Hexagonal Tungsten Bronze Research Articles

Cation-Deficient LixWO3 Surface Coating on Ni-Rich Cathodes Materials for Lithium-Ion Batteries.

In the pursuit to increase the energy density of lithium-ion batteries (LIBs), considerable efforts have focused on developing high-capacity cathode materials. While Ni-rich (Ni ≥ 80 at. %) layered cathode materials are considered a viable commercial option, surface engineering is crucial for enhancing their cycle performance for successful implementation in commercial LIBs. Various functional materials have been explored for effective surface protection and stabilization to reduce interfacial resistance and enhance the structural stability of Ni-rich cathode materials. In this context, we propose a surface coating with a nonstoichiometric lithium hexagonal tungsten bronze (LixWO3) for Ni-rich cathode materials via simple wet-coating. We demonstrate that the distinctive physicochemical properties of LixWO3, such as its high ionic conductivity (∼10-6 S cm-1) and mechanical strength (∼236.0 MPa), are beneficial for enhancing the cycle performance of Ni-rich cathode materials by modulating the interfacial reactions without undesirable loss of reversible capacity. In practice, the LixWO3 surface layer induces a significant reduction in interfacial resistance and effective strain relaxation upon repeated Li+ insertion and extraction. Our findings provide insights into the development of highly reliable Ni-rich cathode materials for high-energy LIBs.

Synthesis of Cs-W-V-O hexagonal tungsten bronzes for the partial oxidation of ethanol to acetaldehyde

Synthesis of Cs-W-V-O hexagonal tungsten bronzes for the partial oxidation of ethanol to acetaldehyde

N-alkylation of amines with alcohols over hydrothermally prepared Nb-W mixed oxides

Hydrothermally synthesized Nb-W mixed oxides (Nb-W-O) have been employed as a cost-effective and recyclable alternative to noble metals and homogeneous catalysts for the N-alkylation of amines with alcohols, with the model reaction of aniline and benzyl alcohol. The synergistic roles of Nb and W and the influence of calcination conditions were explored. Nb-W-O exhibited a linear packing of corner-sharing octahedra, analogous to the hexagonal tungsten bronze (HTB) structure, but without the long-range order of the HTB in the ab plane, characteristic of pseudo-crystalline materials. Calcination at 773 K in an N2 flow preserved the pseudo-crystalline phase and generated additional Brønsted acid sites, associated with reduced tungsten oxide species stabilized by Nb. The primary reaction pathway involved the activation of the benzyl alcohol by Brønsted acid sites, followed by a nucleophilic attack of the formed carbocation by the aniline. Nb-W-O demonstrated superior activity to other solid acids including metal oxides, silica-alumina, and zeolites in the N-alkylation reaction.

Sensitivity of Mild Hydrothermal Synthesis to the Reaction Conditions: Targeting Mixed-Metal Hexagonal Tungsten Bronze Fluorides AxM2+xM3+(1-x)F3 to Investigate Their Magnetic Behavior.

The synthesis of five quaternary hexagonal tungsten bronze (HTB) fluorides, AxM2+xM3+(1-x)F3, (A = Cs and Rb; M2+ = Co2+, Ni2+, and Zn2+; and M3+ = V3+) via a mild hydrothermal route is reported. The crystal structures and compositions were determined using a combination of single-crystal X-ray diffraction, Rietveld refinement of powder X-ray diffraction data, and inductively coupled plasma optical emission spectroscopy analysis. This study highlights the sensitivity of the mild hydrothermal method on the reaction temperature, solvent system, and quantity of starting reagents that directly influence the selective synthesis of kinetically stabilized fluoride materials, including hydrated fluorides, β-pyrochlores, and HTB. The magnetic susceptibility and isothermal magnetization data for all five compounds were collected, which revealed the existence of a strong antiferromagnetic component in these phases. The presence of the Kagome layer in the structure results in geometrical frustration and leads to frustration indexes ranging from 7 to 13 for these compounds.

Topochemical Modulations from 1D Iron Fluoride Precursor to 3D Frameworks.

