Methods for determining and/or adjusting redox-active element concentrations in redox flow batteries
Methods of determining concentrations and/or amounts of redox-active elements at each valence state in an electrolyte solution of a redox flow battery are provided. Once determined, the concentrations and/or amounts of the redox-active elements at each valence state can be used to determine side-reactions, make chemical adjustments, periodically monitor battery capacity, adjust performance, or to otherwise determine a baseline concentration of the redox-active ions for any purpose.
1. A method of determining a vanadium ion concentration in a vanadium redox flow battery, comprising:
step (a), or a combination of steps (b) and (c), wherein
step (a): providing an electrolyte solution comprising V 3+ and V 4+ ; converting the V 3+ to V 4+ ; determining a concentration of each of V 3+ and V 4+ in the electrolyte solution, and determining an amount of V 3+ and V 4+ based on the concentration of each of V 3+ and V 4+ and a volume of the electrolyte solution;
step (b): providing a catholyte solution comprising V 4+ and V 5+ , converting the V 5+ to V 4+ ; determining a concentration of each of V 5+ and V 4+ in the catholyte solution, and determining an amount of V 4+ and V 5+ based on the concentration of each of V 4+ and V 5+ and a volume of the catholyte solution; and
step (c): providing an anolyte solution comprising V 2+ , V 3+ , or both V 2+ and V 3+ ; converting each of the V 2+ and V 3+ , when present, to V 4+ by exposing each of the V 2+ and V 3+ to a first solution containing known concentrations of V 5+ and V 4+ ; determining a concentration of each of V 2+ and V 3+ , when present in the anolyte solution; and determining an amount of V 2+ , V 3+ , or both V 2+ and V 3+ based on the concentration of each of V 2+ , V 3+ , when present, and a volume of the anolyte solution.
2. The method of claim 1 , wherein in step (a), converting the V 3+ to V 4+ comprises exposing the V 3+ to an oxidizing agent comprising a second solution containing known concentrations of V 5+ and V 4+ .
3. The method of claim 1 , wherein in step (a) further comprises using optical absorption spectroscopy to determine the concentration of each of V 3+ and V 4+ in the electrolyte solution.
4. The method of claim 1 , wherein in step (b), converting the V 5+ to V 4+ comprises exposing the V 5+ to a reducing agent selected from the group consisting of Sn 2+ , Fe 2+ , sulfites, phosphites, hypophosphites, phosphorous acid, metal hydrides, metals, metal amalgams, and diboranes.
5. The method of claim 1 , wherein in step (b), converting the V 5+ to V 4+ comprises exposing the V 5+ to a sulfite.
6. The method of claim 1 , wherein in step (b), converting the V 5+ to V 4+ comprises exposing the V 5+ to a reducing agent selected from the group consisting of sugars, alcohols, organic acids, oils, and hydrocarbons.
7. The method of claim 1 , wherein step (b) further comprises using optical absorption spectroscopy to determine the concentration of each of V 5+ and V 4+ in the catholyte solution and step (c) further comprises using optical absorption spectroscopy to determine the concentration of each of V 2+ and V 3+ , when present, in the anolyte solution.