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Tuning quaternary hybride Co–Ni–S–Se composition as a bifunctional electrocatalyst for hydrogen and oxygen evolution reactions
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 6 8 5 e2 7 6 9 4
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Tuning quaternary hybride CoeNieSeSe composition as a bifunctional electrocatalyst for hydrogen and oxygen evolution reactions Xiaojiao Fang a,b, Zegao Wang b,c, Yin Wang b, Shifei Kang b,d, Zaixing Jiang a,b,**, Mingdong Dong b,* a
MIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, 150001, China b Interdisciplinary Nanoscience Center (iNANO), Aarhus University, DK-8000, Aarhus C, Denmark c College of Materials Science and Engineering, Sichuan University, Chengdu, 610065, China d Department of Environmental Science and Engineering, University of Shanghai for Science and Technology, Shanghai, 200093, China
highlights Co0.7Ni0.3S0.5Se1.5 delivers the best activity and durability for HER and OER. The flower-like network of Co0.7Ni0.3S0.5Se1.5 is favorable to a fast kinetic. The metallic nature of catalysts facilitates the transformation of electrons. The pyrite-structure and surface oxides contribute to the improvement.
article info
abstract
Article history:
Developing high-efficiency and earth-abundant electrocatalysts for electrochemical
Received 21 June 2019
water splitting is of paramount importance for energy conversion. Although tremen-
Received in revised form
dous effort has been paid to transition metal (TM) material-based electrocatalysts,
27 August 2019
rational design and controllable synthesis of fine structures to fully utilize the latent
Accepted 30 August 2019
potential of TM materials remain great challenges. We herein report a composition-
Available online 23 September 2019
tuning strategy to achieve rational structure control of quaternary CoeNieSeSe materials through a facile one-pot hydrothermal method, in which earth-abundant Ni is
Keywords:
introduced into a CoSxSe2-x matrix to optimize the morphology and electronic structure
Composition-tuning
of the quaternary electrocatalyst. Because of the introduction of Ni, this novel CoeNieS
Quaternary
eSe quaternary system shows better catalytic activity for water splitting with Tafel
CoeNieSeSe
slopes of 42.1 mV dec1 for hydrogen evolution reaction (HER) and 65.5 mV dec1 for
Hydrogen evolution reaction
oxygen evolution reaction (OER), respectively, compared with its precursor CoeSeSe
Oxygen evolution reaction
ternary system. For stability, there is negligible fading after long-term electrochemical test. Our work not only provides a novel thinking to introduce nickel into CoeSeSe ternary system by a facile hydrothermal synthesis for electrochemical water splitting,
* Corresponding author. ** Corresponding author. MIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, 150001, China. E-mail addresses: [email protected] (Z. Jiang), [email protected] (M. Dong). https://doi.org/10.1016/j.ijhydene.2019.08.257 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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but also this quaternary system realizes bifunctional catalysis and better electrochemical performance relative to the ternary counterpart. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Electrochemical water splitting is considered an efficient way to overcome environmental pollution and the global energy crisis by converting electricity into chemical fuels [1e5]. The electrochemical water splitting process includes two half reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), and both reactions are kinetically sluggish in nature [6,7]. Therefore, developing robust electrocatalysts for water electrolysis with a low Tafel slope and overpotential and excellent long-term stability are of great importance [8,9]. Currently, noble metal-based materials, such as Pt and RuO2/IrO2 have been proven to be the state-ofthe-art electrocatalysts, but their rarity and high cost seriously restrict their applications in industry. CoSe2, a member of transition-metal dichalcogenides (TMDs), has been demonstrated has a conductive metallic nature and low chemisorption energy for H2, enabling catalytic performance for the HER in acidic media [10,11]. Moreover, many recent studies reported the OER activity of CoSe2 after modification, indicating its potential to be used as a bifunctional catalyst [12,13]. Changing the composition of TMbased materials through heteroatom doping is a key approach to improve the electrocatalytic performance by tuning the morphology and electronic structure of materials. Due to the similar crystal structure and ionic radius, TM (Ni, Fe and Mn) and S can be used as substitutes of CoSe2 at cation and anion sites, respectively [14e16]. Although efforts have been made to modify CoSe2, it is still urgent to develop high-performance CoSe2-based HER/OER bifunctional electrocatalysts with high stability and low economic cost. Ternary TMDs have been widely researched as promising candidates for water splitting, yet their performance is still far from satisfactory due to the restrictions of their limited surface active sites and poor durability [17e19]. To the best of our knowledge, there are no studies exploring the potential for using quaternary CoeNieSeSe as a bifunctional HER/OER electrocatalyst. Due to the difference in the atomic radii and the synergistic effects of the constituents, compositional turning of CoSe2 with metal and nonmetal elements simultaneously may open up a new way to synthesize highly efficient electrocatalysts. Herein, we rationally design and synthesize well-ordered 3D quaternary CoeNieSeSe electrocatalysts for the HER and OER by simultaneously engineering the composition and structure of the CoSxSe2-x matrix. Benefiting from the synergistic effects of the appropriate dual tuning of Ni and S dopants in CoSe2, the electronic structure of the catalyst was significantly optimized and more active sites were exposed in the unique 3D structure, thereby leading to a superior bifunctional electrocatalytic performance. The optimized quaternary CoeNieSeSe exhibits robust catalytic activity as a bifunctional catalyst with low Tafel
slopes and overpotentials as well as long-term stability for both the HER and OER. The combination of simultaneously tailoring the chemical composition and structure of the catalyst in this work creates a new approach to develop highly efficient and low-cost bifunctional TM-based electrocatalysts.
Experimental Materials The chemicals in this paper, i.e., nickel chloride (NiCl2, SigmaAldrich, 98%), cobalt nitrate hexahydrate (Co(NO3)2$6H2O, Acros, 98%), Se powder (Se, Sigma-Aldrich, 99.5%), L-cysteine (C3H7NO2S, Sigma-Aldrich, 97%), EDTA-2Na (C10H14N2Na2O8, Sigma-Aldrich, 99%), Nafion perfluorinated resin (SigmaAldrich, 5 wt%), potassium hydroxide (KOH, Merck, 84%), Pt/C (Sigma-Aldrich, 1 wt%), and ethanol (C2H5OH, Sigma-Aldrich, 96%), were used without any further purification.
Synthesis of CoeNieSeSe According to a previous report [20], electrocatalysts exhibit superior long-term stability and catalytic performance when the atomic ratio of S:Se is close to 1:3. Therefore, we fixed the content of S and varied the content of Ni in the CoS0.5Se1.5 system to prepare the final quaternary Co1-xNixS0.5Se1.5 (denoted as CoNiSSe-x) samples by a facile one-pot hydrothermal method. The synthesis process of CoNiSSe-0.3 was as follows: First, 2 mmol Se powder was dissolved in 5 mL of a 20 M KOH solution to obtain Se2. After stirring for 1 h, 24 mmol L-cysteine, 0.3 mmol NiCl2, 0.7 mmol Co(NO3)2$6H2O and 1.6 mmol EDTA-2Na were added to the mixture under constant stirring. Then, the mixture was transferred to a 50 mL Teflonlined stainless autoclave for the hydrothermal reaction at 180 C for 16 h. After the oven cooled to room temperature naturally, the black precipitate was washed with distilled water and ethanol several times by centrifugation and dried at 60 C for 12 h. To investigate the effect of Ni on the electrochemical performance, controlled experiments with different contents of Ni were carried out (x ¼ 0.1, 0.2, and 0.4), and the as-obtained samples were denoted as CoNiSSe-0.1, CoNiSSe0.2, and CoNiSSe-0.4, respectively (shown in Table S1).
Characterization Scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, F30) were employed to determine the morphology and microstructure of the samples. High-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), energy dispersive spectroscopy (EDS), and inductively coupled plasma-
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optical emission spectrometry (ICP-OES, Agilent 730) were used to determine the existed elements and elemental composition. The composition and chemical states of the asobtained catalysts were tested by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Ka). X-ray diffraction (XRD, l ¼ 0.1541 nm, Smartlab-9) with Cu Ka as the X-ray source was used to examine the phase structure of the asprepared samples, and the range was 10e80 . Raman spectroscopy (Horiba LabRAM HR 800) was performed with an excitation light source of 514 nm.
