Vanadium Sites Research Articles

(Invited) Light-Induced Structural Deformations in BiVO4 Photoanodes

Monoclinic scheelite bismuth vandate (BiVO4) stands as one of the best performing thin film metal oxide photoanodes investigated to date. However, these desirable photoelectrochemical performance characteristics are surprising given the driving force for free charge carriers to self-trap as polarons. For the case of electrons, localization at vanadium sites leads to formation of small electron polarons with a large, thermally activated hopping barrier of several hundred meV. For the case of holes, several experimental and theoretical studies have indicated polaron formation. However, despite the importance of photogenerated hole extraction for photoanode function, significant uncertainties remain regarding the degree of hole localization, the associated structural configurations, and the transport barriers. In this work, we apply time resolved optical pump/X-ray diffraction probe experiments to investigate light excitation-induced structural transformations of BiVO4. At low pump fluences, we observe a distinct, non-thermal response that indicates increased symmetry and a partial relaxation of the monoclinic distortion within the BiO8 dodecahedra. We rationalize this finding in terms of the underlying electronic structure of BiVO4, photo-induced depopulation of valence band states, and the nature of the monoclinic-to-tetragonal phase transformation. The resulting structural change is expected to increase hole localization and play a key role in the formation and structure of photo-induced hole polarons within the material. Complimentary electronic transport and optical pump-probe spectroscopies provide additional support for this interpretation and shed light on the underlying mechanisms of polaron formation, transport, and recombination in this promising photoanode material.

Investigation of lattice dynamics, magnetism, and electronic transport in β−Na0.33V2O5

We investigate the electronic and magnetic properties as well as lattice dynamics and spin-phonon coupling of $\beta$-Na$_{0.33}$V$_2$O$_5$ using temperature-dependent Raman scattering, dc-magnetization and dc-resistivity, x-ray photoemission, and absorption spectroscopy. The Rietveld refinement of XRD pattern with space group C2/m confirms the monoclinic structure. The analysis of temperature-dependent Raman spectra in a temperature range of 13--673~K reveals an anharmonic dependence of the phonon frequency and full width at half maximum, which is accredited to the symmetric phonon decay. However, below about 40 K, the hardening of the phonon frequency beyond anharmonicity is attributed to the spin-phonon coupling. Interestingly, the estimated effective magnetic moment $\mu_{\rm eff}=$ 0.63~$\mu_B$ from the magnetization data manifests a mixed-valence state of V ions in 4+ (18$\pm$1\%) and 5+ (82$\pm$1\%). A similar ratio of V$^{4+}$ to V$^{5+}$ is also observed in the x-ray photoemission and x-ray absorption near-edge spectra and that is found to be consistent with the sample stoichiometry. In addition, the V$^{4+}$ ions are distributed between different vanadium (V1 and V3) sites. The analysis of extended x-ray absorption fine structure at different V-sites gives the corresponding V--O bond lengths, which are utilized in the assignment of Raman modes. Moreover, the temperature-dependent resistivity resembles a semiconducting behavior where the charge carrier transport is facilitated by the band conduction at higher temperatures and via hopping $\le$260~K.

Precise Introduction of Single Vanadium Site into Indium-Organic Framework for CO2 Capture and Photocatalytic Fixation.

The capture and fixation of CO2 under mild conditions is a cost-effective route to reduce greenhouse gases, but it is challenging because of the low conversion and selectivity issues. Metal-organic frameworks (MOFs) are promising in the fields of adsorption and catalysis because of their structural tunability and variability. However, the precise structural design of MOFs is always pursued and elusive. In this work, a metal-mixed MOF (SNNU-97-InV) was designed by precisely introducing single vanadium site into the isostructural In-MOF (SNNU-97-In). The single V sites clearly change the interactions between the MOF framework and CO2 molecules, leading to a 71.3% improvement in the CO2 adsorption capacity. At the same time, the enhanced light absorption enables SNNU-97-InV to efficiently convert CO2 into cyclic carbonates (CCs) with epoxides under illumination. Controlled experiments showed that the promoted performance of SNNU-97-InV may be that the V═O site can more easily combine with CO2 and convert them into an intermediate state under illumination, and the possible mechanism was thus speculated.

