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Porous flower-like Mo-doped NiS heterostructure as highly efficient and robust electrocatalyst for overall water splitting
Accepted Manuscript Porous flower-like Mo-doped NiS heterostructure as highly efficient and robust electrocatalyst for overall water splitting
Qiang Jia, Xiaoxia Wang, Shuang Wei, Congli Zhou, Jianmei Wang, Jingquan Liu PII: DOI: Reference:
S0169-4332(19)31164-X https://doi.org/10.1016/j.apsusc.2019.04.165 APSUSC 42473
To appear in:
Applied Surface Science
Received date: Revised date: Accepted date:
26 January 2019 31 March 2019 14 April 2019
Please cite this article as: Q. Jia, X. Wang, S. Wei, et al., Porous flower-like Mo-doped NiS heterostructure as highly efficient and robust electrocatalyst for overall water splitting, Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.04.165
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ACCEPTED MANUSCRIPT
Porous Flower-like Mo-doped NiS Heterostructure as Highly Efficient and Robust Electrocatalyst for Overall Water Splitting
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Qiang Jiaa, Xiaoxia Wanga*, Shuang Weia,b, Congli Zhoua, Jianmei Wanga,c, Jingquan Liu a* College of Materials Science and Engineering, Institute for Graphene Applied Technology
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Innovation, Qingdao University, Qingdao 266071, China.
School of Material Science and Engineering, Ocean University of China.
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School of Life and Environmental Sciences, Deakin University, Geelong, VIC 3217, Australia.
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Email: [email protected] (X. Wang), [email protected] (J. Liu)
Abstract
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The development of electrocatalyst with remarkable activity for hydrogen production
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through water or urea splitting is critical for coping with the energy crisis, but still remains a lot of obstacles. In order to improve the electrocatalytic activity, rational design of nanostructures with rich active sites is an efficient method. Herein, a porous
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flower-like Mo-doped NiS on Ti mesh (Mo-doped p-NiS/Ti) is synthesized through selective etching of Al(OH)3 in Mo-doped NiAl layered double hydroxides, and then
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followed by a facile hydrothermal sulfuration process using sodium sulfide as S source to afford Mo-doped p-NiS/Ti. As a non-noble metal catalyst, Mo-doped p-NiS/Ti catalyst exhibits excellent electrocatalytic performance in electrolyte of 1.0 M KOH or 1.0 M KOH with 0.3 M urea. To drive a current density of 10 mA·cm-2, a small overpotentials of 147.6 mV and 162.3 mV in 1.0 M KOH and 1.0 M KOH with 0.3 M urea are required, respectively, and remarkable durability is observed in 1.0 M KOH or urea water solutions. More significantly, Mo-doped p-NiS/Ti holds promises for potential implementation in the overall water or urea splitting. 1
ACCEPTED MANUSCRIPT Keywords: electrocatalyst, porous, flower-like, hydrogen production 1 Introduction With the growing challenges of environmental pollution and shortage of fossil fuels, the exploration of clean and renewable energy sources has attracted tremendous attentions [1-3]. Hydrogen is regarded as a promising clean fuel owing to its zero
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carbon emission, environmental friendliness and high energy density, while water electrolysis is a convenient and green approach for generating pure and large-scale
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hydrogen [4-7]. However, due to the thermodynamically inherent energy barriers, the
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hydrogen evolution reaction (HER) electrocatalysts are usually needed to increase their efficiency and reduce their onset overpotential. Until now, platinum (Pt) is still
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the most efficient HER catalyst, wheras its high cost and rareness on earth severely limit the scope of practical applications [8-11]. Therefore, developing cost-effective,
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highly efficient, and robust non-noble metal HER catalysis is of great significance for obtaining large-scale and sustainable hydrogen production.
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To date, earth-abundant transition metals containing alloys [12, 13], oxides [14, 15],
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sulfides [16, 17], selenides [18, 19] and phosphides [20] have been widely explored as catalysts for water electrolysis. Among them, transition metal sulfides exhibited excellent catalytic activity for HER, which should be originated from the easily
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generated S-Hads bonds (Hads means H atoms adsorbed on the surface of catalysts). In addition, the existed unsaturated sulfur atoms are beneficial to enhance the catalytic
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activity for HER [21, 22]. However, in order to further improve electrocatalytic performance of transition metal sulfides, three major problems are needed to be seriously tackled. Firstly, much more effort is required to expose more electrochemical active sites on catalyst [23, 24]. Secondly, the instability caused by collapse or aggregation of nanostructure during the electrolytic process severely affects the catalytic performance [25]. Thirdly, absence of effective transport channels for electrolyte impedes the improvement of electrocatalytic performance. To solve these problems, many effective strategies have been reported. For examples, 2
ACCEPTED MANUSCRIPT mesoporous MoS2 foam was synthesized using SiO2 template to engender more exposed edge sites [26]. Nanoporous NiS films were prepared by electrochemical deposition of nickel hexacyanoferrate films (NiHCF) on glassy carbon (GC) via etching/anion exchange procedure, and showed excellent internal conductivity, high active surface area, and lower interfacial resistance [27]. The creation of bimetallic
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catalysts offers a convenience and feasible route to improve the HER performance due to the synergetic effect of between bimetal and increased active sites [23, 28-30].
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For examples, The ternary NiCo2Px nanowires have high catalytic activity for HER,
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reflecting the high efficiency of the bimetallic‐structured phosphide catalyst and derived synergistic effect [31].
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In recent years, two-dimensional (2D) nanostructure catalysts, like MoS2 [32, 33], Co3S4 [34, 35], MXenes [36, 37] et al, have attracted increased interest because their
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unique nanosheets could provide abundant edge catalytic activity sites. However, 2D nanostructure catalysts are easy to stack, resulting in less active sites. In this regard, three-dimensional (3D) nanostructures consisted of 2D nanosheets were designed to
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suppress 2D nanosheets aggregation, expose more active surface and provide effective
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transport channels for electrolyte [38, 39]. Moreover, using metal substrates to support electrocatalysts could both immobilize the electrocatalysts on their substrates
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and increase the capacity of charge transfer [40]. To date, according to this strategy, a few 3D nanostructure catalysts have been successfully prepared. For example, a
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self-supported NiMoS4 nanosheet array on Ti mesh as a 3D cathode showed efficient catalytic activity for HER [41]. Ultrathin porous NiFeV layered double hydroxides (LDHs) nanosheets on nickel foam could be used as bifunctional porous catalyst, which could offer enough space for the transport and diffusion of electrolytes, so only a modest voltage of 1.591 V was required to drive 10 mA·cm-2 for overall water splitting [13]. These results demonstrated that precise designed nanostructures could not only avoid the aggregation of 2D nanosheets but also ensure the transport and diffusion of electrolyte. In order to create more active sites on 2D nanosheets, only the formation of 3D 3
ACCEPTED MANUSCRIPT nanostructures is not enough. Until now, many strategies have been developed to expose more active sites on 2D nanosheets. For example, porous nanosheets of CoP were prepared by selectively etching amphoteric Al(OH)3 layer in CoAl LDH, and this porous structure exposed enormous edge sites, leading to enhanced catalytic activity for HER [42]. 3D porous hierarchical NiMoS nanoflowers were prepared via
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SiO2-assisted hydrothermal process, and provided abundant active sites for the enhanced catalytic efficiency [39]. Therefore, as mentioned above, designing 3D
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porous electrocatalysts consisted of 2D nanosheets could expose more active sites and promote transfer of H3O+ inside the catalysts, and then improve the electrocatalytic
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performance.
