•BaTiO3/TiO2 nanocomposite films were fabricated as photoanodes of DSCs.•Ferroelectric dipole induced electric field would reduce charge recombination.•The highest power conversion efficiency was achieved in 1.0 wt% BaTiO3 addition.BaTiO3/TiO2 nanocomposite films with varied amount of BaTiO3 are fabricated and applied as photoanodes for dye-sensitized solar cells (DSCs) and demonstrated enhanced power conversion efficiency. Ferroelectricity of BaTiO3 in the film after subjected to a annealing process up to 450 °C is examined by Switching Spectroscopy Piezoresponse Force Microscopy (SSPFM). The highest performance is achieved in 1.0 wt% BaTiO3 addition as a result of increased photocurrent density (Jsc) and fill factor (FF), regardless of reduction of dye-loading. Electrochemical impedance spectroscopy (EIS) measurements at different bias voltages (≦Voc) in dark suggest that ferroelectric dipole induced electric field has positive effects on enhancing electron mobility and suppressing charge recombination. Although more detailed experiments are needed in designing of the nanocomposite films for compensating characteristics of dye-loading and electron mobility, introduction of ferroelectric dipole induced electric field into the photoanode would be a good strategy in achieving further improvement of power conversion efficiency of DSCs through improved charge transfer properties.Download high-res image (209KB)Download full-size image

•Mo-doping BiVO4 shows the highest photocurrent (1.91 mA/cm2 at 1.23 V vs. RHE).•The ηsep (74.42%) and ηox (49.25%) are obtained by 3AMo:BV at 1.23 V vs. RHE.•Mo-doped structure is beneficial for the transfer of photogenerated electrons.•Higher photovoltage is beneficial for the charge oxidation kinetics.Monoclinic bismuth vanadate (BiVO4) has been widely applied as a promising n-type semiconductor for photoelectrochemical (PEC) water splitting because of its low cost, environmentally friendly, and relatively narrow band gap of ∼2.4 eV. Here, we developed a facile fabrication of Mo doped BiVO4 photoanode on the fluorine-doped tin oxide substrate by electrodeposition method and used these samples to better understand the doping effect for charge separation and charge oxidation kinetics. Compared with the undoped BiVO4 photoanode, the optimized Mo doped BiVO4 (3AMo:BV) produced a much higher photocurrent of 1.91 mA/cm2 at 1.23 V vs. RHE under AM 1.5G illumination for water oxidation. The results of the photoelectric conversion kinetics for various samples revealed that the charge separation and oxidation kinetics efficiencies for 3AMo:BV sample are 74.42% and 49.25% at 1.23 V vs. RHE, while the values for undoped BiVO4 are 48.04% and 32.98%, respectively. The improved PEC performance for Mo doped BiVO4 is mainly ascribed to the crystal deformation caused by larger tetrahedral ionic VO4 and higher photovoltage generated by the interface of photoanode and electrolyte.Highly charge separation and oxidation kinetics of Mo doped BiVO4 photoanode is fabricated for photoelectrochemical water splitting. After Mo doped in monoclinic BiVO4, the deformation of crystal with VO4 tetrahedron and decrease the energy of transfer of photogenerated electrons thus help to overcome poor electron mobility intrinsically. Moreover, this structure can create a higher photovoltage and enhance the overall photoelectrochemical performance, and the different lines are BV (black), 3SMo:BV (red), 3AMo:BV (blue), and 10AMo:BV (pink), respectively.Download high-res image (249KB)Download full-size image

Hierarchical nickel and cobalt inorganic–organic nanostructures on carbon fiber paper have been fabricated by a one-step hydrothermal approach directed by hexamethylenetetramine (HMT). The morphology and evolution of these single-crystalline inorganic–organic nanosheets–nanowires (IOSW) are investigated. Results show that HMT plays a vital role in the secondary growth of single-crystalline nanowires. Besides, HMT molecules intercalate within the interlayers of inorganic materials, forming single-crystalline organo-LDH (layered double hydroxide) nanosheets and organo-carbonate hydroxide nanowires. Furthermore, the electrode is employed for battery-like energy storage applications. The interconnected nanosheet–nanowire structure affords rich open spaces and efficient passways for ion diffusion and electron transport. A specific capacitance of 1308 F g−1 (1.7 F cm−2, at the current density of 4 mA cm−2) and excellent long-term cycling performance (94% retention after 10 000 cycles) are achieved. The high stability of the electrode could be attributed to the interactions between HMT and nickel/cobalt within the interlayer. In addition, a two-electrode device consisting of nickel and cobalt IOSW as the positive electrode and commercial active carbon as the negative electrode is fabricated and reaches a maximum energy density of 32.3 W h kg−1 (8 A g−1).

