Development of hybrid carbon nanotube electrodes for supercapacitors
Supercapacitors are electrical energy storage devices based on electrochemical double layer capacitance (EDLC). Supercapacitors based on EDLC are electrical energy storage devices that store and release energy by nanoscopic charge separation at the electrochemical interface between an electrode and an electrolyte. As the energy stored is inversely proportional to the thickness of the double layer, these capacitors have an extremely high energy density compared to conventional dielectric capacitors. They are able to store a large amount of charge which can be delivered at much higher power ratings than rechargeable batteries. Supercapacitors are ideal for any application having a short load cycle and high reliability requirement, such as energy recapture sources including load cranes, forklifts, and electric vehicles. Other applications that exploit a supercapacitor’s ability to nearly instantaneously absorb and release power include power levelling for electric utilities and factory power backup. A bank of supercapacitors, for example, can bridge the short time duration between a power failure and the startup of backup power generators. While the energy density of supercapacitors is very high compared to conventional dielectric capacitors, it is still signiﬁcantly lower than batteries or fuel cells. Coupling with batteries (or another power source) is still required for supplying energy for longer periods of time. Thus, there is a strong interest to increase the energy density of supercapacitors to be closer to the energy density of batteries.
In order to increase the capacitance of supercapacitors it is necessary to increase the area of the electrodes.Randomly oriented graphitic carbon-based materials are usually used as current collectors. Recently a lot of investigations have been made using CNTs and graphene as current collectors. So far, the highest obtained values of specific capacitance have been up to 95 Fg−1 for carbon nanotubes and up to 135 Fg−1 for graphene in aqueous KOH and 117 F g−1 in H2SO4. However, it has been shown by Fan et al that higher values of specific capacitance can be obtained with a hybrid CNT/graphene nanostructure. Up to 385 Fg−1 at a scan rate of 10 mVs−1 in aqueous KOH was obtained in the case of CNT/graphene sandwich (CGS) structures with CNT pillars grown in between the graphene layers by CVD approach. A schematic of the nanostructure is shown in the following figure.
But the fabrication process for these nanostructures is very complicated and it is difficult to imagine its implementation into mass production for supercapacitors. In order to produce such CNT/Graphene nanostructures it is necessary to obtain graphene oxide (GO) by chemical exfoliation from natural graphite, convert GO into graphene by the thermal reduction of GO, to form catalyst particles on the surfaces of graphene sheets and synthesize CNTs on the catalyst particles in between the graphene layers. It then takes several weeks for the different chemical and thermo operations. Moreover, the nanostructures obtained by Fan et al were far from ideal as seen in the figure below.
However, BSUIR have developed a chemical vapour deposition (CVD) method for producing high quality ordered hybrid CNT/graphene nanostructures where the graphene layers are allocated on the top of the arrays of vertically allied CNTs. BSUIR’s production process for these nanostructures takes several minutes instead of weeks. Their simple, low-cost, one-step process uses a volatile catalyst and is fully compatible with microelectronics production technology. Based on these unique nanostructures, BSUIR’s CNT/graphene nanostacks demonstrate good electron conductivity, low diffusion resistance to protons/cations, easy electrolyte penetration, and high electroactiveareas. Consequently, they represent promising candidates for the fabrication of high performance supercapacitors with specific capacitance superior to those obtained by Fan et al.
In order to bring the technology a step closer towards application in the clean energy sector, BSUIR need Cleancarb to educate and train them on i) how best to integrate the electrodes into supercapacitor cells and ii) how to perform and analyse comparative vehicle based tests using supercapacitors from Maxwell and Nesscap with BSUIR’s hybrid CNT/graphene electrodes. This will enable BSUIR to have a deeper understanding of the behaviour of the electrodes under typical operating conditions and prove their high performance, reliability and durability.
3-, 4-, and 6-level CNT/PGL nanostacks produced by BSUIR [2,3]