Mesoporous Ultrathin Cobalt Oxides Nanosheets Grown on Carbon

Subscriber access provided by WEBSTER UNIV

Article

Mesoporous Ultrathin Cobalt Oxides Nanosheets Grown on Carbon Cloth as A High-Performance Electrode for Oxygen Evolution Reaction Jian Du, Chao Li, Xilong Wang, Xiaoyue Shi, and Han-Pu Liang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02074 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 4, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Energy Materials

Mesoporous Ultrathin Cobalt Oxides Nanosheets Grown on Carbon Cloth as A High-Performance Electrode for Oxygen Evolution Reaction Jian Du, †, § Chao Li, † Xilong Wang, † Xiaoyue Shi, †, § Han-Pu Liang †, ‡, * †Qingdao

Institute of Bioenergy and Bioprocess Technology, Chinese Academy of

Sciences, Qingdao 266101, P. R. China ‡Center

of Materials Science and Optoelectronics Engineering ， University of

Chinese Academy of Sciences, Beijing 100049, P. R. China University of Chinese Academy of Sciences, Beijing 100049, P. R. China

§

KEYWORDS: Mesoporous ultrathin CoOx nanosheets; Unique 3-D structures; TGA-IR technique; Electrode; Oxygen evolution reaction

ABSTRACT: Developing high-performance and low-cost electrode for oxygen evolution reaction (OER) is a crucial step to produce clean and sustainable energy sources. Herein, we report mesoporous ultrathin cobalt oxides nanosheets (~7 nm) grown on carbon cloth (denoted as CoOx/CC) as an efficient electrode for water oxidation. The electrode was fabricated via a simple calcination of cobalt alkoxyacetate grown on carbon cloth under air atmosphere. The as-synthesized CoOx/CC electrode exhibited better electrocatalytic performance for OER in 1.0 M KOH, with lower overpotential of ~ 263 mV at 10 mA cm-2, lower Tafel slope of 56.07 mV dec-1 and better stability, in comparison with commercial IrO2 and Co3O4

ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

nanoparticles. Also, the OER performances of CoOx/CC-10 are among the best of the reported cobalt oxides electrocatalysts. Detailed characterizations reveal that the interaction between cobalt oxides and CC, unique 3-D structures, ultrathin nature of nanosheets, high specific surface area, nanoporous structures and fast electron transfer rate contributed to the enhanced catalytic activity. More importantly, the synthetic strategy applied here has a great potential application in large-scale production of efficient electrode in water splitting technology. 1. INTRODUCTION Hydrogen fuel, with the highest gravimetric energy density and the only product of non-polluted water compared with other fuels, has been generally expected to be a promising potential energy carrier to relieve the growing global energy crisis and environmental pollution issues.1-3 Electrochemical splitting of water is a green and desirable technique to produce hydrogen. However, the efficiency of the water splitting is largely hindered by the large overpotential in oxygen evolution reaction (OER), which is inherently complex and kinetically sluggish due to the four-electron transfer process.4 Therefore, efficient electrocatalysts are essentially needed to facilitate the oxidative half reaction of OER on anodic electrode. To date, the state-of-the-art catalysts for OER are precious metal oxides (e. g. RuO2 and IrO2), but their scarcity and high cost significantly limited the large-scale application in water splitting technology.5, 6 Recently, there has been a dramatic growth of interest in the development of cobalt-based electrocatalysts for OER.7-16 Particularly, cobalt oxides catalysts have


Page 2 of 35

Page 3 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


been demonstrated as a promising candidate for the water oxidation in an alkaline medium, due to their abundant reserves, thermal stability and low cost.2 In the past decades, Co3O4 and CoO with various morphology and structures has been extensively investigated for OER,17-26 such as Co3O4 nanowires,23 atomic scale CoOx,24 Co3O4 nanowire arrays25 and CoO nanoparticles.26 However, relatively large overpotentials of these reported cobalt oxides were still required to reach the current density of 10 mA cm-2. The main reason for the poor activity is because of the low conductivity and easy aggregation of cobalt oxides, which could decrease the active sites and hamper the transport of electrons or protons during the oxidation process.21 In order to increase the electron transfer rate and prevent the aggregation of cobalt oxides, it is an efficient method to couple cobalt oxides with conductive additives.27 For example, ultrathin Co3O4 nanosheets/reduced graphene oxides,28 Co3O4 nanocrystals grown on N-doped reduced graphene oxide29 and porous graphene sheets wrapped CoO nanoparticles30 have been successfully reported as effective OER electrocatalysts. However, these materials are in the form of powder, which are normally mixed with organic binder prior to being used as working electrode in practice. Unfortunately, the usage of binder usually results in undesirable electrode interfaces between electrolyte and working electrode, such as higher resistance, plenty of dead volume and the peeling of electrocatalysts during gas evolution process.27 Therefore, the development of cobalt oxides onto a kind of proper conductive substrate and directly used as an electrode for OER without the usage of organic binder is particularly desired in practical devices.



Herein, we report a facile approach to fabricate mesoporous ultrathin cobalt oxides nanosheets on carbon cloth (denoted as CoOx/CC) by calcining the nonporous cobalt alkoxyacetate nanosheets precursor grown on CC (denoted as CoEGAc/CC) under air atmosphere. The as-synthesized CoOx/CC electrode could be directly deployed as a working electrode for OER. The electrochemical testing results show that the CoOx/CC electrode requires a low overpotential of 263 mV at the current density of 10 mA cm-2 and low Tafel slope of 56.07 mV dec-1 for OER, which is better than that of commercial IrO2 and Co3O4 nanoparticles under the same testing condition. 2. EXPERIMENTAL SECTION 2.1. Reagents. Cobalt (II) acetate tetrahydrate (C4H6CoO4•4H2O, ACS), commercial IrO2 (99.9% metal basis, Ir≥84.5%) and isopropanol (ACS) were bought from Aladdin Industrial Co., Ltd. Ethylene glycol (AR), sulfuric acid (H2SO4, AR), acetone (AR) and potassium hydroxide (KOH, AR) were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. Poly(N-vinyl-2-pyrrolidone) (PVP, Mw=55000 g mol-1) and Co3O4 nanoparticles (< 50 nm) were bought from Sigma-Aldrich. Carbon cloth (CC) was supplied by Shanghai Hesen Electric Co., Ltd. Nafion solution (5 wt.%) was supplied by DuPont. Ultra-pure deionized water (18.2 MΩ/cm) was used in all experiments. All the chemicals were used as received without further purification. 2.2. Fabrication. Carbon cloth (CC) was sequentially cleaned in acetone and 0.5 M H2SO4 by sonication for 10 min, respectively. Then, CC was washed with ultra-pure water many times to remove the residual chemicals and dried at 100 oC overnight.


