Molecular Facts on the Structure and Dynamics of Electrolyte Species in Cu-Cl Cycle for Hydrogen Generation: An Insight from Molecular Dynamic Simulations.

The Cu complex, which is the key chemical species in well-known Cu-Cl hybrid thermochemical cycles and also in numerous metal hydrometallurgical and sedimentary deposit processes, displays a wide variety of structural and dynamical characteristics that are further complicated by the presence of multiple oxidation states of Cu ions with different coordination chemistries, therefore they are difficult to explore from experiments alone. In this article, an attempt has been made to understand the coordination behavior of the Cu complex using MD simulations. The study provides compelling evidence of the experimentally observed multiple stoichiometries of Cu ions, i.e., 1:6:0, 1:5:1, and 1:4:2 for Cu+:H2O:Cl- and 1:6:0 for Cu2+:H2O:Cl-. The presence of the anionic Cu complex, [Cu+Cl2]-·2H2O, [Cu+Cl2]-·3H2O, [Cu2+Cl3]-·H2O, and [Cu2+Cl3]-·2H2O, was captured in the presence of excess chloride ions. Furthermore, the probability distribution profiles have been estimated to determine the most possible complex in the considered systems. The results establish structural and dynamical reformation of the Cu complex with change in the salt concentration or variation in the solvent medium in which they are dissolved. Moreover, the structure and kinetics of the Cu ions in the Cu-Cl electrolyzer have been explored over a large range of the electric field by extending the simulated systems for varied strengths of the electric fields. It has been observed that with an increase in the strength of the electric field, the water molecules lose their coordination strength with central Cu ions, which, on the other hand, results in a significant change in the structure of the captured complex. The diffusion dynamics of the ions is altered while applying the electric field, which is furthermore modified while increasing the strength of electric field beyond a critical limit. In fact, the diffusion mechanism of the ions was seen to be transformed from Brownian-like to linear motion and then to hopping diffusion with the increasing strength of the electric field. To the best of our knowledge, this is the first time when the multiple oxidation states of the Cu ion are explored using MD simulations, and the coexisting pictures of the multiple coordinations and the solvent effects have been clearly revealed. Also to date, the present article is the first one to report the insights of the structure and the dynamics of the ions in the Cu-Cl electrolyzer over a wide range of the electric field. The present studies will be very helpful in understanding the mechanism involved in numerous metal hydrometallurgical and sedimentary deposit processes and to comprehend the analogies involved in the electrode reactions of the Cu-Cl cycle for hydrogen generation.