In this work, we investigate the fundamental effects contributing to energy storage enhancement in on-chip ferroelectric electrostatic supercapacitors with doped high-k dielectrics. By optimizing energy storage density and efficiency in nanometer-thin stacks of Si:HfO2 and Al2O3, we achieve energy storage density of 90 J/cm(3) with efficiencies up to 90%. We demonstrate for the first time that in such ferroelectric stacks, both negative capacitance and dipole switching contribute to energy density enhancement, with an enhancement of more than 30% when the negative capacitance regime is exploited. These findings lay the groundwork for the design and operation in the most appropriate regime of on-chip energy storage rechargeable devices based on ferroelectric stacks.