MnO2上氧气第一个电子转移步骤的从头计算研究

采用裸露簇和嵌入簇模型, 对β-MnO₂ (001), (110), (111)三个晶面以及O₂在(110)晶面的单址吸附模式(Pauling和Griffths模式), 进行从头计算. 从β-MnO₂ (001), (110), (111)三个晶面的电子结构差异以及O₂在(110)晶面吸附的吸附能、几何结构、集居数以及净电荷数分析得到: (001), (110), (111)三个晶面中(110)晶面的催化活性最高, 其活性顺序为(110)＞(111)＞(001). 氧气在(110)晶面的吸附, Pauling和Griffths两种吸附模式均存在, 属于化学吸附中的离子吸附. 氧气与MnO₂固体间发生了单电子转移, 氧气得到电子被还原成O₂^-, 转移电子属于整个体系, 具有离域性.     

First-principles study of the atomic structures,electronic properties,and surface stability of BaTiO3 (001) and (011) surfaces

The atomic structures, electronic properties, and surface stability of (001) and (011) surfaces of BaTiO₃ are studied by first-principles calculations. Four differently terminated BaTiO₃ surfaces are considered in this study, including (001)-BaO, (001)-TiO₂, (011)-BaTiO, and (011)-O₂ terminations. The relaxations and rumplings are calculated and discussed, finding that the first layer relaxes inwards, while the second layer relaxes outwards for (001) and (110) surfaces. The data obtained for electronic properties show that O_2p states in (001)-BaO/(001)-TiO₂ termination shift to the lower/higher energy region, leading to a wide/narrow band gap. And the new produced surface states are observed in (011) surface terminations, which is mainly attributed to the supplied electrons from outermost surface atoms, even O atoms are oxidized. Furthermore, the (001) surface of BaTiO₃ is found to be more stable than the (011) surface according to the predicted surface energy which is 0.86 and 2.92 J/m² for (001) and (011) surfaces, respectively. Of which, BaO termination is predicted to be more likely to cleavage from the (001) direction than the TiO₂ termination is.     

Enhanced Intrinsic Catalytic Activity of λ‐MnO2 by Electrochemical Tuning and Oxygen Vacancy Generation

Chemically prepared λ‐MnO₂ has not been intensively studied as a material for metal–air batteries, fuel cells, or supercapacitors because of their relatively poor electrochemical properties compared to α‐ and δ‐MnO₂. Herein, through the electrochemical removal of lithium from LiMn₂O₄, highly crystalline λ‐MnO₂ was prepared as an efficient electrocatalyst for the oxygen reduction reaction (ORR). The ORR activity of the material was further improved by introducing oxygen vacancies (OVs) that could be achieved by increasing the calcination temperature during LiMn₂O₄ synthesis; a concentration of oxygen vacancies in LiMn₂O₄ could be characterized by its voltage profile as the cathode in a lithiun–metal half‐cell. λ‐MnO_2?z prepared with the highest OV exhibited the highest diffusion‐limited ORR current (5.5 mA cm^?2) among a series of λ‐MnO_2?z electrocatalysts. Furthermore, the number of transferred electrons (n) involved in the ORR was >3.8, indicating a dominant quasi‐4‐electron pathway. Interestingly, the catalytic performances of the samples were not a function of their surface areas, and instead depended on the concentration of OVs, indicating enhancement in the intrinsic catalytic activity of λ‐MnO₂ by the generation of OVs. This study demonstrates that differences in the electrochemical behavior of λ‐MnO₂ depend on the preparation method and provides a mechanism for a unique catalytic behavior of cubic λ‐MnO₂.     

EPMA of interfaces applied to the solid oxide fuel cell

Chemical interactions at the phase boundaries of materials applied for the solid oxide fuel cell (SOFC) have been studied by EPMA. The chemical reactivity at the interface of La_y-xSr_xMnO₃/ZrO₂-Y₂O₃ is dependent on the stoichiometry (y) and the Sr content (x) of the perovskite. Typical reaction products (zirconates) and a diffusion zone in the ZrO₂–Y₂O₃ have been observed. The extension of cation release (Mn) is related to the increasing chemical activity of Mn oxide in the perovskite by the Sr substitution for La. The wettability of the metal/oxide interface in the anode cermet (Ni/ZrO₂–Y₂O₃) has been found to be influenced by chemical reactions resulting from the applied reducing atmosphere with high carbon activity. The disintegration of ZrO₂–Y₂O₃ in contact with molten Ni or Ni-Ti and Ni-Cr alloys leads to the redeposition of Y₂O₃-enriched oxides and also to Zr-rich intermetallic compounds and eutectics.     

