Interlayer expanded lamellar CoSe2 on carbon paper as highly efficient and stable overall water splitting electrodes

Accepted Manuscript Title: Interlayer expanded lamellar CoSe2 on carbon paper as highly efficient and stable overall water splitting electrodes Author: Yan Zhou Huaqing Xiao Shuo Zhang Yanpeng Li Shutao Wang Zhaojie Wang Changhua An Jun Zhang PII: DOI: Reference:

S0013-4686(17)30843-5 http://dx.doi.org/doi:10.1016/j.electacta.2017.04.084 EA 29341

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

25-1-2017 12-4-2017 17-4-2017

Please cite this article as: Y. Zhou, H. Xiao, S. Zhang, Y. Li, S. Wang, Z. Wang, C. An, J. Zhang, Interlayer expanded lamellar CoSe2 on carbon paper as highly efficient and stable overall water splitting electrodes, Electrochimica Acta (2017), http://dx.doi.org/10.1016/j.electacta.2017.04.084 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Highlights

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One-step solvothermal method to synthesize CoSe2/carbon paper catalytic electrode. CoSe2 nanosheets on carbon paper as highly efficient electrochemical water splitting catalyst. The interlayer of CoSe2 was expanded to offer more electroactive site.

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Interlayer expanded lamellar CoSe2 on carbon paper as highly efficient and stable overall water splitting electrodes†

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Zhaojie Wanga, Changhua Ana, c* and Jun Zhangb*

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Yan Zhoua,b*, Huaqing Xiaoa, Shuo Zhangb, Yanpeng Lib, Shutao Wanga,

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a. College of science, China University of Petroleum, Qingdao, Shandong 266580, People’s Republic of China.

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b. State Key Laboratory of Heavy Oil Processing, College of Science and College of Chemical Engineering, China University of Petroleum, Qingdao, Shandong 266580, People’s Republic of China.

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c. Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, College of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, People’s Republic of China.

Corresponding Authors: Y. Zhou:

[email protected]

C. An:

[email protected]

J. Zhang:

[email protected]

†Electronic Supplementary Information (ESI) available

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Abstract

Water splitting associated with the conversion and storage of renewable energy is considered to be the most significant strategy to create hydrogen. Herein, an efficient

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self-supported electrode was developed by in-situ growth of interlayer expanded lamellar cobalt diselenide (CoSe2) nanosheets (NS) on carbon paper (CP) substrate (CoSe2

NS@CP). The analyses of TEM, SEM and XRD confirmed that the CP is

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homogeneously coated by few stacking layers of interlayer expanded lamellar structured

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CoSe2. This rationally designed nanostructure can provide more active sites for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The

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bifunctional electrode exhibits high electrocatalytic performance with -128 mV vs. RHE onset potential for the HER in 0.5 mol dm-3 H2SO4 and +1521 mV vs. RHE for the OER

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in 1.0 mol dm-3 KOH. Besides, small electrolysis potentials of -201.1 mV vs. RHE and +1636 mV vs. RHE are needed to drive the HER and OER at current density of 100 mA cm-2. Finally, a small overall cell voltage (ca. +1.75 V) was used to drive the water splitting

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reaction in 1.0 mol dm-3 KOH. The CoSe2 NS@CP electrode showed both excellent

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catalytic activity with overall current density of 100 mA cm-2 at 2.13 V and tremendous durability with negligible decrease in potential at a constant current of 20 mA cm-2 for more than 30 hours. Thus this rationally designed electrode material can be readily applied for large-scale water splitting process.

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1. Introduction Water is known as the most essential material for the existence of life and it covers

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71% of Earth’s surface, consisting of two important elements: hydrogen and oxygen. Hydrogen is a highly efficient fuel for obtaining thermal energy,[1, 2]

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whereas oxygen is vital to sustain most terrestrial lives. Therefore, splitting of

water (via electrocatalytic or solar splitting) is a great candidate for the production

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of hydrogen and oxygen.[3-8] Although solar-driven water splitting provides a “green” approach towards the production of hydrogen and oxygen,[9] it was

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limited by the production conditions (exposed under sunlight). In contrast, electrocatalytic water splitting affords higher efficiency as well as broader

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operating conditions.[10] Furthermore, the highly pure hydrogen and oxygen from both approaches can be applied to fuel cells directly without worrying about

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catalysts poisoning.[11]

The electrocatalytic water splitting (also known as the electrolysis of water)

