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Ni3S2 nanostructures as efficient bifunctional electrocatalyst for overall water splitting
Journal of Power Sources 416 (2019) 95–103
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Engineering hybrid CoMoS4/Ni3S2 nanostructures as efficient bifunctional electrocatalyst for overall water splitting
T
Peng Hua, Zhiyuan Jiab, Haibing Chea, Wenyuan Zhoua, Ning Liuc, Fan Lic,∗, Jinshu Wanga,∗∗ a
Key Laboratory of Advanced Functional Materials, Education Ministry of China, School of Materials Science and Engineering, Beijing University of Technology, Beijing, 100124, PR China b Shenyang Research Institute of Chemical Industry, Liaoning, 110021, PR China c College of Environmental & Energy Engineering, Beijing University of Technology, Beijing, 100124, PR China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
Bilayered CoMoS /Ni S electrode • was synthesized by a self-templating 4
3 2
strategy.
Porous CoMoS nanosheets array was • formed from CoMoO nanocolumns. Ni S interlayer greatly increased the • OER activity of CoMoS . Low potential (1.568 V@10 mA cm ) • for overall water splitting was 4
4
3 2
4
−2
achieved.
A R T I C LE I N FO
A B S T R A C T
Keywords: CoMoS4 Ni3S2 Hybrid nanostructure Bifunctional electrocatalyst Water splitting
In overall water splitting, oxygen evolution reaction is a rate-determining step due to its complex reaction processes with sluggish kinetics, so design of bifunctional electrocatalyst with elaborated structure to improve the oxygen evolution reaction performance is critical to their efficient application in overall water splitting. Here bifunctional CoMoS4/Ni3S2 nanostructures are in situ synthesized by a self-templating strategy, and the morphology control of the CoMoS4 outlayer is simultaneously achieved through a unique evolution from CoMoO4 nanocolumns array to hollow CoMoS4 nanosheets array with porous structure. Benefited from the synergistic effect of Ni3S2 interlayer, as obtained electrode exhibits greatly enhanced oxygen evolution activity than single CoMoS4 electrode with well-preserved hydrogen evolution activity. It, together with the three-dimensional topographic structure of CoMoS4 outlayer with high active areas, offers a rather low cell voltage (1.568 V@ 10 mA cm−2) for overall water splitting in a two-electrode system. Accordingly, our results may open new opportunities to explore hybrid nanostructures as efficient bifunctional electrocatalyst for overall water splitting.
1. Introduction Electrochemical water splitting by photoelectrolysis or electrolysis is one of the most promising ways for generating high-purity hydrogen, which is critical to fulfill the need for clean and renewable energy resources alternative to current fossil fuels. Water electrolysis is a thermodynamically unfavored reaction that involved two electrochemical
∗
half-cell reactions: the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) with theoretical potential of 1.23 V [1,2]. In practice, much high overpotential is applied due to the resistance existed in the electrode and its contacted interfaces, arising from the surface redox reactions and interface charges/ions transport (especially the rate-determined OER process) [3,4]. To accelerate its kinetics properties, highly active electrocatalysts are essential to minimize the
Corresponding author. Corresponding author. E-mail addresses: [email protected] (F. Li), [email protected] (J. Wang).
∗∗
https://doi.org/10.1016/j.jpowsour.2019.01.090 Received 20 November 2018; Received in revised form 15 January 2019; Accepted 28 January 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.
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needed to afford a current density of 10 mA cm−2 for over water splitting in a two-electrode system. Thus this work provides a new strategy for exploring efficient bifunctional electrocatalyst with high performance for overall water splitting.
extra overpotential and facilitate the electrochemical process [5,6]. Up to now, noble metal based materials are recognized as the most efficient catalysts and widely employed for HER and OER. But the scarcity and low stability of these noble metal catalysts make an obstacle for their practical applications in a large scale, and development of alternative earth-abundant catalyst materials with high efficient and durable performance for electrochemical hydrogen and oxygen production is highly desired [7,8]. In the search for new HER and OER catalysts alternative to noble metals, different transition-metal based materials, including chalcogenides [9–13], carbides [14,15], phosphides [16–21] etc., and their composites [22–27] have been explored and confirmed efficiently for the HER or/and OER processes. Among these materials, ternary sulfides X-M-S (where X = Co, Ni Cu etc. and M = W or Mo), a novel class of layer-structured materials, have been considered to be the promising candidates as electrochemical active materials due to their unique electric and structural features [28–30]. By comparison to monometal sulfides, ternary sulfides exhibit richer active sites due to chemical environment change by synergistic interaction of bimetallic ions [31–33]. They also present higher electronic conductivity than their corresponding ternary oxides [34], and create more flexible active structure due to the lower electronegativity of sulfur [35], promoting electrocatalytic process from thermodynamic and kinetic aspects. Recent experimental and theoretical works have confirmed their superior performance of HER in solution with a wide range of pH values [36–42]. Few work also explores their potential applications in OER [32,43], and makes them possible to use as bifunctional catalysts for overall water splitting. Constructing bifunctional electrocatalysts with the abilities for efficient HER and OER is benefit for improving the performance of overall water splitting, and further simplifies the watersplitting system in the same electrolyte and decreases the cost, which is critical to their further applications. However, the state-of-the-art bifunctional ternary sulfides electrocatalysts still suffer large overpotential for overall water electrolysis, especially for OER due to the sluggish kinetics of complex four-electron reaction processes. Accordingly, it is highly desired but rather challenging to further improve their OER performance which is critical to their application as efficient electrocatalysts in overall water splitting. Nanocomposites with deliberate structures integrate the excellent properties from their parent materials, thus enhancing their performances for various applications. Here hybrid CoMoS4/Ni3S2 nanostructures was constructed to use as efficient bifunctional electrocatalyst for overall water splitting. The CoMoO4 nanocolumns were firstly synthesized on Ni foam (NF) and then transformed to porous CoMoS4 thin nanonsheets with hollow gaps by anion exchange reaction, together with the formation of Ni3S2 interlayer by the side sulfurization reaction of NF (see the Schematic illustration in Fig. 1). It was found that the Ni3S2 interlayer could greatly increases the OER properties of CoMoS4 due to the well suppressed charges recombination process and increased electrochemically active surface area. The porous and thin CoMoS4 nanosheets outlayer also provides multiple microchannels and surface active sites for electrolyte access and electrochemical reaction occurrence. As a result, superior bifunctional electrocatalytic properties with good stability were achieved under alkali condition, and a low cell voltage of 1.568 V (338 mV higher than the theoretical potential) is
2. Materials and methods 2.1. Synthesis of CoMoO4 nanocolumns arrays on NF All the chemicals are analytical grade and used as received without further purification. The CoMoO4 nanocolumns arrays were firstly synthesized by a solvothermal reaction according to the previous report [44]. In a typical procedure, 1 mmol Co(NO3)2•6H2O and 1 mmol Na2MoO4•2H2O were dissolved in mixed solution of 10 ml DI water and 5 ml ethylene glycol with vigorous stirring for 10 min. As formed solution was transferred into a 25 ml Teflon-lined stainless steel autoclave with a surface-cleaned NF perpendicularly immersed into the solution, and then heated at 180 °C for 12 h. After reaction, the NF was removed from the solution and washed with DI water and ethanol for several times, followed by drying at 60 °C in an oven for further use. 2.2. Synthesis of porous CoMoS4 hollow nanosheets arrays on NF For the synthesis of porous CoMoS4 hollow nanosheets arrays, the CoMoO4 nanocolumns on NF were immersed into 1.8 mmol Na2S solution and hydrothermal treatment at 120 °C for 12 h. As formed products were taken out from the solution and washed several times with DI water and ethanol to remove unreacted residues. The mass loading of the catalyst is about 3.2 mg/cm2. Ni3S2 film was prepared using blank NF as substrate with the same method and parameters. 2.3. Synthesis of MoS2 and CoS2 film on NF MoS2 and CoS2 film on NF were also prepared for comparison of their catalytic activities. MoS2 film was synthesized by directly hydrothermal treatment of 10 mM (NH4)2MoS4 at 150 °C for 12 h with NF immersed into the solution. CoS2 film was synthesize by a two-step hydrothermal strategy. Firstly, the solution of 0.8 M cobalt nitrate and 2 M urea was reacted at 120 °C for 10 h to grow the precursor on NF, then the film was sulfurizing treated at 120 °C for 12 h using thioacetamide as sulfide source. 2.4. Synthesis of Ni3S2/CoMoS4 nanoparticles hybrid film on NF CoMoS4 nanoparticles were firstly synthesized by a precipitation reaction according to previous report [30]. In a typical procedure, 5 mM Co(NO3)2•6H2O and (NH4)2MoS4 solution were mixed together and reacted for 30 min with vigorous stirring. Then the products were washed by water and absolute ethanol for several times, respectively. For preparation of Ni3S2/CoMoS4 nanoparticles hybrid film, 2 mg of as synthesized CoMoS4 nanoparticles were mixed with Nafion solution (25 μl) and water (600 μl), and then drop-cast on NF with a pre-grown Ni3S2 layer. For comparison, the CoMoS4 nanoparticles also deposited on blank NF to investigate the influence of Ni3S2 interlayer on the
