According to the US Environmental Protection Agency, 79% of US greenhouse gas emissions in 2010 were due to the burning of fossil fuels. Researchers are actively seeking alternative energy-conversion systems such as supercapacitors that can bridge the gap between conventional capacitors and rechargeable batteries. Supercapacitors can charge and discharge energy quickly, but they cannot store much energy. They also wear out fast with repeated use, as the materials inside them break down with the constant flow of charge in and out. This is a significant drawback when they are used in devices with long lifetimes, such as hybrid cars.
As reported in the April 19 online edition of Advanced Materials (DOI: 10.1002/adma.201400054), Renzhi Ma and colleagues from the National Institute for Materials Science in Japan have succeeded in preparing superlattice nanocomposites for use in supercapacitors with both high capacity and high power rates. The nanocomposites were prepared by electrostatic hetero-stacking of Co-Al or Co-Ni layered double hydroxide (LDH) nanosheets with graphene oxide nanosheets (i.e., the LDH nanosheets were sandwiched between each other in an alternating sequence on a molecular scale).
(a) X ray diffraction patterns of layered double hydroxide (LDH) nanosheets and rGO (red trace) nanosheets. Indices 001 are basal series of superlattice lamellar composites whereas L 100 and L 110 are in-plane diffraction peaks from LDH nanosheets. (Inset) Schematic illustration of sandwiched LDH nanosheets and graphene. (b) Comparison of typical charge-discharge CD curves; (top) rGO nanosheets and (bottom) nanocomposites of Co-Ni LDH and rGO nanosheet. Reproduced with permission from Adv. Mater. (2014) DOI: 10.1002/adma.201400054. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
X-ray diffraction measurements (see Figure) show that the heteroassembly of LDH nanosheets with GO nanosheets generally produce a basal spacing of ca. 1.2 nm (black trace). Furthermore, the gallery spacing of the lamellar composites could be tuned by modifying the surface charge, and thus the thickness of the GO nanosheets. This is evident in the heteroassembly of LDH with reduced GO (rGO) nanosheets. As demonstrated by the red trace in the figure, a basal spacing of 0.9 nm was obtained, which is consistent with the thickness sum of the LDH nanosheets (0.48 nm) and rGO (0.4 nm). This makes it is easy to modify the interlayer environment and contents of the heteroassembled composites by adjusting the charge density of the GO/rGO nanosheets. Thus, structures can be designed that exhibit both high ion diffusion and electron transport efficiency.
Additional work was conducted to explore the possibility of using this nanomaterial to increase the performance of supercapacitors. Graphene electrodes with an electric double layer (EDL) in supercapacitors have shown an increase in capacitance of ca. 100 farads per gram. More interesting, the overall capacitance was significantly enhanced by the hybridization of conductive graphene oxide nanosheets and the LDH nanosheets. This yielded a high capacitance of up to ca. 650 farads per gram, approximately six times that of pure graphene nanosheets. The direct combination of graphene with the insulating LDH nanosheets thus resulted in an improvement in the charge-transfer efficiency. Such a superfast charging and discharging performance potentially enables a huge energy output within a very short subsecond time scale.
According to the researchers, this work would be of benefit to applications in most electronic device and hybrid cars. Additionally, Ma said they expect that this three-dimensional transition metal/graphene hybrid approach to be effective in developing non-noble metal electrocatalysis for applications such as fuel cells.
