Theoretical expectation and experimental implementation of in-situ Al

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Theoretical expectation and experimental implementation of in-situ Aldoped CoS2 nanowires on dealloying-derived nanoporous intermetallic substrate as an efficient electrocatalyst for boosting hydrogen production Mei Wang, Wenjuan Zhang, Fangfang Zhang, Zhonghua Zhang, Bin Tang, Jinping Li, and Xiaoguang Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04502 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

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ACS Catalysis

Theoretical expectation and experimental implementation of in-situ Al-doped

CoS2

nanowires

on

dealloying-derived

nanoporous

intermetallic substrate as an efficient electrocatalyst for boosting hydrogen production Mei Wang,† Wenjuan Zhang,† Fangfang Zhang,† Zhonghua Zhang,*,§ Bin Tang,† Jinping Li,*,‡ Xiaoguang Wang*,†,‡

†

Laboratory of Advanced Materials and Energy Electrochemistry, Research Institute

of Surface Engineering, Taiyuan University of Technology, Taiyuan, 030024, China ‡

Shanxi Key Laboratory of Gas Energy Efficient and Clean Utilization, Taiyuan,

Shanxi, 030024, China §

School of Materials Science and Engineering, Shandong University, Jingshi Road

17923, Jinan 250061, China

1

ACS Paragon Plus Environment

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ABSTRACT Foreign atom doping is known to not only modify the electronic structure but also improve the intrinsic activity of catalysts. Herein, we fabricate three-dimensional (3D) self-supporting nanoporous cobalt-aluminum intermetallic (np-Co(Al)) through dealloying of Al90Co10 master alloy to in-situ introduce Al element into the precursor. The intrinsic Al dopant is turned out to be insensitive to S vapor, but being beneficial to generate numerous Al-doped CoS2 nanowires (Al-CoS2 NWs) when experienced solid-vapor sulfurization treatment. The density functional theory (DFT) calculations evidence that spontaneous Al-doping in CoS2 crystal could enhance the hybridization between Co d-orbital and S p-orbital near the Fermi level. On the one hand, the Co-sites are more active than that of the S-sites, and the intermediate hydrogen (H*) adsorption free energy (ΔGH*) of Al-CoS2 is only -0.16 eV when adsorbed on the bridge site of Co. On the other hand, the embedded Al can not only facilitate the activity improvement of Co-sites but also greatly activate the inert S-sites in CoS2. An overpotential as small as 86 and 191 mV to reach 10 and 100 mA cm-2 H2-evolving current, a small Tafel slope of 62.47 mV dec-1 as well as long-term operational stability are achieved on the Al-doped CoS2 catalyst. This finding opens up an easily accessible in-situ doping elements route via alloying-dealloying followed by inheritance to nanoporous earth-abundant chalcogenide electrocatalyst for regulating their physicochemical and electrochemical properties, accelerating the development of high performance H2-evolving electrode for electrochemical hydrogen production.

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KEYWORDS: Al-doping, de-alloying, solid-vapor sulfurization, hydrogen evolution reaction, electrocatalysis, DFT calculation

1. INTRODUCTION In the past few decade, the massive usage of traditional fossil fuels, such as coal and crude oil, has brought serious environmental pollution and global warming problems.1 Hydrogen, as a clean, efficient and sustainable energy carrier, is proposed as a promising candidate to replace traditional fossil fuels for the future energy supply.2,3 Among various strategies, electricity-driven water splitting is considered to be an ideal route to produce H2 due to several significant advantages such as abundance of water as raw material, no emission of greenhouse/poisonous gases as well as ultrahigh purity of products. However, the sluggish hydrogen evolution reaction (HER) and overpotential associated substantial energy loss require an efficient electrocatalyst to afford high current at low overpotential toward the HER.4,5 In general, precious metal Pt has been regarded as the best electrocatalyst to accelerate the kinetics in HER because hydrogen adsorption free energy (ΔGH*) on its surface is almost thermodynamic zero. Nevertheless, large-scale commercial applications are greatly impeded by its high cost and scarcity. Therefore, the exploration of low-cost and highly efficient non-noble metal based electrocatalysts with low overpotential and long-term operational stability is highly desirable yet key challenging for electrochemical hydrogen production. Through substantial efforts of ongoing research, many potential catalysts, such as 3


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transition metal phosphides,6-8 selenides,9,10 sulfides11-13 and nitrides14 have been widely put forward and investigated in detail. Among them, experimental synthesis especially via hydro-/solvo-thermal route was mainly conducted in the initial period of design and screening of electrocatalysts towards the HER. Afterward, people paid attention to precisely controlling or modulating the electronic structure and surface topography of materials, aiming to adjusting their energy band structure as well as optimizing the electronic conductivity and thermodynamic H* adsorption/ desorption state. On one hand, energy level engineering is generally employed via heterogeneous atomic doping, achieving favorable d-band center as well as plentiful of desirable active sites toward the HER, such as Ce-doped CoP,15 Mo-doped W18O49,16 Ni-doped CoS2,17 Ni-doped Mo2C,18 N-doped CoS2,19 N-doped WS2,20 P-doped CoS2,21 O-doped MoS2,22 etc. Of course, it is found that the synthesis of these reported catalysts mainly reckoned on common wet-chemically hydrothermal or solvothermal route, namely the “down-to-top” mode. Normally, the addition of another chemical reagent is conducted to realize elemental doping, for example, the nickel chloride hexahydrate for Ni-doping, ammonium hydroxide for N-doping, sodium phosphate for P-doping and so forth. On the other hand, it is also deemed that the geometric configuration of electrocatalyst can not only greatly influence the distribution and density of active sites but also affect the mass transport (i.e., reactant/product diffusion or convection) in the catalytic reaction process. To meet such a requirement, porous electrode with continuous open channels and high active areas is highly appreciated. To date, there are numerous reports on the construction of 4


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self-supporting catalytic electrodes for hydrogen evolution based on traditional foamy metals, such as commercial Ni and Cu foams. However, these traditional metal foams possess pore sizes of mostly hundreds of micrometers. Moreover, the pore wall or ligament merely comprises mono-component. Even if employing a prior surface elemental modification (i.e., sputtering23 and electro-plating24,25), their derivatives are still hard to guarantee the uniformity of decorative layer across the whole dimensions as well as available modulation on porosity range. De-alloying, a selective electro-/chemical etching process, is proven to be a powerful tool to fabricate nanoporous solid, such as metal, alloy and oxides.26-28 De-alloying derived porous metals or alloys not only depict a wide pore size ranging from several tens of nanometers to microns but also possess more uniform composition.29 More importantly, the residual heteroatoms in the as-evolved metal skeleton can be transferred genetically into the lattice of corresponding transition metal compound so as to modulate its inherent electronic structure and thermodynamics of intermediate H* ad/desorption. Al, a rich element in the earth's crust, is generally exploited as sacrificing constitute to construct de-alloying-derived nanoporous solid.30,31 Pristine CoS2, a typical pyrite-type transition metal dichalcogenide, is proven to be an efficient electrocatalyst toward HER. How does it occur if Al as cationic substitution to occupy a part of Co sites in the CoS2 crystal lattice? In the present study, the influence of Al doping on the crystal and electronic structure of CoS2 compound (Al-CoS2) was analyzed by density functional theory (DFT) calculation in detail. Due to the fact that 5


