Control Synthesis of Nickel Selenides and their Multiwalled Carbon

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Control Synthesis of Nickel Selenides and their Multiwalled Carbon Nanotubes Composites as Electrocatalysts for Enhanced Water Oxidation Shuai Chen, Jianli Mi, Peng Zhang, Yonghai Feng, Yang-Chun Yong, and Weidong Shi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09259 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 2018

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The Journal of Physical Chemistry

Control Synthesis of Nickel Selenides and their Multiwalled Carbon Nanotubes Composites as Electrocatalysts for Enhanced Water Oxidation

Shuai Chen,† Jian-Li Mi,*,† Peng Zhang,*,† Yong-Hai Feng,† Yang-Chun Yong,‡ and Wei-Dong Shi§ †Institute

for Advanced Materials, School of Materials Science and Engineering, Jiangsu University,

Zhenjiang 212013, China ‡Biofuels

Institute, School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang

212013, China §School

of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China

ABSTRACT: Development of efficient and noble metal-free catalysts for oxygen evolution reaction (OER) is of great importance for practical energy-related technologies. Herein, one-step hydrothermal method is developed for the synthesis of a series of nickel selenides (NiSe2, NiSe and Ni3Se2) and their multiwalled carbon nanotubes (MWCNTs) nanocomposites as electrocatalysts for OER. Among the NiSe2, NiSe and Ni3Se2 catalysts, NiSe shows the highest activity, exhibiting an overpotential of 389 mV to achieve 10 mA/cm2 and a Tafel slope of 93 mV/dec in 1 M KOH. We further report a facile preparation of nickel selenides nanocrystals (particle size of ~10 nm) grown on MWCNTs. The NiSe/MWCNTs nanocomposite shows improved activity towards OER with an overpotential of 336 mV to reach 10 mA/cm2 and a Tafel slope of 78 mV/dec, which is close to the state-of-the-art RuO2 catalyst. The addition of MWCNTs during 1

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the synthesis not only introduces an abundance of nucleation sites, resulting in small particle sizes of NiSe crystals, but also improves the dispersion of NiSe nanoparticles. The small particle size and good dispersion of NiSe nanoparticles are beneficial to providing abundant active catalytic sites. In addition, the porous and conductive MWCNTs facilitate the electronic transmission and the electrolyte penetration. Furthermore, NiSe/MWCNTs nanocomposite also exhibits good long-term stability.

1. INTRODUCTION To meet the increasing global demands for energy in the coming years, it is eager to explore alternative clean and sustainable renewable energy sources to replace fossil fuels.1,2 Hydrogen, other than being a chemical raw material, is regarded as one of the most potential candidates of clean and renewable energy.3,4 Electrochemical water splitting provides an ideal way to generate sustainable hydrogen energy.5,6 However, the efficiency of water electrolysis is greatly limited by the slow kinetics of the oxygen evolution reaction (OER) because it involves a multistep four-electron process leading to high overpotential for the whole water splitting.7 Therefore, stable and effective electrocatalysts are required to promote the kinetics of OER. Up to date, precious metal oxides such as RuO2 and IrO2 exhibit benchmark electrocatalytic activity toward OER to overcome the large overpotentials,8 but the low abundance, expensive prices and low stability hinder their widespread practical applications.9,10 Consequently, the investigation for efficient, stable and earth-abundant transition metal electrocatalysts for OER is still urgently required.11 Over the past years, Fe,12,13 Co,14,15 and Ni16,17 based materials are considered as the most likely alternatives to conventional noble metal catalysts for OER due to the low price, high elemental abundance, attractive electronic properties, good catalytic activity, and high stability. In particular, Ni-based materials, 2


