Electrodes For Electrochemical Capacitors Research Articles

The effects of vacuum pyrolysis conditions on wood biochar monoliths for electrochemical capacitor electrodes

Vacuum pyrolysis is mostly employed to extract liquid biofuels from biomass in industry, but the potential application of its by-product (biochar) is seldom studied. The wood biochar monoliths were prepared by pyrolyzing cotton rose stem under vacuum conditions and a common nitrogen atmosphere, respectively. The effects of vacuum conditions on the physicochemical properties of biochar monoliths were investigated by various techniques, such as Raman, FTIR, XPS, TG, BET, CO2 adsorption and SEM methods. The results showed that the vacuum pyrolysis biochar, in comparison with nitrogen atmospheric pyrolysis biochar, features a higher degree of graphitization, a richer micro-porosity (57.2% vs. 33.9%), and a more reactive surface. The comparison of electrochemical performance suggested that the electrode prepared by vacuum pyrolysis overtakes that by nitrogen atmosphere pyrolysis with a specific capacitance of 161 F g−1 versus 133 F g−1 and an ease ion delivery capacity. Moreover, 125% of initial capacitance was retained over 10000 cycles at 50 mA cm−2. The vacuum conditions are more favorable for the preparation of biochar as well as the biochar derived activated carbon materials for application as electrodes.

Design of Porous Carbons for Supercapacitor Applications for Different Organic Solvent-Electrolytes

The challenge of optimizing the pore size distribution of porous electrodes for different electrolytes is encountered in supercapacitors, lithium-ion capacitors and hybridized battery-supercapacitor devices. A volume-averaged continuum model of ion transport, taking into account the pore size distribution, is employed for the design of porous electrodes for electrochemical double-layer capacitors (EDLCs) in this study. After validation against experimental data, computer simulations investigate two types of porous electrodes, an activated carbon coating and an activated carbon fabric, and three electrolytes: 1.5 M TEABF4 in acetonitrile (AN), 1.5 M TEABF4 in propylene carbonate (PC), and 1 M LiPF6 in ethylene carbonate:ethyl methyl carbonate (EC:EMC) 1:1 v/v. The design exercise concluded that it is important that the porous electrode has a large specific area in terms of micropores larger than the largest desolvated ion, to achieve high specific capacity, and a good proportion of mesopores larger than the largest solvated ion to ensure fast ion transport and accessibility of the micropores.

Chemically Stabilized and Functionalized 2D‐MXene with Deep Eutectic Solvents as Versatile Dispersion Medium

Abstract2D transition metal carbides and nitrides, namely MXenes, are normally synthesized in acidic solutions and are delaminated in basic solutions. This results in versatile materials with unique physical/chemical properties suitable for various practical applications. However, solution‐based chemical treatments can affect the chemical structures of MXenes, which accelerates the oxidation reactions and degrades their intrinsic properties. Here, long‐term stable Ti3C2Tx dispersion in deep eutectic solvents (DESs) that resisted oxidation degradation for up to 28 weeks is demonstrated. As an anti‐oxidative dispersion medium, DESs helped prevent oxidation of Ti3C2Tx layers due to hydrogen bond accepting and donating molecules passivated surface of the Ti3C2Tx. In addition, DES molecules in bulk solution can also be hydrated in the presence of water, which stabilizes reactive oxygen by forming stable DES‐water cluster. Therefore, the use of DESs enhanced the delamination of the Ti3C2Tx nanosheets, while preventing oxidation of the nanosheets in solution and even in their dried state. As a result, thick and thin films of Ti3C2Tx fabricated using DESs exhibited stable sheet resistance in comparison with pristine‐Ti3C2Tx. In addition, Ti3C2Tx dispersed in DESs can be applied as electrodes for electrochemical capacitors, in which they showed higher chemical stability and better performance than pristine Ti3C2Tx.

