Controlled synthesis of manganese-based heterostructures for supercapacitor application
Tan, Hui Teng
Date of Issue2014
School of Materials Science and Engineering
The invention of SC fills the performance gap between the conventional capacitors and batteries in terms of energy and power densities. Since the scientific breakthrough in electrode material is an answer to the technological bottleneck of SCs, extensive research is recently focusing on the development of advanced functional materials in nanoscale. Although nanoengineering has been regarded as a powerful tool to tailor the intrinsic properties of the electrode materials, incompetence of single-phase electrode nanomaterials handicaps the SCs from realizing their full potential. Hence, the concept of “heterogeneous nanostructures” was implemented to compensate the intrinsic disadvantages of the single-phase nanomaterials via the synergistic effect in hope of making a great leap in electrochemical performances. Herein, this thesis explored the possibility of improving the energy densities of SCs without sacrificing their power density using nanosized manganese-based heterostructures (e.g. MnO, MnOOH, MnO2). The low electronic conductivity and dissolution problems of the MnO have hindered it from further electrochemical enhancement. In response to the aforementioned issues, monodispersed Cu-MnO@carbon nanostructures were synthesized via a facile solution-based method. The electronic conductivity is significantly improved in the presence of metallic Cu. Besides, the unique architecture of Cu-MnO@carbon heterostructures preserves excellent electrical integrity of the electrode via the conductive Cu “nanobridges” that provide efficient electron transfer pathway between the active materials and the current collector. On the other hand, the amorphous carbonaceous shell acts as a protective layer to attain structural integrity and prevent the dissolution of MnO. The incorporation of the carbon-based materials is proved to induce excellent cycling stability of the SCs. In this regard, successful attempt has been made on controlled synthesis of MnOOH NTs on highly conductive GF as flexible SC electrode using a hydrothermal method. As the freestanding electrode excludes the addition of binder, the ESR of the electrode is greatly reduced, giving rise to higher power density. With the NTs structure, the active surface area of the MnOOH can be completely accessible to electrolyte ions besides having shorter charge transport length and greater ability to withstand the structural deformation. Hence, the hollow-structured MnOOH shows great promise in electrochemical system which can be reflected by its high specific capacitance. On the other hand, less capacitance fading is experienced by the MnOOH NTs electrode due to the presence of GF scaffold that contributes to the electronic conductivity and mechanical support. To have further enhancement in energy density, the incorporation of pseudocapacitive materials with rationally designed architecture is of paramount important. In an attempt to explore the possibility of boosting the SC performance using the ternary composites with well-defined morphology, V2O5@MnO2/Fe2O3 core-shell NTs with tunable interior dimensions were synthesized via a facile aqueous-based method. In addition, functionalization of the V2O5@MnO2 NTs with other TMOs results in ternary composites with diverse composition, V2O5@MnO2/M NTs (M = Fe2O3, Co2O3/Co(OH)2, Ni(OH)2). The versatility of this synthetic protocol provides a platform to fabricate complex ternary composites in a more benign way. It is found that the rationally designed tubular morphology endows them with good permeability of electrolyte ions for maximum utilization of the electroactive sites. Hence, high specific capacitance is achieved accompanied by excellent cycling stability. Besides discussion on the fundamental reason that may make the engineered nano-sized manganese-based heterostructures viable candidates for SC application, evaluation of their practicability in device level will be covered as well. In this regard, the manganese-based heterostructures (positive electrodes) were assembled into pouch-type asymmetric SC (using CNTs as the negative electrodes). Preliminary result shows that their power and energy densities are comparable to those commercial products, indicating that they hold great promise as the energy storage device.