Redox bifunctional activities with optical gain of Ni3S2 nanosheets edged with MoS2 for overall water splitting

Journal Pre-proof Redox bifunctional activities with optical gain of Ni3 S2 nanosheets edged with MoS2 for overall water splitting Chengzhong Wang, Xiaodong Shao, Jing Pan, Jingguo Hu, Xiaoyong Xu

PII:

S0926-3373(19)31181-6

DOI:

https://doi.org/10.1016/j.apcatb.2019.118435

Reference:

APCATB 118435

To appear in:

Applied Catalysis B: Environmental

Received Date:

28 August 2019

Revised Date:

22 October 2019

Accepted Date:

13 November 2019

Please cite this article as: { doi: https://doi.org/ This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Redox bifunctional activities with optical gain of Ni 3S2 nanosheets edged with MoS2 for overall water splitting Chengzhong Wanga, Xiaodong Shaoa, Jing Pana, Jingguo Hua, Xiaoyong Xua,b,* a

College of Physics Science and Technology, Yangzhou University, Yangzhou 225002,

China b

Institute of Optoelectronics and Nanomaterials, College of Materials Science and

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Engineering, Nanjing University of Science and Technology, Nanjing 210094, China

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*E-mail addresses: [email protected]

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Graphical abstract

A unique lateral heterostructure of ultrathin Ni3S2 nanosheets terminated with MoS2 edges was designed as efficient and robust bifunctional catalysts with dual optical gains for solar-assisted overall water splitting. 1

Highlights 

A 2D lateral NiMoS heterostructure as novel bifunctional catalysts for overall water splitting



The dual photo-enhanced effects on both HER and OER activities from photoelectron dynamics



A new solution to integrate solar energy into water splitting over semiconducting catalysts

ABSTRACT Exploring cost-efficient catalysts with bifunctional activities of hydrogen evolution

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reaction (HER) and oxygen evolution reaction (OER) holds practical significance for overall water splitting. Herein, for the first time, we report a unique lateral

heterostructure of ultrathin Ni3S2 nanosheets (NSs) edged with MoS2 as an efficient and durable bifunctional catalyst for overall water splitting in alkaline media.

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Benefiting from maximizing functional Mo-S-Ni interfaces that favor the

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chemisorption of hydrogen and oxygen-containing intermediates, the resultant Ni3S2/MoS2 (NiMoS) catalyst exhibits respectively low overpotentials of 78 and 260

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mV at 10 mA cm-2 for HER and OER, superior to the commercial benchmarks (IrO2 and Pt/C), ranking among the best records reported to date. In particular, the NiMoS

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catalyst renders an attractive light-enhanced effect on both HER and OER activities due to photogenerated charge transfer at Mo-S-Ni interface towards redox kinetic

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acceleration. An assembled two-electrode alkaline electrolyzer using NiMoS as bifunctional catalysts can deliver a current density of 10 mA cm−2 at low cell voltage

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of 1.53 V (1.50 V under 1 sun irradiation) with remarkable stability for over 100 h, demonstrating the robust overall water splitting with accessible solar integration.

Keywords: overall water splitting, bifunctional catalyst, transitional metal chalcogenides, heterostructure 2

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1. Introduction Hydrogen (H2) production through water electrolysis powered by renewable electricity has been regarded as an ideal energy strategy for future human being [1,2]. The application of this technique relies on cost-efficient electrocatalysts capable of decreasing the overpotentials of water splitting with H2 evolution reaction (HER) and oxygen (O2) evolution reaction (OER) [3-5]. Although noble metal-based catalysts (e.g., Pt for HER and RuO2/IrO2 for OER) can remarkably reduce reaction

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overpotentials, the scarcity and high cost severely restrict their large-scale application. Therefore, earth-abundant alternatives have been extensively developed based on sulfides, carbides, nitrides, oxides, hydroxides, and phosphides [6-9]. However, most

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of research efforts focused on the development of monofunctional catalyst for either

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OER or HER half-reaction, while few of catalysts reported to date enable bifunctionality with both HER and OER activities in the same pH range [10,11]. This

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is due to that active HER catalysts generally work well in an acidic medium, whereas most OER catalysts run better in a neutral or basic medium [12,13]. Exploring