Exploring the Stability of Single-Atom Catalysts Using the Density Functional Theory-Based Global Optimization Method: H2 Formation on VOx/γ-Al2O3(100)

Single-atom catalysis is indisputably one of the most trending topics in heterogeneous catalysis with distinctive performances in many important chemical reactions. However, surface reconstruction during loading of single-atom targets and its impact on vital reaction steps have rarely been studied. Herein, taking H2 formation, a key step of alkane dehydrogenation, as an example, with genetic algorithm structure search, we reveal the structure–reactivity relationship in the H2 formation on VOx/γ-Al2O3(100). We explore a large number of VOx/γ-Al2O3(100) (x = 0, 1, 2, 3) structures and discover a few reconstructed structures with low formation energies. Since the pristine γ-Al2O3(100) support is highly stable, the production of such structures was completely induced by VOx loading. One VO3/γ-Al2O3(100) structure generates a stable VO4 motif by reconstructing the support surface. Its distinct local chemical environment of vanadium can not only resist the oxygen removal under reductive conditions but can also offer a low-barrier surface reaction channel to H2 formation. Moreover, the hydrogen atom prefers to diffuse into the vanadium site and then couple with another hydrogen instead of direct coupling if the initial state involves two hydrogen atoms both on oxygen sites. Therefore, the H2 formation prefers a surface process that occurs on the vanadium–oxygen pair.

Deep insight into the defect structure, lithium diffusion kinetics and deintercalation study on β-LiVOPO4 cathode material by atomistic simulation method

In this work, atomistic simulation methods including first principles density functional theory (DFT) and molecular dynamics (MD) are applied to study the defect structure, the synergetic motion of lithium ions and the deintercalation mechanism in cathode material β-LiVOPO4. Li+ frenkel defect, Li/V antisite and Li+ vacancies are the main intrinsic defects in the lattice. Three Li+ diffusion paths, including 4c mediated diffusion path on the same edge sharing LiO6 octahedra chain along b-axis, diffusion path between two 4b lithium sites on the nearest neighboring LiO6 octahedra chain along b-axis and vanadium site mediated diffusion path are elaborately revealed by molecular dynamics (MD) and bond-valence-energy-landscape mapping (BVEL). The energy barriers of the Li+ hopping are investigated by nudged elastic band (NEB). Lithium mobility is extremely low for the perfect lattice of β-LiVOPO4 where no impurities and defects are present. Upon the deintercalation of Li+, the lithium ions are activated. The state of charge (SOC) dependency of Li+ diffusion coefficient shows a convex profile with a maximum value at x = 0.5. When the deintercalation level falls within 0.5∼0.75, the obtained energy barriers (0.31–0.38 eV) best match the value obtained by vacancy mediated NEB method, 0.34 eV

Interstitial M+ (M+ = Li+ or Sn4+) Doping at Interfacial BiVO4/WO3 to Promote Photoelectrochemical Hydrogen Production

Doping metals together with heterostructure assemblages is critical to address the challenges encountered while using bismuth vanadate (BVO) to yield improved light-harvesting, charge transfer, and solar-to-hydrogen conversion efficiency. To date, most approaches have focused on substitutional doping using hexavalent metal ions (Mo6+ and W6+) at vanadium or bismuth sites to improve the photoelectrochemical (PEC) performance. Unlike conventional substitution, which produces V-substituted sites that function as hole traps and reduce the activity, herein, we used a simple hydrothermal and metal–organic decomposition approach to introduce interstitial [Li+ or Sn4+]n-doping in BVO (n = 0.25, 0.5, 1.0, 1.5, and 2.0 mM) interfaced with tungsten oxide (WO). The resulting Sn-doped BVO/WO (0.5 mM) shows a reproducible photocurrent density of 1.65 ± 0.07 mA cm–2 and 4.28 ± 0.15 mA cm–2 at 1.23 VRHE for water oxidation and sulfite oxidation, respectively, with a superior quantum efficiency (60% at 470 nm) and long-term durability (>10000 s) under standard AM 1.5 G light irradiation (1 sun). The results show that the Sn-doped BVO/WO exhibited an enhanced PEC performance approximately three times better than that of pristine BVO/WO, thus enabling continuous H2 production (∼800 μmol·cm–2) and highlighting the beneficial role of strategically controlled interstitial dopant concentration. Mott–Schottky analysis revealed an increase in the donor concentration for Li–BVO/WO (∼2.3-fold) and Sn–BVO/WO (3.5-fold), related to the reference BVO/WO photoelectrode. This work highlights the use of low-cost dopants and heterojunction photocatalysts to carry out hydrogen evolution reactions at a significantly improved rate.

