Fabrication of novel hybrid composite La2‐xCoxCuO4 electrode for high performance supercapacitors

Hybrid composites La_2‐xCo_xCuO₄ (x = 0, 0.1, 0.2, and 0.3) are prepared using one‐step simple hydrothermal route as electrodes for supercapacitors. The effect of varying cobalt content on morphological, structural, and electrochemical properties has been explored using X‐ray diffraction, scanning electron microscopy, and cyclic voltammetry, respectively. The structural parameters obtained by X‐ray diffraction showed tetragonal phase of hybrid composite without any evident impurity phases. The analysis of morphological properties suggested a strong correlation with electrochemical properties, for instance, a relationship between fabric porous structures and electrochemically active sites for redox reactions and intercalation/de‐intercalation processes. The hybrid composite electrodes demonstrated high specific capacitance of the order of 1304 F/g at 10 mV/s scan rate and exhibited decreasing trend on increasing scan rate. Hybrid composites were also tested for their ability as an electrode of high performance supercapacitors in different aqueous electrolytes, i. e, KOH, H₂SO₄, and Na₂SO₄ to optimize the best compatible electrolyte. The composite electrode material showed excellent cyclic stability and 98% capacitance retention for 1 A/g after 2000 cycles. The remarkable performance of hybrid composite electrode entails its potential for commercial applications of supercapacitors.     

Graphene‐based materials for flexible electrochemical energy storage

Sustainable development of renewable energy sources is one of the most important themes that humanity faces in this century. Wide use of renewable energy sources will require a drastically increased ability to store electrical energy. Electrochemical energy storage devices are expected to play a key role. With the increased demand in flexible energy resource for wearable electronic devices, great efforts have been devoted to developing high‐quality flexible electrodes for advanced energy storage and conversion systems. Because of its high specific surface area, good chemical stability, high mechanical flexibility, and outstanding electrical properties, graphene, a special allotrope of carbon with two‐dimensional mono‐layered network of sp² hybridized carbon, have been showing great potential in next‐generation energy conversion and storage devices. This review presents the latest advances on the flexible graphene‐based materials for the most vigorous electrochemical energy storage devices, that is, supercapacitors and lithium‐ion batteries. Copyright © 2014 John Wiley & Sons, Ltd.     

Evaluation of fluctuating voltage topology with fuel cells and supercapacitors for automotive applications

To develop a single‐stage power conversion topology in which energy storage devices can be directly coupled, a fluctuating voltage topology is applied, leading to lower cost and more compactness with the absence of DC/DC converters. This paper investigates such a topology for automotive applications where fuel cells are directly connected to the DC bus of the inverter, resulting in fluctuating voltage across the DC bus. Further, a supercapacitor pack is also introduced to maintain the power capacity and voltage stability. The hybridization principle and practical application of such a topology are then discussed in the time domain and frequency domain. Furthermore, the transient power requirement is decomposed to design the size of fuel cells and supercapacitors. Simulation results from the modeling of the fuel cell‐supercapacitor powertrain demonstrate the feasibility and effectiveness of this topology. The supercapacitors can serve as a low‐pass filter for the fuel cells. In conclusion, the peak power requirement can be successfully achieved because of the lowered system impedance, and the fuel cells only need to supply the average power.     

Enhanced hetero‐elements doping content in biomass waste‐derived carbon for high performance supercapacitor

Betel nut wastes are firstly modified with nitric acid/thiourea to fabricate hetero‐element doping carbon (C‐H‐T) for energy storage. C‐H‐T exhibits improved content of O (12.27%), N (2.52%), and S (2.88%) compared with that of purely carbonized carbon with O (9.2%) and N (1.76%). Without nitric acid heat treatment, the carbon materials prepared by hydrothermal treatment with thiourea only get increasing hetero‐elements content of O (10.46%), N (2.9%), and S (0.53%). The similar results have been obtained using urea and melamine as dopants. Due to the synergistic effects of the hetero‐elements containing functional groups, C‐H‐T get a significant enhancement in its electrochemical properties with a high capacitance (423 F g^?1 at 0.5 A g^?1) in KOH electrolyte. C‐H‐T based coin‐type symmetric supercapacitors display maximum energy density of 61.7 Wh kg^?1 and considerate cycling ability with 94% capacitance retention after 10 000 cycles. The fabricated two‐step method can inspire the increase of hetero‐elements content in carbon materials to develop its application in energy storage.     