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hydrogen evolution activity is strongly correlated with the chemisorption energy of intermediate hydrogen onto the electrocatalyst surface,32 the hydrogen adsorption free energy on the {200} facet of Al-CoS2 was calculated to evaluate if Al-doping could reduce the ΔGH* so as to facilitate the H2 generation. The result reveals that the Al-doping plays a key role in stimulating the activity of both active Co-sites and inactive S-sites. Meanwhile, the intermediate H* tends to adsorb on the Co-bridge sites to release hydrogen easily. From Bader charge analysis, the significant electron transfer of Co, S and H also stems from the doped Al. In terms of experiments, the rigid Al-Co (Al9Co2) intermetallic nanoporous framework was obtained via selective etching α-Al phase through de-alloying of Al90Co10 (at.%) plate. After suffering a facile solid-vapor sulfurization toward the dealloying-derived Al-Co intermetallic sample, a self-supported Al-doped CoS2 electrode (Al-CoS2) was constructed. Different from the reports in wet chemical category, herein, we have proposed a nanoporous and hierarchical catalytic electrode, belonging to the typical “top-to-down” synthetic strategy. A dual phasic Al-Co (α-Al + Al9Co2) alloy precursor was designed with one part of Al (α-Al) as a sacrificial role to generate continuous micron-level channels and the other part Al to form intermetallic skeleton networks (Al9Co2). The Al in binary Al9Co2 intermetallic skeleton is insensitive to S vapor but inherited into as-formed CoS2 lattice (Al-CoS2), namely in-situ doping, aiming to optimize the inherent electronic structure and thermodynamics of intermediate H* ad/desorption. Depending on the in-situ Al-doping, highly-dense Al-CoS2 nanowires with superfine diameter were self-grown on the ligament surface 6


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of nanoporous structure, jointly offering the integrated catalyst a larger surface area, more sufficient catalytic sites33 and faster reaction kinetics. The as-evolved hierarchical Al-CoS2 catalyst not only requires overpotentials of only 86 mV and 191 mV (iR-corrected) in 0.5 M H2SO4 to deliver HER current densities of 10 and 100 mA cm-2, but also reveals a small Tafel slope of 62.47 mV dec-1 as well as good physicochemical stability, benefiting from the hybrid functionality between Al dopants, 3D micron-level ligaments/channels and in-situ grown Al-CoS2 nanowire structures. This work not only presents a deeper understanding on the correlation between anion species substitution and improved intrinsic HER catalytic properties for multiple-metal-based electrocatalyst but also offers new insights to better design earth-abundant nanoporous hierarchical HER catalysts with high efficiency and durability.

2. EXPERIMENTAL SECTION Fabrication of nanoporous precursor. Uniform Al90Co10 (at. %) alloy ingot was prepared by co-melting pure Al (99.99 wt. %) and pure Co (99.95 wt. %) in a quartz tube using high-frequency induction furnace, and then ladled into a graphite crucible (Φ 14 mm) under argon atmosphere. Using the wire-cutting machine, targeted alloy slices (Φ14mm×1mm) were got and further polished to a mirror finish. Then, the alloy slices were dealloyed in a 6.0 M NaOH aqueous solution for 40 min to remove dissolvable Al until no gas bubble evolved at ambient temperature (25°C). The as-dealloyed samples were rinsed repeatedly with ethanol and ultra-purified water, 7


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followed by drying at 60°C under vacuum overnight. Solid-vapor sulfurization treatment. The as-dealloyed np-Co(Al) slices were put into a ceramic boat, with ca. 1 g of sulfur powder (Chengdu Kelong Chemical Co. Ltd.) placed 2 cm away from the np-Co(Al) slices at the upstream side. Subsequently, the boat was put into a tube furnace, in which highly-purified Ar flow was continuously purged. The furnace was then heated to 400°C at the speed of 5°C min-1, and kept at this temperature for 1 h. Afterward, the chamber was cooled down to ambient temperature at 2°C min-1. The inert Ar flow was maintained throughout the whole process. As a reference, the pure Co slices were also sulfurized under a same solid-vapor mode. At last, the resultant specimens were washed with ethanol, ultrapure water, and dried in an Ar flow. Physicochemical characterization. X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (DX-2700) equipped with a Cu Kα radiation source (λ=0.154059 nm) at 40 kV and 30 mA. The microscopic morphology and chemical composition were characterized by scanning electron microscope (SEM, S-4800, Hitachi) and energy-dispersive X-ray spectroscopy (EDX). Transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) images were obtained on a JEM-2010F electron microscopy equipped with energy-dispersive X-ray spectroscopy (Quantax-STEM, Bruker). Nitrogen sorption isotherms were measured at 77 K using Micro for TriStar II Plus 2.02 and the specific surface area was calculated based on Brunauer-Emmett-Teller (BET) analysis. The pore size distribution was analyzed using the Barrett-Joyner-Halenda (BJH) method. 8


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X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Scientific K-Alpha X-ray Photoelectron Spectrometer using an Al Kα X-ray source. All of the binding energies were calibrated according to the reference energy of C1s (C1s=284.8eV).

Inductively

coupled

plasma

optical

emission

spectroscopy

(ICP-OES) analysis was conducted on Vista Axial CCD (Varian). And the concentration of element is averaged at least 3 replications. Electrochemical measurements. The electrochemical activities were tested by using CS-350 electrochemical workstation in a typical three-electrode cell equipped with a saturated calomel electrode (SCE) as reference electrode, a graphite rod as counter electrode and as-obtained Al-CoS2 as working electrode. The sulfurized Co slice, commercial CoS2 and commercial Pt/C (20%) were also evaluated as the reference. The preparation procedures of commercial CoS2 and Pt/C (20%) electrode follow our previous work.6,34 The electrocatalytic performance toward the HER was evaluated in acid solution (0.5 M H2SO4, pH=0). Before each electrochemical measurement, the electrolyte was deaerated by N2 bubbling for 30 min. The bubbling was then maintained throughout the electrocatalytic experiment. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 105-10-2 Hz with 10 mV sinusoidal perturbations. For linear sweep voltammetry (LSV) measurements, the scan rate was set at 10 mV s-1. The obtained LSV curves were corrected by the iRs loss compensation according to the following equation:35 Ecorr = Emea − iRs

(1)

Where Ecorr is the corrected potential, Emea is the measured potential and Rs is the 9


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equivalent series resistance determined by electrochemical impedance spectroscopy (EIS). Unless otherwise specified, all potentials are converted to reversible hydrogen electrode (RHE) according to the equation: ERHE = ESCE + 0.241 + 0.059 pH

(2)

