Dihydrophenazine linked porous organic polymers for high capacitance and energy density pseudocapacitive electrodes and devices

Article abstract of DOI:10.1039/d0ta11266f

Redox porous organic polymers are promising pseudocapacitive materials. However, their specific capacitance still needs further elevation. Herein, we report a novel dihydrophenazine derived porous organic polymer, GT-POP-1. The GT-POP-1 based electrode and asymmetric device show high specific capacitance and energy density metrics of 97.1 mF cm^?2and 7.3 μW h cm^?2

Full text of DOI:10.1039/d0ta11266f

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Materials Chemistry A
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Dihydrophenazine linked porous organic polymers
for high capacitance and energy density
pseudocapacitive electrodes and devices†
Cite this: J. Mater. Chem. A, 2021, 9,
4984
a
ab
Xiaohui Tanga and Cheng Gu
*
Huanhuan Zhang,
Received 18th November 2020
Accepted 16th January 2021
Redox porous organic polymers are promising pseudocapacitive materials. However, their specific
capacitance still needs further elevation. Herein, we report a novel dihydrophenazine derived porous
organic polymer, GT-POP-1. The GT-POP-1 based electrode and asymmetric device show high specific
DOI: 10.1039/d0ta11266f
rsc.li/materials-a
capacitance and energy density metrics of 97.1 mF cmꢀ2 and 7.3 mW h cmꢀ2
.
The demand for powering a great variety of electric vehicles, via the aid of conductivity enhancement,11 two dimensionality
portable electronics and, in the near future, wearable health or porosity engineering.5 Although much literature reports
products, keeps growing in our daily lives. Scientic and tech- a capacitance higher than 500 F gꢀ1
,
most redox polymer
5,12
nological improvements relevant to all performance metrics are materials show values much lower.8,10,13–16 In the continuous
highly desirable. Electrochemical energy systems that can pursuit of new compounds that meet the requirement of
provide simultaneous high power and energy density are an competitive high energy density, more novel material designs
urgent global need.1 Pseudocapacitors, which exploit the fast are desirable.
faradaic reactions at the surface of electrodes, are a promising
Apart from the redox active moiety, atoms from other
candidate as they are capable of overcoming the capacity and components should contribute as less as possible to the molar
mass-transfer limitations of electric double-layer capacitors mass, only if they undertake the functions of suppressing
(EDLCs) and batteries, respectively. Regarding the choice of dissolution, distributing charges or building porous structures.
material for pseudocapacitors, novel redox polymers with elec- Guided by this principle, we designed a dihydrophenazine
troactive moieties incorporated into the polymer backbone or (DHPAZ) derived covalent triazine polymer which is synthesized
sidechain have drawn great attention due to their abundant by simple cyclo-trimerization of 4,40-(phenazine-5,10-diyl)
active sites and structural diversity.2–7 They are gradually dibenzonitrile (DHPAZ-2CN) (Fig. 1a). The redox active 5,10-
becoming an important supplement for the conventional tran- diphenyl-5,10-dihydrophenazine core can undergo two stepwise
sition metal oxides and conducting polymers. Yet, the energy redox processes.17–19 The covalent triazine linkage minimizes
density values for most reported redox polymer materials are the molecular weight of inactive groups and helps to build
still not high enough when compared with those for a rigid and porous structure.20,21 The obtained GT-POP-1 elec-
batteries.8–10 According to equation E ¼ 1/2CV2, energy density is trode exhibits a high specic capacitance of 97.1 mF cmꢀ2 (324
proportional to the capacitance and square of the voltage. While F gꢀ1) at a current density of 0.1 mA cmꢀ2. To fabricate super-
voltage can be universally extended by using wide potential capacitor devices with high energy density, an asymmetric
window electrolytes like organic or ionic electrolytes, together supercapacitor with GT-POP-1 as the cathode and activated
with asymmetric device architectures, the elevation in capaci- carbon as the anode (GT-POP-1//AC) is assembled. The GT-POP-
tance is specic and customized. To sum up, increasing the 1//AC device delivers a capacitance of 7.51 mF cmꢀ2 at a current
accessibility of redox sites so as to release the capacitance to the density of 0.1 mA cmꢀ2. The energy density of the GT-POP-1//AC
maximum is the guideline. This strategy is mainly carried out device is 7.3 mW h cmꢀ2 (24.5 W h kgꢀ1) at a power density of
59.7 mW cmꢀ2 (193 W kgꢀ1) and remains almost unchanged at
a high power density of 2987 mW cmꢀ2 (9642 W kgꢀ1). Moreover,
aState Key Laboratory of Luminescent Materials and Devices, Institute of Polymer
Optoelectronic Materials and Devices, South China University of Technology, No.
