Yunhuai Zhang

Ni–Co sulfide nanowires on nickel foam with ultrahigh capacitance for asymmetric supercapacitors

In this paper we synthesize well aligned Ni–Co sulfide nanowire arrays (NWAs) with a Ni–Co molar ratio of 1 : 1 on 3D nickel foam by a facile two-step hydrothermal method. Owing to the low electronegativity of sulfur, Ni–Co sulfide NWAs exhibit a more flexible structure and much higher conductivity compared with Ni–Co oxide NWAs when used as active materials in supercapacitors. The electrochemistry tests show that these self-supported electrodes are able to deliver ultrahigh specific capacitance (2415 F g−1 and 1176 F g−1 at a current density of 2.5 mA cm−2 and 30 mA cm−2, respectively), together with a considerable areal capacitance (6.0 F cm−2 and 2.94 F cm−2 at a current density of 2.5 mA cm−2 and 30 mA cm−2, respectively), and good rate capability. More importantly, the asymmetric supercapacitor, composed of Ni–Co sulfide NWAs as the positive electrode and activated carbon as the negative electrode, reaches up to an energy density of 25 W h kg−1 and a power density of 3.57 kW kg−1 under a cell voltage of 1.8 V. Furthermore, the two assembled supercapacitors in series can power a 3 mm diameter red (2.0 V, 20 mA) round light-emitting diode (LED) indicator for more than 30 minutes after charging separately for a total time of 6 min. The superior electrochemistry capacity demonstrates that the self-standing Ni–Co sulfide nanowire arrays are promising for high-performance supercapacitor applications.

Carbon monoxide annealed TiO2nanotube array electrodes for efficient biosensor applications

Highly ordered titania nanotube (TNT) arrays fabricated by anodic oxidation of titanium foil followed with O2 and CO annealing were employed as matrices for the immobilization of horseradish peroxidase (HRP) and thionine chloride (Th) for biosensor application. The influence of annealing gases on TNT crystallinity, morphology, surface chemistry, and electrochemical properties were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), photoelectron spectroscopy (XPS), and cyclic voltammetry. The results showed that TNT arrays annealed in CO consisted of Ti3+ defects with carbon-doping and exhibited well defined quasi-reversible cyclic voltammetric peaks, indicating enhanced electron transfer and electrical conductivity. When immobilized with HRP and Th, the TNT electrodes were used to sense hydrogen peroxide (H2O2) as investigated by means of UV-Vis absorption spectroscopy, cyclic voltammetry, and amperometry. The results indicated that the biosensor based on a TNT array electrode annealed in CO possessed greatly enhanced properties compared to the as-grown and the O2-annealed TNT array electrodes. The improved biosensor properties of the TiO2nanotube arrays were ascribed to the carbon-doping and the formation of Ti3+ defects.

Ni–Co oxides nanowire arrays grown on ordered TiO2 nanotubes with high performance in supercapacitors

We develop a facile hydrothermal synthesis method to fabricate mesoporous Ni–Co oxides nanowire arrays on ordered TiO2 nanotubes (Ni–Co oxides NWs@TiO2NTs) as a high-capacity pseudocapacitor material. The composite electrode presented the highest specific capacitance of 2353 F g−1 at a charge and discharge current density of 2.5 A g−1 with excellent cycling ability, retaining 95% of the maximum capacitance after 3000 cycles. Even at a high current density of 50 A g−1, the electrode exhibited a specific capacitance of up to 2173 F g−1. In addition, the testing of a (Ni–Co oxides NWs@TiO2NTs)//(Ni–Co oxides NWs@TiO2NTs) symmetric cell yielded a specific capacitance of up to 144 F g−1 at a current density of 4 A g−1. The much improved capacity and cycling stability may be attributed to the superior conductivity and the aligned structure of the Ni–Co oxides NWs and the TiO2NTs substrate, which can improve electron/ion transport and enhance the kinetics of the redox reactions.

