In this work, a new poly(2,2′:5′,2′′-terthiophene) derivative with an electron-rich pendant group was designed using a side-chain engineering approach to prepare high performance redox-active electrode materials for electrochemical energy storage applications. The poly(2-((2-(4-(([2,2′:5′,2′′-terthiophen]-3′-ylmethylene)amino)phenoxy)ethyl)thio)ethanol) (PKAAN) was electrodeposited via a constant potential electrolysis method on stainless steel current collector substrates without any template material or polymeric binder and directly used as a redox-active electrode material. The non-substituted poly(2,2′:5′,2′′-terthiophene) (PTTh) conducting polymer derivative was also prepared to assess the effect of the side chain substituent on the morphological, mechanical and charge storage performance of the PKAAN redox-active electrode material. The morphological features of the PKAAN- and PTTh-based conducting polymer films were monitored by scanning electron microscopy (SEM). Cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques were employed to examine the electrochemical performances of the redox-active electrodes in both 3-electrode and 2-electrode cell configurations. The SEM evaluations revealed that the PKAAN electroactive film possesses a unique 3D and porous morphology offering more effective diffusion pathways for ion transportation during the charge/discharge cycles compared to the PTTh polymeric network. The PKAAN redox-active electrode material achieved superior electrochemical charge storage performance with a gravimetric specific capacitance of 369 F g⁻¹ at a constant current density of 2.5 mA cm⁻² at a potential of 1.85 V in single electrode measurements. Furthermore, the PKAAN electrode exhibited outstanding cycling durability and mechanical stability upon long-term cycling. The PKAAN redox-active electrode material retained 93% of its initial capacitance at the end of 12 500 repeating charge/discharge cycles. The solid-state symmetrical supercapacitor device assembled using PKAAN electrodes delivered a specific capacitance of 205 F g⁻¹, an energy density of 77.5 W h kg⁻¹ and a power density of 750 W kg⁻¹ in an operating voltage window of 1.85 V. In addition to its energy storage behavior, the supercapacitor device demonstrated excellent cycling stability with a capacitance decrease of only 7.5% after 12 500 repeated cycles. The results reveal that the side-chain engineering strategy can be used as a facile and effective way to design conducting polymer-based electrode materials with desired morphological, mechanical and electrochemical features for high-performance energy storage applications.