2D Metal–Organic Framework Derived CuCo Alloy Nanoparticles Encapsulated by Nitrogen-Doped Carbonaceous Nanoleaves for Efficient Bifunctional Oxygen Electrocatalyst and Zinc–Air Batteries

Article abstract of DOI:10.1002/chem.201902389

The development of efficient bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) still remains a challenge in a wide range of renewable energy technologies. Herein, CuCo alloy nanoparticles encapsulated by nitrogen-doped carbonaceous nanoleaves (CuCo-NC) have been synthesized from a Cu(OH)₂/2D leaf-like zeolitic imidazolate framework (ZIF-L)-pyrolysis approach. Leaf-like Cu(OH)₂ is first prepared by the ultrasound-induced self-assembly of Cu(OH)₂ nanowires. The efficient encapsulation of Cu(OH)₂ in ZIF-L is obtained owing to the morphology fitting between the leaf-like Cu(OH)₂ and ZIF-L. CuCo-NC catalysts present superior electrocatalytic activity and stability toward ORR and OER over the commercial Pt/C and IrO₂, respectively, which are further used as bifunctional oxygen electrocatalysts in Zn–air batteries and exhibit impressive performance, with a high peak power density of 303.7 mW cm^?2, large specific capacity of up to 751.4 mAh g^?1 at 20 mA cm^?2, and a superior recharge stability.

Full text of DOI:10.1002/chem.201902389

A Journal of
Accepted Article
Title: 2D Metal-Organic Framework derived CuCo Alloy Nanoparticles
Encapsulated Nitrogen-doped Carbonaceous Nanoleaves
for Efficient Bifunctional Oxygen Electrocatalyst and Zinc-Air
Batteries
Authors: Meiling Huo, Bin Wang, Chaochao Zhang, Shuping Ding,
Haitao Yuan, Zuozhong Liang, Jing Qi, Mingxing Chen, Yang
Xu, Wei Zhang, Haoquan Zheng, and Rui Cao
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To be cited as: Chem. Eur. J. 10.1002/chem.201902389
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DOI: 10.1002/ ((please add manuscript number))
Article type: Full Paper
2D Metal-Organic Framework derived CuCo Alloy Nanoparticles Encapsulated
Nitrogen-doped Carbonaceous Nanoleaves for Efficient Bifunctional Oxygen
Electrocatalyst and Zinc-Air Batteries.
Meiling Huo, †, [a] Bin Wang, †, [a] Chaochao Zhang [a] Shuping Ding,[a] Haitao Yuan,[a]
Zuozhong Liang,[a] Jing Qi,[a] Mingxing Chen,[a] Yang Xu,[a] Wei Zhang,[a] Haoquan
Zheng,*[a] and Rui Cao*[a]
[a]
M. L. Huo, Dr. B. Wang, C. C. Zhang, S. P. Ding, H. T. Yuan, Z. Z. Liang, J. Qi, M.
X. Chen, Y. Xu, Prof. Dr. W. Zhang, Prof. Dr. H. Q. Zheng, Prof. Dr. R. Cao
Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education,
School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an
710119, China. E-mail: ruicao@ruc.edu.cn; zhenghaoquan@snnu.edu.cn
† These authors contributed equally to this work.
Keywords: Metal-organic framework, CuCo alloy nanoparticle, nitrogen-doped carbon,
oxygen electrocatalyst, zinc-air batteries
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Abstract:
The development of efficient bifunctional electrocatalysts for oxygen reduction reaction
(ORR) and oxygen evolution reaction (OER) is still remained a challenge in wide range of
renewable energy technologies. Herein, CuCo alloy nanoparticles encapsulated nitrogen-
doped carbonaceous nanoleaves (CuCo-NC) have been synthesized from a Cu(OH)2/2D leaf-
like zeolitic imidazolate framework (ZIF-L)-pyrolysis approach. Leaf-like Cu(OH)2 is first
prepared by the ultra-sound induced self-assembly of Cu(OH)2 nanowires. The efficient
encapsulation of Cu(OH)2 in ZIF-L is obtained due to the morphology fitting between the
leaf-like Cu(OH)2 and ZIF-L. CuCo-NC catalysts present superior electrocatalytic activity
and stability toward ORR and OER over the commercial Pt/C and IrO2, respectively, which
are further used as bifunctional oxygen electrocatalysts in Zn–air batteries and exhibits
impressive performance, with a high peak power density of 303.7 mW cm−2, large specific
capacity up to 751.4 mAh g−1 at 20 mA cm−2, and a superior rechargeable stability.