O2 Activation with a Sterically Encumbered, Oxygen-Deficient Polyoxovanadate-Alkoxide Cluster.

The isolation of the oxygen-deficient, polyoxovanadate-alkoxide (POV-alkoxide) cluster, [nBu4N][V6O6(OMe)12(MeCN)], and its subsequent reactivity with oxygen (O2), has demonstrated the utility of these assemblies as molecular models for heterogeneous metal oxide catalysts. However, the mechanism through which this cluster activates and reduces O2 to generate the oxygenated species is poorly understood. Currently it is speculated that this POV-alkoxide mediates the four-electron O-O bond cleavage through an O2 bridged dimeric intermediate, a mechanism which is not viable for O2 reduction at solid-state metal oxide surfaces. Here, we report the successful activation and reduction of O2 by the calix-functionalized POV-alkoxide cluster, [nBu4N][(calix)V6O6(OMe)8](MeCN)] (calix = 4-tert-butylcalix[4]arene). The steric hindrance imparted to the open vanadium site by the calix motif eliminates the possibility of cooperative, bimolecular O2 activation, allowing for a comparison of the reactivity of this system with that of the nonfunctionalized POV-alkoxide described previously. Rigorous characterization of the calix-substituted assembly, enabled by its newfound solubility in organic solvent, reveals that the incorporation of the tetradentate aryloxide ligand into the POV-alkoxide scaffold perturbs the electronic communication between the site-differentiated vanadium(III) ion and the cluster core. Collectively, our results provide insight into the physiochemical factors that are important during the O2 reduction reaction at oxygen-deficient sites in reduced POV-alkoxide clusters.

A simultaneous operando FTIR & Raman study of propane ODH mechanism over V-Zr-O catalysts

Simultaneous operando Raman and transmission FTIR spectroscopic measurements have been combined for the study of the structure-activity relationships of V-Zr-O catalysts during propane oxidative dehydrogenation (ODH). The combination of both spectroscopies during reaction allows the identification of reaction intermediates and yields structural information about the catalyst. The experimental results are compared with available data in the literature to propose a reaction mechanism for propane ODH on V-Zr-O catalytic systems, and to identify the role of different oxygen species bound to vanadium sites. The results are consistent with a Mars-van Krevelen mechanism. Besides, they clearly show higher activity of bridging oxygen sites (V-O-V, V-O-Zr) than terminal VO bonds.

Fabrication of reusable polymer nanocomposite films made of thermoplastic polyurethane and modified BiVO4 for photodegradation of Malachite Green

Semiconducting BiVO4, is successfully prepared by solid-state route and attempts are taken to dope vanadium sites by titanium ions to produce BiV1-xTixO4 (x = 5%, 10% and 15%). The products are analyzed using powder X-ray diffraction, Raman spectroscopy, scanning electron microscopy and energy dispersive spectroscopy. The average particle sizes are in the range of 71-82 nm. Using diffuse reflectance spectroscopy, the band gaps are estimated and fall in visible range (2.11-2.41 eV). These nanomaterials are investigated for the photodegradation of malachite green dye solution that can find application in wastewater treatment and their efficiencies are investigated by time-dependent UV-Visible spectroscopy. 15% titanium doped BiVO4 (BVO-15) showed the highest efficiency by completely degrading malachite green in 180 min under visible light irradiation. In such heterogenous processes, separation of the used nanocatalysts remain a challenge. To address this, reusable polymer nanocomposite films are prepared using thermoplastic polyurethane (TPU) to immobilize BVO-15. The polymer nanocomposite film with 0.10 g of catalyst in 1 g of TPU showed maximum efficiency (~ 96%). The powder and film catalysts are investigated for three cycles and found to be effective. Thus, they show remarkable photocatalytic activity, stability and reusability that make them suitable for the wastewater treatment.

Substitutional Vanadium Sulfide Nanodispersed in MoS2 Film for Pt-Scalable Catalyst.

