Electrochemical energy storage systems and methods featuring large negative half-cell potentials

A flow battery comprising:

• a first half-cell comprising:
• a first aqueous electrolyte comprising a first redox active material at a concentration in a range of from 0.75 M to about 2.5 M, and a first carbon electrode in contact with the first aqueous electrolyte, the first electrolyte having a pH in the range of from 8 to 13;
• a second half-cell comprising:
• a second aqueous electrolyte comprising a second redox active material, and a second carbon electrode in contact with the second aqueous electrolyte;
• and
• a separator disposed between the first half-cell and the second half-cell;
• wherein the first half-cell has a half-cell potential ranging between −0.3 V and −0.7 V with respect to a reversible hydrogen electrode, and
• the first redox active material exhibits substantially reversible electrochemical kinetics;
• wherein
• the half-cell potential is the average measured potential of forward and reverse peaks of a cyclic voltammogram of the first aqueous electrolyte, when measured using an ex-situ apparatus using a flat glassy carbon disc electrode at a scan rate of 100 mV/s;
• and wherein
• substantially reversible electrochemical kinetics refers to a condition in which the first aqueous electrolyte exhibits a voltage difference between the anodic and cathodic peaks of less than 0.3 V, when measured by cyclic voltammetry using an ex-situ apparatus using a flat glassy carbon disc electrode at a scan rate of 100 mV/s.