Mechanistic insight into the electrochemical absorption adsorption behaviour of Cd2+ and Na+ on MnO2 in a deionization supercapacitor

A promising electrode material, manganese dioxide (MnO2), has been widely studied for capacitive deionization (CDI) desalination and recently researched for heavy metal removal. However, there are significant differences in the performance of MnO2 in applications for heavy metal removal and desalination. In this paper, the mechanism for different electrochemical processes of Cd2+/Na+ ions on MnO2 electrodes was comprehensively studied. MD simulations show that the diffusion performance of Cd2+ is much slower than that of Na+ in the presence of CdCl−, leading to a lower adsorption rate of Cd2+. Of the two ion storage mechanisms (capacitive and diffusion-controlled progress), capacitance contributions provided 65.6% of the total in the Na+ electrolyte, while diffusion-controlled progress contributed 55.2% in the Cd2+ electrolyte. For diffusion, tunnelling is the main storage mechanism for Na+; instead, Cd2+ undergoes a sequential surface redox process: (1) the combination of Cd2+ and O atoms from Mn-O-Mn results in the transition from Mn4+ to Mn3+, and (2) the H substitution reaction on Mn-O-H is generated from (1). Therefore, the adsorption capacity of Cd2+ is closely related to the chemical state of MnO2 on the surface, which results in worse cycling stability than Na+. •The presence of CdCl+ reduces the migration of Cd2+ on the electrode surface.•Different faradaic processes are responsible for differences in the adsorption of Cd2+ and Na+.•The surface redox mechanism for Cd2+ in MnO2 involves two processes.•The results of this study provide significant insights into the cation storage mechanism of MnO2.