Synthesis of 3D Hexagram-Like Cobalt–Manganese Sulfides Nanosheets Grown on Nickel Foam: A Bifunctional Electrocatalyst for Overall Water Splitting
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The exploration of low-cost and efficient bifunctional electrocatalysts for oxygen evolution reaction and hydrogen evolution reaction through tuning the chemical composition is strongly required for sustainable resources. Herein, we developed a bimetallic cobalt–manganese sulfide supported on Ni foam (CMS/Ni) via a solvothermal method. It has discovered that after combining with the pure Co9S8 and MnS, the morphologies of CMS/Ni have modulated. The obtained three-dimensionally hexagram-like CMS/Ni nanosheets have a significant increase in electrochemical active surface area and charge transport ability. More than that, the synergetic effect of Co and Mn has also presented in this composite. Benefiting from these, the CMS/Ni electrode shows great performance toward hydrogen evolution reaction and oxygen evolution reaction in basic medium, comparing favorably to that of the pure Co9S8/Ni and MnS/Ni. More importantly, this versatile CMS/Ni can catalyze the water splitting in a two-electrode system at a potential of 1.47 V, and this electrolyzer can be efficiently driven by a 1.50 V commercial dry battery.
KeywordsBifunctional electrocatalysts Oxygen evolution reaction Hydrogen evolution reaction Cobalt–manganese sulfides Water splitting
Cobalt–manganese sulfides grown on Ni foam (CMS/Ni) with three-dimensionally hexagram-like nanosheets structure were prepared via a solvothermal method.
As-prepared CMS/Ni shows highly catalytic activity for OER and HER in basic medium and can catalyze water splitting by a 1.50 V dry battery.
Electrocatalytic water splitting has been regarded as the most promising and feasible technology to produce clean hydrogen fuel from aqueous solutions [1, 2, 3]. Hence, efficient electrocatalysts for both the oxygen evolution reaction (OER) at anodes and hydrogen evolution reaction (HER) at cathodes are urgently needed to reduce the energy consumption for overall water splitting [4, 5, 6]. Precious metal oxide (e.g., RuO2, IrO2) and noble metal (e.g., Pt, Ir, Rh) electrocatalysts are so far known as the most efficient electrocatalysts toward OER and HER, respectively, but the high cost and scarcity have limited their widespread application [6, 7]. In this regard, tremendous efforts have been devoted to explore alternatively earth-abundant and cost-effective transition metal materials for OER or HER over the past several decades [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22]. Unfortunately, these electrocatalysts are still not suitable for the real commercial applications. In addition, to simplify the overall water splitting system and cut the cost, developing highly efficient bifunctional electrocatalysts for both OER and HER in the same electrolyte, especially for alkaline electrolyte, has become one of the hottest issues recently . Despite great advances have taken in this field [24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39], it is still in great demand to explore high-performance and non-noble bifunctional electrocatalysts for overall water splitting.
The catalytic activity could improve following the methods of chemical composition tuning and nanostructure modification [40, 41, 42, 43, 44]. On the one hand, to tailor the chemical composition of electrocatalysts, an effective way is doping with foreign atoms into the crystal lattice of materials. Following this way, the multicomponent or composite electrocatalysts would obtain [40, 41, 42, 43, 44, 45, 46, 47, 48, 49]. The formation of different valence and electronic states of metal ions in these composites facilitates the adsorption and desorption of intermediates in the electrocatalysis process, and the synergistic effect between the metal ions is benefit for their catalytic activity. For example, Wu et al.  discovered the different valence states of Ni and synergistic effect between the metal ions in Ni3ZnC0.7, playing an important role in its catalytic activity for HER and OER. Yang et al.  synthesized a Co(II)1−x Co(0) x/3Mn(III)2x/3S nanoparticles combining with B/N-codoped mesoporous nanocarbon. They investigated the formation of different valence and electronic states of Co and Mn ions in facilitating the catalytic activity. On the other hand, optimizing the nanostructure of electrocatalysts can increase the quantity of effectively active sites. Comparing to nanoparticles materials, the electrocatalysts directly supported on conductive substrates, such as Ni foam, Ni mesh, Cu mesh, carbon cloth and carbon paper, with binder free can get to this point easily . Recently, Ni foam has exhibited considerable potential in optimizing the nanostructure of materials [51, 52, 53, 54]. It has a unique three-dimensional (3D) porous structure and high conductivity. For instance, our group  prepared urchin-like sphere arrays Co3O4 supported on 3D Ni foam showing great performance for HER and OER, which is benefit from its urchin-like nanostructure with rich mesopores and low charge-transfer resistance. Hu et al.  developed a Co–Mn carbonate hydroxide (CoMnCH) nanosheet arrays on Ni foam exhibiting superior activity for HER and OER in basic medium.
