This is the pre-peer reviewed version of the following article: Ibupoto Z.H., Tahira A., Tang P., Liu X., Morante J.R., Fahlman M., Arbiol J., Vagin M., Vomiero A.. MoS x @NiO Composite Nanostructures: An Advanced Nonprecious Catalyst for Hydrogen Evolution Reaction in Alkaline Media. Advanced Functional Materials, (2019). 29. 1807562: - . 10.1002/adfm.201807562, which has been published in final form at https://dx.doi.org/10.1002/adfm.201807562. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. Advanced Functional Materials MoS2@NiO composite nanostructures: an advanced nonprecious catalyst for hydrogen evolution reaction in alkaline media --Manuscript Draft-- Manuscript Number: adfm.201807562 Full Title: MoS2@NiO composite nanostructures: an advanced nonprecious catalyst for hydrogen evolution reaction in alkaline media Article Type: Full Paper Section/Category: Keywords: Corresponding Author: Alberto Vomiero Lulea Tekniska Universitet Lulea, SWEDEN Additional Information: Question Response Please submit a plain text version of your Luleå, 26/10/2018 cover letter here. To Dr. Joern Ritterbusch Editor in chief Advanced Functional Materials Re: Submission of the Manuscript entitled “MoS2@NiO composite nanostructures: an advanced nonprecious catalyst for hydrogen evolution reaction in alkaline media” for consideration as an article in Advanced Functional Materials. Dear Dr. Joern Ritterbusch, We are hereby submitting the manuscript entitled “MoS2@NiO composite nanostructures: an advanced nonprecious catalyst for hydrogen evolution reaction in alkaline media” by Zafar Hussain Ibupoto, Aneela Tahira, PengYi Tang, Xianjie Liu, Joan Ramon Morante, Mats Fahlman, Jordi Arbiol, Mikhail Vagin, Alberto Vomiero, to be considered for publication as an article in Advanced Functional Materials. Water electrolysis is one of the most promising ways for oxygen and hydrogen production via simple, cheap and environmentally compatible methodology. For the purpose, noble metal catalysts, i.e. Pt-based electrodes for hydrogen are the most efficient systems. However, they are incompatible with practical exploitation, due to their high cost and scarcity in nature. For this reason, the researchers are pushing the development of alternative catalysts composed of cheap and environmentally friendly materials, to be prepared through simple and scalable methodologies. The main challenge in the field is to be able to produce catalysts, which work for hydrogen production with performance comparable or better than noble metal-based catalysts in terms of onset potential, overpotential, Tafel slope (i.e. the variation of current density with the external applied potential) and long-term stability. Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation Here we propose the synthesis of composite materials for hydrogen evolution reaction (HER) in alkaline media via simple and scalable methodology. While NiO and MoS2 nanostructures have been separately investigated for HER, their poor performance does not allow their practical exploitation. Specifically, the main bottleneck in functionality lies in the low density of active sites in pristine NiO and MoS2, which limits their performance. To overcome this issue we propose here an advanced composite system based on NiO nanostructured film acting as fast charge transporter, coated with a highly porous MoSx layer, which exhibits a high density of catalytically active edges. The combination of the charge transport in NiO and the high catalytic activity of the sulfide top layer makes the proposed heterostructures effective for HER, with 226 mV overpotential to achieve current density of 10 mA/cm2. The Tafel slope of 43 mV dec-1 is the lowest possible in alkaline media for non-precious catalyst to date and it is very close to Pt/C-based catalysts, revealing that the HER kinetics is highly favorable at the surface of the MoS2@NiO composite. In terms of Tafel slope, the performance of the new catalyst is comparable to that of Pt/C in alkaline media. Additionally, the top MoSx layer can protect the oxide under-layer from dissolution, resulting in long-term stability of the system, with current density kept unvaried after 13 hours operations. We are convinced that these results represent a major advancement in the design of functional catalysts as well as newly composite material, and can inspire research in correlated disciplines, where different functionalities (like catalytic activity, charge transport, resistance in harsh environment) are demanded, like, for instance, in photocatalysis, photoelectrochemical cells, lithium ion batteries, or supercapacitors. For all these reasons, we are confident that this manuscript can be of interest for the broad readership of Advanced Functional Materials. Looking forward to your positive evaluation. Yours sincerely, Alberto Vomiero, Ph.D. FRSC (UK), FIMMM, FInstP, FIoN, Marie Curie IOF, Member of the Global Young Academy Swedish Foundations' Consolidator Grant Fellow (http://startinggrant.se/) Associate Editor of Nano Energy (Elsevier) Editorial Board of Scientific Reports (NPG) Associate Editor of the Journal Materials for Renewable and Sustainable Energy (Springer) Chair in Experimental Physics Division of Materials Science Department of Engineering Science and Mathematics Luleå University of Technology SE-971 98 Luleå, Sweden e - mail alberto.vomiero@ltu.se Do you or any of your co-authors have a No. The authors declare no conflict of interest. conflict of interest to declare? Corresponding Author Secondary Information: Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation Corresponding Author's Institution: Lulea Tekniska Universitet Corresponding Author's Secondary Institution: First Author: Zafar Hussain Ibupoto First Author Secondary Information: Order of Authors: Zafar Hussain Ibupoto Aneela Tahira PengYi Tang Xianjie Liu Joan Morante Mats Fahlman Jordi Arbiol Mikhail Vagin Alberto Vomiero Order of Authors Secondary Information: Abstract: The design of the Earth abundant, non-precious, efficient and stable electrocatalysts for efficient hydrogen evolution reaction in alkaline media is a hot research topic in the field of renewable energies. Here we propose a heterostructured system composed of MoSx deposited on NiO nanostructures (MoS2@NiO) as robust catalyst for water splitting. NiO nanosponges are applied as co-catalyst for MoS2 in alkaline media. Both NiO and MoS2@NiO composites are prepared by a hydrothermal method. The NiO nanostructures exhibit sponge-like morphology and are completely covered by the sheet-like MoS2. The NiO and MoS2 exhibit cubic and hexagonal phases, respectively. In the MoS2@NiO composite, the hydrogen evolution reaction experiment in 1M KOH electrolyte resulted in an extremely low overpotential (226 mV) to produce 10 mA cm-2 current density. The Tafel slope for that case is 43 mV/decade, which is the lowest ever achieved for MoS2-based electrocatalyst in alkaline media. The catalyst is highly stable for at least 13 hours, with no decrease in the current density. This simple, cost effective and environmentally friendly methodology can pave the way for exploitation of MoS2@NiO composite catalysts not only for water splitting, but also for other applications such as lithium ion batteries, and fuel cells. Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation Complete Manuscript 1 2 3 4 MoS2@NiO composite nanostructures: an advanced nonprecious catalyst for hydrogen 5 6 evolution reaction in alkaline media 7 8 9 10 Zafar Hussain Ibupotoa, b*, Aneela Tahiraa, PengYi Tangc,d, Xianjie Liue, Joan Ramon Moranted, 11 12 Mats Fahlmane, Jordi Arbiolc,f, Mikhail Vagine, Alberto Vomieroa* 13 14 15 a Division of Material Science, Department of Engineering Sciences and Mathematics, Luleå 16 17 University of Technology, 97187 Luleå, Sweden 18 b 19 Dr. M.A Kazi Institute of Chemistry University of Sindh Jamshoro, 76080, Sindh Pakistan 20 c 21 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, 22 23 Bellaterra, 08193 Barcelona, Catalonia, Spain 24 d 25 Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, Sant Adrià del 26 Besòs, Barcelona 08930, Catalonia, Spain 27 e 28 Department of Physics, Chemistry and Biology, Linkoping University, 58183 Linkoping, 29 30 Sweden 31 f 32 ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Catalonia, Spain 33 34 * Corresponding authors: Zafar Hussain Ibupoto, Alberto Vomiero 35 Email: alberto.vomiero@ltu.se, zafar.ibupoto@ltu.se 36 37 38 Abstract 39 40 The design of the Earth abundant, non-precious, efficient and stable electrocatalysts for efficient 41 42 hydrogen evolution reaction in alkaline media is a hot research topic in the field of renewable 43 44 energies. Here we propose a heterostructured system composed of MoSx deposited on NiO 45 46 nanostructures (MoS2@NiO) as robust catalyst for water splitting. NiO nanosponges are applied 47 as co-catalyst for MoS2 in alkaline media. Both NiO and MoS2@NiO composites are prepared by 48 49 a hydrothermal method. The NiO nanostructures exhibit sponge-like morphology and are 50 51 completely covered by the sheet-like MoS2. The NiO and MoS2 exhibit cubic and hexagonal 52 53 phases, respectively. In the MoS2@NiO composite, the hydrogen evolution reaction experiment in 54 55 1M KOH electrolyte resulted in an extremely low overpotential (226 mV) to produce 10 mA cm- 56 2 57 current density. The Tafel slope for that case is 43 mV/decade, which is the lowest ever achieved 58 for MoS2-based electrocatalyst in alkaline media. The catalyst is highly stable for at least 13 hours, 59 60 with no decrease in the current density. This simple, cost effective and environmentally friendly 61 62 63 64 1 65 1 2 3 4 methodology can pave the way for exploitation of MoS2@NiO composite catalysts not only for 5 6 water splitting, but also for other applications such as lithium ion batteries, and fuel cells. 