First‐principles calculations of the stability and electronic properties of the PbTiO3 (110) polar surface

The structural and electronic properties of five terminations of cubic lead titanate (PbTiO₃) (110) polar surface were investigated by first‐principles total‐energy calculations using a periodic slab model. On the PbTiO termination, an anomalous filling of conduction band was observed, whereas on the O₂ termination, two surface oxygen atoms formed a peroxo group, demonstrating that the electronic structures of the two stoichiometric terminations undergo significant changes with respect to bulk materials. However, for the three nonstoichiometric TiO‐, Pb‐, and O‐terminated surfaces, their electronic structures are very similar to bulk. Charge redistribution results for the five terminations confirmed that electronic structure and surface composition changes are responsible for their polarity compensation. However, which mechanism actually dominates the stabilization process depends upon energetic considerations. A thermodynamic stability diagram suggested that the two stoichiometric terminations are unstable; however, the three nonstoichiometric terminations can be stabilized in some given regions. Furthermore, this study indicates that the very different stabilities and surface states filling behaviors of the PbTiO₃ (110) polar surface with respect to SrTiO₃ and BaTiO₃ ones seem to originate from the partially covalent characteristics of Pb O pairs. © 2008 Wiley Periodicals, Inc. J Comput Chem 2009     

Fabrication and characterization of high performance cathode supported small-scale SOFC for intermediate temperature operation

A high performance small-scale solid oxide fuel cell supported by a microtubular cathode was successfully developed via the extrusion of a (La_0.8Sr_0.2)_0.97MnO₃ cathode support and subsequent surface coating with a (La_0.8Sr_0.2)_0.97MnO₃–Ce_0.9Gd_0.1O_1.95 activation layer followed by Sc₂O₃-doped ZrO₂ electrolyte and NiO–Ce_0.9Gd_0.1O_1.95 anode slurries. The cell was electrochemically evaluated in a humidified hydrogen (3% H₂O) atmosphere, and exhibited a stable open circuit voltage above 1.05 V in the temperature range from 550 to 750 °C. Maximum power densities of 46.5, 163.2 and 452.8 mW cm⁻² were generated at 550, 650 and 750 °C, respectively. The results indicate the realization of a stable and high performance cathode-supported micro SOFC.     

Molybdenum Substitution for Improving the Charge Compensation and Activity of Li2MnO3

Lithium‐rich layer‐structured oxides xLi₂MnO₃? (1?x)LiMO₂ (0<x<1, M=Mn, Ni, Co, etc.) are interesting and potential cathode materials for high energy‐density lithium ion batteries. However, the characteristic charge compensation contributed by O^2? in Li₂MnO₃ leads to the evolution of oxygen during the initial Li⁺ ion extraction at high voltage and voltage fading in subsequent cycling, resulting in a safety hazard and poor cycling performance of the battery. Molybdenum substitution was performed in this work to provide another electron donor and to enhance the electrochemical activity of Li₂MnO₃‐based cathode materials. X‐ray diffraction and adsorption studies indicated that Mo⁵⁺ substitution expands the unit cell in the crystal lattice and weakens the Li?O and Mn?O bonds, as well as enhancing the activity of Li₂MnO₃ by lowering its delithiation potential and suppressing the release of oxygen. In addition, the chemical environment of O^2? ions in molybdenum‐substituted Li₂MnO₃ is more reversible than in the unsubstituted sample during cycling. Therefore molybdenum substitution is expected to improve the performances of the Li₂MnO₃‐based lithium‐rich cathode materials.     

