Electrodes For Electrochemical Capacitors Research Articles

Valorization of agricultural wood wastes as electrodes for electrochemical capacitors by chemical activation with H3PO4 and KOH

Two series of activated carbons were prepared by chemical activation of three different kinds of agricultural waste: cherimoya (Annona cherimola), fig (Ficus carica) and olive (Olea europaea) woods. KOH and H3PO4 were selected as activating agents. The main differences between these trees are their growth rate and the hardness of the wood. Due to these differences, distinct pore structure, surface areas and chemical composition were found for the different activated carbons prepared. Phosphoric acid-activated samples present a high ash content due to the phosphate formation during the activation process. KOH-activated samples are mainly microporous materials with high surface area and pore volumes. On the contrary, when H3PO4 was used as activating agent, activated carbons are meso-microporous materials. Ash content and micropore structure are limiting factors for the application of activated carbons as electrodes for supercapacitors. The best performance corresponds to KOH-activated samples with high capacitance and energy and power densities. The stability of the electrodes, evaluated by charge–discharge cycling, shows that the KOH-activated carbons have a high cyclic stability up to 12,000 cycles. For H3PO4 samples, the capacitance decreases due to their high ash content and the poor stability of the acidic surface groups.

Free-standing interconnected carbon nanofiber electrodes: new structural designs for supercapacitor application

This work aims to develop and characterize a new design of free-standing interconnected carbon nanofiber electrodes for supercapacitor application. The fibers are obtained via carbonization of three components of electrospun nanofiber mats based on a polyacrylonitrile polymer (as a carbon backbone precursor), polyvinylalcohol (as a sacrificial copolymer), and 0–1.0 wt% multi-walled carbon nanotubes. Carbonizing these ternary composites results in fibers with about twice as large in surface area and one order of magnitude higher in electrical conductivity than those obtained by the carbonization of neat polyacrylonitrile and/or binary polyacrylonitrile-0–1.0 wt% carbon nanotube mats. The carbonized polyacrylonitrile-polyvinylalcohol-0.3 wt% carbon nanotube mat reveals the highest surface area and electrical conductivity and best capacitive performance. It exhibits energy and power densities of 27.8 Wh kg−1 and 110.59 kW kg−1, respectively, and cyclic stability of 95% after 2000 charge–discharge cycles at a charging current of 1.0 Ag−1. The nanotubes’ alignment along the fiber’s axis, the formation of fiber–fiber interconnected morphology with more mesopore pollution, and changes in the graphitization degree and defect features of fiber crystallites are the reasons for the observed increase in the electrical conductivity, surface area, and capacitive performance of the carbon fibers. Therefore, the new design represents a potential free-standing carbon nanofiber electrode for future electrochemical double layer capacitor (EDLC) device fabrication.

Preparation of Porous Carbon Nanofibers with Tailored Porosity for Electrochemical Capacitor Electrodes.

Porous carbon electrodes that accumulate charges at the electrode/electrolyte interface have been extensively investigated for use as electrochemical capacitor (EC) electrodes because of their great attributes for driving high-performance energy storage. Here, we report porous carbon nanofibers (p-CNFs) for EC electrodes made by the formation of a composite of monodisperse silica nanoparticles and polyacrylonitrile (PAN), oxidation/carbonization of the composite, and then silica etching. The pore features are controlled by changing the weight ratio of PAN to silica nanoparticles. The electrochemical performances of p-CNF as an electrode are estimated by measuring cyclic voltammetry and galvanostatic charge/discharge. Particularly, the p-CNF electrode shows exceptional areal capacitance (13 mF cm−2 at a current of 0.5 mA cm−2), good rate-retention capability (~98% retention of low-current capacitance), and long-term cycle stability for at least 5000 charge/discharge cycles. Based on the results, we believe that this electrode has potential for use as high-performance EC electrodes.