Sputtered chromium nitride/carbon nanotubes hybrid structure for electrochemical capacitors

Hybrid materials based on transition metal nitrides and carbon nanostructures are attracting intention for high-performance electrochemical energy storage systems. In this work, vertically aligned carbon nanotubes (CNTs) were grown over silicon substrate, followed by direct current plasma-sputtered deposition of chromium nitride (CrN) for different deposition times. The CNTs@ CrN electrodes exhibit vertically aligned cauliflower morphology with different degrees of dispersion on the substrate and were tested as electrodes for electrochemical capacitors (ECs). The areal capacitance of the CNTs@ CrN, as measured in 0.5 M H2SO4 electrolyte, was as high as 156.4 mF cm−2 at a scan rate of 5 mV s−1 with an excellent electrochemical retention of 90% over 10,000 cycles. Furthermore, the areal capacitance was found to increase with CrN thickness increase over the CNTs. This work demonstrates the beneficial use of CNT as template for transition metal nitride deposition and highlights CrN as a promising competitor to other transition metal nitrides already reported for ECs with similar structural morphology.

Exfoliated graphite as electrochemical double layer capacitor electrode: Electrochemical impedance study

Official journal of the Institute of Physics - Sri Lanka. Full text articles are available. The journal also has a website at http://www.ip-sl.org/sljp/The new email for the Editor is eic.ipsl@gmail.com.

Study of the electrophysical properties of nanostructured porous germanium as a promising material for electrodes of electrochemical capacitors

Electrochemical capacitors (ECC) are a fast charging devices, with high power density, capacity and increased life time. Nanostructured semiconductors are now considered as the promising materials for electrodes of such devices due to its conductive properties and effective surface. One of such materials is the porous germanium which can be used as an electrode in electrochemical capacitors. In this article the novel approach based on the method of ion implantation was developed to grow these structures. This method allows to obtain a structures up to 1 μm thick. The object of this work was the investigation of the electrophysical characteristics of samples of nanostructured porous germanium (Ge) depending on the implantation dose and surface morphology. The scientific novelty of this research lies in the search the structures with the highest effective surface area and electronic conductivity, capable of multiplying the energy capacity and specific power of ECC. Methods: The samples of amorphous Ge were grown on dielectric single-crystal substrates of Al2O3. The thickness of samples was 600 and 1000 nm. The magnetron sputtering and ion implantation methods were used to growth these structures. The irradiation with Ge+ ions produced with an energy of 40 keV and the range of implantation doses varied from 2·1016 to 12•1016 ion / cm2. The study of electrical properties was carried out on the Hall installation HL55PC at the NPP KVANT in Moscow. The following parameters were measured: the sheet concentration of carriers in the near-surface layer, electrical resistance, mobility of the charge carriers, Hall coefficient. As a result, the dependences of carriers concentration and their mobility as the function of the implantation dose and thickness of the samples of nanostructured porous germanium were determined, and the results were analyzed. Results: It was found that ion implantation of single-crystal germanium leads to an increase in the carrier concentration in the near-surface layer. To sum up, the most suitable material as an electrode for ECC is the porous germanium with the maximum dose of ion implantation and the largest thickness. The maximum sheet carrier concentration that was obtained in the study for Ge is 1017 cm-2.

Analysis and Electrochemical Capacitance of Single Walled Carbon Nanotubes (10, 4) Synthesized Via Chemical Vapour Deposition Method

Single walled carbon nanotubes are receiving considerable attention in the nanotechnology industry, especially in electronics, as efficient energy storing materials. However, obtaining these materials of desired chirality via chemical vapour deposition method has posed a persistent challenge for over twenty years. In the current report, single walled nanotubes (10, 4) were successfully grown via pyrolysis of C6H14/N2 feedstock on Fe2O3/Al2O3 catalyst. Field emission scanning electron microscopy and high resolution transmission electron microscopy images showed bundled network of single walled carbon nanotubes and resulting analysis of the Raman profile for the sample showed consistency with the results established via Extended Tight Binding model. This was a suggestion that if this method is fully optimized, it may help alleviate the stated global challenge. Specific capacitance (F/g) of 242, 174 and 50, was recorded for the sample in the neutral, acidic and alkaline electrolytes, respectively, also suggesting promising potentials as electrodes for Pseudocapacitor and electrochemical double layer capacitor.

