Krzysztof Jurewicz

Supercapacitors from nanotubes/polypyrrole composites

A novel type of composite electrode based on multiwalled carbon nanotubes (MWNTs) coated with polypyrrole (PPy) has been used in supercapacitors. A homogeneous layer of PPy has been deposited on the nanotubular materials by electrochemical polymerization of pyrrole, that in all cases enhances the specific capacitance. A maximum value of 163 F/g has been obtained for MWNTs prepared at 600 °C and modified by a PPy layer of 5 nm, whereas it is only 50 F/g for the pristine nanotubes. The open entangled network of the nanocomposite favors the formation of a three-dimensional electrical double layer allowing a more effective contribution of the pseudofaradaic properties of PPy.

Nanotubular materials for supercapacitors

Different types of multi-walled (MWNTs) and single-walled nanotubes (SWNTs) have been considered as active electrode materials for the storage of energy in supercapacitors. Due to their unique mesoporosity, these materials have a high ability for the accumulation of charges in the electrode/electrolyte interface. MWNTs supply twice higher values of capacitance in comparison to SWNTs. The nanotubular materials of high purity point out a box-like shape of voltammetry characteristics that proves an entirely electrostatic attraction. Pseudocapacitance effects are observed if metallic particles are present and after additional functionalization of the nanotubes or deposition of conducting polypyrrole (PPy). The value of capacitance obtained from nanotubes modified by PPy reaches 170 F/g, about twice that given either by the nanotubes (ca. 80 F/g) or by pure PPy (ca. 90 F/g). The open entangled network of the nanocomposite seems to favour a better efficiency for the formation of the electrical layer in PPy.

Enhanced capacitance of carbon nanotubes through chemical activation

Abstract Microporosity of pure multi-walled carbon nanotubes (MWNTs) has been highly developed using chemical KOH activation. Depending on the nanotubular material, the burn-off ranged from 20% to 45% after the activation process. At least twofold increase of surface area has been obtained with maximum values of ca. 1050 m 2 / g for KOH/C ratio of 4:1. The activated material still possesses a nanotubular morphology with many defects on the outer walls that give a significant increase of micropore volume, while keeping a noticeable mesoporosity. Such activated MWNTs have been used as electrode material for supercapacitors in alkaline, acidic and aprotic medium. Enhanced values of capacitance were always observed after activation: in some cases it increased almost seven times from 15 F/g (for non-activated nanotubes) to 90 F/g (after chemical activation).

Nanotubular materials as electrodes for supercapacitors

Different modifications of multiwalled carbon nanotubes (MWNTs) have been performed to increase specific capacitance of this material for supercapacitor application. MWNTs used in this work were obtained by decomposition of acetylene at 600 and 700 °C on cobalt catalyst supported on NaY zeolite, silica or produced from a CoxMg1−xO solid solution. Coating of MWNTs by conducting polymers, e.g. polypyrrole (PPy) has been successfully realized and specific capacitance increased from ca. 50 to 180 F/g demonstrating a synergy between the two components of such nanocomposite. The open mesoporous network of well conducting nanotubular material allows an easy access of ions to the electrode/electrolyte interface and more effective contribution of the pseudofaradaic properties of PPy. Long durability (over 2000 cycles) was obtained for such supercapacitor material. The second way of MWNTs modification was their chemical activation by KOH. In this case, a significant enhancement of microporosity has been obtained and specific capacitance of nanotubular material increased from 15 to 90 F/g.

Ammoxidation of brown coals for supercapacitors

The paper reports results of a study on the use of active carbon from ammoxidated brown coal as electrode material for supercapacitors. The raw coal was subjected to demineralisation and ammoxidation by a mixture of ammonia and air, then it was carbonised and activated by steam or carbon dioxide. The electrochemical properties of carbons were determined by galvanostatic and potentiodynamic methods for water electrolytes. The best capacitance parameters of ca. 220 F/g in acidic and ca. 140 F/g in alkaline conditions were obtained for active carbon containing from 2 to 3 wt.% of nitrogen.

