WO3-x Nanoplates Grown on Carbon Nanofibers for an Efficient Electrocatalytic Hydrogen Evolution Reaction

Symmetric Catalyst Design Employing Ir Nanoparticles on Black WO3- x Nanofiber Support for Boosting Water Electrolysis

The efficient evolution of gaseous hydrogen and oxygen from water is required to realize sustainable energy conversion systems. To address the sluggish kinetics of the multielectron transfer reaction, bifunctional catalyst materials for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) should be developed. Herein, a tailored combination of atomically minimized iridium catalysts and highly conductive black WO3- x nanofiber supports are developed for the bifunctional electrolyzer system. Atomic Ir catalysts, particularly those that activate the OER, minimize the utilization of precious metals. The oxygen-deficient black WO3- x NF support, which boosts the HER, offers increased electronic conductivity and favorable nucleation sites for Ir loading. The Ir-black WO3- x NFs exhibit increased double-layer capacitance, a significantly reduced onset potential, lower Tafel slope, and stable cyclability for both the OER and HER, compared to large-sized Ir catalysts loaded on white WO3 nanofibers. This study offers a strategy for developing an optimal catalyst material with suitable supports for high-performance and economical water electrolysis systems for achieving carbon-negative targets.

Functional Nanostructures from Sol-Gel Synthesis Using Keggin Polyoxometallate Phosphotungstic Acid as a Precursor

Subjecting phosphotungstic acid solutions to low pH in combination with introduction of polyvalent cations led to the formation of nanostructured microspheres of approximately 2 μm in size, as shown by scanning electron microscopy, which were almost insoluble and resistant to degradation at neutral and high pH. These microspheres were composed of secondary nanospheres with diameters around 20 nm as revealed by transmission electron microscopy and atomic force microscopy. Investigations of the crystal structure of a potential intermediate of this process, namely, acidic lanthanum phosphotungstate, [La(H2O)9](H3O)3[PW12O40]2(H2O)19, showed a tight network of hydrogen bonding, permitting closer packing of phosphotungstic acid anions, thereby confirming the mechanism of the observed self-assembly process. The new material demonstrated promising electrochemical properties in oxygen evolution reactions with the high stability of the obtained electrode material.

Achieving One Part Per Billion Hydrogen Sulfide (H2S) Level Detection through Optimizing Composition and Crystallinity of Gold-Decorated Tungsten Trioxide (Au-WO3) Nanofibers

As a common environmental pollutant and an important breath biomarker for several diseases, it is essential to develop a hydrogen sulfide gas sensor with a low-ppb level detection limit to prevent harmful gas exposure and allow early diagnoses of diseases in low-resource settings. Gold doped/decorated tungsten trioxide (Au-WO3) nanofibers with various compositions and crystallinities were synthesized to optimize H2S-sensing performance. Systematically experimental results demonstrated the ability to detect 1 ppb H2S with a response value (Rair/Rgas) of 2.01 using a 5 at % Au-WO3 nanofibers with average grain sizes of around 15 nm. Additionally, energy barrier difference of sensing materials in air and nitrogen (ΔEb) and power law exponent (n) were determined to be 0.36 eV and 0.7, respectively, at 450 °C indicating that O- is predominately ionic oxygen species and adsorption of O- significantly altered the Schottky barrier between the grain. Such quantitative analysis provides a comprehensive understanding of H2S detection mechanism.

Optimization Methods of Tungsten Oxide-Based Nanostructures as Electrocatalysts for Water Splitting

Electrocatalytic water splitting, as a sustainable, pollution-free and convenient method of hydrogen production, has attracted the attention of researchers. However, due to the high reaction barrier and slow four-electron transfer process, it is necessary to develop and design efficient electrocatalysts to promote electron transfer and improve reaction kinetics. Tungsten oxide-based nanomaterials have received extensive attention due to their great potential in energy-related and environmental catalysis. To maximize the catalytic efficiency of catalysts in practical applications, it is essential to further understand the structure-property relationship of tungsten oxide-based nanomaterials by controlling the surface/interface structure. In this review, recent methods to enhance the catalytic activities of tungsten oxide-based nanomaterials are reviewed, which are classified into four strategies: morphology regulation, phase control, defect engineering, and heterostructure construction. The structure-property relationship of tungsten oxide-based nanomaterials affected by various strategies is discussed with examples. Finally, the development prospects and challenges in tungsten oxide-based nanomaterials are discussed in the conclusion. We believe that this review provides guidance for researchers to develop more promising electrocatalysts for water splitting.