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including oxides,18 hydroxides,19 oxyhydroxides,20 chalcogenides,21 phosphides,22 nitrides,23 etc, have been extensively studied for OER electrocatalytic activities. Recently, nickel selenides-based catalysts24-26 are reported as non-precious OER catalysts in alkaline environments and have aroused widespread concern due to the variety of morphological structures (nanowires, nanoparticles, nanosheets, and hexagons) and designable electrocatalytic activities. However, bare nickel selenides and other Ni-based catalysts always exhibit low OER activities due to the low electrical conductivity and the limited active catalytic sites.27 Depositing on conductive substrates with 3D porous structure is an effective method to improve the electrocatalytic activity of catalysts. For examples, Ti plate,25 porous gold,28 and nickel foam26 have been used as conductive substrates to support the active nanomaterials. These 3D porous structures used as supports have been proven to have the advantages of providing favorable charge transportation and electrochemical active surface area during the process of OER. Besides, carbon materials such as carbon nanotubes (CNTs) are widely used as conductive supports to provide the high specific surface area and improve the electronic conduction of the modified material.29-31 It can significantly increase the number of catalytic active sites and intrinsic catalytic activity with nanoparticles adhering to the CNT surface. The electrocatalytic activity can be considerably influenced by the chemical composition, crystal structure, particle size and size distribution, electrical conductivity, and so on. There are different types of nickel selenides, including NiSe2, NiSe and Ni3Se2, each of which has been reported as non-precious OER catalyst.25,26,32 However, the comparative study of these materials towards OER activity is still rare and the structure-related property of nickel selenides is unknown. In this work, we report a facile one-step hydrothermal method for the preparation of different nickel selenides (NiSe2, NiSe and Ni3Se2) and their multiwalled carbon nanotubes (MWCNTs) nanocomposites. Detailed structural and morphogical 3



characterizations have been carried out, and an attempt has been made to comparatively study the OER activity of different nickel selenides.

2. EXPERIMENTAL SECTION 2.1. Synthesis of nickel selenides and nickel selenide/MWCNTs nanocomposites. MWCNTs, potassium hydroxide (KOH), sulfuric acid (H2SO4), nitric acid (HNO3), hydrazine hydrate (N2H4·H2O, 80%) and ruthenium oxide (RuO2) were purchased from Sigma-Aldrich. Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), sodium selenite(Na2SeO3) and ethylene diamine tetraacetic acid (EDTA) were purchased from Alfa Aesar. MWCNTs were oxidized by adding 200 mg of MWCNTs into H2SO4 (8 mL) and HNO3 (4 mL) mixture solution in a round bottom flask. The mixture solution was stirred at 70 °C overnight, and diluted with plenty of water. Oxidized MWCNTs were collected and washed repeatedly with deionized water and ethylalcohol by centrifugation and then dried at 60 °C for 6 h. Nickel selenides nanomaterials were synthesized by an improved process of hydrothermal method based on the synthesis proposed by Li’s group.33 In a typical synthesis of NiSe2, molar ratio between Ni(NO3)2·6H2O and Na2SeO3 was chosen to be 1:3, where excessive Na2SeO3 was required in the precursor. 1 g of EDTA was dissolved in 20 mL of distilled water in a Teflon-lined, stainless-steel autoclave of 50 mL capacity. 0.519 g of Na2SeO3 was added under stirring at room temperature for 20 min, and suitable amount of NaOH was added to keep the PH value at about 10. After that, 0.291 g of Ni(NO3)2·6H2O was added to the above mixed aqueous solution. Finally, excessive N2H4·H2O (10 mL) was added drop-wise to the mixed solution. The autoclave was then sealed and heated to 140 °C and 4


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maintained for 20 h. After the reaction, the autoclave was cooled to room temperature naturally. The black product was collected and washed repeatedly with distilled water and absolute ethanol, and then dried at 60 °C for 6 h. For the synthesis of NiSe and Ni3Se2, the molar ratios of Ni(NO3)2·6H2O and Na2SeO3 were 1:1 and 3:2, respectively. The synthesis is similar to that of NiSe2. However, NaOH is not added, and the amounts of distilled water and N2H4·H2O are 30 mL and 3 mL, respectively. The experiments were performed under 120 °C for 12 h. NiSe/MWCNTs, NiSe2/MWCNTs and Ni3Se2/MWCNTs were synthesized under the same reaction conditions of NiSe, NiSe2 and Ni3Se2, respectively. A certain amount of MWCNTs was added for the synthesis of each nickel selenide/MWCNTs sample. Different nickel selenide/MWCNTs samples were prepared and the weight percentages of MWCNTs in nickel selenide/MWCNTs samples were nominally designed to be 5 wt%, 10 wt%, 20 wt%, and 30 wt%, respectively. Physical mixture of NiSe and MWCNTs (NiSe+MWCNTs) was also prepared by sonicating NiSe with certain amount of MWCNTs in distilled water, followed by washing and drying. 2.2 Characterizations. The X-ray diffraction (XRD) was performed on a D8 ADVANCE diffractometer with Cu K radiation (λ = 1.5406 Å). The XRD data were refined using the Rietveld method implemented in the FullProf program.34 The morphology of the products was observed on a Hirachi, S-4800 scanning electron microscope (SEM). High-resolution transmission electron microscopy (HRTEM) images were obtained using a FEI, Tecnai G2 F30 S-TWIN microscope with an accelerating voltage of 300 kV. The chemical compositions were analyzed on the energy-dispersive X-ray spectroscopy (EDS) attached to the FEI Tecnai G2 F30 S-TWIN microscope. X-ray photoelectron spectra (XPS) data were obtained on an EscaLab 250Xi with Al K. 5



