Reversing the Hofmeister Response in Hydrogels via Anion Affinity Chemistry

Hydrogel electrolytes are promising for aqueous energy storage, yet their compatibility with kosmotropic salts remains fundamentally limited by the salting-out effect. In conventional hydrophilic polymer networks, strongly hydrated kosmotropic anions preferentially retain their hydration shells, leading to polymer dehydration and structural instability. Here, we show that this incompatibility can be overcome by reversing the Hofmeister response of the hydrogel through anion-polymer affinity chemistry. Guided by the law of matching water affinities, we introduce chaotropic quaternary ammonium groups (-(CH₃)₃N⁺) into a polymer backbone (ATAC-PEA) to enable selective coordination with kosmotropic SO₄^2- anions. This interaction alters the hydration competition among anions, water, and the polymer network, resulting in a reverse Hofmeister behavior. Kosmotropic anions stabilize the hydrogel swelling state, whereas chaotropic anions induce phase separation. This is evidenced by a 3-fold increase in both the correlation length (ξ) and the polymer bundle radius of gyration (R_g) when compared to the chaotropic counterparts. This hydrogel design regulates water activity while maintaining structural stability, expanding the electrochemical stability window to 2.03 V and enabling Zn anodes to cycle for over 1800 h with ∼99.5% Coulombic efficiency. Furthermore, this design principle extends to seawater-based electrolytes and other aqueous metal chemistries, including lithium, potassium, and magnesium systems. This work establishes a fundamental chemical framework for reversing the Hofmeister response via specific anion-polymer interactions, providing a new design strategy for advanced aqueous electrolytes.