Air activation of charcoal monoliths for capacitive energy storage.

Charcoal monoliths derived from waste wood were activated with air for the application of electrochemical capacitor electrodes and an insight was given into the activation mechanism. The mild air activation is effective and pollution-free compared to the common chemical activation using KOH etc. for the preparation of crack-free carbon monoliths. The activation process was controlled by altering the activation temperature and time, and their effects on the nanostructure of charcoal monoliths were studied. As the activation temperature or time increased, air eroded the defective surface of charcoal layer-by-layer, with the oxygen atoms being introduced by chemisorption and oxidation reactions and removed by dehydration and decomposition reactions. Meanwhile, micro-pores were produced. The electrode activated at 300 °C for 1 h, with a specific surface area of 567 m2 g−1 and a high micro-porosity of 86%, exhibited a specific capacitance of 203 F g−1 and 35.5 F cm−3. Moreover, it presented a higher total capacitance of 3.6 F cm−2 than most reported pellet electrodes. These findings give a reasonable picture of the air activation process and are instructive to prepare activated carbon monoliths under an oxidizing environment.

Mechanisms of charge accumulation in electrochemical systems formed based on of nanoporous carbon and manganese oxide

In this work, the processes occurring in electrochemical systems based on nanoporous carbon material and manganese oxide in an aqueous solution of lithium sulfate are analyzed. Furthermore, it is shows the feasibility of these materials combination cycling as electrodes of a hybrid electrochemical capacitor. The combination of electrode materials with different mechanisms of charge accumulation was determined. Consequently, an increase in the accumulated energy by more than 25% by the formation of an electric double layer and the occurrence of redox reactions based on carbon and manganese oxide respectively. The laboratory sample of an aqueous electrolyte hybrid electrochemical capacitor was formed. Moreover, the laboratory sample is electrochemically stable at an operating voltage of 2 V.

Effect of polyaniline content on the electrochemical behavior of tin oxide/polyaniline composites by solution mixing

The effect of polyaniline (PANI) nanofibers content on the electrochemical properties of composites based on SnO2 nanoflakes was studied. PANI nanofibers and SnO2 nanoflakes were synthesized and characterized (SEM, BET, FTIR, XPS, and XRD) and mixed in solution at different proportions. The resulting composites were characterized electrochemically (CV, EIS, and GCD) to determine their specific capacitance, electric resistance, and cyclic stability to be used as electrochemical capacitor electrodes (ECs). It was found that the SnO2-PANI-(80/20) composite at 5 mVs−1 exhibited higher specific capacitance (CAS = 1.1 Fg−1) than SnO2 (CAS = 0.99 Fg−1); meanwhile, SnO2-PANI-(95/5) at 100 mVs−1 exhibited higher capacitance (CAS = 0.66 Fg−1) than PANI (CAS = 0.42 Fg−1). The GCD results indicate that SnO2, PANI, and their composites presented coulombic efficiencies higher than 90% after 5000 cycles. Also, the highest initial capacitance (C = 0.4 Fg−1, at 2 mAcm−2) was found for SnO2-PANI-(80/20), and the highest final capacitance (C = 0.28 Fg−1, 5000 cycles at 2 mAcm−2) was for SnO2-PANI-(95/5). Finally, PEIS behavior was studied, and the equivalent circuits were proposed, resulting that the contact resistance to charge transfer (RCT) of composites (RCT = 1.61–2.28 Ω) were lower than precursors (SnO2, RCT = 3.66 Ω; PANI, RCT = 4.47 Ω). In addition, it was observed that the capacitance of composites increased, and resistance to charge transfer decreased with the PANI content. Electrode material presents a synergetic effect promoted by the appropriate mixing in solution along with the nanoscale morphologies of the components showing improved specific capacitance, and low both contact and charge transfer resistances, with high coulombic efficiencies (> 90% after 5 k cycles).

