A stable organic dye catholyte for long-life aqueous flow batteries

Article abstract of DOI:10.1039/d0cc05133k

An organic dye, Basic blue 3 (BB3), was reported for the first time as a two-electron catholyte for aqueous redox flow batteries. The exceptional stability of BB3 enabled the full battery to achieve a high capacity retention of >99.991% per cycle during 1500 cycles.

Full text of DOI:10.1039/d0cc05133k

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Conformational control of nonplanar free base porphyrins:
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DOI: 10.1039/D0CC05133K
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A Stable Organic Dye Catholyte for Long-life Aqueous Flow
Battery
Hongbin Li, Hao Fan*, Mahalingam Ravivarma, Bo Hu, Yangyang Feng, Jiangxuan Song*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
An organic dye Basic blue 3 (BB3) was firstly reported as a two-
Benefited from the π-conjugated structure, aza-substituted
electron catholyte for aqueous redox flow battery. The exceptional basic dyes such as phenazine-based organic molecules exhibit
stability of BB3 enabled the full battery to achieve high capacity stable electrochemical reversibility.26-29 However, phenazine-
retention of >99.991% per cycle during 1500 cycles.
based materials were just used in negative pole due to their low
redox potentials. Some researchers have reported a stable
phenothiazine-based catholyte with relatively high potential by
introducing sulfur atom to the core structure.29, 30 These work
bring great enlightenment to the research of catholyte material.
It inspires us that introducing heteroatom atoms such as oxygen
family atoms to stabilized π-conjugated structure is an effective
way to get a new class of redox active materials with increased
redox potential and high stability.
Herein, we reported a highly stable organic dye, Basic blue 3
(BB3), with oxygen atom doped π-conjugated structure, as
catholyte for long-life ARFBs. In this work, BB3 was synthesised
conveniently through an asymmetric cyclization reaction
(Scheme S1, Figure S1). Nitrogen-doped graphene (NG) was
introduced to modify the electrode and efficiently reduce the
peak separation (ΔE) from 308 to 63 mV in cyclic voltammetry
(CV) curves. With acetic acid (HoAc) as cosolvent, the BB3 in
H2SO4-HoAc aqueous solution reached the maximum solubility
of 2.5 M, corresponding to the charge capacity of 134 Ah/L.
Paired with V2+/V3+ (vanadium) as anolyte, the full battery of
BB3/V with NG-modified electrode exhibited a high battery
capacity retention of > 99.991% per cycle or > 99.94% per hour
during 1500 cycles.
Aqueous redox flow batteries (ARFBs) have attracted much
attention as one of the most promising large-scale energy
storage installations1-6 due to their independent energy/power
deployments, low cost, long-term life, and high safety.7,
8
Compared to inorganic metal active species such as vanadium9
and zinc10 in traditional ARFBs, organic redox-active materials,
composed of C, H, O, N and some other Earth-abundant
elements, have lower price.11 Other attractive advantages of
organic redox-active materials are their structure diversity and
tailorability, which make various benefits such as proper redox
potential, fast kinetics, high stability and solubility obtained by
researchers through molecules design engineering.12-15
To date, organic materials utilized as catholyte for ARFBs are
mainly focused on TEMPO,16-19 ferrocene20-22 and
benzoquinone23, 24 derivatives. However, they can not meet the
actual needs of ARFBs for different reasons. TEMPO and
ferrocene derivatives have been exploited as catholyte in
neutral environment. Nevertheless, the low ionic conductivity
of the anion exchange membrane in neutral electrolyte restricts
the rate performance. Moreover, TEMPO derivatives easily
occur irreversible side reactions of disproportionation and ring-
opening, resulting in severe structural damages.25 Although
benzoquinone has high redox potential, its poor
electrochemical stability hinders the application for long-life
battery.23, 24 Therefore, it is urgent to explore organic catholyte
materials with high redox potential, attractive solubility and
exceptional stability.
State Key Laboratory for Mechanical Behavior of Materials, Shaanxi International
Research Center for Soft Matter, Xi’an Jiaotong University, Xi’an 710049, China.
*E-mail: songjx@xjtu.edu.cn; Fanh2018@xitu.edu.cn
Figure 1. (a) Redox memchanism of BB3. (b) Schematic representation of the full battery
of BB3 as cathode paired with V2+/V3+as anolyte.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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The working principle of the BB3/V ARFB is illustrated in Figure (Figure S5). Therefore, it is inferred that heteroatom nitrogen
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1. BB3 is one of typical phenoxazinum derivatives, in which plays an important role in the redoxDpOrI:o1c0e.1s0s39f/oDr0CBCB03513o3nK
three heterocyclic frameworks provide a stably π-conjugated GC@NG.38
system, and p-π electron donating conjugated effect of two
nitrogen atoms makes a steady resonance structure.31 The
heteroatom oxygen increases the electron affinity of the
molecules, and thus improves the redox potentials to meet the
catholyte demand.30 The redox reactions of BB3 take place with
two electrons and two protons in one step to balance the
charge with the extra positive charge locating at the nitrogen
atom of the side ammonium group.32 Compared to single
electron transfer molecules, BB3 with two-electron transfers
can efficiently improve the battery capacity.
