Development of electrodes & electrolytes for high performance and long life supercapacitors

Abstract

This study enhances supercapacitor technology through three primary advancements: First, it introduces a phosphoric acid-based liquid crystal gel electrolyte with enhanced specific capacitance. Second, it examines how water influences the lyotropic liquid crystalline (LLC) mesophase characteristics of C12E23-LiCl-H2O gel electrolytes, revealing their self-healing properties with optimized features for the first time. Lastly, it develops mesoporous Ni0.5Mn0.5Co2O4 electrodes for the first time utilizing the Molten Salt Assisted Assembly technique. These innovations address the shortcomings of traditional supercapacitors, paving the way for adaptable, high-efficiency energy storage solutions suitable for wearable electronics and electric vehicles. In the first part of the thesis study, a novel liquid crystal (LC) gel electrolyte by combining phosphoric acid (H3PO4, PA) and a non-ionic surfactant (NI), optimized at a PA:NI mole ratio of 80:1 (PA-NI80) was developed. A bicontinuous LC mesophase with a lattice distance of 3.9 nm was observed at all mole ratios between 60:1 and 100:1, as confirmed through X-ray diffraction and optical microscopy. The PA-NI80 LC gel achieved a specific capacitance (Cs) of 1128 F g⁻¹ at 0.1 A g⁻¹ using reduced graphene oxide (rGO) symmetric electrodes. Results highlight the pivotal role of the LC gel mesophase in achieving high electrolyte performance. The high viscosities and stable mesophase structures make the PA-NI LC gels suitable for use in flexible and high-performance supercapacitors. The mesophase structure facilitated controlled ion transport via the Grotthuss mechanism, where hydrogen hopping through hydrogen bonds contributed to the gel's high ionic conductivity and superior electrochemical performance. In the second part of this thesis, we systematically investigated the role of water in shaping the lyotropic LC mesophase properties of C12E23-LiCl-H2O gel electrolytes, with a focus on their mechanical and electrochemical performance in supercapacitors. XRD and POM data confirmed the formation of a cubic mesophase at all water concentrations. Rheological analysis suggested quantitative information about gel strength, gelation point, and structural recovery changed with water content and LiCl concentration. The storage modulus increased with decreasing water content, attributed to variations in the quantity and average size of junction points due to system entanglement. The study revealed that excess water molecules break down micellar connections, weakening the gel. Conversely, at low water concentrations, the micellar domains entangle, displaying viscoelastic behavior similar to that of a transitory polymer network. The gel structures exhibited self-healing properties, attributed to their shear-thinning behavior. This property allows the gels to flow under shear stress and self-recover once the stress is removed. By varying water content while maintaining a 5.63:1 LiCl-to-C12E23 molar ratio, we observed that reduced water content significantly enhanced gel rigidity, as evidenced by an increase in storage modulus (G') from 82 Pa for LC1.125(H2O) to 113 Pa for LC0.875(H2O). Structural analyses using XRD revealed cubic mesophases with a unit cell parameter of 13.2 nm across all formulations. Rheological measurements highlighted the gels shear-thinning behavior and self-healing capability, with LC1.125(H2O) achieving full recovery after strain-induced rupture. Electrochemical tests further demonstrated that LC1.125(H2O) displayed a specific capacitance of 316 F·g⁻¹ at 0.25 A·g⁻¹ and retained 80% capacity over 1500 cycles, indicating its suitability for robust energy storage systems. In the last part of this study, to complement the electrolyte, we synthesized mesoporous Ni0.5Mn0.5Co2O4 electrodes using the Molten Salt Assisted Assembly (MASA) method, marking the first application of MASA for ternary metal oxides. This approach enabled the creation of nanoscale active sites and ensured the formation of mesoporous Ni0.5Mn0.5Co2O4, NiCo2O4, and MnCo2O4 with relatively high surface areas. By simply altering the salt type and composition in the initial clear solutions used to prepare the LLC mesophases and applying calcination, MASA facilitated precise control over the electrode material properties. The MASA method allowed for the synthesis of mesoporous Ni0.5Mn0.5Co2O4 with high surface area and enhanced redox behavior. The method involves using metal nitrate salts precursors assembled with non-ionic and ionic surfactants in their lyotropic liquid crystalline phase, followed by high-temperature calcination. Among the synthesized materials, the Ni0.5Mn0.5Co2O4 electrode showed the highest specific capacitance (11.51 F·cm⁻²) and exhibited superior electrochemical activity, with the lowest charge transfer resistance compared to NiCo2O4 and MnCo2O4. The asymmetric supercapacitor, assembled with Ni0.5Mn0.5Co2O4 utilizing the positive electrode and employing activated carbon for the negative electrode, achieved an energy density of 79.52 Wh kg⁻¹. The device exhibited excellent performance, with a low ohmic resistance of 2.348 Ω and a charge transfer resistance of 0.672 Ω. Additionally, it maintained high capacitance even at elevated current densities. The study emphasizes Ni0.5Mn0.5Co2O4, crafted using the MASA technique, as a highly efficient material for asymmetric energy storage devices, attributed to its remarkable specific capacitance, low resistance, and excellent energy density.