Bottom-up fabrication of triazine-based frameworks as metal-free materials for supercapacitors and oxygen reduction reaction

Article abstract of DOI:10.1039/d1ra00043h

Doping porous carbon materials with heteroatoms is an effective approach to enhance the performance in the areas of supercapacitors and the oxygen reduction reaction (ORR). However, most traditional heteroatom-doped metal-free porous carbon materials have random structures and pore distributions with high uncertainty, which is harmful for a deep understanding of supercapacitors and the ORR mechanism. Basing on the molecular design, a series of N, O co-doped porous carbon frameworks (p-PYPZs) has been prepared through the template-free trimerization of cyano groups from our designed and synthesized 2,8-bis(4-isocyanophenyl)-2,3,7,8-tetrahydropyridazino[4,5-g]phthalazine-1,4,6,9-tetraone (PYPZ) monomer and subsequent ionothermal synthesis, which has the advantage that the type, position, content of the heteroatom and the pore structure in the porous carbon material can be regulated. Nitrogen and oxygen atoms introducedviacovalent bond and the hierarchically porous structure endow the material with excellent cycling stability, and 110% capacitance retention after 35?000 cycles in 1 M H₂SO₄. A symmetric supercapacitor was assembled with the material and shows an energy density of 32 W h kg^?1. The material can be applied to the area of oxygen reduction reaction as a metal-free catalyst with an onset potential of 0.85 VversusRHE, indicating the good catalytic ability. The material exhibits excellent methanol crossover resistance and a four-electron pathway mechanism. Results also indicate a positive correlation between the N-Q content and the selectivity of the four-electron pathway. In this paper, the electrochemical properties of materials are regulated at the molecular level, which provides a new idea for further understanding the electrochemical mechanism of energy storage devices.

Full text of DOI:10.1039/d1ra00043h

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Bottom-up fabrication of triazine-based
frameworks as metal-free materials for
Cite this: RSC Adv., 2021, 11, 8384
supercapacitors and oxygen reduction reaction†
b
Ronghan Cao,a Fangyuan Hu,
Tianpeng Zhang,b Wenlong Shao,b Siyang Liub
*
ab
*
and Xigao Jian
Doping porous carbon materials with heteroatoms is an effective approach to enhance the performance in
the areas of supercapacitors and the oxygen reduction reaction (ORR). However, most traditional
heteroatom-doped metal-free porous carbon materials have random structures and pore distributions
with high uncertainty, which is harmful for a deep understanding of supercapacitors and the ORR
mechanism. Basing on the molecular design, a series of N, O co-doped porous carbon frameworks (p-
PYPZs) has been prepared through the template-free trimerization of cyano groups from our designed
and
synthesized
2,8-bis(4-isocyanophenyl)-2,3,7,8-tetrahydropyridazino[4,5-g]phthalazine-1,4,6,9-
tetraone (PYPZ) monomer and subsequent ionothermal synthesis, which has the advantage that the type,
position, content of the heteroatom and the pore structure in the porous carbon material can be
regulated. Nitrogen and oxygen atoms introduced via covalent bond and the hierarchically porous
structure endow the material with excellent cycling stability, and 110% capacitance retention after
35 000 cycles in 1 M H2SO4. A symmetric supercapacitor was assembled with the material and shows an
energy density of 32 W h kgꢀ1. The material can be applied to the area of oxygen reduction reaction as
a metal-free catalyst with an onset potential of 0.85 V versus RHE, indicating the good catalytic ability.
The material exhibits excellent methanol crossover resistance and a four-electron pathway mechanism.
Results also indicate a positive correlation between the N-Q content and the selectivity of the four-
electron pathway. In this paper, the electrochemical properties of materials are regulated at the
molecular level, which provides a new idea for further understanding the electrochemical mechanism of
energy storage devices.
Received 4th January 2021
Accepted 7th February 2021
DOI: 10.1039/d1ra00043h
rsc.li/rsc-advances
state-of-art expensive commercial Pt/C is crucial.10–13 These
challenging issues have drawn considerable attention.
1. Introduction
Porous carbon materials are widely used due to their high
surface areas, chemical stability, low costs, excellent electrical
conductivity, and nontoxic nature.14–16 To enhance the perfor-
mance of porous carbon materials, several strategies have been
implemented. Doping materials with heteroatoms (N, O, S, P,
and B) is one of the effective approaches.17–20 Moreover, owing to
the synergetic effects of heteroatoms, multi-heteroatom doping
is a more effective and versatile approach than using a single
dopant.21,22 The use of different elements in doping has various
effects on material properties in supercapacitors and ORR.
Nitrogen plays a crucial role in these heteroatom-doping
materials. Nitrogen groups affect the inert nature of carbon
materials on the atomic level, and enhance capacitance through
the pseudocapacitance effect.23,24 As for ORR, nitrogen has
a close relationship with the nal performance, and different
nitrogen species have different effects on the catalytic
behavior.25 Guo et al.26 suggested that the carbon atom next to
pyridinic N is responsible for ORR catalysis instead of the
pyridinic N itself. Zhang et al.27 elaborated the point that the
Human beings are facing a series of environment-related issues,
including global warming and the depletion of traditional fossil
fuels. Therefore, sustainable energy storage devices, such as
supercapacitors and efficient energy conversion devices,
including fuel cells and metal-air batteries, are urgently
needed.1–5 Supercapacitors with high energy densities and
specic capacitance are foremost required.6–9 As for fuel cells
and metal-air batteries, developing stable and economical
oxygen reduction reaction (ORR) electrocatalysts to replace the
aState Key Laboratory of Fine Chemicals, Department of Polymer Materials
&
Engineering, Liaoning Province Engineering Research Centre of High Performance
Resins, Dalian University of Technology, Dalian, 116024, China. E-mail: jian4616@
dlut.edu.cn
bSchool of Materials Science and Engineering, Key Laboratory of Energy Materials and
Devices (Liaoning Province), State Key Laboratory of Fine Chemicals, Liaoning
Province Engineering Centre of High Performance Resins, Dalian University of
Technology, Dalian 116024, China. E-mail: hufangyuan@dlut.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra00043h
8384 | RSC Adv., 2021, 11, 8384–8393
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
existence of quaternary N in a graphene layer can provide satisfactory energy storage performance (256 F gꢀ1 at a current
electrons to the p-system, and enhance the nucleophile char- density of 0.1 A gꢀ1) and high cycling stability (110% capaci-
acteristic of the adjacent carbon atom. Such effect enhances the tance retention aer 35 000 cycles) in aqueous electrolyte (1 M
O2 adsorption and catalytic performance. Aryloxy radicals are H2SO4). As for a two-electrode system, p-PYPZ@600 exhibits
responsible for the increase in the paramagnetism, which a capacitance of 121 F gꢀ1 at 0.1 A gꢀ1, energy density of
accompanies the chemical and electrochemical oxidation of 22 W h kgꢀ1 in TEABF4/AN, 74.23 F gꢀ1 at the current density of
carbon materials.28,29 Oxygen doping alone may not have much 0.1 A gꢀ1, and energy density of 32 W h kgꢀ1 in [BMIM][BF4].
