Mxene supported catalyst

1. Journal Pre-proofs
Article
Anchoring Co3O4 nanoparticles on MXene for efficient electrocatalytic
oxygen evolution
Yi Lu, Deqi Fan, Zupeng Chen, Weiping Xiao, Cancan Cao, Xiaofei Yang
PII: S2095-9273(19)30719-4
DOI: https://doi.org/10.1016/j.scib.2019.12.020
Reference: SCIB 916
To appear in: Science Bulletin
Received Date: 22 November 2019
Revised Date: 13 December 2019
Accepted Date: 16 December 2019
Please cite this article as: Y. Lu, D. Fan, Z. Chen, W. Xiao, C. Cao, X. Yang, Anchoring Co3O4 nanoparticles on
MXene for efficient electrocatalytic oxygen evolution, Science Bulletin (2019), doi: https://doi.org/10.1016/j.scib.
2019.12.020
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2. Anchoring Co3O4 nanoparticles on MXene for efficient
electrocatalytic oxygen evolution
Yi Lu1, Deqi Fan1, Zupeng Chen2, Weiping Xiao1*, Cancan Cao1, Xiaofei Yang1,3*
1 College of Science, Institute of Materials Physics and Chemistry, Nanjing Forestry
University, Nanjing 210037, China
2 Institute for Chemical and Bioengineering, Department of Chemistry and Applied
Biosciences, ETH Zürich, CH-8093 Zürich, Switzerland
3 Key Laboratory for Photonic and Electronic Band Materials, Ministry of Education,
School of Physics and Electronic Engineering, Harbin Normal University,
Harbin 150025, China
* Correspondence author: xiaofei.yang@njfu.edu.cn; wpxiao@njfu.edu.cn
Abstract: Rational design and controllable synthesis of efficient electrocatalysts for
water oxidation is of significant importance for the development of promising energy
conversion systems, in particular integrated photoelectrochemical water splitting
devices. Cobalt oxide (Co3O4) nanostructures with mixed valences (Ⅱ
, Ⅲ
) have been
regarded as promising electrocatalysts for the oxygen evolution reaction (OER). They
are able to promote catalytic support of OER but with only modest activity. Here, we
demonstrate that the OER performance of cubic Co3O4 electrocatalyst is obviously
improved when they are anchored on delaminated two-dimensional (2D) Ti3C2 MXene
nanosheets. Upon activation the overpotential of the hybrid catalyst delivers 300 mV at
a current density of 10 mA cm−2 in basic solutions, which is remarkably lower than

3. those of Ti3C2 MXene and Co3O4 nanocubes. The strong interfacial electrostatic
interactions between two components contribute to the exceptional catalytic
performance and stability. The enhanced OER activity and facile synthesis make these
Co3O4 nanocubes-decorated ultrathin 2D Ti3C2 MXene nanosheets useful for
constructing efficient and stable electrodes for high-performance electrochemical water
often limit the reaction rate [2]. Currently, Ru/Ir-based oxide is commonly proposed as
the effective electrocatalysts for OER performance but their practical applications are
restricted by the low abundance and high costs [3]. Developing earth-abundant noble
metal-free electrocatalysts that are efficient and stable in harsh environment is highly
1 Introduction
Past decades have witnessed the rapid emergence of innovative and cost-effective
technologies such as metal-air batteries and fuel cells which need oxygen as the reactant
[1]. Electrochemical water splitting represents a convenient means to produce oxygen
via oxygen evolution reaction (OER), which involves a complex four-electron process
(2H2OO2 + 4H+
+ 4e–
) and the correlations between different elementary reactions
Received: 2019/11/22
Revised: 2019/12/13
Accepted: 2019/12/16
Keywords: Oxygen evolution reaction; Co3O4; MXene; Heterojunctions;
Electrocatalysis; Water splitting
splitting.

