Cation Defects Research Articles

Effect of a-Site Cation Defect on Electrochemical Performance and Stability of La0.8Sr0.2Co0.2Fe0.8O3- δ Cathode

The application of Solid oxide fuel cells (SOFCs) is restricted by the insufficient catalytic activity in oxygen reduction reaction and the impedance increase of cathode in long-term operation at medium and low temperature. In this work, we prepare La0.8Sr0.2Co0.2Fe0.8O3-δ (LSCF) nanofiber cathode material with different A-site defects by electrospinning technology, and systematically explore the influence of A-site cation defect on the electrochemical performance and long-term stability of LSCF cathode. La0.77Sr0.2Co0.2Fe0.8O3-δ, (La0.8Sr0.2)0.97Co0.2Fe0.8O3-δ and La0.8Sr0.17Co0.2Fe0.8O3-δ cathode materials are successfully prepared by controlling the proportion of nitrate input, among which La0.77Sr0.2Co0.2Fe0.8O3-δ obtains the best electrochemical performance, and the polarization impedance is 0.09 Ω·cm2 at 800 ℃.

Cation-deficient T-Nb2O5/graphene Hybrids synthesized via chemical oxidative etching of MXene for advanced lithium-ion capacitors

Orthorhombic niobium pentoxide (T-Nb2O5) is widely acknowledged as a fast pseudocapacitive material. Nevertheless, its application is hindered by the narrow voltage window (1–3 V vs. Li/Li+) that arises from irreversible phase transformation and sluggish kinetics during deep lithiation. Herein, we demonstrate a unique method for introducing Nb vacancies in T-Nb2O5 nanoparticles via amine-assisted oxidative etching of Nb2CTx MXene, providing extra storage sites and improving structural flexibility by introducing cationic defects. Subsequently reduced graphene oxide (rGO) is employed as substrate to disperse T-Nb2O5 nanoparticles and construct T-Nb2O5/rGO nanohybrids. Multiple characterizations and computational simulations demonstrate that the resulting T-Nb2O5/rGO hybrid anode exhibits rapid and stable multi-electron transfer lithium storage. Owing to the enrichment of Nb vacancies and nanoparticle morphology, even when voltage window of 0.01–3 V (vs. Li/Li+) is extended, T-Nb2O5 exhibits a pseudocapacitive mechanism and integrity of partial crystal structure; effectively tackling the structural collapse and sluggish kinetics of T-Nb2O5. Consequently, the T-Nb2O5/rGO anode shows a superior rate capacity (148 mAh/g at 10 A/g) and cycling stability (3000 cycles at 5 A/g). Remarkably, the assembled lithium-ion capacitors achieve a high energy density of 123.7 Wh/kg, a power density of 22.5 kW/kg, and a capacity retention of 83.6% after 20,000 cycles.

Enhanced performance of La1-xFeO3-δ oxygen carrier via A-site cation defect engineering for chemical looping dry reforming of methane

Chemical looping is a process-intensification tool that focuses on the fields of fossil fuel combustion, CO2 capture, hydrocarbon valorization, energy storage, and other energy-related and environmental-related areas. Perovskite-structured LaFeO3, a typical oxygen storage material, shows great potential for chemical looping partial oxidation and H2O/CO2 splitting. Here, we report a simple and efficient strategy to enhance catalytic activity by tuning A-site cation defects in perovskites via a sol-gel method. Systematically studies show that A-site cation defects affect both the physical and chemical properties. With the A-site cation defect engineering, the crystallite size of the La1-xFeO3-δ perovskites is reduced. The catalytic performance for CH4 oxidation and CO2 splitting is enhanced with increasing oxygen vacancy concentration. The La0.93FeO3-δ oxygen carrier exhibits the highest catalytic performance with 88% CO selectivity and 90% methane conversion at 750 °C. In addition, the La0.93FeO3-δ shows higher stability in the successive CH4 partial oxidation/CO2 splitting redox cycles, with almost unchanged CH4 conversion and CO selectivity. All these results demonstrate that the strategy of adjustment of different molar ratios of A/B site in perovskite metal oxides can effectively regulate the oxygen vacancy concentration.

