Novel cobalt-doped molybdenum oxynitride quantum dot@N-doped carbon nanosheets with abundant oxygen vacancies for long-life rechargeable zinc-air batteries

Article abstract of DOI:10.1039/d0ta12414a

Rechargeable zinc-air batteries (ZABs) have emerged as promising alternatives for conventional Li-ion batteries due to their high energy density and low manufacturing cost. However, Pt/C and RuO₂-based conventional rechargeable ZABs are mainly constrained by the sluggish kinetics of oxygen reduction/oxygen evolution reactions (ORR/OER), limiting commercialization possibilities. Herein, a new type of oxygen vacancies enriched cobalt-doped molybdenum oxynitride quantum dot-anchored N-doped carbon nanosheets (V_O-CMON@NCNs) was demonstrated as an advanced air-cathode for long-life rechargeable ZABs. Such V_O-CMON@NCN catalyst has an exceptional ORR performance with a high half-wave potential of 0.857 V and tremendous OER performance with an ultrasmall overpotential of 240 mV at a current density of 10 mA cm^?2, outperforming conventional Pt/C and RuO₂catalysts. As proof of concept, rechargeable ZABs with an optimal V_O-CMON@NCN-800 air-cathode showed an ultrahigh specific capacity of 721.2 mA h g_Zn^?1at a current density of 5 mA cm^?2, a tremendous peak power density of 143.7 mW cm^?2, and ultralong cycling life of 500 h. These consequences suggest that the oxygen vacancies enriched V_O-CMON@NCN can serve as promising bifunctional catalysts for next-generation metal-air batteries and other energy-related applications.

Full text of DOI:10.1039/d0ta12414a

Journal of
Materials Chemistry A
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PAPER
Novel cobalt-doped molybdenum oxynitride
quantum dot@N-doped carbon nanosheets with
abundant oxygen vacancies for long-life
rechargeable zinc–air batteries†
Cite this: J. Mater. Chem. A, 2021, 9,
9092
ab
a
Thanh Tuan Nguyen,
Jayaraman Balamurugan,
Kin-Tak Lau,b
a
ac
*
*
Nam Hoon Kim and Joong Hee Lee
Rechargeable zinc–air batteries (ZABs) have emerged as promising alternatives for conventional Li-ion
batteries due to their high energy density and low manufacturing cost. However, Pt/C and RuO2-based
conventional rechargeable ZABs are mainly constrained by the sluggish kinetics of oxygen reduction/
oxygen evolution reactions (ORR/OER), limiting commercialization possibilities. Herein, a new type of
oxygen vacancies enriched cobalt-doped molybdenum oxynitride quantum dot-anchored N-doped
carbon nanosheets (VO-CMON@NCNs) was demonstrated as an advanced air-cathode for long-life
rechargeable ZABs. Such VO-CMON@NCN catalyst has an exceptional ORR performance with a high
half-wave potential of 0.857 V and tremendous OER performance with an ultrasmall overpotential of
240 mV at a current density of 10 mA cmꢀ2, outperforming conventional Pt/C and RuO2 catalysts. As
proof of concept, rechargeable ZABs with an optimal VO-CMON@NCN-800 air-cathode showed an
Received 23rd December 2020
Accepted 9th March 2021
ultrahigh specific capacity of 721.2 mA h gZn at a current density of 5 mA cmꢀ2, a tremendous peak
ꢀ1
power density of 143.7 mW cmꢀ2, and ultralong cycling life of 500 h. These consequences suggest that
the oxygen vacancies enriched VO-CMON@NCN can serve as promising bifunctional catalysts for next-
generation metal–air batteries and other energy-related applications.
DOI: 10.1039/d0ta12414a
rsc.li/materials-a
batteries (ZABs) have emerged as promising candidates owing
to their high theoretical energy density (1353 W h kgꢀ1),
Introduction
exceptional power density, simple fabrication, most safety, and
low manufacturing cost.7,8 Rechargeable ZABs are primarily
driven by an oxygen reduction reaction (ORR) and oxygen
evolution reaction (OER). However, oxygen reactions (ORR and
OER) at catalyst interfaces show a sluggish kinetics nature that
hinders the rechargeable ZABs in industrial aspects.9 State-of-
the-art noble metals, such as Pt/C and RuO2/IrO2-based cata-
lysts, have been demonstrated as excellent catalysts for ORR and
OER, respectively.10,11 However, the high cost, scarcity in nature,
and low durability have motivated the exploration of highly
active and stable bifunctional non-noble catalysts.12–14 There-
fore, it is challenging to develop highly active, durable, cost-
effective, and non-noble bifunctional catalysts for high-
performance rechargeable ZABs. Transition metal-based cata-
lysts have received immense attraction for both ORR and OER
activities due to their exclusive properties of high abundance in
nature, excellent conductivity, environmental friendliness, and
cost-effectiveness.15–18 Owing to their tunable electronic struc-
tures, transition metal oxides (TMOs), such as Mn2O32.64%
Mo,19 NiFeMo oxyhydroxides,20 and 3DOM-Co@TiOxNy,21 have
been widely studied for energy storage and conversion appli-
cations. Nevertheless, TMOs suffer from low conductivity,
The rapid increases in fossil fuel consumption and environ-
mental pollution worldwide are emergent to nd alternative
renewable energy sources. Green and renewable energy sources,
such as wind, solar, and hydrogen energy, have received
particular attention in academics and industries.1,2 However,
the interruption of renewable energy fails to satises the power
grid's stability requirement. To store energy and release it on
emerging demands, utilizing energy storage devices (ESDs) as
an alternative solution has gained numerous scientic societies'
attention. Most convenient ESDs, such as supercapacitors and
fuel cells, possess high power densities but deprived energy
density,3,4 whereas Li-ion batteries have a high energy density
but low power density.5,6 In this context, rechargeable zinc–air
aAdvanced Materials Institute of Nano Convergence Technology (BK21 FOUR), Dept. of
Nano Convergence Technology, Jeonbuk National University, Jeonju, Jeonbuk, 54896,
Republic of Korea. E-mail: jhl@jbnu.ac.kr; nhk@jbnu.ac.kr
bFaculty of Science, Engineering and Technology, Swinburne University of Technology,
John St., Hawthorn, Melbourne, VIC, 3122, Australia
cCarbon Composite Research Centre, Department of Polymer – Nano Science and
Technology, Jeonbuk National University, Jeonju, Jeonbuk, 54896, Republic of Korea
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ta12414a
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Journal of Materials Chemistry A
which may reduce electron transport kinetics from the active interaction between the oxygen vacancies enriched CMON QDs
catalyst to an electrode substrate. In this context, transition and NCNs frameworks. This study further provides an in-depth
metal phosphides, selenides, and nitrides have recently been understanding of oxygen vacancies in active CMON QDs and
studied as highly efficient bifunctional catalysts for ORR and electrochemical performance toward bifunctional ORR and
OER that could boost electrochemical performances compared OER activities and their application in rechargeable ZABs. The
to their corresponding TMOs.22–24 Among them, metal nitrides present investigations could easily translate for a rational
have attracted considerable interest due to their noble-metal- design of oxygen vacancies enriched metal nitride nanohybrids
like properties and abundant electroactive sites. In particular, for next-generation metal–air batteries and other energy-related
molybdenum nitride is widely used as a highly active catalyst for systems.