Fig. 1. Schematic illustration the formation of hybrid CoMoS4/Ni3S2 nanostructures and used as bifunctional electrodes for overall water splitting. 96
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CoMoS4/Ni3S2 nanostructures were successfully constructed in a large scale on NF by a self-templating strategy. The detailed structure evolution of the CoMoO4 was further confirmed by TEM observation. The starting CoMoO4 shows lamellar structure with large thickness (from the intensive contrast change between the particle and the grid background) in Fig. 3a, and crystalline nature with exposed (002) plane could be further confirmed according to its interplanar space (inset in Fig. 3a). After reaction, the outlayered nanosheets reveals similar size and shape to CoMoO4 precursor as shown in Fig. 3b, and obvious contrast change indicates the decrease in thickness during the transformation process. In addition, as obtained nanosheets exhibit “V” shape with one side connected together, which is well in accordance with the top view of the naonsheets in Fig. 2c and d. Magnified TEM image in Fig. 3c suggests that the nanosheet was composed of ultrathin two dimensional building blocks with porous structure. As formed architecture is beneficial to channeling electrolyte to fully access to the electrode materials and increases surface active sites for electrochemical reaction. Selected-area electron diffraction (SAED) pattern insert in Fig. 3c further corroborated the amorphous nature of as synthesized nanosheets. Elemental mapping was performed in Fig. 3d on a representative nanosheet to illustrate the elemental distribution, and the results clearly demonstrate the uniform distribution of Co, Mo, and S in the obtained products without Ni detected, which well illustrates the completely transformation from CoMoO4 nanocolumns to CoMoS4 hollow nanosheets. To investigate the growth mechanism of the hybrid films, time-dependent experiments were conducted and the products was analyzed by SEM images in Fig. 4. In the initial reaction stage of 2 h, the dense CoMoO4 nanocolumns (Fig. 4a) start to transform to loose nanocolumns due to the anion exchange effect (Fig. 4b). Then the hollow interior could be observed followed by a solid core evacuation in some part of the nanocolumns when the reaction time was prolonged to 6 h as shown in Fig. 4c, and continually enlarged with a longer reaction time (Fig. 4d). During this process, the thickness of the shell decreased obviously with well-preserved two dimensional structure, and finally formed the hollow nanosheets after reaction of 12 h in Fig. 4e. It should be noted that the hollow nanostructures still kept well when the reaction time was further increased to 24 h, illustrating the well adhesive strength of nanosheets film on NF (Fig. 4f). The phase evolution was examined by XRD spectra in Fig. S2. Amorphous structure was firstly obtained at the initial reaction stages of 2 h, indicating the structural transformation from crystalline CoMoO4 to amorphous CoMoS4, and the followed hollowing process could be attributed to the Ostwald ripening mechanism [47,48]. The formation of Ni3S2 was identified at the reaction time of 6 h, and continuously increased with prolonged reaction time. The XPS analysis was used to indicate the chemical species and states on the surface of CoMoS4 outlayer, and Co, Mo, S could be detected from the overview measurement in Fig. 5a. High-resolution XPS spectra in Fig. 5b–d give the detailed chemical states of compositional elements. The Co 2p spectrum (Fig. 5b) includes two main regions of Co 2p2/3 and Co 2p1/2 with binding energy of 778.6 and 794 eV, respectively, which can be well assigned to the Co2+ oxidation state [33]. As for Mo 3d spectrum in Fig. 5c, a doublet located at 231.8 eV and 229 eV could be observed and ascribed to Mo 3d3/2 and 3d5/2, respectively, confirming that Mo is present in its VI oxidation state [41,49,50], which is similar to that in the (MoS4)2- [50,51]. The measured S 2p peak is located at binding energy of 161.6 eV (Fig. 5d), revealing the characteristic of S2− chemical state [39,52]. As indicated above, the XPS results also confirm the existence of single phase CoMoS4 on the outlayer of as obtained hybrid films. In addition, as-proposed strategy also exhibits its potential in synthesis of other sulfide hybrid electrode with controlled nanostructure as shown in Fig. S3. The electrocatalytic performance of as-synthesized catalyst was evaluated using a typical three electrode cell, and the obtained films were directly adopted as working electrode for OER and HER
electrocatalytic activity of CoMoS4. 2.5. Characterization The phase of synthesized products was examined by a SHIMADZU XRD-7000 X-ray diffractometer (Cu Kα radiation) with the 2θ range from 10 to 90°. The morphology analyses were firstly observed by a scanning electron scanning electron microscopy (SEM, Hitachi S4800 N). Detailed structure information and elemental distribution were carried out on a JEM-2010 transmission electron microscopy (TEM) equipped with energy-dispersive X-ray spectroscopy (EDS) at an accelerating voltage of 200 kV. X-ray photoelectron spectra (XPS) was measured on a PHI 5300 ESCA system with a 150 W monochromatic Al Kα line source. 2.6. Electrochemical measurements The electrochemical measurements were conducted on a Bio-logic VMP3 electrochemical workstation by typical three-electrode system. As prepared catalysts grown on NF were used as working electrode. A Ag/AgCl electrode and a carbon rod were used as the reference electrode and the counter electrode, respectively. All the measurements were performed in 1 M KOH aqueous electrolyte at a scan rate of 5 mV S−1. The electrochemical impedance spectroscopy was recorded at the frequency from 0.1 Hz to 100 KHz. The electrochemical doublelayer capacitance measurements were conducted at different scan rates of 20, 40, 60, 80, 100 and 120 mV s−1. The overall water splitting test was performed on a two-electrode system, and the working electrodes were composed of the same CoMoS4/Ni3S2 hybrid film as both cathode and anode. Before measurement, nitrogen gas was blown into the solution to remove residual air. 3. Results and discussion CoMoO4 nanocolumns arrays were firstly grown on NF by solvothermal strategy and used as sacrificial template to fabricate CoMoS4 nanosheets arrays by anion exchange reaction, and the side reaction between NF and excessive Na2S leads to the formation of Ni3S2 interlayer [45,46]. The morphological evolution was analyzed by SEM observation. Low magnified SEM image in Fig. 2a clearly shows that the vertically-aligned CoMoO4 nanocolumns uniformly grow on NF in a large scale, and the nanocolumns were aggregated loosely to form a network structure. Enlarged SEM image in Fig. 2b further illustrates that the CoMoO4 nanocolumns possess lateral length of about several micrometers with width of about 100 nm, and cross section view insert in Fig. 2a reveals the single layer feature of the film. After anion exchange reaction, the synthesized product still uniformly covered on NF from the top view of the film (Fig. 2c), but a hollow space could be clearly observed between two adjusted nanosheets. In addition, from the cross section image insert in Fig. 2c, it could be clearly seen that bilayered structure was obtained after reaction. The surface morphology of the hybrid film was further investigated by magnified SEM image in Fig. 2d, and it is more clearly that the single CoMoO4 nanocolumn has split into two separated CoMoS4 nanosheets with very thin thickness. When the outside nanosheets were exfoliated from the NF, the interlayer composed of nanosized particles with size less than 50 nm could be further observed from SEM characterization (Fig. 2e), and only Ni and S were detected by EDS elemental analysis (Fig. S1), which confirms the formation of nickel sulfide interlayer in the obtained hybrid film. The phase transformation was detected by XRD analysis in Fig. 2f. The crystalline CoMoO4 precursor (curve I, monoclinic, JCPDS 21–0868) has completely disappeared after the sulfuration reaction, which is possibly due to the formation of amorphous CoMoS4, and the weak diffraction peaks could be indexed to the Ni3S2 interlayer (curve II, JCPDS 44–1418). According to above observations, it is reasonably speculated that hybrid 97
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Fig. 2. (a) Low and (b) high magnification SEM images of the CoMoO4 precursors on Ni foam. Top view of (c) low and (d) high magnification SEM images of outlayered CoMoS4 nanosheets arrays. (e) SEM image of in situ synthesized Ni3S2 interlayer, and (f) XRD pattern of the products before and after reaction. The curves I and II in Figure f represent the CoMoO4 precursors and products after the sulfuration reaction, respectively.
