Newly-modeled graphene-based ternary nanocomposite for the magnetophotocatalytic reduction of CO2 with electrochemical performance

The development of CO₂ into hydrocarbon fuels has emerged as a green method that could help mitigate global warning. The novel structured photocatalyst is a promising material for use in a photocatalytic and magneto-electrochemical method that fosters the reduction of CO₂ by suppressing the recombination of electron−hole pairs and effectively transferring the electrons to the surface for the chemical reaction of CO₂ reduction. In our study, we have developed a novel-structured AgCuZnS₂–graphene–TiO₂ to analyze its catalytic activity toward the selective evolution of CO₂. The selectivity of each nanocomposite substantially enhanced the activity of the AgCuZnS₂–graphene–TiO₂ ternary nanocomposite due to the successful interaction, and the selectivity of the final product was improved to a value 3 times higher than that of the pure AgCuZnS₂ and 2 times higher than those of AgCuZnS₂–graphene and AgCuZnS₂–TiO₂ under ultra-violet (UV)-light (λ = 254 nm) irradiation in the photocatalytic process. The electrochemical CO₂ reduction test was also conducted to analyze the efficacy of the AgCuZnS₂–graphene–TiO₂ when used as a working electrode in laboratory electrochemical cells. The electrochemical process was conducted under different experimental conditions, such as various scan rates (mV·s^–1), under UV-light and with a 0.07 T magnetic-core. The evolution of CO₂ substantially improved under UV-light (λ = 254 nm) and with 0.07 T magnetic-core treatment; these improvements were attributed to the facts that the UV-light activated the electron-transfer pathway and the magnetic core controlled the pathway of electron-transmission/prevention to protect it from chaotic electron movement. Among all tested nanocomposites, AgCuZnS₂–graphene–TiO₂ absorbed the CO₂ most strongly and showed the best ability to transfer the electron to reduce the CO₂ to methanol. We believe that our newly-modeled ternary nanocomposite opens up new opportunities for the evolution of CO₂ to methanol through an electrochemical and photocatalytic process.     

R&D considerations for the performance and application of electrochemical capacitors

Research and development (R&D) of electrochemical capacitors is discussed in terms of material characteristics and device performance and testing. Various chemistries and technologies being developed are identified and the status of and problems associated with each of the technologies are discussed. The technologies considered include those using various types of carbon and pseudo-capacitive materials such as metal oxides. What is needed to make the various electrochemical capacitor technologies cost competitive with batteries for different applications and markets is also considered.Electrochemical capacitors (especially double-layer capacitors) are intrinsically high power devices of limited energy storage capability and long cycle life; batteries are basically energy storage devices, which can be designed and used as relatively high power devices with a sacrifice in useable energy storage capacity. Both electrochemical capacitors and high power batteries are designed with thin electrodes, materials having nano-scale characteristics, and a minimum resistance. Much of the research on electrochemical capacitors is concerned with increasing their energy density with the minimum sacrifice in power capability and cycle life for deep discharges. Of special interest has been the development of advanced carbons with specific capacitance (F/g) significantly greater than the present values of 150-200 F/g in aqueous electrolytes and 80-120 F/g in organic electrolytes. Cost continues to be a major obstacle to the development of large markets for electrochemical capacitors particularly for vehicle applications. The development of lower cost carbons appropriate for use in electrochemical capacitors is underway by several speciality carbon suppliers. The goal is to reduce the cost of the carbon to $10-15/kg.     

Influence of cell construction on the electrochemical reduction of nitrate

The electrochemical reduction of nitrates in the weakly alkaline electrolyte, simulating spent solution after regeneration of a strongly basic anion exchanger was studied. Copper was used as the cathode material. The influence of the electrolyser construction, cathode form and process parameters on the current efficiency of the nitrate reduction was followed. Four types of cell construction were used: a plate-electrode cell, a cell with a fluidised bed of inert particles in the inter-electrode space, a packed bed cathode cell and a vertically moving particle bed cell. The highest current efficiency with respect to the nitrate reduction was observed for the vertically moving particle bed reactor. On the other hand, unexpectedly high efficiency was also observed for the simple plate-electrode cell, the disadvantage being very low current density (below 40 A m⁻²) resulting in suitable behaviour. From the point of view of cell efficiency coupled with simplicity of construction and operation, the optimal cell seems to be the cell with the fluidised bed of inert particles in the inter-electrode space.     