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involves hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode. Both cathodic and anodic process need electrocatalysts

to

promote

the

electrode

reactions,

especially

the

four-electron-transfer OER process. Presently, the state-of-art HER and OER catalysts are platinum (Pt) and noble metal oxides such as iridium oxide (IrO2) and ruthenium oxide (RuO2), respectively, but their long-term availability is limited owing to their scarcity and cost.[12-16] Therefore, enormous researches have been devoted to develop low-cost and earth-abundant alternatives to noble metals. Examples

include

transition

metal

chalcognides,[16-25]

pnictides,[26-44]

carbides[45-47] and borides[46, 48] for the HER, and transition metal oxides,[6, 49, 50] hydroxides,[51-53] and phosphates[12, 54] for the OER. Among them, two-dimensional layered nanostructures showed exciting performances towards

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electrochemical catalysed HER and OER owing to their tuneable electronic structures, high catalytic activities and high active surface areas.[55, 56] In order to simplify the electrode preparation procedure, which may significantly reduce the production duration and costs, the development of a catalyst that can concurrently catalyse both cathodic and anodic process is highly desired. Recently,

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CoSe2 has emerged as one of the most active earth-abundant electrocatalyst for both the HER[57, 58] and OER.[59, 60] The metallic property of CoSe2 with

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low-spin 3d electrons of Co is believed to favour the charge transportation along

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the basal plane of the nanosturcture.[60, 61] The lamellar structured CoSe2 with expanded interlayer spacing showed great enhancement towards the HER.[58] The

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assistance of diethylenetriamine (DETA) affords the lamellar CoSe2 nanobelts with metallic structure[62] and layer distortion.[63] These features were found to be responsible for the increased electron transfer capability and catalytic stability,

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thus, may enhance the catalytic activities for both of the HER and OER. Our previous work suggested that the direct growth of supported lamellar structured

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CoSe2 nanosheets on Ti plate showed enhanced catalytic activity for the HER due

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to the direct electron transfer between the electrode and catalyst, compared to the conventional electrode preparation which involved polymer binder to fix the catalysts.[64] Therefore, the self-supported CoSe2 composite electrode is an excellent choice to achieve reasonable outcomes for water electrolysis. In this paper, we prepared carbon paper (CP) supported lamellar structured CoSe2 nanosheets (namely CoSe2 NS@CP) for electrochemical water splitting process. Carbon paper was chosen as the electrode because it provided high electrical conductivity and resistance to corrosion in both acid and basic solutions.[11] The CoSe2 NS@CP electrode afforded 10 mA cm-2 at an potential of -162 mV vs. RHE for the HER in 0.5 mol dm-3 H2SO4 and +1580 mV vs. RHE for the OER in 1.0 mol dm-3 KOH. Furthermore, this 3D lamellar structured catalyst enabled a high-efficiency alkaline water electrolyzer with 100 mA cm-2 at 2.13 V. We demonstrated that without any post-treatment, the self-supported 3D

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CoSe2 NS@CP electrode could be readily applied as a bifunctional catalyst to drive

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the electrochemical overall water splitting reaction.

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2. Experimental 2.1 Reagents

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Carbon paper (thickness = 0.2 mm) was purchased from Shanghai Hesen Technology Co., Ltd., China. DETA and Nafion solution (5%) was obtained from

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Alfa Aesar. Cobalt acetate (Co(CH3COO)2·4H2O), sodium selenite (Na2SeO3) and

ethanol (C2H5OH) were purchased from Sinopharm Chemical Reagent Co., Ltd.,

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Shanghai, China. All the reagents were used as received without further purification. Deionized water (DIW) was obtained from AODLON professional

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water processing system (Qingdao, China) with resistance greater than 18 MΩ.

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2.2 Synthesis of CoSe2 NS@CP

For a typical procedure, CP was rinsed respectively with 6 mol dm-3 HCl solution, ethanol and water prior to further experiments. 0.8 mmol Co(CH3COO)2·4H2O

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and 0.8 mmol Na2SeO3 were dissolved into a mixture of 20 mL DIW and 20 mL

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DETA (volume ratio of VDETA:VDIW = 1:1). After the mixture was sonicated 1 h, it was transferred to a Teflon-lined autoclave with the capacity of 50 mL. Then, a piece of CP (2 × 3 cm2) was immerged in the solution. The autoclave was sealed and heated at 180 oC for 14 h. When the autoclave was cooled to room temperature naturally, the product was washed with water and ethanol several times. The final sample was collected after being dried in a vacuum desiccator.

2.3 Synthesis of CoSe2 nanobelts (NBs) The synthetic procedure of CoSe2 NBs was similar to that of CoSe2 NS@CP hybrid, except the solvothermal process was carried out without CP.

2.4 Characterization

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The crystalline structures of the samples were confirmed with powder X-ray diffraction (XRD) by using a Philips X'Pert diffractometer with Cu Kα radiation (λ = 0.15418 nm). Transmission electron microscopy (TEM) and high-resolution transmission electron microscope (HRTEM) images were collected on a JEM-2100UHR transmission microscope (JEOL, Japan) operated at 200 kV.