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low-cost bifunctional catalysts with favorable compatibility of HER and OER activities in the same electrolyte environment is of great significance for full water

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splitting in practice, yet remains a formidable challenge. Recently, the earth-abundant transition-metal chalcogenides (TMCs) are emerging

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as a class of excellent OER catalysts in alkaline media because the 3d transition-metal intermediates with various valence states serve as the superior active sites [14-16]. However, the TMCs usually exhibit unsatisfactory HER activity in alkaline media due to

the

sluggish

kinetics

of

initial

water

dissociation

(Volmer

step,

H2O + e –  OH – + H* ) [17], which hampers their applicability in overall water

splitting. To address this issue, Feng et al. creatively proposed a smart strategy of 4

integrating Ni3S2 catalysts with nano-sized MoS2 for synergistic adsorption of both OER and HER-involving intermediates, resulting in an overall water-splitting activity [18]. Recently, Yang and Lin et al. fabricated the heterogeneous MoS2/Ni3S2 nanorods (NDs) and nanosheets (NSs), respectively, where the hetero-interface contribution to electrochemical bifunctionality enabled enhanced overall water splitting [19,20]. Furthermore, the covalent doping methods, such as Co-/Ni-doping in MoS2 NSs and Mo-doping in Ni3S2 NSs, were successively demonstrated to create the bifunctional

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OER and HER activities in alkaline media [21-23]. Therefore, further along the line, the rational engineering of covalent Mo-S-Ni coupling is promising and desirable to

develop low-cost TMCs as high-activity bifunctional catalysts for overall water

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splitting.

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Herein, we report for the first time a novel Ni3S2/MoS2 (NiMoS) heterostructure of ultrathin Ni3S2 NSs edged with MoS2 that is synthesized on Ni foam (NF) via one-pot

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hydrothermal route as an excellent bifunctional water-splitting catalyst, in which abundant Mo-S-Ni coupling interfaces are generated with preferred exposure of

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unsaturated Mo-S edges. Such engineering of heterogeneous assembly induces significantly enhanced activities of both HER and OER, especially with optical gains,

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leading to an impressive overall water-splitting performance in alkaline media. The resultant NiMoS catalyst exhibits low OER and HER overpotentials of 260 and 78

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mV at 10 mA cm-2, which outperforms the state-of-the-art IrO2 and Pt/C catalysts, respectively. The unique light-enhanced effect on both HER and OER activities comes from the photogenerated charge transfer from excited Ni3S2 to modulate redox dynamics. An assembled two-electrode electrolyzer using NiMoS catalyst needs a quite low cell voltage of only 1.53 V (1.50 V under 1 sun irradiation) to deliver a current density of 10 mA cm-2 with excellent stability for over 100 h in 1.0 M KOH, 5

accompanied by stoichiometric O2 and H2 generation with near unity Faraday efficiency.

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2. Results and discussion

Fig. 1. Fabrication and morphology of NiMoS grown on NF. (a) Schematic illustration of synthetic process. (b,c) SEM images of bare NF and NiMoS grown NF with their amplified profiles (insets). (d) TEM image, (e) HRTEM image with corresponding FFT pattern (inset), and (f) elemental mappings of NiMoS exfoliated from NF. (g) XRD pattern, (h) Ni 2p-core and (i) Mo 3d-core XPS spectra of NiMoS 6

with Ni3S2 and MoS2 benchmarks. The NiMoS heterostructure is prepared via a facile and scalable one-pot hydrothermal method, which is illustrated schematically in Fig. 1a (see the experimental details in the Supplementary Material). Briefly, certain amount of Na2MoO4·2H2O and C2H5NS are dissolved into deionized water as the Mo and S sources, and then the cleaned NFs are immersed into the solution to provide the Ni source, meanwhile serve as the substrates which load products. After hydrothermal

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reaction at 200 ℃ for 21 h, the final products supported on NFs are directly used as

OER and HER electrodes. Especially, the heterogeneous assembly between Ni3S2 and MoS2 can be modulated by adjusting the ratio of Mo and S sources (Fig. S1); a unique

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heterostructure of Ni3S2 NSs decorated laterally with MoS2 edges forms when the

