High Current Stability Research Articles

Field emission performance of vertically aligned carbon nanotubes and their improved enhancement factors by the simple solution treatment

In this study, the vertically aligned carbon nanotubes (VACNTs) are fabricated on substrate via a simply spraying chemical vapor deposition method. The relationship between the field emission performance and the aspect ratio of the VACNTs is systematically studied. The optimal growth conditions of the VACNTs with outstanding field emission performance are obtained. Meanwhile, the field emission properties of the VACNTs are further improved via a simple acetone-densification treatment. After the densification, the turn-on electric field and threshold electric field could decrease to 0.83 V/μm and 2.45 V/μm, respectively, and the field enhancement factor will increase to 26,588. In addition, the field emission current stability of the VACNTs is also strengthened. The improvement of field emission performance could be resulted from the reduction of field shielding effect after tip densification. The high current density and good stability demonstrate that the densified VACNTs are the promising and highly efficient field electron emitters.

Modification of various carbons with various metal oxides and noble metal compositions as electrocatalysts for ethanol oxidation

ABSTRACT The modification of various carbons with various metal oxides and noble metal composites was performed to prepare electrocatalysts, and the catalysts were further investigated for ethanol electro-oxidation. The catalyst activity and stability were studied by cyclic voltammetry (CV) and chronoamperometry (CA), respectively. First, various metal oxides (e.g., TiO2, CeO2 and MnO2) were prepared on a graphene oxide (GO) support. Pt on TiO2 with a GO support (Pt/TiO2-GO) provided a higher current density and long-term stability than the other catalysts. Second, TiO2 was prepared on various carbons (e.g., graphite (G), graphite oxide (GTO), GO and polydopamine (PDA)-modified GO (PDA-GO)), and the corresponding electrocatalytic activities were determined. Pt/TiO2-PDA-GO showed the highest activity and stability for oxidation. Consequently, TiO2-PDA-GO was chosen as a support for further studies on the optimal composition of bimetallic catalysts (xPtyPd/TiO2-PDA-GO). 4Pt2Pd/TiO2-PDA-GO showed the highest performance for ethanol oxidation. The prepared M/TiO2-C catalysts were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). The results showed that the dispersed nanoparticles were loaded on the modified carbon supports. From electrochemical measurements, the prepared electrocatalysts showed improved electrocatalytic activities in terms of low onset potential and high current density and stability.

Aprotic and Protic Ionic Liquids as Electrolytes for Organic Radical Polymers

In this study we systematically investigated the impact of aprotic and protic ionic liquids on the electrochemical behavior of poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA) based electrodes. We showed that the use of aprotic ILs based on the FSI− anion enables the realization of electrodes able to display high capacity, good capacity retention at high current density (42% at 50 C), low self-discharge and high stability during float tests. We also reported, for the first time, that it is possible to successfully use PTMA-based electrodes in combination with PILs. We showed that the PTMA electrodes display good capacity but a limited cycling stability in these ionic liquids. This investigation confirms that ionic liquids are very interesting candidates for the realization of high performance PTMA-based electrodes. A deeper understanding of the interactions taking place between ionic liquids and organic active materials will be required to finely tune the properties of these promising electrolytes for an application in organic radical polymer-based energy storage systems.

Well-aligned periodic germanium nanoisland arrays with large areas and improved field emission performance induced by femtosecond laser

Si-based field emitters with high and stable current densities have gained increasing attention owing to their relevant compatibility with other silicon-based microelectronic and photonic devices. Currently, well-aligned Ge nanostructure arrays can be used in fabrication of Si-based field emitters with excellent performance. In this study, large-area well-aligned Ge nanoisland arrays are induced by femtosecond laser (120 fs, 800 nm, 1 kHz). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman analyses, and X-ray photoelectron spectroscopy (XPS) reveal that well-aligned Ge nanoisland arrays with good quality. Field emission (FE) measurements indicate Ge nanoisland arrays with high emission current densities and good stability. Overall, the excellent FE properties of the proposed Ge nanoisland arrays will make them potential for applications in high-performance field emitters integrated with Si nanodevices.

Tuning Pt-skinned PtAg nanotubes in nanoscales to efficiently modify electronic structure for boosting performance of methanol electrooxidation

The modification of electronic structure can significantly affect electrocatalytic activity. An architecture art of Pt-skinned PtAg bimetallic nanotubes is successfully synthesized, delivering much higher catalytic activity and better stability toward methanol electrooxidation than PtAg bimetallic nanoparticles and commercial Pt/C catalysts. Theoretical studies reveal that the Pt skin on PtAg bimetallic nanotubes prominently optimize the electronic structure of Pt to greatly enhance the dissociative adsorption of methanol while increasing CO poisoning resistance for fast electrode kinetics, high catalytic current density and stability. This work offers a low Pt loading but highly active anode catalyst for direct methanol fuel cells, demonstrating that rationally tuning the electronic structure by well-controlling surface morphology in nanoscales could open new opportunities to greatly improve the electrocatalytic properties.

