Chemical Engineering Journal 468 (2023) 143607
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Chemical Engineering Journal
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Transformation of a new polyoxometalate into multi-metal active sites on
ZIF-derived carbon nanotubes as bifunctional cathode catalyst and
dendrite-free anode coating for Zn-air batteries
Jiaqi Niu a, Chaoyao Geng a, Xiaoqiang Liu a, *, Anthony P. O’Mullane b, *
a
Henan Joint International Research Laboratory of Environmental Pollution Control Materials, College of Chemistry and Molecular Sciences, Henan University, Kaifeng,
Henan Province 475004, PR China
School of Chemistry and Physics, Queensland University of Technology (QUT), Brisbane, QLD 4001, Australia
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Polyoxometalate
ORR/OER catalytic performance
Zn-air battery
Zinc dendrite
Density functional theory calculation
To improve the kinetics of the ORR/OER at the cathode and inhibit dendrite formation on the anode of a Zn-Air
battery (ZAB), a new polyoxometalate (POM) with a basic structrual unit of [Zn{P4Mo6}2] was designed and
converted to well-separated Zn/Mo2C catalytic sites on Co, N doped carbon nanotubes (Co-NCNTs) derived from
ZIF-67. This conversion was achieved at a relatively low pyrolysis temperature of 600 ◦ C, which circumvented
the collapse of catalyst structures, burial of catalytic sites and volatilization of metals. The experimental results
demonstrate that the pyrolysis of [Zn{P4Mo6}2] can etch Co-NCNTs-800 to produce a porous structure with an
enlarged specific surface area, thus increasing the number of catalytic sites and promoting the transfer of
electrons/ions. The high-valence Mo atoms from the POM precursor are inclined to hybridize with O active
intermediates produced during ORR/OER, which can improve electron exchange and the ORR/OER perfor­
mance. Furthermore, the multi-catalytic sites (Zn and Mo2C, etc.) produced from the [Zn{P4Mo6}2] precursor can
not only facilitate ORR/OER at the battery cathode, but also inhibit the formation of zinc dendrites by forming a
large number of nucleation sites on the battery anode. Accordingly, the ZABs assembled with the POM-derived
catalyst exhibit a high open circuit voltage of 1.506 V and a peak power density of 223.54 mW cm− 2. Moreover,
the assembled all-solid coin cell ZABs also display high capacity and long-term charge–discharge stability.
Furthermore, density functional theory calculations demonstrate that the synergy between Zn/Mo2C and CoNCNTs active sites mainly contributes to the superior ORR/OER catalytic performance.
1. Introduction
In recent years, the ever-increasing energy demands of our society
has caused enormous pressure on both the environment and the world’s
natural resources. As a type of prospective next-generation energy
storage device, rechargeable Zn-air batteries (ZABs) are being developed
to ease the emerging energy crisis owing to their cost effectiveness, easy
preparation, low toxicity, excellent safety and large energy density [13]. However, two main factors still restrict the practical application of
ZABs. One is the poor kinetics of both the oxygen reduction reaction
(ORR) and oxygen evolution reaction (OER) at the air electrode of ZABs,
and the other one is dendrite growth on the zinc anode during long-term
cycling. For the first problem, although Pt-based and Ru/Ir-oxide ma­
terials are considered as excellent catalysts for the ORR and OER,
respectively, their high cost, poor stability, and weak poison resistance
have severely limited their large-scale applications in electrochemical
devices [4]. Meanwhile, it has been reported that dendrite growth on the
zinc anode can be effectively inhibited by coating it with selected metal
organic framework (MOF)-derived materials [5-9]. However, the
application of MOF-derived materials to inhibit dendrite growth on the
Zn anode has suffered from problems such as the limited choice of
suitable MOFs and the severe agglomeration of their calcination prod­
ucts [10]. Considering the above issues, ingenious design and prepara­
tion of a stable, economically viable and efficient electrocatalyst with
both bifunctional catalytic activity for ORR/OER and strong suppression
of dendrite growth will certainly contribute to the development of
practical ZABs.
To date, most of the non-noble metal cathode catalysts for ZABs
* Corresponding authors.
E-mail addresses: liuxq@henu.edu.cn (X. Liu), anthony.omullane@qut.edu.au (A.P. O’Mullane).
https://doi.org/10.1016/j.cej.2023.143607
Received 20 March 2023; Received in revised form 3 May 2023; Accepted 16 May 2023
Available online 19 May 2023
1385-8947/© 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).
J. Niu et al.
Chemical Engineering Journal 468 (2023) 143607
belong to M-N-C materials, which are normally prepared by calcining
MOFs or mixtures of metal hydroxides and nitrogen doped carbon [1113]. Nevertheless, high temperature calcination will often destroy
catalyst structures, bury catalytic active sites and volatilize some metals.
In particular, the choice of suitable MOFs or metal hydroxides for pre­
paring M-N-C catalysts is also limited. As a type of polynuclear metal­
–oxygen complexes, polyoxometalates (POMs) have great potential to
replace the above type MOFs or metal hydroxide precursors for pre­
paring oxygen catalysts due to their relatively low calcination temper­
atures, abundant electrocatalytic active sites, multi-metal types, easy
preparation and low cost [14-16]. Meanwhile, the metal centers in
POMs can also be substituted by other transition metals, which would
further strengthen their versatile electrocatalytic activity for different
reactions. Indeed, the high-efficiency electrocatalytic capability of
POMs for the ORR/OER has been recently demonstrated. For example, a
{Co4MoxWy} electrocatalyst was prepared by calcining a Mo incorpo­
rated POM precursor of [Co4(H2O)2(PW9O34)2]10- [17]. At 10 mA cm− 2,
the OER overpotential of the Mo-containing electrocatalyst was 188 mV
lower than that of the catalyst without Mo, demonstrating that the
doped Mo could effectively promote OER performance.
Moreover, POMs and their calcination products have been success­
fully applied for the construction of energy devices due to their excellent
electrocatalytic activity. For example, Wang et al. synthesized an effi­
cient bifunctional electrocatalyst by loading Keggin-type POMs onto
nitrogen doped carbon paper derived from rice paper and used it as an
air cathode for assembling ZABs [18]. The assembled Zn-air battery
exhibited an open circuit voltage of 2.09 V, a peak power density of 349
mW cm− 2, and a specific energy density of 1009 Wh⋅kg− 1 when dis­
charged at 20 mA cm− 2. In addition, Goodenough’s group prepared a
novel IrMn/Fe3Mo3C catalyst with Mo7O24 as the Mo source, which
exhibited superior electrocatalytic properties over Pt/C and Ir/C, as well
as excellent performance in Zn–air batteries [19]. However, only a
limited number of POMs or their derivatives (the majority of them
belong to Kegging and Mo7O24) have been successfully used in batteries
[20]. Accordingly, we explored other types of POMs as cathode catalyst
precursors to assemble rechargeable Zn–air batteries to further expand
the application range of POM families.
