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

• Mass changes in MnO 2 electrode in Zn 2+ /Mn 2+ 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. 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 ZnSO 4 and 2 M ZnSO 4 + 0.1 M MnSO 4 ). Monitoring of electrode mass variation during potential cycling provides direct evidence that redox processes in MnO 2 electrodes co-occur with intercalation of protons and zinc ions. Combined CV and EQCM studies reveal that electrodeposited films of MnO 2 are unstable in 2 M ZnSO 4 electrolyte. The repeated potential cycling in Zn-containing electrolytes leads to rapid deterioration of electrode capacity in the few initial cycles due to the Zn 2+ insertion into subsurface structures of MnO 2 and blocking of electroactivity of MnO 2 film on Au substrate. On the other hand, reversible processes of intercalation of protons and zinc ions occur in 2 M ZnSO 4 + 0.1 M MnSO 4 electrolyte. Two main steps of mass increase during the discharging process, taking place at 1.4 V (vs. Zn/Zn 2+ ) 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 H 3 O + ), whereas the second step of mass increase is mainly associated with formation of surface compounds like zinc sulfate hydroxide salts.