The kinetics properties of as synthesized products for OER process were explored by Tafel plots derived from polarization curves in Fig. 6a. As indicated in Fig. 6b, CoMoS4/Ni3S2 electrode exhibits a Tafel slope of 63 mV dec−1, which is lower than that of the CoS2, MoS4 and Pt/C electrodes, respectively, implying a more rapid OER rate of as synthesized products. Kinetics mechanism for the enhanced catalytic activity were further investigated by electrochemical impedance spectroscopy (EIS) in Fig. 6c, which illustrates the interface resistance of the reaction process. The hybrid electrode exhibits a much smaller semicircular diameter compared to other binary sulfides, implying well suppressed resistance of working electrodes. These findings suggest that the porous and thin sheet-like nanostructure of the obtained electrocatalyst provides large surface active site and multichannels for redox reaction and mass diffusion, leading to a significant increase of charge separate efficiency and in turn improves the OER properties. The long-term stability test was also assessed by measuring the time-dependent current density curve under fixed current density of 10 mA cm−2. As shown in Fig. 6d, the current density of the obtained catalyst displays a slight decay from the initial 9.93 mA cm−2 to 9.28 after 10 h test, revealing a high durability of 93.5%. Since OER represents the major barrier for water splitting due to its high theoretical potential with sluggish kinetics, the superior performance of as prepared material indicates its promising prospect as excellent OER electrocatalyst for water splitting.
electrocatalysis process. As references, CoS2 (Fig. S4), MoS2 (Fig. S5), Pt/C and Ni electrodes were also tested under the same condition. Fig. 6a represents the typical linear sweep voltammetry (LSV) of different samples towards OER process. It should be noted that the LSV curve of CoMoS4/Ni3S2 electrode was scanned from negative direction at scan rate of 1 mV S−1 to avoid the interference of Ni oxidation reaction [2,53]. Compared to inactive bare Ni, three sulfide electrodes all exhibit obvious OER catalytic activity at positive potentials. CoMoS4/ Ni3S2 hybrid electrode reveals much sharp raised catalytic current and lower onset potential than pristine CoS2 and MoS2, which could be ascribed to the synergistic effect Co and Mo ions in ternary sulfides [54]. Importantly, the overpotential required to reach current density of 10 mA cm−2 for CoMoS4/Ni3S2 is only 200 mV, which is much lower than 332 mV, 382 mV and 501 mV for CoS2, MoS2 and Pt/C, respectively, suggesting its superior OER catalytic activity. A comparative summary of OER activities of various electrocatalysts in alkaline conditions was further shown in Table S1 to reveal the advanced properties of as obtained hybrid electrocatalyst. For the OER process of transitionmetal phosphides and sulfides, it is important to investigate the structural evolution of the catalysts [55–57]. In our work, the phase and morphology of hybrid electrocatalyst after OER testing were analyzed and shown in Fig. S6. It could be concluded that no obvious change in morphology and phase were observed during OER process. 98
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Fig. 3. Typical TEM images of (a) CoMoO4 precursor and (b) obtained CoMoS4 nanosheets well-illustrated the morphology evolution. (c) magnified TEM and (d) EDS elemental mapping of a well preserved CoMoS4 hollow nanosheet.
a current density of 10 mA cm−2. Although CoS2 and MoS2 electrodes are also capable of HER catalysis, much higher overpotential of 328 and 478 mV were necessary to drive current density of 10 mA cm−2. In sharp contrast, CoMoS4/Ni3S2 electrode reveals excellent HER activity compared to binary sulfides, 158 mV overpotential is sufficient to
The HER activity of the obtained hybrid electrode was evaluated together with CoS2, MoS2 and commercial Pt/C electrode for references. Fig. 7a presents the LSV curves of different electrodes in H2 saturated 1.0 M KOH solution. It can be concluded that Pt/C exhibits the best HER activity and only required an overpotential of 76 mV to reach
Fig. 4. SEM images of products obtained at different reaction times: (a) 0 h, (b) 2 h, (c) 6 h, (d) 9 h, (e) 12 h and (f) 24 h. 99
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Fig. 5. (a) Overall XPS spectrum and high resolution spectra of (b) Co 2p, (c) Mo 3d and (d) S 2p of obtained product.
Fig. 6. Electrocatalytic activities of different electrodes for OER in O2 saturated 1.0 M KOH solution: (a) polarization curves, (b) corresponding Tafel plots, (c) Nyquist plots with the potential of 1.47 V versus RHE and (d) time-dependent current density curves under fixed current density of 10 mA cm−2.
deliver 10 mA cm−2, much lower than that for CoS2 and MoS2. The Tafel plots of different samples were displayed in Fig. 7b. Commercial Pt/C exhibits the smallest Tafel slope of 95 mV dec−1. While the hybrid electrode presents a Tafel slope of 169 mV dec−1, which is also smaller than those of CoS2 (188 mV dec−1) and MoS2 (198 mV dec−1), suggesting well suppressed interface resistance of HER process. A comparative summary of HER activities of various electrocatalysts in alkaline conditions was shown Table S2, which further illustrates the superior properties of as obtained products. The favorable kinetics
process of the obtained products could be further confirmed by the EIS spectra measurement under HER operation condition as shown in Fig. 7c. The time-dependent current density curve reveals a current decay of 7.1% after 10 h measurement (Fig. 7d), implying its well durability for electrocatalysis HER under alkali condition. The amounts of Ni3S2 have obvious influences on the HER performances of the obtained products. The catalytic activities of products obtained at different sulfuration time were also tested and shown in Fig. S7. It could be observed that the current densities were firstly increased 100
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Fig. 7. Electrocatalytic activities of different electrodes for HER in H2 saturated 1.0 M KOH solution: (a) polarization curves, (b) corresponding Tafel plots, (c) Nyquist plots with the potential of −120 mV versus RHE and (d) time-dependent current density curves under fixed current density of 10 mA cm−2.
