Stanford team develops asymmetrical supercapacitors based on graphene hybrid materials; high energy and power densities
Supercapacitors in aqueous solutions are low-cost energy storage devices with high cycling stability and fast charging and discharging capabilities, but suffer from low energy densities. A team at Stanford University has addressed this challenge by fabricating asymmetrical supercapacitors using a RuO2/graphene hybrid material combined with a Ni(OH)2/graphene electrode.
The resulting supercapacitor showed a high energy density of ~48 Wh/kg at a power density of ~0.23 kW/kg, and a high power density of ~21 kW/kg at an energy density of ~14 Wh/kg, all with a 1.5V cell voltage operating in low-cost and safe (non-inflammable) 1M KOH aqueous solutions.
Such pairing up of metal-oxide/graphene and metal-hydroxide/graphene hybrid materials for asymmetrical supercapacitors represents a new approach to high performance energy storage, the team reports in a paper in the journal Nano Research.
The Ni(OH)2/graphene hybrid was synthesized by a two-step solution phase approach, resulting in single-crystalline hexagonal Ni(OH)2 nanoplates (thickness <10 nm) on chemically exfoliated graphene sheets that are ideal for high capacity supercapacitor applications.
The RuO2/graphene hybrid was also made in two steps, resulting in small RuO2 nanoparticles (< 10 nm diameter) selectively grown on graphene sheets.
There are two types of supercapacitors: electrical double layer (EDL) capacitors based on ion adsorption, and pseudocapacitors based on electrochemical redox reactions, the team notes in a paper.
The latter generally exhibits much higher specific capacitances than the former and has been used to build asymmetrical supercapacitors with improved energy and power densities. Nevertheless, the energy densities of supercapacitors operating in low cost and safe aqueous solutions remain low compared to batteries.Wang et al.
The new asymmetrical supercapacitor delivered higher energy and power densities than some of the best reported aqueous based supercapacitors including ones based on RuO2 (one of the highest energy capacity supercapacitor materials). This is the first time that nickel hydroxide and ruthenium oxide nanomaterials are paired up to produce supercapacitors with high energy and power densities, the Stanford researchers noted.
Hailiang Wang, Yongye Liang, Tissaphern Mirfakhrai, Zhuo Chen, Hernan Sanchez Casalongue, Hongjie Dai (2011) Advanced Asymmetrical Supercapacitors Based on Graphene Hybrid Materials. Nano Res doi: 10.1007/s12274-011-0129-6