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Integrated Hierarchical Cobalt Sulfide/Nickel Selenide Hybrid Nanosheets as An Efficient 3D Electrode for Electrochemical and Photoelectrochemical Water Splitting Yang Hou, Ming Qiu, Gyutae Nam, Min-Gyu Kim, Tao Zhang, kejun liu, xiaodong zhuang, Jaephil Cho, Chris Yuan, and Xinliang Feng Nano Lett., Just Accepted Manuscript • Publication Date (Web): 06 Jun 2017 Downloaded from http://pubs.acs.org on June 7, 2017

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Integrated Hierarchical Cobalt Sulfide/Nickel Selenide Hybrid Nanosheets as An Efficient 3D Electrode for Electrochemical and Photoelectrochemical Water Splitting

Yang Hou,1,4† Ming Qiu,2† Gyutae Nam,3 Min Gyu Kim,5 Tao Zhang,1 Kejun Liu,1 Xiaodong Zhuang,1 Jaephil Cho,3 Chris Yuan6 and Xinliang Feng1* 1

Center for Advancing Electronics Dresden (cfaed) & Department of Chemistry and Food Chemistry, Technische Universitaet Dresden, 01062 Dresden, Germany E-mail: [email protected] 2

Institute of Nanoscience and Nanotechnology, College of Physical Science and Technology, Central China Normal University, Wuhan 430079, China

3

Department of Energy Engineering, School of Energy and Chemical Engineering,

Ulsan National Institute of Science and Technology (UNIST), 44919, Ulsan, Republic of Korea 4

College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China

5

Beamline Division, Pohang Accelerator Laboratory, Pohang, Kyungbuk, 37673, Republic of Korea

6

Department of Mechanical and Aerospace Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States †These authors contributed equally to this work.

ACS Paragon Plus Environment

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ABSTRACT Developing highly active electrocatalysts for photoelectrochemical water splitting is critical to bring solar/electrical-to-hydrogen energy conversion processes into reality. Herein, we report a 3D hybrid electrocatalyst which is constructed through in situ anchoring of Co9S8 nanosheets onto the surface of Ni3Se2 nanosheets vertically aligned on an electrochemically exfoliated graphene foil. Benefiting from the synergistic effects between Ni3Se2 and Co9S8, the highly conductive graphene support, and large surface area, the novel 3D hybrid electrode delivers superior electrocatalytic activity toward water reduction in alkaline media, featuring overpotentials of -0.17 and -0.23 V to achieve current densities of 20 and 50 mA cm−2, respectively, demonstrating an electrocatalytic performance on the top of the Ni3Se2- and Co9S8based electrocatalysts as reported in literature. Experimental investigations and theoretical calculations confirm that the remarkable activity of the obtained material results from the unique 3D hierarchical architecture and interface reconstruction between Ni3Se2 and Co9S8 through Ni-S bonding, which leads to charge redistribution and thus lowers the energy barrier of hydrogen desorption in the water splitting process. Further integration of the 3D hybrid electrode with a macroporous silicon photocathode enables highly active and sustainable sunlight-driven water splitting in both basic media and real river-water. The overall water splitting with 10 mA cm−2 at a low voltage of 1.62 V is achieved using our hybrid as both anode and cathode catalysts, which surpasses that of the Ir/C-Pt/C couple (1.60 V) for sufficiently high overpotentials.

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KEYWORDS: 3D hierarchical architecture, Strong coupling effect, Earth-abundant hybrid catalyst, Electrocatalysis, Photoelectrocatalysis, Water splitting

Photoelectrochemical (PEC) water splitting into hydrogen and oxygen using solar energy is considered as one of the most promising pathways for renewable energy conversion.1-3 Under solar irradiation, photoexcited electron–hole pairs are generated in semiconductors, and the photogenerated charge carriers are then separated and transferred

to

the

electrode

interface,

which

participates

in

the

water

oxidation/reduction reactions.4-6 As the charge transfer in the semiconductor materials determines the generation of separated electron–hole pairs, recent studies have focused on improving the charge transfer in the bulk semiconductors.7-9 So far, little attention has been paid to improving the surface reaction kinetics at the semiconductor/liquid interface, which also plays a crucial role in PEC water splitting.10,11 To enhance the surface reaction kinetics, few efforts have been made to develop highly active and stable electrocatalysts/cocatalysts that can serve as electron (or hole) traps to promote charge separation at the interfaces.12-15 At present, Pt- and Ir/Ru-based materials represent the most efficient electrocatalysts for PEC hydrogen 3


