Oxygen Vacancy Formation Energy Research Articles

Tailored Platinum Group Metal/Spinel Oxide Catalysts for Dynamically Enhanced Methane Oxidation.

A combined experimental and molecular modeling study identifies a family of spinel oxides that in combination with PGM (platinum group metals) provide enhanced methane oxidation activity. With a reduction in greenhouse gas (GHG) emissions urgently needed, there is renewed interest in the use of natural gas vehicles (NGVs) and engines (NGEs) for transportation, commerce, and industrial applications. NGVs and NGEs emit less CO2 than their petroleum-derived counterparts but may emit uncombusted methane, an even more potent GHG. For stoichiometric engines, methane oxidation catalysts containing PGM and spinel oxide in layered architectures offer increased methane oxidation activity and lower light-off temperatures (T50). The reducible spinel oxide has direct and indirect roles that are effectively described by the bulk oxygen vacancy formation energy (Evac). We apply density functional theory (DFT) to identify several earth-abundant, cobalt-rich spinel oxides with favorable Evac, shown to correlate with dynamic oxygen storage capacity (DOSC) and CO and H2 oxidation activity. We experimentally rank-order the DFT-identified spinel oxides in combination with Pt+Pd for their methane oxidation activity measurements, under both time-invariant and modulated feed conditions. We show good agreement between the activity and the DFT-computed reducibility of the spinel oxide. The findings suggest spinel reducibility is a key factor in achieving enhanced low-temperature methane conversion, enabled through a balance of methane activation on the PGM sites and subsequent oxidation of the intermediates and byproducts on spinel oxides. In agreement with its computationally predicted Evac, NiCo2O4 was confirmed to have the highest DOSC and lowest T50 among the tested spinel samples.

Li/Ni Intermixing: The Real Origin of Lattice Oxygen Stability in Co‐Free Ni‐Rich Cathode Materials

AbstractThe Co‐free Ni‐rich layered cathode materials with excellent structural stability and low‐cost in Ni‐rich family are emerging as the promising candidates for the next generation of high‐energy density cathodes. Previously, Mn is generally regarded as the structural stabilizer in Co‐free Ni‐rich cathodes, while the role of inevitable Li/Ni intermixing is underestimated. Herein, the study reveals that the real origin of the lattice oxygen/structure stability of Co‐free Ni‐rich cathodes is dominated more by Li/Ni intermixing than the widely accepted Mn. In the Li/Ni intermixing configuration, the intermixed Ni ions can suppress the oxidation of lattice oxygen by increasing the formation energy of oxygen vacancy and reducing charge compensation. Besides, the Li vacancy formed via the delithiation of intermixed Li in transition metal layer can serve as O2 capture sites. This dual stabilization mechanism is proposed and proved to be effectively in enhancing the reversibility of lattice oxygen and the stability of material structural. This work sheds light on the mechanism of stabilizing lattice oxygen through Li/Ni intermixing, which provides new insights for designing better batteries.

Enhancement of light absorption and oxygen vacancy formation in CeO2 by transition metal doping: A DFT study

It has been demonstrated in previous experimental and computational work that doping CeO2 with transition metals is an effective way of tuning its properties. However, each previous study on CeO2 doping has been limited to a single or a few dopants, making it difficult to establish a definitive strategy to modify the properties of CeO2 through doping. In this paper, we systematically study the formation energies, structural stability and electronic properties of CeO2 doped with the entire range of the ten 3d transition metals using density functional theory (DFT) calculations at the hybrid level. Particularly, the formation energies of oxygen vacancies, and their effects on electronic properties, are also considered. These results could provide a meaningful guide for the design of CeO2-based materials with improved visible-light photocatalytic and catalytic performance.