Among transition metal dichalcogenides (TMdCs) as alternatives for Pt‐based catalysts, metallic‐TMdCs catalysts have highly reactive basal‐plane but are unstable. Meanwhile, chemically stable semiconducting‐TMdCs show limiting catalytic activity due to their inactive basal‐plane. Here, metallic vanadium sulfide (VSn) nanodispersed in a semiconducting MoS2 film (V–MoS2) is proposed as an efficient catalyst. During synthesis, vanadium atoms are substituted into hexagonal monolayer MoS2 to form randomly distributed VSn units. The V–MoS2 film on a Cu electrode exhibits Pt‐scalable catalytic performance; current density of 1000 mA cm−2 at 0.6 V and overpotential of −0.08 V at a current density of 10 mA cm−2 with excellent cycle stability for hydrogen‐evolution‐reaction (HER). The high intrinsic HER performance of V–MoS2 is explained by the efficient electron transfer from the Cu electrode to chalcogen vacancies near vanadium sites with optimal Gibbs free energy (−0.02 eV). This study provides insight into ways to engineer TMdCs at the atomic‐level to boost intrinsic catalytic activity for hydrogen evolution.

Ambient-Temperature Hydrogen Storage via Vanadium(II)-Dihydrogen Complexation in a Metal-Organic Framework.

The widespread implementation of H2 as a fuel is currently hindered by the high pressures or cryogenic temperatures required to achieve reasonable storage densities. In contrast, the realization of materials that strongly and reversibly adsorb hydrogen at ambient temperatures and moderate pressures could transform the transportation sector and expand adoption of fuel cells in other applications. To date, however, no adsorbent has been identified that exhibits a binding enthalpy within the optimal range of -15 to -25 kJ/mol for ambient-temperature hydrogen storage. Here, we report the hydrogen adsorption properties of the metal-organic framework (MOF) V2Cl2.8(btdd) (H2btdd, bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin), which features exposed vanadium(II) sites capable of backbonding with weak π acids. Significantly, gas adsorption data reveal that this material binds H2 with an enthalpy of -21 kJ/mol. This binding energy enables usable hydrogen capacities that exceed that of compressed storage under the same operating conditions. The Kubas-type vanadium(II)-dihydrogen complexation is characterized by a combination of techniques. From powder neutron diffraction data, a V-D2(centroid) distance of 1.966(8) Å is obtained, the shortest yet reported for a MOF. Using in situ infrared spectroscopy, the H-H stretch was identified, and it displays a red shift of 242 cm-1. Electronic structure calculations show that a main contribution to bonding stems from the interaction between the vanadium dπ and H2 σ* orbital. Ultimately, the pursuit of MOFs containing high densities of weakly π-basic metal sites may enable storage capacities under ambient conditions that far surpass those accessible with compressed gas storage.

Effects of bismuth doping on the physical and electrochemical properties of Na3V2-xBix (PO4)3/C (0 ≤ x ≤ 0.07) composite cathode materials

Na3V2-xBix(PO4)3/C (0 ≤ x ≤ 0.07) samples were prepared by a facile sol-gel method. Dependence of their physical and electrochemical properties on the amount of Bi3+ doping are investigated by X-ray diffraction (XRD) analysis, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma optical emission spectrometer (ICP-OES), impedance spectroscopy (EIS), cyclic voltammetry (CV), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HRTEM). It is found that Bi3+ is substituted on the vanadium site without damage to the structure of Na3V2(PO4)3 compound. The Na3V1.97Bi0.03(PO4)3/C cathode materials demonstrate superior rate capability and delivered specific capacity of 116.3 and 77.4 mAh∙g−1 at charge-discharge of 0.1 C and 20 C (1 C =117.6 mA∙g−1), respectively. Circulating at a high current rate of 20 C, the discharge capacity of 67.3 mAh∙g−1 can be still reached, and the capacity can remain at 86.68% over 100 cycles. Na3V2-xBix(PO4)3/C composites possess an excellent electrochemical characteristic, which mainly ascribes to its nano-sized particles, stability in structure, and enhancement in ionic and electronic conductivity arose from Bi3+ doping.

Atomically Dispersed Vanadium Sites Anchored on N-Doped Porous Carbon for the Efficient Oxidative Coupling of Amines to Imines.

