Vanadium Substitution Research Articles

Understanding the Relation Between Intrinsic Parameters of Substituents and Physical‐Chemical Properties of NVP

AbstractNASICON‐type Na3V2(PO4)3 (NVP) is regarded as an intriguing cathode material choice for sodium ion batteries (SIBs) due to its cycling stability and relatively high capacity. However, its voltage and electronic conductivity still need to be improved for larger‐scale fast‐charging applications (e.g. electric vehicles and mobile phones). In this work, we investigate the influence of Vanadium (V) substitution by other environmentally friendly, cheap, and/or high‐valent transition metal (TM) elements on the electrochemical performance of NVP. Density functional theory calculation was used to study the volume change, voltage, conductivity, and redox mechanism during charge/discharge of different compositions. It is found that a substitution of 50% of V by Mn, Mo or W ions resulting in Na3VMn(PO4)3 (NVMnP), Na3VMo(PO4)3 (NVMoP), and Na3VW(PO4)3 (NVWP) significantly alters the cathode materials’ physical and chemical properties, notably decreasing the band gap. In particular, NVMnP has lesser than 1 eV theoretical band gap and provides a higher voltage, while NVWP a much lower voltage in comparison to NVP. This means that NVMnP and NVWP can be promising cathode and anode materials respectively. This work also establishes a relation between fundamental properties of substituents (i.e. ionization energy and ionic size) and the overall performance of NVP.

Effect of vanadium doping on structural, dielectric, magnetic, and ferroelectric properties of Ba0.95La0.05TiO3 ceramics

The polycrystalline different Ba0.95La0.05Ti1-xVxO3 (where, x = 0.00 to 0.12) ceramics were synthesized using solid-state reaction method. The toroid- and pellet-shaped samples were sintered at 1200°C temperature for 4 h. According to the outcomes of the X-ray diffraction (XRD) investigation, each sample possesses a tetragonal crystal structure that includes a few impurity peaks. Extra Raman modes at 327 cm−1 (for x = 0.06 to 0.12) may be attributed to localized phonon vibrations in the vicinity of a (substitutional) defect or as a result of the relaxation of Raman selection rules due to defect-induced disorder. The density showed a progressive increase as the concentration of V increases, reaching its maximum at the x = 0.06 sample, after which it drops. The x = 0.06 sample displayed the highest bulk density, with a value of ρB = 4.01 g/cm3. Scanning Electron Micrograph (SEM) showed gradual agglomeration of the grain with V-content due to magnetic dipole interaction between one particle and other neighboring particles and exposed the porous nature of the samples. The TEM image demonstrates that the particles are well crystalline. For all of the compositions, the dielectric constant was larger in lower frequency areas and remained constant in higher frequency region. The sample with x = 0.06 showed a maximum dielectric constant of 1140 at 102 Hz. AC conductivity increased with increasing frequency. The Nyquist plot suggested an appearance of grain boundary (bulk) property of the material. The constant value of the real part of complex initial permeability over long range of frequency indicated the magnetic stability of the materials. For ceramics with x = 0.03, the largest real part of the initial permeability of 17.29 is evident. From the M − H loop, at room temperature, feeble ferromagnetism was observed with vanadium substitution in BLT ceramics. It is found that the highest saturation magnetization for the x = 0.03 sample is 2.61 emu/g. For the composition of x = 0.06, the highest polarization of 2.80 μC/cm2 is observed.

Tuning the electronic structure of monolayer MoS2 towards metal like via vanadium doping

Doping of two-dimensional layered semiconducting materials is becoming pivotal in tailoring their electronic properties, enabling the development of advanced electronic and optoelectronic devices, where the selection of dopant is important. Here, we demonstrate the potential of substitutional vanadium (V) doping in monolayer molybdenum disulfide (MoS2) lattice in different extents leading to tunable electronic and optoelectronic properties. We found that low-level V doping (∼1 at.%) induces p-type characteristics in otherwise n-type monolayer MoS2, whereas medium-level doping (∼5 at.%) leads to an ambipolar semiconductor. Degenerately doped MoS2 (∼9 at.%) facilitates a transition from semiconducting towards metallic (metal-like) with reduced electrical resistivity (∼4.5 Ωm of MoS2 to ∼2.2×10−5Ωm), low activation energy for transport (∼11 meV), and electric field independent drain current in field effect transistor–based transfer characteristics. A detailed temperature- and power-dependent photoluminescence study along with density functional theory–based calculations in support unravels the emergence of an excitonic transition at ∼850 nm with its intensity dependent on the amount of vanadium. This study shows the potential of V doping in MoS2 for generating multifunctional two-dimensional materials for next generation electronics, optoelectronics, and interconnects with systematic control over its electronic structure in a wide range. Published by the American Physical Society 2024

