No abstract

The ever-increasing energy demand and global warming caused by fossil fuels push for the exploration of sustainable and eco-friendly energy sources. Waste thermal energy has been considered as one of the promising candidates for sustainable power generation as it is abundantly available everywhere in our daily lives. Recently, thermo-electrochemical cells based on the temperature-dependent redox potential have been intensely studied for efficiently harnessing low-grade waste heat. Despite considerable progress in improving thermocell performance, no attempt was made to develop electrode materials from renewable precursors. In this work, we report the synthesis of a porous carbon electrode from mandarin peel waste through carbonization and activation processes. The influence of carbonization temperature and activating agent/carbon precursor ratio on the performance of thermocell was studied to optimize the microstructure and elemental composition of electrode materials. Due to its well-developed pore structure and nitrogen doping, the mandarin peel-derived electrodes carbonized at 800 °C delivered the maximum power density. The areal power density (P) of 193.4 mW m−2 and P/(ΔT)2 of 0.236 mW m−2 K−2 were achieved at ΔT of 28.6 K. However, KOH-activated electrodes showed no performance enhancement regardless of activating agent/carbon precursor ratio. The electrode material developed here worked well under different temperature differences, proving its feasibility in harvesting electrical energy from various types of waste heat sources.

Thermocells, also called thermogalvanic cells, are a promising technology that can efficiently harvest low-grade waste heat with direct thermoelectric conversion. The cells operate under a temperature difference, with one electrode on the hot side and the other on the cold side. The recently developed electrodes, such as porous carbon materials and pin-structured electrodes, have led to a temperature gradient even inside one of the electrodes. However, it still remains an open question of what temperature difference determines the open-circuit voltage of thermocells. Here, we investigated the effective voltage of a thermocell with thick electrodes. The temperature difference that determines the voltage turned out to be the smallest temperature difference between anode and cathode electrodes, the average temperature difference, or in between, depending on the internal resistances of the cell. We also verified the validity of normalized power density estimated from the open-circuit voltage. In addition, a strategy was demonstrated to improve the power density of a thermocell that consists of thick electrodes. The results provided here would help devise high-performance thermocells with optimized electrode structures.