Page 4 of 35

Page 5 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.312 g C4H6CoO4•4H2O and 0.24 g PVP was added into a solution of 40 mL ethylene glycol and 0.8 mL ultra-pure deionized water under stirring until completely dissolved. A piece of CC (1 cm * 2 cm) was immersed in the obtained solution and kept at 180 oC for 3 h in a Teflon-lined stainless steel autoclave. After cooling to room temperature naturally, the CC was taken out by a tweezer, rinsed with ethanol several times and then dried at room temperature. To prepare CoOx/CC electrode, the as-synthesized cobalt alkoxyacetate/CC was placed in a quartz boat and calcined at 300 oC for a certain of time at a heating rate of 2 oC min-1 in a furnace under air atmosphere. CoOx/CC electrode was obtained after cooling to room temperature. 2.3. Materials characterization. Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advanced diffractometer using Cu Kα radiation. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) spectroscopy were carried out on Hitachi S-4800 microscope. Transmission electron microscopy (TEM) images were performed on FEI Tecnai T20. Atomic force microscopy (Multimode 8, AFM) with PFT-QNM mode was carried out to characterize the surface morphology of CoOx-10 nanosheets scraped from the electrode. Fourier transform infrared spectroscopy (FT-IR) spectra were obtained on Varian 800 FT-IR spectrometer using the potassium bromide (KBr) pellet technique. X-ray photoelectron spectroscopy (XPS) was collected on a K-Alpha X-ray photoelectron spectrometer (Thermo Scientific) with Al Kα micro focused monochromated X-ray source. The specific surface

area

and

the

pore

size

distribution

were

determined

by

N2

adsorption/desorption isotherms on Autosorb-iQ adsorption analyzer at 77 K. Prior to



the measurements, the samples were degassed at 110 oC for 5 h under vacuum. The content of Co grown on CC was measured by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Agilent 5110). CoEGAc nanosheets and porous CoOx nanosheets for nitrogen adsorption/desorption analysis were prepared with the absence of carbon cloth in the autoclave. Thermal gravimetric analysis (TGA) was conducted on a thermal gravimetric analyzer (TA instrument, Q5000IR) to investigate the weight changes of sample as a function of temperature under inert gas atmosphere. He purge gas (BIP, Air products) was introduced at a flow rate of 10 mL min-1 in all experiment. Restek triple filter was used to remove the possible oxygen and moisture in the helium gas before it is fed to TGA. In the case of TGA experiment, the temperature was increased to 400 °C at a rate of 20 °C min-1 under helium at a flow rate of 25 mL min-1. An on-line infrared spectrometer (NICOLET 380, Thermo Scientific) was used to simultaneously analyze the exhausted gas from the TGA furnace in the in situ thermal gravimetric analysis (TGA) - infrared spectroscopy (IR) technique. The transmission line, connecting TGA with infrared spectrometer and glass flow cell inside the infrared spectrometer were kept at the temperature of 120 °C to prevent water condensation. X-ray absorption fine structure spectroscopy (XAFS) measurement of Co K-edge was conducted on the 06ID superconducting wiggler sourced hard X-ray microanalysis beamline at the Beijing Synchrotron Radiation Facility (BSRF). Temperature programmed reduction (TPR) experiments were performed on a Micromeritics Auto Chem II 2920 instrument to determine catalyst reduction temperature-time profiles with a dilute gas mixture of 10% H2 in Ar.


Page 6 of 35

Page 7 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Catalysts were heated from 50 oC to 500 oC in a 10% H2/Ar gas flow with a ramp rate of 2 oC min-1. Co dispersion was measured by H2 chemisorption measurements on Micromeritics Auto Chem II 2920. The samples were reduced at 350 oC for 10 min under a gas mixture of 10% H2 in Ar. Following reduction, the temperature was cooled down to 100 oC and the hydrogen adsorption was measured using the pulse chemisorption method. Co dispersion was calculated assuming that the stoichiometry of hydrogen atom on cobalt atom is 1:1. 2.4.

Electrochemical

measurement.

Electrochemical

measurements

were

performed via a three-electrode system at 25 oC using a CHI 660E electrochemical workstation. Hg/HgO (1.0 M KOH) electrode and Pt foils was used as reference and counter electrodes, respectively. CoOx/CC was directly used as working electrode. The solution resistance caused by ohmic potential drop (iR) was corrected by the electrochemical impedance spectroscopy (EIS) plot.31 EIS experiments were performed in the frequency range of 100 kHz to 0.1 Hz at 1.65 V vs. RHE with a perturbation of 5 mV. All measured potential vs. Hg/HgO was converted to the reversible

hydrogen

electrode

(RHE)

via

the

Nernst

equation:

ERHE=EHg/HgO+0.0591*pH+0.098. The electrolyte was 1.0 M KOH aqueous solution. Linear sweep voltammetry (LSV) was recorded at a scan rate of 5 mV s-1 with the iR-correction. For comparison, commercial IrO2 and Co3O4 nanoparticles were tested under the same conditions with the loadings of 300 μg cm-2 and 500 μg cm-2, respectively. To load commercial IrO2 catalyst on carbon cloth, 6 mg catalyst and 10 μL of Nafion solution (5 wt.%) were firstly added in 990 μL of absolutely ethanol



solution and sonicated for 10 min. Next, 50 μL of suspension was dropped on carbon cloth with a geometric area of 1 cm * 1 cm. To load commercial Co3O4 catalyst on carbon cloth, 10 mg catalyst and 10 μL Nafion solution (5 wt.%) were dispersed in 990 μL absolutely ethanol by ultrasonication. Next, 50 μL of suspension was dropped on carbon cloth with a geometric area of 1 cm * 1 cm. Tafel slopes were calculated from the corresponding LSV curves by plotting overpotential (η) against log (J). The stability of the catalyst was assessed by chronopotentiometry at a constant current density of 10 mA cm-2. Cyclic voltammetry (CV) was performed for 500 cycles with a scan rate of 200 mV s-1 in the potential region from 1.4 V to 1.55 V vs. RHE. To measure double-layer capacitance (Cdl), the CV measurement with different scan rates ranging from 30-80 mV s-1 was carried out in a non-faradaic potential region from 0.9 to 0.98 V vs. RHE. The Cdl was estimated by plotting the △J=(Ja-Jc) at 0.94 V vs. RHE against the scan rate. Where, Ja and Jc are the anodic and cathodic current densities, respectively. The linear slope was twice of double layer capacitance Cdl. The obtained Cdl can be converted into electrochemical active surface area (ECSA) using the formula: ECSA=Rf*S, Rf=Cdl/Cs, in which S stands for the surface area of CC (1 cm2). In this study, the specific capacitance value (Cs) for Co3O4 is 60 μF cm-2 according to the literature.32 2.5. Calibration of the reference electrode. The calibration of the Hg/HgO reference electrode was conducted in the standard three electrode system following a method by Zhang et al.11 Briefly, Hg/HgO electrode was used as reference electrode and Pt foils were used as working and counter electrodes. The electrolyte (1.0 M


Page 8 of 35

Page 9 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


KOH) was pre-purged with high purity H2 overnight and continuously bubbled with H2 during calibrations. LSV was run between hydrogen evolution and oxidation and the potential associated with zero current density was recorded. The LSV curve was shown in Figure S21. It can be found that the potential of zero current density was about 0.902 V. The measured pH value of 1.0 M KOH at 25 oC is 13.6. Thus, the as-calculated potential of zero current density 0.098+0.0591*pH=0.9017 V, which is similar to the measured potential. 3. RESULTS AND DISCUSSION

Scheme 1. Schematic illustration for the fabrication process of mesoporous ultrathin CoOx nanosheets on carbon cloth (CC). The mesoporous ultrathin CoOx nanosheets on CC (CoOx/CC) were synthesized by a two-step process and the fabrication process was illustrated in Scheme 1. Firstly, the nonporous cobalt alkoxyacetate (CoEGAc) nanosheets on CC were fabricated by a simple one-pot hydrothermal synthesis. Secondly, the as-synthesized nonporous