2.3. Electrochemical measurements. Electrochemical measurements were carried out using a rotating disk working electrode made of glassy carbon (PINE, 5 mm diameter) connected to a CHI 760E electrochemical workstation (Chenhua Instrumental Co., Shanghai China). Plantinum wire and Ag/AgCl were used as counter and reference electrodes, respectively. To prepare the working electrode with the catalysts loaded, 4 mg of catalyst powder was dispersed in 960 μL of water and ethanol solution (3:1, v/v), followed by the addition of 40 μL of Nafion (5 wt%) under sonication to form a homogeneous ink. 10 μL of suspension was dropped onto the glassy carbon rotating disk, leading to the catalyst loading about 0.2 mg/cm2. The as-prepared working electrode was dried at room temperature. O2-saturated 1 M KOH solution was used as the electrolyte at room temperature. All measured potentials were referenced to Ag/AgCl (measured) and converted to the reversible hydrogen electrode (RHE) scale according to the equation of E (RHE) = E (Ag/AgCl) + 1.023 V. All currents presented were corrected with a compensation level of 95% by eliminating iR drop of the solution resistance. Polarization curves were obtained using linear sweep voltammetry (LSV) at the scan rate of 10 mV/s by rotating disk electrode (RDE) at 1225 rpm. Tafel slope (b) was derived by fitting the portion the Tafel curve according to Tafel equation (η = a + b logj). To estimate the electrochemical double-layer capacitance (Cdl), cyclic voltammograms (CV) measurements were taken by sweeping the potential across the non-faradic region between 0.97 V and 1.07 V vs RHE (iR-uncompensated) at various scan rates (20, 40, 60, 80, 100, 120, 140, 160, 180, and 200 mV/s). Electrochemical impedance spectroscopy (EIS) experiments were carried out in the frequency range of from 104 Hz to 10-2 Hz, and the amplitude of 10 mV. The stability was examined by continuous CV tests between 1.22 V and 1.67 V vs RHE (iR-uncompensated). Time dependent current density of the catalyst was performed at a potential of 1.57 V (iR-uncompensated) for 24 h in O2-saturated 1 M KOH solution 6


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with the catalyst loading of ~ 0.5 mg/cm2. To prevent the catalyst falling off from the glassy carbon electrode, carbon fiber paper electrode was used for the time dependent experiment.

3. RESULTS AND DISCUSSION Figure 1 shows the three commonly observed crystal structures of nickel selenides: cubic NiSe2 with space group of Pa-3, hexagonal NiSe with space group of P63/mmc, and rhombohedral Ni3Se2 with space group of R32. Figure 2 shows the Rietveld refinement results of the XRD patterns of NiSe2, NiSe and Ni3Se2 prepared by hydrothermal synthesis. We have found that the synthesis of phase pure NiSe2 is relative difficult compared with that of NiSe and Ni3Se2. A higher reaction temperature of 140 °C and a longer reaction time of 20 h were applied for the synthesis of NiSe2, and there is still a small amount of NiSe coexisted in the NiSe2 sample. Pure NiSe and Ni3Se2 samples can be easily obtained at 120 °C for 12 h. The calculated unit cell parameters from the Rietveld refinements are a = b = c = 5.956(1) Å for cubic NiSe2, a = b = 3.625(1) Å, c = 5.319(1) Å for hexagonal NiSe, and a = b = 6.020(1) Å, c = 7.257(1) Å for rhombohedral Ni3Se2, respectively. The crystallite sizes were calculated by the Scherrer equation through fitting the peaks by a Lorentz function and averaged from different reflections (a silicon standard was used to account for the instrumental broadening). The crystallite sizes are around 33, 25 and 26 nm for NiSe2, NiSe and Ni3Se2, respectively, indicating that NiSe2 has a relatively larger average crystallite size than NiSe and Ni3Se2. The representative SEM images are shown in Figure 2d-f. The particle sizes shown in the SEM images are relatively larger than those calculated from the XRD indicating that the particles are highly aggregated. Moreover, it is seen that NiSe2 shows larger particle sizes than NiSe and Ni3Se2, indicating different nucleation and growth kinetics of NiSe2. The different synthesis condition such as 7



larger amount of N2H4·H2O used, relative higher reaction temperature and longer reaction time for NiSe2 also affect the crystallization kinetics, resulting the larger particle size.

