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.

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.

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.

This paper reports the electrochemical behaviour of supercapacitor carbon electrodes operating in different aqueous solutions modified by various redox-active species (hydroxybenzenes, bromine derivatives and iodide). Three dihydroxybenzenes with varying stereochemistry, i.e., –OH substitution, have been considered as electrolyte additives (0.38 mol L⁻¹) in acidic, alkaline and neutral solutions. High capacitance values have been obtained, especially for the acidic and alkaline solutions containing 1,4-dihydroxybenzene (hydroquinone). Bromine derivatives of dihydroxybenzenes were also considered as the additive in alkaline solution for use as a supercapacitor electrolyte, and a significant increase in capacitance value was observed. The redox couple investigated next was an iodide/iodine system, where 2 mol L⁻¹ NaI aqueous electrolyte was utilized. In this case, the most promising faradaic contribution during capacitor operation was achieved. In particular, stable capacitance values from 300–400 F g⁻¹ have been confirmed by long-term galvanostatic cycling (over 100 000 cycles), cycling voltammetry and floating. The mechanism of pseudocapacitance phenomena was discussed and supported by electrochemical and physicochemical measurements, e.g., in situ Raman spectroscopy.