effect on the performance, but a signicant increase is observed With regard to ORR, p-PYPZ@700 shows a four-electron domi-
when another heteroatom, such as nitrogen, is added to the nant pathway and excellent methanol crossover resistance in an
carbon material.30 In aqueous electrolytes, nitrogen and oxygen- alkaline electrolyte. Moreover, the bottom-up design is an
containing functional groups boost the efficiency of ion transfer by ingenious approach for the facile synthesis of dened
enhancing the property of hydrophily and provide extra pseudo- heteroatom-doped carbon materials with hierarchical pore
capacitance through the reversible redox reaction.31–33 Meanwhile, structures for energy storage and electrocatalysts.
the fabrication of the hierarchically porous distribution is of
paramount importance. Micropores increase the number of
exposed active sites, and mesopores facilitate the mass transport in
supercapacitors and fuel cells.34–38 However, most traditional
2. Experimental
2.1 Synthesis of PYPZ and p-PYPZs
heteroatom-doped metal-free porous carbon materials have
random structures and pore distributions with high uncertainty,
which render the understanding of ORR and supercapacitor
mechanisms difficult.39 Therefore, materials with precise hetero-
All of the starting materials were obtained from commercial
suppliers and used without further purication unless other-
wise indicated.
The synthesis of a monomer containing nitrogen and oxygen
heteroatoms, 2,8-bis(4-isocyanophenyl)-2,3,7,8-tetrahydropyridazino
[4,5-g]phthalazine-1,4,6,9-tetraone (PYPZ), has the following proce-
atom and pore distribution structures should be explored.
The development of porous organic frameworks (POFs)
enables a controllable fabrication that meets the above-
dures: (1) 2.25 g of 1H,3H-benzo[1,2-c:4,5-c0]difuran-1,3,5,7-tetraone
mentioned requirements. Among all kinds of POFs, covalent
and 3.355 g of (4-isocyanophenyl)hydrazine were added to a three-
triazine frameworks (CTFs), which were rst developed by
neck ask with 175 ml glacial acetic acid. (2) The mixture was stir-
red for 5 h under 125 ꢁC, and then cooled down to room temper-
ature. (3) The nal product was obtained aer ltration, washed
with absolute ethyl alcohol, and dried under vacuum. Finally,
Antonietti and Thomas et al.,40 possess robust thermal and
chemical stability, large specic surface areas, and high N contents.
The thermal and chemical stabilities are benecial for the struc-
tural maintenance aer high-temperature carbonization.41–44 Large
4.165 g of a faint yellow product was obtained (yield: 74.3%).
specic surface areas have large advantages in terms of the prop-
The nal products were obtained by mixing the precursor
erties related to the electrochemical energy storage and electro-
monomer PYPZ and anhydrous ZnCl2 in a molar ratio of 1 : 5,
catalysis for the reasons mentioned above. More importantly, the
and transferring the mixture to a quartz tube ampoule in
heteroatoms introduced to the materials can be adjusted by using
a glovebox (argon with <0.1 ppm water and <0.1 ppm oxygen).
ZnCl2 acted as the catalyst and solvent simultaneously. The
different monomers, and the dened pore sizes can be acquired by
selecting monomers with different sizes.
Based on a molecular design, we designed a monomer con-
taining nitrogen and oxygen heteroatoms, namely, 2,8-bis(4-
isocyanophenyl)-2,3,7,8-tetrahydropyridazino[4,5-g]
phthalazine-1,4,6,9-tetraone (PYPZ). The high N content of the
monomer enables the production of N-rich nal products. The
polymerization product CTFs (p-PYPZs) were synthesized
through the high-temperature ionothermal polymerization of
the nitril precursor by using ZnCl2 as a solvent and catalyst. The
following carbonization procedures afford materials with
signicantly enhanced charge transfer and mass transport
properties. The holes formed through the ionothermal method
are more homogeneously distributed, and have more regular
ampoule was thenꢀe1vacuated and sealed rapidly, and heated at
ꢁ
ꢁ
a rate of 5 C min to 550 C and maintained for 30 h. Even-
tually, the resulting black powder was dispersed in a 5% HCl
solution, and deionized water for the removal of impurities.
Then, the nal p-PYPZ@550 product was obtained by drying the
ꢁ
solution under vacuum at 100 C for 24 h. The samples were
denoted as p-PYPZ@x, where x represents the treatment
temperature. Other products, p-PYPZ@600, p-PYPZ@650, and p-
PYPZ@700, were synthesized under similar ꢁconditions, but
higher temperatures (600 ꢁC, 650 ꢁC, and 700 C, respectively).
2.2 Material characterization
pore sizes than other porous carbons synthesized through KOH All of the materials synthesized above were characterized by
activation or template-assistance methods. In summary, the various methods, as follows. The 1H-NMR spectrum was tested
strategy used for polymerization determines the structure of to detect the purity of the monomer. Mass spectrometry (MS)
products, and structure determines the nal performance. The spectra were measured on a LTQ Orbitrap XL mass spectrom-
as-synthesized nitrogen, oxygen co-doped p-PYPZ products have eter. Thermogravimetric analysis (TGA) was conducted on
several merits as follows: uniform distribution of oxygen and a Mettler TGA/SDTA851 TGA thermal analysis meteꢀr1in a owing
nitrogen heteroatoms, high nitrogen content, oxygen content, nitrogen atmosphere at a heating rate of 20 ꢁC min from 30 ꢁC
ꢁ
hierarchical porous structures, high specic surface area, and to 800 C. Thermal parameters of the products were measured
easy to acquire without using templates. p-PYPZ@600 exhibits using a NETZSCH DSC 204 differential scanning calorimetry
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 8384–8393 | 8385

View Article Online
RSC Advances
Paper
(DSC) instrument in owing nitrogen at a different heating rate electrode system. The specic capacitance was calculated
from 25 ꢁC to 400 ꢁC. The Fourier transform infrared (FT-IR) using the following equations:
spectra were acquired by a Bruker Equinox 55 spectrometer.
