Lithium Manganese Oxide in an Aqueous Electrochemical System for Low-Grade Thermal Energy Harvesting

Abstract

Low-grade heat (<100 °C) is abundant but mostly wasted because its utilization requires efficient energy harvesting systems with low cost and high efficiency. The thermally regenerative electrochemical cycle is a promising strategy to harvest low-grade heat, which exploits the dependence of electrochemical potential on temperature. In each cycle between low and high temperature, the electrochemical cell is charged at a lower voltage and discharged at a higher voltage, therefore converting heat to electricity. The temperature coefficient (α) of the system is a key parameter to determine the conversion efficiency, and the α value of the full cell will be maximized if those of the cathode and the anode have opposite signs. Previously, most studies have worked on electrode materials with negative α. In this study, we focused on lithium manganese oxide (LMO), a widely used lithium-ion battery cathode material, showing a positive α of 0.62 mV K–1 and stable performance in an aqueous electrolyte. We demonstrate an electrochemical system consisting of an LMO cathode and a copper hexacyanoferrate anode in the Li+ and K+ hybrid electrolyte for low-grade heat harvesting. The α of the full cell is 1.061 mV K–1 and the heat-to-electricity conversion efficiency can reach 1.8% in the temperature range of 10–40 °C. The relative efficiency reaches 18.8% of the theoretical Carnot limit, which is enhanced to 27% with the assumption of 50% heat recuperation. This work may open new opportunities for studies on the electrode materials with positive α and hybrid electrolyte systems with both a positive-α and a negative-α material for efficient low-grade heat harvesting.