First‐principles study of the (001) surface of cubic PbTiO3

We present first‐principles calculations on the (001) surfaces of cubic PbTiO₃ with PbO and TiO₂ terminations. The cleavage energy, surface energy, surface grand potential, surface relaxation and surface electronic structure have been investigated by using the projector‐augmented wave method under generalized gradient approximation (GGA). The results show that surface energy of a TiO₂‐terminated surface is a little lower than that of a PbO‐terminated one, thus allowing both terminations to coexist. The PbO‐termination is stable in O‐ and Pb‐rich environments, while on the contrary, the TiO₂‐termination is stable in O‐ and Pb‐poor conditions. In addition, the surface rumpling S of a PbO‐terminated surface is slightly larger than that of a TiO₂‐terminated one. The relaxations dominantly take place on the outermost three layers, and an oscillatory (? + ?) damping (|Δd₁₂ | > | Δd₂₃ | > | Δd₃₄|) relaxation phenomenon appears for both terminations. The band gaps of both PbO‐ and TiO₂‐terminations are slightly lower than that of the bulk. Moreover, the DOS curves of each layer show that for the TiO₂‐termination, the top of the valence band of the first and third TiO₂ layers moves toward Fermi level. Copyright © 2008 John Wiley & Sons, Ltd.     

Effect of Bi0.75Y0.25O1.5 electrolyte additive in collector layer to properties of bilayer composite cathodes of solid oxide fuel cells based on La(Sr)MnO3 and La(Sr)Fe(Co)O3 compounds

The sintering features, electroconductivity, and electrochemical characteristics of bilayer electrodes with functional composite layers based on La(Sr)MnO₃ (LSM) and La(Sr)Fe(Co)O₃ with LSM collector layer and Bi(Y)O_1.5 (YDB) electrolyte additive in contact with Ce (Sm)O₂(SDC), La(Sr)Ga(Mg)O₃, and Zr(Sc)O₂ electrolytes were studied. YDB additive to the electrode collector layer was shown to produce a positive effect to the properties of the studied electrode systems. The maximum electrochemical activity and electroconductivity was observed for the electrodes with 5 wt % of YDB electrolyte additive in the collector layer. Thus, electroconductivity of electrodes is almost doubled and 100 mV cathode overvoltage current density is increased by 30% at the temperatures of 800 to 900°C and up to 10-fold at 650 to 700°C. The collector layer sintering temperature of bilayer electrodes can be reduced from 1150 to 1000°C without loss of electrochemical activity. The service life tests (about 1200 h) of composite electrodes with LSM2-SDC functional layer and 90% LSM2 + 10% YDB collector layer in contact with SDC electrolyte showed the time dependences of polarization resistance tending to saturation and described with damped exponent. Original Russian Text ? N.M. Bogdanovich, D.I. Bronin, G.K. Vdovin, I.Yu. Yaroslavtsev, B.L. Kuzin, 2009, published in Elektrokhimiya, 2009, Vol. 45, No. 4, pp. 486–494.     

Porous Mn2O3: A Low‐Cost Electrocatalyst for Oxygen Reduction Reaction in Alkaline Media with Comparable Activity to Pt/C

Preparing nonprecious metal catalysts with high activity in the oxygen reduction reaction (ORR) can promote the development of energy conversion devices. Support‐free porous Mn₂O₃ was synthesized by a facile aerosol‐spray‐assisted approach (ASAA) and subsequent thermal treatment, and exhibited ORR activity that is comparable to commercial Pt/C The catalyst also exhibits notably higher activity than other Mn‐based oxides, such as Mn₃O₄ and MnO₂. The rotating ring disk electrode (RRDE) study indicates a typical 4‐electron ORR pathway on Mn₂O₃. Furthermore, the porous Mn₂O₃ demonstrates considerable stability and a good methanol tolerance in alkaline media. In light of the low cost and high earth abundance of Mn, the highly active Mn₂O₃ is a promising candidate to be used as a cathode material in metal–air batteries and alkaline fuel cells.     