contributes to the active sites of CoMoS4 toward OER process. The influence of Ni3S2 interlayer on the electrocatalytic activity of CoMoS4 was also investigated in our work. In controlled experiments, Ni3S2 layer was firstly grown on NF by in situ sulfuration reaction (Fig. S9), then CoMoS4 powders were deposited on it to form CoMoS4/Ni3S2 bilayer electrode. For comparison, CoMoS4 powders were also deposited on blank NF, and their electrocatalytic performance toward OER and HER were evaluated as shown in Figs. S10a and b. The bilayer hybrid electrode exhibits much higher OER current density than single CoMoS4 film with 80 mV decrease of overpotential to reach current density of 10 mA cm−2, and slightly increased HER current densities were also observed. These results reveal that Ni3S2 interlayer could greatly increase the OER activity of CoMoS4 with well-preserved HER performance. EIS spectra in Figs. S10c and d further reveals that the much favorable charge transfer efficiency for CoMoS4 could be obtained by adding Ni3S2 interlayer, which enhances the electrochemical reaction processes in the aspect of kinetics properties, and in turn accelerates redox reaction at the electrode/electrolyte interface. Similar findings were also reported in NF-Ni3S2@Ni(OH)2‐graphene sandwich structure electrode and Cu2O/NiO/Cu2MoS4 hybrid film electrode [65,66]. In addition, the CoMoS4/Ni3S2 nanoparticles hybrid electrode shows smaller current densities both for OER and HER than CoMoS4/ Ni3S2 nanosheets arrays hybrid electrode, and these results suggest that in situ growth of the electrode materials with controlled nanostructures on conductive substrate could significantly enhances their electrochemical performances due to effectively increased electrochemically active sites with well suppressed interface resistance. To get a better understand of the inherent mechanism of improved electrocatalytic performance of hybrid electrocatalyst, the electrochemically active surface area (ECSA) was determined through electrical double-layer capacitance measurement in 1 M KOH electrolyte. Figs. S11 and S12 represents the capacitive currents densities collected at different scan rates for CoMoS4 with and without a pre-grown Ni3S2
with the prolonged reaction time to 12 h. Then the HER activity decreased sharply with increased amount of Ni3S2 in as obtained hybrid electrocatalyst when reaction time was further increased to 24 h. In addition, as obtained products also exhibited superior HER activity in acid solution. Fig. S8 presents the LSV curves of CoMoS4/Ni3S2 electrode in 0.5 M H2SO4. It could be seen that higher polarization current could be obtained with rather low overpotential of 93 mV at 10 mA cm−2 than that in KOH solution. These results well illustrate the feasibility of as synthesized electrocatalyst applied in solution with a wide range of pH values. For bifunctional electrocatalyst, it is highly desired to identify the active sites for HER and OER, respectively. Because the electrochemical reactions are mainly occurred at the surface of the electrocatalyst, the HER and OER of as synthesized bilayered electrode are exclusively occurred at the surface of the CoMoS4 outlayer. For monometal sulfides such as MoS2, it is generally believed that the most of the active sites for HER locate at the Mo-edges of 2D structure rather than S-edges [31]. When the Co metal ions are introduced to the MoS2, the incorporated Co ions tend to located at the S-edge, which are also catalytically active for HER due to the decreased Gibbs free energy for adsorbed atomic hydrogen [58]. Thus the Mo-edges and Co-binding S-edges are both availably active sites for HER. While for OER, the origin of electrocatalytic activity of CoMoS4 are seldom reported. Recently Li and Wang investigated different materials’ active sites toward OER and HER from experiments and theoretical calculations [59–62]. It was found that quaternary bimetallic phosphosulphide is inactive to OER due to the high theoretical overpotential, and the in-situ formed oxyhydroxide during electrochemical process act as real active sites for OER [63]. Chu et al. also reported that CoSx on Co3O4 surface could facilitate the formation of Cobalt oxyhydroxide, which serves as surface active sites for OER [64]. Considering the Cobalt hydroxide is possibly in-situ generated at the surface of CoMoS4 during electrocatalysis process in alkali solution [32], it is reasonably speculated that oxyhydroxide also 101
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Fig. 8. Comparison of overall water splitting using a two-electrode system composed of different electrocatalysts as both the cathode and anode in 1 M KOH: (a) polarization curve and (b) time-dependent current density curve of as obtained hybrid electrodes under fixed current density of 10 mA cm−2.
with thin thickness. It was found that the existence of Ni3S2 interlayer formed by the pathway sulfurization reaction of Ni greatly increases the OER activity of CoMoS4, and the obtained hybrid electrode delivers overpotentials of 200 mV and 158 mV to reach current density of 10 mA cm−2 for OER and HER, respectively. Furthermore, a cell voltage of 1.568 V is achieved to afford a current density of 10 mA cm−2 for over water splitting in a two-electrode system, highlighting the advance of our proposed strategy to rational design of efficient electrodes with superior electrochemical properties.
layer. It could be observed that the CoMoS4/Ni3S2 electrode exhibits higher capacitive currents densities than pure CoMoS4 electrodes at each scan rate. In addition, the capacitance values were calculated according to the linear dependence of the current densities on the scan rates, 0.0059 and 0.0037 mF cm−2 for CoMoS4/Ni3S2 and pure CoMoS4, respectively, well consistent with their electrocatalytic performance for OER and HER. The considerably higher capacitance of CoMoS4/Ni3S2 electrode well suggested the enhanced electrochemical surface area by synergistic effect of Ni3S2 interlayer and, correspondingly, enhanced its electrocatalytic activity significantly [2]. Furthermore, the ECSA of the obtained CoMoS4 hollow nanosheets coupled with Ni3S2 interlayer was further investigated as shown in Fig. S13, and the capacitance value was further increased to 0.03301 mF cm−2. These results well confirmed that the electrochemical performances of the obtained hybrid electrode could be further enhanced by direct growth of the electrode materials on conductive substrate with controlled nanostructures. In overall water electrolysis process, OER is a rate-determining step due to its sluggish kinetics ascribed to its complex four-electron oxidation process [3], thus as obtained catalyst with enhanced OER activity is expected to exhibit excellent performance in overall water electrolysis. To assess the bifunctional properties of electrocatalytic overall water splitting, a two-electrode electrolyzer was constructed using same electrocatalysts as both the cathode and anode, respectively. From the LSV in Fig. 8a, it could be observed that symmetric CoMoS4/ Ni3S2 || CoMoS4/Ni3S2 system exhibits the highest water electrolysis properties among all the sample. A cell voltage of 1.568 V is needed to afford a current density of 10 mA cm−2 for overall water splitting with obvious gas evolution on both electrodes (Fig. 8b, inset), only 338 mV higher than the theoretical potential toward the water splitting reaction. A further comparison is shown in Table S3, and as synthesized products exhibit a much lower overpotential for water splitting than most of recently reported bifunctional electrode materials. The durability measurement was also conducted under fixed current density of 10 mA cm−2 in Fig. 8b, and the result well suggests the long-term stability of the obtained hybrid electrode, which could be further confirmed by the well preserved morphology of hybrid electrodes after a long-term stability test (Fig. S14). These findings indicate the superior performance of our CoMoS4/Ni3S2 as a promising and comparable candidate for overall electrocatalytic water splitting in alkaline solution.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 11575228, 91634118, 51472009); The National Natural Science Found for Innovative Research Groups (No. 51621003); Beijing Natural Science Foundation (No. 2182010, 2151001) and the Beijing Municipal High Level Innovative Team Building Program (No. IDHT20170502). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpowsour.2019.01.090. References [1] C. Tang, N. Cheng, Z. Pu, W. Xing, X. Sun, Angew. Chem. Int. Ed. 54 (2015) 9351–9355. [2] A. Sivanantham, P. Ganesan, S. Shanmugam, Adv. Funct. Mater. 26 (2016) 4661–4672. [3] X. Li, X. Hao, A. Abudula, G. Guan, J. Mater. Chem. 4 (2016) 11973–12000. [4] M. Fang, G. Dong, R. Wei, J.C. Ho, Adv. Energy Mater. 7 (2017) 1700559. [5] H. Vrubel, X. Hu, Angew. Chem. Int. Ed. 51 (2012) 12703–12706. [6] T.R. Cook, D.K. Dogutan, S.Y. Reece, Y. Surendranath, T.S. Teets, D.G. Nocera, Chem. Rev. 110 (2010) 6474–6502. [7] T. Wang, H. Xie, M. Chen, A. D'Aloia, J. Cho, G. Wu, Q. Li, Nano Energy 42 (2017) 69–89. [8] Y. Yan, B.Y. Xia, B. Zhao, X. Wang, J. Mater. Chem. 4 (2016) 17587–17603. [9] R. Miao, B. Dutta, S. Sahoo, J. He, W. Zhong, S.A. Cetegen, T. Jiang, S.P. Alpay, S.L. Suib, J. Am. Chem. Soc. 139 (2017) 13604–13607. [10] Z. Fang, L. Peng, Y. Qian, X. Zhang, Y. Xie, J.J. Cha, G. Yu, J. Am. Chem. Soc. 140 (2018) 5241–5247. [11] Y. Li, P. Hasin, Y. Wu, Adv. Mater. 22 (2010) 1926–1929. [12] Z. Fang, L. Peng, H. Lv, Y. Zhu, C. Yan, S. Wang, P. Kalyani, X. Wu, G. Yu, ACS Nano 11 (2017) 9550–9557. [13] T. Tian, L. Huang, L. Ai, J. Jiang, J. Mater. Chem. 5 (2017) 20985–20992. [14] C. Tang, W. Wang, A. Sun, C. Qi, D. Zhang, Z. Wu, D. Wang, ACS Catal. 5 (2015) 6956–6963. [15] X. Peng, L. Hu, L. Wang, X. Zhang, J. Fu, K. Huo, L.Y.S. Lee, K.-Y. Wong, P.K. Chu, Nano Energy 26 (2016) 603–609. [16] J.A. Vigil, T.N. Lambert, B.T. Christensen, J. Mater. Chem. 4 (2016) 7549–7554. [17] B. You, N. Jiang, M. Sheng, S. Gul, J. Yano, Y. Sun, Chem. Mater. 27 (2015) 7636–7642. [18] Z. Zhao, D.E. Schipper, A.P. Leitner, H. Thirumalai, J.-H. Chen, L. Xie, F. Qin,
4. Conclusion In summary, a novel self-templating strategy was developed to directly grow CoMoS4 nanonsheets arrays coupled with Ni3S2 interlayer on NF by anion exchange of CoMoO4, together with the sharply morphology evolution from nanocolumn splitting into porous nanosheets 102
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Engineering hybrid CoMoS4/Ni3S2 nanostructures as efficient bifunctional electrocatalyst for overall water splitting
T
Peng Hua, Zhiyuan Jiab, Haibing Chea, Wenyuan Zhoua, Ning Liuc, Fan Lic,∗, Jinshu Wanga,∗∗ a
Key Laboratory of Advanced Functional Materials, Education Ministry of China, School of Materials Science and Engineering, Beijing University of Technology, Beijing, 100124, PR China b Shenyang Research Institute of Chemical Industry, Liaoning, 110021, PR China c College of Environmental & Energy Engineering, Beijing University of Technology, Beijing, 100124, PR China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
Bilayered CoMoS /Ni S electrode • was synthesized by a self-templating 4
3 2
strategy.
