A contemporary description of the Carnot cycle featured by chemical work from equilibrium: The electrochemical Carnot cycle

Abstract

The emerging electrochemical heat engines featured by chemical work output necessitate a contemporary description of the Carnot cycle, as it was previously described in terms of volume work. Starting from the quasi-static process and relaxation time, this study recognizes the risk to the consistency of thermodynamic logic caused by multiple relaxation times in the same process. To address this issue, a rigorous description of the Carnot cycle featured by chemical work, namely, the electrochemical Carnot cycle, is presented. Using an ideal mixture capable of phase transitions and chemical reactions as the working fluid, a one-to-one correspondence between the destruction and restoration of the equilibrium in each process is achieved in the electrochemical Carnot cycle. This description effectively avoids interference from other relaxation times when determining equilibrium. In practical terms, the electrochemical Carnot cycle could be implemented by available heat exchangers and electrochemical cells with flow battery structures. Furthermore, an investigation of existing electrochemical heat engines reveals that the cycle configuration of electrochemical Brayton cycle is more thermodynamically rational than that of thermally regenerative electrochemical cycle. This study is expected to provide guidance for the cycle construction of electrochemical heat engines and inspire further thinking in this field.