The ever worsening energy depletion and global warming issues call for not only urgent development of clean alternative energies and emission control of global warming gases, but also more advanced energy storage and management devices. Supercapacitors, offering transient but extremely high powers, are probably the most important next generation energy storage device. [1] To boost the specific capacitance of supercapacitors, the specific surface area of the electrode materials needs to be as high as possible to promote the electric double-layer capacitances and to accommodate a large amount of superficial electroactive species to participate in faradaic redox reactions. In addition, suitable pore sizes, 2-5 nm, of the porous electrode materials are critical to ease the mass transfer of electrolytes within the pores for fast redox reactions and double-layer charging/discharging. [2][3][4][5] Aerogels are a class of mesoporous materials possessing highly specific surface areas and porosities, [6] from which promising applications in a wide range of areas have been investigated. [7][8][9][10] They are composed of 3D networks of nanoparticles with an average pore size of several nanometers, adjustably falling within the optimal pore sizes of 2-5 nm. Consequently, aerogels are a promising candidate for supercapacitor applications.As to the electrode material, electroactive materials possessing multiple oxidation states/structures that enable rich redox reactions for pseudocapacitance generation are desirable for supercapacitors. Transition metal oxides are such a class of materials that have drawn extensive and intensive research attention in recent years. Among them, RuO 2 is the most prominent one with a specific capacitance as high as 1580 F g À1 , [11] probably the highest ever reported. The commercialization of RuO 2 based supercapacitors, however, is not promising because of the high cost and rareness of Ru. Spinel nickel cobaltite (NiCo 2 O 4 ) is a low-cost, environmentally friendly transition metal oxide, which has been employed in electrocatalytic water splitting (oxygen evolution) [12][13][14] and lithium ion batteries. [15,16] Its application in supercapacitors, however, received much less attention. [16,17] Nickel cobaltite has been reported to possess a much better electronic conductivity, at least two orders of magnitude higher, and higher electrochemical activity than those of nickel oxides and cobalt oxides.[18] It is expected to offer richer redox reactions, including contributions from both nickel and cobalt ions, than the two corresponding single component oxides and is a potential cost-effective alternative for RuO 2 .Based on the above considerations, one would expect nickel cobaltite aerogels, with anticipated good electronic conductivity, low diffusion resistance to protons/cations, easy electrolyte penetration, and high electroactive areas to be a promising candidate for the construction of next-generation, ultrahighperformance supercapacitors. Traditionally, aerogels are prepared with sol-gel...

Nickel cobaltite, a low cost and an environmentally friendly supercapacitive material, is deposited as a thin nanostructure of 3–5 nm nanocrystals into carbon aerogels, a mesoporous host template of high specific surface areas and high electric conductivities, with a two‐step wet chemistry process. This nickel cobaltite/carbon aerogel composite shows ultrahigh specific capacitances of around 1700 F g−1 at a scan rate of 25 mV s−1 within a potential window of −0.05 to 0.5 V in 1 M NaOH solutions. The composite also possesses an excellent high rate capability manifested by maintaining specific capacitances above 800 F g−1 at a high scan rate of 500 mV s−1, and an outstanding cycling stability demonstrated by a negligible 2.4% decay in specific capacitances after 2000 cycles. The success is attributable to the fuller utilization of nickel cobaltite for pseudocapacitance generation, made possible by the composite structure enabling well exposed nickel cobaltite to the electrolyte and easy transport of charge carriers, ions, and electrons, within the composite electrode.

Manganese oxide/carbon aerogel composite electrodes are prepared by a self‐limiting anodic‐electrochemical deposition of manganese oxide into a binder‐enriched carbon aerogel layer, drop‐cast on a graphite substrate, using 0.1 M Mn(CH3COO)2·4H2O as the electrolyte. Manganese oxide grows in the form of thin nanofibers along the backbone of the carbon aerogel, leaving adequate working space for the electrolyte and enabling a fuller extent of the utilization of the manganese oxide to make the composite an outstanding supercapacitor electrode material. The manganese oxide is determined to be Mn3O4 with the Raman spectroscopy and high‐resolution transmission electron microscopy. The rectangularity of the cyclic‐voltammogram loops of the composite electrode is excellent and remains that way for scan rates up to a very‐high value of 500 mV s−1, indicating extremely good redox reversibility and cycle efficiency. At a scan rate of 25 mV s−1, the specific capacitance, as measured in 0.5 M Na2SO4 for a potential window of 0.1–0.9 V vs. Ag/AgCl, reaches a maximum value of 503 F g−1 and experiences only a negligible decay of less than 1% at the 6000th cycle, implying an extraordinary cycling stability. The cycling efficiency is as high as 98% at a current density of 8 A g−1 cm−2, showing an almost‐ideal capacitive behavior. The power density reaches 48.5 kW kg−1 and the energy density 21.6 W h kg−1 at a scan rate of 500 mV s−1, well above the specifications of current state‐of‐the‐art supercapacitors.

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