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1. Introduction
Rechargeable metal–air batteries have attracted great interest as a promising solution for
energy-storage due to its high theoretical energy density, high security, and low cost.1-8
Precious metal-based electrocatalysts have shown high activities for oxygen evolution
reactions (OER) and oxygen reduction reactions (ORR). However, their poor stability and
high cost limit the commercialization of metal–air batteries.9-19 Therefore, developing
efficient catalysts with low cost and electrocatalytic activity toward both OER and ORR is
still one of the main challenges for metal–air batteries and other renewable energy
technologies.20-33
The different electronegativity between carbon and nitrogen makes nitrogen-doped
carbon (NC) materials potential electrocatalysts, which have been demonstrated to exhibit
excellent catalytic properties.22, 34-41 After encapsulating transition metals in N-doped carbons
(M-NC), M-NC materials show enhanced electrocatalytic activities toward ORR/OER, which
are attributed to the synergetic effects between the transition metal nanoparticles (NPs) and
the NC shell.11, 34, 36, 42-54 Compared to monometallic system, bimetallic or trimetallic alloys
have shown promising catalytic activity, and sometimes unique catalytic activity, which
monometallic systems do not have.11, 40, 55-62 Among these systems, Cu based bimetallic alloy
electrocatalysts attracted a lot of attentions, since Cu compounds exhibit biomimetic
chemistry with O2.63-64 However, Cu-based NC materials suffer from irreversible fusion and
self-aggregation during pyrolysis at high-temperature, which is caused by its large diffusion
constant at the same temperature. 11, 65-67 Therefore, it is still challenging to prepare Cu-based
NC electrocatalysts with homogenous distribution of Cu species, which prevents the loss in
electrocatalytic capabilities.
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Zeolitic imidazolate frameworks (ZIFs) are a class of metal-organic frameworks, which
have been considered as potential precursors for NC materials due to large surface areas and
abundant N species.11, 68-74 Recently, Zheng and co-workers reported that a Cu, Co@NC
derived from ZIF-67 and Cu(OH)2. Instead of CuCo alloy nanoparticles, Cu and Co
nanoparticles are separately but homogenously distributed in NC framework.75 More recently,
Fe, Cu@NC was derived from ZIF-8 and used as electrocatalysts. Fe was homogeneously
distributed throughout the entire architecture, while Cu was distributed in the form of
dispersed nanoparticles throughout NC nanoparticle.76 To date, it is still difficult to prepare a
Cu based bimetallic alloy@NC based electrocatalysts for both ORR and OER.
Here, we fabricated a CuCo alloy NPs-encapsulated NC materials (CuCo@NC). ZIF-L-
Co possesses a two-dimensional (2D) leaf-like morphology and a 2D layer-by-layer stacked
structure as well. Guest molecules between the layers can be removed during pyrolysis.77-78
Leaf-like Cu(OH)2 has been synthesized by the ultra-sound induced self-assembly of
Cu(OH)2 nanowires. Thus, we propose that it is possible to fully confine the Cu species in this
2D layered framework, which prevents irreversible fusion that is commonly found in a three-
dimensional (3D) continuous framework during pyrolysis. CuCo@NC exhibits superior OER
and ORR bifunctional activity and impressive zinc-air performance. Specifically, when
integrated into Zn-air batteries, a high peak power density of 303.7 mW cm−2, large specific
capacity up to 751.4 mAh g−1 at 20 mA cm−2, small charge-discharge voltage gap of about
0.84 V at 2 mA cm−2, and a superior rechargeable stability are observed, which is prominently
superior to commercial Pt/C and Ir/C mixture catalyst.
2. Results and Discussion
2.1. Characterization of CuCo@NC electrocatalysts
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Figure 1. Schematic illustration of the preparation of CuCo@NC.