C ¼ It/mV
(1)
Samples were analyzed using pellets prepared by compressing
a dispersed mixture of the sample and KBr powder. For the
PYPZ monomer and p-PYPZs, elemental analysis (EA, a Flash
EA1112 analyzer) was carried out to determine the elemental
content in the materials. As for p-PYPZs, nitrogen sorption/
desorption isotherms of the samples were measured at 77 K
on an Autosorb iQ (Quantachrome) analyzer. Before sorption
measurements, the samples were degassed at 120 ꢁC under
vacuum for 30 h. The Brunauer–Emmett–Teller (BET) method
and density functional theory (DFT) pore model were utilized to
calculate the specic surface area and pore size distribution
respectively. The X-ray diffraction (XRD) spectrum was obtained
C ¼ 2It/mV
(2)
Herein, eqn (1) applied to the calculation of the three-electrode
system, while eqn (2) was used for the two-electrode system. C is
the specic capacitance of the electrode (F gꢀ1), I is the
discharge current (A), t is the discharge time (s), m is the mass of
the active material (g), and V is the discharge voltage (V). The
energy density (E, W h kgꢀ1) and the power density (P, W kgꢀ1
were calculated from the galvanic discharging curve by using
the following equations:
)
by a SmartLab 9KW Rigaku at the scanning rate of 20ꢁ minꢀ1
.
E ¼ CV2/(8 ꢂ 3.6)
P ¼ 3600E/t
(3)
(4)
Raman spectra were taken by DXR Microscope. X-Ray photo-
electron spectroscopy (XPS) was conducted to observe the
surface functional groups and bonding characterization of the
composites via an ESCALAB 250 device with a base pressure of 1
ꢂ 10ꢀ9 mbar and Al Ka X-ray radiation. The morphologies and
the structure of the as-obtained products were observed by 2.4 Electrocatalytic measurements
scanning electron microscopy (SEM) (Quanta-450, FEI), trans-
For the establishment of a catalyst ink, 4 mg of the catalyst
material, 700 ml of ethyl alcohol, 250 ml of H2O, and 50 ml of
Naon117 solution were mixed evenly through 1 h of ultrasonic
treatment. The slurry of the active material was then coated and
dried on a glassy carbon electrode with a geometrical surface
area of 0.196 cm2 (all of the current densities in the curves of the
electrochemical measurements were normalized to the glassy
carbon geometric area). An ink suspension (15 ml) was loaded on
a freshly polished glassy carbon electrode with a catalyst
loading of 300 mg cmꢀ2. For comparison, Pt/C (Alfa Aesar,
20 wt% Pt on carbon black) was also fabricated on a glassy
carbon electrode with a loading density of 150 mg cmꢀ2 with the
same procedure. A three-electrode cell was constructed using
Ag/AgCl, Pt, and Teon-coated glassy carbon as the reference,
counter, and working electrodes, respectively. All electro-
chemical measurements in this work were performed using
a BioLogic Potentiostat (Model A VMP-3) at room temperature
(ꢃ24 ꢁC). All of the potentials reported in this work were
referred to the RHE, based on the Nernst equation.
mission electron microscopy (TEM) (Eindhoven, The Nether-
lands, 120 kV), and high-resolution transmission electron
microscopy (HR-TEM). The elemental composition of the
materials was studied via SEM in cooperation with energy
dispersive spectroscopy (EDS).
2.3 Electrochemical measurements
To check the energy storage properties of the products, three-
electrode system and two-electrode system measurements
were performed. The working electrodes were fabricated as
follows: (1) for the three-electrode system, a p-PYPZ lm pressed
on a titanium mesh was used as the working electrode, a plat-
inum sheet (10 mm ꢂ 20 mm) was used as the counter elec-
trode, and an aqueous Ag/AgCl was used as the reference
electrode. The electrolyte was 1 M H2SO4 solution, and these
three electrodes were immersed into the electrolyte to form the
three-electrode testing system; (2) for the two-electrode system,
the materials were prepared by mixing p-PYPZ (80 wt%, 32 mg),
carbon black (10 wt%, 4 mg), and polytetrauoroethylene
(10 wt%, 4 mg) evenly. Hereaeꢁr, the as-prepared thin lm was
dried at the temperature of 80 C under vacuum for 12 h, and
E(RHE) ¼ E(Ag/AgCl) + 0.0592 ꢂ pH + 0.1957V
(5)
The electrolyte (0.1 M KOH) was saturated by bubbling
then cut into a circular shape with a diameter of 10 mm. Two p- oxygen and nitrogen for at least 30 min before the test. Elec-
PYPZ electrodes with the same weight were selected as the trochemical catalytic activities of the materials were evaluated
symmetrical working electrodes, and assembled in a coin cell in oxygen and nitrogen saturated 0.1 M KOH electrolytic solu-
with tetraethylammonium tetrauoroborate in an acetonitrile tion by rotating disk electrode (RDE), and then rotating ring
solution (TEABF4/AN) or 1-butyl-3-methylimidazolium tetra- disk electrode (RRDE) via a rotating electrode system (Pine) to
uoroborate ([BMIM][BF4]) with glassy ber as the separator.
