Direct Conversion of Low-Grade Heat to Electricity Based on Transition Metal Coordination Complexes

Abstract

Thermo-electrochemical cells (or thermocells), which could convert thermal energy to electrical power, are promising for waste heat utilization. Steady and continuous electricity can be generated through thermocells as long as there is a temperature gradient. The commercialization of thermocells is presently limited by low power output, energy conversion efficiency and high capital investment.Liquid-based thermo-electrochemical cells have great potential for large-scale industrialization. Transition metal (Fe, Ni, Co, Ru, Cr etc.) redox pairs, as well as their coordination complexes, have shown excellent thermo-electrochemical properties. The corresponding electromotive force is linked to the entropy changes of the redox reactions. Further, the thermodynamic and kinetics of the redox reactions are significantly influenced by the type of coordinating ligands, solvents and electrodes materials. Therefore, the design of an efficient thermo-electrochemical cell with outstanding thermal-to-electrical power output requires careful design and consideration of all components within the cell.In this study, transition metal coordination redox couples have been investigated for thermo-electrochemical energy conversion. Iron and cobalt are screened out for their spin states and low cost. By tuning the chelating ligands of transition metals in various solvents, increased Seebeck coefficient and fast ion diffusion were obtained, resulting in significant improvement of the performances in cell voltage and power density. Figure 1