Integrating various devices to achieve high-performance energy storage systems to satisfy various demands in modern societies become more and more important. Electrical double-layer capacitors (EDLCs), one kind of the electrochemical capacitors, generally provide the merits of high charge-discharge rates, extremely long cycle life, and high efficiency in electricity capture/storage, leading to a desirable device of electricity management from portable electronics to hybrid vehicles or even smart grid application. However, the low cell voltage (2.5-2.7 V in organic liquid electrolytes) of EDLCs lacks the direct combination of Li-ion batteries (LIBs) and EDLCs for creating new functions in future applications without considering the issue of a relatively low energy density. Here we propose a guideline, "choosing a matching pair of electrode materials and electrolytes", to effectively extend the cell voltage of EDLCs according to three general strategies. Based on the new strategy proposed in this work, materials with an inert surface enable to tolerate a wider potential window in commercially available organic electrolytes in comparison with activated carbons (ACs). The binder-free, vertically grown graphene nanowalls (GNW) and nitrogen-doped GNW (NGNW) electrodes respectively provide good examples for extending the upper potential limit of a positive electrode of EDLCs from 0.1 to 1.5 V (vs Ag/AgNO3) as well as the lower potential limit of a negative electrode of EDLCs from -2.0 V to ca. -2.5 V in 1 M TEABF4/PC (propylene carbonate) compared to ACs. This newly designed asymmetric EDLC exhibits a cell voltage of 4 V, specific energy of 52 Wh kg(-1) (ca. a device energy density of 13 Wh kg(-1)), and specific power of 8 kW kg(-1) and ca. 100% retention after 10,000 cycles charge-discharge, reducing the series number of EDLCs to enlarge the module voltage and opening the possibility for directly combining EDLCs and LIBs in advanced applications.

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This work demonstrates a novel electrochemical procedure to enhance the specific capacitance (C S ) of activated carbon (AC) in organic electrolytes for the electrical double-layer capacitors (EDLCs). An irreversible oxidation process which provides pseudocapacitance occurs on AC when the electrode potentials are more positive than 0 V in the propylene carbonate (PC) electrolyte containing 1 M tetraethylammonium tetrafluoroborate (TEABF 4 ). This modification has been optimized by repeating 5 times of 100 chargedischarge cycles between −1.9 and 0.5 V in the fresh electrolyte. The surface of modified AC has been identified to contain N, B, and F, probably resulting from the repeated anchoring of electrolyte. An asymmetric supercapacitor (ASC) consisting of the above modified-AC positive electrode and an as-received AC negative electrode shows the specific energy and power of 18 Wh kg −1 and 6.5 kW kg −1 at 5 A g −1 with a cell voltage of 2.6 V. This ASC also shows excellent charge-discharge stability in TEABF 4 /PC from the 1.9% decay in the cell capacitance after 10,000-cycle stability test at 5 A g −1 between 0 and 2.6 V, revealing superior performances.

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