The waste thermal/heat energy harvesting can contribute to more sustainable and efficient energy systems. The high energy harvesting and storage are indispensable with considering ferroelectric/antiferroelectric materials. In this work, Pb0.99Nb0.02[(Zr0.57Sn0.43)(0.92)Ti-0.08](0.98)O-3(PNZST) is studied for high energy density (harvesting and storage). The Olsen cycle is performed on the materials for direct thermal/waste heat to electrical energy conversion. A large energy harvesting density of 1.0 MJ/m(3)per cycle was obtained at temperatures of 303 and 403 K, cycling the electric field between 1 and 9 kV/mm. An estimated high energy harvesting is a result of the polarization change due to the ferroelectric (rhombohedral) to antiferroelectric (tetragonal) phase transition. The high energy storage of 0.9 J/m(3)and energy efficiency of 81% were obtained at 403 K under an electric field of 9 kV/mm. The results will enrich our understanding of PNZST materials that offer high-performance energy harvesting and storage-based applications. This work can also be helpful in improving energy harvesting density via phase transition behaviors.