As a common problem in ZABs, dendrite growth mainly results from
the occurrence of the hydrogen evolution reaction (HER) at the zinc
anode and electrode corrosion by OH– ions in alkaline electrolyte, which
can produce a thick “dead zinc” (Zn(OH4)2-, etc.) passivation layer
which hinders the reversible and ordered conversion between Zn2+ and
Zn. In addition, the uneven charge distribution caused by the nonuniform wetting of the zinc anode with electrolyte is another reason
for the formation of zinc dendrites [21]. It has been reported that
immobilization of a porous nanomaterial layer with large numbers of
nucleation sites on the Zn anode could be an effective strategy to inhibit
dendrite growth [22]. For example, ZnO nanorods were uniformly
grown on graphene oxide (GO), which was then coated on a zinc anode
to improve the reversible conversion between Zn and ZnO, and restrict
dendrite growth to a critical state via the multinuclear sites of the ZnOGO layer [23]. It should be pointed out that POMs can also form multinucleation sites on a Zn anode due to the existence of various and
abundant metal sites inside their structures.
Herein, a mixture of ZIF-67 and melamine was initially transformed
at 800 ◦ C to form Co, N-doped carbon nanotubes (Co-NCNTs-800). Then,
a newly designed Zn incorporated molybdate of [Zn{P4Mo6}2] was
prepared and co-pyrolysed with Co-NCNTs-800 at 600 ◦ C to produce
multi-metal active sites (Zn and Mo2C nanoparticles) on Co-NCNTs-800.
The main advantages of this new POM as a precursor are described as
follows: (1) As a nanoscale etcher, [Zn{P4Mo6}2] can produce many
pores or defects on the substrate material (Co-NCNTs-800) due to its
inherent acidity and the metal nanoparticles generated during the py­
rolysis, thus promoting the contact between electrolyte and electrode
material and the transfer of electrons/ions [20]. (2) It is reported that
high-valence Mo atoms from the POM precursor are inclined to
hybridize with O active intermediates produced during ORR/OER,
which can therefore improve electron exchange and the ORR/OER
performance [24]. (3) The multi-catalytic sites (Zn and Mo2C, etc.)
produced from the [Zn{P4Mo6}2] precursor can not only facilitate ORR/
OER at the battery cathode, but also inhibit the formation of zinc den­
drites by forming a large number of nucleation sites on the battery anode
[22]. Moreover, the reasons for adopting this two-step calcination
method for preparing Zn/Mo2C@Co-NCNTs are elaborated as follows:
(1) The conversion of ZIF-67/ melamine at 800 ◦ C to Co-NCNTs-800 can
provide a support with large specific surface area to alleviate the ag­
gregation of catalytic sites. (2) The calcination of the mixture of [Zn
{P4Mo6}2] and Co-NCNTs-800 at a relatively low temperature (600 ◦ C)
produced abundant multiple active sites, which can not only improve
the exposure of catalytic sites, but also circumvent the collapse of the
nanostructures that occurs at high pyrolysis temperatures. Owing to the
above reasons, the ZABs assembled with Zn/Mo2C@Co-NCNTs as both
the cathode catalyst and the anode coating showed excellent opencircuit voltage, power density and long-term cycle stability.
2. Experimental section
2.1. Synthesis of Zn/Mo2C@Co-NCNTs
To synthesize Co-NCNTs-800, 0.30 g of ZIF-67 (the detailed synthesis
of ZIF-67 is provided in Supporting Information 1.2) and 1.00 g of
melamine were dispersed in ethanol aqueous solution (VCH3OH: VH2O =
1: 1), and then placed in a 60 ◦ C water bath until the solvent was
completely volatilized. The residual solid powders were ground for 30
min until the two components were thoroughly mixed, and then placed
in an oven to dry at 60 ◦ C. Afterwards, the dried mixture of ZIF-67/
melamine was placed in a porcelain boat, which was then pyrolysed
inside a tubular furnace with a stepwise temperature increase of 5 ◦ C
min− 1 and retained in 800 ◦ C and nitrogen atmosphere for 120 min.
Finally, the as-prepared Co-NCNTs-800 (240 mg) and [Zn{P4Mo6}2] (60
mg) (see Supporting Information 1.1 for the detailed synthesis of [Zn
{P4Mo6}2]) were added to a ethanol aqueous solution (VCH3OH: VH2O =
1:1) at 60 ◦ C and stirred until the solvent was completely volatilized.
Subsequently, the mixture was vigorously ground for 30 min, before it
was placed in a porcelain boat and pyrolysed inside a tubular furnace in
600 ◦ C and nitrogen atmosphere for 2 h at an increasing rate of 5 ◦ C
min− 1 to obtain Zn/Mo2C@Co-NCNTs.
2.2. Characterization of Zn/Mo2C@Co-NCNTs
X-ray diffraction (XRD) spectra were acquired at D8 Advance
diffractometer (Bruker Co. Ltd., Germany). Scanning electron micro­
scopic (SEM) and transmission electron microscopic (TEM) pictures
were obtained with Carl Zeiss Field Emission SEM (Zeiss, Germany) and
JEM-2100 TEM (Nippon Electronics Corporation, Japan), respectively.
X-ray photoelectron spectroscopy (XPS) was conducted at a 250XI X-ray
photoelectron spectrometer (Thermo Escalab, USA). Raman spectra
were obtained from a RENISHAW INVIA Laser Microscopic Raman
Spectrometer (Renishaw, UK). Thermogravimetric analysis was con­
ducted at a TGA/DSC3 + Simultaneous Thermal Analyzer (METTLER
TOLEDO, Switzerland). The contact angle measurement was conducted
at a fully automatic contact angle measuring instrument SCI6000E
(Shanghai Zhongchen Technology Equipment Co., Ltd., China) with an
original water droplet of 5 μL. Elemental analysis was performed on a
CSONH elemental analyzer (CS844).
2.3. Electrochemical characterization of Zn/Mo2C@Co-NCNTs
Electrochemical test was carried out using CHI760E workstation (CH
Instrument, China) connected with a Rotating Ring Disk Electrode
Apparatus (BAS Co. Ltd. Japan). A catalyst-modified 3 mm glassy carbon
rotating disk electrode (RDE) or rotating ring-disk electrode, Hg/HgO
2
J. Niu et al.
Chemical Engineering Journal 468 (2023) 143607
and Pt stick was used as working electrode, reference and auxiliary
electrode, respectively.
To prepare a catalyst modified working electrode, catalyst (3.5 mg)
was initially dispersed in 1.0 mL solution (850 μL H2O + 100 μL
CH3CH2OH + 50 μL Nafion), and ultrasonically vibrated for 60 min until
a uniform slurry (3.5 mg mL− 1) was formed. Then, the slurry was
applied on a RDE or RRDE, and dried to obtain a modified working
electrode (catalyst loading 0.4 mg cm− 2). For comparison, the mixed Pt/
C (20 w.t.%) + RuO2 catalyst was modified on a RDE or RRDE via the
same procedure.
To evaluate the ORR and OER performance, cyclic voltammograms
(CVs) were collected in a 0.1 M KOH with scan speed of 0.05 V⋅s− 1, and
linear sweep voltammograms (LSVs) were acquired in O2-saturated 0.1
M KOH with scan speed of 0.01 V⋅s− 1 within 400–2025 rpm rotation
range.
