To address the global challenge of intermittent renewable energy, vanadium redox flow batteries are developed as a key technology for grid-scale energy storage. In the positive half-cell of this battery, the discharge reaction is given by:
VO2+(aq)+2H+(aq)+e−→VO2+(aq)+H2O(l) \text{VO}_2^+(\text{aq}) + 2\text{H}^+(\text{aq}) + e^- \rightarrow \text{VO}^{2+}(\text{aq}) + \text{H}_2\text{O}(\text{l}) VO2+(aq)+2H+(aq)+e−→VO2+(aq)+H2O(l)As the battery discharges and reactants are depleted, how do the relative concentrations of the vanadium species change, and what is the corresponding effect on the cell's electromotive force (EMF)?
The concentration ratio [VO2+][VO2+]\frac{[\text{VO}_2^+]}{[\text{VO}^{2+}]}[VO2+][VO2+] decreases, which increases the cell EMF.
The concentration ratio [VO2+][VO2+]\frac{[\text{VO}_2^+]}{[\text{VO}^{2+}]}[VO2+][VO2+] increases, which decreases the cell EMF.
The concentration ratio [VO2+][VO2+]\frac{[\text{VO}_2^+]}{[\text{VO}^{2+}]}[VO2+][VO2+] decreases, which decreases the cell EMF.
The concentration ratio [VO2+][VO2+]\frac{[\text{VO}_2^+]}{[\text{VO}^{2+}]}[VO2+][VO2+] remains constant, keeping the cell EMF constant until complete depletion.