Synthesis of 3D Hexagram-Like Cobalt–Manganese Sulfides Nanosheets Grown on Nickel Foam: A Bifunctional Electrocatalyst for Overall Water Splitting

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1 Nano-Micro Lett. (218) 1:6 ARTICLE Synthesis of 3D Hexagram-Like Cobalt Manganese Sulfides Nanosheets Grown on Nickel Foam: A Bifunctional Electrocatalyst for Overall Water Splitting Jingwei Li 1. Weiming Xu 1. Jiaxian Luo 1. Dan Zhou 1. Dawei Zhang 1. Licheng Wei 1. Peiman Xu 1. Dingsheng Yuan 1 Received: 13 August 217 / Accepted: 17 September 217 / Published online: 13 October 217 Ó The Author(s) 217. This article is an open access publication Highlights Cobalt manganese sulfides grown on Ni foam () with three-dimensionally hexagram-like nanosheets structure were prepared via a solvothermal method. As-prepared shows highly catalytic activity for OER and HER in basic medium and can catalyze water splitting by a 1.5 V dry battery. Abstract 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 () via a solvothermal method. It has discovered that after combining with the pure Co 9 S 8 and MnS, the morphologies of have modulated. The obtained three-dimensionally hexagram-like 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 electrode shows great performance toward hydrogen evolution reaction and oxygen evolution reaction in basic medium, comparing favorably to that of Power Overall water splitting // 5 wt% Pt/C/Ni//RuO 2 /Ni Ni//Ni the pure Co 9 S 8 /Ni and MnS/Ni. More importantly, this versatile can catalyze the water splitting in a twoelectrode system at a potential of 1.47 V, and this electrolyzer can be efficiently driven by a 1.5 V commercial dry battery. Keywords Bifunctional electrocatalysts Oxygen evolution reaction Hydrogen evolution reaction Cobalt manganese sulfides Water splitting Electronic supplementary material The online version of this article (doi:1.17/s ) contains supplementary material, which is available to authorized users. & Dingsheng Yuan tydsh@jnu.edu.cn 1 School of Chemistry and Materials Science, Jinan University, Guangzhou 51632, People s Republic of China

2 6 Page 2 of 1 Nano-Micro Lett. (218) 1:6 1 Introduction Electrocatalytic water splitting has been regarded as the most promising and feasible technology to produce clean hydrogen fuel from aqueous solutions [1 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 6]. Precious metal oxide (e.g., RuO 2, IrO 2 ) 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 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 [23]. Despite great advances have taken in this field [24 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 [4 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 [4 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. [42] discovered the different valence states of Ni and synergistic effect between the metal ions in Ni 3 ZnC.7, playing an important role in its catalytic activity for HER and OER. Yang et al. [5] synthe sized a Co(II) 1-x Co() x/3 Mn(III) 2x/3 S 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 [41]. Recently, Ni foam has exhibited considerable potential in optimizing the nanostructure of materials [51 54]. It has a unique three-dimensional (3D) porous structure and high conductivity. For instance, our group [32] prepared urchinlike sphere arrays Co 3 O 4 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. [41] developed a Co Mn carbonate hydroxide (CoMnCH) nanosheet arrays on Ni foam exhibiting superior activity for HER and OER in basic medium. Based on the above mentioned, a cobalt manganese sulfides composite with a unique nanostructure was synthesized to exhibit an efficient catalytic performance for OER and HER. However, to the best of our knowledge, the cobalt manganese sulfide with efficient catalystic performance has never been reported. Herein, the 3D hexagram-like was prepared via a simple two-step hydrothermal method (Scheme 1). Firstly, the CO 3 2- and OH - ions were released by the hydrolysis of urea and gradually co-precipitated with Co 2? and Mn 2? ions to form the 3D hexagram-like precursor CoMn-LDH/Ni. Subsequently, the 3D hexagram-like CMS/ Ni nanosheets were synthesized after a sulfurization process employing the thioacetamide (TAA) as the sulfur sources. The electrochemical catalytic activity of this composite was evaluated by linear sweep voltammetry, electrochemical impedance spectroscopy and chronoamperometry. The electrocatalytic performance of for HER and OER is optimized with respect to those of the pure Co 9 S 8 /Ni and MnS/Ni. The was designed as an efficient electrolyzer for overall water splitting. Co 2+ Mn 2+ urea H 2 O NH 4 F I II TAA, 12 H 2 O 2 12, 6 h, 8 h Ni foam Scheme 1 Scheme of the synthesis procedure for CoMn-LDH/Ni

