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Hierarchical NiCo2S4@NiFe LDH Heterostructures Supported on Nickel Foam for Enhanced Overall-Water-Splitting Activity Jia Liu, Jinsong Wang, Bao Zhang, Yunjun Ruan, Lin Lv, Xiao Ji, Kui Xu, Ling Miao, and Jianjun Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 23 Mar 2017 Downloaded from http://pubs.acs.org on March 24, 2017

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ACS Applied Materials & Interfaces

Hierarchical NiCo2S4@NiFe LDH Heterostructures Supported on Nickel Foam for Enhanced Overall-Water-Splitting Activity Jia Liu, Jinsong Wang, Bao Zhang, Yunjun Ruan, Lin Lv, Xiao Ji, Kui Xu, Ling Miao*, Jianjun Jiang* School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, PR China

* Corresponding author E-mail: [email protected]; [email protected]

ACS Paragon Plus Environment


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Abstract Low cost and high efficient bifunctional electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) are intensively investigated for overall water splitting. Herein, we combined experimental researches with first-principles calculations based on density functional theory (DFT) to engineer the NiCo2S4@NiFe LDH heterostructures interface for enhancing overall water splitting activity. The DFT calculations exhibit a strong interaction and charge transfer between NiCo2S4 and NiFe LDH, which change the interfacial electronic structure and surface reactivity. The calculated chemisorption free energy of hydroxide (∆EOH) reduces from 1.56 eV for pure NiFe LDH to 1.03 eV for the heterostructures, indicating the dramatic improvement of OER performance, while the chemisorption free energy of hydrogen (∆EH) maintains almost invariable. By using the facile hydrothermal method, NiCo2S4 nanotubes, NiFe LDH nanosheets and NiCo2S4@NiFe LDH heterostructures are prepared on nickel foam, of which the corresponding experimental OER overpotentials are 306 mV, 260 mV and 201 mV at 60 mA cm-2, respectively. These results are good agreement with the theoretical predictions. Meanwhile, the HER performance has a little improvement with overpotential about 200 mV at 10 mA cm-2. Due to the OER performance has a dramatic improvement, exhibit an enhanced overall water splitting activity of the NiCo2S4@NiFe LDH heterostructures with a low voltage of 1.6 V. Keywords: Heterostructures; Hydrogen evolution reaction; Oxygen evolution reaction; Density functional theory; Overall water splitting


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1. Introduction Due to the high energy density, low weight, high abundance, and environment friendly, hydrogen has attracted much attention to serve as an ideal energy source to replace fossil fuels.1, 2 As one of the energy technologies to generate hydrogen fuel, the electrocatalytic water splitting has been widely regarded as a promising and sustainable approach of producing clean hydrogen fuel from aqueous solutions.3, 4 The water splitting reaction consists of two half reactions, namely hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).5, 6 At present, most of state-ofart electrocatalysts are noble metals such as Pt-group metals, Ir-based and Ru-based materials. It is necessary to design an earth-abundant, highly active, highly durable electrocatalysts as efficient electrode materials for water splitting to substitute the scarcity and expensiveness of these noble metal based catalysts.7-9 For the HER electrocatalysts, the traditional transition metal sulfides,10, selenides,12, 13 and phosphides6, 14

11

are the promising candidates, and much effort has

been devoted to improving their HER performance.15-18 Meanwhile the transition metal oxides19-21 and hydroxide22-26

have emerged as highly active catalyst for the

OER. However, to accomplish high overall water splitting performance, the coupling of HER and OER catalysts in the alkaline solution by using the same catalyst as both anode and cathode is a great challenge. The current universal methods, likely, Doping,27 introducing vacancy,28 and straining29 are efficiently to decrease the half reaction overpotential. Nevertheless, these often result in contradictory integration of the two kinds of catalysts and lead to inferior overall water splitting performance.



