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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]