ORR and OER due to its high electrical conductivity and excel-
lent catalytic activities.25–27 However, due to the low surface area
and fabrication difficulties of the pure molybdenum nitride
phase, it is challenging to achieve desired catalytic activities. In
Experimental
Synthesis of VO-CMON@NCN nanohybrids
this context, doping with another metal has been recognized as
a new strategy to enhance catalytic activities towards ORR and
OER.22,28–30 Similar to the insertion of metal cations, enrichment
of oxygen vacancies is also a benecial route to boost the cata-
lytic activities by altering electronic structures.31–34 For example,
oxygen vacancies enriched metal nitride (VO-MeN) with
numerous active sites has been successfully fabricated through
a simple and cost-effective method to enhance the OER and
ORR activities.35,36 Nevertheless, the electrochemical activities
of the VO-MeN catalysts have not been satisfactory due to the
deprived electron-transport properties across the nanoparticles.
The introduction of conducting carbon nanosheets (CNs)
networks into VO-MeN can enhance the electron-transport
kinetics and catalyst activities.16,37 Compared to CNs, N-doped
carbon nanosheets (NCNs) show numerous electroactive sites
due to a high charge polarization between nitrogen and carbon,
thus improving the electron-transport kinetics.13,38,39 The mini-
mization of the VO-MeN particle size is another novel approach
to enhance the electrochemical performance due to the
increased number of electroactive sites to form superstructures
effectively.
All chemicals were purchased from Sigma-Aldrich and used
directly without further purication. In a typical process, 0.25 g
glucose and 5 g dicyandiamide were dissolved in 200 mL
deionized (DI) water. Aer that, 0.02 mmol of Co(NO3)2$6H2O
and 0.02 mmol of (NH4)6Mo7O24$4H2O were slowly added to the
above mixture with constant stirring at 80 ꢁC until the water
completely evaporated. The as-obtained powder was pyrolyzed
at different temperatures (700, 800, and 900 ꢁC) under a mixture
of Ar/H2 gas atmosphere (ratio 90 : 10). Samples were denoted
as VO-CMON@NCN-700, VO-CMON@NCN-800, and VO-
CMON@NCN-900, respectively, for further characterization. For
comparison, we have pyrolyzed the reaction mixture only under
an Ar atmosphere instead of an Ar/H2 mixture gas, and the nal
material was denoted as T-CMON@NCN-800.
Physical characterizations
The morphological and structural analyses of the as-obtained
materials were investigated using a eld emission scanning
electron microscope (FE-SEM; JEOL JSM-6701F, Japan) and
a transmission electron microscopy (TEM; JEM-ARM200F,
Japan). An X-ray diffractometer (Rigaku Ultima IV; Cu Ka radi-
ation; l ¼ 0.154 nm) was used to accumulate the X-ray diffrac-
tion (XRD) patterns at the Center for University-Wide Research
Facilities (CURF) in Jeonbuk National University. An EDAX
(SUPRA 40 VP; Carl Zeiss, Germany) and an inductively coupled
plasma-atomic emission spectroscope (ICP-AES; J-A1100;
Jarrell-Ash Company, Japan) were used to examine the
elemental compositions. The roughness and thickness of the
as-obtained materials were measured by an atomic force
microscope (AFM; Park System, XE-100). The surface area and
pore size structure of the as-obtained materials were projected
from a BET (Micromeritics ASAP 2020 at ꢂ77 K). The elemental
compositions and oxidation states of the as-obtained materials
were recorded with X-ray photoelectron spectroscopy (XPS;
Theta Probe; Thermo Fisher Scientic, UK) in the Jeonju center
of KBSI. The electron paramagnetic resonance (EPR) spectra
were collected on Bruker EMX plus 6/1 spectrometer equipped
with a dual-mode cavity (ER4116DM).
Herein, we proposed a new approach to design oxygen
vacancies enriched cobalt-doped molybdenum oxynitride
quantum dots (QDs) hybridized with NCNs (VO-CMON@NCNs)
that could be employed as robust bifunctional ORR and OER
catalysts to substitute Pt/C and RuO2 with low-cost and high-
performance rechargeable ZABs. To the best of our knowl-
edge, there has been no report on
a VO-CMON@NCN
nanohybrid-based bifunctional catalyst for ORR and OER and
their application in rechargeable ZABs. The effect of the oxygen
vacancies on the active CMON QDs was systematically investi-
gated in this present study. The optimal VO-CMON@NCN-800
catalyst showed excellent ORR performance with a high half-
wave potential (E1/2) value of 0.857 V and tremendous OER
performance with an ultrasmall overpotential of 240 mV at 10
mA cmꢀ2, outperforming the conventional Pt/C and RuO2
catalysts.40–42 Most impressively, rechargeable ZABs with VO-
CMON@NCN air-cathode exhibited high open-circuit voltage
(OCV) of 1.435 V, a high peak power density of 143.7 mW kgꢀ1
,
and ultralong cycle life of 500 h, outperforming the benchmark
Pt/C + RuO2 air-cathode-based rechargeable ZAB and recently
Electrochemical analyses
reported ZABs in the literature.43,44 Such high electrochemical The Gamry Reference 3000 (Gamry Instruments, USA) electro-
performance of VO-CMON@NCN is due to its large specic chemical workstation was used to investigate all electro-
surface area, numerous porous networks, and high synergistic chemical measurements. To prepare the working electrode,
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5 mg of the as-obtained nanohybrids and 10 mL of Naon used as the air-cathode, polished Zn foil as the anode, and PVA-
(5 wt%) were dispersed in 1 mL water and ethanol (ratio of 1 : 1 KOH was used as the gel electrolyte. The gel electrolyte was
by volume). Then, the mixture was sonicated for half an hour. prepared by dissolving PVA and 6 M KOH and 0.2 M Zn(ac)2 at
Aer that, 10 mL of ink solution was drop cast onto the rotating 90 ꢁC to obtain the homogeneous solution.45 Aer that, the gel
disk with a glassy carbon electrode (GCE) for electrochemical mixture was poured on the Petri-disc and kept in the freezer to
performance. Hg/HgO and graphite rod were used as the attain the gel-electrolyte. The acrylic tape (3 M) was used to seal
reference and counter electrodes, respectively, and an aqueous the rechargeable QSS-ZABs. The i–v polarization and power
solution of 0.1 M KOH was used as the electrolyte. The ORR density measurements of the rechargeable ZABs were carried
measurement was carried out in O2 saturated 0.1 M KOH with out from 0.2 to 0.4 A. A Maccor 4300 (USA) was employed to
different rotation speeds from 400 to 2800 rpm at a xed scan perform the discharge/charge of both liquid and QSS
rate of 10 mV sꢀ1. The CV measurement was performed in N2 rechargeable ZABs at current densities of 5 and 20 mA cmꢀ2
and O2 saturated 0.1 M KOH at a xed scan rate of 50 mV sꢀ1
.