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evolution reactions (HERs) and oxygen evolution reactions (OERs), respectively; however, the high cost and scarcity of these noble metals hamper their large-scale applications. Although some earth-abundant-metal electrocatalysts have been recently developed,15-17 only a few of them have been used in PEC water splitting systems with such issues as chemical incompatibility, material instability, inefficient charge transfer across the interfaces between the photoelectrode and electrocatalyst, etc.15-19 Moreover, practical implementation of these materials as bifunctional electrodes in an integrated electrolyzer for overall water splitting are often hindered by the disparity between the required electrolytes because their best working conditions often mismatch.2,20-25 Non-noble metal chalcogenides, typically nickel selenide (Ni3Se2) nanosheets, have attracted enormous attention recently due to their high catalytic activity and abundance in nature.26,27 However, the poor electronic conductivity and instability of Ni3Se2 significantly impedes its practical application in commercial alkali electrolyzer systems. Construction of Ni-based hybrid nanostructures possessing strong coupling effects between different components based on their advantages, is an effective pathway to enhance the overall electrochemical water splitting activity and stability of the active materials.28,29 Metallic cobalt sulfide (Co9S8) nanosheets are one of the most ideal candidates for the fabrication of Ni-based (e.g., Ni3Se2 nanosheets) hybrids owing to the high electrical conductivity associated with their unique layered structure.30-32

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Herein, we integrated Co9S8 nanosheets with vertically aligned Ni3Se2 nanosheets on electrochemically exfoliated graphene (EG) foil for the construction of a 3D hybrid electrode (EG/Ni3Se2/Co9S8) for both electrochemical and PEC water splitting in alkaline media. In such a hybrid system, the Co9S8 nanosheets, with a diameter of ∼ 50 nm, are tightly grafted on the Ni3Se2 nanosheets, which are perpendicular to the EG foil, with 10 nm thickness and ∼ 170 nm lateral size. Benefiting from the large surface area (83 m2 g−1), highly conductive support of the EG, strong interfacial coupling and interface reconstruction between the Ni3Se2 and the grafted Co9S8 through Ni-S bonding, and the unique 3D hierarchical structure, the resulting EG/Ni3Se2/Co9S8 hybrid electrode exhibits remarkable activity for catalyzing HERs in alkaline media, affording current densities of 20 and 50 mA cm−2 at overpotentials of -0.17 and -0.23 V, respectively. Furthermore, EG/Ni3Se2/Co9S8-based photocathode fabricated by the integration of EG/Ni3Se2/Co9S8 with a macroporous silicon (MSi) photoelectrode efficiently catalyses hydrogen generation from basic aqueous and real river-water. Using EG/Ni3Se2/Co9S8 as bifunctional electrocatalysts, overall water splitting at 10 mA cm−2 in alkaline media is achieved at 1.62 V with excellent durability. A two-step procedure was used to fabricate EG/Ni3Se2/Co9S8, as illustrated in Figure S1. First, the Ni3Se2 nanosheet arrays were hydrothermally grown on an EG foil produced through an electrochemical exfoliation treatment of graphite foil in (NH4)2SO4 electrolyte.33 Then, the Co9S8 nanosheets were grafted onto the surface of the EG/Ni3Se2 through a facile solvothermal method, to form the 3D EG/Ni3Se2/Co9S8 hybrid. 5


