Rapid advancements in modern electronics have been starved of further breakthroughs to achieve high-energy, large-power, and long-running energy storage devices. Carbon-based supercapacitors (CSs) are promising large-power systems that can store electrical energy at the interface between the carbonaceous electrode surface and adsorbed electrolyte layer. However, commercial CSs using activated carbons suffer from limited energy densities in the organic electrolytes owing to sluggish mass diffusion and restricted charge accumulation. To overcome these issues, significant efforts have been devoted toward increasing the energy storage of CSs by the exploration of both large-capacitance electrodes and high-potential electrolytes. This paper reviews the recent advances made in the two core components (i.e., electrodes and electrolytes) of CSs. Firstly, we describe the involved energy storage mechanisms of CSs, followed by a brief overview of the key factors affecting the electrochemical performances. Then, novel design concepts have been summarized that can be used to fabricate carbon-based electrodes, such as microporous carbons, mesoporous carbons, hierarchical porous carbons, functionalized carbons, and carbon composites. Further, tailoring of the geometrical morphologies, pore structures, and surface functionalities can be discussed in pursuit of large-capacitance electrodes. Furthermore, various types of aqueous and nonaqueous electrolytes, such as water-in-salt, organic media, ionic liquids, and (quasi-)solid-state electrolytes, have been systematically investigated to settle the water hydrolysis concern of traditional electrolytes. Finally, the challenges in maximizing the synergistic effect between the electrodes and electrolytes are summarized, and we have proposed the future outlook for the development of advanced CSs.

Quinones are attractive redox-active starting materials due to their excellent electron transfer kinetics and outstanding redox reversibility, while amines featuring high electron-pair donicity reinforce the redox behavior of quinones and simultaneously introduce redox groups themselves. Herein, a novel quinone-amine route is designed to fabricate N, O codoped porous carbon nanospheres (PCNs) by direct carbonization–activation of a quinone-amine polymer which serves as an “all-in-one” precursor for the carbon source and self-dopant. The synthesis features simplicity, high efficiency and scale-up potential, based on the polymerization of p-benzoquinone and 3,3′-diaminobenzidine in ethanol under mild conditions, without any template, complicated technique or tedious procedure. PCNs integrate the superiorities of uniform spherical geometry, superior pore structure, and ultrahigh surface area and N/O content. A typical PCN electrode exhibits a high capacitance (376 F g⁻¹ at 0.5 A g⁻¹ in a three-electrode system) in KOH electrolyte. The assembled supercapacitor based on the PCN electrode and KOH electrolyte delivers a capacitance of 72.5 F g⁻¹ and an energy output of 6.5 W h kg⁻¹. More importantly, a constructed symmetric supercapacitor using an ionic liquid gel electrolyte gives an ultrahigh energy density up to 65.8 W h kg⁻¹, along with superb cycling stability. This study presents a more convenient and competitive route to design functionalized carbons for highly efficient energy storage.

Significantly boosting the energy densities of supercapacitors without compromising their power densities is of paramount importance for practical applications, but still faces great challenges. Herein, we report an ultrahigh-energy-density solid-state supercapacitor enabled by synergistical design of a N, O co-doped honeycomb porous carbon (HPC) electrode and an ionogel electrolyte. HPC is synthesized through the co-assembly of melamine/formaldehyde with silica spheres, and shows an ultrahigh surface area (2379 m² g⁻¹) coupled with a 3D interconnected macro-, meso- and microporous structure, and high-level redox-active N/O dopants (6.90 and 10.17 wt%). Benefiting from such merits, the HPC electrode yields an extremely high capacitance of 533 F g⁻¹ at 0.5 A g⁻¹ in an alkaline electrolyte, together with superior cycling stability with 92.1% capacitance retention after 20 000 cycles at 5 A g⁻¹. HPC assembled supercapacitors deliver energy outputs of 12.8 and 26.6 W h kg⁻¹ using KOH and Na₂SO₄ electrolytes, respectively. More attractively, a HPC-fabricated all-solid-state symmetric device based on the use of a well-designed, polymer-gel supported ionic liquid electrolyte achieves an ultrahigh energy density of 94.1 W h kg⁻¹, which is the highest value among those of previously reported supercapacitors of the same type, and an excellent cycling stability (91.5% retention over 10 000 cycles). This study highlights promising prospects of developing solid-state energy storage systems of high energy-power supply.