Surface Reconstruction of Covellite CuS Nanocrystals for Enhanced OER Catalytic Performance in Alkaline Solution.

Oxygen evolution reaction (OER) is one of the important half-reactions in energy conversion equipment such as water-spitting devices, rechargeable metal-air batteries, and so on. It is beneficial to develop efficient and low-cost catalysts that understand the reaction mechanism of OER and analyze the reconstruction phenomenon of transition metal sulfide. Interestingly, copper sulfide and cuprous sulfide with the same components possess different reconstruction behaviors due to their different metal ion valence states and different atomic arrangement modes. Because of a unique atomic arrangement sequence and certain cationic defects, the reconstruction phenomenon of CuS nanomaterials are that S2- is firstly oxidized to SO4 2- and then Cux + is converted into CuO via Cu(OH)2 . In addition, the specific "modified hourglass structure" of CuS with excellent conductivity is easier to produce intermediates. Compared with Cu2 S, CuS exhibits excellent OER activity with a lower overpotential of 192mV at 10mA cm-2 and remarkable electrochemical stability in 1.0m KOH for 120h. Herein, this study elucidates the reconstruction modes of CuS and Cu2 S in the OER process and reveals that CuS has a stronger CuS bond and a faster electronic transmission efficiency due to "modified hourglass structure," resulting in faster reconstruction of CuS than Cu2 S.

High‐Density Cationic Defects Coupling with Local Alkaline‐Enriched Environment for Efficient and Stable Water Oxidation

AbstractThe inferior activity and stability of non‐noble metal‐based electrocatalysts for oxygen evolution reaction (OER) seriously limit their practical applications in various electrochemical energy conversion systems. Here we report, a drastic nonequilibrium precipitation approach to construct a highly disordered crystal structure of layered double hydroxides as a model OER catalyst. The unconventional crystal structure contains high‐density cationic defects coupled with a local alkaline‐enriched environment, enabling ultrafast diffusion of OH− ions and thus avoiding the formation of a local acidic environment and dissolution of active sites during OER. An integrated experimental and theoretical study reveals that high‐density cationic defects, especially di‐cationic and multi‐cationic defects, serve as highly active and durable catalytic sites. This work showcases a promising strategy of crystal structure engineering to construct robust active sites for high‐performance oxygen evolution in an alkaline solution.

High-Density Cationic Defects Coupling with Local Alkaline-Enriched Environment for Efficient and Stable Water Oxidation.

The inferior activity and stability of non-noble metal-based electrocatalysts for oxygen evolution reaction (OER) seriously limit their practical applications in various electrochemical energy conversion systems. Here we report, a drastic nonequilibrium precipitation approach to construct a highly disordered crystal structure of layered double hydroxides as a model OER catalyst. The unconventional crystal structure contains high-density cationic defects coupled with a local alkaline-enriched environment, enabling ultrafast diffusion of OH- ions and thus avoiding the formation of a local acidic environment and dissolution of active sites during OER. An integrated experimental and theoretical study reveals that high-density cationic defects, especially di-cationic and multi-cationic defects, serve as highly active and durable catalytic sites. This work showcases a promising strategy of crystal structure engineering to construct robust active sites for high-performance oxygen evolution in an alkaline solution.

Cationic defects coupled with trace Pt under the assistance of corrosive engineering for efficient hydrogen electrocatalysis with large current density

Exploiting efficient and durable samples toward electrocatalytic hydrogen evolution reaction (HER) with large current density is critical for commercializing hydrogen energy. Herein, Ni, Fe layered double hydroxide with cationic defects and trace Pt (Pt/D-NiFe LDH) on NiFe foam is constructed under the assistance of corrosion engineering and alkaline etching at ambient conditions. The porous nanostructure, leaf-like morphology and coexistence of defects and Pt endow the as-designed sample presents a remarkable electrocatalytic activity for HER in alkaline media with low overpotentials with 28, 163 and 217 mV to drive 10, 500 and 1000 mA cm−2. Remarkably, the catalyst enables the two-electrode electrolyzer with low voltages 1.65 and 1.72 V to attain 500 and 1000 mA cm−2. It also can be powered by solar, thermal and wind sustainable energies. The as-synthesized Pt/D-NiFe LDH has excellent catalytic activities and stability in alkaline seawater and neutral electrolytes with small overpotentials.