Single-atom catalysts effectively integrate the respective advantages of homogeneous and heterogeneous catalysts and are a pioneering research frontier in catalysis by virtue of their maximized utility of metal atoms and distinct atomic configuration. However, development of such catalysts is still in the early stages. Herein, atomically dispersed vanadium (V) sites that are coordinated by N atoms and inlaid within N-incorporated porous carbon networks were prepared through a top-down strategy by annealing a V-containing metal-organic framework, NH2-MIL-101(V), followed by acid etching. The resulting V-N-C-600 catalyst exhibits unexpected catalytic reactivity, selectivity, and robust stability for the direct aerobic oxidation of benzylamine to generate N-benzylidene benzylamine with molecular oxygen under mild conditions. The turnover frequency reaches 53.9 h-1, which is much superior to those achieved over the commercial V2O5 and state-of-the-art non-noble metal heterogeneous catalysts reported in the literature. Kinetic analysis reveals a low activation energy barrier (37 kJ mol-1) for the benzylamine oxidation and indicates that a carbocationic intermediate is involved in the reaction mechanism. The synergistic effect between the isolated V single-atomic sites and N-doped hierarchically porous carbon network boosts the performance of V-N-C-600. Moreover, V-N-C-600 exhibits a wide generality for the efficient synthesis of a set of symmetrical imines, unsymmetrical imines, and imine derivatives.

Importance of Crystallographic Sites on Sodium-Ion Extraction from NASICON-Structured Cathodes for Sodium-Ion Batteries.

The V4+/V3+ (3.4 V) redox couple has been well-documented in cathode material Na3V2(PO4)3 for sodium-ion batteries. Recently, partial cation substitution at the vanadium site of Na3V2(PO4)3 has been actively explored to access the V5+/V4+ redox couple to achieve high energy density. However, the V5+/V4+ redox couple in partially substituted Na3V2(PO4)3 has a voltage far below its theoretical voltage in Na3V2(PO4)3, and the access of the V5+/V4+ redox reaction is very limited. In this work, we compare the extraction/insertion behavior of sodium ions from/into two isostructural compounds of Na3VGa(PO4)3 and Na3VAl(PO4)3, found that, by DFT calculations, the lower potential of the V5+/V4+ redox couple in Na3VM(PO4)3 (M = Ga or Al) than that in Na3V2(PO4)3 is because of the extraction/insertion of sodium ions through the V5+/V4+ redox reaction at different crystallographic sites, that is, sodium ions extracting from the Na(2) site in Na3VM(PO4)3 while from the Na(1) site in Na3V2(PO4)3, and further evidenced that the full access of the V5+/V4+ redox reaction is restrained by the excessive diffusion activation energy in Na3VM(PO4)3.

Decoupling the metal insulator transition and crystal field effects of VO2

VO2 is a highly correlated electron system which has a metal-to-insulator transition (MIT) with a dramatic change of conductivity accompanied by a first-order structural phase transition (SPT) near room temperature. The origin of the MIT is still controversial and there is ongoing debate over whether an SPT induces the MIT and whether the Tc can be engineered using artificial parameters. We examined the electrical and local structural properties of Cr- and Co-ion implanted VO2 (Cr-VO2 and Co-VO2) films using temperature-dependent resistance and X-ray absorption fine structure (XAFS) measurements at the V K edge. The temperature-dependent electrical resistance measurements of both Cr-VO2 and Co-VO2 films showed sharp MIT features. The Tc values of the Cr-VO2 and Co-VO2 films first decreased and then increased relative to that of pristine VO2 as the ion flux was increased. The pre-edge peak of the V K edge from the Cr-VO2 films with a Cr ion flux ≥ 1013 ions/cm2 showed no temperature-dependent behavior, implying no changes in the local density of states of V 3d t2g and eg orbitals during MIT. Extended XAFS (EXAFS) revealed that implanted Cr and Co ions and their tracks caused a substantial amount of structural disorder and distortion at both vanadium and oxygen sites. The resistance and XAFS measurements revealed that VO2 experiences a sharp MIT when the distance of V–V pairs undergoes an SPT without any transitions in either the VO6 octahedrons or the V 3d t2g and eg states. This indicates that the MIT of VO2 occurs with no changes of the crystal fields.

Modified H2 V3 O8 to Enhance the Electrochemical Performance for Li-ion Insertion: The Influence of Prelithiation and Mo-Substitution.