Porous CoMoS nanosheets array was • formed from CoMoO nanocolumns. Ni S interlayer greatly increased the • OER activity of CoMoS . Low potential (1.568 V@10 mA cm ) • for overall water splitting was 4
4
3 2
4
−2
achieved.
A R T I C LE I N FO
A B S T R A C T
Keywords: CoMoS4 Ni3S2 Hybrid nanostructure Bifunctional electrocatalyst Water splitting
In overall water splitting, oxygen evolution reaction is a rate-determining step due to its complex reaction processes with sluggish kinetics, so design of bifunctional electrocatalyst with elaborated structure to improve the oxygen evolution reaction performance is critical to their efficient application in overall water splitting. Here bifunctional CoMoS4/Ni3S2 nanostructures are in situ synthesized by a self-templating strategy, and the morphology control of the CoMoS4 outlayer is simultaneously achieved through a unique evolution from CoMoO4 nanocolumns array to hollow CoMoS4 nanosheets array with porous structure. Benefited from the synergistic effect of Ni3S2 interlayer, as obtained electrode exhibits greatly enhanced oxygen evolution activity than single CoMoS4 electrode with well-preserved hydrogen evolution activity. It, together with the three-dimensional topographic structure of CoMoS4 outlayer with high active areas, offers a rather low cell voltage (1.568 V@ 10 mA cm−2) for overall water splitting in a two-electrode system. Accordingly, our results may open new opportunities to explore hybrid nanostructures as efficient bifunctional electrocatalyst for overall water splitting.
1. Introduction Electrochemical water splitting by photoelectrolysis or electrolysis is one of the most promising ways for generating high-purity hydrogen, which is critical to fulfill the need for clean and renewable energy resources alternative to current fossil fuels. Water electrolysis is a thermodynamically unfavored reaction that involved two electrochemical
∗
half-cell reactions: the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) with theoretical potential of 1.23 V [1,2]. In practice, much high overpotential is applied due to the resistance existed in the electrode and its contacted interfaces, arising from the surface redox reactions and interface charges/ions transport (especially the rate-determined OER process) [3,4]. To accelerate its kinetics properties, highly active electrocatalysts are essential to minimize the
Corresponding author. Corresponding author. E-mail addresses: [email protected] (F. Li), [email protected] (J. Wang).
∗∗
https://doi.org/10.1016/j.jpowsour.2019.01.090 Received 20 November 2018; Received in revised form 15 January 2019; Accepted 28 January 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.
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needed to afford a current density of 10 mA cm−2 for over water splitting in a two-electrode system. Thus this work provides a new strategy for exploring efficient bifunctional electrocatalyst with high performance for overall water splitting.
extra overpotential and facilitate the electrochemical process [5,6]. Up to now, noble metal based materials are recognized as the most efficient catalysts and widely employed for HER and OER. But the scarcity and low stability of these noble metal catalysts make an obstacle for their practical applications in a large scale, and development of alternative earth-abundant catalyst materials with high efficient and durable performance for electrochemical hydrogen and oxygen production is highly desired [7,8]. In the search for new HER and OER catalysts alternative to noble metals, different transition-metal based materials, including chalcogenides [9–13], carbides [14,15], phosphides [16–21] etc., and their composites [22–27] have been explored and confirmed efficiently for the HER or/and OER processes. Among these materials, ternary sulfides X-M-S (where X = Co, Ni Cu etc. and M = W or Mo), a novel class of layer-structured materials, have been considered to be the promising candidates as electrochemical active materials due to their unique electric and structural features [28–30]. By comparison to monometal sulfides, ternary sulfides exhibit richer active sites due to chemical environment change by synergistic interaction of bimetallic ions [31–33]. They also present higher electronic conductivity than their corresponding ternary oxides [34], and create more flexible active structure due to the lower electronegativity of sulfur [35], promoting electrocatalytic process from thermodynamic and kinetic aspects. Recent experimental and theoretical works have confirmed their superior performance of HER in solution with a wide range of pH values [36–42]. Few work also explores their potential applications in OER [32,43], and makes them possible to use as bifunctional catalysts for overall water splitting. Constructing bifunctional electrocatalysts with the abilities for efficient HER and OER is benefit for improving the performance of overall water splitting, and further simplifies the watersplitting system in the same electrolyte and decreases the cost, which is critical to their further applications. However, the state-of-the-art bifunctional ternary sulfides electrocatalysts still suffer large overpotential for overall water electrolysis, especially for OER due to the sluggish kinetics of complex four-electron reaction processes. Accordingly, it is highly desired but rather challenging to further improve their OER performance which is critical to their application as efficient electrocatalysts in overall water splitting. Nanocomposites with deliberate structures integrate the excellent properties from their parent materials, thus enhancing their performances for various applications. Here hybrid CoMoS4/Ni3S2 nanostructures was constructed to use as efficient bifunctional electrocatalyst for overall water splitting. The CoMoO4 nanocolumns were firstly synthesized on Ni foam (NF) and then transformed to porous CoMoS4 thin nanonsheets with hollow gaps by anion exchange reaction, together with the formation of Ni3S2 interlayer by the side sulfurization reaction of NF (see the Schematic illustration in Fig. 1). It was found that the Ni3S2 interlayer could greatly increases the OER properties of CoMoS4 due to the well suppressed charges recombination process and increased electrochemically active surface area. The porous and thin CoMoS4 nanosheets outlayer also provides multiple microchannels and surface active sites for electrolyte access and electrochemical reaction occurrence. As a result, superior bifunctional electrocatalytic properties with good stability were achieved under alkali condition, and a low cell voltage of 1.568 V (338 mV higher than the theoretical potential) is
2. Materials and methods 2.1. Synthesis of CoMoO4 nanocolumns arrays on NF All the chemicals are analytical grade and used as received without further purification. The CoMoO4 nanocolumns arrays were firstly synthesized by a solvothermal reaction according to the previous report [44]. In a typical procedure, 1 mmol Co(NO3)2•6H2O and 1 mmol Na2MoO4•2H2O were dissolved in mixed solution of 10 ml DI water and 5 ml ethylene glycol with vigorous stirring for 10 min. As formed solution was transferred into a 25 ml Teflon-lined stainless steel autoclave with a surface-cleaned NF perpendicularly immersed into the solution, and then heated at 180 °C for 12 h. After reaction, the NF was removed from the solution and washed with DI water and ethanol for several times, followed by drying at 60 °C in an oven for further use. 2.2. Synthesis of porous CoMoS4 hollow nanosheets arrays on NF For the synthesis of porous CoMoS4 hollow nanosheets arrays, the CoMoO4 nanocolumns on NF were immersed into 1.8 mmol Na2S solution and hydrothermal treatment at 120 °C for 12 h. As formed products were taken out from the solution and washed several times with DI water and ethanol to remove unreacted residues. The mass loading of the catalyst is about 3.2 mg/cm2. Ni3S2 film was prepared using blank NF as substrate with the same method and parameters. 2.3. Synthesis of MoS2 and CoS2 film on NF MoS2 and CoS2 film on NF were also prepared for comparison of their catalytic activities. MoS2 film was synthesized by directly hydrothermal treatment of 10 mM (NH4)2MoS4 at 150 °C for 12 h with NF immersed into the solution. CoS2 film was synthesize by a two-step hydrothermal strategy. Firstly, the solution of 0.8 M cobalt nitrate and 2 M urea was reacted at 120 °C for 10 h to grow the precursor on NF, then the film was sulfurizing treated at 120 °C for 12 h using thioacetamide as sulfide source. 2.4. Synthesis of Ni3S2/CoMoS4 nanoparticles hybrid film on NF CoMoS4 nanoparticles were firstly synthesized by a precipitation reaction according to previous report [30]. In a typical procedure, 5 mM Co(NO3)2•6H2O and (NH4)2MoS4 solution were mixed together and reacted for 30 min with vigorous stirring. Then the products were washed by water and absolute ethanol for several times, respectively. For preparation of Ni3S2/CoMoS4 nanoparticles hybrid film, 2 mg of as synthesized CoMoS4 nanoparticles were mixed with Nafion solution (25 μl) and water (600 μl), and then drop-cast on NF with a pre-grown Ni3S2 layer. For comparison, the CoMoS4 nanoparticles also deposited on blank NF to investigate the influence of Ni3S2 interlayer on the
