Flow batteries incorporating a nitroxide compound within an aqueous electrolyte solution
Flow batteries can include a first half-cell containing a first aqueous electrolyte solution. a second half-cell containing a second aqueous electrolyte solution, and a separator disposed between the first half-cell and the second half-cell. The first aqueous electrolyte solution contains a first redox-active material, and the second aqueous electrolyte solution contains a second redox-active material. At least one of the first redox-active material and the second redox-active material is a nitroxide compound or a salt thereof. Particular nitroxide compounds can include a doubly bonded oxygen contained in a ring bearing the nitroxide group, a doubly bonded oxygen appended to a ring bearing the nitroxide group, sulfate or phosphate groups appended to a ring bearing the nitroxide group, various heterocyclic rings bearing the nitroxide group, or acyclic nitroxide compounds.
1. A flow battery comprising:
a first half-cell containing a first aqueous electrolyte solution;
a second half-cell containing a second aqueous electrolyte solution; and
a separator disposed between the first half-cell and the second half-cell;
wherein at least one of the first redox-active material and the second redox-active material comprises a nitroxide compound having a structure of:
or a salt form thereof;
wherein:
R 1 -R 4 are, independently, optionally substituted C 1 -C 10 straight chain or branched alkyl;
each R 5 is, independently, H; optionally substituted alkyl, alkenyl, alkynyl, aryl, aralkyl, heterocyclyl, or heteroaryl; C 2 -C 6 polyol; C(═O)R 6 ; C(═O)OR 6 ; C(═O)NR 6 R 6 ; OR 6 ; O(C═O)R 6 ; SR 6 ; S(═O)R 6 ; S(═O) 2 R 6 ; NR 6 R 6 ; NR 6 (C═O)R 6 ; NR 6 (C═O)NR 6 R 6 ; (CH 2 ) 1-10 CO 2 H; (CH 2 ) 1-10 (CHOH)CO 2 H; CH 2 (OCH 2 CH 2 ) x OCH 3 ; CH(OH)CH 2 OH; halogen; cyano; sulfonyl; or perfluoroalkyl;
each R 6 is, independently, H; optionally substituted alkyl, alkenyl, alkynyl, aryl, aralkyl, heterocyclyl, or heteroaryl; perfluoroalkyl; (CH 2 ) 1-10 CO 2 H; (CH 2 ) 1-10 (CHOH)CO 2 H; (CH 2 CH 2 O) x CH 3 ; CH 2 (OCH 2 CH 2 ) x OCH 3 ; CH(OH)CH 2 OH; or C 2 -C 6 polyol;
optional double bonds are present;
a and b are, independently, 1 or 2; and
x is an integer in a range between 0 and about 100;
A 1 is C(═O), S(═O), S(═O) 2 , P(═O)R 7 , or P(═O)OR 7 ; and
R 7 is an optionally substituted alkyl or aryl group.
2. The flow battery of claim 1 , wherein each R 5 is H, and R 1 -R 4 are each methyl.
3. The flow battery of claim 1 , wherein A 1 is S(═O), S(═O) 2 , P(═O)R 7 , or P(═O)OR 7 .
4. The flow battery of claim 3 , wherein A 1 is S(═O).
5. The flow battery of claim 1 , wherein the nitroxide compound is present in only the first aqueous electrolyte solution.
6. The flow battery of claim 5 , wherein the first aqueous electrolyte solution is a positive electrolyte solution and the first half-cell is a positive half-cell of the flow battery.
7. The flow battery of claim 6 , wherein the second half cell is a negative half-cell of the flow battery, and the second redox-active material is a coordination compound.
8. The flow battery of claim 7 , wherein the coordination compound has a formula of:
D g M(L 1 )(L 2 )(L 3 )
wherein:
M is a transition metal or main group metal;
D is ammonium, tetraalkylammonium, an alkali metal ion, or any combination thereof;
g is an integer in a range between 0 and 6; and
L 1 , L 2 and L 3 are ligands.
9. The flow battery of claim 8 , wherein M is a titanium cation.
10. The flow battery of claim 8 , wherein at least one of L 1 , L 2 and L 3 is a catecholate ligand or a substituted catecholate ligand.
11. The flow battery of claim 1 , wherein the salt form is an alkali metal salt form, a combination of alkali metal salt forms, an ammonium salt form, a tetraalkylammonium salt form, or any combination thereof.