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 2 65 1 2 3 4 Text 5 6 Hydrogen production by cost-effective electrochemical water splitting is one of the most 7 8 promising approaches to confront the energy crisis and to obtain clean fuels with high energy 9 10 density. Hydrogen evolution reaction (HER) aims at meeting the stringent criteria for entering the 11 12 energy market, through fabrication of electro-catalysts guaranteeing highly efficient hydrogen 13 14 production.1,2 However, the best-known electro-catalysts for hydrogen production are Pt and Pt- 15 derivatives, which hinder the possibility of application of such technology due to their high cost 16 17 and scarcity.3,4 Therefore, the development of environmentally friendly, low cost and Earth- 18 19 abundant catalysts to replace noble metals is mandatory to realize large scale electro-catalytic 20 21 hydrogen production.5 22 23 To obtain efficient water splitting at a practical level, the electrocatalysts for HER must work either 24 25 in acidic or in alkaline electrolytes at low overpotential.6 To develop simple and cost effective 26 functional catalysts for electrochemical water splitting, the latest research directions involve the 27 28 investigation of Mo-based nanomaterials,7 including Mo2C, 8, 9, 10, 11, 12 MoN,13 MoS2 ,14 and related 29 30 compounds,15,16,17 aiming at competing with Pt in terms of catalytic efficiency in HER. A 31 32 successful strategy is the deposition of Mo compounds on various conducting substrates with high 33 34 specific surface area, including carbon nanosheets,18 and nanotubes,19,20 which prevent aggregation 35 of the nanocomposites, keeping the effect of the high density of their active sites. 36 37 HER performance of Mo-compounds is critically dependent on the crystalline planes exposed to 38 39 the catalytic activity. For example, in MoS2, (10-10) edges are highly catalytically active, while 40 41 (0001) basal planes are inert.21 This observation is driving the research on the development of new 42 43 materials with limited flat surfaces and increased density of edge sites, such as crystalline,22, 23, 24, 44 25 26, 27, 28 45 amorphous compounds and hybrid systems.29, 30, 31 Researchers also reported recently 46 that strain induced by patterned gold nanocones can activate the basal plane of monolayer 2H- 47 48 MoS2 for HER.32 However, despite the promising results, Mo-compounds are still below the 49 50 expectations in terms of functional properties, compared to Pt. 51 52 In parallel to Mo-compounds, other novel electro-catalysts were recently introduced for HER 53 54 based on Earth-abundant metal chalcogenides.2 A successful strategy to boost functional 55 56 performances is the application of heterostructured nanomaterials: for example, decoration with 57 Ni/NiO nanoparticles of CoSe2 nanobelts (on a stable glassy carbon electrode) demonstrate 58 59 improved HER,33 mainly due to the chemical coupling effect of CoSe2 and of the anchored Ni/NiO. 60 61 62 63 64 3 65 1 2 3 4 Besides this, the fabrication of catalysts for HER typically needs several successive steps, making 5 6 the development of a functional electrocatalyst with enhanced HER activity in alkaline media for 7 8 industrial application a very challenging task to date. NiO electrocatalyst exhibits poor HER 9 10 performance, due to both the low electrical conductivity and the poor catalytic activity. 11 12 For these reasons, aiming at increasing the density of active sites and the electrical conductivity, 13 14 we propose the development of a MoS2@NiO composite nanostructure, targeting the fabrication 15 of a highly stable and efficient electrocatalyst for HER in alkaline media. Herein, we report a new 16 17 class of heterostructure materials as robust catalyst for HER in 1M KOH at room temperature. The 18 19 electrocatalyst is based on a MoS2@NiO composite structure that reveal an efficient HER activity 20 21 due to synergetic effect between MoS2 and NiO as supporting co-catalyst material. This composite 22 23 electrode exhibits excellent performance for HER with a Tafel slope (43 mV/decade) comparable 24 25 to that of commercial Pt/C and compatible with practical applications. Importantly, the presented 26 electrocatalyst is highly stable with negligible loss of potential for almost 10 hours under operation. 27 28 These results can be of importance not only for water splitting but also for other processes and 29 30 fields of energy sector, in which new electro-catalysts can improve device functionality, like, for 31 32 instance, in lithium ion batteries, supercapacitors and fuel cells. 33 34 Figure 1(a) shows the morphological features of NiO, revealing a porous and flower like shape. 35 After the deposition of the MoS2 layer (Figure 1(b)), the composite nanostructures have porous 36 37 nanoparticle morphology. SEM/TEM images of pristine MoS2 (Figure S1) indicate a nanosheet- 38 39 like morphology. 40 41 XRD analysis (Figure 1) of pure NiO confirms its cubic phase (diffraction pattern well matching 42 43 the JCPDS card no. 96-101-0382). After MoSx deposition, the MoS2@NiO composite exhibits 44 45 reflections fully compatible with NiS cubic phase (JCPDS card no. 96-591-0138), in addition to 46 reflections from the NiO cubic phase. 