Nanostructured H2V3O8 is a promising high‐capacity cathode material, suitable not only for Li+ but also for Na+, Mg2+, and Zn2+ insertion. However, the full theoretical capacity for Li+ insertion has not been demonstrated experimentally so far. In addition, improvement of cycling stability is desirable. Modifications like substitution or prelithiation are possibilities to enhance the electrochemical performance of electrode materials. Here, for the first time, the substitution of vanadium sites in H2V3O8 with molybdenum was achieved while preserving the nanostructure by combining a soft chemical synthesis approach with a hydrothermal process. The obtained Mo‐substituted vanadate nanofibers were further modified by prelithiation. While pristine H2V3O8 showed an initial capacity of 223 mAh g−1 and a retention of 79 % over 30 cycles, combining Mo substitution and prelithiation led to a superior initial capacity of 312 mAh g−1 and a capacity retention of 94 % after 30 cycles.

Insight into Diffusion Mechanism in Cathode Materials NaVPO5 and NaVFPO4 for Sodium Ion Batteries: DFT Investigation

In transition metal-based cathode materials, the oxidation and reduction of transition metal ions cause local distortion in the crystal structure during intercalation and de-intercalation, resulting in the formation of a quasiparticle, named small polaron. Since a strong binding energy of 500 meV between an alkali ion and a small polaron was reported[1], the polaron would migrate simultaneously with the diffusion of the alkali ions[2]. Using the density functional method, we investigate the crystal, electronic structure, and electrochemical properties of orthorhombic NaxVPO5 and NaxVPO4F, focusing on the diffusion mechanism of a Na ion accompanied with polaron. The diffusion of Na ions can be described as a process of the complex of the Na vacancy and accompanying positive polaron at high Na concentrations and as that of the Na ion and negative polaron at low Na concentrations. When a Na vacancy is introduced to fully occupied structure (x=1), the positive small polaron prefers locating at one of the two first nearest neighbour (1NN) vanadium sites V1NN to the Na vacancy. Similarly, after a Na ion is inserted to fully unoccupied structures (x=0), the negative polaron is formed at a V1NN site of Na ion. Three elementary diffusion processes (EPDs), including the single, crossing,and parallel diffusion processes are explored. The single process occurs when the polaron that strongly binding with the Na vacancy stays at the same position during the Na vacancy diffusion. On the other hand, the parallel (crossing) diffusion process takes place when polaron hops from a V1NN site to the adjacent V1NN site along the direction paralleled (crossed) to Na diffusion path. It is found that the [010] direction is preferable for Na ions diffusing during charging or discharging processes. At a high Na concentration regime, the activation energy E a gains approximately 400 meV on all three EPDs in NaVPO5, whereas it lowers to 190 meV, 320 meV and 345 meV for single, crossing and parallel EDPs, respectively, in NaVPO4F. These values of E a indicate that the effect of polaron migration is negligibly weak in NaVPO5 at high Na concentrations. At low Na concentrations, such a effect of negative polaron migration on Na ion diffusion is considerably substantial in both materials. While the parallel diffusion process is less preferred (Ea = 830 meV and 570 meV for VPO5 and VPO4F, respectively), the single and crossing processes can occur with same activation energy of about 630 meV and 530 meV for VPO5 and VPO4F, respectively. In general, energy barriers required for diffusion in NaVPO4F are notably lower than those in NaVPO5. Compared with some common cathode materials such as the olivines, it is expected that NaVOPO4 and NaVPO5 perform as good as LiFePO4 for LIBs.Figure 1: Diffusion pathway of Na vacancy–positive polaron complex along the [010] direction in de-intercalation. The brown and green octahedra indicate the 1NN and 2NN VO6 groups relative to the Na vacancy, respectively. The red, green and blue balls illustrate the trace of the crossing, single and parallel diffusion processes, respectively. Curved arrows illustrate the migration directions of the polaron in each EDP. [1]T. Maxisch, F. Zhou and G. Ceder, Phys. Rev. B: Condens. Matter Mater. Phys., 73, 104301 (2006). [2]V. A. Dinh, J. Nara, and T. Ohno, Appl. Phys. Express, 5,045801(2012). Figure 1

Unexpected discovery of magnesium-vanadium spinel oxide containing extractable Mg2+ as a high-capacity cathode material for magnesium ion batteries