DFT Calculations. All DFT calculations were conducted by using the Vienna ab initio simulation package (VASP) suite that is based on the projector augmented wave (PAW) approach and the plane wave basis set.36,37 The structures of Al-CoS2 and CoS2 were optimized using the Perdew-Burke-Ernzerhof38 to approximate the exchange-correlation effects, with all models fully relaxed in the self-consistent accuracy of 10-5 eV. For the plane-wave basis restriction, the cut-off energy was set as 450 eV, and the k-points were taken under Monkhorst-Pack for the Brillouin-zone integration. Band structure calculations were performed along the following paths, connecting the high-symmetry points: X (0.5, 0, 0.5), G (0, 0, 0), L (0.5, 0.5, 0.5), W (0.5, 0.25, 0.75) and K (0.375, 0.375, 0.75) in the k-space.39 For Al-doped CoS2, we used one Al atom to replace one Co atom. The activity toward hydrogen evolution reaction was reflected by the Gibb's free energy change (ΔGH*) and the values of ΔGH* were obtained from the formula below.32 (3)

𝛥𝐺𝐻 ∗ = 𝛥𝐸𝐻 ∗ + 𝛥𝐸𝑍𝑃𝐸 ―𝑇𝛥𝑆

Where 𝛥𝐸𝐻 ∗ is the hydrogen binding energy. 𝛥𝐸𝑍𝑃𝐸 is the zero-point energy change, which can be obtained from vibrational frequency calculation as implemented in VASP. 𝑇𝛥𝑆 is estimated to be 0.24 eV to consider the entropy change at room temperature. Hence, the formula (3) can be simplified as follow 10


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(4)

𝛥𝐺𝐻 ∗ = 𝛥𝐸𝐻 ∗ +0.24

3. RESULTS and DISCUSSION Theoretical analysis. For the convenience of calculation, the crystal structure models of pristine CoS2 and Al-CoS2 are hypothetically put forward, that is Co32S64 and Al25Co7S64, respectively (Figure S1). As illustrated in Figure 1 (a, b), the Al introduction leads to the replacement of Co atoms (atom radii, 1.26 Å) by Al atoms with larger atom radii (1.43 Å). On one hand, there is no conspicuous variation in the crystal structure when compared with the prototype. On the other hand, owing to the different ionic radius of guest and host metal ions,23 the bond distance of Co-S decreases from 2.307 Å in CoS2 (Figure 1c) to 2.206 Å in Al-CoS2 (Figure 1d) while the bond distance between Al and S increases to 2.509 Å. To gain fundamental understanding on the influence of foreign Al in-situ doping, we systematically explore the electronic structure and surface adsorption energy of H atoms on CoS2 with and without Al dopants via first-principle calculations. The electronic band structures of pristine CoS2 and Al-doped CoS2 are illustrated in Figure 2 (a, b). It indicates that the CoS2 depicts inherent metallic nature, which would favor the electron transfer process in catalysis, being same as that of previously reported.40 As for the Al-CoS2, it appears much closer band states near the Fermi level as compared to CoS2, attesting more intrinsic metallicity and higher conductivity. Moreover, it is in accord with the detailed partial density of states (PDOS) for CoS2 (Figure S2) and Al-CoS2 (Figure S3). Especially, the PDOS around the Fermi level 11


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for CoS2 is mainly donated by the S p-orbital, while the Co d-orbital for Al-CoS2 is getting more intensive than S p-orbital and Al d-orbital (Figure 2c). These results demonstrate that the embedded Al can enhance hybridization between Co d-orbital and S p-orbital,41 and thus significantly promote charge transfer in the material.14 It is generally recognized that the higher electron density at the Fermi level leads to higher conductivity,42 which is also consistent with the results of PDOS calculation herein. It is observed that the Fermi level of Al-CoS2 (1.8727 eV) is much higher than the value for CoS2 (1.2853 eV), revealing that the doping of Al evidently enhances the electron transfer.43 As shown in Figure 2d and Table S1, the electron densities at Co atoms and H atoms are both reduced in Al-doped CoS2, along with significant increase at the neighboring S atoms. A smaller change in Bader charge for Co atom is also observed, pointing to a lower intermediate adsorption energy at Co site.15 The Bader charge analysis further proves the assumption that the presence of Al dopant could change the electronic state of Co, S and H due to the electron transfer between the atoms. The HER process on the catalyst surface involves an initial H+ state, an intermediate H* state, and 1/2 H2 as the final product. Generally, the activity of a catalyst toward HER is governed by the Gibb’s free energy for H* adsorption step (ΔGH*).44,45 To ensure a fast proton/electron-transfer step coupled with the following hydrogen desorption process, the value of ΔGH* should be close to zero because either too negative or too positive value of ΔGH* reflects too strong or too weak binding of H,46 which are disadvantageous to HER kinetics. Herein, we have examined four possible sites on Al-CoS2 (marked in Figure 1d), that is Co-top site (S1), Co-hollow 12


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site (S2), Co-bridge site (S3) and S-top site (S4) for H adsorption. As a contrast, the ΔGH* at Co-top site (S1') and S-top site (S2') of CoS2 are also calculated (Figure 1c). Note that the ΔGH* on Co site of pristine CoS2 (S1') is deduced to be 0.44 eV (Figure 2e), which is in accordance with the recent reports.47-49 The large positive ΔGH* value represents a too easy desorption (or too weak adsorption) capability of H* on Co (S1') of pristine CoS2. With introducing Al dopants, the ΔGH* value at different Co site is determined to be -0.24 eV (S1), -0.19 eV (S2) and -0.16 eV (S3), respectively. It reveals the Al-doping in CoS2 can indeed significantly change the electronic structure of pristine CoS2 and thus generate a moderate H* desorption/adsorption trend on Co site. Specifically, the intermediate H* prefers to adsorb on the Co-bridge sites (S3) so as to release hydrogen more easily, thus boosting the HER activity of Co-sites. Meanwhile, according to Figure 2f, the ΔGH* at S sites of Al-CoS2 is calculated to be 0.45 eV, being more favorable than that of pristine CoS2 (1.31 eV). It is also noticeable that, there exists a large ΔGH* gap between Co site (0.44 eV) and S site (1.31 eV) on pristine CoS2. Due to the beneficial neutral ΔGH* rule, it manifests the intermediate H* adsorption/desorption preferably occurs on Co site relative to S site in the HER process, suggesting the S sites on CoS2 are originally inactive. By contrast, it is surprising that both Co site and S site are largely activated toward the HER in virtue of the fact that implanted Al atoms can effectively decrease not only the ΔGH* value but also the ΔGH* gap of neighboring Co and S sites on Al-CoS2. In a word, though Al atoms itself are not expected to work as active sites, but rather serve as an effective dopant to profitably adjust the electronic structure and hydrogen adsorption Gibbs 13