2.9. DFT calculations
DFT calculation was carried out with CASTEP module implemented
in Material Studio. We adopted the plane wave basis with a cutoff energy
of 400 eV and a self-consistent field (SCF) tolerance of 1 × 10-6 eV. A
(1 0 1) crystal surface of Mo2C (PDF#35–0787) and a supercell consist­
ing of the graphene unit cells partially replaced with N, Zn and Co atoms
were chosen with a vacuum region of 15 Å in the vertical direction. The
free energies of H2 and H2O were calculated by placing them in a
10*10*10 cubic lattice, respectively.
3. Results and discussion
3.1. Characterization of Zn/Mo2C@Co-NCNTs
Firstly, after the careful design and control of the additional metal
elements, a new type of POM containing Mo and Zn was hydrothermally
synthesized with a molecular formula of (Zn2BBTZ)[Zn(BBTZ)]2[Zn
(P4Mo6O31H7)2]⋅5H2O (Abbr. [Zn{P4Mo6}2]), as displayed by Scheme 1
(a) and Fig. S1. Next, a series of characterizations were conducted to
confirm the successful preparation of [Zn{P4Mo6}2]. First, the experi­
mental X-ray diffraction (XRD) pattern (Fig. 1(a)) obtained from the
molecular structure information of [Zn{P4Mo6}2] (Tables S1-2 in sup­
porting information) is consistent with the simulated XRD data from the
Cambridge crystal structure database (see Supporting Information 1.7
for details) within the range of 0 − 50◦ . This verified that [Zn{P4Mo6}2]
was synthesized with high phase purity. Furthermore, the appearance of
vibrational peaks belonging to Mo = O, Mo-O-Mo and P-O groups in the
Fourier transform infrared spectrum (Fig. S2) also confirmed the suc­
cessful preparation of [Zn{P4Mo6}2]. In addition, the thermal stability of
[Zn{P4Mo6}2] was studied by thermogravimetric analysis (Fig. S3). It
was found that there were two stages of weight loss with an increase of
temperature. The first weight loss of 3.98% before 219.8 ◦ C is attributed
to the volatilization of water in the POM molecule; while at 220–800 ◦ C,
[Zn{P4Mo6}2] collapses, and P and Mo elements gradually volatilized in
the forms of P2O5 and MoO3 [14]. This also explains why the P
component disappeared after calcining the [Zn{P4Mo6}2] precursor at
600 ◦ C.
The preparation process for Zn/Mo2C@Co-NCNTs is displayed in
Scheme 1(b). Initially, ZIF-67 nanocrystals were prepared and then
calcined together with melamine, which is rich in carbon and nitrogen,
at 800 ◦ C to obtain Co/N co-doped carbon nanotubes (Co-NCNTs-800).
Then a homogeneous mixture of [Zn{P4Mo6}2] and Co-NCNTs-800 ([Zn
{P4Mo6}2]@Co-NCNTs-800) was calcined at 600 ◦ C to obtain Zn and
Mo2C nanoparticles (NPs) that were well separated and uniformly
dispersed on the Co-NCNTs-800 nanotubes.
Subsequently, XRD spectra were collected to confirm the crystallinity
and phase purity of the obtained catalysts. Firstly, Fig. S4 indicates that
every peak position of ZIF-67 is completely consistent with that of the
simulated data, proving the successful preparation of ZIF-67. The XRD
pattern of Co-NCNTs-800 (Fig. 1(b)) presents typical peaks at 2θ =
44.86◦ and 52.13◦ , which correspond to the Co (1 1 1) and (2 0 0) planes,
respectively. Moreover, the XRD pattern of the 600 ◦ C calcination
product of [Zn{P4Mo6}2] (Zn/Mo2C-600) displays several typical peaks
at 2θ values of 44.17◦ and 51.38◦ , belonging to the Zn (1 1 0) and (1 0 2)
crystal planes, respectively, and the other peaks at 2θ values of 37.42◦ ,
52.16◦ , 64.87◦ and 76.13◦ , correspond to the (1 0 1) (1 0 2), (1 1 0) and
(2 0 1) planes of Mo2C, respectively. In addition, the XRD pattern of the
600 ◦ C calcined product of [Zn{P4Mo6}2]/Co-NCNTs-800 presents
almost all the typical peaks attributed to Zn (1 1 0), Co (1 1 1) and Mo2C
(1 0 1) planes, indicating the formation of Zn and Mo2C NPs on CoNCNT-800.
Next, X-ray photoelectron spectroscopy (XPS) was used to confirm
the element composition and valence state of Zn/Mo2C@Co-NCNTs. The
C 1s, O 1s and survey scan XPS spectra of the target catalyst material of
Zn/Mo2C@Co-NCNTs are shown in Fig. S5. Fig. 1(c) shows that the N 1s
2.4. Assembly and test of traditional ZABs
The traditional ZABs were assembled with air electrodes, polished
zinc plates (thickness of 0.5 mm), separators and electrolytes (6 M KOH
+ 0.2 M Zn(CH3COO)2). Air electrode catalysts were prepared by
initially dispersing 10 mg of Zn/Mo2C@Co-NCNTs or commercial Pt/C
+ RuO2 (Pt/C: RuO2 = 1: 1) catalyst in 950 μL CH3CH2OH + 50 μL
Nafion solution for 1 h to obtain an uniform slurry, respectively. Then, 2
mg of slurry was cast on a composite substrate (carbon paper + water­
proof film + nickel foam) and dried to obtain an air electrode (mass
loading 2 mg⋅cm− 2). The charge–discharge polarization curves were
measured by LSV on a CHI760E. Cyclic stability test with each cycle
consisting of discharge and charge for 10 min respectively and galva­
nostatic method were measured on a LANHE CT3002A test system.
2.5. Preparation of the coated Zn electrode
A mixture of Zn/Mo2C@Co-NCNTs powder, poly(vinylidene fluo­
ride) and acetylene black (7: 2: 1) was dispersed in N-methyl-2-pyrro­
lidone before it was coated on bare Zn disks (15 mm diameter, 50 μm
thickness) by doctor blading method, and then dried at 60 ◦ C.
2.6. Assembly of Coated/Bare Zn symmetric cells
CR2032-type symmetric cells were assembled by connecting two
Coated Zn or two Bare Zn foil disks to evaluate the striping/plating
performance. The cycling stability test of the Zn foil disks was performed
at 0.5, 1, 2 and 5 mA⋅cm− 2, respectively with a fixed capacity of 1
mAh⋅cm− 2.
2.7. Preparation of polyvinyl alcohol (PVA) gel electrolyte
Firstly, 5 g PVA was added into 20 mL deionized water, and stirred in
95 ◦ C for 30 min. Subsequently, 5 mL mixed solution containing 18 M
potassium hydroxide and 0.2 M Zn(CH3COO)2 was poured in the above
solution, continually stirred at 95 ◦ C for 30 min, and the resulting
transparent sol was placed in a refrigerator for freeze dry. Before use,
PVA gel electrolyte was taken out, thawed and soaked in 4 M KOH for
12 h.
2.8. Assembly of all-solid-state coin ZABs
To assemble CR2032-type coin cells, a mixed catalyst containing
20% w.t. % Pt/C and RuO2 (1:1) was immobilized on a carbolic paper to
fabricate an air electrode (1.5 cm diameter) with a loading of ~ 2.0 mg
cm− 2. A polished Bare Zn foil or Coated Zn foil was applied as Zn anode,
and PVA electrolyte was used as the solid electrolyte.