.
For the rotating ring-disk electrode (RRDE), the ring potential
was set to 0.5 V (vs. Hg/HgO) to oxidize intermediately. The
linear sweep voltammetry (LSV) was performed to examine the
OER activity of the as-obtained catalysts at a xed scan rate of
1 mV sꢀ1. All of the potentials were obtained from ORR, and the
OER studies were converted into reversible hydrogen electrodes
(RHE) with the Nernst equation (ERHE ¼ EHg/HgO + 0.059 ꢃ pH +
0.197). The EIS measurement was carried out at a frequency
range from 0.01 to 100 000 Hz with an amplitude of 5 mV at the
open circuit potential.
Results and discussion
Materials characterization
The VO-CMON@NCN was synthesized by a simple, scalable, and
cost-effective in situ one-step pyrolysis process, and is illustrated
in Fig. 1. First, mixed metal precursors, dicyandiamide, and
glucose were dissolved in DI water and vigorously stirred at
80 ꢁC overnight. The mixture was then pyrolyzed at various
temperatures from 700 ꢁC to 900 ꢁC to achieve enriched oxygen
vacancies with high electrochemical performance. For
comparison, a monometallic VO-MON@NCN nanohybrid was
also prepared under similar experimental conditions. During
the pyrolysis process, an electrostatic interaction was developed
Fabrication of rechargeable zinc–air batteries
For the fabrication of liquid-based rechargeable ZABs, the between dicyandiamide and the mixed metal, resulting in
optimal VO-CMON@NCN-800 ink was coated onto a carbon oxygen vacancies enriched CMON QDs integrated with highly
cloth and employed as the air-cathode, polished Zn foil was porous N-doped carbon nanosheets. The number of oxygen
used as the anode, and 6 M KOH with 0.2 M Zn(ac)2 was used as vacancies could be controlled by varying the argon and
the electrolyte. The total amount of the air-cathode material was hydrogen gas ratio and pyrolysis temperature in the present
about 2.1 mg cmꢀ2, and the active area of the electrode was investigation. It should be noted that the formation of the
around 1.0 ꢃ 1.0 cm2. For the exible rechargeable QSS-ZAB, oxygen vacancies enhanced the reversibility of the catalyst, and
the optimal VO-CMON@NCN-800 ink-coated carbon cloth was boost the activities and durability of the ORR and OER.
Fig. 1 Schematic illustration for the rational design and fabrication of VO-CMON@NCN nanohybrids, and their application in rechargeable ZABs.
9094 | J. Mater. Chem. A, 2021, 9, 9092–9104
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The XRD patterns of VO-CMON@NCNs with different pyrol-
Journal of Materials Chemistry A
The morphological characteristics of all as-obtained mate-
ysis temperatures are displayed in Fig. S1.† For the mono- rials were examined by FE-SEM and TEM analyses, and the
metallic VO-MON@NCN-800, the typical XRD pattern showed results are shown in Fig. 2 and S2.† The FE-SEM image of VO-
three strong peaks at 37.8, 43.0, and 64.1ꢁ, corresponding to the CMON@NCN-800 revealed that the ultrasmall VO-CMON QDs
(112), (200), and (204) lattice planes of the tetragonal Mo2N were homogeneously anchored onto highly porous and crum-
crystal (ICDD #00-025-1368). When the Co atom was doped into pled NCNs. The FE-SEM image further showed the numerous
the monometallic VO-MON@NCN-800, the XRD pattern clearly nanoporous networks throughout VO-CMON@NCN-800
showed diffraction peaks at 40.1, 42.6, 44.3ꢁ corresponding to (Fig. 2a). They could enhance the electroactive surface area,
the (122), (031), and (131) planes, respectively, fully matching and improve the ORR and OER activities. The homogeneous
with the Co2Mo3N phase (ICDD #04-010-6426).46 Notably, the distribution of the Co, Mo, O, N, and C elements in VO-
intensities of these peaks were relatively low. It might be CMON@NCN-800 was further conrmed by SEM-EDS color
because the ultra-small nanoparticles were integrated into mapping (Fig. S3†). In contrast, the FE-SEM and HR-TEM
NCNs, reecting the successful encapsulation of the VO-CMON images of VO-CMON@NCN-700 and VO-CMON@NCN-900
QDs into NCNs frameworks. Besides, there was a broad peak at showed that the bulk sizes of CMON with different shapes
25.7ꢁ, which corresponded to the (002) plane of the N-doped were supported onto the NCN frameworks, which may reduce
CNs,47,48 demonstrating the successful formation of NCNs in the catalytic activities and durability of the catalysts. These
the VO-CMON@NCN nanohybrids. Most remarkably, the results demonstrate that the optimal synthesis conditions could
optimal VO-CMON@NCN-800 catalyst showed well-resolved enrich the oxygen vacancies and simultaneously lead to an
peaks compared to the other comparative catalysts pyrolyzed effective nitridation process. The thermal stability of all as-
at 700 and 900 ꢁC (i.e., VO-CMON@NCN-700 and VO- obtained materials was carried out by thermogravimetric anal-
ꢁ
CMON@NCN-900). This might be due to a highly crystalline ysis (TGA) in the temperature range of 40 to 800 C under an
lmꢁwith pure phase formation at an optimum temperature of atmosphere of air (Fig. S4†). A large weight loss occurred at
800 C.
a temperature in the range of 400 to 500 ꢁC due to the
Fig. 2 Morphological analysis of VO-CMON@NCN-800: (a) SEM image, (b) TEM image (inset shows the corresponding particle size distributions),
(c) HR-TEM image, and (d) STEM-EDS color mapping of VO-CMON@NCN-800 with respect to the Co, Mo, O, N, and C elements.