As illustrated in Figure 1, Cu(OH)2 nanowires were first synthesized. Then, ultra-sound
was used to induce the formation of leaf-like Cu(OH)2 via self-assembly. Cu(OH)2@ZIF-L
was synthesized by mixing pre-prepared Cu(OH)2 with Co(NO3)2 and 2-methylimidazole
(Hmim) at ambient temperature. After pyrolysis at designed temperatures under argon (Ar)
atmosphere, CuCo@NC was obtained. As shown in transmission electron microscopy (TEM)
images, Cu(OH)2 nanowires with 10 nm in diameter were successfully synthesized (Figure
S1). When simply mixing the Cu(OH)2 nanowires, Co(NO3)2 and 2-methylimidazole,
Cu(OH)2 nanowires were not encapsulated in ZIF-L-Co. Instead, two separated phases,
Cu(OH)2 nanowires and ZIF-L-Co, were observed in the final product. Therefore, a further
treatment of these nanowires in ultra-sound for 2 h was carried out. Leaf-like Cu(OH)2 was
formed via self-assembly (Figure 2a). As shown in Figure 2b, scanning electron microscopy
(SEM) image shows that Cu(OH)2@ZIF-L-Co nanosheets display a leaf-like morphology,
which is similar to that of traditional ZIF-L-Co nanosheets. The thickness of the leaf-like
Cu(OH)2@ZIF-L-Co is approximately 450 nm, which is determined from the edge of the
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o
material (Figure 2b, insert). After pyrolysis at 900 C under Ar, the material keeps the leaf-
like morphology, while its thickness decreases sharply to 180 nm (Figure 2c). There are pores,
carbon nanotubes, and metal nanoparticles with size of 10-90 nm in the CuCo@NC
framework (Figure 2d and 2e). As shown in the high-resolution TEM (Figure 2e), the metal
nanoparticles were encapsulated in shelled NC multi-layered structure. The NC layers coated
on metal nanoparticles prevent further aggregation from the formation of bigger nanoparticles.
The interplanar distances of 0.36 nm and 0.208 nm from HRTEM image are indexed to the
(002) plane of graphitic carbon and the (111) plane of Co, respectively.
Figure 2. a) TEM image of leaf-like Cu(OH)2. b) SEM image of Cu(OH)2@ZIF-L. c) SEM
image, d) TEM image and e) HRTEM image of CuCo@NC. f) XRD patterns of leaf-like
Cu(OH)2, ZIF-L and Cu(OH)2@ZIF-L. g) XRD patterns of Co@NC and CuCo@NC. h)
STEM and the corresponding EDS elemental mapping results of CuCo@NC. The scale bar in
h) is 50 nm. i) Thermogravimetric analysis of Cu(OH)2@ZIF-L and Cu(OH)2@ZIF-67.
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The powder X-ray diffraction (PXRD) pattern of Cu(OH)2@ZIF-L-Co shows diffraction
peaks with high intensity, indicating the good crystallinity of the material and the formation
of ZIF-L-Co (Figure 2f). The energy dispersive X-ray spectra (EDX) demonstrated that
Cu(OH)2 nanoleaves were well-scattered in ZIF-L nanosheets (Figure S2 and S3). PXRD
pattern of CuCo@NC material has the same diffraction peaks as those of Co@NC (Figure 2g).
We proposed that the Cu and Co formed CuCo alloy nanoparticles due to the similar unit cell
and atomic arrangement of their face-centered-cubic (fcc) structure. As shown in high-angle
annular dark field-scanning transmission electron microscopy (HAADF-STEM) image and
the corresponding EDS mapping of CuCo@NC, the N element was homogenously distributed
throughout the whole nanoleaf (Figure S5), while Cu and Co were specifically located on the
metal nanoparticles (Figure 2h). When zooming in an individual metal nanoparticle, Cu and
Co elements were well-distributed, demonstrating the formation of CuCo alloy NPs. In the
reported work using ZIF-67 instead of ZIF-L-Co, Cu and Co nanoparticles are separately but
homogenously distributed in NC framework derived from ZIF-67.75 To further investigate the
difference in ZIF-L-Co and ZIF-67 used in this system, thermogravimetric analyses (TG) of
Cu(OH)2@ZIF-L-Co and Cu(OH)2@ZIF-67 in N2 were performed (Figure 2i). Compared to
o
ZIF-67, ZIF-L-Co has an extra weight loss at 250-300 C, which was caused by the weight
loss of Hmim between crystalline layered structure. The layered structure became dense after
o
removal of these Hmim molecules at 450-500 C, which happened before the decomposition
of Cu(OH)2. Therefore, the Cu species were confined in nonporous 2D layered framework,
which prevents irreversible fusion and give rise to the formation of CuCo alloy in the NC
framework.
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Figure 3. a) N2 adsorption/desorption isotherms of Co@NC, CuCo@NC and its
correspongding to precursors. b) Raman spectra of Co@NC and CuCo@NC. c) XPS survey
spectra of Co@NC and CuCo@NC. High-resolution XPS spectra of d) Co 2p, e) Cu 2p, and f)
N 1s of CuCo@NC composite. High-resolution XPS spectra of N 1s of g) Co@NC composite
and h) NC composite. i) Different percentages of N species in CuCo@NC, Co@NC and NC.