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A field-emission scanning electron microscopy (FESEM) image of EG/Ni3Se2/Co9S8 is presented in Figure 1, in which the aligned Ni3Se2/Co9S8 nanosheets can be observed to grow uniformly on the surface of the EG foil. The lateral size of the Ni3Se2 nanosheets (large nanosheets) is approximately 170 nm, with thickness approximately at 10 nm. Densely packed Co9S8 nanosheets (small nanosheets), with ∼ 50 nm diameter and 5 nm thickness, are tightly anchored on the Ni3Se2 nanosheets, forming a 3D hierarchical structure. The average thicknesses of the Ni3Se2 and Co9S8 nanosheets were determined by atomic force microscopy (AFM) to be ∼ 10.8 and 5.7 nm, respectively (Figure S2). The EDX spectrum indicates the coexistence of Co, S, Ni, Se, C, and O elements (inset of Figure 1b), and elemental mapping images of EG/Ni3Se2/Co9S8 clearly demonstrate that all elements are uniformly distributed throughout the whole hybrid without distinctive phase separation (Figure S3). Highresolution transmission electron microscopy (HRTEM) images verify the intimate contact of the Ni3Se2 and Co9S8 phases, and the interfaces between these phases can be clearly identified. Resolved lattice fringes of 0.30, 0.18, 0.20, and 0.29 nm, which are indexed to the (110) plane of Ni3Se2 and the (440), (511), (222) planes of Co9S8, respectively, are revealed in Figure 1c-1d. X-ray diffraction (XRD) patterns evidence the co-existence of EG, Ni3Se2, and Co9S8 in the hybrid (Figure S4). The Raman spectrum of EG/Ni3Se2/Co9S8 contains typical peaks corresponding to Ni3Se2 and Co9S8 and characteristic peaks for the D and G bands of EG at 1,336 and 1,600 cm−1, respectively (Figure 1e and Figure S5). The low ID/IG peak intensity ratio of the hybrid indicates a low degree of defects in the EG.20 The molar content of Co9S8 in the EG/Ni3Se2/Co9S8 hybrid was determined to be 6


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approximately 22.7% by inductively coupled plasma–optic emission spectrometry (ICP-OES). X-ray photoelectron spectroscopy (XPS) of EG/Ni3Se2/Co9S8 reveals the presence of C, Ni, Co, S, Se, and O elements with a Co/S atomic ratio of 9: 8.5 in the hybrid (Figure S6-S7). N2 sorption analysis of EG/Ni3Se2/Co9S8 shows a typical type IV isotherm with a distinct hysteresis loop, suggesting the existence of slit mesopores generated from sheet-like aggregation.20 The Brunauer–Emmett–Teller (BET) surface area is determined to be 83 m2 g−1 (Figure 1f), higher than those of EG/Ni3Se2 (29 m2 g−1) and EG/Co9S8 (37 m2 g−1) prepared under the same experimental conditions, respectively (Figure S8-S9). The high surface area with porous structure can provide abundant exposed active sites and facilitate mass transport during the electrochemical reactions.34 A contact angle of 0° was identified for EG/Ni3Se2/Co9S8, suggesting its super hydrophilic nature (Figure S10), which may facilitate electrolyte ion trapping and access to the active surface of the hybrid electrode.

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(b)

(a) Co9S8 20 nm Ni3Se2

O Ni Co

Ni

C Se S

Co Ni

200 nm 0

4

100 nm

8 KeV 10

6

(d)

Interface

Co9S8 (222) 0.29 nm

Co9S8 (440) 0.18 nm

Ni3Se2 (110) 0.30 nm

Ni3Se2 (110) 0.30 nm

2 nm

5 nm 240

-1

G

Volume absorbed (cm g )

(e)

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Co9S8 (511) 0.20 nm

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EG/Ni3Se2/Co9S8

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Ni3Se2

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BET surface area: 83 m g 0 0.0

0.2

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Relative pressure (P/P0)

Raman shift (cm )

Figure 1. Characterization of EG/Ni3Se2/Co9S8. (a-b) FESEM images, (c-d) HRTEM images, (e) Raman spectrum, (f) N2 adsorption isotherm and corresponding pore size distribution (inset) of EG/Ni3Se2/Co9S8. Inset: corresponding EDX spectrum and enlarged FESEM image of EG/Ni3Se2/Co9S8 (b).

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The HERs electrocatalytic activity of EG/Ni3Se2/Co9S8 was first assessed in 1.0 M KOH solution. All reported potentials were converted to values versus RHE, and a commercial Pt/C catalyst was also examined for comparison. As expected, the Pt/C catalyst exhibited superior HERs activity, whereas the EG demonstrated a poor performance (Figure 2a). In comparison to the activities of EG/Ni3Se2 and EG/Co9S8, EG/Ni3Se2/Co9S8 showed excellent catalytic activity with a low onset potential of 0.16 V, a large current density of 114.6 mA cm−2, and a high mass activity of 45.8 mA mg−1 at -0.3 V (Figure S11). The obtained current density value of 114.6 mA cm−2 is even larger than the sum of the current densities for EG/Ni3Se2 and EG/Co9S8, suggesting a synergic enhancement among EG, Ni3Se2, and Co9S8 in the hybrid. In addition, the EG/Ni3Se2/Co9S8 hybrid only requires low potentials of -0.17 and -0.23 V to achieve current densities of 20 and 50 mA cm−2, respectively. These overpotentials for EG/Ni3Se2/Co9S8 are comparable to the best reported values for all existing Ni3Se2- and Co9S8-based materials and superior to those of well-known, nonprecious-metal HERs catalysts (molybdenum compounds, first-row transition metal catalysts, carbon-based materials, etc., Table S1). And we also give the comparison of Pt wire and graphite rod as the counter electrode (Figure S12), which indicates that the excellent catalytic performance of EG/Ni3Se2/Co9S8 indeed reflects the intrinsic high catalytic activity of EG/Ni3Se2/Co9S8 instead of Pt contamination.35 The Tafel plot of EG/Ni3Se2/Co9S8 gives a value of 83 mV decade−1 (Figure 2b), which is much smaller than that determined for EG (354 mV decade−1), EG/Ni3Se2 (171 mV decade−1), and EG/Co9S8 (96 mV decade−1), suggesting the outstanding intrinsic HERs kinetics of the EG/Ni3Se2/Co9S8. The exchange current density of the 9