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Fig. 3 (a) The XPS survey and deconvolution of (b) Co 2p, (c) Mo 3d, (d) O 1s, (e) N 1s + Mo 3p, and (f) C 1s for VO-CMON@NCN-800; (g) EPR; (h)
Raman spectra, and (i) BET (inset show the pore size distribution) of VO-CMON@NCN-800 and T-CMON@NCN-800.
decomposition of the NCNs frameworks. The calculated crystalline nature (inset of Fig. 2c). Moreover, the presence of
amount of carbonaceous species was about 35, 37, and 38% for the Co, Mo, O, N, and C elements in the VO-CMON@NCN-800
VO-CMON@NCN-700,
CMON@NCN-900, respectively, which are entirely consistent EDS also conrmed the successful formation of VO-CMON
with EDAX and ICP-AES observations (Table S1†). QDs in the nal VO-CMON@NCN-800 nanohybrid through our
VO-CMON@NCN-800,
and
VO- was further proved by STEM-EDS analysis (Fig. 2c). The STEM-
The TEM images further revealed that CMON QDs were simple and cost-effective strategy. The thickness and roughness
homogeneously embedded into the NCNs frameworks (Fig. 2b). of all as-obtained materials were measured with an AFM, and
The particle size distribution clearly showed that CMON QDs are shown in Fig. S5.† The thickness of VO-CMON@NCN-800
had an ultrasmall size with an average diameter of 1.6 ꢄ 0.4 nm was around 1.4 nm, which is thinner than VO-CMON@NCN-
(inset of Fig. 2b). The high-resolution TEM (HR-TEM) image 700 (1.6 nm) and VO-CMON@NCN-900 (1.7 nm), and is in
further proved that highly crystalline CMON QDs were effec- good agreement with TEM and SEM observations.
tively anchored onto NCNs. During the pyrolysis process,
The XPS measurement was carried out to understand the
dicyandiamide was decomposed, generating the N species and elemental compositions and bonding nature of the as-obtained
effectively forming metal nitride QDs, as well as high doping materials and results shown in Fig. 3a–f and S6.† The survey
of N into the carbon framework, which could improve the XPS spectrum of VO-CMON@NCN-800 displayed typical peaks
catalytic activities and durability. The highly porous N-doped at ꢂ782.6, 531.1, 397.1, 285.8, and 233.9 eV that correspond to
graphitic C framework can completely protect the CMON QDs the Co, O, N, C, and Mo elements, respectively (Fig. 3a). It is well
and avoid aggregation during a long-term cycling stability test. known that the doping amount of N in the host materials could
The VO-CMON@NCN-800 possesses a d-spacing of ꢂ0.222 and crucially enhance the ORR and OER activities. As a result, the
ꢂ0.211 nm, corresponding to the (122) and (031) planes, amount of N content for VO-CMON@NCN-800 was high as
respectively (Fig. 2c). The magnied HR-TEM image clearly 14.3%, which is much higher than the other comparative
showed that the VO-CMON@NCN-800 possesses a highly materials of VO-CMON@NCN-800 (13.1%) and VO-
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Fig. 4 Electrochemical performance of the as-obtained catalysts: (a) LSV of all as-obtained catalysts at a fixed rotation speed of 1600 rpm, (b)
corresponding Tafel plots, (c) LSV curves of the optimal VO-CMON@NCN-800 catalyst at different rotation speeds from 400 to 2800 rpm, and
(d) corresponding K–L plots. (e) RRDE measurement, (f) percentage of peroxide in the total oxygen reduction products of all catalysts, (g) LSV for
both OER and ORR of VO-CMON@NCN-800 and Pt/C, (h) LSV, and (i) EIS of the all as-obtained catalysts. All electrochemical measurements
were conducted in 0.1 M KOH.
CMON@NCN-900 (13.7%), demonstrating that dicyandiamide replaced by oxygen. Subsequently, the enriched oxygen vacan-
was a useful N precursor with an optimum pyrolysis tempera- cies were formed. These rich oxygen vacancies could help to
ꢁ
ture of 800 C. As shown in Fig. 3b, the Co 2p deconvolution enhance the electron transport behaviors and catalyst activities
spectrum of VO-CMON@NCN-800 exhibited typical peaks at of VO-CMON@NCN-800. The N 1s and Mo 3p deconvolution
ꢂ779.1 eV and 781.9 eV corresponding to Co 2p3/2, whereas the spectra displayed typical peaks at ꢂ397.2, 400.1, 402.6, and
42
peaks at ꢂ794.1 eV and 797.5 eV were related to Co 2p1/2
.
403.9 eV, corresponding to the pyridinic N, pyrrolic N, Me–N,
Furthermore, two broad peaks at ꢂ786.1 and 802.8 eV were graphitic N, and oxidized N, respectively.50 Also, the standard
illustrated as satellites of Co 2p3/2 and Co 2p1/2, respectively. peaks positioned at 394.6 and 398.7 eV correspond to Mo 3p3/2
,
,
The Mo 3d deconvolution spectrum showed the prominent whereas the peaks at 412.2 and 416.1 eV are related to Mo 3p1/2
peaks positioned at ꢂ235.8 and 232.7 eV, which corresponded respectively. Notably, the high contents of pyridinic N and
to the Mo 3d3/2, and the peaks at 231.9 and 228.6 eV related to pyrrolic could increase the catalytic activities of VO-
N
Mo 3d5/2, respectively. The O 1s were deconvoluted into CMON@NCN-800. Moreover, VO-CMON@NCN-800 holds about
different peaks at ꢂ530.3, 531.1, 532.2, and 533.1 eV, corre- 27.7% of pyridinic N, which is relatively higher than VO-
sponding to Co–O–N, Mo–O–N, N–C–O, and C–O–C, respec- CMON@NCN-700 (24.3%) and VO-CMON@NCN-900 (25.1%)
tively. The peak that appeared at 530.8 eV represented the (Fig. S7†). Such high pyridinic N could boost the ORR activities
oxygen vacancies with high relative intensity.36,49 In contrast, the and durability. The additional peak was observed at 401.2 eV,
XPS spectra of the VO-CMON@NCN-700 and VO-CMON@NCN- indicating that Me–N bonding in the VO-CMON@NCN-800
900 materials further conrm the successful formation nanohybrid was successfully formed. The high-resolution C 1s
bonding of Co and Mo with the O, N, and C elements in the spectrum was deconvoluted into multiple peaks at ꢂ284.4,
NCNs frameworks. During the pyrolysis process, dicyandiamide 284.9, 285.5, 286.4, 288.2, and 291.2 eV related to the C–C, C]C,
was decomposed, liberating numerous N atoms that could be C]N, C]O, COOH, and p–p* bonds, respectively. Specically,
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Fig. 5 Electrochemical performances of the liquid-based rechargeable ZABs: (a) schematic illustration of the assembled liquid-based
rechargeable ZAB with the VO-CMON@NCN-800 air-cathode, (b) i–v polarization and power density curves of the liquid-based rechargeable
ZABs. (c) Specific capacity of the liquid-based rechargeable ZABs at a current density of 5 mA cmꢀ2. (d) Cycling stabilities of the rechargeable
ZABs with VO-CMON@NCN-800 and Pt/C + RuO2 air-cathodes, and (e) the photos of the open-circuit voltage, powered LEDs, and configu-
rations of the series-connected rechargeable ZABs, respectively.