A review of performance optimization of MOF‐derived metal oxide as electrode materials for supercapacitors

Metal‐organic frameworks (MOFs), as new class of porous materials, are constructed by inorganic metal centers and bridging organic links. Recently, MOFs have been proved to be effective templates for preparing metal oxides with large surface areas and controlled shape by directly annealing in air. There are lots of reports about metal‐organic framework‐derived metal oxides as electrode materials for supercapacitors. Metal‐organic framework‐derived metal oxides can offer higher capacitances compared with that prepared by other synthetic methods, likely attribute to high surface areas and optimal pore sizes. However, at present, the specific capacitances of MOF‐derived metal oxides received are far lower than theoretical values, and the cycle numbers could not meet practical demands. Accordingly, much effort has been made to improve the performance by further modifying MOFs. Thus, this paper focused on the advances in performance optimization of MOF‐derived metal oxide as electrode materials for supercapacitors as follows:

1. Dual metal MOF‐derived binary metal oxides. Metal oxides with 2 metal cations possess better electrical conductivity and richer redox active sites than single metal oxides.
2. Metal‐organic framework‐derived carbon/metal oxide composites (MO@C) or graphene/MOF‐derived graphene/metal oxide composites. Doping carbon not only facilitate transportation of electrodes but also contribution to extra double‐layer capacitance.
3. Hybrid MOF‐derived metal oxide composites (MO@MO). Metal oxide composites can produce some synergistic effects that the individuals cannot provide.
4. Metal‐organic framework‐derived metal oxides with a hollow structure. The Hollow structure could shorten ion diffusion distance and adapt to volume expansion generated during the ion intercalated/extracted process.

     

Flexible sodium‐ion supercapacitor based on polypyrrole/carbon electrode by use of harmless aqueous electrolyte for wearable devices

With the emergence of various wearable devices, supercapacitors have gained immense attention because of their fast response rates. However, most supercapacitors use hazardous electrolyte materials, such as H₂SO₄, KOH, and acetonitrile. Leakage of these types of electrolytes during use would be very harmful to human skin. Therefore, a supercapacitor that does not employ hazardous materials is an attractive option for use in the energy‐storage components of wearable devices. Herein, we successfully demonstrate a Na‐ion supercapacitor (NISC) with a polypyrrole/carbon‐coated heat‐treated carbon felt electrode and an aqueous 0.4 M NaCl electrolyte, which is not harmful. Furthermore, our NISC with polypyrrole/carbon‐coated heat‐treated carbon felt exhibits a high specific capacitance (31.09 F g^?1) and a fast response rate (chargeable at 0.5‐s intervals). The proposed NISC with no harmful materials in the electrolyte has an excellent response rate. It will establish useful guidelines for the energy‐storage components in wearable devices Copyright © 2017 John Wiley & Sons, Ltd.     

Effect of matrix content on the performance of carbon paper as an electrode for PEMFC

Porous conducting carbon fiber‐based composite paper is used as an electrode backing in the fuel cell assembly. It not only acts as a channel through which the reactant and product gases pass to and from the bipolar plate and the catalyst site but also helps in the flow of electrons. In order to perform its role efficiently, it should have sufficient strength, high electrical conductivity, and ideal porous structure. Carbon paper has been fabricated, which builds up the required composite properties. Studies have been conducted to optimize the fiber/matrix ratio in the carbon paper, while ensuring the perfect combination of porosity, mechanical strength, and electrical conductivity for an electrode in a proton electrolyte membrane fuel cells. Detail physico‐mechanical and electrochemical characterizations further ascertain that the fiber/matrix ratio plays an important role in tuning the composite properties. The polarization curve of the unit proton exchange membrane (PEM) fuel cell (with an effective electrode area 4 cm²) shows a peak power density of 916 mW/cm² for the sample with fiber/matrix ratio of 65:35, which is almost the same as the commercially available sigracet gas diffusion layer (SGL) carbon paper tested under similar conditions. Further, proportionally enlarging the electrode area to 100 cm² shows that the carbon paper not only shows almost repeatable results in a given set up but also scales up.     

