Multifunction polyethylene glycol for hydrogen evolution inhibition of aqueous iron ion battery

• Polyethylene glycol (PEG) as a multifunctional electrolyte additive is identified. • PEG addition altered H 2 O’s hydrogen-bonding network and Fe 2+ solvation structure. • PEG’s preferential adsorption on iron induces uniform Fe 2+ deposition. • Effectively inhibited the hydrogen evolution reaction (HER) and side reactions. Iron ion batteries have great potential for large-scale energy storage in power grids due to their low cost. However, the cycle lives of Fe ion batteries are poor due to the low Coulombic efficiency of the Fe plating/stripping, ascribing to the occurrence of the undesired hydrogen evolution. This paper addresses these issues by introducing a multi-hydroxy polymer (polyethylene glycol, PEG) as an additive in the electrolyte. The modified electrolyte alters the solvation structure of Fe 2+ and reduces the coordination number and reaction activity for H 2 O molecules, suppressing hydrogen evolution on the Fe anode. Additionally, the preferential adsorption of PEG on the Fe surface to occupy more active sites and facilitates the uniform deposition of Fe 2+ on the anode surface, thus improving the cycle stability and cycle life. Consequently, the symmetric cell with a PEG-added electrolyte exhibits excellent cycling performance (with cycling times of 1200 h at current densities of 0.1 mA cm −2 ), outstanding interfacial stability, and a dendrite-free morphology. Furthermore, the fabricated full cell (Fe||IPB) demonstrates remarkable cycling stability, indicating that the electrolyte engineering provides a promising direction for designing highly reversible and stable aqueous iron ion batteries.