In addition, another important method can be derived from the mechanism of urea
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fuel cell to obtain hydrogen energy. Compared to water splitting, electrolysis of alkaline urea aqueous solution is a more energy-efficient process for hydrogen
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production because urea is a kind of species that can be easily oxidized electrically. The thermodynamic cell voltage is 0.37 V for urea electrolysis, which is much lower
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than 1.23 V for water electrolysis under the standard conditions [43, 44]. Therefore,
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the urea oxidation reaction (UOR) is an ideal replacement to OER towards efficient hydrogen production. Moreover, urea is cheap, non-toxic, and non-flammable, and as we known, urea-rich wastewater is mainly derived from fertilizer and urine-rich
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animal excreta, so urea electrolysis is conducive to environmental restoration [45, 46]. Therefore, designing 3D porous catalysts with 2D nanosheets can be used for
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hydrogen production via urea electrolysis at less energy consumption. Herein, we report the preparation of a porous flower-like molybdenum doped NiS on Ti mesh (denoted as Mo-doped p-NiS/Ti), which was utilized as a highly efficient electrocatalyst for water or urea splitting. Through etching of Al(OH)3 in Mo-doped NiAl layered double hydroxides (denoted as Mo-doped NiAl-LDH), the porous structure can be readily obtained, and after the subsequent sulfuration process, Mo-doped p-NiS/Ti was successfully synthesized. The synthesized Mo-doped p-NiS/Ti electrocatalyst exhibits more active sites, larger specific surface area and 4
ACCEPTED MANUSCRIPT synergistic effect comes from multiple components, which can greatly promote the electrocatalytic performance for water or urea electrolysis. Low overpotential of 147.6 mV at 10 mA·cm-2 for HER in 1.0 M KOH electrolyte solution were observed, and the corresponded Tafel slope was 88.1 mV·dec-1. In electrolyte of 1.0 M KOH with 0.3 M urea, the overpotential and corresponded Tafel slope were 162.3 mV at 10
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mA·cm-2 and 87.8 mV·dec-1 for HER. In addition, the Mo-doped p-NiS/Ti electrode showed long-term durability in both 1.0 M KOH and 1.0 M KOH with 0.3 M urea
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electrolyte. Meanwhile, using Mo-doped p-NiS/Ti as bifunctional catalyst, overall water splitting (1.0 M KOH) or urea splitting (1.0 M KOH with 0.3 M urea) were also
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2 Results and discussion
Scheme 1. Schematic illustration for the stepwise preparation of Mo-doped p-NiS/Ti catalyst. The detailed synthesis process of the flower-like Mo-doped p-NiS/Ti is illustrated in Scheme 1. Ti mesh was used as substrate, which could provide high electrical conductivity and strong mechanical robustness. Homogeneous metal ions (Ni2+, Mo6+, 5
ACCEPTED MANUSCRIPT Al3+) could continuously deposited on the acid-treated Ti mesh during the hydrothermal process to obtain a thin film of flower-like Mo-doped NiAl-LDH on Ti mesh (denoted as Mo-doped NiAl-LDH/Ti). After reacting with NaOH, the Al(OH)3 ingredent in the Mo-doped NiAl-LDH nanosheets could be removed to afford porous Mo-doped Ni-LDH (denoted as Mo-doped Ni-LDH/Ti). Through a sulfuration process
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using Na2S as S source, Mo-doped Ni-LDH could be converted into Mo-doped p-NiS
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with inherited flower-like morphology.
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Fig. 1 (a) XRD pattern of Mo-doped p-NiS/Ti. SEM images of (b) Mo-doped NiAl-LDH/Ti (c) Mo-doped Ni-LDH and (d-f) Mo-doped p-NiS/Ti with different magnifications. (g) SEM image and the corresponding elemental mapping images of Mo-doped p-NiS/Ti. Fig.1a shows the X-ray diffraction (XRD) pattern of Mo-doped p-NiS/Ti. The diffraction peaks of Ti mesh (JCPDS No. 44-1294) and NiS were clearly observed, the diffraction peaks at 18.4°, 30.3°, 32.2°, 35.7°, 37.3°, 40.45° and 48.8° could be 6
ACCEPTED MANUSCRIPT corresponded to (110), (101), (300), (021), (220), (211) and (131) of NiS phase (JCPDS No. 12-0041). In addition, different crystal phase of NiS (JCPDS No. 02-1280) were observed at 30.17° and 46.03°, which could be corresponded to (100) and (102) planes [47-49]. Besides NiS, the diffraction peaks at 9.9°, 22.5°, 27.9° and 30.7° could be corresponded to (120), (340), (401) and (640) of Mo17O47 phase
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(JCPDS No. 13-0345) [50]. It was further analyzed by ICP-OES that Mo atoms accounted for 0.038% in Mo-doped p-NiS/Ti. Due to the low content of Mo, the MoS2
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was hardly observed in the XRD measurement.
The morphologies of as-prepared products were investigated by SEM analysis. It
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could be seen that, after the hydrothermal process, the bare Ti mesh (Fig. S1) was uniformly covered by the flower-like Mo-doped NiAl-LDH nanosheets (Fig. 1b and
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S2a, 2b). After the removal of Al(OH)3 from the Mo-doped NiAl-LDH nanosheets by an etching process, the nanosheets became thinner and porous (Fig. 1c and S2c, 2d),
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and the flower-like structure was successfully retained. Ultimately, the Mo-doped p-NiS/Ti was obtained via hydrothermal sulfuration process using Na2S as sulphur
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source (Fig. 1d-f), and the element mapping images of Mo-doped p-NiS/Ti revealed
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the existence of uniformly distributed Ni, Mo, O and S elements (Fig. 1g). These results indicated that Mo element has been successfully doped into the nanosheets. As shown in Fig. S3a-c, after sulfuration process, flower-like Mo-doped porous NiS was
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generated on the surface of Ti mesh, and consisted of a lot of very thin 2D nanosheets. In comparison, Mo-doped NiAl-LDH was directly sulfurated without the etching of
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Al(OH)3 (denoted as Mo-doped NiAlS/Ti) by the same method, and the morphology is shown in Fig. S4(a-c). Obviously, the nanosheets were thicker than Mo-doped p-NiS/Ti and no porous feature was observed. From the element mapping images (Fig. S4d), Ni, Mo, Al, and S elements were evenly distributed in the nanosheets of Mo-doped NiAlS/Ti.
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Fig. 2 TEM images of the Mo-doped p-NiS/Ti with (a) whole flower-like structure
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and (b) the magnified image of the area as square-labeled in a. (c, d) HRTEM images of Mo-doped p-NiS nanosheets on Ti mesh.
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The morphology and nanostructure of as-prepared Mo-doped p-NiS/Ti were further studied by TEM and HRTEM. Before sulfuration, the flower-like Mo-doped porous
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Ni LDH composed of 2D nanosheets had rough transparent surface (Fig. S5a, b), and after sulfurization, this flower-like structure was inherited (Fig. 2a, 2b). As shown in Fig. 2c, many NiS nanoparticles around 3-5 nm were clearly observed, and the interplanar spacing of 0.22 nm, 0.24 nm, 0.29 nm and 0.19 nm (Fig. 2d) corresponded well to the crystal planes of the (211), (220), (101) and (102) of NiS [49], which was consistent with the XRD result (Fig. 1a). The lattice fringes with interplanar spacing of 0.23 nm and 0.62 nm corresponded to the (104) and (002) plane of the MoS2 [51, 52]. 8
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Fig. 3 XPS spectra of Mo-doped p-NiS/Ti: Survey spectrum (a), narrow spectra for (b) Ni 2p, (c) Mo 3d and (d) S 2p, respectively.
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The X-ray photoelectron spectroscopy (XPS) measurement can illustrate the surface chemical state of Mo-doped p-NiS/Ti.