High performance hybrid supercapacitors with LiNi1/3Mn1/3Co1/3O2/activated carbon cathode and activated carbon anode

Hybrid supercapacitors have been studied as a next generation energy storage device that combines the advantages of supercapacitors and batteries. One important challenge of hybrid supercapacitors is to improve energy density (8.9–42 Wh/kg) with maintaining excellent power density (800–7989 W/kg) and cyclability (98.9% after 9000 cycles). Herein, we demonstrate an approach to design hybrid supercapacitors based on LiNi_1/3Mn_1/3Co_1/3O₂ (NMC)/activated carbon (AC) cathode and AC anode (NMC/AC//AC). The NMC/AC//AC hybrid supercapacitors shows outstanding electrochemical performances due to the enhanced energy and power densities. These findings suggest that the NMC/AC cathode is an effective method for high performance hybrid supercapacitors.     

Gel combustion synthesized NiMoO4 anchored polymer nanocomposites as a flexible electrode material for solid state asymmetric supercapacitors

Recent research has focused on the search for new electrode materials to improve the specific capacitance of supercapacitors. Conductive polymers and metal oxides have been extensively tested as electrode materials for supercapacitors. Incorporating both conductive polymers and metal oxides into a composite provides excellent results for the electrochemical performance of supercapacitors. In this present work, we have fabricated the nanoscale α-NiMoO₄ particles that enwrapped on electronically conducting polymer nanocomposites (PNCs) based on Polyvinyl alcohol (PVA)/Poly(vinyl) pyrrolidone (PVP) for supercapacitor applications. The different concentrations of PVA/PVP with α-NiMoO₄ loaded polymer nanocomposites were developed by using a solution casting method. All the polymer nanocomposites have been subjected to Scanning Electron Microscopy (SEM), Fourier Transforms Infrared (FTIR), X-ray diffraction (XRD), and electrochemical studies. The prepared PNCs surface morphology has been acquired as a non-uniform rod-like structure. The electrochemical performances of the prepared PNCs have been investigated and the resultant value of the maximum specific capacitance is 15.56 F g⁻¹ for 1 wt % of α-NiMoO₄ nanoparticles(NPs) loaded polymer blended electrode at a scan rate of 5 mVs⁻¹. The prepared PNCs exhibit 97.12% of columbic efficiency studied by using two electrode systems at room temperature in an aqueous electrolyte solution of 3 M KOH. From these investigation, it has been revealed that the PVA/PVP/α-NiMoO₄ composites could be portable and flexible electrodes for energy storage applications.     

Metal-organic frameworks as highly efficient electrodes for long cycling stability supercapacitors

Among a large variety of energy storage technologies, supercapacitors possess special advantages such as rapid charge/discharge, high power density, safety, and environmental friendliness to meet the requirement of specific applications. The common electrode materials of supercapacitors, including porous carbon, conductive polymers, and metal oxides/hydroxides, have their own benefits and drawbacks in energy density and stability. Owing to the big surface area and controllable porosity, the metal-organic frameworks (MOFs) have been explored as important candidates for supercapacitor applications. This mini-review focuses on the recent advances of MOF-based materials including pristine MOFs, MOFs composite materials, and MOF-derived materials in the development of long cycling life supercapacitors. The devices discussed here mean those with capacitive retention rates of more than 90% after 10,000 cycles and high energy density. In addition, we also describe the fundamental knowledge of supercapacitors, highlight the stabilization mechanism of MOFs, and propose the strategies to enhance the stability of MOF-based supercapacitor electrodes.     

Synthesis of biomass porous carbon materials from bean sprouts for hydrogen evolution reaction electrocatalysis and supercapacitor electrode

Biomass carbon porous materials have attracted more attention for their green, renewable and simple preparation process. They have a large specific surface area and abundant pore structure. Their structural characteristics make them expose more active sites and allow the electrolyte ions to transfer quickly, which indicates they have great application prospects in hydrogen evolution reaction (HER) and supercapacitors. In the present work, bean sprout (BS) was used as the carbon source because of its nitrogen self-doped characteristic and advantages of low-cost, simple, mature production process. The high temperature carbonization method without physical and/or chemical activation was used to synthesize the carbon materials. And different pyrolysis temperature (500–900 °C) was investigated in this work. The as-prepared BS-800 were used as the bifunctional electrode materials. The measurement results showed that BS-800 exhibited the high activity for HER and high specific capacitance for supercapacitors.     