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Mo/S ratio at 2:9, and its loading mass on NF is approximately 8.6 mg cm-2. A commercial NF cleaned by HCl solution shows the bare surface in the scanning

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electron microscopy (SEM, Fig. 1b); after the growth reaction, its macroporous structure remains while entire surface is uniformly covered with nano-sized product

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(Fig. 1c and Fig. S2). The transmission electron microscopy (TEM) image, as shown in Fig. 1d, reveals that the product consists of lamellar NSs of 40–70 nm in size. The

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individual NS shows the ultrathin texture (electron-beam transparent) and layered edge (noted by white arrows). Interestingly, the in-plane and edge lattice fringes with

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different d-spacing of 0.28 and 0.62 nm discerned in the high-resolution TEM (HRTEM, Fig. 1e) image can be typically indexed to Ni3S2 (110) and MoS2 (002) facets, respectively.[10] The corresponding fast Fourier transform (FFT) pattern (inset) also evidences the heterogeneous composition. Moreover, energy-dispersive X-ray spectroscopy (EDS) elemental mappings (Fig. 1f) clearly show the central body distribution of Ni element and the edge distribution of Mo element, verifying further 7

the lateral heterostructure of NiMoS NSs. The X-ray diffraction (Fig. 1g) and Raman (Fig. S3) spectra further determine the heterogeneous NiMoS composition. The X-ray photoelectron spectroscopy (XPS) survey spectrum (Fig. S4a) reveals the chemical composition of Ni, Mo, and S, consistent with the EDS result. In the high-resolution XPS profiles (Fig. S4b-d), the S 2p-core characteristic peaks at 161.9 and 162.3 eV and the satellite peak at 168.5 eV are ascribed to the S2- states coexisted in MoS2 and Ni3S2 [24]. The Ni 2p-core and Mo 3d-core XPS profiles (Fig. 1h and i) display the

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binding energies at 873.9, 855.6, 233.3 and 228.0 eV, which can be indexed to Ni 2p1/2, Ni 2p3/2, Mo 3d3/2, and Mo 3d5/2, respectively [25]. More importantly, these

characteristic peak positions appear obviously to shift compared with those of pure

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Ni3S2 and MoS2, especially for Mo 3d3/2, which indicates the strong electronic interaction by Mo-S-Ni covalent bonds at Ni3S2/MoS2 interface [19]. The low-fraction

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MoS2 serves as the edges of heterogeneous NSs, in which the terminal Mo atoms are

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more sensitive to the interfacial effect and more likely to be oxidized, and thus the Mo 3d-core XPS profile occurs the more distinct shift and emerges the Mo6+ signal. Note that the Ni3S2 is a non-layered crystal and unusually appears here in two-dimensional

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(2D) lamellar structure probably due to the unidirectional diffusion of Ni ions from NF surface toward the solution (Table S1) [6]; moreover, MoS2 selectively grows

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around Ni3S2 NSs as the active HER edges meanwhile not masking the active OER

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sites from 3d-Ni centers. The specific surface areas were analyzed by the Brunauer-Emmett-Teller (BET) adsorption data, as shown in Table S2, the NiMoS on NF presents a BET surface area higher than those of Ni3S2 on NF and bare NF. In view of creating large specific surface area and strong covalent bonding interaction for OER and HER sites, such a unique assembly is reasonably expected to favor the bifunctional activities of overall water splitting. 8

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Fig. 2. HER performance of NiMoS catalyst in 1.0 M KOH. (a) Polarization curves

and (b) Tafel plots of NF, Pt/C, MoS2, Ni3S2 and NiMoS supported on NFs. (c)

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Amperometric j  t curves under chopped 1 sun illumination at different potentials,

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(d) long-term chronoamperometric stability test with a digital photo of H2 bubbling (inset), and (e) comparison of polarization curves in dark and light fields before and

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after stability test for NiMoS catalyst on NF.

