  Searching for performant multiferroic materials attracts general research
interests in energy science as they have been increasingly exploited as the
conversion media among thermal, electric, magnetic and mechanical energies by
using their temperature-dependent ferroic properties. Here we report a material
development strategy that guides us to discover a reversible phase-transforming
ferroelectric material exhibiting enduring energy harvesting from small
temperature differences. The material satisfies the crystallographic
compatibility condition between polar and nonpolar phases, which shows only
2.5C thermal hysteresis and high figure of merit. It stably generates 15uA
electricity in consecutive thermodynamic cycles in absence of any bias fields.
We demonstrate our device to consistently generate 6uA/cm2 current density near
100C over 540 complete phase transformation cycles without any electric and
functional degradation. The energy conversion device can light up a LED
directly without attaching an external power source. This promising material
candidate brings the low-grade waste heat harvesting closer to a practical
realization, e.g. small temperature fluctuations around the water boiling point
can be considered as a clean energy source.
