A Cost‐Effective Supercapacitor Material of Ultrahigh Specific Capacitances: Spinel Nickel Cobaltite Aerogels from an Epoxide‐Driven Sol–Gel Process

Abstract

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...

Toward Aerogel Electrodes of Superior Rate Performance in Supercapacitors through Engineered Hollow Nanoparticles of NiCo2O4

“…Particularly, transitional metal oxide (TMO) electrodes possess multiple oxidation states that enable rich redox reactions for excellent pseudocapacitance and thus have attracted wide research attention in recent years 4, 5, 6, 7, 8. As a typical TMO electrode material, spinel nickel cobaltite (NiCo 2 O 4 ) is attracting much attention due to its environmentally benign nature and high theoretical capacity (higher than 2000 F g −1 ) 9, 10.…”

“…Apparently, when the sweep rate is increased, the current density increases, and the position of anodic peak shifts positively to higher potential due to polarization but keeping similar curve shape,28 indicating that the fast and reversible redox reactions occur at the electrode/electrolyte interface 29, 30. Even at 50 mV s −1 scan rate (Figure 4a), the CV curve still shows a pair of redox peaks, since the carbon joints connected to the hollow NiCo 2 O 4 NPs are beneficial to hopping electrons and could accelerate redox reactions 4. To investigate the specific capacitance and the rate performance of the Ni‐Co‐12 aerogel, GCD measurements were performed in a 6.0 m KOH solution at different current densities.…”

Hierarchically nanostructured transition metal oxides for supercapacitors

“…Nickel cobaltite exhibits excellent electrochemical performance because of richer redox reaction between nickel and cobalt ions [118]. A variety of nickel cobaltite materials with different morphologies, such as NPs [119], nanowires [120,121], urchin-like shape [122], and hollow spheres [123], have been studied as electrode materials for SCs.…”

Hybrid system of nickel–cobalt hydroxide on carbonised natural cellulose materials for supercapacitors

“…Another method is the substitution of nickel by other metals to enhance conductivity; cobalt is usually used because it shares a similar crystal structure with Ni [17]. The co-deposition of Ni and Co can reach a high supercapacitance of 1920 F g −1 , consistently higher than the counterpart of Ni or Co modified alone [14,[17][18][19][20][21]. Intensive research has focussed on the structure of Ni-Co compounds.…”