monitor the formation of the intermediate peroxide species. CV
Cyclic voltammograms (CV), galvanostatic charge/discharge measurements were performed in an N2 or O2 saturated 0.1 M
(GCD) curves, and electrochemical impedance spectroscopy KOH solution under a potential range from 0 to 1 V at a scan
(EIS) were recorded via a BioLogic Potentiostat (Model A VMP-3) rate of 10 mV sꢀ1. Linear sweep voltammogram (LSV) curves
at room temperature. The potential window ranged from ꢀ0.2 were obtained in an O2 saturated 0.1 M KOH solution. The test
to 0.8 V in 1 M H2SO4 for the three-electrode system, and 0 to was scanned under the potential from 0.1 to 1 V at a scan rate of
2.3 V in TEABF4/AN, 0 to 3.5 V in [BMIM][BF4] for the two- 5 mV sꢀ1, and the N2 background current was subtracted to
8386 | RSC Adv., 2021, 11, 8384–8393
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
eliminate the contributions of the capacitive currents. While isocyanophenyl)hydrazine. Then, PYPZ was polymerized for the
using RRDE to detect the H2O2 yield, the ring potential was set preparation of p-PYPZs with a facile ionothermal strategy. PYPZ
to 1.2 V vs. RHE to oxidize the H2O2 transferred from the glassy was characterized through Fourier transform infrared (FT-IR)
carbon disk electrode. The H2O2 yield rate and the electron spectroscopy, 1H NMR spectroscopy, elemental analysis (EA),
transfer number (n) were calculated by the following equation: TGA, and DSC (Fig. 1f, S1 and Table S1†). MS calculated for
C
24H12N6O4, 448.26; found, 448.23; anal. calcd for C24H12N6O4:
H2O2 (%) ¼ 200 ꢂ (Ir/N)/(Id + Ir/N)
n ¼ 4 ꢂ Id/(Id + Ir/N)
(6)
(7)
C, 64.29; H, 2.7; N, 18.74; found: C, 64.13; H, 2.73; N, 18.78.
According to FT-IR spectroscopy for the monomer in Fig. 1f, the
signals at 2221 and 1680 cmꢀ1 indicate the existence of the
cyano and carbonyl groups in the PYPZ monomer. The strong
and intense characteristic cyano stretching band at 2221 cmꢀ1
disappeared, and the peaks of the characteristic triazine
appeared at 1250 and 1550 cmꢀ1. This nding indicates that p-
PYPZs were obtained successfully via cyano polymerization. The
TGA curves in Fig. S2† and DSC curves in Fig. S3† indicate that
where, Id and Ir are the disk and ring currents, respectively, and
N is the ring collection efficiency determined to be 0.37.
3. Results and discussion
3.1 Structure characterization
ꢁ
the monomer is stable below 350 C. Fig. S4† shows the good
thermal stability of p-PYPZs with a high carbon yield at 800 ꢁC.
Such good thermal characteristics can be attributed to the
triazine-based framework of the materials.
Fig. 1a illustrates the ionothermal strategy for the polymeriza-
tion of p-PYPZs. The monomer PYPZ was rst synthesized by
using 1H,3H-benzo[1,2-c:4,5-c0]difuran-1,3,5,7-tetraone and (4-
Fig. 1 (a) Schematic of the synthesis of p-PYPZs; (b) SEM image of p-PYPZ@600; (c–e) SEM-EDS mapping images of p-PYPZ@600; (f) FT-IR
spectra of PYPZ and p-PYPZs; (g) Raman spectra of p-PYPZs; and (h) XRD patterns of p-PYPZs.
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 8384–8393 | 8387

View Article Online
RSC Advances
Paper
Raman spectroscopy is a conventional and effective method
for characterizing the degree of disorder of carbonaceous
materials.45 The D-band (defect) located at 1347 cmꢀ1 and G-
band (graphite) located at 1602 cmꢀ1 are the two main peaks
in Fig. 1g. The intensity ratios of the D and G bands reect the
degree of material defect. The intensity ratios (ID/IG) of the p-
PYPZs were 0.96, 0.98, 1.00, and 1.02 for p-PYPZ@550, p-
PYPZ@600, p-PYPZ@650, and p-PYPZ@700, respectively. The
defects increased when the reaction temperature increased
from 550 ꢁC to 700 ꢁC. The degree of crystallization in all
samples was determined on the basis of the XRD patterns.
Fig. 1h shows no intense peak and two broad peaks at 24ꢁ and
44ꢁ, corresponding to the (002) and (101) lattice planes of
amorphous carbon.46 The result indicates that the products
exhibited the dominant features of the amorphous carbon
structures. SEM was used in intuitively observing the
morphology of p-PYPZs. The images shown in Fig. S5† are
consistent with the conclusion above; that is, the number of
fragmental products appearing at high polymerization
temperatures increases.
Fig. 2 (a) Nitrogen adsorption/desorption isotherm and (b) pore size
distributions for p-PYPZs; (c) Overall XPS analysis scanned spectrum of
p-PYPZs; (d) high resolution N 1s XPS spectra of p-PYPZs.
3.5 wt%, and XPS shows the nitrogen content decreases from
8.1 to 3.46 at% when the polymerization temperatures were
According to the TEM and HR-TEM images (Fig. 1a, S6 and
S7†), the p-PYPZs possess vermicular pore structures. The pore
structures were further determined using N2 adsorption/
desorption isotherm curves, and the BET method was used.
Their specic surfaces were determined at 77 K. The density
functional theory (DFT) method was used in calculating the
pore size distributions. The results of the specic surface area,
total pore volume, and average pore size are summarized in
Table S4.† The porosity of the resultant product was inuenced
by the reaction temperature because the surface area of p-PYPZs
increased from 981 m2 gꢀ1 to 1464 m2 gꢀꢁ 1 when the polymeri-
ꢁ
ꢁ
changed from 550 C to 700 C, consistent with that from EA.
The complex N 1s spectra of the p-PYPZ could be further
deconvoluted into three typical peaks as follows: 398.3 eV (N-6),
399.6 eV (N-5), and 400.7 eV (N-Q), which correspond to
pyridinic N (or triazine N), pyrrolic N (or carbazole N), and
graphitic N, respectively.48 The contents of different nitrogen
species were inuenced by the reaction temperature. The
results are shown in Fig. 2d. The content of N-Q increased with
the reaction temperature. This phenomenon was attributed to
the conversion of the pyridinic N and pyrrolic N to graphitic N
as the pyrolysis temperature increases.49 The hydrophilicity
properties of p-PYPZs were measured by the contact angles test.
The result showed that the wettability of the material is
improved with the increment total content of oxygen and
nitrogen atoms (Fig. S9†).