47 48 To probe the chemical composition, X-ray photoelectron spectroscopy (XPS) studies were carried 49 50 out. Survey scans were used to obtain the elemental composition (see Tables S1-S3 for XPS 51 52 quantitative analysis). In the MoS2@NiO sample, the Mo:S:O:Ni atomic ratio measured from XPS 53 54 is equal to 26.5:34.0:36.5:3.0. The low Ni concentration is ascribed to the small signal coming 55 56 from the underlying NiO layer, suggesting a nearly complete coverage of the NiO nanostructures 57 by the MoS2 layer. The high concentration of oxygen (36.5% at.) indicates that Mo is partially 58 59 60 61 62 63 64 4 65 1 2 3 4 oxidized, as indicated by TEM, suggesting that the NiO underlying layer is covered by a mixture 5 6 of molybdenum sulfides and oxide. 7 8 In Figure 1(e) the S2p core level spectrum for MoS2@NiO is reported. The MoS2@NiO contains 9 10 two spin-split doublets (S2p3/2 and S2p1/2) with a 4:1 ratio. The high intensity S2p3/2 peak is situated 11 12 at ~161.5 eV and the second lower intensity feature is located at ~163.0 eV. The main S2p core 13 14 level feature for the composite system can be assigned to S2- .34, 35, 36 It is suggested that the shift 15 of the S2p core level to lower binding energy from ~162.2 eV to ~161 eV is indicative of a 16 17 transition from 2H to 1T phase of MoS2. In Figure 1(f) the Mo3d and S2 core level spectrum for 18 19 MoS2@NiO is reported. The Mo3d spectrum of the MoS2@NiO sample contains three spin-split 20 21 doublets (Mo3d5/2 and Mo3d3/2) with the Mo3d5/2 peaks centered at ~228.7 eV (54%), ~230.0 eV 22 23 (24%) and ~232.4 eV (22%). The feature at 228.7 eV can be assigned to Mo4+ and is compatible 24 25 with the binding energy of the 1T phase of MoS2.34 The higher binding energy features are assigned 26 to Mo5+ and Mo6+ respectively, in good agreement with literature values.35 As the Mo5+ and Mo6+ 27 28 features are not present in stoichiometric MoS2 films, we expect that they originate from MoOy or 29 30 MoSxOy regions in the films. For MoS2@NiO the relative concentration of the oxygen-derived 31 32 (Mo5+, Mo6+) components is about 50%, in good agreement with the stoichiometric values 33 34 previously reported. 35 The chemical composition, the morphology and crystalline features were also analyzed via high 36 37 angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) combined 38 39 with electron energy loss spectroscopy and high-resolution transmission electron microscopy 40 41 (HRTEM) for both the samples (Figure 2). The pure NiO sample is composed of Ni and O and no 42 43 other impurity was detected. Furthermore, HRTEM confirms that the material crystallizes in the 44 45 cubic NiO phase, [FM3-M]-Space group 225, with lattice parameters of a = b= c = 0.4179 nm, and 46 47 α = β ＝ γ ＝90°, as visualized along the [101] direction. 48 49 In the MoS2@NiO sample, the presence of Ni, O, Mo, and S was confirmed in the composite. The 50 HRTEM and the corresponding FFT spectrum indicate that the nanoplates crystallize in the cubic 51 52 NiO phase, [FM3-M]-Space group 225, with lattice parameters of a = b= c = 0.4179 nm, and α = 53 54 β = γ = 90° as visualized along the [110] direction. The detailed structure of the NiO/MoSx 55 56 interface and the corresponding FFT spectrum indicate that it crystallizes in the hexagonal MoS2 57 58 phase, [P63/MMC]-Space group 194, with lattice parameters of a = b = 0.3165 nm, c = 1.2295 nm, 59 and α = β = 90°, γ = 120° as visualized along the [1-21-3] direction. 60 61 62 63 64 5 65 1 2 3 4 These results are in good agreement with XRD and XPS. 5 6 Figure 3(a) shows the polarization curves obtained through linear sweep voltammetry (LSV) 7 8 electrochemical mode for the various electrocatalysts: pristine GCE, MoS2, NiO, MoS2@NiO, and 9 10 benchmark 20%Pt/C at scan rate of 5 mV/s in 1M KOH solution saturated with N2 gas. The GCE 11 12 shows HER activity at higher potential (570 mV vs RHE), compared to all the other samples. 13 14 The nanostructured MoS2 exhibits HER activity still at high potential (508 mV vs RHE), which 15 16 can probably due to the fact that MoS2 tends to agglomerate during HER experiment, resulting in 17 low density of active edges and thus in poor HER performance. The catalytic activity of NiO is 18 19 also limited and HER activity is reported at high potential (535 mV vs RHE). The composite 20 21 system, instead, exhibits HER activity at low dynamic potential: the MoS2@NiO composite system 22 23 achieves 10 mA/cm2 current density at 406 mV overpotential. This behavior can be assigned to 24 25 the synergetic effect of the MoS2 nanosheets on the surface of NiO nanostructures, which increases 26 27 the density of active edges (thanks to MoS2), and the fast charge transfer guaranteed by the NiO 28 29 nanostructures. The overpotential for the MoS2@NiO electrocatalyst is close or lower than the 30 reported electrocatalysts such CoP/CC (500 mV),37 N, P-G (700 mV),38 MoSx (540 mV),25 Co- 31 32 S/FTO (480 mV),39 and Co-NR CNTs (450 mV).40 The overpotential of noble Pt/C catalyst is 33 34 about 110 mV, of course lower than MoS2@NiO, but still the reported results for a nonprecious 35 36 and earth abundant electrocatalyst can be valuable to develop new nanomaterials for water spitting. 