We show that non-equilibrium thermodynamics theory for surfaces combined with electrochemical impedance spectroscopy can be used to derive the excess surface concentrations of reactants and products of an electrochemical reaction at an electrode. We predict the equivalent circuit for a postulated reaction using this theory, and derive expressions for the excess surface concentrations. The method is illustrated with experimental data for the following hydride reaction to hydrogen at a Zn anode in a molten eutectic mixture of LiCl and KCl at 673 K:
The results support a two-step mechanism for hydrogen evolution via the hydrogen atom. We calculate the excess surface concentrations of the hydride ions and the hydrogen atoms at the metal surface, and find that the hydride ions cover a fraction of the surface while the hydrogen atoms are present in large excess. The excess surface concentration of the hydride ions varies largely with the polarized state of the surface, and so does its mean activity coefficient at the surface. The results contribute to a better understanding of the system in question. The method is general and is expected to give similar information for other electrodes.
We study a reacting mixture (2F ↔ F2) in a temperature gradient. We had previously used boundary-driven non-equilibrium molecular dynamics (NEMD) simulations to study this system, and found that the reaction was close to local chemical equilibrium in temperature gradients up to 1012 K/m. Using the condition of local chemical equilibrium, we show that the heat of transfer of the reacting mixture is equal to minus the enthalpy of the reaction. The fact that the sign of the heat of transfer is determined by the type of reaction adds insight to the discussion of the origin of the sign.
We review recent efforts aimed at analyzing energy transduction processes in biological systems from the perspective of mesoscopic non-equilibrium thermodynamics. The inherent nonlinear nature of many of these systems, which undergo activated processes, has over the years impeded the use of classical non-equilibrium thermodynamics for their description, because this theory accounts only for the linear regime of these processes.
The diffculty of putting non-equilibrium thermodynamics methods into a broader scope has recently been overcome. It has been shown that if one assumes local equilibrium at short time and length scales, in the mesoscale domain, the limitation of only providing linear laws can be removed and Arrhenius type nonlinear laws can be derived. The new theory proposed here provides a scenario under which transformations taking place in chemical and biological processes can be studied. We show in this paper how the theory can be applied to describe energy conversion processes in molecular motors and pumps and conclude that both systems can be studied by means of this common framework.