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EG/Ni3Se2/Co9S8 was calculated at 0.27 mA cm−2, which is higher than those of EG/Ni3Se2 (0.22 mA cm−2) and EG/Co9S8 (0.05 mA cm−2). The lower charge-transfer resistance observed for EG/Ni3Se2/Co9S8 relative to EG/Ni3Se2 and EG/Co9S8 implies that the hybridization accelerated the charge-transfer process of EG/Ni3Se2/Co9S8 for the HERs (Figure S13). The metallic Co9S8 nanosheets improves the electrical conductivity of the catalyst on the supports and ensures rapid electron transport between the semiconducting Ni3Se2 nanosheets (Figure S14).36 The EG/Ni3Se2/Co9S8 hybrid delivers a stabilized catalytic HERs current density of ∼ 102 mA cm−2 at -0.30 V over 10 h (inset of Figure 2c). Also, the cathodic current density showed no significant decay (Figure 2c) after 1,000 cycles, suggesting its robust catalytic activity. The effect of different loadings of the Co9S8 nanosheets on the HERs activity was also investigated and is summarized in Figure S15-S16. An optimal molar ratio (Ni3Se2/Co9S8) of 1: 1 was identified in this work.

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0.4

-2

Current density (mA cm )

-10

-20

0

Potential (V vs. RHE)

-2

0

-30 -60

EG EG/Ni3Se2

-90

EG/Co9S8

(a)

EG/Ni3Se2/Co9S8 -120 -0.4

-0.3

-0.2

-0.1

0.0


-30

Pt/C EG/Ni3Se2/Co9S8

-40 -0.9

-0.6

-0.3

0.3

(b)

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55 mV decade (Pt/C)

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EG/Ni3Se2/Co9S8

0

2

4

6

8

10

Time (h)

-80

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0.5

pH = 14.0

H

0.25 +

0.00

H +e

1/2 H2 (Ni site) Ni3Se2/Co9S8 Ni3Se2

-0.50

-120 -0.9

-0.6

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Reaction Coordinate

0.0


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(d)

∗

-

Initial After 1,000 cycles

Stage 0 0 eV

-1.6

Co9S8

0.0

Stage 0 0 eV

Stage 1 -1.41 eV

Ni3Se2

0.0

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(e)

Stage 1 -0.41 eV

-0.6 -1.2

Gas -0.02 eV

Stage 2 -0.30 eV

-0.8

Gas -0.28 eV

Stage 2 -0.99 eV

-1.0 -2.0

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-2

(S site) Ni3Se2/Co9S8

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Co9S8

(c)

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83 mV decade (EG/Ni3Se2/Co9S8)

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-0.3 V

∆GH* (eV)

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Ni3Se2/Co9S8

Stage 2 -1.07 eV

Stage 1 -1.69 eV

Gas -2.16 eV

Reaction Coordinate

Figure

2.

Electrocatalytic

EG/Ni3Se2/Co9S8.