Preparation of the working electrode To avoid absorbing the environmental moisture, 3 mg of the as-prepared CoNiSSe-x powder was weighed and dissolved in 200 mL of 17:3 v/v water/ethanol solution in a glove box. Then, 20 mL Nafion was added and the mixture was ultrasonicated for 1 h to obtain a homogeneous catalyst ink. After that, 5 mL of the ink was drop-cast on glassy carbon electrodes (GC, 3 mm in diameter), which have already been polished with sand paper (5000 mesh) and alumina powder (1.0 and 0.1 mm). The catalyst films were dried at room temperature naturally and the loading were about 0.96 mg cm2. Moreover, a Pt/C working electrode was prepared as a control.
Electrochemical measurements All the electrochemical measurements were performed with a standard three-electrode cell using a CHI660E (CHI Instruments, Shanghai Chenhua Instrument Corp., China) at room temperature (shown in Fig. S1). The GC with the catalysts served as the working electrode, and a graphite rod and a Ag/AgCl electrode (in 3 M KCl solution) were adopted as the counter and reference electrodes, respectively. All the potentials reported in this paper are calibrated to a reversible hydrogen electrode (RHE), and all the data are without iR compensation. Solution of 0.5 M H2SO4 and 1 M KOH were used as the electrolytes for the HER and OER, respectively. N2 was bubbled through the electrolyte for approximately 30 min before measurements to eliminate dissolved O2 and maintain a fixed Nernst potential. Polarization curves were measured by linear sweep voltammetry (LSV), and the scan rate was 5 mV s1. The frequency of the electrochemical impedance spectroscopy (EIS) tests ranged from 101 Hze105 Hz centered at 0.138 V (vs. RHE) for the HER and at 1.60 V (vs. RHE) for the OER. A sinusoidal voltage with an amplitude of 10 mV was applied. The electrochemical double-layer capacitance (Cdl) was used to evaluate the effective electrode surface area of the catalysts, and it was measured using cyclic voltammetry (CV) between þ0.35 and þ0.45 V (vs. RHE) for the HER and from 0.22 to 0.12 V (vs. RHE) for the OER. The scanning rates were 20, 40, 60, 80, 120 and 160 mV s1. The long-term durability tests were performed using continuous cycling at a scan rate of 100 mV s1 from 0.2 to 0 V (vs. the RHE) for the HER and from 1.02 to 1.58 V (vs. the RHE) for the OER for 1000 cycles. The chronopotentiometric measurements were also carried out to test the stability of the catalysts for 5000 s at a voltage of 0.141 V (vs. the RHE) for the HER and 1.56 V (vs. the RHE) for the OER.
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Results and discussion Characterization of the CoNiSSe-x materials XRD was used to determine the structural characteristics of the as-prepared CoNiSSe-x. As shown in Fig. 1(a), the peaks located at 30.3 , 34.7 and 38.2 can be indexed to the (200) lattice plane of CoSe2, and the peaks located at 45.8 and 52.0 are indexed to the (221) lattice plane of CoSe2 [21,22]. Almost all the characteristic peaks in the pattern well match the pyrite structure of CoSe2 with a cubic morphology (JCPDS # 0080414) and without any other secondary peaks related to NiSx, NiSex and CoSx, indicating that doping did not change the crystal structure of CoSe2 [23,24]. Furthermore, almost all the peak positions in the pattern of CoNiSSe-x continuously downshifted as the Ni content increased, indicating the successful doping of Ni in the sample, and the lattice expansion could be attributed to the partial substitution of larger nickel ions for cobalt ions. The relative intensity of the CoSe2 diffraction peaks gradually increase from x ¼ 0.1 to 0.3, suggesting an increase in the cubic CoSe2 loading in the composites. When the x value increased to 0.4, there was a phase conversion in CoSe2 from the thermodynamically stable cubic phase to the orthorhombic phase (JCPDS # 089-2003). Therefore, the appropriate x value of Ni in CoNiSSe-x is 0.3, which results in the desired pure cubic phase. Raman spectra were employed to further investigate the change in the molecular vibration of the as-obtained samples. As shown in Fig. 1(b), the peaks at 154 and 189 cm1 are ascribed to the stretching mode of chalcogenide atoms in Niand Co-chalcogenides, respectively. The peaks at 465, 510, and 670 cm1 are assigned to the Eg, F12g, and F22g modes of CoeO from the oxidation of Co [25]. Notably, a slight hypsochromic shift occurred for the peak at 189 cm1 as the content of Ni increased, especially in CoNiSSe-0.3. In general, the crystal structure of CoNiSSe-0.3 is basically in agreement with our design of an ideal quaternary CoeNieSeSe electrocatalyst. Furthermore, XPS measurements were carried out to investigate the chemical state and composition of CoNiSSe0.3. The high-resolution XPS spectrum of Co 2p (Fig. 1(c)) displays two main deconvoluted peaks located at 779.0 and 793.8 eV, corresponding to Co 2p3/2 and Co 2p1/2, respectively, for the Co-chalcogenide bond. The spin-orbit splitting value of 2p3/2 and 2p1/2 reaches 14.8 eV, implying the coexistence of Co2þ and Co3þ, which is in good agreement with a previous report [26]. The characteristic peaks of Co are blue-shifted by 0.8 eV from those of metallic Co and indicate that the doping of S influences the electronic structure of CoSe2. The peaks located at 781.3 and 797.7 eV are assigned to partially oxidized bonding on the surface of CoNiSSe-0.3 as a result of exposed to air. The satellite peaks at 785.5 and 802.6 eV are due to the shake-up excitation of high-spin Co2þ. The results indicate that the cobalt ions in CoNiSSe-0.3 have a d7 electronic structure in the form of t62ge1g. This electronic configuration is believed to be desirable for superior catalysts because it is close to the optimal eg filling of 1.2 for a paramagnetic configuration [27,28]. The Ni 2p XPS spectrum (Fig. 1(d)) reveals that the peaks of Ni 2p3/2 and Ni 2p1/2 are located at 853.6 and 871.9 eV, respectively, corresponding not only to the Ni-
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Fig. 1 e XRD patterns (a) and Raman (b) spectra of the as-prepared quaternary compounds. High-resolution XPS spectra of the Co 2p region (c), Ni 2p region (d), S 2p region (e) and Se 3d region (f) of CoNiSSe-0.3.
chalcogenide bond but also to the NieO bond from the surface oxide phase. The spin-orbit splitting value of 2p3/2 and 2p1/2 is 18.3 eV, indicating the coexistence of Ni2þ and Ni3þ. Similarly, the peaks of Ni were blue-shifted by 0.9 eV from those of neutral Ni at 852.7 and 871.0 eV. The peaks at bonding energies of 856.3 and 879.8 eV are assigned to the satellite peaks of Ni 2p3/2 and Ni 2p1/2, respectively [29]. The blue-shifts of Co and Ni confirmed the successful doping of more electronegative S in CoNiSSe-0.3. It is well known that electrocatalysts solely act as precursors in the OER process, and the true catalysts are the oxide/oxyhydroxide species created via surface oxidation. According to the XPS results of Co and Ni, the presence of Co3þ, Ni3þ and corresponding oxides will be favorable for the superior OER activity of CoNiSSe-0.3. In Fig. 1(e), two characteristic peaks of S 2p3/2 (160.7 eV) and S 2p1/2 (162.2 eV) confirm the existence of S2, which can be attributed to previously active HER species. In addition, a fitted peak at approximately 165.9 eV can be ascribed to sulfur oxide in CoNiSSe-0.3 [30]. The Se 3d spectrum (Fig. 1(f)) was resolved into two main peaks at 54.7 and 55.8 eV, corresponding to Se 3d5/2 and Se 3d3/ 2, respectively, and indicating the bond between Se anions and Co or Ni cations. The peak at 59.7 eV can be attributed to SeOx on the surface [31]. The XPS survey of CoNiSSe-0.3 (shown in Fig. S2) demonstrates the coexistence of Co, Ni, S, and Se and proves the successful doping of Ni and S in CoSe2. SEM was used to investigate the morphology of the asprepared samples. Fig. 2(a) and Fig. S3 indicate that the selfassembled flower-like CoNiSSe-0.3 is composed of uniformly distributed 2D nanosheets, and its large exposed specific surface area was expected to increase the number of active sites [32]. Additionally, the morphology of the other three quaternary compounds with different Ni/Co ratios was also characterized by SEM. Other Ni contents in CoeNieSeSe did not induce self-assembly of nanosheets, suggesting that an
appropriate content of Ni would determine the distinct structure and avoid aggregation and restacking of nanosheets. It should be noted that due to the special morphology of CoNiSSe-0.3, the ability of the electrolyte to permeate the structure during the electrocatalytic reaction would be accelerated [33,34]. As shown in Fig. 2(b), TEM further confirms the regular lamellar structure of CoNiSSe-0.3, which can promote the exchange of intermediates and transformation of electrons during water electrolysis. Fig. 2(c) displays the high-resolution TEM (HRTEM) image of CoNiSSe-0.3. The d-spacing of the CoNiSSe-0.3 lattice fringes is 0.181 nm, which is larger than that of CoSe2 in the (221) plane. The subtle change in the crystal lattice arises from the different radius of the dopant [35]. This result is in accordance with the XRD results. EDS elemental mapping under STEM mode showed a uniform distribution of Co, Ni, S, and Se in CoNiSSe-0.3 (Fig. 2(d)), indicating the successful substitution of Ni and S. The existed elements and composition were determined by EDS (Table S2) and ICP-OES (Table S3), and both of the test results show similar atomic ratio of Co:Ni:S:Se. Notably, the average atomic ratio of Co:Ni:S:Se in CoNiSSe-0.3 was estimated to be about 7: 3: 5: 15, indicating that 30% of the Co sites were substituted by Ni and 25% of the Se sites were substituted by S. Therefore, we have rationally designed and synthesized quaternary CoNiSSe-0.3 by considering the differences in atomic size and electronegativity of Co, Ni, S, and Se. The characteristic results confirmed the atomic-level substitution of Ni and S and that the electronic structure of CoSe2 was tuned while maintaining cubic phase CoSe2. Since it was reported that heteroatom doping is favorable for improving electrocatalytic performance [36,37], an improved electrochemical performance of the as-prepared CoNiSSe-0.3 can be expected.