Topochemical reactions are powerful pathways to modify inorganic extended structures, but the present approaches are limited by the degrees of freedom to tune the structural connectivity and dimensionality. In this work, we unveil a new topochemical bottom-up approach to tailor three-dimensional (3D) iron fluoride frameworks from the same one-dimensional (1D) FeF3.3H2O (IF) precursor upon reacting with iodide-based reagents (AI; A+ = Na+, K+, and NH4+). To elucidate their formation mechanism, a series of topochemical reactions are performed by varying the concentration of precursors, reaction temperatures, and durations, and their corresponding products are analyzed through X-ray diffraction, nuclear magnetic resonance, and Mössbauer spectroscopy techniques. Although the lower molar ratio of AI:IF (≈0.25:1.0) produces the same hexagonal tungsten bronze (HTB)-type AxFeF3, the topochemical reactions with higher AI:IF ratios yield weberite-Na1.95Fe2F7, tetragonal tungsten bronze (TTB)-K0.58FeF3, and pyrochlore-NH4Fe2F6 phases. Our density functional theory calculations attribute the formation of iron fluoride phases to their thermodynamic stability. Moreover, kinetics also play an important role in enabling weberite and HTB fluorides with high purity, while the pyrochlore-NH4Fe2F6 retains a minor HTB-(NH4)xFeF3 impurity. Overall, this work shows a new possibility of modulating the low-dimensional precursor to attain 3D frameworks with different structural arrangements via topochemical approaches.

Continuous Solar Energy Conversion Windows Integrating Zinc Anode-Based Electrochromic Device and IoT System.

Integration of solar cells and electrochromic windows offers crucial contributions to green buildings. Solar-charging zinc anode-based electrochromic devices (ZECDs) present opportunities for addressing the solar intermittency issue. However, the limited energy storage capacity of ZECDs results in wasted harnessing of solar energy as well as overcharging. Herein, spectral-selective dual-band ZECDs that continuously transport solar energy to indoor appliances by remotely controlling the repeated bleached-tinted cycles during the daytime, are reported. Hexagonal phase cesium-doped tungsten bronze (h-Cs0.32WO3, CWO) nanocrystals are adopted for dual-band ZECDs due to their independent control ability of near-infrared (NIR) and visible (VIS) light transmittance (∆T=73.0%, 700nm; ∆T = 83.7%, 1200nm) and excellent cycling stability (0.8% optical contrast decay at 1200nm after 10000 cycles). The prototype device (i.e., CWO//Zn//CWO) delivers extraordinary thermal insulation capability, displaying a 10°C difference between "bright" and "dark" modes. Furthermore, an Internet of Things (IoT) controller to control the NIR and VIS lights of the CWO//Zn//CWO window wirelessly with a smartphone, empowering the continuous discharging of the solar-charged window during the daytime remotely, is developed. Such windows represent an intriguing potential technology whose future impact on green buildings may be substantial.

Exploiting the Reactivity of Metal Trifluoroacetates to Access Alkali-Niobium(V) Oxyfluorides.

Motivated by the lack of facile routes to alkali-niobium(V) oxyfluorides KNb2O5F and CsNb2O5F, we investigated the reactivity of alkali trifluoroacetates KH(tfa)2 and CsH(tfa)2 (tfa = CF3COO-) toward Nb2O5 in the solid state. Tetragonal tungsten bronze KNb2O5F and pyrochlore CsNb2O5F were obtained by simply reacting the corresponding trifluoroacetate with Nb2O5 at 600 °C under air, without the need for specialized containers or a controlled atmosphere. Thermolysis of KH(tfa)2 in the presence of Nb2O5 yielded single-phase polycrystalline KNb2O5F. By contrast, the reaction between CsH(tfa)2 and Nb2O5 produced a mixture of CsNb2O5F and a new oxyfluoride of formula CsNb3O7F2, whose crystal structure was solved using powder X-ray and electron diffraction. CsNb3O7F2 (space group P6/mmm) belongs to the family of hexagonal tungsten bronzes and features an open-framework structure consisting of corner-sharing Nb(O,F)6 octahedra with hexagonal channels occupied by Cs+ ions. Isomorphous RbNb3O7F2 was obtained upon reacting RbH(tfa)2 with Nb2O5. Synthetic optimization enabled the preparation of RbNb3O7F2 and CsNb3O7F2 as single-phase polycrystalline solids at 500 °C under flowing synthetic air. Both oxyfluorides were found to be semiconductors with a band gap of ≈3.5 eV. The discovery of these two oxyfluorides highlights the importance of probing the reactivity of solids whose full potential as fluorinated precursors is yet to be realized.