Fabrication of Ru/WO3-W2N/N-doped carbon sheets for hydrogen evolution reaction

Recent experimental analysis indicates WO₃-based nanostructures exhibit poor hydrogen evolution reactivity, particularly in alkaline medium, arising from the low electron transfer rate. It is imperative to tune the composition and structure of WO₃ to boost the cleavage of H-OH bond. Here, we construct Ru/WO₃-W₂N/N-doped carbon sheets (Ru/WO₃-W₂N/NC) using m-WO₃ nanosheets as precursors with the aid of RuCl₃, Tris (hydroxymethyl) aminomethane, and dopamine. Structural investigation reveals the formation of N-doped carbon sheets, Ru nanoparticles, and WO₃-W₂N. As a result, hydrogen evolution reactivity is greatly improved on Ru/WO₃-W₂N/N-doped carbon sheets with 64 mV at 10 mA/cm² in 1 mol/L (M) KOH, outperforming most of WO₃-based electrocatalysts in previous literatures. Meanwhile, it facilitates the generation of H₂ in 0.5 M H₂SO₄ with the excellent activity of 110 mV at 10 mA/cm². Our work provides an efficient strategy to tailor the electronic structure of WO₃ to catalyze acidic and alkaline hydrogen evolution reaction.

Synthesis and Physical Characteristics of Undoped and Potassium-Doped Cubic Tungsten Trioxide Nanowires through Thermal Evaporation

We report an efficient method to synthesize undoped and K-doped rare cubic tungsten trioxide nanowires through the thermal evaporation of WO₃ powder without a catalyst. The WO₃ nanowires are reproducible and stable with a low-cost growth process. The thermal evaporation processing was conducted in a three-zone horizontal tube furnace over a temperature range of 550-850 °C, where multiple substrates were placed at different temperature zones. The processing parameters, including pressure, temperature, type of gas, and flow rate, were varied and studied in terms of their influence on the morphology, aspect ratio and density of the nanowires. The morphologies of the products were observed with scanning electron microscopy. High resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction studies were conducted to further identify the chemical composition, crystal structure and growth direction of the nanostructures. Additionally, the growth mechanism has been proposed. Furthermore, we investigated the potassium doping effect on the physical properties of the nanostructures. Photoluminescence measurements show that there were shorter emission bands at 360 nm and 410 nm. Field emission measurements show that the doping effect significantly reduced the turn-on electric field and increased the enhancement factor. Furthermore, as compared with related previous research, the K-doped WO₃ nanowires synthesized in this study exhibited excellent field emission properties, including a superior field enhancement factor and turn-on electric field. The study reveals the potential of WO₃ nanowires in promising applications for sensors, field emitters and light-emitting diodes.

Engineering Coupled NiSx -WO2.9 Heterostructure as pH-Universal Electrocatalyst for Hydrogen Evolution Reaction

Exploiting highly active and low-cost materials as pH-universal electrocatalysts for the hydrogen evolution reaction (HER) and achieving high-purity hydrogen fuel is highly desirable but remains challenging. Herein, a novel type of coupled heterostructure was designed by simple electrodeposition followed by a sulfurization treatment. This hierarchical structure was composed of nickel sulfides (NiS, NiS₂ , denoted as NiS_x ) and oxygen-deficient tungsten oxide (WO_2.9 ), which was directly grown on nickel foam (NF) as self-supporting electrodes (NiS_x -WO_2.9 /NF) for HER over a wide pH range. The systematic experimental characterizations confirmed that the material had abundant catalytic active sites, fast interfacial electron transfer ability, and strong electronic interaction, resulting in the optimized reaction kinetics for HER. Consequently, the NiS_x -WO_2.9 /NF catalyst required low overpotentials of 96 and 117 mV to reach current densities of 50 and 100 mA cm^-2 in an alkaline medium, outperforming most of the reported non-noble metal-based materials. Moreover, this self-supported electrode exhibited impressive performance over a wide pH range, only requiring 220 and 304 mV overpotential at 100 mA cm^-2 in 0.5 m H₂ SO₄ and 1 m phosphate-buffered saline electrolytes. This work may offer a new approach to the development of advanced pH-universal electrodes for hydrogen production.

Structure and Photoluminescence of WO3-x Aggregates Tuned by Surfactants

The optoelectronic properties of transition metal oxide semiconductors depend on their oxygen vacancies, nanostructures and aggregation states. Here, we report the synthesis and photoluminescence (PL) properties of substoichiometric tungsten oxide (WO_3-x) aggregates with the nanorods, nanoflakes, submicro-spherical-like, submicro-spherical and micro-spherical structures in the acetic acid solution without and with the special surfactants (butyric or oleic acids). Based on theory on the osmotic potential of polymers, we demonstrate the structural change of the WO_3-x aggregates, which is related to the change of steric repulsion caused by the surfactant layers, adsorption and deformation of the surfactant molecules on the WO_3-x nanocrystals. The WO_3-x aggregates generate multi-color light, including ultraviolet, blue, green, red and near-infrared light caused by the inter-band transition and defect level-specific transition as well as the relaxation of polarons. Compared to the nanorod and nanoflake WO_3-x aggregates, the PL quenching of the submicro-spherical-like, submicro-spherical and micro-spherical WO_3-x aggregates is associated with the coupling between the WO_3-x nanoparticles and the trapping centers arising from the surfactant molecules adsorbed on the WO_3-x nanoparticles.