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Fig. 2 e SEM image (a), TEM image (b), HRTEM image (c), STEM image and STEM-EDS elemental mapping (d) of CoNiSSe-0.3.
Electrochemical HER performance To investigate the electrocatalytic activity of CoNiSSe-x, the electrochemical performance was evaluated with a standard three-electrode configuration in 0.5 M H2SO4 and 1 M KOH solutions. Fig. 3(a) shows the HER polarization curves of the as-obtained samples and commercial Pt/C in a N2-saturated 0.5 M H2SO4 solution at a sweep rate of 5 mV s1. It is obvious that CoNiSSe-0.3 delivers the best catalytic activity among the quaternary catalysts as it requires the lowest potential to obtain the same current density. To reach a current density of 10 mA cm2, the potential of only 138 mV is applied to CoNiSSe-0.3, which is much lower than that of CoNiSSe-0.1 (204 mV), CoNiSSe-0.2 (186 mV), and CoNiSSe-0.4 (182 mV). The results demonstrate a more rapid water reduction reaction when using flower-like CoNiSSe-0.3. These results demonstrate that the proper substitution of Co with Ni could significantly improve the electrocatalytic performance. The Tafel slope, which is calculated from the Tafel equation, is considered an intrinsic characteristic of a catalyst. As shown in Fig. 3(b), the Tafel slope of CoNiSSe-0.3 is 42.1 mV dec1, which indicates that it follows the Volmer-Heyrovsky´ mechanism and that the Heyrovsky´ reaction is the ratelimiting step (Hads þ H3Oþ þ ee / H2 þ H2O) [38]. This value is much smaller than that of CoNiSSe-0.1 (78.4 mV dec1), CoNiSSe-0.2 (77.1 mV dec1) and CoNiSSe-0.4 (53.8 mV dec1). The catalytic performance was greatly improved when the Ni/ Co atomic ratio increased to 3:7. A further increase in the content of Ni (i.e., Ni/Co atomic ratio of 2:3) did not result in better catalytic activity, demonstrating that the optimized Ni content leads to the highest reaction rate and kinetics. Moreover, it has been reported that the synergistic effect of different components can also influence the surface structure to expose more active sites and tune the intrinsic electric properties of hybrid materials, thus improving the electrocatalytic activity [39,40]. In this case, the optimized Ni/Co atom ratio illustrates the synergistic effect between nickel and cobalt atoms.
The improved HER activity of CoNiSSe-0.3 can be also attributed to large active surface area from morphology, which can facilitate penetration of the electrolyte in the HER process [41]. The double layer capacitance (Cdl) was tested to evaluate the electrochemical surface area (ECSA) and roughness factor via CV in the voltage range of 0.35e0.45 V (vs. the RHE) without a redox process at different scan rates (shown in Fig. S4). As shown in Fig. 3(c), the Cdl value of CoNiSSe-0.3 is 8.28 mF cm2, which is approximately 2.3 times, 2.0 times and 1.1 times higher than that of CoNiSSe-0.1, CoNiSSe-0.2 and CoNiSSe-0.4, respectively. The results indicate that appropriate Ni doping can increase the Cdl of CoS0.5Se1.5 and that a larger ECSA provides more active sites, which largely contributes to its superior electrocatalytic activity [42]. In addition, the high ECSA of CoNiSSe-0.3 suggests that the standing 2D nanosheets in the 3D flower-like structure can promote the penetration and release of H2 during the water-splitting process. Therefore, the exposure and utilization of active sites on the electrochemical surface of CoNiSSe-0.3 were improved and the catalytic performance was enhanced. EIS measurements were carried out at a potential of 138 mV (vs. the RHE) to understand the interfacial reactions and electrode kinetics. As shown in Fig. 3(d), the semicircle, which corresponds to lower frequency and associates with Faradic effect, includes the interfacial charge transfer resistance (Rct) and Cdl. The Rct obtained from the Nyquist plots shows that the Rct of CoNiSSe-0.3 (39 U) is much lower than that of CoNiSSe-0.1, CoNiSSe-0.2, and CoNiSSe-0.4. Combined with the Cdl results shown in Fig. 3(c), we can conclude that proper doping of Ni can facilitate charge transfer and the combination of adsorbed hydrogen atoms and electrons, resulting in larger double-layer capacitance and fast kinetics for hydrogen evolution [43e45]. The stability and durability of electrocatalysts are challenging and important factors in the practical applications of high-performance electrolysis. For this reason, continuous CV and chronopotentiometric tests were performed. Fig. 3(e) shows the polarization curves of CoNiSSe-0.3 after the 1st and
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Fig. 3 e Polarization curves of the as-prepared samples and Pt/C in a 0.5 M H2SO4 solution at a scan rate of 5 mV s¡1 (a) and the corresponding Tafel plots (b). Linear fitting of the capacitive current density of the as-prepared samples vs. scan rate (c); the corresponding Nyquist plots under ¡0.138 V (vs. RHE) (d). Stability tests of CoNiSSe-0.3 with the initial polarization curve and the curve after 1000 cycles (e); Time dependence of the cathodic current density of CoNiSSe-0.3 held at ¡0.141 V (vs. RHE) for 5000 s (f).
1,000th CV cycles between 0.2 and 0 V (vs. RHE) at a scan rate of 100 mV s1. Compared with the initial state, the polarization curve after 1000 cycles exhibits almost no degradation, implying that CoNiSSe-0.3 has high structural durability in the continuous CV process. The superior durability of CoNiSSe-0.3 could be ascribed to its high crystallization, flower-like structure, and oxygen-free composition, all of which help to avoid the dissolution of catalysts in H2SO4 solution. In addition, CoNiSSe-0.3 presents high stability in the chronopotentiometric measurement when tested at a constant potential of 0.141 V (Fig. 3(f)). It is obvious that the current density stayed at approximately 12 mA cm2 even after 5000 s. The negligible degradation of the HER performance could be attributed to the catalyst falling off from the electrode due to the ample hydrogen gas bubbles generated and abruptly liberated from the electrodes [46]. Notably, the standard serrated shape of current density could be attributed to the
periodic accumulation and release of H2 bubble. Thus, the excellent durability and stability of CoNiSSe-0.3 were ascertained by CV and chronopotentiometric measurements for the HER, and it its performance superior to that of many Cobased and Ni-based electrocatalysts [47e49].