CoEGAc/CC was converted into porous CoOx/CC by a simple calcination process at 300 oC for 10 min (denoted as CoOx/CC-10) and for 120 min (denoted as CoOx/CC-120) under air atmosphere. The optical image of as-synthesized CoOx/CC-10 and bare CC electrode is given in Figure S1, from which it is evident that the carbon cloth electrode turns black after mesoporous cobalt oxide formed on the CC. The as-synthesized CoEGAc/CC is characterized by Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD). As evident from the FT-IR spectrum in Figure S2, two sharp peaks are located at 2852 cm-1 and 1095 cm-1, which could be assigned to the stretching vibration of –CH2 and –C-OH groups in C2H5O2anions. Two absorption bands at 1636 cm-1 and 1351 cm-1 could be attributed to the stretching vibrations of COO- groups. Therefore, these bands confirm the formation of CoEGAc/CC, which is in agreement with CoEGAc reported by Chakroune et al.33 and Cao et al.34 In addition, the XRD pattern of CoEGAc/CC in Figure S3 shows that there is only one sharp peak at 10.4 o, which was a typical feature from the coordination of organic molecular with Co (II) cations.35 The strong peak at 26.3 o and weak peak at 54.4 o could be assigned to the diffraction peaks of bare CC. The XRD result further confirms the formation of CoEGAc/CC. The pyrolysis process of CoEGAc precursor under air atmosphere was investigated by a thermogravimetric analysis (TGA) - infrared spectroscopy (IR) technology. It is evident from the TGA profile of CoEGAc in Figure S4a that the precursor of CoEGAc could be completely oxidized at the temperature above 300 oC in the


Page 10 of 35

Page 11 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


presence of air when the curve of weight loss is stable. The 3-dimensional (3-D) image of TGA-IR spectrum of exhaust gas from CoEGAc in Figure S4b shows the presence of absorption bands associated with carbon dioxide and water vapor, indicated by arrows, which suggests that the CoEGAc precursor has been oxidized into cobalt oxide, carbon dioxide and water vapor. In the case of CoEGAc/CC, the TGA profile in Figure S5 shows that the obvious weight loss of CoEGAc begins at 229 oC and decrease quickly to reach a relatively constant weight at 270 oC. When the temperature is above 325 oC, it starts losing weight. Therefore, the target temperature of 300 oC was chosen to calcine the CoEGAc/CC under air atmosphere, which could completely convert the CoEGAc precursor into cobalt oxides.



Figure 1. Various SEM images of (a-c) CoEGAc nanosheets and (d-f) mesoporous CoOx-10 nanosheets on CC at different magnifications. (g-i) EDS elemental mapping images of CoOx/CC-10. (j) TEM images of CoEGAc and (k) mesoporous CoOx-10 nanosheets scraped from CC and corresponding (l) HRTEM image of CoOx-10 nanosheets. The morphology of CoEGAc/CC, CoOx/CC-10 and CoOx/CC-120 was studied by scanning electron microscopy (SEM). The low-magnification SEM image in Figure 1a shows that the CoEGAc nanosheets are formed uniformly and grown vertically on carbon fiber of CC. Upon closer observation on the SEM images in Figure 1b-c, it can be seen that the thickness of the nanosheets is about 7 nm and the surface is smooth. After calcination at 300 oC for 10 min, CoEGAc nanosheets could be oxidized into mesoporous CoOx nanosheets. The low-magnification SEM image in Figure 1d reveals that the morphology of CoOx nanosheets on CC remained nearly unchanged. However, the SEM image in Figure 1e shows that the surface of nanosheets is rough due to the presence of many interconnected nanoparticles. The thickness of the nanosheets is around 7 nm. Upon careful observation of the nanosheets in Figure 1f, it can be found that these irregular nanoparticles are interconnected with each other to form the mesoporous ultrathin nanosheets. In addition, the AFM image in Figure S6 shows that the thickness of the CoOx nanosheets is about 6-6.5 nm, which is very close to the SEM results in Figure 1b and 1c. In the case of CoOx-120/CC, the SEM images in Figure S7a-b show that CoOx-120/CC keeps the similar morphology and mesoporous sheet-structure to CoOx/CC-10. In addition, the energy-dispersive X-ray


Page 12 of 35

Page 13 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


spectroscopy (EDS) elemental mapping images of CoOx/CC-10 in Figure 1g-i confirms the presence of Co, O and C elements, which are uniformly distributed on the carbon fiber of CC. The nanostructure of as-synthesized CoEGAc, CoOx-10 and CoOx-120 nanosheets scraped from CC was further investigated by transmission electron microscopy (TEM). As evident from the TEM image in Figure 1j, it is clear that the surface of CoEGAc nanosheets is smooth. In the case of CoOx-10, after the calcination for 10 min, the TEM image of nanosheets in Figure 1k confirms the formation of mesoporous structure with irregular pores on the nanosheets. Furthermore, the high-resolution transmission electron microscopy (HRTEM) image in Figure 1l shows that the interplanar spacing of visible lattice fringes is 0.244 nm, which can be attributed to the (311) plane of Co3O4. In the case of CoOx-120, it is evident from the TEM images in Figure S7c-d that the CoOx-120 nanosheets remained similar mesoporous structure but with larger irregular nanoparticles on the surface. Based on the analyses of SEM and TEM images of CoEGAc/CC precursor, CoOx/CC-10 and CoOx/CC-120, it can be envisaged that the morphology and the thickness of the nanosheets grown on CC maintain no obvious changes, except for the formation of nanoporous structures during the calcination process. Furthermore, the porous structure of as-synthesized CoEGAc and samples after calcination for 10 min and 120 min were characterized by Brunauer-Emmett-Teller (BET) gas sorptometry. As shown in Figure S8a, all samples exhibit a typical type IV adsorption isotherm.36,

37

According to BET analysis, the specific surface area of



samples calcined for 10 min and 120 min are 78.3 m2 g-1 and 68.3 m2 g-1, respectively. However, the specific surface area of CoEGAc precursor is 35.5 m2 g-1. In addition, the corresponding BJH (Barrett-Joyner-Halenda) pore size distribution in Figure S8b shows that the average pore diameter of CoOx-10 and CoOx-120 is 3.8 nm and 4.3 nm, respectively, further suggesting the formation of mesoporous structures. Besides, the formation of numerous mesopores is beneficial for the diffusion of OH- and evolution of oxygen when compared with that of electrocatalysts in the form of powder. Moreover, it appears that prolonging the calcination time could decrease the specific surface area and increase the average pore size of nanosheets. The crystal structure of CoEGAc/CC, CoOx/CC-10 and CoOx/CC-120 was further characterized by the X-ray diffraction (XRD) and the results are compared in Figure 2a. It is clearly found that both the patterns of CoOx/CC-10 and CoOx/CC-120 show the presence of two diffraction peaks located at 36.5 º and 42.4 º, which could be attributed to the (111) and (200) planes of CoO phase according to the JCPDS No. 43-1004. The peaks located at 36.8 º, 44.8 º, 59.3 º and 65.2 º could be ascribed to the (311), (400), (511) and (440) planes of Co3O4 phase according to the JCPDS No. 43-1003. All these diffraction peaks indicate the formation of Co3O4 and CoO phases on CC. It should be noted that the signals from CoEGAc are absent. To further analyze the oxidation state of Co in CoOx on CC, X-ray photoelectron spectroscopy (XPS) was performed. As shown in Figure 2b, the XPS survey scans of CoEGAc/CC, CoOx/CC-10 and CoOx/CC-120 show the presence of Co, O and C elements.