Figure 1. Crystal structures of (a) NiSe2, (b) NiSe, and (c) Ni3Se2.

Figure 2. (a-c) Rietveld refinements with observed, calculated and difference XRD patterns of NiSe2, NiSe, and Ni3Se2, respectively. (d-f) SEM images of NiSe2, NiSe, and Ni3Se2, respectively.

The OER electrocatalytic performance of NiSe2, NiSe and Ni3Se2 nanocatalysts was investigated in O2-saturated 1 M KOH at room temperature. Figure 3a shows the polarization curves of different samples after iR correction. The oxidation peaks around 1.37 V (vs RHE) is a Ni2+ to Ni3+ redox reaction couple.35 The results show that NiSe exhibits the best activity towards OER, and it requires an overpotential of 389 8


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mV to attain the current density of 10 mA/cm2. NiSe2 and Ni3Se2 require the overpotentials of 427 mV and 469 mV, respectively, to reach 10 mA/cm2. The Tafel plots of the three catalysts are presented in Figure 3b. The Tafel slope increases in the order of NiSe (93 mV/dec) < Ni3Se2 (120 mV/dec) < NiSe2 (136 mV/dec), indicating the fastest OER kinetics of NiSe. The electrocatalytic activity for OER can be considerably influenced by the chemical composition and crystal structure. It is suggested that the efficient OER catalysts should be designed by considering the bond energies of the intermediates formed and the electronic structure through modification of the chemical composition and crystal structure of the catalyst.36,37 Se heteroatoms in nickel selenides catalysts do not have any direct influence in enhancing the OER kinetics. However, because of the negative charge localized on Se sites and the cumulative 3d-2p repulsion between the metal d-band center and Se d-band center, the delivery of the dioxygen molecule may become faster, resulting in nickel selenides attractive OER catalysts. From the crystal structures, it can be seen that Ni atoms have different coordination environments in the structures of NiSe2, NiSe and Ni3Se2. As obtained from the XRD refinements, in NiSe2, Ni atom is coordinated by six Se atoms to form six Ni-Se bonds with the lengths of 2.468(1) Å. However, in NiSe, Ni atom is coordinated by six Se atoms and other two Ni atoms, and the bond lengths are 2.480(1) Å and 2.660(1) Å for Ni-Se and Ni-Ni, respectively. And in Ni3Se2, Ni atom is coordinated by four Se and four Ni atoms, with two Ni-Se bonds of 2.387(1) Å, two Ni-Se bonds of 2.429(1) Å, two Ni-Ni bonds of 2.390(1) Å, and two Ni-Ni bonds of 2.660(1) Å. As the suggested catalytic mechanisms of nickel selenides, it is expected that the polymorphs with higher Se content are the possibly higher active catalysts due to the increased number of Ni-Se bonds and the enhanced cumulative 3d-2p repulsion. Nevertheless, direct comparison of the OER intrinsic catalytic property of the catalysts is difficult due to the variation in particle size of the materials. As expected, the 9



experimental results show that NiSe with higher Se content has better OER electrocatalytic performance than Ni3Se2, where NiSe and Ni3Se2 have similar particle sizes as suggested by the XRD and SEM results. However, it is suggested that NiSe shows better OER activity than NiSe2 as a result of the increased number of active catalytic sites ascribed to the smaller particle size.

Figure 3. (a) Polarization curves of NiSe, NiSe2 and Ni3Se2 under 1 M KOH at a scan rate of 10 mV/s, and (b) corresponding Tafel plots.