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free energy (ΔGH*) of CoS2 so as to facilitate the design of new class of highly active, low-cost and earth-abundant HER electrocatalyst. Microstructure analysis. Based on the first-principle calculation guidance, we first got nanoporous Co(Al) (np-Co(Al)) intermetallic solid via de-alloying route as the precursor. Afterward, by virtue of a simple solid-vapor sulfurization reaction, a three-dimensional (3D) self-supported nanoporous Al-CoS2 hierarchical material was acquired as rigid HER catalytic electrode. Optical images of alloy ingot, slice, post-dealloyed and sulfurized samples are shown in Figure S4. The master alloy slice, Al90Co10 (at. %), comprises Al9Co2 (JCPDS NO. 97-005-7598) and α-Al (JCPDS NO. 04-003-7059) phases (Figure S5a). After selective leaching α-Al at ambient temperature, most of the α-Al phase vanished but Al9Co2 intermetallic retained (Figure S5b). Figure S6 (a,b) shows the typical SEM images of Al90Co10 slice under a back scattered electron mode, in which two contrasted phases are also observed, as marked by A and B, respectively. EDX elemental mappings in Figure S6 (c,d) clearly indicate that the B region is a Co-containing phase while the continuous A region is mainly composed of Al. The EDX spectra in Figure S6 (e,f) demonstrate the atomic ratio of Al/Co in A region and B region is close to 1:0 and 9:2, being identical to the stoichiometric ratio of α-Al and Al9Co2, respectively. In combination with the above results, it is concluded that the A phase is α-Al and B phase is Al9Co2 in Al90Co10 alloy. Moreover, the α-Al phase exhibits a continuous contour throughout the entire slice. By contrast, the Al9Co2 phase shows a scattered topography with sizes ranging from several to dozens of 14


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microns. After selective leaching α-Al at ambient temperature, the resultant porous Al9Co2 intermetallic depicts the characteristic size of ligament at ca. 10-20 microns (Figure S7(a,b)). It is noteworthy that the consecutive channel and dispersive solid skeleton match well with the initial dual phasic pattern of master Al90Co10 (Figure S6). Thus, it further verifies that the dealloying of continuous α-Al contributes to the formation of nanoporous Co(Al) structure. Observed from Figure S7 (c,d), both Al and Co elements roughly exhibit a homogeneous distribution throughout the ligament surface of np-Co(Al). And the corresponding EDX spectrum (Figure S7e) further confirms the co-existence of Al and Co with atom ratio of 9:2, in agreement with the above-mentioned phase analysis. As shown in Figure 3a, powder X-ray diffraction (XRD) revealed that the main diffraction lines of as-sulfurized np-Co(Al) sample matches well with that of CoS2 (JCPDS NO. 97-005-3068). However, a slightly positive shift of the main diffraction peaks is observed relative to the standard lines, due to the incorporation of Al atoms into the lattice of CoS2 by replacing partial of Co atoms. Except for the active Al-CoS2 as the main phase, a tiny of buried Al9Co2 as minor phase could also be monitored within the XRD detection range because it is buried in the in-depth/core part and surrounded with the Al-CoS2 matrix. Differing from the micron-level porous precursor, the as-sulfurized pure Co slice contains a mixture of Co3S4 (JCPDS NO. 97-003-1095) and Co9S8 phases (JCPDS NO. 00-019-0364) (Figure 3b). The formation of sulfur-rich Al-CoS2 phase on the porous precursor, on the one hand, should benefit from the widespread continuous channels which facilitates sufficient 15


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exposure of micron-level ligament to sulfur vapors, allowing more S bonded with Co. On the other hand, although Al cannot bond with S, the existence of Al widens the pristine Co-Co bond, increasing the mutual reaction tendency between Co and S. As observed in Figure 4a, the as-obtained Al-CoS2 still maintains its integrated porous feature and the size of ligament slightly increases to 20-30 μm after sulfurization. When zoomed in the surface section of as-sulfurized ligament, quantities of vertically aligned hair-like Al-CoS2 nanowires (NWs) were uniformly self-grown on the surface of ligament (Figure 4b). From the magnified view, the diameter of Al-CoS2 NWs is located at the range of 2-5 nm whereas the length can attain to several microns, and the area density of NWs is found to be as large as ca. 1000 roots per μm2 (Figure 4c). Such hierarchical topography comprising both continuous 3D micron-level ligament-channel network and high-density hair-like NWs on ligament surface, not only can provide more active sites for the adsorption of intermediate H* but also is good for releasing the detached bubbles during HER process, thus effectively reducing the overpotential so as to enhance the HER performance. The corresponding EDX elemental mapping (Figure 4d) verifies the uniform elemental distribution of Al, Co and S on the surface of Al-CoS2. The TEM image of one single Al-CoS2 NW further confirms its diameter of ca. 5 nm (Figure 4e). Apparently, the well-resolved lattice fringes are observed in the HRTEM image (Figure 4f). And the distances between two adjacent fringes is measured to be 2.72 Å, slightly smaller than that of the facet (200) of CoS2 (2.76 Å) due to the Al-doping. The specific surface area and pore size distribution of samples are also assessed by N2 16


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adsorption/desorption measurements. As displayed in Figure S8 (a,c), the adsorption/desorption isotherms correspond well to the type IV isotherm with a H3-type hysteresis loop, indicating the mesoporous structures of the samples. The Brunauer-Emmett-Teller (BET) surface areas of Al-CoS2 and CoS2 are 4.65 and 2.57 m2 g-1, respectively. The larger BET surface area of Al-CoS2 verifies that the micron-level ligament/channel topography as well as self-supporting high-density hair-like NWs should be beneficial for increasing the surface areas of rigid electrode. Figure

S8

(b,d)

present

the

pore-size

distribution

curves

using

the

Barrett-Joyner-Halenda (BJH) method. It suggests that the average pore sizes are in the range of 2-10 nm, confirming the mesoporous nature of Al-CoS2 and CoS2. By contrast, when employing the same solid-vapor sulfurization toward pure Co slice, the surface of as-synthesized Co3S4&Co9S8 catalyst is found to be covered with two kinds of particles (Figure S9). The one exhibits larger diameters of ca. 3-5 μm (Figure S9a) and sparse NWs/nano-ribbon (NR) forest was also self-grown on the surface (Figure S9b). The other one exhibits a closely-packed pattern with a mean size as small as ca. 100 nm (Figure S9c), being widespread under the embedded larger ones. Similar to that of 3D Al-CoS2, the uniform elemental distribution of Co and S is also depicted on the Co3S4&Co9S8 surface (Figure S9d-f). It is worth noting that the widespread Al dopants together with large curvature of micron-level ligament can be conducive to the highly-dense growth of NWs. According to previous reports,50-53 the growth of NWs in a vapor-solid growth mode can be accelerated by the density of defects on the substrate, serving as the nucleation sites. In the present case, the 17


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widespread defects stemming from dealloying-derived Al-doping inside micron-level porous substrate can serve as the nucleation sites for NW growth, which play an important role in lowering the energy barrier of NWs growth. Owing to quantities of nuclei on the micron-level ligament surface, a self-sacrifice growth of high density Al-CoS2 nanowires take places via consuming more sulfur vapors. XPS measurements are conducted to analyze the superficial chemical state of Al-CoS2 and CoS2 samples. Comparison of full survey spectrum at the binding energy range of 60～80 eV illustrates the existence of Al 2p

