A Rechargeable Zinc-Copper Battery Based on the Combination of a Monovalent Cationic Exchange Membrane and an Inert Charge Shuttle
Authors: Jameson Alexander , Khazaeli Ali , Barz Dominik P.J. 
Published: May 2020

To reach their full potential, renewable and intermittent energy sources such as wind and sun need to be coupled with a cost effective energy management system that is able to convert, store, and delivery energy as required. The Daniell element is one of the earliest non-rechargeable batteries. It is based on abundant, cost-effective and environmentally-benign materials and the chemistry relies on reduction of copper at the positive electrode and oxidation of zinc at the negative electrode. Since both electrodes are immersed in different electrolytes, they have to be separated by a porous barrier or a salt-bridge in order to prevent the self-discharge of the battery. However, these separators are not selective and copper ions migrate over time to the negative electrode and deposit on the surface of the zinc electrode which reduces shelf time and makes recharging of the element impossible.
In this work, we report on a novel Zinc-Copper battery design which is based on the application of a selective cation exchange membrane in conjunction with an unreactive sodium background electrolyte which acts as a charge shuttle. The membrane is impermeable for the copper (and zinc) ions but allows the sodium ions to migrate between the two half-cells to maintain the electro-neutrality.
We first tested the suitability of different zinc-based electrolytes, including zinc sulfate, zinc chloride, and zinc nitrate. At equal electrolyte conductivities, we found a higher potential for the zinc deposition in sulfate electrolytes compared to that in chloride electrolytes. It was also observed that the zinc electrode passivated due to the formation of zinc oxide (ZnO) when operated in a zinc nitrate solution. In terms of the electrodeposition of copper, sulfate and nitrate-based electrolytes showed a nearly equal potential. However, chloride-based copper electrolytes shifted the potential to more negative values due to the stabilization of the monovalent copper ion intermediate.
The membrane selectivity was evaluated by monitoring the crossover of copper ions into the zinc half-cell for various concentrations of copper in the copper half-cell. After seven days of open circuit operation, we measured copper concentrations below 10 ppm in the zinc half-cell when there was 0.1 M of copper sulfate in the copper half-cell. When the copper half-cell was filled with 0.5 M copper sulfate, the effect of crossover was evident by a blue tint of the electrolyte and the Zn electrode was covered with a layer of black/red deposit.
Hence, we chose a conservative catholyte composition of 0.1 M copper sulfate and 1M sodium sulfate in the battery. We performed discharge measurements where we varied the discharge current density and measured the voltage over time. Discharge continued until a cell voltage of 0 V was reached, which corresponds to a complete consumption of the copper ions in the catholyte. Indeed, it turned out that we utilized only around 3 to 6 % of the zinc electrode and less than 1 % of the copper electrode during discharge. These very low values indicate that the present system could also be favorably-designed as a hybrid redox (flow) battery. Hence, for the current preliminary cell design it is seems reasonable to normalize the current with the mass of the initial copper ion content since it is the limiting electro-active species. At 0.5 mA/cm2 (421 mA/g), the battery was discharged for around 17 hours before all of the copper ions were consumed resulting in a specific capacity of 763 mAh/g. The theoretical charge content based on the initial copper concentration was 843 mAh/g giving a faradaic yield of 90.5 %. The capacity and faradaic yield of the discharge at 1 mA/cm−2 (842 mA/g) were 583 mAh/g and 69.2 %, respectively.
The recharge-ability of the battery was investigated over 100 charge-discharge cycles at a current density of ±0.5 mA/cm2. The average capacity was 530 mAh/g while the average exergy efficiency was 74 %. The deviations over the cycle numbers are less than 5 % of the average values. In other words, there was no clear increase or decrease during operation, indicating a steady performance. Afterwards, the zinc electrode was characterized using Energy-dispersive X-ray spectroscopy. Only a very small amount of deposited copper was found, indicating a negligible copper ion cross over. This verifies that the combination of a cost-effective monovalent cation exchange membrane along with an unreactive background electrolyte imparts recharge-ability to the Daniell cell.

Figure 1