4. desirable.
Cobalt-based nanostructured materials have usually been discussed as efficient
electrocatalysts for OER owing to their tunable morphological features and varied
degree of crystallinity [4-10]. However, the innate poor electrical conductivity and low
active surface area of the most common oxide, Co3O4, hinder the further improvement
of its OER performance. Considering the shortcomings of pristine Co3O4, researchers
persistently attempted to integrate Co3O4 nanostructures with a variety of conductive
substrates (e.g., carbon nanofibers, porous carbon, and reduced graphene oxide) to
improve electrical conductivity and thus achieving optimal electrocatalytic activity [11-
13]. Layered two-dimensional transition metal carbides (2D MXenes) simultaneously
possess superior metallic conductivity, outstanding hydrophilicity and diverse chemical
functionalization of the surfaces, making them promising candidates for highly efficient
electrocatalysis [14, 15]. Moreover, ultrathin delaminated MXene nanosheets with
negatively-charged surface and ultralow work function offer an attractive platform for
constructing confined hybrid electrocatalysts with well-organized nanostructures and
strong interfacial interactions, favoring the optimization of active centers of
electrocatalysts [16, 17]. Thus, it is highly presumable that the precise hybridization of
Co3O4 nanomaterials with 2D MXene nanosheets would be beneficial for the improved
OER, since the unique 2D topography of MXene could substantially shorten the
pathway for mass diffusion and charge transfer, and also obtain heterojunction
electrocatalysts with highly exposed active sites.
In our work, we propose a new type of efficient hybrid electrocatalyst for OER by
synergistically integrating zero-dimensional (0D) Co3O4 nanoparticles with exfoliated
few-layer 2D Ti3C2 MXene nanosheets. The 0D Co3O4/2D Ti3C2 MXene

5. heterojunctions are fabricated by combining in situ electrostatic assembly with a
solvothermal approach. Co3O4 nanoparticles are evenly anchored on Ti3C2 MXene
nanosheets via strong interfacial interactions and electronic coupling. In 1 mol L–1 KOH
solution, the heterojunction Co3O4/MXene (denoted as CM) was employed as a model
to evaluate the OER activity and to unveil the activity origin. Impressively, CM (1:0.1)
is capable of achieving a much lower overpotential of 300 mV at a current density of 10
mA cm−2
compared to those of Co3O4 nanopaticles and Ti3C2 MXene nanosheets. It is
also notable that the hybrid electrocatalyst CM (1:0.1) shows a lower Tafel slope than
those of bulk 0D Co3O4 and 2D Ti3C2 MXene, implying a much faster OER kinetics
occurred on the hybrid. The results demonstrate that the superior OER performance on
the CM (1:0.1) hybrid could be attributed to the synergistic enhancing effects between
conductive MXene and the main electrocatalyst Co3O4. The finding of this study
highlights the great promise of ultrathin 2D MXene-based heterojunction nanostructures
in electrocatalytic water splitting.
2 Experimental Section
2.1 Material synthesis
The synthesis of Ti3C2 MXene flakes was adopted from a reported method [18]. For the
preparation of CM nanocomposites, 10 mL of ethanol suspension with 10 mg MXene
was treated by ultrasonication for 10 min and then 30 mL ethanol anhydrous with 100
mg Co(Ac)24H2O were added to the solution under magnetic stirring at 80 o
C with N2
protection for 10 h. Furthermore, the mixed solution was transferred into a 50 mL
Teflon-lined stainless steel autoclave for solvothermal treatment at 150 oC for 3 h. The
CM (1:0.1) hybrid was collected by high-speed centrifugation, washed repeatedly with
ethanol and water, the precipitates were eventually freeze-dried overnight. The CM