Effect of different coupling agents in the doping of graphite oxide with 3–3′ diaminobenzidine: textural, structural and electrical properties

The doping reactions of graphite oxide (GO) with 3-3′-diaminobenzidine (DAB) were studied using N, N′-dicyclohexylcarbodiimide (DCC), cyanuric chloride (CC) and hexafluorophosphate (HATU) as coupling agents. The bifunctionality of the coupling agents aid to interact GO functional groups with amino groups of DAB without being part of the final product. The doped materials (d-GO) and GO were characterized by thermogravimetric analysis, x-ray diffraction, FTIR/Raman spectroscopy, x-ray photoelectron, high-resolution electron microscopy and cyclic voltammetry. The GO-HATU material was more thermally stable than other graphitic material, with at 10% weight loss at 300 °C, this thermal stability is related to a more difficult intramolecular physisorbed water removal process than the other d-GO materials. GO-CC and GO-HATU materials presented 8.2 and 8.0 Å of interlayer spacing, which was associated with a good oxidation-doping process. Besides, these two materials showed modifications in the vibrations by FTIR technique, corresponding to epoxy and hydroxyl groups of the GO being more susceptible to react with the amino groups. Moreover, ID/IG ratio calculated by Raman Spectroscopy presents the following trend 0.70, 0.94, 0.97 and 1.04 for GO, GO-CC, GO-DCC and GO-HATU, respectively, this increase is related with a major disorder during the doping process. XPS analysis shows C–N and N=C bands for high resolution of C 1s and N 1s, respectively, for d-GO materials. This possibly suggests the formation of benzimidazoles during the oxidation-doping process, this generates a similar -non-lattice and -lattice oxygen amount for O 1s related to crosslinking between the functional groups of GO and DAB which improve the electronic mobility between the surface and the bulk of the final graphitic material. Finally, the obtained d-GO materials were investigated as a working electrode for electrochemical capacitors and all of them showed typical capacitive behaviour.

Electrochemical performance of Ce/Ni-MOFs electrode

A novel electrochemical capacitor electrode based on Ce/Ni-MOFs was prepared. During this process, we introduced hydrothermal method. However improper reaction conditions would cause the collapse of crystal structure which will do harm to the performance of the electrode. We explored the option reaction conditions including temperature and end urance time. Under the option condition, we performed a series of electrochemical testing and surface characterizations to explore and electrochemical performance and cycle life. We obtain the Ce/Ni-MOF electrode with the capacitance of 584F/g and the retention of 95.5% after 1000 cycles of charging and discharging test. This indicate that Ce/Ni-MOFs might be a promising supercapacitor electrode for future use.

Bipolar electrochemical capacitors using double-sided carbon nanotubes on graphite electrodes

The electrochemical capacitor (EC) is a key enabler for the miniaturized self-powered systems expected to become ubiquitous with the advent of the internet-of-things (IoT). Vertically aligned carbon nanotubes (VACNTs) on graphite holds promise as electrodes for compact and low-loss ECs. However, as with all ECs, the operating voltage is low, and miniaturization of higher voltage devices necessitates a bipolar design. In this paper, we demonstrate a bipolar EC using graphite/VACNTs electrodes fabricated using a joule heating chemical vapor deposition (CVD) setup. The constructed EC contains one layer of double-sided VACNTs on graphite as bipolar electrode. Compared to a series connection of two individual devices, the bipolar EC has 22% boost in volumetric energy density. More significant boost is envisaged for stacking more bipolar electrode layers. The energy enhancement is achieved without aggravating self-discharge (71.2% retention after 1 h), and at no sacrifice of cycling stability (96.7% over 50000 cycles) owing to uniform growth of VACNTs and thus eliminating cell imbalance problems.

Effect of ruthenium on superior performance of cellulose nanofibers/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)/ruthenium oxide/ionic liquid actuators

