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Facile synthesis of CoNi2S4 and CuCo2S4 with different morphologies as prominent catalysts for hydrogen evolution reaction
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Facile synthesis of CoNi2S4 and CuCo2S4 with different morphologies as prominent catalysts for hydrogen evolution reaction Yuancai Ge a, Jingjie Wu b, Xiaowei Xu a, Mingxin Ye a,*, Jianfeng Shen a,** a b
Institute of Special Materials and Technology, Fudan University, 200433, Shanghai, China Department of Materials Science and NanoEngineering, Rice University, 6100 Main Street, Houston, TX 77005, USA
article info
abstract
Article history:
In this paper, a facile one-pot method was employed to synthesize CoNi2S4 and CuCo2S4 with
Received 18 May 2016
various morphologies. The required overpotentials for CoNi2S4 nanorod and CuCo2S4 cluster
Received in revised form
are only 111 mV and 135 mV, respectively, to reach the current density of 10 mA/cm2. The
27 July 2016
Tafel slopes for CoNi2S4 nanorod (53 mV/dec) and CuCo2S4 cluster (63 mV/dec) are lower than
Accepted 15 August 2016
that for binary systems of CoS2 and NiS2. The CoNi2S4 nanorod shows better long-term
Available online xxx
durability than CuCo2S4 cluster during the galvanostatic electrolysis. Overall these ternary sulfides exhibit promising activity and fair stability for hydrogen evolution reaction.
Keywords:
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
CoNi2S4 CuCo2S4 Ternary sulfides Hydrogen evolution reaction
Introduction With an ultra-high energy density of 140 MJ/kg, hydrogen is regarded as the cleanest chemical fuel for sustainable energy [1]. Production of hydrogen from water splitting is one of the most promising methods for renewable energy application [2,3]. Typically, noble metals, such as platinum, are widely used as the cathodic material to generate hydrogen due to their superb activity toward HER. However, the poor abundance and high cost of platinum have hampered its largescale application for HER [4]. Hence, replacement of platinum with earth abundant and lower cost elements is full of practical significance. Extensive effort has been devoted to develop alternative catalyst with non-precious metals. For
example, many studies have been directed to layered transition-metal dichalcogenides (LTMDs), such as WS2, MoS2, MoSe2 and WSe2, whose edge sites have performed excellent catalytic activity toward HER [5e10]. Jin's group had reported the successful fabrication of 1T-MoS2 via lithium intercalation with superb HER activity (a low overpotential of 187 mV/dec to reach a current density of 10 mA/cm2 and small Tafel slope of 43 mV/dec) [11]. Although 1T phase of MoS2 and WS2 has been found to own higher basal plane activities than their 2H counterparts, the 1T structure is metastable that is easily transformed to a more stable 2H structure with inert basal plane sites [12e14]. So far the approach to increase more stable active sites of TMDs in a massive way still remains a great challenge [15,16].
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (M. Ye). http://dx.doi.org/10.1016/j.ijhydene.2016.08.096 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Ge Y, et al., Facile synthesis of CoNi2S4 and CuCo2S4 with different morphologies as prominent catalysts for hydrogen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.08.096
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On the other hand, metal chalcogenide, metal phosphide and their alloys such as Ni3S2, CoS2, NiS2, CoSe2, NiSe2, NiSe and CoPS have emerged as another family of advanced HER catalysts [17e27]. Especially, CoS2 exhibits fast electron transfer and high conductivity due to its intrinsically metallic property [28e30]. Ramakrishna et al. prepared flexible film combining CoS2 nanosheet with graphene and carbon nanotube to form binder-free 3D structure, which demonstrated superior HER activity with a small overpotential (142 mV) at 10 mA/cm2 [21]. Jin et al. prepared a series of pyrite films containing FeS2, CoS2, NiS2 and their alloys through an electron-beam evaporation treatment [22]. It was demonstrated that CoS2 could reach the current density of 10 mA/ cm2 under an overpotential of 192 mV and its Tafel slope was only 52 mV/decade. They had also reported the more thermal neutral hydrogen adsorption of pyrite phase CoPS was more favorable for HER. The required overpotential was only 48 mV to reach a current density of 10 mA/cm2 with a small Tafel slope of 56 mV/decade for CoPS nanoplates [27]. Ternary metal sulfides, such as CuCo2S4, CoNi2S4 and NiCo2S4 have been previously applied as active electrode materials for energy storage, but only few reports for HER [31e35]. To the best of our knowledge, the HER catalysts based on Co0.59Ni0.41S2, Cu2MoS4, NiCo2S4 and Ni0.33Co0.67S2 have been reported recently [22,36e42]. The work reported by Lu et al. demonstrated excellent electrochemical performance of NiCo2S4 fabricated on carbon cloth in alkali solution for full water splitting [38]. The overpotential to reach a current density of 100 mA/cm2 was only 305 mV in 1.0 M KOH. In addition, Zheng et al. has revealed that the HER activity of bimetallic Ni0.33Co0.67S2 is much higher than single Co- or Ni- based sulfide catalysts [39]. As reported previously, CoNi2S4 was inclined to form needle-like morphology or sheet by depositing cobalt and nickel precursors on various substrates followed an anioneexchange reaction [33,35]. In other works, the CuCo2S4 and CoNi2S4 nanoparticles could be formed in a one-pot solvothermal method in the absence of tertbutanol (TBA) [32,34]. Herein, we report a facile solvothermal approach to synthesize morphology controlled CuCo2S4 cluster and CoNi2S4 nanorod with an assistance of TBA molecules through a onepot method. In this work, the CuCo2S4 cluster and CoNi2S4 nanorod with a large fraction of active sites were for the first time applied to HER. Compared with planar electrode, the micro- and nano-morphology of the CuCo2S4 cluster and CoNi2S4 nanorod could provide more active sites and facilitate the diffusion of the electrolyte [20,43]. Furthermore, their stability could be strengthened notably by promoting the release of H2 bubbles from the electrode surface [44]. In contrast to CoS2 and NiS2, the better HER activities of CuCo2S4 cluster and CoNi2S4 nanorod might ascribe to the increment of active sites for catalysis resulting from the introducing of different metal elements and special morphologies.
