Carbon based composites for supercapacitor electrodes

Abstract

Nowadays, renewable energy is being developed to satisfy the increasing demand for energy all over the world. In addition, energy storage devices are being designed for energy to be stored more stably and for more convenient and efficient use. The supercapacitor, as one of the main storage devices, is applied in many different application fields, like electric vehicles and portable electronic products, due to their high power density and quick-charging properties. However, the low energy density of supercapacitors limits further applications, when compared with batteries. The electric properties of supercapacitors mainly rely on the characteristics of the electrode materials. This research work is aimed at designing carbon based materials with high energy density and long cycle life for supercapacitor electrodes. At present, there are three main materials which are used as electrode materials, carbon based materials, conducting polymers and transition metal oxides. However, these materials show certain flaws when they are fabricated as electrodes for supercapacitors. Carbon based material has high conductivity, but the specific capacitance is extremely low due to the ions only electrostatic adsorption and desorption. Conducting polymers can produce pseudocapacitance due to redox reactions, but their chemical stability is poor during the charging and discharging cycles. The specific capacitance of carbonaceous materials also can be improved by modifying metal oxides, but the high resistance of metal oxides restricts the charging rate. This work investigates the capacitive performance of carbon based composites in aqueous solution and its capacitive performance improvement by electrochemical approaches. In the first part, the research focuses on capacitive performance of composites of multiwalled carbon nanotubes (MWCNTs) and the conducting polymer, poly-3,4-ethlyenedioxythiophene-polystyrene sulfonate (PEDOT-PSS), in aqueous solution. It is found that specific capacitance decreases due to the degradation of PEDOT-PSS during continuous cycling. In this case, the MWCNTs/PEDOT-PSS composite just behaves as a double-layer capacitor without pseudo-capacitance from PEDOT-PSS when the specific capacitance (Csp) gets stable. In the second part, the Csp of the MWCNTs/PEDOT-PSS composite is improved by high dynamic potential treatment in dilute acids. The Csp of treated MWCNTs/PEDOT-PSS composites is ~2.5 times higher than that of untreated composites, and the drop of Csp in cyclability for the treated electrodes is 0.8%, and much less than that for the untreated electrodes (16%). In the third part, three-dimension composites fabricated from electrochemical reduced graphene oxides (ecrGO) and MWCNTs are developed and used as binder-free electrodes for supercapacitors in aqueous systems. MWCNTs, used as spacers, intercalate between graphene sheets so that the ions could diffuse to the surface of the graphene sheets more efficiently, thereby increasing the capacitance of the electrodes. The ecrGO/MWCNTs composite exhibits a high Csp of 177 F g⁻¹ after 1000 cycles of charge/discharge at a discharge current density of 1 A g⁻¹, with retention of 93% after 4000 cycles of charge/discharge at varying current densities. In the last part, MWCNTs decorated with silver nanoparticles (AgNPs, diameter of ~15 nm) and manganese oxides (MnOx) are designed and used for supercapacitors with PEDOT-PSS as a binder. The MWCNTs/AgNPs/MnOx electrode has a significantly high specific capacitance of 120 F g⁻¹, compared to the MWCNTs/AgNPs electrode or MWCNTs/MnOx electrode, but its specific capacitance decreases to 45 F g⁻¹ after 100 cycles of charging and discharging. The poor capacitive stability of electrodes decorated with silver might be the result of silver flaking from the electrode during the redox reaction between the Ag and Ag₂SO₄.