The development of high performance energy storage devices like rechargeable Li-ion batteries (LIBs) and supercapacitors (SCs) is becoming increasingly important to enable efficient utilization of clean and renewable energy sources, such as wind and solar energies. Graphene, a two-dimensional (2D) form of carbon atoms with a hexagonal lattice structure, has been considered one of the most desirable materials for application in LIBs and SCs due to its outstanding properties, especially the high electrical conductivity and large surface area. One of the most promising ways to utilize the inherently large surface area of graphene to the fullest extent is to construct a 3D hierarchical porous structure. Such a structure of graphene can help improve the mass and electron transport kinetics during the charge/discharge processes as well as provide opportunities for better design of electrode components in a 3D architecture. This work is dedicated to developing composite electrode materials made from two different types of 3D graphene, graphene aerogel (GA) and graphene foam (GF), for high performance LIBs and SCs.
Bi-functional nanocomposites consisting of ultrafine, cobalt carbonate (CC)-based nanoneedles and 3D porous GA (CC/GA) are in situ synthesized based on a one-step hydrothermal route followed by freeze drying. The optimized composite electrodes deliver excellent reversible capacity of 1150 mAhg-1
at 100 mAg-1
with an almost 100% Coulombic efficiency as anode in LIB which is among the best of similar composite anodes embedded in a graphene substrate. The stable multi-step electrochemical reactions of the CC compound as well as the low charge resistance in the GA conductive networks are responsible for such exceptional Li storage capability. Moreover, the modified composite electrodes with an optimized composition and microstructure also present a remarkable specific capacitance of 1134 F g-1
at 1 A g-1
as the SC electrode. This value is among the highest capacitances of cobalt compound electrodes with or without nanocarbons reported so far. The electrode also delivers exceptional rate performance and cyclic stability benefiting from the pseudocapacitive characteristics of CC-based active material and the highly conductive, interconnected 3D-structured GA. The ex situ XPS analysis identifies the reversible redox reactions of cobalt cations during charge/discharge cycles as the electrochemical mechanism responsible for the high pseudocapacitive properties of CC-based electrodes.
The applications of GF as another type of 3D graphene in LIBs and SCs are also studied in this work. High quality GFs produced by a CVD method are used as a conductive substrate to deposit MnO2
polypyrrole (PPy) core-shell arrays. The freestanding MnO2
-PPy/GF electrode exhibits a high reversible capacity of 945 mAh g-1
at 0.1 A g-1
after 150 cycles as well as excellent rate capability. Such rational assembly of an electroactive material in the core and a highly conductive polymer as the shell on a conductive GF substrate with a 3D structure offers a potential solution to designing novel MnO2
-based electrodes with much enhanced electrochemical performance. The in-situ TEM examination is also employed to investigate the conversion reaction taking place in α-MnO2
during lithiation without and in the presence of the PPy conductive coating.
Benefiting from the unique feature of GFs in designing freestanding electrodes, hierarchical NiCo2
arrays consisting of a mesoporous NiCo2
nanowire core and a cross-linked MnO2
nanosheet shell are grown on the GF substrate for SC application. The electrode exhibits an exceptional gravimetric specific capacitance of 2577 F g-1
at 1 A g-1
along with high capacitance retention of 94.3% after 5000 cycles. To test the electrode in real-world application, an asymmetric device with the NiCo2
/GF positive electrode and the CNT/GF negative electrode is assembled which delivers a maximum energy density of 55.1 W h kg-1
at a power density of 187.5 W kg-1
. The encouraging findings can be regarded as a general approach to assemble ternary hybrids for future design of active materials.
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