(a)

performance

Polarization

curves

and of

EG,

mechanistic EG/Ni3Se2,

study

of

EG/Co9S8,

EG/Ni3Se2/Co9S8, and Pt/C for the HER. (b) The corresponding Tafel plots. (c) Polarization curves of EG/Ni3Se2/Co9S8 before and after 1,000 CV cycles. (d) DFTcalculated free-energy diagrams of the HER for Co9S8, Ni3Se2, and Ni3Se2/Co9S8 11


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(with S-site and Ni-site). (e) Free-energy pathways of the HER of Co9S8, Ni3Se2, and Ni3Se2/Co9S8 hybrid. Inset: chronoamperometry measurement of EG/Ni3Se2/Co9S8 at 0.3 V without iR correction. All experiments were carried out in 1.0 M KOH. To elucidate the reasons behind the high catalytic performance of the EG/Ni3Se2/Co9S8 for HERs, the free energies of atomic hydrogen adsorption (∆GH*) on Co9S8, Ni3Se2, and the interface of the Ni3Se2/Co9S8 hybrid nanostructure were calculated utilizing the first principles method within the framework of density function theory (DFT, see supporting information for details). Considering that the S and Ni sites are typical catalytic centers in transition metal chalcogenides for HERs,37,38 the S site of Co9S8, Ni site of Ni3Se2, and S and Ni sites of the interface of the Ni3Se2/Co9S8 hybrid were evaluated as the active sites of the electrocatalysts. The HER process can be described as follows: the initial hydrogen (H+ + e−) is adsorbed onto the binding site of the catalyst as an intermediate adsorbed H*, and the molecular hydrogen (1/2 H2) is then released from the active site.39,40 According to the basic Sabatier principle, a ∆GH* ≈ 0 is suggested as the ideal free energy change between the adsorbed H* on the catalyst and product (1/2 H2), which indicates that lower ∆GH* values allow the protons to strongly bond to the surface of the catalyst and lead to fast HER kinetics.41 First, the hydrogen adsorption on the active sites of Co9S8, Ni3Se2, and the interface of the Ni3Se2/Co9S8 hybrid was investigated. The optimized structural parameters of the crystals (unit cells of Co9S8 and Ni3Se2) compared with those determined in other works can be found in Table S2. As shown in Figure 2d, the free energies of atomic

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hydrogen adsorption, which directly determine the hydrogen evolution activities, were 0.42 and -0.31 eV for the S site of Co9S8 and Ni site of Ni3Se2, respectively, according to the Tafel reactions. This result can be explained by the metal d-orbital electron contribution and bonding/anti-bonding orbital determinations.42-45 For the S and Ni sites on the Ni3Se2/Co9S8 hybrid, the free energies calculated were 0.28 and -0.12 eV, respectively, which indicate higher catalytic activities than those of the S site of Co9S8 and Ni site of Ni3Se2, correspondingly, thus boosting their electrocatalytic performance towards HERs. Next, the DFT calculation-derived HER pathway is shown in Figure 2e. The S site of the Ni3Se2/Co9S8 hybrid was the most energetically favorable site for the first H adsorption among the Ni3Se2, Co9S8, and Ni3Se2/Co9S8 hybrid. According to the Volmer-Heyrovsky route, four stages were investigated: stages 0, 1, and 2 and the gas stage (see supporting information for details). There were energy barriers of 1.11 and 0.53 eV for Co9S8 and the Ni3Se2/Co9S8 hybrid from stage 1 to stage 2, which means that overpotentials were required to aid continuous H adsorption. For the Ni3Se2, the H spontaneously adsorbed on the surface. During the desorption process, there were energy barriers of 0.28 and 0.68 eV for Co9S8 and Ni3Se2, respectively, for 1/2 H2 release from the corresponding active sites, and the H2 was spontaneously desorbed from the surface of the Ni3Se2/Co9S8 hybrid, indicating that the hybrid had a much lower energy barrier compared with those of Ni3Se2 and Co9S8 for the HER. The representative schematic models (possible adsorption sites of H*) of the VolmerHeyrovsky route for DFT calculations are depicted in Figure 3a, and other models can be found in the supplementary information (Figure S17-S21). In Figure S17, the 13


