Krzysztof Fic

Carbon nanotubes and their composites in electrochemical applications

This paper presents various applications of carbon nanotubes as components of electrode materials for such electrochemical use as electrochemical capacitors, fuel cells, hydrogen electrosorption and accumulators. Generally, carbon nanotubes give exceptional improvement of electrode performance due to their mesoporous and well conducting networks. The cell resistance is drastically reduced and the transport of ions is greatly enhanced. In addition to their good conductivity, carbon nanotubes can be flexible and stretchable which is crucial for cyclability of electrodes, especially if volumetric changes of electrode material occur during operation. Consequently, they serve as excellent support for conducting polymers (e.g. polyaniline, polypyrrole) and metal oxides (e.g. MnO2) giving attractive capacitor electrodes. The presence of nanotubes in carbon precursor rich in heteroatoms, e.g. nitrogen from melamine or oxygen, also supplies an interesting carbonized composite with a good charge propagation for supercapacitor use. Furthermore, carbon nanotubes have been tested for hydrogen electrosorption giving a very moderate hydrogen capacity (0.27 wt.%). However, modification of nanotubes can enhance hydrogen storage. On the other hand, carbon nanotubes can serve as an excellent additive to many electrode materials for improvement of conductivity and cell performance. They could be a good support of the catalytic particles for fuel cell application as well.

Novel insight into neutral medium as electrolyte for high-voltage supercapacitors

This paper is focused on neutral aqueous medium, i.e.lithium, sodium and potassium sulfate solutions in a wide range of concentrations (0.1–2.5 mol L−1) as promising electrolytes for electrochemical capacitors because they are cheap, non-corrosive and allow applying diverse current collectors. These properties make the capacitor assembling process much easier and cheaper. Additionally, such electrolytes are electrochemically stable and environmentally friendly. Electrochemical investigations carried out especially for 1 mol L−1Li2SO4 aqueous solution confirmed the possibility of efficient capacitor work in a wider voltage range, i.e. even at 2.2 V without any significant capacitance fade during 15 000 cycles. The physicochemical properties of ions (i.e. solvation, diffusion or mobility) and their influence on the capacitor electrochemical behaviour are considered.

Electrochemistry Serving People and Nature: High‐Energy Ecocapacitors based on Redox‐Active Electrolytes

Positive Poles: A new type of electrochemical capacitor with two different aqueous solutions, separated by a Nafion membrane is described. High capacitance values as well as excellent energy/power characteristics are reported and discussed. The neutral character of the applied electrolytes makes this capacitor an environmentally friendly, easy to assemble, and cost-effective device for energy storage.

Unusual energy enhancement in carbon-based electrochemical capacitors

Electrochemical capacitors – also called supercapacitors – represent a relatively new electricity storage system applied for harvesting energy and delivering high power pulses for short periods. The main developed technology is based on charging an electrical double-layer (EDL) at the electrode–electrolyte interface of high surface area carbons. The main disadvantages of the latter are a relatively low energy density and safety issues related to the use of organic electrolytes. This paper describes alternative solutions where pseudocapacitive contributions play together with the EDL capacitance in a protic aqueous electrolyte, giving rise to a possible enhancement of energy density. It first reviews traditional solutions by using materials that are able to undergo fast faradic redox reactions, e.g. oxides and electrically conducting polymers (ECPs). Since the surface of materials essentially plays in this case, the realized capacitance values are much lower than theoretically expected; additionally the working voltage is generally lower than 1 V and the stability of some of these materials, e.g. ECPs, is relatively low. The realization of composites with carbon nanotubes which play as three-dimensional conductive supports contributes to the enhancement of performance of oxides and ECPs. A new source of pseudocapacitance involving a redox electrolyte is demonstrated by high surface area carbons. The latter is based on the interface formed by species with a large variety of oxidation states, e.g. iodine, bromine and vanadium, and gives rise to capacitance values as high as 250 F g−1 at 1 A g−1 for 1 mol L−1 KI aqueous solution. The last pseudocapacitive phenomenon involves electrochemical hydrogen storage in the negative electrode of carbon–carbon capacitors in neutral aqueous electrolytes, e.g. alkali sulphates. The voltage (and consequently the energy density) is considerably enhanced by comparison with systems in basic or acidic electrolytes owing to an important overpotential for di-hydrogen evolution at the negative electrode; besides a pseudocapacitive contribution related to hydrogen storage appears at the highest voltage values.

