Synthesis and characterization of marula nut derived carbon and modified manganese fluorophosphates for electrochemical energy storage applications

Abstract

In this thesis, six (6) different electrode materials for supercapacitor applications were created and studied. Electrode materials are one of the most important components in supercapacitors. The increase in energy density requires require electrode materials that have high surface area, conductivity, redox properties. Hence, in this study, the electrochemical characteristics of nitrogen doped activated carbons (N-ACs) material made from marula nuts as a carbon source and melamine as a nitrogen source were studied in both symmetric and asymmetric capacitors. Again, potassium and lithium intercalation in manganese fluorophosphate (MFP) were studied in asymmetric capacitor. Ceria oxide (CeO2) was used to form a composite with MFP and the electrode material was studied in asymmetric capacitor. All of the synthetic materials' morphology, crystallinity, defects, elemental composition, and structural porosity were examined. Additionally, electrochemical analysis of all materials in different electrolytes were ran to obtain current response, charge and discharge time and charge transfer resistance using two (2) and three (3) electrode systems respectively. Firstly, marula nutshell waste, was used as a source of carbon to form activated carbons (ACs) doped with nitrogen. The high surface area and nitrogen present on the material resulted in energy density that is high (17.2 Wh/kg). Secondly, a novel chemical intercalation synthesis method based on potassium hydroxide-induced hydrothermal process was used to create alkali ion-containing triplite MFP. Research findings in the study indicate that d-spacing increase when intercalating potassium into the triplite structure. An asymmetric supercapacitor was fabricated using potassium intercalated material (K+/MFP) as a cathode and N-ACs as anode in alkaline and neutral electrolytes. Higher energy density (26.9 Wh/kg) was observed in 1 M KNO3 than 6 M KOH with 25.2 Wh/kg. Thirdly, assembly of asymmetric supercapacitor of N-ACs and Li+/MFP was done. The electrode material exhibited energy of 18.8 Wh/kg in 1 M LiFSTI with capacitance retention of 69.5% after 15 000 cycles. N-ACs and MFP-CeO2 was assembled in 1 M K2SO4. The material gave energy of 23.8 Wh/kg with specific power of 525.6 Wh/kg. The MFP-CeO2//N ACs had a good cycling stability of 69.8% after 15 000 cycles.