Ultrafast photoresponse of vertically oriented TMD films probed in a vertical electrode configuration on Si chips

Integrated photodetectors based on transition metal dichalcogenides (TMDs) face the challenge of growing their high-quality crystals directly on chips or transferring them to the desired locations of components by applying multi-step processes. Herein, we show that vertically oriented polycrystalline thin films of MoS₂ and WS₂ grown by sulfurization of Mo and W sputtered on highly doped Si are robust solutions to achieve on-chip photodetectors with a sensitivity of up to 1 mA W^-1 and an ultrafast response time in the sub-μs regime by simply probing the device in a vertical arrangement, i.e., parallel to the basal planes of TMDs. These results are two orders of magnitude better than those measured earlier in lateral probing setups having both electrodes on top of vertically aligned polycrystalline TMD films. Accordingly, our study suggests that easy-to-grow vertically oriented polycrystalline thin film structures may be viable components in fast photodetectors as well as in imaging, sensing and telecommunication devices.

Topochemical Transformation of Two-Dimensional VSe2 into Metallic Nonlayered VO2 for Water Splitting Reactions in Acidic and Alkaline Media

The engineering of the structural and morphological properties of nanomaterials is a fundamental aspect to attain desired performance in energy storage/conversion systems and multifunctional composites. We report the synthesis of room temperature-stable metallic rutile VO₂ (VO₂ (R)) nanosheets by topochemically transforming liquid-phase exfoliated VSe₂ in a reductive Ar-H₂ atmosphere. The as-produced VO₂ (R) represents an example of two-dimensional (2D) nonlayered materials, whose bulk counterparts do not have a layered structure composed by layers held together by van der Waals force or electrostatic forces between charged layers and counterbalancing ions amid them. By pretreating the VSe₂ nanosheets by O₂ plasma, the resulting 2D VO₂ (R) nanosheets exhibit a porous morphology that increases the material specific surface area while introducing defective sites. The as-synthesized porous (holey)-VO₂ (R) nanosheets are investigated as metallic catalysts for the water splitting reactions in both acidic and alkaline media, reaching a maximum mass activity of 972.3 A g^-1 at -0.300 V vs RHE for the hydrogen evolution reaction (HER) in 0.5 M H₂SO₄ (faradaic efficiency = 100%, overpotential for the HER at 10 mA cm^-2 = 0.184 V) and a mass activity (calculated for a non 100% faradaic efficiency) of 745.9 A g^-1 at +1.580 V vs RHE for the oxygen evolution reaction (OER) in 1 M KOH (overpotential for the OER at 10 mA cm^-2 = 0.209 V). By demonstrating proof-of-concept electrolyzers, our results show the possibility to synthesize special material phases through topochemical conversion of 2D materials for advanced energy-related applications.

Mechanistic insights into dual active sites in Au@W18O49 electrocatalysts for hydrogen evolution reaction

Electrocatalytic hydrogen evolution reaction (HER) for water splitting is promising to replace fossil fuels. The high-efficient electrocatalyst with multiple functional sites is indispensable but challenging. Herein, urchin-like Au@W18O49 electrocatalyst with...

Chemically Activating Tungsten Disulfide via Structural and Electronic Engineering Strategy for Upgrading the Hydrogen Evolution Reaction

Both improving the intrinsic activity and activating basal plane sites of the layered metal dichalcogenides are desirable to enhance their electrocatalytic performance for energy storage and conversion. Herein, we present palladium (Pd)-doped tungsten disulfide (WS₂) epitaxially sheathed around linear tungsten oxide for the hydrogen evolution reaction (HER). The Pd doping is evidenced to tune the electronic structure of WS₂ for activating basal sites of WS₂, while the unique core-shell structure facilitates charge transfer. The as-prepared Pd-WS₂/W₃O with 5.65 wt % Pd content exhibits a small overpotential of only 54 mV at -10 mA cm^-2 and superior stability in the acidic electrolyte, which are superior to that of the 5 wt % Pt/C benchmark and are unprecedented in the reported WS₂-based electrocatalysts. Theoretical results have revealed that Pd substituting for W in coordination with four S atoms is thermodynamically stable, and the in-plane S atoms adjacent to the doped Pd represent new catalytic active centers for promoting hydrogen adsorption. This work provides a new multiscale structural and electronic engineering strategy for improving the catalytic performance of transition-metal dichalcogenides.