3 Nano-Micro Lett. (218) 1:6 Page 3 of Experimental Sections 2.1 Materials MnCl 2 4H 2 O, Co(NO 3 ) 2 6H 2 O, urea and NH 4 F were purchased from Aladdin. Ni foam was purchased from Kunshan Electronic Limited Corporation. All chemicals were directly used without any purification. 2.2 Synthesis of CoMn-LDH/Ni Typically, 198 mg MnCl 2 4H 2 O, 582 mg Co(NO 3 ) 2 6H 2 O, 18 mg urea and 37 mg NH 4 F were dissolved into a beaker containing 4 ml distilled water and 1 ml absolute ethanol to form a homogeneous solution under stirring for 1 min. A piece of Ni foam (3 9 3cm 2 ) 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 (8 ml). The autoclave was sealed and heated at 12 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 6 C. 2.3 Synthesis of To obtain the, 4 mg thioacetamide was dissolved in 5 ml deionized water. Then, the clean solution was transferred into an autoclave containing a piece of prepared CoMn-LDH/Ni. After heating at 12 C for 12 h, the product was taken out and severally washed by absolute ethanol and deionized water for four times. Finally, the was obtained after dehydrating in an oven at 6 C overnight. The mass loading of CMS is 4.1 mg cm -2. For comparison, the Co 9 S 8 /Ni and MnS/Ni were also synthesized through the same processes without adding the Mn 2? or Co 2? ions. 2.4 Materials Characterization The X-ray diffraction (XRD) patterns were tested by a MSAL-XD2 X-ray diffractometer with Cu Ka radiation (k = Å). The scanning electron microscopy (SEM) was performed by a Philips SEM-XL3S microscope operated at 15 kv. High-resolution transmission electron microscope (HRTEM, JEOL JEM-21F) coupled with an energy-dispersive X-ray spectroscopy (EDS) analyzer was carried out with an accelerating voltage of 2 kv. The nitrogen sorption isotherms were carried out by a Micromeritics TriStar 3 Analyzer at 77 K. The X-ray photoelectron spectroscopy (XPS) was analyzed by an ESCALab Electrochemical Measurements All the electrochemical measurements were carried out in a conventional three-electrode system. The and other obtained samples were used as the working electrodes. Pt foil and Hg/HgO electrodes were separately used as the counter and reference electrodes. The 1. mol L -1 KOH was used as the electrolyte. Linear sweep voltammetry (LSV) was analyzed with a scan rate of 2 mv s -1. All potentials in this work were calibrated by the Nernst equation: E RHE ¼ E Hg=HgO þð:98 þ :59 phþ V ð1þ g ¼ E RHE 1:23 V ð2þ where g is the overpotential. In addition, the Tafel slope was modeled by Tafel equation: g ¼ blogj þ a ð3þ where g is the overpotential, b is the Tafel slope, j is the current density and a is a constant. Chronoamperometry measurements were tested at a static potential, and the electrochemical impedance spectroscopy (EIS) was also performed in the frequency range from 1 khz to 1 mhz with an amplitude of 5 mv. The electrochemically active surface areas (ECSAs) were evaluated by the electrochemical double-layer capacitance (C dl ) via collecting cyclic voltammograms (CVs). The different CV cycles (5, 1, 15, 2, 3, and 5 mv s -1 ) were tested in the non- Faradaic potential region from.124 to.224 V vs RHE to determine the C dl. 3 Results and Discussion 3.1 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 Co 9 S 8 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 (44) planes of Co 9 S 8 (No ). Meanwhile, the typical diffraction peaks of MnS located at 34.3 and 49.3 are matched well with the (2) and (22) planes of cubic MnS (No ). Interestingly, the XRD pattern of CMS reveals the crystal structures of the Co 9 S 8 and MnS are still maintained in CMS after modulating with them as a composite. Moreover, the EDS analysis reveals the atomic

