Teng Zhai

Hydrogenated TiO2 nanotube arrays for supercapacitors.

We report a new and general strategy for improving the capacitive properties of TiO(2) materials for supercapacitors, involving the synthesis of hydrogenated TiO(2) nanotube arrays (NTAs). The hydrogenated TiO(2) (denoted as H-TiO(2)) were obtained by calcination of anodized TiO(2) NTAs in hydrogen atmosphere in a range of temperatures between 300 to 600 °C. The H-TiO(2) NTAs prepared at 400 °C yields the largest specific capacitance of 3.24 mF cm(-2) at a scan rate of 100 mV s(-1), which is 40 times higher than the capacitance obtained from air-annealed TiO(2) NTAs at the same conditions. Importantly, H-TiO(2) NTAs also show remarkable rate capability with 68% areal capacitance retained when the scan rate increase from 10 to 1000 mV s(-1), as well as outstanding long-term cycling stability with only 3.1% reduction of initial specific capacitance after 10,000 cycles. The prominent electrochemical capacitive properties of H-TiO(2) are attributed to the enhanced carrier density and increased density of hydroxyl group on TiO(2) surface, as a result of hydrogenation. Furthermore, we demonstrate that H-TiO(2) NTAs is a good scaffold to support MnO(2) nanoparticles. The capacitor electrodes made by electrochemical deposition of MnO(2) nanoparticles on H-TiO(2) NTAs achieve a remarkable specific capacitance of 912 F g(-1) at a scan rate of 10 mV s(-1) (based on the mass of MnO(2)). The ability to improve the capacitive properties of TiO(2) electrode materials should open up new opportunities for high-performance supercapacitors.

Flexible solid-state supercapacitors based on carbon nanoparticles/MnO2 nanorods hybrid structure.

A highly flexible solid-state supercapacitor was fabricated through a simple flame synthesis method and electrochemical deposition process based on a carbon nanoparticles/MnO(2) nanorods hybrid structure using polyvinyl alcohol/H(3)PO(4) electrolyte. Carbon fabric is used as a current collector and electrode (mechanical support), leading to a simplified, highly flexible, and lightweight architecture. The device exhibited good electrochemical performance with an energy density of 4.8 Wh/kg at a power density of 14 kW/kg, and a demonstration of a practical device is also presented, highlighting the path for its enormous potential in energy management.

H-TiO2@MnO2//H-TiO2@C Core–Shell Nanowires for High Performance and Flexible Asymmetric Supercapacitors

A flexible solid-state asymmetric supercapacitor device with H-TiO(2) @MnO(2) core-shell NWs as the positive electrode and H-TiO(2) @C core-shell NWs as the negative electrode is developed. This device operates in a 1.8 V voltage window and is able to deliver a high specific capacitance of 139.6 F g(-1) and maximum volumetric energy density of 0.30 mWh cm(-3) with excellent cycling performance and good flexibility.

High energy density asymmetric quasi-solid-state supercapacitor based on porous vanadium nitride nanowire anode.

To push the energy density limit of asymmetric supercapacitors (ASCs), a new class of anode materials is needed. Vanadium nitride (VN) holds great promise as anode material for ASCs due to its large specific capacitance, high electrical conductivity, and wide operation windows in negative potential. However, its poor electrochemical stability severely limits its application in SCs. In this work, we demonstrated high energy density, stable, quasi-solid-state ASC device based on porous VN nanowire anode and VOx nanowire cathode for the first time. The VOx//VN-ASC device exhibited a stable electrochemical window of 1.8 V and excellent cycling stability with only 12.5% decrease of capacitance after 10,000 cycles. More importantly, the VOx//VN-ASC device achieved a high energy density of 0.61 mWh cm(-3) at current density of 0.5 mA cm(-2) and a high power density of 0.85 W cm(-3) at current density of 5 mA cm(-2). These values are substantially enhanced compared to most of the reported quasi/all-solid-state SC devices. This work constitutes the first demonstration of using VN nanowires as high energy anode, which could potentially improve the performance of energy storage devices.

Stabilized TiN Nanowire Arrays for High-Performance and Flexible Supercapacitors

Metal nitrides have received increasing attention as electrode materials for high-performance supercapacitors (SCs). However, most of them are suffered from poor cycling stability. Here we use TiN as an example to elucidate the mechanism causing the capacitance loss. X-ray photoelectron spectroscopy analyses revealed that the instability is due to the irreversible electrochemical oxidation of TiN during the charging/discharging process. Significantly, we demonstrate for the first time that TiN can be stabilized without sacrificing its electrochemical performance by using poly(vinyl alcohol) (PVA)/KOH gel as the electrolyte. The polymer electrolyte suppresses the oxidation reaction on electrode surface. Electrochemical studies showed that the TiN solid-state SCs exhibit extraordinary stability up to 15,000 cycles and achieved a high volumetric energy density of 0.05 mWh/cm(3). The capability of effectively stabilizing nitride materials could open up new opportunities in developing high-performance and flexible SCs.