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average values of the H bond on the S site of Co9S8 and Ni site of Ni3Se2 were 1.36 and 1.43 Å, respectively. For the Ni3Se2/Co9S8 hybrid, the average values of the H bond on S and Ni sites of Ni3Se2/Co9S8 were 1.38 and 1.56 Å, respectively (Figure 3a). The H bond on the Ni site of Ni3Se2 was shorter than that on the Ni site of the hybrid. The lengthened H bond indicates that the S and Ni sites of Ni3Se2/Co9S8 can bind the protons strongly and thus accelerate the HER kinetics. From the free energy and reaction pathway diagrams (Figure 2d-2e), the Ni site showed better catalytic activity than the S site. Meanwhile, the formed Ni-S bonds showed high electrocatalytic performance in the HER.35 The population distributions of the surface atoms of the Co9S8, Ni3Se2, and Ni3Se2/Co9S8 hybrid nanostructures are presented in Figure 3b. For the S atoms of the Ni3Se2/Co9S8 hybrid, the electron acceptance was less (approximately -0.30 ~ -0.20 eV) than that on Co9S8; for the Ni atoms of the Ni3Se2/Co9S8 hybrid, the electron acceptance was slightly low compared with that of Ni3Se2. The above results indicate that the ability of the surface reconstruction and the strong electron-electron interactions between Ni3Se2 and Co9S8 to change the surface hybrid states (S-Ni-Se and Se-Co-S) were the main reasons for the modulation of the free energies for atomic hydrogen adsorption on the surface of the Ni3Se2/Co9S8.46 Interestingly, a few Se atoms adjacent to the Ni atoms of the Ni3Se2/Co9S8 hybrid transitioned to reduction states but showed oxidation states on the surface of pure Ni3Se2. This state change may have made the Se atoms near the hybrid interface potential catalytic active sites and helped to lower the Coulombic potential for atomic

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hydrogen adsorption on the adjacent Ni atoms for HERs, which may be one of the main reasons behind the enhanced electrocatalytic performance. The strong coupling interactions at the interface of the Ni3Se2/Co9S8 hybrid were confirmed by XPS studies. The Ni 2p peaks of the EG/Ni3Se2/Co9S8 exhibited a positive shift (855.8 eV) to a higher binding energy compared to the EG/Ni3Se2 (855.1 eV, Figure 3c), confirming the existence of strong electronic interactions between Ni3Se2 and Co9S8 in the hybrid.47 The Co9S8 chemically interacts with Ni3Se2 by forming an Ni-S bond (Figure S7), which is consistent with the theoretical studies. To further confirm the existence of Ni-S bonding between Ni3Se2 and Co9S8, we performed an extended X-ray absorption fine structure (EXAFS) analysis of the Ni Kedge (Figure 3d). According to the radial distribution function profile of the Ni Kedge, Ni3Se2/Co9S8 has Ni-S bond lengths between Ni3Se2 and Co9S8 of approximately 1.8 Å, while both Ni3Se2 and the physical mixture of the Ni3Se2 and Co9S8 samples exhibit Ni-Se bond lengths of over 2 Å. This agrees well with the XPS results, indicating that the desired electrocatalyst possesses Ni-S bonding not only on the surface of the electrocatalyst but also in the entire bulk structure. Due to the existence of the hybrid structure, the X-ray absorption near edge structure (XANES) spectra of Ni3Se2/Co9S8 differs from those of the Ni-Se bonding (Figure S22). In addition, the Co K-edge radial distribution function of all samples exhibits the same bonding length but slightly different particle sizes (Figure 3e). The Ni3Se2/Co9S8 exhibits larger particle sizes due to the stronger coupling interactions between the Ni3Se2 and Co9S8 than the other samples. Note that all samples in the Co XANES spectra indicate similar charge states regardless of the coupling interactions. 15


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Moreover, there is an obvious FTIR downshift in the case of EG/Ni3Se2/Co9S8 compared to those of Ni3Se2 and Co9S8 (Figure S23), further suggesting the strong coupling

interactions

between

Ni3Se2

and

Co9S8.48

On

the

whole,

the

EG/Ni3Se2/Co9S8 delivers superior catalytic performance, which benefit from synergistic effects: unique 3D hierarchical structure that ensures the sufficient exposure and better utilization of electroactive sites, and facilitates electrolyte penetration/diffusion; strong interfacial coupling and interface reconstruction between Ni3Se2 and grafted Co9S8 by forming an Ni-S bond which results in the charge redistribution between the Ni3Se2 and the Co9S8 and thus lowers the adsorption energy of the reactant and product; together with highly conductive support of EG for efficient charge transfer.

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(a)

(c)

Ni 2p3/2

Ni 2p Ni 2p1/2

Intensity (a.u.)

(b)

EG/Ni3Se2/Co9S8 EG/Ni3Se2 890

880

870

860

850

Binding energy (eV) 12

12

(d)

(e) FT [k χ(k)] (a.u.)

Ni3Se2/Co9S8 Ni3Se2

8

Co9S8 Ni3Se2/Co9S8

8

Physical mixture

3

Physical mixture

3

FT [k χ(k)] (a.u.)

