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In this work, we have designed, characterized, and optimized an inertial-based rotational vibration energy harvester device which can effectively harvest energy from vibrations at ultra-low frequency (0.02 Hz). The device is mainly composed of a twist-driving system, a pawl-ratchet clutch system, and a magnet-coil transduction system. With the twist-driving system, the linear motion from vibration can be converted to high-speed rotation of the disk. The pawl-ratchet clutch system is designed to deliver and store the kinetic energy during vibration and keep rotating inertially during the interval between two impacts. The magnet-coil transduction system is applied to transfer the kinetic energy to electricity. The device can achieve a high maximum energy conversion efficiency of 97.2% with a single compression. A high power of 6 mW has been harvested when the device is excited by the continuous compression at low …