A new type of orthoborates: ASr4La3(BO3)6 (A = Li,Na)

Two new rare earth containing orthoborate crystals ASr₄La₃(BO₃)₆ (A = Li, Na) have been obtained by spontaneous nucleation from high-temperature melts of A₂O–SrO–La₂O₃–B₂O₃–AF. X-ray diffraction analyses show that they both crystallize in the rhombohedral space group R-3 with cell parameters of a = 12.309(7) Å, c = 9.316(7) Å and a = 12.4049(13) Å, c = 9.348(2) Å for the Li and Na compounds respectively. Similar to the large A′₆MM′(BO₃)₆ family, these compounds are all related to the structure of Sr₃Y(BO₃)₃ with La and Sr statistically occupy the Sr site, and the alkaline elements and remaining Sr enter the ordered Y1 and Y2 sites, which can be approximately represented as (La_2.91Sr_3.09)(La_0.09Sr_0.91)Li[B₆O₁₈] and (La_2.85Sr_3.15)(La_0.15Sr_0.85)Na[B₆O₁₈]. The characteristic of the structure is that the La/Sr and isolated BO₃ groups form a network with tunnels along the c-axis where the alkaline A and Sr ions alternatively reside. The optical transmission spectrum shows that the ultraviolet absorption edge of NaSr₄La₃(BO₃)₆ crystal is about 193 nm and Raman spectra reveal that both crystals possess sharp peaks at 930 cm⁻¹.     

Thermal processes in the systems with Li-battery cathode materials and LiPF6 -based organic solutions

Thermodynamic instability of positive electrodes (cathodes) in Li-ion batteries in humid air and battery solutions results in capacity fading and batteries degradation, especially at elevated temperatures. In this work, we studied thermal interactions between cathode materials Li₂MnO₃, xLi₂MnO₃ ^.(1???x)Li(MnNiCo)O₂,LiNi_0.33Mn_0.33Co_0.33O₂, LiNi_0.4Mn_0.4Co_0.2O₂, LiNi_0.8Co_0.15Al_0.05O₂ LiMn_1.5Ni_0.5O₄, LiMn(or Fe)PO₄, and battery solutions containing ethylene carbonate (EC) or propylene carbonate (PC), dimethyl carbonate (DMC) or ethylmethyl carbonate (EMC) and LiPF₆ salt in the temperature range of 40–400 °C. It was found that these materials are stable chemically and well performing in LiPF₆-based solutions up to 60 °C. The thermal decomposition of the electrolyte solutions starts >180 °C. The macro-structural transformations of cathode materials upon exothermic reactions were studied by transmission electron microscopy (TEM), X-ray difraction (XRD) and Raman spectroscopy. Differential scanning calorimetry (DSC) studies have shown that the exothermic reactions in the temperature range of 60–140 °C lead to partial decomposition of both the cathode material and electrolyte solution. The systems thus formed consisted of partially decomposed solutions and partially chemically delithiated cathode materials covered by reactions products. Thermal reactions terminate and this system reaches equilibrium at about 120 °C. It remains stable up to the beginning of the solution decomposition at about 180 °C. The increased content of surface Li₂CO₃ is found to significantly affect the thermal processes at high temperature range due to extensive exothermic decomposition at low temperatures.     

Pt adsorption on the PbTiO3(110) polar surface: a density functional theory study

The monolayer (ML) and submonolayer Pt on both terminations of PbTiO₃(110) polar surface have been studied by using density functional theory (DFT) with projector‐augmented wave(PAW) potential and a supercell approach. The most favored ML Pt arrangements on PbTiO and O₂ terminations are the hollow site and the short‐bridge site, respectively. By examining the geometries of different ML arrangements, we know that the dominant impetus for stability of the favored adsorption site for PbTiO termination is the Pt–Ti interaction (mainly from covalent bonding), while that for O₂ termination is the Pt–O interaction (mainly from ionic bonding). In addition, the appearance of the gap electronic states in the outermost layers of each termination indicates that a channel for charge transfer between adsorbed layer and substrate is formed. Moreover, the interface hybridization between Pt 5d and O 2p orbitals is also observed, especially for ML Pt on O₂ termination. The stability sequences for various arrangements of 1/2 ML Pt adsorption conform well with those of ML Pt adsorption, and the most stable arrangement is energetically more favorable than the corresponding ML coverage in the view of adsorption energy maximization. The behavior, i.e. the increase in adsorption energy with decrease in coverage, indicates that Pt? Pt interactions weaken those between Pt and the substrate. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of synthesis method on oxygen adsorption/desorption properties of La–Sr–Co–Fe–O perovskite-type oxide