Experimental Materials Cu (NO3)2$3H2O, Co (NO3)2$6H2O, Ni (NO3)2$6H2O, thiourea, ammonium, iso- propyl alcohol (IPA) and TBA were of
research purity and purchased from Sinopharm Chemical. All chemical reagents were used without further purification.
Synthesis of CoNi2S4 nanorod The synthesis of CoNi2S4 followed a previously reported route with some modifications [34]. First, 2 mmol cobalt nitrate hexahydrate, 4 mmol nickel nitrate hexahydrate and 18 mmol thiourea was dissolved in 40 mL deionized water and 60 mL TBA. Then 5 mL ammonium was added dropwisely and the mixture was transferred to 150 mL Teflonlined autoclave. The solvothermal reaction was maintained at 180 C for 12 h. After that, the autoclave was cooled down to room temperature and the black precipitate was washed with deionized water and ethanol for three times and dried at 60 C for 24 h in vacuum.
Synthesis of CuCo2S4 cluster CuCo2S4 was synthesized through a facile one-pot solvothermal method as described above for CoNi2S4 with minor modifications. Briefly, 2 mmol copper nitrate hydrate, 4 mmol cobalt nitrate hexahydrate, and 10 mmol thiourea was dissolved in 40 mL deionized water and 60 mL TBA. Then 5 mL ammonium was added to the solution dropwisely and allowed to react following the steps as mentioned above.
Characterization Powder X-ray (PXRD) diffraction was employed to characterize the as-obtained samples with a Bruker D8 Advance X-ray diffractometer by using Cu Ka radiation at a scan rate of 8 / min in a 2q ranging from 5 to 90 . X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi) experiment was investigated to further confirm the chemical state and the composition of the samples by using an Al Ka source 1846.6 eV anode. And the data of XPS spectra were calibrated to the standard by using the C 1s line corrected to 284.8 eV. The software Auger Scan was employed to subtract the background and fit the curves. The morphology of the samples was observed by a field emission scanning electron microscope (FESEM, Tescan MAIA3 XMH) at an acceleration voltage of 5 kV. Transmission electron microscope (TEM, JEOL JEM-2100 LaB6) was conducted under an acceleration voltage of 200 kV and all samples were dispersed in ethanol via sonication for 4 h before testing.
Electrochemistry measurements To fabricate the working electrode, the catalyst ink was made up by mixing 4 mg CoNi2S4 or CuCo2S4, 30 mL nafion solution (5wt%) with 1 mL IPA and sonicated for 30 min to form homogeneous black ink. Then 10 mL of as-prepared catalyst ink was loaded on the glass carbon electrode (GCE) of 3 mm in diameter. The loading of the catalyst was controlled to be 2.6 mg/cm2. Then, the electrodes were dried at 60 C for 24 h in vacuum before electrochemical tests.
Please cite this article in press as: Ge Y, et al., Facile synthesis of CoNi2S4 and CuCo2S4 with different morphologies as prominent catalysts for hydrogen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.08.096
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All of the electrochemical experiments were investigated in a standard three-electrode setup and 0.5 M H2SO4 was used as the electrolyte. To calibrate all results to reversible hydrogen electrode (RHE), a polished platinum plate was used as the working electrode, saturated calomel electrode (SCE) as the reference electrode and platinum plate as the counter electrode with continuous H2 flow [45]. A scan rate of 0.1 mV/s was chosen to perform the linear sweep voltammetry (LSV) recorded by an Autolab PGSTAT302N electrochemical workstation (Metrohm AG, Switzerland), and the value of overpotential at which the current density was zero was chosen to be the thermodynamic potential (vs. SCE) where hydrogen electrode reaction took place. Fig. S1 had exhibited the value of thermodynamic potential of 279 mV in 0.5 M H2SO4. Therefore, E (RHE) ¼ E (SCE) þ 279 mV in this work. To investigate the electrochemical properties of CoNi2S4 nanorod and CuCo2S4 cluster, LSV, chronopotentiometry and electrochemical impedance spectroscopy (EIS) tests were employed with a platinum plate as the counter electrode and a SCE as the reference electrode. All results were obtained by testing the working electrodes containing catalysts with a scan rate of 5 mV/s. N2 flowing was maintained to deprive the dissolved oxygen during the tests. All potentials reported were after correction of ohmic losses via iR compensation.
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Results and discussion Compositional and structural characterization It's widely accepted that the morphology of the catalysts has substantial influence on the HER property. In this work, the morphologies of CoNi2S4 nanorod and CuCo2S4 cluster were mainly controlled by TBA concentration due to its special chemical structure and high viscosity through a one-pot solvothermal treatment. The FESEM image of CoNi2S4 nanorod and CuCo2S4 cluster observed in Fig. 1(a) and (c) illustrate that the sizes of both samples are hundred nanometer in diameter and these morphologies are in agreements with previous report [28]. Taking the elemental mapping images of CoNi2S4 nanorod and CuCo2S4 cluster (Fig. S2) into consideration, it was obvious that the elements of Ni, Co, Cu and S were dispersed homogeneously in each specimen. The detailed strategies for fabrication of the CuCo2S4 cluster and CoNi2S4 nanorod are exhibited in Scheme 1. When the ratio of TBA and deionized water was 3:2, water-in-oil (W/O) type nanoemulsion was produced by self-assembly. Then, the drifting cobalt ions, nickel ions and copper ions would fill the clearance of the nanoemulsions to form nucleus. After adding ammonium drop wisely, a large amount of metal hydroxide
Fig. 1 e (a) SEM images of CoNi2S4 nanorod, (b) HRTEM image of CoNi2S4 nanorod, (c) SEM image of CuCo2S4 cluster and (d) HRTEM image of CuCo2S4 cluster. Please cite this article in press as: Ge Y, et al., Facile synthesis of CoNi2S4 and CuCo2S4 with different morphologies as prominent catalysts for hydrogen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.08.096
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Scheme 1 e Schematic illustration for the synthesis of CoNi2S4 nanorod and CuCo2S4 cluster in this paper. (a) Synthesis CoNi2S4 nanorod and CuCo2S4 cluster through a one-pot solvothermal treatment at 180 C for 12 h, (b) The formation of CoNi2S4 nanorod and CuCo2S4 cluster by filling the gaps of the water/TBA emulsion and (c) Removing the emulsion through repeated washing and thoroughly drying under vacuum.