Page 14 of 35

Page 15 of 35

(a)

(b) Co 2p

JCPDS No. 43-1003, Co3O4

CoEGAc/CC CoOx/CC-10 CoOx/CC-120

O 1s

Intensity (a. u.)

Intensity (a. u.)

CoOx/CC-10 CoOx/CC-120 Bare CC

C 1s

JCPDS No. 43-1004, CoO

20

30

40

50

60

70

80

2 Theta (degrees)

(c)

1000

800

600

400

200

Binding energy (eV)

(d)

2p3/2

2+ 3+

2+ 3+

2+

2+

Intensity (a. u.)

Intensity (a. u.)

2p3/2 2p1/2 2p1/2

2+

2p1/21/2 2p 2+

3+

2+

Co /Co =100/0 815 815

(e)

Binding energy (eV)

3+

2p3/2

Intensity (a. u.)

3+

2p1/2

2+

3+ 2+

3+

805

800

1000

2p1/2

2+

Co /Co =1.5/2 810

3+

(f)

2p3/2

815

2+

815 815 810 810 805 805 800 800 795 795 790 790 785 785 780 780 775 775 770 770

Binding energy (eV)

2+

3+

3+

Co /Co =1.5/2

810 805 800 795 790 785 780 775 770 810 805 800 795 790 785 780 775 770

2+

0

3+

2+

3+

Co /Co =1/2 795

790

785

780

Binding energy (eV)

775

770

Intensity (a. u.)

10

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


815

810

805

800

795

790

785

780

775

770

Binding energy (eV)

Figure 2. (a) XRD patterns of CoOx/CC-10, CoOx/CC-120 and bare CC. (b) XPS survey scans and high-resolution XPS Co 2p spectra of (c) CoEGAc/CC, (d) CoOx/CC-10, (e) CoOx/CC-120 and (f) CoOx/CC-10 after stability testing. The high-resolution XPS Co 2p spectrum of CoEGAc/CC is shown in Figure 2c. Two major peaks are located at binding energies of ~ 780.8 eV and ~ 796.6 eV, which can be assigned to the Co 2p3/2 and Co 2p1/2 of Co2+ oxidation state, along with two


800


shake-up satellite peaks at ~ 785.6 and ~ 802.5 eV. Figure 2d-e compare the high-resolution XPS Co 2p spectra of CoOx/CC-10 and CoOx/CC-120. It is evident that two additional peaks are present and located at 779.3 and 794.4 eV in both spectra, which are assigned to Co3O4 phase. This result is in agreement with that reported by Qiao et al.25 It should be mentioned that the theoretical ratio of Co2+/Co3+ for pure Co3O4 is 1/2. However, the ratios of Co2+/Co3+ in the case of CoOx/CC-10 and CoOx/CC-120 are both 1.5/2, which is higher than 1/2, indicating the possible presence of additional Co2+, that is CoO, in both CoOx/CC-10 and CoOx/CC-120. In addition, the similar ratio of Co2+/Co3+ in CoOx/CC-10 and CoOx/CC-120 suggests that the presence of CoO was not affected by the calcination time.

Figure 3. (a) In situ 3-dimensional (3-D) image of TGA-IR spectra and (b) an individual IR spectrum of the TGA exhaust gas at 300 oC of the mixture of commercial Co3O4 nanoparticles with 13C powder at a heating rate of 20 oC min-1.


Page 16 of 35

Page 17 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


All of the above characterizations manifest that mesoporous ultrathin cobalt oxides nanosheets grown on CC have been successfully fabricated. To study the mechanism for the formation of CoOx on CC, one control experiment was carried out, in which CoEGAc nanosheets were calcinated at 300 oC for 10 min with the absence of CC. The corresponding XRD pattern in Figure S9 shows the presence of broad peaks at 31.27 o, 36.84 o, 44.80 o, 59.35

o

and 65.23 o, which could be assigned to the (220),

(311), (400), (511) and (440) planes of Co3O4 phase. No peaks associated with CoO phase are observed. Therefore, it seems that the formation of additional CoO in the case of both CoOx/CC-10 and CoOx/CC-120 could be due to the presence of CC, which might reduce Co3O4 at high temperature. In order to understand this assumption, a

13C

labelling experiment was carried out, in which an in situ TGA-IR

technique was deployed to investigate the reduction reaction of a mixture of

13C

powder and commercial Co3O4 particles. Figure S10 and Figure 3a show the TGA profile and 3-dimentional (3-D) image of the in situ IR spectra of the TGA exhaust gas from the mixture of Co3O4 and 13C powder, respectively. The TGA profile shows a weight loss of 1.8%. Furthermore, it is evident from the 3-D image in Figure 3a that the bands associated with 13CO2 are present at the temperature of approximately 270 oC,

indicating the occurrence of reaction between Co3O4 particles and

13C

powder.

Figure 3b gives an individual IR spectrum of the exhaust gas at 300 oC. It is evident that two strong peaks associated with

13CO

2

are present at 2297 and 2266 cm-1,

indicating the formation of 13CO2 from the reaction between Co3O4 and 13C. It should be noted that the calcination temperature at present work is 300 oC, at which the



reduction reaction between ultrathin Co3O4 nanosheets and carbon fiber could happen. Therefore, it seems from the control experiment that the formation of CoO on CC could be attributed to the reaction between the Co3O4 and CC at the contact section. (b)500

80

CoOx/CC-10 CoOx/CC-120 Co3O4/CC

60

IrO2/CC Bare CC

40

20

0 0.9

1.1

1.2

1.3

1.4

1.5

Potential (V vs. RHE)

1.6

1.7

dec 56.07 mV

0.2 0.2

300 250

-10 -120 x/CC /CC x O o O C Co

-2

-1

V dec -1 87.53 m3 mV dec 87.5 -1 dec 64.63 mV3 mV dec-1 -1 .6 64 V dec 56.07 m -1

0.3 0.3

350

(d)1.1 0.8

CoOx/CC-10 CoOx/CC-10 CoOx/CC-120 CoOx/CC-120 Co O4/CC -1 IrO32/CC c IrO2/CC mV de 112.43

0.4

400

200

1.8

△ density j (mA cm ) cm-2) Current (mA

Overpotential (V) (V) Overpotential

0.5 (c) 0.5

1.0

-2

40 mA cm -2 20 mA cm -2 10 mA cm

450

Overpotential (mV)

-2

Current density (mA cm )

(a)

0.10.4 0.6 0.8 1.0 1.2 0.30 0.45 0.60 0.75 0.90 1.05 1.20 1.35 1.50 -2

CoOx/CC-10 CoOx/CC-120 Co3O4/CC

0.2 0.8

1.0

= C dl

-2

0.8 0.6

F cm 1m 5 . 5 -2 C dl= F cm .90 m 3 = C dl

0.4 0.2 -2

Cdl=0.28 mF cm

0.0 40

50

60

-1

70

80

cm mF 1 5 5.