1 2 3 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

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4

4

0

0 0

2

4

6

0

R (angstrom)

2

4

R (angstrom)

Figure 3. Mechanistic study of EG/Ni3Se2/Co9S8 for HER. (a) Chemisorption models of the H, 2H and H2 intermediates on Ni3Se2/Co9S8 hybrid. Yellow = S, Pink = Co, Blue = Ni, Orange = Se, White = H. (b) Population distribution on the Co9S8, 17


6

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1 2 3 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

Ni3Se2, and Ni3Se2/Co9S8 hybrid nanostructures. (c) High-resolution Ni 2p XPS spectra of EG/Ni3Se2 and EG/Ni3Se2/Co9S8. (d) Ni K-edge k3-weighted EXAFS spectra of Ni3Se2/Co9S8, Ni3Se2, and their physical mixture. (e) Co K-edge k3weighted EXAFS spectra of Ni3Se2/Co9S8, Co9S8, and their physical mixture. Encouraged by the excellent HERs results, the capability of the EG/Ni3Se2/Co9S8 electrocatalyst for solar-driven PEC water splitting was tested by integrating it with an MSi photocathode in 1.0 M KOH solution under simulated AM 1.5G solar irradiation (100 mA cm−2, Figure S24). Compared with that of EG/Ni3Se2/Co9S8, a 300 mV positive shift in the onset potential was observed for MSi/EG/Ni3Se2/Co9S8 (Figure 4a), highlighting the cooperative interactions between the MSi photocathode and EG/Ni3Se2/Co9S8 electrocatalyst. A remarkable shift in the onset potential from -0.08 V for MSi to 0.14 V for MSi/EG/Ni3Se2/Co9S8 with improved photocurrent density was observed (Figure S24). The positive shift of ～ 220 mV suggested that the EG/Ni3Se2/Co9S8 served as efficient cocatalyst promoted rapid interfacial charge transfer across the photoelectrode/electrolyte interface and decreased the reaction kinetic barrier for HERs,8 thus increasing the overall eﬃciency of PEC water splitting. Electrochemical impedance spectroscopy (EIS) exhibited that the introduction of EG/Ni3Se2/Co9S8 or Ni3Se2/Co9S8 caused signiﬁcant decrease in the radius of arc compared with MSi both in dark and under irradiation (Figure 4b). The radius of the MSi/EG/Ni3Se2/Co9S8 was the smallest among all above MSi-based photoelectrodes. In the Nyquist diagram, the smaller radius is the lower charge transfer resistance at the electrode/electrolyte interface.49 These results revealed that the presence of

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EG/Ni3Se2/Co9S8 remarkably enhanced photoexcited electrons transfer from the MSi surface to the electrolyte with the concomitant evolution of H2 by suppressing the charge recombination. Based on the band diagrams27,32 and density of states (Figure S25) profiles, the Ni3Se2/Co9S8 hybrid can be verified as n-type material. The n-type hybrid/p-MSi heterojunction is a typical type-II junction that the photogenerated electrons on the conduction band of MSi can be transferred to the Ni3Se2/Co9S8 hybrid through the EG (electron transport ‘‘highway’’), and then the electrons are consumed by the reduction of adsorbed H+ ions in the electrolyte to H2, promoting the effective charge separation. Of course, direct transfer of some photogenerated electrons from MSi to Ni3Se2/Co9S8 is possible (Figure S25).50 Thus, it is not surprising that Ni3Se2/Co9S8 alone as cocatalyst also can promote the PEC activity of MSi for HERs (Figure 4b and Figure S24). For practical use, we also successfully conducted PEC HERs in real river-water using MSi/EG/Ni3Se2/Co9S8 as a photocathode (Figure S26). This result suggested that the MSi/EG/Ni3Se2/Co9S8 had excellent activity and stability for PEC river-water splitting.

