Surface Modification of a NiS2 Nanoarray with Ni(OH)2 toward

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Surface Modification of a NiS2 Nanoarray with Ni(OH)2 toward Superior Water Reduction Electrocatalysis in Alkaline Media Ling Zhang,† Ibrahim Saana Amiinu,‡ Xiang Ren,† Zhiang Liu,§ Gu Du,∥ Abdullah M. Asiri,⊥ Baozhan Zheng,*,† and Xuping Sun*,† †

College of Chemistry, Sichuan University, Chengdu 610064, China State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China § College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, China ∥ Chengdu Institute of Geology and Mineral Resources, Chengdu 610064, China ⊥ Chemistry Department, King Abdulaziz University, Jeddah 21589, Saudi Arabia

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S Supporting Information *

to accelerate the Volmer step because the edges of Ni(OH)2 sites can facilitate H2O dissociation and produce H* intermediates, which adsorb on the metal HER catalyst and subsequently generate H2. Inspired by this, we anticipate that hybridization of Ni(OH)2 with non-noble-metal sulfides is an efficient method toward enhanced HER performance in alkali media. Here, we report our recent effort toward this direction by the electrodeposition of amorphous Ni(OH)2 on NiS2 on titanium mesh [Ni(OH)2−NiS2/TM] to greatly enhance its HER activity in alkaline media. Ni(OH)2−NiS2/TM behaves as a highperformance and durable HER catalyst electrode with the need for an overpotential of 90 mV to deliver 10 mA cm−2 in 1.0 M KOH. Density functional theory (DFT) calculations show that H2O dissociation free energy of Ni(OH)2−NiS2 decreases obviously and hydrogen adsorption free energy is optimized significantly. NiS2/TM was converted from α-Ni(OH)2 (Figures S1−S3) via sulfuration reaction. The X-ray diffraction (XRD) pattern for sulfide (Figure 1a) is consistent with a NiS2 standard pattern (JCPDS 11-0099).20 Other strong peaks are diffraction peaks of titanium from the TM (JCPDS 44-1294). After electrodeposition of Ni(OH)2, the intensity of NiS2-related diffraction characteristic peaks decreases and no Ni(OH)2-related peak is observed, confirming the formation of amorphous Ni(OH)2. The XRD pattern of directly electrodeposited Ni(OH)2/TM further suggests the amorphous structure of Ni(OH)2. Parts b−d of Figure 1 show the X-ray photoelectron spectroscopy (XPS) spectra for Ni(OH)2−NiS2/TM. In the Ni 2p region (Figure 1b), the spectrum exhibits two major peaks at 855.7 and 873.3 eV assigned to Ni 2p3/2 and Ni 2p1/2,21,22 respectively. The spin− orbit splitting energy between the two Ni 2p peaks is 17.6 eV, indicating the appearance of Ni(OH)2.22 The other two peaks (879.4 and 861.5 eV) are the satellite peaks of NiO, which imply the existence of surface oxidation on NiS2.21 In the S 2p spectrum (Figure 1c), the peak at 163.2 and 164.6 eV are ascribed to typical S22− species.23 It is possible to identify that another peak at 167.8 eV is assigned to the presence of SO32−/SO42− species on the surface of NiS2 derived from exposure to air.24 The peaks for S 2p

ABSTRACT: Interface engineering has been demonstrated to be effective in promoting hydrogen evolution reaction (HER) in an alkaline solution. Herein, we report that the HER activity of a NiS2 nanoarray on a titanium mesh (NiS2/TM) in alkaline media is greatly boosted by the electrodeposition of Ni(OH)2 onto NiS2 [Ni(OH)2− NiS2/TM]. Ni(OH)2−NiS2/TM only needs an overpotential of 90 mV to deliver 10 mA cm−2 in 1.0 M KOH. Density functional theory calculations confirm that Ni(OH)2−NiS2 has a lower water dissociation free energy and a more optimal hydrogen adsorption free energy than NiS2.

T

he increasing depletion of fossil fuels and associated environmental effects have prompted the search for clean alternatives as energy carriers.1,2 Hydrogen (H2) is an ideal alternative with high energy output and combustion products free of carbon.3,4 Water splitting in an alkaline solution is a promising way for pure H2 generation.5 However, the main challenge of electrochemical water splitting in an alkaline solution is the high overpotential required for hydrogen evolution reaction (HER).6,7 Platinum (Pt) is considered to be the best catalyst toward the HER in alkaline electrolyzers but suffers from high cost and scarcity.8 Many efforts have been focused on the development of cost-effective HER electrocatalysts in basic conditions based on earth-abundant elements,9−16 but their performance is still unsatisfactory, and thus enhancing their catalytic activity in alkaline media still remains challenging. The fundamental HER mechanism in alkali media typically consists of an initial hydrogen intermediate formation (H*, Volmer step) and concomitant molecular H2 generation (Heyrovsky or Tafel step).17,18 Compared with the same process in acidic media, the Volmer step in alkali media is more difficult because of the slow kinetics of the initial H2O dissociation.18 Therefore, promoting the H2O dissociation process is crucial to improving the HER catalytic activity of electrocatalysts in alkaline media. Recently, some researchers reported that Ni(OH)2 in metal catalysts can promote H2O dissociation18,19 © 2017 American Chemical Society

