Bipolar Nitronyl Nitroxide Small-Molecule for an All-Organic Symmetric Redox-Flow
Battery
Tino Hagemannab, Jan Winsbergab, Bernhard Häuplerab, Tobias Janoschkaab, Jeremy J.
Gruberc, Andreas Wildab, Ulrich S. Schubertab,*
a
Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University
Jena, Humboldtstrasse 10, 07743 Jena, Germany
E-mail: [email protected]
b
Center for Energy and Environmental Chemistry Jena (CEEC Jena), Friedrich Schiller
University Jena, Philosophenweg 7a, 07743 Jena, Germany
#
Current address: Department of Chemical Engineering, 158 Fenske Laboratory, The
Pennsylvania State University, University Park, PA 16802
EXPERIMENTS
Electrochemical characterization
The radical content of 4 was determined via X-Band electron paramagnetic resonance (EPR)
spectroscopy. The measurements were conducted on an EMXmicro CW-EPR spectrometer
from Bruker using powdered samples as well as 10−4 M solutions in toluene. Samples were
investigated at room temperature and data handling was done on the Bruker Xenon software
package, version 1.1b86. The SpinCount™ software module was used for quantitative
measurements. The spectrometer was calibrated using TEMPO (99% (HPLC) purity, SigmaAldrich Chemie GmbH) as a reference standard. The total spin count provided in this
contribution is the average of four measurements. Cyclic voltammetry (CV) and rotating disc
electrode (RDE) measurements were conducted on a Model SP-50 potentiostast/galvanostat
(Bio-Logic, France), with a glassy carbon disc electrode (diameter 5 mm), an AgNO3/Ag
reference electrode for organic-based electrolytes and a platinum wire counter electrode. The
rotation speed was controlled externally by a Model CTV 101 ring-disk electrode system
(Radiometer analytica, France). Evaluation of the RDE analysis via Levich plot (limiting
current ilimvs. square root of rotation speed ω) yields the diffusion coefficient D by using Levich
2
1
1
equation, 𝑖𝑙𝑖𝑚 = 0.62𝑛𝐹𝐴𝐷 3 𝜔 2 𝜐 −6 𝑐0 ,with n = 1, Faraday’s constant F = 96,485 C mol−1,
electrode surface of 0.2 cm2, the solutions kinematic viscosity v = 4.48×10−7 m2 s−1 at 298.3 K1
and the bulk concentration c0 of the redox-active nitronyl nitroxide units.
200
150
Intensity
100
50
0
-50
-100
-150
-200
3425
3450
3475
3500
3525
3550
3575
Field / G
Figure S1. EPR spectrum of 4 in solid state. In contrast to the spectrum measured in 10 −4 M toluene solution
(Figure 1a), the powder spectrum displays only a single line without hyperfine structure due to the high
concentration of the nitronyl nitroxide radicals and the resulting line broadening.
0.8
0.6
Current / mA
(b)
0 rpm
100 rpm
225 rpm
625 rpm
900 rpm
1200 rpm
1600 rpm
2000 rpm
2500 rpm
3000 rpm
3600 rpm
4200 rpm
0.4
0.2
Limiting current / A
(a)
0.0
7.0x10-4
6.0x10-4
5.0x10-4
4.0x10-4
3.0x10-4
2.0x10-4
1.0x10-4
0.0
0.1
0.2
0.3
0.4
0.5
0.6
slope = (3.28 ± 0.02) x 10-5 A rad-1/2 s1/2
2
0.7
4
2.5x104
2.0x104
1.5x104
(d)
0.36 V
0.37 V
0.38 V
0.39 V
0.40 V
0.41 V
0.42 V
8
10
12
14
16
18
20
22
-3.0
-3.1
log |ik|
Current-1 / A-1
(c)
6
(Rotation rate)1/2 / rad1/2 s-1/2
Potential vs. AgNO3/Ag / V
-3.2
-3.3
1.0x104
-3.4
5.0x103
slope = (6.838 ± 0.068) V-1
log |i0| = -3.264 ± 0.001
-3.5
0.0
0.05
0.10
0.15
0.20
0.25
0.30
(Rotation rate)-1/2 (s1/2 rad-1/2)
0.35
-0.03
-0.02
-0.01
0.00
0.01
0.02
0.03
Overpotential / V
Figure S2: (a) Rotating disk electrode measurement of 4 (2×10−3 mol L−1), 0.1 M TBAPF6 in acetonitrile, scan
rate 5 mV s−1, at rotating rates from 0 to 4200 rpm, (b) Levich-plot from the obtained limiting currents at 650 mV
vs. AgNO3/Ag; application of Levich-equation yields a diffusion coefficient D = 4.74×10−6 cm2 s−1, (c) KouteckýLevich plot for different overpotentials yielding the mass-transfer-independent current ik, (d) Tafel plot yielding
k0 = 1.42×10−2 cm s−1 and α = 0.59.
