We demonstrate numerically an efficient vibrational energy harvester based on a triboelectric mechanism. The energy harvester consists of a clamped-clamped beam with center mass to enable the impact between the triboelectric layers subjected to external vibrations. The lower electrode is aluminum covered with a polydimethylsiloxane (PDMS) layer and the top electrode is an aluminum foil. Upon contact, electric charges are generated and alternative current flows between the upper and lower electrodes. We report the frequency bandwidth gets wider with a hardening behavior introduced by the impact nonlinearity in the structure. We then investigate the effect of the surface charge density on the output voltage, current, and power. The output voltage and power are as large as 1:73 V, 3 mW, respectively with 0:4 g vibrational amplitude and 30 mC=m2 surface charge density. The frequency bandwidth ranged between 5-18 Hz.