In order to further improve the electrocatalytic properties of nickel selenides, nickel selenide/MWCNTs nanocomposites were synthesized by a simple one-step hydrothermal synthesis where MWCNTs were added during the synthesis. First, a series of NiSe/MWCNTs nanocomposites with different contents of MWCNTs (5 wt%, 10 wt%, 20 wt%, and 30 wt%) were prepared. The XRD patterns of NiSe/MWCNTs composites with different contents of MWCNTs are shown in Figure S1a. It clearly shows the characteristic diffraction peaks of NiSe indicating the formation of NiSe phase in the composites. The diffraction peak around 26° belongs to the (002) reflection of graphite. The crystallite size of NiSe in NiSe/MWCNTs (20 wt%) calculated by the Scherrer equation is about 15 nm, which is much smaller than that of bare NiSe sample. This could be explained by the fact that the supporting MWCNTs introduce an abundance of nucleation sites, and therefore MWCNTs play an important role in control of the particle size 10


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and size distribution of the catalysts during the synthesis. The morphology of NiSe/MWCNTs (20 wt%) was investigated by TEM (Figure 4 and Figure S2). Figure 4a shows that NiSe/MWCNTs are consisted of aggregated NiSe nanoparticles and one dimensional carbon nanotubes. The aggregated NiSe nanoparticles are interwoven with MWCNTs. The typical size of aggregated NiSe nanocrystals is around 10 nm as shown in Figure 4b and Figure S2a, which agrees with the XRD results. Besides the aggregated nanoparticles, many NiSe nanocrystals are adhered to or embedded in the MWCNTs (Figure 4c, Figure S2b and S2c), forming a synergistic interplay at the interfaces between MWCNTs and NiSe nanocrystals. The HRTEM image (Figure 4d) clearly shows the well-resolved 2D diffraction fringers which can be assigned to the [2-21] view direction of NiSe crystal structure. EDS analysis shows that the atom ratio of Ni to Se is about 1:1.06, indicating that NiSe nanocrystals have a good stoichiometry in the NiSe/MWCNTs composite.

Figure 4. (a-c) Typical TEM images of NiSe/MWCNTs (20 wt%). (d) HRTEM image of a NiSe

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nanocrystal and the detailed atomic arrangement illustrated with a view direction of [2-21].

XPS was used to further confirm the chemical composition and the valence of the surface elements of NiSe/MWCNTs (20 wt%) composite. The XPS survey spectrum (Figure 5a) demonstrates the presence of C, O, Ni and Se elements in the NiSe/MWCNTs (20 wt%). The C element mainly originates from the MWCNTs and the O element is probably from the surface oxygen functional groups. Figure 5b shows the C 1s spectrum and the peaks located at 284.8 eV and 285.4 eV can be assigned to the chemical bonds of C-C and C-O, respectively.38 Figure 5c shows the XPS spectrum of Ni 2p. The peaks at 856.8 eV and 874.6 eV correspond to Ni 2p1/2 and Ni 2p3/2, respectively, indicating the Ni2+ oxidation.39 The peaks at 862.1 eV and 880.4 eV are ascribed to the satellite structures. In Figure 5d, the peak 54.7 eV is associated with Se 3d indicating the formation of Ni-Se bonds. The broad peak near 59.6 eV suggests the surface oxidation state of Se.39 All the characterizations indicate the successful synthesis of NiSe/MWCNTs nanocomposite.

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Figure 5. XPS spectra of the NiSe/MWCNTs (20 wt%) catalyst: (a) survey spectrum, (b) C 1s, (c) Ni 2p, and (d) Se 3d.

The influence of MWCNTs contents on the OER activities of NiSe/MWCNTs nanocomposites was investigated. The LSV curves of NiSe/MWCNTs with various contents of MWCNTs (5 wt%, 10 wt%, 20 wt%, and 30 wt%) are shown in Figure S1b. It can be seen that compared with the bare NiSe sample, NiSe/MWCNTs nanocomposites exhibit improved electrocatalytic performance. The overpotentials of 389 mV, 381 mV, 374 mV, 336 mV, and 363 mV are required to reach 10 mA/cm2 for NiSe, NiSe/MWCNTs (5 wt%) NiSe/MWCNTs (10 wt%), NiSe/MWCNTs (20 wt%), and NiSe/MWCNTs (30 wt%), respectively. The OER activity of NiSe/MWCNTs nanocomposites becomes better with the increasing MWCNTs content from 0 to 20 wt%. On one hand, the conductivity of the catalyst will be improved with the increasing content of MWCNTs, and this facilitates the faster electronic transmission. On the other hand, with the increasing MWCNTs content, the dispersion of NiSe nanoparticles will be improved, providing more active catalytic sites in the catalyst. However, when the MWCNTS content reaches 30 wt%, the OER activity decreases. It can be attributed to the reduced density of active catalytic sites because of the decreasing NiSe content in the catalyst. The results show that NiSe/MWCNTs (20 wt%) catalyst exhibits the best OER activity among the studied nanocomposites. Figure 6a shows the polarization curves of MWCNTs, NiSe, NiSe/MWCNTs (20 wt%), NiSe+MWCNTs (20 wt%), and commercial RuO2. The current density of MWCNTs is nearly zero within the investigated potential window, while NiSe exhibits an overpotential of 389 mV to reach a current density of 10 mA/cm2, confirming that the active catalytic sites are from NiSe. Compared with NiSe, the 13