Ni 2p, Mo 3d and S 2p all existed in
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the survey spectrum of the Mo-doped p-NiS/Ti (Fig. 3a). As shown in Fig. 3b, two peaks of Ni 2p1/2 and Ni 2p3/2 at the binding energies (BEs) of 873.6 eV and 856.3 eV were observed, respectively, confirming the presence of Ni2+ [41, 53]. Meanwhile, two shakeup satellites (abbreviated as “Sat”) at 879.9 eV and 862.1 eV confirmed the existence of Ni-O bond [54]. Fig. 3c showed the binding energies of Mo 3d with one peak at 235.7 eV for Mo 3d3/2 and another peak at 232.8 eV for Mo 3d5/2, respectively, which can be attributed to the presence of Mo with VI oxidation state [41, 55]. A broad peak at 226-229 eV can be assigned to S 2s. In the S 2p region (Fig. 3d), 9
ACCEPTED MANUSCRIPT the S 2p3/2 and S 2p1/2 orbital peaks were observed at 162.1 eV and 163.2 eV, respectively, indicating the presence of terminal S2-, which was effective for HER activity [56]. The peak at high binding energy of S 2p at 169.1 eV revealed the presence of oxysulfide species in the Mo-doped p-NiS/Ti samples, which indicated the existence of trace metal oxides in Mo-doped p-NiS/Ti [57, 58]. That is, compared
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to sulfurization under high temperature like annealing, hydrothermal sulfurization process could not proceed completely. However, the flower-like structure could be
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easily destroyed or aggregated using annealing sulfurization process, and then reduce
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the catalytic performance.
Fig. 4 The electrochemical surface areas (ECSA) of Mo-doped p-NiS/Ti and Mo-doped NiAlS/Ti was estimated. Cyclic voltammetry curves of (a) Mo-doped p-NiS/Ti and (b) Mo-doped NiAlS/Ti with different scanning rates. (c) Plot of capacitive currents density (janodic − jcathodic) at -0.1 V vs. SCE as a function of various scan rates for Mo-doped p-NiS/Ti and Mo-doped NiAlS/Ti (d) Nitrogen adsorption-desorption isotherms of Mo-doped p-NiS/Ti and Mo-doped NiAlS/Ti. 10
ACCEPTED MANUSCRIPT Cyclic voltammetry (CV) was carried out for Mo-doped p-NiS/Ti and Mo-doped NiAlS/Ti electrodes in a potential window of -150 mV to -50 mV vs SCE at various scan rate of 10, 20, 40, 60, 80, 100, and 120 mV·s-1 (Fig. 4a and 4b). Cdl is usually used to evaluate the electrochemical surface area (ECSA). As shown in Fig. 4c linear slopes equivalent to the double layer capacitance (Cdl) value were obtained by plotting
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capacitive current density (janodic − jcathodic) at -0.1 V vs. SCE against scan rates. The capacitive current of Mo-doped p-NiS/Ti was much larger than that of Mo-doped
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NiAlS/Ti at the same scan rate. The double-layer capacitances (Cdl) of Mo-doped
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p-NiS/Ti and Mo-doped NiAlS/Ti were derived to be 4.93 and 2.12 mF·cm-2, respectively, which implied that Mo-doped p-NiS/Ti had more active sites than
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Mo-doped NiAlS/Ti. Brunauer-Emmett-Teller (BET) were used to measure the specific surface area of Mo-doped p-NiS/Ti and Mo-doped NiAlS/Ti (Fig. 4d), and
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the results indicated that the Mo-doped p-NiS/Ti had a larger specific surface area
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(12.171 m2/g) than that of Mo-doped NiAlS/Ti (9.005 m2/g).
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Fig. 5 LSV curves of Mo-doped p-NiS/Ti, Mo-doped NiAlS/Ti, p-NiS/Ti, pure Ti mesh, and Pt/C in electrolyte of (a) 1.0 M KOH and (c) 1.0 M KOH with 0.3 M urea at a scan rate of 5 mV·s–1. The Tafel slopes of Mo-doped p-NiS/Ti, Mo-doped NiAlS/Ti, p-NiS/Ti, pure Ti mesh, and Pt/C in electrolyte of (b) 1.0 M KOH and (d) 1.0 M KOH with 0.3 M urea. Time-dependent current density curves of Mo-doped p-NiS/Ti in electrolyte of (e) 1.0 M KOH and (f) 1.0 M KOH with 0.3M urea. The HER activity of Mo-doped p-NiS/Ti electrode was investigated by a typical three-electrode system with a scan rate of 5 mV·s-1 in electrolytes of 1.0 M KOH and 12
ACCEPTED MANUSCRIPT 1.0 M KOH with 0.3 M urea. For comparison, the electrocatalytic performance of Mo-doped NiAlS/Ti, p-NiS/Ti, pure Ti mesh, and commercial Pt/C (20 wt.% Pt) were all investigated under the same conditions. Fig. 5a showed the linear sweep voltammetry (LSV) curves (after iR correction) of Mo-doped p-NiS/Ti, Mo-doped NiAlS/Ti, p-NiS/Ti, pure Ti mesh, and Pt/C samples in 1.0 M KOH. It can be seen
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that Pt/C showed the best activity towards HER, however the bare Ti mesh exhibited negligible current density. Mo-doped p-NiS/Ti exhibited an overpotential of 147.6 mV
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to drive 10 mA·cm-2 in 1.0 M KOH, which is much lower than those of Mo-doped
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NiAlS/Ti (214.8 mV) and p-NiS/Ti (164.5 mV), respectively. This result demonstrated that the removal of Al(OH)3 and Mo doping were both important
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contributors to the enhanced HER activity of Mo-doped p-NiS/Ti. Moreover, the Mo-doped Ni-LDH and Mo-doped NiAl-LDH/Ti required large overpotentials of
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283.4 and 315.6 mV for driving 10 mA·cm-2 (Fig. S6a), respectively, which indicated that the sulfuration process was vital and essential to promote HER activity. The HER kinetics were estimated by their corresponding Tafel plots. As shown in
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Fig. 5b, Mo-doped p-NiS/Ti presented obviously lower Tafel slopes of 87.8 mV·dec-1
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than those of Mo-doped NiAlS/Ti and p-NiS/Ti, which were 101.5 and 116.7 mV·dec-1, respectively.
In contrast,
Mo-doped Ni-LDH/Ti
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NiAl-LDH/Ti showed much large Tafel slopes of 121.6 and 136.5 mV·dec-1 (Fig. S5b), respectively. The lower Tafel slope of Mo-doped p-NiS/Ti corresponds to better
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electron transfer ability compared to other samples. Therefore, Mo-doped p-NiS/Ti electrode exhibited excellent catalytic performance for HER, which should result from the special 3D flower-like structure, porous characteristic, and synergistic effects of bimetallic elements (Ni and Mo). In addition, Mo doping could lead to the formation of MoS2 nannocrystals in the nanosheets, and the self-generated lattice defects of MoS2 should largely contribute to the enhanced HER performance. Fig. 5c shows the LSV curves of Mo-doped p-NiS/Ti, Mo-doped NiAlS/Ti, p-NiS/Ti, pure Ti mesh and commercial Pt/C in 1.0 M KOH with 0.3 M urea. Again, compared to other samples, Mo-doped p-NiS/Ti showed obvious low overpotentials 13
ACCEPTED MANUSCRIPT (162.3 mV) at 10 mA·cm-2. In comparison, Mo-doped NiAlS/Ti, p-NiS/Ti, Mo-doped Ni-LDH/Ti and Mo-doped NiAl-LDH/Ti (Fig. S6c) required the larger overpotentials of 180.2, 151.6, 286.3, and 296.7 mV for achieving the same current density. As shown in Fig. 5d, the reaction kinetics for HER were estimated by their corresponding Tafel plots. Mo-doped p-NiS/Ti presented a smaller Tafel slope (87.8 mV·dec-1) than that of Mo-doped NiAlS/Ti (115.1 mV·dec-1), p-NiS/Ti (94.7 mV·dec-1), Mo-doped
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Ni-LDH/Ti (119.5 mV·dec-1) and Mo-doped NiAl-LDH/Ti (90.86 mV·dec-1) (Fig.
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S6d).