ꢁ
zation temperature increased from 550 C to 700 C. As illus-
trated in Fig. 2a, under increasing reaction temperature, the
isotherm curves gradually changed from type I (p-PYPZ@550
and p-PYPZ@600) to type IV (p-PYPZ@650 and p-PYPZ@700),
and the total pore volume increased from 0.65 cm3 gꢀ1 to 0.93
cm3 gꢀ1. The N2 adsorption/desorption isotherms of p-PYPZs
showed high absorption at a low relative pressure (P/P0), indi-
cating the existence of numerous micropores (Fig. 2b). The
3.2 Electrochemical performance of the supercapacitor
formation of hysteresis loops is related to the capillary Owing to their high specic surface areas and unique hierar-
condensation and evaporation in mesopores, and therefore chical pore structures, the as-synthesized materials are prom-
conrmed the coexistence of mesoporous structures.47 Fig. 2b ising candidates as metal-free porous carbon materials for
shows the pore size distribution plots obtained through DFT. supercapacitors. The prepared materials were tested using
The samples had a similar pore size distribution with a micro- a three-electrode cell in 1 M H2SO4 acidic aqueous electrolyte for
pore distribution peak between 0.5 and 1.5 nm, and a mesopore the evaluation of their electrochemical performance through
peak between 2 and 3 nm. The proportion of micropores CV, galvanostatic charge/discharge (GCD), electrochemical
increased with polymerization temperature, and micropores impedance spectroscopy (EIS), and long cycle life test. The CV
contribute more to the specic surface area. Therefore, a higher curves of p-PYPZs in Fig. 3a were tested at a scan rate of 10 mV
temperature will lead to a larger specic surface area, which is sꢀ1. The curves had irregular rectangle shapes and broad redox
consistent with the result in Table S4.†
bumps at approximately 0.1–0.6 V, which are the results of the
The EDS mapping images show the even distribution of redox reactions between the electrolytes and nitrogen- and
nitrogen (pink) and oxygen (green) atoms in the p-PYPZ@600 oxygen-containing functional groups. Another conclusion that
skeleton (Fig. 1b–e). The elemental chemical composition and can be drawn from the CV curves is that p-PYPZ@600 exhibited
bonding congurations of p-PYPZs were explored through XPS the largest integrated area under a potential window of ꢀ0.2–
and EA. As shown in Fig. S8,† three element species existed in p- 0.8 V. This conclusion indicates that the specic capacity of p-
PYPZs, and their contents are listed in Table S1 and S2.† The EA PYPZ@600 was higher than that of the other p-PYPZs materials
test shows that the nitrogen content decreased from 8.85 wt% to under the same measurement conditions.
8388 | RSC Adv., 2021, 11, 8384–8393
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
Fig. 3 Electrochemical performance of p-PYPZs in 1.0 M H2SO4 aqueous solution: (a) CV curves at 10 mV sꢀ1; (b) GCD curves at 1 A gꢀ1; (c)
specific capacitance of p-PYPZs at different current densities; (d) Nyquist plots of p-PYPZs with frequency ranging from 100 kHz to 10 mHz; (e)
cycling performance of the p-PYPZ@600 electrode at 10 A gꢀ1 for 35 000 cycles (inset: GCD curves for the first and last 10 cycles; contact angles
before and after 35 000 cycles).
The GCD curves of p-PYPZs (Fig. 3b) were performed at 1, 2, 5, and 10 A gꢀ1). The small mesopores facilitate the fast ion
1 A gꢀ1, and reect the same trend of capacitance as the results transport through the porous structures, especially when a large
from CV. p-PYPZ@600 shows the longest charge–discharge time current density is employed.51 The as-synthesized p-PYPZs
at 1 A gꢀ1, consistent with the largest integrated area of CV possessed a good rate capacitance property due to the existence
curves, and had the best capacitance characteristics. The of the small mesopores.
specic capacitance of different materials was calculated by the
The conductive and diffusion behavior of the materials were
discharge slope in the GCD curves, which were obtained at measured using the EIS technique (Fig. 3d) in 1 M H2SO4
different current densities (Fig. S10†). p-PYPZ@600 also solution at a frequency ranging from 100 kHz to 10 mHz. The
exhibited superior capacitance of 256 F gꢀ1 at 0.1 A gꢀ1, which is equivalent circuit for the impedance spectra and the tting data
higher than p-PYPZ@550 (241 F gꢀ1), p-PYPZ@650 (212 F gꢀ1), is shown in Fig. S10.† The Nyquist plots of p-PYPZs clearly
and p-PYPZ@700 (142 F gꢀ1). The good supercapacitor perfor- exhibited vertical lines in the low-frequency area, indicating
mance can be attributed to the high content of heteroatoms and a low ionic diffusion resistance. In the high-frequency region,
proper hierarchical pore structure. On the one hand, the the intercept of the Nyquist plots with the Z0 axis represented
pyrrolic nitrogen, pyridinic nitrogen, and quinine-type O can the equivalent series resistance (Rs). Furthermore, the Rs of p-
provide pseudocapacitance through faradaic redox reactions in PYPZ@550, p-PYPZ@600, p-PYPZ@650, and p-PYPZ@700 were
acidic electrolytes. On the other hand, the phenol-type O can 0.25, 0.21, 0.38 and 0.21 U, respectively. The small Rs of p-
enhance the carbon wettability and improve the inltration of PYPZ@600 and p-PYPZ@700 can be attributed to the combined
the electrolyte.50 Thus, the appropriate presence of different effect of both graphitization degree and the N content. The
types of nitrogen and oxygen is important for enhancing the charge transfer resistance (Rct) can be obtained from the
supercapacitor properties. Fig. S10† displays the GCD curves of diameter of the semicircle in the high-frequency region. Rct
different materials at increasing current densities (0.1, 0.2, 0.5, decreased from 2.67, 1.25, 0.81 to 0.62 U for p-PYPZ@550, p-
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 8384–8393 | 8389

View Article Online
RSC Advances
Paper
PYPZ@600, p-PYPZ@650, and p-PYPZ@700, respectively. The not rectangular, indicating inferior electrochemical perfor-
small Rct indicated that it was conducive to transfer electrons on mance compared with the performance of the organic electro-
the surface of the electrode material. The electrochemical lyte. The specic capacitance in Fig. S12(d)† was calculated
stability is another crucial parameter determining the nal long using the GCD technique at different current densities in
cycle life of a material. Hence, the materials were tested at [BMIM][BF4]. The device had a specic capacitance of 74 F gꢀ1 at
a current density of 10 A gꢀ1 for 35 000 cycles, and the result 0.1 A gꢀ1. The Ragone plots shown in Fig. S12† indicate that the
shows that the specic capacitance has an increasing tendency p-PYPZ@600-based supercapacitor can deliver a high-energy
rather than a declining tendency aer 35 000 cycles. This result density of up to 32 W h kgꢀ1 at 0.1 A gꢀ1. As the power
can be attributed to the covalent-based heteroatom-doped density increased to 8845 W kgꢀ1 at 10 A gꢀ1, the energy density
structure, which effectively prevented the loss of heteroatoms was maintained at about 11 W h kgꢀ1
from the electrode, and ensured an efficient charge transfer. Such In summary, the enhanced electrochemical performance of
.