Vanadium embedded in monolayer silicene: Energetics and proximity-induced magnetism

Using the density-functional theory approach, including Hubbard U correction, we investigate the defect structures consisting of vanadium (V) atoms embedded in a monolayer silicene. Specifically, we consider V–V atom pairs in antiferromagnetic (AFM), ferromagnetic (FM), and non-magnetic states, which are embedded in substitutional and interstitial sites. We determine the ground-state structures, formation and binding energies, electronic structures, induced magnetization, as well as the spin-exchange coupling between the V–V pair. For the substitutional vanadium atom pair, the stability of the AFM and FM spin configurations depends on the sublattice sites in which the V atoms are sited. When the V pair is located on a similar sublattice site type, the AFM spin alignment is more energetically favored, whereas when the pair is located in a different sublattice site, the FM interactions are more stable. However, the relative stability of the AFM or FM configurations changes rapidly as the separation between the V pair increases. Regarding the interstitial-hole V–V pair configurations, the most stable structure is when the pair is at the nearest-neighbor hole sites and is in an FM alignment. Also, at larger separations, the AFM or FM hole configurations are approximately degenerate in energy. Furthermore, we elucidate on the Ruderman–Kittel–Kasuya–Yosida, direct-exchange, and the superexchange interaction mechanisms in the vanadium-embedded silicene. In addition, we estimate a Curie temperature (Tc) of up to ∼500 K for a silicene structure containing a V pair in the FM spin alignment. Such a high Tc, in addition to the stability of the material, suggests that vanadium-embedded silicene is a potential candidate material for spintronic device applications.

Rational optimization of substituted α‐MnO2 cathode for aqueous zinc‐ion battery

AbstractDensity functional theory (DFT) is utilized to explore the effects of increasing concentrations of vanadium (V) and chromium (Cr) substitution on the discharge of α‐MnO2 cathode in hydrated zinc‐ion batteries (ZIBs). During H+‐intercalation and Zn2+‐intercalation, Cr‐substitution proves to be more effective than V‐substitution in promoting discharge behaviors. Transitioning from Mn0.875Cr0.125O2, Mn0.75Cr0.25O2 to Mn0.625Cr0.375O2 is found to consistently enhance the discharge voltage, along with improved tunnel structure retention and volume expansion suppression. In comparison, the promoting effect of increasing V‐substitution is relatively small at initial discharge stages and leads to degradation at later stages, primarily due to an increased concentration of unstable Mn2+ ion. The superior effect of Cr‐substitution is attributed to the unique atomic and electronic structures of substituted Cr4+ and reduced Cr3+ ions during discharge. These ions serve as active electron acceptors to limit the formation of Mn3+ and Mn2+ ions, and as anchors to stabilize the α‐MnO2 framework and intercalated H+/Zn2+ ions, respectively. Our study highlights fine‐tuning through substitution to enhance the performance of α‐MnO2‐based cathode materials in ZIBs.

Investigating the structural, electronic, and magnetic properties of Cd1-xVxTe: Insights from First-Principles calculations

In our study, we aim to investigate the structural, electronic, and magnetic properties of CdVTe, a diluted magnetic semiconductor with a Cadmium telluride (CdTe) structure. To achieve this, we employ the full potential linear augmented plane wave method implemented in the CASTEP code, which allows us to accurately simulate the behavior of the system. To describe the electronic exchange and correlation effects, we adopt the generalized gradient approximation (GGA) within the density functional theory (DFT) framework. This choice of methodology ensures reliable and accurate calculations of the electronic and magnetic properties of Cd1-xVxTe. Our results reveal that the substitution of Vanadium (V) into the Cadmium (Cd) site of the CdTe lattice does not alter the zinc blende crystal structure, demonstrating the stability and structural integrity of the system. Furthermore, our calculations demonstrate that the introduction of Vanadium impurities leads to spin polarization within the material, resulting in the emergence of a magnetic moment. Notably, we find that a Vanadium concentration of approximately x≈0.12 exhibits the strongest magnetic properties, characterized by a significant magnetic moment. These findings provide valuable insights into the behavior and potential applications of CdVTe as a diluted magnetic semiconductor, shedding light on the interplay between structural, electronic, and magnetic properties in this material.