The design and fabrication of sustainable and efficient electrocatalyst for water splitting are crucial for rechargeable metal-air batteries and regenerative fuel cells. In this report, a highly stable and active NiCo-selenide electrocatalyst was successfully prepared by a facile two-step solvothermal approach. In 0.1 M KOH alkaline electrolyte solution, the novel NiCo-selenide electrocatalyst afforded the current density of 10 mA cm−2 at a lower overpotential of 393 mV, a smaller Tafel slope of ~89 mV dec−1 and better stability, compared with NiCo-based oxide and sulfide.

Nickel and cobalt oxides, sulfides, selenides are widely studied as active materials in the application of energy storage and enhanced capacity are obtained as the atomic number of the non-metal element increase. In this study, Nickel and cobalt oxides (NiCoO2), sulfides and selenides are prepared via ion-exchange method and they are investigated as electrode materials for aqueous battery applications. From the perspective of electrochemistry, a detailed study of the fundamental relationships between composition, properties, and electrochemical performance are carried out. The results reveal that the enhanced charge capacity is associated with a more active property of nickel and cobalt as well as higher diffusivity of electrolyte after introducing different anions (S or Se). However, the reversible specific capacity decreases for sulfides and selenides and X-ray photoelectron spectroscopy results after long-term cycling tests indicate that the dissolution of S and Se may be responsible for the severe degrading in charge capacity.

Because of the potential application in photoelectrochemical cells for water splitting, the synthesis of BiFeO3 (BFO) is receiving increasing attention. A simple, low-cost drop-casting route was used to synthesize Ti ions doped BiFeO3 thin films and the effects of the Ti ions doping on their crystal structure, morphology, and photoelectrocatalytic activity had been studied. X-ray diffraction results of Ti-BFO films showed that the doping of Ti ions lead to the distortion of crystal structure. Photoelectrochemical measurements indicated that the photoelectrocatalytic activities of BFO film doped with 5 % Ti ions showed the best performance. The UV–Vis absorption spectra of Ti-BFO films showed that the absorption wavelength of BFO doped with 5 % Ti had obvious shift and the band gap decreased. From the results of Mott–Schottky (M–S) plots, we calculated the positions of valence band (VB) and conduction band (CB) for Ti-BFO film. The outcomes showed that the band gap structure of BFO samples doped with 5 % Ti had more positive CB and more negative VB. The calculated photogenic charge carrier concentrations of BFO and Ti (0.05)-BFO were 5.07 × 1019 and 6.32 × 1019 cm−3, respectively. The reduced band gap and increased photogenic charge carrier concentration are the major factors contributing to the enhancement of photoelectrocatalytic performances of Ti-BFO films.

The synergetic effects of mixed transition metal compounds have demonstrated great potential in improving the electrochemical performance of materials. In this work, large scale of ammonium/bimetallic phosphates (NH4CoxNi1−xPO4·H2O) with layered nanostructures are synthesized by a facile one-step solvothermal method. The tuning effects of Co/Ni molar ratios on the crystal structure and morphology of NH4CoxNi1−xPO4·H2O are systematically studied by XRD, FTIR, SEM, TEM. It is revealed that the synergistic effects of cobalt and nickel may significantly influence the crystal structure and morphologies of NH4CoxNi1−xPO4·H2O, exposing more CoO6 and NiO6 octahedra active sites for redox reactions, improving the surface coverage of the redox species. Electrochemistry tests prove that the sample with a Co/Ni ratio of 2:3 which has layered nanoplate structure and abundant uniform pores, and exhibits the highest specific capacitance of 1567 F g−1 at 1 A g−1 and long-term cycling stability of 97.6 % compared with single-component ammonium nickel or cobalt phosphate hydrate. An asymmetric cell is constructed using NH4CoxNi1−xPO4·H2O (Co/Ni 2:3) and activated carbon with an operation potential from 0 to 1.65 V. This asymmetric device presented a high energy density of 37.4 Wh kg−1 at a power density of 826 W kg−1. More importantly, the device also displayed a good long-term cycling stability with 91.2 % capacity retention after 5000 cycles.