Synergistically Integrating Nickel Porous Nanosheets with 5d Transition Metal Oxides Enabling Efficient Electrocatalytic Overall Water Splitting

An integration hydrogen adsorption benign component such as a metal with an oxygen-containing reactant adsorption benign component such as metal oxide allows for efficient overall water splitting in alkaline solutions and yet remains a considerable challenge. Herein, 5d transition metal oxide WO₂ and WO₃ (denoted as WO_x) nanoparticles are purposely integrated with a porous Ni nanosheet array grown on nickel foam (NF) to design a strongly coupled Ni/WO_x/NF porous nanosheet array electrocatalyst. Through the anion exchange of Ni(OH)₂ nanosheets with tungstate, followed by hydrogenation treatment, abundant Ni/WO_x interfaces with strong coupling interaction are generated. Benefiting from the strong synergies between Ni and WO_x and the unique nanostructure, Ni/WO_x/NF only requires the overpotentials of 42 mV for hydrogen evolution reaction (HER) and 395.7 mV for oxygen evolution reaction (OER) to achieve the current densities of 10 and 100 mA cm^-2, respectively. Furthermore, the Ni/WO_x/NF can achieve a current density of 10 mA cm^-2 at a low cell voltage of 1.54 V in a two-electrode system. This work opens a novel avenue for the design of high-performance but low-cost electrocatalysts for overall water splitting.

Ultra-Thin SnS2-Pt Nanocatalyst for Efficient Hydrogen Evolution Reaction

Transition-metal dichalcogenides (TMDs) materials have attracted much attention for hydrogen evolution reaction (HER) as a new catalyst, but they still have challenges in poor stability and high reaction over-potential. In this study, ultra-thin SnS₂ nanocatalysts were synthesized by simple hydrothermal method, and low load of Pt was added to form stable SnS₂-Pt-3 (the content of platinum is 0.5 wt %). The synergistic effect between ultra-thin SnS₂ rich in active sites and individual dispersed Pt nanoclusters can significantly reduce the reaction barrier and further accelerate HER reaction kinetics. Hence, SnS₂-Pt-3 exhibits a low overpotential of 210 mV at the current density of 10 mA cm⁻². It is worth noting that SnS₂-Pt-3 has a small Tafel slope (126 mV dec⁻¹) in 0.5 M H₂SO₄, as well as stability. This work provides a new option for the application of TMDs materials in efficient hydrogen evolution reaction. Moreover, this method can be easily extended to other catalysts with desired two-dimensional materials.

Electrospinning metal Phosphide/Carbon nanofibers from Phytic Acid for hydrogen evolution reaction catalysts

This paper reports a general electrospinning method to prepare various metal phosphide/carbon nanofibers composite for electrochemical hydrogen evolution reaction (HER) catalysts. An earth-abundant organic acid-phytic acid is successfully incorporated into a conventional electrospinning precursor as the phosphorus source, and continuous nanofibers can be obtained through spinning. After heat treatment, metal phosphide/carbon composite nanofibers can be obtained, with fine phosphide nanoparticles well dispersed on the surface of an interconnected carbon backbone network. Such fibrous structures offer fast charge transfer pathways and enlarged active surface area, which are beneficial for electrocatalysts. As a result, enhance HER catalytic activity can be achieved.

In situ interfacial engineering of nickel tungsten carbide Janus structures for highly efficient overall water splitting

Regulating chemical bonds to balance the adsorption and disassociation of water molecules on catalyst surfaces is crucial for overall water splitting in alkaline solution. Here we report a facile strategy for designing Ni₂W₄C-W₃C Janus structures with abundant Ni-W metallic bonds on surfaces through interfacial engineering. Inserting Ni atoms into the W₃C crystals in reaction progress generates a new Ni₂W₄C phase, making the inert W atoms in W₃C be active sites in Ni₂W₄C for overall water splitting. The Ni₂W₄C-W₃C/carbon nanofibers (Ni₂W₄C-W₃C/CNFs) require overpotentials of 63 mV to reach 10 mA cm^-2 for hydrogen evolution reaction (HER) and 270 mV to reach 30 mA cm^-2 for oxygen evolution reaction (OER) in alkaline electrolyte, respectively. When utilized as both cathode and anode in alkaline solution for overall water splitting, cell voltages of 1.55 and 1.87 V are needed to reach 10 and 100 mA cm^-2, respectively. Density functional theory (DFT) results indicate that the strong interactions between Ni and W increase the local electronic states of W atoms. The Ni₂W₄C provides active sites for cleaving H-OH bonds, and the W₃C facilitates the combination of H_ads intermediates into H₂ molecules. The in situ electrochemical-Raman results demonstrate that the strong absorption ability for hydroxyl and water molecules and further demonstrate that W atoms are the real active sites.

