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International audience; Electrochemical capacitors (ECs), including electrical-double-layer capacitors and pseudocapacitors, feature high power densities but low energy densities. To improve the energy densities of ECs, redox electrolyte-enhanced ECs (R-ECs) or supercapbatteries are designed through employing confined soluble redox electrolytes and porous electrodes. In R-ECs the energy storage is based on diffusion-controlled faradaic processes of confined redox electrolytes at the surface of a porous electrode, which thus take the merits of high power densities of ECs and high energy densities of batteries. In the past few years, there has been great progress in the development of this energy storage technology, particularly in the design and synthesis of novel redox electrolytes and porous electrodes, as well as the configurations of new devices. Herein, a full-screen picture of the fundamentals and the state-of-art progress of R-ECs are given together with a discussion and outlines about the challenges and future perspectives of R-ECs. The strategies to improve the performance of R-ECs are highlighted from the aspects of their capacitances and capacitance retention, power densities, and energy densities. The insight into the philosophies behind these strategies will be favorable to promote the R-EC technology toward practical applications of supercapacitors in different fields.

Battery-like supercapacitors feature high power and energy densities as well as long-term capacitance retention. The utilized capacitor electrodes are thus better to have large surface areas, high conductivity, high stability, and importantly be of binder free. Herein, vertically aligned carbon nanofibers (CNFs) coated boron-doped diamonds (BDD) are employed as the capacitor electrodes to construct battery-like supercapacitors. Grown via a thermal chemical vapor deposition technique, these CNFs/BDD hybrid films are binder free and own porous structures, resulting in large surface areas. Meanwhile, the containment of graphene layers and copper metal catalysts inside CNFs/BDD leads to their high conductivity. Electric double layer capacitors (EDLCs) and pseudocapacitors (PCs) are then constructed in the inert electrolyte (1.0 M H2SO4 solution) and in the redox-active electrolyte (1.0 M Na2SO4 + 0.05 M Fe(CN)63-/4-), respectively. For assembled two-electrode symmetrical supercapacitor devices, the capacitances of EDLC and PC devices reach 30 and 48 mF cm-2 at 10 mV s-1, respectively. They remain constant even after 10 000 cycles. The power densities are 27.3 kW kg-1 and 25.3 kW kg-1 for EDLC and PC devices, together with their energy densities of 22.9 Wh kg-1 and 44.1 Wh kg-1, respectively. The performance of formed EDLC and PC devices is comparable to market-available batteries. Therefore, the vertically aligned CNFs/BDD hybrid film is a suitable capacitor electrode material to construct high-performance battery-like and industry-orientated supercapacitors for flexible power devices.