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Scanning electron microscopy (SEM) images were acquired using a Hitachi S-4800

field emission scanning electronic microscopy (FESEM). X-ray photoelectron

using

an

Al

Kα

(1486.6

eV)

photon

source.

Fourier

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spectrometer

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spectroscopy (XPS) analysis was performed on a VG ESCALABMK II

Transform-Infrared (FTIR) spectra were measured by using a Thermo Nicole FTIR spectrometer (NEXUS, USA). Atomic absorption spectroscopy（AAS） was

All

electrochemical

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2.5 Electrochemical measurements

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collected on a Contr AA700 (Germany).

measurements

were

conducted

on

a

CHI

660E

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electrochemical workstation (CH Instruments, Inc., Shanghai) with a standard

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three-electrode configuration. The HER performance was evaluated in 0.5 mol dm-3 H2SO4 solution using the as-fabricated CoSe2 NS@CP as the working electrode with exposed area of 2 cm2 (two sides with 1 cm2 on each side), a Pt plate served as the counter electrode, and an Ag/AgCl/3M KCl as the reference electrode. For comparison, the HER performance of a bare CP plate and commercially Pt/C (20% Pt) or CoSe2 NBs supported on glassy carbon working electrode (GCE: 4 mm in diameter, loading density 0.16 mg/cm2) were also measured. The oxygen was removed from electrolyte by bubbling N2 for 30 min prior to the starting of each HER test. The electrode potentials were converted to reverse hydrogen electrode (RHE) by Equation 1.

ERHE = EAg/AgCl + 0. 059pH + E0Ag/AgCl

(E0Ag/AgCl = 0.209 V)

(1)

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The OER performance was evaluated in the same three-electrode cell in 1.0 mol dm-3 KOH solution except that an Hg/HgO was used as the reference electrode. In this case, a bare CP and CoSe2 NBs supported on glassy carbon working electrode were measured for comparison. The electrode potentials were

(E0Hg/HgO = 0.098 V)

(2)

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ERHE = EHg/HgO + 0. 059pH + E0Hg/HgO

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converted to RHE by Equation 2.

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The full electrolyzer configuration was assembled using two identical CoSe2 NS@CP electrodes and measured in a two-electrode cell in 1.0 mol dm-3 KOH.

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For other electrochemical characterization experiments, linear sweep voltammetry (LSV) and cyclic voltammetry (CV) were performed in 0.5 mol dm-3 H2SO4 or 1.0

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mol dm-3 KOH solution, respectively. The polarization curves were replotted as potential (η) versus logarithm of current (j) to obtain the Tafel plots for assessing

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the HER and OER kinetics of catalysts under investigation. The electrochemical impedance measurements were carried out in the same configuration at DC

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potential of -0.24 V vs. RHE from 105 to 0.1 Hz in 0.5 mol dm-3 H2SO4.

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3. Results and discussion 3.1 Structural characterization of CoSe2 NS@CP

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The morphologies of both CoSe2 NBs and CoSe2 NS@CP nanocomposite obtained by SEM and TEM are shown in Fig. 1. The support-free CoSe2 shows

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nanobelt-like morphology with an average width of 200-300 nm and length up to several tens of micrometers (Fig. 1a). However, after grown on CP, CoSe2 exists in

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a form of densely and perpendicularly packed nanosheets, which are short compared with CoSe2 NBs. The thickness of CoSe2 nanosheets were ~20 nm.

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(Fig. 1b, 1c and 1d). Accordingly, the thin nanosheets can provide sufficient active sites to enhance the efficiency of electron transfer and catalytic activity for HER

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and OER since the increment of exposed catalytic active sites of Co2+.[63] The HRTEM image (Fig. 1e) taken from a typical region of CoSe2 NS@CP nanocomposite shows clear grain boundaries and the lattice fringes of (210) planes

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with a spacing of 0.27 nm, indicating the sample grown preferentially along [210]

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direction. The CoSe2 NS is composed of several layers with single layer thickness of 0.57 nm (Fig. 1f). It is worth noting that the interlayer distance along (001) is ca. 1.05 nm for the as-synthesized CoSe2 (Fig. 1g), showing a layer expansion compared to conventional sample (JCPDS No.53-0449). According to literature reports, the expanded interlayer is attributed to the intercalation of protonated DETA.[62] The increased edge exposure in expanded layers is completely different from other mesostructured 2D counterparts, which can be favourable for enhanced catalytic activity. Accordingly, lamellar structured CoSe2 NS with expanded interlayers were successfully synthesized with the assistance of DETA. Scheme 1 summarizes the formation of CoSe2 NS with the interaction of DETA. The CoSe2 NS is assembled by few layers of CoSe2 with thinckness of 0.57 nm and interlayer distance between CoSe2 layers to be 1.05 nm. Since the primitive crystal of CoSe2 consists with 0.57 nm in unit cell height, it is reasonable that the lamellar

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structured CoSe2 NS can be actually regarded as well separated single layers, which

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is believed to increase the number of active sites, thus enhanced catalytic activities.

