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[Display omitted] • A copolymer hydrogel was fabricated to harvest oceanic energy. • Ionic concentration was main driving force for continuous electricity generation. • This device produced a power density of 89.4 µW cm−2 in real seawater. • The series connection of six devices could power commercial electronic devices. Micro/nano fluidic electricity generation systems based on solid–liquid interface interaction to harvest energy from water in nature offer a potential green and clean electricity generation method. However, the performances of electricity generation were limited in salt solutions. In this paper, we prepared an extraordinary-performance hydrogel through ultraviolet polymerization for the access of ion concentration energy in seawater. This copolymer hydrogel made of AAm/AA (acrylamide/acrylic acid) provides a negatively charged three-dimensional transport network with micro/nano porous channels, which exhibits extraordinary ion selectivity in high ion concentration solutions. The experimental results show that this electricity generation device with intrinsic micro/nano pores can generate a continuous open-circuit voltage of 0.37 V, a short-circuit current of 116 μA, and a power density of 89.4 µW/cm2 from seawater (the Yellow Sea in China). The output current and power can be maximally more than 100 times and 50 times higher than the existing results. Through the series connection of six devices, the overall output is sufficient to supply stable power to directly light up small commercial electronic devices. Our study presents a new research idea for the development of low-cost, environmentally friendly, and self-powered devices. [ABSTRACT FROM AUTHOR]