Growing concern over global warming is driving research into new materials for clean, alternative energy systems. Now, scientists led by Shih-Yuan Lu and Chi-Chang Hu at the National Tsing-Hua University in Taiwan1 have developed a low-cost method for manufacturing one of the most important next-generation devices for energy storage and management — supercapacitors.

Supercapacitors can rapidly and reversibly store large amounts of charge due to the electrical double layer mechanism. When voltage is applied to a mixture of a liquid electrolyte and a solid electrode, parallel layers of oppositely charged ions form along the electrode surface. Using electrodes with as high a surface area as possible, many such charge layers can be formed. The extraordinary power densities of supercapacitors have yielded numerous ‘green’ applications, such as in hybrid vehicles and for storage of wind or solar energy.

To boost supercapacitor storage even higher, scientists are turning to materials such as ruthenium oxide (RuO2). Because RuO2 can form multiple oxidation states — cations with different degrees of positive charge — the material has enormous capacitive capabilities. However, scarcity and high cost have prevented the commercial use of RuO2. Instead, Lu and Hu's team used a material called nickel cobaltite (NiCo2O4) for their supercapacitor electrodes — an environmentally friendly substance that combines the oxidation states of two metal ions to give a cost-effective alternative to RuO2.

Fig. 1: A current density–voltage curve for a nickel cobaltite aerogel with ultra-high specific capacitances up to 1400 F/g, superimposed over a transmission electron microscopy image of the aerogel.

To form NiCo2O4 into a high-surface-area electrode, the scientists created an ‘aerogel’. First, a solution of nickel and cobalt ions in ethanol was mixed with a reactive compound called an epoxide to give a metal–oxygen gel. The solvent was then removed by drying under supercritical carbon dioxide to produce an aerogel containing a three-dimensional network of nanoparticles and pores of 2–5 nm in size (Fig. 1). According to Lu, this pore size is optimal for transferring ions within the supercapacitor electrode.

After heating at 200 °C, the NiCo2O4 aerogel achieved extremely high capacitance nearly equal to that of RuO2, and was stable for over 2,000 charge/discharge cycles. The researchers are currently developing low-cost composite aerogels, which are expected to exhibit even higher capacitances because of their improved electronic conductivities.