Intensified electrochemical hydrogen storage capacity of multi-walled carbon nanotubes supported with Ni nanoparticles

The electrochemical hydrogen storage properties of Ni-supported multi-walled carbon nanotube (Ni/MWCNT) electrodes were investigated using charge/discharge (C&D) and cyclic voltammetry (CV) techniques. Nickel NPs were deposited on the MWCNT surface, which was first chemically oxidized by H₂SO₄ and HNO₃ (3:1, v/v). Hydrogen storage was carried out by using the Ni/MWCNT electrode as the working electrode in the electrochemical cell. A set of various current densities were applied to the cell to produce (C&D) cycles, and it became optimum corresponding to 1.5 mA current. According to the electrochemical test results, the highest electrochemical discharge capacity of 1625 mAh g^?1 was obtained for the electrode with ratio of 4:1 (MWCNTs to Ni) in the initial cycle, which corresponded to 6.07 wt% H₂. The storage capacity was increased and reached to 4909 mAh g^?1 (18.34 wt% H₂) after 20 cycles, and the electrode maintained the specific capacity as cycling continued. Thus, the Ni/MWCNT electrode displays an excellent cycle stability and a high capacity reversibility. CV measurements also showed that the electrochemical adsorption and desorption amount of hydrogen was increased by Ni loading onto the CNTs and indicated that the electrochemical hydrogen adsorption of the electrode has an activated period.     

Electrodeposition of platinum–nickel alloy nanocomposites on polyaniline-multiwalled carbon nanotubes for carbon monoxide redox

An electrochemical method was developed to deposit platinum (Pt)–nickel (Ni) alloy nanocomposites on polyaniline-multiwalled carbon nanotubes (Pt–Ni/PAN/MWCNTs). The material was characterized by various methods including field emission scanning electron microscope, X-ray diffraction, X-ray photoelectron spectroscopy, and electrochemical techniques. An appreciably improved catalysis toward oxidation of carbon monoxide (CO) was observed at the Pt–Ni/PAN/MWCNTs nanocomposites (real ratio of Pt–Ni of 17:1), which was interpreted by a mechanism based on the bifunctional catalysis. The successful preparation of Pt–Ni/PAN/MWCNTs nanocomposites opens a new path to synthesize the promising catalysts for CO.     

碳纳米管上原位沉积钯-钴-镍纳米颗粒及其对乙醇氧化的电活性

以水合肼为还原剂,在水和乙醇的混合溶液中制备多壁碳纳米管(MWCNT)负载的纳米镍(Ni/MWCNT)和纳米镍钴(Ni-Co/MWCNT)颗粒,然后将它们分别与氯化钯溶液反应,形成的钯纳米颗粒原位沉积在MWCNT表面,从而得到MWCNT负载的Pd-Ni/MWCNT和Pd-Ni-Co/MWCNT催化剂。SEM和TEM图像显示,MWCNT上的催化剂颗粒是由5~10 nm的小颗粒团聚而成的30~100 nm的大颗粒,三金属催化剂的粒径比双金属的粒径小,在MWCNT上的分散度更高。ICP和EDS分析显示,Pd直接还原并包覆在纳米镍和纳米镍钴表面;采用循环伏安和计时电流技术,研究了催化剂在碱性溶液中对乙醇氧化的电催化活性,结果表明,Pd-Ni-Co/MWCNT催化剂对乙醇氧化具有强的电催化活性,乙醇氧化对应的峰电流密度达101.8 mA·cm^-2,并且催化剂催化活性稳定。     

碳纳米管上原位沉积钯-钴-镍纳米颗粒及其对乙醇氧化的电活性

以水合肼为还原剂,在水和乙醇的混合溶液中制备多壁碳纳米管(MWCNT)负载的纳米镍(Ni/MWCNT)和纳米镍钴(Ni-Co/MWCNT)颗粒,然后将它们分别与氯化钯溶液反应,形成的钯纳米颗粒原位沉积在MWCNT表面,从而得到MWCNT负载的PdNi/MWCNT和Pd-Ni-Co/MWCNT催化剂。SEM和TEM图像显示,MWCNT上的催化剂颗粒是由5~10 nm的小颗粒团聚而成的30~100 nm的大颗粒,三金属催化剂的粒径比双金属的粒径小,在MWCNT上的分散度更高。ICP和EDS分析显示,Pd直接还原并包覆在纳米镍和纳米镍钴表面;采用循环伏安和计时电流技术,研究了催化剂在碱性溶液中对乙醇氧化的电催化活性,结果表明,Pd-Ni-Co/MWCNT催化剂对乙醇氧化具有强的电催化活性,乙醇氧化对应的峰电流密度达101.8 m A·cm~(-2),并且催化剂催化活性稳定。     

