A method for producing a hydrogel according to the present disclosure includes: (A1) a step of preparing an object to be treated including a water-soluble cellulose-based compound and water; (B1) a step of heating the object to be treated so as to separate water from the object to be treated; and (C1) a step of cooling the object to be treated that has acquired an increased content percentage of the cellulose-based compound through the treatment in step (B1), and a series of steps from step (B1) to step (C1) are repeated until the content percentage of the cellulose-based compound in the object to be treated reaches 10% by mass or more.

A low cost, sandwich-structured thin film composite (TFC) anion exchange membrane for redox flow batteries, fuel cells, electrolysis, and other electrochemical reaction applications is described. The sandwich-structured TFC anion exchange membrane comprises a microporous substrate membrane, a first hydrophilic ionomeric polymer coating layer on the surface of the microporous substrate layer, a cross-linked protonated polyamine anion exchange polymer coating layer on top of the first hydrophilic ionomeric polymer coating layer, and a second hydrophilic ionomeric polymer protective layer on top of the cross-linked protonated polyamine anion exchange polymer coating layer. Methods of making the TFC anion exchange membrane comprises a microporous substrate membrane and redox flow battery system incorporating the TFC anion exchange membrane comprises a microporous substrate membrane are also described.