381 Wushan Road, Tianhe Distinct, Guangzhou, 510640, P. R. China. E-mail:
gucheng@scut.edu.cn
bGuangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates,
South China University of Technology, No. 381 Wushan Road, Tianhe Distinct,
Guangzhou, 510640, P. R. China
stable cycling performance is achieved with 84% capacitance
retention aer 2000 charge–discharge cycles at 0.2 mA cmꢀ2
.
The synthesis of GT-POP-1 was conducted by cyclo-
trimerization of DHPAZ-2CN with triic acid (TfOH) as the
catalyst (Fig. 1a, ESI†).22,23 To conrm the formation of GT-POP-
1, Fourier Transform Infrared Spectroscopy (FTIR) analysis was
conducted (Fig. 1b). The emergence of two vibration peaks at
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ta11266f
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Fig. 1 (a) Synthesis route for GT-POP-1 and schematic demonstration of its reversible redox processes in an Et4NBF4 electrolyte; (b) FTIR spectra
of GT-POP-1 and its precursor DHPAZ-2CN; (c) N2 and CO2 adsorption/desorption isothermal curves of GT-POP-1 at 77 and 195 K, respectively;
(d) PXRD pattern and (e) SEM image of the as-prepared GT-POP-1 powder.
1674 and 1321 cmꢀ1 indicated the formation of triazine rings, structure with pore sizes of 0.34 and 0.38 nm under CO2 and
while the disappearance of the characteristic nitrile stretching N2, respectively (Fig. S3†). The powder X-ray diffraction (PXRD)
band at 2230 cmꢀ1 veried the complete cyclization of 1,4- pattern indicated the amorphous nature of GT-POP-1. The
dicyanobenzene.24–26 The new peak at 1254 cmꢀ1 together with broad band at 2q ¼ 22ꢁ can be attributed to p–p stacking
those at 1112 cmꢀ1 and 1082 cmꢀ1 may be ascribed to the (Fig. 1d). Scanning electron microscopy (SEM) images also
characteristic vibration of S]O and C–F from residual TfOH, showed the porous and amorphous morphology of GT-POP-1
respectively.27 The 13C NMR spectra further conrmed the (Fig. 1e). High-resolution transmission electron microscopy
presence of carbon from the C]N linkage in the triazine unit by (HR-TEM) revealed the highly porous textiles (Fig. S4†). Ther-
showing a new peak at 178 ppm (Fig. S1†). The chemical envi- mogravimetric (TG) analysis showed that GT-POP-1 remains
ꢁ
ronments of typical elements including C, N, F and S, stable before decomposition above 300 C (Fig. S5†).