Low-Current-Density Magnetic Tunnel Junctions for STT-RAM Application Using MgO$_{{x}}$ N$_{\text{1}-{x}}\,\,({x}=\text{0.57}$ ) Tunnel Barrier

High switching speed, endurance, and low-current-based perpendicular magnetic tunnel junction (p-MTJ) memory is attracting wide interest as a key promising candidate for next-generation spintronic memory technology. p-MTJ-based spin-transfer torque RAM (STT-RAM) has been extensively investigated, and despite the promise, there is concern about the high switching current density and low stability with regard to scaling. In this work, the current controllability of p-MTJ in iron (Fe)-enriched Co20Fe60B20 with a newly designed MgO x N1– x tunnel layer is systematically investigated, with the expectation that the introduction of N minimizes the oxidation of Fe to improve the performance of the device. A facile, plasma-based oxynitridation (MgO x = 0.57N1– x =0.43) of MgO through RF-sputter deposition serves as a reliable procedure to establish a tunnel barrier for an MTJ structure fabricated with ~300-nm diameter and pinned with synthetic antiferromagnetic (SAF) [Co/Pt] ${_{n}}$ multilayer stack. Current-controlled tunneling magnetoresistance (TMR) up to ~65% was observed at room temperature (RT) with ultralow switching current density ( ${J}_{c}$ ) of 136 ± 17 kA/cm2. TMR along with tunnel conductance ( ${g}$ ( ${V}$ )) was measured to be highly stable in the read-bias regime (−200 to +200 mV) for MgO x N1– x as compared to the reported MgO barrier. The analogous MgO x N1– x -based MTJ structures were modeled using the nonequilibrium Green’s function (NEGF) with appropriate tunnel barrier parameters and incorporating modulated barrier height as compared with the MgO barrier. The current–voltage characteristics of the modeled device showed close agreement with experimental data indicating high spin current. Based on the field-induced magnetization analysis, the macro-magnetic reversal analysis suggests the free-layer switching duration of ~3 ns. These observations show the strong candidature of MgO x N1– x ( ${x} = {0.57}$ ) MTJs for STT-RAM device application.

Graphene oxide selenium nanorod composite as a stable electrode material for energy storage devices

Selenium (Se), a member of the sulfur family, is considered as a potential alternative cathode material due to its high electronic conductivity and comparable volumetric capacity density to sulfur. Herein, we report on a simple one-step procedure to prepare trigonal selenium nanorods (t-Se) on graphene oxide (GO) without using any surfactant. The developed method does not utilize any harmful chemicals or harsh experimental conditions, and is, therefore, friendly from the environmental and economic point of views. We discovered that well-dispersed Se nanorods can be prepared at 20 mM Se under heating at 85 °C for 3 h. The successful growth of t-Se rods on GO sheets was confirmed by X-ray diffraction (crystalline structure), transmission electron microscopy (morphology), Raman spectroscopy (crystalline form and defects), and XPS technique (elemental composition). The Se nanorod-loaded GO composite (GO–Se) displayed a promising specific capacity of 405.5 mAh g−1 at a high current density (10 A g−1) and high cycling stability with a negligible capacity decay over 1000 cycles. Importantly, the GO–Se electrode delivered a high reversible capacity of 471.5 mAh g−1 at 0.5 A g−1. The improved electrical performance of the GO–Se composite could be attributed to the electrical conductivity and unique architecture of reduced graphene, which effectively enhances the utilization of active Se rods and significantly augment the electronic conductivity of the GO–Se electrode. The present study demonstrates that a hybrid system of 1D t-Se impregnated GO composite could be a promising cathode material for long-life batteries and other storage devices.