Electrochemical OER performances Considering that TMD-based compounds are known to be bifunctional electrocatalysts, we also investigated the catalytic activities of CoNiSSe-x for the OER. Similar to its HER performance, CoNiSSe-0.3 exhibits the best OER activity compared with that of other quaternary catalysts. At an overpotential as low as 370 mV, the current density of CoNiSSe-0.3 reaches 10 mA cm2, which further increases rapidly to 50 mA cm2 at 1.68 V and is comparable to that of the best reported Ni-based electrocatalysts (shown in Fig. 4(a))
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[50e52]. Furthermore, the enhanced OER activity of CoNiSSe0.3 is confirmed by its smaller Tafel slope (65.5 mV dec1) than that of CoNiSSe-0.1 (122.9 mV dec1), CoNiSSe-0.2 (93.7 mV dec1) and CoNiSSe-0.4 (88.1 mV dec1), revealing that proper Ni substitution also leads to higher kinetics and reaction rate for the OER (shown in Fig. 4(b)). Notably, both the overpotentials at a current density of 10 mA cm2 and the Tafel slopes present the same tendency as those for the HER, exhibiting a unique “V-type” plot with the Ni content. Compared to those for the HER, the reduction of the Tafel slopes upon varying the content of Ni is relatively high, suggesting that the Ni doping in CoNiSSe-x has more influence on the reaction kinetics and rate of the OER compared with the HER. In KOH solution, the existing M(OH)x is oxidized to produce oxide layers when a positive voltage is applied. The oxide layers become the actual OER
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electrocatalytic sites, as demonstrated by many previous reports [53,54]. The CoeNieSeSe underneath the oxide layers maintains the electrical conductivity between the active oxide layers and the electrode, thus improving the electrocatalytic performance. To estimate the ECSA, the Cdl was obtained by the CV method in a non-Faradaic potential range of 0.22 to 0.12 V (vs. the RHE) in 1 M KOH solution (shown in Fig. S5). As presented in Fig. 4(c), the Cdl values are 116 mF cm2, 132 mF cm2, 317 mF cm2 and 163 mF cm2 for CoNiSSe-0.1, CoNiSSe-0.2, CoNiSSe-0.3, and CoNiSSe-0.4, respectively. The similar Cdl tendency for the OER and HER indicates that the catalyst has a larger ECSA and more active sites when the value of x increased to 0.3, while further increasing the Ni content did not result in better catalytic performance. Therefore, the better catalytic performance of CoNiSSe-0.3 can be ascribed to
Fig. 4 e Polarization curves of as-prepared samples in a 1 M KOH solution at a scan rate of 5 mV s¡1 (a) and the corresponding Tafel plots (b). Linear fitting of the capacitive current density of as-prepared samples vs. scan rate (c); the corresponding Nyquist plots under 1.60 V (vs. RHE) (d). Stability tests of CoNiSSe-0.3 with the initial polarization curve and the one after 1000 cycles (e); time dependence of the anodic current density of CoNiSSe-0.3 held at 1.56 V (vs. RHE) for 5000 s (f).
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Table 1 e Comparison of the HER or OER activity of various catalysts. Catalyst HER CoSe2/CP CoS2/RGO CoS2 CoSe2/DETA MoS2/CoSe2 Co0.7Ni0.3S0.5Se1.5 OER Co2P/Mo2C/Mo3Co3C@C CoSe Co/rGO Ni0.3Co0.74Se NiCoP/RGO Co0.7Ni0.3S0.5Se1.5
Onset potential (mV vs. RHE) J ¼ 1 mA cm2
Overpotential (mV vs. RHE) J ¼ 10 mA cm2
Tafel slope (mV dec1)
Ref.
e e 137 e 11 78 e 360 e 277 e 278
137 138 187 149 68 138 362 e 455 397 270 370
42.1 48.2 87 46.8 36 42.1 82 74.7 81.6 76 65.7 65.5
27 44 46 60 61 This work 13 25 39 40 59 This work
its large ECSA due to the flower-like morphology, which was tuned by the Ni content. Rct was evaluated by EIS (shown in Fig. 4(d)), and similar to the EIS results for the HER, CoNiSSe-0.3 exhibited the smallest Rct value of the samples of approximately 85 U. This reveals that tuning the electronic structure of TMD-based catalysts by optimizing the content of Ni can robustly facilitate the charge transfer process at the interface of catalysts and electrolyte. The substantially decreased Rct of CoNiSSe-0.3 can be attributed to the surface reactivity of various hydroxides with active O* intermediates. The OER stability of CoNiSSe-0.3 was also tested by successive CV and chronopotentiometry measurements. As shown in Fig. 4(e), compared with the 1st LSV curve, negligible degradation in the polarization curves for the overpotential range from 370 to 372 mV was observed at 10 mA cm2 after the 1,000th cycle, suggesting that CoNiSSe-0.3 had high structural durability during the continuous CV process. Fig. 4(f) shows that there is little deterioration in the OER performance after 5000 s, and this could be ascribed to the oxidation of TMD in KOH medium, which has been reported in the literature [55,56]. Similar to the serrated shape of current density shown in Fig. 3(f), the periodic fluctuations were also caused by the alternative process of O2 bubble accumulation and release. Thus, the high durability and stability of CoNiSSe0.3 were determined by CV and chronopotentiometric measurements for the OER.
Comparison of electrochemical performance and improvement mechanism To better understand the performance advantages of the asprepared quaternary CoNiSSe-0.3 materials, we compared the HER and OER properties of the quaternary materials with those of other CoSe2-based materials. As shown in Table 1, CoNiSSe-0.3 performs well for both the HER and OER, indicating that the designed flower-like quaternary CoeNieSeSe materials with excellent and balanced properties can be used as HER/OER bifunctional electrocatalysts with high stability and low economic cost. According to a previous report, it is believed that pyritestructured CoNiSSe-x also has a similar HER mechanism and the number of active sites as other electrocatalysts with the pyrite-structure, not only metals (Co and Ni) but also
nonmetals (Se and S) in edged sites may take part in the HER process [57]. Moreover, it was reported that the OER reactivity of hydroxide monolayers and metal oxide thin films of firstrow TMs follow the order Fe < Co < Ni, as determined by the strength of the MOH bond [58,59]. The higher catalytic activity of the dopant Ni can be attributed to the Ni hydroxide or oxide layer being more active than the Co layer. Therefore, we suggest that the pyrite-structure and coexistence of CoeNieSeSe and amorphous oxide phases are responsible for the superior bifunctional electronic activity. Meanwhile, the specific flower-like morphology of CoNiSSe0.3 not only provides abundant surface active sites but also improves the long-term stability by reducing structural stress [60,61]. Benefiting from the unique structure and composition, the optimal CoNiSSe-0.3 shows a remarkably enhanced catalytic performance for the HER and OER. The activity of CoNiSSe-0.3 was better than or comparable to that of other Co-based catalysts. Due to the low cost and earth abundance of Ni, CoNiSSe-0.3 is a promising candidate for commercial applications.
Conclusion We have successfully fabricated flower-like Co0.7Ni0.3S0.5Se1.5 by a facile one-step hydrothermal method. CoNiSSe-0.3 has been confirmed as a robust and bifunctional catalyst with overpotentials of 138 mV and 370 mV to drive a current density of 10 mA cm2 for the HER in acidic medium and the OER in alkaline medium, respectively. Its robust electrocatalytic performance can be attributed to its unique structure and special composition: (1) The 3D flower-like conductive network formed by 2D nanosheets greatly improves the electrolyte penetration and the transfer of electrons, and results in a more favorable reaction kinetic process. (2) The typical metallic nature of Co-based and Nibased catalysts ensures efficient transfer of electrons between the surface of the catalysts and the electrodes. (3) The maintained pyrite-structure and the oxides on the surface of CoNiSSe-0.3 are favorable for enhancing catalytic performance. Our work not only develops a high-performance bifunctional catalyst for water electrolysis but also paves the way for engineering the composition and structure of new materials.
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Acknowledgement This research was supported by grants from the Danish National Research Foundation, the AUFF-NOVA project from Aarhus Universitets Forskningsfond and the EU H2020RISE 2016-MNR4S Cell project. Zaixing Jiang acknowledges financial support from National Program for Support of Top-notch Young Professionals, National Natural Science Foundation of China (No. 51773049), China Aerospace Science and Technology Corporation-Harbin Institute of Technology Joint Center for Technology Innovation Fund (HIT15-1A01), Shanghai Academy of Spaceflight Technology Fund (SAST2017-126), Harbin city science and technology projects (2013DB4BP031 and RC2014QN017035), China Postdoctoral Science Foundation (No. 201003420, No.20090460067), HIT Research Institute (Zhao Yuan) of New Materials and Intelligent Equipment Technology Co., Ltd. Scientific and Technological Cooperation and Development Fund (No.2017KJHZ002). Zegao Wang thanks the supporting by the Fundamental Research Funds for the Central Universities, China (YJ201893) and State Key Lab of Advanced Metals and Materials, China (Grant No. 2019-Z03).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.08.257.