appeared. During the solvothermal treatment, CoNi2S4 nanorod and CuCo2S4 cluster grown along the surface of the TBA chains. It's worth noting some nanosheets were shown in Fig. S3 which might ascribe to the appearance of some metal layered double hydroxides (MLDHs). However, the amount of the nanosheets is little as evidence by the unremarkable the peaks of MLDHs in PXRD. The crystal structures of CoNi2S4 nanorod and CuCo2S4 cluster were further revealed by HRTEM in Fig. 1(b) and (d). The resolved lattice fringes of 0.212 nm in Fig. 1(b) belongs to the (4 0 0) plane of CoNi2S4 while 0.211 nm in Fig. 1(d) for the (0 0 4) plane of CuCo2S4 is in accordance with the results of PXRD. The results demonstrated in Fig. S4 further exhibit the clear evidence that the amount of CoNi2S4 and CuCo2S4 nanoparticles would increase rapidly in the absence of TBA molecules. Hence, the special morphologies of CoNi2S4 nanorod and CuCo2S4 cluster are mainly related to using TBA and water as the solvents which have fortified their catalytic performances with more active sites by increasing exposed surface area for HER. Fig. 2 shows the PXRD patterns of CuCo2S4 cluster and CoNi2S4 nanorod obtained through solvothermal reaction. The
Fig. 2 e The PXRD patterns of CoNi2S4 nanorod, CuCo2S4 cluster, CoS2, NiS2 and their standard PXRD patterns.
patterns of CuCo2S4 and CoNi2S4 are in good agreement with previous reports [31,46,47]. The peaks at 26.6 , 31.3 , 38.0 , 47.0 , 50.0 and 54.8 correspond to the (0 2 2), (1 1 3), (0 0 4), (2 2 4), (1 1 5) and (0 4 4) planes of the carrollite phase of CuCo2S4 structure, respectively. The peaks at 26.9 , 31.5 , 38.3 , 50.3 and 55.1 of CoNi2S4 correspond to the (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 0 0) diffraction planes, respectively. Also, Fig. 2 illustrated the PXRD pattern of pyrite-structure CoS2 (JCPDS 65-3322) and NiS2 (JCPDS 65-3325) as control specimen. It was obvious that all peaks of CoNi2S4 nanorod and CuCo2S4 cluster were in consistent with standard PXRD patterns of the cubic phase of CoNi2S4 (JCPDS 24-0334) and the carrollite structure phase of CuCo2S4 (JCPDS 42-1450). Moreover, the different PXRD patterns of these catalysts suggested that the CoNi2S4 nanorod and CuCo2S4 cluster were not simply the alloys of the metal pyrite phase. XPS was used to verify the valence and chemical compositions of CuCo2S4 cluster and CoNi2S4 nanorod, as plotted in Figs. 3 and 4. In Fig. 3 (b), the Cu 2p spectrum can be fitted into two chemical states. The binding energy at 932.7 and 954 eV are assigned to Cuþ and the binding energy at 935 and 957 eV are assigned to Cu2þ as previously reported [48]. In Figs. 3 (c) and Fig. 4 (b), the spineorbit doublets centered at 780 eV, 796.2 eV are assigned to Co 2p 3/2, Co 2p 1/2 which indicate the existence of Co2þ, Co3þ and two shake-up satellites. Similarly, the Ni 2p spectrum can also be fitted into two spin-orbit doublets of Ni2þ, Ni3þ centered at 855 eV and 875.2 eV with two satellites in Fig. 4 (c). Figs. 3 (d) and Fig. 4 (d) present the state for S 2p 3/2 that the peak centered at 162.5 eV agrees with the metal-sulfur bonds (CoeS, NieS and CueS). While the peak centered at 169.0 eV is assigned to the higher oxidation state of Sulfur (S4O6 2 ) due to the partial oxidation of the specimens. Therefore, the states of each element are revealed by XPS and the consequences are in good agreements with PXRD.
Please cite this article in press as: Ge Y, et al., Facile synthesis of CoNi2S4 and CuCo2S4 with different morphologies as prominent catalysts for hydrogen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.08.096
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Fig. 3 e (a) Survey spectrum of CuCo2S4 (b) Cu 2p, (c) Co 2p and (d) S 2p.
Fig. 4 e (a) Survey spectrum of CoNi2S4 (b) Co 2p, (c) Ni 2p and (d) S 2p.