The prepared NiMoS supported on NF was firstly evaluated as HER catalyst in 1.0

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M KOH solution by a standard three-electrode configuration with a graphite rod counter electrode and a saturated Ag/AgCl reference electrode, respectively. For

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comparison, bare NF, MoS2, Ni3S2 and commercial Pt/C supported on NFs with the same mass loading were also tested. Fig. 2a shows their HER polarization curves with iR correction. As shown, the NiMoS exhibits a superior HER activity that is comparable to Pt/C, and better than pure MoS2, Ni3S2 and bare NF. In detail, the ultralow overpotentials ( 10 and 100 ) of 78 and 145 mV can be achieved by NiMoS to deliver the current densities of 10 and 100 mA cm-2, respectively, which are close 9

to that of Pt/C (10 = 46 mV, 100 = 165 mV). The corresponding Tafel plots (Fig. 2b) show that the NiMoS renders an impressive Tafel slop of approximately 68 mV dec-1, which is considerably lower than those of single components (132 mV dec-1 for MoS2 and 104 mV dec-1 for Ni3S2) and approaches that of Pt/C (66 mV dec-1). The HER in alkaline media proceeds in two steps as following: the first step is a discharge step (Volmer reaction); the second step is either the ion and atom reaction (Heyrovsky reaction) or the atom combination reaction (Tafel reaction).19 Tafel slope, an

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important indicator of catalytic kinetics, can be used to probe the elementary steps,

and the HER kinetic model suggests that the Volmer, Heyrovsky and Tafel reactions dominate distinct Tafel slopes of around 120, 40 or 30 mV dec-1, respectively. The

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significant decrease in Tafel slope to 68 mV dec-1 may indicate the kinetic acceleration in the Volmer step toward HER for NiMoS catalyst. In addition, the electrochemical

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impedance spectroscopy (EIS) analyses (Fig. S5) indicate that the NiMoS presents a

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much lower charge-transfer resistance than those of pure Ni3S2 and MoS2, suggesting the favorable HER kinetic process over NiMoS catalyst. These performance

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parameters make the present NiMoS heterostructure rank among the best records for nonprecious-metal-based HER catalysts reported so far (Table S3) [26-28]. Fig. 2c

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shows the chronoamperometry ( j  t ) curves under intermittent 1 sun illumination (100 mW cm-2, AM 1.5) at several typical potentials. In particular, the current

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densities display good reversible response with the switched illumination, which suggests an interesting light-enhanced catalytic HER activity. Note that the photocurrent response is proportional to the applied potential, indicating the enhanced HER activity under illumination is mainly ascribed to the photogenerated electron injection rather than the photo-thermal effect. Moreover, the effect of light wavelengths on current response is displayed in Fig. S6, where the photocurrent 10

response window is in consistent with the optical adsorption range of semiconducting Ni3S2, and thus it can be concluded that the hot electron transfer from photo-excited Ni3S2 can be responsible for light-enhanced HER performance. The long-term chronoamperometry was examined to evaluate the stability of NiMoS during high-current HER process at a constant potential of -150 mV. As shown in Fig. 2d, the NiMoS exhibits the excellent durability with negligible current change and vigorous effervescence (bottom-left inset) over 15 h. The polarization curves before and after

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the stability testing (Fig. 2e) remain almost unchanged, further confirming the robust HER activities in alkaline medium whether in dark or light fields. For the HER in alkaline media, the kinetics is determined through a subtle balance between the initial

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water dissociation and the subsequent chemisorption of the intermediates (OH— and H*) on the catalyst surface [29]. The undercoordinated Mo-S edge sites possess the

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suitable adsorption strength for H* species similar to Pt sites [30], but the slow —

species restrict the HER

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water-dissociation step and the strong adsorption of OH

activity of Mo-S coordinated catalysts in alkaline media [31]. Recent theoretical

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studies confirmed that the covalent Mo-S-Ni coordination could effectively reduce the kinetic energy barrier of the initial water-dissociation step and tune the chemisorption

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of hydrogen-containing intermediates [32,33], which may account for the enhanced HER kinetics of NiMoS in alkaline media. In addition, the photogenerated electron

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transfer from excited Ni3S2 can enrich hot electrons for active Mo-S edge sites into HER, rendering a valuable light-assisted HER dynamics, similar to the hot electron injection from photo-excited plasmons or semiconductors to HER-active sites [34-36].