Sulfur-incorporated,porous graphene films for high performance flexible electrochemical capacitors

Graphene and its derivatives are considered potential electrode materials for flexible electrochemical capacitors (f-ECs), but their capacitive performances have to be improved for practical applications. Herein, we demonstrate fabrication of flexible sulfur (S)-incorporated reduced graphene oxide (SRGO) electrodes obtained by pyrolyzing free-standing film consisting of benzyl disulfide-functionalized graphene oxides at 900 °C. The effect of S incorporation on morphology and chemical structure of SRGO were investigated by various microscopic and spectroscopic methods. Incorporation of S and the crumpled and porous morphology of SRGO electrodes improve capacitive performance of f-ECs; SRGO f-ECs show a specific capacitance of 140.8 F/g at 1 A/g, rate capability of 91.5% retention, and cyclic performance of 93.4% after 1000 charge/discharge cycles at 4 A/g. Impressively, SRGO f-ECs exhibit excellent electrochemical and mechanical durability after 1000 charge/discharge cycles at a bending angle of 120° with values that greatly exceed those of conventional RGO-based f-ECs. This study provides a fundamental foundation of the correlation between S composition of carbon nanomaterials and their electrochemical (or surface) properties.     

Energy and power performance of electrochemical double-layer capacitors based on molybdenum carbide derived carbon

Cyclic voltammetry, constant current charge/discharge, and electrochemical impedance spectroscopy have been applied to establish the electrochemical characteristics for electric double-layer capacitor (EDLC) consisting of the 1 M (C₂H₅)₃CH₃NBF₄ electrolyte in acetonitrile and micro/mesoporous carbon electrodes prepared from Mo₂C, noted as C(Mo₂C). The N₂ sorption (total BET specific surface area (S_BET ≤ 1855 m² g⁻¹), micropore area (S_micro ≤ 1823 m² g⁻¹), total pore volume (V_tot ≤ 1.399 m³ g⁻¹) and pore size distribution (average NLDFT pore width d_NLDFT ≥ 0.89 nm) values obtained have been correlated with the electrochemical characteristics for EDLCs (region of ideal polarizability (ΔV = 3.0 V), characteristic time constant (τ_R = 1.05 s), gravimetric capacitance (C_m ≤ 143 F g⁻¹)) dependent strongly on the C(Mo₂C) synthesis temperature. High gravimetric energy (35 Wh kg⁻¹) and gravimetric power (237 kW kg⁻¹) values, normalised to the total active mass of both C(Mo₂C) electrodes, synthesised at T_synt = 800 °C, have been demonstrated at cell voltage 3.0 V and T = 20 °C.     

Pore characteristics and electrochemical performance of ordered mesoporous carbons for electric double-layer capacitors

Three types of ordered mesoporous carbon materials with different pore characteristics have been synthesized via different routes. Whatever the synthesis route was, triblock copolymer was employed as both a carbon precursor and a structure-directing agent. The relationship between the capacitances of carbon electrodes and their pore characteristics was elucidated in detail. The material C-P exhibits the lowest resistance and highest specific capacitance value of exceeding 170 F/g among these carbon materials, which can be due to not only high surface area but also its appropriate pore size distribution. In addition, the noteworthy is that the maintenance of specific capacitance with increasing current load for each sample is better than that for general activated carbons, where larger mesopores and high mesopore fraction play important roles in the rate capability.     

Development of new nanocomposite based on nanosized-manganese oxide and carbon nanotubes for high performance electrochemical capacitors

We report the synthesis of a new composite electrode based on nanosized-manganese oxide and carbon nanotubes (CNTs) by electrophoretic deposition of CNTs on a stainless steel (SS) substrate followed by direct spontaneous reduction of MnO₄⁻ ions to MnO₂ to form the multi-scaled SS-CNT-MnO₂ electrode. The resulting material was characterized by scanning electron microscopy, energy dispersive X-ray analysis, cyclic voltammetry and galvanostatic charge-discharge in a 0.65 M K₂SO₄ aqueous solution. The binderless SS-CNT-MnO₂ nanocomposite electrode shows a very high specific capacitance of 869 F/g of CNT-MnO₂ and good stability during long galvanostatic charge-discharge cycling. To the best of our knowledge, this is one of the highest capacitance for manganese oxide electrode ever reported. In addition to its applicability in electrochemical capacitors, this methodology could be extended to develop other high performance nanocomposite material electrodes based on carbon nanotubes and metal oxide for the future generation of electrochemical power sources.     