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Available online at www.sciencedirect.com
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Tuning quaternary hybride CoeNieSeSe composition as a bifunctional electrocatalyst for hydrogen and oxygen evolution reactions Xiaojiao Fang a,b, Zegao Wang b,c, Yin Wang b, Shifei Kang b,d, Zaixing Jiang a,b,**, Mingdong Dong b,* a
MIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, 150001, China b Interdisciplinary Nanoscience Center (iNANO), Aarhus University, DK-8000, Aarhus C, Denmark c College of Materials Science and Engineering, Sichuan University, Chengdu, 610065, China d Department of Environmental Science and Engineering, University of Shanghai for Science and Technology, Shanghai, 200093, China
highlights Co0.7Ni0.3S0.5Se1.5 delivers the best activity and durability for HER and OER. The flower-like network of Co0.7Ni0.3S0.5Se1.5 is favorable to a fast kinetic. The metallic nature of catalysts facilitates the transformation of electrons. The pyrite-structure and surface oxides contribute to the improvement.
article info
abstract
Article history:
Developing high-efficiency and earth-abundant electrocatalysts for electrochemical
Received 21 June 2019
water splitting is of paramount importance for energy conversion. Although tremen-
Received in revised form
dous effort has been paid to transition metal (TM) material-based electrocatalysts,
27 August 2019
rational design and controllable synthesis of fine structures to fully utilize the latent
Accepted 30 August 2019
potential of TM materials remain great challenges. We herein report a composition-
Available online 23 September 2019
tuning strategy to achieve rational structure control of quaternary CoeNieSeSe materials through a facile one-pot hydrothermal method, in which earth-abundant Ni is
Keywords:
introduced into a CoSxSe2-x matrix to optimize the morphology and electronic structure
Composition-tuning
of the quaternary electrocatalyst. Because of the introduction of Ni, this novel CoeNieS
Quaternary
eSe quaternary system shows better catalytic activity for water splitting with Tafel
CoeNieSeSe
slopes of 42.1 mV dec1 for hydrogen evolution reaction (HER) and 65.5 mV dec1 for
Hydrogen evolution reaction
oxygen evolution reaction (OER), respectively, compared with its precursor CoeSeSe
Oxygen evolution reaction
ternary system. For stability, there is negligible fading after long-term electrochemical test. Our work not only provides a novel thinking to introduce nickel into CoeSeSe ternary system by a facile hydrothermal synthesis for electrochemical water splitting,
* Corresponding author. ** Corresponding author. MIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, 150001, China. E-mail addresses: [email protected] (Z. Jiang), [email protected] (M. Dong). https://doi.org/10.1016/j.ijhydene.2019.08.257 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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but also this quaternary system realizes bifunctional catalysis and better electrochemical performance relative to the ternary counterpart. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Electrochemical water splitting is considered an efficient way to overcome environmental pollution and the global energy crisis by converting electricity into chemical fuels [1e5]. The electrochemical water splitting process includes two half reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), and both reactions are kinetically sluggish in nature [6,7]. Therefore, developing robust electrocatalysts for water electrolysis with a low Tafel slope and overpotential and excellent long-term stability are of great importance [8,9]. Currently, noble metal-based materials, such as Pt and RuO2/IrO2 have been proven to be the state-ofthe-art electrocatalysts, but their rarity and high cost seriously restrict their applications in industry. CoSe2, a member of transition-metal dichalcogenides (TMDs), has been demonstrated has a conductive metallic nature and low chemisorption energy for H2, enabling catalytic performance for the HER in acidic media [10,11]. Moreover, many recent studies reported the OER activity of CoSe2 after modification, indicating its potential to be used as a bifunctional catalyst [12,13]. Changing the composition of TMbased materials through heteroatom doping is a key approach to improve the electrocatalytic performance by tuning the morphology and electronic structure of materials. Due to the similar crystal structure and ionic radius, TM (Ni, Fe and Mn) and S can be used as substitutes of CoSe2 at cation and anion sites, respectively [14e16]. Although efforts have been made to modify CoSe2, it is still urgent to develop high-performance CoSe2-based HER/OER bifunctional electrocatalysts with high stability and low economic cost. Ternary TMDs have been widely researched as promising candidates for water splitting, yet their performance is still far from satisfactory due to the restrictions of their limited surface active sites and poor durability [17e19]. To the best of our knowledge, there are no studies exploring the potential for using quaternary CoeNieSeSe as a bifunctional HER/OER electrocatalyst. Due to the difference in the atomic radii and the synergistic effects of the constituents, compositional turning of CoSe2 with metal and nonmetal elements simultaneously may open up a new way to synthesize highly efficient electrocatalysts. Herein, we rationally design and synthesize well-ordered 3D quaternary CoeNieSeSe electrocatalysts for the HER and OER by simultaneously engineering the composition and structure of the CoSxSe2-x matrix. Benefiting from the synergistic effects of the appropriate dual tuning of Ni and S dopants in CoSe2, the electronic structure of the catalyst was significantly optimized and more active sites were exposed in the unique 3D structure, thereby leading to a superior bifunctional electrocatalytic performance. The optimized quaternary CoeNieSeSe exhibits robust catalytic activity as a bifunctional catalyst with low Tafel
slopes and overpotentials as well as long-term stability for both the HER and OER. The combination of simultaneously tailoring the chemical composition and structure of the catalyst in this work creates a new approach to develop highly efficient and low-cost bifunctional TM-based electrocatalysts.
Experimental Materials The chemicals in this paper, i.e., nickel chloride (NiCl2, SigmaAldrich, 98%), cobalt nitrate hexahydrate (Co(NO3)2$6H2O, Acros, 98%), Se powder (Se, Sigma-Aldrich, 99.5%), L-cysteine (C3H7NO2S, Sigma-Aldrich, 97%), EDTA-2Na (C10H14N2Na2O8, Sigma-Aldrich, 99%), Nafion perfluorinated resin (SigmaAldrich, 5 wt%), potassium hydroxide (KOH, Merck, 84%), Pt/C (Sigma-Aldrich, 1 wt%), and ethanol (C2H5OH, Sigma-Aldrich, 96%), were used without any further purification.
Synthesis of CoeNieSeSe According to a previous report [20], electrocatalysts exhibit superior long-term stability and catalytic performance when the atomic ratio of S:Se is close to 1:3. Therefore, we fixed the content of S and varied the content of Ni in the CoS0.5Se1.5 system to prepare the final quaternary Co1-xNixS0.5Se1.5 (denoted as CoNiSSe-x) samples by a facile one-pot hydrothermal method. The synthesis process of CoNiSSe-0.3 was as follows: First, 2 mmol Se powder was dissolved in 5 mL of a 20 M KOH solution to obtain Se2. After stirring for 1 h, 24 mmol L-cysteine, 0.3 mmol NiCl2, 0.7 mmol Co(NO3)2$6H2O and 1.6 mmol EDTA-2Na were added to the mixture under constant stirring. Then, the mixture was transferred to a 50 mL Teflonlined stainless autoclave for the hydrothermal reaction at 180 C for 16 h. After the oven cooled to room temperature naturally, the black precipitate was washed with distilled water and ethanol several times by centrifugation and dried at 60 C for 12 h. To investigate the effect of Ni on the electrochemical performance, controlled experiments with different contents of Ni were carried out (x ¼ 0.1, 0.2, and 0.4), and the as-obtained samples were denoted as CoNiSSe-0.1, CoNiSSe0.2, and CoNiSSe-0.4, respectively (shown in Table S1).
Characterization Scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, F30) were employed to determine the morphology and microstructure of the samples. High-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), energy dispersive spectroscopy (EDS), and inductively coupled plasma-
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 6 8 5 e2 7 6 9 4
optical emission spectrometry (ICP-OES, Agilent 730) were used to determine the existed elements and elemental composition. The composition and chemical states of the asobtained catalysts were tested by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Ka). X-ray diffraction (XRD, l ¼ 0.1541 nm, Smartlab-9) with Cu Ka as the X-ray source was used to examine the phase structure of the asprepared samples, and the range was 10e80 . Raman spectroscopy (Horiba LabRAM HR 800) was performed with an excitation light source of 514 nm.