N2 adsorption/desorption isotherms of ZIF-L, Cu(OH)2@ZIF-L, Co@NC, and
CuCo@NC are shown in Figure 3a. The adsorption amounts of N2 in ZIF-L and
Cu(OH)2@ZIF-L are very low, indicating that both ZIF-L and Cu(OH)2@ZIF-L possess no
porous structure. This result can be explained by the layer-by-layer crystalline structure of
ZIF-L, which is filled by Hmim molecules and has no space for the adsorption of N2
molecules. BET specific surface areas of Co@NC, and CuCo@NC after annealing are
significantly higher than those of corresponding precursors. BET specific surface area of
CuCo@NC (~ 326 m2 g−1) is more than twice comparable to that of the Co@NC (~ 162 m2
g−1). The introduce of Cu(OH)2 into ZIF-L causes the increased porosity of final CuCo@NC
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product, which is favorable for mass transfer and beneficial for electrocatalysis. The
characteristic G and D bands in the Raman spectra were located at 1600 and 1350 cm−1,
respectively79 (Figure 3b). The G band was assigned to the sp2 graphitic carbons, while the D
band indicated the defective sites of disordered sp3 carbons in materials, which promoted
electron transfer and provided more catalytic centers.80-83 The intensity ratio between D and G
bands (ID/IG) of CuCo@NC was calculated to be 1.05, which was larger than that of Co@NC
catalyst (0.71), indicating the formation of more defective carbons in CuCo@NC.
XPS measurements were further performed to analyze the surface chemical compositions
of obtained samples. The survey spectra of CuCo@NC demonstrated the existence of C, N,
Co, and Cu in CuCo@NC, indicating the doping of Cu element in CuCo@NC (Figure 3c).
The XPS of CuCo@NC in Co 2p3/2 region showed three main XPS peaks located at 778.8,
780.4, and 782.1 eV from the core photoelectron lines, which were assigned to the metallic
Co, CoOx/CoCxNy, and CoNy, respectively (Figure 3d). Satellite peak of Co 2p3/2 at 785.2 eV
was observed, which was similar to those of Co@NC (Figure S6).75, 84-85 In the high-
resolution Cu 2p3/2 XPS spectra of CuCo@NC, the peaks located at 933.0 and 934.7 eV were
assigned to the Cu(0)/Cu(I) and Cu(II) species, respectively (Figure 3e). The Cu 2p satellite
peak at 944.2 eV can be attributed to Cu(II) species in CuCo@NC due to unfilled electron
state of Cu 3d9 orbitals.75, 86-87
The N contents in CuCo@NC, Co@NC, and NC are calculated to be 3.83, 3.71 and 1.04
at%, respectively, which are much higher than that in Co encapsulated N-doped carbon
74
derived from ZIF-L. NC was derived from ZIF-L-Zn at 900 ℃ in Ar, which has the same
topological structure as ZIF-L-Co and is used as a reference. We further investigated the
contents of different types of N in these materials (Figure 3f-i). The high-resolution N1s
spectra of Co@NC and CuCo@NC are deconvoluted into three peaks, which are assigned to
graphitic N (402.7 eV), pyrrolic N (401.2 eV) and pyridinic N (398.9 eV).88-89 The
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percentages of the different types of N in materials were calculated based on the integral areas
of corresponding peaks (Figure 3i). CuCo@NC had the highest content of pyridinic N and
graphitic N (2.6%) compared to that of Co@NC (2.1%) and NC (0.5%). The low content of
NC may be attributed to the leaf-like morphology of NC, which gives rise to an easy loss of N
at high temperature. As previously reported, the graphitic N and the pyridinic N are active
sites for the ORR and OER.26, 28, 90-91 92Combining with Raman measurements, the addition of
Cu(OH)2 into ZIF-L-Co not only introduces Cu in CuCo alloy NPs but also gives rise to more
defective sites in electrocatalysts, which are also expected to promote electron transfer and
improve the electrocatalysis.
Figure 4. a) CV polarization curves of CuCo@NC in O2- or Ar-saturated 0.1 M KOH. b)
LSV curses for the samples on an RDE at 1600 rpm in an O2-saturated 0.1 M KOH. c) LSV
curves of CuCo@NC at different rotation speeds. d) Corresponding K-L plots of CuCo@NC
at different potentials. e) Tafel slope values at low overpotential regions for CuCo@NC,
Co@NC and Pt/C. f) LSV curves of CuCo@NC before and after 1000 cycles in O2-saturated