embedded in GT-POP-1 or from residual TfOH, were also
We systematically investigated the electrochemical proper-
investigated by X-ray photoelectron spectroscopy (XPS) ties of the GT-POP-1 electrode. First of all, cyclic voltammetry
(Fig. S2†). The C 1s spectrum could be deconvoluted into four (CV) measurements were conducted to characterize the vol-
peaks with binding energies of 284.6, 285.6, 288.2 and 291.8 eV, tammetry response in a three-electrode conguration, with the
ascribed to C]C in the benzene ring, C–N in the DHPAZ ring, active material as the working electrode, Pt wire as the auxiliary
C]N in the triazine ring, and quaternary carbon in residual electrode and Ag+/Ag as the reference electrode. At a slow scan
TfOH, respectively. The deconvoluted peaks in the N 1s spec- rate of 10 mV sꢀ1, the GT-POP-1 exhibited two pairs of reversible
trum at 399.6, 400.2 and 400.8 eV are attributed to N]C in the redox bands in succession at about ꢀ0.5–0.5 V and 0.5–1.25 V
triazine ring, tertiary amine and protonated tertiary amine in during the anodic scan and ꢀ0.5–0.25 V and 0.25–1.0 V during
DHPAZ due to TfOH, respectively. The appearance of S 2p and F the cathodic scan, which were ascribed to the 5,10-diphenyl-
1s in the sample was arbitrarily attributed to TfOH. By quanti- 5,10-dihydrophenazine moiety losing or regaining its rst and
tatively comparing the amount of introduced S (or F) with that second electrons, respectively (Fig. 2a).29 Each band had ne
of inherent N, about 0.40 M (or 0.46 M) TfOH per mole of peak structures, probably because of the various environments
DHPAZ was trapped in the polymer. The porosity was detected of the dihydrophenazine moiety in the polymer; for example,
by gas sorption measurements. As shown in Fig. 1c, N2 uptake at the protonated DHPAZ by TfOH. To verify the effect of the
77 K was neglectable. However, a substantial amount of CO2 residual TfOH on the electrode performance, I–V conductivity
(121 mg gꢀ1) uptake was observed at 195 K, implying the polar tests were conducted (Fig. S6†). The calculated electronic
properties of the GT-POP-1 surface which selectively adsorbs conductivity of GT-POP-1 is only 1.8 ꢂ 10ꢀ8 S cmꢀ1, which can
polar CO2 and excludes nonpolar N2.28 The Brunauer–Emmett– hardly have any enhancement effect on the performance. The
Teller (BET) surface area was calculated to be 320 m2 gꢀ1
.
reduction properties of GT-POP-1 were also investigated. During
Evaluation of the pore size distribution was performed accord- the potential scan of 0–ꢀ2 V, an irreversible reduction peak
ing to the slit-pore non-local-density functional theory (NLDFT) appeared at ꢀ1.55 V, ascribed to the reduction of the triazine
model, verifying that GT-POP-1 has a micropore-dominated moiety. The shoulder peak at ca. ꢀ0.75 V was probably caused
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Fig. 2 (a) Anodic and cathodic scans of the GT-POP-1 electrode at a scan rate of 10 mV sꢀ1; (b) cyclic voltammetry responses at different scan
rates. The scan rates were (from in to out) 10, 20, 50, 100, 200 and 400 mV sꢀ1; (c) currents of redox peaks as a function of scan rate, the slope of
the fitting line gives the b value; (d) k1, k2 analysis eqn (2) at a scan rate of 10 mV sꢀ1. These shaded regions show the contribution of pseu-
docapacitance as a function of potential; (e) galvanostatic charge/discharge curves and (f) capacitance retention and coulombic efficiency at
current densities of 0.2, 0.5, 1.0, and 2.0 mA cmꢀ2. The electrolyte was 1.0 M Et4NBF4 in propylene carbonate.
by the reduction of the trace H2O/O2 complex in GT-POP-1. The GT-POP-1 electrode exhibited a high specic areal capacitance
anodic CV curves at difference scan rates are shown in Fig. 2b. of 97.1 mF cmꢀ2 (324 F gꢀ1) at a current density of 0.2 mA cmꢀ2
,
As the scan rate increased, the current accordingly increased. which is among the top values reported for various porous
The ne redox peaks merged into a broad band. The log(i) versus organic polymer electrodes (Table 1). As the current density
log(n) plot of the forward anodic (ox1, ox2) and backward increased, obvious capacitance loss was observed. When the
cathodic (red1, red2) peak current response is plotted in Fig. 2c, current density increased tenfold at 2.0 mA cmꢀ2, 41.5% of the
to investigate the charge storage kinetics. According to eqn (1), maximum capacitance remained, probably due to the rapid loss
of the pseudocapacitance component as discussed in the
i ¼ anb
(1)
kinetic analysis.
