A Continuously Operated Electrochemical System Driven by Low-Grade Thermal Energy

Developing clean energy technologies is becoming increasingly crucial nowadays in order to meet the ever-growing energy demand and achieve the goal of carbon neutral. Beside renewable energy utilization, great attentions have also been drawn to harvest energy from various low- to medium-grade heat sources. The low-grade heat sources at the temperature of lower than 100 °C are ubiquitous in environment, industrial power plants, solar and geothermal field, which however bring great challenges to developing efficient and cost-effective technologies due to the distributed nature and narrow temperature differences with ambient. Compared with organic Rankine cycle, solid-state thermoelectric generator and liquid-state thermogalvanic cells, the thermally regenerative electrochemical cycle (TREC) system has been considered as a promising technology with high energy conversion efficiency for such heat sources. TREC exploits the temperature coefficient of the material in electrochemical cells and realizes the heat-to-electricity conversion by charging at a lower voltage and discharging at a higher voltage. However, the charging process not only requires additional energy consumption, but also causes discontinuity of power generation. Here, we present a continuously operated TREC system for directly converting low-grade heat to electricity. Two identical electrochemical cells are combined in a unit and always operate at different temperatures, continuously generating electricity by periodical temperature alternation. No additional electricity is required for the cell regeneration. The concept is demonstrated with copper hexacyanoferrate cathode and Cu/Cu²⁺ anode, which is able to achieve an energy conversion efficiency of 5.60% (equivalent to 45.21% of Carnot efficiency) when operated between 10 °C and 50 °C. Even at an ultralow temperature difference of 10 °C from room temperature, the energy conversion efficiency can be 2.06%. The system is also successfully demonstrated for powering the external device, with the maximum output efficiency performance achieved at a matched resistance. Such system allows great freedom of selection of electrode materials with large temperature coefficient without considering their electrochemical potential, which is flexible, scalable and cost-effective, advancing the practical application of continuously operated TREC system for direct low-grade thermal energy conversion.