3
J. Niu et al.
Chemical Engineering Journal 468 (2023) 143607
Scheme 1. Preparation process of (a) [Zn{P4Mo6}2] polyoxometalate and (b) Zn/Mo2C@Co-NCNTs.
spectrum is divided into several typical peaks of pyridine N (398.89 eV),
pyrrolic N (400.79 eV), graphitic N (401.85 eV) and oxide N (404.09
eV), and their content were calculated to be 44.51%, 21.75%, 23.85%
and 9.89%, respectively, as shown in Table S3. Previous studies reported
that pyrrole N provide electrons for π conjugated systems and enhance
the nucleophilicity of adjacent carbon atoms to improve the adsorption
of O2, thereby facilitating the kinetics of the ORR [25-28]. Also, as an
electron-absorbing group, pyridine N could promote the adsorption of O
species (OH*, OOH*) to facilitate the OER [25,29]. Meanwhile, pyridine
N and pyrrole N are inclined to form metal-N to stabilize the active
centers, which could also increase ORR/OER catalytic efficiency
[30,31]. Fig. 1(d) clearly shows that the XPS peak of Mo 3d is divided
into two peaks at 232.45 eV and 235.53 eV, attributed to Mo2+ 3d5/2 and
Mo2+ 3d3/2, respectively, suggesting the possible formation of Mo2C. In
Fig. 1(e), the Zn 2p XPS spectrum contains double peaks belonging to
2p3/2 (1022.06 eV) and 2p1/2 (1044.96 eV). In addition, the highresolution XPS spectrum of Co 2p displays two satellite peaks at
786.89 eV and 803.43 eV, two Co0 2p3/2 peaks at 778.15 eV and 794.01
eV and two Co2+ 2p1/2 peaks at 780.94 eV and 796.03 eV (Fig. 1(f)).
Because Co2+ contains empty orbitals and pyridine-N has a lone pair
electrons, they can form Co-N observed at 783.98 eV and 799.11 eV to
promote the formation and adsorption of OOH* and O* intermediates,
and thus improve the ORR [25,30]. Furthermore, Raman spectra were
collected to explore the defect concentration and graphitization degree
on the prepared materials. Fig. 1g displays indicative carbon peaks at
1370 (D band) and 1600 cm− 1 (G band) of various materials, where we
can see that Zn/Mo2C@Co-NCNTs has the largest IG/ID ratio (0.648),
implying that it has the most defect structures, higher electrical con­
ductivity and potentially the highest catalytic performance among the
prepared catalyst materials [25]. Next, nitrogen adsorption–desorption
experiments were carried out to obtain the specific surface area and pore
size distribution of Zn/Mo2C@Co-NCNTs. Fig. 1h indicates that the
recorded isotherm of Zn/Mo2C@Co-NCNTs belongs to a typical IV type
with a surface area of 487.26 m2⋅g− 1. Meanwhile, the average pore size
of Zn/Mo2C@Co-NCNTs is distributed within 1–6 nm (Fig. 1i), sug­
gesting a mesoporous structure. In addition, the specific surface area/
average pore size of Co-NCNTs-800 and Na2MoO4/ZnO/Co@NCNTs800 (Supporting Information 1.6 for the detailed synthesis) are 250.94
m2⋅g− 1/ 2–10 nm and 91.23 m2⋅g− 1/2–14 nm (Fig. S6), respectively.
The above results strongly indicate that [Zn{P4Mo6}2] is an ideal pre­
cursor to prepare porous catalyst materials with a large specific surface
area through pyrolysis and the etching effect of POMs. The larger the
specific surface area is, more catalytic active sites are available, which
should result in better ORR/OER catalytic performance [32]. In addi­
tion, the ICP-AES and CHONS elemental analysis provided the content of
each element in this material, as presented in Table S4.
Subsequently, scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) were used for studying the microstructure of
the synthesized materials. Figs. S7(a-b) indicate the hexagon structure of
ZIF-67 (diameter ~100–200 nm). After being mixed with a certain
proportion of melamine, ZIF-67 nanocrystals are coated with a semitransparent layer (Fig. S8) without any obvious change in their
morphology and size. As indicated by Fig. 2(a), with the aid of mel­
amine, the 800 ◦ C pyrolysis process converted ZIF-67 to nanotubes (CoNCNTs-800) with an average length of ~300–400 nm and a diameter of
~50 nm. In comparison, ZIF-67 without melamine was also calcined at
800 ◦ C, which only resulted in the collapse of its structure (Fig. S7(c)).
Additionally, a mixture of ZIF-67 and melamine was pyrolysed at 700 ◦ C
and 900 ◦ C, respectively. Fig. S9(a) shows that most of the ZIF-67 could
not be transformed to carbon nanotubes at 700 ◦ C, while the 900 ◦ C
pyrolysis process produced nanotubes with a very uneven size ranging
from ~100 nm to 1 μm (Fig. S9(b)), demonstrating that the pyrolysis
temperature greatly affects the formation of carbon nanotubes.
Then, a catalyst precursor mixture of [Zn{P4Mo6}2] and Co-NCNTs800 (Fig. S10) was calcined at 500, 600 and 700 ◦ C, and the obtained
products were characterized by SEM and TEM. At 500 ◦ C, most of [Zn
{P4Mo6}2] could not be transformed into Zn and Mo2C NPs, as indicated
by Fig. S11(a), which is quite similar to Fig. S9. On the contrary, the SEM
image of the 600 ◦ C calcined product (Fig. 2(b)) verifies that highdensity NPs were produced and well dispersed on Co-NCNTs-800,
implying the successful conversion of [Zn{P4Mo6}2] to Mo2C and Zn
NPs. It is worth noting that the calcination of individual [Zn{P4Mo6}2]
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J. Niu et al.
Chemical Engineering Journal 468 (2023) 143607
Fig. 1. (a) XRD patterns of the synthesized and simulated [Zn{P4Mo6}2]. (b) XRD patterns of Co-NCNTs-800, Zn/Mo2C-600 and Zn/Mo2C@Co-NCNTs. XPS spectra
of (c) N 1s, (d) Mo 3d, (e) Zn 2p and (f) Co 2p of Zn/Mo2C@Co-NCNTs. (g) Raman spectra of Zn/Mo2C-600, Co-NCNTs-800 and Zn/Mo2C@Co-NCNTs. (h) N2
absorption/desorption isotherms at 77 K and (i) pore size distribution of Zn/Mo2C@Co-NCNTs.
