Rechargeable batteries as energy storage devices for the electric grid are advantageous to bridge the intermittency in the daily energy demand and production by renewable energy sources such as solar microgrids. Another important consideration to satiate this energy landscape is the need for inexpensive, safe, and non-toxic rechargeable battery technology and chemistry. Zinc aqueous (alkaline) batteries are one of the prime candidates that satisfies the aforementioned criterion and has thus warranted a lot of scientific and technological research for commercial applicability. There has been considerable work conducted by the researchers in this field to bolster the cycle life and active material utilization of zinc in alkaline systems. However, zinc is thermodynamically susceptible to hydrogen gas evolution in alkaline solutions as witnessed by its Pourbaix diagram. On a device level, this affects the columbic efficiency due to the deleterious side reaction, which in turn negatively impacts the energy density of the battery. Water-in-salt electrolytes (WiSE) are a recently developed class of electrolytes that directly address this shortcoming of dilute electrolytes due to the modified solvation structure of water which lowers the activity of free water molecules in solution.

In this work, we show a type of non-toxic and cheap acetate-based WiSE that has lower hydrogen gassing rates when compared to traditional alkaline potassium hydroxide-based electrolytes. Specifically, we quantify hydrogen gas evolution at the zinc electrode-electrolyte interface using the rotating disk electrode technique at different overpotentials to show the remarked improvement in gassing observed. At overpotentials of 100, 300 and 500 mV, 27 m (molal) potassium acetate is observed to have gassing rates of up to 16 times less than that of 25 weight percent potassium hydroxide. Using the Koutecky-Levich equation, mass-transfer and kinetic parameters for the hydrogen evolution reaction were also determined. The results establish the applicability of this class of WiSE in reducing gassing on zinc and thus, improving the overall efficiency of zinc-based aqueous batteries.