Please cite this article in press as: Ge Y, et al., Facile synthesis of CoNi2S4 and CuCo2S4 with different morphologies as prominent catalysts for hydrogen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.08.096
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Electrochemical activity and stability toward HER The polarization curves were recorded to quantify the HER performance of these two ternary sulfide catalysts. Both CoNi2S4 nanorod and CuCo2S4 cluster exhibits very low onset overpotential to initiate the HER (Fig. 5a). To reach the benchmark of current density of 10 mA/cm2, CoNi2S4 nanorod require an overpotential of 110 mV, a slightly lower than that for CuCo2S4 cluster (135 m V), but still higher than that for Pt/C catalysts (less than 50 mV) [49,50]. By plotting the logarithm of the current density against the overpotential, Tafel plots (h ¼ b log j þ a, where j was the current density, h was the overpotential and b was the Tafel slope) are obtained as depicted in Fig. 5(b). The Tafel slopes for CoNi2S4 nanorod and CuCo2S4 cluster are extracted as 53 mV/dec and 63 mV/dec, respectively. As a comparison, the Pt/C shows a characteristic Tafel slope of 30 mV/dec. The mechanism of HER in aqueous solution is widely accepted to proceed through a proton discharge step (Volmer reaction) to electrochemical desorption step (Heyrovsky reaction) or chemical recombination step (Tafel reaction) [51,52]. According to the Tafel slopes, it strongly suggests HER proceed via a Volmer-Heyrovsky route for both CoNi2S4 nanorod and CuCo2S4 cluster with Heyrovsky route as the most possible rate determining step [53,54]. In addition, CoNi2S4 nanorod has exhibited faster HER kinetics than CuCo2S4 cluster. The EIS analysis was conducted to provide in-depth HER kinetics of CoNi2S4 nanorod and CuCo2S4 cluster. And the data are fitted into a semicircle by using simplified Randles equivalent circuit. The obtained result shows CoNi2S4 nanorod has a
charge transfer resistance of 46 U at 100 mV, drastically smaller than 71 U for CuCo2S4 cluster, suggesting a faster electron transferring pathway for CoNi2S4 (Fig. 5d). The capacitance derived from the CPE fitting value indicates CoNi2S4 nanorod (2.1 105 F) have around two times larger effective surface area than CuCo2S4 cluster (1.10 105 F). To evaluate the double-layer capacitance of CoNi2S4 nanorod and CuCo2S4 cluster, the result obtained from Cyclical voltammetry (CV) scans (Fig. S5) has also confirmed that the active surface area of CoNi2S4 nanorod (7.22 mF/cm2) is larger than CuCo2S4 cluster (2.21 mF/cm2) [55,56]. For a fair comparison, the exchange current density and Tafel slope are normalized to the relative surface areas (Table S1). The normalized current density (j0 normalized) of CoNi2S4 nanorod and CuCo2S4 cluster are 7.3 mA and 16 mA, respectively. That means CoNi2S4 nanorod and CuCo2S4 cluster share the comparable exchange current density which can stand for the intrinsic activity. Thus, the decent HER property of CoNi2S4 nanorod is mainly attributed to its intrinsic faster charge transfer and its larger amount of active sites. The long-term stabilities of as prepared samples were evaluated by applying chrono-potentiometry method. In the total 10 h continuous running at 10 mA/cm2, CoNi2S4 nanorod exhibited negligible degradation (Fig. 5c). However, the overpotential of CuCo2S4 cluster increases from 137 to 190 mV after 10 h. Fig. S6 had illustrated the SEM images of CoNi2S4 nanorod and CuCo2S4 cluster to evaluate the changes in morphology after long-term electrochemical experiments. The superb stabilities of CoNi2S4 nanorod and CuCo2S4 cluster were confirmed without obvious aggregation and stack after 10 h running in 0.5M H2SO4.
Fig. 5 e (a) Polarization curves, (b) Tafel plots, (c) durability test of CoNi2S4 nanorod and CuCo2S4 cluster and (d) EIS Nyquist plots and fitted data of CoNi2S4 nanorod and CuCo2S4 cluster (solid line). Please cite this article in press as: Ge Y, et al., Facile synthesis of CoNi2S4 and CuCo2S4 with different morphologies as prominent catalysts for hydrogen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.08.096
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The HER activities of CoNi2S4 nanorod and CuCo2S4 cluster were also measured in 1M KOH and neutral phosphate buffer solution (Fig. S7). Their required overpotentials were 184 and 224 mV for CoNi2S4 nanorod and CuCo2S4 cluster to reach the current density of 10 mA/cm2 in neutral phosphate buffer solution. To reach the current density of 10 mA/cm2 in 1 M KOH, the overpotentials were higher for both CoNi2S4 nanorod (146 mV) and CuCo2S4 cluster (156 mV) than that in 0.5 M H2SO4.
[10]
[11]
[12]
Conclusion [13]
In conclusion, a facile solvothermal method has been employed to fabricate CoNi2S4 nanorod and CuCo2S4 cluster. Compared to their monometallic sulfides such as CoS2 and NiS2(Table S2), both CoNi2S4 nanorod and CuCo2S4 cluster exhibited higher current densities, lower overpotentials and better durability for water reduction. The admirable HER activity of CoNi2S4 nanorod and CuCo2S4 cluster may be attributed to their enhanced conductivity and large electrochemical surface area. This work opens up a potential opportunity to design ternary sulfides for as advanced HER catalysts.
Appendix A. Supplementary data Supplementary data related to this chapter can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.08.096.
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Please cite this article in press as: Ge Y, et al., Facile synthesis of CoNi2S4 and CuCo2S4 with different morphologies as prominent catalysts for hydrogen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.08.096