Molecular coordination induced high ionic conductivity of composite electrolytes and stable LiF/Li3N interface in long-term cycling all-solid-state lithium metal batteries

Composite polymer/oxide electrolyte (CPE) is a promising candidate for all-solid-state lithium metal batteries (ASSLMBs), but controlling the interactions between components in CPEs to achieve high efficient Li+-transport and stable CPE/electrode interface is a challenge. Here, we report a mesoporous black LaxCoO3-δ nanofiber (NF) film with tunable cationic defects to regulate the molecular coordination with PEO/LiTFSI matrix and realize stable operations of ASSLMBs over 1000 cycles. By tuning the A-site defects from x = 1.0 to 0.8, the LaxCoO3-δ NFs show gradually enhanced coordination with PEO/LiTFSI, achieving a high ionic conductivity of 2.5 × 10−4 S/cm at 30 °C. Moreover, the defective La0.8CoO3-δ can catalyze the formation of stable LiF/Li3N interfacial layers, which possess high Li+-conduction kinetics and interfacial energy, thus enabling stable battery operations even at high currents. As a result, the LiFePO4-based ASSLMBs show a long life of >1000 cycles at 0.2 C with a capacity retention of >75%, and they can deliver a high discharge capacity of 142 mA h g−1 at 1 C with a capacity retention of 92.2% over 300 cycles. Importantly, this molecular coordination strategy is still effective when extended to other defective oxides, thus providing a universal method to design high-performance CPEs.

Understanding the Effect of Single Atom Cationic Defect Sites in an Al2O3 (012) Surface on Altering Selenate and Sulfate Adsorption: An Ab Initio Study.

Adsorption is a promising under-the-sink selenate remediation technique for distributed water systems. Recently it was shown that adsorption induced water network re-arraignment control adsorption energetics on the (012) surface. Here, we aim to elucidate the relative importance of the water network effects and surface cation identity on controlling selenate and sulfate adsorption energy using density functional theory calculations. Density functional theory (DFT) calculations predicted the adsorption energies of selenate and sulfate on nine transition metal cations (Sc-Cu) and two alkali metal cations (Ga and In) in the (012) surface under simulated acidic and neutral pH conditions. We find that the water network effects had larger impact on the adsorption energy than the cationic identity. However, cation identity secondarily controlled adsorption. Most cations decreased the adsorption energy weakening the overall performance, the larger Sc and In cations enabled inner-sphere adsorption in acidic conditions because they relaxed outward from the surface providing more space for adsorption. Additionally, only Ti induced Se selectivity over S by reducing the adsorbing selenate to selenite but not reducing the sulfate. Overall, this study indicates that tuning water network structure will likely have a larger impact than tuning cation-selenate interactions for increasing adsorbate effectiveness.

Formation Energy Profiles of Oxygen Vacancies at Grain Boundaries in Perovskite‐Type Electroceramics

Oxygen vacancy formation energies play a major role in the electric field‐assisted abnormal grain growth of technologically relevant polycrystalline perovskite phases. The underlying effect on the atomic scale is assumed to be a redistribution of cationic and anionic point defects between grain boundaries (GBs) and the bulk interior regions of the grains due to different defect formation energies in the structurally different regions, accompanied by the formation of space charge zones. Using atomistic calculations based on classical interatomic potentials, optimized structures of the symmetric tilt GBs Σ5(210)[001] and Σ5(310)[001], and of the asymmetric tilt GB (430)[001]||(100)[001] in the electroceramic perovskite materials SrTiO3, BaTiO3, and BaZrO3, are derived and discussed. Profiles of oxygen vacancy formation energies across those GBs are presented and their dependence on composition and GB type is discussed.