37 38 These results indicate superior performance for the composite system than for the pure NiO and 39 40 MoS2 catalysts. 41 Entering into the investigation of the HER process, the HER kinetics can be evaluated from the 42 43 linear regions of the Tafel plot, through fitting the LSV curves with the Tafel equation [Eq. (1)]: 44 45 46 47 η  b log ( j )  a (1) 48 49 50 51 Where a is related to the exchange current density (j0) and b represents the Tafel slope. In alkaline 52 53 conditions, the HER kinetics most likely takes place via the formation of hydrogen adsorbed 54 55 intermediates (Hads). The formation of Hads involves electron transfer via the discharge of water by 56 following the Volmer step [Eq. (2)]: 57 58 59 60 H2O + e-  Hads + OH_ (2) 61 62 63 64 6 65 1 2 3 4 5 6 The next step in HER is either through a second electron transfer (known as Heyrovsky step [Eq. 7 8 (3)]): 9 10 11 12 Hads + H2O + e_  H2 + OH_ (3) 13 14 15 16 or by the Tafel step [Eq. (4)]: 17 18 19 Hads + Hads  H2 (4) 20 21 22 23 Generally, a Tafel slope of 120, 40, and 30 mV/decade correspond to the Volmer, Heyrovsky, or 24 25 Tafel step as the rate-limiting and determining step in the HER kinetics, respectively.41, 42, 43 The 26 27 Tafel slope of 43 mV/decade suggests that the Heyrovsky step is determining the rate in 1M KOH 28 for the MoS2@NiO composite nanostructures as shown in Figure 3(b). The Tafel slope for the NiO 29 30 (46 mV/decade) and the pristine MoS2 (44 mV/decade) indicates that also these samples follow 31 32 the Heyrovsky mechanism, and that the reaction rate is limited by the electrochemical desorption 33 34 of the hydrogen gas. The Tafel slope for the GCE is 105 mV/decade, in which the rate of reaction 35 36 is limited by the Volmer step and induces slower HER activity in this sample. The Tafel slope of 37 38 Pt/C noble catalyst is 30 mV/decade, indicating that the electrochemical reaction is limited by 39 adsorption (Tafel step). The values of the Tafel slopes for MoS2@NiO, NiO and MoS2 (40 40 41 mV/decade) are relative low, almost one half of the reported Ni-based and other electrocatalysts 42 43 in the alkaline media, including NiO/Ni-carbon nanotubes (82 mV/decade),41 Ni3S2 nanoparticles 44 45 (97 mV/decade),44 CoP nanowires on carbon cloth (129 mV/decade),37 NiCu nanoalloys (116 46 47 mV/decade),45 Ni-Mo/Cu nanowires (107 mV/decade),46 and electrodeposited Ni nanoparticles 48 49 (88 mV/decade).47 By considering the Tafel slope of the electrocatalyst in this study in alkaline 50 media with respect to reported catalysts, we have produced the best known electrocatalyst (from 51 52 this point of view) for hydrogen evolution reaction in alkaline media, which is very important for 53 54 industrial applications. The smaller the Tafel value, the faster is the HER kinetics and the better 55 56 for hydrogen production. 57 58 Most of the studies in the literature on MoS2-based catalysts for HER use Ni foam and other 59 60 conducting substrates, which somewhat impairs their validity, since the foam itself is catalytically 61 62 63 64 7 65 1 2 3 4 active. In the present study, instead, the use of powders deposited on GCE prevents the problem 5 6 of the Ni foam, improving the reliability of the results. It is generally accepted that Ni-based 7 8 electrocatalysts can be a basis for the future generation of efficient catalysts in alkaline media. We 9 10 demonstrated Ni-based composites with efficiencies close to that of benchmarking Ni-based 11 12 electrocatalysts.41 13 14 To gain additional information about the HER mechanism, we calculated the electrochemical 15 surface area by simple cyclic voltammetry versus Ag/AgCl (Figure 4 (a-f)), from the slope of the 16 17 linear fit of the average current density versus the scan rate. We obtained almost similar values for 18 19 NiO, MoS2 and MoS2@NiO, i.e. (1.46 ± 0.02)10-4 F/cm2 for pure MoS2, (1.45E-4 ± 0.03)10-4 20 21 F/cm2 for NiO, and (1.47E-4 ± 0.03)10-4 F/cm2 for MoS2@NiO. From these results, we can 22 23 deduce that no significant increase in surface area is found in the composite catalysts. The 24 25 improvement in HER activity should come from the synergetic effects between MoS2 and NiO 26 27 nanostructures in the composite form and not from an increase in specific surface area. 28 Figure 5(a) shows the HER polarization curves of the MoS2@NiO sample before and after the 29 30 stability test. The composite catalyst is highly stable, without any current density loss. We 31 32 performed the stability test through chronopotentiometry for the MoS2@NiO composite for 13 33 34 hours. We found a gradual shift in the voltage from 490 mV to 510 mV to maintain the current 35 36 density at 10 mA/cm2 the first two hours, and no potential drop was observed after the two-hour 37 38 transient for the remaining eleven hours. 39 Electrochemical impedance spectroscopy was utilized to quantify the surface phenomena and the 40 41 kinetics of HER on the developed catalysts (Figure 6). NiO showed two orders of magnitude 42 43 higher values of total capacitance (Figure 6A) visible at low frequencies in comparison with MoS2. 