Preparation of the working electrode To avoid absorbing the environmental moisture, 3 mg of the as-prepared CoNiSSe-x powder was weighed and dissolved in 200 mL of 17:3 v/v water/ethanol solution in a glove box. Then, 20 mL Nafion was added and the mixture was ultrasonicated for 1 h to obtain a homogeneous catalyst ink. After that, 5 mL of the ink was drop-cast on glassy carbon electrodes (GC, 3 mm in diameter), which have already been polished with sand paper (5000 mesh) and alumina powder (1.0 and 0.1 mm). The catalyst films were dried at room temperature naturally and the loading were about 0.96 mg cm2. Moreover, a Pt/C working electrode was prepared as a control.
Electrochemical measurements All the electrochemical measurements were performed with a standard three-electrode cell using a CHI660E (CHI Instruments, Shanghai Chenhua Instrument Corp., China) at room temperature (shown in Fig. S1). The GC with the catalysts served as the working electrode, and a graphite rod and a Ag/AgCl electrode (in 3 M KCl solution) were adopted as the counter and reference electrodes, respectively. All the potentials reported in this paper are calibrated to a reversible hydrogen electrode (RHE), and all the data are without iR compensation. Solution of 0.5 M H2SO4 and 1 M KOH were used as the electrolytes for the HER and OER, respectively. N2 was bubbled through the electrolyte for approximately 30 min before measurements to eliminate dissolved O2 and maintain a fixed Nernst potential. Polarization curves were measured by linear sweep voltammetry (LSV), and the scan rate was 5 mV s1. The frequency of the electrochemical impedance spectroscopy (EIS) tests ranged from 101 Hze105 Hz centered at 0.138 V (vs. RHE) for the HER and at 1.60 V (vs. RHE) for the OER. A sinusoidal voltage with an amplitude of 10 mV was applied. The electrochemical double-layer capacitance (Cdl) was used to evaluate the effective electrode surface area of the catalysts, and it was measured using cyclic voltammetry (CV) between þ0.35 and þ0.45 V (vs. RHE) for the HER and from 0.22 to 0.12 V (vs. RHE) for the OER. The scanning rates were 20, 40, 60, 80, 120 and 160 mV s1. The long-term durability tests were performed using continuous cycling at a scan rate of 100 mV s1 from 0.2 to 0 V (vs. the RHE) for the HER and from 1.02 to 1.58 V (vs. the RHE) for the OER for 1000 cycles. The chronopotentiometric measurements were also carried out to test the stability of the catalysts for 5000 s at a voltage of 0.141 V (vs. the RHE) for the HER and 1.56 V (vs. the RHE) for the OER.
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Results and discussion Characterization of the CoNiSSe-x materials XRD was used to determine the structural characteristics of the as-prepared CoNiSSe-x. As shown in Fig. 1(a), the peaks located at 30.3 , 34.7 and 38.2 can be indexed to the (200) lattice plane of CoSe2, and the peaks located at 45.8 and 52.0 are indexed to the (221) lattice plane of CoSe2 [21,22]. Almost all the characteristic peaks in the pattern well match the pyrite structure of CoSe2 with a cubic morphology (JCPDS # 0080414) and without any other secondary peaks related to NiSx, NiSex and CoSx, indicating that doping did not change the crystal structure of CoSe2 [23,24]. Furthermore, almost all the peak positions in the pattern of CoNiSSe-x continuously downshifted as the Ni content increased, indicating the successful doping of Ni in the sample, and the lattice expansion could be attributed to the partial substitution of larger nickel ions for cobalt ions. The relative intensity of the CoSe2 diffraction peaks gradually increase from x ¼ 0.1 to 0.3, suggesting an increase in the cubic CoSe2 loading in the composites. When the x value increased to 0.4, there was a phase conversion in CoSe2 from the thermodynamically stable cubic phase to the orthorhombic phase (JCPDS # 089-2003). Therefore, the appropriate x value of Ni in CoNiSSe-x is 0.3, which results in the desired pure cubic phase. Raman spectra were employed to further investigate the change in the molecular vibration of the as-obtained samples. As shown in Fig. 1(b), the peaks at 154 and 189 cm1 are ascribed to the stretching mode of chalcogenide atoms in Niand Co-chalcogenides, respectively. The peaks at 465, 510, and 670 cm1 are assigned to the Eg, F12g, and F22g modes of CoeO from the oxidation of Co [25]. Notably, a slight hypsochromic shift occurred for the peak at 189 cm1 as the content of Ni increased, especially in CoNiSSe-0.3. In general, the crystal structure of CoNiSSe-0.3 is basically in agreement with our design of an ideal quaternary CoeNieSeSe electrocatalyst. Furthermore, XPS measurements were carried out to investigate the chemical state and composition of CoNiSSe0.3. The high-resolution XPS spectrum of Co 2p (Fig. 1(c)) displays two main deconvoluted peaks located at 779.0 and 793.8 eV, corresponding to Co 2p3/2 and Co 2p1/2, respectively, for the Co-chalcogenide bond. The spin-orbit splitting value of 2p3/2 and 2p1/2 reaches 14.8 eV, implying the coexistence of Co2þ and Co3þ, which is in good agreement with a previous report [26]. The characteristic peaks of Co are blue-shifted by 0.8 eV from those of metallic Co and indicate that the doping of S influences the electronic structure of CoSe2. The peaks located at 781.3 and 797.7 eV are assigned to partially oxidized bonding on the surface of CoNiSSe-0.3 as a result of exposed to air. The satellite peaks at 785.5 and 802.6 eV are due to the shake-up excitation of high-spin Co2þ. The results indicate that the cobalt ions in CoNiSSe-0.3 have a d7 electronic structure in the form of t62ge1g. This electronic configuration is believed to be desirable for superior catalysts because it is close to the optimal eg filling of 1.2 for a paramagnetic configuration [27,28]. The Ni 2p XPS spectrum (Fig. 1(d)) reveals that the peaks of Ni 2p3/2 and Ni 2p1/2 are located at 853.6 and 871.9 eV, respectively, corresponding not only to the Ni-
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Fig. 1 e XRD patterns (a) and Raman (b) spectra of the as-prepared quaternary compounds. High-resolution XPS spectra of the Co 2p region (c), Ni 2p region (d), S 2p region (e) and Se 3d region (f) of CoNiSSe-0.3.
chalcogenide bond but also to the NieO bond from the surface oxide phase. The spin-orbit splitting value of 2p3/2 and 2p1/2 is 18.3 eV, indicating the coexistence of Ni2þ and Ni3þ. Similarly, the peaks of Ni were blue-shifted by 0.9 eV from those of neutral Ni at 852.7 and 871.0 eV. The peaks at bonding energies of 856.3 and 879.8 eV are assigned to the satellite peaks of Ni 2p3/2 and Ni 2p1/2, respectively [29]. The blue-shifts of Co and Ni confirmed the successful doping of more electronegative S in CoNiSSe-0.3. It is well known that electrocatalysts solely act as precursors in the OER process, and the true catalysts are the oxide/oxyhydroxide species created via surface oxidation. According to the XPS results of Co and Ni, the presence of Co3þ, Ni3þ and corresponding oxides will be favorable for the superior OER activity of CoNiSSe-0.3. In Fig. 1(e), two characteristic peaks of S 2p3/2 (160.7 eV) and S 2p1/2 (162.2 eV) confirm the existence of S2, which can be attributed to previously active HER species. In addition, a fitted peak at approximately 165.9 eV can be ascribed to sulfur oxide in CoNiSSe-0.3 [30]. The Se 3d spectrum (Fig. 1(f)) was resolved into two main peaks at 54.7 and 55.8 eV, corresponding to Se 3d5/2 and Se 3d3/ 2, respectively, and indicating the bond between Se anions and Co or Ni cations. The peak at 59.7 eV can be attributed to SeOx on the surface [31]. The XPS survey of CoNiSSe-0.3 (shown in Fig. S2) demonstrates the coexistence of Co, Ni, S, and Se and proves the successful doping of Ni and S in CoSe2. SEM was used to investigate the morphology of the asprepared samples. Fig. 2(a) and Fig. S3 indicate that the selfassembled flower-like CoNiSSe-0.3 is composed of uniformly distributed 2D nanosheets, and its large exposed specific surface area was expected to increase the number of active sites [32]. Additionally, the morphology of the other three quaternary compounds with different Ni/Co ratios was also characterized by SEM. Other Ni contents in CoeNieSeSe did not induce self-assembly of nanosheets, suggesting that an
appropriate content of Ni would determine the distinct structure and avoid aggregation and restacking of nanosheets. It should be noted that due to the special morphology of CoNiSSe-0.3, the ability of the electrolyte to permeate the structure during the electrocatalytic reaction would be accelerated [33,34]. As shown in Fig. 2(b), TEM further confirms the regular lamellar structure of CoNiSSe-0.3, which can promote the exchange of intermediates and transformation of electrons during water electrolysis. Fig. 2(c) displays the high-resolution TEM (HRTEM) image of CoNiSSe-0.3. The d-spacing of the CoNiSSe-0.3 lattice fringes is 0.181 nm, which is larger than that of CoSe2 in the (221) plane. The subtle change in the crystal lattice arises from the different radius of the dopant [35]. This result is in accordance with the XRD results. EDS elemental mapping under STEM mode showed a uniform distribution of Co, Ni, S, and Se in CoNiSSe-0.3 (Fig. 2(d)), indicating the successful substitution of Ni and S. The existed elements and composition were determined by EDS (Table S2) and ICP-OES (Table S3), and both of the test results show similar atomic ratio of Co:Ni:S:Se. Notably, the average atomic ratio of Co:Ni:S:Se in CoNiSSe-0.3 was estimated to be about 7: 3: 5: 15, indicating that 30% of the Co sites were substituted by Ni and 25% of the Se sites were substituted by S. Therefore, we have rationally designed and synthesized quaternary CoNiSSe-0.3 by considering the differences in atomic size and electronegativity of Co, Ni, S, and Se. The characteristic results confirmed the atomic-level substitution of Ni and S and that the electronic structure of CoSe2 was tuned while maintaining cubic phase CoSe2. Since it was reported that heteroatom doping is favorable for improving electrocatalytic performance [36,37], an improved electrochemical performance of the as-prepared CoNiSSe-0.3 can be expected.