2.0

1.5

1.0

-2

-1

-0.2 0.6 -0.4 0.5 -0.6 0.4 -0.8 0.3

0.5

d

0.0 2.0

0.90 30

40

0.92

50

0.94

60

0.96 70

-1 Potential (V vs. RHE) Scan rate (mV s )

0.98 80

CoOx/CC-10 Co3O4/CC IrO2/CC

1.8 1.7 1.6 1.5 1.4

30

C

cm mF 30-80 mV=s3.90 C l

0.0 0.7

1.9

-2

/C Co 3O 4

-2

0.4 0.9

(f) 2.0 Potential (V vs. RHE)

(e) 1.2

/CC IrO 2

CoOx/CC-10 CoOx/CC-120

1.0 0.6

Log (J/mA cm -2) Log (J/mA cm )

Ja-Jc (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 35

0

20

Scan rate (mV s )

40

60

80

100 120 140 160 180

Time (h)

Figure 4. (a) LSV polarization curves of CoOx/CC-10, CoOx/CC-120, commercial IrO2/CC, Co3O4/CC and bare CC at a scan rate of 5 mV s-1 for OER in 1.0 M KOH with iR-correction. (b) Corresponding overpotentials at various current densities. (c) Tafel slopes derived from the LSV curves. (d) CVs of CoOx/CC-10 from 30 to 80 mV


Page 19 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


s-1 at 0.94 V vs. RHE. (e) The capacitive current as a function of the scan rate for CoOx/CC-10, CoOx/CC-120 and Co3O4 nanoparticles. (f) Galvanostatic curves of electrocatalysts at a current density of 10 mA cm-2. The as-synthesized CoOx/CC-10 and CoOx/CC-120 are directly used as the working electrode for OER without the usage of extra substrate or organic binder. Their electrochemical performances were evaluated in a standard three-electrode system at 25 oC in 1.0 M KOH solution at a scan rate of 5 mV s-1 with iR-correction. For comparison, commercial IrO2 and Co3O4 nanoparticles, the SEM images of which are given in Figure S11 and Figure S12, respectively, were loaded onto CC (denoted as IrO2/CC and Co3O4/CC) and tested under the same conditions. The N2 adsorption/desorption isotherms of commercial Co3O4 nanoparticles is given in Figure S13, from which a specific surface area of 40.2 m2 g-1 is obtained. The Co contents of CoOx/CC-10

and

CoOx/CC-120

were

measured

by

inductively

coupled

plasma-optical emission spectroscopy (ICP-OES) and the detailed results are shown in Table S1. Figure 4a shows the linear sweep voltammetry (LSV) polarization curves of CoOx/CC-10, CoOx/CC-120, IrO2/CC, Co3O4/CC and bare CC. It is clearly observed that bare CC substrate shows negligible catalytic activity for OER during the potential region of 1.0-1.7 V. The oxidation peak at around 1.30 V has been tentatively attributed to the oxidation of Co3+ to Co4+ according to the reported literature.25 CoOx/CC-10 exhibits the most negative onset potential among these samples, indicating the outstanding OER performance. Figure 4b compares the corresponding overpotentials at different current density of electrodes. It could be



found that the required overpotentials to drive the given current density of 10, 20 and 40 mA cm-2 are 263, 291 and 312 mV, respectively, in the case of CoOx/CC-10 electrode. In contrast, higher overpotentials for CoOx/CC-120, IrO2/CC and Co3O4/CC are needed to realize the same current density. Furthermore, the catalytic kinetics for OER was evaluated by Tafel slopes. As reported, a low Tafel slope means that the catalyst requires a low overpotential to produce the required current, implying the good catalytic activity.38 It is clearly seen from Figure 4c that the Tafel slope value of CoOx/CC-10 is 56.07 mV dec-1, which is the lowest among 64.63 mV dec-1 for CoOx/CC-120, 87.53 mV dec-1 for IrO2/CC and 112.43 mV dec-1 for Co3O4/CC. In the case of CoOx/CC-120, the higher Tafel slope could be due to the larger nanoparticles in nanosheets and lower specific surface area, which lengthened the electron transfer pathway and decreased the electron transfer rate.39 The highest Tafel slope value of Co3O4 nanoparticles might be caused by the severely aggregation (Figure S12). Thus, it seems that CoOx/CC-10 electrode possesses the fastest electron transport kinetics. Moreover, the enhanced electron transport of CoOx/CC-10 was also inferred from electrochemical impedance spectroscopy (EIS) in Figure S14. It is clearly found that CoOx/CC-10 has the lowest charge transfer resistance (Rct) at OER-occurring potential (1.65 V) as compared with that of Co3O4/CC and CoOx/CC-120, implying its high feasibility in shuttling charge during OER process.40 In order to better compare the electrochemical activities of CoOx/CC-10 with reported cobalt oxides, a series of catalytic activities of cobalt oxides was summarized and listed in Table S2. It is evident that the overpotential of


Page 20 of 35

Page 21 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


263 mV for CoOx/CC-10 at the current density of 10 mA cm-2 demonstrates the outstanding performance for OER among these reported cobalt oxides. The double layer capacitance (Cdl) in a non-Faradaic potential region was measured to assess the electrochemical active surface area (ECSA) of the as-synthesized electrode by the cyclic voltammograms (CVs) (Figure 4d, Figure S15, Figure S16). The as-calculated Cdl and ECSA results are displayed in Figure 4e and Table S3, respectively. Obviously, both of the Cdl and ECSA values for the CoOx/CC-10 (5.51 mF cm-2, 91.8 cm2) are significantly larger than that of CoOx/CC-120 (3.90 mF cm-2, 65 cm2) and Co3O4/CC (0.28 mF cm-2, 4.7 cm2). The improved ECSA for CoOx/CC-10 electrode is beneficial to the exposure of catalytic active sites and the contact of electrolyte with electrocatalyst. This result is in agreement with the BET result. In addition, it should be mentioned that the decrement of ECSA for CoOx/CC-120 might be attributed to the formation of larger Co3O4 nanoparticles on nanosheets due to the prolonged calcination time. The long-term stability testing is also an important parameter for energy conversion and storage system.41,42 CV measurement was carried out for the investigation of the stability. Figure S17 shows the LSV curves of CoOx/CC-10 electrode before and after 500 cycles. Slight attenuation in current density are observed. The EIS plots in Figure S18 exhibit that the Rct increased after 500 cycles, indicating the decrement of catalytic activity. Moreover, chronopotentiometric measurements of CoOx/CC-10, IrO2/CC and Co3O4/CC at a constant current density of 10 mA cm-2 were carried out and the detailed results are given in Figure 4f. It is evident that CoOx/CC-10 exhibits



a relatively stable potential during the 180 h electrochemical testing with a small potential increase of ~3.1%, whereas IrO2/CC shows a larger potential rising of 7.2% after the 20 h stability testing. It also should be noted that Co3O4/CC required larger potential than CoOx/CC-10 to reach the current density of 10 mA cm-2 during the stability testing. These results manifest the obvious advantages of CoOx/CC-10 electrode for OER in comparison with IrO2/CC and Co3O4/CC. After stability testing, the SEM image in Figure S19a demonstrates that the ultrathin CoOx nanosheets are still vertically grown on CC without obvious changes, indicating the robust 3-D structure. The Co loading amount on CC still (Table S1) remains stable. However, the high-magnification SEM image in Figure S19b reveals that the nanoparticles in the nanosheets slightly become bigger when compared with that of initial CoOx/CC-10 (Figure 1f). The TEM images in Figure S20 further confirm that CoOx nanosheets still remain nanoporous structures without obvious aggregation. In addition, the high-resolution Co 2p XPS spectrum of CoOx/CC-10 after electrochemical stability testing (Figure 2f) shows that the ratio of Co2+/Co3+ decreased from 1.5/2 to 1/2, indicating the oxidation of Co2+ into Co3+. Thus, it seems that the small increase in potential of CoOx/CC-10 might be caused by the enlarged nanoparticles in nanosheets and the lowering Co2+/Co3+ ratio.