By taking advantage of two different electrode materials,
asymmetric supercapacitors can extend their operating voltage
window, thus providing a solution to the energy storage limi-
tation of symmetric supercapacitors.37 Herein, asymmetric
capacitors were assembled with GT-POP-1 as the cathode and
active carbon (AC) as the anode. AC showed a nearly rectangular
current response in the potential range 0–ꢀ2.0 V at a scan rate
of 10 mV sꢀ1 (Fig. 3a). The asymmetric device of GT-POP-1//AC
will have a voltage as high as 3 V in theory. CV scans at different
voltage cut-offs were conducted to make sure of the best
working voltage. As the voltage increases, the current of the
device increases, and broad redox peaks appear indicating that
the contribution of pseudocapacitance originated from dihy-
drophenazine (Fig. 3b). The optimized voltage of GT-POP-1 was
2.8 V. The galvanostatic charge/discharge behaviours at
different current densities are shown in Fig. 3c. The specic
capacitance of the device at different current densities was
calculated from the discharge curve. The capacitance retention
or rate stability of the asymmetric device was calculated to be
95% (from 7.51 mF cmꢀ2 at 0.1 mA cmꢀ2 to 7.13 mF cmꢀ2 at 4.4
mA cmꢀ2) (Fig. 3d). Electrochemical impedance spectroscopy
where a and b are dimensionless coefficients, i is the peak
current density (mA cmꢀ2) and n is the scan rate (mV sꢀ1).
Typically, when b ¼ 0.5, the redox process is controlled by semi-
innite diffusion, while b ¼ 1 indicates capacitive behaviour.30,31
The b values of four redox peaks lie in the range of 0.7–0.75,
indicating a mixed-control process. The current response is
a
combination of capacitor-like and diffusion-controlled
behaviours; consequently, the capacitance is contributed by
the EDLC and pseudo-capacitance. The fraction of each
component was calculated according to eqn (2),
i ¼ k1n + k2n1/2
(2)
where k1 and k2 are constants at a certain voltage. At a scan rate
of 10 mV sꢀ1, pseudocapacitance contributes 71% of the total
capacitance. As the scan rate increases, the pseudo fraction
gradually decreases, along with the EDLC component gradually
increasing. At a scan rate of 400 mV sꢀ1, the pseudo contribu-
tion was only 28%; the EDLC capacitance became predominant
(Fig. S7†). To calculate the specic capacitance, galvanostatic
charge/discharge tests were performed, as shown in Fig. 2d. The
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Table 1 Comparison of capacitance for selected supercapacitor electrodes based on porous organic polymers
BET surface area
Materials
(m2 gꢀ1
)
Pore size (nm)
Electrolyte
Maximum capacitance
Current density
Ref.
[TEMPO]100%-NiP-COF
Aza-MOFs@COFs
Benzil-CMP
Hex-aza-COF-3
p(TPA-EDOT)
5.2
1.4
0.7
0.1 M Bu4NClO4
1 M Et4NBF4
1 M H2SO4
167 F gꢀ1
0.1 A gꢀ1
0.2 Aꢀc1mꢀ2
1 A g
10
32
33
5
34
9
8
35
16
365
149
124
—
20.35 mF cmꢀ2
189 F gꢀ1
ꢃ0.7
—
1 M H2SO4
663 F gꢀ1
1 A gꢀ1
—
—
0.2
0.1 M Bu4NPF6
0.1 M Bu4NPF6
1 M H2SO4
1 M phosphate buffer
3 M H2SO4
151.2 mF cmꢀ2
3.0 mF cmꢀ2
48 ꢄ 10 F gꢀ1
416 F gꢀ1
1 mA cmꢀ2
150 mA cmꢀ2
10 mV sꢀ1
0.5 A gꢀ1
3.3 mA cmꢀ2
DAAQ-TFP-COF
—
435
369
1734
TpPa-(OH)2-COF
TpOMe-DAQ
0.10ꢃ0.18
2.3
1600 mF cmꢀ2
169 F gꢀ1
Dq1Da1TpCOF
PDI-triptycene
GT-POP-1
804
185
320
2.0–2.3
ꢃ3
1 M H2SO4
1 M Na2SO4
1 M Et4NBF4
111 F gꢀ1
1.56 mA cmꢀ2
0.2 A gꢀ1
14
36
352 F gꢀ1
0.34
97.1 mF cmꢀ2
324 F gꢀ1
0.1 mA cmꢀ2
0.5 A gꢀ1
This work
(EIS) was also conducted to probe the interface and bulk well as electrical resistance of the electrodes, charge transfer
conditions. The EIS curve exhibits a typical frequency response resistance (Rct) between the electrolyte and the electrode inter-
of supercapacitor devices with a semi-circle in the high face in parallel with the capacitor (C) related to the electrode
frequency region and a straight line in the low frequency region surface capacitance and Warburg diffusion impedance (W)
(Fig. 3e). This response behaviour can be tted into the equiv- indicating the ion and mass transfer process. An inductor
alent circuit of [LRs(RctC)W], which consists of internal resis- element (L) was also added in the circuit to show the oscillation
tance (Rs) including solution resistance of the bulk electrolyte as impedance caused by the coiled wire in the very high frequency
Fig. 3 (a) CV curves of the AC cathode and GT-POP-1 anode, scan rate 50 mV sꢀ1; (b) CV curves at different cut off voltages of 0.5, 1.0, 2.0, and
3.0 V; (c) galvanostatic charge/discharge at different current densities of 0.1, 0.2, 0.5, 1.0, and 2.0 mA cmꢀ2; (d) capacitance retention and
coulombic efficiency calculated from the discharge curves; (e) Nyquist plots (inset is the enlarged part in the high frequency region); (f) Ragone
plots and (g) repeated charge/discharge stability (capacitance retention and coulombic efficiency h) of the GT-POP-1//AC asymmetric
supercapacitor.
