The Interactions Between Chlorides and Zn(001) Surfaces in Zinc/Bromine Flow Battery Electrolytes

Abstract

Chloride-based supporting electrolytes are conventionally used in zinc/bromine (Zn/Br) and other flow batteries to ensure good conductivity during charge/discharge operation. This work presents preliminary results from experimental and periodic density functional studies of the interactions between chlorides and electrodeposited zinc generated during the charging phase of the Zn/Br system. Scanning electron microscopy and X-ray diffraction analysis show that even relatively low concentrations (0.5 M) of ZnCl2 supporting electrolyte influences zinc electrodeposition behaviour during the charging phase, with a strong preference for the (004) orientation. Molecular modelling carried out in this work provides a foundation for understanding how chlorides present in the supporting electrolyte interact with and alter the electrodeposited zinc surface in Zn/Br flow batteries. Chloride strongly binds above the Zn(001) surface with binding energies of 260–288 kJ mol-1 at 0.11–0.33 ML coverage. Chloride binding lowers the Zn(001) surface uniformity, with higher coverage resulting in lower binding energies. These findings warrant further investigation of the influence and behaviour of supporting electrolytes on the primary electrolyte and at the electrode-electrolyte interface. While this work focuses on Zn/Br systems, the findings are also potentially applicable to other types of RFBs which utilise chloride-containing electrolytes.