(a)
(b) -1.0x10-4
0.1
0.0
Current / mA
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
-2.2
-2.0
-1.8
-1.6
-1.4
-1.2
-1.0
-0.8
-2.0x10-4
Limiting current / A
0 rpm
100 rpm
225 rpm
625 rpm
900 rpm
1200 rpm
1600 rpm
2000 rpm
2500 rpm
3000 rpm
3600 rpm
4200 rpm
-0.1
-3.0x10-4
-4.0x10-4
-5.0x10-4
-6.0x10-4
-7.0x10-4
slope = (-2.78 ± 0.31) x 10-5 A rad-1/2 s1/2
-8.0x10-4
-0.6
Potential vs. AgNO3/Ag / V
2
4
6
8
10
12
14
16
18
20
22
(Rotation rate)1/2 / rad1/2 s-1/2
Figure S3:(a) Rotating disk electrode measurement of 4 (2.1×10−3 mol L−1), 0.1 M TBAPF6 in acetonitrile, scan
rate 5 mV s−1, at different rotating rates from 0 to 4200 rpm, (b) Levich-plot from the obtained limiting currents at
2000 mV vs. AgNO3/Ag; application of Levich-equation yields a diffusion coefficient D = 3.45×10−6 cm2 s−1.
BATTERIE TESTS
Cell assembly
The static laboratory cell was designed and constructed in a flat cell type with a membrane
active area of 5 cm2 (JenaBatteries GmbH, Germany). See Figure S4 for a detailed overview.
The graphite felt (2.25 × 2.25 × 0.4 cm3, sigracell® GFA6 EA, SGLCarbon, Germany), as well
as the anion-exchange membrane fumasep® FAP-PK-3130 (FuMA-Tech, Germany) were cut
into appropriate pieces. Electrochemical impedance spectroscopy (EIS) and charge/discharge
tests were conducted on a VMP3 potentiostat/galvanostat (Bio-Logic, France). To study the
impact of the current density on the battery performance, dynamic measurements were
performed. The electrolyte was circulated between the electrochemical cell and the storage
tanks with a peristaltic pump (Hei-FLOW Value 01 Multi, Heidolph, Germany). Typically, 4
or 10 mL of electrolyte were used with a flow rate of 10 mL min−1. All measurements were
carried out at 25 °C under argon atmosphere in a glove box. The batteries were
charged/discharged with constant current and the resulting potential was measured over time.
Cut-off voltages of 1.10 V and 1.95 V were employed.
Figure S4: Schematic representation of the 5 cm2 test cell. One half-cell consists of a frame a), PTFE block with
hose connections and rubber seal b), graphite current collector c), Teflon sealing d), PTFE flow frame e), Teflon
sealing f), surface enhancing graphite felt g). Both half cells are separated by an anion-exchange membrane h).
Figure S5: Photograph of a laboratory RFB (5 cm2), comprising the electrochemical cell, two reservoir tanks and
a peristaltic pump, used for charge/discharge experiments.