Study of the pore size effect on the charge storage of hydrous RuO2 nanoparticles supported within the pores of activated carbon

Hydrous ruthenium oxide (RuO2) nanoparticles are hybridized inside the pores of activated carbon (AC) for preparing electrochemical capacitor electrodes. The hybridization consists of the adsorption of an organoruthenium complex and subsequent heat treatment to reduce the complex into Ru nanoparticles within the AC pores, followed by the electrochemical oxidation of the resulting Ru nanoparticles into hydrous RuO2 nanoparticles. Two kinds of AC with only micropores and both micropores and mesopores of ~4 nm are examined to study the effects of pore sizes on the charge/discharge characteristics of the AC/RuO2 hybrids. Since the introduction of hydrous RuO2 nanoparticles is not accompanied by the volumetric expansion of the AC particles, the volumetric capacity is enhanced in comparison with the pristine AC. However, the capacitance retention at high charge/discharge rates is decreased by micropores or an excessive amount of the hydrous RuO2 nanoparticles. This method is free from the use of any organic solvent, concomitant solvent removal and drying process, or reducing and oxidizing reagents, and therefore, can be used to synthesize porous carbon/metal oxide nanoparticle hybrids for versatile applications.

From wood to thin porous carbon membrane: Ancient materials for modern ultrafast electrochemical capacitors in alternating current line filtering

Ultrafast electrochemical capacitors with alternating current line filtering function have attracted growing attention owing to their potential to replace the state-of-the-art bulky aluminum electrolyte capacitors. In spite of rapid advance recently involving nanomaterials as electrode building units, it remains largely unexplored how to structurally and chemically engineer electrodes out of renewable resource with competitive or better rate performance. Herein, wood as a renewable resource was used to fabricate highly conductive, robust, porous thin carbon membranes as free-standing electrodes for ultrafast electrochemical capacitors. Transformation of wood slice to carbon membrane proceeds via wet-chemical treatment of wood slices and subsequent morphology-maintaining carbonization by spark plasma sintering. Judiciously combining high conductivity, characteristic porous architecture with low tortuosity and high continuity, and the ultrathin thickness down to 20 μm, the carbon membrane-based electrochemical capacitor exhibits excellent frequency response with efficient 120 Hz filtering (phase angle = −83.5o). Compared to the latest electrodes for line filtering application that are fabricated from carbon nanotubes, graphene, and MXene, the wood-derived carbon membranes possess a competitive specific areal capacitance of up to 509.7 µF cm−2, and extremely low resistance-capacitance constant of 164.7 µs, plus the inexpensive scalable fabrication strategy.

Improvement of the Pseudocapacitive Performance of Cobalt Oxide-Based Electrodes for Electrochemical Capacitors

Cobalt oxide nanopowders are synthesized by the pyrolysis of aerosol particles of water solution of cobalt acetate. Cobalt nanopowder is obtained by subsequent reduction of obtained cobalt oxide by annealing under a hydrogen atmosphere. The average crystallite size of the synthesized porous particles ranged from 7 to 30 nm, depending on the synthesis temperature. The electrochemical characteristics of electrodes based on synthesized cobalt oxide and reduced cobalt oxide are investigated in an electrochemical cell using a 3.5 M KOH solution as the electrolyte. The results of electrochemical measurements show that the electrode based on reduced cobalt oxide (Re-Co3O4) exhibits significantly higher capacity, and lower Faradaic charge–transfer and ion diffusion resistances when compared to the electrodes based on the initial cobalt oxide Co3O4. This observed effect is mainly due to a wide range of reversible redox transitions such as Co(II) ↔ Co(III) and Co(III) ↔ Co(IV) associated with different cobalt oxide/hydroxide species formed on the surface of metal particles during the cell operation; the small thickness of the oxide/hydroxide layer providing a high reaction rate, and also the presence of a metal skeleton leading to a low series resistance of the electrode.