4 6 Page 4 of 1 Nano-Micro Lett. (218) 1:6 ratio of Co, Mn and S is *.9:.1:1. 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.:8. and 1.:1., respectively. Figure S3 presents the SEM images of Co9S8/Ni and MnS/Ni. The nanosheet structures of Co9S8/Ni and MnS/Ni are still retained a rough surface with respect to their precursors. However, the morphologies are modified after combining with Co9S8/Ni and MnS/Ni. It can be observed in Fig. 1a, b that the 3D hexagram-like nanosheets were obtained after sulfonating the CoMn-LDH/Ni (Fig. S4), and each hexagram is entirely covered by uniform nanosheets (Fig. 1c). The TEM image shown in Fig. 1d reveals the 3D hexagram-like nanosheets which are made up of solid nanorods. Figure 1e shows the HRTEM image, where the distinct lattice fringes of d =.3 nm and d =.191 nm are corresponding to (311) and (511) crystal plane of Co9S8, respectively, and (a) the d =.261 nm is attributed to (2) crystal plane of MnS. This result indicates that the CMS is a composite material, which is comprised by the interrelated Co9S8 and MnS. Additionally, it can be seen that the elements of Co, Mn and S are uniformly dispersed in, as shown in Fig. 1f. The Co 2p XPS spectra of in Fig. 1g reveal two distinct peaks at 781. and ev corresponding to the Co 2p3/2 and Co 2p1/2, respectively, with two related satellite peaks at and 82.9 ev. These are the characteristic peaks of Co2? [5], and the other two peaks at and ev are assigned to the metallic Co [5]. 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, which may be attributed to its low content in. As Mn 2p XPS spectra of shown in Fig. 1h, the peak located at (b) (c) 2 μm (d) 1 μm (e) 2 nm (f) d=.3 nm Co9S8 (311) Co d=.261 nm MnS (2) d=.191 nm Co9S8 (551) 2 nm.5 μm Mn Co2+ * (i) Mn2+ S 2p Mn 2p Intensity (a.u.) Intensity (a.u.) Co 2p (h) Co2+ Co Co * Mn2+ Intensity (a.u.) (g) S Mn2+ * CMS 81 CMS Binding energy (ev) CMS Binding energy (ev) Binding energy (ev) 16 Fig. 1 The characterization results of : a c SEM images in different magnifications, d TEM image, e HRTEM image, f elemental mapping images and g i XPS spectra

5 Nano-Micro Lett. (218) 1:6 Page 5 of ev confirms the oxidized Mn 3? species in [5, 55], while there is no Mn 3? detected from XPS spectra of MnS/Ni counterpart (Fig. S5b). Simultaneously, Mn 2? reveals with two characteristic peaks at bending energies of ev (Mn 2p 3/2 ) and ev (Mn 2p 1/2 ). The above results are similar to Yang s work [5]. The occurrence of metallic Co and Mn 3? in can be explained by the higher reduction potential of Co 2? /Co (Co 2?? 2e -? Co, V vs. NHE) comparing to Mn 2? /Mn (Mn 2?? 2e -? Mn, V vs. NHE). This means that the Co 2? as oxidizer is easier to be reduced from Co 2? to metallic Co than the same reaction of Mn 2?. The reduction in Co 2? to metallic Co would result in the oxidation of Mn 2? to Mn 3? at the same time (Mn 2? - e -? Mn 3?, 1.5 V vs. NHE) [56]. The different valance states of metal cations in are benefit for improving the catalytic performance [42, 5]. Figure 1i shows the peaks of S 2p 3/2 and S 2p 1/2 are located at and ev, respectively, which are derived from metal sulfur bonds [5]. Furthermore, a peak at ev with its satellite peak at 17. ev is attributed to the superficial oxidation of in air [55]. 3.2 Hydrogen Evolution Activity The electrocatalytic activity of toward HER was characterized by LSV measurement with a scan rate of 2mVs -1. For comparison, the Co 9 S 8 /Ni, MnS/Ni, bare Ni foam and commercial 5 wt% Pt/C coated on Ni foam (5 wt% Pt/C/Ni [3], loading 4.1 mg cm -2 ) were also evaluated. As the polarization curves illustrated in Fig. 2a, the 5 wt% Pt/C/Ni possesses the most excellent HER performance with a near-zero onset potential. The reveals a much smaller onset potential at - 88 mv and larger HER current than those of Co 9 S 8 /Ni, MnS/Ni and bare Ni foam. This result highlights the catalytic activity of has significantly enhanced after combining Co 9 S 8 / Ni with MnS/Ni as a composite, which may be ascribed to the synergistic effect of Co and Mn. The HER performance of is also comparable with the CoMn-LDH/Ni, as (a) mv dec 1 25 Co 9 S 8 /Ni mv dec 1 MnS/Ni 3 Ni 5wt% Pt/C/Ni. 3.7 mv dec Log (j/ma cm ) (b) Co 9 S 8 /Ni MnS/Ni 5wt% Pt/C/Ni 14.1 mv dec 1 J (ma cm 2 ) (c) Co 9 S 8 /Ni MnS/Ni 24. mf cm mf cm mf cm Scan rate (mv s 1 ) (d) j / ma cm 2.14 V Initial 2 After 1 25 cycles E/V vs. RHE Time (h) Fig. 2 a Polarization curves, b Tafel plots, c plots of the current density for, Co 9 S 8 /Ni, MnS/Ni at potential of.174 V against the scan rates and d chronoamperometric curve (the inset is polarization curves before and after 1 CV cycles)