a structure ensured the stability of the capacitance, and the p-PYPZ@600 can be ascribed to the synergistic effects. First,
increasing tendency was mainly due to the active process occurring abundant heteroatoms in the materials not only act as func-
aer the inltration of the electrolyte. The contact angles gure in tional groups for pseudocapacitive performance, but also tune
the inset of Fig. 3e showed the improved hydrophilicity property of the hydrophilicity and polarity of the carbon frameworks. More
p-PYPZ@600 before and aer 35 000 cycles. The GC curves in the importantly, the heteroatoms exist in the materials via covalent
inset in Fig. 3e were symmetrically triangular in shape aer 35 000 bonds, which contribute to the stability. Second, the hierarchical
cycles, which also indicates the outstanding cycling performance pore structure provided the rapid electrochemical kinetics process,
of p-PYPZ@600. The cycling stability of p-PYPZ@600 was better as well as the exposure of the surface area for adsorbing the
than that of other reported heteroatom-doped carbon electrodes charged ions. All of these good properties are attributed to the
(Table S5†). In addition, the preparation method is much easier precise structure by bottom-up method and molecular design.
than that of other reported materials.
All of the above results illustrated that p-PYPZ@600 exhibi-
ted excellent long-term durability as the supercapacitor elec-
3.3 Electrocatalytic properties in ORR
trode material, and the good performance of p-PYPZ@600 was The sluggish kinetic process of the cathodic oxygen reduction
mainly due to the combined advantages of the dened hierar- reaction in fuel cells greatly hinders the commercial application
chical pore structure, high specic surface area, good conduc- of the process. Therefore, a four-electron pathway-dominant
tivity and large amount of stable surface functional groups.
efficient ORR catalyst is of vital importance. The presence
Given the good three-electrode performance of p-PYPZ@600, of N and O greatly affects the electroneutrality of carbon atoms,
we further evaluated the electrochemical properties of p- and promotes catalytic reaction. The large difference in the
PYPZ@600 by assembling symmetric two-electrode super- electronegativity among N, O, and C promotes the adsorption of
capacitors. The energy density of a supercapacitor requires the oxygen reactant.52 Therefore, in this study, the ORR
urgent improvement, and thus symmetric supercapacitors use performance of p-PYPZs was evaluated using a rotating disk
the organic electrolyte TEABF4/AN with a potential window of electrode (RDE) and rotating ring disk electrode (RRDE) method
2.3 V. A wide potential window can greatly enhance the energy in 0.1 M KOH. Fig. S14† shows the linear sweep voltammetry
density properties of the supercapacitors.
(LSV) curves collected at a rotation speed of 1600 rpm by RDE. The
Fig. 4 shows the performance of p-PYPZ@600 in TEABF4/AN. synthesized p-PYPZ@700 has an onset potential of 0.86 V vs. RHE
As shown in Fig. 4a, p-PYPZ@600 exhibited rectangular CV and a diffusion limiting current density of 4.8 mA cmꢀ2, which is
curves and triangular-shaped GCD curves in TEABF4/AN, indi- better than other p-PYPZs, indicating a better mass transport
cating enhanced capacitive performance with facilitated elec- ability and better catalytic activity of p-PYPZ@700 on ORR. The CV
tron and ion propagation process. The specic capacitance was curves in Fig. S13† show a reduction peak at 0.8 V in O2-saturated
calculated using the discharge slope in GCD curves at different 0.1 M KOH, whereas no such peak was found under N2-saturated
current densities in TEABF4/AN. In Fig. 4d (inset), the specic solutions, suggesting an efficient conversion of oxygen. The low
capacitance of p-PYPZ@600 is 121 F gꢀ1 at the current density of overpotential can be comparable to that of Pt/C for about 0.87 V.
0.1A gꢀ1. The Ragone plot is a link between the power and
The electron transfer number and peroxide yield rate are the
energy densities, and is a useful indicator for the performance two crucial parameters reecting the selectivity property of
of the supercapacitor devices. The two-electrode device showed a catalyst. The two-electron pathway produces peroxide as
a high energy density of up to 22 W h kgꢀ1 at 0.1 A gꢀ1, and a catalytic product in contrast to the four-electron pathway,
10 W h kgꢀ1 at 10 A gꢀ1 with a power density of 5737 W kgꢀ1
.
which results in water products. Peroxide generated from a two-
Fig. 4e shows the high stability of p-PYPZ@600 (only 10% electron pathway limits the practical application of the mate-
capacitance decay aer 15 000 cycles at 10 A gꢀ1) in TEABF4/AN, rials in fuel cells, impairing the ESI† or membrane and
indicating a promising application prospect.
poisoning the cells. Therefore, RRDE was used in analyzing
The ionic liquid as 1-butyl-3-methylimidazolium tetra- these properties. By calculating the data of the RRDE test in
uoroborate ([BMIM][BF4]) was used as the electrolyte in the Fig. 5a, we obtained the nal results in Fig. 5b. The average
two-electrode system. We aimed to broaden the voltage window electron transfer number value of p-PYPZ@700 was calculated
to 3.5 V. Fig. S12† shows the performance of p-PYPZ@600 in to be in the range of 3.5–3.7 under the potential of 0.2–0.6 V,
[BMIM][BF4]. The shapes of the CV curves for p-PYPZ@600 were and was the highest among the values of p-PYPZs. Additionally,
8390 | RSC Adv., 2021, 11, 8384–8393
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
Fig. 4 Electrochemical performance of p-PYPZ@600 in the two-electrode system with TEABF4/AN: (a) CV curves at different scan rates; (b) GCD
curves at 0.1 A gꢀ1 and 0.5 A gꢀ1; (c) GCD curves at 1, 2, 5, and 10 A gꢀ1; (d) Ragone plots (inset: specific capacitance at different current densities); (e)
long-term cycling tests of the p-PYPZ@600 electrode with the electrolyte of TEABF4/AN (inset: GCD curves for the first and last 10 cycles).
the peroxide yield rate decreased from 60% for p-PYPZ@550 to crossover and high selectivity for electrocatalytic oxygen
20% for p-PYPZ@700. To summarize, p-PYPZ@700 showed an reduction. This phenomenon indicates a promising prospect of
efficient four-electron reaction process similar to commercial the application in the methanol fuel cell.