There has been a growing interest in utilizing La–Sr–Co–Fe–O perovskite-type oxide for efficient high temperature oxygen adsorption applications and oxygen removal process. In this paper, we focus our attention on the analysis of the determinants of the synthesis methods of La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF) powders for the oxygen adsorption/desorption applications. To this aim, LSCF powders were successfully synthesized by different synthesis routes using polymerized complex and citrate methods. The effects of synthesis methods on the structure, particle size, specific surface area, oxygen adsorption/desorption kinetics, and oxygen uptake capacities of LSCF perovskite-type oxides were investigated. The oxygen adsorption/desorption capacities and kinetics of the LSCF oxides increase with increasing (1) the temperature from 700 to 900 °C and (2) the surface area observed at a given temperature. Collectively, the experimental observations suggest that particle sizes may play an important role in oxygen uptake capacities and adsorption/desorption kinetics.     

Theoretical insights into the effect of surface structure and anion ordering on the properties of SrTaO2N for photocatalytic water splitting

As one of the representative perovskite-type oxynitride photocatalysts, SrTaO₂N has the ability to split water in the visible-light region. It was found that the surface modification and interfacial design of SrTaO₂N-based materials are closely related to the photocatalytic activity, but the microscopic mechanisms of these experimental phenomena are not well understood. In this work, we have utilized density functional theory (DFT) calculations to investigate the effect of anion ordering and exposed terminations on the electronic structures, optical absorption, water adsorption and the mechanisms of water oxidation and reduction reactions of SrTaO₂N. Our results indicate that cis configurations are more stable than trans configurations. The anion ordering has an important effect on the band gap and optical absorption coefficient. The terminations with exposed Ta atoms are more stable and have bigger work functions than those with exposed Sr atoms possibly due to the bonding ionicity and surface dipoles. The dissociative adsorption of water is energetically more favorable than the molecular adsorption on most surfaces. The highly active sites of hydrogen evolution reaction (HER) are the exposed nonmetal atoms. Terminations with exposed Sr and N atoms have lower overpotentials (0.70–0.77 V) of oxygen evolution reaction (OER) than others. They are comparable to the calculated results of common photocatalysts, such as Co₃O₄ and TiO₂. This study sheds light on the relationship between the termination structure with different anion orders and the photocatalysis-related properties of SrTaO₂N at a molecular level, which provides guidance for constructing highly active photocatalytic materials.     

Synthesis and characterization of n=5, 6 members of the La4Srn−4TinO3n+2 series with layered structure based upon perovskite

Layered compounds have been synthesized and structurally characterized for the n=5 and 6 members of the perovskite-related family La₄Sr_n−4Ti_nO_3n+2 by combining X-ray diffraction and transmission electron microscopy. Their structure can be regarded as comprising [(La,Sr)₅Ti₅O₁₇] and [(La,Sr)₆Ti₆O₂₀] perovskite blocks joined by crystallographic shears along the a-axis, with consecutive blocks shifted by 1/2 [100]_p. The n=5 member is similar to the previously reported n=5 member of other A_nB_nO_3n+2-related series. The n=6 member, which has only been briefly reported in other systems previously, is also a well-behaved member of this A_nB_nO_3n+2 series.     

Importing Antibonding-Orbital Occupancy through Pd−O−Gd Bridge Promotes Electrocatalytic Oxygen Reduction

