In this study, we report for the first time a simple method that directly converts heat into electrical energy at sub-zero ambient temperatures. The thermo-electrochemical cell was constructed with negative temperature coefficient (NTC) carbon nanotube-vanadium oxide (CNT-VO (x) ) self-heating cathode, which provided thermal energy through an induced Joule effect. The electrical energy was obtained by creating in situ temperature difference between the electrodes (Delta T) and with subsequent redox reactions. A decrease in the cell resistance with an increase in the Delta T, and enhanced electrical energy conversion through a charge-transfer mechanism (i.e., Faradaic redox reaction) was observed. In addition, the advantage of using NTC CNT-VO (x) cathode as a self-heating source at various Delta T (i.e., without the support of any external source) in a thermo-electrochemical system for sub-zero temperature energy conversion is presented.