6. (1:0.4), CM (1:1) and CM (1:10) hybrids were also prepared by regulating the mass of
MXene by controlling the weight ratio of Co(Ac)24H2O to MXene with 1:0.4, 1:1 and
1:10. For comparison, pristine Co3O4 was prepared by using the same methods in the
absence of MXene.
2.2 Material Characterization
X-ray diffraction (XRD) measurements were conducted by using a Rigaku Ultima IV
X-ray diffractometer with 2 ranging from 20 to 70. X-ray photoelectron
spectroscopy (XPS) was carried out on a Thermo Scientific ESCALAB 25O Xi
equipment. The morphology and nanostructure of as-prepared samples were examined
by scanning electron microscope (SEM, FEI NovaNano SEM 450) and transmission
electron microscope (TEM, FEI Tecnai G2 F30 S-TWIN). The energy-dispersive X-ray
spectroscopy (EDX) was also collected to reveal elemental distributions of the samples.
2.3 Electrochemical testing
Electrochemical OER measurements were carried out in 1.0 mol L–1 KOH solution on
CHI-760E equipment using the three-electrode system. For the preparation of working
electrode, 5 mg catalyst were mixed with 1 mL isopropanol/Nafion (1 ‰ Nafion)
solution followed by ultrasonication for 10 min to form a homogeneous ink. 16 μL of
the catalyst dispersion (namely, 80 μg catalysts) was then loaded onto a glassy carbon
electrode with 5 mm diameter and then dried naturally for the polarization
measurements. The reversible hydrogen electrode (RHE) was used as reference
electrode and the carbon rod was used as the counter electrode. The linear sweep
voltammetry (LSV) and cyclic voltammetry (CV) curves were recorded in 1.0 mol L–1
KOH aqueous solution at the sweep rate of 5 and 50 mV s–1. The long-term durability of
the catalyst was investigated by CV for 600, 1200 and 2000 cycles in 1.0 mol L–1 KOH

7. aqueous solution.
3 Results and discussion
The scheme of MXene nanosheets production and CM nanohybrid fabrication are
shown in Fig. 1. Specifically, the accordion-like Ti3C2 structures were prepared by
selective chemical etching of Al element layers from bulk Ti3AlC2 phase with HF. Ti3C2
MXene nanosheets were subsequently obtained by further exfoliation Ti3C2 with DMSO
intercalation under ultrasonic conditions. Then, a certain amount of Co(Ac)2 was
dispersed in Ti3C2 MXene ethanol solution under N2 protection and Co3O4 were in-situ
grown on the surface of Ti3C2 MXene nanosheets under solvothermal conditions,
forming the CM hybrids. By precisely controlling the weight ratio of Co(Ac)2 to Ti3C2
MXene with 1:0.1, 1:0.4, 1:1 and 1:10, the CM (1:0.1), CM (1:0.4), CM (1:1) and CM
(1:10) hybrids were obtained.
The morphological features of Ti3C2 MXene nanosheets are shown in Fig. 2. SEM
images of etched Ti3C2 MXene (Fig. 2a and b) indicated that the MXene was well
delaminated and exhibited “accordion-like” morphology. TEM image showed that the
as-prepared Ti3C2 MXene was stacked with nanosheets of a few layers (Fig. 2c). The
lattice distance of Ti3C2 MXene phase was measured to be around 0.26 nm by high-
resolution transmission electron microscope (HRTEM) in Fig. 2d, corresponding to the
(100) plane of Ti3C2. Aggregated Co3O4 particles with cube-like morphology were
observed in the absence of 2D Ti3C2 MXene nanosheets during the solvothermal
synthesis, and the particle size is nonuniform ranging from 200 to 400 nm (Fig. 3a and
b). In contrast, much smaller and more uniform Co3O4 nanoparticles (around 50 nm)
were found to decorate on the surface of MXene in the CM (1:0.1) nanohybrid (Fig. 3c
and d).

