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Graphical abstract The NA-LDH-OA-2//AC ASC device deliver a high energy density of 40.26 Wh kg−1 at a power density of 943 W kg−1, and good cycling performance. Highlights • The NA-LDH-OA ultrathin nanosheets were synthesized by two-phase solvothermal method. • Excellent cycling stability of 94.5% capacitance retention after 5000 cycles. • The asymmetric supercapacitor achieved a high energy density of 40.26 Wh kg−1 at a power density of 943 W kg−1. Abstract Using sodium oleate as surfactant and intercalating agent, a regulated Ni-Al layered double hydroxide (NA-LDH-OA) nanosheets with a high interlayer space were prepared by two-phase method combined with a short reflux process and mild solvothermal reaction. In the process of electrochemical reaction, the electron transport is accelerated by the higher base spacing of the NA-LDH-OA ultrathin nanosheets. The as-prepared material exhibited a high specific capacity (1.040 C cm−2 at a current density of 1.68 mA cm−2) and good cycling performance (capacity retention of 88.25% after 2000 cycles) in a three-electrode system. Moreover, an aqueous asymmetric supercapacitor (ASC) was assembled using the synthesized NA-LDH-OA nanosheets as the positive electrode and activated carbon (AC) as the negative electrode. The aqueous ASC device (NA-LDH-OA-2//AC) achieved a high energy density of 40.26 Wh kg−1 at a power density of 943 W kg−1, and a good cycling performance of 94.5% retention after 5000 cycles. These results demonstrated that the NA-LDH-OA nanosheets possess the potential for upcoming energy storage devices. [ABSTRACT FROM AUTHOR]

[Display omitted] • A new method for preparation of HA-K by alkali quenching of sludge was proposed. • Chlorination roasting and quenching can control the pore structure of carbon. • The specific capacitance of the prepared N, O porous carbon is 395F g−1. • This study provides a new idea for the harmless and resource utilization of sludge. Municipal sludge (MS) pyrolysis is often used to produce carbon materials for supercapacitors, however, the conventional heating process limits the improvement of its performance. In this paper, a facile method for the preparation of potassium humate (HA-K) by co-pyrolysis of MS and alkali was proposed for the first time, and then it was mixed with sodium chloride for roasting and quenching to obtain high performance HA-K carbon materials for supercapacitors. Compared with the electrode without chlorination roast quenching, the sample of HA-K-650-M−Q (the sample calcined at 650°C and quenched at 200°C) exhibited higher specific capacitance (395F g−1 at 1 A g−1) and enhanced energy density (32.28 Wh kg−1 at a power density of 2805 W kg−1). After 10,000 cycles at 10 A g−1, the capacitance retention rate reached 87%. This remarkable performance is due to its proper N- and O- self-doping and hierarchical porous geometry, which fundamentally promotes mass and charge transfer in the reaction. Therefore, this study provides a feasible method to solve the problem of sludge value-added and design suitable for the practical application of carbon materials. [ABSTRACT FROM AUTHOR]

• A universal approach for the preparation of hexagonal ring-like LDH was proposed. • A series of hexagonal ring-like LDH were synthesized by a universal NaOA-assisted TTS method. • The formation mechanism of nanoring was systematically studied. • As-prepared ring-like CoAlNi LDH exhibited enhanced supercapacitor performance than that of the plate-like LDHs. Hexagonal ring-like structure of ternary-component layered double hydroxide (LDH) could be promising materials for advanced energy storage and conversion systems. However, it is still a great challenge to present a universal and cost-effective approach to fabricate ternary-component LDH nanomaterials with ring-like micro-/nano-structure. In this work, a universal sodium oleate (NaOA)-assisted two-step two-phase solvothermal (TTS) method is developed to synthesize hexagonal ring-like structure of ternary-component LDH nanomaterials. This strategy is generic for synthesis of ring-like LDHs with different element components, such as CoAlNi, MgAlNi, and MgAlCo. The formation mechanism of nanorings constructed by TTS method is further systematically studied, which can be ascribed to the dissolution-recrystallization process driven by the solubility product of different reactant and Ostwald ripening effect. The ring-like CoAlNi LDH exhibited enhanced supercapacitor performance with a specific capacitance of 1339.6 F g−1 at 1 A g−1. Its asymmetric supercapacitor device exhibited high energy density (50.3 Wh kg−1 at a power density of 800 W kg−1) and stable cycling performance (about 82.2% capacitance retention over 3000 cycles). All of the electrochemical results of ring-like CoAlNi LDH are superior to that of pristine CoAlNi LDH nanosheets. The present work makes a significant contribution to the design and synthesis of ternary-component LDH with a unique hexagonal ring-like nano-architecture, as well as their potential applications in electrochemical energy storage. [ABSTRACT FROM AUTHOR]