processing environments, and consequently, their incorporation strongly influences electronic and electrical properties. Therefore, a deeper understanding of the interaction of these species with the semiconductor surface and bulk defects is necessary for enabling the development of devices based on them, such as photovoltaic and photocatalytic systems and fuel cells. It is shown here, through the analysis of the reported surface work function values and substitutional bulk O-defect energies, that the surface Fermi level of semiconductors with physisorbed O2 lies universally at approximately −5.1 eV below the vacuum level. Similarly, the results show that the energy of substitutional bulk O-related amphoteric defects incorporated during the crystal growth also has a universal energy of ∼−5.0 eV with respect to the vacuum level for most semiconductors investigated. It is shown that the process of “surface transfer doping” involving an adsorbed water film on the semiconductor surface is likely responsible for the universal alignment of oxygen levels.