Synthesis of BiVO4 nanoflake array films for photoelectrochemical water oxidation

Because of the potential for application in photoelectrochemical cells for water splitting, the synthesis of nanostructured BiVO4 is receiving increasing attention. Here we report a simple new drop-casting method for the first time to synthesize un-doped and doped bismuth vanadate (BiVO4) nanoflake array films. Synthesis parameters such as the amount of polyethylene glycol 600 (PEG-600) and the precursor solution drying time are investigated to optimize the films for photoelectrochemical water oxidation. The BiVO4 films consisting of nanoflakes with an average thickness of 20 nm and length of 2 μm were synthesized from a precursor solution containing Bi3+, V3+ and PEG-600 with a Bi:V: PEG-600 volume ratio of 2:2:1, dried at 135 °C for 55 min. Photoelectrochemical measurements show that the BiVO4 nanoflake array films have higher photoelectrochemical activity than the BiVO4 nanoparticle films. Additionally, the nanoflake arrays were tested after incorporating W and Mo to enhance the photoelectrochemical activity. The 2% W, 6% Mo co-doped BiVO4 nanoflake array films demonstrate the best photoelectrochemical activity with photocurrent densities about 2 times higher than the un-doped BiVO4 nanoflake films and greater than the photocurrents of individually Mo doped or W doped BiVO4 films. The origin of enhanced photoelectrochemical activity for the co-doped film may be due to the improved conductivity through the BiVO4 or slightly enhanced water oxidation kinetics.

Ultrathin single-crystalline vanadium pentoxide nanoribbon constructed 3D networks for superior energy storage

A new 3D V2O5@PPy network built from numerous ultrathin, flexible and single-crystalline nanoribbons was successfully fabricated by a combined hydrothermal, freeze-drying and nanocasting process. Such a unique network can not only provide a high surface area for enhancement of electrolyte/electrode interactions, and reduce the diffusion length of ions, but also efficiently maintain the high electrical conductivity. As a result, this network exhibits high capacitance, excellent rate capability and good charge–discharge stability for energy storage. An asymmetric supercapacitor based on a 3D V2O5@PPy network as the cathode material further delivers high energy density and high power density. We expect that our work presents an efficient approach to design and produce various 3D architectures built from nanoribbons or nanosheets for energy storage and other applications.

Branched ultra-fine nickel oxide/manganese dioxide core–shell nanosheet arrays for electrochemical capacitors

We demonstrate the fabrication of ultra-fine hierarchical NiO@MnO2 core–shell nanosheet arrays on carbon fiber paper via a facile and two-step hydrothermal method as the electrode for supercapacitors. Owing to the unique hierarchical core–shell heterostructured nanosheet array configuration and synergetic contribution from the NiO core and MnO2 shell, the as-prepared hybrid nanosheet arrays exhibited a high specific capacitance of 494.8 F g−1 and good rate capability, which are better than those of the obtained individual component of the NiO nanosheets. More importantly, the asymmetric supercapacitor, composed of NiO@MnO2 core–shell nanosheet arrays as the positive electrode and activated carbon as the negative electrode delivers a specific capacitance of 86.3 F g−1 and reaches up to an energy density of 30.6 W h kg−1 and power density of 400 W kg−1. The improved performance of the NiO@MnO2 core–shell nanosheet arrays demonstrates that rational design of electrode structure is an effective way to fabricate a high-performance electrode for supercapacitors.

Hydrogen-treated BiFeO3 nanoparticles with enhanced photoelectrochemical performance

Enhancing charge separation and collection in semiconducting photoelectrodes is a key issue in the area of photoelectrochemical applications. Herein, we report that the photoelectrochemical performance of BiFeO3 nanoparticles can be significantly enhanced by treatment with hydrogen atmosphere at elevated temperature for different time periods. Compared with pristine BiFeO3 nanoparticles, the hydrogen-treated BiFeO3 nanoparticles showed substantial improvement of intrinsic features from structural, optical and electronic aspects. These hydrogen-treated BiFeO3 nanoparticles exhibit an unexpected red shift, which was attributed to the formation of sub-band-gap states between the conduction and valence bands, resulting from oxygen vacancies due to the hydrogen treatment. However, the highly concentrated defects in BiFeO3 nanoparticles can act as charge annihilation centers, which is not conducive to the separation of photo-generated carriers. The hydrogen-treated BiFeO3 prepared at 300 °C for 15 min exhibited optimal photoelectrochemical performance. More than three times higher photocurrent density was achieved compared with pristine BiFeO3, demonstrating its potential in practical application.

Vertically aligned cobalt oxide nanowires on graphene networks for high-performance lithium storage.