6 6 Page 6 of 1 Nano-Micro Lett. (218) 1:6 shown in Fig. S6a, b. Noticeably, the drives a high current density of 1 ma cm -2 at an overpotential of 217 mv, which is lower than 419 mv for Co 9 S 8 /Ni and 56 mv for MnS/Ni. Such an overpotential of is superior to most of the recently reported bifunctional electrocatalysts listed in Table S1. The efficient catalytic activity of is also supported by the Tafel slopes in Fig. 2b. The Tafel slope of is 48.2 mv dec -1 which is obviously lower than 81.1 mv dec -1 for Co 9 S 8 /Ni and 14.1 mv dec -1 for MnS/Ni, implying great HER kinetics and catalytic activity. In addition, the EIS measurements of, Co 9 S 8 /Ni and MnS/Ni were measured at a static potential of -.33 V to further elucidate the charge transport of the as-prepared materials. As exhibited in Fig. S7, the shows lower resistance of.98 X than the pure Co 9 S 8 /Ni (1.8 X) and MnS/Ni (2.14 X), suggesting an improvement of conductivity. The lower charge-transfer resistance of may be attributed to its uniquely 3D hexagram-like nanosheets structure contacting with the electrolyte efficiently, and the metallic Co generated in CMS (see Fig. 1g) after integrating with two Co 9 S 8 and MnS to be the composite [5]. On the other hand, the superior activity of for HER, in comparison with that of the pure Co 9 S 8 /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, Co 9 S 8 /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. mf cm -2 for Co 9 S 8 /Ni and 13. mf cm -2 for MnS/Ni are tremendously increase to 56.3 mf cm -2 for, implying the has more effective active sites, as shown in Fig. 2c. The HER stability of was further evaluated at a constant potential of -.14 V. As shown in Fig. 2d, the reveals a great stability with a negligible decay of the current density after 1 h continuous measurements. Simultaneously, the LSV polarization curve tested at 1 mv s -1 after 1 cycles is similar to the first cycle, as the inset in Fig. 2d. The superior durability of is benefit from its high structure stability because the hexagram-like structure for just exhibits a little aggregation after stability measurement in Fig. S9a. The excellent catalytic performance and great stability highlight the great potential of for practical application. 3.3 Oxygen Evolution Activity The electrocatalytic activity of for OER was also characterized by LSV measurement with a scan rate of 2mVs -1. The Co 9 S 8 /Ni, MnS/Ni, bare Ni foam and commercial RuO 2 coated on Ni foam (RuO 2 /Ni [3], loading 4.1 mg cm -2 ) were evaluated for comparison. Figure 3a shows the polarization curves of the as-prepared samples, in which the oxidation peaks from 1.33 to 1.41 V are ascribed to the transition from M 2? to M 3? [3]. Owing to the intense oxidation peaks, the overpotential for materials to generate the anodic current of 1 ma cm -2 for OER is not accurate. Therefore, we report the overpotentials at 1 ma cm -2 here. As observed, a current density of 1 ma cm -2 is easily reached at the overpotential of 298 mv for, better than those of Co 9 S 8 /Ni (439 mv), MnS/Ni (492 mv), RuO 2 /Ni (568 mv) and even most of the other recently reported bifunctional materials exhibited in Table S1. The above results suggest the efficient OER activity of, which is also superior to the CoMn-LDH/Ni (Fig. S6c, d). A tremendous improvement of catalytic performance for with respect to its monometallic counterparts can also be supported by its smallest Tafel slope. As shown in Fig. 3b, the smallest Tafel slopes of 43.9 mv dec -1 for with respect to Co 9 S 8 /Ni (71.5 mv dec -1 ), MnS/Ni (1.3 mv dec -1 ) and RuO 2 /Ni (132.4 mv dec -1 ) imply the great OER kinetics activity. The stability of for OER was calculated at a static potential of 1.46 V in 1. mol L -1 KOH solution. Figure 3c presents a negligible decrease in current density after 1 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 1 cycles is similar to the initial cycle. In particular, it can be seen that the morphology of slightly changes after stability testing in Fig. S9b. All the above results indicate that the possesses superior HER and OER catalytic activity comparing to the pure Co 9 S 8 /Ni and MnS/Ni, which could be involved the following factors: (1) The obtained 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 O 2,H 2 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, 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, 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, 5, 58].