Unlocking Quasi‐Monophase Behavior in NASICON Cathode to Drive Fast‐Charging Toward Durable Sodium‐Ion Batteries

AbstractThe NASICON cathode, Na3V2(PO4)3, has garnered significant attention due to its robust framework with fast Na+ migration. To expand its application scenarios by diversified electronic reaction, the substitution of vanadium with cost‐effective and abundant redox elements is a special research topic. Nevertheless, in terms of reducing toxicity, increasing Na content and widening voltage range, the V4+/5+ redox couple in Na4FeV(PO4)3 often accompanies asymmetric and irreversible electrochemical reactions that pose a dilemma for capacity and structural stability, especially at high currents. Herein, in this work, Na4FeV1/3Ti2/3(PO4)3 (NFVT) has achieved highly reactive of multiple electron transfer (Ti2+/3+, Fe2+/3+, and V3+/4+/5+) by utilizing the redox reaction with quasi‐monophase behavior, and it can reserve great capacity retention after 3,000 cycles. More competitively, its boosting kinetics makes the fast‐charging characteristic, just requiring only 3.63 min to reach 80% state of charge at 2 C. The rapid ion/electron transport dynamics can achieve the decay of only 0.043% per cycle by unlocking the quasi‐monophase behavior in the framework of NFVT full cells. The present study provides a fresh perspective on designing cathode materials with fast‐charging capabilities for sodium‐ion batteries.

Reactivity control of nitrate-incorporating octadecavanadates by changing the oxidation state and metal substitution.

Clarification and control of the active sites at the atomic/molecular level are important to develop nanocatalysts. The catalytic performance of two oxidation states of nitrate-incorporating octadecavanadates, [V18O46(NO3)]5- (V18) and [V18O46(NO3)]4- (V18ox), and a copper-substituted one, [Cu2V16O44(NO3)]5- (Cu2V16), in selective oxidation was investigated. Both V18 and V18ox possessed the same double-helical structures and one of two tetravalent vanadium sites of V18 was oxidized in V18ox. The comparison of the mobility of the incorporated nitrate reveals that tetravalent vanadium centres show stronger interaction with the incorporated anions than pentavalent ones. The oxidation reaction with V18ox proceeded more smoothly with tert-BuOOH as an oxidant than that with V18. The reactivity and selectivity of the oxidation of 2-cyclohexen-1-ol were different among the derivatives. V18ox showed the higher reactivity with 72% selectivity to epoxide. With V18, reactivity was lower but higher selectivity to epoxide was achieved. In the presence of Cu2V16, 2-cyclohexen-1-one was selectively obtained with 81% selectivity. The order of the reactivity for cyclooctene was V18ox, V18 and Cu2V16. These results shows that the cap part of the double-helix acts as the active site. Even though the vanadium-oxygen species exhibit the same structures, the catalytic properties can be controlled by changing the valence of vanadium and metal substitution.

The role of vanadium substitution in the oxygen sublattice disorder of Ba7Nb4MoO20-based hexagonal perovskite oxide-ion conductors.