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n-Acevedo M, Stone ML, Schmidt JR, Thomas JG, Ding Q, [27] Caba Chang H, et al. Efficient hydrogen evolution catalysis using ternary pyrite-type cobalt phosphosulphide. Nat Mater 2015;14:1245e51. [28] Ren R, Faber MS, Dziedzic R, Wen Z, Jin S, Mao S, et al. Metallic CoS2 nanowire electrodes for high cycling performance supercapacitors. Nat Nanotechnol 2015;26:494001. [29] Liang Y, Liu Q, Luo Y, Sun X, He Y, Asiri AM. Zn0.76Co0.24S/ CoS2 nanowires array for efficient electrochemical splitting of water. Electrochim Acta 2016;190:360e4. [30] Liu T, Liang Y, Liu Q, Sun X, He Y, Asiri AM. Electrodeposition of cobalt-sulfide nanosheets film as an efficient electrocatalyst for oxygen evolution reaction. Electrochem Commun 2015;60:92e6. [31] Tang Jianhua, Ge Yuancai, Shen Jianfeng, Ye Mingxin. Facile synthesis of CuCo2S4 as a novel electrode material for ultrahigh supercapacitor performance. Chem Commun 2015;52:1509e12. [32] Du W, Zhu Z, Wang Y, Liu J, Yang W, Qian X, et al. One-step synthesis of CoNi2S4 nanoparticles for supercapacitor electrodes. RSC Adv 2014;4:6998e7002. [33] Hu W, Chen R, Xie W, Zou L, Qin N, Bao D. CoNi2S4 nanosheet arrays supported on nickel foams with ultrahigh capacitance for aqueous asymmetric supercapacitor applications. ACS Appl Mater Inter 2014;6:19318e26. [34] Tang J, Shen J, Li N, Ye M. One-pot tertbutanol assisted solvothermal synthesis of CoNi2S4/reduced graphene oxide nanocomposite for high-performance supercapacitors. Ceram Int 2015;41:6203e11. [35] Chen L, Zhou Y, Dai H, Yu T, Liu J, Zou Z. One-step growth of CoNi2S4 nanoribbons on carbon fibers as platinum-free counter electrodes for fiber-shaped dye-sensitized solar cells with high performance: polymorph-dependent conversion efficiency. Nano Energy 2015;11:697e703. [36] Liu T, Asiri AM, Sun X. Electrodeposited Co-doped NiSe2 nanoparticles film: a good electrocatalyst for efficient water splitting. Nanoscale 2016;8:3911e5. [37] Tran PD, Nguyen M, Pramana SS, Bhattacharjee A, Chiam SY, Fize J, et al. Copper molybdenum sulfide: a new efficient electrocatalyst for hydrogen production from water. Energ Environ Sci 2012;5:8912. [38] Liu D, Lu Q, Luo Y, Sun X, Asiri AM. NiCo2S4 nanowires array as an efficient bifunctional electrocatalyst for full water splitting with superior activity. Nanoscale 2015;7:15122e6. [39] Peng Z, Jia D, Al-Enizi AM, Elzatahry AA, Zheng G. From water oxidation to reduction: homologous Ni-Co based nanowires as complementary water splitting electrocatalysts. Adv Energy Mater 2015;5:1402031. [40] Deng Jiao, Li Haobo, Xiao Jianping, Tu Yunchuan, Deng Dehui, Yang Huaixin, et al. Triggering the electrocatalytic hydrogen evolution activity of the inert twodimensional MoS2 surface via single-atom metal doping. Energy Environ Sci 2015;8:1594e601. [41] Tran PD, Chiam SY, Boix PP, Ren Y, Pramana SS, Fize J, et al. Novel cobalt/nickeletungsten-sulfide catalysts for electrocatalytic hydrogen generation from water. Energy Environ Sci 2013;6:2452e9.
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Please cite this article in press as: Ge Y, et al., Facile synthesis of CoNi2S4 and CuCo2S4 with different morphologies as prominent catalysts for hydrogen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.08.096
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Facile synthesis of CoNi2S4 and CuCo2S4 with different morphologies as prominent catalysts for hydrogen evolution reaction Yuancai Ge a, Jingjie Wu b, Xiaowei Xu a, Mingxin Ye a,*, Jianfeng Shen a,** a b
Institute of Special Materials and Technology, Fudan University, 200433, Shanghai, China Department of Materials Science and NanoEngineering, Rice University, 6100 Main Street, Houston, TX 77005, USA
article info
abstract
Article history:
In this paper, a facile one-pot method was employed to synthesize CoNi2S4 and CuCo2S4 with
Received 18 May 2016
various morphologies. The required overpotentials for CoNi2S4 nanorod and CuCo2S4 cluster
Received in revised form
are only 111 mV and 135 mV, respectively, to reach the current density of 10 mA/cm2. The
27 July 2016
Tafel slopes for CoNi2S4 nanorod (53 mV/dec) and CuCo2S4 cluster (63 mV/dec) are lower than
Accepted 15 August 2016
that for binary systems of CoS2 and NiS2. The CoNi2S4 nanorod shows better long-term
Available online xxx
durability than CuCo2S4 cluster during the galvanostatic electrolysis. Overall these ternary sulfides exhibit promising activity and fair stability for hydrogen evolution reaction.
Keywords:
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
CoNi2S4 CuCo2S4 Ternary sulfides Hydrogen evolution reaction
Introduction With an ultra-high energy density of 140 MJ/kg, hydrogen is regarded as the cleanest chemical fuel for sustainable energy [1]. Production of hydrogen from water splitting is one of the most promising methods for renewable energy application [2,3]. Typically, noble metals, such as platinum, are widely used as the cathodic material to generate hydrogen due to their superb activity toward HER. However, the poor abundance and high cost of platinum have hampered its largescale application for HER [4]. Hence, replacement of platinum with earth abundant and lower cost elements is full of practical significance. Extensive effort has been devoted to develop alternative catalyst with non-precious metals. For
example, many studies have been directed to layered transition-metal dichalcogenides (LTMDs), such as WS2, MoS2, MoSe2 and WSe2, whose edge sites have performed excellent catalytic activity toward HER [5e10]. Jin's group had reported the successful fabrication of 1T-MoS2 via lithium intercalation with superb HER activity (a low overpotential of 187 mV/dec to reach a current density of 10 mA/cm2 and small Tafel slope of 43 mV/dec) [11]. Although 1T phase of MoS2 and WS2 has been found to own higher basal plane activities than their 2H counterparts, the 1T structure is metastable that is easily transformed to a more stable 2H structure with inert basal plane sites [12e14]. So far the approach to increase more stable active sites of TMDs in a massive way still remains a great challenge [15,16].