Ni-based bimetallic alloys have superior physiochemical characteristics compared to monometallic Ni. In this study, a new type of low cost bimetallic NimCon (n + m = 4) electrocatalysts with high active surface were synthesized on Ti substrate through a hydrogen evolution assisted electrodeposition method. The as-prepared NimCon were characterized by XRD, EDS, and SEM. It was revealed that the composition, surface morphology, as well as the crystal phase structure of the bimetallic NimCon electrocatalysts were significantly changed with the increased content of cobalt. Electrochemical measurements showed that the bimetallic NimCon catalysts, compared with the monometallic Ni, have superior catalytic activity and stability toward the methanol electrooxidation reaction. Additionally, Ni2Co2 sample presented the highest oxidation current density and the best durability. The mechanism study based on electrochemical experiments and density functional theory based calculations showed that the doping of Co in NimCon can signally improve the surface coverage of the redox species, weaken the CO adsorption, as well as adjust the CH3OH adsorption. Such understanding is of important directive significance to design efficient nonprecious catalysts.Keywords: carbon monoxide; density functional theory; electrocatalysis; fuel cells; methanol oxidation; NiCo catalyst

Dimensionality and rational design of electrode architectures play a crucial role in determining materials' fundamental properties and the electrochemical performance of supercapacitor. For a proof-of-concept, Ni–Co layered double hydroxides (LDH), NiCo2O4 and NiCo2S4 nanosheets supported on carbon fiber paper (CFP) substrate are prepared by simple hydrothermal methods in this work. When tested as the pseudo-capacitor positive electrode, the self-support nanosheets on CFP demonstrate good performance and rate capability as well as excellent cycling life, which contributes to the unique hierarchical nanosheets structure supported on 3D conductive CFP substrate with open permeable channels, facilitating electrolyte penetration and ensuring more efficient ion diffusion and faster electron transport. The asymmetric supercapacitor based on pseudocapacitance of both electrodes is further first realized by using NiCo2S4 nanosheets and FeOOH nanorods as positive and negative materials, respectively. The obtained device can deliver a maximum power density of 8.6 kW kg−1 and energy density of 45.9 Wh kg−1 and even after 10000 reversible cycles at a cell voltage of 1.6 V in aqueous electrolyte, there still retained 86.4% of its initial capacitance.

Previous work demonstrates that Fe2O3 is a promising negative electrode material. Its biochemical performance is strongly influenced by the morphology. Hence, Fe2O3 with nanoparticles, nanosheets and nanorods morphologies are synthesized by hydrothermal method respectively. The relationship between morphology and electrochemical property, particularly in terms of specific capacitance and rate capability are investigated. Detail cyclic voltammetry analysis reveals that the relative contributions of surface capacitive and inner intercalation capacitance change with the morphology. Different contribution of surface relative to inner charge storage leads to different specific capacitance and rate capability. Additionally, Fe2O3 nanosheets delivers a remarkable specific capacitance of 279.9 F g−1 (419.8 mF cm−2) at the scan rate of 5 mV s−1 ascribing to the inner intercalation effect. Moreover, electrochemical tests in neutral and alkaline solution show that the capacitive behavior of Fe2O3 is closely related to the potential window and different electrolyte leads to different contribution of surface relative to inner charge storage. These studies should be useful for elucidating the mechanism and morphological effects on charge storage for Fe2O3-based electrode and provide a direction to enhance its capacitance.