Magnesium ion batteries (MIBs) have attracted significant attention as a promising candidate for the next generation energy storage system owing to their large volumetric capacity and abundant resource. Currently, several researchers have focused on Mg-free materials to be used as cathode in MIBs; But the exploration of Mg-rich electrode materials will promote the development of magnesium batteries towards obtaining more flexible MIBs. Herein, a Mg(Mg0.5V1.5)O4 is investigated as a potential cathode material, in which the Mg2+ can be extracted and the fast Mg2+ reaction kinetics. Benefited from the exceptional cathode, the Mg battery shows a high reversible specific capacity of 250 mA h g−1 at 100 mA g−1 and capacity retention of 100 mA h g−1 after 500 cycles at high rate of 1 A g−1. The excellent rate capability and desirable cycling performance obtained herein outperformed those of previously reported magnesium spinel oxides. It is also demonstrated that Mg2+ extraction/insertion mechanism of Mg(Mg0.5V1.5)O4 is related to the coexistence of two-phase process and single-phase solid solution reaction through a series of systematic in situ/ex situ characterizations. X-ray absorption near edge structure (XANES) demonstrates that the valence state of vanadium changes and the octahedral symmetry of vanadium site varies due to the extraction of Mg2+ from spinel Mg(Mg0.5V1.5)O4. Significantly, the cathode containing extractable Mg2+ can be coupled with Mg-free anode materials (Na2Ti3O7) to assemble a full cell in Mg(TFSI)2/acetonitrile electrolyte, displaying a discharge capacity of 102 mA h g−1 after 100 cycles at 50 mA g−1. The encouraging results show that the Mg(Mg0.5V1.5)O4 is a promising electrode material, which paves ways for the development and further improvements of MIBs.

Insights into the elevated electrochemical performance and kinetic characteristics of magnesium-substituted Na3V2−xMgx(PO4)3/C with superior rate capability and long lifespan

Na3V2(PO4)3 has been pursued to be a prospective cathode for SIBs because of its outstanding structural stability. Nevertheless, the extensive application of Na3V2(PO4)3 is impeded by its poor electronic conductivity and inferior Na+ migration ability. Herein, a promising Mg-doped Na3V2−xMgx(PO4)3/C composite is prepared by a facile carbon-thermal reduction route. The substitution of magnesium onto vanadium site downsizes the particle, providing shorter pathway for the migration of Na+ and electron. The introduction of Mg2+ generates beneficial holes to facilitate the electronic diffusion efficiently. A superior electrochemical performance of Na3V1.93Mg0.07(PO4)3/C sample can be achieved due to the multiple synergetic effects. It delivers a high specific capacity of 113.5 mAh g−1 at 0.1 C. A high reversible capacity of 95 mAh g−1 can be obtained at 10 C rate, and the retention is 84.6% after 1000 cycles. Moreover, a comprehensive GITT analysis is conducted to give a better understanding of the elevated electrochemical properties for Na3V1.93Mg0.07(PO4)3/C: The migration of Na+ suffers from the intense interactions arising from the phase transfer during the potential plateau (~ 3.4 V). The minimum values of DNa+ at ~ 3.4 V for Na3V1.93Mg0.07(PO4)3/C are one order magnitude higher than that of undoped sample, implying the improved kinetics from magnesium substitution.

Photoelectrochemical Decomposition of Lignin Model Compound on a BiVO4 Photoanode.

The photoelectrochemical decomposition of lignin model compounds at a BiVO4 photoanode is demonstrated with simulated sunlight and an applied bias of 2.0 V. These prototypical lignin model compounds are photoelectrochemically converted into the corresponding aryl aldehyde and phenol derivatives in a single step with conversion of up to ≈64 % over 20 h. Control experiments suggest that vanadium sites are electrocatalytically active, which precludes the need for a redox mediator in solution. This feature of the system is corroborated by a layer of V2 O5 deposited on BiVO4 serving to boost the conversion by 10 %. Our methodology capitalizes on the reactive power of sunlight to drive reactions that have only been studied previously by electrochemical or catalytic methods. The use of a BiVO4 photoanode to drive lignin model decomposition therefore provides a new platform to extract valuable aromatic chemical feedstocks using solar energy, electricity and biomass as the only inputs.