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Recently, several researches focused on utilizing heterostructures to improve the overall water splitting. Zhao et al.30 designed hierarchical NiFe/NiCo2O4/Ni foam by electrodeposition method as bifunctional catalysts, which showed highly efficient whole cell water splitting. Furthermore, Zhang et al.31 presented the interface engineering of MoS2/Ni3S2 heterostructures as bifunctional electrocatalysts, which exhibited a good overall water splitting performance. Geng et al.32 demonstrated NiFe-LDH ultrain sheets grown on NiCo2O4 nanowire arrays as an efficient bifunctional catalyst toward both HER and OER reaction. Feng et al.33 reported s ternary hybrid that is constructed by in situ growth of Cobalt selenide nanosheets vertically oriented on exfoliated graphene foil, with subsequent deposition of NiFe LDH by a hydrothermal treatment. The resulting 3D hierarchical hybrid exhibits excellent electrocatalytic activity for overall water splitting in alkaline conditions. Based on the above-reported results, it can be concluded that interface engineering would be a highly-efficiency approaches to reconcile HER and OER electrocatalysts to achieve efficiently overall water splitting. Herein, we combined experimental research with first-principles calculations based on density functional theory (DFT) to engineer a heterostructural interface with high catalytic activity between the NiCo2S4 nanotubes and NiFe LDH nanosheets for overall water splitting. The DFT calculation shows that chemisorption free energy of hydroxide at the interface between NiCo2S4 and NiFe LDH has an obvious reduction compared to that of individual component, which should be a high electrocatalytic region for enhanced OER performance. Moreover, the NiCo2S4@NiFe LDH


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heterostructures synthesized by a three-step hydrothermal method exhibits a low overpotential of 201 mV at 60 mA cm-2 and the overall water splitting voltage is 1.6 V at the current density of 10 mA cm-2. Hence, the experimental performance of the NiCo2S4@NiFe LDH heterostructures matches well with the computational prediction, indicating the interface engineering of heterostructures could be a prospective strategy for efficient overall water splitting.

2. Experimental section 2.1. Theoretical Simulation Our calculations are performed using VASP code, based on density-functional theory (DFT).34,

35

The exchange correlation functional and projector augmented-

wave (PAW) of Perdew-Burke-Ernzerhof (PBE) with van der Waals correction DFTD3 were adopted.36, 37 A plane wave cutoff of 500eV and a k-point mesh of 5×5×1 in the Monkhorst Pack sampling are used, where the self-consistent convergence of the total energy is 1.0×10-4 eV per atom. The construction with a 20 Å vacuum region in the z direction to minimize the interactions between adjacent image cells by relaxed via the conjugate-gradient method until the forces on all atoms were smaller than 0.04 eV/Å.

2.2. Synthesis of NiCo2S4 nanotube arrays on foam The NiCo2S4 nanotube arrays on Ni foam were synthesized by a three-step hydrothermal reaction. Firstly, to fabricate the precursor, 4 mmol nickel chloride



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hexahydrate (NiCl2·6H2O), 8 mmol cobalt chloride hexahydrate (CoCl2·6H2O) and 12 mmol urea were mixed together in 60 ml deionized (DI) water, then stirring for 15 min to obtained a clear solution. The above solution was transferred into Teflon-lined stainless-cleaned 100 ml autoclave. Before using the Ni foam as a substrate to in situ grow the arrays, it was washed with hydrochloric acid, acetone, ethanol and DI water to remove the NiO layer from the surface. Then, the surface-cleaned Ni foam (2 cm × 4 cm) was immersed into the solution. The hydrothermal reaction was conducted at 120 ℃ for 10 h. After reaction, the Ni foam was taken out, rinsed with DI water to remove any unreacted residues. Secondly, 0.3 g sodium sulfide nonahydrate (Na2S·9H2O) was added into 60 ml deionized water with stirring for 10 min to obtain a homogeneous solution, the above Ni foam was immersed into the solution. Then the solution was transferred into Teflon-lined stainless-cleaned 100 ml autoclave and maintained at 180 ℃ for 6 h. After cooling to room temperature, the Ni foam was wash several times with DI water and ethanol, respectively, followed by drying at 60 ℃.

2.3. Synthesis of heterostructures NiCo2S4 @ NiFe LDH on Ni foam 1 mmol ferric nitrate nonahydrate (Fe(NO3)3·9H2O), 3 mmol nickel nitrate hexahydrate (Ni(NO3)2·6H2O) and 5 mmol urea was dissolved into 60 ml DI water with stirring for 15 min, then the as-obtained NiCo2S4 nanotube arrays on Ni foam were immersed into the solution and transferred the solution into a 100 ml Teflon-


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lined stainless steel autoclave. The reaction completed at 120 ℃ for 10 h. Finally, NiCo2S4@NiFe LDH arrays on Ni foam was cleaned by DI water and ethanol, respectively, and vacuum-dried overnight. In order to contrast with NiFe LDH arrays on Ni foam, a surface-cleaned Ni foam was immersed into the above solution, and then the product was washed with DI water, ethanol, then dried over-night.