Page 22 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H2 consumption (a. u.)

Page 23 of 35

CoOx/CC-10 Co3O4

100

200

300

400

500

o

Temperature ( C)

Figure 5. H2-TPR profiles of Co3O4 nanoparticles and CoOx/CC-10. Figure 5 shows the H2-TPR profiles of Co3O4 nanoparticles and CoOx/CC-10. It is evident that two similar reduction peaks are present at both profiles. The first weak peak represents the reduction of Co3O4 to CoO and the second pronounced peak is attributed to the reduction of CoO to metallic Co. In the case of CoOx/CC-10, it should be mentioned that both the reduction peaks shift to slightly higher temperatures. This slightly higher reduction temperature could be tentatively attributed to the formation of interaction between cobalt oxides and CC support, which is more difficult to reduce,43-45 during fabrication process CoOx/CC-10. The interaction formed during the calcination process could contribute to the stability of CoOx/CC-10 during the OER process. Analysis of the Co dispersion has been conducted by H2 pulse chemisorption and the results are included in Table S4. The Co dispersion of CoOx/CC-10 is about 0.929%, which is nearly 2 times higher than that of 0.361% for Co3O4 nanoparticles, indicating that CoOx/CC-10 electrode provides more active sites for OER.


0.2

ACS Applied Energy8Materials

7680

Page 24 of 35

6 0.6

4

7721

-0.2

10

E

Normalized intensit

Normaliz

0.4 0.0 7740

7722

7800

2

7723

Energy (eV)

0

7860

0

1

2

3

Energy (eV)

(b)1520

-10 0.2

0.0 -15 -0.2 0

E

D

0.8

4 77406

7722

7723

Energy (eV)

87800 10

12 7860

14 5

Co-O

12 0 10 -58

00 0

14

2

1

4

2

10

6

R

3

8

B R (A) (Å)

4

10

5

12

6

14

CoOx/CC-10 Commercial Co3O4

15 3χ(k) oscillation K-edgeCoOx/CC-10 EXAFS k Co3O4 nanoparticles 10

Figure 6. (a) Co

6

CoOx/CC-10 CoOx/CC-10Co O Commercial 3 4 Co3O4 nanoparticles

Co-Co

k (ÅB-1)(eV) Energy 15

5

6 -10 4 -152

0.6

7721

2 7680

FT (k3χ(k))

18 10 16

5 1.0

0.8 0 0.6 -5 0.4

4

R (A)

CoOx/CC-10 CoOx/CC-10 CoO Onanoparticles nanoparticles Co 3344

15 1.4 10 1.2

Normalized intensity

F

k3χ(k) (Arb.intensity Units) Normalized

(a) 1.6

functions and (b) the

5

D

5 corresponding FT curves of the as-synthesized CoOx/CC-10 and commercial Co3O4 F

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.6

0

nanoparticles. -5

0 -5 -10

-10

For further insights into the excellent -15 electrocatalytic performance of the -15

0

2

4

6

8

10

12

14

0

2

4

6

8

10

12

14

as-synthesized CoOx/CC electrode, the electronic structure B of catalyst was B investigated by X-ray absorption fine structure spectroscopy (XAFS). Figure 6a shows the Co K-edge k3χ(k) oscillation curves of CoOx/CC-10 and commercial Co3O4 nanoparticles, from which a little difference in amplitude is observed. This result implied the different Co local atomic arrangements between CoOx/CC-10 and commercial Co3O4 nanoparticles. Moreover, the corresponding Fourier transform (FT) curves in Figure 6b show that there were two major peaks at 1.92 Å and 2.86 Å in both of samples, which could be attributed to the Co-O and Co-Co coordination, respectively. It is clearly found that the intensity of these peaks in CoOx/CC-10 is reduced in comparison with commercial Co3O4 nanoparticles, indicating the lower coordination number of Co atoms. This is related to the electronic structural difference between them. It should be noted that the particle size of commercial


Page 25 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Co3O4 nanoparticles is about 20-30 nm (Figure S11), whereas the thickness of as-synthesized CoOx nanosheets is about 7 nm. Thus, the lower coordinative number of as-synthesized ultrathin CoOx nanosheets suggests that the ultrathin nature of the nanosheets results in the presence of a large amount of dangling bonds in the surface Co octahedron (CoO6-x) with a structural distortion, facilitating the interfacial electron transfer rate and thereby increasing the electrocatalytic activity.46 Based on the above experimental results, the high electrochemical performance of CoOx/CC-10 for OER could be attributed to the following reasons. Firstly, the growth of cobalt oxides on the conductive CC without the usage of binder forms interaction between cobalt oxides and CC, improving the intrinsic conductivity, electron transfer rate and Co dispersion. Secondly, the 3-D structures constructed by the vertical growth of CoOx nanosheets on CC could increase the efficient contact of electrocatalysts with electrolyte and the formation of mesopores facilitates the diffusion of OH- and evolution of oxygen.47 Thirdly, the ultrathin and mesoporous structures of CoOx nanosheets decrease the length of electron transfer path and endow the electrode with high specific surface area.48 Fourthly, the ultrathin nature of CoOx nanosheets endows the electrode with numerous dangling bonds in the surface of catalyst to accelerate the electron transport. Benefiting from these unique characteristics, the as-synthesized CoOx/CC electrode exhibits high electrocatalytic activity for OER. 4. CONCLUSIONS



In summary, mesoporous ultrathin cobalt oxides nanosheets grown on CC have been successfully synthesized. The presence of CoO and Co3O4 on CC was confirmed by XRD and XPS techniques and the plausible formation mechanism was studied using a 13C labelling experiment with the assistance of the in situ TGA-IR technique. It is speculated that CoO was formed at the contact section between nanosheets and CC and the rest part of nanosheets was Co3O4. The as-synthesized CoOx/CC electrode was directly deployed as a working electrode for OER in 1.0 M KOH solution. The electrochemical testing result shows that the CoOx/CC electrode exhibits excellent catalytic performances for OER in alkaline solution with lower overpotential of 263 mV at the current density of 10 mA cm-2, lower Tafel slope of 56.07 mV dec-1 and better stability in comparison with commercial IrO2 and Co3O4 nanoparticles on CC. The improved electrocatalytic activity of CoOx/CC electrode could be attributed to unique 3-D structures of mesoporous ultrathin cobalt oxides nanosheets vertically grown on CC, leading to fast electron transfer rate, well dispersion and distribution, high specific surface area and numerous coordinately unsaturated active sites. The present strategy to fabricate large efficient electrode is very promising for the large-scale application in water splitting technology. AUTHOR INFORMATION Corresponding Author * Email: [email protected] Notes


Page 26 of 35

Page 27 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The authors declare no competing financial interest. ACKNOWLEDGMENT H.-P. Liang is thankful for support from the “Hundred Talent Program” of Chinese Academy of Sciences (RENZI[2015] 70HAO, Y5100619AM), DICP&QIBEBT UN201804, Dalian National Laboratory for Clean Energy (DNL), CAS, and director innovation fund (QIBEBT SZ201801). REFERENCES (1) Han, L.; Dong, S.; Wang, E. Transition-Metal (Co, Ni, and Fe)-based electrocatalysts for the water oxidation reaction. Adv. Mater. 2016, 28, 9266-9291. (2) Suen, N. T.; Hung, S. F.; Quan, Q.; Zhang, N.; Xu, Y. J.; Chen, H. M. Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives. Chem. Soc. Rev. 2017, 46 337-365. (3) Shang, X.; Liu, Z.; Zhang, J.; Dong, B.; Zhou, Y.; Qin, J.; Wang, L.; Chai, Y.; Liu, C. Electrochemical corrosion engineering for Ni-Fe oxides with superior activity towards water oxidation. ACS Appl. Mater. Inter. 2018, 10, 42217-42224. (4) Anantharaj, S.; Ede, S. R.; Sakthikumar, K.; Karthick, K.; Mishra, S.; Kundu, S. Recent trends and perspectives in electrochemical water splitting with an emphasis on sulfide, selenide, and phosphide catalysts of Fe, Co, and Ni: a review. ACS Catal. 2016, 6, 8069-8097. (5) Asefa, T. Metal-free and noble metal-free heteroatom-doped nanostructured carbons as prospective sustainable electrocatalysts. Accounts Chem. Res. 2016, 49, 1873-1883.