Preparation of ZnO resistive switching film and its corrosion behavior in 3.5 wt.% NaCl solution

ZnO resistive switching films were prepared on the surface of SS304 using hydrothermal method combined with heat treatment. The surface and cross-sectional morphology, composition, oxygen vacancy content and semiconductor type of ZnO films were characterized. The corrosion resistance and resistance switching properties of ZnO films were determined by electrochemical methods. The oxygen vacancy formation energy, surface adsorption energy and diffusion barrier of ZnO system were calculated using first-principles. The results show that the samples with 0.5 mol/L Zn2+ and 4 mol/L NaOH have the best performance, and its corrosion resistance efficiency can reach more than 99.5 %. Calculations show that the ZnO film containing oxygen vacancies still prevents the entry of Cl−, and applied electric field promotes the formation of oxygen vacancies and conductive filaments. Thus, polarization-immersion treatment can adjust the oxygen vacancy content in film, which enables film to switch between high and low resistance states, realizing the cyclic resistance-switching performance of film and greatly extending service life.

Defective oxygen inert phase stabilized high-voltage nickel-rich cathode for high-energy lithium-ion batteries

Pushing layered cathode to higher operating voltage can facilitate the realization of high-energy lithium-ion batteries. However, the released oxygen species initiate materials surface upon highly delithiated states will react severely with electrolyte, accelerating the structure deterioration and triggering the thermal degradation. Here we propose an inert phase of La2Mo2O9 with abundant oxygen vacancies (about 41%) by regulating the annealing temperature to engineer the cathode interface beyond conventional modifications. By employing LiNi0.8Co0.1Mn0.1O2 as a model system and extending to higher voltage-operated LiCoO2 and Li-rich cathode, we demonstrate that the introduced lanthanum and molybdenum ions will transfer electrons to enhance the surface oxygen electronegativities, thus served as “oxygen anchor” to alleviate oxygen evolution. Furthermore, the possible released oxygen can be operando captured and reserved by β-phase La2Mo2O9 depositor for the intrinsic high oxygen vacancy formation energy. The reaction involving oxygen species with electrolyte is fundamentally diminished, thus effectively mitigate the structure deterioration and elevate the electrochemical performances, enabling a 1.5-Ah pouch-type full cell to exhibit negligible 6.0% capacity loss after 400 cycles.

Exploring the Latent Chemical Space of Oxygen Vacancy Formation Energy by a Machine Learning Ensemble

Exploring the Latent Chemical Space of Oxygen Vacancy Formation Energy by a Machine Learning Ensemble

Experimental study and first-principles calculation of enhanced electrochemical properties of Bi0.7Sr0.3FeO3-δ (BSFO) – La0.8Sr0.2MnO3-δ (LSM) composite cathodes for intermediate-temperature solid oxide fuel cells

In order to enhance the electrochemical performance of cobalt-free Bi0.7Sr0.3FeO3-δ (BSFO) cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs), a composite cathode consisting of BSFO and La0.8Sr0.2MnO3-δ (LSM) is developed. The optimized polarization resistance of the composite cathode (0.11 Ω cm2 at 800 °C) is found to be only 61 % that of pure BSFO (0.18 Ω cm2 at 800 °C), indicating a significant improvement in performance. The first-principles calculation indicates that the enhanced electrochemical performance is mainly due to the domain of low oxygen vacancy formation energies of BSFO-LSM-L interfaces (Evac, −3.63 eV) and LSM (Evac, −3.11 eV). The anode-supported single cell of NiO-SDC/SDC/BSFO－LSM shows a peak power density of 825.83 mW cm−2 at 800 °C using wet H2 (∼3 vol% H2O) as fuels and ambient air as oxidants. In addition, the single cell exhibits a good stability at the electric current density 400 mA cm−2 for 50 h measured at 650 °C.