Electrospun CNF Supported Ceramics as Electrochemical Catalysts for Water Splitting and Fuel Cell: A Review

With the per capita growth of energy demand, there is a significant need for alternative and sustainable energy resources. Efficient electrochemical catalysis will play an important role in sustaining that need, and nanomaterials will play a crucial role, owing to their high surface area to volume ratio. Electrospun nanofiber is one of the most promising alternatives for producing such nanostructures. A section of key nano-electrocatalysts comprise of transition metals (TMs) and their derivatives, like oxides, sulfides, phosphides and carbides, etc., as well as their 1D composites with carbonaceous elements, like carbon nanotubes (CNTs) and carbon nanofiber (CNF), to utilize the fruits of TMs’ electronic structure, their inherent catalytic capability and the carbon counterparts’ stability, and electrical conductivity. In this work, we will discuss about such TM derivatives, mostly TM-based ceramics, grown on the CNF substrates via electrospinning. We will discuss about manufacturing methods, and their electrochemical catalysis performances in regards to energy conversion processes, dealing mostly with water splitting, the metal–air battery fuel cell, etc. This review will help to understand the recent evolution, challenges and future scopes related to electrospun transition metal derivative-based CNFs as electrocatalysts.

1D WO3 Nanorods/2D WO3-x Nanoflakes Homojunction Structure for Enhanced Charge Separation and Transfer towards Efficient Photoelectrochemical Performance

Designing and fabricating photoelectrodes with low carrier recombination, high carrier transfer, and high light-capture capability is of great significance for achieving effective photoelectrochemical (PEC) water splitting. Herein, for the first time, 2D nonstoichiometric WO_3-x nanoflakes (NFs) were vertically grown by hydrothermal synthesis on 1D WO₃ nanorods (NRs) obtained by a hydrothermal method and high-temperature annealing (HTA). In this 1D HTA-WO₃ /2D WO_3-x photoanode, the 2D WO_3-x NFs with active areas could maximize light harvesting, and the unique 1D/2D homojunction structure could improve the carrier-separation efficiency. At the same time, the 1D WO₃ NRs with high aspect ratio were more beneficial to charge transfer after HTA. As expected, the 1D HTA-WO₃ /2D WO_3-x photoanode yielded an enhanced photocurrent density of 0.98 mA cm^-2 at 1.23 V versus reversible hydrogen electrode, which is approximately 3.16 times that of pristine WO₃ . The improvement could be attributed to the synergistic effect of HTA and the homojunction structure in the 1D HTA-WO₃ /2D WO_3-x photoanode, which could effectively improve carrier separation and transfer. Furthermore, this work may provide a promising strategy for the design and fabrication of semiconductor-based photoelectrodes.

Oxygen deficiency introduced to Z-scheme CdS/WO3-x nanomaterials with MoS2 as the cocatalyst towards enhancing visible-light-driven hydrogen evolution

An oxygen deficiency modified Z-scheme CdS/WO3-x nanohybrid with MoS2 as the cocatalyst was synthesized by a microwave hydrothermal method and was used for photocatalytic hydrogen production under visible light irradiation. Loadings of WO3-x and MoS2 as well as the synthesis time of the microwave-assisted hydrothermal process were optimized, and the physicochemical and optical properties of the as-prepared photocatalysts were characterized by various techniques. Results showed that the material with 30 wt% of WO3-x, 0.1 wt% of MoS2 and a preparation time of 120 minutes exhibited the most desirable morphology and structure for hydrogen production. The maximum hydrogen production of 2852.5 μmol g-1 h-1 was achieved, which was 5.5 times that of pure CdS (519.1 μmol g-1 h-1) and 1.5 times that of CdS/30 wt% WO3-x (1879.0 μmol g-1 h-1), and the external quantum efficiency (EQE) reached 10.0% at 420 nm. The improvement of photocatalytic performance could be attributed to the Z-scheme formed between CdS and WO3-x and MoS2 as an electron trap. It is worth mentioning that the size of the composite had a negative correlation with the H2 production rate.

Enhanced catalytic degradation by using RGO-Ce/WO3 nanosheets modified CF as electro-Fenton cathode: Influence factors, reaction mechanism and pathways

Development of an efficient cathode in advanced oxidation process is an important challenge. In this work, we synthesized a low-cost, high-catalytic-active and stable reduced graphene oxide (RGO)-Ce/WO₃ nanosheets (RCW) to modify carbon felt (CF) as cathode to degrade ciprofloxacin (CIP) in electro-Fenton process. Compared to traditional heterogeneous electro-Fenton process, carbon black was substituted by RGO and poly tetra fluoroethylene was avoided to be used as binder. We found that RCW/CF cathode reached about 100% degradation efficiency of CIP after 1 h and 98.55% mineralization degree after 8 h. Meanwhile, it had a very high current density, about 2.5 times that of CF. RCW/CF cathode produced more O₂^-, H₂O₂ and OH via one-electron reduction process (O₂→O₂- →H₂O₂). The modified cathode kept a stable performance for high CIP degradation efficiency during 5 cycles. The introduction of RGO could promote electron transfer, and the adding of Ce into the WO₃ lattice provided superior conditions for the adsorption and activation of oxygen molecules, thus promoting the formation of active oxygen species on the surface of RCW. This novel RCW/CF composite is an efficient and promising electrode for removal of CIP in the wastewater.