Pt/C. The polymerization temperature can greatly affect the ORR
activities and the selectivity of the reaction pathway.
Table S3† lists the contents of different nitrogen species in p-
PYPZs. The content of N-Q increased with temperature. The
increase was caused by the conversion of N-6 and N-5 to N-Q.
Fig. 5d shows the positive correlation between the N-Q content
and the four-electron pathway selectivity. The result can be
attributed to the contribution of N-Q atoms to improving the
conductivity, and providing strong physicochemical interactions
and adsorption of the oxygen reactant. Thus, the high content of N-
Q leads to the best ORR performance of p-PYPZ@700.
The methanol-tolerance capacity of p-PYPZ@700 was tested
through the chronoamperometric measurement and Pt/C as
a reference (Fig. 5c). Methanol (10 ml) was added into 200 ml
0.1 M KOH electrolyte at 100 s. The current density of Pt/C
declined sharply due to the electrochemical oxidization of
methanol. However, such phenomenon did not occur on p-
PYPZ@700, suggesting its excellent tolerance to methanol
Fig. 5 (a) RRDE voltammogram curves of p-PYPZs samples and Pt/C
recorded in 0.1 M KOH at 1600 rpm and 5 mV sꢀ1; (b) electron transfer
number and peroxide yield rate calculated from the RRDE tests; (c)
methanol-tolerance test for p-PYPZ@700 and commercial Pt/C; (d)
electron transfer number n and N-Q content.
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 8384–8393 | 8391

View Article Online
RSC Advances
Paper
All of these properties showed p-PYPZ@700 to be an excellent
candidate as the metal-free ORR catalyst, and implied that N-Q
plays an important role in the catalytic ORR reaction process.
3 G. Wang, L. Zhang and J. Zhang, Chem. Soc. Rev., 2012, 41,
797–828.
4 B. Liang, K. Li, Y. Liu and X. Kang, Chem. Eng. J., 2019, 358,
1002–1011.
5 Y. Liang, Y. Li, H. Wang and H. Dai, J. Am. Chem. Soc., 2013,
135, 2013–2036.
4. Conclusion
A series of multi-heteroatom porous carbon frameworks (p-
PYPZs) was designed and synthesized with a bottom-up method
through the trimerization of the molecular designed monomer,
PYPZ. The as-synthesized p-PYPZs show high ORR activities and
energy storage capacitance owing to the abundant N, O
heteroatoms, large surface areas, and microporous and meso-
porous structures. The large surface area and hierarchical
6 C. Guan, X. Li, Z. Wang, X. Cao, C. Soci, H. Zhang and
H. J. Fan, Adv. Mater., 2012, 24, 4186–4190.
7 Q. Wu, L. Yang, X. Wang and Z. Hu, Adv. Mater., 2020, 32,
e1904177.
8 M. Nawwar, R. Poon, R. Chen, R. P. Sahu, I. K. Puri and
I. Zhitomirsky, Carbon Energy, 2019, 1, 124–133.
9 Y. Yuan and J. Lu, Carbon Energy, 2019, 1, 8–12.
porous structures can shorten the length of ion diffusion and 10 Y. Huang, Y. Wang, C. Tang, J. Wang, Q. Zhang, Y. Wang and
enlarge the effective surface area. The electrochemical perfor- J. Zhang, Adv. Mater., 2019, 31, e1803800.
mance of p-PYPZs was evaluated in an aqueous electrolyte with 11 M. Wang, Y. Li, J. Fang, C. J. Villa, Y. Xu, S. Hao, J. Li, Y. Liu,
a three-electrode system. The as-made p-PYPZ@600 exhibits
a specic capacitance of 256 F gꢀ1 at 0.1 A gꢀ1, and a superb
C. Wolverton, X. Chen, V. P. Dravid and Y. Lai, Adv. Energy
Mater., 2019, 10, 1902736.
cycling stability with a capacity retention of 110% aer 35 000 12 C. X. Zhao, B. Q. Li, J. N. Liu and Q. Zhang, Angew. Chem., Int.
cycles in 1 M H2SO4. The material shows high stability as Ed., 2020, 60(9), 4448–4463.
a symmetric two-electrode supercapacitor (only 10% capaci- 13 S. Liu, Z. Li, C. Wang, W. Tao, M. Huang, M. Zuo, Y. Yang,
tance decay aer 15 000 cycles at 10 A gꢀ1) in TEABF4/AN and
K. Yang, L. Zhang, S. Chen, P. Xu and Q. Chen, Nat.
Commun., 2020, 11, 938.
a high energy density of 32 W h kgꢀ1 at 0.1 A gꢀ1 in [BMIM][BF4].
Such good electrochemical stability can be attributed to the 14 G. He, G. Yan, Y. Song and L. Wang, Front. Chem., 2020, 8,
homogeneous distribution of N, O heteroatoms at the atomic 226.
level. In addition, p-PYPZ@700 possesses good ORR catalysis 15 H. Lu, L. Zhuang, R. R. Gaddam, X. Sun, C. Xiao, T. Duignan,
ability, excellent methanol tolerance, and the four-electron
transfer dominant pathway, which is relative to the content of N-
Z. Zhu and X. S. Zhao, J. Mater. Chem. A, 2019, 7, 22579–
22587.
Q. The effect of different heteroatoms and different types of 16 B. Liu, M. Yang, D. Yang, H. Chen and H. Li, J. Power Sources,
nitrogen should be claried for the further enhancement of the 2020, 456.
electrochemical and electrocatalytic performance, which highly 17 C. C. Jianhua Hou, F. Idrees and X. Ma, ACS Nano, 2015, 9,
depends on the precise control of the structure of the materials. 2556–2564.
Therefore, the facile synthesis route and the conception of the 18 L. L. Z. Jongwoo Han, S. Lee, J. Oh, Kyoung-Seok Lee,
bottom-up strategy show the superiority of the prepared materials,
and such approach can be extended to other cyano-containing
J. R. Potts, J. Ji, X. Zhao, R. S. Ruoff and S. Park, ACS Nano,
2013, 7, 19–26.
monomers to tune the structure of the carbonaceous materials 19 A. M. P. Denisa Hulicova-Jurcakova, O. I. Poddubnaya,
´
´
´
by changing the element composition of the monomer.
F. Suarez-Garcıa, J. M. D. Tascon and G. Qing Lu, J. Am.
Chem. Soc., 2009, 131, 5026–5027.