8. TEM examination was conducted to further confirm the microstructure and element
distribution of CM (1:0.1) nanohybrid (Fig. 4). Microscopic study (Fig. 4a and b)
demonstrates again the uniform distribution of Co3O4 nanoparticles on Ti3C2 MXene
surface. Such 0D/2D heterostructure architecture is essential for significant reduction of
the pathway for mass diffusion and strong bonding between two components, which
promotes the charge transfer during electrochemical reactions. The lattice spacings of
the formed Co3O4 nanoparticles were determined to be 0.244 and 0.202 nm by HRTEM
(Fig. 4c), corresponding to (311) and (400) faces, respectively. The scanning
transmission electron microscope-high angle annular dark field (STEM-HAADF)
analysis further verifies the uniform distribution of Co3O4 on MXene in CM (1:0.1)
hybrid (Fig. 4d).
XRD measurement was conducted to analyze the crystal structure of the obtained
samples (Fig. 5a). The absence of the characteristic peak at 39o corresponding to the
(104) planes of unetched Ti3AlC2, in the formed MXene indicates the successful
elimination of Al layers [19]. The peaks of Co3O4 nanoparticles were also observed in
CM (1:0.1), which also demonstrate the successful hybrid of Co3O4 nanoparticles with
MXene. EDX spectrum of the hybrid CM (1:0.1) further reveals the presence of Co and
Ti (Fig. 5b). The exact content of Co element in the CM (1:0.1) was determined by the
inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis and the
concentration of Co was 4.34105
ppm. XPS spectra of CM (1:0.1) and Co3O4 were
further collected to verify chemical compositions and oxidization states. The survey
spectra of CM (1:0.1) showed the presence of Ti, Co, O, C and F elements (Fig. 5c).
High-resolution Ti 2p XPS spectra in Fig. 5d demonstrated three pair of peaks related to
Ti-C (462.7 and 455.4 eV), Ti-Ti (461.6 and 457.1 eV), and Ti-O (464.6 and 459.0 eV),

9. respectively [20-23]. Co 2p XPS spectra of Co3O4 and CM (1:0.1) are shown in Fig. 5e
and f. In the case of CM (1:0.1), the peaks located at 796.27 and 781.13 eV are
correlated with Co2+
2p1/2 and Co2+
2p3/2, while peaks at 794.67 and 779.62 eV are
connected with Co3+
2p1/2 and Co3+
2p3/2, which are slightly shifted to higher binding
energies in comparison to pristine Co3O4, implying the facilitated charge transfer from
0D Co3O4 nanoparticles to 2D Ti3C2 MXene nanosheets.
The electrocatalytic OER performance was evaluated using standard three-electrode
system in 1.0 mol L–1 KOH aqueous solution. Fig. 6a showed the CV curves of MXene,
Co3O4 and CM (1:0.1). Two pairs of oxidation/reduction peak were observed on CM
(1:0.1), corresponding to the oxidation and reduction of Co2+/Co3+ and Co3+/Co4+,
whereas only one pair peak and no peak were found on Co3O4 and MXene. The LSV
plot in Fig. 6b demonstrated a relatively low overpotential (300 mV vs. RHE) for CM
(1:0.1) at a current density of 10 mA cm–2, which is remarkably lower than that of
Co3O4 (390 mV) and MXene. This result is among the best cobalt oxide-containing
materials in terms of OER performance (Table 1). Moreover, the CM (1:0.1) also
displayed much lower Tafel slope (118 mV dec–1
) than those of Co3O4 (153 mV dec–1
)
and MXene (442 mV dec–1) (Fig. 6c), indicating a much faster OER kinetics in CM
(1:0.1) hybrid catalyst originating from the synergistic effects between 2D MXene and
0D Co3O4. The presence of 2D MXene nanosheets prevents the aggregation of Co3O4
nanoparticles and facilitates the charge transfer from Co3O4 to MXene for OER
processes. The CV curves for MXene, Co3O4 and CM (1:0.1) at various scan rates in the
in the non-Faraday region (0.4-0.6 V) were shown in Fig. S1 (online). The double-layer
capacitance (Cdl) values are obtained by calculating half the slope derived by linear
fitting the current (ja–jc) at 0.5 V - scan rate plots (Fig. 6d). As expected, CM(1:0.1)