Carbon nanotubes for flexible energy storage devices--A review

碳纳米管具有本征sp²共价结构所带来的优异导电,导热及力学性能,并在锂电池,超级电容器等储能体系中具有广泛的应用前景.本文回顾了碳纳米管柔性储能的背景,介绍了碳纳米管优异的性能,独特的结构.这种一维的管状结构既可以作为柔性电极中的支撑骨架,也可以构建优越的长程导电网络,提供能源存储活性位点,进而提高柔性器件的性能.本文详细评述近年来碳纳米管在柔性超级电容器,锂离子电池,锂硫电池等能源存储元件应用中的一些最新进展和研究热点,其中柔性超级电容器,锂离子电池方面研究比较成熟,而锂硫电池尚在起步阶段,预期会取得快速进步.文章介绍了电极构建途径及性能评估方法,展示了碳纳米管基柔性储能器件的进展,并展望了其未来发展方向.     

The methanol-air fuel cell: A selective review of methanol oxidation mechanisms at platinum electrodes in acid electrolytes

Over the past few years there has been a resurgence of interest in methanol oxidation in acid electrolytes, where platinum group metals are the only practical catalysts. The recent literature concerning the adsorption and oxidation processes occurring at platinum in acid solutions is reviewed. The overall model based on contemporary data assumes that methanol adsorption follows Langmuir kinetics at low surface coverages and Elovich kinetics at higher values. The ‘poisoning’ intermediate (probably COH) is susceptible to an ageing process, rendering it less active, and its ultimate removal is achieved via a chemical reaction with an electrosorbed water molecule.Bimetallic catalysts which adsorb both water and methanol at low potentials are much more active than platinum alone. Modern, highly-dispersed catalysts on carbon supports have been reported with high specific activities. However, for commercial viability (particularly with regard to automotive applications) considerable improvements are still required.     

C,N co-doped Co3O4 hollow spheres prepared via hydrothermal reaction for improved electrochemical activity

Superior electrode materials play a key role on the electrochemical performance for the lithium-ion batteries and supercapacitors. The Co₃O₄-based materials are promising electrode materials due to their high specific capacity and energy density. However, the poor cycle performance limits their applications during the process of the commercialization for the lithium-ion batteries and supercapacitors. Because of the poor cycle stability, C, N co-doped Co₃O₄ hollow spheres are successfully prepared and used as electrode materials for the lithium-ion batteries and supercapacitors. Via the C, N co-doping process, the electronic conductivity is greatly improved. Moreover, the hollow structure could ensure the structural stability during the electrochemical process. As a result, the cycle performance and specific capacity are greatly improved when the C, N co-doped Co₃O₄ composites are used as electrode materials for the lithium-ion batteries and supercapacitors.     

Catalytic control of automotive NOx: a review

This article summarizes several technical studies reported in the literature on catalytic conversion technology to control pollution due to automotive exhausts with specific focus on NO_x reduction. While simple theoretical reactions are stated, the review concisely presents the various techniques available with their specifications and performances. Noble‐metal converters, in spite of their proven‐technology advantage, are considered expensive while zeolite‐based catalysts are preferred today as increasingly more research findings have made this technology more mature. Conclusion and recommendations on specific applications have been presented as well. Copyright © 1999 John Wiley & Sons, Ltd.     

Composite of manganese dioxide impregnated in porous hollow carbon spheres for flexible asymmetric solid‐state supercapacitors

Compared with symmetric supercapacitors, asymmetric supercapacitors are been widely applied in energy storage devices because of delivering an impressible energy density. Herein, a simple temple strategy was used to fabricate the porous hollow carbon spheres (PHCS) with high specific surface area of 793 m² g^?1, large pore volume of 1.0 cm³ g^?1 and pore size distribution from micropores to mesopores, serving as the capacitive electrodes of asymmetric supercapacitors. Subsequently, manganese dioxide (MnO₂) was impregnated into the PHCS to form a faradic electrode with a promising performance, owing to a synergistic effect between high capacity MnO₂ and conductive PHCS. Furthermore, the flexible asymmetric solid‐state devices were constructed with PHCS anode, PHCS@MnO₂ cathode, and PVA/LiCl electrolyte, extending a voltage window up to 1.8 V. The extensive voltage window would lead to an increased energy density. In our case, the flexible asymmetric sandwich exhibit excellent electrochemical performance in terms of a high energy density capacity of 26.5 W·h kg^?1 (900 W kg^?1) and superior cycling performance (10 000 cycles). Therefore, the developed strategy provides a strategy to achieve the PHCS‐based composites for the application in the asymmetric solid‐state supercapacitors, which will enable a widely field of flexible energy storage devices.     