Synthesis and Characterization of Vanadium Doped Hexagonal Rubidium Tungsten Bronze

Attempts have been made to synthesize vanadium doped rubidium hexagonal tungsten bronzes with the nominal composition RbxVyW1-yO3 (x = 0.30, 0.25, and 0.0 ≤ y ≤ x). The samples were synthesized using the solid state synthesis method in a silica glass tube under vacuum at (10-2 Torr) 700°C. The X-ray diffraction measurements reveal that a pure hexagonal tungsten bronze (HTB) phase can be formed with a 60% replacement of W5+ by V5+. The systematic incorporation of vanadium into the HTB lattice, as well as the reduction of V/W-O bond lengths in the xy plane and the lengthening of these bonds in the crystallographic c direction, are also revealed by Rietveld structure refinement of XRD data. The XRD results are supported by FTIR absorption spectra of the oxidized phases. Furthermore, an absorption signature develops as a function of y and exhibits a considerable increase in intensity with the eventual replacement of W5+ by V5+, showing a large decrease in the metallic like contribution and revealing the compounds' nonmetallic nature. Elemental analysis indicates good agreement with nominal values, demonstrating that vanadium was systematically incorporated into the RbxVyW1-yO3 system. Dhaka Univ. J. Sci. 72(1): 77-84, 2024 (January)

Absorption peak decomposition of an inhomogeneous nanoparticle ensemble of hexagonal tungsten bronzes using the reduced Mie scattering integration method

Recent optical analyses of cesium-doped hexagonal tungsten bronze have accurately replicated the absorption peak and identified both plasmonic and polaronic absorptions in the near-infrared region, which have been exploited in various technological applications. However, the absorption peaks of tungsten oxides and bronzes have not generally been reproduced well, including those of the homologous potassium- and rubidium-doped hexagonal tungsten bronzes that lacked evidence of polaronic subpeaks. The present study reports a modified and simplified Mie scattering integration method which incorporates the ensemble inhomogeneity effect and allows precise peak decomposition and determination of the physical parameters of nanoparticles. The decomposed peaks were interpreted in terms of electronic structures, screening effect, and modified dielectric functions. The analysis revealed that the plasma frequencies, polaron energies, and the number of oxygen vacancies decrease in the dopant order Cs → Rb → K. The coexistence of plasmonic and polaronic excitations was confirmed for all the alkali-doped hexagonal tungsten bronzes.

Fabrication of copper porphyrin-ruthenium pincer complex coupled polymer/Cs0.33WO3 Z-scheme composite photocatalyst for highly efficient CO2 conversion

The photocatalytic system that simulates the principle of photosynthesis is considered one of the most promising technologies for converting CO2 into chemical fuels. Herein, copper tetra(4-carboxyphenyl)porphyrin (CuPor)-ruthenium 2,6-bis(5-amino-benzimidazol-2-yl)pyridine pincer complex (RuN3) coupled polymer (CuPor-RuN3) is in-situ polymerized on hexagonal tungsten bronze Cs0.33WO3 (CsWO) nanoparticles to construct a novel CuPor-RuN3/CsWO composite photocatalyst with cascade charge transfer mechanism, where the inorganic–organic Z-scheme heterojunction at the CuPor-RuN3/CsWO interface and the Z-scheme molecular junction (-[CuPor-RuN3]-) within the polymer facilitate photoexcited electrons transferring from CuPor units to RuN3 ones, and finally enriching on Ru sites for boosting the CO2 reduction reaction (CO2RR). Under visible light irradiation and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole serving as electron donor, the resultant CuPor-RuN3/CsWO delivers average CO/CH4 yields up to 4645/89 μmol g-1h−1, corresponding to an overall photoactivity of 10002 μmol g-1h−1, which is 20 times higher than that of the single polymer. These results provide new ideas for exploring high-performance artificial photosynthesis system to convert CO2 into chemical fuels.

