Conductive wrapping greatly improves supercapacitor performance of graphene/MnO2 electrodes; applicable to a range of advanced battery electrode materials
Researchers at Stanford University led by Professors Yi Cui and Zhenan Bao have developed a “conductive wrapping” method that greatly improves the supercapacitor performance of hybrid graphene/MnO2 (GM)-based nanostructured electrodes. This approach, they conclude, is also applicable to a wide range of insulating energy storage electrode materials such as sulfur, LiMnPO4, and silicon in lithium-ion batteries.
In a paper published in the ACS journal Nano Letters, they report that by three-dimensional (3D) conductive wrapping of graphene/MnO2 nanostructures with carbon nanotubes or conducting polymer, specific capacitance of the electrodes (considering total mass of active materials) has substantially increased by 20% and 45%, respectively, with values as high as 380 F/g achieved. The ternary composite electrodes also exhibited excellent cycling performance with >95% capacitance retention over 3,000 cycles.
Supercapacitors offer a number of attractive attributes, including high power capability, long cycle lifetime, and fast charge and discharge rates. However, the energy storage density of existing supercapacitors is limited—generally an order of magnitude lower than that of batteries, according to the team. Thus, improving the energy density while maintaining the high power density and cycling stability for supercapacitor devices remains a primary research focus in the field.
Pseudocapacitive transition-metal oxides such as RuO2, NiO, and MnO2, have been studied extensively as active electrode materials for supercapacitors owing to their high energy density and large charge- transfer-reaction pseudocapacitance which is based on fast and reversible redox reactions at the electrode surface, resulting in much higher specific capacitance exceeding that of carbon-based materials using electric double layer charge storage.
Although MnO2 is considered to be the most attractive oxide material owing to high abundance of Mn, low cost, and environmental friendliness, the poor conductivity of MnO2 (10-5–10-6 S/cm) remains a major challenge and limits the rate capabilities for high power performance, thus hindering its wide applications in energy storage systems.
...To realize many practical applications that require large capacitance and high energy storage, the high mass loading of active MnO2 materials usually leads to the increased electrode resistance and the decreased specific capacitance, because MnO2 becomes densely packed with limited electrochemically active surface area, resulting in only a very thin top layer (up to a few hundreds of nanometers) of oxide nanomaterials participating in the charge storage process.
To solve these critical problems, we developed a “three- dimensional (3D) conductive wrapping” method to rationally design ternary systems based on graphene/MnO2/CNT (GMC) and graphene/MnO2/poly(3,4-ethylenedioxythiophene) poly- (styrenesulfonate) (PEDOT:PSS) (GMP) composites for high-performance electrochemical electrodes...An ultra-thin layer of single-walled CNTs (SWNTs) or conducting polymer that wraps around graphene/MnO2 three-dimensionally not only provides an additional electron transport path besides the graphene layer underneath MnO2 nanomaterials but actively participates in the charge storage process as both can contribute to the energy storage of the whole film via electric double layer capacitance or pseudocapacitance.
Such [a] 3D conductive wrapping approach would provide a promising design direction for optimizing the electrochemical performance of insulating metal-oxide based electrode materials and could be generally applicable to many promising but challenging energy storage electrode materials in which the electron transport limits the device performance.—Yu et al.
The researchers prepared GM textile electrodes using a two-step solution-based coating process they had recently developed; conductive wrapping to form a GMC system involved simply dipping the electrodes into a SWNT (single wall nanotube) ink solution and subsequently drying in a vacuum oven at 100 °C for 10 minutes.
Similarly, they prepared GMP-based electrodes by coating GM materials with a commercially available PEDOT:PSS solution via the “dip and dry” method.
Such a 3D conductive wrapping approach represents an effective and convenient technique to improve the specific capacitance and rate capability of oxide-based supercapacitors and can be applicable to a wide range of insulating energy storage electrode materials such as sulfur, LiMnPO4, and silicon in lithium-ion batteries. For example, conductive polymer can be used for wrapping sulfur cathode materials to enhance electrode conductivity and contain polysulfide intermediates, therefore minimizing polysulfide dissolution and improving the performance of Li-S batteries.—Yu et al.
Guihua Yu, Liangbing Hu, Nian Liu, Huiliang Wang, Michael Vosgueritchian, Yuan Yang, Yi Cui, and Zhenan Bao (2011) Enhancing the Supercapacitor Performance of Graphene/MnO2 Nanostructured Electrodes by Conductive Wrapping. Nano Letters DOI: 10.1021/nl2026635