Preparation and electrochemical properties of nanofiber poly(2,5-dihydroxyaniline)/activated carbon composite electrode for supercapacitor

In this study, poly(2,5-dihydroxyaniline) (PDHA) was successfully prepared by electrochemical method on the surface of active carbon (AC) electrodes. The physical and electrochemistry properties of PDHA/AC composite electrode compared with pure AC electrode were investigated by scanning electronic microscope (SEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy, cycle life test. From SEM, PDHA presents nanofiber network morphology. The diameter of the nanofiber PDHA is about 200–300 nm. PDHA/AC composite electrode shows redox peaks in CV curve and voltage plateaus in galvanostatic charge–discharge curve, and all these indicate that PDHA/AC composite electrode has more advantages. The maintenance of the capacitance compared to initial cycle capacitance of composite electrode is about 90% during the charge–discharge cycles. In conclusion, The PDHA/AC composite electrode shows much higher specific capacitance (958 F g⁻¹), better power characteristics, longer cycle life. Therefore, PDHA/AC composite electrodes were more promising for application in capacitor. This can be attributed to the introduction of nanofiber PDHA. The effect and role of PDHA in the composite electrodes were also discussed in detail.     

Enhancement of the electrochemical reduction of oxygen at platinum by nickel underpotential deposition

Polycrystalline Pt electrode was modified by underpotential deposition (upd) of nickel. The modification was performed by potential cycling in phosphate buffer pH 7. 0 containing NiSO₄, in which hydrogen and nickel upd processes were well separated. The maximum Ni upd coverage was found to be 0.3. Oxygen reduction was studied at bare and nickel upd-modified Pt. It was found that the reaction rate increased with increasing Ni upd coverage. At θ(Ni)=0.3, the current density was a factor of 2 higher compared to bare Pt (at the potential of 0.85 V). The capacitance of the electrode interface was determined in potential-relaxation experiments following interruption of the polarization current. It was found that the pseudocapacitance owing to a coverage by the adsorbed reaction intermediates was higher on the Ni-modified Pt surface than on bare Pt, which resulted in higher reaction rate. The influence of Ni adatoms on the surface coverage by the reaction intermediates was attributed to the inhibition of OH adsorption on Pt by OH ligands attached on neighboring Ni atoms.     

多壁碳纳米管负载Pd-Ni电催化剂对乙二醇的电催化氧化

采用微波加热还原法制备了不同化学计量比的Pd-Ni复合多壁碳纳米管(Pd-Ni/MWCNT)催化剂. 通过X射线衍射(XRD)、透射电镜(TEM)和能量散射谱(EDS)等微结构和组成表征表明, 所合成的催化剂中Pd-Ni合金具有较小的纳米颗粒以及较好的分散程度. 循环伏安(CV)、线性扫描(LSV)、计时电流方法(CA)和交流阻抗(EIS)等测试表明, Pd-Ni(3:1)/MWCNT催化剂对乙二醇的电化学氧化具有较高的催化活性.     

电化学阴极沉积制备氧化镍/碳纳米管复合电极的准电容特性

A novel type of composite electrode based on multiwalled carbon nanotubes coated with nano nickel oxide particles has been used in supercapacitors. Nickel oxide cathodically deposited from Ni(NO3)2 solution with carbon nanotubes as the matrix exhibited large pseudocapacitance of 25F/g in 6 mol/L KOH. The morphology of composites was examined by scanning electron microscope (SEM). To characterize the CNTs/nickel oxide composite electrode, a charge discharge cycling test for measuring specific capacitance, cyclic voltammetry, and ac impedance test is executed. The nickel oxide composite exhibiting excellent pseudocapacitive behavior(i.e.high reversibility, high specific capacitance, and low self discharge rate) has been demonstrated to be a potential candidate for the application of electrochemical supercapacitors.     

Evaluating the electrochemical capacitance of surface-charged nanoparticle oxide coatings

While transition metal oxides have been thoroughly investigated as coatings for electrochemical capacitors due to their pseudocapacitance, little work has been done investigating other oxide coatings. There exists a whole class of nanoporous oxides typically synthesized by sol-gel chemistry techniques that have very high differential capacitance. This high differential capacitance has been attributed to the surface potential of these materials and the close approach of counterions near the surface of these oxides. This study focuses on investigating the electrochemical capacitance of non-transition metal oxide nanoparticle coatings when deposited on supporting electrodes. Here, we show that, by adding coatings of SiO(2), AlOOH, TiO(2), and ZrO(2) nanoparticles to graphite support electrodes, we can increase the electrochemical capacitance. We also show that the measured electrochemical capacitance of these oxide-coated electrodes directly relates to the electrophoretic mobility of these materials with the lowest values in capacitance occurring at or near the respective isoelectric pH (pH(IEP)) of each oxide.     