20 X. Zhao, Q. Zhang, C.-M. Chen, B. Zhang, S. Reiche, A. Wang,
Conflicts of interest
¨
T. Zhang, R. Schlogl and D. Sheng Su, Nano Energy, 2012, 1,
There are no conicts to declare.
624–630.
21 J. P. Paraknowitsch, J. Zhang, D. Su, A. Thomas and
M. Antonietti, Adv. Mater., 2010, 22, 87–92.
22 T. Li, W. Zhang, L. Zhi, H. Yu, L. Dang, F. Shi, H. Xu, F. Hu,
Z. Liu, Z. Lei and J. Qiu, Nano Energy, 2016, 30, 9–17.
Acknowledgements
The authors acknowledge the support from the National Key Research
and Development Program (No. 2018YFB1107500), Liao Ning Revital- 23 F. o. B. Elzbieta Frackowiak, Carbon, 2002, 40, 1775–1787.
ization Talents Program (XLYC1907144), the National Natural Science 24 T. Lin, I.-W. Chen, F. Liu, C. Yang, H. Bi, F. Xu and F. Huang,
Foundation of China (No. 51503024), Dalian Youth Science and
Technology Star Project Support Program (No. 2017RQ104).
Science, 2015, 350, 1508–1513.
´
´
´
25 J. Quılez-Bermejo, E. Morallon and D. Cazorla-Amoros,
Carbon, 2020, 165, 434–454.
26 R. S. Donghui Guo, C. Akiba, S. Saji, T. Kondo and
J. Nakamura, Science, 2018, 351, 361–365.
References
1 C. Wei, R. R. Rao, J. Peng, B. Huang, I. E. L. Stephens, 27 L. Zhang, C. Y. Lin, D. Zhang, L. Gong, Y. Zhu, Z. Zhao, Q. Xu,
M. Risch, Z. J. Xu and Y. Shao-Horn, Adv. Mater., 2019, 31,
e1806296.
2 B. C. Kim, J.-Y. Hong, G. G. Wallace and H. S. Park, Adv.
Energy Mater., 2015, 5, 1500959.
H. Li and Z. Xia, Adv. Mater., 2019, 31, e1805252.
28 W. H. Jun Xu and R. L. McCreery, J. Electroanal. Chem., 1996,
235–242.
29 C. Paliteiro, J. Electroanal. Chem., 1987, 147–159.
8392 | RSC Adv., 2021, 11, 8384–8393
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
30 M. J. Mostazo-Lopez, D. Salinas-Torres, R. Ruiz-Rosas, 41 L. Hao, S. Zhang, R. Liu, J. Ning, G. Zhang and L. Zhi, Adv.
E. Morallon and D. Cazorla-Amoros, Materials, 2019, 12,
1346.
31 T. Lin, I.-W. Chen, F. Liu, C. Yang, H. Bi, F. Xu and F. Huang,
Science, 2015, 350, 1508–1513.
Mater., 2015, 27, 3190–3195.
42 Y. Cao, Y. Zhu, X. Chen, B. S. Abraha, W. Peng, Y. Li,
G. Zhang, F. Zhang and X. Fan, Catal. Sci. Technol., 2019,
9, 6606–6612.
32 S. Zhang, S. Tsuzuki, K. Ueno, K. Dokko and M. Watanabe, 43 S. Cao, B. Li, R. Zhu and H. Pang, Chem. Eng. J., 2019, 355,
Angew. Chem., Int. Ed., 2015, 54, 1302–1306. 602–623.
33 D. Zhu, J. Jiang, D. Sun, X. Qian, Y. Wang, L. Li, Z. Wang, 44 Y. Zheng, S. Chen, H. Song, H. Guo, K. A. I. Zhang, C. Zhang
X. Chai, L. Gan and M. Liu, J. Mater. Chem. A, 2018, 6,
12334–12343.
34 G. Y. J. Chmiola, Y. Gogotsi, C. Portet, P. Simon and
P. L. Taberna, Science, 2018, 313, 1760–1763.
35 G. A. Ferrero, K. Preuss, A. B. Fuertes, M. Sevilla and
M. M. Titirici, J. Mater. Chem. A, 2016, 4, 2581–2589.
and T. Liu, Nanoscale, 2020, 12, 14441–14447.
45 F. Su, C. K. Poh, J. S. Chen, G. Xu, D. Wang, Q. Li, J. Lin and
X. W. Lou, Energy Environ. Sci., 2011, 4, 717–724.
46 H. Yu, L. Shang, T. Bian, R. Shi, G. I. Waterhouse, Y. Zhao,
C. Zhou, L. Z. Wu, C. H. Tung and T. Zhang, Adv. Mater.,
2016, 28, 5080–5086.
36 B. Guo, R. Ma, Z. Li, S. Guo, J. Luo, M. Yang, Q. Liu, 47 L. Jiao, G. Wan, R. Zhang, H. Zhou, S. H. Yu and H. L. Jiang,
T. Thomas and J. Wang, Nano-Micro Lett., 2020, 12, 20. Angew. Chem., Int. Ed., 2018, 57, 8525–8529.
37 Z. Chen, S. Ye, S. D. Evans, Y. Ge, Z. Zhu, Y. Tu and X. Yang, 48 W. Yu, S. Gu, Y. Fu, S. Xiong, C. Pan, Y. Liu and G. Yu, J.
Small, 2018, 14, e1704015. Catal., 2018, 362, 1–9.
38 Y. Qiu, M. Hou, J. Gao, H. Zhai, H. Liu, M. Jin, X. Liu and 49 G. H. Tiva Shari, X. Jia and T. Wagberg, ACS Nano, 2012, 6,
L. Lai, Small, 2019, 15, e1903836. 8904–8912.
39 Z. L. Yuxi Xu, X. Huang, Y. Wang, Yu Huang and X. Duan, 50 F. Hu, J. Wang, S. Hu, L. Li, G. Wang, J. Qiu and X. Jian,
Adv. Mater., 2013, 25, 1–6. Nanoscale, 2016, 8, 16323–16331.
40 P. Kuhn, M. Antonietti and A. Thomas, Angew. Chem., Int. 51 M. Kim, I. Oh and J. Kim, J. Power Sources, 2015, 282, 277–
Ed., 2008, 47, 3450–3453.
285.
52 T. Oh, K. Kim and J. Kim, J. Energy Chem., 2019, 38, 60–67.
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 8384–8393 | 8393