EIS and relaxation times analysis of the electric double layer construction on Pt cathode surface in MgCl2 molten salt hydrate

The electric double layer on Pt cathode surface in the process of cathodic polarization was analyzed with cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), Raman spectroscopy, and Scanning electron microscope (SEM) in magnesium chloride (MgCl2) solution and MgCl2 molten salt hydrate (MSH). The effects of KCl on the structure of MgCl2 molten salt hydrate and electrode reaction were discussed. EIS data were fitted to propose the equivalent electric circuits to simulate the structure of electric double layers on Pt electrodes. Distribution of relaxation times (DRT) analysis was estimated to identify each process for MgCl2 solution and MgCl2 molten salt hydrate on the Pt cathode. The results show that the solution transforms into MSHs as the water content in the solution decreases. The solution and molten salt exhibit different double-layer properties. The movement of hydrated ions has greater resistance in MSHs which leads to the reduction of electrode reaction rate. The DRT and EIS results demonstrate that H+ and hydrated Mg2+ are simultaneously adsorbed on the Pt surface to form a double electric layer in MgCl2 solution or MSHs. In the MgCl2 MSHs, H+, H2O, and compact hydrated Mg2+ on the Pt electrode surface form a double layer. KCl plays an important role in changing the structure of MgCl2 molten salt hydrate, increasing the active H2O, and offering H+ adsorbed on the Pt surface.