Study of the mesopore size effect on the electrochemical capacitor behaviors of mesoporous carbon/quinone derivative hybrids

A quinone derivative, 2,5-dichloro-1,4-benzoquinone (DCBQ), is hybridized with MgO-templated mesoporous carbons (MgO-Cs) with the average pore sizes of 5, 10, and 30 nm through gas-phase adsorption. The resulting hybrids are used for aqueous electrochemical capacitor electrodes due to the reversible redox reaction of DCBQ within the mesopores when using an aqueous H2SO4 electrolyte. This hybridization method precisely controls the weight ratio of porous carbons and DCBQ, and therefore, facilitates the discussion of the correlations between the amount of adsorbed DCBQ and the electrochemical capacitor behaviors of the MgO-C/DCBQ hybrids. Irrespective of the pore size of the MgO-Cs, the volumetric capacitance increases with increasing DCBQ content at current densities of 0.05−0.5 A g−1 because the hybridization of the redox-active DCBQ with porous carbons is not accompanied by the volumetric expansion of porous carbon particles. However, the DCBQ contents above 6 mmol per 1 g of MgO-C results in the decrease of the capacitance retention in the high current density region of ~10 A g−1. Not all the hybridized DCBQ molecules contribute to the capacitance enhancement, and the increase of the DCBQ content leads to a gradual decrease of the utilization ratio. In addition, the average pore sizes of 5 and 10 nm do not show the significant difference in the utilization ratio, but the MgO-C with the average pore size of 30 nm shows a decrease of the utilization ratio in comparison with those of the other MgO-Cs. This hybridization is applicable to any porous carbons, and the commercial activated carbon containing mesopores is appropriate for preparing high-performance electrochemical capacitors, due to its low-cost, large surface area for accommodating a large amount of DCBQ, and the absence of large mesopores as large as 30 nm.

Symmetric electrochemical capacitor based on PPy obtained via MnO2 reactive template synthesis

The present work explores the synthesis of sulphate-polypyrrole (PPy/SO42−) using MnO2 as a reactive template. Samples were characterized by different techniques, including SEM, CHNS, XRD, and XPS, which confirmed that MnO2 acted as a reactive template during PPy synthesis. The materials obtained were characterized by cyclic voltammetry and electrochemical impedance spectroscopy techniques to evaluate their electrochemical properties as electrodes for electrochemical capacitors (ECs electrodes). Py50/M50 electrode exhibited the highest specific capacitance value being 1215 mF cm−2 (or 125 F g−1) at 5 mV s−1. According to PEIS analysis, the enhancement of capacitive performance is attributed to the elongated shape morphology of Py50/M50 electrode that improves the ionic diffusion and reduces the charge transfer resistance on its surface. Furthermore, a symmetric ECs assembly using two identical pieces of Py50/M50 electrodes exhibited a maximum areal energy and power density of 13.5 mW h cm −2 and 324 mW cm−2, respectively. These results suggest a new route of synthesis of PPy/SO42− with elongated shape morphology to produce electrodes for ECs with high electrochemical performance.

Scalable spray-coated graphene-based electrodes for high-power electrochemical double-layer capacitors operating over a wide range of temperature

Advancements in electrochemical double-layer capacitor (EDLC) technology require the concomitant use of novel efficient electrode materials and viable electrode manufacturing methods. Cost-effectiveness, scalability and sustainability are key-drivers for fulfilling product development chain accepted by worldwide legislations. Herein, we report a scalable and sprayable “green” electrode material-based ink based on activated carbon and single-/few-layer graphene (SLG/FLG) flakes. We show that, contrary to commercial reduced graphene oxide, defect-free and flat SLG/FLG flakes reduce the friction of ions over the electrode films, while spray coating deposition of our ink maximises the electrolyte accessibility to the electrode surface area. Sprayed SLG/FLG flakes-based EDLCs display superior rate capability performance (e.g., specific energies of 31.5, 23.7 and 12.5 Wh kg−1 at specific powers of 150, 7500 and 30000 W kg−1, respectively) compared to both SLG/FLG flakes-free devices and commercial-like EDLCs produced by slurry-coating method. The use of SLG/FLG flakes enables our sprayed EDLCs to operate in a wide range of temperature (−40/+100°C) compatible with ionic liquid/organic solvent-based electrolytes, overcoming the specific power limits of AC-based EDLCs. A prototype EDLCs stack consisting of multiple large-area EDLCs, each one displaying a capacitance of 25 F, demonstrates the industrial potential of our technology.