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Fig. 2 e SEM image (a), TEM image (b), HRTEM image (c), STEM image and STEM-EDS elemental mapping (d) of CoNiSSe-0.3.
Electrochemical HER performance To investigate the electrocatalytic activity of CoNiSSe-x, the electrochemical performance was evaluated with a standard three-electrode configuration in 0.5 M H2SO4 and 1 M KOH solutions. Fig. 3(a) shows the HER polarization curves of the as-obtained samples and commercial Pt/C in a N2-saturated 0.5 M H2SO4 solution at a sweep rate of 5 mV s1. It is obvious that CoNiSSe-0.3 delivers the best catalytic activity among the quaternary catalysts as it requires the lowest potential to obtain the same current density. To reach a current density of 10 mA cm2, the potential of only 138 mV is applied to CoNiSSe-0.3, which is much lower than that of CoNiSSe-0.1 (204 mV), CoNiSSe-0.2 (186 mV), and CoNiSSe-0.4 (182 mV). The results demonstrate a more rapid water reduction reaction when using flower-like CoNiSSe-0.3. These results demonstrate that the proper substitution of Co with Ni could significantly improve the electrocatalytic performance. The Tafel slope, which is calculated from the Tafel equation, is considered an intrinsic characteristic of a catalyst. As shown in Fig. 3(b), the Tafel slope of CoNiSSe-0.3 is 42.1 mV dec1, which indicates that it follows the Volmer-Heyrovsky´ mechanism and that the Heyrovsky´ reaction is the ratelimiting step (Hads þ H3Oþ þ ee / H2 þ H2O) [38]. This value is much smaller than that of CoNiSSe-0.1 (78.4 mV dec1), CoNiSSe-0.2 (77.1 mV dec1) and CoNiSSe-0.4 (53.8 mV dec1). The catalytic performance was greatly improved when the Ni/ Co atomic ratio increased to 3:7. A further increase in the content of Ni (i.e., Ni/Co atomic ratio of 2:3) did not result in better catalytic activity, demonstrating that the optimized Ni content leads to the highest reaction rate and kinetics. Moreover, it has been reported that the synergistic effect of different components can also influence the surface structure to expose more active sites and tune the intrinsic electric properties of hybrid materials, thus improving the electrocatalytic activity [39,40]. In this case, the optimized Ni/Co atom ratio illustrates the synergistic effect between nickel and cobalt atoms.
The improved HER activity of CoNiSSe-0.3 can be also attributed to large active surface area from morphology, which can facilitate penetration of the electrolyte in the HER process [41]. The double layer capacitance (Cdl) was tested to evaluate the electrochemical surface area (ECSA) and roughness factor via CV in the voltage range of 0.35e0.45 V (vs. the RHE) without a redox process at different scan rates (shown in Fig. S4). As shown in Fig. 3(c), the Cdl value of CoNiSSe-0.3 is 8.28 mF cm2, which is approximately 2.3 times, 2.0 times and 1.1 times higher than that of CoNiSSe-0.1, CoNiSSe-0.2 and CoNiSSe-0.4, respectively. The results indicate that appropriate Ni doping can increase the Cdl of CoS0.5Se1.5 and that a larger ECSA provides more active sites, which largely contributes to its superior electrocatalytic activity [42]. In addition, the high ECSA of CoNiSSe-0.3 suggests that the standing 2D nanosheets in the 3D flower-like structure can promote the penetration and release of H2 during the water-splitting process. Therefore, the exposure and utilization of active sites on the electrochemical surface of CoNiSSe-0.3 were improved and the catalytic performance was enhanced. EIS measurements were carried out at a potential of 138 mV (vs. the RHE) to understand the interfacial reactions and electrode kinetics. As shown in Fig. 3(d), the semicircle, which corresponds to lower frequency and associates with Faradic effect, includes the interfacial charge transfer resistance (Rct) and Cdl. The Rct obtained from the Nyquist plots shows that the Rct of CoNiSSe-0.3 (39 U) is much lower than that of CoNiSSe-0.1, CoNiSSe-0.2, and CoNiSSe-0.4. Combined with the Cdl results shown in Fig. 3(c), we can conclude that proper doping of Ni can facilitate charge transfer and the combination of adsorbed hydrogen atoms and electrons, resulting in larger double-layer capacitance and fast kinetics for hydrogen evolution [43e45]. The stability and durability of electrocatalysts are challenging and important factors in the practical applications of high-performance electrolysis. For this reason, continuous CV and chronopotentiometric tests were performed. Fig. 3(e) shows the polarization curves of CoNiSSe-0.3 after the 1st and
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Fig. 3 e Polarization curves of the as-prepared samples and Pt/C in a 0.5 M H2SO4 solution at a scan rate of 5 mV s¡1 (a) and the corresponding Tafel plots (b). Linear fitting of the capacitive current density of the as-prepared samples vs. scan rate (c); the corresponding Nyquist plots under ¡0.138 V (vs. RHE) (d). Stability tests of CoNiSSe-0.3 with the initial polarization curve and the curve after 1000 cycles (e); Time dependence of the cathodic current density of CoNiSSe-0.3 held at ¡0.141 V (vs. RHE) for 5000 s (f).
1,000th CV cycles between 0.2 and 0 V (vs. RHE) at a scan rate of 100 mV s1. Compared with the initial state, the polarization curve after 1000 cycles exhibits almost no degradation, implying that CoNiSSe-0.3 has high structural durability in the continuous CV process. The superior durability of CoNiSSe-0.3 could be ascribed to its high crystallization, flower-like structure, and oxygen-free composition, all of which help to avoid the dissolution of catalysts in H2SO4 solution. In addition, CoNiSSe-0.3 presents high stability in the chronopotentiometric measurement when tested at a constant potential of 0.141 V (Fig. 3(f)). It is obvious that the current density stayed at approximately 12 mA cm2 even after 5000 s. The negligible degradation of the HER performance could be attributed to the catalyst falling off from the electrode due to the ample hydrogen gas bubbles generated and abruptly liberated from the electrodes [46]. Notably, the standard serrated shape of current density could be attributed to the
periodic accumulation and release of H2 bubble. Thus, the excellent durability and stability of CoNiSSe-0.3 were ascertained by CV and chronopotentiometric measurements for the HER, and it its performance superior to that of many Cobased and Ni-based electrocatalysts [47e49].