Oxygen vacancy-induced plasmonic and polaronic excitations in rubidium- and potassium-doped hexagonal tungsten bronzes

Potassium- and rubidium-doped hexagonal tungsten bronzes (K- and Rb-HTBs) were examined in comparison with cesium-doped hexagonal tungsten bronze (Cs-HTB) for their crystal structure and dielectric functions via spectrophotometry, spectroscopic ellipsometry, X-ray diffraction, and X-ray photoelectron spectroscopy. A substantial amount of oxygen vacancy (VO) was detected in all HTBs via chemical analysis, which increased depending on the dopant K → Rb → Cs. A minute (201) peak appeared for K- and Rb-HTBs, indicating symmetry transition from P63/mcm to P6322 due to the pseudo Jahn–Teller distortion; this suggests that K- and Rb-HTBs contain lesser conduction electrons than Cs-HTB. Double-peaked loss functions were decomposed using the Drude−Lorentz theory, and the associated plasma frequency, polaron energies, and W5+ fractions increased as the dopants changed, K → Rb → Cs, with increasing VO content. The origin of the decreased VO content in K- and Rb-HTBs was discussed in relation to water-related defects on prismatic planes.

Rational Cationic Disorder in Hexagonal Cesium Tungsten Bronze Nanoparticles for Infrared Absorption Materials.

Hexagonal cesium tungsten bronze (Cs0.33WO3) nanoparticles (NPs) have attracted attention for their potential applications in near-infrared (NIR) absorbing materials. However, the insufficient Cs doping in Cs0.33WO3 NPs has limited their NIR absorbing capabilities and practical stability. In this study, we demonstrate the transition pathway from intermediate W-defective Cs0.33WO3 NPs synthesized by flame spray pyrolysis to cationic (Cs, W)-disordered Cs0.33WO3 NPs prepared through appropriate heat treatments. Direct atomic observations reveal the basal shear and prismatic (Cs, W)-defective planes, which contributed to the disorder of full Cs doping in Cs0.33WO3 NPs. The obtained Cs0.33WO3 NPs with cationic disorder exhibited enhanced practical performance compared with conventional Cs0.33WO3 NPs. Therefore, the developed approach that regulates cationic disorder enables the rational design of defective metal oxides for a variety of applications, including NIR absorbing materials.

Highly Reversible Iron Fluoride Conversion Cathodes Enabled by Deep‐Eutectic Solvent Method and Heterostructure Design

AbstractConversion‐reaction metal fluoride cathodes have the advantages of low cost, environmental friendliness, and high energy density. However, their tailored synthesis and structure design under safe, facile, and scalable conditions have always been a puzzle, which limits the popularization of conversion‐type cathodes in lithium metal batteries. To address these challenges, here an iron fluoride heterostructure with FeF2 and FeF3 binary phases is proposed and prepared by deep eutectic solvent method under the assistance of cobalt cation as an oxidization promoter. The compact heterogeneous contact between tetragonal FeF2 and hexagonal tungsten bronze FeF3 phases enables the promotion and stabilization of interface charge transfer and topotactic conversion reaction. The narrower band gap of FeF2, higher theoretical capacity and thermodynamic potential of FeF3 synergistically contribute to the balance between cycling reversibility and capacity. The heterostructure cathode with an optimized molar ratio of FeF2/FeF3 can realize a highly reversible capacity of ≈520 mAh g−1 in the first tens of cycles and 305 mAh g−1 after 200 cycles. The typical two‐stage reaction plateaus are well preserved even under the high current density of 1000 mA g−1. The green synthesis strategy and heterostructure fluoride design open a new horizon for the development of high‐energy‐density metal fluoride batteries.