Ba7Nb4MoO20-based hexagonal perovskite derivatives are promising oxygen-ion conductors for solid electrolytes in solid-oxide fuel cells and electrolysers. A thorough understanding of chemical substitution and its impact on structural features conducive to high ionic conductivity is fundamental for decreasing the operation temperature of such devices. Here, a new 7H polytype-based composition, namely Ba7Nb3.9-x V x Mo1.1O20.05, is investigated to assess the effect of vanadium substitution. Structural changes upon V incorporation are studied using X-ray and neutron diffraction, as well as 51V and 93Nb solid-state nuclear magnetic resonance spectroscopy. For the undoped composition at room temperature, two distinct oxygen sites (O1 and O5) are found along the palmierite-like layer, corresponding to a mix of four- and six-fold coordination for adjacent M2 cations. At high temperature (527 °C), reorganization of oxygen results in the major occupation of O1 and four-fold (tetrahedral) coordination of the M2 cations. The same rearrangement is observed upon V-substitution, but already at room temperature. From 51V NMR, we identified a tetrahedral coordination for V5+ cations, indicating their preferential occupation of the M2 site. This preferential occupation by V5+ cations is correlated with increasing tetrahedral coordination of Nb5+ cations as observed from 93Nb NMR. Altogether, these observations indicate that V-substitution impacts the oxygen sublattice so as to mimic the high-temperature structure. Additionally, BVSE calculations demonstrate a decreasing energy barrier for O2- migration associated with the presence of vanadium in the structure. This conclusion corroborates the hypothesis that vanadium's propensity for a lower coordination number is beneficial for promoting high O2- mobility in this promising class of oxide-ion conductors.

Tailoring titanosilicate molecular sieves by vanadium substitution for humidity sensing applications

There is a great interest in developing humidity sensors with enhanced sensitivity at all relative humidity (RH) ranges. To explore this, vanadium (V)-incorporated titanosilicate microporous thin films were successfully fabricated to investigate their performances as humidity sensors. The V-precursor was directly added to obtain V/V+Ti atomic of 0.1 (V0.1ETS-10) to 0.2 (V0.2ETS-10) and 0.3 (V0.3ETS-10), and characterized by FE-SEM, XRD, Raman, UV-Vis, and XPS analyses. The ETS-10 sensor displayed high sensitivity (1.11 × 104) and linearity (R2=0.9818) at room temperature across medium (32–67 %) and high (>67 %) relative humidity (RH), with rapid response (∼8 s) and recovery times (∼35 s). The sensor with an optimized amount of vanadium doping (V0.2ETS-10), showed a significant improvement in the performance in the low RH region (8–32 %) with enhanced sensitivity (1.75 × 104) and linearity (R2=0.9914). The results of our study indicate that the formation of Ti3+ with the emergence of V4+ and V5+ states resulted in enhanced sensitivity at low RH levels. Based on the obtained results, a plausible shift in the humidity sensing mechanism evolves from chemisorbed intracrystalline water to physisorbed intercrytalline and surface water with increasing RH. However, higher vanadium content led to structural alteration and impurities resulting in a notable reduction in sensitivity (1.17 × 103) and linearity (R2=0.8927).

Influence of Vanadium substitution on electronic, thermoelectric and optical response of Cu2O

Cu2O semiconductor attained much research interest due to excellent electronic and optical response. In this work, Vanadium-doped Cu2O compositions were studied for electronic, thermoelectric, and optical response using density functional theory. The thin films were experimentally fabricated using the chemically derived spin coating method. The x-ray diffraction analysis revealed the growth of crystalline thin films with cubic structure having space-group 224-Pn-3m. The scanning electron micrographs exhibit the uniform grain growth with well-defined grain boundaries for pure Cu2O films. Density of states spectra display the maxima for O-2p and Cu-3d while V-3d states occupied the conduction band. The value of the experimental band gap for pure Cu2O is estimated as 2.02 eV and found to decrease with V-doping. The Seebeck coefficient and specific heat are found to increase with the increment in V-doping content due to thermal fluctuations. A steady increase is observed in real epsilon with the increase in energy and dopant concentration.

Modulated Magnetic, Electronic, and Optical Properties of Vanadium‐Doped LaAlO3: A First‐Principles Study

The optoelectronic and magnetic features of La1−xVxAlO3 (x = 12.5, 25, 50, 75%) perovskites are investigated within density functional theory (DFT) using generalized gradient approximation by Perdew–Burke–Ernzerhof (PBE–GGA). The effect of different percentages of vanadium (V) substitution on the properties of LaAlO3 is studied and stability is confirmed through formation energies. The spin‐polarized band structure of La1−xVxAlO3 (x = 12.5, 25, 50, 75%) elucidates their half‐metallic ferromagnetic character. Furthermore, the optical performance of the given compounds is explored by optical parameters like dielectric function, optical conductivity, refractive index, reflectivity, absorption, and extinction coefficients. The obtained results make these compounds suitable for optoelectronic applications. It is observed that the determined magnetic moment in La1−xVxAlO3 is mainly due to the V‐3d states. The regulated reduction in bandgap (Eg) with the upsurge of V doping and half‐metallic ferromagnetic nature makes them appropriate for spintronic and magnetic devices.