To evaluate the catalytic stability of Mo-doped p-NiS/Ti, long-term stability tests
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were conducted in 1.0 M KOH and 1.0 M KOH with 0.3 M urea under a constant potential of -1.27 V vs. SCE. In 1.0 M KOH, a slight decrease of catalytical activity
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for Mo-doped p-NiS/Ti after a long period of 17 h (Fig. 5e), suggesting the excellent stability as HER catalyst. Meantime, Mo-doped p-NiS/Ti could still keep 83.8% of its
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initial current density after long-term electrolysis for 10 h in 1.0 M KOH with 0.3 M urea (Fig. 5f). These results demonstrated that excellent electrochemical stability of
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Mo-doped p-NiS/Ti towards the long-term HER process in 1.0 M KOH or 1.0 M
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KOH with 0.3 M urea. The XPS measurement was performed for Mo-doped p-NiS/Ti after the stability test in the electrolyte of 1.0 M KOH with 0.3 M urea to detect the change of chemical state. As shown in Fig. S7, the peaks of the Ni 2p, Mo 3d, and S
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2p still existed, however, the peak intensities of Mo and S (Fig. S7c, 7d) were
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obviously reduced compared to those before the stability test (Fig. 3c and 3d) [41].
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Fig. 6 (a) Polarization curves and the corresponded (b) Tafel plots for Mo-doped
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p-NiS/Ti, Mo-doped NiAlS/Ti and RuO2 with a scan rate of 5 mV/s for OER in
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electrolyte of 1.0 M KOH. (c) Polarization curves and (d) the corresponded Tafel plots recorded for Mo-doped p-NiS/Ti, Mo-doped NiAlS/Ti and RuO2 with a scan rate of 5
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mV/s for UOR in electrolyte of 1.0 M KOH with 0.3 M urea. The anode reaction activity of Mo-doped p-NiS/Ti, Mo-doped NiAlS/Ti and RuO2
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were also tested at a scan rate of 5 mV/s in a conventional three-electrode setup under different electrolytes of 1.0 M KOH and 1.0 M KOH with 0.3 M urea. As shown in Fig. 6a, the OER activity of Mo-doped p-NiS/Ti catalyst outperformed that of Mo-doped NiAlS/Ti and RuO2 in 1.0 M KOH. To drive 50 mA·cm-2 current density, the potential of 1.62 V was required for Mo-doped NiS/Ti, meanwhile, the potential of Mo-doped p-NiAlS/Ti and RuO2 was 1.66 V and 1.79 V, respectively, which clearly evidenced the better OER activity for Mo-doped p-NiS/Ti. The OER kinetics was also elucidated by their corresponding Tafel plots (Fig. 6b). The Tafel slopes of Mo-doped 15
ACCEPTED MANUSCRIPT p-NiS/Ti, Mo-doped NiAlS/Ti and RuO2 were calculated to be 185, 225 and 67.5 mV·dec-1, respectively, suggesting better kinetics of OER for Mo-doped p-NiS/Ti. The UOR activity of Mo-doped p-NiS/Ti and Mo-doped NiAlS/Ti were measured in an electrolyte of 1.0 M KOH with 0.3 M urea. As shown in Fig. 6c, to obtain 50 mA·cm-2 current density, the potentials of 1.53 V and 1.65 V for Mo-doped p-NiS/Ti
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and Mo-doped NiAlS/Ti were required, respectively. These results suggested that UOR in 1.0 M KOH with 0.3 M urea is easier to occur than OER in 1.0 M KOH
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electrolyte. The UOR kinetics were studied by their corresponding Tafel plots (Fig. 6d), and the Tafel slopes of Mo-doped p-NiS/Ti and Mo-doped NiAlS/Ti were 12.9
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and 25.1 mV·dec-1, respectively, implying the more favorable UOR kinetics for
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Mo-doped p-NiS/Ti electrode.
Fig. 7. (a) Polarization curves of water electrolysis for Mo-doped p-NiS/Ti, Mo-doped NiAlS/Ti at a scan rate of 5 mV/s in 1.0 M KOH. (b) Polarization curves of electrolysis for Mo-doped p-NiS/Ti at a scan rate of 5 mV/s in 1.0 M KOH and 1.0 M 16
ACCEPTED MANUSCRIPT KOH with 0.3 M urea solution. (c) CP (chronopotentiometry) curve for Mo-doped p-NiS/Ti at a constant current density of 20 mA/cm2 in 1.0 M KOH with 0.3 M urea solution. The inset image shows the photograph of cathode (HER) and anode (UOR) and during alkaline electrolysis. (d) SEM image of Mo-doped p-NiS/Ti after water electrolysis for 18 h in 1.0 M KOH with 0.3 M urea solution.
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We further investigated the electrocatalytic performance of Mo-doped p-NiS/Ti in a constructed two-electrode system utilizing Mo-doped p-NiS/Ti as both cathode for
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HER and anode for OER or UOR (denoted as Mo-doped p-NiS/Ti||Mo-doped
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p-NiS/Ti) in 1.0 M KOH or 1.0 M KOH with 0.3 M urea, respectively. For comparison, a two-electrode system of Mo-doped NiAlS/Ti||Mo-doped NiAlS/Ti was
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assembled to investigate the electrocatalytic performance, too. As shown in Fig. 7a, Mo-doped p-NiS/Ti||Mo-doped p-NiS/Ti system showed better performance with cell
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voltage of 1.92 V to acquire current density of 10 mA·cm-2 in 1.0 M KOH, while cell voltage of 1.94 V was needed to obtain current density of 10 mA·cm-2 for Mo-doped NiAlS/Ti||Mo-doped NiAlS/Ti system. In electrolyte of 1.0 M KOH with 0.3 M urea,
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a cell voltage 1.95 V was needed to drive 10 mA·cm-2 for Mo-doped NiS/Ti||Mo-doped NiS/Ti system (Fig. 7b). The long-term durability of Mo-doped NiS/Ti||Mo-doped NiS/Ti system was tested in 1.0 M KOH with 0.3 M urea (Fig. 7c).
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After 20 h of continuous urea electrolysis testing, the catalytic activity of the Mo-doped NiS/Ti||Mo-doped NiS/Ti system was virtually unchanged, and the
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flower-like structure was maintained as evidenced by SEM analysis (Fig. 7d), which suggested that the Mo-doped p-NiS/Ti had excellent long-term durability for full urea splitting.
The excellent electrocatalytic activity and stability of the Mo-doped p-NiS/Ti electrode for H2 production both in water and urea electrolyte can be attributed to the following factors. (1) The 3D flower-like structure consisting of thin and porous 2D sheets provides a larger specific surface area and more exposed active sites. Moreover, this unique structure is conducive to the diffusion of electrolytes. (2) The strong 17
ACCEPTED MANUSCRIPT connection between the conductive Ti mesh and 3D flower-like structure ensures mechanical robustness and good charge transfer capacity between them, leading to excellent long-term durability. (3) The Mo-doping can generate more active sites and then enhance the catalytic performance. In addition, the sulfurization process provides much higher electrocatalytic activity compared to their hydroxide, indicating that the
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sulfurization is an essential process to further improve the electrocatalysis activity.
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3 Conclusion
The Mo-doped p-NiS on Ti mesh was successfully synthesized by selective etching of
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amphoteric Al(OH)3 in Mo-doped NiAl-LDH/Ti and the subsequent sulfuration process. The Mo-doped p-NiS/Ti with unique and stable 3D flower-like structure
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exhibited excellent catalytic activity and long-term durability in 1.0 M KOH and 1.0 M KOH with 0.3 M urea electrolyte. Only 147.6 mV and 162.3 mV was required to
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achieve current density of 10 mA·cm-2 for HER in 1.0 M KOH and 1.0 M KOH with 0.3 M urea. The excellent activity performance comes from rich active sites provided
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by high specific surface area, synergistic effect of the binary metal components, and
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the nature of Mo-doping. In addition, Mo-doped p-NiS/Ti could be used as both cathode and anode for electrochemical water or urea splitting. This work is expected to help develop other non-precious metal porous electrocatalyst materials for
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applications in water and urea splitting.
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Acknowledgements
This work was supported by Qingdao Innovation Leading Talent Program and the First-Class Disciplines funding of Shandong Province.
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ACCEPTED MANUSCRIPT Highlights • Porous flower-like Mo-doped NiS is highly efficient electrocatalyst for overall water splitting. • Obtaining a porous structure by a simple selective etching method.
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• The Mo-doping can enhance the catalytic performance.
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• Mo-doped p-NiS/Ti is used in a two-electrode system for overall water or urea
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splitting.