the C]N bond further validated the effective doping of N into CMON@NCN-800 in the sp2 hybrid carbon frameworks,
the NCN frameworks. Fig. 3g shows the EPR spectra of VO- whereas the G band represents an in-plane stretching of the C–C
CMON@NCN-800 and T-CMON@NCN-800. The signal intensi- bond in graphitic carbon. The ID/IG ratio of the VO-
ties in the EPR spectra represent the oxygen vacancies concen- CMON@NCN-800 was ꢂ1.08, which is much higher than T-
tration of the as-obtained materials. Remarkably, the VO- CMON@NCN-800 (ꢂ0.93). It was due to the effective encapsu-
CMON@NCN-800 displayed the highest intensity when lation of the oxygen vacancies enriched CMON QDs into the
compared to T-CMON@NCN-800, indicating the electron trap- NCNs framework with more defects. Such defects could
ping at the oxygen vacancies.51,52 The results further demon- enhance the electroactive site and durability of VO-
strated that VO-CMON@NCN-800 possesses the abundant CMON@NCN-800. The VO-CMON@NCN-800 displayed a G
oxygen vacancies in a controlled pyrolysis process.
band at ꢂ1592 cmꢀ1, which was relatively higher than the G
Moreover, the Raman spectra of the optimal VO- band of T-CMON@NCN-800 (1581 cmꢀ1), demonstrating
CMON@NCN-800 catalyst exhibited peaks at 1354 cmꢀ1 and a highly synergistic interaction between the active CMON QDs
1584 cmꢀ1 corresponding to the D and G bands, respectively and NCNs support.38
(Fig. 3h).53 The D band arises from a disordered structure of VO-
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Fig. 6 Electrochemical performance of rechargeable QSS-ZABs: (a) schematic illustration of the assembled rechargeable QSS-ZAB and series-
connected flexible QSS ZABs supplying power to the yellow LED, (b) cycling stabilities of QSS ZABs with VO-CMON@NCN-800 and Pt/C + RuO2
air-cathodes at a current density of 5 mA cmꢀ2, and (c) the OCV and different states of rechargeable QSS-ZAB, and (d) the charge–discharge
curves of rechargeable QSS-ZAB with different bending and twisting states.
The N2 adsorption–desorption analysis validated the porous hierarchical porous structure with enriched oxygen vacancies in
nature of the as-obtained VO-CMON@NCN-700, VO- the optimal VO-CMON@NCN-800 provided a large specic
CMON@NCN-800, VO-CMON@NCN-900, and T-CMON@NCN- surface area, numerous micropores, and excellent electrical
800 nanohybrids (Fig. 3i and S8†). The as-obtained nano- conductivity. Thus, we expect VO-CMON@NCN-800 to be
hybrids showed a type I isotherm curve with a concave shape to utilized as a bifunctional catalyst for ORR and OER activities,
the relative pressure (P/P0). The calculated Brunauer–Emmett– and their application in rechargeable ZABs to boost the elec-
Teller (BET) surface area of VO-CMON@NCN-800 was as high as trochemical performances.
ꢂ875.3 m2 gꢀ1, which is relatively larger than VO-CMON@NCN-
700 (ꢂ772.3 m2 gꢀ1), VO-CMON@NCN-700 (ꢂ695.7 m2 gꢀ1), and
Bifunctional catalytic activities of VO-CMON@NCN
nanohybrids
T-CMON@NCN-800 (ꢂ768.4 m2 gꢀ1). The BJH analysis further
revealed that VO-CMON@NCN-800 had unique pores with an
ORR performance. The ORR activity of the as-obtained
average diameter of about 1.92 nm (inset of Fig. 3i and S8†),
catalysts was measured using a rotating disk electrode (RDE)
which is much smaller than VO-CMON@NCN-700 (3.94 nm), T-
in 0.1 M KOH. The cyclic voltammetry (CV) measurement was
CMON@NCN-900 (4.17 nm), and T-CMON@NCN-800 (2.31
nm), indicating its microporous nature that could improve the
catalytic activities of VO-CMON@NCN-800. These results
suggest that the optimal synthesis conditions could produce
a large specic surface area and exclusive microporous nature,
thus enhancing the OER and ORR activities. This study used
a facile, scalable, and cost-effective in situ method to produce
performed in N2 and O2-saturated 0.1 M KOH at a xed scan rate
of 50 mV sꢀ1 at a potential range from 0.2 to 1.2 V vs. RHE
(Fig. S9†). In the N2 saturated 0.1 M KOH, the typical CV curve
exhibited a capacitive behavior. Remarkably, the CV displayed
a distinct peak at ꢂ0.853 V vs. RHE in O2-saturated 0.1 M KOH,
demonstrating an unusual ORR activity. The LSV curves of the
as-obtained catalysts were obtained at a xed rotation speed of
VO-CMON QDs anchored NCN frameworks to avoid the
1600 rpm, and the results are shown in Fig. 4a. The optimal VO-
conventional toxic ammonia gas synthesis route. The
CMON@NCN-800 catalyst held superior ORR performance with
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a higher onset potential of ꢂ0.89 V, which is comparable to only 84.5%, demonstrating its outstanding durability. More-
conventional Pt/C (ꢂ0.96 V). The half-wave potential (E1/2) is the over, the LSV curves of the VO-CMON@NCN-800 catalyst at
most critical parameter when evaluating the ORR activity. The 1600 rpm exhibited almost similar E1/2 aer 10 000 CV cycles,