Charge storage performance of doped carbons prepared from polyaniline for supercapacitors

Doped carbons have been prepared from polyaniline for supercapacitors. The morphology of samples has been characterized by scanning electron microscope, the surface chemical composition of samples has been investigated by X-ray photoelectron spectroscopy, and the surface area of samples has been calculated by Brunauer–Emmett–Teller measurement. Electrochemical properties have been studied by cyclic voltammograms, galvanostatic charge/discharge, and electrochemical impedance spectroscopy measurements in 6 mol L⁻¹ potassium hydroxide. Their charge storage performance has been evaluated, and the effect of nitrogen atomic functionalities on the pseudocapacitive property has been studied. The experimental results have proved two mechanisms of energy storage in doped carbons: double-layer formation and pseudocapacitance. The overall specific capacitance of doped unactivated carbon is mainly attributed to pseudocapacitance, that of doped activated carbon prepared by physical activation is attributed to the synergic effect of pseudocapacitance and double-layer capacitance, but that of doped activated carbon prepared by chemical activation is mainly attributed to double-layer capacitance.     

三元复合材料PANI／Ag／MWNT的电化学行为和比电容

通过原位聚合的方式在银纳米粒子/多壁碳纳米管（Ag/MWCNT）复合材料的表面成功聚合苯胺单体制备了聚苯胺/银纳米粒子/多壁碳纳米管（PANI/Ag/MWCNT）三元复合材料苯．通过对三元复合材料的结构以及表面形貌进行分析,表明聚苯胺层完全包覆了Ag/MWCNT复合材料,形成了核壳式结构．同时银纳米粒子则以单质晶体的形态存在于多壁碳纳米管与聚苯胺层之间．三元复合材料电极在1 mol/L的KOH溶液中具有极低的阻抗,而与聚苯胺电极相比,这些复合材料电极则表现出更低的电阻、更高的电化学活性和更好的循环稳定性．尤其是当苯胺和Ag：MWCNTs质量比为5：5时,该复合材料电极在0.25 A/g的电流密度下表现出最大的比电容值为160 F/g．     

Pt–Ni alloy nanoparticles supported on multiwalled carbon nanotubes for methanol oxidation in alkaline media

The Pt–Ni alloy nanoparticles with different Pt/Ni atomic ratios supported on functionalized multiwalled carbon nanotubes surface were synthesized via an impregnation-reduction method. The nanocatalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrochemical techniques. XRD demonstrated that Pt was alloyed with Ni. TEM showed that the Pt–Ni alloy nanoparticles were uniformly dispersed on the multiwalled carbon nanotubes (MWCNTs) surface, indicating appropriate amount of Ni in Pt–Ni alloy which facilitates the dispersion of nanoparticles on the MWCNT surface. XPS revealed that the Pt 4f peak in Pt–Ni/MWCNT (4:1) catalyst shifted to a lower binding energy compared with Pt/MWCNT catalyst, and nickel oxides/hydroxides such as NiO, Ni(OH)₂, and NiOOH were on the surface of Pt–Ni nanoparticles. Electrochemical data based on cyclic voltammetry and chronoamperometric curves indicated that Pt–Ni (4:1) alloy nanoparticles exhibited distinctly higher activity and better stability than those of Pt/MWCNTs toward methanol oxidation in alkaline media.     

Sulfide-enhanced electrochemical capacitance of cobalt hydroxide on nanofibered parent substrate