at 600 ◦ C without the Co-NCNTs-800 support resulted in severe nano­
particle agglomeration (Fig. S12), verifying the importance of the CoNCNTs-800 support in the synthesis of the target catalyst. However,
almost no NPs were observed on Co-NCNTs-800 in Fig. S11(b) at 700 ◦ C,
indicating that any nanoparticles that were generated were either
vaporized or not formed at this temperature. Subsequently, the fine
structure of Zn/Mo2C@Co-NCNTs was researched with high-resolution
TEM (HR-TEM). The 0.268, 0.206 and 0.228 nm spacings respectively
correspond to Zn (1 1 0), Co (1 0 1) and Mo2C (1 0 1) planes (Fig. 2(c)),
further confirming the formation of Zn and Mo2C NPs on the Co doped
NCNTs. In Fig. 2(d), the TEM image and EDS images (Mo, Zn and C)
demonstrate that the Zn and Mo2C NPs are dispersed separately on the
Co-NCNTs, while EDS images of N and Co show that these two elements
are uniformly doped into the Zn/Mo2C@Co-NCNTs. The selected area
electron diffraction pattern of Zn/Mo2C@Co-NCNTs (insert in Fig. 2(d))
indicates a polycrystalline structure of the prepared composite with
multiple components. On the contrary, when Na2MoO4 and ZnO (two
raw materials for synthesis of [Zn{P4Mo6}2]) were simply mixed with
Co-NCNTs-800 and calcined at 600 ◦ C, the calcination product (Fig. S13)
was highly aggregated and only a small number of nanoparticles were
generated on the material. By comparing Fig. S13 with Fig. 2(b-d), it is
clearly seen that the morphology of the calcination product of [Zn
{P4Mo6}2] is much better than that of a Na2MoO4/ZnO mixture, which
verified the superiority of using [Zn{P4Mo6}2] POM as the catalyst
precursor.
3.2. Electrocatalytic performance of Zn/Mo2C@Co-NCNTs
After confirming the formation of the prepared catalyst materials, we
also tested their ORR/OER bifunctional properties via various electro­
chemical techniques. Fig. 3(a) shows that 20 w.t.% Pt/C, Zn/Mo2C@CoNCNTs, Co-NCNTs-800 and Zn/Mo2C-600 exhibit reduction peaks at
0.867, 0.855, 0.827 and 0.823 V, respectively in KOH solution purged
with O2. In contrast, these reduction peaks cannot be found in the cyclic
voltammograms (CVs) of the four catalyst materials in N2 purged solu­
tion. The small reduction peak potential difference between Zn/
Mo2C@Co-NCNTs and Pt/C indicates similar ORR catalytic properties of
the two materials. Furthermore, the CVs of the calcined products of ZIF67 (Fig. S14) at 700, 800 and 900 ◦ C and [Zn{P4Mo6}2]@Co-NCNTs-800
(Fig. S15) calcined at 500, 600 and 700 ◦ C verify that the best ORR
performance was achieved at 800 ◦ C for the former and 600 ◦ C for the
latter. As shown in the above TEM and SEM images, the optimal mor­
phologies were also obtained at these two pyrolysis temperatures for the
above two catalyst precursors, respectively. Afterwards, linear sweep
voltammetry (LSV) was carried out under identical test conditions with
the above CVs to validate the catalytic activity for the ORR at the ob­
tained catalysts. Fig. 3(b) shows that the initial voltage (Eonset, 0.918 V)
and half-wave potential (E1/2, 0.838 V) of Zn/Mo2C@Co-NCNTs are
significantly higher than those of Co-NCNTs-800 (Eonset = 0.882 V, E1/2
= 0.803 V) and Zn/Mo2C-600 (Eonset = 0.884 V, E1/2 = 0.799 V), sug­
gesting that the ORR behaviour of Zn/Mo2C@Co-NCNTs is better than
that of the other two catalyst materials due to a synergistic effect.
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Chemical Engineering Journal 468 (2023) 143607
Fig. 2. (a) SEM images of Co-NCNTs-800 and (b) Zn/Mo2C@Co-NCNTs. (c-d) HR-TEM, TEM and EDS images of Zn/Mo2C@Co-NCNTs. The inset in (d): selected-areaelectron-diffraction of Zn/Mo2C@Co-NCNTs.
Subsequently, the ORR mechanism for Zn/Mo2C@Co-NCNTs was
studied by LSV at different rotation speeds. Fig. 3(c) shows that the
diffusion current density (jL) is proportional to the rotational speed,
implying that the ORR on Zn/Mo2C@Co-NCNTs obeys first-order reac­
tion kinetics. From the Koutecky-Levich equation, an average ~4etransfer of ORR is confirmed on the target catalyst at different voltages,
as displayed in Fig. 3(d). In addition, Fig. 3(d) also suggests that only a
very small amount of H2O2 (<5%) was produced on Zn/Mo2C@CoNCNTs during the ORR, further verifying that O2 was nearly completely
reduced to H2O via a 4e- mechanism.
Because ORR/OER bifunctional catalytic activity is a prerequisite for
an air electrode catalyst of ZABs, the OER performance of Zn/Mo2C@CoNCNTs was also investigated in O2 purged alkaline electrolyte by LSV.
The OER performance of a material is usually benchmarked by the
voltage value at 10 mA cm− 2 (Ej = 10) on a LSV curve, and Ej = 10 is
inversely related to the OER performance [33,34]. As shown in Fig. 3(e),
the Ej = 10 of Zn/Mo2C@Co-NCNTs (1.543 V) is significantly smaller
than that of Co-NCNTs-800 (1.629 V) and Zn/Mo2C-600 (1.623 V), and
very close to RuO2 (1.505 V), demonstrating the excellent OER activity
of Zn/Mo2C@Co-NCNTs. Moreover, Figs. S16–17 demonstrate that the
best OER catalytic activity can be obtained with the 800 ◦ C calcination
product of ZIF-67 (Co-NCNTs-800) and the 600 ◦ C calcination product of
[Zn{P4Mo6}2]@Co-NCNTs-800 (Zn/Mo2C@Co-NCNTs). As a compari­
son, Fig. S18 shows that the ORR and OER performance of Na2MoO4/
ZnO/Co@NCNTs-800 catalytic activities are far inferior to those of Zn/
Mo2C@Co-NCNTs derived from the [Zn{P4Mo6}2] precursor, further
proving the advantages of this new polyoxometalate precursor. In
addition, the OER kinetics of these catalysts were investigated by
determining the Tafel slope (Fig. 3(f)) obtained from Fig. 3(e). It is found
that the Tafel slope of Zn/Mo2C@Co-NCNTs (97 mV dec-1) is signifi­
cantly lower than that of Co-NCNTs-800 (173 mV dec-1) and Zn/Mo2C600 (125 mV dec-1), indicating fast intrinsic OER kinetics on Zn/
Mo2C@Co-NCNTs. Furthermore, the Nyquist plot (Fig. 3(g)) shows that
the semicircular diameter of Zn/Mo2C@Co-NCNTs (64.84 Ω) is smaller
than that of the other catalysts including Pt/C + RuO2 (87.66 Ω), Co-
NCNTs-800 (95.38 Ω) and Zn/Mo2C-600 (91.77 Ω), suggesting the
fastest electron transfer rate at Zn/Mo2C@Co-NCNTs.