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (M. Ye). http://dx.doi.org/10.1016/j.ijhydene.2016.08.096 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Ge Y, et al., Facile synthesis of CoNi2S4 and CuCo2S4 with different morphologies as prominent catalysts for hydrogen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.08.096
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On the other hand, metal chalcogenide, metal phosphide and their alloys such as Ni3S2, CoS2, NiS2, CoSe2, NiSe2, NiSe and CoPS have emerged as another family of advanced HER catalysts [17e27]. Especially, CoS2 exhibits fast electron transfer and high conductivity due to its intrinsically metallic property [28e30]. Ramakrishna et al. prepared flexible film combining CoS2 nanosheet with graphene and carbon nanotube to form binder-free 3D structure, which demonstrated superior HER activity with a small overpotential (142 mV) at 10 mA/cm2 [21]. Jin et al. prepared a series of pyrite films containing FeS2, CoS2, NiS2 and their alloys through an electron-beam evaporation treatment [22]. It was demonstrated that CoS2 could reach the current density of 10 mA/ cm2 under an overpotential of 192 mV and its Tafel slope was only 52 mV/decade. They had also reported the more thermal neutral hydrogen adsorption of pyrite phase CoPS was more favorable for HER. The required overpotential was only 48 mV to reach a current density of 10 mA/cm2 with a small Tafel slope of 56 mV/decade for CoPS nanoplates [27]. Ternary metal sulfides, such as CuCo2S4, CoNi2S4 and NiCo2S4 have been previously applied as active electrode materials for energy storage, but only few reports for HER [31e35]. To the best of our knowledge, the HER catalysts based on Co0.59Ni0.41S2, Cu2MoS4, NiCo2S4 and Ni0.33Co0.67S2 have been reported recently [22,36e42]. The work reported by Lu et al. demonstrated excellent electrochemical performance of NiCo2S4 fabricated on carbon cloth in alkali solution for full water splitting [38]. The overpotential to reach a current density of 100 mA/cm2 was only 305 mV in 1.0 M KOH. In addition, Zheng et al. has revealed that the HER activity of bimetallic Ni0.33Co0.67S2 is much higher than single Co- or Ni- based sulfide catalysts [39]. As reported previously, CoNi2S4 was inclined to form needle-like morphology or sheet by depositing cobalt and nickel precursors on various substrates followed an anioneexchange reaction [33,35]. In other works, the CuCo2S4 and CoNi2S4 nanoparticles could be formed in a one-pot solvothermal method in the absence of tertbutanol (TBA) [32,34]. Herein, we report a facile solvothermal approach to synthesize morphology controlled CuCo2S4 cluster and CoNi2S4 nanorod with an assistance of TBA molecules through a onepot method. In this work, the CuCo2S4 cluster and CoNi2S4 nanorod with a large fraction of active sites were for the first time applied to HER. Compared with planar electrode, the micro- and nano-morphology of the CuCo2S4 cluster and CoNi2S4 nanorod could provide more active sites and facilitate the diffusion of the electrolyte [20,43]. Furthermore, their stability could be strengthened notably by promoting the release of H2 bubbles from the electrode surface [44]. In contrast to CoS2 and NiS2, the better HER activities of CuCo2S4 cluster and CoNi2S4 nanorod might ascribe to the increment of active sites for catalysis resulting from the introducing of different metal elements and special morphologies.
Experimental Materials Cu (NO3)2$3H2O, Co (NO3)2$6H2O, Ni (NO3)2$6H2O, thiourea, ammonium, iso- propyl alcohol (IPA) and TBA were of
research purity and purchased from Sinopharm Chemical. All chemical reagents were used without further purification.
Synthesis of CoNi2S4 nanorod The synthesis of CoNi2S4 followed a previously reported route with some modifications [34]. First, 2 mmol cobalt nitrate hexahydrate, 4 mmol nickel nitrate hexahydrate and 18 mmol thiourea was dissolved in 40 mL deionized water and 60 mL TBA. Then 5 mL ammonium was added dropwisely and the mixture was transferred to 150 mL Teflonlined autoclave. The solvothermal reaction was maintained at 180 C for 12 h. After that, the autoclave was cooled down to room temperature and the black precipitate was washed with deionized water and ethanol for three times and dried at 60 C for 24 h in vacuum.
Synthesis of CuCo2S4 cluster CuCo2S4 was synthesized through a facile one-pot solvothermal method as described above for CoNi2S4 with minor modifications. Briefly, 2 mmol copper nitrate hydrate, 4 mmol cobalt nitrate hexahydrate, and 10 mmol thiourea was dissolved in 40 mL deionized water and 60 mL TBA. Then 5 mL ammonium was added to the solution dropwisely and allowed to react following the steps as mentioned above.
Characterization Powder X-ray (PXRD) diffraction was employed to characterize the as-obtained samples with a Bruker D8 Advance X-ray diffractometer by using Cu Ka radiation at a scan rate of 8 / min in a 2q ranging from 5 to 90 . X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi) experiment was investigated to further confirm the chemical state and the composition of the samples by using an Al Ka source 1846.6 eV anode. And the data of XPS spectra were calibrated to the standard by using the C 1s line corrected to 284.8 eV. The software Auger Scan was employed to subtract the background and fit the curves. The morphology of the samples was observed by a field emission scanning electron microscope (FESEM, Tescan MAIA3 XMH) at an acceleration voltage of 5 kV. Transmission electron microscope (TEM, JEOL JEM-2100 LaB6) was conducted under an acceleration voltage of 200 kV and all samples were dispersed in ethanol via sonication for 4 h before testing.
Electrochemistry measurements To fabricate the working electrode, the catalyst ink was made up by mixing 4 mg CoNi2S4 or CuCo2S4, 30 mL nafion solution (5wt%) with 1 mL IPA and sonicated for 30 min to form homogeneous black ink. Then 10 mL of as-prepared catalyst ink was loaded on the glass carbon electrode (GCE) of 3 mm in diameter. The loading of the catalyst was controlled to be 2.6 mg/cm2. Then, the electrodes were dried at 60 C for 24 h in vacuum before electrochemical tests.
Please cite this article in press as: Ge Y, et al., Facile synthesis of CoNi2S4 and CuCo2S4 with different morphologies as prominent catalysts for hydrogen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.08.096
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All of the electrochemical experiments were investigated in a standard three-electrode setup and 0.5 M H2SO4 was used as the electrolyte. To calibrate all results to reversible hydrogen electrode (RHE), a polished platinum plate was used as the working electrode, saturated calomel electrode (SCE) as the reference electrode and platinum plate as the counter electrode with continuous H2 flow [45]. A scan rate of 0.1 mV/s was chosen to perform the linear sweep voltammetry (LSV) recorded by an Autolab PGSTAT302N electrochemical workstation (Metrohm AG, Switzerland), and the value of overpotential at which the current density was zero was chosen to be the thermodynamic potential (vs. SCE) where hydrogen electrode reaction took place. Fig. S1 had exhibited the value of thermodynamic potential of 279 mV in 0.5 M H2SO4. Therefore, E (RHE) ¼ E (SCE) þ 279 mV in this work. To investigate the electrochemical properties of CoNi2S4 nanorod and CuCo2S4 cluster, LSV, chronopotentiometry and electrochemical impedance spectroscopy (EIS) tests were employed with a platinum plate as the counter electrode and a SCE as the reference electrode. All results were obtained by testing the working electrodes containing catalysts with a scan rate of 5 mV/s. N2 flowing was maintained to deprive the dissolved oxygen during the tests. All potentials reported were after correction of ohmic losses via iR compensation.