Two approaches—substrate nanostructuring and incorporation of sulfide—were studied with the aim to increase electrochemical capacitance of cobalt (hydro)oxide. A fiber structure of cobalt was deposited electrochemically with the fibers in the order of tens of nanometers in thickness and hundreds of nanometers in length. Cobalt hydroxide film was formed on the nanostructured substrate by anodic polarization in an alkaline solution. The hydroxide formation and its electrochemical capacitance have been studied by cyclic voltammetry in conjunction with the electrochemical quartz crystal microbalance (EQCM). An irreversible behavior was typical of the first anodic polarization cycle; it turned gradually to a reversible one during subsequent cycling. EQCM measurements indicated exponential electrode mass growth during the first cycle, with subsequent transition to a quasipassive state. The redox transitions Co(II) → Co(III) → Co(IV), which determine pseudocapacitance, did not cause remarkable electrode mass change. The electrochemical capacitance of the nanofiber sample was found up to five times higher when compared to that formed on conventional cobalt (abraded surface). Specifics of “per 1 g” evaluation of capacitance performance is discussed. Measurements showed that about 10% of the entire hydroxide structure took part in the capacitive process. The capacitance value determined per 1 g of active Co(OH)₂ was in agreement with the limiting value predicted by the Faraday’s law (2,421 F g⁻¹) sulfide-enhanced system with 18% CoS exhibited up to three times higher capacitance when compared to that of the sulfide-free counterpart. The system shows promise for practical applications due to its low cost and technical simplicity.     

Development of a voltammetric procedure for assay of thebaine at a multi-walled carbon nanotubes electrode: quantification and electrochemical studies

The study of electrochemical behavior and determination of thebaine (THEB), an opiate alkaloid, is described on a multi-walled carbon nanotube (MWCNT) modified glassy carbon electrode by adsorptive stripping voltammetry and electrochemical impedance spectroscopy. The results indicated that MWCNT electrodes remarkably enhance electrocatalytic activity toward the oxidation of THEB in a wide pH range of 2.0–10.0, and it shows two irreversible and diffusion-controlled anodic peaks. Then, a sensitive, simple, and time-saving cyclic voltammetric procedure was developed for the analysis of THEB in human urine samples. Under optimized conditions, the oxidation peak has two linear dynamic ranges of 1.0–80.0 and 100.0–600.0 μM, with detection limit of 0.23 μM and a precision of <4% (relative standard deviation for eight analysis).     

Preparation of hexagonal nanoporous nickel hydroxide film and its application for electrochemical capacitor

Nanoporous nickel hydroxide film has been successfully electrodeposited on titanium substrate from nickel nitrate dissolved in the aqueous domains of the hexagonal lyotropic liquid crystalline phase of Brij 56. Low-angle X-ray diffraction (XRD), transmission electron microscopy (TEM), and atomic force microscopy (AFM) studies show that the film has a regular nanostructure consisting of a hexagonal array of cylindrical pores with a repeat center-to-center spacing of about 7 nm. Preliminary electrochemical studies are carried out using cyclic voltammetry (CV) and chronopotentiometry technology. A maximum specific capacitance of 578 F g⁻¹ could be achieved for the nanoporous Ni(OH)₂ film electrode, suggesting its potential application in electrochemical capacitors.     

Electrochemical performance of multi-walled carbon nanotube composite electrodes is enhanced with larger diameters and reduced specific surface area

Multi-walled carbon nanotubes (MWCNT) have been widely used to fabricate composite electrodes due to their electrochemical properties. MWCNTs can be fabricated by various approaches and a range of MWCNT types and sizes has been developed. This study focuses on understanding the influence MWCNT diameter and specific surface area has on the electrochemical properties of a composite electrode. MWCNTs with fixed length range and diameters ranging from 10–20 to 50–80 nm were examined in this study. The amount of MWCNT utilised to fabricate the electrodes was identical and above the percolation threshold. MWCNT electrodes fabricated with larger diameters showed enhanced thermodynamic and kinetic properties towards common redox species which covered surface-insensitive, surface-sensitive and adsorption-based processes. Overall, these findings indicate that the number of strands of MWCNT alone is not essential for enhanced conductivity in composite materials but other geometric parameters play important roles.     

Multi-wall carbon nanotube counter electrodes for dye-sensitized solar cells prepared by electrophoretic deposition

In this study, electrophoretic deposition (EPD) was employed to fabricate multi-wall carbon nanotube (MWCNT) counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). Firstly, raw MWCNTs were functionalized by means of an acid mixture solution and then subjected to EPD. The results obtained from Raman spectroscopy, Fourier transform infrared spectroscopy, field-emission scanning electron microscope, and cyclic voltammogram demonstrated that the defects and open ends on the MWCNTs can be obtained via chemical functionalization and thus facilitate the enhancement in the electrocatalytic activity for I₃⁻ reduction of MWCNT CEs. In addition to optimizing chemical functionalization of MWCNTs surface, the optimal thickness of MWCNT CEs prepared by EPD was also investigated. Additionally, consecutive cyclic voltammetric tests demonstrated that the MWCNT CE fabricated by EPD possessed excellent electrochemical stability. In comparison with MWCNT CEs fabricated by tape-casting approach, MWCNT CEs prepared by EPD presented a superior adhesion between MWCNT deposits and conducting glass substrates. Therefore, MWCNT CEs fabricated by EPD can be of great potential for use in low-cost plastic DSSCs.     

