Graphite stays solid at standard pressure up to 3900K (source: wikipedia https://en.wikipedia.org/wiki/Carbon https://en.wikipedia.org/wiki/Carbon) so you get ~

Graphite stays solid at standard pressure up to 3900K (source: wikipedia https://en.wikipedia.org/wiki/Carbon https://en.wikipedia.org/wiki/Carbon) so you get ~3600K of ΔT from standard temperature of ~300K before you have to deal with a liquid or gas. Coupled with being denser than water (but having lower specific heat than liquid or solid water), my napkin is telling me you get about 10x the heat capacity in J/m³ with graphite instead of water.