44 45 Consistently, the capacitance of NiO film (Figure 6B) visible as ordinate value of transition 46 47 towards the saturation was two orders of magnitude higher than for MoS2.48 These effects illustrate 48 49 the higher concentration of charge carriers in NiO film in comparison with MoS2. The mutual 50 integration of MoS2 and NiO led to the monotonous transition of electrocapacitive properties 51 52 between the pristine films. 53 54 The presence of two time-resolved processes visible in the Bode plot (e.g. on the spectra of 55 56 composite film, Figure 6E) was modelled by two RC elements in the simplest unified equivalent 57 58 circuit (Inset in Figure 6) developed for an electrode covered with a damaged (porous) coating 49 59 Since the boundary between the layers is not ideally smooth, due to increased surface roughness 60 61 62 63 64 8 65 1 2 3 4 of the porous film, a quantitative analysis of the electrode impedance response requires a more 5 6 complicated, distributed circuit model featuring constant phase elements (CPE) rather than pure 7 8 capacitors. The equivalent circuit (Inset in Figure 6) providing the best fit consists of the solution 9 10 resistance Rs and two combined R-CPE units (I and II). A single set of parameters has been used 11 12 to simultaneously fit the real and imaginary parts of the impedance over the frequency range from 13 14 1.25 Hz to 50 kHz. A value of the fitting quality parameter 2 of  0.001 obtained for all spectra 15 16 indicates a very good fit. The first R-CPE unit showed smaller RC values than the second one 17 illustrating the faster kinetics of the first process. Therefore, the first R-CPE unit might be assigned 18 19 to fast electronic transport connected with faradaic phenomena of HER (where RI – charge transfer 20 21 resistance proportional to reversed rate constant of electrode reaction, CPE I – double layer 22 23 capacitance of GCE at the bottom of pores), while the second R-CPE unit might be assigned to the 24 25 slower ionic transport (where RII – resistance of the film established via pores, CPEII – film 26 capacitance).49 Consistently with raw signal evaluation, the NiO film showed a 46 times larger 27 28 value of capacitance (ca. 46 F) in comparison with MoS2 (1 F), while the composite film showed 29 30 an intermediate value (3 F). Coherently, the MoS2 film showed 4 times lager film resistance than 31 32 NiO, as shown in Figure 6D. The capacitance of double layer established on glassy carbon on the 33 34 bottom of pores revealed a smaller alteration on different film material (0.8 F, 4.5 F and 0.7 F 35 36 for MoS2, NiO and the composite film, respectively) illustrating the minor morphology change at 37 38 GCE/film interface as shown in Figure 6E. Importantly, the NiO film showed a significantly 39 40 smaller charge transfer resistance (15 ) in comparison with pristine MoS2 and the composite 41 42 films (4.1 k and 475 , respectively) as shown in Figure 6C. This result explains the fastest 43 44 HER rate on NiO. The composite film revealed a transitional value of HER rate between the two 45 pristine films NiO and MoS2. 46 47 In summary, we developed a novel architecture composed of NiO semiconducting nanostructures 48 49 covered by a MoSx layer through the simple excessive sulfurization method, and we tested it for 50 51 HER in alkaline media. The NiO and MoS2 in composite structure exhibit cubic and hexagonal 52 53 crystalline features, respectively. The composite system is purely composed of Ni, O, Mo and S 54 elements with no other impurity. The synthetic strategy is simple and cost effective and can be 55 56 used at large scale for the production of functional materials. The chemical sulfurization process 57 58 results in increased density of active sites in the composite catalysts. The hybrid catalyst 59 60 demonstrated better HER performance (including stability and durability) compared to pristine 61 62 63 64 9 65 1 2 3 4 MoS2 electrocatalyst, achieving low overpotential for HER, as demonstrated by the extremely low 5 6 Tafel slope and high current densities, which has never been reported before for MoS2-based 7 8 catalysts in alkaline media. The obtained lowest Tafel slope value (43 mV/decade), the excellent 9 10 durability and stability are the prominent features of the new nonprecious electrocatalyst and make 11 12 it a good candidate for precious metal free catalysis in HER. These results can be useful for not 13 14 only water splitting, but also in a broader range of applications, like for instance oxygen reduction 15 reactions, supercapacitors, lithium ion batteries, gas sensing and fuel cells. 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 10 65 1 2 3 4 Experimental Methods 5 6 Synthesis of NiO nanostructures and their composites with MoSx. 7 8 Nickel chloride hexahydrate, hexamethylenetetramine, ammonium phosphomolydate hydrate, L- 9 10 cystein, potasium hydroxide, and 5% Nafion were purchased from Sigma Aldrich Stockholm 11 12 Sweden. All chemicals were of analytical grade and used without further purification. 