The present work explores in detail the effect of alkaline-basic electrolytes on the capacitance performance of biomass-derived carbonaceous materials used as electrodes in symmetric supercapacitors. The proof-of-concept is demonstrated by two commercial carbon products (YP-50F and YP-80F, Kuraray Europe GmbH, Vantaa, Finland), obtained from coconuts. The capacitance performance of YP-50F and YP-80F was evaluated in three types of basic electrolytes: 6 M LiOH, 6 M NaOH and 6 M KOH. It was found that the capacitance performance of YP-50F improved in the following order: NaOH < LiOH < KOH; Meanwhile, for YP-80F, the order changes to LiOH < NaOH < KOH. After 1000 cycles, the cycling stability of both YP-50F and YP-80F increased in the order NaOH < LiOH < KOH. This order of performance improvement is determined by both the electrolyte conductivity and the interaction between the functional groups of carbonaceous materials and alkaline electrolytes. The reactivity of the functional groups was assessed by postmortem SEM/EDS and X-ray photoelectron spectroscopy (XPS) analyses of the electrodes after prolonged cycling.

For the development and optimization of solid-state symmetrical supercapacitors, herein, we propose using carbon-based electrodes and sodium- and lithium-form Aquivion electrolyte membranes, which serve as the separator and electrolyte. Carbon xerogels, synthesized using microwave-assisted sol-gel methodology, with designed and controlled properties were obtained as electrode materials. Commercial activated carbon (YP-50F, “Kuraray Europe” GmbH) was used as the active material for comparison. Notably, the developed solid-state symmetrical supercapacitors provide sufficiently high specific capacitances of 105–110 F g⁻¹ at 0.2 A g⁻¹, along with an energy density of 4.5 Wh kg⁻¹ at 300 W kg⁻¹, and a voltage window of 0–1.2 V in aqueous environments, also demonstrating an excellent cycling stability for up to 10,000 charge/discharge cycles. These results can demonstrate the potential applications of carbon xerogel as the active electrode material and cation exchange membrane as the electrolyte in the development of solid-state supercapacitor devices.

The creation of supercapacitors with superior energy density and power capabilities is critical for advanced energy storage solutions. Ionic liquid electrolytes offer a promising alternative in this respect. However, improving their cycle stability and efficiency is a complex task requiring extensive research and significant effort. The high viscosity of ionic liquids (ILs) limits their lifetime, but this can be mitigated by increasing the temperature or adding solvents. In this research, the electrochemical performance of symmetric activated carbon supercapacitors with 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF₄) and different ratios of acetonitrile (ACN) as electrolytes were investigated. Long-term galvanostatic charge/discharge tests, impedance studies, and cyclic voltammetry were performed at temperatures between 24 to 60 °C. The addition of ACN to the ionic liquid increased electrochemical stability and reduced internal resistance, with the best performance observed at a 1:2 volume ratio of EMIMBF₄ to ACN. This supercapacitor exhibited 87% cyclic stability after 5000 charge/discharge cycles in the voltage range of 0.05–2.8 V and a current rate of 1 Ag⁻¹. It also achieved an energy density of 23 Whkg⁻¹ and a power density of 748 Wkg⁻¹. The supercapacitors were stable at elevated temperatures up to 60 °C, showing no degradation after operation under various thermal conditions.

This study aims to quantify the synergistic effect of Ni²⁺ and Mn²⁺ ions on the capacitive performance of oxide, hydroxide and phosphate electrodes in alkaline electrolytes. Three types of phases containing both nickel and manganese in a ratio of one-to-one were selected due to their stability in alkaline media: oxides with ilmenite and spinel structures (NiMnO₃ and Ni_1.5Mn_1.5O₄); hydroxides with layered structures (β-Ni_1/2Mn_1/2(OH)₂); and phosphates with olivine and maricite structures (LiNi_1/2Mn_1/2PO₄ and NaNi_1/2Mn_1/2PO₄). In the mixed hydroxides and phosphates, Ni²⁺ and Mn²⁺ ions randomly occupied one crystallographic site, whereas in the ilmenite oxide, a common face was shared by the Ni²⁺ and Mn⁴⁺ ions. The electrochemical parameters of the Ni–Mn compositions were evaluated in asymmetric hybrid supercapacitor cells working with alkaline electrolytes and activated carbon as a negative electrode. A comparative analysis of oxides, hydroxides and phosphates enabled us to differentiate the effects of nickel and manganese ions, structures and morphologies on their capacitive performance. Thus, the best performed electrode was predicted. The electrode composition should simultaneously contain Ni and Mn ions, and their morphologies should comprise spherical aggregates. This was an ilmenite NiMnO₃, which delivers high energy and power density (i.e., 65 W h kg⁻¹ at 3200 W kg⁻¹) and exhibits a good cycling stability (i.e., around 96% after 5000 cycles at a current load of 240 mA g⁻¹).

A MnO₂/AgNP nanocomposite was synthesized using a sonochemical method and investigated as an electrode material in a solid-state hybrid supercapacitor. Aquivion’s sodium and lithium electrolyte membrane serves as an electrolyte and separator. For comparison, MnO₂ was used as the active material. The developed supercapacitor containing a carbon xerogel as a negative electrode, the MnO₂/AgNP composite as a positive electrode and a Na⁺-exchange membrane demonstrated the highest performance characteristics. These results indicate that the incorporation of silver nanoparticles into the MnO₂ structure is a prospect for obtaining an active composite electrode material for solid-state supercapacitors.

Mechanically and thermally stable novel gel polymer electrolytes (GPEs) have been prepared and applied in supercapacitor cells. Quasi-solid and flexible films were prepared by solution casting technique and formulated by immobilization of ionic liquids (ILs) differing in their aggregate state. A crosslinking agent and a radical initiator were added to further stabilize them. The physicochemical characteristics of the obtained crosslinked films show that the realized cross-linked structure contributes to their improved mechanical and thermal stability, as well as an order of magnitude higher conductivity than that of the non-crosslinked ones. The obtained GPEs were electrochemically tested as separator in symmetric and hybrid supercapacitor cells and showed good and stable performance in the investigated systems. The crosslinked film is suitable for use as both separator and electrolyte and is promising for the development of high-temperature solid-state supercapacitors with improved capacitance characteristics.