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Qiang Jia, Xiaoxia Wang, Shuang Wei, Congli Zhou, Jianmei Wang, Jingquan Liu PII: DOI: Reference:
S0169-4332(19)31164-X https://doi.org/10.1016/j.apsusc.2019.04.165 APSUSC 42473
To appear in:
Applied Surface Science
Received date: Revised date: Accepted date:
26 January 2019 31 March 2019 14 April 2019
Please cite this article as: Q. Jia, X. Wang, S. Wei, et al., Porous flower-like Mo-doped NiS heterostructure as highly efficient and robust electrocatalyst for overall water splitting, Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.04.165
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ACCEPTED MANUSCRIPT
Porous Flower-like Mo-doped NiS Heterostructure as Highly Efficient and Robust Electrocatalyst for Overall Water Splitting
a
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Qiang Jiaa, Xiaoxia Wanga*, Shuang Weia,b, Congli Zhoua, Jianmei Wanga,c, Jingquan Liu a* College of Materials Science and Engineering, Institute for Graphene Applied Technology
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Innovation, Qingdao University, Qingdao 266071, China.
School of Material Science and Engineering, Ocean University of China.
c
School of Life and Environmental Sciences, Deakin University, Geelong, VIC 3217, Australia.
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b
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Email: [email protected] (X. Wang), [email protected] (J. Liu)
Abstract
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The development of electrocatalyst with remarkable activity for hydrogen production
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through water or urea splitting is critical for coping with the energy crisis, but still remains a lot of obstacles. In order to improve the electrocatalytic activity, rational design of nanostructures with rich active sites is an efficient method. Herein, a porous
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flower-like Mo-doped NiS on Ti mesh (Mo-doped p-NiS/Ti) is synthesized through selective etching of Al(OH)3 in Mo-doped NiAl layered double hydroxides, and then
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followed by a facile hydrothermal sulfuration process using sodium sulfide as S source to afford Mo-doped p-NiS/Ti. As a non-noble metal catalyst, Mo-doped p-NiS/Ti catalyst exhibits excellent electrocatalytic performance in electrolyte of 1.0 M KOH or 1.0 M KOH with 0.3 M urea. To drive a current density of 10 mA·cm-2, a small overpotentials of 147.6 mV and 162.3 mV in 1.0 M KOH and 1.0 M KOH with 0.3 M urea are required, respectively, and remarkable durability is observed in 1.0 M KOH or urea water solutions. More significantly, Mo-doped p-NiS/Ti holds promises for potential implementation in the overall water or urea splitting. 1
ACCEPTED MANUSCRIPT Keywords: electrocatalyst, porous, flower-like, hydrogen production 1 Introduction With the growing challenges of environmental pollution and shortage of fossil fuels, the exploration of clean and renewable energy sources has attracted tremendous attentions [1-3]. Hydrogen is regarded as a promising clean fuel owing to its zero
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carbon emission, environmental friendliness and high energy density, while water electrolysis is a convenient and green approach for generating pure and large-scale
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hydrogen [4-7]. However, due to the thermodynamically inherent energy barriers, the
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hydrogen evolution reaction (HER) electrocatalysts are usually needed to increase their efficiency and reduce their onset overpotential. Until now, platinum (Pt) is still
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the most efficient HER catalyst, wheras its high cost and rareness on earth severely limit the scope of practical applications [8-11]. Therefore, developing cost-effective,
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highly efficient, and robust non-noble metal HER catalysis is of great significance for obtaining large-scale and sustainable hydrogen production.
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To date, earth-abundant transition metals containing alloys [12, 13], oxides [14, 15],
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sulfides [16, 17], selenides [18, 19] and phosphides [20] have been widely explored as catalysts for water electrolysis. Among them, transition metal sulfides exhibited excellent catalytic activity for HER, which should be originated from the easily
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generated S-Hads bonds (Hads means H atoms adsorbed on the surface of catalysts). In addition, the existed unsaturated sulfur atoms are beneficial to enhance the catalytic
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activity for HER [21, 22]. However, in order to further improve electrocatalytic performance of transition metal sulfides, three major problems are needed to be seriously tackled. Firstly, much more effort is required to expose more electrochemical active sites on catalyst [23, 24]. Secondly, the instability caused by collapse or aggregation of nanostructure during the electrolytic process severely affects the catalytic performance [25]. Thirdly, absence of effective transport channels for electrolyte impedes the improvement of electrocatalytic performance. To solve these problems, many effective strategies have been reported. For examples, 2
ACCEPTED MANUSCRIPT mesoporous MoS2 foam was synthesized using SiO2 template to engender more exposed edge sites [26]. Nanoporous NiS films were prepared by electrochemical deposition of nickel hexacyanoferrate films (NiHCF) on glassy carbon (GC) via etching/anion exchange procedure, and showed excellent internal conductivity, high active surface area, and lower interfacial resistance [27]. The creation of bimetallic
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catalysts offers a convenience and feasible route to improve the HER performance due to the synergetic effect of between bimetal and increased active sites [23, 28-30].
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For examples, The ternary NiCo2Px nanowires have high catalytic activity for HER,
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reflecting the high efficiency of the bimetallic‐structured phosphide catalyst and derived synergistic effect [31].
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In recent years, two-dimensional (2D) nanostructure catalysts, like MoS2 [32, 33], Co3S4 [34, 35], MXenes [36, 37] et al, have attracted increased interest because their
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unique nanosheets could provide abundant edge catalytic activity sites. However, 2D nanostructure catalysts are easy to stack, resulting in less active sites. In this regard, three-dimensional (3D) nanostructures consisted of 2D nanosheets were designed to
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suppress 2D nanosheets aggregation, expose more active surface and provide effective
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transport channels for electrolyte [38, 39]. Moreover, using metal substrates to support electrocatalysts could both immobilize the electrocatalysts on their substrates
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and increase the capacity of charge transfer [40]. To date, according to this strategy, a few 3D nanostructure catalysts have been successfully prepared. For example, a
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self-supported NiMoS4 nanosheet array on Ti mesh as a 3D cathode showed efficient catalytic activity for HER [41]. Ultrathin porous NiFeV layered double hydroxides (LDHs) nanosheets on nickel foam could be used as bifunctional porous catalyst, which could offer enough space for the transport and diffusion of electrolytes, so only a modest voltage of 1.591 V was required to drive 10 mA·cm-2 for overall water splitting [13]. These results demonstrated that precise designed nanostructures could not only avoid the aggregation of 2D nanosheets but also ensure the transport and diffusion of electrolyte. In order to create more active sites on 2D nanosheets, only the formation of 3D 3
ACCEPTED MANUSCRIPT nanostructures is not enough. Until now, many strategies have been developed to expose more active sites on 2D nanosheets. For example, porous nanosheets of CoP were prepared by selectively etching amphoteric Al(OH)3 layer in CoAl LDH, and this porous structure exposed enormous edge sites, leading to enhanced catalytic activity for HER [42]. 3D porous hierarchical NiMoS nanoflowers were prepared via
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SiO2-assisted hydrothermal process, and provided abundant active sites for the enhanced catalytic efficiency [39]. Therefore, as mentioned above, designing 3D
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porous electrocatalysts consisted of 2D nanosheets could expose more active sites and promote transfer of H3O+ inside the catalysts, and then improve the electrocatalytic
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performance.
