Passive tuning phenomenon with a sliding mass on a vibrating beam has been observed and studied in the literature. Such a phenomenon can be extended to self-resonant energy harvesting, where the natural frequency can be favorably adjusted to the excitation frequency for enhanced energy harvesting. In this paper, we consider the nonlinear dynamic coupling of a piezoelectric clamped-clamped beam with sliding mass and study experimentally and numerically how these nonlinear interactions affect the performance of the energy harvester. We derive the mathematical model using the extended Hamilton principle. The governing equations of motion are obtained as three coupled nonlinear partial differential equations. The Galerkin method is employed to obtain a reduced order model. Our mathematical formulation is validated via experiments and the results show very good agreement between the simulation and the experiment. Parametric studies are carried out to examine how key parameters affect the performance of the energy harvester. The findings suggest that a passively tuned mechanism with a small sliding mass can increase the power output even when the excitation frequency is far off the original resonance.