Figure 1 SEM images of CoSe2 NBs (a) and CoSe2 NS@CP (b and c) and TEM (d) and HRTEM (e, f and g) of CoSe2 NS from CoSe2 NS@CP.

Synthetic procedures are curial for fabricating efficient electrocatalysts with appropriate structures. Detailed experimental analysis revealed that the volume ratio of solvents, loading amounts of CoSe2 and reaction time were directly responsible

for

the

formation

of

CoSe2

nanosheets

morphology

and

mesostructures. Since DETA plays an important role in the formation of a layer-expanded nanostructures, the favorable DETA to DIW volume ratios (VDETA : VDIW) for the growth of lamellar structured nanosheets were examined by tuning the volume ratio to be 4:1; 2:1; 1:1; 1:2 and 1:4. As shown in Fig. 1b and Fig. S2, the VDETA : VDIW of 1:1 favours the growth of CoSe2 nanosheets. Accordingly, the

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optimal volume ratio of solvents was then fixed as 1:1 in the forthcoming

Sche

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experiments.

me 1 A cartoon formation of CoSe2 NS@CP.

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The effects of CoSe2 loading amounts on the CP were also studied. When the weight percentage of CoSe2 to CP was 0.32 wt% (Fig. S3a), the growth of CoSe2 nanosheets were scarce. Whereas overgrowth of CoSe2 nanosheets and aggregation of small spherical particles adhered to nanosheets can be clearly observed in Fig. S3b when the weight percentage raised to 0.76 wt%. Consequently, 0.63 wt% were found to be the optimal weight percentage of CoSe2 to CP since the uniformly aligned CoSe2 nanosheets grown on CP surface was observed (Fig 1b). Furthermore, the morphology of CoSe2 was also related to reaction time. The time-dependent experiments showed that insufficient reaction time (i.e. 10 h) caused scarce growth of CoSe2 (Fig. S4a) and prolonged reaction time led to severe aggregation of CoSe2 (i.e. 18 h) shown in Fig. S4b. Therefore, 14 h reaction was appropriate for obtaining lamellar structured CoSe2 nanosheets.

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Phase composition of the as-prepared sample (CoSe2 NS@CP), pure CoSe2 NBs, and commercial CP were examined by XRD. As shown in Fig. 2a, the pattern of CP matches the standard graphite-2H data (JCPDS 00-041-1487). The two strong diffraction patterns at 26.4o and 54.5o can be indexed to the (002) and (004) planes of graphite. Two new peaks with duple relationship appear at

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low-angle region for both CoSe2 NBs and CoSe2 NS@CP, which can be indexed to the (001) and (002) reflections for CoSe2 with lattice spacings of 1.08 and 0.55

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nm, compared with the standard primitive phase CoSe2 (JCPDS 09-0234; added in

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M

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standard primitive phase CoSe2 (JCPDS 09-0234).

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the bottom). The other wide-angle reflections match the same pattern as the

Figure 2 (a) XRD patterns for CP, CoSe2 NBs and CoSe2 NS@CP and (b) Infrared spectrum for CoSe2 NS@CP synthesized with and without water.

The broadening width of all reflections and the disappearance of some diffraction peaks were associated with the ultrathin characteristic of the nanosheets. Compared with the d spacing of 0.27 nm of the pristine CoSe2, a lateral view of the obtained nanosheets along the direction of the strips is parallel to (210), in which the interlayer distance can be estimated to be 1.08 nm. The increased d spacing may be attributed to intercalation of protonated DETA into two CoSe2 layers according to Yu’s work.[62] An FT-IR spectrum shows the typical vibration bands of -CH2-, -NH2, C-N and -NH of CoSe2 NS@CP prepared from DETA/water mixture (1:1) and pure DETA, which shows the presence of DETA in CoSe2 (Fig. 2b). The weak intensity of all vibration peaks and the lack of

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several other bands are attributed presumably to the relatively low content of DETA in the nanosheets. XPS was used to characterize the composition of CoSe2 NS@CP electrode. The coexistence of DETA is evidenced by the appearance of N 1s peak on XPS survey spectra (Fig. S5). The high resolution region of Se and Co peaks are shown

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in Fig. 3. In Fig. 3a, two shoulder observed on the strong peaks at 778.7 eV and 793.8 eV are ascribed to the electron-binding energy of Co 2p3/2 and 2p1/2,

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respectively, which are corresponded to Co2+ cations in CoSe2.[65, 66] The Se 3d

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spectrum exhibits two contributions, 3d5/2 and 3d3/2, located at respectively 54.6 eV and 55.3 eV (Fig. 3b), which can be assigned to CoSe2.[66] The peak at 59.0 eV

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is assigned to the oxidized Se and the peaks at 60.1 eV and 61.0 eV are attributed

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to the Co 3p3/2 and Co 3p1/2.