Enhanced rate performance and high current cycle stability of LiNi0.8Co0.1Mn0.1O2 by sodium doping

Sodium ions have been successfully doped into LiNi0.8Co0.1Mn0.1O2 (NCM811) by solid phase method. The influences of Na doping on the electrochemical properties of the electrode at high current intensities were studied. The experimental results show that 3% Na modified LiNi0.8Co0.1Mn0.1O2 has superior rate performance and cycling behavior due to the cation mixing is suppressed and the structural stability is enhanced by Na doping. The capacity retention of Li0.97Na0.03Ni0.8Co0.1Mn0.1O2 is 64.33% after 100th cycles at 10 C (1 C = 200 mA · g–1) in the potential range from 2.8 V to 4.3 V, which is greatly higher than the pristine NCM (only 35.01%). Li0.97Na0.03Ni0.8Co0.1Mn0.1O2 also shows excellent rate performance of 140.5 mAh · g–1 at 10 C, and the discharge capacity of 3% Na-NCM is higher than that of the pristine NCM when the current intensity is returned from 10 C to 0.5 C. The analysis of the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements confirm that the Na doping plays an important role in inhibiting the electrochemical polarization of LiNi0.8Co0.1Mn0.1O2 and improving the diffusion of lithium ions.

Evaluation of Electromagnetic Behavior of No-Insulation REBCO Pancake Coil With Multiple Defects

We previously developed a high-magnetic-field REBCO coil for use in medical devices such as medical accelerators and high-magnetic-field MRI. The use of high-temperature superconducting (HTS) coils in such applications necessitates their miniaturization while providing high magnetic fields. However, when the current density is improved by reducing the copper-stabilization-layer thickness in the conventional insulated HTS coil, the thermal stability is lowered, which leads to a tradeoff between high current density and high thermal stability. Therefore, we examine no-insulation (NI) coils that allow compatibility between high current density and thermal stability. Recent research has indicated that NI coils with multiple defects can afford a stable and high critical current (Ic) and operate without serious problems, which makes it possible to produce coils wound with commercial REBCO tapes containing several defects along the tape, which in turn makes fabrication economically viable. However, there are many unknown aspects of the current, magnetic field, and heat behaviors of NI coils with defects, particularly when large currents flow through them. Here, in order to examine the behavior of a multiple-defect NI coil subjected to a large current at the temperature of ~30 K via application of the conduction cooling method, we use a combination of the partial element equivalent circuit method and the finite-element method to investigate the detailed current and thermal behaviors. Our results indicate that defective NI coils can afford a critical current Ic value of ~80% of that of defect-free NI coils, which makes our findings valuable in terms of contributing to further developments in HTS coil applications.

Synergistic storage of lithium ions in defective anatase/rutile TiO2 for high-rate batteries

Fabrication of heterostructured materials is a strategy to boost the charge-transfer kinetics and the performance of high-rate lithium storage. Here, a facile, low-temperature method for the synthesis of high-area TiO2 nanospheres containing both anatase and rutile phases is described. The as-prepared materials contain a high concentration of oxygen vacancies facilitating electron conduction in the anatase phase and theoretical calculations provide evidence of a low energy barrier for Li+ transport in the rutile phase. The synergy between the two phases renders the shared conduction of electrons through anatase and Li+ ions via rutile at high-current rates, leading to the anodes that outperform the alternate TiO2 systems when the combination of capacity at high current densities and cycle stability are considered, displaying a capacity of 95.4 mAh g−1 at 10 A g−1 and a 97.2% retention of capacity over 500 cycles at 1 A g−1.

Ge-doped ZnO nanowire arrays as cold field emitters with excellent performance

Elemental doping is an efficient strategy to modify the electronic and optoelectronic properties of semiconductor materials. In this work, a high-quality Ge-doped tapered ZnO nanowire array was prepared via a simple chemical vapor deposition approach. The I–V and light emission behaviors were investigated based on a single nanowire with good electrical conductivity. The Kelvin probe force microscopy and ultraviolet photoelectron spectroscopy characterizations indicate that the doped nanowires possess a largely reduced work function relative to that of the pure ZnO nanostructures. These advantages allow Ge-doped ZnO nanowire arrays to deliver excellent field emission performance, including low turn-on and threshold fields, high emission current and long-term stability.

Robust electrochemistry of black TiO2 as stable and high-rate negative electrode for lithium-ion batteries

Electrochemically stable black TiO2 composed of Ti3+ ions and oxygen vacancies is successfully synthesized by a facile and economic sol–gel method followed by calcination in nitrogen atmosphere at 400 °C for 2 h. Several physicochemical techniques are probed to validate the desired state of the obtained material. The material is formed in a pure state with an average size of 10 nm. The electrochemical studies are conducted for its use as negative electrode for Li-ion batteries. At high current rate of 5 C, the electrodes deliver a high discharge capacity of 226 mA h g−1 even after 150 cycles. Similarly, the electrodes also deliver discharge capacities of 197 and 153 mA h g−1 at current rates of 7 C and 10 C, respectively. The robust electrochemical properties of black TiO2 including large specific capacities at high current rates and high stability are ascribed to the formation of defective structure, conductive Ti3+ ions and oxygen vacancies.