High solubility of the redox active materials is desired to
achieve high capacity of the full battery. As calculated via the
capacity formula in supporting information, ARFBs with organic
electrolyte require an equivalent electron concentration of at
least 1.5 M to reach a competitive capacity with all-vanadium
ARFBs (~40 Ah/L). In this work, BB3 receives a maximum
solubility of 2.5 M in mixed solvent (H2O+HoAc volume ratio of
1:1, containing 3.5 M H2SO4) measured by UV-vis (Figure S2).
Figure 2. (a) SEM image of the morphology of pure NG. (b) XPS spectrum and (c) N1s high
Considering its ability to store two electrons per molecule, this
solubility corresponds to a theoretical equivalent electron
concentration of 5.0 M and charge capacity of 134 Ah/L. The
high solubility can be attributed to the hydrophilic tertiary
amines group at both side chains of BB3. In addition, HoAc has
been demonstrated as an useful cosolvent,30 which can
significantly increase the solubility of BB3. Fourier-transform
infrared spectroscopy (FTIR) spectra further proves that there is
only some negligible structure change for BB3 in the mixed
supporting solution (Figure S3).
CV curve was performed with bare glass carbon (bare GC)
electrode in electrochemical workstation to investigate the
electrochemical reversibility of BB3. As shown in Figure S4, the
redox process of BB3 is acidity dependent due to the
protonation/deprotonation of BB3 involved in the redox
reaction. The result indicates an apparent CV curve with a redox
potential of 0.54 V vs SHE in 3.5 M H2SO4 as supporting solution.
ΔE from CV curve is an useful parameter to estimate the redox
dynamics of active species.33 The CV curve of BB3 with a large
ΔE reveals sluggish redox kinetics, which might lead to a
decrease of energy efficient by creating large over potentials in
the full battery.33, 34
In order to overcome the drawback of the sluggish kinetics,
various nanomaterials have been used to catalytic the redox
process of the active species.35-37 In this work, we introduced
NG as an electrocatalyst to modify the bare GC electrode,
named as GC@NG. The three-dimensional morphology of NG
can supply more active sites for redox reaction (Figure 2a).38 The
high concentration of nitrogen atoms (6.5 at.%, Figure 2b-c) is
in favour of superior catalytic activity for the BB3 redox
reaction. As shown in Figure 2d, the ΔE significantly reduces to
63 mV from 308 mV, indicating a remarkable catalytic effect.
Previous work have shown that nanocarbon materials could
effectively catalyze the redox process.39-41 However, in this
work, there is no obvious improvement obtained for the BB3
redox process on pure graphene modified bare GC electrode
resolution spectra of pure NG. (d) CV curves of 0.05 M BB3 in 3.5 M H2SO4 under 0.1 V/s
on bare GC and GC@NG, respectively.
The dynamic parameters of BB3 during redox process were
measured by linear sweep voltammetry (LSV) on a rotating disk
electrode device with the rotations from 400 to 2500 rpm. The
LSV curves were obtained by sweeping from the positive to
negative potentials on bare GC (Figure S6) and GC@NG (Figure
S7) electrodes, respectively. For bare GC electrode, typical
sigmoidal LSV curves show one wave at ~0.45 V (Figure S6a),
indicating a one-step reaction in the reduction process of BB3.
In contrast, GC@NG electrode shows two waves at ~0.40 and
~0.52 V in its LSV curves. The low potential wave is rotation-
dependent, representing the reduction process of BB3. The
higher one is rotation-independent (Figure S7a-b), suggesting
the adsorption process of BB3 on GC@NG, which is further
confirmed by the LSV measurements from the reversed
sweeping direction (Figure S8). The disappearance of the high
potential wave at ~0.52 V in the positive sweeping (Figure S8,
S7c-d) could be attributed to the consumption of absorbed BB3.
Clearly, the LSV measurement performed under none-
adsorption condition is a better way to determine the diffusion
coefficient (D0) and redox constant rate (k0) of redox materials.
According to the Levich equation,33 the D0 of BB3 in the bulk
solution is 1.1×10-7 cm2 s-1. The redox constant rate k0 of BB3 on
GC@NG calculated through Koutecky-Levich equation is
2.87×10-3 cm s-1, which is about 1.5 times greater than that on
the bare GC electrode (1.95×10-3 cm s-1), revealing that NG
effectively facilitates the BB3 redox reaction.