Despite various electrochemically active materials, such as metals, metal oxides and sulfides, which have been widely utilized for lithium storage, these materials still encounter unsatisfied electrochemical performances including low reversible capacity, slow charge-discharge capability and poor cycle performance. Here, we demonstrate a simple approach to fabricate one-dimensional CoO nanowires vertically aligned on a 3D graphene network (denoted as a 3D CoO/graphene network) via a wet chemistry process. The resulting CoO/graphene network possesses an interconnected graphene network, hierarchical pores and a carpet-like structure. This unique network can (1) facilitate the easy access of the electrolyte, (2) prevent the aggregation of CoO nanowires, (3) accommodate the volume change of CoO during the cycle processes, (4) maintain a high electrical conductivity for the overall electrode and (5) give rise to a high content of CoO in the composite (∼92 wt%). As a result, the 3D CoO/graphene network can be directly used as an anode material without any binder or conductive additives for lithium storage, and it exhibits a high capacity of 857 mAh g(-1), an excellent rate capability and good cycle performance. We believe that such a simple but efficient protocol will provide a new pathway for the fabrication of various 3D metal or metal oxide-graphene networks for wide applications in such fields as energy storage, sensors and catalysts.

Electric field tuned MoS2/metal interface for hydrogen evolution catalyst from first-principles investigations

Understanding the interfacial properties of catalyst/substrate is crucial for the design of high-performance catalyst for important chemical reactions. Recent years have witnessed a surge of research in utilizing MoS2 as a promising electro-catalyst for hydrogen production, and field effect has been employed to enhance the activity (Wang et al 2017 Adv. Mater. 29, 1604464; Yan et al 2017 Nano Lett. 17, 4109-15). However, the underlying atomic mechanism remains unclear. In this paper, by using the prototype MoS2/Au system as a probe, we investigate effects of external electric field on the interfacial electronic structures via density functional theory (DFT) based first-principles calculations. Our results reveal that although there is no covalent interaction between MoS2 overlayer and Au substrate, an applied electric field efficiently adjusts the charge transfer between MoS2 and Au, leading to tunable Schottky barrier type (n-type to p-type) and decrease of barrier height to facilitate charge injection. Furthermore, we predict that the adsorption energy of atomic hydrogen on MoS2/Au to be readily controlled by electric field to a broad range within a modest magnitude of field, which may benefit the performance enhancement of hydrogen evolution reaction. Our DFT results provide valuable insight into the experimental observations and pave the way for future understanding and control of catalysts in practice, such as those with vacancies, defects, edge states or synthesized nanostructures.Understanding the interfacial properties of catalyst/substrate is crucial for the design of high-performance catalyst for important chemical reactions. Recent years have witnessed a surge of research in utilizing MoS2 as a promising electro-catalyst for hydrogen production, and field effect has been employed to enhance the activity (Adv. Mater. 2017, 29, 1604464; Nano Lett. 2017, 17, 4109). However, the underlying atomic mechanism remains unclear. In this paper, by using the prototype MoS2/Au system as a probe, we investigate effects of external electric field on the interfacial electronic structures via density functional theory (DFT) based first-principles calculations. Our results reveal that although there is no covalent interaction between MoS2 overlayer and Au substrate, an applied electric field efficiently adjusts the charge transfer between MoS2 and Au, leading to tunable Schottky barrier type (n-type to p-type) and decrease of barrier height to facilitate charge injection. Furthermore, we predict that the adsorption energy of atomic hydrogen on MoS2/Au to be readily controlled by electric field to a broad range within a modest magnitude of field, which may benefit the performance enhancement of hydrogen evolution reaction. Our DFT results provide valuable insight into the experimental observations and pave the way for future understanding and control of catalysts in practice, such as those with vacancies, defects, edge states or synthesized nanostructures.

DFT studies on the interaction of PtxRuyMz (M = Fe, Ni, Cu, Mo, Sn, x + y + z = 4, x ≥ 1, y ≥ 1) alloy clusters with O2

The reaction mechanism of O2 dissociation on PtxRuyMz (M = Fe, Ni, Cu, Mo, Sn, x + y + z = 4, x ≥ 1, y ≥ 1) alloy catalysts have been investigated with density functional theory calculations in this work. For bare alloy clusters, bimetallic clusters are more stable than the ternary alloy clusters. The geometries of the PtxRuyMz–O2 system, O–O bond stretching frequency and electronic-structure details have been investigated. The energies of O2 adsorption on PtRu clusters are slightly higher than those on PtxRuyMz clusters, and the more charge transfer to O2 from the metal cluster, the higher O2 the adsorption energy obtains. The reaction barriers show that the catalytic performance of trimetallic clusters are better than those of bimetallic clusters, and Pt2RuM clusters exhibit superior catalytic activity for O2 dissociation. The different performance of these alloy clusters for O2 dissociation is scrutinised with aid of molecular orbital and natural bond orbital population analysis.