Electrochemical heat engines can exploit the temperature dependence of the electrode potential in redox reactions to convert low-grade heat into electricity efficiently. However, the energy conversion mechanism from the level of thermodynamic cycles still remained to be clarified, resulting in ossification in cycle configuration and unclear performance improvement methodology. This study carried out a progressive study on the electrochemical heat engines from ideal to actual and then to extended applications, and explored the collaborative integration method of thermoelectric conversion and energy storage. Starting from the basic concepts of classical thermodynamics, the non-consistency of thermodynamic logic caused by the coexistence of multiple relaxation phenomena was recognized, and the ideal cycle, electrochemical Carnot cycle, was determined. Using an ideal mixture capable of phase transitions and chemical reactions as the working fluid, the separate transfer of heat and chemical work ensured a one-to-one correspondence between the destruction and restoration of equilibrium in each process, thus avoiding interference of other relaxation phenomena on the determination of equilibrium states. The electrochemical Brayton cycle (EBC) with strong thermodynamic rationality was proposed. A porous electrode model coupled with mass transfer, reaction kinetics and temperature for EBC was established to assess its performance. Optimization results show that the power density and exergy efficiency of electrochemical Brayton cycle under a given temperature difference of 50°C were 5.28W m^-2 and 20.6%, respectively. The exergy efficiency was 3.2% higher than that of thermally regenerative electrochemical cycle under the same conditions. To overcome the disadvantage of low power density, the integration of the electrochemical Brayton cycle with flow battery energy storage was proposed, the EBC-VFB integrated system performed a maximum normalized thermal efficiency of 3.0%，or gained an energy efficiency increase of 3.5%, which provided a solution for the integration of energy storage and low-grade heat harvesting.

Electrochemical heat engine,energy storage,low-grade heat harvesting,thermoelectric conversion