VO-CMON@NCN-800 catalyst displayed E1/2 as high as ꢂ0.857 V which is superior to the Pt/C catalyst, further conrming its
vs. RHE, similar to Pt/C (ꢂ0.877 V). This is much higher than outstanding ORR durability (Fig. S13†). To further examine and
other comparative catalysts, such as VO-CMON@NCN-700 understand the mechanism of the active sites and the stability
(ꢂ0.826 V), VO-CMON@NCN-900 (ꢂ0.833 V), or VO- of the catalyst, morphological analysis was performed aer ORR
MON@NCN-800 (ꢂ0.829 V), and superior to recently reported measurements. The SEM and HR-TEM images clearly showed
non-noble ORR catalysts (Table S2†). Interestingly, VO- that the VO-CMON@NCN-800 nanohybrid retained its nano-
CMON@NCN-800 received a high limiting current of 6.88 mA structure without any change, further demonstrating its
cmꢀ2, which is comparable to conventional Pt/C (6.93 mA cmꢀ2
)
exceptional stability towards ORR (Fig. S14†). Besides, the XPS
(Fig. 4a). Tafel slopes were generally used to analyze the kinetic was conducted aer ORR performances presented in Fig. S15.†
rates of the catalyst toward ORR. The Tafel slopes of all as- The pyridinic N peak increased, while the pyrrolic N and
obtained catalysts are illustrated in Fig. 4d. The optimal VO- graphitic N contents decreased because N was oxidized during
CMON@NCN-800 catalyst showed an ultrasmall Tafel slope the ORR process, which caused the reduction of the N amount
value of ꢂ69.7 mV decꢀ1, which is similar to Pt/C (ꢂ62.9 mV in the catalyst. Also, pyrrolic N is less stable than pyridinic N
decꢀ1), much smaller than VO-CMON@NCN-700 (80.3 mV and graphitic N, and more vulnerable to oxidation.37 Also, the
decꢀ1), VO-CMON@NCN-900 (88.4 mV decꢀ1), and VO- reaction of the C atom next to the pyridinic N and –OH caused
MON@NCN-800 (109.1 mV decꢀ1), indicating that the VO- the conversion of pyridinic N to pyridonic N (400.7 eV) aer
CMON@NCN-800 catalyst held superior ORR kinetics. To ORR performances, illustrating a higher active site of C atoms
further investigate the ORR activities of the as-obtained cata- near the pyridinic N.13 The small shiing of N 1s XPS exhibited
lysts, LSV was performed at different rotation speeds from 400 an extremely stable catalyst under ORR measurements. More-
to 2800 rpm in O2-saturated 0.1 M KOH, and the results are over, the bonding of –OH could shi the binding energy of N 1s
shown in Fig. 4c and S10.† When the rotating speed was from 398.9 to 400.3 eV (Fig. S15e†). The XPS of O 1s and C 1s
increased from 400 to 2800 rpm, the diffusion-limiting current aer ORR revealed the –OH group due to the oxygen reduction
density was signicantly increased, indicating the enhance- process. The intensity of the C–OH bonding for the non-
ment of O2 diffusion toward the electrode surface. Koutecky– aromatic hydrocarbon (532.1 eV) decreased. In contrast, the
Levich (K–L) plots between jꢀ1 and uꢀ1/2 were derived from the aromatic hydrocarbon (533.4 eV) increased, illustrating that the
LSV of VO-CMON@NCN-800 with various potential from 0.3 to –OH group preferred to attach the aromatic carbon aer the
0.7 V (Fig. 4d). The corresponding number of electrons (n) ORR process (Fig. S15d†). The increasing intensity of O 1s aer
transferred was 3.9, indicating that the VO-CMON@NCN-800 ORR performances due to peroxide formation may lead to the
catalyst held a four-electron ORR pathway (O2 + 4eꢀ + 2H2O oxidation of the carbon species and stabilization of N defects on
/ 4OHꢀ).
the graphene framework (Fig. S15e†). Besides, the XPS of C 1s
The RRDE was performed to understand the ORR kinetics of aer ORR performances displayed a higher C–OH bonding
the optimal VO-CMON@NCN-800 catalyst, and illustrated in intensity (285.5 eV) (Fig. S15f†). Such high intensity of graphitic
Fig. 4e. As received, the VO-CMON@NCN-800 catalyst displayed C (284.5 eV) demonstrated a highly stable carbon under ORR
a ring current density of 6.88 mA cmꢀ2, which is very close to Pt/ performances. Moreover, the high intensity at 286.4 eV showed
C (6.94 mA cmꢀ2). The percentages of H2O2 for the as-obtained that there was plenty of C–N, which is fully consistent with
catalysts were calculated during the ORR performances at the high-resolution N 1s XPS.
a potential range of 0.3 to 0.7 V vs. RHE, and are shown in
OER performance. For the successful application as an air-
Fig. 4f. These results revealed that the VO-CMON@NCN-800 cathode in rechargeable ZABs, it is essential to examine the
catalyst showed an H2O2 yield as low as 6.1 to 23.3%, which is OER performance. Fig. 4g displays the OER polarization curves
comparable to conventional Pt/C (7.9 to 9.1%). To examine such of all as-obtained catalysts. The optimal VO-CMON@NCN-800
conict of undesirable effects by fuel oxidation, we measured catalyst showed an ultrasmall onset potential of 1.42 V vs.