Figure 3 XPS spectra of CoSe2 NS@CP for (a) Co and (b) Se region.

3.2 CoSe2 NS@CP electrocatalytic performance towards HER In order to identify the most efficient CoSe2 NS@CP electrode, the variation of DETA to water volume ratio, loading amount of catalysts and reaction time were investigated. Since DETA is an essential reagent on the formation of lamellar structure with interlayer expanded CoSe2 nanosheets, the volume ratio of DETA to water should be crucially affect the HER catalytic activity, that is, the excessive DETA may reduce the conductivity of CoSe2 NS@CP. The HER activities of

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CoSe2 NS@CP with different solvent ratios were shown in Fig. S6. It is clear that the volume ratio of 1:1 delivered the best HER performance with the potential of -201.1 mV vs. RHE at a current density of 100 mA cm-2. Accordingly, the volume ratio of 1:1 was used subsequently for the synthesis of CoSe2 NS@CP. Additionally, the different loading of catalysts suggested that the loading

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amount of 0.63 wt% was the optimal condition in this system (Fig. S7). This may be caused by the optimal growth amount of CoSe2 nanosheets on the CP

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substrate, since the CoSe2 and CP play different roles in the HER, with CoSe2 is

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responsible for the HER catalysis whereas the CP is responsible for charge transportation. Finally yet importantly, the reaction time of HER electrocatalytic

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performance was further examined. Fig. S8 indicates that the reaction for 14 h was optimal in case of the achievement of most efficient CoSe2 NS@CP electrocatalyst. Therefore, the following electrochemical analysis was performed

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using the optimized CoSe2 NS@CP electrode, with 1:1 solvent volume ratio, 0.63 wt% loading percentage and 14 h reaction duration. By comparing with all the

d

corresponding LSV polarization curves of HER, it is clear that the best catalytic

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activity occurs only on the complete lamellar structured CoSe2 nanosheets. Indeed, lamellar CoSe2 nanosheets favors the donation and linear transportation of electrons along the basal plane. Besides, the specific lamellar structure with more exposed defects could also provide larger number of active sites.

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Figure 4 (a) LSV polarization curves of HER for Pt/C, CoSe2 NS@CP, CP plate and CoSe2 NBs in 0.5 mol dm-3 H2SO4 at scan rate of 5 mV s-1. (b) the corresponding Tafel plots for the various catalysts derived from (a). (c) LSV polarization curves recorded at CoSe2 NS@CP electrode with a sweep rate of 5 mVs-1 before and after 500 consecutive cycles between -0.4 V and +0.3 V vs. RHE

d

at 100 mV s-1 (with iR compensated). (d) Time-dependent current density curve of the CoSe2

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NS@CP under a static potential of -0.16 V vs. RHE for 24 h.

The HER electrocatalytic performance of the CoSe2 NS@CP electrode was evaluated in 0.5 mol dm-3 H2SO4. Commercial Pt/C, bare CP and CoSe2 NBs were used for comparison. Fig 4a shows the LSV polarization curves of different catalytic HER electrodes at a scan rate of 5 mV s-1. Pt/C undoubtedly exhibits the highest catalytic activity towards HER with near 0 V vs. RHE onset potential, whereas the bare CP shows negligible catalytic current even at high electrode potential (i.e. up to -0.4 V vs. RHE). Noticeably, CoSe2 NBs also exhibited very low catalytic activity towards HER, suggesting that the presence of substrate is essential for the enhanced electrocatalytic activity. CoSe2 NS@CP electrode has low onset potential of -128 mV vs. RHE (Fig. S1a) and potential of -162 mV vs. RHE at catalytic current density of 10 mA cm-2 and -201.1 mV vs. RHE at 100 mA

Page 17 of 32

cm-2. Fig. 4b illustrates the Tafel slope of the same catalytic electrodes presented in Fig. 4a. The Tafel slope of 38.1 mV dec-1 for Pt/C exhibits the expected Volmer-Heyrovsky reaction pathway, with 4.6RT/3F = 38 mV dec-1 at room temperature (where R, T and F are ideal gas constant, temperature and Faraday constant, respectively).[56] The CoSe2 NS@CP electrode shows similar slope of

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38.7 mV dec-1, indicating the same Volmer-Heyrovsky pathway as the Pt/C

electrocatalyst. This fast electrode kinetics and Tafel slope is comparable with

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other reported earth-abundant HER catalysts.[23, 24, 45] In comparison, the