0.1 M KOH solution.
As bifunctional electrocatalysts for ORR and OER, the ORR electrocatalytic activity of
CuCo@NC was first tested by cyclic voltammetry (CV) curves in O2-saturated 0.1 M KOH
solution compared to that in Ar-saturated solution (Figure 4a). The CV of CuCo@NC shows
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a stronger cathodic peak compared to that of Co@NC and Cu@NC (Figure S7), indicating its
significantly efficient ORR performance. The doping amount of Cu(OH)2 in ZIF-L-Co
precursors would affect the state of Cu species, and the corresponding state of Co and the
defective structure in the final CuCo-NC materials. The linear sweep voltammetry (LSV)
polarization curves of CuCo@NC with different amounts of Cu(OH)2 in ZIF-L-Co precursors
were performed using a rotating disk electrode (RDE). The onset potential (Eonset) and the
half-wave potential (E1/2) of CuCo@NC increase with increasing Cu(OH)2 contents in ZIF-L-
Co and reach the highest values (Eonset = 0.945 V, E1/2 = 0.866 V) when doping 11 at%
Cu(OH)2 nanoleaves in ZIF-L-Co. When the Cu(OH)2 doping further increases to 20 at%,
their ORR activities decrease (Figure S8). The PXRD pattern of CuCo@NC with 20 at% Cu
doping shows additional sharp diffraction peaks, which can be indexed as Cu nanoparticles
instead of CuCo alloys (Figure S9). Therefore, the CuCo alloy structure is proposed to be
important for the enhanced ORR performance.
The LSV curses for the NC, Cu@NC, Co@NC, CuCo@NC, and commercial Pt/C were
then performed (Figure 4b). Among the non-precious metal catalysts, CuCo@NC exhibited
the highest ORR activities (Eonset = 0.945 V, E1/2 = 0.866 V), which was higher than the NC,
Co@NC and Cu@NC catalysts and comparable to Pt/C catalyst. Therefore, CuCo alloy NPs
in NC is beneficial for the ORR performance, which is better than non-metal NC and
monometallic NC. SEM images of Cu@NC, Co@NC, CuCo@NC after ORR performance
tests show no significant change (Figure S10), indicating good structural stability of catalysts.
The electrocatalysts synthesized by simply annealing physically mixed leaf-like Cu(OH)2 and
ZIF-L-Co showed a lower catalytic activity than CuCo@NC, indicating the importance of
doping process (Figure S11). Polarization curves for CuCo@NC and Pt/C electrodes were
recorded at rotation speeds from 400 to 2025 rpm (Figure 4c and S12). The Koutecky–Levich
(K–L) plots from the kinetics diffusion control area exhibited a good parallelism and linearity,
indicating the similar electron transfer number and the first order with respect to the
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concentration of dissolved O2 at different potentials (Figure 4d).93-94 The rotating ring-disk
electrode (RRDE) technique was performed to investigate the mechanism. The electron
transfer number for Co@NC and CuCo@NC catalysts are approaching the ideal value of 4.0,
which are similar to that of the Pt/C (Figure S13 and S14). CuCo@NC had the smallest Tafel
slope of 63 mV dec−1 (Figure 4e). The stability of catalyst was conducted with repetitive RDE
voltammetry experiment. After 1000 cycles, the LSV of CuCo@NC catalyst is almost the
same as that of initial state, while there is a decrease of E1/2 (10 mV) for Pt/C catalyst before
and after 1000 cycles (Figure 4f). Comparied with Pt/C catalyst, the CuCo@NC material
exhibits higher resistance to the methanol cross-over effect in oxygen-saturated 0.1M KOH
(Figure S15). Thus, CuCo@NC catalyst has superior stability.
Figure 5. a) LSV curves of various samples for oxygen evolution in 1.0 M KOH solution. b)
Tafel slopes for CuCo@NC, Co@NC, and RuO2. c) CV curves of CuCo@NC at various scan
rates of in the range of 0.2-0.3 V vs. Ag/AgCl. d) Capacitance currents plotted versus scan
rates of CuCo@NC, Co@NC, and Cu@NC. e) Impedance diagrams for CuCo@NC, Co@NC,
and Cu@NC. f) Chronopotentiometry curves for CuCo@NC, Co@NC, and RuO2.