54

for Al-CoS2 and the absence

of Al 2p for CoS2 (Figure 5a). Besides, the typical peaks of Co, S, O and C can also be observed in Al-CoS2 and CoS2. Among them, C and O should be ascribed to the carbon contamination and superficial oxidation owing to air contact.7 For the high resolution scan, the peak at 74.1 eV in Al 2p region belongs to Al (III) species (Figure 5b).55 In the Co 2p spectrum (Figure 5c), the peaks at 778.7 and 793.7 eV are assigned to Co 2p3/2 and Co 2p1/2, respectively, being match well with previous reports on Coδ+-S materials.56,57 The total ratio of Coδ+ for Al-CoS2 (31.9 %) is larger than that for CoS2 (18.6 %), being consistent with above theoretical results of the electron transfer from Al to Co (Table S2). Furthermore, the peaks at 781.6 and 797.6 eV reflect the presence of more than one chemical state of Co element in Al-CoS2, such as a minor amount of Al2CoO4 because of the surface oxidation.58 With regards to the S region (Figure 5d), the signals at 162.6 and 163.8 eV are attributed to the binding energies (BEs) of S 2p3/2 and S 2p1/2 for CoS2, respectively, which belongs to the typical metal-sulfur bonds.59 And the peak at 168.7 eV should represent S-O bond 18


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ACS Catalysis

arising from surface air exposure.60 It’s surprising that the S 2p peaks of Al-CoS2 are all red shifted to 163.0, 164.1 and 169.1 eV, which are similar to that of Co 2p peaks. It demonstrates that there has been an increasing of the electronic interaction between Co and S in Al-CoS2. Thus, combining the above analysis, it verifies the widespread existence of interactive neighboring chemical bond and electron transfer between Al, Co and S, reflecting the successful formation of Al-CoS2 hybrid electrocatalyst. Electrocatalytic performance analysis. By virtue of linear sweep voltammetry (LSV), the electrochemical activity of self-supported nanoporous Al-CoS2 electrode was evaluated in 0.5 M H2SO4 using a three electrode set-up with a scan rate of 10 mV s-1 (Figure 6a). Under the same condition, the electrocatalytic performances of non-sulfurized np-Co(Al), Co3S4&Co9S8 and commercial Pt/C (20%) were also examined as the reference. As expected, the state-of-the-art Pt/C electrocatalyst exhibits excellent activity with negligible overpotential toward HER while the non-sulfurized np-Co(Al) has poor performance with a significant onset potential of 104 mV, yielding a current density of 1 mA cm-2. It is worth noting that the as-obtained Al-CoS2 and Co3S4&Co9S8 catalysts give onset overpotentials of merely 37 mV and 72 mV, respectively. Furthermore, the Al-CoS2 affords cathodic current densities of 10, 50 and 100 mA cm-2 at overpotentials of 86 mV (η10), 148 mV (η50) and 191 mV (η100), respectively, outperforming those of Co3S4&Co9S8 (η10=139 mV, η50=279 mV and η100=363 mV) and non-sulfurized np-Co(Al) (η10=239 mV, η50=370 mV and η100=445 mV) (Table S3). Meanwhile, the as-revealed catalytic performance of Al-CoS2 is even better than many of other previously-reported multiple component 19


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catalysts, such as MoS2/CoS2/CC (η10=87 mV, η100=261 mV),61 Ni2.3%-CoS2/CC (η10=181 mV, η100=240 mV),62 Zn-Co-S/TM (η10=188 mV, η100=382 mV),63 etc. (Table S4). The corresponding Tafel plots according to the Tafel equation (𝜂 = 𝑎 +b ∙ log 𝑗, where 𝜂 is the overpotential after 𝑖𝑅 correction, 𝑎 is the Tafel constant, 𝑏 is the Tafel slope, and j is the current density) are shown in Figure 6b. In principle, a lower Tafel slope indicates a certain increase of applied overpotentials triggers a more rapid increase of cathodic HER current.64 Herein, Al-CoS2 exhibits a Tafel slope as small as 62.47 mV dec-1, which is lower than the other tested catalysts except for Pt/C (31.81 mV dec-1), manifesting a faster HER kinetics on Al-CoS2. According to the classic theory on the mechanism of hydrogen evolution, three principle steps for converting H+ to H2 have been proposed for the HER in acidic media.65 The first step is commonly discharge step (Equation (5)), which is followed by either an electrochemical desorption step (Equation (6)) or a recombination step (Equation (7)) to give H2. Discharge step (Volmer reaction): H3O+ + e- → Hads + H2O, b = 2.303RT/αF≈120 mV dec-1

(5)

Desorption step (Heyrovsky reaction): Hads + H3O+ + e- → H2 + H2O, b = 2.303RT/(1+α)F≈40 mV dec-1

(6)

Recombination step (Tafel reaction): Hads + Hads → H2, b = 2.303RT/2F≈30 mV dec-1

(7)

Due to the fact that the Al-CoS2 yields a Tafel slope of 62.47 mV dec-1, the HER 20


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process on the Al-CoS2 should follow a Volmer-Heyrovsky mechanism. Besides, the exchange current density (j0) is also a kinetic parameter to assess the intrinsic electron transfer rate on the catalyst surface. By extrapolating the Tafel plot to zero overpotential, the j0 value of Al-CoS2 is calculated to be 0.409 mA cm-2, being not only higher than the reference Co3S4&Co9S8 (~0.132 mA cm-2) but also superior to many of other previous reports such as CoS2 (~3.53×10-3 mA cm-2),66 (Fe0.48Co0.52)S2 (~9.59×10-4 mA cm-2),66 Cu-0-CoS2 (~4×10-2 mA cm-2),47 etc (Table S4). Turnover frequency (TOF) is another important factor to evaluate the intrinsic activity toward the HER, namely the number of H2 molecules evolved per second per active site (Supplementary Note 1, Supporting Information). Figure 6c shows the dependence of TOF values of Al-CoS2 electrode on applied overpotentials and the mutual comparison with the other widely-reported non-noble HER catalysts. To reach the η value of 100, 150 and 200 mV, the TOFs of Al-CoS2 are determined to be 0.048, 0.164 and 0.354 s-1, respectively. It demonstrates that the Al-CoS2 is indeed superior to most of the other HER catalysts including CoS2/RGO (0.16 s-1 at η of 200 mV),67 MoP (0.024 s-1 at η of 100 mV),68 Ni-Co2P/NCNTs (0.1 s-1 at η of 171 mV),69 Cu-Co2P/NCNTs (0.1 s-1 at η of 192 mV), hH-MoS2 (0.11 s-1 at η of 200 mV),70 NiCo2Px (0.021 s-1 at η of 100 mV)71 and so forth. Moreover, the Al-CoS2 at the thermodynamic zero potential can also generate a TOF value of 1.29×10-3 s-1, being close to the MoP|S (1.21×10-3 s-1)68 and MoSx@Mo2C-1:24 (1.04×10-3 s-1).72 Figure 6d represents the Nyquist plots of Al-CoS2, Co3S4&Co9S8 and np-Co(Al) at an overpotential of -100 mV in the frequency range of 105-10-2 Hz. After fitting with 21