Regulation of oxygen vacancy concentration over ZnCr oxide in bifunctional catalyst for the direct conversion of syngas to light olefins

Metal oxides play a crucial role in catalytic activity of bifunctional catalyst for directly converting syngas into light olefins. A series of ZnCr oxides were synthesized with varying concentrations of precipitant and different atmospheres of precipitate treatment via co-precipitation method. The characterization results indicate that suitable concentration of precipitant and hydrogen calcination contribute to the generation of more oxygen vacancies that can promote the chemical adsorption of CO. Density function theory calculations confirmed that the adsorption energy of CO could be decreased with increasing concentration of oxygen vacancies, and that the formation energy of oxygen vacancies was lower under the hydrogen atmosphere as compared to the oxygen atmosphere. The catalytic activity demonstrated that more oxygen vacancies promoted the chemisorption and conversion of CO. The ZnCr oxide obtained through hydrogen calcination exhibited the highest concentration of oxygen vacancy and achieved a C2=-C4= selectivity of 86 % and a CO conversion of 43 % at 4 MPa and 5400 mL g−1h−1 combined with SAPO-34 zeolite. These findings suggest that increasing oxygen vacancy concentration on the surface of metal oxides is an effective approach for designing high-performance catalysts for directly converting syngas into light olefins.

ABO3 perovskite as well as BaF2, SrF2 and CaF2 bulk and surface F-center first principles predictions

We, at the ab initio level, simulated the rearrangement magnitudes of the adjacent neighboring ions, surrounding the (100) surface F-center in ABO3 perovskite matrixes. They are noticeably greater than the respective ionic shift magnitudes of the adjacent neighboring ions surrounding the bulk F-center. In ABO3 perovskites, the electron charge is noticeably better bounded on the inside of the bulk oxygen vacancy, as interior the respective (100) surface vacancy. The oxygen vacancy formation energy, located on the (100) surface of ABO3 perovskites, as a rule, is smaller as in the bulk. This slight energy distinction encourages the oxygen vacancy segregation from the ABO3 perovskite bulk to their (100) surfaces. The ABO3 complex oxide (100) surface F-center generated defect levels are positioned nearer to the (100) surface CB bottom than the bulk F-center generated respective defect levels. In contrary, the BaF2, SrF2 and CaF2, both, surface and bulk F-center charges are well localized inside the fluorine vacancy. The ionic rearrangement magnitudes of the adjacent neighboring ions, surrounding the surface and bulk F-centers in BaF2, SrF2 and CaF2 matrixes, are much smaller regarding the respective situation in ABO3 perovskites.

Doped Mn modulates the local charge distribution of cobalt-based spinel catalysts to promote the availability of ligand lattice oxygen for complete oxidation of methane

Doping other metals such as Mn, Ni, In, et al. is known as an effective strategy to improve the environmental catalytic performance of Co3O4. In our works, Mn ions were successfully incorporated into the lattice of Co3O4 via the oxalic acid synchronous complexation method. The Mn ions preferentially occupy octahedrally coordinated Co3+ sites and constructing the Mn-O-Co structure. In addition, theoretical calculation further verified that the enhanced catalytic performance of Co5Mn1 originated from highly reactive lattice oxygen coordinated with both Mn and Co ions. Significantly, Mn ions transfer more electrons to lattice oxygen. This oxygen sites have the lower oxygen vacancy formation energy (EOv = 1.94 eV), allowing the lattice oxygen to participate in the reaction following the MvK mechanism in the high temperature range. Collectively, this work is helpful to understand the mechanism of effect of Mn ion doping on the physical and chemical properties of Co-based catalysts.

Cation-ordered Ni-rich positive electrode material with superior chemical and structural stability enabled by atomic substitution for lithium-ion batteries

