Systems and methods of decoupled hydrogen generation using energy-bearing redox pairs
Described herein are systems and methods of hydrogen generation and electrolyte regeneration as independent operations in separate redox flow cells. The operations can be decoupled by using an energy-bearing redox pair that electrochemically bears energy facilitating flexible, efficient hydrogen generation. In one example, the hydrogen generation redox flow cell can include a liquid, energy-bearing electrolyte solution in which at least one species of an energy-bearing redox pair is dissolved, to decouple the hydrogen evolution reaction from the reaction at the opposite electrode (e.g., the oxygen evolution reaction of conventional direct water electrolysis). Each species of the energy-bearing redox pair is associated with a standard electrode potential within the water electrolysis window.
1. A system of hydrogen generation comprising:
A first redox flow cell generating hydrogen and having:
A positive electrode side oxidizing an energy-bearing redox species dissolved in a liquid, energy-bearing, electrolyte solution to an oxidized state, wherein the liquid, energy-bearing electrolyte solution comprises an aqueous acid solution;
A negative electrode side comprising a H + /H 2 half-cell; and
A proton permeable membrane separating the positive electrode and negative electrode sides; and
A second redox flow cell regenerating the liquid, energy-bearing electrolyte solution and having:
A negative electrode side reducing the energy-bearing redox species to a reduced state;
A positive electrode side comprising an oxygen evolution reaction (OER) half-cell;
A proton permeable membrane separating the positive electrode and negative electrode sides;
A controller operably connected to the first and second redox flow cells and configured to select for operation of the first redox flow cell alone, the second redox flow cell alone, or concurrently both based on an energy-market condition;
wherein the energy-bearing redox species is associated with a reversible redox reaction having a standard electrode potential within a water electrolysis voltage window for the electrolyte solution, and wherein the first and second redox flow cells are decoupled for independent operation one from another; and
a circulation sub-system configured to transfer a first, liquid, energy-bearing, electrolyte solution comprising the energy-bearing redox species from the first redox flow cell to the second, and configured to transfer a second, liquid, energy-bearing, electrolyte solution comprising the energy-bearing redox species from the second redox flow cell to the first.
2. The system of claim 1 , wherein the circulation sub-system further comprises a first storage container configured to store a portion of the first liquid, energy-bearing, electrolyte solution and a second storage container configured to store a portion of the second liquid, energy-bearing, electrolyte solution.
3. The system of claim 1 , wherein energy-market condition comprises price of energy supply, energy demand, power grid health, H 2 price, H 2 demand, time of day, weather conditions, or a combination thereof.
4. The system of claim 1 , wherein the energy-bearing redox species in the reduced and oxidized states comprise Fe 2+ and Fe 3+ , respectively.
5. The system of claim 1 , wherein the energy-bearing redox species comprises iodine, vanadium, bromine, chlorine, TEMPO, respectively.
6. The system of claim 1 , wherein the aqueous acid solution comprises a dissolved acid having a concentration greater than or equal to 2 M, 4 M, 6 M, 8 M, or 10 M.
7. The system of claim 1 , wherein the liquid, energy-bearing electrolyte solution comprises the energy-bearing redox species in an amount greater than or equal to 0.5 M, 1 M, 1.5 M, 2 M, 3M, 4 M, 5 M, or 8 M.