3 Experimental Sections
MnCl2·4H2O, Co(NO3)2·6H2O, urea and NH4F were purchased from Aladdin. Ni foam was purchased from Kunshan Electronic Limited Corporation. All chemicals were directly used without any purification.
3.2 Synthesis of CoMn-LDH/Ni
Typically, 198 mg MnCl2·4H2O, 582 mg Co(NO3)2·6H2O, 180 mg urea and 37 mg NH4F were dissolved into a beaker containing 40 mL distilled water and 10 mL absolute ethanol to form a homogeneous solution under stirring for 10 min. A piece of Ni foam (3 × 3 cm2) which was cleaned by sonication sequentially in 3 mol L−1 HCl solution and absolute ethanol for 15 min each was immersed into the above solution, and then, the mixture was transferred into an autoclave (80 mL). The autoclave was sealed and heated at 120 °C for 6 h. After cooling down to room temperature, the precursor CoMn-LDH/Ni (XRD pattern is exhibited in Fig. S1) was taken out and washed by deionized water for four times and dried at 60 °C.
3.3 Synthesis of CMS/Ni
To obtain the CMS/Ni, 400 mg thioacetamide was dissolved in 50 mL deionized water. Then, the clean solution was transferred into an autoclave containing a piece of prepared CoMn-LDH/Ni. After heating at 120 °C for 12 h, the product was taken out and severally washed by absolute ethanol and deionized water for four times. Finally, the CMS/Ni was obtained after dehydrating in an oven at 60 °C overnight. The mass loading of CMS is 4.1 mg cm−2. For comparison, the Co9S8/Ni and MnS/Ni were also synthesized through the same processes without adding the Mn2+ or Co2+ ions.
3.4 Materials Characterization
The X-ray diffraction (XRD) patterns were tested by a MSAL-XD2 X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å). The scanning electron microscopy (SEM) was performed by a Philips SEM-XL30S microscope operated at 15 kV. High-resolution transmission electron microscope (HRTEM, JEOL JEM-2100F) coupled with an energy-dispersive X-ray spectroscopy (EDS) analyzer was carried out with an accelerating voltage of 200 kV. The nitrogen sorption isotherms were carried out by a Micromeritics TriStar 3000 Analyzer at 77 K. The X-ray photoelectron spectroscopy (XPS) was analyzed by an ESCALab250.
3.5 Electrochemical Measurements
4 Results and Discussion
4.1 Structure and Morphology of Materials
The XRD patterns of the as-prepared samples were performed using their powders scraped down from the Ni foam. As shown in Fig. S2a, the XRD pattern of Co9S8 exhibits the cubic crystalline phase with diffraction peaks at 29.8°, 31.1°, 47.6°, and 51.9°, which are severally corresponding to the (311), (222), (511), and (440) planes of Co9S8 (No. 02-1459). Meanwhile, the typical diffraction peaks of MnS located at 34.3° and 49.3° are matched well with the (200) and (220) planes of cubic MnS (No. 65-2919). Interestingly, the XRD pattern of CMS reveals the crystal structures of the Co9S8 and MnS are still maintained in CMS after modulating with them as a composite. Moreover, the EDS analysis reveals the atomic ratio of Co, Mn and S is ~0.9:0.1:1.0 in the composite (Fig. S2b), while the Cu elemental is coming from copper mesh. Figure S2c, d shows the atomic ratios of Co9S8 and MnS are ~9.0:8.0 and 1.0:1.0, respectively.
The Co 2p XPS spectra of CMS/Ni in Fig. 1g reveal two distinct peaks at 781.0 and 797.3 eV corresponding to the Co 2p 3/2 and Co 2p 1/2, respectively, with two related satellite peaks at 786.3 and 802.9 eV. These are the characteristic peaks of Co2+ , and the other two peaks at 778.2 and 793.2 eV are assigned to the metallic Co . Nevertheless, there is not metallic Co presented on the Co 2p XPS spectra of Co9S8/Ni counterpart (Fig. S5a). Moreover, there are no diffraction peaks of metallic Co presenting in the XRD pattern of CMS/Ni, which may be attributed to its low content in CMS/Ni. As Mn 2p XPS spectra of CMS/Ni shown in Fig. 1h, the peak located at 643.3 eV confirms the oxidized Mn3+ species in CMS/Ni [50, 55], while there is no Mn3+ detected from XPS spectra of MnS/Ni counterpart (Fig. S5b). Simultaneously, Mn2+ reveals with two characteristic peaks at bending energies of 641.6 eV (Mn 2p 3/2) and 653.8 eV (Mn 2p 1/2). The above results are similar to Yang’s work . The occurrence of metallic Co and Mn3+ in CMS/Ni can be explained by the higher reduction potential of Co2+/Co (Co2+ + 2e− → Co, − 0.277 V vs. NHE) comparing to Mn2+/Mn (Mn2+ + 2e− → Mn, − 1.18 V vs. NHE). This means that the Co2+ as oxidizer is easier to be reduced from Co2+ to metallic Co than the same reaction of Mn2+. The reduction in Co2+ to metallic Co would result in the oxidation of Mn2+ to Mn3+ at the same time (Mn2+ − e− → Mn3+, 1.5 V vs. NHE) . The different valance states of metal cations in CMS/Ni are benefit for improving the catalytic performance [42, 50]. Figure 1i shows the peaks of S 2p 3/2 and S 2p 1/2 are located at 161.6 and 162.7 eV, respectively, which are derived from metal–sulfur bonds . Furthermore, a peak at 168.8 eV with its satellite peak at 170.0 eV is attributed to the superficial oxidation of CMS/Ni in air .