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range of 10 000 to 1 M Hz (Fig. S8†). The tting result had an methodology. Cheng Gu: conceiving and directing the research,
error less than 10% (Table S1†). According to the tting results, funding acquisition, writing – review & editing.
the device has a contact and charge transfer resistance of 6.9 U,
indicating good contact between all interfaces. In the low
frequency region, the straight line of GT-POP-1 is inclined to the Conflicts of interest
imaginary axis, indicating a capacitive behaviour; this is also
There are no conicts to declare.
conrmed in the Bode plot where GT-POP-1 has a phase angle of
nearly 90ꢁ in the low frequency regions (Fig. S7†).30 Specic
energy and power density are crucial parameters for super-
Acknowledgements
capacitor commercialization. The asymmetric device exhibited
the highest energy density of 7.3 mW h cmꢀ2 (24.5 W h kgꢀ1) at
This work was supported by the National Natural Science
Foundation of China (Nos, 21975078 and 21905098), the China
Postdoctoral Science Foundation Grant (2018M643067), the
Natural Science Foundation of Guangdong Province
a power density of 59.3 mW cmꢀ2 (193 W kgꢀ1) and showed high
rate stability with the energy density remaining unchanged even
when the power density increased to 2987 mW cmꢀ2 (9642 W
kgꢀ1) (Fig. 3f). The energy density is among the top values re-
(2019B030301003), the 111 Project, the Fundamental Research
ported in many state-of-the-art porous polymer based super-
Funds for the Central Universities, and the Thousand Youth
capacitors reported recently, even comparable to some
Talents Plan.
transition metal oxide based pseudocapacitors (Table S2†).38–42
Besides, the asymmetric device also showed good cycling
stability with a capacitance retention of 84% (67%) aer 2000
(5000) galvanostatic charge/discharge cycles (Fig. 3g).
Notes and references
An analogue polymer CTF-400 was also synthesized by the
ionothermal reaction with ZnCl2 as the catalyst and the solvent
at 400 ꢁC (Fig. S10–S12†). The supercapacitor CTF-400//AC only
provided a maximum capacitance of 3.5 mF cmꢀ2 at a current
density of 0.05 mA cmꢀ2. This comparison demonstrates that
the synthesis method using TfOH as the catalyst maintains the
redox-activity of the incorporated DHPAZ moiety and maxi-
mizes the pseudocapacitance contribution.
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A highly redox active dihydrophenazine derived porous organic
polymer, GT-POP-1, was synthesized with triic acid as the
catalyst. Cyclo-trimerization of the DHPAZ-2CN precursor yields
polymers with as few inactive groups as possible. The covalent
triazine linkages also contribute to building a rigid and porous
structure. Kinetic study revealed that the capacitance was
mainly contributed by pseudocapacitance. The electrochemical
tests revealed that the incorporation of redox-active dihy-
drophenazine endowed the GT-POP-1 electrode with a high
specic capacitance of 97.1 mF cmꢀ2 (324 F gꢀ1). By virtue of the
high capacitance and wide voltage window of the asymmetric
supercapacitor, the GT-POP-1//AC device shows a high energy
density of 7.3 mW h cmꢀ2 (24.5 W h kgꢀ1) at a power density of
59.3 mW cmꢀ2 and retains excellent rate stability even when the
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