Photoactive TiO2-V2O5-WO3 film composites immobilized in titanium phosphate matrix by plasma electrolytic oxidation

V-, W-containing oxide-phosphate layers on titanium were obtained by plasma electrolytic oxidation (PEO) for 5 min at anode current density i = 0.08 A/cm2 in electrolytes containing 0.05 mol/L Na6P6O18 and 0.1 mol/L (Na2WO4 + NaVO3). The effect of replacing vanadate with tungstate (NaVO3:Na2WO4 = 0:1, 1:3, 1:1, 3:1, and 1:0) was studied on voltage–time responses, PEO coatings surface morphology, composition, optical and photocatalytic properties. The coatings were investigated by X-ray diffraction analysis, scanning electron microscopy, energy-dispersive X-ray microanalysis, X-ray photoelectron spectroscopy, and diffuse reflectance spectroscopy. Under experimental conditions, amorphous coatings with a thickness of 18–27 μm and a surface porosity of 1.5–3.6% were formed. Results show that vanadium (up to 10.5 at. %) and tungsten (up to 5.2 at. %) are incorporated into surface layers in proportion to their concentrations in electrolyte. The main mechanism for the incorporation of V and W into coatings is assumed to be thermolysis of vanadungstophosphate heteropolyoxoanions, while high concentrations of polyphosphate species contribute to the amorphization of coatings and the formation of a titanium phosphate matrix. The band gap of V-containing composites decreases from 2.22 to 1.97 eV as tungsten replaces vanadium. For W-containing composites (V-free ones), the band gap is 3.19 eV. All formed coatings exhibit photocatalytic activity in the degradation of methyl orange (10 mg/LMO, 10 mmol/L H2O2) in a neutral medium (pH 6.8) under visible and UV irradiation. The gradual substitution of vanadium for tungsten in the coatings leads to a decrease in MO degradation (from 87 to 50% and from 32 to 9% under 3-h UV and visible irradiation) with a simultaneous increase in their durability due to a decrease in vanadium leaching. An electrolyte with Na2WO4:NaVO3 = 1:3 is optimal for the formation of photoactive and stable composites. When using composites obtained in this electrolyte, MO conversions are 83 and 31% under UV and visible irradiation, respectively. The mechanism of the photocatalytic action of WO3-V2O5-TiO2 composites is considered, and the role of hydrogen peroxide is discussed.

Development of modulation, pairing mechanism, and slip system with optimum vanadium substitution at Bi-sites in Bi-2212 ceramic structure