A two-dimensional cation-deficient Ti0.87O2 artificial protection layer for stable sodium metal anodes

The practical application of sodium (Na) metal batteries is still hampered by uncontrolled Na dendrite growth and unstable solid-electrolyte interphase (SEI), which lead to poor cycling stability and even serious safety concerns. Here, we present a dendrite-free and highly stable Na metal anode via directly transferring a cation-deficient Ti0.87O2 nanosheet constructed artificial layer onto Na surface. Profiting from the sodiophilic nature and high Young's modulus, the Ti0.87O2 layer can seamlessly connect to the Na surface and avoid the layer fracture caused by Na volume changes. In addition, the existence of cation defects enables Ti0.87O2 layer with strong Na+ adsorption ability that pre-uniforms the Na+ flux. Further, the Ti0.87O2 protection layer has high ionic conductivity but poor electronic conductivity; this allows Na+ quasi self-diffusion but restricts Na plating under the protection layer. With these advantages, the Ti0.87O2 layer stabilizes the electrolyte/electrode interface and guarantees dendrite-free plating/stripping of Na. As a result, significantly improved performance of the symmetric batteries and Na3V2(PO4)3 based full cells has been obtained. Importantly, the Ti0.87O2 layer can also protect Li anodes and further enhance their performance. Our work presents a new versatile strategy that brings metal based anodes one step closer to being a viable technology.

Functional organic cation induced 3D-to-0D phase transformation and surface reconstruction of CsPbI3 inorganic perovskite

Efficiency and stability are the main research focuses for perovskite solar cells. Inorganic perovskites like CsPbI3 possess higher chemical stability than those with organic A-site cations, while they also exhibit higher defect density. Nonetheless, it is highly challenging to induce orderly secondary arrangement or reconstruction of inorganic perovskites with reduced defects because of their unique chemical properties. In this work, in-situ three-dimension-to-zero-dimension (3D-to-0D) phase transformation and surface reconstruction on CsPbI3 film is achieved as induced by a functional organic cation, benzyldodecyldimethylammonium (BDA), a process of which that is similar to phase-transfer catalysis. With the help of BDABr salt treatment, 0D Cs4PbI6 perovskites are secondarily formed along CsPbI3 grain boundaries with Cs-related cationic defects passivated, yielding structures of higher stability. The BDA-CsPbI3 films exhibit reduced non-radiative recombination and promoted charge transfer, leading to inorganic perovskite solar cells with a high power conversion efficiency of 20.63% and good operational stability.

Structural, Optical, Electric and Magnetic Characteristics of (In1-xGdx)2O3 Films for Optoelectronics.

After (In1-xGdx)2O3 powder with a wide x range of 0 to 10 at.% was chemically produced, (In1-xGdx)2O3 thin films were evaporated under ultra-vacuum using an electron beam apparatus. We investigated the influence of the Gd doping concentration on the magnetic, optical, electrical, and structural properties of the resultant In2O3 deposits. The produced Gd-doped In2O3 films have a cubic In2O3 structure without a secondary phase, as shown by the X-ray diffraction results. Additionally, the chemical analysis revealed that the films are nearly stoichiometric. A three-layer model reproduced the spectroscopic ellipsometer readings to determine the optical parameters and energy gap. The Egopt changed toward the lower wavelength with growing the Gd doping in (In1-xGdx)2O3 films. The Egopt in the (In1-xGdx)2O3 films was observed to increase from 3.22 to 3.45 eV when the Gd concentration climbed. Both carrier concentration and hall mobility were found during the Hall effect studies. It was possible to construct the heterojunction of Ni (Al)/n-(In1-xGdx)2O3/p-Si/Al. At voltages between -2 and 2 volts, investigations into the dark (cutting-edge-voltage) characteristics of the produced heterojunctions were made. The oxygen vacancies and cationic defects in the lattice caused by the uncompensated cationic charges resulted in significant magnetism and ferromagnetic behavior in the undoped In2O3 films. The (In1-xGdx)2O3 films, however, displayed faint ferromagnetism. The ferromagnetism seen in the (In1-xGdx)2O3 films was caused by oxygen vacancies formed during the vacuum film production process. Metal cations created ferromagnetic exchange interactions by snatching free electrons in oxygen.