Rapid Synthesis of AxWO3 (A = H, Na, K) Tungsten Bronze Materials with Strong Near‐Infrared Absorption and Enhanced Photoelectric Anticorrosion Properties

The intrinsic reduction method is extended for the synthesis of tungsten bronzes (TBs), and monoclinic phase hydrotungsten bronze (HWO), tetragonal phase sodium tungsten bronze (NaWO) and hexagonal phase potassium tungsten bronze (KWO) are obtained. The introduction of cations brought a large number of localized state free electrons, which endowed TBs with strong near‐infrared (NIR) absorption and achieved light energy utilization covering almost the entire solar spectrum in the form of localized surface plasmon resonance (LSPR) and small polariton transfer. The strong NIR absorption enhanced the corrosion protection performance of tungsten bronze coatings, especially the HWO coating, which had an increased impedance value of 859.5% compared to the pure epoxy resin. The excellent corrosion resistance of TBs inhibitors can be derived from the strong NIR absorption enhanced photocathodic protection and local magnetic field, which improved the charge transfer resistance of the coating by nearly two orders of magnitude and greatly inhibited the charge movement in the electrochemical corrosion reaction. This study discovered the contribution of oxygen vacancy‐enhanced NIR absorption to the electrochemical anticorrosion properties, expanding the application area of TBs and making an important contribution to innovative anticorrosion strategies.

Influence of the Precursor Structure on the Formation of Tungsten Oxide Polymorphs.

Understanding material nucleation processes is crucial for the development of synthesis pathways for tailormade materials. However, we currently have little knowledge of the influence of the precursor solution structure on the formation pathway of materials. We here use in situ total scattering to show how the precursor solution structure influences which crystal structure is formed during the hydrothermal synthesis of tungsten oxides. We investigate the synthesis of tungsten oxide from the two polyoxometalate salts, ammonium metatungstate, and ammonium paratungstate. In both cases, a hexagonal ammonium tungsten bronze (NH4)0.25WO3 is formed as the final product. If the precursor solution contains metatungstate clusters, this phase forms directly in the hydrothermal synthesis. However, if the paratungstate B cluster is present at the time of crystallization, a metastable intermediate phase in the form of a pyrochlore-type tungsten oxide, WO3·0.5H2O, initially forms. The pyrochlore structure then undergoes a phase transformation into the tungsten bronze phase. Our studies thus experimentally show that the precursor cluster structure present at the moment of crystallization directly influences the formed crystalline phase and suggests that the precursor structure just prior to crystallization can be used as a tool for targeting specific crystalline phases of interest.

Synthesis and characterization of AsO[(W,Mo)O3]13, a new (6)-intergrowth tungsten bronze (ITB)

Crystals of a new arsenic tungsten-molybdenum oxide, AsO[(W,Mo)O3]13, have been grown through vapour phase transport at high temperature (1123 ​K). The crystal structure was investigated by combining electron and single-crystal X-ray diffraction techniques, electron microprobe chemical analysis and X-ray absorption spectroscopy. Single-crystal XRD yielded the orthorhombic unit-cell parameters a ​= ​25.0895, b ​= ​7.3061, c ​= ​3.9089 ​Å. Although electron diffraction indicated a doubled translation along [001], superstructure reflections were undetectable with X-ray diffraction so that only the average structure (sub-cell) was determined. The most reliable structural model was obtained in the P222 space group (R1 ​= ​6.96%). It exhibits an octahedral framework which forms six-octahedra thick perovskite slabs (PTB) alternated to single hexagonal tungsten bronze (HTB) modules, indicating that the compound corresponds to a (6)-ITB phase. As3+ was found to be disordered on two off-centred, symmetry-related sites into the hexagonal channels with the typical pyramidal AsO3 geometry. This is in agreement with the results of the extended X-ray absorption fine structure that indicate an As–O bond distance of 1.77 ​Å. The ordered distribution of As within the tunnels, likely related to the doubling of periodicity along [001], could not be determined.