Subsequently, the OER tests of CuCo@NC, Cu@NC, Co@NC, and commercial RuO2
were evaluated using a three-electrode system in 1 M KOH (Figure 5a). A potential of 1.568
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V versus the reversible hydrogen electrode (vs. RHE) for CuCo@NC catalyst was needed to
achieve a current density of 50 mA cm−2. For the Co@NC catalyst and the state-of-art RuO2,
higher potentials are needed to achieve a current density of 50 mA cm−2 (1.625 V vs. RHE for
Co@NC and 1.603 V vs. RHE for the state-of-art RuO2) than that of CuCo@NC catalyst. The
Tafel slope of CuCo@NC catalyst is the smallest (95 mV dec−1) compared to that of Co@NC
(133 mV dec−1) and RuO2 (98 mV dec−1), confirming the superior electrochemical OER
performance of CuCo@NC electrode (Figure 5b). The electrochemical surface area (ECSA)
was calculated based on electrochemical double-layer capacitance (Cdl) at the non-Faraday
region (Figure 5c, 5d and S16).95 The Cdl of CuCo@NC (5.226 mF) catalyst exhibited more
than four times higher than that of Co@NC (1.148 mF) and Cu@NC (0.607 mF). The more
exposed electrocatalytic active sites for OER lead to the superior catalytic performance of the
CuCo@NC catalyst. The electrochemical impedance spectra of the electrocatalysts were
shown in Figure 5e. As shown in impedance curves of CuCo@NC, Co@NC and Cu@NC, the
second semicircles indicate that the CuCo@NC has the lowest charge transfer resistance
among all electrocatalysts. The alloying of Co and Cu contributes to the increased electrical
conductivity and further the enhanced activity of the CuCo@NC. Furthermore, the durability
of the catalysts is essential to be measured. The chronopotentiometry curve of CuCo@NC
showed no significant change during continuous electrolysis and attained a current density,
which was higher than that of commercial RuO2 and Co@NC, verifying excellent OER
activity and its good stability (Figure 5f).
The superior ORR and OER performances of the CuCo@NC could be ascribed to the
multiple synergistic effects. First, the presence of metallic Cu might favor the charge transport
between the surface of the catalyst and the collector, thus endowing the catalyst with a high
electrocatalytic activity. Second, the pyrolysis of Cu(OH)2 in ZIF-L-Co at a high temperature
caused the formation of more pores, which benefits the exposure of more electrochemical
active sites. Third, benefiting from the unique 2D structure of the CuCo@NC and the alloy
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forming of Cu and Co, the electrocatalytic activity might be remarkably promoted. Fourth, the
graphitic N and the pyridinic N are active sites for the ORR and OER, which were coupled
with defective sites certified by Raman spectra. Furthermore, the coordination between Co
atoms and N atoms in N doped carbon could also tune the adsorption of the rate-determining
intermediate (OOH*), which is beneficial for the bifunctional electrocatalytic activity of the
NC@Co nanocage catalysts toward ORR and OER.71 All issues above endow the catalyst
with a high electrocatalytic activity.
Figure 6. Application of bifunctional CuCo@NC in Zn-air batteries. a) The schematic of Zn-
air batteries. b) Charge/discharge polarization and power density curves. c) Discharge curves
at a constant current density of 20 mA cm−2. d) Charge-discharge cycling curves at 2 mA
cm−2. e) Long cycling test.
The promising bifunctional performance of CuCo@NC makes it possible to construct
rechargeable Zn-air batteries, which were further evaluated its application potentials. The
anode was a Zn foil, while the air electrode was a catalyst-coated carbon cloth/gas diffusion
layer, with a loading of 0.75 mg cm−2 (Figure 6a). To ensure the reversible charge reactions
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in anode, 0.2 M ZnCl2 was added in 6.0 M KOH as electrolyte. The mixture of Pt/C and Ir/C
(1: 1 of mass ratio) was applied as a control sample for comparison.
The Zn-air battery catalyzed by CuCo@NC exhibits an evidently shrunken charge-
discharge voltage gap compared to Pt/C + Ir/C (Figure 6b). Moreover, the peak power density
of the one using CuCo@NC air electrode is 303.7 mW cm−2, 6-times that of Pt/C + Ir/C,
indicating a superior bifunctional activity and rechargeability. Notably, the Zn-air battery
using CuCo@NC could be operated under extremely high current densities, with nearly 300
mA cm−2 for charge and over 400 mA cm−2 for discharge, which are obviously higher than the
current densities of Pt/C + Ir/C (55.1 and 127.3 mA cm−2 for charge and discharge,
respectively). When galvanostatically discharging at 20 mA cm−2, a considerably larger
discharge voltage of around 1.23 V was delivered by the Zn-air battery based on CuCo@NC
in comparison with Pt/C + Ir/C (Figure 6c). Moreover, no obvious degradation was observed.
The specific capacity at 20 mA cm−2 for CuCo@NC based Zn-air battery is calculated to be
751.4 mAh g−1, which is normalized to consumed Zn. It is higher than that of Pt/C + Ir/C
(720.5 mAh g−1) and corresponding to 91.6% of the theoretical value (820 mAh g−1). All
these metrics are one of the best results among reported non-precious-metal catalysts, and
even superior to that of the commercial noble metal counterpart, revealing the superior
bifunctional activity and rechargeability (Table S1).