the chronoamperometric response of the VO-CMON@NCN-800 RHE, which is comparable to benchmark RuO2 (1.40 V vs. RHE),
catalyst with methanol addition. When 10 mL of methanol and much lower than those of other counterparts. Most
was quickly injected into 150 mL of 0.1 M KOH, the VO- importantly, the optimal VO-CMON@NCN-800 catalyst exhibi-
CMON@NCN-800 catalyst predominantly held the current with ted an ultralow overpotential of 240 mV at a current density of
minimum changes. In contrast, the Pt/C catalyst showed high 10 mA cmꢀ2, which is much lower than that for benchmark
sensitivity towards the intermediate fuel oxidation, resulting in RuO2 (220 mV), VO-CMON@NCN-700 (270 mV), VO-
a dramatic current loss. This further conrmed that the NCN CMON@NCN-900 (300 mV), and pure VO-MON@NCN-800 (320
frameworks could protect the active VO-CMON QDs from mV), and superior to the recently reported non-noble OER
methanol oxidation (Fig. S11†). The long-term ORR durability of catalysts (Table S3†). As shown in Fig. S16,† the Tafel slope value
the VO-CMON@NCN-800 catalyst was then investigated using of the VO-CMON@NCN-800 catalyst was as low as 74.4 mV
chronoamperometry (CA) and LSV techniques. As shown in decꢀ1, which is much lower than that for the conventional RuO2
Fig. S12,† the VO-CMON@NCN-800 catalyst held 97.4% of its catalyst (69.5 mV decꢀ1) and other comparative catalysts. These
current density aer 40 h, whereas the Pt/C catalyst retained results further demonstrated that the VO-CMON@NCN-800
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catalyst holds ultrafast OER kinetics. Electrochemical imped- proton transport kinetics for OER. Consequently, the optimal
ance spectroscopy (EIS) was performed to investigate the as- VO-CMON@NCN-800 catalyst is expected to achieve high-
obtained catalysts' conductivity, and the results are shown in performance rechargeable ZABs with ultralong cycle life. The
Fig. 4h. By tting with the equivalent circuit, the VO- synergistic effect of the bimetallic nanohybrids directly inu-
CMON@NCN-800 catalyst exhibited an ultrasmall charge- ences the interaction of the catalyst surface towards the oxygen
transfer resistance (Rct) of 0.36 U, which is much lower than reactions. The enormous differences in the compound's cata-
conventional RuO2 (0.59 U), VO-CMON@NCN-700 (0.46 U), VO- lytic performance are robustly affected by the electronic char-
CMON@NCN-900 (0.44 U), and VO-MON@NCN-800 (0.60 U). acteristic nanostructures and the bond strength of the
Such ultrasmall Rct value of the VO-CMON@NCN-800 catalyst catalysts.59,60 By controlling the synthesis conditions, the
could lead to improved catalytic activities and enhanced geometries and electronic nanostructures of the VO-CMON QDs
electron-transport kinetics. To determine the electrochemically were optimized. Moreover, the chemical bonding can change
active surface areas (ECSAs) of the as-obtained catalysts, CV the substantial distance between the Co and Mo atoms.
measurement was carried out in a non-faradaic region at Therefore, they radically change the catalytic performances. The
various sweep rates of 10 to 100 mV sꢀ1 in an interval of 10 mV monometallic nanohybrids required more vacant coordination
sꢀ1. The VO-CMON@NCN-800 catalyst held a double-layer sites for the catalytic activations, which drastically reduced the
capacitance (Cdl) as high as 59.9 mF cmꢀ2 (Fig. S17†). These activity. Therefore, the electronic interactions between the
results suggested that the VO-CMON@NCN-800 catalyst had the bimetallic nanohybrid components could drastically boost the
highest ECSA value because the VO-CMON QDs could be catalytic activities towards ORR and OER.61 On the other hand,
homogeneously anchored onto the NCN frameworks without the oxygen vacancies could improve the oxygen absorbance,
any aggregation. The turnover frequency (TOF) represents the enhancing the ORR and OER activities and durability.
number of O2 molecules generated per active site per unit
time.54 To further understand the inherent activity of the as-
obtained catalysts, the TOFs of the as-obtained catalysts were
Rechargeable ZABs performances with VO-CMON@NCN-800
air-cathode
calculated and examined. The VO-CMON@NCN-800 catalyst
possessed the highest TOF value of 0.162 sꢀ1 at an overpotential
Liquid-based rechargeable ZABs. Benetting from both ORR
of 260 mV, which was much higher than VO-CMON@NCN-700 and OER activities of VO-CMON@NCN-800, rechargeable ZABs
(0.0636 sꢀ1), VO-CMON@NCN-900 (0.0367 sꢀ1), and recently were fabricated and are illustrated in Fig. 5a. The rechargeable
reported non-noble OER catalysts (Fig. S18†).55,56 It should be ZABs were assembled using VO-CMON@NCN-800 coated carbon
noted that the active sites were mainly attributed to the exposed cloth (CC) as an air-cathode, commercial Zn foil as an anode,
electroactive surface area, size effects, distribution, and struc- and 6 M KOH with 0.2 M Zn(ac)2 as a liquid electrolyte. For
ture of the catalysts. To examine the durability of the VO- comparison, we also assembled ZAB with the Pt/C + RuO2
CMON@NCN-800 catalyst, chronoamperometry and CV anal- couple as an air-cathode. Subsequently, the i–v polarization and
yses were carried out. The optimal VO-CMON@NCN-800 catalyst power density curve of VO-CMON@NCN-800 and the Pt/C +
retains the initial current with a negligible loss aer continuous RuO2 air-cathodes-based ZABs are shown in Fig. 5b. The liquid-
40 h in the chronoamperometry test, demonstrating an based rechargeable ZAB with VO-CMON@NCN-800 air-cathode
outstanding OER durability and superiority to the conventional exhibited a high peak power density of 143.7 mV cmꢀ2 at
RuO2 (Fig. S19†). Furthermore, the LSV of the VO-CMON@NCN- a current density of 179.5 mA cmꢀ2, outperforming the ZAB with
800 catalyst displayed minor changes aer 10 000 CV cycles Pt/C + RuO2 air-cathode (103.6 mV cmꢀ2 at 125.4 mA cmꢀ2) and
(Fig. S20†), further demonstrating its exceptional electro- the state-of-the-art air-cathodes-based ZABs in the literature
chemical durability. To understand the catalyst's reversible (Table S4†). Furthermore, the liquid-based rechargeable ZAB
oxygen activities, LSV tests were performed for bifunctional with the VO-CMON@NCN-800 air-cathode exhibited a high
ꢀ1
ORR and OER activities in a 0.1 M KOH (Fig. 4i). The potential specic capacity of 721.2 mA h gZn
(energy density of
difference between ORR (EORR(1/2)) and OER (EOER(10)) was 841 W h kgꢀ1) at a current density of 5 mA cmꢀ2, which was
calculated and represented as DE ¼ EOER(10) ꢀ EORR(1/2). The much higher than ZAB with the Pt/C + RuO2 air-cathode (640.5
optimal VO-CMON@NCN-800 catalyst showed a lowest DE value mA h gZnꢀ1; energy density 715 W h kgꢀ1) and superior to the
of 0.896 V, outperform than Pt/C + RuO2 couple (1.15 V) and recently reported state-of-the-art air-cathodes based ZABs
recently reported non-noble bifunctional catalysts (Fig. 4i),57,58 (Fig. 5c and Table S4†). These results prove that the VO-
demonstrating its superior bifunctional ORR and OER CMON@NCN-800 air-cathode based rechargeable ZAB holds
activities.