physical mixture NiSe+MWCNTs (20 wt%) shows improved OER activity with an overpotential of 359 mV for 10 mA/cm2. The addition of MWCNTs enhances the conductivity of the catalyst and accelerates the electron transfer rate. The one step synthesized nanocomposite NiSe/MWCNTs (20 wt%) shows even better OER activity, indicating that NiSe growing on MWCNTs is more efficient than physically mixed NiSe+MWCNTs in improving catalytic activity for water oxidation. NiSe/MWCNTs (20 wt%) only requires an overpotential of 336 mV to reach a current density of 10 mA/cm2, which is comparable to that of RuO2 (332 mV). Despite of the small catalyst loading (0.2 mg/cm2), the overpotential of NiSe/MWCNTs (20 wt%) is also better than previously reported Ni(OH)2/MWCNTs (~430 mV for 5 mA/cm2),19 and comparable to NiO/C (335 mV for 10 mA/cm2),40 NiSe/nickel foam (~390 mV for 50 mA/cm2),24 and CoTe2 (357 mV for 10 mA/cm2).41 As shown in Figure 6b, the corresponding Tafel slopes derived from the LSV curves of NiSe, NiSe/MWCNTs (20 wt%), NiSe+MWCNTs (20 wt%), and RuO2 are 93 mV/dec, 78 mV/dec, 116 mV/dec, and 86 mV/dec, respectively. The above results indicates that NiSe/MWCNTs (20 wt%) exhibits promising OER activities comparable to that of RuO2. The improved activity towards OER of NiSe/MWCNTs (20 wt%) can be interpreted as follows. As suggested above, the addition of MWCNTs during the synthesis introduces an abundance of nucleation sites, resulting in small particle sizes of NiSe crystals in NiSe/MWCNTs samples. Moreover, the addition of MWCNTs prevents the agglomeration of NiSe nanoparticles during the synthesis and other processes, and significantly improves the dispersion of NiSe nanoparticles. The small particle size and good dispersion of NiSe nanoparticles are favorable to providing abundant active catalytic sites. In addition, the highly porous and conductive interconnected MWCNTs not only facilitate the fast electronic transmission, but also provide the fast channels for ion transfer and easy access to the electrolyte penetration. And the synergistic interfaces between MWCNTs 14


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and NiSe nanocrystals can further improve the charge transfer. Similarly, the catalysts of NiSe2/MWCNTs (20 wt%) and Ni3Se2/MWCNTs (20 wt%) show improved activity towards OER compared with NiSe2 and Ni3Se2, respectively, as shown in Figure S3 and S4. The overpotentials of 388 mV and 361 mV are required for NiSe2/MWCNTs (20 wt%) and Ni3Se2/MWCNTs (20 wt%), respectively, to reach the current density of 10 mA/cm2. The Tafel slopes are 116 mV/dec, and 67 mV/dec for NiSe2/MWCNTs (20 wt%) and Ni3Se2/MWCNTs (20 wt%), respectively. Catalytic stability is crucial for the commercial application of electrocatalysts. Figure 6c and Figure S5 show the polarization curves for OER of NiSe/MWCNTs (20 wt%) and commercial RuO2 recorded before and after 1000 and 2000 cycles of CV scans. It can be seen that the polarization curves after 1000 and 2000 CV cycles have no apparent change compared with the initial curve, implying excellent long-term cycling stability of NiSe/MWCNTs (20 wt%). Furthermore, time dependence of current density for NiSe/MWCNTs (20 wt%) was performed on carbon fiber paper electrode at a potential of 1.57 V vs RHE (iR-uncompensated). As shown in Figure 6d, no significant change is observed after 24 hours of continuous testing. The results indicate the promising long-term practical application of NiSe/MWCNTs nanocomposite.