Fig. 1. Schematic illustration the formation of hybrid CoMoS4/Ni3S2 nanostructures and used as bifunctional electrodes for overall water splitting. 96
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CoMoS4/Ni3S2 nanostructures were successfully constructed in a large scale on NF by a self-templating strategy. The detailed structure evolution of the CoMoO4 was further confirmed by TEM observation. The starting CoMoO4 shows lamellar structure with large thickness (from the intensive contrast change between the particle and the grid background) in Fig. 3a, and crystalline nature with exposed (002) plane could be further confirmed according to its interplanar space (inset in Fig. 3a). After reaction, the outlayered nanosheets reveals similar size and shape to CoMoO4 precursor as shown in Fig. 3b, and obvious contrast change indicates the decrease in thickness during the transformation process. In addition, as obtained nanosheets exhibit “V” shape with one side connected together, which is well in accordance with the top view of the naonsheets in Fig. 2c and d. Magnified TEM image in Fig. 3c suggests that the nanosheet was composed of ultrathin two dimensional building blocks with porous structure. As formed architecture is beneficial to channeling electrolyte to fully access to the electrode materials and increases surface active sites for electrochemical reaction. Selected-area electron diffraction (SAED) pattern insert in Fig. 3c further corroborated the amorphous nature of as synthesized nanosheets. Elemental mapping was performed in Fig. 3d on a representative nanosheet to illustrate the elemental distribution, and the results clearly demonstrate the uniform distribution of Co, Mo, and S in the obtained products without Ni detected, which well illustrates the completely transformation from CoMoO4 nanocolumns to CoMoS4 hollow nanosheets. To investigate the growth mechanism of the hybrid films, time-dependent experiments were conducted and the products was analyzed by SEM images in Fig. 4. In the initial reaction stage of 2 h, the dense CoMoO4 nanocolumns (Fig. 4a) start to transform to loose nanocolumns due to the anion exchange effect (Fig. 4b). Then the hollow interior could be observed followed by a solid core evacuation in some part of the nanocolumns when the reaction time was prolonged to 6 h as shown in Fig. 4c, and continually enlarged with a longer reaction time (Fig. 4d). During this process, the thickness of the shell decreased obviously with well-preserved two dimensional structure, and finally formed the hollow nanosheets after reaction of 12 h in Fig. 4e. It should be noted that the hollow nanostructures still kept well when the reaction time was further increased to 24 h, illustrating the well adhesive strength of nanosheets film on NF (Fig. 4f). The phase evolution was examined by XRD spectra in Fig. S2. Amorphous structure was firstly obtained at the initial reaction stages of 2 h, indicating the structural transformation from crystalline CoMoO4 to amorphous CoMoS4, and the followed hollowing process could be attributed to the Ostwald ripening mechanism [47,48]. The formation of Ni3S2 was identified at the reaction time of 6 h, and continuously increased with prolonged reaction time. The XPS analysis was used to indicate the chemical species and states on the surface of CoMoS4 outlayer, and Co, Mo, S could be detected from the overview measurement in Fig. 5a. High-resolution XPS spectra in Fig. 5b–d give the detailed chemical states of compositional elements. The Co 2p spectrum (Fig. 5b) includes two main regions of Co 2p2/3 and Co 2p1/2 with binding energy of 778.6 and 794 eV, respectively, which can be well assigned to the Co2+ oxidation state [33]. As for Mo 3d spectrum in Fig. 5c, a doublet located at 231.8 eV and 229 eV could be observed and ascribed to Mo 3d3/2 and 3d5/2, respectively, confirming that Mo is present in its VI oxidation state [41,49,50], which is similar to that in the (MoS4)2- [50,51]. The measured S 2p peak is located at binding energy of 161.6 eV (Fig. 5d), revealing the characteristic of S2− chemical state [39,52]. As indicated above, the XPS results also confirm the existence of single phase CoMoS4 on the outlayer of as obtained hybrid films. In addition, as-proposed strategy also exhibits its potential in synthesis of other sulfide hybrid electrode with controlled nanostructure as shown in Fig. S3. The electrocatalytic performance of as-synthesized catalyst was evaluated using a typical three electrode cell, and the obtained films were directly adopted as working electrode for OER and HER
electrocatalytic activity of CoMoS4. 2.5. Characterization The phase of synthesized products was examined by a SHIMADZU XRD-7000 X-ray diffractometer (Cu Kα radiation) with the 2θ range from 10 to 90°. The morphology analyses were firstly observed by a scanning electron scanning electron microscopy (SEM, Hitachi S4800 N). Detailed structure information and elemental distribution were carried out on a JEM-2010 transmission electron microscopy (TEM) equipped with energy-dispersive X-ray spectroscopy (EDS) at an accelerating voltage of 200 kV. X-ray photoelectron spectra (XPS) was measured on a PHI 5300 ESCA system with a 150 W monochromatic Al Kα line source. 2.6. Electrochemical measurements The electrochemical measurements were conducted on a Bio-logic VMP3 electrochemical workstation by typical three-electrode system. As prepared catalysts grown on NF were used as working electrode. A Ag/AgCl electrode and a carbon rod were used as the reference electrode and the counter electrode, respectively. All the measurements were performed in 1 M KOH aqueous electrolyte at a scan rate of 5 mV S−1. The electrochemical impedance spectroscopy was recorded at the frequency from 0.1 Hz to 100 KHz. The electrochemical doublelayer capacitance measurements were conducted at different scan rates of 20, 40, 60, 80, 100 and 120 mV s−1. The overall water splitting test was performed on a two-electrode system, and the working electrodes were composed of the same CoMoS4/Ni3S2 hybrid film as both cathode and anode. Before measurement, nitrogen gas was blown into the solution to remove residual air. 3. Results and discussion CoMoO4 nanocolumns arrays were firstly grown on NF by solvothermal strategy and used as sacrificial template to fabricate CoMoS4 nanosheets arrays by anion exchange reaction, and the side reaction between NF and excessive Na2S leads to the formation of Ni3S2 interlayer [45,46]. The morphological evolution was analyzed by SEM observation. Low magnified SEM image in Fig. 2a clearly shows that the vertically-aligned CoMoO4 nanocolumns uniformly grow on NF in a large scale, and the nanocolumns were aggregated loosely to form a network structure. Enlarged SEM image in Fig. 2b further illustrates that the CoMoO4 nanocolumns possess lateral length of about several micrometers with width of about 100 nm, and cross section view insert in Fig. 2a reveals the single layer feature of the film. After anion exchange reaction, the synthesized product still uniformly covered on NF from the top view of the film (Fig. 2c), but a hollow space could be clearly observed between two adjusted nanosheets. In addition, from the cross section image insert in Fig. 2c, it could be clearly seen that bilayered structure was obtained after reaction. The surface morphology of the hybrid film was further investigated by magnified SEM image in Fig. 2d, and it is more clearly that the single CoMoO4 nanocolumn has split into two separated CoMoS4 nanosheets with very thin thickness. When the outside nanosheets were exfoliated from the NF, the interlayer composed of nanosized particles with size less than 50 nm could be further observed from SEM characterization (Fig. 2e), and only Ni and S were detected by EDS elemental analysis (Fig. S1), which confirms the formation of nickel sulfide interlayer in the obtained hybrid film. The phase transformation was detected by XRD analysis in Fig. 2f. The crystalline CoMoO4 precursor (curve I, monoclinic, JCPDS 21–0868) has completely disappeared after the sulfuration reaction, which is possibly due to the formation of amorphous CoMoS4, and the weak diffraction peaks could be indexed to the Ni3S2 interlayer (curve II, JCPDS 44–1418). According to above observations, it is reasonably speculated that hybrid 97
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Fig. 2. (a) Low and (b) high magnification SEM images of the CoMoO4 precursors on Ni foam. Top view of (c) low and (d) high magnification SEM images of outlayered CoMoS4 nanosheets arrays. (e) SEM image of in situ synthesized Ni3S2 interlayer, and (f) XRD pattern of the products before and after reaction. The curves I and II in Figure f represent the CoMoO4 precursors and products after the sulfuration reaction, respectively.