By pairing with V2+/V3+ as anolyte, two BB3/V ARFBs were
constructed with 0.05 M BB3 in 3.5 M H2SO4 as catholyte using
bare carbon paper (bare CP) and NG modified CP (CP@NG) as
working electrode, respectively. Compared with bare CP based
battery, the CP@NG based battery displays lower
overpotentials and higher battery capacity, even at high current
densities (Figure 3a, b). Since the resistances of the two
2 | J. Name., 2012, 00, 1-3
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batteries are comparable (Figure S9), the performance slight chemical shift of the specific proton, comVpieawreAdrtictleoOntlhinee
DOI: 10.1039/D0CC05133K
difference can be attributed to NG catalyst. The effect of NG on initial structure (Figure S12). All the experiment results and
performance is also apparent in the rate capability test (Figure analysis prove that the highly soluble BB3 possesses an
3c). The Coulombic efficiencies (CE) of two batteries are over excellent electrochemical stability for ARFB application. The
99%. The battery with CP@NG exhibits much better energy high stability is mainly attributed to the π-conjugated system
efficiency (EE) than that using bare CP (Figure 3d). For example, provided by the three heterocyclic frameworks of the BB3 core
the CP@NG based battery delivers ~70% EE under 60 mA/cm2 structure. Corresponding proportion capacities are exhibited
charge and discharge current density, much higher than that when the catholyte concentration increases from 0.05 M to 0.5
(~40%) of the bare CP based battery. The long-time cycling M and 1.6 M in the full battery test, respectively (Figure S13,
stability of the 0.05 M BB3/V ARFBs were conducted at 40 S14). That means, the electrochemical process is independent
mA/cm2. The CP@NG based battery exhibits a weak declination with the solution concentration.
in capacity during 1500 cycles testing (Figure 3e). The variation
In summary, we firstly reported phenoxazinium derivative BB3
tendency is similar to the battery using bare CP in limited 100 as a promising catholyte for ARFBs. The BB3 exhibits a two-
cycles (Figure S10). Therefore, the introduced NG is electron, two-proton redox reaction in one step on a redox
independent of the battery lifetime. The CP@NG based battery potential of 0.54 V vs SHE in 3.5 M H2SO4. The BB3 has a
displays high capacity retention of >99.991% per cycle or solubility as high as 2.5 M in mixed acid solution, corresponding
>99.94% per hour during 1500 cycles, which is outstanding to the theoretical charge capacity of 134 Ah/L. The exceptional
among reported catholytes for ARFBs (Table S1).
stability of BB3 enables the full BB3/V battery with CP@NG
electrode to achieve a high capacity retention of >99.991% per
cycle or >99.94% per hour during 1500 cycles, as well as higher
energy efficiency and higher battery capacity than those of the
cell using bare CP. This work of phenoxazinium derivative
utilized as reodx-acitve material will widen the road of the
development of organic catholyte material for high
performance ARFBs.
Acknowledgements
This work is financially supported by grants from the National
Natural Science Foundation of China (Nos. 21875181 and
51802256), Natural Science Basic Research Program of Shaanxi
(Program No.2019JLP-13), Shaanxi Key Research and
Development Project (No. 2019TSLGY07-05) and 111 Project 2.0
(BP2018008).
Conflicts of interest
There are no conflicts to declare.
Notes and references
1.
A. R. Dehghani-Sanij, E. Tharumalingam, M. B. Dusseault
and R. Fraser, Renewable and Sustainable Energy Reviews,
2019, 104, 192-208.
Figure 3. BB3/V ARFBs charge-discharge voltage profiles with catholyte composed of
0.05 M BB3 dissolved in 3.5 M H2SO4 (a) with bare CP and (b) with CP@NG as electrode
at different current densities ranging from 20 to 100 mA/cm2. Rate capability comparison
of bare CP based battery and CP@NG based battery show (c) discharge capacity and (d)
EE and CE, respectively. (e) Charge and discharge capacity and CE of the BB3/V ARFB with
0.05 M BB3 using CP@NG as the cathode electrode at 40 mA/cm2 for 1500 cycles.
2.
3.
4.
S. Koohi-Fayegh and M. A. Rosen, Journal of Energy
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Y. Feng, C. Zhang, X. Jiao, Z. Zhou and J. Song, Energy
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Q. Liu, Y. Liu, X. Jiao, Z. Song, M. Sadd, X. Xu, A. Matic, S.
Xiong and J. Song, Energy Storage Materials, 2019, 23,
105-111.
S. Xiong, Y. Liu, P. Jankowski, Q. Liu, F. Nitze, K. Xie, J. Song
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The average CE is over 99%, suggesting that the side reaction
was negligible and the membrane crossover of the redox
species during cycling was at a low rate.42 CV curve of the post-
cycled catholyte shows no obvious peak current loss and redox
potential shift relative to the initiate one (Figure S11). In
addition, the post-mortem 1H NMR spectrum exhibits negligibly
5.
6.
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