This study proposes novel actuators based on cellulose nanofibre/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)/hydrous ruthenium oxide/ionic liquid (CNF/PEDOT:PSS/RuO2/IL) electrodes and the poly(vinylidene fluoride-co-hexafluoropropylene)/IL (PVdF(HFP)/IL) electrolyte, and investigates the effect of hydrous ruthenium oxide on the electrochemical and electromechanical properties of the proposed actuators. The proposed system contains an electrochemical capacitor electrode, which acts as both a faradaic capacitor (FC) and a small electrostatic double layer capacitor (EDLC). This hybrid capacitor is based on the FC (RuO2) mechanism, and a base polymer (PEDOT:PSS) and CNF skeleton replace carbon nanotubes (CNTs). Therefore, this device functions differently from traditional CNT/PVdF(HFP))/IL electrode actuators, which are only used as EDLC units, and CNF/PEDOT:PSS/IL (without RuO2) actuators, where PEDOT:PSS plays the role of both FC and base polymer. Devices built using the proposed actuators exhibit higher strains and greater maximum generated stresses than those exhibited by devices based on CNF/PEDOT:PSS/IL (i.e. without RuO2). Surprisingly, the synergism obtained by combining RuO2 and PEDOT:PSS is considerably greater than the enhancement achieved using PEDOT:PSS. The frequency dependences of the displacement responses of the CNF/PEDOT:PSS/RuO2/IL electrode actuators were then measured and successfully simulated using a double layered charging kinetic model. The developed films are novel, robust, and flexible, and they exhibit potential for use as actuator materials.

Supercapacitive Performances of Ternary CuCo2S4 Sulfides.

Currently, ternary CuCo2S4 sulfides are intensively investigated as electrode materials for electrochemical capacitors due to their low cost, high conductivity, and synergistic effect. The research of CuCo2S4 materials for energy storage has gradually grown from 2016. The supercapacitive performances of CuCo2S4 electrodes for electrochemical capacitors are briefly reviewed in this work. The structure, morphology, and particle size of CuCo2S4 are related to the synthesis conditions and electrochemical performances. The thin films of CuCo2S4 nanostructures deposited on conductive substrates and their composites both show better properties than single CuCo2S4. CuCo2S4 and its composites reveal large potential for asymmetric capacitors, delivering high energy densities. However, there is still much new space remaining for future research. The possible development directions, challenges, and opportunities for CuCo2S4 materials are also discussed.

Optimized Electrolytic Carbon and Electrolyte Systems for Electrochemical Capacitors

AbstractElectrochemical reduction of a molten Li2CO3−Na2CO3−K2CO3 eutectic is used to produce highly amorphous carbon with considerable oxygen functionalization in a process focusing on the conversion of CO2 to value‐added carbon. Electrochemical characterization of materials using multiple techniques in different electrolytes allowed for the optimization of these materials as electrochemical capacitor electrodes. Variation in capacitive performance of the investigated materials has been provided based on their physical characteristics. The synthesized carbons are hybrid materials, showing both pseudocapacitive and electric double layer contributions to the total performance. Annealing under nitrogen at 800 °C is shown to widen pores in the carbon material, resulting in an increase in medium scan rate (5–10 mV.s−1) capacitance. A stable specific capacitance of 425 F.g−1 is obtained at 5 mV s−1 in 0.5 M Na2SO4 after 1000 cycles.

Hierarchical porous carbon derived from eucalyptus-bark as a sustainable electrode for high-performance solid-state supercapacitors

Conversion of waste biomass into porous carbon, with improved energy and power, for use as an electrode for electrochemical double layer capacitors.

Selection of activated carbon for the production of electrodes of electrochemical capacitors

Selection of activated carbon for the production of electrodes of electrochemical capacitors

Properties of electrochemical double-layer capacitors with carbon-nanotubes-on-carbon-fiber-felt electrodes

Carbon nanotube (CNT) layers deposited on carbon fiber cloth (CFC) materials have been studied as electrodes of electrochemical double layer capacitors (EDLCs), in particular, the electrochemical performance and cycle stability of symmetric EDLCs in an organic electrolyte (tetraethyl-ammonium-fluoroborate in acetonitrile). Due to the large surface area of carbon-fibers, the CNT mass loading can be as high as 18 mg/cm2 which is magnitudes larger than that of what can be deposited on aluminium or nickel metal sheets. The area normalized double layer capacitance of CNT/CFC electrodes in the above organic electrolytes were found to be in the range of 100–400 mF/cm2, and the specific capacitances were 18–48 F/g. These latter values are below the achievable values of single-wall CNT of 80 F/g; the lower values can be attributed to the presence of multi-walled CNTs of some quantities, having lower accessible surface area. The energy density of CNT/CFC supercapacitors is 0.8–1.5 Wh/kg, while the power density varies between 5 and 20 kW/kg calculated on electrode level. Excellent cycling stability of EDLCs built with CNT-on carbon felt electrodes has been demonstrated up to 1 million cycles, which is due to the inert nature of substrate causing the absence of corrosion process and high mass load of CNT.