The kinetics properties of as synthesized products for OER process were explored by Tafel plots derived from polarization curves in Fig. 6a. As indicated in Fig. 6b, CoMoS4/Ni3S2 electrode exhibits a Tafel slope of 63 mV dec−1, which is lower than that of the CoS2, MoS4 and Pt/C electrodes, respectively, implying a more rapid OER rate of as synthesized products. Kinetics mechanism for the enhanced catalytic activity were further investigated by electrochemical impedance spectroscopy (EIS) in Fig. 6c, which illustrates the interface resistance of the reaction process. The hybrid electrode exhibits a much smaller semicircular diameter compared to other binary sulfides, implying well suppressed resistance of working electrodes. These findings suggest that the porous and thin sheet-like nanostructure of the obtained electrocatalyst provides large surface active site and multichannels for redox reaction and mass diffusion, leading to a significant increase of charge separate efficiency and in turn improves the OER properties. The long-term stability test was also assessed by measuring the time-dependent current density curve under fixed current density of 10 mA cm−2. As shown in Fig. 6d, the current density of the obtained catalyst displays a slight decay from the initial 9.93 mA cm−2 to 9.28 after 10 h test, revealing a high durability of 93.5%. Since OER represents the major barrier for water splitting due to its high theoretical potential with sluggish kinetics, the superior performance of as prepared material indicates its promising prospect as excellent OER electrocatalyst for water splitting.
electrocatalysis process. As references, CoS2 (Fig. S4), MoS2 (Fig. S5), Pt/C and Ni electrodes were also tested under the same condition. Fig. 6a represents the typical linear sweep voltammetry (LSV) of different samples towards OER process. It should be noted that the LSV curve of CoMoS4/Ni3S2 electrode was scanned from negative direction at scan rate of 1 mV S−1 to avoid the interference of Ni oxidation reaction [2,53]. Compared to inactive bare Ni, three sulfide electrodes all exhibit obvious OER catalytic activity at positive potentials. CoMoS4/ Ni3S2 hybrid electrode reveals much sharp raised catalytic current and lower onset potential than pristine CoS2 and MoS2, which could be ascribed to the synergistic effect Co and Mo ions in ternary sulfides [54]. Importantly, the overpotential required to reach current density of 10 mA cm−2 for CoMoS4/Ni3S2 is only 200 mV, which is much lower than 332 mV, 382 mV and 501 mV for CoS2, MoS2 and Pt/C, respectively, suggesting its superior OER catalytic activity. A comparative summary of OER activities of various electrocatalysts in alkaline conditions was further shown in Table S1 to reveal the advanced properties of as obtained hybrid electrocatalyst. For the OER process of transitionmetal phosphides and sulfides, it is important to investigate the structural evolution of the catalysts [55–57]. In our work, the phase and morphology of hybrid electrocatalyst after OER testing were analyzed and shown in Fig. S6. It could be concluded that no obvious change in morphology and phase were observed during OER process. 98
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Fig. 3. Typical TEM images of (a) CoMoO4 precursor and (b) obtained CoMoS4 nanosheets well-illustrated the morphology evolution. (c) magnified TEM and (d) EDS elemental mapping of a well preserved CoMoS4 hollow nanosheet.
a current density of 10 mA cm−2. Although CoS2 and MoS2 electrodes are also capable of HER catalysis, much higher overpotential of 328 and 478 mV were necessary to drive current density of 10 mA cm−2. In sharp contrast, CoMoS4/Ni3S2 electrode reveals excellent HER activity compared to binary sulfides, 158 mV overpotential is sufficient to
The HER activity of the obtained hybrid electrode was evaluated together with CoS2, MoS2 and commercial Pt/C electrode for references. Fig. 7a presents the LSV curves of different electrodes in H2 saturated 1.0 M KOH solution. It can be concluded that Pt/C exhibits the best HER activity and only required an overpotential of 76 mV to reach
Fig. 4. SEM images of products obtained at different reaction times: (a) 0 h, (b) 2 h, (c) 6 h, (d) 9 h, (e) 12 h and (f) 24 h. 99
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Fig. 5. (a) Overall XPS spectrum and high resolution spectra of (b) Co 2p, (c) Mo 3d and (d) S 2p of obtained product.
Fig. 6. Electrocatalytic activities of different electrodes for OER in O2 saturated 1.0 M KOH solution: (a) polarization curves, (b) corresponding Tafel plots, (c) Nyquist plots with the potential of 1.47 V versus RHE and (d) time-dependent current density curves under fixed current density of 10 mA cm−2.
deliver 10 mA cm−2, much lower than that for CoS2 and MoS2. The Tafel plots of different samples were displayed in Fig. 7b. Commercial Pt/C exhibits the smallest Tafel slope of 95 mV dec−1. While the hybrid electrode presents a Tafel slope of 169 mV dec−1, which is also smaller than those of CoS2 (188 mV dec−1) and MoS2 (198 mV dec−1), suggesting well suppressed interface resistance of HER process. A comparative summary of HER activities of various electrocatalysts in alkaline conditions was shown Table S2, which further illustrates the superior properties of as obtained products. The favorable kinetics
process of the obtained products could be further confirmed by the EIS spectra measurement under HER operation condition as shown in Fig. 7c. The time-dependent current density curve reveals a current decay of 7.1% after 10 h measurement (Fig. 7d), implying its well durability for electrocatalysis HER under alkali condition. The amounts of Ni3S2 have obvious influences on the HER performances of the obtained products. The catalytic activities of products obtained at different sulfuration time were also tested and shown in Fig. S7. It could be observed that the current densities were firstly increased 100
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Fig. 7. Electrocatalytic activities of different electrodes for HER in H2 saturated 1.0 M KOH solution: (a) polarization curves, (b) corresponding Tafel plots, (c) Nyquist plots with the potential of −120 mV versus RHE and (d) time-dependent current density curves under fixed current density of 10 mA cm−2.