Ni-rich layered oxides are considered as the most promising cathode materials for lithium-ion batteries (LIBs) due to their high specific capacity and low cost. However, the disordered Li/Ni mixing greatly affects their structural stability and electrochemical performance, thus hindering their wide application in commercial LIBs. Aiming to inhibit the premature death of Ni-rich layered oxides originated from the disordered Li/Ni mixing, atomic substitution of transition metal ions in LiNi0.83Co0.11Mn0.06O2 with high-valence metal molybdenum ions (denoted as Mo-NCM) is proposed in this work. The structure evolution of the as-prepared Mo-NCM caused by molybdenum substitution and the stability of Mo-NCM cathodes are comprehensively studied. Integrating experimental characterization with calculation results, it is found that the partial substitution of transition metal ions with Mo6+ ions can induce cation ordering and promote Li+ diffusion dynamics. Moreover, Mo6+ cations can act as the pillar to prevent local collapse and structural distortion, thus maintaining structural stability of the material. Furthermore, the formation of strong Mo-O bond between molybdenum and oxygen element can stabilize lattice oxygen and enhance the chemical stability of Ni-rich cathode. In addition, molybdenum modification can effectively reduce the exchange energy of Li+/Ni2+ and increase oxygen vacancy formation energy, further enhancing structural stability and promoting the kinetics of Li+ diffusion, and eventually enhancing the electrochemical performance of Mo-NCM based Li-ion batteries. The capacity retention of LIBs with 1 mol% Mo-NCM cathode can reach 98.67% after 200 cycles at 1C. This study provides a new insight into enhancing the chemical and structural robustness of Ni-rich cathode electrode materials.

Unravelling the Impact of Metal Dopants and Oxygen Vacancies on Syngas Conversion over Oxides: A Machine Learning-Accelerated Study of CO Activation on Cr-Doped ZnO Surfaces.

As a critical component of the OX-ZEO composite catalysts toward syngas conversion, the Cr-doped ZnO ternary system can be considered as a model system for understanding oxide catalysts. However, due to the complexity of its structures, traditional approaches, both experimental and theoretical, encounter significant challenges. Herein, we employ machine learning-accelerated methods, including grand canonical Monte Carlo and genetic algorithm, to explore the ZnO(1010) surface with various Cr and oxygen vacancy (OV) concentrations. Stable surfaces with varied Cr and OV concentrations were then systematically investigated to examine their influence on the CO activation via density functional theory calculations. We observe that Cr tends to preferentially appear on the surface of ZnO(1010) rather than in its interior regions and Cr-doped structures incline to form rectangular islands along the [0001] direction at high Cr and OV concentrations. Additionally, detailed calculations of CO reactivity unveil an inverse relationship between the reaction barrier (Ea) for C-O bond dissociation and the Cr and OV concentrations, and a linear relationship is observed between OV formation energy and Ea for CO activation. Further analyses indicate that the C-O bond dissociation is much more favored when the adjacent OVs are geometrically aligned in the [1210] direction, and Cr is doped around the reactive sites. These findings provide a deeper insight into CO activation over the Cr-doped ZnO surface and offer valuable guidance for the rational design of effective catalysts for syngas conversion.

Interface-type tunable oxygen ion dynamics for physical reservoir computing

Reservoir computing can more efficiently be used to solve time-dependent tasks than conventional feedforward network owing to various advantages, such as easy training and low hardware overhead. Physical reservoirs that contain intrinsic nonlinear dynamic processes could serve as next-generation dynamic computing systems. High-efficiency reservoir systems require nonlinear and dynamic responses to distinguish time-series input data. Herein, an interface-type dynamic transistor gated by an Hf0.5Zr0.5O2 (HZO) film was introduced to perform reservoir computing. The channel conductance of Mott material La0.67Sr0.33MnO3 (LSMO) can effectively be modulated by taking advantage of the unique coupled property of the polarization process and oxygen migration in hafnium-based ferroelectrics. The large positive value of the oxygen vacancy formation energy and negative value of the oxygen affinity energy resulted in the spontaneous migration of accumulated oxygen ions in the HZO films to the channel, leading to the dynamic relaxation process. The modulation of the channel conductance was found to be closely related to the current state, identified as the origin of the nonlinear response. In the time series recognition and prediction tasks, the proposed reservoir system showed an extremely low decision-making error. This work provides a promising pathway for exploiting dynamic ion systems for high-performance neural network devices.