4.2 Hydrogen Evolution Activity
On the other hand, the superior activity of CMS/Ni for HER, in comparison with that of the pure Co9S8/Ni and MnS/Ni, results from the significant increase in electrochemical active surface areas (ECSAs) to expose more accessible catalytic active sites. The ECSAs of CMS/Ni, Co9S8/Ni and MnS/Ni were measured by the capacitance measurements through cyclic voltammograms in a non-Faradaic at different scan rates (Fig. S8). The ECSA of an electrocatalyst is proportional to its C dl value. It can be seen that the C dl values of 24.0 mF cm−2 for Co9S8/Ni and 13.0 mF cm−2 for MnS/Ni are tremendously increase to 56.3 mF cm−2 for CMS/Ni, implying the CMS/Ni has more effective active sites, as shown in Fig. 2c.
The HER stability of CMS/Ni was further evaluated at a constant potential of − 0.14 V. As shown in Fig. 2d, the CMS/Ni reveals a great stability with a negligible decay of the current density after 10 h continuous measurements. Simultaneously, the LSV polarization curve tested at 100 mV s−1 after 1000 cycles is similar to the first cycle, as the inset in Fig. 2d. The superior durability of CMS/Ni is benefit from its high structure stability because the hexagram-like structure for CMS/Ni just exhibits a little aggregation after stability measurement in Fig. S9a. The excellent catalytic performance and great stability highlight the great potential of CMS/Ni for practical application.
4.3 Oxygen Evolution Activity
The stability of CMS/Ni for OER was calculated at a static potential of 1.46 V in 1.0 mol L−1 KOH solution. Figure 3c presents a negligible decrease in current density after 10 h continuing OER measurement, indicating the superior durability. Additionally, this result can be further confirmed by the LSV curves in Fig. 3d, because the polarization curve after 1000 cycles is similar to the initial cycle. In particular, it can be seen that the morphology of CMS/Ni slightly changes after stability testing in Fig. S9b.
All the above results indicate that the CMS/Ni possesses superior HER and OER catalytic activity comparing to the pure Co9S8/Ni and MnS/Ni, which could be involved the following factors: (1) The obtained CMS/Ni has a uniquely 3D hexagram-like nanosheet structure, which not only provides a large electrochemical active surface areas (ECSAs) to expose more accessible catalytic active sites, but also contacts with the electrolyte efficiently and facilitates the transportation of O2, H2 bubbles [28, 32, 57]; (2) the 3D hexagram-like CMS directly supported on Ni foam substrate enhances the structure stability and improves electrons transport ability of CMS/Ni, which are beneficial to improving the catalytic activity; and (3) the XPS results (shown in Fig. 1g, h) indicate that the different valence states of Co and Mn are presented in the CMS/Ni, which can facilitate the adsorption and desorption of intermediates in the electrocatalysis process. The synergistic effect between Co and Mn is helpful for the catalytic activity [42, 49, 50, 58].
4.4 Overall Water Splitting
In summary, a simple anion exchange method was employed to successfully prepare 3D hexagram-like CMS/Ni. The 3D hexagram-like CMS/Ni nanosheets have large electrochemical active surface area to expose more active sites and low charge-transfer resistance. Noticeably, the synergetic effect of Co and Mn is also presented in this composite. Consequently, it exhibits superior catalytic activity in basic medium with low overpotentials of 217 mV for HER and 298 mV for OER to reach a current density of 100 mA cm−2. More importantly, the assembled CMS/Ni//CMS/Ni device for overall water splitting can be driven by a 1.50 V dry battery, indicating the great potential for practical applications. Therefore, this work provides a scalable method to synthesize bi- or multimetallic sulfide composites and extends the preparation of the other novel electrocatalysts for water splitting.
This work was supported by National Natural Science Foundation of China (21576113 and 21376105) and Foshan Innovative and Entrepreneurial Research Team Program (No. 2014IT100062).
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