Additives for fluorenone/fluorenol based aqueous redox flow batteries
Aqueous anolytes for redox flow batteries are disclosed. The anolytes include a fluorenone-fluorenol derivative, an additive comprising an organic compound including one or more proton acceptor groups, an alkali metal hydroxide, and water. The additive functions as a homogeneous organocatalyst and may increase the current density of an aqueous redox flow battery including the anolyte.
1 . An aqueous composition, comprising:
an aqueous anolyte comprising
(i) a fluorenone/fluorenol derivative or a salt thereof having a structure according to any one of formulas I-III
where Q 1 -Q 4 independently are CH, C(R 1 ) or N, wherein 0, 1, or 2 of Q 1 -Q 4 are N, Q 5 -Q 8 independently are CH, C(R 2 ), or N, wherein 0, 1, or 2 of Q 5 -Q 8 are N, Y is C═O or C(H) OH,
each R 1 and R 2 independently is an electron withdrawing group,
n is an integer >1, and
x and y independently are 0, 1, 2, 3, or 4, where if none of Q 1 -Q 8 is N, then x and y are not 0, and
(a) R 1 and R 2 are different electron withdrawing groups, or
(b) the compound comprises two different R 1 or two different R 2 groups, or
(c) both (a) and (b);
(ii) an additive, wherein the additive is an organic compound including one or more proton acceptor groups;
(iii) an alkali metal hydroxide; and
(iv) water.
2 . The aqueous composition of claim 1 , wherein:
(i) the additive comprises two or more proton acceptor groups; or
(ii) wherein the additive has a pKa of from 9 to 17; or
(iii) both (i) and (ii).
3 . The aqueous composition of claim 1 , wherein the proton acceptor groups comprise —OH, —N(R A )R B , —O − Z, or any combination thereof, where Z is a counterion with a +1 charge, and R A and R B independently are H, unsubstituted C 1 -C 6 alkyl, or substituted C 1 -C 6 alkyl.
4 . The aqueous composition of claim 1 , wherein the additive comprises an oligosaccharide, a sugar alcohol, a glycol, guanidinium chloride, an amine, or any combination thereof.
5 . The aqueous composition of claim 1 , wherein the additive comprises a cyclodextrin, D-sorbitol, diglycol, guanidinium chloride, or any combination thereof.
6 . The aqueous composition of claim 1 , wherein:
(i) the additive is present in the anolyte at a concentration of from 0.05 M to 2 M; or
(ii) the additive is present in the anolyte at a concentration of from 1.5 N to 2 N with respect to proton acceptor groups; or
(iii) both (i) and (ii).
7 . The aqueous composition of claim 1 , wherein the additive comprises β-cyclodextrin.
8 . The aqueous composition of claim 7 , wherein the anolyte comprises 0.07 M to 0.1 M β-cyclodextrin.
9 . The aqueous composition of claim 1 , wherein each R 1 and R 2 independently is —SO 3 Z, —CO 2 Z, —CF 3 , —NO 2 , —CN, or —OH, wherein each Z independently is a counterion with a +1 charge.
10 . The aqueous composition of claim 1 , wherein the fluorenone/fluorenol derivative has a structure according to any one of formulas IA-IC, IIA-IIB, or IIIA-IIIC:
11 . The aqueous composition of claim 1 , wherein the fluorenone/fluorenol derivative comprises
or any combination thereof, where Y is C═O or C(H)OH, and each Z independently is a counterion with a +1 charge.
12 . The aqueous composition of claim 11 , wherein the additive comprises a cyclodextrin, D-sorbitol, diglycol, guanidinium chloride-, diglyme, or any combination thereof.
13 . The aqueous composition of claim 12 , wherein the additive comprises β-cyclodextrin.
14 . The aqueous composition of claim 1 , wherein:
(i) the fluorenone/fluorenol derivative is present in the anolyte at a concentration of 0.5 M to 1.5 M; or
(ii) the additive is present in the anolyte at a concentration of from 1.5 N to 2 N with respect to proton acceptor groups; or
(iii) the alkali metal hydroxide is present in the anolyte at a concentration of from 0.1 M to 6 M; or
(iv) any combination of two or more of (i), (ii), and (iii).
15 . The aqueous composition of claim 1 , consisting essentially of:
the fluorenone/fluorenol derivative;
the additive;
the alkali metal hydroxide; and
water.
16 . An aqueous electrolyte system for a redox flow battery system, comprising:
an aqueous anolyte comprising an aqueous composition according to claim 1 ; and
an aqueous catholyte comprising water and an electrochemically active material.
17 . The aqueous electrolyte system of claim 16 , wherein the aqueous catholyte comprises:
K 4 Fe(CN) 6 , K 3 Fe(CN) 6 , or a combination thereof; and
water.
18 . The aqueous electrolyte system of claim 16 , wherein the additive comprises a cyclodextrin, D-sorbitol, diglycol, guanidinium chloride-, diglyme, or any combination thereof.
19 . The aqueous electrolyte system of claim 16 , wherein:
(i) the fluorenone/fluorenol derivative comprises
or any combination thereof, where Y is C═O or C(H)OH, and each Z independently is a counterion with a +1 charge; or
(ii) the additive comprises a cyclodextrin; or
(iii) both (i) and (ii).
20 . A redox flow battery system, comprising:
the aqueous electrolyte system of claim 16 ;
an anode;
a cathode; and
a separator.