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the equivalent circuit model (Inset of Figure 6d), the parameters are summarized in Table S5. It is known that the Rs represents the ohmic resistance arising from electrolyte and all contact, and the Rct depicts the charge transfer resistance for HER at the electrode/electrolyte interface. A smaller Rct value indicates much faster charge transfer kinetics.73 It is obvious that, the Rs value of Al-CoS2 is merely ca. 0.223 Ω, meaning a smaller ohmic loss or better electrical conductivity of the electrode, which is consistent with the theoretical calculations. Moreover, the Rct of Al-CoS2 (0.781 Ω) is also lower than that of Co3S4&Co9S8 (ca. 7.424 Ω) and np-Co(Al) (ca. 29.822 Ω). The EIS result undoubtedly verifies a faster charge transport rate on the abundant active edge sites of Al-CoS2, being in line with the sequence of the polarization curves and Tafel slopes. To further explore the HER kinetics, EIS investigations on Al-CoS2 are carried out at different applied overpotentials. As illustrated by the Nyquist plots in Figure S10a, the Rct is as large as 60.489 Ω at an overpotential of 0 mV while its value sharply decreases to 10.197, 0.781 and 0.198 Ω at -50, -100 and -150 mV, respectively (Figure S10b and Table S6). It indicates that the HER charge transfer kinetics can be significantly accelerated upon the increasing of overpotentials. Here, the Tafel slope can also be derived from the plot of overpotential versus log (1/Rct) (inset in Figure S10b), which is 60.36 mV dec-1, slightly smaller than that extracted from the polarization curve (62.47 mV dec-1). It can more accurately reflect the charge transfer kinetics due to the elimination of catalyst resistance. In order to evaluate electrochemical active surface area (ECSA) of the electrode, electrochemical double-layer capacitances (Cdl) are first analyzed by successive CVs 22


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at different scan rates (Figure 6e).56,74 Through plotting △j ( △j = janode －jcathode) at 0.15 V against the scan rates, the Cdl for Al-CoS2 is determined to be 48 mF cm-2 (Figure 6f), which is larger than that of Co3S4&Co9S8 (16 mF cm-2, Figure S11) as well as other reported counterparts such as Cu3P-CoP/CC (18.2 mF cm-2),75 NiCoP/NF (4.9 mF cm-2)76 and MoP@RGO (15.3 mF cm-2).77 Afterward, the ECSA can be estimated from the value of Cdl according to the following formula:78 𝐸𝐶𝑆𝐴 =

C𝑑𝑙 C𝑠

C𝑑𝑙

(8)

= 40 μF cm ―2 per cm2

𝐸𝐶𝑆𝐴

where Cs is the specific capacitance of a flat surface that could be in the range of 20-60 μF cm-2, and 40 μF cm-2 is chosen as a moderate value in this study as previous literatures did.79 Herein, the ECSA of Al-CoS2 is calculated to be 1200 cm2, which is the largest value when compared with the other advanced catalysts, such as NiCo2S4 (126.9 cm2),80 CoSe2 NP/CP (352.5 cm2)81 and MoS2@MoP (1040 cm2).82 And the Al-CoS2 also possesses a superior HER performance with the overpotential (η10) of only 86 mV, far less than those of NiCo2S4 (η10=247mV), CoSe2 NP/CP (η10=137mV) and MoS2@MoP (η10=108mV). These results manifest that the Al-CoS2 acquires a remarkable ECSA to facilitate the HER catalytic process on its surface owing to the co-operation of bi-continuous micron-level ligament/channel as well as highly dense hair-like NWs on the ligament network. The electrochemical stability is another key criterion for an advanced electrocatalyst. The stability of Al-CoS2 toward the HER was probed by accelerated degradation test (ADT) for 5000 cycles at a potential scan rate of 50 mV s-1. Figure 6g shows that the polarization curves before and after ADT test depict no obvious 23


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change, verifying that the Al-CoS2 is a stable HER catalyst. Meanwhile, from a practical point of view, superior durability under a constant current density/potential is more meaningful. According to the η-t curve, it is found that the performance of Al-CoS2 possesses a negligible degradation even after continuous HER operation at a constant current of 10 mA cm-2 for 12 hours. Meanwhile, the j-t profile under a static overpotential of -100 mV (Figure S12a) verifies the Al-CoS2 electrode maintains its current density at around 10 mA cm-2 over 12h. Upon increasing the overpotential from -100 to -200 mV in chronoamperometric test (Figure S12b), the current density of Al-CoS2 electrode increases accordingly but rapidly gets stabilized. When the applied overpotential comes back to -150 and -100 mV, the current density is still well restored. Additionally, a certain amount of electrolyte was sampled after 12 h and 24 h of galvanostatic electrolysis at 10 mA cm-2. As illustrated in Table S7, the ICP analysis verifies the dissolution rates of Al and Co element after 24 h of durability test in 0.5 M H2SO4 are merely 1.04 % and 0.44 %, respectively. Then, the dissolution rate diminishes gradually with the increase of electrolysis duration. On the one hand, it evidences the fact that most of the Al inherited from Al9Co2 were successfully doped into the as-formed CoS2 lattice rather than maintenance in the form of easily-corrosive metallic species. On the other hand, these results unanimously demonstrate that the Al-CoS2 electrode possesses preferable stability and durability toward the acid-type HER. As illustrated in Figure S13, the morphologies and mean compositions of Al-CoS2 before and after extended stability and durability tests are also unchanged, 24


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manifesting that the overall nanoporous hierarchical Al-CoS2 framework is highly acid-stable. To further assess the superficial chemical state of post-tested Al-CoS2, XPS measurement is also employed (Figure S14). As compared to the spectra of pristine Al-CoS2, the peaks at 778.3 and 793.4 eV in the Co 2p spectrum and the peaks at 162.7 and 164.0 eV in the S 2p spectrum all slightly shift to negative direction, indicating the binding energy of CoS2 becomes a bit smaller. It evidences that the electronic interaction between Co and S should be slightly weakened after electrochemical HER, during which a trace of active material may be dissolved in the acid electrolyte, finally leading to a slight attenuation of HER performance along solid/electrolyte interface. However, the Al 2p spectrum is relatively flat, reflecting the superficial Al2CoO4 could be dissolved from the superficial surface of Al-CoS2 after the long-term durability test. Furthermore, we have also examined the total composition of Al-CoS2 NWs before and after long-term HER test. Firstly, enough amount of in-situ grown Al-CoS2 NWs have been gathered from the surface of pristine and post-HER tested electrode, respectively. Through thoroughly dissolved in harsh HNO3 (20 mL), the ICP analysis was employed (Table S8). The difference of the detected Al content between the as-prepared pristine and post-HER Al-CoS2 should derive from the dissolution of easily-corrosive metallic species during the long-term HER process in 0.5 M H2SO4 solution. It proves that about 95.1% of Al in Al9Co2 intermetallic has been in-situ doped into CoS2 lattice whereas only a tiny of Al (ca. 4.9%) may not dope into CoS2 but form the interrelated easily-corrosive species (i.e., Al2CoO4) on the surface. 25