Preparation and Electrochemical Properties of Porous Carbon Nanofiber Electrodes Derived from New Precursor Polymer: 6FDA-TFMB.

Porous carbon nanofibers (CNFs) with high energy storage performance were fabricated with a single precursor polymer, 6FDA-TFMB, without the use of any pore-generating materials. 6FDA-TFMB was synthesized, electrospun, and thermally treated to produce binder-free CNF electrodes for electrochemical double-layer capacitors (EDLCs). Highly porous CNFs with a surface area of 2213 m2 g−1 were prepared by steam-activation. CNFs derived from 6FDA-TFMB showed rectangular cyclic voltammograms with a specific capacitance of 292.3 F g−1 at 10 mV s−1. It was also seen that CNFs exhibit a maximum energy density of 13.1 Wh kg−1 at 0.5 A g−1 and power density of 1.7 kW kg−1 at 5 A g−1, which is significantly higher than those from the common precursor polymer, polyacrylonitrile (PAN).

High Power Energy Storage via Electrochemically Expanded and Hydrated Manganese-Rich Oxides.

Understanding the materials design features that lead to high power electrochemical energy storage is important for applications from electric vehicles to smart grids. Electrochemical capacitors offer a highly attractive solution for these applications, with energy and power densities between those of batteries and dielectric capacitors. To date, the most common approach to increase the capacitance of electrochemical capacitor materials is to increase their surface area by nanostructuring. However, nanostructured materials have several drawbacks including lower volumetric capacitance. In this work, we present a scalable “top-down” strategy for the synthesis of EC electrode materials by electrochemically expanding micron-scale high temperature-derived layered sodium manganese-rich oxides. We hypothesize that the electrochemical expansion induces two changes to the oxide that result in a promising electrochemical capacitor material: (1) interlayer hydration, which improves the interlayer diffusion kinetics and buffers intercalation-induced structural changes, and (2) particle expansion, which significantly improves electrode integrity and volumetric capacitance. When compared with a commercially available activated carbon for electrochemical capacitors, the expanded materials have higher volumetric capacitance at charge/discharge timescales of up to 40 s. This shows that expanded and hydrated manganese-rich oxide powders are viable candidates for electrochemical capacitor electrodes.

Structure, functional composition and electrochemical properties of nitrogen-doped multi-walled carbon nanotubes synthesized using Co–Mo, Ni–Mo and Fe–Mo catalysts

We studied the effect of Co–Mo/MgO, Ni–Mo/MgO, and Fe–Mo/MgO catalysts on the structure and functional composition of nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs) synthesized by chemical vapor deposition method. Polyoxomolybdates containing Co–Mo, Ni–Mo, Fe–Mo сores were used as the catalyst precursors and acetonitrile was the source of carbon and nitrogen for the growth of N-MWCNTs in argon atmosphere at 900 °C. The Ni–Mo/MgO catalyst promoted the formation of N-MWCNTs with the smallest outer diameter of 8 nm and the highest content of embedded nitrogen of 2.7 at.% with a predominance of pyridinic nitrogen form. Co–Mo/MgO and Fe–Mo/MgO catalysts yielded thicker N-MWCNTs with an average outer dimeters of 12 and 15 nm and lower nitrogen content of 2 and 1.5 at.%, respectively. N-MWCNTs were tested as electrodes in electrochemical capacitors and Li-ion batteries. A higher efficiency of the N-MWCNTs synthesized using Ni–Mo/MgO catalyst confirmed the advantage provided by a high content of nitrogen in the nanotube walls.

Nanoconfinement Effects on Enhanced Reversibility of Redox Reactions Coupled with an Irreversible Chemical Process by Electrolysis Acceleration in Nanoporous Carbon Electrodes for a Redox-Enhanced Electrochemical Capacitor

Redox-enhanced electrochemical capacitors (Redox-ECs) in which electrons are stored and released by redox reactions of organic molecules either dissolved in an electrolyte or adsorbed on an electro...