Electrochemical storage of hydrogen in activated carbons

Reversible storage of hydrogen in microporous active carbon was performed by galvanostatic electrodecomposition of water either in 6 M KOH or in 1 M H2SO4 electrolytic solution. The most indicative proof of hydrogen insertion into carbon is a significant decrease (from 700 to 1000 mV) of the rest potential of the carbon electrode after the charge process, as well as a distinct discharge plateau during the oxidation process. Capacity of the stored hydrogen was estimated from the Faradaic charge passed during hydrogen oxidation. The uptake of ca. 2 wt.% reached in the alkaline medium is at least four times higher than the values obtained for the same material under pressure conditions. This enhancement is interpreted by the electrolytic formation of nascent hydrogen that penetrates easily in the useful porosity of the carbon nanostructure.

Synergy of components in supercapacitors based on nanotube/polypyrrole composites

Multiwalled nanotubes (MWNTs) composites with electrodeposited polypyrrole (PPy) have been proposed as electrode materials for supercapacitors. The total capacitance of the nanocomposite combines a pure electrostatic attraction of ions and the electrochemical redox reactions of the -conjugated system of PPy. A synergy effect has been found between the two components of the nanocomposite. The mesoporous network of the nanotubular material supplies very good transport conditions for ions, and PPy enhances its conducting properties. A high ability of charge accumulation in such volumetric capacitor was obtained with capacitance values of 165 F/g of material.

Correlation of hydrogen capacity in carbon material with the parameters of electrosorption

AbstractElectrochemical storage of hydrogen in activated carbon material has been investigated using different parameters of cathodic polarization. It has been proven that application of short galvanostatic pulses could be efficient for hydrogen storage in microporous carbon material. Charging current loads from 50 mA g−1 to 32 A g−1 have been used showing correlation between hydrogen capacity, time of charging and electrical efficiency. The anodic charge equivalent to electrooxidation of 1.0 wt% of hydrogen can be already reached after 90 s of cathodic polarization. Temperature effect has been also evaluated and a gradual increase of hydrogen capacity with a better pronounced oxidation plateau was obtained at higher temperatures. Reversible electrosorption of hydrogen is a useful reaction in supercapacitor performance and it might have a potential application for a negative electrode of supercapacitor as well as reversibly operating electrode in the secondary cell.

Electrical capacitance of fibrous carbon composites in supercapacitors

The aim of this work was the application of active carbon composites as electrode material for supercapacitors. We have produced and investigated composites from viscose cellulose fibers impregnated with novolak and resolic resins. Composition and porous structure of the composites were described and electrochemical properties determined by galvanostatic and potentiodynamic methods. Dependence of electrical capacitance on treatment procedure and some of the structural parameters was confirmed. The use of novolak resin for activation with carbon dioxide was more advantageous. Positive electrode revealed better performance in acidic conditions (185 F/g) while negative electrode in alkaline conditions (160 F/g).

Electrochemical Application of Carbon Nanotubes

Different types of nanotubes have been used for storage of energy, e.g. lithium insertion and electrochemical capacitors. In both cases the electrochemical properties of nanotubes are dominated by their mesoporous character mainly due to the entanglement and a presence of a central canal. The most promising electrochemical application of carbon nanotubes seems to be their usage for supercapacitors. Capacitance values of pristine nanotubes varied widely from 10 to 80 F/g depending mainly on their microtexture, presence of central canal, amorphous carbon and catalyst impurities. Special modification has been performed to increase the capacitance values. For instance, a coating of nanotubes by conducting polymers, e.g. polypyrrole (PPy) supply pseudocapacitive effects connected with quick faradaic reactions. The open entangled network of the nanotubular composite seems to form a volumetric electrochemical capacitor where the charge has a threedimensional distribution. On the other hand, a significant increase of capacitance was reached by development of nanotubes specific surface area, i.e. micropore volume, through chemical KOH activation. Modified nanotubes and their nanocomposites with PPy supplied capacitance values of 80 to 170 F/g, hence, they are attractive electrode materials for supercapacitors.