Inexpensive metal-free organic redox flow battery (ORBAT) for grid-scale storage

A flow battery comprising:

• a positive electrode;
• a positive electrode electrolyte including water and a first redox couple that includes a first organic compound, the positive electrode electrolyte flowing over and contacting the positive electrode, the positive electrode electrolyte having a pH from 0.5 to 6.5;
• a negative electrode;
• a negative electrode electrolyte including water and a second redox couple, the negative electrode electrolyte flowing over and contacting the negative electrode, the negative electrode electrolyte having a pH from 0.5 to 6.5, the second redox couple including a second organic compound having formula 7, a reduction product of the second organic compound being oxidized to the second organic compound during discharge:
• wherein:
• R8-R15 are each independently selected from the group consisting of H, —SO3H, and electron donating groups with the proviso that at least one of R8-R15 is hydroxyl, NH2, —NHR2, N(R2)2, pyridinyl, imidazoyl, or pyrroyl, where R2 is H or C1-10 alkyl;
• and
• a polymer electrolyte membrane interposed between the positive electrode and the negative electrode.