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Obviously, the amount of actual Al-doping is consistent with the Al25Co7S64 structure model. Although the surface oxidized Al(III) species coupled with superficial Al and Co sites suffered a leaching during the HER process, it cannot degrade the activity of catalyst because most of the Al sites are still embedded in robust CoS2 lattice to modulate its inherent electronic structure and thermodynamics of intermediate H* ad/desorption. The Faraday efficiency (FE) is also monitored during potentiostatic electrolysis at an overpotential of 200 mV in 0.5 M H2SO4. It reveals that the amount of experimentally quantified H2 gas is in accordance with the theoretical value calculated on the basis of the charge transferred, representing a high Faraday efficiency close to 100% on Al-CoS2 electrode (Figure S15). Figure 6h presents the volcano plots of log i0 (exchange current density) as a function of ΔGH* for Al-CoS2 (red pentagram), pure CoS2 (blue triangle) and some of the state-of-the-art HER catalysts (black rhombuses) cited from the previously reported literatures.32 It is observed that the position of Al-CoS2 is much closer to the volcano peak than those of pure CoS2 and the other non-noble metals, further verifying that the Al-doped CoS2 electrode has excellent HER performance and can employ as an efficient 3D self-supported cathode for HER in acidic media. To confirm the Co-sites and S-sites are both activated as theoretically expected on Al-CoS2 crystal, we further identify the contributing role of sulfur and cobalt sites on both Al-CoS2 and commercial CoS2. It is well known that the thiocyanate ion (SCN-) is prone to form stable coordinated complex with metal ion in acidic media and thus 26


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ACS Catalysis

block the metal-sites, while the dodecanethiol can adsorb on the surface of catalyst to stimulate the dissociation of metal-S bonds and effectively suppress the S-sites.83,84 The polarization curves from commercial CoS2 before and after poisoning its Co-sites and S-sites are presented in Figure 7a. As observed, the HER performance was greatly degraded when blocking the Co-sites (having an increase of ca. 105 mV for η100). To the contrary, it almost cannot impair the HER performance when fully poisoning the S-sites (Figure 7a and c). It elucidates that the addition of SCN- rather than dodecanethiol could alter the polarization curve (toward deteriorative side) when compared with the situation without adding SCN- or dodecanethiol. In other words, the pure CoS2 lattice comprises the catalytically active Co sites and catalytically inert S sites. As shown in Figure 7b, a distinct situation is presented for the Al-CoS2 after blocking its Co-sites and S-sites, respectively. Once 10 mM NaSCN is added to block the Co-sites, the value of η100 significantly increases from 191 mV to 406 mV. Similarly, the η100 increases from 191 mV to 310 mV when the S-sites are poisoned by dodecanethiol. It means that either SCN- or dodecanethiol can lead to the variation of polarization curves (toward deteriorative side) when compared with non-poisoned Al-CoS2. That is Co-sites and S-sites are both catalytically active toward the HER on Al-CoS2 surface. Combining the DFT expectation and experimental implementation, we would expect that this kind of novel Al-doped micron-level ligament/channel and self-supported hierarchical CoS2 catalyst (Al-CoS2) not only acquires optimized electronic state and hydrogen adsorption capability but also possesses a large amount of active sites and mass transport channels, and could be a promising choice for 27


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applications in electrochemical water splitting toward commercial hydrogen production.

4. CONCLUSIONS In summary, we have proposed and validated a class of highly active Al-doped cobalt sulfide catalyst for boosting electrochemical hydrogen evolution reaction. DFT calculations clearly manifest that the incorporation of Al into the CoS2 lattice results in obvious changes of charge distribution on the catalyst, finally achieving the decrease of hydrogen adsorption Gibbs free energy (ΔGH*). Meanwhile, Al-doping can improve the activity of CoS2 by activating the Co-sites, as well as stimulating the inert S-sites. Experimental test further verifies the resultant Al-CoS2 electrode holds superior HER performance (i.e., requiring overpotentials of only 86 mV and 191 mV to deliver 10 and 100 mA cm-2 H2-evolving current, a small Tafel slope of 62.47 mV dec-1 as well as long-term stability and durability) relative to a number of previously reported non-noble catalysts in acidic electrolyte. There is no doubt that the large electrochemical active surface area of Al-CoS2 owing to both typical micron-level ligament/channel and hierarchical self-supporting configuration could shorten the ion diffusion paths in electrolyte so as to accelerate the charge transfer toward the HER. From the perspective of electrode assembly, in-situ architecture of Al-CoS2 nanowires further avoids the detachment of active material, being favorable for the physical and electrochemical stability. In a word, this study opens up a new field via in situ incorporation of metal dopants (alloying-dealloying) for hierarchical chalcogenide 28


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compound electrocatalyst toward electrochemical hydrogen generation.

ASSOCIATED CONTENT Supporting Information Additional DFT results, optical photographs, XRD patterns, SEM images, cyclic voltammograms, N2 adsorption-desorption isotherm, EIS, plots of capacitive current versus scan rate, j-t profiles, EDX spectra, XPS spectra, plots of Faraday efficiency, details of TOF calculation and Tables S1-S8.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected]. cn (X.W.). *E-mail: [email protected] (J.L.). *E-mail: [email protected] (Z.Z) ORCID Xiaoguang Wang: 0000-0002-0141-0701 ORCID Jinping Li: 0000-0002-2628-0376 ORCID Zhonghua Zhang: 0000-0002-2883-4459

Notes The authors declare no competing financial interest.

29


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ACKNOWLEDGEMENTS This work is financially supported by National Natural Science Foundation of China

(21878201),

Natural

Science

Foundation

of

Shanxi

Province

(201801D121059), Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi “OIT”, Special/Youth Foundation of Taiyuan University of Technology (1205-04020203/ SC18100330), Program for Shanxi Key Laboratory of Gas Energy Efficient and Clean Utilization (201705D111002), and Research Project Supported by Shanxi Scholarship Council of China (2017-034).

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REFERENCES (1) Das, D.; Nanda, K. K. One-Step, Integrated Fabrication of Co2P Nanoparticles Encapsulated N, P Dual-doped CNTs for Highly Advanced Total Water Splitting. Nano Energy 2016, 30, 303-311. (2) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of Electrocatalysts for Oxygen- and Hydrogen-Involving Energy Conversion Reactions. Chem. Soc. Rev. 2015, 44, 2060-2086. (3) Turner, J. A. Sustainable Hydrogen Production. Science 2004, 305, 972-974. (4) Li, M.; Liu, X.; Xiong, Y.; Bo, X.; Zhang, Y.; Han, C.; Guo, L. Facile Synthesis of Various Highly Dispersive CoP Nanocrystal Embedded Carbon Matrices as Efficient Electrocatalysts for the Hydrogen Evolution Reaction. J. Mater. Chem. A 2015, 3, 4255-4265. (5) Zheng, Y.; Jiao, Y.; Jaroniec, M.; Qiao, S. Z. Advancing the Electrochemistry of the Hydrogen-Evolution Reaction through Combining Experiment and Theory. Angew. Chem. Int. Ed. 2015, 54, 52-65. (6) Wang, X.; Kolen'ko, Y. V.; Bao, X. Q.; Kovnir, K.; Liu, L. One-Step Synthesis of Self-Supported Nickel Phosphide Nanosheet Array Cathodes for Efficient Electrocatalytic Hydrogen Generation. Angew. Chem. Int. Ed. 2015, 54, 8188-8192. (7) Wu, T.; Pi, M.; Zhang, D.; Chen, S. 3D Structured Porous CoP3 Nanoneedle Arrays as an Efficient Bifunctional Electrocatalyst for the Evolution Reaction of Hydrogen and Oxygen. J. Mater. Chem. A 2016, 4, 14539-14544. (8) Tang, C.; Gan, L.; Zhang, R.; Lu, W.; Jiang, X.; Asiri, A. M.; Sun, X.; Wang, J.; Chen, L. Ternary FexCo1-xP Nanowire Array as a Robust Hydrogen Evolution Reaction Electrocatalyst with Pt-like Activity: Experimental and Theoretical Insight. Nano Lett. 2016, 16, 6617-6621. (9) Xu, R.; Wu, R.; Shi, Y.; Zhang, J.; Zhang, B. Ni3Se2 Nanoforest/Ni foam as a Hydrophilic, Metallic, and Self-Supported Bifunctional Electrocatalyst for Both H2 and O2 Generations. Nano Energy 2016, 24, 103-110. (10) Liu, T.; Liu, Q.; Asiri, A. M.; Luo, Y.; Sun, X. An Amorphous CoSe Film 31


