Understanding H 2 Evolution Electrochemistry to Minimize Solvated Water Impact on Zinc-Anode Performance.

H 2 evolution is the reason for poor reversibility and limited cycle stability with Zn-metal anodes, and impedes practical application in aqueous zinc-ion batteries (AZIBs). Here, using a combined gas chromatography experiment and computation, it is demonstrated that H 2 evolution primarily originates from solvated water, rather than free water without interaction with Zn 2+ . Using linear sweep voltammetry (LSV) in salt electrolytes, H 2 evolution is evidenced to occur at a more negative potential than zinc reduction because of the high overpotential against H 2 evolution on Zn metal. The hypothesis is tested and, using a glycine additive to reduce solvated water, it is confirmed that H 2 evolution and "parasitic" side reactions are suppressed on the Zn anode. This electrolyte additive is evidenced to suppress H 2 evolution, reduce corrosion, and give a uniform Zn deposition in Zn|Zn and Zn|Cu cells. It is demonstrated that Zn|PANI (highly conductive polyaniline) full cells exhibit boosted electrochemical performance in 1 M ZnSO 4 -3 M glycine electrolyte. It is concluded that this new understanding of electrochemistry of H 2 evolution can be used for design of relatively low-cost and safe AZIBs for practical large-scale energy storage.