13 14 Synthesis of NiO nanostructures 15 NiO nanostructures were obtained by two steps. Firstly, nickel hydroxide nanostructures were 16 17 prepared by mixing 2.37 g of nickel chloride hexahydrate and 1.41 g of hexamethylenetetramine 18 19 in 100 mL of distilled water. A homogenous solution was obtained by constant stirring for 30 20 21 mints. Then growth solution was covered with an aluminum foil and left into preheated electric 22 23 oven at 95 C for 5 hours. After the completion of growth time, nickel hydroxide nanostructures 24 25 were obtained by filtration and followed by washing several times with deionized water and dried 26 at room temperature for overnight. In the second step, nickel hydroxide phase was converted into 27 28 NiO by calcination at 450 C in air for 3 hours. 29 30 Synthesis of MoS2@NiO composite nanostructures 31 32 After the growth of Ni oxide nanostructures, the MoSx layer was deposited on them. A quantity of 33 34 0.3 grams of NiO nanomaterial was immersed in the growth solution of 0.175 g ammonium 35 36 phosphomolybdate hydrate and 0.275 g of L-cystein in 50 mL of distilled water, each in a separate 37 38 Teflon vessel of capacity of 100 mL. The Teflon vessel was sealed in a stainless-steel autoclave at 39 200 C for 20 hours. A thick layer of MoSx was deposited on nickel oxides, giving the final 40 41 structure of the composite materials. 42 43 Characterization. 44 45 The morphology of nanostructures was evaluated by JEOL scanning electron microscope (SEM) 46 47 operated at 15 KV. The crystalline structure and phase purity of pristine NiO and MoS2@NiO 48 49 composite nanostructures was investigated by X-ray powder diffraction using a Philips powder 50 diffractometer associated with CuKα radiation (λ = 1.5418 Å) at generator voltage of 45 kV and a 51 52 current of 45 mA. The X-ray photoelectron spectroscopy (XPS) experiments were performed using 53 54 a Scienta ESCA 200 spectrometer in ultrahigh vacuum at a base pressure of 10−10 mbar with a 55 56 monochromatic Al (K alpha) x-ray as a source of photons with 1486.6 eV. The XPS experimental 57 58 methodology was set in such a way so that the full width at half maximum of the clean Au 4f7/2 59 60 line was 0.65 eV. All spectra were obtained at a photoelectron takeoff angle of 0° (normal 61 62 63 64 11 65 1 2 3 4 emission). All the samples for HRTEM and STEM were prepared via a mechanical process, as 5 6 published elsewhere. S1 HRTEM and STEM images have been collected through a FEI Tecnai F20 7 8 field emission gun microscope with a 0.19 nm point-to-point resolution at 200 kV equipped with 9 10 an embedded Quantum Gatan Image Filter for EELS analyses. The collected images were analyzed 11 12 by means of Gatan Digital Micrograph software. 13 14 Electrolysis of water in alkaline media. 15 All HER experiments were carried on a Solartron analytical potentiostat using three-electrode 16 17 system cell assembly in 1M KOH solution saturated with N2 gas. The catalyst ink was prepared 18 19 by 10 mg of each catalyst in 2 mL of distilled water and 100 µL of 5% Nafion and mixture was 20 21 sonicated in ultrasonic bath for 15 mints in order to get a homogenous catalyst ink. A glassy 22 23 carbon electrode (3 mm in diameter) was applied as working electrode for deposition of 10 µL of 24 25 catalysts ink by drop casting method and used as working electrode. The modified glassy carbon 26 electrode was dried with flow of N2 gas at room temperature. An Ag/AgCl with saturated KCl as 27 28 reference electrode and platinum mesh was used as counter electrode respectively. Linear sweep 29 30 voltammetry was employed at the scan rate of 5 mV/s for HER characterization in alkaline media. 31 32 Electrochemical Impedance Spectroscopy (EIS) experiment was performed in the frequency range 33 34 of 50 kHz to 1.25 Hz with onset potential of 100 mV and an amplitude of 10 mV at the reference. 35 36 The capacitance double layer was used to estimate active surface area by cyclic voltammetry 37 against Ag/AgCl. The potential vs RHE was calibrated to RHE through Nernst equation. ERHE 38 39 =EAg/AgCl+0.059pH+EoAg/AgCl. 40 41 42 43 Acknowledgements. 44 45 AV and ZHI acknowledge Knut and Alice Wallenberg Foundation and Kempe Foundation for 46 47 partial funding. JA and PYT acknowledge funding from Generalitat de Catalunya 2017 SGR 327 48 and the Spanish MINECO project VALPEC (ENE2017-85087-C3). ICN2 acknowledges support 49 50 from the Severo Ochoa Programme (MINECO, Grant no. SEV-2013-0295). ICN2 and IREC are 51 52 funded by the CERCA Programme / Generalitat de Catalunya. Part of the present work has been 53 54 performed in the framework of Universitat Autònoma de Barcelona Materials Science PhD 55 56 program. 57 58 59 60 61 62 63 64 12 65 1 2 3 4 Figure Captions 5 6 7 8 Figure 1. (a, b) SEM images of the nickel oxide nanostructures before (a) and after (b) deposition 9 of the MoSx layer, respectively. (c, d) XRD patterns of bare NiO (c) and MoS2@NiO (d) 10 11 nanostructures. (e) XPS S2p core level spectra and (f) S2s and Mo3d core level spectra of the 12 MoS2@NiO composite. In (e) and (f) the measured spectra are represented by dots, while the 13 dashed and solid lines are fitting curves. 