In addition, another important method can be derived from the mechanism of urea
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fuel cell to obtain hydrogen energy. Compared to water splitting, electrolysis of alkaline urea aqueous solution is a more energy-efficient process for hydrogen
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production because urea is a kind of species that can be easily oxidized electrically. The thermodynamic cell voltage is 0.37 V for urea electrolysis, which is much lower
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than 1.23 V for water electrolysis under the standard conditions [43, 44]. Therefore,
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the urea oxidation reaction (UOR) is an ideal replacement to OER towards efficient hydrogen production. Moreover, urea is cheap, non-toxic, and non-flammable, and as we known, urea-rich wastewater is mainly derived from fertilizer and urine-rich
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animal excreta, so urea electrolysis is conducive to environmental restoration [45, 46]. Therefore, designing 3D porous catalysts with 2D nanosheets can be used for
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hydrogen production via urea electrolysis at less energy consumption. Herein, we report the preparation of a porous flower-like molybdenum doped NiS on Ti mesh (denoted as Mo-doped p-NiS/Ti), which was utilized as a highly efficient electrocatalyst for water or urea splitting. Through etching of Al(OH)3 in Mo-doped NiAl layered double hydroxides (denoted as Mo-doped NiAl-LDH), the porous structure can be readily obtained, and after the subsequent sulfuration process, Mo-doped p-NiS/Ti was successfully synthesized. The synthesized Mo-doped p-NiS/Ti electrocatalyst exhibits more active sites, larger specific surface area and 4
ACCEPTED MANUSCRIPT synergistic effect comes from multiple components, which can greatly promote the electrocatalytic performance for water or urea electrolysis. Low overpotential of 147.6 mV at 10 mA·cm-2 for HER in 1.0 M KOH electrolyte solution were observed, and the corresponded Tafel slope was 88.1 mV·dec-1. In electrolyte of 1.0 M KOH with 0.3 M urea, the overpotential and corresponded Tafel slope were 162.3 mV at 10
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mA·cm-2 and 87.8 mV·dec-1 for HER. In addition, the Mo-doped p-NiS/Ti electrode showed long-term durability in both 1.0 M KOH and 1.0 M KOH with 0.3 M urea
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electrolyte. Meanwhile, using Mo-doped p-NiS/Ti as bifunctional catalyst, overall water splitting (1.0 M KOH) or urea splitting (1.0 M KOH with 0.3 M urea) were also
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investigated.
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2 Results and discussion
Scheme 1. Schematic illustration for the stepwise preparation of Mo-doped p-NiS/Ti catalyst. The detailed synthesis process of the flower-like Mo-doped p-NiS/Ti is illustrated in Scheme 1. Ti mesh was used as substrate, which could provide high electrical conductivity and strong mechanical robustness. Homogeneous metal ions (Ni2+, Mo6+, 5
ACCEPTED MANUSCRIPT Al3+) could continuously deposited on the acid-treated Ti mesh during the hydrothermal process to obtain a thin film of flower-like Mo-doped NiAl-LDH on Ti mesh (denoted as Mo-doped NiAl-LDH/Ti). After reacting with NaOH, the Al(OH)3 ingredent in the Mo-doped NiAl-LDH nanosheets could be removed to afford porous Mo-doped Ni-LDH (denoted as Mo-doped Ni-LDH/Ti). Through a sulfuration process
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using Na2S as S source, Mo-doped Ni-LDH could be converted into Mo-doped p-NiS
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with inherited flower-like morphology.
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Fig. 1 (a) XRD pattern of Mo-doped p-NiS/Ti. SEM images of (b) Mo-doped NiAl-LDH/Ti (c) Mo-doped Ni-LDH and (d-f) Mo-doped p-NiS/Ti with different magnifications. (g) SEM image and the corresponding elemental mapping images of Mo-doped p-NiS/Ti. Fig.1a shows the X-ray diffraction (XRD) pattern of Mo-doped p-NiS/Ti. The diffraction peaks of Ti mesh (JCPDS No. 44-1294) and NiS were clearly observed, the diffraction peaks at 18.4°, 30.3°, 32.2°, 35.7°, 37.3°, 40.45° and 48.8° could be 6
ACCEPTED MANUSCRIPT corresponded to (110), (101), (300), (021), (220), (211) and (131) of NiS phase (JCPDS No. 12-0041). In addition, different crystal phase of NiS (JCPDS No. 02-1280) were observed at 30.17° and 46.03°, which could be corresponded to (100) and (102) planes [47-49]. Besides NiS, the diffraction peaks at 9.9°, 22.5°, 27.9° and 30.7° could be corresponded to (120), (340), (401) and (640) of Mo17O47 phase
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(JCPDS No. 13-0345) [50]. It was further analyzed by ICP-OES that Mo atoms accounted for 0.038% in Mo-doped p-NiS/Ti. Due to the low content of Mo, the MoS2
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was hardly observed in the XRD measurement.
The morphologies of as-prepared products were investigated by SEM analysis. It
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could be seen that, after the hydrothermal process, the bare Ti mesh (Fig. S1) was uniformly covered by the flower-like Mo-doped NiAl-LDH nanosheets (Fig. 1b and
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S2a, 2b). After the removal of Al(OH)3 from the Mo-doped NiAl-LDH nanosheets by an etching process, the nanosheets became thinner and porous (Fig. 1c and S2c, 2d),
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and the flower-like structure was successfully retained. Ultimately, the Mo-doped p-NiS/Ti was obtained via hydrothermal sulfuration process using Na2S as sulphur
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source (Fig. 1d-f), and the element mapping images of Mo-doped p-NiS/Ti revealed
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the existence of uniformly distributed Ni, Mo, O and S elements (Fig. 1g). These results indicated that Mo element has been successfully doped into the nanosheets. As shown in Fig. S3a-c, after sulfuration process, flower-like Mo-doped porous NiS was
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generated on the surface of Ti mesh, and consisted of a lot of very thin 2D nanosheets. In comparison, Mo-doped NiAl-LDH was directly sulfurated without the etching of
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Al(OH)3 (denoted as Mo-doped NiAlS/Ti) by the same method, and the morphology is shown in Fig. S4(a-c). Obviously, the nanosheets were thicker than Mo-doped p-NiS/Ti and no porous feature was observed. From the element mapping images (Fig. S4d), Ni, Mo, Al, and S elements were evenly distributed in the nanosheets of Mo-doped NiAlS/Ti.
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Fig. 2 TEM images of the Mo-doped p-NiS/Ti with (a) whole flower-like structure
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and (b) the magnified image of the area as square-labeled in a. (c, d) HRTEM images of Mo-doped p-NiS nanosheets on Ti mesh.
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The morphology and nanostructure of as-prepared Mo-doped p-NiS/Ti were further studied by TEM and HRTEM. Before sulfuration, the flower-like Mo-doped porous
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Ni LDH composed of 2D nanosheets had rough transparent surface (Fig. S5a, b), and after sulfurization, this flower-like structure was inherited (Fig. 2a, 2b). As shown in Fig. 2c, many NiS nanoparticles around 3-5 nm were clearly observed, and the interplanar spacing of 0.22 nm, 0.24 nm, 0.29 nm and 0.19 nm (Fig. 2d) corresponded well to the crystal planes of the (211), (220), (101) and (102) of NiS [49], which was consistent with the XRD result (Fig. 1a). The lattice fringes with interplanar spacing of 0.23 nm and 0.62 nm corresponded to the (104) and (002) plane of the MoS2 [51, 52]. 8
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Fig. 3 XPS spectra of Mo-doped p-NiS/Ti: Survey spectrum (a), narrow spectra for (b) Ni 2p, (c) Mo 3d and (d) S 2p, respectively.
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The X-ray photoelectron spectroscopy (XPS) measurement can illustrate the surface chemical state of Mo-doped p-NiS/Ti.
Ni 2p, Mo 3d and S 2p all existed in
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the survey spectrum of the Mo-doped p-NiS/Ti (Fig. 3a). As shown in Fig. 3b, two peaks of Ni 2p1/2 and Ni 2p3/2 at the binding energies (BEs) of 873.6 eV and 856.3 eV were observed, respectively, confirming the presence of Ni2+ [41, 53]. Meanwhile, two shakeup satellites (abbreviated as “Sat”) at 879.9 eV and 862.1 eV confirmed the existence of Ni-O bond [54]. Fig. 3c showed the binding energies of Mo 3d with one peak at 235.7 eV for Mo 3d3/2 and another peak at 232.8 eV for Mo 3d5/2, respectively, which can be attributed to the presence of Mo with VI oxidation state [41, 55]. A broad peak at 226-229 eV can be assigned to S 2s. In the S 2p region (Fig. 3d), 9
ACCEPTED MANUSCRIPT the S 2p3/2 and S 2p1/2 orbital peaks were observed at 162.1 eV and 163.2 eV, respectively, indicating the presence of terminal S2-, which was effective for HER activity [56]. The peak at high binding energy of S 2p at 169.1 eV revealed the presence of oxysulfide species in the Mo-doped p-NiS/Ti samples, which indicated the existence of trace metal oxides in Mo-doped p-NiS/Ti [57, 58]. That is, compared
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to sulfurization under high temperature like annealing, hydrothermal sulfurization process could not proceed completely. However, the flower-like structure could be
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the catalytic performance.
