(Invited) On Non-equilibrium Thermodynamics in Electrochemical Systems

Abstract

Much of our understanding of physical behavior of materials is based on the concept of equilibrium, which lies at the heart of classical thermodynamics, condensed matter physics, and modern reaction kinetics. If a thermodynamic system is in equilibrium conditions, which is the situation when an energy system (e.g., a fuel cell or a battery) is under open circuit voltage, the surface and bulk of the electrode are only subject to fluctuation of thermodynamic qualities. For the cases that are not at equilibrium, but are close to it, Onsager established linear reciprocal relationships between flux and thermodynamic force for a thermodynamic system in nonequilibrium states. These linear relationships are manifested in transport phenomena, which are non-equilibrium processes, such as ion diffusion and heat conduction.When an electrochemical reaction takes place at an electrode of a fuel cell, electrolyzer, or battery, the thermodynamic system is far away from equilibrium. Therefore, the thermodynamic states of the surface and bulk of an electrode are subject to external thermodynamic forces. As a result, in an active electrode, the electrochemical reaction on the surface causes all thermodynamic variables to change in both the surface and the bulk. In this talk, I will use solid oxide cells and lithium-ion batteries as an example to address three questions related to materials in non-equilibrium thermodynamic states: (i) how do fast kinetics and high current in an operating electrochemical cell affect the thermodynamic states of its material constituents, (ii) whether or not the state of non-equilibrium can remain stable with constant flow of matter and energy, and (iii) what is the origin that governs activity and stability in solid oxide cells?