Sugar alcohol blends of galactitol and mannitol and compositions comprising such blends are disclosed as phase change materials (PCMs). A method of forming carbon nanotubes on a carbon substrate is described. Carbon substrates with carbon nanotubes, in particular, conformal layers of carbon nanotubes on carbon substrates, are also disclosed, as are methods of making and using these materials. Thermal storage units are also provided. The thermal storage units can comprise a heat exchange path through which a heat exchange medium flows, and a thermal storage medium in thermal contact with the heat exchange path.
STATEMENT OF GOVERNMENT SUPPORT
 This invention was made with government support under Grant No. DE-AR0000178 awarded by the Advanced Research Projects Agency Energy, a division of the US Department of Energy. The government has certain rights in the invention.