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Fig. 3. OER performance of NiMoS catalyst in 1.0 M KOH. (a) Polarization curves

and (b) Tafel plots of NF, Pt/C, MoS2, Ni3S2 and NiMoS supported on NFs. (c)

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Amperometric j  t curves under chopped 1 sun illumination at different OER potentials, (d) long-term chronoamperometric stability test with a digital photo of O2

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bubbling (inset), and (e) comparison of polarization curves in dark and light fields

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before and after stability test for NiMoS catalyst on NF. On the other hand, we also explored the OER performance of NiMoS catalyst in 1.0

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M KOH. The iR-corrected OER polarization curves are shown in Fig. 3a, in which the NiMoS displays the earliest onset and the fastest growth of the current with the

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applied potential when compared to MoS2, Ni3S2 and bare NF. The ultralow overpotential of only 260 mV is required to deliver the current density of 10 mA cm-2 for NiMoS, versus the higher ones of 360 mV for MoS2 and 340 mV for Ni3S2. The enhanced OER activity of NiMoS is further confirmed by the smallest Tafel slope (59 mV dec-1), compared with those of other tested counterparts (Fig. 3b). The alkaline OER involves four discrete electron transfer steps with distinct Tafel slopes. A Tafel 12

slope reduced down to 59 mV dec-1 for NiMoS by one order of magnitude indicates the retrocession of rate-determining step to accelerate the OER process [37]. Noticeably, the baseline commercial IrO2 catalyst loaded on NF shows an overpotential of 310 mV with a Tafel slope of 66 mV dec–1, which are higher than those of NiMoS. The demonstrated OER activity of NiMoS is much better than that of single Ni3S2 and MoS2, even outperforms precious metal IrO2, making present NiMoS catalyst comparable or superior to most of the recently reported OER catalysts (Table

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S4) [38,39]. The excellent OER kinetics can also be confirmed by the best charge-transfer capacity in NiMoS electrode in comparison with MoS2 and Ni3S2 counterparts (Fig. S7). Moreover, the reproducible photocurrent response to instant 1

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sun illumination at OER potentials determines the light-assisted catalytic OER performance (Fig. 3c). The photocurrent response is found to accord with the optical

from

the

charge

excitation

over

semiconductor.

The

long-term

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derives

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absorption band of Ni3S2 (Fig. S8), indicating that the light-enhanced OER activity

chronoamperometry test that can render a steady catalytic current with negligible decay (Fig. 3d) reveals the excellent OER durability of NiMoS in alkaline medium,

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and thereby the post-test polarization curves maintain the initial levels of catalytic activities whether in dark or light fields (Fig. 3e). It should be noted that the anodic

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peak near 1.38 V in the OER polarization curves corresponds to the oxidation

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transition from NiII to NiIII/IV species [40]; the similar anodic oxidation peak usually occurs prior to Ni-based OER catalysis in alkaline solution [41-43]. Thus the formed high-valence NiIII/IV species are generally considered to be the origin of OER catalytic activity. The anodic peak on Ni oxidation is very prominent for NiMoS in Fig. 3a and obviously increases upon illumination in Fig. 3e, underscoring the accelerating formation of OER-active NiIII/IV sites, which may account for the superior OER 13

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activity and its enhancement upon light irradiation in NiMoS.

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Fig. 4. Overall water-splitting performance of NiMoS as bifunctional catalysts in 1.0 M KOH. (a) Polarization curves of two-electrode electrolyzer assembled by and

Pt/C-IrO2

couples.

(b)

Gas

evolution

amounts

over

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NiMoS-NiMoS

two-NiMoS-electrode electrolyzer at cell voltage of 1.62 V in dark and light fields

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with theoretical values calculated based on current densities at 100% Faraday efficiency. (c) Long-term chronoamperometry stability test over two-NiMoS-

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electrode electrolyzer with chopped illumination at cell voltage of 1.62 V; the inset is schematic illustration of two-NiMoS-electrode electrolyzer. In order to further verify the bifunctionality of NiMoS catalyst, we constructed a two-electrode alkaline electrolyzer using NiMoS coating NFs as both the anode and cathode for overall water splitting, as illustrated in Fig. S9. The polarization curves in Fig. 4a show that the NiMoS affords a current density of 10 mA cm-2 at a cell voltage 14

of 1.53 V, while the Pt/C-IrO2 coupled electrolyzer requires a cell voltage of 1.63 V at 10 mA cm-2. Impressively, when the two NiMoS electrodes are simultaneously illuminated, the current displays the faster growth with an earlier onset and reaches 10 mA cm-2 at only 1.50 V, identifying an outstanding performance toward overall water splitting which outperforms most of congeneric bifunctional catalysts reported recently (Table S5) [44,45]. Stoichiometric evolutions of H2 and O2 with a ratio of virtually two match well to full water-splitting reaction, as evidenced by the gas