The excellent electrocatalytic performance of Zn/Mo2C@Co-NCNTs
is partially caused by a large electrochemical surface area (ECSA), which
can be estimated by double-layer capacitance (Cdl) measurements. It is
observed from Fig. S19 that Zn/Mo2C@Co-NCNTs (11.98 mF cm− 2)
exhibits a significantly larger Cdl than Co-NCNTs-800 (Cdl = 4.98 mF
cm− 2) and Zn/Mo2C-600 (Cdl = 6.48 mF cm− 2), implying that the target
catalyst has the largest ECSA and thus the most active sites among the
studied materials [33]. Afterwards, the methanol resistance of Zn/
Mo2C@Co-NCNTs was investigated through the variation of the current
in a i-t curve with the addition of methanol. From Fig. 3(h), it is observed
that after injecting methanol into the electrolyte at 470 s, the current of
Zn/Mo2C@Co-NCNTs barely changed, while that of Pt/C fluctuated
dramatically, showing greater methanol tolerance of the former than the
latter which would be beneficial for direct methanol fuel cell applica­
tions. Next, the CV scans of the prepared catalysts were performed be­
tween 0 and 2 V (vs. RHE) to evaluate their ORR/OER binary catalytic
property, which is inversely related to the ΔE value (ΔE = Ej=10(OER) E1/2(ORR)). Fig. 3(i) shows that Zn/Mo2C@Co-NCNTs has a ΔE value of
0.702 V, similar as the mixed catalyst of Pt/C and RuO2 (0.695 V), and
much smaller than Co-NCNTs-800 (0.926 V) and Zn/Mo2C-600 (0.919
V), which further verifies the excellent ORR/OER activity of Zn/
Mo2C@Co-NCNTs. The stability of Zn/Mo2C@Co-NCNTs for both the
ORR and OER were also verified by chronoamperometry. Fig. S20 shows
that 93.59 % of the initial current value was achieved at Zn/Mo2C@CoNCNTs, while 89.20 % of the initial current value was obtained at Pt/C
after 20 h of ORR stability test. Similarly, Zn/Mo2C@Co-NCNTs main­
tained 93.24 % of the initial current after 20 h of OER stability test,
whereas RuO2 only maintained 84.91 %. Additionally, OER and ORR
LSVs of Zn/Mo2C@Co-NCNTs in Fig. S21, indicate that the polarization
curves only changed slightly after 30000 s stability test for both the
oxygen reactions.
To confirm the structural stability of Zn/Mo2C@Co-NCNTs, the
morphology and phase composition of the catalyst after 30000 s of ORR/
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Chemical Engineering Journal 468 (2023) 143607
Fig. 3. (a) CV and (b) LSV of different catalysts in 0.1 M O2(solid line)/N2(dotted line) saturated KOH recorded at a RDE (1600 rpm). (c) LSV curves of Zn/
Mo2C@Co-NCNTs at 400, 625, 900, 1225, 1600 and 2025 rpm and their K-L plots (inset). (d) Electron transfer number and H2O2 production rate of the ORR at Zn/
Mo2C@Co-NCNTs calculated by RRDE. (e) OER LSVs for various materials and (f) their Tafel plots. (g) Nyquist plots for various materials. (h) Methanol resistance
test of Zn/Mo2C@Co-NCNTs and Pt/C. (i) ORR/OER LSVs for various materials.
OER tests were characterized by SEM, XRD and XPS. Fig. S22 shows that
the catalyst retains its original structure after the ORR/OER chro­
noamperometric tests, proving its good structural stability. In addition,
the XRD peak positions of the catalyst did not change either after 30000
s of ORR/OER tests (Fig. S23), which also verified the stability of the
catalyst. Furthermore, the XPS results (Fig. S24) indicate that after the
ORR/OER tests, all the elements of the catalyst still exist, and the
valence state and composition of each element have not changed
significantly. In summary, the above experimental results demonstrated
the excellent stability of Zn/Mo2C@Co-NCNTs.
the four oxygen intermediates on the Zn/Mo2C@Co-NCNTs-Co model,
which is composed of an upper Co/Zn-N4 graphene carbon layer and a
lower Mo2C (1 0 1) crystal plane layer. In addition, the adsorption con­
figurations of oxygen intermediates on Co-NCNTs-800, Zn/Mo2C-600,
and Zn/Mo2C@Co-NCNTs-Zn are also presented in Figs. S25-S27. The
density of states (DOS) calculation results (Fig. 4(b)) show that there is
no band gap in Co-NCNTs-800, Zn/Mo2C-600 and Zn/Mo2C@CoNCNTs, and that the electronic occupation state of Zn/Mo2C@CoNCNTs near the Fermi level is significantly increased compared with
that of Co-NCNTs-800 and Zn/Mo2C-600, indicating good electrical
conductivity and electrocatalytic activity of Zn/Mo2C@Co-NCNTs.
Moreover, Fig. 4(c) indicates that the free energies of all oxygen
intermediates on the four material models show a decreasing trend
during the ORR at U = 0 V, indicating spontaneous reactions. Mean­
while, free energy of the ORR last transition state (OH*) on the Zn/
Mo2C@Co-NCNTs-Co model (1.314 V) is lower than that on Zn/
Mo2C@Co-NCNTs-Zn (1.385 V), Co-NCNTs-800 (1.364 V) and Zn/
Mo2C-600 (1.560 V), implying that Co in Zn/Mo2C@Co-NCNTs is the
most favorable adsorption site for oxygen intermediates. Moreover,
Fig. 4(c) also shows that the rate-determining step (RDS) for the ORR at
all the models is the conversion of OOH* to O*. From Fig. 4(d), a ther­
modynamic limit voltage of 0.855 V for the RDS step on Zn/Mo2C@CoNCNTs-Co was obtained, and is obviously larger than 0.375 V on CoNCNTs-800, 0.284 V on Zn/Mo2C-600 and 0.665 V on Zn/Mo2C@CoNCNTs-Zn, suggesting that Zn/Mo2C@Co-NCNTs can promote the
3.3. Electrocatalytic mechanism based on DFT calculations
The above experimental results demonstrate that the Zn/Mo2C@CoNCNTs catalyst has excellent bifunctional ORR/OER activity and sta­
bility. To further confirm this conclusion and explore the intrinsic syn­
ergistic mechanism between the different catalytic active sites, four
models of Co-NCNTs-800, Zn/Mo2C-600, Zn/Mo2C@Co-NCNTs-Zn (the
adsorption site is Zn) and Zn/Mo2C@Co-NCNTs-Co (the adsorption site
is Co) were constructed, and the free energy and adsorption energy of
each reaction intermediate were calculated. Generally, both ORR and
OER under alkaline conditions include four basic steps, that is, ORR
proceeds by generating O2*, OOH*, O* and OH* (* represents the active
site) successively on the active sites, while the OER follows the exact
reverse direction [35-37]. Fig. 4(a) shows the transformation process of
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Chemical Engineering Journal 468 (2023) 143607
Fig. 4. (a) Graphical description of transformation of oxygen species on Zn/Mo2C@Co-NCNTs during ORR. (b) Densities of states (DOS) of surface atoms on CoNCNTs-800, Zn/Mo2C-600 and Zn/Mo2C@Co-NCNTs. (c) Free energy diagram for ORR and OER on Co-NCNTs-800, Zn/Mo2C-600 and Zn/Mo2C@Co-NCNTs-Co
at U = 0 V and 1.23 V, respectively. Free energy diagram and thermodynamic limiting potentials of (d) ORR and (e) OER on different catalysts.