CoSe2 NBs and bare CP exhibit large Tafel slope of 67.2 mV dec-1 and 154.3 mV

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dec-1, respectively. It is worth mentioning that it is difficult to predict the catalytic reaction pathway for CoSe2 NBs electrode at this stage since the assumption of

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direct electron transfer from electrode (or substrate) to catalyst is rather invalid. The Tafel plot is also useful to predict the exchange current density, which is

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another important factor to evaluate the HER catalytic performances of electrodes. The exchange current density (j0) for CoSe2 NS@CP is 1.58×10-3 mA

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cm-2, which is about 14 times larger than that of CoSe2 NBs electrode. Yet the

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Pt/C still shows the state-of-art performance towards HER with j0 = 0.05 mA cm-2. The potential at I = 10 mA cm-2, Tafel slope and exchange current density for different electrodes are summarized in Table 1. Accelerated degradation test (ADT) was performed in order to test the stability of CoSe2 NS@CP electrode for HER reactions. The test was performed by cyclic voltammetry (CV) at 100 mV s-1 for 500 cycles. As shown in Fig. 4c, slight deviation was observed after 500 cycles compared to the first cycle, suggesting that reasonable stability for CoSe2 NS@CP electrode towards HER. Similar conclusion can be derived by the characterization of stability using i-t curve. As shown in Fig. 4d, the current density remained approximately 7 mA cm-2 after 24 h at a constant potential of 0.16 V vs. RHE. Low current decrement indicates a highly stable catalytic electrode for HER. The morphological and XRD characterization were subsequently performed after 24 h of electrochemical HER. As shown in Fig. S9, 2D nanosheet structure barely changed after 24 hours of electrolysis. XRD data (Fig. S10) demonstrated the

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characteristic peaks of (210) and (221) plane of CoSe2, suggesting the crystal structure of CoSe2 remained unchanged during HER. Thus, the CoSe2 NS@CP electrode demonstrated excellent stability in electrochemical HER.

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Table 1 kinetic data of CoSe2 NBs and CoSe2 NS@CP for HER

3.3 CoSe2 NS@CP electrocatalytic performance towards OER

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Electrocatalysis of the OER is much more difficult than the HER since the former process involves a four proton-coupled-electron-transfer process and an O-O

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bond formation. Therefore, the OER process are usually challenging both

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thermodynamically and kinetically. Thus, OER usually determines the overall rate of water electrolysis. Similar to the HER experiments, the effects of synthetic conditions of CoSe2 NS@CP towards OER were investigated in 1 mol dm-3 KOH aqueous solution. The optimal volume ratio between DETA and water was 1:1, with OER potential of +1636 mV vs. RHE at 100 mA cm-2 (Fig. S11). The loading percentages of CoSe2 NS on CP were 0.14 wt%, 0.32 wt%, 0.63 wt%, 0.76 wt% and 0.87 wt%, whereas 0.63 wt% performed the best OER catalytic activity (Fig. S12). Fig. S13 shows the comparison of OER activities over CoSe2 NS@CP obtained at different reaction durations, showing that the sample obtained at 14 h has the highest activity. It is worth noting that the optimal synthetic conditions of CoSe2 NS@CP for OER activity is identical to HER activity, suggesting that the fast and efficient charge transport is important no matter what catalytic processes. Consequently, the following OER tests were also carried out with the optimized CoSe2 NS@CP electrode.

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Figure 5 (a) LSV polarization curves of OER for CoSe2 NS@CP, CP plate and CoSe2 NBs in 1 mol dm-3 KOH at scan rate of 5 mV s-1. (b) the corresponding Tafel plots for the various catalysts derived from (a). (c)LSV polarization curves recorded from CoSe2 NS@CP with a sweep rate of 5 mVs-1 before and after 500 consecutive cycles between +0.9 V and +1.8 V vs. RHE at 100 mV s-1

d

(iR compensated). (d) Time-dependent current density curve of the CoSe2 NS@CP under a static

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potential of +1.5 V vs. RHE for 24 h.

The polarization curve of CoSe2 NS@CP towards the OER was compared with pure CP and CoSe2 NBs. As shown in Fig. 5a, the pure CP and CoSe2 NBs electrodes show negligible OER catalytic activity, whereas CoSe2 NS@CP exhibits excellent performance with a low onset potential of +1521 mV vs. RHE (Fig. S1b), and low electrode potential of +1580 mV vs. RHE and +1636 mV vs. RHE at 10 mA cm-2 and 100 mA cm-2 catalytic current density, respectively. It can be concluded that the catalytic activity of electrocatalysts depends not only on their structure, but the electron transfer mediator (supporting substrate) is also important. Furthermore, the electrode kinetics was evaluated by Tafel plots, as shown in Fig. 5b. Compared with the large Tafel slopes of pure CP (397.3 mV dec-1) and CoSe2 NBs (292.1 mV dec-1), the CoSe2 NS@CP electrode shows much