contributes to the active sites of CoMoS4 toward OER process. The influence of Ni3S2 interlayer on the electrocatalytic activity of CoMoS4 was also investigated in our work. In controlled experiments, Ni3S2 layer was firstly grown on NF by in situ sulfuration reaction (Fig. S9), then CoMoS4 powders were deposited on it to form CoMoS4/Ni3S2 bilayer electrode. For comparison, CoMoS4 powders were also deposited on blank NF, and their electrocatalytic performance toward OER and HER were evaluated as shown in Figs. S10a and b. The bilayer hybrid electrode exhibits much higher OER current density than single CoMoS4 film with 80 mV decrease of overpotential to reach current density of 10 mA cm−2, and slightly increased HER current densities were also observed. These results reveal that Ni3S2 interlayer could greatly increase the OER activity of CoMoS4 with well-preserved HER performance. EIS spectra in Figs. S10c and d further reveals that the much favorable charge transfer efficiency for CoMoS4 could be obtained by adding Ni3S2 interlayer, which enhances the electrochemical reaction processes in the aspect of kinetics properties, and in turn accelerates redox reaction at the electrode/electrolyte interface. Similar findings were also reported in NF-Ni3S2@Ni(OH)2‐graphene sandwich structure electrode and Cu2O/NiO/Cu2MoS4 hybrid film electrode [65,66]. In addition, the CoMoS4/Ni3S2 nanoparticles hybrid electrode shows smaller current densities both for OER and HER than CoMoS4/ Ni3S2 nanosheets arrays hybrid electrode, and these results suggest that in situ growth of the electrode materials with controlled nanostructures on conductive substrate could significantly enhances their electrochemical performances due to effectively increased electrochemically active sites with well suppressed interface resistance. To get a better understand of the inherent mechanism of improved electrocatalytic performance of hybrid electrocatalyst, the electrochemically active surface area (ECSA) was determined through electrical double-layer capacitance measurement in 1 M KOH electrolyte. Figs. S11 and S12 represents the capacitive currents densities collected at different scan rates for CoMoS4 with and without a pre-grown Ni3S2
with the prolonged reaction time to 12 h. Then the HER activity decreased sharply with increased amount of Ni3S2 in as obtained hybrid electrocatalyst when reaction time was further increased to 24 h. In addition, as obtained products also exhibited superior HER activity in acid solution. Fig. S8 presents the LSV curves of CoMoS4/Ni3S2 electrode in 0.5 M H2SO4. It could be seen that higher polarization current could be obtained with rather low overpotential of 93 mV at 10 mA cm−2 than that in KOH solution. These results well illustrate the feasibility of as synthesized electrocatalyst applied in solution with a wide range of pH values. For bifunctional electrocatalyst, it is highly desired to identify the active sites for HER and OER, respectively. Because the electrochemical reactions are mainly occurred at the surface of the electrocatalyst, the HER and OER of as synthesized bilayered electrode are exclusively occurred at the surface of the CoMoS4 outlayer. For monometal sulfides such as MoS2, it is generally believed that the most of the active sites for HER locate at the Mo-edges of 2D structure rather than S-edges [31]. When the Co metal ions are introduced to the MoS2, the incorporated Co ions tend to located at the S-edge, which are also catalytically active for HER due to the decreased Gibbs free energy for adsorbed atomic hydrogen [58]. Thus the Mo-edges and Co-binding S-edges are both availably active sites for HER. While for OER, the origin of electrocatalytic activity of CoMoS4 are seldom reported. Recently Li and Wang investigated different materials’ active sites toward OER and HER from experiments and theoretical calculations [59–62]. It was found that quaternary bimetallic phosphosulphide is inactive to OER due to the high theoretical overpotential, and the in-situ formed oxyhydroxide during electrochemical process act as real active sites for OER [63]. Chu et al. also reported that CoSx on Co3O4 surface could facilitate the formation of Cobalt oxyhydroxide, which serves as surface active sites for OER [64]. Considering the Cobalt hydroxide is possibly in-situ generated at the surface of CoMoS4 during electrocatalysis process in alkali solution [32], it is reasonably speculated that oxyhydroxide also 101
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Fig. 8. Comparison of overall water splitting using a two-electrode system composed of different electrocatalysts as both the cathode and anode in 1 M KOH: (a) polarization curve and (b) time-dependent current density curve of as obtained hybrid electrodes under fixed current density of 10 mA cm−2.
with thin thickness. It was found that the existence of Ni3S2 interlayer formed by the pathway sulfurization reaction of Ni greatly increases the OER activity of CoMoS4, and the obtained hybrid electrode delivers overpotentials of 200 mV and 158 mV to reach current density of 10 mA cm−2 for OER and HER, respectively. Furthermore, a cell voltage of 1.568 V is achieved to afford a current density of 10 mA cm−2 for over water splitting in a two-electrode system, highlighting the advance of our proposed strategy to rational design of efficient electrodes with superior electrochemical properties.
layer. It could be observed that the CoMoS4/Ni3S2 electrode exhibits higher capacitive currents densities than pure CoMoS4 electrodes at each scan rate. In addition, the capacitance values were calculated according to the linear dependence of the current densities on the scan rates, 0.0059 and 0.0037 mF cm−2 for CoMoS4/Ni3S2 and pure CoMoS4, respectively, well consistent with their electrocatalytic performance for OER and HER. The considerably higher capacitance of CoMoS4/Ni3S2 electrode well suggested the enhanced electrochemical surface area by synergistic effect of Ni3S2 interlayer and, correspondingly, enhanced its electrocatalytic activity significantly [2]. Furthermore, the ECSA of the obtained CoMoS4 hollow nanosheets coupled with Ni3S2 interlayer was further investigated as shown in Fig. S13, and the capacitance value was further increased to 0.03301 mF cm−2. These results well confirmed that the electrochemical performances of the obtained hybrid electrode could be further enhanced by direct growth of the electrode materials on conductive substrate with controlled nanostructures. In overall water electrolysis process, OER is a rate-determining step due to its sluggish kinetics ascribed to its complex four-electron oxidation process [3], thus as obtained catalyst with enhanced OER activity is expected to exhibit excellent performance in overall water electrolysis. To assess the bifunctional properties of electrocatalytic overall water splitting, a two-electrode electrolyzer was constructed using same electrocatalysts as both the cathode and anode, respectively. From the LSV in Fig. 8a, it could be observed that symmetric CoMoS4/ Ni3S2 || CoMoS4/Ni3S2 system exhibits the highest water electrolysis properties among all the sample. A cell voltage of 1.568 V is needed to afford a current density of 10 mA cm−2 for overall water splitting with obvious gas evolution on both electrodes (Fig. 8b, inset), only 338 mV higher than the theoretical potential toward the water splitting reaction. A further comparison is shown in Table S3, and as synthesized products exhibit a much lower overpotential for water splitting than most of recently reported bifunctional electrode materials. The durability measurement was also conducted under fixed current density of 10 mA cm−2 in Fig. 8b, and the result well suggests the long-term stability of the obtained hybrid electrode, which could be further confirmed by the well preserved morphology of hybrid electrodes after a long-term stability test (Fig. S14). These findings indicate the superior performance of our CoMoS4/Ni3S2 as a promising and comparable candidate for overall electrocatalytic water splitting in alkaline solution.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 11575228, 91634118, 51472009); The National Natural Science Found for Innovative Research Groups (No. 51621003); Beijing Natural Science Foundation (No. 2182010, 2151001) and the Beijing Municipal High Level Innovative Team Building Program (No. IDHT20170502). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpowsour.2019.01.090. References [1] C. Tang, N. Cheng, Z. Pu, W. Xing, X. Sun, Angew. Chem. Int. Ed. 54 (2015) 9351–9355. [2] A. Sivanantham, P. Ganesan, S. Shanmugam, Adv. Funct. Mater. 26 (2016) 4661–4672. [3] X. Li, X. Hao, A. Abudula, G. Guan, J. Mater. Chem. 4 (2016) 11973–12000. [4] M. Fang, G. Dong, R. Wei, J.C. Ho, Adv. Energy Mater. 7 (2017) 1700559. [5] H. Vrubel, X. Hu, Angew. Chem. Int. Ed. 51 (2012) 12703–12706. [6] T.R. Cook, D.K. Dogutan, S.Y. Reece, Y. Surendranath, T.S. Teets, D.G. Nocera, Chem. Rev. 110 (2010) 6474–6502. [7] T. Wang, H. Xie, M. Chen, A. D'Aloia, J. Cho, G. Wu, Q. Li, Nano Energy 42 (2017) 69–89. [8] Y. Yan, B.Y. Xia, B. Zhao, X. Wang, J. Mater. Chem. 4 (2016) 17587–17603. [9] R. Miao, B. Dutta, S. Sahoo, J. He, W. Zhong, S.A. Cetegen, T. Jiang, S.P. Alpay, S.L. Suib, J. Am. Chem. Soc. 139 (2017) 13604–13607. [10] Z. Fang, L. Peng, Y. Qian, X. Zhang, Y. Xie, J.J. Cha, G. Yu, J. Am. Chem. Soc. 140 (2018) 5241–5247. [11] Y. Li, P. Hasin, Y. Wu, Adv. Mater. 22 (2010) 1926–1929. [12] Z. Fang, L. Peng, H. Lv, Y. Zhu, C. Yan, S. Wang, P. Kalyani, X. Wu, G. Yu, ACS Nano 11 (2017) 9550–9557. [13] T. Tian, L. Huang, L. Ai, J. Jiang, J. Mater. Chem. 5 (2017) 20985–20992. [14] C. Tang, W. Wang, A. Sun, C. Qi, D. Zhang, Z. Wu, D. Wang, ACS Catal. 5 (2015) 6956–6963. [15] X. Peng, L. Hu, L. Wang, X. Zhang, J. Fu, K. Huo, L.Y.S. Lee, K.-Y. Wong, P.K. Chu, Nano Energy 26 (2016) 603–609. [16] J.A. Vigil, T.N. Lambert, B.T. Christensen, J. Mater. Chem. 4 (2016) 7549–7554. [17] B. You, N. Jiang, M. Sheng, S. Gul, J. Yano, Y. Sun, Chem. Mater. 27 (2015) 7636–7642. [18] Z. Zhao, D.E. Schipper, A.P. Leitner, H. Thirumalai, J.-H. Chen, L. Xie, F. Qin,
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