Nanocarbon-based thermocells involving aqueous potassium ferro/ferricyanide electrolyte are investigated as an alternative to conventional thermoelectrics for thermal energy harvesting. The dependencies of power output on thermocell parameters, such as cell orientation, electrode size, electrode spacing, electrolyte concentration and temperature, are examined to provide practical design elements and principles. Observation of thermocell discharge behavior provides an understanding of the three primary internal resistances (i.e., activation, ohmic and mass transport overpotentials). The power output from nanocarbon thermocells is found to be mainly limited by the ohmic resistance of the electrolyte and restrictions on mass transport in the porous nanocarbon electrode due to pore tortuosity. Based on these fundamental studies, a comparison of power generation is conducted using various nanocarbon electrodes, including purified single-walled and multi-walled carbon nanotubes (P-SWNTs and P-MWNTs, respectively), unpurified SWNTs, reduced graphene oxide (RGO) and P-SWNT/RGO composite. The P-SWNT thermocell has the highest specific power generation per electrode weight (6.8 W/kg for a temperature difference of 20 °C), which is comparable to that for the P-MWNT electrode. The RGO thermocell electrode provides a substantially lower specific power generation (3.9 W/kg).