Present study focuses extensively on the change in electrical, superconducting and microhardness parameters with partial substitution of trivalent V+3 impurities replacing Bi+3 ions in Bi-2212 ceramic compound with the aid of dc electrical resistivity and microhardness test measurements. Experimental findings, calculation results, and phenomenological discussions provide that the optimum vanadium substitution level is found to be x = 0.01 in the Bi2.0-xVxSr2.0Ca1.1Cu2.0Oy (Bi-2212) ceramic system for the highest conductivity, crystallinity quality, superconducting, and mechanical performance features depending on the decreased microscopic structural problems. All the findings are wholly verified by scanning electron microscopy (SEM) and X-Ray diffraction (XRD) analyses. The dc electrical measurements indicate that the optimum vanadium ions support the pairing mechanism for the formation of new polaronic states in the clusters of microdomains, and hence expand superconducting energy gap due to the enhancement of amplitude part of pair wave function in the spin-density wave systems. The excess vanadium content degrades all the basic thermodynamics and quantum mechanical quantities mentioned due to the stress-induced phase transformation. Numerically, the Bi-2212 advanced ceramic matrix prepared by the optimum vanadium impurity is noticed to present the smallest residual resistivity value of 0.08 mΩ cm, room temperature resistivity value of 8.84 mΩ cm, and broadening degree of 0.36 K. Similarly, the ceramic material is found to possess the highest residual resistivity ratio of 3.05, carrier concentration number of 0.153041, critical transition offset and onset value of 84.66 K and 85.02 K, respectively. Besides, the microhardness findings reveal that the same compound with the least sensitivity to the applied test loads exhibits the largest Hv value of 4.799 GPa, Young's moduli of 393.303 GPa, yield strength of (0.969 GPa), and elastic stiffness coefficient of 15.5574 (GPa)7/4 under the applied test load of 0.245 N. The XRD investigations show that the presence of optimum vanadium impurity supports the formation of a high superconducting phase, c-axis length, and average crystallite size. All the findings are morphologically confirmed by the SEM images. It is found that the crystallographically best crystallinity quality and view of surface morphology is observed for the optimum vanadium substitution level. All in all, new higher properties for the conductivity, crystallinity quality, surface morphology, superconducting, and microhardness parameters based on the optimum vanadium replacement encourage the Bi-2212 crystal system to use in much more application places.

Optimizing the electrochemical activity and understanding the reaction mechanism of Li3.27FeII0.19FeIII0.81V(PO4)3 cathode material for lithium-ion batteries

This work reports the synthesis of a NaSICON-type structure material, a cost-efficient electrode for lithium-ion batteries, by the substitution of vanadium with cost-effective and eco-friendly iron. Li3.27FeII0.19FeIII0.81V(PO4)3 was synthesized using sol-gel followed by a calcination at 450 °C under argon. XRD has confirmed the successful preparation of a compound isostructural to Li3V2(PO4)3 which crystallizes in a monoclinic structure with the P21/c space group. The electrochemical properties of the novel phosphate were explored at different cut-off voltage windows (2.0–4.2 V, 2.8–4.5 V, 1.4–4.5 V, and 1.0–4.6 V), and using three electrolytes (LP30, LP40, and LP50). The best electrochemical performance was obtained at C/2 over 1.4–4.5 V voltage range and using LP40 delivering a reversible capacity of 150 mAh g−1 after 55 cycles with coulombic efficiency and capacity retention of 99% and 79%, respectively. 57Fe Mӧssbauer spectroscopy evidenced the contribution of Fe2+/Fe3+ redox couple upon the electrochemical reaction. Magnetic measurements revealed the oxidation of the transition metals during the charge process. XAFS study around V and Fe at K-edge revealed the successful substitution of Fe in the V site with a reorganization of the local structure at short and medium range order.

Impact of vanadium substitution on structural, magnetic, microwave absorption features and hyperfine interactions of SrCo hexaferrites

The Co-V co-substituted nano Sr-hexaferrite, Sr0.5Co0.5VxFe12−xO19 (0.00 ≤ x ≤ 0.08) (V→SrCo (0.00 ≤ x ≤ 0.08) NHFs), were manufactured by citrate combustion route. The microstructure, morphology, magnetic features, hyperfine interactions and oxidation state and chemical composition of products explained by XRD, SEM with EDX, HR-TEM, VSM, Mossbauer spectrometry, and XPS, respectively. The DXRD (Average crystallite size) of products were assessed via Scherrer formula as within the range of 37 and 83 nm. The hexagonal morphology and chemical composition of the products was presented by TEM, HR-TEM, SEM and EDX analyses. The XPS analysis provided both the composition and oxidation states of prepared NHFs. Mossbauer spectra showed that the s electron density around Fe3+ ions of all sites except 12k influenced V3+ content. The coercivity data indicates that all samples denote a magnetically hard material at both low and high temperatures. Ms, Mr and nB fluctuate with respect to dopant concentration and attain their highest magnitudes for the V→SrCo (x = 0.04) NHFs. A SQR of around 0.5 is achieved at each temperature, reflecting a relatively high large anisotropy field. The field-cooling (FC) curves of all samples yield a drop in magnetization with increasing temperature. Their irreversibility temperature is detected to be beyond 325 K. Both M-H and M-T imply the ferrimagnetic behavior of different samples. Microwave properties were measured between 2 and 18 GHz as S11-S21 parameters. It demonstrated non-linear behavior of the microwave properties vs. V concentration. The origin of the reflection losses in NHVs can be explained by the dipole polarization processes.