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Figure 6. (a) Polarization curves for OER of MWCNTs, NiSe, NiSe/MWCNTs (20 wt%), NiSe+MWCNTs (20 wt%), and commercial RuO2. (b) Tafel plots of NiSe, NiSe/MWCNTs (20 wt%), NiSe+MWCNTs (20 wt%), and RuO2 derived from the polarization curves. (c) Polarization curves for OER of NiSe/MWCNTs (20 wt%) recorded before and after 1000 and 2000 cycles of cyclic voltammetry scans. (d) Time dependence of current density of NiSe/MWCNTs (20 wt%) at a potential of 1.57 V for 24 h in O2-saturated 1 M KOH solution. The catalysts were deposited on carbon fiber paper with the catalyst loading of ~ 0.5 mg/cm2.

Electrochemical surface area (ECSA) is also a general factor to evaluate the electrochemical activity. We use electrochemical double-layer capacitance (Cdl) to represent the ECSA. Figure 7a and 7b show the CV cycles of NiSe/MWCNTs (20 wt%) and NiSe, respectively, taken between 0.97 V and 1.07 V vs RHE at

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different scan rates (20 ~ 200 mV/s). Linear relationships between the scan rates and current densities are obtained and the Cdl values are the slopes of the linear lines as shown in Figure 7c. The Cdl value of 1.10 mF/cm2 is obtained for NiSe/MWCNTs (20 wt%), which is about 22 times as much as that of NiSe (0.05 mF/cm2). It suggests that NiSe/MWCNTs (20 wt%) has a much larger ECSA and more active sites than NiSe for OER. Electrochemical impedance spectroscopy (EIS) experiments were carried out in the frequency range of 104 Hz to 10-2 Hz at the amplitude of 10 mV to study the electrode kinetics. Figure 7d shows the Nyquist plots of NiSe/MWCNTs (20 wt%) and NiSe. It can be seen that NiSe/MWCNTs (20 wt%) has a smaller semicircle than NiSe in the high frequency region, suggesting that NiSe/MWCNTs (20 wt%) has a smaller electron transfer resistance at the interface between the catalyst and electrolyte. The results further confirm the improved catalytic activity by compositing MWCNTs with NiSe nanoparticles.

Figure 7. (a-b) cyclic voltammograms of (a) NiSe/MWCNTs (20 wt%) and (b) NiSe recorded between

17



0.97 V and 1.07 V vs RHE (iR-uncompensated) at various scan rates. (c) Dependence of the current density on the scan rate. The current densities j = (ja − jc)/2 at 1.02 V are plotted as a function of the scan rates, where ja and jc are the anodic and cathodic current densities, respectively. The Cdl values are the slopes of the linear fits. (d) Nyquist plots of NiSe/MWCNTs (20 wt%) and NiSe recorded on a current density of 1 mA/cm2 for OER.

4. CONCLUSIONS Nickel selenides (NiSe, NiSe2 and Ni3Se2) nanoparticles were successfully synthesized by a simple hydrothermal method. It reveals that the electrocatalytic activity for OER is considerably influenced by the chemical composition, crystal structure and crystallite size. It is found that NiSe exhibits the best activity towards OER among NiSe, NiSe2 and Ni3Se2. The addition of MWCNTs to prepare nickel selenide/MWCNTs nanocomposites can further improve the catalytic activity attributed to the enhanced number of catalytic active sites, improved conductivity and easy electrolyte penetration. The excellent catalytic activity and good long-term stability of nickel selenide/MWCNTs composites make them good candidates for alternative to conventional noble metal catalysts for OER.

ASSOCIATED CONTENT Supporting Information Additional SR-PXRD patterns, polarization curves, and TEM images of NiSe/MWCNTs nanocomposites; Polarization curves and Tafel plots NiSe2/MWCNTs (20 wt%) and Ni3Se2/MWCNTs (20 wt%); Time dependence of current density of NiSe/MWCNTs (20 wt%). This information is available free of charge via 18


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the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 51401089), and the Six Talent Peaks Project of Jiangsu Province (2016-XCL-016).

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Table of Contents Graphic

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Control Synthesis of Nickel Selenides and their Multiwalled Carbon

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