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Dark

-1

-2

-3 Graphite/EG/Ni3Se2/Co9S8 MSi/EG/Ni3Se2/Co9S8

-4 -0.8

(b)

-0.4

0.0


-40

-80

-Z'' (ohm)

(a)

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9000

-2


-2

0

0

Dark

6000 Dark 3000

Solar Dark Solar Solar

0

-120

0.4

0

2500

5000

Z (ohm)

7500

10000

1.8

24

(d)


EG/Ni3Se2/Co9S8 (+)//EG/Ni3Se2/Co9S8 (-)

-2


1 2 3 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

Photocurrent density (mA cm )

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Pt/C (+)//Pt/C (-) Ir/C (+)//Pt/C (-)

18

10 mA cm

-2

1.7

(c) 12

1.6

6

EG/Ni3Se2/Co9S8 (+)//EG/Ni3Se2/Co9S8 (-) Ir/C (+)//Pt/C (-)

1.5 1.2

1.4

1.6

1.8

2


4

6

Time (h)

8

10

Figure 4. Electrochemical and PEC performance of MSi/EG/Ni3Se2/Co9S8. (a) Polarization curves for MSi/EG/Ni3Se2/Co9S8 and graphite/EG/Ni3Se2/Co9S under dark and simulated sunlight irradiation. (b) EIS Nyquist plots of MSi (○, □), MSi/Ni3Se2/Co9S8 (△, ▽), and MSi/EG/Ni3Se2/Co9S8 (◇, ☆) at a bias of -0.2 V under dark and simulated sunlight irradiation. (c) Polarization curves of EG/Ni3Se2/Co9S8 (+ and -), Pt/C (+)//Pt/C (-), and Ir/C (+)//Pt/C (-) for overall water splitting in a two-electrode configuration (not iR-corrected). (d) Chronopotentiometry curves of EG/Ni3Se2/Co9S8 and Ir/C (+)//Pt/C (-) under a current density of 10 mA cm−2 without iR correction. All experiments were carried out in 1.0 M KOH. Finally, considering that the transition metal sulfides/selenides have been identified as potential bifunctional catalysts,20,51 a two-electrode setup (uncompensated iR-drop) 20


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using 3D EG/Ni3Se2/Co9S8 as both the anode and cathode was assembled to investigate its performance for overall water splitting in 1.0 M KOH solution (Figure 4c). Remarkably, the EG/Ni3Se2/Co9S8 hybrid exhibited outstanding performance for overall water splitting with an overpotential of 1.62 V to afford 10 mA cm−2, which surpasses that of the Ir/C-Pt/C couple (1.60 V) for sufficiently high overpotentials (large current densities) and those of reported bifunctional electrocatalysts for overall water splitting (Table S3). Meanwhile, the EG/Ni3Se2/Co9S8 electrode could withstand continuous electrolysis for at least 10 h with less degradation than the benchmarking combination of the Ir/C-Pt/C couple at a current density of 10 mA cm−2 (Figure 4d). After the long-term tests, XPS studies in the Co 2p region showed no obvious changes in the chemical composition, confirming the robustness of the 3D hybrid catalyst (Figure S27). In summary, we demonstrated a novel 3D hybrid electrocatalyst consisting of Co9S8 nanosheets strongly coupled with vertical Ni3Se2 nanosheets grown on EG. Benefiting from the unique 3D hierarchical architecture, large surface area, and synergistic effects between Ni3Se2 and Co9S8, the obtained EG/Ni3Se2/Co9S8 hybrid exhibits remarkable catalytic activity and excellent stability toward HERs in alkaline solutions, which are superior to almost all existing Ni3Se2- and Co9S8-based catalysts. Theoretical studies demonstrated that the strong coupling interactions and interface reconstruction between Ni3Se2 and Co9S8 through Ni-S bonding contribute to the excellent catalytic activity of the hybrid, well aligned with the experimental results. The EG/Ni3Se2/Co9S8 hybrid could be readily integrated with an MSi photocathode to enable highly active solar-driven PEC water splitting in both basic media and real 21


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river-water. When used in an alkaline water electrolyzer, the EG/Ni3Se2/Co9S8 hybrid is stable with better overall water splitting activity than that of the commercial Pt/CIr/C couple at high overpotentials. This work provides a new route for the design of efficient and robust 3D hybrid electrodes for a variety of electrochemical and PEC applications, such as nitrogen fixation and CO2 reduction. ACKNOWLEDGMENT This work was financially supported by the ERC Grant 2DMATER, UP-GREEN, the CFAED, and the EC under the Graphene Flagship (No. CNECT-ICT-604391). We acknowledge the use of the facilities in the Dresden Center for Nanoanalysis (DCN) at TUD. SUPPORTING INFORMATION AVAILABLE Experimental details for the material preparation, characterization, electrochemical and photoelectrochemical property testing, along with additional supporting data. This material is available free of charge via the Internet at http://pubs.acs.org. NOTES The authors declare no competing financial interest. REFERENCES (1)

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