2.4. Characterization The phase formation was identified using powder XRD (Philips X’pert PRO; Cu Kα, λ = 0.1524 nm). The morphologies of the catalysts were observed by scanning electron microscope (SEM FEI Quanta 200, FESEM JEOL JSM-7100F). The X-ray photoelectron spectroscopy (XPS) spectra were measured on Kratos AXIS Ultra DLD-600W XPS system equipped with a monochromatic Al Kα (1486.6 eV) as X-ray source.

2.5. Electrochemical Measurements All electrochemical tests were performed by using CHI760e electrochemical workstation. The platinum wire was served as the counter electrode, an Ag/AgCl (saturated KCl solution) was used as the reference electrode, and the synthesized NiCo2S4@NiFe LDH/NF was utilized as working electrode. All electrochemical were performance in 1 M KOH aqueous electrolyte which was deaerated with high-purity



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argon. For the OER performance, all linear sweep voltammograms (LSV) were measured from 0 to 0.7 V versus saturated Ag/AgCl at a scan rate of 2 mV s-1. And the HER performance, LSV were measured from -0.8 to -1.4 V versus saturated Ag/AgCl at a scan rate of 5 mV s-1. Chronoamperometric measurements were performed at corresponding potential to deliver a current density of 10 mA cm-2 for 10 h. The potential converted to RHE scale using equation ERHE=EAg/AgCl + 0.197 + 0.059 × PH, where the ERHE is the potential referred to RHE and EAg/AgCl is the measured potential against Ag/AgCl reference electrode. The overpotentials were obtained from the intersection of the tangents of LSV current and the polarization curve baseline. The electrochemically active surface areas (ECSAs) can be estimated from the electrochemical

double

layer

capacitance

(Cdl)

through

collecting

cyclic

voltammograms (CVs), CV tests with different scan rates (2, 4, 6 and 8 mV·s-1) in the potential range from 0 to 0.05 V versus Ag/AgCl. The electrochemical impedance spectroscopy (EIS) measurements were carried out in 1.0 M KOH solution, with the frequency range from 100 kHz to 0.1 Hz.

3. Results and Discussion The atomic structure schematic diagram of the NiCo2S4@NiFe LDH heterostructures is shown in Fig. 1a and Fig. S1. The interaction between the NiFe LDH (0 0 1) surface and the NiCo2S4 (1 0 0) surface can be evaluated by the interfacial adhesion energy defined as = − − , where , and is the energy of the NiFe LDH (0 0


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1) adsorbing on NiCo2S4 (1 0 0) surface, the energy of the NiFe LDH (0 0 1) surface, and the energy of NiCo2S4 (1 0 0) surface, respectively. The calculated adhesion energy is -1.43eV, which indicates a strong interaction energy between the NiFe LDH (0 0 1) surface and the NiCo2S4 (1 0 0) surface. The chemisorption free energies of hydrogen (∆EH) and hydroxide (∆EOH) on the (001) surface of NiFe LDH and the NiCo2S4@NiFe-LDH are shown in Fig. 1b. NiFe LDH exhibits a poor catalytic activity for HER and OER because of a high chemisorption free energy ∆EH and ∆EOH of 1.32eV and 1.56 eV, respectively, which is in accordance with previous reports. Considering the formed interface between NiFe LDH and NiCo2S4, ∆EH possesses a small increment while ∆EOH shows significant decrease. Accordingly, formed interface facilitates the rupture of the O-H bonds of the H2O molecule to accelerate the OER process, and the effect for HER is negligible simultaneously. It is consistent with recent experiments that heterostructures present significant increasing on HO-chemisorption energy. To clarify the effect of the constructed interfaces for the chemisorption of hydrogen and oxygen-containing intermediates, the Bader charges at their interfaces are analyzed to identify the interlayered electronic interaction between NiFe LDH and NiCo2S4 (Fig. 1c). And the results shown that the charge redistribution in the form of 0.28 e− transfer from NiFe LDH to NiCo2S4, which leads to an electron-rich region on NiCo2S4 and a hole-rich region on NiFe LDH sheet, causing the Fermi energy movement. Therefore, the interface between NiFe LDH and NiCo2S4 has significant effects on the activity of electrocatalysts. The density of states (DOS) and project density of states (PDOS) of NiFe LDH