Ageing phenomena in high-voltage aqueous supercapacitors investigated by in situ gas analysis

High-voltage aqueous electrolyte based supercapacitors (U > 1.23 V) attract significant attention for next-generation high power, low cost and environmentally friendly energy storage applications. Cell ageing is however markedly pronounced at elevated voltages and results in accelerated overall performance fade and increased safety concerns. Online electrochemical mass spectrometry, combined with cell pressure analysis, is for the first time shown to provide a powerful means for in situ investigation of degradation mechanisms in aqueous electrolyte/carbon based supercapacitors. The activated carbon electrodes possess high specific surface area and oxygen-based surface functionalities (mainly phenol, lactone and anhydride groups), which are oxidized already at a cell voltage of 0.6 V to provoke the evolution of minor amounts of CO and CO2. Noticeable water decomposition starts at a high voltage of 1.6 V with the evolution of H2 on the negative electrode and carbon corrosion on the positive electrode with the generation of predominantly CO. In this paper we also report that short-term cycling leads to partly reversible gas evolution/consumption side-reactions giving negligible capacitance. On the other hand, long-term cycling causes irreversible side-reactions, deteriorates the electrochemical performance, and increases the internal pressure of the cell. Repeated cycling (U < 2 V) is confirmed as a more harmful technique for the electrode integrity compared to the voltage holding in a floating test. In situ gas analysis is shown to provide valuable insights into the electrochemical cell ageing aspects, such as the nature and potential onsets of side-reactions, hence paving the way for fundamental understanding and mitigating the performance and safety loss of high-energy aqueous supercapacitors.

Lithium rhenium(VII) oxide as a novel material for graphite pre-lithiation in high performance lithium-ion capacitors

The electrochemical reversibility of lithium extraction from lithium rhenium oxide (Li5ReO6, LReO) has been studied using standard 1 mol L−1 LiPF6 in EC/DMC electrolyte. An irreversible capacity of 410 mA h g−1 was observed below 4.3 V vs. Li/Li+ (close to the total theoretical capacity of 423 mA h g−1). Owing to this huge irreversible capacity, LReO could be included as a sacrificial material in the positive activated carbon electrode of a lithium-ion capacitor (LIC) to be used for pre-lithiating the graphite negative electrode. After the pre-lithiation step, the hybrid lithium-ion capacitor constituted of Li doped graphite and activated carbon as negative and positive electrodes, respectively, was cycled at current densities from 0.25 A g−1 to 0.65 A g−1. The LIC system demonstrated excellent capacitance stability up to 5000 cycles in the voltage range 2.2–4.1 V. The energy and power densities calculated per total mass of both electrodes reached 40 W h kg−1 and 0.5 kW kg−1, respectively. Hence, by using LReO, the prelithiation of graphite can be considerably simplified in comparison to traditional LIC systems, while enabling safer operating conditions owing to the absence of metallic lithium.

