Second law analysis of overpotentials in a lithium-mediated electrochemical ammonia production cell

• Second law analysis conducted on a Li-mediated ammonia production cell. • Minimizing irreversibilities during operation improves system performance. • Developed a thermodynamic framework to model the overpotential response of the cell. • Quantified irreversibilities from activation, ohmic and electrodeposition processes. • Identified the cell’s operating regions where entropy generation is minimized. This article presents a thermodynamic framework based on the second law of thermodynamics for predicting overpotential irreversibilities in a lithium-mediated electrochemical cell for ammonia synthesis. It analyzes experimental data from prior studies, in terms of the overpotential entropy generation, and identifies regions where entropy generation is minimized. Overpotential models for the activation kinetics, nucleation, mass transport, and ohmic resistance of the electrochemical cell are developed as functions of the operating current density. Analysis of potentiodynamic and impedance data was used to determine the key electrochemical parameters of the system. A differential analysis identified the charge transfer coefficients, and improvements were found when comparing its correlation coefficient to that obtained through logarithmic analysis in a previous study. The predictive model allows for minimizing the entropy generation attributed to the thermodynamic overpotentials, wherein the second law captures the irreversibilities in the electrochemical system. The new model is applied to a lithium-mediated electrochemical cell with high Faradaic efficiency, and the entropy generation of the system is evaluated within the framework. Predicted results are compared against past experimental data. The approach analyses entropy generation across various overpotential components, including ohmic losses, activation and nucleation energy barriers, as well as mass transport limitations during electrodeposition. By considering these overpotential contributions to the overall entropy generation, the study provides new insight into thermodynamic irreversibilities of lithium-mediated electrochemical systems and proposes ways to reduce them. The paper advances the theoretical understanding of electrochemical irreversibilities in lithium-mediated electrochemical systems and provides a practical tool for assessing and improving the performance of electrochemical cells including with nucleation and electrodeposition.