A novel design has been invented for redox flow batteries. Different from the single-membrane, double-electrolyte redox flow battery as a basic structure, the design of the present invention involves double-membrane (one cation exchange membrane and one anion exchange membrane), triple-electrolyte (one electrolyte in contact with the negative electrode, one electrolyte in contact with the positive electrode, and one electrolyte positioned between and in contact with the two membranes) as the basic characteristic. The cation exchange membrane is used to separate the negative or positive electrolyte and the middle electrolyte, and the anion exchange membrane is used to separate the middle electrolyte and the positive or negative electrolyte. This particular design physically isolates, but ionically connects, the negative electrolyte and positive electrolyte. The physical isolation offers a great freedom in choosing redox pairs in the negative electrolyte and positive electrolyte, making high voltage of redox flow batteries possible. The ionic conduction not only makes the design functional, but also drastically reduces the overall ionic crossover between negative electrolyte and positive one, leading to high columbic efficiency.
 This invention was made with government support under Grant Nos. DE-AR000009 and DE-AR0000346 awarded by ARPA-E project of the U.S. Department of Energy. The government has certain rights in the invention.