Charge Storage Mechanism of LixWO3 Hexagonal Tungsten Bronze in Aqueous Electrolytes

The electrochemical behavior of the lithium hexagonal tungsten bronze, LixWO3, is investigated herein. The material was synthesized at a low temperature under hydrothermal conditions, yielding nanorod-like particles with growth along the c-axis. Upon cycling in a 5 M LiNO3 aqueous electrolyte, a specific capacity of 71 C.g−1 was obtained at 2 mV.s−1, corresponding to a charge/discharge cycle of 10 min. The charge storage mechanism was elucidated using various complementary techniques, such as electrochemical quartz crystal microbalance (EQCM) and synchrotron operando X-ray absorption spectroscopy (XAS). A desolvation process upon Li+ intercalation into the lattice of the material was evidenced, accompanied by a reversible reduction/oxidation of tungsten cations in the crystal structure upon charge/discharge cycling.

Resolving old problems with layered polytungstates related to hexagonal tungsten bronze: phase formation, structures, crystal chemistry and some properties.

A 60-year-old problem with the atomic arrangements and exact compositions of alkali polytungstates related to hexagonal tungsten bronze (HTB) was solved. The systems A2WO4-WO3 (A = K, Rb) were restudied and the average monoclinic layered structures of stoichiometric polytungstates A4W11O35 (A = K, Rb, Cs, Tl) and A2W7O22 (A = K, Rb, Cs) were first successfully determined. The structures resemble those of "MoW11O36" and "MoW14O45" (J. Graham and A. D. Wadsley, Acta Crystallogr., 1961, 14, 379-383) and are derived from HTB by breaking into slabs parallel to (100) due to the ordered omission of some [WO]∞ chains along the hexagonal tunnels. The slabs in A4W11O35 (A = Cs, Tl) and A2W7O22 (A = Rb, Cs) are mutually shifted by the a/2 HTB unit cell axis. These data mainly confirmed our preliminary structural models of HTB-like alkali polytungstates (S. F. Solodovnikov, N. V. Ivannikova, Z. A. Solodovnikova and E. S. Zolotova, Inorg. Mater., 1998, 34, 845-853) and revealed a new similar thallium polytungstate. The structures of the HTB-like polytungstates and related compounds form a homologous series of layered complex oxides or fluorides An+2-xM3n+2X9n+8 where n = 2, 3 and 4 are equal to the numbers of HTB hexagonal tunnels across the polytungstate slab width for Tl2W4O13, A4W11O35 and A2W7O22 (A = K, Rb, Cs or Tl), respectively. The structures of the HTB-like polytungstates seem to intergrow with HTB-type AxWO3 to form, in particular, higher homologues of the series. Our group-supergroup analysis, measurements of nonlinear optical activity and electrical conductivity, and calculations of the bond-valence site energy barriers indicate possible ferroelectric/ferroelastic properties and moderate 2D oxide-ion mobility within the HTB-type slabs of the studied polytungstates.

In situ facile one-step solvothermal synthesizing hexagonal ammonium tungsten bronze (NH4)0.33WO3 to tap NIR light absorption ability by controlling crystallinity

Tungsten bronze (TB) has potentially high visible light transparency and excellent near-infrared (NIR) shielding ability. To fully dig out the concealed optical properties of TB with specific chemical components, in this work, hexagonal ammonium tungsten bronze (h-ATB) (NH4)0.33WO3 was fabricated by a one-step solvothermal reaction. Through X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and ultraviolet–visible–near infrared (UV–vis–NIR) spectroscopy to characterize h-ATB, which unraveled that controlling crystallinity could adjust the optical property of h-ATB. In particular, when the crystallinity of the h-ATB is over 95%, it exhibits the attractive NIR light shielding capability, which is in line with the predicted optical property via the first principle calculation based on density functional theory (DFT). Given the result, this work can provide a feasible research idea to explore the hidden optical properties of TB.