Besides rechargeability, the stability of Zn-air batteries is critical for practical
applications. When galvanostatically cycled at 2 mA cm−2, a small charge-discharge voltage
gap of about 0.84 V was detected for the Zn-air battery using CuCo@NC air electrode (1.19
V for discharge and 2.03 V for charge), with a round-trip energy efficiency of 58% (Figure
6d). The voltage gap is smaller than that of Pt/C +Ir/C and can be further decreased by around
30 mV after 14 cycle test, which is attributed to a surface activation of zinc anode during the
dissolution/deposition process of the initial cycling. Furthermore, the Zn-air battery using
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10.1002/chem.201902389
Chemistry - A European Journal
CuCo@NC exhibits a stable charge and discharge voltage throughout 100 h cycling test,
revealing excellent stability (Figure 6e). The performance degradation during long time
cycling is caused by the reaction of the alkaline electrolyte and CO2 in atmosphere, which can
be regenerated when refreshing electrolytes.
3. Conclusion
CuCo alloy nanoparticles encapsulated nitrogen-doped carbonaceous nanoleaves have
been prepared by pyrolysis of a 2D leaf-like Cu(OH)2/ZIF-L material. The CuCo-NC
catalysts present superior electrocatalytic activity and stability toward ORR and OER over the
commercial Pt/C and IrO2, respectively. The high efficiency of the CuCo-NC catalysts in both
ORR and OER is attributed to the synergistic effects of CuCo alloy, high surface area and
high N content. The constructed Zn-air battery using CuCo-NC catalysts showed impressive
performance, with a peak power density of 303.7 mW cm−2, a large specific capacity of 751.4
mAh g−1 at 20 mA cm−2, a small charge-discharge voltage gap of about 0.84 V at 2 mA cm−2,
and a good rechargeable stability. This work opens up new opportunities for design and
construction of bifunctional oxygen electrocatalysts using a 2D leaf-like Cu(OH)2/ZIF-L
material for energy conversion and storage applications.
4. Experimental Section
Methods and materials: Co(NO3)2·6H2O (99.99%), CuCl2 (99%), and Hmim (98%) were
purchased from the Energy Chemical. NaOH (98%) was purchased from Sinopharm Group
Chemical Reagent Co., Ltd. Polyethylene glycol (Mw= 20000) was purchased from Tianjin
Kemiou Chemical Reagent Co., Ltd. All chemicals were used without further purification.
Synthesis of leaf-like Cu(OH)2: Cu(OH)2 nanowires were synthesized as precursors. In a
typical synthesis condition, 180 mg of copper chloride (CuCl2) and 200 mg of polyethylene
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glycol (Mw = 20000) were dissolved in 200 mL of deionized water under stirring. Then 1.2
mL of deionized water containing 288 mg of sodium hydroxide was added dropwise into the
above solution. The mixed solution was kept at room temperature for 30 min under stirring.
The obtained Cu(OH)2 nanowires were centrifuged (10000 rpm, 5 min), washed with
deionized water for several times, and finally dried in vacuum freeze dryer. Then, leaf-like
Cu(OH)2 was prepared using the synthetic ratio of per 50 mg of Cu(OH)2 nanowires dispersed
in 10 mL deionized water under ultrasonication for 2 h.
Synthesis of Cu(OH)2@ZIF-L nanosheets: Typically, Cu(OH)2 nanoleaves (12.5-100
mg) and Co(NO3)2·6H2O (0.58 g) were mixed in 40 mL of deionized water, then sonicated for
30 min at room temperature. Afterwards, 40 mL of deionized water containing 1.3 g of Hmim
was added into the above solution under stirring. The mixture was stirred at room temperature
for 4 h. The obtained purple precipitates were collected by centrifugation (10000 rpm, 5 min),
washed with methanol for several times, and dried in vacuum freeze dryer for further
application. The composite without adding Cu(OH)2 nanoleaves was prepared by a procedure
similar to that for preparing Cu(OH)2@ZIF-L nanosheets.
Synthesis of CuCo@NC nanosheets: The quartz boat containing Cu(OH)2@ZIF-L (200
mg) powders was placed in a tube furnace under Ar flow without heating for 30 min to
remove air. Carbonization of the composite was performed under Ar at 900 °C for 2 h with a
heating ramp of 5 °C·min−1. Furthermore, all comparative samples, such as Co@NC, NC, m-
CuCoNC and m-CuNC were pyrolyzed under the same program (900 °C for 2 h).