10. showed a much higher Cdl (16 mF cm−2
) than those of Co3O4 (3.5 mF cm−2
) and MXene
(0.293 mF cm−2
). The tremendous increase of Cdl in CM-0.1 indicated that the presence
of hydrophilic and conductive MXene nanosheets may assist the easy access of aqueous
electrolyte into the catalytically active surfaces.
In addition, the effects of the weight ratio between Co3O4 and MXene were also
studied. Fig. 7a compared CV curves of CM (1:0.1), CM (1:0.4), CM (1:1) and CM
(1:10) hybrid catalysts. The current densities of oxidation and reduction of Co2+/Co3+
and Co3+/Co4+ were increased upon the decrease of MXene ratio, the CM (1:0.1) hybrid
exhibited a higher current density than those of CM (1:0.4), CM (1:1) and CM (1:10)
hybrids. The overpotentials at a current density of 10 mA cm–2 were determined to be
300 mV (CM (1:0.1)), 410 mV (CM (1:0.4)), 440 mV (CM (1:1)) and 530 mV (CM
(1:10)), respectively by LSV plots (Fig. 7b). Moreover, the CM (1:0.1) also displayed a
lower Tafel slope than those of CM (1:0.4), CM (1:1) and CM (1:10) hybrid catalysts
(Fig. 7c). These results indicate that the introduction of a certain amount of MXene is
beneficial to accelerate electron transfer between Co3O4 and MXene, while excessive
MXene is dentrimental for OER due to the lack of active sites on the surface of MXene.
The durability was also evaluated by comparing the overpotential at the current density
of 10 mA cm–2 after 600, 1200 and 2000 cycles (Fig. 7d). The overpotential was
attenuated only 60 mV after 2000 cycles continuous measurement in the case of CM
(1:0.1). The SEM images was performed for CM (1:0.1) after 2000 cycles durability
tests and the Co3O4 nanoparticles were well reserved, suggesting the good stability of
the hybrid catalyst (Fig. S2 online).
4 Conclusions
In summary, we have demonstrated that 0D Co3O4/2D Ti3C2 MXene heterojunctions

11. with intimate interfacial coupling could be successfully fabricated and employed as
stable, and highly efficient electrocatalysts for OER, which demonstrate an extremely
low overpotential of 300 mV at a current density of 10 mA cm−2 and also, a lower Tafel
slope than those of bulk materials. Experimental observations reveal that the presence of
2D MXene nanosheets can not only regulate the morphology and interface of the hybrid
electrocatalyst, but also accelerate the interfacial electron transport property, which
endow CM heterojunctions with enhanced electrical conductivity, rich active sites and
shortened pathway for charge transfer, leading to the highly improved electrocatalytic
activity. The study provides a valuable guideline for the rational design and construction
of robust, high-performance MXene-based hybrid electrocatalysts for sustainable
energy conversion.
Conflicts of interest
The authors declare that they have no conflict of interest.
Acknowledgments
We gratefully acknowledge financial support from the National Natural Science
Foundation of China (21975129), Natural Science Foundation of Jiangsu Province
(BK20180777, BK20190759), Natural Science Foundation of Jiangsu Higher Education
Institutions of China (18KJB430018, 19KJB430003), Scientific Research Foundation
for Advanced Talents (CXL2018046) and Science Innovation Foundation for Young
Scientists (CX2018012), Nanjing Forestry University. This work was also supported by
Student’s Platform for Innovation and Entrepreneurship Training Program in Jiangsu

12. Province (201810298029Z) and Student’s Platform for Innovation and Entrepreneurship
Training Program (2018NFUSPITP602), Nanjing Forestry University.
Author contributions
Deqi Fan and Cancan Cao carried out the experiments. Yi Lu designed the catalysts and
wrote the draft paper with input from all authors. Weiping Xiao and Xiaofei Yang
developed the electrocatalytic system and supervised the project. Zupeng Chen polished
the language of the paper. All authors analyzed and discussed the data. The manuscript
was prepared with contributions from all the authors.
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Yi Lu received her Ph.D. degree from Nanjing Tech University in 2017. She
subsequently joined the faculty of Nanjing Forestry University. Her research interests
focus on the design and controllable synthesis of functional nanomaterials for
sustainable energy conversion.

