fig3

Figure 3. (A) Hydrogel network and functional additives including redox couples, salt ions, and additive polymers. (B) General routes of utilizing gradient energy with the aid of hydrogel materials. (B₁) Phase-change hydrogel serves as a local concentration regulator to suppress or promote redox reactions at the electrode and electrolyte interfaces. (B₂) Either dry polyelectrolyte adsorbing water from moist air or wet polyelectrolyte being thermally activated can release charged mobile ions to establish a concentration gradient across the hydrogel. (B₃) Introduction of functional additives into the hydrogel can selectively localize anions but facilitate the ions with the opposite charge, consequently building the concentration gradient. (B₄) Hydrogel made of nano/microchannels (or designed physical constraints) works as an ion-selective membrane. In (B₁-B₄) the hydrogel performs in different length scales (i.e., at the interface or in bulk).