14 15 Figure 2. (a) EELS chemical composition maps obtained from the red rectangled area of the ADF- 16 17 STEM micrograph. Individual Ni (red), O (green) maps and their composite, (b) Left Top: low 18 magnification TEM image showing the nanostructure. Right top: the magnified HRTEM image 19 shows the detail parts of left top TEM. Left bottom: HRTEM micrograph showing the twin 20 boundary located at the red squared region and the corresponding FFT spectrum indicates that the 21 material crystallizes in the cubic NiO phase. Right bottom: HRTEM image showing the detailed 22 23 structure of the blue squared region, which is also visualized from the [101] direction, as confirmed 24 by the FFT spectrum. (c) EELS chemical composition maps obtained from the red rectangle area 25 of the ADF-STEM micrograph for the NiO/MoSx composite. Individual element maps and their 26 composites are reported, (d) NiO/MoSx system. Left top: low magnification TEM image showing 27 28 the distribution of the NiO/MoSx nanocomposite. Middle and Right top: HRTEM micrograph 29 showing the structure of the NiO nanoplate. Left bottom: HRTEM image of the red squared region 30 and the corresponding FFT spectrum. Right bottom: detailed structure of the interface of the blue 31 squared region and the corresponding FFT spectrum. 32 33 34 Figure 3. (a) Linear sweep voltammetry (LSV) HER polarization curves in N2 saturated 1M KOH 35 at 25 C. (b) Calculated Tafel slopes for the different samples. 36 37 Figure4. (a-f). Electrochemical surface area estimated by cyclic voltammetry (a, c, e) against 38 39 Ag/AgCl as reference electrode in 1M KOH saturated with N2 gas, and linear fit (b, d, f) for pure 40 NiO (a, b), pure MoS2 (c, d) and MoS2@NiO (e, f). 41 42 Figure 5. (a) HER polarization curves before and after 13 hours stability test in alkaline media. 43 (b) Chronopotentiometry stability in alkaline media for 13 hours at the current density of 10 44 45 mA/cm2. 46 47 Figure 6. A. The effect of film composition on total capacitance, 6B. The frequency dependencies 48 of the total capacitance calculated from impedance spectra acquired on pristine (MoS2 and NiO as 49 50 black and blue, respectively) and a composite films (red); onset potential of HER (100 mV), 10 51 mV amplitude, 1M KOH, 6C Impedance spectra acquired on pristine (MoS2 and NiO as black and 52 blue, respectively) and a composite films (red); solid lines – results of the fitting with equivalent 53 circuit (Inset in D-E); onset potential of HER (100 mV), 10 mV amplitude, 1M KOH. 54 55 56 57 58 59 60 61 62 63 64 13 65 1 2 3 4 Figure 1 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 14 65 1 2 3 4 Figure 2 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 15 65 1 2 3 4 Figure 3 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 16 65 1 2 3 4 Figure 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 17 65 1 2 3 4 Figure 5 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 18 65 1 2 3 4 Figure 6 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 19 65 1 2 3 4 MoS2@NiO composite nanostructures: an advanced nonprecious catalyst for hydrogen 5 6 evolution reaction in alkaline media 7 8 9 10 Zafar Hussain Ibupotoa, b*, Aneela Tahiraa, PengYi Tangc,d, Xianjie Liue, Joan Ramon Moranted, 11 12 Mats Fahlmane, Jordi Arbiolc,f, Mikhail Vagine, Alberto Vomieroa* 13 14 15 a Division of Material Science, Department of Engineering Sciences and Mathematics, Luleå 16 17 University of Technology, 97187 Luleå, Sweden 18 b 19 Dr. M.A Kazi Institute of Chemistry University of Sindh Jamshoro, 76080, Sindh Pakistan 20 c 21 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, 22 23 Bellaterra, 08193 Barcelona, Catalonia, Spain 24 d 25 Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, Sant Adrià del 26 Besòs, Barcelona 08930, Catalonia, Spain 27 e 28 Department of Physics, Chemistry and Biology, Linkoping University, 58183 Linkoping, 29 30 Sweden 31 f 32 ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Catalonia, Spain 33 34 * Corresponding authors: Zafar Hussain Ibupoto, Alberto Vomiero 35 Email: alberto.vomiero@ltu.se, zafar.ibupoto@ltu.se 36 37 38 39 Supporting information 40 41 Table S1. Elemental atomic composition for NiO/MoSx films obtained from XPS survey scans. 42 Mo S O Ni 43 NiO/MoSx 26.5% 34% 36.5% 3% 44 45 46 Table S2. Curve fitting parameters obtained for the S2p core level of NiO/MoSx films. A Shirley 47 48 background and mixed Gaussian/Lorentzian functions were used with the 2p3/2-2p1/2) kept at 1.2 49 eV and the intensity ratio at 2:1. 50 S2p3/2 (#1) Rel. Conc. (#1) S2p3/2 (#2) Rel. Conc. (#2) 51 NiOMoSx 161.5 eV 78% 163.1 eV 22% 52 53 54 Table S3. Curve fitting parameters obtained for the S2s and Mo3d core levels of, NiO/MoSx films. 55 A Shirley background and mixed Gaussian/Lorentzian functions were used with the 3d5/2-2d3/2) 56 57 kept at 3.13 eV and the intensity ratio at 3:2. 58 #1 #2 #3 59 Mo3d5/2 Rel. Conc. Mo3d5/2 Rel. 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