Surface engineered MXene with multi-electroactive sites for developing durable and efficient water-splitting electrolyzer

Combining multilayered structure, tunable physicochemical, and surficial properties, two-dimensional Ti3C2Tx MXene has been extensively investigated for their potential in field of noble-metal free electrocatalysis. However, easy aggregation of MXene nanosheets and their thermodynamic instability drastically reduce active sites and, hence, decline overall water-splitting efficiency. In this regard, approaches of surface engineering of MXene by introducing large-sized dopants have been acknowledged to overcome aforementioned issues. Considering this, herein, we have proposed a strategy for tuning MXene surface by substitutional vanadium doping, which induced additional accessible electrochemically active surface sites (V0+, V1+/2+, V3+, V4+, and V5+), along with inherent MXene sites (Ti2+, Ti3+, and Ti4+). This also inhibited inevitable self-restacking with minimal inherent aqueous oxidation. Vanadium doping has created abundant intimate heterointerface networks favoring electronic redistribution on conducting surface of MXene, thereby resulting in highly active sites, predominantly with low-valence (V0+ and Ti2+) and high-valence (V5+ and Ti4+) for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Benefitting from surficial synergistic features, optimized sample revealed superior bifunctionality along with long-term durability to drive both HER and OER with overpotentials of 78 and 175 mV. Also, an assembled water-splitting system revealed a low cell voltage of 1.48 V. Thus, this work provides significant insight to significantly and synchronously enhance surface utilization of MXene with abundant electrochemical active sites, crucial for designing high-performance water-splitting electrolyzer.

The evolution of structure–property relationship of P2-type Na0.67Ni0.33Mn0.67O2 by vanadium substitution and organic electrolyte combinations for sodium-ion batteries

The evolution of structure–property relationship of P2-type Na0.67Ni0.33Mn0.67O2 by vanadium substitution and organic electrolyte combinations for sodium-ion batteries

Isovalent substitution of vanadium in LiFePO4: Evolution of monoclinic α-Li3Fe2(PO4)3 phase

We report the synthesis and characterization of isovalent substitution of vanadium at phosphorus site in orthorhombic lithium iron phosphate (LiFePO4) nanoparticles by non-aqueous sol–gel method. Detailed analysis of X-ray diffraction patterns reveals the evolution of monoclinic α-Li3Fe2(PO4)3 phase and its formation is favored for the vanadium content higher than 20%. A continuous increase in unit cell volume of LiFePO4 is observed with increase in vanadium content from x = 0.05 to 0.20, followed by rapid decrease above x = 0.20, due to the creation of a greater number of Li-ion vacancies. FTIR studies demonstrate that the wavenumbers of various absorption maxima and the covalency strength parameter do not show a variation with x, thus confirm the substitution of vanadium in the form of VO4. Our results through Mössbauer and electron paramagnetic resonance spectroscopy unambiguously demonstrate the enhancement in the formation of α-Li3Fe2(PO4)3 phase for vanadium content higher than 20%.

High-temperature properties of the solid solution BaNi2(V1−xPxO4)2

The polycrystalline samples of BaNi2(V1−xPxO4)2 (x ​= ​0–1) were prepared via a solution combustion synthesis. The study of the crystal structure shows that the substitution of vanadium (5+) cations by smaller phosphorus (5+) cations leads to compression of the [Ni2(V1−xPxO4)2]∞ layers in the ab plane with a decrease in the unit cell volume of BaNi2(V1−xPxO4)2. As a result, the BaO12 polyhedra are elongated along the c axis. The melting point increases monotonically with increasing phosphorus concentration. The linear dependence of the relative elongation (ΔL/L0) of samples on temperature after 500 ​°C becomes S-shaped. The electrical conductivity (σ) of BaNi2(V1−xPxO4)2 was measured in the range of 300–900 ​°C. Electron transfer in BaNi2(V1−xPxO4)2 established to occur along the Ni–O–Ni chains and is explained by the formation of Ni3+.