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lower Tafel slope of 34.5 mV dec-1, suggesting fast electrode kinetics of CoSe2 NS@CP electrode towards OER. The detailed kinetic information of catalyst is summarized in Table 2. It is obvious that the existence of CP support significantly accelerates the electron transfer between the electrode and catalyst, thus increase the electrode kinetics dramatically. Similar to the HER experiments, ADT was

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performed for the OER catalytic process (Fig. 5c). Slight deviation of anodic current was observed after 500 consecutive cycles. The small deviation of anodic

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current can be attributed to the physically absorbed O2 gas bubbles, resulting in

blockage of active sites of the catalyst, thus slow the electrode kinetics. Fig. 5d

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shows that the current density remained at 7.5 mA cm-2 at static potential of +1.50 V vs. RHE for 24 h. The almost flat line suggests that the CoSe2 NS@CP exhibited

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excellent stability towards OER. Fig. S14 shows the SEM image of CoSe2 NS@CP after 24 hours of electrochemical OER at 1.50 V vs. RHE. Although slight

M

deformation was observed, the 2D nanosheet morphology remained, suggesting reasonable stability of CoSe2 NS@CP in OER. The crystal structure of CoSe2 after

d

OER was further confirmed by XRD (Fig. S15), suggesting a stable crystal

Ac ce pt e

structure of CoSe2 during OER.

Table 2 kinetic data of CoSe2 NBs and CoSe2 NS@CP for OER

3.4 Electrochemical characterization of CoSe2 NS@CP electrode The electrochemical characterization of CoSe2 NS@CP electrode was performed in order to understand the relationship between effective surface area or surface conductivity and the catalytic activities. It is known that CV at non-Faradic potential windows can be used to determine the double layer capacitance (Cdl),

Page 21 of 32

which can be related to the specific electrode area.[67] As shown in Fig. 6a and b, the current densities of CoSe2 NS@CP and CoSe2 NBs increase with increasing scan rates. The corresponding plot of current density against scan rates (Fig. 6c) provide insight information of Cdl, with 1078 µF cm-2 and 24 µF cm-2 over CoSe2 NS@CP and CoSe2 NBs respectively. Since Cdl is directly proportional to surface

ip t

area, therefore it is clear that the effective surface area of CoSe2 NS@CP is nearly 45 folds larger than CoSe2 NBs, implying large availability of active surface areas

cr

of CoSe2 NS@CP toward catalytic reactions. It was mentioned previously that the

us

excellent catalytic activity of CoSe2 NS@CP is attributed to not only the specially designed lamellar structure with expanded interlayer spacing, but also the fast-electron-transfer between CP and CoSe2 NS. This conclusion was further

an

confirmed by EIS tests. The Nyquist plot in Fig. 6d shows that CoSe2 NS@CP provides a much smaller charge transfer resistance (Rct = 2.3 Ω) compared with

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that of CoSe2 NBs, indicating fast electron transfer kinetics. The Nyquist plot of pure CP was also performed and shown in the same graph, but due to the nature

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of conductive substrate itself, there is no semicircle observed.

Page 22 of 32

Figure 6 (a) CV curves of CoSe2 NS@CP with variable scan rates. (b) CV curves of CoSe2 NBs with variable scan rates. (c) the corresponding current density against scan rate plots for CoSe2 NS@CP (a) and CoSe2 NBs (b). (d) EIS Nyquist plots of CoSe2 NS@CP, CoSe2 NBs and CP. All electrochemical analysis were performed in 0.5 mol dm-3 H2SO4 using a three electrode configuration.

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3.5 Overall electrochemical water splitting over CoSe2 NS@CP electrocatalyst The aforementioned analysis (i.e. HER and OER) suggests that the self-supported

cr

CoSe2 NS@CP electrode can readily be served as both HER and OER

electrocatalysts. Therefore, it is worth to examine its bifunctionality in the same

us

apparatus. Fig. 7a shows the polarization curve of the HER and OER in 1.0 mol dm-3 KOH on CoSe2 NS@CP electrode. The onset potential towards HER was

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-147 mV vs. RHE and OER was +1460 mV vs. RHE. Accordingly, the overall electrolysis potential (ηo) can be defined as ηo = 147 mV + 1460 mV = 1607 mV.