Laser-Assisted Lattice Recovery of Graphene by Carbon Nanodot Incorporation.

Producing highly oriented graphene is a major challenge that constrains graphene from fulfilling its full potential in technological applications. The exciting properties of graphene are impeded in practical bulk materials due to lattice imperfections that hinder charge mobility. A simple method to improve the structural integrity of graphene by utilizing laser irradiation on a composite of carbon nanodots (CNDs) and 3D graphene is presented. The CNDs attach themselves to defect sites in the graphene sheets and, upon laser-assisted reduction, patch defects in the carbon lattice. Spectroscopic experiments reveal graphitic structural recovery of up to 43% and electrical conductivity four times larger than the original graphene. The composites are tested as electrodes in electrochemical capacitors and demonstrate extremely fast RC time constant as low as 0.57 ms. Due to their low defect concentrations, the reduced graphene oxide-carbon nanodot (rGO-CND) composites frequency response is sufficiently fast to operate as AC line filters, potentially replacing today's electrolytic capacitors. Using this methodology, demonstrated is a novel line filter with one of the fastest capacitive responses ever reported, and an aerial capacitance of 68.8 mF cm-2 . This result emphasizes the decisive role of structural integrity for optimizing graphene in electronic applications.

Mesoporous Zinc–Nickel–Cobalt nanocomposites anchored on graphene as electrodes for electrochemical capacitors

This study reports a facile, environmentally-friendly and scalable liquid-phase exfoliation of HOPG process combined with a simple solvothermal method, consequently with calcination for the synthesis of highly porous graphene/Zinc–Nickel–Cobalt (ZNC) nanosheets arrays. The crystallographic and morphological information of the nanostructures were evaluated by X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) Transmission electron microscope (TEM), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscope (HR-TEM). Given its unique architecture and composition, the nanocomposites were analysed as electrode materials for electrochemical capacitor. Additionally, different graphene/(ZNC) nanocomposites composed of various mass loading of graphene to ZNC were prepared under the same synthesis conditions and the results revealed that the mass loading of the precursors significantly influenced the electrochemical behaviour of the active electrode materials. The electrochemical performance of the graphene-(ZNC) electrode prepared at a mass loading of 0.3 of graphene and labelled G3-(ZNC), exhibited fascinating specific capacitance of 502 Fg-1 at current density of 0.2 Ag-1 and with excellent cycling stability of 82% capacitance retention after 2600 cycles.

Immobilization of Polyiodide Redox Species in Porous Carbon for Battery-Like Electrodes in Eco-Friendly Hybrid Electrochemical Capacitors.

Hybrid electrochemical capacitors have emerged as attractive energy storage option, which perfectly fill the gap between electric double-layer capacitors (EDLCs) and batteries, combining in one device the high power of the former and the high energy of the latter. We show that the charging characteristics of the positive carbon electrode are transformed to behave like a battery operating at nearly constant potential after it is polarized in aqueous iodide electrolyte (1 mol L−1 NaI). Thermogravimetric analysis of the positive carbon electrode confirms the decomposition of iodides trapped inside the carbon pores in a wide temperature range from 190 °C to 425 °C, while Raman spectra of the positive electrode show characteristic peaks of I3− and I5− at 110 and 160 cm−1, respectively. After entrapment of polyiodides in the carbon pores by polarization in 1 mol L−1 NaI, the positive electrode retains the battery-like behavior in another cell, where it is coupled with a carbon-based negative electrode in aqueous NaNO3 electrolyte without any redox species. This new cell (the iodide-ion capacitor) demonstrates the charging characteristics of a hybrid capacitor with capacitance values comparable to the one using 1 mol L−1 NaI. The constant capacitance profile of the new hybrid cell in aqueous NaNO3 for 5000 galvanostatic charge/discharge cycles at 0.5 A g−1 shows that iodide species are confined to the positive battery-like electrode exhibiting negligible potential decay during self-discharge tests, and their shuttling to the negative electrode is prevented in this system.

