Suppressing H 2 evolution by using a hydrogel for reversible Na storage in Na 3 V 2 (PO 4 ) 3 .

We report a low-cost hydrogel electrolyte by adding 3 wt% poly(acrylate sodium) (PAAS) into 1 M Na 2 SO 4 aqueous electrolyte, which achieves a widened electrochemical stability window (ESW) of 2.45 V on stainless steel current collector from 2.12 V in 1 M Na 2 SO 4 aqueous electrolytes (AE). Moreover, the H 2 evolution potential reaches -1.75 V vs. Ag/AgCl on titanium current collector. The results reveal that the polymer network structure of PAAS has the ability to interact with water molecules and thus the hydrogen evolution reaction can be limited effectively, which broadens the ESW of aqueous electrolyte and allows the reversible Na-ion intercalation/deintercalation of Na 3 V 2 (PO 4 ) 3 as an anode material in aqueous electrolyte reported for the first time.