Multivariate Control of Effective Cobalt Doping in Tungsten Disulfide for Highly Efficient Hydrogen Evolution Reaction

Tungsten Disulfide (WS₂) is considered to be a promising Hydrogen Evolution Reaction (HER) catalyst to replace noble metals (such as Pt and Pd). However, progress in WS₂ research has been impeded by the inertness of the in-plane atoms during HER. Although it is known that microstructure and defects strongly affect the electrocatalytic performance of catalysts, the understanding of such related catalytic origin still remains a challenge. Here, we combined a one-pot synthesis method with wet chemical etching to realize controlled cobalt doping and tunable morphology in WS₂. The etched products, which composed of porous WS₂, CoS₂ and a spot of WO_x, show a low overpotential and small Tafel slope in 0.5 M H₂SO₄ solution. The overpotential could be optimized to −134 mV (at 10 mA/cm²) with a Tafel slope of 76 mV/dec at high loadings (5.1 mg/cm²). Under N₂ adsorption analysis, the treated WS₂ sample shows an increase in macropore (>50 nm) distributions, which may explain the increase inefficiency of HER activity. We applied electron holography to analyze the catalytic origin and found a low surface electrostatic potential in Co-doped region. This work may provide further understanding of the HER mechanism at the nanometer scale, and open up new avenues for designing catalysts based on other transition metal dichalcogenides for highly efficient HER.

Advanced electrospun nanomaterials for highly efficient electrocatalysis

We highlight the recent developments of electrospun nanomaterials with controlled morphology, composition and architecture for highly efficient electrocatalysis.

Crystalline Ru0.33 Se Nanoparticles-Decorated TiO2 Nanotube Arrays for Enhanced Hydrogen Evolution Reaction

Nowadays, the state-of-the-art electrocatalysts for hydrogen evolution reaction (HER) are platinum group metals. Nonetheless, Pt-based catalysts show decreased HER activity in alkaline media compared with that in acidic media due to the sluggish dissociation process of H₂ O on the surface of Pt. With a cost 1/25 that of Pt, Ru demonstrates a favorable dissociation kinetics of absorbed H₂ O. Herein, crystalline Ru_0.33 Se nanoparticles are decorated onto TiO₂ nanotube arrays (TNAs) to fabricate Ru_0.33 Se @ TNA hybrid for HER. Owing to the large-specific surface area, Ru_0.33 Se nanoparticles are freely distributed and the particle aggregation is eliminated, providing more active sites. The contracted electron transport pathway rendered by TiO₂ nanotubes and the synergistic effect at the interface significantly improve the charge transfer efficiency in the hybrid catalyst. Compared with Ru_0.33 Se nanoparticles deposited directly on the Ti foil (Ru_0.33 Se/Ti) or carbon cloth (Ru_0.33 Se/CC), Ru_0.33 Se @ TNA shows an enhanced catalytic activity with an overpotential of 57 mV to afford a current density of 10 mA cm^-2 , a Tafel slope of 50.0 mV dec^-1 . Furthermore, the hybrid catalyst also exhibits an outstanding catalytic stability. The strategy here opens up a new synthetic avenue to the design of highly efficient hybrid electrocatalysts for hydrogen production.

Electronic Structure Control of Tungsten Oxide Activated by Ni for Ultrahigh-Performance Supercapacitors

Tuning the electron structure is of vital importance for designing high active electrode materials. Here, for boosting the capacitive performance of tungsten oxide, an atomic scale engineering approach to optimize the electronic structure of tungsten oxide by Ni doping is reported. Density functional theory calculations disclose that through Ni doping, the density of state at Fermi level for tungsten oxide can be enhanced, thus promoting its electron transfer. When used as electrode of supercapacitors, the obtained Ni-doped tungsten oxide with 4.21 at% Ni exhibits an ultrahigh mass-specific capacitance of 557 F g^-1 at the current density of 1 A g^-1 and preferable durability in a long-term cycle test. To the best of knowledge, this is the highest supercapacitor performance reported so far in tungsten oxide and its composites. The present strategy demonstrates the validity of the electronic structure control in tungsten oxide via introducing Ni atoms for pseudocapacitors, which can be extended to other related fields as well.

Nonstoichiometric tungsten oxide residing in a 3D nitrogen doped carbon matrix, a composite photocatalyst for oxygen vacancy induced VOC degradation and H2 production

An oxygen vacancy rich WO_3−x based composite material with 3D nitrogen doped carbon as a matrix is synthesized and it exhibits photocatalytic VOC removal and H₂ production activities.

