Ion-Valence Engineering in Moisture-Enabled Electric Generator: Mechanisms, Materials, and Hybrid Systems

Moisture-enabled electric generator (MEG) represents an emerging paradigm for harvesting ubiquitous atmospheric humidity as a sustainable energy source. This review comprehensively analyzes the electrochemical mechanisms underpinning MEG, focusing on ion migration as the critical driver of power generation. Devices are systematically categorized by charge carriers, including protons, cations, and anions, and elucidate material design strategies that optimize ion transport through functional group gradients, heterojunctions, and nanofluidic confinement. Breakthroughs in graphene derivatives, biomaterials, metal-organic frameworks (MOFs), and active electrodes have enabled voltages >1.5 V and power densities >100 µW cm⁻², overcoming early limitations of low output and instability. Furthermore, synergistic integration with supercapacitors, batteries, and photonic systems mitigates intermittency and enhances energy autonomy. Despite progress, challenges persist in long-term gradient stability and concurrent high voltage/current delivery. Future directions are critically outlined, including multi-ion cooperative transport and bioinspired moisture harvesting, to position MEG as a viable solution for distributed micro-energy applications.