Ionically conductive asymmetric composite membrane for electrochemical energy system applications

A method of preparing an ionically conductive asymmetric composite membrane comprising:

• applying a layer of an aqueous solution comprising a water-soluble hydrophilic ionomeric polymer to one surface of a microporous substrate membrane;
• drying the coated membrane to form an ionically conductive composite membrane comprising a nonporous water-soluble hydrophilic ionomeric polymer coating layer on the microporous substrate membrane;
• and
• converting the nonporous water-soluble hydrophilic ionomeric polymer coating layer into an asymmetric hydrophilic ionomeric polymer coating layer comprising a porous layer with a nonporous layer on top;
• wherein the microporous substrate membrane is made from a polymer different from the hydrophilic ionmeric polymer.

A redox flow battery system, comprising:

• at least one rechargeable cell comprising a positive electrolyte, a negative electrolyte, and an ionically conductive asymmetric composite membrane positioned between the positive electrolyte and the negative electrolyte, the positive electrolyte in contact with a positive electrode, and the negative electrolyte in contact with a negative electrode, wherein the ionically conductive asymmetric composite membrane comprises a microporous substrate membrane with an asymmetric hydrophilic ionomeric polymer coating layer thereon, wherein the coating layer made of a hydrophilic ionomeric polymer, wherein the coating layer comprises a porous layer with a nonporous layer thereon, and wherein the microporous substrate membrane is made from a polymer different from the hydrophilic ionomeric polymer;
• the positive electrolyte consisting essentially of FeCl2 at a concentration of 1.0-4.5 M, NH4Cl at a concentration of 1.0-4.0 M or KCl at a concentration of 1.0-3.0 M, HCl at a concentration of 0.05-2.5 M, and glycine at a concentration of 0.01-3.0 M, optionally boric acid at a concentration of 0.01-1.0 M, and optionally FeCl3 at a concentration of 0.1-1.0 M;
• and
• the negative electrolyte consisting essentially of FeCl2 at a concentration of 1.0-4.5 M, NH4Cl at a concentration of 1.0-4.0 M or KCl at a concentration of 1.0-3.0 M, optionally boric acid at a concentration of 0.01-1.0 M, optionally glycine at a concentration of 0.01-3.0 M, and optionally FeCl3 at a concentration of 0.1-1.0 M.