A flow battery comprising: a positive electrode ; a positive electrode electrolyte including water and a first redox couple , the positive electrode electrolyte flowing over and contacting the positive electrode , the first redox couple including a first organic compound, the first organic compound including a first moiety having formula 1 in conjugation with a second moiety having formula 2 and a reduction product of the first organic compound, the first organic compound being reduced during discharge: wherein: Y 1 and Y 2 are each independently O or NR; and R is H or carbon atom; a negative electrode ; a negative electrode electrolyte including water and a second redox couple , and a polymer electrolyte membrane interposed between the positive electrode and the negative electrode , said negative electrode electrolyte flowing over and contacting the negative electrode , the second redox couple including a second organic compound, a reduction product of the second organic compound being oxidized to the second organic compound during discharge, characterized in that said second organic compound has the following formula 7: wherein: R 8 -R 15 are each independently selected from the group consisting of H, -SO 3 H, and electron donating groups with the proviso that at least one of R 8 -R 15 is hydroxyl, NH 2 , - NHR2, N(R 2 ) 2 , pyridinyl, imidazoyl, or pyrroyl, where R 2 is H or C 1-10 alkyl. a positive electrode ; a positive electrode electrolyte including water and a first redox couple , the positive electrode electrolyte flowing over and contacting the positive electrode , the first redox couple including a first organic compound, the first organic compound including a first moiety having formula 1 in conjugation with a second moiety having formula 2 and a reduction product of the first organic compound, the first organic compound being reduced during discharge: wherein: Y 1 and Y 2 are each independently O or NR; and R is H or carbon atom; a negative electrode ; a negative electrode electrolyte including water and a second redox couple , and a polymer electrolyte membrane interposed between the positive electrode and the negative electrode , said negative electrode electrolyte flowing over and contacting the negative electrode , the second redox couple including a second organic compound, a reduction product of the second organic compound being oxidized to the second organic compound during discharge, characterized in that said second organic compound has the following formula 7: wherein: R 8 -R 15 are each independently selected from the group consisting of H, -SO 3 H, and electron donating groups with the proviso that at least one of R 8 -R 15 is hydroxyl, NH 2 , - NHR2, N(R 2 ) 2 , pyridinyl, imidazoyl, or pyrroyl, where R 2 is H or C 1-10 alkyl. Y 1 and Y 2 are each independently O or NR; and R is H or carbon atom; a negative electrode ; a negative electrode electrolyte including water and a second redox couple , and a polymer electrolyte membrane interposed between the positive electrode and the negative electrode , a negative electrode ; a negative electrode electrolyte including water and a second redox couple , and a polymer electrolyte membrane interposed between the positive electrode and the negative electrode , said negative electrode electrolyte flowing over and contacting the negative electrode , the second redox couple including a second organic compound, a reduction product of the second organic compound being oxidized to the second organic compound during discharge, characterized in that said second organic compound has the following formula 7: wherein: R 8 -R 15 are each independently selected from the group consisting of H, -SO 3 H, and electron donating groups with the proviso that at least one of R 8 -R 15 is hydroxyl, NH 2 , - NHR2, N(R 2 ) 2 , pyridinyl, imidazoyl, or pyrroyl, where R 2 is H or C 1-10 alkyl. <EOS>