Received: September 26, 2017 Published: November 7, 2017 13651

DOI: 10.1021/acs.inorgchem.7b02466 Inorg. Chem. 2017, 56, 13651−13654

Communication

Inorganic Chemistry

of NiS2 gives lattice fringes with a distance of 0.27 nm, assigned to the NiS2(200) facet (Figure 2e). Figure 2f shows an ambiguous crystal lattice, which confirms that Ni(OH)2 on the NiS2 surface is amorphous. The electrocatalytic activity of Ni(OH)2−NiS2/TM toward the HER was examined in 1.0 M KOH. Figure 3a shows linear-

Figure 1. (a) XRD patterns of Ni(OH)2−NiS2/TM, NiS2/TM, and electrodeposited Ni(OH)2/TM. (b) Ni 2p, (c) S 2p, and (d) O 1s XPS spectra of Ni(OH)2−NiS2/TM.

in Ni(OH)2−NiS2/TM show negative shifts compared with those of NiS2/TM (Figure S4). Such negative shifts suggest strong electron interactions between NiS2 and Ni(OH)2,25 which may cause an activity change of NiS2 around the surface. The O 1s spectrum (Figure 1d) shows two O-atom contributions. The peak at 531.2 eV is attributed to the O atom in OH−.22,26,27 The peak at 532.7 eV is ascribed to adsorbed water on the catalyst surface.28 Scanning electron microscopy (SEM) images show that the entire surface of TM is covered with a NiS2 nanosheet array (Figure S5a). A closer view (inset in Figure 2a) concludes that

Figure 3. (a) LSV curves of Ni(OH)2−NiS2/TM with different electrodeposition times. (b) LSV curves of Pt/C, Ni(OH)2−NiS2/TM, NiS2/TM, and bare TM for HER. (c) Tafel plots of Pt/C, Ni(OH)2− NiS2/TM, and NiS2/TM. (d) LSV curves for Ni(OH)2−NiS2/TM before and after 1000 cycles and i−t curve of Ni(OH)2−NiS2/TM under an overpotential of 220 mV.

sweep-voltammetry (LSV) curves. The Ni(OH)2−NiS2/TM samples prepared with different electrodeposited times show HER activities superior to those of NiS2/TM, and the sample obtained for 60 s [named as Ni(OH)2−NiS2/TM] exhibits the most efficient activity. Figure 3b compares the HER activities of bare TM, NiS2/TM, Ni(OH)2−NiS2/TM, and Pt/C coated on TM. It is seen that Pt/C exhibits the best HER catalytic activity, while the TM is inactive. Ni(OH)2−NiS2/TM demands an overpotential of 90 mV to deliver 10 mA cm−2, 101 mV smaller than that for NiS2/TM (η10 mA cm−2 = 191 mV). This overpotential outperforms most reported NiS2 and other nickelbased HER catalysts in alkaline electrolytes (Table S1). Figure 3c shows Tafel plots for the as-prepared electrodes. The Tafel plot slopes of Ni(OH)2−NiS2/TM, NiS2/TM, and Pt/C are 89, 169, and 72 mV dec−1, respectively, suggesting markedly favorable HER kinetics for Ni(OH)2−NiS2/TM. Figure S8 displays a chronopotentiometric curve for Ni(OH)2−NiS2/TM obtained at a multicurrent step from 40 to 400 mA cm−2. The potentials are well kept throughout the current density change steps from 40 to 400 mA cm−2, demonstrating the excellent conductivity, rapid mass transport, and mechanical robustness of Ni(OH)2− NiS2/TM.29 This improvement is ascribed to the fact that the edges of Ni(OH)2 sites facilitate H2O dissociation and produce H* intermediates, which then adsorb on NiS2 and recombine into H219 (Figure S9). We evaluated the stability of Ni(OH)2− NiS2/TM by cyclic voltammetry scanning from −0.25 to −0.15 V versus RHE. After 1000 cycles, the LSV curve of Ni(OH)2− NiS2/TM appears to nearly overlap with the initial one (Figure 3d). The i−t (current density vs time) curve of Ni(OH)2−NiS2/ TM under an overpotential of 220 mV shows no obvious loss of the current density after 27 h of electrolysis (Figure 3d). Moreover, Ni(OH)2−NiS2/TM still remains the integral morphology after the test (Figure S10), further demonstrating

Figure 2. SEM images for (a) NiS2/TM and (b) Ni(OH)2−NiS2/TM. TEM and HRTEM images of (c and e, respectively) NiS2 and (d and f, respectively) Ni(OH)2−NiS2 nanosheet.