16. Weiping Xiao obtained her Ph.D. degree from Huazhong University of Science and
Technology in 2018. Then she joined the College of Science, Nanjing Forestry
University as a talented young scholar. Her current research interests focus mainly on
the fabrication of nanostructured materials for electrocatalytic applications.
Xiaofei Yang is a full professor of Materials Chemistry at Nanjing Forestry University.
He completed his Ph.D. degree in the School of Chemistry at the University of Leeds,
UK in 2009. He was a Max Planck Postdoctoral Fellow at the Max Planck Institute of
Colloids and Interfaces, Germany with Prof. Markus Antonietti in 2014. His current
research interests focus on advanced catalytic materials for energy conversion and
environmental remediation.

17. Figures Captions
Fig. 1 (Color online) Schematic illustration of the preparation process of CM hybrids.
Fig. 2 (Color online) (a) Low-magnification and (b) high-magnification SEM images, (c)
TEM and (d) HRTEM images of HF-etched layed Ti3C2 MXene nanosheets.
Fig. 3 (Color online) (a) Low-magnification and (b) high-magnification SEM images of
cubic-like Co3O4; (c) low-magnification and (d) high-magnification SEM images of
CM (1:0.1).
Fig. 4 (Color online) (a) Low-magnification, (b) high-magnification TEM, (c) HRTEM
images and (d) STEM-HAADF analysis of CM (1:0.1).
Fig. 5 (Color online) (a) XRD patterns of MXene nanosheets, Co3O4 nanoparticles, and
CM (1:0.1); (b) EDX spectrum of CM (1:0.1); (c) the survey spectra and (d) high-
resolution XPS spectra of Ti 2p on CM (1:0.1); (e, f) the comparative high-resolution
XPS spectra of Co 2p on Co3O4 and CM (1:0.1).
Fig. 6 (Color online) (a) CV, (b) LSV curves, and (c) Tafel plots on MXene, Co3O4 and
CM (1:0.1). CV and LSV curves were measured in 1 mol L–1 KOH solution at a scan
rate of 50 and 5 mV s–1
, respectively. (d) Double-layer capacitance (Cdl) for MXene,
Co3O4 and CM (1:0.1) obtained by calculating the slope from current density of CV
curves at 0.5 V plotted against scan rate.
Fig. 7 (Color online) The comparison of (a) CV, (b) LSV curves, and (c) Tafel plots of
CM (1:0.1), CM (1:0.4), CM (1: 1) and CM (1:10) hybrid catalysts. (d) The LSV curves
of the CM (1:0.1) catalyst after CV test for 600, 1200 and 2000 cycles in 1.0 mol L–1
KOH aqueous solution.

18. Table
Table 1 Comparisons between the CM hybrid and recently reported electrocatalysts
based on cobalt oxides or MXene for OER.
Composition
Electrolyte KOH
Ej10 (V vs.
η (V) References
(mol L–1) RHE)
CM (1:0.1) 1 1.53 0.30 This work
Co3O4-C/rGO-W 0.1 1.61 0.38 [24]
Co3O4/CNTs 0.1 1.55 0.32 [25]
Graphene/Co/Co3O4 0.1 1.53 0.30 [26]
Co(OH)2/graphene oxide 2 1.65 0.42 [27]
2D CoBDC/MXene 0.1 1.64 0.41 [28]
NiCoS/Ti3C2Tx 1 1.60 0.37 [29]
Hierarchical Zn-doped
CoO
1 1.52 0.29 [30]
NiFe-LDH/MXene/NF 1 1.52 0.29 [31]
Co3O4/CN 1 1.53 0.30 [32]
CoxOy/CN 0.1 1.66 0.43 [33]
Co3O4/N-rmGO 0.1 1.54 0.31 [34]