Electrochemical studies: A CHI 760E Electrochemical Analyzer (CH Instruments) and a
Pine Modulated Speed Rotator (Pine Research Instrumentation, Inc.) were used to carry out
all electrochemical experiments of ORR performance at ambient temperature. Typically，all
electrochemical measurements were carried out in a 0.1 M KOH solution and performed with
a three-electrode configuration, using a RDE (area of 0.196 cm2) or a RRDE (area of 0.247
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Chemistry - A European Journal
cm2) electrode as the working electrode, a Pt wire as the counter electrode, and an saturated
Ag/AgCl as the reference electrode. All potentials converted into RHE with the conversion E
(vs. RHE) = E (vs. Ag/AgCl) + 0.197 V + 0.059 × pH.
For the fabrication of the working electrode, 5 mg of the catalyst was dispersed in 450
μL of isopropanol, 20 μL of deionized water and 10 μL of 5 wt% Nafion aqueous solution.
The mixed solution was sonicated for 1h to form a homogeneous suspension. Then 20 µL of
the catalysts was drop-casted onto the RDE electrode (catalyst loading: ~ 0.4 mg cm−2) and
dried naturally to form a homogeneous membrane. For the ORR, before the measurement, use
Ar/O2 flow to bubble the electrolyte at least 30 minutes. The electrochemical experiments
were conducted in O2-saturated 0.1 M KOH for ORR. The CV experiments were carried out
electrolyte solutions with a scan rate of 10 mV s−1 at room temperature. The LSV of RDE
measurements were performed at various rotating rate from 400 to 2025 rpm with a scan rate
of 5 mV s−1. RRDE measurements were performed with a scan rate of 5 mV s−1 at room
temperature and the ring-electron potential was set to 1.5 V vs RHE. The electron transferred
number (n) and the hydrogen peroxide yield (H2O2 %) can be calculated according to the
following formulas:
is the disk current; is the ring current; N is the current collection efficiency of the Pt
ring (~ 0.39 with the LSV measurement in K3Fe[CN]6 solution).
For OER, all electrochemical measurements were performed with a three-electrode
configuration and carried out in a 1.0 M KOH solution at room temperature. A glassy carbon
(GC, area of 0.07 cm2) electrode, a Pt wire, and an saturated Ag/AgCl were used as the
18
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10.1002/chem.201902389
Chemistry - A European Journal
working electrode, as the auxiliary electrode, and as the reference electrode, respectively.
Typically, 2 mg of the catalyst was added in 30 µL of the Nafion solution (5 wt%, DuPont)
and suspended in 1 mL of water–ethanol solution (vol. ratio of 2: 1). The solution was
sonicated for 1 h to form a homogeneous slurry. Afterwards, 5 µL of the as-prepared catalysts
were loaded onto a GC electrode. The Tafel plots were measured by carrying out the LSV at a
scanning rate of 5 mV s−1.
For Zn-air battery, a polished Zn foil with thickness of 0.25 mm, 6.0 M KOH solution
(containing 0.2 M ZnCl2), catalyst-coated carbon cloth/gas diffusion layer (CC/GDL) were
used as anode, as electrolyte, and as air electrode, respectively. The carbon cloth (1.5 ×1.5
cm2, WOS 1009, CeTech) was attached on GDL through a hot-press method. Then a certain
volume of catalyst ink was dropped on CC/GDL to reach a catalyst loading of 0.75 mg cm−2.
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
We are grateful for support from the “Thousand Talents Program” of China, the National
Natural Science Foundation of China (Grant Nos. 21601118, 21101170, 21573139, and
21773146), the Fundamental Research Funds for the Central Universities (GK201903033),
and the Research Funds of Shaanxi Normal University.
Received: ((will be filled in by the editorial staff))
Revised: ((will be filled in by the editorial staff))
Published online: ((will be filled in by the editorial staff))
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2D Metal-Organic Framework derived CuCo Alloy Nanoparticles Encapsulated
Nitrogen-doped Carbonaceous Nanoleaves for Efficient Bifunctional Oxygen
Electrocatalyst and Zinc-Air Batteries.
Keyword: metal-organic framework, CuCo alloy nanoparticle, nitrogen-doped carbon,
oxygen electrocatalyst, zinc-air batteries.
M. L. Huo, B. Wang, C. C. Zhang, S. P. Ding, H. T. Yuan, Z. Z. Liang, J. Qi, M. M. Chen, Y.
Xu, W. Zhang, H. Q. Zheng,* and R. Cao*
26
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