In this presentation we will describe a new approach for conversion of heat into chemical energy, stored in a battery. A thermo-electrochemical system with a redox flow battery (RFB) is proposed to allow the conversion of heat at relatively low temperature into chemical energy. The electroactive species of the battery are copper ions in an organic solvent mixture containing acetonitrile as a complexing agent, resulting in a cell voltage of 1.2 V. Water is avoided because its presence reduces considerably the cell voltage. This non-aqueous hybrid redox flow battery can be utilized for electrochemical energy storage, and the efficiency of the system for this purpose will be discussed.

More interestingly, the battery can be also charged with heat. For thermal charging, the Cu(I) complex with acetonitrile can be destabilized by a heat source of around 170°C (or more). At these temperatures all the acetonitrile is distilled off and the destabilized Cu(I) species disproportionates into metallic Cu and Cu(II) species dissolved in the electrolyte. Following separation and recovery of the distilled acetonitrile the system can be discharged again. Now the chemical energy stored in the battery be converted into electricity with the Cu oxidation and Cu(II) reduction in the different half cells.

In this presentation we shall discuss the electrochemistry of copper in acetonitrile and propylene carbonate mixtures, the thermodynamic properties of the copper electrolytes as well as the heat required for the thermal regeneration, as has been evaluated by differential scanning calorimetry. Additionally, we will present our latest results with non-aqueous copper slurry electrolytes and report the efficiency of the full cycling consisting of electrochemical discharge followed by thermal charge.

The theoretical efficiency of the heat-to-power conversion has been evaluated as 30%, close to the Carnot efficiency with T_cold at 298 K and T_hot at 443 K. The losses of the cell will decrease the practical efficiency, so further improvement of the flow battery is required.