excellent energy storage capability. These series-connected
These tremendous bifunctional activities of VO- rechargeable ZABs successfully powered the blue/red and
CMON@NCN-800 could be ascribed to the following aspects: (i) numerous red LEDs (inset of Fig. 5c, e, and S21†), further vali-
the homogeneously anchored VO-CMON QDs into the NCN dating their practical applicability. To examine the durability of
frameworks could increase the electroactive sites; (ii) the high our liquid-based rechargeable ZAB, the successive charge–
synergistic effect between the active VO-CMON QDs and the discharge was carried out at current densities of 5 and 20 mA
NCN support could boost the durability of the VO-CMON@NCN- cmꢀ2. The ZAB with VO-CMON@NCN-800 air-cathode exhibited
800 catalyst, and (iii) the active VO-CMON QDs supplied an ultralong cycle life at 5 mA cmꢀ2 over 500 h, with a slight
a successful four-electron pathway for ORR and enhanced increase in the potential gap from 0.852 V to 0.863 V (Fig. 5d). In
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contrast, the liquid-based rechargeable ZAB with conventional transport kinetics. Therefore, the present VO-CMON@NCN-
Pt/C + RuO2 air-cathode maintained a potential window up to 800 air-cathode-based QSS-ZABs possesses tremendous power
170 h only. Aer that, it showed a signicant change in the densities and ultralong cycle life, and has great promise for
potential (Fig. 5d). Such exceptional cycling stability of our industrial energy storage systems as hybrid electric vehicles and
liquid-based rechargeable ZAB is because the VO-CMON@NCN- wearable electronics.
800 air-cathode has high ORR and OER activities. Excitingly, our
liquid-based rechargeable ZAB showed outstanding durability
Conclusions
at a high current density of 20 mA cmꢀ2, preserving a small
potential gap of 1.415 V aer 150 h successive charge–discharge
cycles, outperforming the rechargeable Pt/C + RuO2 based ZABs
and recently reported ZABs in the literature (Fig. S21 and Table
For the rst time, we have successfully established VO-
CMON@NCN nanohybrids through a facile, scalable, and cost-
effective, and single-step in situ pyrolysis process. Due to the
quantum size effects and enriched oxygen vacancies, the
optimal VO-CMON@NCN-800 catalyst displayed high catalytic
activities and durability for both ORR and OER, outperforming
conventional Pt/C and RuO2 catalysts. Most importantly, the VO-
CMON@NCN-800 air-cathode based rechargeable ZABs ꢀe1xhibi-
ted an ultrahigh specic capacity of 721.2 mA h gZn and
a tremendous peak power density of 143.7 mW cmꢀ2, which is
superior to the conventional Pt/C + RuO2 and recently reported
state-of-the-art air-cathodes based ZABs. The current strategy
for fabricating hierarchical VO-CMON@NCN nanohybrids could
be considered a cost-effective approach to achieving multi-
functional catalysts for industrial rechargeable ZABs with
ultralong cycle life.
S4†). Fig. 5e displayed
a series-connected liquid-based
rechargeable ZABs with an open-circuit voltage (OCV) of
2.807 V, successfully illuminating numerous red LEDs. To the
best of our knowledge, such outstanding durability over 500 h
has not been reported until now. Such high-performance of the
VO-CMON@NCN-800 air-cathode was due to numerous oxygen
vacancies and high synergistic interaction between the VO-
CMON QDs and NCN frameworks, thus suppressing the active
VO-CMON QDs from aggregation or corrosion during a pro-
longed cycling stability test.
Quasi solid-state rechargeable ZABs. To further investigate
the practical applicability toward industrial aspects, we con-
structed exible rechargeable QSS-ZAB (with VO-CMON@NCN-
800 air-cathode and illustrated in Fig. 6a). Instead of the
liquid electrolyte, we used PVA-KOH with Zn(ac)2 lm as the gel
electrolyte. The series-connected rechargeable QSS-ZAB could
power a yellow LED for more than 1 h with different bending
and twisting circumstances. To examine the cycling stability of
the rechargeable QSS-ZABs, we have carried out the charge–
discharge curves at a xed current density of 5 mA cmꢀ2
(Fig. 6b). The stable potential was shown in both ORR and OER
over 250 h, demonstrating that the QSS-ZAB held an ultralong
cycle life. In contrast, the Pt/C + RuO2 based rechargeable QSS-
ZABs showed a high increment of potentials in both ORR and
OER aer 80 h. The rechargeable QSS-ZAB with VO-
CMON@NCN-800 air-cathode displayed a high OCV of 1.409 V
even at different bending and twisting circumstances (Fig. 6c).
To further conrm the exible nature of our rechargeable QSS-
ZAB, charge–discharge tests were performed under various
bending and twisting states at a xed current density of 5 mA
cmꢀ2 (Fig. 6d). The results demonstrated the high exibility and
mechanical durability of the VO-CMON@NCN-800 based
rechargeable QSS-ZAB. These results further prove that the VO-
CMON@NCN-800 has excellent potential to be utilized as
a highly active air-cathode for exible QSS-ZABs and next-
generation electronic vehicles. The superior bifunctional cata-
lytic activities of VO-CMON@NCN-800 originated from the
following aspects: (i) complete embedded active VO-CMON QDs
into NCN frameworks could help suppress aggregation during
the prolonged charge–discharge process, therefore enhancing
the cycling stability, (ii) highly synergistic interaction between
VO-CMON QDs and NCN frameworks could enhance the ORR
and OER activities, and improve the electrochemical perfor-
mance of the rechargeable ZABs, (iii) enriched oxygen vacancies
are promoted by optimizing the synthesis protocol, leading to
the increased number of active sites and better electron-
Author contributions
T. T. N.: conceptualization, data curation, formal analysis,
investigation, methodology, writing – original dra. J. B.:
conceptualization, methodology, writing – review & editing. K.
T. L.: validation, methodology, supervision, writing – review &
editing. N. H. K.: writing – review & editing, funding acquisition,
supervision. J. H. L.: writing – review & editing, funding acqui-
sition, supervision.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This research was supported by the Basic Science Research
Program (2017R1A2B3004917) and the Regional Leading
Research Center Program (2019R1A5A8080326) through the
National Research Foundation (NRF) funded by the Ministry of
Science and ICT of the Republic of Korea.
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