Nano-sized Mo- and Nb-doped TiO2 as anode materials for high energy and high power hybrid Li-ion capacitors

Influence of Li concentration on structural, morphological and electrochemical properties of anatase-TiO2 nanoparticles

Lithium-doped anatase-TiO2 nanoparticles (LixTi1-xO2 NPs, x = 0, 0.05, 0.10, 0.15 and 0.20) could be synthesized by a simple sol-gel process. X-ray diffraction (XRD) results displayed the tetragonal (space group: I41/amd) of polycrystalline TiO2 anatase phase. The spectroscopy results of Raman and FT-IR confirmed the anatase phase of TiO2 through the specific modes of metal oxides vibration in the crystalline TiO2. Surfaces micrographs by scanning electron microscope (SEM) of agglomerated LixTi1-xO2 NPs showed a spongy like morphology. Transmission electron microscope (TEM) illustrated a cuboidal shape of dispersed NPs with particle size distributed in a narrow range 5-10 nm. Bruanauer Emmett-Teller (BET) results showed the increased surface area of LixTi1-xO2 NPs with increasing Li content. LixTi1-xO2 NPs (x = 0.05-0.20) working electrodes illustrated a pseudocapacitive behavior with excellent electrochemical properties through the whole cycles of GCD test. Interestingly, Li0.1Ti0.9O2 NPs electrode illustrated a high performance in terms of maximum specific capacitance 822 F g-1 at 1.5 A g-1 in 0.5 M Li2SO4 electrolyte, with excellent capacitive retention 92.6% after 5000 cycles GCD test.

Phosphorus-doped TiO2mesoporous nanocrystals for anodes in high-current-rate lithium ion batteries

Limited by the intrinsic low electronic conductivity and inferior electrode kinetics, the use of TiO2as an anode material for lithium ion batteries (LIBs) is hampered. Nanoscale surface-engineering strategies of morphology control and particle size reduction have been devoted to increase the lithium storage performances. It is found that the ultrafine nanocrystal with mesoporous framework plays a crucial role in achieving the excellent electrochemical performances due to the surface area effect. Herein, a promising anode material for LIBs consisting of phosphorus-doped TiO2mesoporous nanocrystals (P-TMC) with ultrafine size of 2-8 nm and high specific surface area (234.164 m2g-1) has been synthesized. It is formed through a hydrothermal process and NaBH4assisted heat treatment for anatase defective TiO2(TiO2-x) formation followed by a simple gas phosphorylation process in a low-cost reactor for P-doping. Due to the merits of the large specific surface area for providing more reaction sites for Li+ions to increase the storage capacity and the presence of oxygen vacancies and P-doping for enhancing material's electronic conductivity and diffusion coefficient of ions, the as-designed P-TMC can display improved electrochemical properties. As a LIB anode, it can deliver a high reversible discharge capacity of 187 mAh g-1at 0.2 C and a good long cycling performance with ∼82.6% capacity retention (101 mAh g-1) after 2500 cycles at 10 C with an average capacity loss of only 0.007% per cycle. Impressively, even the current rate increases to 100 times of the original rate, a satisfactory capacity of 104 mAh g-1can be delivered, displaying good rate capacity. These results suggest the P-TMC a viable choice for application as an anode material in LIB applications. Also, the strategy in this work can be easily extended to the design of other high-performance electrode materials with P-doping for energy storage.

Toward Enhanced Electrochemical Performance by Investigation of the Electrochemical Reconstruction Mechanism in Co2V2O7 Hexagonal Nanosheets for Hybrid Supercapacitors

As for hybrid supercapacitors, it is important to enhance the long cycling performance and high specific capacitance. In this paper, cobalt vanadate (Co₂V₂O₇) hexagonal nanosheets on nickel foam are manufactured by a facile hydrothermal method and then transformed into numerous smaller size interconnected hierarchical nanosheets without any shape change via electrochemical reconstruction. Benefiting from the favorable architecture of hierarchical nanosheets via electrochemical reconstruction, the Co₂V₂O₇ hexagonal nanosheet electrode exhibits a remarkable long cycling performance with 272% specific capacitance retention after 100,000 cycles at a current density of 5 A g^-1 and then displays an increasing specific capacitance of 1834 F g^-1 (tested at 1 A g^-1). Furthermore, an aqueous hybrid supercapacitor device based on the Co₂V₂O₇ hexagonal nanosheet electrode exhibits a high energy density of 35.2 Wh kg^-1 at a power density of 1.01 kW kg^-1 and an excellent cyclic stability with 71.4% capacitance retention after 10,000 cycles at 5 A g^-1. These results offer a practicable pathway for enhancing the electrochemical properties of other metal oxides through electrochemical reconstruction.

