Phil McKenna, writing for NOVA Next:
The incredible energy density of [phase-change materials] is a result of something known as the heat of fusion. In the case of melting ice, thermal energy is required to break hydrogen bonds between individual molecules. When water freezes, thermal energy is released as new bonds form.
And:
In recent years, research on PCMs has moved beyond water and ice to a vast number of new materials with a wide array of melting temperatures and applications. Some of the new materials will help keep us cool, while others will store energy at temperatures so high they would make us melt. Collectively, they will help us use existing energy supplies more efficiently, which may ultimately keep the entire planet from overheating.