Fig. 4 The electrochemical surface areas (ECSA) of Mo-doped p-NiS/Ti and Mo-doped NiAlS/Ti was estimated. Cyclic voltammetry curves of (a) Mo-doped p-NiS/Ti and (b) Mo-doped NiAlS/Ti with different scanning rates. (c) Plot of capacitive currents density (janodic − jcathodic) at -0.1 V vs. SCE as a function of various scan rates for Mo-doped p-NiS/Ti and Mo-doped NiAlS/Ti (d) Nitrogen adsorption-desorption isotherms of Mo-doped p-NiS/Ti and Mo-doped NiAlS/Ti. 10
ACCEPTED MANUSCRIPT Cyclic voltammetry (CV) was carried out for Mo-doped p-NiS/Ti and Mo-doped NiAlS/Ti electrodes in a potential window of -150 mV to -50 mV vs SCE at various scan rate of 10, 20, 40, 60, 80, 100, and 120 mV·s-1 (Fig. 4a and 4b). Cdl is usually used to evaluate the electrochemical surface area (ECSA). As shown in Fig. 4c linear slopes equivalent to the double layer capacitance (Cdl) value were obtained by plotting
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capacitive current density (janodic − jcathodic) at -0.1 V vs. SCE against scan rates. The capacitive current of Mo-doped p-NiS/Ti was much larger than that of Mo-doped
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NiAlS/Ti at the same scan rate. The double-layer capacitances (Cdl) of Mo-doped
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p-NiS/Ti and Mo-doped NiAlS/Ti were derived to be 4.93 and 2.12 mF·cm-2, respectively, which implied that Mo-doped p-NiS/Ti had more active sites than
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Mo-doped NiAlS/Ti. Brunauer-Emmett-Teller (BET) were used to measure the specific surface area of Mo-doped p-NiS/Ti and Mo-doped NiAlS/Ti (Fig. 4d), and
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the results indicated that the Mo-doped p-NiS/Ti had a larger specific surface area
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(12.171 m2/g) than that of Mo-doped NiAlS/Ti (9.005 m2/g).
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Fig. 5 LSV curves of Mo-doped p-NiS/Ti, Mo-doped NiAlS/Ti, p-NiS/Ti, pure Ti mesh, and Pt/C in electrolyte of (a) 1.0 M KOH and (c) 1.0 M KOH with 0.3 M urea at a scan rate of 5 mV·s–1. The Tafel slopes of Mo-doped p-NiS/Ti, Mo-doped NiAlS/Ti, p-NiS/Ti, pure Ti mesh, and Pt/C in electrolyte of (b) 1.0 M KOH and (d) 1.0 M KOH with 0.3 M urea. Time-dependent current density curves of Mo-doped p-NiS/Ti in electrolyte of (e) 1.0 M KOH and (f) 1.0 M KOH with 0.3M urea. The HER activity of Mo-doped p-NiS/Ti electrode was investigated by a typical three-electrode system with a scan rate of 5 mV·s-1 in electrolytes of 1.0 M KOH and 12
ACCEPTED MANUSCRIPT 1.0 M KOH with 0.3 M urea. For comparison, the electrocatalytic performance of Mo-doped NiAlS/Ti, p-NiS/Ti, pure Ti mesh, and commercial Pt/C (20 wt.% Pt) were all investigated under the same conditions. Fig. 5a showed the linear sweep voltammetry (LSV) curves (after iR correction) of Mo-doped p-NiS/Ti, Mo-doped NiAlS/Ti, p-NiS/Ti, pure Ti mesh, and Pt/C samples in 1.0 M KOH. It can be seen
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that Pt/C showed the best activity towards HER, however the bare Ti mesh exhibited negligible current density. Mo-doped p-NiS/Ti exhibited an overpotential of 147.6 mV
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to drive 10 mA·cm-2 in 1.0 M KOH, which is much lower than those of Mo-doped
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NiAlS/Ti (214.8 mV) and p-NiS/Ti (164.5 mV), respectively. This result demonstrated that the removal of Al(OH)3 and Mo doping were both important
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contributors to the enhanced HER activity of Mo-doped p-NiS/Ti. Moreover, the Mo-doped Ni-LDH and Mo-doped NiAl-LDH/Ti required large overpotentials of
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283.4 and 315.6 mV for driving 10 mA·cm-2 (Fig. S6a), respectively, which indicated that the sulfuration process was vital and essential to promote HER activity. The HER kinetics were estimated by their corresponding Tafel plots. As shown in
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Fig. 5b, Mo-doped p-NiS/Ti presented obviously lower Tafel slopes of 87.8 mV·dec-1
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than those of Mo-doped NiAlS/Ti and p-NiS/Ti, which were 101.5 and 116.7 mV·dec-1, respectively.
In contrast,
Mo-doped Ni-LDH/Ti
and
Mo-doped
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NiAl-LDH/Ti showed much large Tafel slopes of 121.6 and 136.5 mV·dec-1 (Fig. S5b), respectively. The lower Tafel slope of Mo-doped p-NiS/Ti corresponds to better
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electron transfer ability compared to other samples. Therefore, Mo-doped p-NiS/Ti electrode exhibited excellent catalytic performance for HER, which should result from the special 3D flower-like structure, porous characteristic, and synergistic effects of bimetallic elements (Ni and Mo). In addition, Mo doping could lead to the formation of MoS2 nannocrystals in the nanosheets, and the self-generated lattice defects of MoS2 should largely contribute to the enhanced HER performance. Fig. 5c shows the LSV curves of Mo-doped p-NiS/Ti, Mo-doped NiAlS/Ti, p-NiS/Ti, pure Ti mesh and commercial Pt/C in 1.0 M KOH with 0.3 M urea. Again, compared to other samples, Mo-doped p-NiS/Ti showed obvious low overpotentials 13
ACCEPTED MANUSCRIPT (162.3 mV) at 10 mA·cm-2. In comparison, Mo-doped NiAlS/Ti, p-NiS/Ti, Mo-doped Ni-LDH/Ti and Mo-doped NiAl-LDH/Ti (Fig. S6c) required the larger overpotentials of 180.2, 151.6, 286.3, and 296.7 mV for achieving the same current density. As shown in Fig. 5d, the reaction kinetics for HER were estimated by their corresponding Tafel plots. Mo-doped p-NiS/Ti presented a smaller Tafel slope (87.8 mV·dec-1) than that of Mo-doped NiAlS/Ti (115.1 mV·dec-1), p-NiS/Ti (94.7 mV·dec-1), Mo-doped
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Ni-LDH/Ti (119.5 mV·dec-1) and Mo-doped NiAl-LDH/Ti (90.86 mV·dec-1) (Fig.
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S6d).
To evaluate the catalytic stability of Mo-doped p-NiS/Ti, long-term stability tests
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were conducted in 1.0 M KOH and 1.0 M KOH with 0.3 M urea under a constant potential of -1.27 V vs. SCE. In 1.0 M KOH, a slight decrease of catalytical activity
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for Mo-doped p-NiS/Ti after a long period of 17 h (Fig. 5e), suggesting the excellent stability as HER catalyst. Meantime, Mo-doped p-NiS/Ti could still keep 83.8% of its
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initial current density after long-term electrolysis for 10 h in 1.0 M KOH with 0.3 M urea (Fig. 5f). These results demonstrated that excellent electrochemical stability of
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Mo-doped p-NiS/Ti towards the long-term HER process in 1.0 M KOH or 1.0 M
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KOH with 0.3 M urea. The XPS measurement was performed for Mo-doped p-NiS/Ti after the stability test in the electrolyte of 1.0 M KOH with 0.3 M urea to detect the change of chemical state. As shown in Fig. S7, the peaks of the Ni 2p, Mo 3d, and S
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2p still existed, however, the peak intensities of Mo and S (Fig. S7c, 7d) were
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obviously reduced compared to those before the stability test (Fig. 3c and 3d) [41].