(6) Qin, Q.; Li, P.; Chen, L.; Liu, X. Coupling bimetallic oxides/alloys and N-doped carbon nanotubes as tri-functional catalysts for overall water splitting and zinc-air batteries. ACS Appl. Mater. Inter. 2018, 10, 39828-39838. (7) Wang, J.; Cui, W.; Liu, Q.; Xing, Z.; Asiri, A. M.; Sun, X. Recent progress in cobalt-based heterogeneous catalysts for electrochemical water splitting. Adv. Mater. 2016, 28, 215-230. (8) Karthick, K.; Anantharaj, S.; Karthik, P. E.; Subramanian, B.; Kundu, S. Self-assembled molecular hybrids of CoS-DNA for enhanced water oxidation with low cobalt content. Inorg. Chem. 2017, 56, 6734-6745. (9) Liu, J.; Ke, J.; Li, Y.; Liu, B.; Wang, L.; Xiao, H.; Wang, S. Co3O4 quantum dots/TiO2 nanobelt hybrids for highly efficient photocatalytic overall water splitting. Appl. Catal. B- Environ. 2018, 236, 396-403. (10) Subbaraman, R.; Tripkovic, D.; Chang, K. C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M. Trends in activity for thewater electrolyser reactions on 3d M(Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts. Nat. Mater. 2012, 11, 550-557. (11) Zhang, B.; Zheng, X.; Voznyy, O.; Comin, R.; Bajdich, M.; Garcia-Melchor, M.; Han, L.; Xu, J.; Liu, M.; Zheng, L.; Garcia de Arquer, F. P.; Dinh, C. T.; Fan, F.; Yuan, M.; Yassitepe, E.; Chen, N.; Regier, T.; Liu, P.; Li, Y.; De Luna, P.; Janmohamed, A.; Xin, H. L.; Yang, H.; Vojvodic, A.; Sargent, E. H. Homogeneously dispersed multimetal oxygen-evolving catalysts. Science 2016, 352, 333-337. (12) Hong, T.; Liu, Z.; Zheng, X.; Zhang, J.; Yan, L. Efficient photoelectrochemical


Page 28 of 35

Page 29 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


water splitting over Co3O4 and Co3O4/Ag composite structure. Appl. Catal. B-Environ. 2017, 202, 454-459. (13) Ma, Y.-Y.; Wu, C.-X.; Feng, X.-J.; Tan, H.-Q.; Yan, L.-K.; Liu, Y.; Kang, Z.-H.; Wang, E.-B.; Li, Y.-G. Highly efficient hydrogen evolution from seawater by a low-cost and stable CoMoP@C electrocatalyst superior to Pt/C. Energ. Environ. Sci. 2017, 10, 788-798. (14) Liu, Y.; Xiao, C.; Lyu, M.; Lin, Y.; Cai, W.; Huang, P.; Tong, W.; Zou, Y.; Xie, Y. Ultrathin Co3S4 nanosheets that synergistically engineer spin states and exposed polyhedra that promote water oxidation under neutral conditions. Angew. Chem. Int. Edit. 2015, 54, 11231-11235. (15) Huang, J.; Chen, J.; Yao, T.; He, J.; Jiang, S.; Sun, Z.; Liu, Q.; Cheng, W.; Hu, F.; Jiang, Y.; Pan, Z.; Wei, S. CoOOH nanosheets with high mass activity for water oxidation. Angew. Chem. Int. Edit. 2015, 54, 8722-8727. (16) Yang, J.; Zhu, G.; Liu, Y.; Xia, J.; Ji, Z.; Shen, X.; Wu, S. Fe3O4-decorated Co9S8 nanoparticles in situ grown on reduced graphene oxide: a new and efficient electrocatalyst for oxygen evolution reaction. Adv. Funct. Mater. 2016, 26, 4712-4721. (17) Anantharaj, S.; Reddy, P. N.; Kundu, S. Core-oxidized amorphous cobalt phosphide nanostructures: an advanced and highly efficient oxygen evolution catalyst. Inorg. Chem. 2017, 56, 1742-1756. (18) Ye, Z.; Qin, C.; Ma, G.; Peng, X.; Li, T.; Li, D.; Jin, Z. Cobalt-iron oxide nanoarrays supported on carbon fiber paper with high stability for electrochemical



oxygen evolution at large current densities. ACS Appl. Mater. Inter. 2018, 10, 39809-39818. (19) Xiao, Z.; Wang, Y.; Huang, Y.-C.; Wei, Z.; Dong, C.-L.; Ma, J.; Shen, S.; Li, Y.; Wang, S. Filling the oxygen vacancies in Co3O4 with phosphorus: an ultra-efficient electrocatalyst for overall water splitting. Energ. Environ. Sci. 2017, 10, 2563-2569. (20) Dou, Y.; Liao, T.; Ma, Z.; Tian, D.; Liu, Q.; Xiao, F.; Sun, Z.; Ho Kim, J.; Dou, S. X. Graphene-like holey Co3O4 nanosheets as a highly efficient catalyst for oxygen evolution reaction. Nano Energy 2016, 30, 267-275. (21) Jin, H.; Wang, J.; Su, D.; Wei, Z.; Pang, Z.; Wang, Y. In situ cobalt-cobalt oxide/N-doped carbon hybrids as superior bifunctional electrocatalysts for hydrogen and oxygen evolution. J. Am. Chem. Soc. 2015, 137, 2688-2694. (22) Rosen, J.; Hutchings, G. S.; Jiao, F. Ordered mesoporous cobalt oxide as highly efficient oxygen evolution catalyst. J. Am. Chem. Soc. 2013, 135, 4516-4521. (23) Wang, Y.; Zhou, T.; Jiang, K.; Da, P.; Peng, Z.; Tang, J.; Kong, B.; Cai, W.-B.; Yang, Z.; Zheng, G. Reduced mesoporous Co3O4 nanowires as efficient water oxidation electrocatalysts and supercapacitor electrodes. Adv. Energy Mater. 2014, 4, 1400696. (24) Dou, S.; Dong, C.-L.; Hu, Z.; Huang, Y.-C.; Chen, J.-l.; Tao, L.; Yan, D.; Chen, D.; Shen, S.; Chou, S.; Wang, S. Atomic-scale CoOx species in metal–organic frameworks for oxygen evolution reaction. Adv. Funct. Mater. 2017, 27, 1702546.