Concurrent Top and Buried Surface Optimization for Flexible Perovskite Solar Cells with High Efficiency and Stability

AbstractAlthough much progress is made toward enhancing the efficiency of perovskite solar cells (PSCs), their operational reliability, particularly their mechanical stability, which is a crucial factor for flexible PSCs (f‐PCSs), has not attracted sufficient attention. The defects in the perovskite layer, especially on the top and the buried surface of the perovskite layer, can induce perovskite fracture, highly limiting the performance of f‐PSCs. Herein, a novel multifunctional organic salt, metformin hydrochloride, which can passivate cationic and anionic defects, is incorporated on both the top and buried surfaces of perovskite layer to suppress defects. As a result, a power conversion efficiency (PCE) of 24.40% for rigid PSCs and a PCE of 22.04% for f‐PSCs are achieved. Simultaneously, the device can retain 90% and 80% of the initial efficiency after 1000 h of light illumination and 10 000 bending cycles, respectively, showing excellent operational stability. This study may provide a global way to design a passivation strategy and fabricate flexible perovskite solar cells with high efficiency and stability.

Effect of intrinsic point defects on the catalytic and electronic properties of Cu2WS4 single layer: Ab initio calculations

The challenges imposed by climate change require the continued improvement and identification of materials for the development of green technologies. Point defect engineering is a promising technology for producing green hydrogen by taking advantage of catalytic hydrogen evolution reactions. In this work, we investigate the role of anionic and cationic vacancy point defects, as well as the nature of the active sites, in the catalytic activation of ${\mathrm{Cu}}_{2}{\mathrm{WS}}_{4}$ single layers. The stability of the pristine and defective structures of ${\mathrm{Cu}}_{2}{\mathrm{WS}}_{4}$ has been thoroughly investigated using density-functional theory calculations. A deep analysis of the formation enthalpy indicates that the Cu vacancy is the chemically most favorable vacancy. However, the calculated adsorption energy indicates that the presence of such vacancies slightly enhances the hydrogen evolution reaction. In contrast, the formation of an S vacancy considerably magnifies the same reaction in ${\mathrm{Cu}}_{2}{\mathrm{WS}}_{4}$ single layers.

Unveiling Structural Defects by 139La NMR and Raman Spectroscopies at the Origin of Surface Stability for the Design of Cerium-Based Catalysts

Cerium-based complex oxides are essential materials well-suited for numerous applications related to heterogeneous catalysis and electrocatalysis. However, the impact of structural defects on the thermal stability of the surface has still to be elucidated. Probing lanthanum environments by 139La NMR and analyzing, for the first time in parallel, the associated defects by Raman spectroscopy allow for a better view of the structural defects at the surface and in the bulk for samples annealed at T = 600 and 1200 °C, respectively. Moreover, we propose another indexing of the Raman spectra of cerium-based compounds with a fluorite structure by considering two tetrahedral environments around the cationic defects. We evidence the existence of an unusual isolated La pseudo-cubic site, in a series of cerium-, zirconium-, and lanthanum-based oxides. It is found to be stabilized at the subsurface farther from surface’s Zr atoms and oxygen vacancies. La3+ ions in the bulk are preferentially associated with bulk Zr4+ cations as in the La2Zr2O7 ternary compound. When this peculiar La3+ environment surrounded exclusively by Ce4+ cubic sites is thermally stable, the specific surface area remains interesting for catalytic application at high temperatures. However, enhancing the La and Zr contents tends to increase the association of La3+ and Zr4+ ions in clusters and induce a loss of surface area.

Ni, Co, and Yb Cation Co-doping and Defect Engineering of FeOOH Nanorods as an Electrocatalyst for the Oxygen Evolution Reaction.