Quasi physisorptive two dimensional tungsten oxide nanosheets with extraordinary sensitivity and selectivity to NO2

Attributing to their distinct thickness and surface dependent physicochemical properties, two dimensional (2D) nanostructures have become an area of increasing interest for interfacial interactions. Effectively, properties such as high surface-to-volume ratio, modulated surface activities and increased control of oxygen vacancies make these types of materials particularly suitable for gas-sensing applications. This work reports a facile wet-chemical synthesis of 2D tungsten oxide nanosheets by sonication of tungsten particles in an acidic environment and thermal annealing thereafter. The resultant product of large nanosheets with intrinsic substoichiometric properties is shown to be highly sensitive and selective to nitrogen dioxide (NO₂) gas, which is a major pollutant. The strong synergy between polar NO₂ molecules and tungsten oxide surface and also abundance of active surface sites on the nanosheets for molecule interactions contribute to the exceptionally sensitive and selective response. An extraordinary response factor of ∼30 is demonstrated to ultralow 40 parts per billion (ppb) NO₂ at a relatively low operating temperature of 150 °C, within the physisorption temperature band for tungsten oxide. Selectivity to NO₂ is demonstrated and the theory behind it is discussed. The structural, morphological and compositional characteristics of the synthesised and annealed materials are extensively characterised and electronic band structures are proposed. The demonstrated 2D tungsten oxide based sensing device holds the greatest promise for producing future commercial low-cost, sensitive and selective NO₂ gas sensors.

Conductive Tungsten Oxide Nanosheets for Highly Efficient Hydrogen Evolution

Exploring efficient and economical electrocatalysts for hydrogen evolution reaction is of great significance for water splitting on an industrial scale. Tungsten oxide, WO₃, has been long expected to be a promising non-precious-metal electrocatalyst for hydrogen production. However, the poor intrinsic activity of this material hampers its development. Herein, we design a highly efficient hydrogen evolution electrocatalyst via introducing oxygen vacancies into WO₃ nanosheets. Our first-principles calculations demonstrate that the gap states introduced by O vacancies make WO₃ act as a degenerate semiconductor with high conductivity and desirable hydrogen adsorption free energy. Experimentally, we prepared WO₃ nanosheets rich in oxygen vacancies via a liquid exfoliation, which indeed exhibits the typical character of a degenerate semiconductor. When evaluated by hydrogen evolution, the nanosheets display superior performance with a small overpotential of 38 mV at 10 mA cm^-2 and a low Tafel slope of 38 mV dec^-1. This work opens an effective route to develop conductive tungsten oxide as a potential alternative to the state-of-the-art platinum for hydrogen evolution.

Noble-metal-free tungsten oxide/carbon (WOx/C) hybrid manowires for highly efficient hydrogen evolution

Developing active, stable, and low-cost electrocatalysts to generate hydrogen is a great challenge in the fields of chemistry and energy. Nonprecious metal catalysts comprised of inexpensive and earth-abundant transition metals are regarded as a promising substitute for noble metal catalysts used in hydrogen evolution reaction (HER), but are still practically unfeasible mainly due to unsatisfactory activity and durability. Here we report a facile two-step preparation method for WO_x nanowires with high concentration of oxygen vacancies (OVs) via calcination of W-polydopamine compound precursors. The resulting hybrid material possesses a uniform and ultralong 1D nanowires structure and a rough and raised surface, which can effectively improve the specific surface area. The products exhibit excellent performance for H₂ generation: the required overpotentials for 1 and 10 mA cm^-2 are 18 and 108 mV, the Tafel slope is 46 mV/decade, and the electrochemically active surface area is estimated to be ∼77.0 m² g^-1. After 1000 cycles, the catalyst works well without significant current density drop. Our experimental results verified metallic transition metal oxides as superior non-Pt electrocatalysts for practical hydrogen evolution reactions.

The effects of exfoliation, organic solvents and anodic activation on the catalytic hydrogen evolution reaction of tungsten disulfide

The rational design of transition metal dichalcogenide electrocatalysts for efficiently catalyzing the hydrogen evolution reaction (HER) is believed to lead to the generation of a renewable energy carrier. To this end, our work has made three main contributions. At first, we have demonstrated that exfoliation via ionic liquid assisted grinding combined with gradient centrifugation is an efficient method to exfoliate bulk WS₂ to nanosheets with a thickness of a few atomic layers and lateral size dimensions in the range of 100 nm to 2 nm. These WS₂ nanosheets decorated with scattered nanodots exhibited highly enhanced catalytic performance for HER with an onset potential of -130 mV vs. RHE, an overpotential of 337 mV at 10 mA cm^-2 and a Tafel slope of 80 mV dec^-1 in 0.5 M H₂SO₄. Secondly, we found a strong aging effect on the electrocatalytic performance of WS₂ stored in high boiling point organic solvents such as dimethylformamide (DMF). Importantly, the HER ability could be recovered by removing the organic (DMF) residues, which obstructed the electron transport, with acetone. Thirdly, we established that the HER performance of WS₂ nanosheets/nanodots could be significantly enhanced by activating the electrode surface at a positive voltage for a very short time (60 s), decreasing the kinetic overpotential by more than 80 mV at 10 mA cm^-2. The performance enhancement was found to arise primarily from the ability of a formed proton-intercalated amorphous tungsten trioxide (a-WO₃) to provide additional active sites and favourably modify the immediate chemical environment of the WS₂ catalyst, rendering it more favorable for local proton delivery and/or transport to the active edge site of WS₂. Our results provide new insights into the effects of organic solvents and electrochemical activation on the catalytic performance of two-dimensional WS₂ for HER.