desorption of OOH* thereby facilitating better ORR behaviour on Zn/
Mo2C@Co-NCNTs-Co than on the other models. Meanwhile, as shown in
Fig. 4(e), the thermodynamic limit voltages for the RDS of the OER on
Co-NCNTs-800, Zn/Mo2C-600, Zn/Mo2C@Co-NCNTs-Zn and Zn/
Mo2C@Co-NCNTs-Co are 1.765, 1.824, 1.668 and 1.610 V, respectively,
indicating superior OER performance on Zn/Mo2C@Co-NCNTs-Co over
that on the other models. The DFT calculation confirms that the synergy
between Zn/Mo2C with Co-NCNTs active sites greatly contributes to the
outstanding ORR/OER performance on Zn/Mo2C@Co-NCNTs-Co.
in batteries. As a comparison, a mixed catalyst of Pt/C + RuO2 was
immobilized on an air electrode to construct a benchmark battery. Fig. 5
(a) and its inset show that the target battery (1.506 V) has a larger open
circuit voltage (OCV) than the benchmark battery (1.455 V). In addition,
from the discharge curves in Fig. 5(b), the peak power density of the
target battery (~233.54 mW cm− 2) is much higher than its benchmark
counterpart (~198.14 mW cm− 2). Furthermore, the specific capabilities
of the target battery (894.314 mAh g-1
Zn) and the benchmark battery
− 2
(741.936 mAh g-1
(Fig. 5(c)). Then, a
Zn) were obtained at 5 mA cm
long-term charge–discharge test (Fig. 5(d)) was conducted at the same
current density as the above to assess cycling stability. Fig. 5(e) indicates
that the initial voltage variation (0.562 V) and cycling efficiency (70.04
%) of the target battery only increased by 0.126 V and decreased by 5.99
%, respectively after 300 h of cycling, while the benchmark battery
3.4. ZABs performance based on Zn/Mo2C@Co-NCNTs
Next, a liquid ZAB was fabricated using Zn/Mo2C@Co-NCNTs as a
cathode catalyst (Fig. S28) to confirm its practical catalytic performance
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Chemical Engineering Journal 468 (2023) 143607
Fig. 5. (a) Open circuit voltages of the ZABs assembled with Zn/Mo2C@Co-NCNTs (target battery) or Pt/C + RuO2 (benchmark battery) catalyst. (b) Charge/
discharge polarization curves and the derived power density curves of the target battery and benchmark battery. (c) Specific capacities of the target battery and
benchmark battery. (d) Long-term discharge–charge cycling curves of the target battery and benchmark battery and (e) The enlarged view of 0–1 and 299–300 cycles
of the discharge–charge cycling curve for the target battery. (f) Photograph of a red LED screen (2.0 V) powered by two target ZABs in series. Galvanostatic curves of
Bare Zn and Coated Zn symmetric cells at (g) 0.5 mA cm− 2 and (h) 1 mA cm− 2 with a battery capacity of 1 mAh cm− 2. (For interpretation of the references to colour
in this figure legend, the reader is referred to the web version of this article.)
symmetric cells fluctuated abruptly after only 30 h (0.5 mA‧cm− 2) and
68 h (1 mA‧cm− 2) of galvanostatic plating/stripping test, respectively,
resulting in the short circuiting of the symmetric cells. However, the
coated Zn symmetric cells could withstand more than 100 h of stripping/
plating cycles without any obvious change in the polarization voltages.
In addition, the zinc plating/stripping behavior was also evaluated at
higher current densities, and the coated Zn still maintained excellent
cycling behavior at 2 mA‧cm− 2 and 5 mA‧cm− 2 (Fig. S30). The
morphology of Zn anode surface after the testing (Fig. S31) was not
obviously changed compared with that before the test, implying a
reversible stripping/plating behavior of Zn ions and an effective sup­
pression of Zn dendrite growth on the Zn/Mo2C@Co-NCNTs coated Zn
anode.
To further explore the dendrite inhibition effect of Zn/Mo2C@CoNCNTs, an all-solid-state Zn-air coin cell (Fig. 6(a) and Fig. S29(b)) was
assembled with a bare Zn foil (the top of Fig. 6(b)) or a Zn/Mo2C@CoNCNTs coated Zn foil (the top of Fig. 6(c)) as the anode, and a com­
mercial mixed catalyst of 20% Pt/C + RuO2 as the air electrode catalyst.
After 100 h of charge–discharge cycling stability test, the bottom of
Fig. 6(b) shows that a severely corroded bare Zn anode is coated with
deteriorated rapidly after 100 h of testing. The above results demon­
strate that Zn/Mo2C@Co-NCNTs is superior to a noble metal mixed air
catalyst for ZABs. Furthermore, a red LED screen (2.0 V) of “Zn-air”
pattern can be lighted up by two connected target batteries (Fig. 5(f)).
Table S5 shows that the performance of the Zn/Mo2C@Co-NCNTs based
battery is comparable to those of previously reported rechargeable
ZABs, indicating the great application prospects of Zn/Mo2C@CoNCNTs in energy storage devices.
The significance of this study lies in the design and application of Zn/
Mo2C@Co-NCNTs as both the air electrode catalyst for assembling a ZAB
and the anode coating for preventing dendrite growth. After charac­
terizing the morphology, composition and properties of the prepared
catalysts, we then studied its ability to suppress dendrite growth in
detail. Firstly, a CR2032-type symmetric cell (Fig. S29(a)) was assem­
bled by connecting two coated Zn or two bare Zn foil disks to assess zinc
plating/stripping performance on both the coated Zn and bare Zn anode.
Fig. 5(g) and 5(h) show the galvanostatic curves of coated/bare Zn
symmetric coin cells at 0.5 mA‧cm− 2 and 1 mA‧cm− 2 where the capacity
of both cells was controlled at a fixed value of 1 mAh‧cm− 2. From both
figures, it is observed that the polarization voltages of the bare Zn
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Chemical Engineering Journal 468 (2023) 143607
Fig. 6. (a) The assembly parts of an
all-solid-state coin cell. Photographs
of (b) a Bare Zn and (c) a Coated Zn
foil before (top) and after (bottom)
100 h of charge and discharge cycling
test. SEM images of (d) a Bare Zn and
(e) a Coated Zn foil before (top) and
after (bottom) 100 h of charge and
discharge cycling test. (f) Stripping/
plating process on Bare and Coated Zn
anodes. (g) OCVs of, (h) Discharge
polarization curves and the derived
power density curves of, (i) Specific
capacities of, (j) Long-term dis­
charge–charge cycling curves of full
coin cells with 20% Pt/C + RuO2
catalyst as the air cathode, and Coated
Zn (red trace) or Bare Zn (black trace)
as the anode. (For interpretation of
the references to colour in this figure
legend, the reader is referred to the
web version of this article.)
large numbers of white particles, implying the formation of a large
amount of dendrites on its surface. On the contrary, the coated Zn anode
surface remains nearly unchanged after the same test (the bottom of
Fig. 6(c)), which indicates significant dendrite suppression with the Zn/
Mo2C@Co-NCNTs coating. Fig. 6(d) demonstrates that the smooth bare
Zn anode (the top SEM image) became very coarse and a large number of
deposits could be observed on it after 100 h of cycling test, indicating the
severe accumulation of Zn dendrites on the Zn anode (the bottom SEM
image). In contrast, Fig. 6(e) shows that morphology of the coated Zn
anode barely changed following the same test, meaning that the depo­
sition of Zn dendrites was inhibited by the coating of Zn/Mo2C@CoNCNTs. Furthermore, Fig. S32 shows the morphology of a Na2MoO4/
ZnO/Co@NCNTs-800 coated Zn foil after a plating/stripping cycling test
at 1 mA cm− 2 to further confirm the advantages of using [Zn{P4Mo6}2]
as the calcination precursor. The results indicate that although the
growth of dendrites was inhibited to a certain extent (50–100 μm), the
inhibition effect of Na2MoO4/ZnO/Co@NCNTs-800 is still not as good
as that of Zn/Mo2C@Co-NCNTs, which restrains the growth of dendrites
to only 10 μm (Fig. 6d).
