posted on 2021-12-14, 16:36authored byYan Jing, Eric M. Fell, Min Wu, Shijian Jin, Yunlong Ji, Daniel A. Pollack, Zhijiang Tang, Dian Ding, Meisam Bahari, Marc-Antoni Goulet, Tatsuhiro Tsukamoto, Roy G. Gordon, Michael J. Aziz
Water-soluble
anthraquinones (AQs) hold great promise serving as
redox-active species in aqueous organic flow batteries. Systematic
investigations into how the properties of redox molecules depend on
the water-solubilizing groups (WSGs) and the way in which they are
bound to the redox core are, however, still lacking. We introduce
WSGs linked to anthraquinone by CC bonds via a cross-coupling
reaction and convert CC to C–C bonds through hydrogenation.
The anthraquinone and the WSGs are connected via (un)branched chains
with (un)saturated bonds. We investigate the influence of chains and
ionic ending groups on the redox potentials of the molecules and identify
three important trends: (1) The electron-withdrawing ending groups
can affect the redox potentials of AQs with two unsaturated hydrocarbons
on the chains through π-conjugation. (2) For chains with two
(un)saturated straight hydrocarbons, WSGs increase the redox potentials
of the AQs in the order PO32– < CO2– < SO3–. (3) AQs with (un)saturated chains at high pH possess desirably
low redox potentials, high solubilities, and high stability. Disproportionation
leads to the formation of anthrone, which can be regenerated to anthraquinone.
Tautomerization results in the saturation of alkene chains, stabilizing
the structure. We utilize these observations to identify a potentially
low-cost and long-lifetime negative electrolyte that demonstrates
a temporal fade rate as low as 0.0128%/day when paired with a potassium
ferrocyanide positive electrolyte.