Preparation Of Supercapacitors Based On New Carbon Materials And Study On Their Energy Storage

Supercapacitor, the function of which ranges between that of common capacitor and chemical batteries, is aband-new energy storaged device. It features such perior qualities as larged is charge power, large capacitance of farad grede, higher energy, wider operating temperature range, longer service life, exemption from maintenance, economy, and environmental protection. The most common electrode materials in supercapacitors are carbon-based active materials. Among them, carbon nanotubes (CNTs) is one of the promising energy storage materials used in supercapacitors on account of their unique mesoporous network with high accessible surface area, excellent electrical conductivity and chemical stability. However, the pure CNTs have low double-layer capacitance, which are not up to the standard of the commercial application in supercapacitors. While, some metal oxides and electronically conducting polymer possess huge faradaic capacitance. But these pseudo-capacitive species alone are not ideal used as advanced capacitors materials because of their structural disability. Thus, surface modification of the CNTs with these electroactive species propose a facile solution to the capacitor limitation of the CNTs and the structural defects of these electroactive species. Among these pseudocapacitors, polyaniline (PANI) is a particularly promising polymer used in the supercapacitors due to its low cost, redox reversibility, easy synthesis and relatively high capacitance. Unfortunately, its low electronic conductivity and poor cycling stability hinder its wide application in the supercapacitors. Therefore, a combination of PANI with CNTs can improve dramatically the energy storage performance of supercapacitors based on the CNTs or PANI alone. Graphene has been considered as an ideal electrode materials for supercapacitors because of its extremely high theoretical surface area (2630 m2·g-1) and a superior theoretical capacitance (550 F-g-1). Recently, transition metal hexacyanoferrates (MHCF, M represents as Fe, Co, Ni, et al), a zeolite-like Prussian blue coordination compound, attract increasing interest in application as supercapacitors due to their unique structure, low cost, reversible redox ability and superior environmental benignancy. In order to improve the hybrids excellent energy storage capacity and cycling stability, using an in situ co-precipitation strategy to prepare different nanocubes supported on the CNTs and reduced graphene oxide (rGO). The main points were described as follows:(1) An easy and cost-effective approach was developed to deposit polyaniline (PANI) on a single graphitized multiwalled carbon nanotube (GMWCNT) grafted with poly(4-vinylpyridine) (P4VP) by in situ chemical polymerization.The PANI nanowires grow on the surface of GMWCNTs to shape as caterpillar with a core shell structure. In contrast, the surface is not modified GMWCNTs due to poor dispersion will polymerize into bundles in the water. For PANI/P4VP-g-GMWCNT hybrids, the maximum specific capacitance is 2130 F-g-1 at a discharge density of 1 A-g-1 in 0.5 M H2SO4 solution with a 92.2% capacitance retention over 1000 discharge/charge cycles, whereas PANI/GMWCNT hybrids have a specific capacitance up to 1510 F-g-1, which will decay by 20.6% under some condition. Thus, PANI/P4VP-g-GMWCNT hybrids exhibit an enhanced performance as compared to PANI/GMWCNT hybrids on specific capacitance, rate capability and cyclic stability, which is attributed to the unique caterpillar shape with core shell structure and the full coverage of PANI on the surface of GMWCNTs.(2) NiHCF can be uniformly deposited into the matrix of poly(4-vinylpyridine) grafted onto multiwalled carbon nanotubes (MWCNTs), and form coaxial NiHCF/MWCNT nanocables with MWCNTs as core and NiHCF as shell. A different NiHCF content was loaded onto MWCNTs, and their electrochemical behavior and capacitive performance were determined. The as-prepared NiHCF/P4VP-g-MWCNT hybrids can possess a huge specific capacitance up to 1035 F-g-1 at a discharge density of 0.05 A-g-1. These capacitance will decay by 10% at 25.6 A-g-1. They remain 92.7% of its initial capacitance at 2 A·g-1 after 10000 discharge/charge cycles. Their energy density can approach 56.2 Wh·kg-1 at a power density of 10 W·kg-1.(3) A facile co-precipitation strategy was developed to prepare nickel hexacyanoferrate nanocubes (NiHCF NBs) supported on the reduced graphene oxide (rGO) in the presence of poly(diallyldimethylammonium chloride) (PDDA). The NiHCF NBs are uniformly deposited on the rGO by electrostatic interaction. Their size can be tuned from 10 nm to 85 nm by changing their content from 32.6% to 68.2%. The specific capacitance of NiHCF/PDDA/rGO hybrids reaches up to 1320 F·g-1 at a discharge density of 0.2 A·g-1, more than twice that of the pure NiHCF, as well as slight capacitance decay by 15% at 0.2 A·g-1 and excellent cycling stability with 87.2% of its initial capacitance after 10000 discharge/charge cycles. More importantly, NiHCF/PDDA/rGO hybrids exhibit an ultrahigh energy density of 58.7 Wh·kg-1 at the power density of 80 W·kg-1. The superior storage energy performance of NiHCF/PDDA/rGO hybrids, such as high specific capacitance, good rate capacity and long cycling stability, positions them as a promising candidate for supercapacitor materials.