Studies on novel electrode materials for energy applications

Abstract

Conducting polymers (CPs) are attractive electrode materials for super capacitor applications owing to their low cost, ease of synthesis, and high pseudo capacitance. However, their practical utility is constrained by poor cyclic stability arising from structural degradation during repeated charge/discharge cycling, a narrow operating potential window in aqueous electrolytes, and the difficulty of tuning their physico chemical properties through functionalization. This thesis addresses these limitations through two complementary strategies: the covalent functionalization of polyaniline (PANI) with heteroatom-doped reduced graphene oxide, and the rational design of a new, readily functionalizable conjugated polymer family. In the first part of this work, PANI was covalently attached to nitrogen-doped reduced graphene oxide (N-mrGO) through an amide linkage. The N-mrGO acted as a soft template that stabilized PANI chain growth, reduced chain entanglement, and suppressed rGO sheet aggregation, while nitrogen doping improved conductivity and introduced additional redox activity. The optimized composite, N-mrGO-PANI5, exhibited a specific capacitance of 748.97 F g⁻¹ in a three-electrode configuration. A symmetric supercapacitor device fabricated using this material delivered high specific capacitance, an energy density of 34.07 Wh kg⁻¹, a power density of 800 W kg⁻¹, and retained 97.64% of its capacitance after 5000 charge/discharge cycles. Building on this, the system was further improved by co-doping the rGO with sulfur and nitrogen (SN-mrGO), introducing additional non-covalent interactions and pseudocapacitive contributions. The optimized SN-mrGO-PANI7.5 composite showed enhanced performance, and the corresponding symmetric device achieved a specific capacitance of 464 F g⁻¹ at 1 A g⁻¹, an energy density of 41.28 Wh kg⁻¹, a power density of 799.3 W kg⁻¹, and 99.35% capacitance retention after 5000 cycles, demonstrating that covalent heteroatom-doped rGO functionalization is an effective and robust strategy for improving PANI-based electrodes. In the second part of this work, a new family of conducting polymers based on a 4- (phenyldiazenyl)aniline (P4PDA) backbone was designed and synthesized via a simple, scalable one-pot polymerization in aqueous acidic medium. The azo linkage within the backbone functions as a proton sink and electron-withdrawing unit, shifting charge storage toward intrinsic backbone redox processes rather than interfacial water-driven Faradaic reactions. This design enabled a stable operating potential window of approximately 2.5 V in aqueous acidic electrolytes (1 M H₂SO₄, 1 M MSA), extending to 2.7 V in 1 M LiClO₄ — apotential window uncommon for conducting polymers in aqueous systems. Functional group versatility was demonstrated by introducing -OH, -OMe, -COOH, and -SH substituents (yielding POH, POME, PCOOH, and PSH, respectively, alongside the unsubstituted parent polymer PH) directly during synthesis, without multistep reactions or expensive reagents. Detailed structural, spectroscopic, thermal, and electrochemical characterization, supported by DFT calculations of the polymerization mechanism, established clear structure-property relationships linking substituent chemistry to charge-carrier behavior, conductivity, and electrochemical response. A preliminary symmetric supercapacitor device fabricated using PH demonstrated stable capacitive behavior across the full 2.5 V window. Additionally, the polymer family exhibited frequency-dependent dielectric behavior and absorption-dominated electromagnetic interference (EMI) shielding effectiveness, with performance strongly dependent on the substituent present. Collectively, this thesis demonstrates a clear progression from improving an established conducting polymer through composite engineering to the rational design of a new, inherently more stable and multifunctional conducting polymer platform, establishing design strategies relevant to next-generation aqueous energy storage and electromagnetic shielding materials.