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Fig. 6 (a) Polarization curves and the corresponded (b) Tafel plots for Mo-doped
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p-NiS/Ti, Mo-doped NiAlS/Ti and RuO2 with a scan rate of 5 mV/s for OER in
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electrolyte of 1.0 M KOH. (c) Polarization curves and (d) the corresponded Tafel plots recorded for Mo-doped p-NiS/Ti, Mo-doped NiAlS/Ti and RuO2 with a scan rate of 5
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mV/s for UOR in electrolyte of 1.0 M KOH with 0.3 M urea. The anode reaction activity of Mo-doped p-NiS/Ti, Mo-doped NiAlS/Ti and RuO2
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were also tested at a scan rate of 5 mV/s in a conventional three-electrode setup under different electrolytes of 1.0 M KOH and 1.0 M KOH with 0.3 M urea. As shown in Fig. 6a, the OER activity of Mo-doped p-NiS/Ti catalyst outperformed that of Mo-doped NiAlS/Ti and RuO2 in 1.0 M KOH. To drive 50 mA·cm-2 current density, the potential of 1.62 V was required for Mo-doped NiS/Ti, meanwhile, the potential of Mo-doped p-NiAlS/Ti and RuO2 was 1.66 V and 1.79 V, respectively, which clearly evidenced the better OER activity for Mo-doped p-NiS/Ti. The OER kinetics was also elucidated by their corresponding Tafel plots (Fig. 6b). The Tafel slopes of Mo-doped 15
ACCEPTED MANUSCRIPT p-NiS/Ti, Mo-doped NiAlS/Ti and RuO2 were calculated to be 185, 225 and 67.5 mV·dec-1, respectively, suggesting better kinetics of OER for Mo-doped p-NiS/Ti. The UOR activity of Mo-doped p-NiS/Ti and Mo-doped NiAlS/Ti were measured in an electrolyte of 1.0 M KOH with 0.3 M urea. As shown in Fig. 6c, to obtain 50 mA·cm-2 current density, the potentials of 1.53 V and 1.65 V for Mo-doped p-NiS/Ti
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and Mo-doped NiAlS/Ti were required, respectively. These results suggested that UOR in 1.0 M KOH with 0.3 M urea is easier to occur than OER in 1.0 M KOH
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electrolyte. The UOR kinetics were studied by their corresponding Tafel plots (Fig. 6d), and the Tafel slopes of Mo-doped p-NiS/Ti and Mo-doped NiAlS/Ti were 12.9
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and 25.1 mV·dec-1, respectively, implying the more favorable UOR kinetics for
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Mo-doped p-NiS/Ti electrode.
Fig. 7. (a) Polarization curves of water electrolysis for Mo-doped p-NiS/Ti, Mo-doped NiAlS/Ti at a scan rate of 5 mV/s in 1.0 M KOH. (b) Polarization curves of electrolysis for Mo-doped p-NiS/Ti at a scan rate of 5 mV/s in 1.0 M KOH and 1.0 M 16
ACCEPTED MANUSCRIPT KOH with 0.3 M urea solution. (c) CP (chronopotentiometry) curve for Mo-doped p-NiS/Ti at a constant current density of 20 mA/cm2 in 1.0 M KOH with 0.3 M urea solution. The inset image shows the photograph of cathode (HER) and anode (UOR) and during alkaline electrolysis. (d) SEM image of Mo-doped p-NiS/Ti after water electrolysis for 18 h in 1.0 M KOH with 0.3 M urea solution.
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We further investigated the electrocatalytic performance of Mo-doped p-NiS/Ti in a constructed two-electrode system utilizing Mo-doped p-NiS/Ti as both cathode for
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HER and anode for OER or UOR (denoted as Mo-doped p-NiS/Ti||Mo-doped
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p-NiS/Ti) in 1.0 M KOH or 1.0 M KOH with 0.3 M urea, respectively. For comparison, a two-electrode system of Mo-doped NiAlS/Ti||Mo-doped NiAlS/Ti was
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assembled to investigate the electrocatalytic performance, too. As shown in Fig. 7a, Mo-doped p-NiS/Ti||Mo-doped p-NiS/Ti system showed better performance with cell
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voltage of 1.92 V to acquire current density of 10 mA·cm-2 in 1.0 M KOH, while cell voltage of 1.94 V was needed to obtain current density of 10 mA·cm-2 for Mo-doped NiAlS/Ti||Mo-doped NiAlS/Ti system. In electrolyte of 1.0 M KOH with 0.3 M urea,
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a cell voltage 1.95 V was needed to drive 10 mA·cm-2 for Mo-doped NiS/Ti||Mo-doped NiS/Ti system (Fig. 7b). The long-term durability of Mo-doped NiS/Ti||Mo-doped NiS/Ti system was tested in 1.0 M KOH with 0.3 M urea (Fig. 7c).
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After 20 h of continuous urea electrolysis testing, the catalytic activity of the Mo-doped NiS/Ti||Mo-doped NiS/Ti system was virtually unchanged, and the
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flower-like structure was maintained as evidenced by SEM analysis (Fig. 7d), which suggested that the Mo-doped p-NiS/Ti had excellent long-term durability for full urea splitting.
The excellent electrocatalytic activity and stability of the Mo-doped p-NiS/Ti electrode for H2 production both in water and urea electrolyte can be attributed to the following factors. (1) The 3D flower-like structure consisting of thin and porous 2D sheets provides a larger specific surface area and more exposed active sites. Moreover, this unique structure is conducive to the diffusion of electrolytes. (2) The strong 17
ACCEPTED MANUSCRIPT connection between the conductive Ti mesh and 3D flower-like structure ensures mechanical robustness and good charge transfer capacity between them, leading to excellent long-term durability. (3) The Mo-doping can generate more active sites and then enhance the catalytic performance. In addition, the sulfurization process provides much higher electrocatalytic activity compared to their hydroxide, indicating that the
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sulfurization is an essential process to further improve the electrocatalysis activity.
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3 Conclusion
The Mo-doped p-NiS on Ti mesh was successfully synthesized by selective etching of
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amphoteric Al(OH)3 in Mo-doped NiAl-LDH/Ti and the subsequent sulfuration process. The Mo-doped p-NiS/Ti with unique and stable 3D flower-like structure
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exhibited excellent catalytic activity and long-term durability in 1.0 M KOH and 1.0 M KOH with 0.3 M urea electrolyte. Only 147.6 mV and 162.3 mV was required to
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achieve current density of 10 mA·cm-2 for HER in 1.0 M KOH and 1.0 M KOH with 0.3 M urea. The excellent activity performance comes from rich active sites provided
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by high specific surface area, synergistic effect of the binary metal components, and
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the nature of Mo-doping. In addition, Mo-doped p-NiS/Ti could be used as both cathode and anode for electrochemical water or urea splitting. This work is expected to help develop other non-precious metal porous electrocatalyst materials for
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applications in water and urea splitting.
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Acknowledgements
This work was supported by Qingdao Innovation Leading Talent Program and the First-Class Disciplines funding of Shandong Province.
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ACCEPTED MANUSCRIPT Facile synthesis and superior electrochemical performances of CoNi2S4/graphene nanocomposite suitable for supercapacitor electrodes, J. Mater. Chem. A 2 (2014)
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ACCEPTED MANUSCRIPT Highlights • Porous flower-like Mo-doped NiS is highly efficient electrocatalyst for overall water splitting. • Obtaining a porous structure by a simple selective etching method.
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• The Mo-doping can enhance the catalytic performance.
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• Mo-doped p-NiS/Ti is used in a two-electrode system for overall water or urea
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splitting.
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