Stability, Scale-up, and Performance of Quantum Dot Solar Cells with Carbonate-Treated Titanium Oxide Films

A novel yet simple approach of carbonate (CBN) treatment of TiO₂ films is performed, and quantum dot solar cells (QDSCs) with high power conversion efficiencies (PCEs), reasonably good stabilities, and good fill factors (FFs) are fabricated with TiO₂-CBN films. The ability of carbonate groups to passivate defects or oxygen vacancies of TiO₂ is confirmed from a nominally enhanced band gap, a lowered defect induced fluorescence intensity, an additional Ti-OH signal obtained after carbonate decomposition, and a more capacitive low frequency electrochemical impedance behavior achieved for TiO₂-CBN compared to untreated TiO₂. A large area QDSC of 1 cm² with a TiO₂-CBN/CdS/Au@PAA (poly(acrylic acid)) photoanode delivers an enhanced PCE of 4.32% as opposed to 3.03% achieved for its analogous cell with untreated TiO₂. Impedance analysis illustrates the role of carbonate treatment in increasing the recombination resistance at the photoanode/electrolyte interfaces and in suppressing back-electron transfer to the electrolyte, thus validating the superior PCE achieved for the cell with carbonate-treated TiO₂. QDSCs with the configuration TiO₂-CBN/CdS/Au@PAA-polysulfide/SiO₂ gel-carbon-fabric/WO_3-x and active areas of 0.2-0.3 cm² yield efficiencies in the range of 5.16 to 6.3%, and the average efficiency of the cells is 5.9%. The champion cell is characterized by the following photovoltaic parameters: J_SC (short circuit current density), 11.04 mA cm^-2; V_OC (open circuit voltage), 0.9 V; FF, 0.63; and PCE, 6.3%. Stability tests performed on this cell show that dark storage has a less deleterious effect on cell performance compared to extended illumination. In dark, the PCE of the cell dropped from 5.69 to 5.52%, and under prolonged continuous irradiance of 5 h, it decreased from 5.91 to 4.83%. A scaled-up QDSC with the same architecture of 4 cm² size showed a PCE of 1.06%, and the demonstration of the lighting of a LED accomplished using this cell exemplifies that this cell can be used for powering electronic devices that require low power.

Morphology and Structure Engineering in Nanofiber Reactor: Tubular Hierarchical Integrated Networks Composed of Dual Phase Octahedral CoMn2 O4 /Carbon Nanofibers for Water Oxidation

1D hollow nanostructures combine the advantages of enhanced surface-to-volume ratio, short transport lengths, and efficient 1D electron transport, which can provide more design ideas for the preparation of highly active oxygen evolution (OER) electrocatalysts. A unique architecture of dual-phase octahedral CoMn₂ O₄ /carbon hollow nanofibers has been prepared via a two-step heat-treatment process including preoxidation treatment and Ostwald ripening process. The hollow and porous structures provide interior void spaces, large exposed surfaces, and high contact areas between the nanofibers and electrolyte and the morphology can be engineered by adjusting the heating conditions. Due to the intimate electrical and chemical coupling between the oxide nanocrystals and integrated carbon, the dual-phase octahedral CoMn₂ O₄ /carbon hollow nanofibers exhibit excellent OER activity with overpotentials of 337 mV at current density of 10 mA cm^-2 and Tafel slope of 82 mV dec^-1 . This approach will lead to the new perception of design issue for the nanoarchitecture with fine morphology, structures, and excellent electrocatalytic activity.

Non-Noble Metal-based Carbon Composites in Hydrogen Evolution Reaction: Fundamentals to Applications

Hydrogen has been hailed as a clean and sustainable alternative to finite fossil fuels in many energy systems. Water splitting is an important method for hydrogen production in high purity and large quantities. To accelerate the hydrogen evolution reaction (HER) rate, it is highly necessary to develop high efficiency catalysts and to select a proper electrolyte. Herein, the performances of non-noble metal-based carbon composites under various pH values (acid, alkaline and neutral media) for HER in terms of catalyst synthesis, structure and molecular design are systematically discussed. A detailed analysis of the structure-activity-pH correlations in the HER process gives an insight on the origin of the pH-dependence for HER, and provide guidance for future HER mechanism studies on non-noble metal-based carbon composites. Furthermore, this Review gives a fresh impetus to rational design of high-performance noble-metal-free composites catalysts and guide researchers to employ the established electrocatalysts in proper water electrolysis technologies.