On the electrochemical activation of alkali-treated soft carbon for advanced electrochemical capacitors

The electrochemical activation process of the so-called “alkali-treated soft carbon” (ASC) has been examined in organic electrolyte solutions. SEM observation demonstrated that the edge plane of graphene structure of the ASC particle becomes rough after the activation, and XRD measurements indicated that the average lattice constant of graphene stacking in ASC increases after the activation process. Ex-situ ⁷Li NMR measurements proved that the insertion of cation (Li⁺) into the pore structure of ASC is associated with the activation process in the electrolyte dissolving Li salt. The pore-size distribution determined from N₂-gas adsorption for ASC electrodes before and after the electrochemical activation indicates that the pore structure becomes developed after the electrochemical polarization, especially in the pore-diameter range of 2–10 nm. A schematic model of the activation process has been presented, which includes electrochemical insertion of ions into the inside of the ASC.     

Effects of charge redistribution on self-discharge of electrochemical capacitors

The effect of charge redistribution on the self-discharge profile of porous carbon (Spectracarb 2225) electrodes is examined. A model pore based on the de Levie transmission line circuit is used to show that self-discharge due purely to charge redistribution results in the same self-discharge profile as that expected for an activation-controlled self-discharge mechanism (the potential falls linearly with log t), thus the linear log time profile is not characteristic of an activation-controlled mechanism. The addition of a hold step reduces the amount of charge redistribution in porous carbon electrodes, although the hold time required to minimize the charge redistribution is much longer than expected, with electrodes which have undergone a 50 h hold time still evidencing charge redistribution effects. The time required for the charge redistribution through the porous electrode is also much greater than predicted, likely requiring tens of hours. This highlights the importance of the charge redistribution in self-discharge of systems using porous electrodes, such as electrochemical capacitors.     

Influence of the textual properties of activated carbon nanofibers on the performance of electric double-layer capacitors

To investigate the relationship between textural properties and electrochemical properties, activated carbon nanofibers were manufactured using an electrospinning process followed by chemical activation using KOH or NaOH. The specific surface area of the KOH-activated carbon nanofibers was higher than that of NaOH-activated carbon nanofibers; however, the total pore volume and mesopore volume of the NaOH-activated carbon nanofibers were greater than those of the KOH-activated carbon nanofibers when the same number of moles of KOH and NaOH were used. The specific capacitances increased as the specific surface area and pore volume of the activated carbon nanofibers were increased. However, the specific capacitance obtained at a high scan rate (50 mV/s) and the retained capacitance of the activated carbon nanofibers increased with increasing total pore and mesopore volume, especially for mesopores with diameters of 2–4 nm.     

Effect of graphene nanosheets on electrochemical performance of Li4Ti5O12 in lithium-ion capacitors

In order to improve the electrochemical performance of lithium titanium oxide, Li₄Ti₅O₁₂ (LTO), for the use in the lithium-ion capacitors (LICs) application, LTO/graphene composites were synthesized through a solid state reaction. The composite exhibited an interwoven structure with LTO particles dispersed into graphene nanosheets network rather than an agglomerated state pristine LTO particles. It was found that there is an optimum percentage of graphene additives for the formation of pure LTO phase during the solid state synthesis of LTO/graphene composite. The effect of graphene nanosheets addition on electrochemical performance of LTO was investigated by a systemic characterization of galvanostatic cycling in lithium and lithium-ion cell configuration. The optimized composite exhibited a decreased polarization upon cycling and delivered a specific capacity of 173 mA h g⁻¹ at 0.1 C and a well maintained capacity of 65 mA h g⁻¹ even at 20 C. The energy density of 14 Wh kg⁻¹ at a power density of 2700 W kg⁻¹ was exhibited by a LIC full cell with a balanced mass ratio of anode to cathode along with a superior capacitance retention of 97% after 3000 cycles at a current density of 0.4 A g⁻¹. This boost in reversible capacity, rate capability and cycling performance was attributed to a synergistic effect of graphene nanosheets, which provided a short lithium ion diffusion path as well as facile electron conduction channels.     

Effect of synthesis routes on the performance of hydrated Mn(IV) oxide-exfoliated graphite composites for electrochemical capacitors

MnO₂ · nH₂O-EG composites for electrochemical capacitors were prepared using commercially available low cost exfoliated graphite (EG) as a conductive substrate, and (a) potassium permanganate and (b) manganese(II) acetate solutions by two different routes. Method (1) was addition of EG to (a), followed by 1 h stirring and then slow addition of (b), and in Method (2) the solutions (a) and (b) were swapped. Using Method (1) submicron or smaller sized MnO₂ · nH₂O particles having mesopores were formed, whereas Method (2) produced lumps of aggregated particles of several tens microns without mesopores, though specific surface areas were not very different and both were similar by XRD. Although EG alone showed only about 2 F g⁻¹, the capacitance per net amount of MnO₂ in 1 mol L⁻¹ Na₂SO₄ solution increased proportionally with EG content and was always larger by Method (1) than Method (2), that is, the utilization ratio of MnO₂ increased with EG content and the effect was more prominent in the case of Method (1). The results indicated that EG is a good conductive material for MnO₂ · nH₂O electrochemical capacitors if appropriate synthesis processes are used. The performance of the composites strongly depends on synthesis method, even using the same raw materials. It was suggested that the morphology of the products was a primary factor leading to different performance rather than composition.     