Page 30 of 35

Page 31 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(25) Zhu, Y. P.; Ma, T. Y.; Jaroniec, M.; Qiao, S. Z. Self-templating synthesis of hollow Co3O4 microtube arrays for highly efficient water electrolysis. Angew. Chem. Int. Edit. 2016, 5, 1324-1328. (26) Liu, T.; Guo, Y.-F.; Yan, Y.-M.; Wang, F.; Deng, C.; Rooney, D.; Sun, K.-N. CoO nanoparticles embedded in three-dimensional nitrogen/sulfur co-doped carbon nanofiber networks as a bifunctional catalyst for oxygen reduction/evolution reactions. Carbon 2016, 106, 84-92. (27) Tang, C.; Wang, H. F.; Zhang, Q. Multiscale principles to boost reactivity in gas-involving energy electrocatalysis. Account Chem. Res. 2018, 4, 881-889. (28) Chen, Y.; Hu, J.; Diao, H.; Luo, W.; Song, Y. F. Facile preparation of ultrathin Co3O4/nanocarbon composites with greatly improved surface activity as a highly efficient oxygen evolution reaction catalyst. Chem-Eur. J. 2017, 23, 4010-4016. (29) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction. Nat. Mater. 2011, 10, 780-786. (30) Zhao, Y.; Sun, B.; Huang, X.; Liu, H.; Su, D.; Sun, K.; Wang, G. Porous graphene wrapped CoO nanoparticles for highly efficient oxygen evolution. J. Mater. Chem. A 2015, 3, 5402-5408. (31) Yang, X.; Li, H.; Lu, A.; Minb, S.; Idriss, Z.; Hedhili, M.; Huangb, K.; Idriss, H.; Li, L. Highly acid-durable carbon coated Co3O4 nanoarrays as efficient oxygen evolution electrocatalysts. Nano Energy 2016, 25, 42-50. (32) Jeon, H.; Jee, M.; Kim, H.; Ahn, S.; Hwang, Y.; Min, B. Simple chemical



solution deposition of Co3O4 thin film electrocatalyst for oxygen evolution reaction. ACS Appl. Mater. Inter. 2015, 7, 24550-24555. (33) Chakroune, N.; Viau, G.; Ammar, S.; Jouini, N.; Gredin, P.; Vaulay, M. J.; Fiévet, F. Synthesis, characterization and magnetic properties of disk-shaped particles of a cobalt alkoxide: CoII(C2H4O2). New J. Chem. 2005, 29, 355-361. (34) Cao, A. M.; Hu, J. S.; Liang, H. P.; Song, W. G.; Wan, L. J.; He, X. L.; Gao, X. G.; Xia, S. H. Hierarchically structured cobalt oxide (Co3O4): the morphology control and its potential in sensors. J. Phys. Chem. B 2006, 110, 15858-15863. (35) Poul, L.; Jouini, N.; Fiévet, F. Layered hydroxide metal acetates (metal ) zinc, cobalt, and nickel): elaboration via hydrolysis in polyol medium and comparative study. Chem. Mater. 2000, 12, 3123-3132. (36) Chang, J.; Lv, Q.; Li, G.; Ge, J.; Liu, C.; Xing, W. Core-shell structured Ni12P5/Ni3(PO4)2 hollow spheres as difunctional and efficient electrocatalysts for overall water electrolysis. Appl. Catal. B-Environ. 2017, 204, 486-496. (37) Song, W.; Ren, Z.; Chen, S. Y.; Meng, Y.; Biswas, S.; Nandi, P.; Elsen, H. A.; Gao, P. X.; Suib, S. L. Ni- and Mn-promoted mesoporous Co3O4: A stable bifunctional catalyst with surface-structure-dependent activity for oxygen reduction reaction and oxygen evolution reaction. ACS Appl. Mater. Inter. 2016, 8, 20802-20813. (38) Du, J.; Wang, X.; Li, C.; Liu, X.; Gu, L.; Liang, H. Hollow Rh nanoparticles with nanoporous shell as efficient electrocatalyst for hydrogen evolution reaction. Electrochim. Acta 2018, 282, 853-859.


Page 32 of 35

Page 33 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(39) Yang, J.; Zhu, J.; Xu, J.; Zhang, C.; Liu, T. MoSe2 nanosheet array with layered MoS2 heterostructures for superior hydrogen evolution and lithium storage performance. ACS Appl. Mater. Inter. 2017, 9, 44550-44559. (40) Ryu, J.; Jung, N.; Jang, J.; Kim, H. J.; Yoo, S. In situ transformation of hydrogen-evolving CoP nanoparticles: toward efficient oxygen evolution catalysts bearing dispersed morphologies with Co-oxo/hydroxo molecular units. ACS Catal. 2015, 5, 4066-4074. (41) Pan, Y.; Sun, K.; Liu, S.; Cao, X.; Wu, K.; Cheong, W. C.; Chen, Z.; Wang, Y.; Li, Y. Core−shell ZIF-8@ZIF-67-derived CoP nanoparticle-embedded N ‑ doped carbon nanotube hollow polyhedron for efficient overall water splitting. J. Am. Chem. Soc. 2018, 140, 2610-2618. (42) Anantharaj, S.; Ede, S. R.; Karthick, K.; Sam Sankar, S.; Sangeetha, K.; Karthik, P. E.; Kundu, S. Precision and correctness in the evaluation of electrocatalytic water splitting: revisiting activity parameters with a critical assessment. Energy Environ. Sci. 2018, 11, 744-771. (43) Keyser, M. M.; Prinsloo, F. F. Loading of cobalt on carbon nanofibers. Stu. Surf. Sci. Catal. 2007, 163, 45-73. (44) Cheng, K.; Subramanian, V.; Carvalho, A.; Ordomsky, V.; Wang, Y.; Khodakov, A. The role of carbon pre-coating for the synthesis of highly efficient cobalt catalysts for Fischer–Tropsch synthesis. J. Catal. 2016, 337, 260-271.



(45) Bitter, J. H.; van der Lee, M. K.; Slotboom, A. G. T.; van Dillen, A. J.; de Jong, K. P. Synthesis of highly loaded highly dispersed nickel on carbon nanofibers by homogeneous deposition–precipitation. Catal. Lett. 2003, 89, 139-142. (46) Chen, X.; Liu, B.; Zhong, C.; Liu, Z.; Liu, J.; Ma, L.; Deng, Y.; Han, X.; Wu, T.; Hu, W.; Lu, J. Ultrathin Co3O4 layers with large contact area on carbon fibers as high-performance electrode for flexible Zinc–Air battery integrated with flexible display. Adv. Energy Mater. 2017, 18, 1700779. (47) Ma, T.; Dai, S.; Jaroniec, M.; Qiao, S. Metal-organic framework derived hybrid Co3O4-carbon porous nanowire arrays as reversible oxygen evolution electrodes. J. Am. Chem. Soc. 2014, 136, 13925−13931. (48) Malik, B.; Anantharaj, S.; Karthick, K.; Pattanayak, D. K.; Kundu, S., Magnetic CoPt nanoparticle-decorated ultrathin Co(OH)2 nanosheets: an efficient bi-functional water splitting catalyst. Catal. Sci. Technol. 2017, 7, 2486-2497.


Page 34 of 35

Page 35 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents Graphic


Mesoporous Ultrathin Cobalt Oxides Nanosheets Grown on Carbon

Recommend Documents