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Behaves as an Active and Stable Full Water-Splitting Electrocatalyst under Strongly Alkaline Conditions. Chem. Commun. 2015, 51, 16683-16686. (11) Hu, C.; Zhang, L.; Zhao, Z. J.; Li, A.; Chang, X.; Gong, J. Synergism of Geometric

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Se-(NiCo)Sx/(OH)x

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Nonprecious

Intermetallic

Al7Cu4Ni

Nanocrystals

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Seamlessly

Integrated

in

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Splitting. Nano Lett. 2016, 16, 7718-7725. (77) Zhang, G.; Wang, G.; Liu, Y.; Liu, H.; Qu, J.; Li, J. Highly Active and Stable Catalysts of Phytic Acid-Derivative Transition Metal Phosphides for Full Water Splitting. J. Am. Chem. Soc. 2016, 138, 14686-14693. (78) Chen, Y. Y.; Zhang, Y.; Jiang, W. J.; Zhang, X.; Dai, Z.; Wan, L. J.; Hu, J. S. Pomegranate-Like

N,P-Doped

Mo2C@C

Nanospheres

as

Highly

Active

Electrocatalysts for Alkaline Hydrogen Evolution. ACS Nano 2016, 10, 8851-8860. (79) Wang, X.; Xu, Y.; Rao, H.; Xu, W.; Chen, H.; Zhang, W.; Kuang, D.; Su, C. Novel Porous Molybdenum Tungsten Phosphide Hybrid Nanosheets on Carbon Cloth for Efficient Hydrogen Evolution. Energy Environ. Sci. 2016, 9, 1468-1475. (80) Sheng, G.; Chen, J.; Li, Y.; Ye, H.; Hu, Z.; Fu, X. Z.; Sun, R.; Huang, W.; Wong, C. P. Flowerlike NiCo2S4 Hollow Sub-Microspheres with Mesoporous Nanoshells Support Pd Nanoparticles for Enhanced Hydrogen Evolution Reaction Electrocatalysis in Both Acidic and Alkaline Conditions. ACS Appl. Mater. Interfaces 2018, 10, 22248-22256. (81) Kong, D.; Wang, H.; Lu, Z.; Cui, Y. CoSe2 Nanoparticles Grown on Carbon Fiber Paper: An Efficient and Stable Electrocatalyst for Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2014, 136, 4897-4900. (82) Wu, A.; Tian, C.; Yan, H.; Jiao, Y.; Yan, Q.; Yang, G.; Fu, H. Hierarchical MoS2@MoP Core-Shell Heterojunction Electrocatalysts for Efficient Hydrogen Evolution Reaction over a Broad pH Range. Nanoscale 2016, 8, 11052-11059. (83) Yin, J.; Fan, Q.; Li, Y.; Cheng, F.; Zhou, P.; Xi, P.; Sun, S. Ni-C-N Nanosheets as Catalyst for Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2016, 138, 14546-14549. (84) Makarova, M.; Okawa, Y.; Aono, M. Selective Adsorption of Thiol Molecules at Sulfur Vacancies on MoS2(0001), Followed by Vacancy Repair via S-C Dissociation. J. Phys. Chem. C 2012, 116, 22411-22416.

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Figure captions:

Figure 1 Schematic crystal structure of (a) CoS2 and (b) Al-CoS2. (c) Atomic bond length of Co-S in CoS2. (d) Atomic bond length of Co-S and Al-S in Al-CoS2. Co: blue, S: yellow, Al: purple.

Figure 2 The electronic band structures of (a) CoS2 and (b) Al-CoS2. (c) The PDOS plots for CoS2 and Al-CoS2: Co d-orbital (black line), S p-orbital (red line) and Al d-orbital (blue line). (d) The calculated Bader charges of Co (green bar), S (orange bar) and H (brown bar) atoms for CoS2 and Al-CoS2, respectively. The hydrogen adsorption Gibb’s free-energy diagram of CoS2 and Al-CoS2 on the (e) Co-sites and (f) S-sites.

Figure 3 XRD patterns of (a) Al-CoS2 and (b) Co3S4&Co9S8 catalysts.

Figure 4 (a) Low- and (b, c) high-magnification SEM images of Al-CoS2. (d) The corresponding EDX elemental mapping of Al, Co and S. (e) TEM and (f) HRTEM images taken from a single Al-CoS2 nanowire.

Figure 5 XPS spectra of Al-CoS2 and CoS2 electrodes. (a) Survey scan, (b) Al 2p, (c) Co 2p, (d) S 2p.

Figure 6 (a) LSV curves of Al-CoS2, Co3S4&Co9S8, np-Co(Al) and Pt/C catalysts in 0.5 M H2SO4 at a scan rate of 10 mV s-1. (b) Tafel plots. (c) TOF values of Al-CoS2 (red line) along with the other recently reported catalysts. (d) Nyquist plots for Al-CoS2, Co3S4&Co9S8 and np-Co(Al) catalysts measured at an overpotential of -100 mV (The inset is the corresponding equivalent circuit for fitting). Scatters: experimental data; line: fitting curves. (e) CV curves showing the capacitive behaviors of electrochemical double layer of Al-CoS2. (f) The capacitive currents at 40


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ACS Catalysis

the middle of potential window as a function of scan rate. (g) LSV curves of Al-CoS2 measured before and after 5000 ADT continuous cycles, and the η-t profile recorded at a constant cathodic current of 10 mA cm-2 for 12 h. (h) Volcano plots of log i0 as a function of ΔGH* for CoS2 (blue triangle), Al-CoS2 (red pentagram) and some of common non-noble metal catalysts (black rhombuses).

Figure 7 LSV curves obtained for (a) commercial CoS2 and (b) Al-CoS2 before and after poisoning their Co-sites and S-sites, respectively. The overpotential values (η100) comparison for the HER between on (c) CoS2 and (d) Al-CoS2 before and after blocking their Co-sites and S-sites, respectively.

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Figures:

Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Table of Content:

Through theoretical expectation and experimental implementation, in-situ Al-doped CoS2 nanowires grown on dealloying-derived nanoporous intermetallic substrate acts as efficient catalyst for boosting electrochemical hydrogen production.

TOC Figure

49


Theoretical expectation and experimental implementation of in-situ Al

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