Water oxidation is a critical step in water splitting to make hydrogen fuel. We report a hybrid material consisting of NiCoO2 nanowires grown on carbon fiber paper (denoted as NiCoO2@CFP) as a highly efficient oxygen evolution reaction (OER) electrocatalyst. In 0.1 M KOH, the highly nanostructured NiCoO2 catalyst presents a small overpotential of ∼0.303 V at a current density of 10 mA cm−2 and a low Tafel slope of ∼57 mV dec−1, comparable to the commercial precious RuO2 catalyst. Such a good performance for OER may be attributed to the NaCl-type structure of the NiCoO2 nanowires and CFP substrate, which can boost the formation of active Ni-Co layered hydroxide/oxyhydroxide species during the catalysis process. Additionally, the aligned 3D structure of NiCoO2@CFP plays an important role in the high catalytic activity for OER. These merits combined with the satisfactory stability of the hybrid material indicate that it is a promising material for water oxidation.

•CoNiO/TiO2NTs was synthesized by a facile hydrothermal method.•The addition of Ni to CoO could improve its conductivity.•CoNiO/TiO2NTs nanocomposite material is a 3D structure.•It delivers a high areal capacity when used as lithium-ion battery anode material.CoNiO nanowire arrays loaded on TiO2 nanotubes (CoNiO/TiO2NTs) are synthesized by a hydrothermal method and used firstly as an anode material for lithium-ion batteries. The morphology, structure and composition of the composite are characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The EDS patterns display the atomic ratio of Co to Ni is 0.41:0.59 with accuracy of more than 99%. SEM images show that the diameters of these nanowires range from 10 to 40 nm and the average length approximately 1 μm. Electrochemical characterizations are performed in a three-electrode system to determine the capacity, cyclic stability and to investigate the reaction mechanism. As an anode material for lithium-ion batteries, the CoNiO/TiO2NTs nanocomposite delivers a high areal capacity of 362 μAh cm−2 (1097 mAh g−1, 0.33 mg cm−2) after 40 discharge/charge cycles at a current density of 0.2 mA cm−2 (about 606 mA g−1). EIS results show that addition of Ni to the CoO could increase the conductivity of the composite significantly and improve the kinetic behavior during discharge–charge process. The present finding provides a kind of nanostructure fabrication that might be applied in supercapacitor and solar cells, etc.

•The dominant pathways for methanol dehydrogenation on Pt7 and PtNi clusters are compared.•The C–H scission for methanol dehydrogenation is more favorable on Pt7 cluster.•On PtNi bimetallic cluster, the O–H breaking for methanol oxidation is more easily.•CO dissociates more preferable on PtNi bimetallic cluster than that on Pt7.Density functional theory based calculations have been employed to investigate structures and properties of coupled tetragonal pyramid (CTP) Pt7 based Pt(7−x)Nix (x = 1, 2, 3) bimetallic clusters, and the reaction mechanism of methanol dehydrogenation to CO on Pt7 and PtNi bimetallic clusters. The models chosen to catalyze the methanol are Pt7 (CTP, quintet) cluster and Pt5Ni2 (I) cluster (two Pt atoms in the bottom of Pt7 (CTP) are replaced by Ni atoms) which is the most stable structure among all the isomers of Pt(7−x)Nix (x = 1, 2, 3). The methanol dehydrogenation on Pt7 (CTP) cluster preferentially proceeds along the pathway of CH3OH → CH2OH → CH2O → CHO → CO, while on Pt5Ni2 (І) the pathway of CH3OH → CH3O → CH2O → CHO → CO is more favorable. In addition, the complete dehydrogenation product of methanol, CO, can more easily dissociate from Pt5Ni2 (I) than that on Pt7. Electronic configuration analysis shows that charge transfer from Ni to Pt and results in increase of the electron density in Pt 5d orbitals. Moreover, the density of states (DOS) at Fermi level of clusters reduces gradually as the increase of the doped Ni atoms and this improves the catalytic activity for methanol decomposition.