The active-site density, intrinsic activity, and durability of Pd−based materials for oxygen reduction reaction (ORR) are critical to their application in industrial energy devices. This work constructs a series of carbon-based rare-earth (RE) oxides (Gd₂O₃, Sm₂O₃, Eu₂O₃, and CeO₂) by using RE metal–organic frameworks to tune the ORR performance of the Pd sites through the Pd−RE_xO_y interface interaction. Taking Pd−Gd₂O₃/C as a representative, it is identified that the strong coupling between Pd and Gd₂O₃ induces the formation of the Pd−O−Gd bridge, which triggers charge redistribution of Pd and Gd₂O₃. The screened Pd−Gd₂O₃/C exhibits impressive ORR performance with high onset potential (0.986 V_RHE), half-wave potential (0.877 V_RHE), and excellent stability. Similar ORR results are also found for Pd−Sm₂O₃/C, Pd−Eu₂O₃/C, and Pd−CeO₂/C catalysts. Theoretical analyses reveal that the coupling between Pd and Gd₂O₃ promotes electron transfer through the Pd−O−Gd bridge, which induces the antibonding-orbital occupancy of Pd−*OH for the optimization of *OH adsorption in the rate-determining step of ORR. The pH-dependent microkinetic modeling shows that Pd−Gd₂O₃ is close to the theoretical optimal activity for ORR, outperforming Pt under the same conditions. By its ascendancy in ORR, the Pd−Gd₂O₃/C exhibits superior performance in Zn-air battery as an air cathode, implying its excellent practicability.     

The low temperature synthesis of metal oxides by novel hydrazine method

The hydroxide, oxalate and citrate precursors of the metal oxides such as γ-Fe₂O₃, (MnZn)Fe₂O₄, Cu(K)Fe₂O₄, BaTiO₃, La(Sr)MnO₃, La(Sr)AlO₃, La/Gd(Ca/Ba/Sr)CoO₃, and anatase TiO₂ on modifications with the hydrazine decompose at low temperatures give single phase oxides of superior properties, while the complexes without such modification require higher temperatures for achieving the phases. The hydrazine released at lower temperatures reacts with the oxygen in the atmosphere, N₂H₄+O₂→N₂+2H₂O; ΔH=−625 kJ mol⁻¹, and liberates enormous energy that is sufficient for the oxidative decomposition of the complexes now devoid of hydrazine. Such extra energy is not available in the case of the precursors without such modifications. The reaction products of hydrazine oxidation provide desired partial pressure of moisture needed for the stabilization of γ-Fe₂O₃. Also, the nitrogen that is formed in the reaction of hydrazine with oxygen gets trapped in the lattice of TiO₂ giving yellow color nitrogen doped TiO_2−xN_x photocatalyst. Thus, hydrazine method of preparation has many advantages in the preparation of metal oxides of superior properties.     

Hexagonal La2O3 Nanocrystals Chemically Coupled with Nitrogen-Doped Porous Carbon as Efficient Catalysts for the Oxygen Reduction Reaction

The construction of nano-scale hybrid materials with a smart interfacial structure, established by using rare earth oxides and carbon as building blocks, is essential for the development of economical and efficient catalysts for oxygen reduction reactions (ORRs). In this work, hexagonal La₂O₃ nanocrystals on a nitrogen-doped porous carbon (NPC) derived from crop radish, served as building bricks, are prepared by chemical precipitation and then calcination at elevated temperatures. The obtained La₂O₃/NPC hybrid exhibits a very high ORR activity with a half-wave potential of 0.90 V, exceeding that of commercial Pt/C (0.83 V). Both DFT theoretical and experimental results have verified that the significantly enhanced catalytic performance is ascribed to the formation of the C−O−La covalent bonds between carbon and La₂O₃. Through the covalent bonds, electrons can transfer from the carbon to La₂O₃ and occupy the unfilled e_g orbital of the La₂O₃ phase. This results in the accelerated adsorption of active oxygen and the facilitated desorption of the surface hydroxides (OH_ad⁻), thereby promoting the ORR over the catalyst.     

Electrode reactions of manganese oxides for secondary lithium batteries

Nanorods of MnO₂, Mn₃O₄, Mn₂O₃ and MnO are synthesized by hydrothermal reactions and subsequent annealing. It is shown that though different oxides experience distinct phase transition processes in the initial discharge, metallic Mn and Li₂O are the end products of discharge, while MnO is the end product of recharge for all these oxides between 0.0 and 3.0 V vs. Li⁺/Li. Of these 4 manganese oxides, MnO is believed the most promising anode material for lithium ion batteries while MnO₂ is the most promising cathode material for secondary lithium batteries.