Exploring the electronic properties and oxygen vacancy formation in SrTiO3 under strain

SrTiO3 (STO) thin films are widely used as substrates in oxide-based devices, and although STO is one of the most studied materials, both experimentally and computationally in its bulk form, the potential for strain engineering of STO thin-films is largely ignored. In this work, we perform Density Functional Theory (DFT) calculations to investigate the tuning of the structural and electronic properties of STO thin films under uni- and biaxial strain. We find that for TiO2-terminated STO slab, the band gap changes due to strain are more significant than that of the SrO-terminated surface because the band gap is determined by the TiO2 layers closer to the surface. Furthermore, we also find that the formation energy of oxygen vacancies depends on the type of strain applied and the position of oxygen vacancies. These findings demonstrate the large and hitherto underexplored perspectives of using strain engineering to modulate the structural and electronic properties of perovskite film materials for a wide variety of energy and electronic applications.

Design of a perovskite oxide cathode for a protonic ceramic fuel cell

High catalytic activity, low-cost and stable cathode in a temperature range 550–700 °C is essential for the development of protonic ceramic fuel cells (PCFCs). Doping nickel into perovskite La0.5Sr0.5MnO3-δ(LSM) is designed as a cobalt-free cathode based on theoretical calculations and experiments. La0.5Sr0.5Mn0.9Ni0.1O3-δ (LSMNi) as cathode shows higher proton conductivity and ORR activity than the undoped LSM. The PCFCs with LSMNi exhibit low polarization resistance and high peak power density 1.1 W cm−2 at 700 °C. The density functional theory simulations indicate that doping with nickel decreases the oxygen vacancy formation energy and promotes the formation of hydroxide defects. The decrease in proton transfer energy barriers and hydration energy improves the proton conductivity. The improved performance is attributed to fast proton transfer and rapid kinetics of oxygen reduction on the surface of LSMNi. This work provides a novel approach to design cobalt-free cathode for a protonic ceramic fuel cell.

In-situ exsolution of PrO2−x nanoparticles boost the performance of traditional Pr0.5Sr0.5MnO3-δ cathode for proton-conducting solid oxide fuel cells

By synthesizing the nominal PrxSr0.5MnO3-δ materials (x=0.5, 0.6, 0.7, 0.8), new Pr0.5Sr0.5MnO3-δ (PSM50)+PrO2-x composite cathodes for proton-conducting solid oxide fuel cells (SOFCs) were developed. The structure analysis and morphology observations verified the exsolution of PrO2-x particles, and the amount of exsolved PrO2-x increased with the amount of Pr in PrxSr0.5MnO3-δ. An H-SOFC with a Pr0.7Sr0.5MnO3-δ (PSM70) cathode enabled the highest reported fuel cell output for H-SOFCs with manganate cathodes. The construction of a PSM50/PrO2 heterostructure interface can reduce the formation energy of oxygen vacancies, hence accelerating the cathode oxygen reduction reaction (ORR) kinetics, as confirmed by oxygen diffusion and surface exchange experiments. The excellent electrochemical performance was combined with its good chemical stability against CO2 and H2O, allowing a stable operation of the cell for over 100 hours, indicating that PSM70, which was in fact PSM50+PrO2-x, was a highly efficient and durable cathode material for H-SOFCs.

High-entropy perovskite oxides for direct solar-driven thermochemical CO2 splitting

The global energy crisis and climate change have fueled interest in solar-driven thermochemical CO2 splitting as a potential solution. However, conventional materials like CeO2 encounter limitations attributable to their low solar absorptivity and CO yield (even at high reduction temperatures), exerting a detrimental influence on both solar energy utilization and process efficiency. This study proposes high-entropy A-site doped perovskites as a potential solution for efficient solar thermochemical CO2 splitting. The increased configurational entropy enhances the solar thermal CO2 splitting cycles by decreasing oxygen vacancy formation energy and lattice oxygen migration energy barrier. This is supported by experimental results, where Sm0.25Sr0.25Ca0.25La0.25MnO3 achieved a maximum CO yield of 764.76 μmol g−1 with low temperature difference between the reduction and oxidation steps, marking an important progress in solar thermal CO2 splitting cycles. The study demonstrates the potential of high-entropy perovskites for direct solar energy harvesting with high spectrum absorption coefficients and sustainable CO2 utilization.