A Combination of EPR, Microscopy, Electrophoresis and Theory to Elucidate the Chemistry of W- and N-Doped TiO2 Nanoparticle/Water Interfaces

The doping of TiO2-based nanomaterials for semiconductor-sensitised photoreactions has been a practice extensively studied and applied for many years. The main goal remains the improvement of light harvesting capabilities under passive solar irradiation, that in the case of undoped TiO2 is limited and restricted to relatively low latitudes. The activity and selectivity of doped TiO2 photocatalysts are generally discussed on the basis of the modified band structure; energetics of intrinsic or extrinsic band gaps including trapping states; redox potentials of band edges, including band bending at solid/fluid interfaces; and charge carriers scavenging/transfer by/to adsorbed species. Electron (and hole) transfer to adsorbates is often invoked to justify the formation of highly reactive species (e.g., HO. from water); however, a complete description of the nanoparticle surface chemistry dictating adsorption/desorption events is often missing or overlooked. Here, we show that by employing a surface electrochemical triple-layer (TLM) approach for the nanoparticles/water interface, in combination with electron paramagnetic resonance spectroscopy (EPR), transmission electron microscopy and electrophoretic measurements, we can elucidate the surface chemistry of doped TiO2 nanoparticles and link it to the nature of the dopants. Exemplifying it for the cases of undoped, as well as W- and N-doped and codoped TiO2 nanoparticles, we show how surface charge density; surface, Stern and ζ potentials; surface acidity constants; and speciation of surface sites are influenced by the nature of the dopants and their loading.

Surface Characteristic Effect of Ag/TiO2 Nanoarray Composite Structure on Supercapacitor Electrode Properties

Ag-ion-modified titanium nanotube (Ag/TiO2-NT) arrays were designed and fabricated as the electrode material of supercapacitors for electrochemical energy storage. TiO2 nanotube (NT) arrays were prepared by electrochemical anodic oxidation and then treated by Ag metal vapor vacuum arc (MEVVA) implantation. The Ag amount was controlled via adjusting ion implantation parameters. The morphology, crystallinity, and electrochemistry properties of as-obtained Ag/TiO2-NT electrodes were distinguished based on various characterizations. Compared with different doses of Ag/TiO2-NTs, the electrode with the dose of 5.0 × 1017 ions·cm-2 exhibited much higher electrode capacity and greatly enhanced activity in comparison to the pure TiO2-NTs. The modified electrode showed a high capacitance of 9324.6 mF·cm-3 (86.9 mF·g, 1.2 mF·cm-2), energy density of 82.8 μWh·cm-3 (0.8 μWh·g, 0.0103 μWh·cm-2), and power density of 161.0 mW·cm-3 (150.4 μW·g, 2.00 μW·cm-2) at the current density of 0.05 mA. Therefore, Ag/TiO2-NTs could act as a feasible electrode material of supercapacitors.

Highly crystalline Nb-doped TiO2 nanospindles as superior electron transporting materials for high-performance planar structured perovskite solar cells

Planar-structured perovskite solar cells (PSCs) have received tremendous attention due to their high power conversion efficiency (PCE), simple process and low-cost fabrication. A compact thin film of electron transport materials (ETMs) plays a key role in these PSCs. However, the traditional ETMs of PSCs, TiO₂ nanoparticulate films, suffer from low conductivity and high trap state density. Herein, we exploited TiO₂ nanospindles as a compact ETM in planar PSCs for the first time, and achieved an efficient device with a PCE of 19.1%. By optimization with Nb doping into the TiO₂ nanospindles, the PCE of the PSC was further improved up to 20.8%. The carrier transfer and collection efficiency were significantly improved after Nb⁵⁺ doping, revealed by Mott–Schottky (MS) analysis, space charge limited current (SCLC), photoluminence (PL), time-resolved photoluminence (TRPL) spectra, electrochemical impedance spectra (EIS) and so forth. Moreover, the hysteresis behavior was effectively inhibited and the stability was significantly enhanced. This work may provide a new avenue towards the rational design of efficient ETMs for perovskite solar cells.

Highly crystalline Nb:TiO₂ nanospindles have been successfully exploited as efficient ETMs in planar perovskite solar cells, achieving a power conversion efficiency of 20.8%, superior to that of the undoped one (19.1%).

Ultrafast-Charging and Long-Life Li-Ion Battery Anodes of TiO2-B and Anatase Dual-Phase Nanowires

Ideal lithium-ion batteries (LIBs) should possess a high power density, be charged extremely fast (e.g., 100C), and have a long service life. To achieve them all, all battery components, including anodes, cathodes, and electrolytes should have excellent structural and functional characteristics. The present work reports ultrafast-charging and long-life LIB anodes made from TiO₂-B/anatase dual-phase nanowires. The dual-phase nanowires are fabricated with anatase TiO₂ nanoparticles through a facile and cost-effective hydrothermal process, which can be easily scaled up for mass production. The anodes exhibit remarkable electrochemical performance with reversible capacities of ∼225, 172, and 140 mAh g^-1 at current rates of 1C, 10C, and 60C, respectively. They deliver exceptional capacity retention of not less than 126 and 93 mAh g^-1 after 1000 cycles at 60C and 100C, respectively, potentially worthwhile for high-power applications. These values are among the best when the high-rate capabilities are compared with the literature data for similar TiO₂-based anodes. The Ragone plot confirms both the exceptionally high energy and power densities of the devices prepared using the dual-phase nanowires. The electrochemical tests and operando Raman spectra present fast electrochemical kinetics for both Li⁺ and electron transports in the TiO₂ dual-phase nanowires than in anatase nanoparticles due to the excellent Li⁺ diffusion coefficient and electronic conductivity of nanowires.