Continuous fast Fourier transform admittance voltammetry as a new approach for studying the change in morphology of polyaniline for supercapacitors application

In this study, continuous fast Fourier transform admittance voltammetry (CFFTAV) was used to study and characterize the surface morphology of polyaniline (PANI) on glassy carbon (GC) electrodes. Four polymer films with various thicknesses (0.5 μm to 11 μm) were synthesized by an electrochemical method. A new modified square wave voltammetry (SWV) method based on application of a discrete Fast Fourier Formation (FFT) method, background subtraction and two-dimensional integration of the electrode response over a selected potential range and time window was used. Moreover, the electrode response could be calculated by measuring the changes in SW voltammogram (or admittance). Results showed that by using the CFFTAV method, changes in the porosity of PANI and the behavior of PANI formation in H2SO4 solution were investigated more quantitatively than when using scanning electron microscopy (SEM), cyclic voltammetry (CV), charge–discharge (CD) and impedance spectroscopy (EIS) methods. By monitoring the electrode response, the kinetics for reaching steady state condition was studied. It was found that with the increasing thickness of polymer film from 0.5 μm to 11 μm, the accessible porosity decreased by up to three times. Furthermore, dimensions of nanochannels in the polymer film decreased with increasing the thickness. Moreover, maximum potential for ion insertion increased from 324 mV to 365 mV. Capability of electrodes for use as supercapacitor materials was tested by CV, CD and EIS, and the calculated capacity of electrodes was equal to 620 F g−1 and 247 F g−1 for thinnest and thickest polymer films respectively.

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.

Towards sustainable power sources: chitin-bound carbon electrodes for electrochemical capacitors

Chitin – a naturally occurring biopolymer – was employed for the first time as a binder for carbon electrodes and studied in electrochemical capacitors. Chitin-bound electrodes have shown excellent performance in neutral aqueous electrolytes up to 5 A g−1 current load with a capacitance retention of ca. 80% of the initial value for mild regimes. This study reports on the electrochemical behaviour of commercially available activated carbon (Supra 30 NORIT) bound with chitin (10% wt.) in the form of pellets, operating in two different aqueous electrolytes, i.e. 1 M Li2SO4 and 1 M KI solutions. It has been found that, for the 1 M Li2SO4 solution, the carbon electrodes demonstrate a moderate capacitance value of 65 F g−1 at 1 A g−1 current density. In 1 M KI solution merging electrical double-layer capacitance and faradaic contribution of the iodide/iodine redox couple, at the same current load, the capacitance was 175 F g−1 and it significantly increased with cycling to 260 F g−1 in the case of the chitin binder, and 300 F g−1 for the PTFE-bound electrodes taking into account the total charge supplied during capacitor discharging. Moreover, for the 1 M Li2SO4 solution, the chitin-bound electrodes display slightly better charge propagation than the PTFE-bound ones, whereas for the 1 M KI solution, the energy of the capacitor has been improved by 1 W h kg−1. Considering the rather negative impact of the commonly used binding fluoropolymers on the environment, chitin may become a great alternative for the development of cheap and environmentally benign electrochemical capacitors, while preserving their mechanical and electrochemical performance. Additionally, fluorine-based (e.g. PVDF or PTFE) electrodes are more hydrophobic and thus electrolyte penetration into the bulk of electrodes is unfavoured. It is noteworthy that the formation of a chitin complex with electrochemically generated iodine, which has a tendency to leave the system, may enhance the reversibility of the iodide/iodine redox couple and improve both the capacitance value as well as the cycle life.

Electrochemical performance of silicon nanostructures in low-temperature ionic liquids for microelectronic applications

Successful implementation of silicon nanostructures as suitable electrodes for microcapacitors performing at low temperatures (up to −40 °C) has been realized. In order to avoid the freezing of the electrolyte solution, two ionic liquids with low freezing points have been synthesized and applied as the electrolyte. Silicon nanotrees selected as the most useful electrode material have been combined with the mixture of two miscible ionic liquids (EMIM TFSI + AMIM TFSI). This allowed the operational voltage equal to 3 V to be achieved. The specific capacitance of 377 μF cm−2 resulted in 2 mJ cm−2 energy output. Furthermore, all electrode materials subjected to the investigation were able to deliver the energy even at high frequency (120 Hz). Hence, the application in microelectronics has been considered.