Stable high capacity cycling of Li metal via directed and confined Li growth with robust composite sponge

The practical applications of high capacity metallic Li electrodes is severely hindered by the uncontrollable Li nucleation and growth that lead to poor cycle life and growth of unsafe dendrites. Here we demonstrate the design of porous but robust sponges with Cu nanoparticles and graphene and their applications for enabling Li electrodes with stable cycling under high capacity and high current conditions. We discovered that the Cu nanoparticles decorated on graphene effectively direct the heterogeneous nucleation of Li whereas the graphene frameworks provide ample spaces for confining Li growth. These sponges are able to host at least 60 mAh cm−2 Li without forming dendrites and allow stable Li plating/stripping with low overpotentials for hundreds of cycles even at a high current of 7 mA cm−2. The sponges have remarkable mechanical strengths and exhibited minimal structural deformation after aggressive plating/stripping cycles. These superior properties enable Li metal batteries with substantially improved high current cyclic stability and rate performance, as demonstrated with both LiNi0.4Co0.2Mn0.4O2 and LiFePO4 cathodes.

CZTS Decorated on Graphene Oxide as an Efficient Electrocatalyst for High-Performance Hydrogen Evolution Reaction.

Cu2ZnSnS4 (CZTS) was synthesized by the sonochemical method using 2-methoxyethanol as the solvent and subsequently decorated onto graphene oxide (GO synthesized by the modified Hummers’ method) using two different approaches such as in situ growth and ex situ synthesis followed by deposition. Preliminary characterizations indicated that the synthesized CZTS belongs to the kesterite structure with a sphere-like morphology. The in situ-synthesized CZTS/GO (I-CZTS/GO) composite is used as an efficient electrocatalyst for hydrogen evolution reaction (HER) which revealed superior electrocatalytic activity with a reduced overpotential (39.3 mV at 2 mA cm–2), Tafel slope (70 mV dec–1), a larger exchange current density of 908 mA cm–2, and charge transfer resistance (5 Ω), significantly different from pure CZTS. Besides, the I-CZTS/GO composite exhibits highest HER performance with high current stability of which shows no noticeable degradation after i–t amperometry. The catalytic activity demonstrates that the I-CZTS/GO composite could be a promising electrocatalyst in hydrogen production from their cooperative interactions.

Electroforming and resistive reversible switching effect in polycrystalline fullerene C60 films

It has been found that the conductivity of a polycrystalline fullerene C60 thin film increases by several orders of magnitude after lengthy exposure to an electric field of ∼ 106 V/cm. This effect is called electroforming. After electroforming, the current-voltage characteristic shows high current stability and reproducibility of results. An increase in capacitance and a decrease in resistance by 4 orders is observed in a narrow voltage range of 0.1-1 V, which is the effect of reversible resistive switching. In fields with a strength of 105-106 V/cm, the I-V characteristic of molded films is determined by the current with a limited space charge. Trap concentrations are defined as Nt-1 ≈ 2.3· 1021 m−3 and Nt-2 ≈ 5.8·1020 m−3 and their depth Et ≈ 0.17 eV for polycrystalline C60 films with thicknesses of 250 and 500 nm, respectively.

Graphitic carbon nitride nanosheets coated with Ni2CoS4 nanoparticles as a high-rate electrode material for supercapacitor application

In this study, Ni2CoS4 nanoparticles were coated on the surface of graphitic carbon nitride (g-C3N4) nanosheets using a hydrothermal process. The composition and morphology of the nanocomposite thus obtained were characterized using different spectroscopic techniques. When subjected to cyclic voltammetry and constant current chronopotentiometry in alkaline media, the charge storage properties and electrochemical performance of the as-prepared Ni2CoS4@g-C3N4 nanocomposite were found to be superior not only to those of Ni2CoS4 nanoparticles and their monometallic counterparts (Co3S4@g-C3N4 and Ni3S4@g-C3N4) but also to those of g-C3N4 nanosheets, Ni1.5Co1.5S4@g-C3N4, and NiCo2S4@g-C3N4. Moreover, the Ni2CoS4@g-C3N4 nanocomposite exhibited a high charge storage capacity of 246 mAh g−1 or 2210 F g−1 at a discharge current of 1 A g−1, an excellent rate capability (over 86% retention at 40 A g−1), a high cycling stability (85% of charge storage capacity retention after 4000 charge-discharge cycles), and a high columbic efficiency of 99% at a discharge current of 40 A g−1. Based on the results obtained, such hybrid nanocomposites may be claimed to be good candidates for use as electrode materials in energy storage devices due to their high performance at high current densities, good stability, and high columbic efficiency.

