In this paper, we study underwater energy harvesting from bending vibrations of an annular ionic polymer metal composite (IPMC). The IPMC is clam-ped to a moving base, which harmonically excites the IPMC to vibrate along its fundamental axisymmetric mode shape. We model the IPMC as a thin plate, and the interaction with the surrounding fluid is described through a distributed hydrodynamic loading which is responsible for a marked reduction of the resonance spectrum through added mass effect. IPMC sensing is described though a lumped circuit model, comprising a resistor, a capacitor, a Warburg impedance, and a voltage source controlled by the mechanical curvature. In a series of experiments, the IPMC electrodes are shunted with a resistor, which is parametrically varied together with the excitation frequency to elucidate power harvesting. We also assess the effect of the IPMC geometry on power harvesting, by systematically varying the inner radius of the annulus to encompass five different configurations. Experimental results suggest that IPMC geometry has a primary role on its vibrations, whereby increasing the inner radius results into a significant increase of the resonance frequency. Such an increase does, in turn, produce a robust improvement in power harvesting that increases of two orders of magnitude as the ratio between the inner and the outer radius varies from 0.23 to 0.5.