Revealing the Adsorption Behavior of Nitrogen Reduction Reaction on Strained Ti2 CO2 by a Spin-Polarized d-band Center Model.

Electrocatalytic reduction of dinitrogen to ammonia has attracted significant research interest. Herein, it reports the boosting performance of electrocatalytic nitrogen reduction on Ti2 CO2 MXene with an oxygen vacancy through biaxial tensile strain engineering. Specifically, tensile strain modified electronic structures and formation energy of oxygen vacancy are evaluated. The exposed Ti atoms with additional electron states near the Fermi level serve as active site for intermediate adsorption, leading to superior catalytic performance (Ulimit = -0.44V) under 2.5% biaxial tensile strain through a distal mechanism. However, the two sides of the "Sabatier optimum" in volcano plot are not limited by two different electronic steps, but are induced by the diverse adsorption behaviors of intermediates. Crucially, the "Sabatier optimum" results from the different response speeds of the adsorption energy for *N2 and *NNH to strains. Moreover, the authors observe conventional d-band adsorption for *N2 and *NNH, non-linear adsorption for *NNH2 , and abnormal d-band adsorption for *N, *NH, *NH2 , and *NH3 , which can be explained by the competition between attractive orbital hybridization and repulsive orbital orthogonalization with the spin-polarized d-band model, which further clarifies the contributions of 3σ → dz2 and dxz /dyz → 2π* to the overall population of bonding and anti-bonding states.

A highly efficient Sb-doped La0.5Sr0.5FeO3-δ cathode for protonic ceramic fuel cells

The La0.5Sr0.5FeO3-δ (LSF) material is modified with Sb doping to improve the performance of conventional LSF cathode for protonic ceramic fuel cells (PCFCs). The Sb cations can be inserted into the LSF lattice to develop a new composition La0.5Sr0.5Fe0.9Sb0.1O3-δ (LSFSb), which reduces the formation energy of oxygen vacancies and lowers the proton migration energy of the Sb-doped material when compared to the Sb-free LSF. Additional experimental findings reveal that Sb doping enhances the proton/oxygen transport and surface exchange kinetics. The fuel cell with the LSFSb cathode has a substantially higher peak power density of 933 mW cm−2 at 700 °C than the LSF cell, which only reaches 606 mW cm−2 under the same testing conditions. Using the LSFSb composite cathode further improved the fuel-cell performance, resulting in a promising fuel cell performance of 1388 mW cm−2, which is high for LSF-based PCFCs, thus demonstrating that LSFSb is a suitable cathode for PCFCs.

Ab initio characterization of hybrid MOF-MXenes surfaces: The case of Cu-pyridyl on Ti2CO2

Hybrid MOF-MXenes appear as an appealing system for electrocatalytic reactions. The combination of a MXene substrate with a metal-embedded organic 2D network prevents the metal sintering and provides the material with unique redox surface properties. In this work we characterize by periodic density functional theory the geometry, energetics, and electronic structure of a superstructure containing copper atoms, pyridine, and 1,3,5-tris(pyridyl)benzene molecules supported on a Ti2CO2 substrate. The adsorption of each individual adsorbate is energetically favorable, and their coadsorption stabilizes the overall system, with an electron transfer from the adsorbates to the support and a charge of ∼+0.68 |e| for Cu. The (co)adsorption systems are found to be more reducible than the bare support, as observed in the work function values and the oxygen vacancy formation energy. Interestingly, the formation of oxygen vacancies close to the position of Cu is found to be the most energetically favorable, inducing the migration of the Cu atom to fill the vacancy and become negatively charged, ∼−0.40 |e|. This effect may play an important role in electrocatalytic processes by (de)stabilizing intermediates adsorbed on Cu sites, as in the ORR reaction.