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evolving amounts versus reaction time (Fig. 4b). The faraday efficiencies for HER and OER are estimated by comparing with theoretical gas evolutions to be both close

to 100%. The long-term stability test was conducted by the chronoamperometry

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measurement. As shown in Fig. 4c, the measured current remains almost constant

along with good reversible behavior upon the switched illumination. And the

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electrolyzer steadily operates overall water splitting over 100 h, guarantying the

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excellent stability of NiMoS as bifunctional catalysts in alkaline medium. Meanwhile, both two NiMoS electrodes generate a large amount of gas bubbles, observable by the naked eye (Movie S1). Moreover, the morphologies of NiMoS catalysts after test

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show no obvious variation compared with that before use (Fig. S10). These results not only demonstrate the applicability of NiMoS as the bifunctional catalysts but also

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propose the valuable capability of directly integrating renewable solar energy into

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water-splitting dynamics.

Herein, the HER and OER bifunctionality with optical gain in constructed NiMoS

heterostructure is experimentally verified; however, their origins are difficult to be precisely identified. In alkaline media, the HER kinetics is determined through the initial water dissociation and the concomitant chemisorption of hydrogen-containing intermediates (OH —

and H*) [29]; the OER kinetics is controlled by the 15

chemisorption and dissociation for OH— and oxygen-containing intermediates (OH* and OOH*) [46]. The recent density functional theory (DFT) calculation has revealed that the covalent Mo-S-Ni coordination could effectively facilitate the dissociation of O-H bonds in H2O molecules, OH* and OOH* intermediates [18,22]. Meanwhile, the undercoordinated Mo-S edges and high-valence NiIII/IV species have been well recognized as active HER and OER sites that possess suitable binding affinities to H*, OH* and OOH* intermediates, respectively [30,42]. Thus the superior bifunctionality

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in NiMoS should be ascribed to the abundant Mo-S-Ni covalent interface that

facilitates the water-dissociation step and the synergistic chemisorption abilities for both hydrogen and oxygen-containing intermediates. Moreover, the light-enhanced

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water-splitting activities can be attributed to the photoelectron transfer from excited

Ni3S2 to HER-active Mo-S edge sites and the accelerating formation of OER-active

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NiIII/IV sites, respectively.

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3. Conclusions

In summary, we design a 2D lateral heterostructure of ultrathin Ni3S2 NSs

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terminated with MoS2 edges as a robust bifunctional catalyst for overall water splitting. The resultant NiMoS catalyst exhibits the exceptionally high activity and

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stability for both HER and OER in alkaline media, comparable to and even superior to the commercial benchmarks (Pt/C and IrO2). As for overall water splitting, a cell

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voltage of only 1.53 V is required to achieve a current density of 10 mA cm-2, surpassing the most current bifunctional catalysts and even the IrO2-Pt/C couple. Especially, the light-assisted strategy to enhance synchronously HER and OER activities is proposed in NiMoS catalyst, enabling accessible solar integration into overall water splitting. Such superior catalytic performance is ascribed to the nano-assembly of active components with Mo-S-Ni coupling interfaces that favor 16

binding affinities to both hydrogen and oxygen-containing intermediates; moreover, the light-enhanced effect is associated with the charge contribution from photo-excited Ni3S2 component. We believe that this work here will promote the development of water-splitting bifunctional catalysts based on transitional metal chalcogenides. Conflict of Interest Form The manuscript was written through contributions of all authors. All authors have

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given approval to the final version of the manuscript. All authors declare no competing financial interest.

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Acknowledgements

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This work was supported by the National Natural Science Foundation of China (Nos. 11974303 and 11574263), the Open Project of Key Laboratory of Ministry of

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Industry and Information Technology in Nanjing University of Science and Technology, the Qing-Lan Project of Jiangsu Province and the Advanced Talent

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Development Plan of Yangzhou University.

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Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at

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***.

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