Freestanding 3D nanostructured composite material with improved capacitive performance

Smart-MEMPHIS partner Chalmers recently published results on improved materials for supercapacitors.

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Supercapacitor is an efficient energy storage device with wide applications such as in consumer electronics and electric vehicles. Depending on the working mechanism, supercapacitors can be categorized as electrical double layer (EDL) capacitors and pseudocapacitors. EDL capacitors primarily work on physical separation of charge, and advantage in good power capability and long cycle life, while the energy density is quite limited with pure carbon electrodes. On the other hand, pseudocapacitors store energy through faradic reactions and have much higher energy density than EDL capacitors, however, the power performance is usually restricted by the low electrical conductivity of pseudocapacitive electrodes (e.g. MnO2). It is challenging to achieve an electrode material which can deliver high energy output at high rate.

Recently, a pseudocapacitive composite material electrode combining the advantages of both EDL and pseudocapacitive materials was fabricated by Chalmers. This material features in deposition of MnO2 nanorods on both sides of nitrogen doped carbon nanofiber (NCNF) scaffold. The resulting material is a freestanding MnO2‖NCNF‖MnO2 3D nanostructure with a morphology inherited from the NCNF scaffold. NCNFs are highly conductive and thus provide fast electron transport. In addition, since the pseudocapacitive behavior occurs only at or near the electrode surface, the thin layer of MnO2 formed on the scaffold surface is used with high efficiency. As a consequence, this composite material shows much improved capacitive performance compared to both pure NCNF and MnO2 materials. At a very high current density of 15 A/g, the nanostructured composite can still deliver an impressive specific capacitance of 84.3 F/g and specific power density of 23.3 kW/kg (based on the overall material mass), which means that a capacitor containing 10 g of this material as electrode can be fully charged in 2 seconds and power a LED flashlight for more than 1 hour. This corresponds to approximately 80% of the specific capacitance obtained at a current density of 1 A/g.

The demonstration of enhanced pseudocapacitive capacitive performance of MnO2 through the rational design of a MnO2‖conductive scaffold‖MnO2 3D nanostructure can be extended to include other pseudocapacitive materials as well. This is a very beneficial concept for the development of robust pseudocapacitors capacitors with high energy density at high rate to achieve power performance.efficient energy storage systems for certain applications, e.g. electric vehicles and self-powered devices.

The results were presented at Nature Conference on Materials for Energy 2016 on June 13th in Wuhan, China.

SEM images of pure NCNF scaffold surface (a), NCNF/MnO2 surface (b), NCNF/MnO2 cross-section, and the electrochemical performance of NCNF/MnO2 composite material (d-f).

SEM images of pure NCNF scaffold surface (a), NCNF/MnO2 surface (b), NCNF/MnO2 cross-section, and the electrochemical performance of NCNF/MnO2 composite material (d-f).

Author(s):

Qi Li, Chalmers

Posted on September 7, 2016 .