Tungsten oxide and carbide composite synthesized by hot filament chemical deposition as electrodes in aqueous-based electrochemical capacitors

Abstract Developing more sustainable, low cost and feasible energy storage devices is a keystone of aerospace, automobile, and electronics industries. In research new materials and techniques that allow their production in large scale are studied by researchers around the world. Regarding materials, supercapacitors, for instance, may have carbon based materials on electrodes, owing to their chemical stability, low cost, and safety; however, their specific capacitance is lower when compared to other materials. Additionally, materials that may provide higher capacitance–due to faradaic or redox reactions – are also studied. Within this group, transition metal oxides are potential candidates for that, once faradaic behavior may provide higher capacitance values; however, not cyclability in most cases. Thus, this work presents a composite material with tungsten carbide (W2C) and tungsten oxide species (WxOy), which showed both faradaic behavior and cyclability. This composite material was synthesized by deposition of tungsten using a technique known as hot filament chemical vapor deposition (HFCVD). In this process, sublimation of tungsten occurred under H2 constant flow; thus, reducing the metallic tungsten; while oxide species were formed by spontaneous passivation. Experimental results indicate that the efficiency improved from 59% to 85% for first and 10, 000th cycles, respectively, with boost of specific capacitance of 160%. In sum, the material reported herein presents the unique aspect of improving its properties during cyclability. Therefore, such feature and synthesis process may enable its use as renewable energy storage devices.

Gravimetric and Volumetric Changes of Carbon Electrodes in Electrochemical Capacitors with Aqueous Electrolytes

Ionic fluxes at carbon electrode/electrolyte interface have been already well described for organic medium. This description allowed for comprehensive understanding of the phenomena occuring during electrochemical capacitor operation. However, the processes on-going in aqueous electrolytes still need to be investigated, since several accompanying phenomena (eg. hydration, corrosion) might occur in this medium. Our attention is primarily focused on the performance of carbon electrodes operating with pH-neutral aqueous electrolytes, allowing a relatively high capacitor voltage to be reached. Apart from typical capacitive storage, our interest is focused also on hybrid mechanisms, delivered by application of redox-active electrolytes, based on iodides or pseudohalides. This paper will provide the insights into the ionic fluxes at the electrode/electrolyte interface, monitored by Quartz Crystal Electrochemical Microbalance, Scanning Electrochemical Microscope and Dilatometry techniques. Preliminary results indicate that the situation at the interface is somehow different than one might expect. For instance, the changes for positive and negative electrode are significantly different, although from electrochemical point of view they appear to follow the conventional cation/anion adsorption manner. In the case of redox-active electriolytes, the situation is even more complicated. This paper will provide the results from three different techniques - of different modus operandi but allowing for complementary observations and conclusions. Finally, the goal of this work is to understand and identify the phenomena at the interface, mostly in order to prevent the parasitic or detrimental processes and prolong the capacitor lifetime.

Characterization Capacitive Desalinzation Cell By Using the FeIII(CN)6 3-/FeII(CN)6 4- Redox Couple As an Electrochemical Probe