Electrochemical OER performances Considering that TMD-based compounds are known to be bifunctional electrocatalysts, we also investigated the catalytic activities of CoNiSSe-x for the OER. Similar to its HER performance, CoNiSSe-0.3 exhibits the best OER activity compared with that of other quaternary catalysts. At an overpotential as low as 370 mV, the current density of CoNiSSe-0.3 reaches 10 mA cm2, which further increases rapidly to 50 mA cm2 at 1.68 V and is comparable to that of the best reported Ni-based electrocatalysts (shown in Fig. 4(a))
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[50e52]. Furthermore, the enhanced OER activity of CoNiSSe0.3 is confirmed by its smaller Tafel slope (65.5 mV dec1) than that of CoNiSSe-0.1 (122.9 mV dec1), CoNiSSe-0.2 (93.7 mV dec1) and CoNiSSe-0.4 (88.1 mV dec1), revealing that proper Ni substitution also leads to higher kinetics and reaction rate for the OER (shown in Fig. 4(b)). Notably, both the overpotentials at a current density of 10 mA cm2 and the Tafel slopes present the same tendency as those for the HER, exhibiting a unique “V-type” plot with the Ni content. Compared to those for the HER, the reduction of the Tafel slopes upon varying the content of Ni is relatively high, suggesting that the Ni doping in CoNiSSe-x has more influence on the reaction kinetics and rate of the OER compared with the HER. In KOH solution, the existing M(OH)x is oxidized to produce oxide layers when a positive voltage is applied. The oxide layers become the actual OER
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electrocatalytic sites, as demonstrated by many previous reports [53,54]. The CoeNieSeSe underneath the oxide layers maintains the electrical conductivity between the active oxide layers and the electrode, thus improving the electrocatalytic performance. To estimate the ECSA, the Cdl was obtained by the CV method in a non-Faradaic potential range of 0.22 to 0.12 V (vs. the RHE) in 1 M KOH solution (shown in Fig. S5). As presented in Fig. 4(c), the Cdl values are 116 mF cm2, 132 mF cm2, 317 mF cm2 and 163 mF cm2 for CoNiSSe-0.1, CoNiSSe-0.2, CoNiSSe-0.3, and CoNiSSe-0.4, respectively. The similar Cdl tendency for the OER and HER indicates that the catalyst has a larger ECSA and more active sites when the value of x increased to 0.3, while further increasing the Ni content did not result in better catalytic performance. Therefore, the better catalytic performance of CoNiSSe-0.3 can be ascribed to
Fig. 4 e Polarization curves of as-prepared samples in a 1 M KOH solution at a scan rate of 5 mV s¡1 (a) and the corresponding Tafel plots (b). Linear fitting of the capacitive current density of as-prepared samples vs. scan rate (c); the corresponding Nyquist plots under 1.60 V (vs. RHE) (d). Stability tests of CoNiSSe-0.3 with the initial polarization curve and the one after 1000 cycles (e); time dependence of the anodic current density of CoNiSSe-0.3 held at 1.56 V (vs. RHE) for 5000 s (f).
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Table 1 e Comparison of the HER or OER activity of various catalysts. Catalyst HER CoSe2/CP CoS2/RGO CoS2 CoSe2/DETA MoS2/CoSe2 Co0.7Ni0.3S0.5Se1.5 OER Co2P/Mo2C/Mo3Co3C@C CoSe Co/rGO Ni0.3Co0.74Se NiCoP/RGO Co0.7Ni0.3S0.5Se1.5
Onset potential (mV vs. RHE) J ¼ 1 mA cm2
Overpotential (mV vs. RHE) J ¼ 10 mA cm2
Tafel slope (mV dec1)
Ref.
e e 137 e 11 78 e 360 e 277 e 278
137 138 187 149 68 138 362 e 455 397 270 370
42.1 48.2 87 46.8 36 42.1 82 74.7 81.6 76 65.7 65.5
27 44 46 60 61 This work 13 25 39 40 59 This work
its large ECSA due to the flower-like morphology, which was tuned by the Ni content. Rct was evaluated by EIS (shown in Fig. 4(d)), and similar to the EIS results for the HER, CoNiSSe-0.3 exhibited the smallest Rct value of the samples of approximately 85 U. This reveals that tuning the electronic structure of TMD-based catalysts by optimizing the content of Ni can robustly facilitate the charge transfer process at the interface of catalysts and electrolyte. The substantially decreased Rct of CoNiSSe-0.3 can be attributed to the surface reactivity of various hydroxides with active O* intermediates. The OER stability of CoNiSSe-0.3 was also tested by successive CV and chronopotentiometry measurements. As shown in Fig. 4(e), compared with the 1st LSV curve, negligible degradation in the polarization curves for the overpotential range from 370 to 372 mV was observed at 10 mA cm2 after the 1,000th cycle, suggesting that CoNiSSe-0.3 had high structural durability during the continuous CV process. Fig. 4(f) shows that there is little deterioration in the OER performance after 5000 s, and this could be ascribed to the oxidation of TMD in KOH medium, which has been reported in the literature [55,56]. Similar to the serrated shape of current density shown in Fig. 3(f), the periodic fluctuations were also caused by the alternative process of O2 bubble accumulation and release. Thus, the high durability and stability of CoNiSSe0.3 were determined by CV and chronopotentiometric measurements for the OER.
Comparison of electrochemical performance and improvement mechanism To better understand the performance advantages of the asprepared quaternary CoNiSSe-0.3 materials, we compared the HER and OER properties of the quaternary materials with those of other CoSe2-based materials. As shown in Table 1, CoNiSSe-0.3 performs well for both the HER and OER, indicating that the designed flower-like quaternary CoeNieSeSe materials with excellent and balanced properties can be used as HER/OER bifunctional electrocatalysts with high stability and low economic cost. According to a previous report, it is believed that pyritestructured CoNiSSe-x also has a similar HER mechanism and the number of active sites as other electrocatalysts with the pyrite-structure, not only metals (Co and Ni) but also
nonmetals (Se and S) in edged sites may take part in the HER process [57]. Moreover, it was reported that the OER reactivity of hydroxide monolayers and metal oxide thin films of firstrow TMs follow the order Fe < Co < Ni, as determined by the strength of the MOH bond [58,59]. The higher catalytic activity of the dopant Ni can be attributed to the Ni hydroxide or oxide layer being more active than the Co layer. Therefore, we suggest that the pyrite-structure and coexistence of CoeNieSeSe and amorphous oxide phases are responsible for the superior bifunctional electronic activity. Meanwhile, the specific flower-like morphology of CoNiSSe0.3 not only provides abundant surface active sites but also improves the long-term stability by reducing structural stress [60,61]. Benefiting from the unique structure and composition, the optimal CoNiSSe-0.3 shows a remarkably enhanced catalytic performance for the HER and OER. The activity of CoNiSSe-0.3 was better than or comparable to that of other Co-based catalysts. Due to the low cost and earth abundance of Ni, CoNiSSe-0.3 is a promising candidate for commercial applications.
Conclusion We have successfully fabricated flower-like Co0.7Ni0.3S0.5Se1.5 by a facile one-step hydrothermal method. CoNiSSe-0.3 has been confirmed as a robust and bifunctional catalyst with overpotentials of 138 mV and 370 mV to drive a current density of 10 mA cm2 for the HER in acidic medium and the OER in alkaline medium, respectively. Its robust electrocatalytic performance can be attributed to its unique structure and special composition: (1) The 3D flower-like conductive network formed by 2D nanosheets greatly improves the electrolyte penetration and the transfer of electrons, and results in a more favorable reaction kinetic process. (2) The typical metallic nature of Co-based and Nibased catalysts ensures efficient transfer of electrons between the surface of the catalysts and the electrodes. (3) The maintained pyrite-structure and the oxides on the surface of CoNiSSe-0.3 are favorable for enhancing catalytic performance. Our work not only develops a high-performance bifunctional catalyst for water electrolysis but also paves the way for engineering the composition and structure of new materials.
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Acknowledgement This research was supported by grants from the Danish National Research Foundation, the AUFF-NOVA project from Aarhus Universitets Forskningsfond and the EU H2020RISE 2016-MNR4S Cell project. Zaixing Jiang acknowledges financial support from National Program for Support of Top-notch Young Professionals, National Natural Science Foundation of China (No. 51773049), China Aerospace Science and Technology Corporation-Harbin Institute of Technology Joint Center for Technology Innovation Fund (HIT15-1A01), Shanghai Academy of Spaceflight Technology Fund (SAST2017-126), Harbin city science and technology projects (2013DB4BP031 and RC2014QN017035), China Postdoctoral Science Foundation (No. 201003420, No.20090460067), HIT Research Institute (Zhao Yuan) of New Materials and Intelligent Equipment Technology Co., Ltd. Scientific and Technological Cooperation and Development Fund (No.2017KJHZ002). Zegao Wang thanks the supporting by the Fundamental Research Funds for the Central Universities, China (YJ201893) and State Key Lab of Advanced Metals and Materials, China (Grant No. 2019-Z03).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.08.257.
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