(a)
(b)
3.0
2.5
60
-lm(Z) / Ohm
2.0
-lm(Z) / Ohm
70
1.5
1.0
0.5
0.0
50
40
30
20
10
-0.5
-1.0
0
-1.5
-10
12
13
14
15
16
17
18
0
10
20
R (Z) / Ohm
30
40
50
60
70
R (Z) / Ohm
Figure S6. Nyquist plot of a electrochemical impedance spectroscopy (EIS) measurement of a pumped 5 cm2 test
cell with 0.1 M 4, 0.5 M TBAPF6 in acetonitrile, (a) between 100 mHz and 200 kHz at 0 V in potentiostatic mode
prior cycling, (b) between 100 mHz and 400 kHz at 0 V in potentiostatic mode after cycling.
As example, the area-specific resistance of the pumped flow cell with 0.1 mM 4, 0.5 M TBAPF6
in acetonitrile before and after cycling was calculated. With a cell area of 5 cm2 the area-specific
resistance before the cycling is 2.58 Ohm and after cycling 1.91 Ohm. The decrease of the cell
resistance is caused by the fact that the utilized membrane needs some time to reach the ideal
operational conditions.
(a)
2.0
(b)
2.5
0.50
1.6
1.4
1.2
0.40
0.35
1.5
0.30
Charging capacity
Discharging capacity
Coulombic efficiency
1.0
0.25
0.20
0.5
Coulombic efficiency
2.0
Capacity / mA h
Cell potential / V
0.45
1.8
0.15
1.0
0.0
5000
6000
7000
8000
9000
Time / s
10000
11000
12000
0.10
0
1
2
3
4
5
6
7
8
9
10
11
Number of cycle
Figure S7. Charge/discharge experiments in a pumped RFB test cell with 0.5 M 4, 1.13 M TBAPF6 in acetonitrile,
(a) exemplary 2nd to 5th charge/discharge cycle at a current density of 0.4 mA cm−2, (b) charge and discharge
capacity and columbic efficiency at a current density of 0.4 mA cm−2 over 10 charge/discharge cycles. At this high
active material concentration the charged species precipitated in the graphite felts, impairing the material transport
and finally impeding the discharging process.
CALCULATION OF THE ACTIVE MATERIAL COSTS
The utilized bipolar organic charge-storage material was obtained in four synthesis steps. The
starting material 2,3-dimethyl-2,3-dinitrobutane can be purchased at prices in the range of $1
to 5 per kg, 1-chloro-2-[2-[2-(2-chloroethoxy)ethoxy]ethoxy]ethane for $1 per kg and the 4hydroxybenzaldehyde for $0.1 per kg. All wholesale prices were obtained from Alibaba.com
for a purchase at industrial scale. In the following synthesis steps further materials were used:
Tin for $10 per kg, hydrochloric acid for $0.2 per kg, K2CO3 for $0.82 per kg, DMF for $0.65
per kg, THF for $1 per kg, CH2Cl2 for $0.35 per kg, m-chloroperbenzoic acid for $0.09 per kg,
NaHCO3 for $0.18 per kg and NaIO4 for $0.5 per kg. In combination with an addition of 20%
for the incidental synthesis cost and a cell voltage of 1.53 V costs of the active material of $725
per kW h is determined, which is siginificantly higher than the price of $81 kW h for the
vanadium systems.2 However, the calculated price for the active material 4 can be reduced
considerably through an optimized synthesis route and the utilization of cheaper materials. For
example to synthesize the diamino compound 1, 2,3-dimethyl-2,3-dinitrobutane was
completely reduced with Sn/conc. HCl. With the high tin price ($10 per kg) in mind catalytic
hydrogenation or other reducing agents, like Zn ($3 per kg), iron(III)acetylacetonate ($1 per
kg) or hydrazine hydrate ($1 to 1.8 per kg) may be used for a scale up.
REFERENCES
1.
DDBST GmbH. http://www.ddbst.com/en/EED/PCP/VSK_C3.php. Accessed 10
August 2016.
2.
Huskinson, B., Marshak, M. P., Suh, C., Er, S., Gerhardt, M. R., Galvin, C. J., Chen,
X., Aspuru-Guzik, A., Gordon, R. G., Aziz, M. J. A metal-free organic-inorganic
aqueous flow battery. Nature 505, 195-198 (2014).