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Therefore, the potential of electrolysis of water was selected to be greater than 1.6 V accordingly. The electrolyser with two-electrode configuration showed that the

d

electrolysis starts at 1.75 V and the current density increased dramatically after the

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electrolysis potential (Fig. 7b). The steep slope suggests that large current density can be achieved at a relatively low cell potential. Indeed, the cell shows extremely small electrolysis potential at 2.13 V to achieve current density of 100 mA cm-2. Normally, electrochemical water splitting process is limited by the slow four-electron-transfer OER process, which can cause a large gradient. The steep slope of our catalytic electrode indicates fast anodic process, thus it is highly desirable for large-scale water splitting. The capability of the water electrolyzer was examined at various current densities from 10 mA cm-2 to 100 mA cm-2, as shown in Fig. 7c. The cell voltage increases accordingly as the increment of current density. It is worth noting that the potential stabilize rapidly after each increment of current density, indicating outstanding stability. When the current density was dropped to 20 mA cm-2 from 100 mA cm-2, the cell potential restored well and almost has the identical cell potential to the former 20 mA cm-2 current density. As mentioned in the previous

Page 23 of 32

paragraph, 1.6 V was calculated to be the minimal cell voltage to drive electrolysis process. In order to test the performance of electrolyzer at the minimal cell voltage, 1.6 V was applied between the two identical electrodes. As shown in Fig. 7d, even at the minimal cell voltage, large amounts of gas bubbles were observed at both electrodes (shown as a video in the supplementary video). Thus, the

ip t

bifunctional CoSe2 NS@CP electrode showed fabulous catalytic performance

towards electrochemical overall water splitting. Furthermore, the chemical stability

cr

of CoSe2 NS@CP during overall water splitting process was confirmed by atomic

us

absorption spectroscopy. Trace amount of Co was detected and remained at low concentration level in electrolyte solution (Table S1), suggesting good chemical stability of CoSe2 NS@CP electrode during overall water splitting process in alkali

an

condition.

In addition, according to the previous published papers, the valence of Co and

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Se could be changed after HER and OER process.[19, 68] Therefore, XPS analysis were acquired for both electrodes after the overall water splitting process for 24

d

hours. As shown in Fig. S16a, the anode showed high valence Co3+ appeared after

Ac ce pt e

electrolysis for 24 hours. As previously reported, the real catalytic active material for OER process was the CoOOH formed on the surface.[69-71] This was evidenced by the appearance of Co3+ with the increased peak intensity of SeOx (Fig. S16b).[19] Besides, the appearance of satellite peaks in Fig S16a indicates the formation of Co2+, which is due to the formation of Co(OH)2 or CoO on the electrode surface.[72] Since the CoSe2 NS@CP cathode was kept under reduced condition, no Co3+ peaks were observed (Fig. S17a). The appearance of satellite peaks is caused by the formation of Co2+ (i.e. Co(OH)2 or CoO), which is reasonable since the catalytic process was performed under alkaline condition. Again, as shown in Fig. S17b, the increased peak intensity for SeOx could be attributed to the oxidation of Se at the electrode surface under strong alkaline condition.[19] Thereby, it can be concluded that both Co2+ and Co3+ is appeared after OER process in alkaline condition whereas only Co2+ appeared after HER

Page 24 of 32

process. The OER process was attributed to CoOOH. Se was further oxidised

d

M

an

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cr

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after both HER and OER process due to the strong alkaline conditions.

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Figure 7 (a) Polarization curve recorded in a three-electrode configuration in a wide potential widow (-0.9 V to +2.4 V vs. RHE) showing the CoSe2 NS@CP toward both HER and OER. (b) Polarization curve recorded at a scan rate of 5 mV s-1 in a two-electrode configuration. (c) Chronopotentiometric curve of water electrolysis at several different current densities. (d) A digital photograph showing the evolution of H2 and O2 gas from the electrodes at 1.6 V cell voltage. 1.0 mol dm-3 KOH aqueous solution was used as electrolyte for water splitting tests.

Page 25 of 32

Conclusions In conclusion, interlayer expanded CoSe2 NS on carbon paper as a self-support catalytic electrode were successfully fabricated via a solvothermal method. We

ip t

demonstrated that DETA played vital role in the formation of lamellar structure

of CoSe2 with interlayer spacing expanded to 1.05 nm and 0.57 nm thickness on

cr

each layer. The expanded interlayer spacing and special structure led to enhanced

us

catalytic activities. This rationally designed CoSe2 NS@CP electrode showed excellent catalytic activity towards both HER and OER. Besides, this catalytic electrode was applied in a two-electrode electrolyser and showed excellent

an

performance for electrochemical overall water splitting, with only 2.13 V of cell voltage needed to drive 100 mA cm-2 electrolysis current density. Finally, the

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durability tests showed that the CoSe2 NS@CP electrode exhibited remarkable

Ac ce pt e

more than 30 hours.

d

stability with negligible increase of potential at current density of 20 mA cm-2 for

Page 26 of 32

Acknowledgements The authors thank the financial supports by National Natural Science Foundation of China (Grant No. 21471160) and Fundamental Research Funds for the Central

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Universities (14CX02154A, 16CX05014A, 16CX05016A, 14CX05037A and

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d

M

an

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cr

15CX05045A).

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