the NiS2 nanosheet array is well-aligned and vertically grown. After electrodeposition, the nanosheets with a height of 5.1 μm (Figure S6) become rough (Figure 2b). Energy-dispersive X-ray (EDX) elemental mapping images (Figure S7) suggest a uniform distribution of Ni, S, and O elements in the nanosheets. The transmission electron microscopy (TEM) images (Figure 2c,d) demonstrate that NiS2 and Ni(OH)2−NiS2/TM are typical flakelike structures. The high-resolution TEM (HRTEM) image 13652

DOI: 10.1021/acs.inorgchem.7b02466 Inorg. Chem. 2017, 56, 13651−13654

Communication

Inorganic Chemistry its robust nature. Ni(OH)2−NiS2/TM has an Faradic efficiency (FE) of 97.5% for electrochemical H2 production (Figure S11). The catalytic HER activity in alkali media is considered to be controlled by the activation energy for H2O dissociation as well as the adsorption and rate of recombination of the H* intermediates, implying H2O dissociation is the first ratedetermining step.19,30,31 Therefore, to gain more details on the synergistic effect of the composite structure [Ni(OH)2−NiS2] toward HER, a series of DFT calculations were performed using model structures (Figure S12). Therefore, the HER reaction pathways (Figure 4a), including the initial H2O dissociation into

Ni(OH) 2 and NiS 2 at the interface of Ni(OH) 2 −NiS 2 significantly improves on the sluggish HER kinetics of NiS2 and then promotes reduction of the energy barrier for effective H2O dissociation in the initial step as well as in subsequent adsorption of H* intermediates for efficient H2 production. In summay, electrodeposition of amorphous Ni(OH)2 on a NiS2 nanoarray is effective toward greatly boosting the HER activity of a NiS2 nanoarray in alkali. Such a Ni(OH)2−NiS2/TM is highly efficient and durable with the need for an overpotential of 90 mV to deliver 10 mA cm−2 in 1.0 M KOH. The superior catalytic activity is due to cooperation between NiS2 and Ni(OH)2, which enhances the alkaline HER of NiS2/TM. This study offers a universal strategy toward making nanoarray electrodes with metal hydroxide−sulfide interfaces34 for applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02466. Experimental section, structural model, Tables S1 and S2, schematic diagrams, SEM images, XRD pattern, XPS spectra, EDX elemental mapping images, chronopotentiometric curve, and FE data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.Z.). *E-mail: [email protected] (X.S.). ORCID

Figure 4. (a) Mechanism of H2O activation, formation of intermediate H*, and recombination for H2 generation on Ni(OH)2−NiS2. (b) Calculated hydrogen binding energy diagram. (c) Calculated reaction energy diagram and corresponding catalyst surface structure and reaction mechanism (inset) at different stages of H2O dissociation toward H2 generation. The blue dotted circles indicate the state of preferential adsorption of an intermediate on the catalyst surface.

Ibrahim Saana Amiinu: 0000-0003-4426-7893 Abdullah M. Asiri: 0000-0001-7905-3209 Baozhan Zheng: 0000-0003-4060-5912 Xuping Sun: 0000-0001-5034-1135 Notes

The authors declare no competing financial interest.

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a H* intermediate (Volmer step) and H2 generation (Tafel or Heyrovsky step), were evaluated. The calculated free energies of hydrogen adsorption (ΔGH*) onto Ni(OH)2, NiS2, and Ni(OH)2-NiS2 surfaces are −0.402, −0.345, and −0.143 eV, respectively (Figure 4b). This indicates that the coupling effect of the two phases results in an optimum hydrogen adsorption strength on Ni(OH)2 −NiS 2. To better understand the synergistic effect of the catalyst, the H2O dissociation path was also evaluated. As shown in Figure 4c, NiS2 exhibits very weak adsorption strength for OH* intermediates with a free energy of ΔGH2O = 0.837. Such a high value is undesirable for effective H2O dissociation into H* intermediates and will render the HER kinetic process sluggish. Previous studies have proven that Ni(OH)2 acts as a favorable active component for the effective adsorption of OH* intermediates, suggesting it to be a good promotor for H2O dissociation.32,33 Interestingly, by coupling Ni(OH)2 with NiS2, the composite structure [Ni(OH)2−NiS2] exhibits a significantly reduced ΔGH2O value of 0.101 eV. This indicates that Ni(OH)2 effectively stimulates cleavage of H−OH bonds to produce H* intermediates, which then adsorb on the active site of neighboring NiS2 at the Ni(OH)2−NiS2 interface. The electron redistribution on the Ni(OH)2−NiS2 interface also enhances reduction of the free energy for intermediate H* adsorption at the interface. Therefore, the synergy between

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 21575137) and Science and Technology Innovation Foundation of Jilin Province for Talents Cultivation (Grant 20150519014JH).

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DOI: 10.1021/acs.inorgchem.7b02466 Inorg. Chem. 2017, 56, 13651−13654