Nanogenerators for energy harvesting: A holistic review of mechanisms, materials, and emerging applications

Continuously increasing demand for compact, decentralized and sustainable energy solutions has increased interest in nanogenerators, which offer promising avenues for harvesting ambient energy across diverse environments. This study presents an in-depth review of all major types of nanogenerators, i.e., piezoelectric, triboelectric, pyroelectric, thermoelectric, electromagnetic, and hybrid systems, emphasizing their working principles, materials, synthesis techniques, device architectures, and performances. Special attention is given to recent advancements in cutting-edge materials, including 2D materials, MXenes, conductive polymers, perovskites, biodegradable and biocompatible composites, and porous nanostructures, that have significantly enhanced energy conversion efficiency, flexibility, and multifunctionality. A critical comparison of fabrication methods, scalability, and durability is provided to guide future research. A wide range of applications of nanogenerators, encompassing implantable and wearable medical devices, human-machine interfaces, Internet of Things nodes, soft robotics, and autonomous sensor systems, are discussed. By systematically integrating insights from material science, device engineering, and applied technology, this review offers a perspective on the current status, challenges, and future potential of nanogenerators as next-generation self-powered systems.