A Rechargeable Zn-Cu Battery Using a Selective Cation Exchange Membrane
The Daniell cell (Zn-Cu battery) offers several potential advantages over current battery technologies, but is held back by the problems of copper ion (Cu2+) crossover. We resolved this through employing a selective cation exchange membrane as the separator. The membrane allows sodium ions (Na+) to maintain ion transfer between the zinc and copper half-cells while preventing Cu2+ from entering the zinc half-cell. We observed that at equal electrolyte conductivities, the potential of zinc deposition is larger in sulphate electrolytes than in chloride electrolytes. Zinc nitrate electrolytes lead to the formation of zinc oxide (ZnO) which passivated the zinc electrode surface. In copper electrolytes, sulphate and nitrate electrolytes possessed nearly equal potentials for electrodeposition. Chloride-based copper electrolytes possessed large negative potentials for copper deposition due to the stabilization effect of chloride (Cl-) on the monovalent Cu+ intermediate species. Tafel analyses revealed that zinc deposition and dissolution had near identical kinetics in sulphate and chloride electrolytes when they were present at equal conductivites. We also observed through cyclic voltammetry and Tafel analyses that there is a small increase in the kinetics of both the zinc and copper electrochemical reactions as the active ion concentration was increased. Low levels of Cu2+ did not have a significant effect on the zinc electrode performance, but at 1000 ppm concentration of Cu2+, the overpotentials for zinc deposition and dissolution were much larger. It was observed at open-circuit that at low concentrations of catholyte, only very little Cu2+ was able to cross the membrane. Recharging the battery without any Na+ in the electrolyte was not possible. When 1.0 M sodium sulphate (Na2SO4) was added to the catholyte the battery was able to be successfully cycled 100 times with no noticeable losses in performance and only small amounts of Cu2+ crossing over. High coulombic efficiencies were observed at low current drains.