Two-electron redox active molecules with high capacity and energy density for energy storage applications

1. A non-aqueous redox flow battery comprising:

a catholyte comprising a two-electron redox active compound of Formula I,

wherein:

each of R 1 -R 8 is independently H, OR 9 , NO 2 , F, Cl, Br, I, unsubstituted alkyl, substituted or unsubstituted heteroalkyl, substituted or unsubstituted aryl, —N(R 9 ) 2 , —OP(R 9 ), —OP(OR 9 ) 3 , —C(═O)R 10 , or —S(═O) 2 R 1′ ;

E 1 is S;

E 2 is NR 11 ;

each R 9 is H, substituted or unsubstituted alkyl, substituted or unsubstituted heteroalkyl, substituted or unsubstituted haloalkyl, or substituted or unsubstituted aryl;

each R 10 is H, —N(R 9 ) 2 , substituted or unsubstituted alkyl, substituted or unsubstituted haloalkyl, or substituted or unsubstituted aryl; and

R 11 is substituted or unsubstituted alkyl, substituted or unsubstituted heteroalkyl, substituted or unsubstituted haloalkyl, or substituted or unsubstituted aryl;

a non-aqueous liquid anolyte; and

a separator positioned between the anolyte and the catholyte, wherein the separator is impermeable to the two-electron redox active compound of Formula I,

wherein the non-aqueous redox flow battery is configured to operate at a voltage sufficient to access a second electron redox couple of the compound of Formula (I); and

with the provisos that:

when R 1 -R 8 are all H, then R 11 is not methyl, ethyl, propyl, acetyl, CF 3 , or CH 2 CHFCF 3 ; and

when E 2 is NCH 2 CH 3 then R 7 is not Cl.

2. The non-aqueous redox flow battery of claim 1 , wherein each of R 1 -R 8 is independently H, F, Cl, Br, or I.

3. The non-aqueous redox flow battery of claim 1 , wherein the compound of Formula I is 10-(2-methoxyethyl)-10H-phenothiazine.

4. The non-aqueous redox flow battery of claim 1 , wherein the active compound is present in the catholyte from about 0.001 M to about 5.0 M.

5. The non-aqueous redox flow battery of claim 1 further comprising a solvent and an electrolyte salt.

6. The non-aqueous redox flow battery of claim 5 , wherein the electrolyte salt comprises LiBr, LiI, LiSCN, LiBF 4 , LiAlF 4 , LiPF 6 , LiA S F 6 , LiClO 4 , Li 2 SO 4 , LiB(Ph) 4 , LiAlO 2 , Li[N(FSO 2 ) 2 ], Li[SO 3 CH 3 ], Li[BF 3 (C 2 F 5 )], Li[PF 3 (CF 2 CF 3 ) 3 ], Li[B(C 2 O 4 ) 2 ], Li[B(C 2 O 4 )F 2 ], Li[PF 4 (C 2 O 4 )], Li[PF 2 (C 2 O 4 ) 2 ], Li[CF 3 CO 2 ], Li[C 2 F 5 CO 2 ], Li[N(CF 3 SO 2 ) 2 ], Li[C(SO 2 CF 3 ) 3 ], Li[N(C 2 F 5 SO 2 ) 2 ], Li[CF 3 SO 3 ], Li 2 B 12 X 12-n H n , (Li 2 B 10 X 10-n′ H n′ , LiS x″ , (LiS x″ R 30 ) y , (LiSe x″ R 30 ) y , or a lithium alkyl fluorophosphate; and further wherein X is a halogen, n is an integer from 0 to 12, n′ is an integer from 0 to 10, x″ is an integer from 1 to 20, y is an integer from 1 to 3, and R 30 is H, alkyl, alkenyl, aryl, ether, F, —CF 3 , —COCF 3 , —SO 2 CF 3 , —SO 2 CH 3 , or —SO 2 F.

7. The non-aqueous redox flow battery of claim 5 , wherein the electrolyte salt is present in the catholyte from about 0.1 M to about 2.0 M.

8. The non-aqueous redox flow battery of claim 5 , wherein the solvent is an aprotic solvent.

9. The non-aqueous redox flow battery of claim 1 further comprising an anode, wherein the anode comprises Li, Na, or Mg.

10. The non-aqueous redox flow battery of claim 1 , wherein the anolyte comprises viologen or quinoxaline.

11. A method of operating a redox flow battery, the method comprising:

providing a redox flow battery comprising a catholyte comprising a two-electron redox active compound of Formula I;

an anolyte comprising a redox active species; and

a separator positioned between the anolyte and the catholyte, wherein the separator is impermeable to the two-electron redox active compound of Formula I; and

operating the redox flow battery at a voltage sufficient to access the second electron redox couple of the compound of Formula I;

wherein:

the compound of Formula I is represented as:

each of R 1 -R 8 is independently H, OR 9 , NO 2 , F, Cl, Br, I, unsubstituted alkyl, heteroalkyl, aryl, —N(R 9 ) 2 , —OP(R 9 ),), —OP(OR 9 ) 3 , —C(O)R 10 , or S(O) 2 R 10 ;

E 1 is S;

E 2 is NR 11 ;

each R 9 is H, alkyl, heteroalkyl, haloalkyl, or aryl;

each R 10 is H, —N(R 12 ) 2 , alkyl, haloalkyl, or aryl; and

R 11 is alkyl, heteroalkyl, haloalkyl, or aryl; with the provisos that:

when R 1 -R 8 are all H, then R 11 is not methyl, ethyl, propyl, acetyl, CF 3 , or CH 2 CHFCF 3 ; and

when E 2 is NCH 2 CH 3 then R 7 is not Cl.

12. The method of claim 11 , wherein the compound of Formula I is 10-(2-methoxyethyl) 10H-phenothiazine, 10-(2,2,2-trifluoroethyl)-10H-phenothiazine, or a combination thereof.