Low efficiencies and costly electrode materials have limited harvesting of thermal energy as electrical energy using thermo-electrochemical cells (or "thermocells"). We demonstrate thermocells, in practical configurations (from coin cells to cells that can be wrapped around exhaust pipes), that harvest low-grade thermal energy using relatively inexpensive carbon multiwalled nanotube (MWNT) electrodes. These electrodes provide high electrochemically accessible surface areas and fast redox-mediated electron transfer, which significantly enhances thermocell current generation capacity and overall efficiency. Thermocell efficiency is further improved by directly synthesizing MWNTs as vertical forests that reduce electrical and thermal resistance at electrode/substrate junctions. The efficiency of thermocells with MWNT electrodes is shown to be as high as 1.4% of Carnot efficiency, which is 3-fold higher than for previously demonstrated thermocells. With the cost of MWNTs decreasing, MWNT-based thermocells may become commercially viable for harvesting low-grade thermal energy.

Graphene, a two-dimensional (2-D) nanostructure of carbon, has attracted a great deal of attention since it was experimentally discovered in 2004. 1 Like carbon nanotubes, graphene sheets possess a high surface area to volume ratio and extraordinary electronic transport properties. 2 These properties make graphene very promising for many applications such as solar cells, sensors, batteries, supercapacitors, and hydrogen storage. 3,4 Carbon materials are widely used in lithium batteries, for example, disordered carbon, 5,6 hierarchically porous carbon monoliths, 7 and acid treated graphite. 8 The nanostructuring of electrode materials is a promising strategy to further improve the capacity of batteries. 9 Among various carbon nanostructures, carbon nanotubes (CNTs) have been widely studied as electrodes for lithium batteries since their unique structure should allow rapid insertion/removal of lithium ions. 10,11 Another active research direction in advanced batteries is to make batteries flexible, which could lead to important applications such as in wearable power sources.

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