3D SnO2 nanoflowers on Ti foil have been synthesized by a simple hydrothermal method without using any surfactant or catalyst. The synthesized 3D SnO2 nanostructures are characterized by X-ray diffraction, field emission scanning electron microscopy and X-ray photoelectron spectroscopy. The FESEM results showed that the nanoflower consists of multiple nanopetals of width ∼5 nm and each SnO2 nanoflower is ∼3 μm in diameter. The time-dependent morphology evolution of SnO2 nanoflowers was studied in detail, and a possible growth mechanism was proposed. In addition, high dispersed Ni nanoparticles was loaded on SnO2 nanoflowers applied in supercapacitor. The electrochemical properties of the 3D Ni/SnO2 nanoflowers were investigated by a series of electrochemical tests.

SnO2@carbon-doping TiO2 nanotube arrays (SnO2@C-TiO2NTs) were synthesized by hydrothermal method and evaluated for lithium ion insertion and photoelectrochemical activity. The composite electrode prepared for 5 h as anode materials for lithium-ion batteries exhibited much improved electrochemical performance due to the aligned pore structure and the synergistic effect of the electrode. A capacity of 142 μA h cm−2 can be obtained after 50 discharge/charge cycles at a high current density of 200 μA cm−2. Moreover, UV–vis results of the sample showed stronger absorption intensity in the range of 200–800 nm compared with bare TiO2NTs. The composite electrode displayed the maximum photocurrent density of 1.80 mA cm−2. This is attributed to the heterojunction formed at the interface between SnO2 and TiO2NTs resulting the enhance charge separation efficiency. Eletrochemical impendence spectroscopy (EIS) also shows that SnO2@C-TiO2 NTs has a noticeably lower charge-transfer resistance.Highlights► SnO2 nanoparticels synthesized by hydrothermal method were loaded on carbon doping TiO2NTs. ► SnO2@C-TiO2NTs showed optimal electrochemical and photoelectrochemical properties due to the heterojunction formed at the interface and the synergistic effect of the electrode.

Highly ordered TiO2 nanotube arrays (TiO2NTs) evenly modified by Ni–Cu nanoparticles were successfully prepared by potential step method. Their morphologies, structures, and alloy composition were characterized by FESEM, XRD and EDS, respectively. The as-prepared Ni–Cu/TiO2NTs electrodes were employed for non-enzymatic glucose detection in alkaline electrolyte and showed better electro-catalytic activity compared with Ni/TiO2NTs and Cu/TiO2NTs electrodes. Factors that affected the electrocatalysis of the electrodes were examined and optimized. Consequently, a sensitive amperometric electrode of glucose was achieved under 0.6 V vs. Ag/AgCl with a high sensitivity (1590.9 μA mM−1 cm−2), low detection limit (5 μM) and wide linear range from 10 μM to 3.2 mM (R2 = 0.993). Furthermore, the oxidable species such as ascorbic acid and uric acid showed no significant interference in determination of glucose. The experiment results revealed a very good reproducibility and high stability for the proposed Ni–Cu/TiO2NTs electrodes.

A novel nonenzymatic glucose sensor was developed based on the Ni nanoparticles-loaded TiO2 nanotube arrays (Ni-NPs/TiO2NTs). The Ni-NPs/TiO2NTs nanocomposites were prepared first by anodization of Ti foil, followed by pulsed electrodeposition method (PED) and characterized by SEM, XRD and XPS respectively. The results showed that spherical Ni nanoparticles were well dispersed and embedded in the TiO2NTs. The electrochemical measurements presented that the activated Ni-NPs with a diameter of 40 nm displayed high electrocatalytic activity for the oxidation of glucose and a good sensitivity of 700.2 μA mM−1 cm−2 at applied potential 0.6 V, with detection limit of 2 μM (S/N = 3), and good linear range (from 0.004 to 4.8 mM). Such high sensitivity was attributed to the large surface area of the highly dispersed nanoparticles for electrocatalytic reaction and the fast electron transfer in the TiO2NTs electrode. The good analytical performance, low cost and simple preparation method make this novel electrode material promising for the development of effective glucose nonenzymatic glucose sensor.