Electrocatalytic water splitting is a feasible technology that can produce hydrogen from renewable sources. The oxygen evolution reaction (OER), which has a slower kinetics and higher overpotential than the hydrogen evolution reaction, is the bottleneck that limits the overall water splitting. It is essential to develop efficient OER catalysts to reduce the anode overpotential. Herein, Ni,Co,Yb-FeOOH nanorod arrays grown directly on a carbon cloth are synthesized by a simple one-step hydrothermal method. The doped Ni2+ and Co2+ can occupy Fe2+ and Fe3+ sites in FeOOH, increasing the concentration of oxygen vacancies (VO), and the doped Yb3+ with a larger ionic radius can occupy the interstitial sites, which leads to more edge dislocations. VO and edge dislocations greatly enrich the active sites in FeOOH/CC. In addition, density functional theory calculations confirm that doping of Ni2+, Co2+, and Yb3+ modulates the electronic structure of the main active Fe sites, bringing its d-band center closer to the Fermi level and reducing the Gibbs free energy change of the rate-determining step of the OER. When the current density reaches 10 mA cm-2, the overpotential of Ni,Co,Yb-FeOOH/CC is only 230.9 mV, and the Tafel slope is 22.7 mV dec-1. In particular, a mechanism of multi-cation doping synergistic interaction with the oxygen vacancy and edge dislocation to enhance the OER catalytic activity of the material is proposed.

Iodide/triiodide redox shuttle-based additives for high-performance perovskite solar cells by simultaneously passivating the cation and anion defects.

Halide perovskite solar cells (PSCs) have received remarkably increasing interests due to their facile fabrication procedures, use of cost-effective raw materials, and high power conversion efficiencies (PCEs) during the past 10 years. Nevertheless, the state-of-the-art organic-inorganic PSCs suffer from high defect concentration and inferior humid/thermal stability, significantly restricting the widespread applications of PSCs. More specifically, point defects including metallic lead (Pb0) and halide iodine (I0) are easily generated in Pb/I-based PSCs during fabrication processes and operational conditions due to the inferior interaction between the anions and cations in halide perovskites and promote detrimental carrier recombination and ion migration, leading to inferior PCEs and durability of the PSCs. Herein, to tackle the above-mentioned issues, iodide/triiodide (I-/I3-) redox shuttles as a new additive were introduced to simultaneously passivate the cation and anion defects of methylammonium lead iodide (MAPbI3)-based PSCs. In particular, I-/I3- redox shuttles play a vital role in regenerating the cation (Pb0) and anion (I0) defects through the redox cycles of Pb0/Pb2+ and I0/I-. Consequently, the cell with an optimized amount of I-/I3- additive generated a superior PCE of 20.4%, which was 12% higher than the pristine device (18.2%). Furthermore, the introduction of the I-/I3- additive remarkably improved the humid and thermal stability of MAPbI3-based PSCs. This work manifests the importance of the design of redox shuttle-based additives to boost the efficiency and durability of organic-inorganic PSCs.

The role of cationic defects in boosted lattice oxygen activation during toluene total oxidation over nano-structured CoMnOx spinel

This study firstly revealed that toluene oxidation activity can be considerably improved by cationic defects rather than oxygen defects over nano-structured CoMnOx.

Constructing Cation Vacancy Defects on NiFe-LDH Nanosheets for Efficient Oxygen Evolution Reaction

Active site exposure and intrinsic catalytic performance are considered important aspects of oxygen evolution reaction catalyst design. In this work, the coordination capacity of tributylphosphine is utilized to construct cationic vacancy defects on NiFe-LDH nanosheets. As-prepared defective NiFe-LDH nanosheets show not only the optimization of the exposure ability of the active site but also the intrinsic catalytic capacity is improved by construction of cationic vacancy defect to tune local electronic structure. The x-ray photoelectron spectroscopy results revealed that after reconstruction of the prepared d-NiFe-LDH, high-valence Ni and Fe can stably appear on the surface of the material. The presence of high-valence Ni and Fe is considered to be the main reason to improve the intrinsic catalytic capacity of catalysts. Finally, d-NiFe-LDH nanosheets show excellent catalytic performance (η 10 = 243 mV) and remarkable long-term stability.