More importantly, the possible formation and inhibition mechanism
of zinc dendrites on the Zn anode was elaborated as follows. (1) As
demonstrated by Fig. 6(f), dendritic growth is usually associated with
unordered deposition of Zn ions, the hydrogen evolution reaction (HER),
and corrosion by concentrated OH– electrolyte on the Zn anode [38,39].
More specifically, during the discharge process, the occurrence of the
HER will lead to an imbalance of the local distribution of OH– on the zinc
anode surface, and the alkaline electrolyte (OH–) will also corrode the
bare zinc anode. Both of them will generate Zn(OH)24 ions to form a Zn
hydroxide passivation layer on the Zn anode, impeding the conversion of
Zn2+ and producing dendrites [40]. In this work, Zn/Mo2C@Co-NCNTs
is tightly and smoothly coated on the zinc anode surface (Fig. S33),
which can protect the zinc anode from being corroded by OH–, cover the
H2 generation sites to inhibit the HER (Fig. S34) and promote the or­
dered deposition of Zn. (2) Moreover, the Zn/Mo2C@Co-NCNTs coating
forms large numbers of nucleation sites on the zinc anode. According to
the Ostwald Ripening effect [39], the Zn(OH)24 ions generated during
the discharge process can be confined under a critical passivation size by
the nucleation sites even after long-term cycling, which will greatly
impede dendrite growth on the Zn anode. (3) In addition, an uneven
charge distribution caused by the non-uniform wetting of the zinc anode
is another main reason to produce Zn dendrites [41]. The wettability of a
modified electrode surface is positively correlated to the hydrophilicity
of the immobilized electrode material, which could be evaluated by
measuring the dynamic contact angle of a water droplet (5 μL) on the
modified electrode. As shown in Fig. S35, the contact angle of the coated
Zn anode (41.2◦ ) is much smaller than that of the bare Zn anode (73.6◦ ),
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Chemical Engineering Journal 468 (2023) 143607
implying better wettability of the former. The good wettability and
porosity of the Zn/Mo2C@Co-NCNTs coating will greatly promote the
ordered stripping/plating of Zn2+/Zn, and significantly inhibit dendritic
growth on the zinc anode.
Subsequently, the performance of the above two solid-state ZABs
with bare and coated Zn anodes was evaluated to further verify the
practical dendrite inhibition effect of the Zn/Mo2C@Co-NCNTs coating.
In Fig. 6(g), the OCV of the coin battery based on the coated anode
(1.384 V) is significantly larger than that of the battery based on the bare
anode (1.306 V). The polarization curves in Fig. S36 also imply that the
charge–discharge performance of the coin cell based on the coated
anode is significantly superior over that of the bare anode cell. In Fig. 6
(h), maximum power densities of coated Zn coin cell (255.796 mW
cm− 2) and bare Zn coin cell (219.707 mW cm− 2) were obtained. Next,
the galvanostatic discharge curves (Fig. 6(i)) indicate that the coated Zn
coin cell has an energy density of 764.613 mAh g-1
Zn, obviously larger
− 2
than the bare Zn coin cell (318.143 mAh g-1
Zn) at 5 mA cm . Following
that, Fig. 6(j) displays that the ΔE value of the coated Zn coin cell
increased by only 0.012 V, while the round trip efficiency barely
changed after 100 h of galvanostatic testing. Furthermore, the long-term
cycling tests of ZABs at different current densities (0.5, 1 and 2 mA
cm− 2) also demonstrate the excellent stability of Zn/Mo2C@Co-NCNTs
(Fig. S37). On the contrary, the ΔE value of the bare Zn coin battery
started to change abruptly after only 20 h of cycling, mainly due to the
severe hindrance of charge transport due to passivation dendrites.
Additionally, after 100 h of charge–discharge cycling test, the Zn(OH)2
crystal peak can be found in Fig. S38 (XRD pattern of bare Zn) due to
existing dendrites, while that of the coated Zn anode only shows the Zn
crystal peak after Zn/Mo2C@Co-NCNTs was scraped off from the anode,
proving the effective inhibition of dendrite growth by the coating ma­
terial. In summary, the above results confirm that a strong suppression
of dendrite formation can be achieved by the Zn/Mo2C@Co-NCNTs
coating.
Methodology, Software, Validation, Visualization, Writing – original
draft. Chaoyao Geng: Data curation, Software, Validation. Xiaoqiang
Liu: Formal analysis, Methodology, Project administration, Resources,
Supervision, Writing – review & editing. Anthony P. O’Mullane:
Formal analysis, Project administration, Supervision, Writing – review &
editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgments
This work was financially supported by the National Natural Science
Foundation of China (No. 62071169), AOM gratefully acknowledges
support from the Australian Research Council (DP180102869).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.cej.2023.143607.
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4. Conclusion
In conclusion, a newly designed [Zn{P4Mo6}2] polyoxometalate was
synthesized and pyrolysed in the presence of Co/N doped CNTs origi­
nated from a ZIF-67/melamine mixture. It was demonstrated that a
600 ◦ C calcination temperature could convert the polyoxometalate to
well-dispersed Zn and Mo2C NPs on the Co-NCNTs. The [Zn{P4Mo6}2]
precursor can provide active sites to catalyze the ORR/OER on the ZAB
cathode, form abundant nucleation sites to inhibit the formation of Zn
dendrites on the ZAB anode, and promote the generation of a porous
structure with large specific surface area. All of these effects are
attributed to the [Zn{P4Mo6}2] precursor which have been demon­
strated to be much superior to those from a mixed Na2MoO4/ZnO pre­
cursor. The optimum catalyst displays ideal binary catalytic capability
for the ORR/OER with a low ΔE (0.704 V), ascribed to the synergy be­
tween Zn/Mo2C and Co-NCNTs active sites, which was supported by
DFT calculations. This synergy can significantly reduce the reaction
barrier of oxygen intermediates, thereby accelerating the ORR/OER
reaction kinetics. The assembled liquid ZABs showed a power density of
223.54 mW cm− 2, a high energy density of 894.3 Wh kg− 1, and a roundtrip efficiency of 64.05% after 300 h of charge–discharge cycling.
Equally important, when it was used as the anode coating, the assembled
symmetric cell exhibited a plating/stripping stability for 100 h. The fullsolid coin battery could maintain excellent charge–discharge capability
for 100 h, while the coin battery with the bare Zn anode could only
withstand 20 h of cycling. This simple, efficient, and broadly applicable
strategy will greatly promote the development and practical application
of highly efficient and stable ZABs.
CRediT authorship contribution statement
Jiaqi
Niu:
Conceptualization,
Data
curation,
Investigation,
11
J. Niu et al.
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