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Results and discussion Compositional and structural characterization It's widely accepted that the morphology of the catalysts has substantial influence on the HER property. In this work, the morphologies of CoNi2S4 nanorod and CuCo2S4 cluster were mainly controlled by TBA concentration due to its special chemical structure and high viscosity through a one-pot solvothermal treatment. The FESEM image of CoNi2S4 nanorod and CuCo2S4 cluster observed in Fig. 1(a) and (c) illustrate that the sizes of both samples are hundred nanometer in diameter and these morphologies are in agreements with previous report [28]. Taking the elemental mapping images of CoNi2S4 nanorod and CuCo2S4 cluster (Fig. S2) into consideration, it was obvious that the elements of Ni, Co, Cu and S were dispersed homogeneously in each specimen. The detailed strategies for fabrication of the CuCo2S4 cluster and CoNi2S4 nanorod are exhibited in Scheme 1. When the ratio of TBA and deionized water was 3:2, water-in-oil (W/O) type nanoemulsion was produced by self-assembly. Then, the drifting cobalt ions, nickel ions and copper ions would fill the clearance of the nanoemulsions to form nucleus. After adding ammonium drop wisely, a large amount of metal hydroxide
Fig. 1 e (a) SEM images of CoNi2S4 nanorod, (b) HRTEM image of CoNi2S4 nanorod, (c) SEM image of CuCo2S4 cluster and (d) HRTEM image of CuCo2S4 cluster. Please cite this article in press as: Ge Y, et al., Facile synthesis of CoNi2S4 and CuCo2S4 with different morphologies as prominent catalysts for hydrogen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.08.096
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Scheme 1 e Schematic illustration for the synthesis of CoNi2S4 nanorod and CuCo2S4 cluster in this paper. (a) Synthesis CoNi2S4 nanorod and CuCo2S4 cluster through a one-pot solvothermal treatment at 180 C for 12 h, (b) The formation of CoNi2S4 nanorod and CuCo2S4 cluster by filling the gaps of the water/TBA emulsion and (c) Removing the emulsion through repeated washing and thoroughly drying under vacuum.
appeared. During the solvothermal treatment, CoNi2S4 nanorod and CuCo2S4 cluster grown along the surface of the TBA chains. It's worth noting some nanosheets were shown in Fig. S3 which might ascribe to the appearance of some metal layered double hydroxides (MLDHs). However, the amount of the nanosheets is little as evidence by the unremarkable the peaks of MLDHs in PXRD. The crystal structures of CoNi2S4 nanorod and CuCo2S4 cluster were further revealed by HRTEM in Fig. 1(b) and (d). The resolved lattice fringes of 0.212 nm in Fig. 1(b) belongs to the (4 0 0) plane of CoNi2S4 while 0.211 nm in Fig. 1(d) for the (0 0 4) plane of CuCo2S4 is in accordance with the results of PXRD. The results demonstrated in Fig. S4 further exhibit the clear evidence that the amount of CoNi2S4 and CuCo2S4 nanoparticles would increase rapidly in the absence of TBA molecules. Hence, the special morphologies of CoNi2S4 nanorod and CuCo2S4 cluster are mainly related to using TBA and water as the solvents which have fortified their catalytic performances with more active sites by increasing exposed surface area for HER. Fig. 2 shows the PXRD patterns of CuCo2S4 cluster and CoNi2S4 nanorod obtained through solvothermal reaction. The
Fig. 2 e The PXRD patterns of CoNi2S4 nanorod, CuCo2S4 cluster, CoS2, NiS2 and their standard PXRD patterns.
patterns of CuCo2S4 and CoNi2S4 are in good agreement with previous reports [31,46,47]. The peaks at 26.6 , 31.3 , 38.0 , 47.0 , 50.0 and 54.8 correspond to the (0 2 2), (1 1 3), (0 0 4), (2 2 4), (1 1 5) and (0 4 4) planes of the carrollite phase of CuCo2S4 structure, respectively. The peaks at 26.9 , 31.5 , 38.3 , 50.3 and 55.1 of CoNi2S4 correspond to the (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 0 0) diffraction planes, respectively. Also, Fig. 2 illustrated the PXRD pattern of pyrite-structure CoS2 (JCPDS 65-3322) and NiS2 (JCPDS 65-3325) as control specimen. It was obvious that all peaks of CoNi2S4 nanorod and CuCo2S4 cluster were in consistent with standard PXRD patterns of the cubic phase of CoNi2S4 (JCPDS 24-0334) and the carrollite structure phase of CuCo2S4 (JCPDS 42-1450). Moreover, the different PXRD patterns of these catalysts suggested that the CoNi2S4 nanorod and CuCo2S4 cluster were not simply the alloys of the metal pyrite phase. XPS was used to verify the valence and chemical compositions of CuCo2S4 cluster and CoNi2S4 nanorod, as plotted in Figs. 3 and 4. In Fig. 3 (b), the Cu 2p spectrum can be fitted into two chemical states. The binding energy at 932.7 and 954 eV are assigned to Cuþ and the binding energy at 935 and 957 eV are assigned to Cu2þ as previously reported [48]. In Figs. 3 (c) and Fig. 4 (b), the spineorbit doublets centered at 780 eV, 796.2 eV are assigned to Co 2p 3/2, Co 2p 1/2 which indicate the existence of Co2þ, Co3þ and two shake-up satellites. Similarly, the Ni 2p spectrum can also be fitted into two spin-orbit doublets of Ni2þ, Ni3þ centered at 855 eV and 875.2 eV with two satellites in Fig. 4 (c). Figs. 3 (d) and Fig. 4 (d) present the state for S 2p 3/2 that the peak centered at 162.5 eV agrees with the metal-sulfur bonds (CoeS, NieS and CueS). While the peak centered at 169.0 eV is assigned to the higher oxidation state of Sulfur (S4O6 2 ) due to the partial oxidation of the specimens. Therefore, the states of each element are revealed by XPS and the consequences are in good agreements with PXRD.