Recently, a desalination technique based on porous carbon has been reported [1], merging together desalination process with electrochemical energy storage. Thanks to this new strategy, an important step forward, respect to other water desalination technologies, has been achieved. The principle is based on the capacitive adsorption of Na+ and Cl- ions in the pores of a porous activated carbon, constituting the active material of an electrochemical double layer capacitor electrode. Differently from conventional EDLC, the active material is made in the present case as slurries flowed through an electrochemical cell to achieve a continuous desalination of seat water; such system has been named Electrochemical Flow Capacitor (ELC) [2], [3]. The objective of the study is to improve the energy efficiency of the desalination process by optimizing the cell design and by selecting the proper activated carbon. To do so, we characterized a desalinization cell using different type of electrodes using the FeIII(CN)6 3-/FeII(CN)6 4- redox couple as an electrochemical probe. Our approach allowed to define a dimensionless number θ, which is proportional to the flow rate and inversely proportional to the potential scan rate, giving a guideline to sort out different operating regimes in the flow cell. Two operating regimes have been evidenced: a permanent regime for θ > 60, allowing for higher and efficient charge adsorption, and a non-permanent domain for θ < 60. Références : [1] M. E. Suss, S. Porada, X.Sun, P. M. Biesheuvel, J. Yoon and V. Presser, Energy Environ Sci., 20015, 8, 2296-2319. [2] K. B. Hatzell, E. Iwama, A. Ferris, B. Daffos, K. Urita, T. Tzedakis,F. Chauvet, P. Taberna, Y. Gogotsi, P. Simon, Electrochemistry Communications 43 (2014) 18–21. [3] Y. Gendel, A. K. E. Rommerskirchen, O. David, M. Wessling, Electrochemistry Communications 46 (2014) 152–156.

Unexpected High Rate Capability Enhancement By MnO2 Incorporation in Cellulose-Derived Carbon Nanofiber Electrodes for Electrochemical Capacitors

Electrochemical capacitors (ECs) can provide ultra-long cycle life and ultra-fast energy delivery, which are impossible to reach by most battery technologies. Therefore, ECs are irreplaceable in applications such as energy harvesting systems and grid power buffer [1, 2]. The compositing between carbon and manganese oxides (denoted here as carbon/MnO2) is a popular strategy in the materials research community for getting higher energy densities of EC electrodes [3, 4]. MnO2 is well acknowledged for its high pseudocapacitance due to surface-confined redox processes that take place during charging/discharging, but also known for its poor intrinsic electrical conductivity (0.1 to 1 mS/m) that restricts high power applications [5]. The combination of carbon and MnO2 circumvents the limitation of MnO2 by taking advantage of good rate capability endowed by superior electrical conductivity of carbon. Normally, the fast rate capability (in terms of a retention value of capacitance at high current density relative to that at low current density) of the composite is enhanced compared to pure MnO2, but still remains inferior to pure carbon [6]. However, a greatly enhanced rate capability (78.2%, the ratio of 15 A/g capacitance to 0.5 A/g capacitance) compared to the original carbon (10.2%) was observed after the incorporation of MnO2 into a cellulose-derived nitrogen-doped carbon nanofibers (NCNF) system presented here. In the present study, the composite material (NCNF/MnO2) was prepared via the carbonization of electrospun cellulose and subsequent reaction between carbon nanofibers and permanganate (MnO4 -) in a liquid phase under a hydrothermal condition. The above processes generate a physical blend of the supporting NCNF matrix and MnO2. The unexpected high rate capability enhancement in this system can be explained by a significant change in carbon surface functional groups, specifically, the increased fraction of phenolic groups and the decreased fraction of carboxylic groups. This study presents not only a high performance electrode material for ECs (over 110 F/g specific capacitance, 78.2% rate capability and 96.3% capacitance retention over 50 000 cycles), but also highlights the critical role of carbon surface modification that is induced by the compositing processes, especially in the process deployed in this work, i.e. the electroless deposition of MnO2 on carbon via reaction with MnO4 - in liquid media.

Synthesis and Electrochemical Properties of CMK-3 with Particles of Nickel, Cobalt and Copper

Mesostructured carbon material CMK-3 for electrodes of electrochemical capacitors was obtained by the method of template synthesis. In order to increase the capacitance characteristics, impregnation of metal ions (Co, Ni, and Cu) into the structure of mesoporous carbon CMK-3 was carried out. The structure of the obtained materials was studied by X-ray diffraction and gas adsorption. The study by TEM showed that highly dispersed, nanosized particles are metal oxides Co, Ni and Cu with the size of 30-50 nm. The particles are uniformly distributed inside the carbon material. Electrochemical characteristics were studied in aqueous electrolytes (1M KCl and 1M KOH). It has been established that the impregnation of metal ions increases in the specific capacity of the mesoporous carbon material by about 30 % (from 110 to 156 F/g) in KOH