EQCM study of intercalation processes into electrodeposited MnO2 electrode in aqueous zinc-ion battery electrolyte

•Mass changes in MnO2 electrode in Zn2+/Mn2+ electrolyte are monitored directly.•Molar mass of species transferred in (1.6–1.3) V range supports H+ intercalation.•Mass transfer measurements confirm reversible formation and dissolution of ZHS. [Display omitted] In this work manganese oxide films were obtained by electrodeposition and their electrochemical and mass transfer processes in aqueous zinc-ion battery electrolyte were studied by cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM). Cyclic voltammograms and corresponding mass variation curves of manganese oxide during charge-discharge processes were examined simultaneously on Au-coated quartz crystal electrodes. The investigations were conducted in aqueous electrolytes of different composition (2 M ZnSO4 and 2 M ZnSO4 + 0.1 M MnSO4). Monitoring of electrode mass variation during potential cycling provides direct evidence that redox processes in MnO2 electrodes co-occur with intercalation of protons and zinc ions. Combined CV and EQCM studies reveal that electrodeposited films of MnO2 are unstable in 2 M ZnSO4 electrolyte. The repeated potential cycling in Zn-containing electrolytes leads to rapid deterioration of electrode capacity in the few initial cycles due to the Zn2+ insertion into subsurface structures of MnO2 and blocking of electroactivity of MnO2 film on Au substrate. On the other hand, reversible processes of intercalation of protons and zinc ions occur in 2 M ZnSO4 + 0.1 M MnSO4 electrolyte. Two main steps of mass increase during the discharging process, taking place at 1.4 V (vs. Zn/Zn2+) and in the potential range (1.3–1.0) V were demonstrated by EQCM. The first step of mass increase is mainly related to the intercalation of H+ (as H3O+), whereas the second step of mass increase is mainly associated with formation of surface compounds like zinc sulfate hydroxide salts.