Co3O4-nanoparticle-entrapped nitrogen and boron codoped mesoporous carbon as an efficient electrocatalyst for hydrogen evolution.

Co3O4-nanoparticle-entrapped nitrogen and boron codoped mesoporous carbon was synthesized via the molten salt method. Melamine formaldehyde resin (MF resin) was used as the nitrogen and carbon precursor, and boric acid was utilized as the boron precursor. Furthermore, cobalt chloride was used as the cobalt precursor and the template for the formation of mesopores, which could also be removed and partly recovered by acid washing. The characterization results revealed that the as-obtained samples possessed mesoporous structures, with high cobalt, boron, and nitrogen content values. For the sample of Co0.65B0.3NC800, the atomic content values of Co, N, and B are 2.3%, 8.87%, and 8.67%, respectively. Moreover, the carbonation temperature and the amount of salt template could both affect the mesoporous structures of the final samples and then affect the electrocatalytic activities for the hydrogen evolution reaction (HER). When the carbonation temperature was 800 °C, the sample of Co0.65B0.3NC800 showed superior performance for the HER under basic conditions, with high current density, low overpotential, and good stability.

RGO supported NiWO4 nanocomposites for hydrogen evolution reactions

Reduced graphene oxide supported nickel tungstate (rGO/NiWO4) nanocomposites (NCs) were synthesized by molten salts followed by hydrothermal treatment at 120 °C. XRD, Raman, FTIR and electron microscopic studies reveal the formation of rGO/NiWO4-NCs. The prepared rGO/NiWO4-NCs exhibit enhanced hydrogen evolution reactions (HER) than pure NiWO4 nanoparticles. The nanocomposites as the electro-catalysts deliver low onset potential (−1.35 V), low Tafel value (250 mV/dec), high current density (∼40 mA/cm2) and good stability for HER in alkaline medium compared to pure NiWO4.

Boosting CH3OH Production in Electrocatalytic CO2 Reduction over Partially Oxidized 5 nm Cobalt Nanoparticles Dispersed on Single-Layer Nitrogen-Doped Graphene.

Herein, we successfully synthesized partially oxidized 5 nm cobalt nanoparticles dispersed on a single-layer nitrogen-doped graphene (SL-NG) (denoted as PO-5 nm Co/SL-NG) catalyst by a unique and simple one-pot synthesis strategy, which was efficiently applied for highly selective electrocatalytic reduction of carbon dioxide to methanol in 0.1 mol dm-3 aqueous NaHCO3 medium under mild conditions, reaching the maximum faradaic efficiency (FE) of 71.4% for methanol at -0.90 V versus saturated calomel electrode (SCE), possessing a strong electrocatalytic current density of 4 mA cm-2 and a high yield of 1.10 mmol dm-3 h-1, and the corresponding overpotential is as low as 280 mV. Moreover, at -1.0 V versus SCE, a high current density of 10 mA cm-2 can be obtained, and the FE for methanol still remains 23.2%. Notably, the proposed catalyst exhibits prominent stability after 10 h electroreduction of CO2, and the morphology, particle size, structure, and element contents of the catalyst almost remain stable. This work first provides an advanced PO-5 nm Co/SL-NG for selective electroreduction of carbon dioxide into methanol, which simultaneously possesses the merits of high current density, low overpotential, high selectivity, superior FE, and good stability, outperforming most reported electrocatalysts.

Development of high-precision DC power supply prototype for HEPS-TF

This power supply prototype is developed for the quadrupole magnets in the storage ring of HEPS. The project requires a high current stability of these power supplies, in order to master the key technology of this type of power supply in advance, a prototype is made in the project of HEPS test facility. Modularization is used in the prototype’s topology and a combination of digital and analog control is adopted in the closed-loop control. Three control loops are used to improve the output current’s quality. In order to improve the current stability, the key devices in the circuit are under thermostatical controlled. The acceptance test results for the NDRC are given in details, which show that all the parameters are better than the design indexes. After a lot of experiments and tests, all the parameters of this prototype passed the acceptance test. Among the results, the stability of this prototype achieved 5.2 ppm, the repeatability is 3.2 ppm, and the low-frequency voltage ripples are lower than the requirements of Physicists. The success of this prototype means that the requirements of power supplies in physical design can be met.