Direct electron transfer of cytochrome C and its electrocatalytic properties on multiwalled carbon nanotubes/ciprofloxacin films

In this study, stable and homogenous thin films of multiwalled carbon nanotubes (MWCNTs) were obtained on conducting surface using ciprofloxacin (CF, fluoroquinolone antibiotic) as an effective-dispersing agent. Further, MWCNTs/CF film modified electrodes (glassy carbon and indium tin oxide-coated glass electrode) are used successfully to study the direct electrochemistry of proteins. Here, cytochrome C (Cyt-C) was used as a model protein for investigation. A MWCNTs/CF film modified electrode was used as a biocompatible material for immobilization of Cyt-C from a neutral buffer solution (pH 7.2) using cyclic voltammetry (CV). Interestingly, Cyt-C retained its native state on the MWCNTs/CF film. The Cyt-C adsorbed MWCNTs/CF film was characterized by scanning electron microscopy (SEM), UV–visible spectrophotometry (UV-vis) and CV. SEM images showed the evidence for the adsorption of Cyt-C on the MWCNTs/CF film, and UV–vis spectrum confirmed that Cyt-C was in its native state on MWCNTs/CF film. Using CV, it was found that the electrochemical signal of Cyt-C was highly stable in the neutral buffer solution and its redox peak potential was pH dependent. The formal potential (−0.27 V) and electron transfer rate constant (13 ± 1 s⁻¹) were calculated for Cyt-C on MWCNTs/CF film modified electrode. A potential application of the Cyt-C/MWCNTs/CF electrode as a biosensor to monitor H₂O₂ has been investigated. The steady-state current response increases linearly with H₂O₂ concentration from 2 × 10⁻⁶ to 7.8 × 10⁻⁵ M. The detection limit for determination of H₂O₂ has been found to be 1.0 × 10⁻⁶ M (S/N = 3). Thus, Cyt-C/MWCNTs/CF film modified electrode can be used as a biosensing material for sensor applications.     

Nanocrystalline nickel cobalt hydroxides/ultrastable Y zeolite composite for electrochemical capacitors

A novel nanocomposite of Co(OH)₂−Ni(OH)₂ and ultrastable Y molecular sieves was synthesized by an improved chemical precipitation method for electrochemical capacitors. The Co(OH)₂−Ni(OH)₂/ultrastable Y zeolite (USY) composite and its microstructure were characterized by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. Electrochemical characterization was performed by cyclic voltammetry and galvanostatic charge–discharge measurements. The results show that Co(OH)₂−Ni(OH)₂/USY microstructure applied for the electrochemical energy storage has displayed superior capacitive performance. The effect of heat treatment conditions on specific capacitance properties was also systemically explored. Upon annealing at 250 °C, the maximum specific capacitance was up to 479 F/g (or 1,710 F/g after correcting for the weight percent of Co(OH)₂−Ni(OH)₂ phase). Annealing temperatures higher than 250 °C may cause the hydroxide to form oxide phase and decrease the surface activity of the oxide, thereby leading to a decline of the specific capacitance.     

The effect of ‘crystallinity’ and structural disorder on the electrochemical performance of substituted nickel hydroxide electrodes

β_bc-Nickel hydroxide exhibit non-uniform broadening reflections in their PXRD pattern due to the presence of structural disorder. β_bc-Nickel hydroxide electrodes with smaller crystallite size and structural disorder reversibly exchanges 0.9e/Ni. Co/Zn/Ca/Cd-substituted β_bc-nickel hydroxide samples also display non-uniform broadening of reflections in their powder X-ray diffraction patterns with smaller crystallite size and exchanges 0.7–0.8e/Ni. Hydrothermal treatment of β_bc-nickel hydroxide slurry at 170 °C results in an ordering of reflections in their powder X-ray diffraction pattern with an increased crystallite size. Crystalline β-nickel hydroxide electrode reversibly exchanges 0.3–0.4e/Ni. Hydrothermal-treated Co/Zn/Ca/Cd-substituted β_bc-nickel hydroxide slurries at 170 °C display sharp reflections with similar crystallite size and electrochemical activities as that of crystalline β-nickel hydroxide. This clearly demonstrates that partial substitution of Co/Zn/Ca/Cd in the nickel hydroxide matrix does not show any dramatic improvement in their electrochemical activity at 25–30 °C. Structural disordered material with smaller crystallite size delivers electrochemical activity close to theoretical capacity.