Recent progress in Ni-PPy based supercapacitor energy storage: a comprehensive review on structural insights, fabrication, and electrochemical mechanisms

The rising global demand, coupled with the limitations of conventional resources and renewable energy inefficiencies, has accelerated the need for advanced energy storage solutions. Supercapacitors (SCs) have emerged as a promising alternative to traditional electrochemical batteries (BTs), offering long cycle life and rapid charge-discharge capabilities. Their performance likely depends on the choice and design of electrode material, where nickel (Ni)-based compounds and polypyrrole (PPy) have demonstrated significant potential in advancing SC technology. Ni-based materials, such as nickel oxides and hydroxides, provide excellent theoretical capacitance, are cost-effective and eco-friendly, yet face limitations in rate capability and conductivity. In contrast, PPy, known for its flexibility and ease of synthesis, is an alternative for excellent physicochemical properties and adaptability in flexible SCs. This review outlines the structure, properties, and synthesis of PPy that includes chemical oxidative, electrochemical polymerization, and others. Along with the discussion on energy storage mechanisms in SCs, types of SCs (PCs, EDLC, and HCs) and electrochemical analysis techniques (GCD, CV, EIS, and energy and power density). Furthermore, the research explores the development and electrochemical performance of Ni and Ni-PPy composite electrodes for SCs, highlighting their potential for innovative applications.