Recent progress in the application of carbon materials to supercapacitors and lead-carbon batteries

新型炭材料是电化学储能领域中非常重要的一类储能材料,目前广泛应用于各种电化学储能器件.本文综述了具有电容特性的高比表面积炭材料在超级电容器与铅炭电池中的应用.采用不同的方法合成具有高比表面积的新型炭材料作为超级电容器电极材料,能够得到较高的比容量.适量高比表面积的炭材料应用于铅酸电池负极,形成铅炭电池,极大地提高了电池的储能特性.论文最后探讨了新型炭材料在超电容以及铅炭电池中应用的发展方向.     

Construction of MnO2 nanosheets@graphenated carbon nanotube networks core-shell heterostructure on 316L stainless steel as binder-free supercapacitor electrodes

Carbon nanotubes are regarded as typical and promising electrode materials in supercapacitors. However, small specific capacitance of carbon nanotubes restricts the practical application in high energy density devices. Herein, MnO₂ nanosheets@graphenated carbon nanotube networks are synthesized directly on 316L stainless steel as binder-free electrodes for high-performance supercapacitors. Graphenated carbon nanotube networks are grown in-situ on stainless steel by chemical vapor deposition method followed by annealing treatment. Subsequently, MnO₂ nanosheets are uniformly deposited on graphenated carbon nanotube networks to construct core-shell heterostructure based on the facile hydrothermal reaction using KMnO₄ as the precursor. Core carbon nanotube networks can offer a stable structural backbone and shell MnO₂ nanosheets can shorten diffusion paths of ions. The MnO₂ nanosheets@graphenated carbon nanotube networks exhibit a high specific capacitance of 575.4 F g⁻¹ (areal capacitance of 274 mF cm⁻²) at the current density of 0.5 mA cm⁻² and good cycling stability (93% of capacity retention after 6000 cycles), due to the synergistic effects between pseudocapacitive MnO₂ nanosheets and conductive carbon nanotube networks. The developed synthetic strategy offers design guidelines for the construction of advanced binder-free electrodes for high-performance supercapacitors.     

Geometrical effects on ionic diffusion in carbon-carbon symmetric supercapacitors

There is a great need to understand the structural dependence of supercapacitors for the development of advanced electrode materials that can be used to improve the electrochemical performance and structural integrity. In this work, we investigate the effects of the size and pore width/size of activated carbon microspheres (ACMSs) with diameter in a range of 2.7 ± 0.54 to 9.45 ± 1.81 μm and average pore width in a range of 1.60 to 2.0 nm on the electrochemical impedance characteristics of carbon-carbon symmetrical supercapacitors. Both the size and pore width/size of the ACMSs have negligible effects on contact resistance. The activation energy for the migration/diffusion of electrolyte ions in the supercapacitors made from the ACMSs of nearly same porous structure increases nonlinearly with the increase of the average size of the ACMSs, which is likely associated with overscreening effect. The activation energy for the migration/diffusion of electrolyte ions in the supercapacitors made from the ACMSs of nearly same size increases first and then decreases as the pore width/size increases.     

Synthesis of engineered graphene nanocomposites coated with NiCo metal-organic frameworks as electrodes for high-quality supercapacitor

Metal-organic frameworks (MOF) are a novel generation of promising electrodes for supercapacitors. The synthesis of glucose modified, high-quality graphene (HQG) as a platform for the nucleation and growth of bimetallic MOF nanosheets (NiCo-MOF) is reported. The –COOH and –OH groups formed on the surface, thus enhance the interaction between the HQG and NiCo-MOF. This heterostructure is used as an electrode in the preparation of asymmetric supercapacitors with a capacity of 244 F/g and an energy density of 76.3 W h/g. The nanocomposite also exhibits a high specific capacitance of 4077.3 F/g at a current density of 2.5 A/g. Therefore, new approaches have been introduced for the fabrication of efficient supercapacitors, and a step forward has been taken for the development of other electrical applications, including batteries, fuel cells, catalysts, and types of sensors.