7 Nano-Micro Lett. (218) 1:6 Page 7 of (a) Co 9 S 8 /Ni MnS/Ni RuO 2 /Ni Ni (b) Co 9 S 8 /Ni MnS/Ni RuO 2 /Ni mv dec mv dec mv dec mv dec Log (j/ma cm 2 ) (c) 1.46 V (d) Initial After 1 cycles Time (h) Fig. 3 a Polarization curves, b Tafel plots, c chronoamperometric curve and d polarization curves before and after 1 CV cycles 3.4 Overall Water Splitting Based on the excellent activity toward OER and HER, the was directly assembled as the cathodic and anodic electrodes in two-electrode cell (//) in 1. mol L -1 KOH electrolyte for overall water splitting. As shown in Fig. 4a, the LSV curve of the cell indicates that water is electrolyzed only at the voltage of 1.47 V to 5 // 5 wt% Pt/C/Ni//RuO 2 /Ni Ni//Ni (a) (b) Volume (ml) Collected the evoluted H 2 and O 2 H 2 evolution experimental Theoretical O 2 evolution experimental Theoretical Time (min) 1 ma cm Time (h) Fig. 4 a Polarization curves of //, 5 wt Pt/C/Ni//RuO 2 /Ni and Ni//Ni for overall water splitting (the inset is a device for overall water splitting). b Chronopotentiometry curve of // (the inset is volume of O 2 and H 2 generated for theoretically calculated and experimentally measured versus time)

8 6 Page 8 of 1 Nano-Micro Lett. (218) 1:6 generate H 2 and O 2 bubbles. This result is supported by a device, which is driven by a 1.5 V dry battery (the inset in Figs. 4a and S1). For comparison, the 5 wt% Pt/C/Ni, RuO 2 /Ni and bare Ni foam were also used as the twoelectrode electrolyzers. A current density of 1 ma cm -2 for // can be achieved at a cell voltage of 1.6 V, equaling to that of 1.6 V for 5 wt% Pt/C/Ni// RuO 2 /Ni. Although this cell voltage (1.6 V) of // is higher than 1.52 V for Cu@CoS x /CF Cu@CoS x /CF [59], 1.45 V for MoO x /Ni 3 S 2 /NF//MoO x / Ni 3 S 2 /NF [6] and 1.53 V for NixCo 3-x S 4 /Ni 3 S 2 /NF//Ni x- Co 3-x S 4 /Ni 3 S 2 /NF [61], it is superior to that of NiCo 2 S 4 NW/NF//NiCo 2 S 4 NW/NF (1.68 V) [25], FeNi 3 N/NF// FeNi 3 N/NF (1.62 V) [27], Ni/NiP//Ni/NiP (1.61 V) [3], Co 3 O 3 O (1.64 V) [32], Ni 2.5 Co.5 Fe/NF// Ni 2.5 Co.5 Fe/NF (1.62 V) [35], Ni(OH) 2 /NF//Ni(OH) 2 /NF (1.68 V) [36], Co 1 Mn 1 CH/NF//Co 1 Mn 1 CH/NF (1.68 V) [41], Co(S.71 Se.29 ) 2 //Co(S.22 Se.78 ) 2 (1.63 V) [43] and the two-electrode electrolyzers listed in Table S1. In addition, the chronopotentiometry curve of //CMS/ Ni was measured at a constant current of 1 ma cm -2 for 2 h, exhibiting a great stability for // in Fig. 4b. The volume time plots for generating O 2 and H 2 suggest that the Faradaic efficiency of // electrolyzer is nearly 1% (see the inset in Fig. 4b and experimental details are shown in SI). 4 Conclusions In summary, a simple anion exchange method was employed to successfully prepare 3D hexagram-like CMS/ Ni. The 3D hexagram-like 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 1 ma cm -2. More importantly, the assembled // device for overall water splitting can be driven by a 1.5 V dry battery, indicating the great potential for practical applications. 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