Hydrothermally Enhanced Tungsten-Tungsten Bronze (W-MxWO3) Electrodes for Pseudocapacitive Energy Storage

Hexagonal tungsten bronzes (h-MxWO3) are an emergent class of pseudocapacitive energy storage material: a metastable phase arranged in a hexagonal crystal structure that enhances the oxide’s rapid intercalation mechanism with impressive capacitances of over 200 F/g and excellent cyclability figures reported. A symmetrical pseudocapacitive device composed of h-MxWO3 electrodes negates the need for expensive and problematic rare minerals such as cobalt and lithium instead using low-cost tungsten whilst offering more efficient high-rate energy storage than commercial lithium-ion batteries.Hexagonal tungsten bronze has shown some promising capacitances in literature in conventional electrode configurations, i.e. composite structures with binder support materials which increase electrode mass impacting specific capacitance. If the bronze was instead grown directly upon a current-collecting substrate, this would negate the need for additional electrode components, with the entire electrode mass participating in storing charge. The present work explores this concept of a novel binder-free electrode design where the nanostructured tungsten-bronze (active material) is grown directly upon a tungsten-based substrate (current collector). This simple design offers potentially higher specific capacitances than conventionally fabricated electrodes.Nanostructured W/h-MxWO3 electrodes were fabricated in a twostep process. Firstly, tungsten foil was oxidised in hot nitric acid, forming a seed layer of monoclinic-WO3. A second step saw the hexagonal bronze deposited upon substrates through a hydrothermal dehydration reaction of sodium tungstate (Na2WO4) where the effect of the directing agent (NaCl, Na2SO4, (NH4)2SO4& Rb2SO4) on the deposited nanostructure was investigated. To assess the benefit of the acid pretreatment step, vis-à-vis a pristine untreated tungsten foil, the hydrothermal reaction was completed upon both a pristine tungsten foil and an acid treated foil. The nanostructured surface deposited from the hydrothermal reaction was characterised by SEM, and the crystal phase identified by XRD. The electrochemical behaviour was analysed using a Biologic SP-300 potentiostat in a 3-electrode cell: as-synthesised foils employed as working electrode; Pt Wire as counter electrode; a saturated calomel electrode (SCE) as reference; and 0.5M H2SO4 as electrolyte. The performance of devices fabricated from two symmetrical as-synthesised electrodes was also characterised using both the potentiostat and a NEWARE battery cycling test system in a two-electrode cell configuration.XRD analysis confirmed that hexagonal-tungsten bronzes were successfully deposited upon substrates irrespective of the substrate pretreatment. The acid pretreatment generally resulted in larger masses of bronze being deposited upon the substrate, but this did not necessarily translate to superior electrochemical performance. The choice of directing agent present in hydrothermal synthesis drastically altered the nature of the deposited layer as may be seen in the provided figure of as-synthesised electrodes (figure b-d). Sodium directed samples formed powder-like deposits (figure b), ammonium sulphate produced larger colloidal bronze layers (figure c) and rubidium sulphate developed glossy smooth sheets (figure d). Additionally, the ratio of tungstate ions (WO4 2-) to directing agent cations (M+) was identified as being critical to obtaining the hexagonal phase. The mechanical stability of the deposited bronze proved problematic, with the layer prone to delamination and disintegration from a variety of mechanisms. A bespoke electrode holder, device testing apparatus and handling procedure was therefore developed to enable repeatable electrode characterization.The cyclic voltammetry response of the hydrothermally deposited layers was typical of an intercalation system, though an initial pseudocapacitive behaviour deteriorated to a battery-like response at sweeprates above 20 mV/s. Galvanostatic charge-discharge testing displayed a significant pseudocapacitive response over limited potential windows; moving beyond these limits produced steep non-linear curves indicating a departure from pseudocapacitive charge transfer processes. The charging at current densities above 1 A/g occurred over rapid time scales showing the electrodes’ suitability for high power applications. Preliminary 3-electrode capacitance results are encouraging with specific capacitances of over 100 Fg-1 by dry mass. The tunnelated M+ ion in the bronze is found to greatly influence the electrochemical performance of the electrode.This work reports the first example of hexagonal tungsten bronze being successfully grown directly on tungsten substrates for application as energy storage electrodes. The binder free electrodes were found to deliver promising performance characteristics in both 3-electrode analysis and in a two-electrode symmetrical device. The performance so far is hampered by the mechanical fragility of the electrode structures, while scale-up of the manufacturing process may prove challenging. The observed mechanism of deposition of hexagonal tungsten bronze from solution to substrate is of interest for future work, as is the significance of tunnelated M+ cations to the hexagonal bronze (h-MxWO3) structure, stability, and electrode performance (figure a). Figure 1