## 1 Introduction  
The generation and storage of clean and renewable energy has recently been one of the key, yet rather complicated, research topics [1; 2; 3]. High demand for green energy generation and its efficient storage is driven by the scarcity of conventional energy sources and the ongoing environmental degradation. While numerous types of renewable energy such as solar, wind and hydropower, are perceived as the new generation of energy sources, electrochemical energy storage, i.e., batteries and supercapacitors are predominant in the energy transmission and storage solutions segments [4; 5; 6]. Currently, such devices are being implemented and used in a wide range of industries, from small portable electronics to military and aerospace applications. However, the development of new types and improvement of existing energy storage devices has not passed unnoticed in the scientific circles in various countries. Among numerous scientific studies, attention should be paid to the key significance of electrochemical devices, which are either called supercapacitors or electrochemical capacitors with a double electrical layer, or ultracapacitors. These are energy storage systems that can be used on their own or in conjunction with modern electrochemical batteries to improve the overall efficiency of the power system, to minimize disturbances to the power grid, as electricity buffers in electric vehicles (efficient collection and return of braking energy and fast charging energy buffering), energy buffers for renewable energy sources, etc. [8; 9; 10; 11; 12]. It should be added that the combination of batteries and supercapacitors makes it possible to release a large volume of electric energy rapidly, leading to very high specific power values [13; 14]. The supercapacitors have specific power almost five times higher than the corresponding value for conventional lead-acid batteries [10]. Therefore, if high power consumption is required by any system, supercapacitors are superior to the batteries. In addition, supercapacitors can be incorporated into a hybrid energy storage system along with a battery, which will lead to a reduction in the size of such a system while meeting energy and power requirements, as well as to an enhancement of the battery life of such a power system [15]. In systems that include some portable electronic devices, supercapacitors can be used as the primary power source, as they can be recharged quickly. It is also important that many supercapacitors can operate over a wide temperature range (\(-40\) to \(+85\)\({}^{\circ}\)C), while conventional batteries are typically used over a narrower temperature range [16]. Since the electrode materials used in supercapacitors are subject to marginally small phase and/or chemical changes during charge-discharge processes, it is believed that supercapacitors can exhibit very high cycling (lifetime) [13]. The activated carbon (AC) materials are the most widely used materials for making the electrodes of supercapacitors with both aqueous and non-aqueous electrolytes [1,17]. This is due to their well-developed porous structure, good electrical conductivity, low production costs and, above all, they are environmentally friendly. The operational principle of supercapacitors made of AC materials is based on the principle of movement of ions under the influence of an applied electric field (Figure 1). At first, the ions are chaotically scattered in the electrolyte. Under the influence of a potential difference applied to the supercapacitor plates, the ions begin to move towards the electrodes--the cations move towards the cathode and the anions towards the anode. The ions enter the porous AC structure and reach the carbon surface. At this time instant, the surface of the carbon becomes charged with the opposite sign. In this manner, a double electrical layer is formed on the AC material surface of each electrode. Commonly used raw materials for AC material manufacturing are wood, coal, oil derivative waste, peat and polymers.