Oxygen‐Deficient Hematite Nanorods as High‐Performance and Novel Negative Electrodes for Flexible Asymmetric Supercapacitors

Oxygen-deficient α-Fe2 O3 nanorods with outstanding capacitive performance are developed and demonstrated as novel negative electrodes for flexible asymmetric supercapacitors. The asymmetric-supercapacitor device based on the oxygen-deficient α-Fe2 O3 nanorod negative electrode and a MnO2 positive electrode achieves a maximum energy density of 0.41 mW·h/cm(3) ; it is also capable of charging a mobile phone and powering a light-emitting diode indicator.

Facile synthesis of large-area manganese oxide nanorod arrays as a high-performance electrochemical supercapacitor

Large-area manganese oxide nanorod arrays (MONRAs) and herringbones (MOHBs) were successfully synthesized on F-doped SnO2 coated glass (FTO) substrates by a simple electrochemical method. Cyclic voltammetry (CV) and galvanostatic charge/discharge measurements demonstrated that the MONRAs and MOHBs exhibited excellent specific capacitance and good cycling stability in 0.5 M Na2SO4 aqueous solution. For example, the specific capacitance of the MONRAs achieves as high as 660.7 F g−1 at a scan rate of 10 mV s−1 and 485.2 F g−1 at a current density of 3 A g−1, respectively. Furthermore, the presented method may be extended to allow similar MONRs with a specific capacitance of 583.6 F g−1 to grow on flexible Ti foil, which may have great potential application in fabricating flexible supercapacitors.

Polyaniline and Polypyrrole Pseudocapacitor Electrodes with Excellent Cycling Stability

Conducting polymers such as polyaniline and polypyrrole have been widely used as pseudocapacitive electrode materials for supercapacitors. However, their structural instability resulting from repeated volumetric swelling and shrinking during charge/discharge process has been a major hurdle for their practical applications. This work demonstrates a simple and general strategy to substantially enhance the cycling stability of conductive polymer electrodes by deposition of a thin carbonaceous shell onto their surface. Significantly, carbonaceous shell-coated polyaniline and polypyrrole electrodes achieved remarkable capacitance retentions of ∼95 and ∼85% after 10,000 cycles. Electron microscopy studies revealed that the presence of ∼5 nm thick carbonaceous shell can effective prevent the structural breakdown of polymer electrodes during charge/discharge process. Importantly, the polymer electrodes with a ∼5 nm thick carbonaceous shell exhibited comparable specific capacitance and pseudocapacitive behavior as the bare polymer electrodes. We anticipate that the same strategy can be applied for stabilizing other polymer electrode materials. The capability of fabricating stable polymer electrodes could open up new opportunities for pseudocapacitive devices.

Solid‐State Supercapacitor Based on Activated Carbon Cloths Exhibits Excellent Rate Capability

Activated carbon cloth is used as an electrode, achieving an excellent areal capacitance of 88 mF/cm(2) (8.8 mF/g) without the use of any other capacitive materials. Significantly, when it is incorporated as part of a symmetric solid-state supercapacitor device, a remarkable charge/discharge rate capability is observed; 50% of the capacitance is retained when the charging rate increases from 10 to 10,000 mV/s.

Oxygen vacancies promoting photoelectrochemical performance of In 2 O 3 nanocubes

This work reports a facile method for preparing the new photoactive In2O3 films as well as their implementation in photoelectrochemical (PEC) application. We firstly investigated the relationship between oxygen vacancies (VO) and PEC performance and revealed a rule between them. We found that the optimized In2O3−n sample yielded a photocurrent density up to 3.83 mA/cm2 in 1 M Na2SO4 solution under the solar illumination. It also gave efficiency as high as 75% over 400 nm in the incident-photon-to-current-conversion efficiency (IPCE) spectrum, which is the best value for an In2O3 photoanode reported. Moreover, the PEC performance of these films is enhanced as the VO increased and then decreased with further increasing VO. This two-side effect means VO can favor the photoelectron activation, or act as recombination centers to prohibit the generation of photocurrent. Making highly photoactive In2O3 nanostructures in this work will open up new opportunities in various areas.