Ni nanoparticles were loaded on TiO2 nanotube arrays (Ni/TiO2NTs) via pulsed electrodeposition method and employed as catalyst for methanol oxidation in alkaline media. The as-prepared Ni/TiO2NTs catalyst was characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical analyzer. The electrochemical studies showed that Ni/TiO2NTs under illumination exhibited higher catalytic activity and stronger poisoning-tolerance for methanol oxidation than that in the dark. The effect of illumination which improved catalytic activity of Ni/TiO2NTs were attributed to the p–n junction formed at the interface between Ni(OH)2 and TiO2NTs resulting in the photo-generated holes flow into Ni(OH)2, thus, the transmitted holes boosted the oxidation of Ni(OH)2 and produced the OH to oxidize methanol and intermediates that absorbed on the catalyst. The present study shows Ni/TiO2NTs can act as a promising candidate for the anode catalyst in photo assist direct methanol fuel cells (PDMFCs) application.Highlights► Ni nanoparticles loaded on TiO2NTs as catalyst for methanol electro-oxidation ► Catalytic activity was evaluated both in the dark and under illumination. ► Catalytic activity was improved under illumination duo to the p–n junction formed.

In this paper, highly ordered titania nanotube (TNT) arrays fabricated by anodization were annealed at different temperatures in CO to create different concentrations of surface defects. The samples were characterized by SEM, XRD and XPS. The results showed different concentrations of Ti3+ defects were doped in TNT arrays successfully. Furthermore, after co-immobilized with horseradish peroxidase (HRP) and thionine chloride (Th), TNT arrays was employed as a biosensor to detect hydrogen peroxide (H2O2) using an amperometric method. Cyclic voltammetry results and UV–Vis absorption spectra presented that with an increase of Ti3+ defects concentration, the electron transfer rate and enzyme adsorption amount of TNT arrays were improved largely, which could be ascribed to the creation of hydroxyl groups on TNT surface due to dissociative adsorption of water by Ti3+ defects. Annealing in CO at 500 °C appeared to be the most favorable condition to achieve desirable nanotube array structure and surface defects density (0.27%), thus the TNT arrays showed the largest adsorption amount of enzyme (9.16 μg/cm2), faster electron transfer rate (1.34 × 10−3 cm/s) and the best response sensitivity (88.5 μA/mM l−1).

In this paper, highly-ordered TiO2 nanotube (TNT) electrodes fabricated by anodization at 20 V in 0.1 M F−-based solution were annealed in O2, N2 and CO respectively. The surface properties of the TiO2 electrodes after annealing treatment by different gases were studied by means of photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electrochemical properties of the TNT electrodes were investigated by cyclic voltammetry, steady-state polarization and photocurrent response measurements. The results showed that Tin+ (n = 0–3) cations and oxygen vacancies existed in the TNT electrode after annealing in CO, leading to a very efficient electron transfer rate of 1.34 × 10− 3 cm/s. Steady-state polarization measurement and photocurrent response demonstrated that the electrode potential of oxygen evolution reaction (OER) reduced by 20% and the photocurrent response increased by 50% for CO-annealed TNT electrode compared with O2-annealed TNT electrode.

•Liquid-phase exfoliation of layered ammonium phosphates successfully.•Exfoliated 4D sample exhibits a very stable cycling performance over 10000 cycles.•The relation between exfoliation time and energy storage performance is obtained.Liquid-phase exfoliation is an effective method to fabricate thin nanosheet materials with layered structure. In this paper, we successfully synthesized NH4Co0.4Ni0.6PO4·H2O nanosheets with the thickness of about tens of nanometers through liquid-phase exfoliation under ultrasonication. Specifically, the formamide solution was chosen as the exfoliation solvent due to it is capable of entering the interlayer of bulk materials and rapidly swelling between layers. Electrochemistry tests revealed that the optimized exfoliation time was 4 days. The asymmetric battery type energy storage device AC//NH4Co0.4Ni0.6PO4·H2O nanosheets showed an energy density of 10.8 Wh kg−1 at the power density of 703.1 W kg−1. More importantly, the long-term cycle stability measurements demonstrated that NH4Co0.4Ni0.6PO4·H2O nanosheets after exfoliation had an ultra-long cycling life that still retained 102.3% of specific capacitance even 10000 continuous charge/discharge cycles.