Please cite this article in press as: Ge Y, et al., Facile synthesis of CoNi2S4 and CuCo2S4 with different morphologies as prominent catalysts for hydrogen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.08.096
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Fig. 3 e (a) Survey spectrum of CuCo2S4 (b) Cu 2p, (c) Co 2p and (d) S 2p.
Fig. 4 e (a) Survey spectrum of CoNi2S4 (b) Co 2p, (c) Ni 2p and (d) S 2p.
Please cite this article in press as: Ge Y, et al., Facile synthesis of CoNi2S4 and CuCo2S4 with different morphologies as prominent catalysts for hydrogen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.08.096
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Electrochemical activity and stability toward HER The polarization curves were recorded to quantify the HER performance of these two ternary sulfide catalysts. Both CoNi2S4 nanorod and CuCo2S4 cluster exhibits very low onset overpotential to initiate the HER (Fig. 5a). To reach the benchmark of current density of 10 mA/cm2, CoNi2S4 nanorod require an overpotential of 110 mV, a slightly lower than that for CuCo2S4 cluster (135 m V), but still higher than that for Pt/C catalysts (less than 50 mV) [49,50]. By plotting the logarithm of the current density against the overpotential, Tafel plots (h ¼ b log j þ a, where j was the current density, h was the overpotential and b was the Tafel slope) are obtained as depicted in Fig. 5(b). The Tafel slopes for CoNi2S4 nanorod and CuCo2S4 cluster are extracted as 53 mV/dec and 63 mV/dec, respectively. As a comparison, the Pt/C shows a characteristic Tafel slope of 30 mV/dec. The mechanism of HER in aqueous solution is widely accepted to proceed through a proton discharge step (Volmer reaction) to electrochemical desorption step (Heyrovsky reaction) or chemical recombination step (Tafel reaction) [51,52]. According to the Tafel slopes, it strongly suggests HER proceed via a Volmer-Heyrovsky route for both CoNi2S4 nanorod and CuCo2S4 cluster with Heyrovsky route as the most possible rate determining step [53,54]. In addition, CoNi2S4 nanorod has exhibited faster HER kinetics than CuCo2S4 cluster. The EIS analysis was conducted to provide in-depth HER kinetics of CoNi2S4 nanorod and CuCo2S4 cluster. And the data are fitted into a semicircle by using simplified Randles equivalent circuit. The obtained result shows CoNi2S4 nanorod has a
charge transfer resistance of 46 U at 100 mV, drastically smaller than 71 U for CuCo2S4 cluster, suggesting a faster electron transferring pathway for CoNi2S4 (Fig. 5d). The capacitance derived from the CPE fitting value indicates CoNi2S4 nanorod (2.1 105 F) have around two times larger effective surface area than CuCo2S4 cluster (1.10 105 F). To evaluate the double-layer capacitance of CoNi2S4 nanorod and CuCo2S4 cluster, the result obtained from Cyclical voltammetry (CV) scans (Fig. S5) has also confirmed that the active surface area of CoNi2S4 nanorod (7.22 mF/cm2) is larger than CuCo2S4 cluster (2.21 mF/cm2) [55,56]. For a fair comparison, the exchange current density and Tafel slope are normalized to the relative surface areas (Table S1). The normalized current density (j0 normalized) of CoNi2S4 nanorod and CuCo2S4 cluster are 7.3 mA and 16 mA, respectively. That means CoNi2S4 nanorod and CuCo2S4 cluster share the comparable exchange current density which can stand for the intrinsic activity. Thus, the decent HER property of CoNi2S4 nanorod is mainly attributed to its intrinsic faster charge transfer and its larger amount of active sites. The long-term stabilities of as prepared samples were evaluated by applying chrono-potentiometry method. In the total 10 h continuous running at 10 mA/cm2, CoNi2S4 nanorod exhibited negligible degradation (Fig. 5c). However, the overpotential of CuCo2S4 cluster increases from 137 to 190 mV after 10 h. Fig. S6 had illustrated the SEM images of CoNi2S4 nanorod and CuCo2S4 cluster to evaluate the changes in morphology after long-term electrochemical experiments. The superb stabilities of CoNi2S4 nanorod and CuCo2S4 cluster were confirmed without obvious aggregation and stack after 10 h running in 0.5M H2SO4.
Fig. 5 e (a) Polarization curves, (b) Tafel plots, (c) durability test of CoNi2S4 nanorod and CuCo2S4 cluster and (d) EIS Nyquist plots and fitted data of CoNi2S4 nanorod and CuCo2S4 cluster (solid line). Please cite this article in press as: Ge Y, et al., Facile synthesis of CoNi2S4 and CuCo2S4 with different morphologies as prominent catalysts for hydrogen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.08.096
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The HER activities of CoNi2S4 nanorod and CuCo2S4 cluster were also measured in 1M KOH and neutral phosphate buffer solution (Fig. S7). Their required overpotentials were 184 and 224 mV for CoNi2S4 nanorod and CuCo2S4 cluster to reach the current density of 10 mA/cm2 in neutral phosphate buffer solution. To reach the current density of 10 mA/cm2 in 1 M KOH, the overpotentials were higher for both CoNi2S4 nanorod (146 mV) and CuCo2S4 cluster (156 mV) than that in 0.5 M H2SO4.
[10]
[11]
[12]
Conclusion [13]
In conclusion, a facile solvothermal method has been employed to fabricate CoNi2S4 nanorod and CuCo2S4 cluster. Compared to their monometallic sulfides such as CoS2 and NiS2(Table S2), both CoNi2S4 nanorod and CuCo2S4 cluster exhibited higher current densities, lower overpotentials and better durability for water reduction. The admirable HER activity of CoNi2S4 nanorod and CuCo2S4 cluster may be attributed to their enhanced conductivity and large electrochemical surface area. This work opens up a potential opportunity to design ternary sulfides for as advanced HER catalysts.
Appendix A. Supplementary data Supplementary data related to this chapter can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.08.096.
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Please cite this article in press as: Ge Y, et al., Facile synthesis of CoNi2S4 and CuCo2S4 with different morphologies as prominent catalysts for hydrogen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.08.096