Operating characteristics of aqueous magnetite electrochemical capacitors

Operating characteristics, including capacitance, leakage current, operating potential range, cycling stability and open-circuit self-discharge behaviours, of the magnetite (Fe₃O₄) supercapacitor, containing 10 wt % carbon black as conductive additive, in aqueous electrolytes of Na₂SO₃, KOH and Na₂SO₄ were investigated. Although the capacitance of the oxide was found to depend heavily on electrolyte composition, the self-discharge mechanism in these electrolytes appeared to be the same. Reduction in the dissolved oxygen content (DOC) of the electrolyte reduced the leakage current and profoundly improved the cycling stability. In particular, Na₂SO_3(aq) gives the highest capacitance, nearly 30 F (g-Fe₃O₄)^–1 or 80 F cm^–2 of actual surface area, with an operation range of 1.1 V based on a leakage current less than 0.1 mA F^–1, and the electrode showed no deterioration after 10⁴ cycles under a DOC < 0.1 ppm.     

掺水量对聚吡咯高分子膜电化学性能的影响

在0.1 mol/L吡咯+0.2 mol/L Na Cl O4的乙腈溶液中加入不同含量[0,3%,6%,9%(体积分数)]的水,采用循环伏安法制备了聚吡咯(PPy)高分子膜。利用扫描电子显微镜(SEM)对其表面形貌进行了观察,利用恒电流充放电曲线研究了其电化学性能。比较了不同电流密度下不锈钢/聚吡咯(SS/PPy)比电容和能量密度,当电流密度由1 m A/cm2增大到5 m A/cm2时,PPy(6%)电极的比电容下降幅度最小,下降了21.6%。     

Influence of thin film properties on the electrochemical performance of diamond electrodes

The electrochemical properties of nanocrystalline diamond films grown by microwave-enhanced chemical vapour deposition from a helium–hydrogen–methane gas phase mixture on Ti substrates are explored. A range of important redox systems are examined in aqueous solution including the oxidation of hydroquinone and ascorbic acid, and the electrodeposition and stripping of Au and Cu. Compared to boron-doped diamond materials, the nanodiamond is found to be a highly active electrode material, with low overpotentials and high adherence of metallic electrodeposits, for the redox systems studied.     

掺水量对聚吡咯高分子膜电化学性能的影响

在0.1 mol/L吡咯+0.2 mol/L Na Cl O4的乙腈溶液中加入不同含量[0,3%,6%,9%(体积分数)]的水,采用循环伏安法制备了聚吡咯(PPy)高分子膜。利用扫描电子显微镜(SEM)对其表面形貌进行了观察,利用恒电流充放电曲线研究了其电化学性能。比较了不同电流密度下不锈钢/聚吡咯(SS/PPy)比电容和能量密度,当电流密度由1 m A/cm2增大到5 m A/cm2时,PPy(6%)电极的比电容下降幅度最小,下降了21.6%。     

Influence of oxidation level on capacitance of electrochemical capacitors fabricated with carbon nanotube/carbon paper composites

Gaseous oxidation of carbon papers (CPs) decorated with carbon nanotubes (CNTs) with varying degrees of oxidation was conducted to investigate the influence of surface oxides on the performance of electrochemical capacitors fabricated with oxidized CNT/CP composites. The oxidation period was found to significantly enhance the O/C atomic ratio on the composites, and the increase in oxygen content upon oxidation is mainly contributed by the formation of CO and C-O groups. The electrochemical behavior of the capacitors was tested in 1 M H₂SO₄ within a potential of 0 and 1 V vs. Ag/AgCl. Both superhydrophilicity and specific capacitance of the oxidized CNT/CP composites were found to increase upon oxidation treatment. A linearity increase of capacitance with O/C ratio can be attributed to the increase of the population of surface oxides on CNTs, which imparts excess sites for redox reaction (pseudocapacitance) and for the formation of double-layer (double-layer capacitance). The technique of ac impedance combined with equivalent circuit clearly showed that oxidized CNT/CP capacitor imparts not only enhanced capacitance but also a low equivalent series resistance.