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and NiCo2S4@NiFe LDH heterostructures are exhibited in Fig. 2. The overlaps of porbital of O atom and the d-orbital of Fe and Ni indicates p-d hybridization, resulting from the formed O-Fe and O-Ni bonds. The major contributions for DOS near Fermi levels are attributed to localized Fe-3d and Ni-3d orbitals. With the formation of interface between NiCo2S4 and NiFe LDH, Ni-3d and O-2p orbitals do not change obviously except for the decreasing of the d-bands center, thus the ∆GH is higher compare with that of NiFe LDH. Moreover, the PDOS of Fe-3d possesses a significant change due to the effect of interface, after coupling NiFe LDH with NiCo2S4. The Fe-3d orbitals are fully occupied from partially occupied state, and Bader charges for Fe ions change from +1.4 to +1.8, resulting stronger chemisorption free energies of hydroxide for enhanced OER performance. Figure.3a displays the schematic illustration of in situ growth of NiCo2S4@NiFe LDH heterostructures on a Ni foam substrate using a hydrothermal method. In the first step, Ni-Co-carbonates hydroxide nanowire arrays was in situ grown on NF through a simple hydrothermal method, and a pink thin film was uniformly deposited onto the skeleton of NF. Next, by the anion-exchange reaction, the Ni-Co-carbonates hydroxide nanowire arrays converted into spinel NiCo2S4 nanotube arrays, through a second hydrothermal method by using sodium sulfide as a sulfur source. During the sulfidation process, the active sulfide ions (S2-) released from sodium sulfide upon hydrolyzation reaction, then S2- anions in the solution exchanged with CO3-and OHanions in the Ni-Co-carbonates hydroxide to form a thin layer of Ni-Co sulfides at the surface of nanowire arrays. As the reaction progress, the inner Ni-Co-carbonates


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hydroxide spontaneously outward diffused to external surface and the sulfide ions inward diffused through the thin layer. Because of the diffusion rate of these ions, the starting nanowires become hollow nanotubes gradually.38, 39 And the pink precursor turned to black NiCo sulfides after hydrothermal reaction. Finally, the NiFe LDH nanosheets were coated on the NiCo2S4 nanotube arrays to form the NiCo2S4@NiFe LDH heterostructures by a third hydrothermal reaction, and yellow production can be observed on the NF. The smooth surface of NiCo2S4 changes into rough after the final reaction process, showing that NiFe LDH could grow on the surface of NiCo2S4 nanotube arrays. To investigate the morphology of the as-synthesized NiCo2S4@NiFe LDH/NF heterostructures, the SEM images of the catalysts are shown in Fig. 4, and the bare Ni foam shows in Fig. S2. It is clearly that the surface of the NiCo2S4 nanotube arrays is smoothly (Fig. 4a, b). After further hydrothermal, the surface of the NiCo2S4 nanotube arrays are very rough (Fig. 4c), many nanosheets are uniformly growing on theses nanotubes. It indicates that the interaction between NiCo2S4 nanotube and hydroxide nanosheets is actually exists. This unique structure would promote the performance of overwater splitting. Further structural information and morphology evolution are carried out by TEM. The TEM images of NiCo2S4 nanotube arrays show in Fig. 4d and e, the diameter of the NiCo2S4 nanotubes is about 200 nm and the wall thickness is confirmed to be 25 nm. In addition, the high-resolution transmission electron microscopy (HRTEM) image is shown in Fig. 4f, where the visible lattice fringes with a spacing of about 0.52 nm and 0.277 nm, attributing to the (111) and (311) planes of



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the cubic phase. Furthermore, the TEM images of the NiCo2S4@NiFe LDH/NF heterostructures reveal that the NiFe LDH nanosheets are grow on NiCo2S4 nanotube arrays, and the size of these hydroxide nanosheets is about several tens of nanometers. The lattice sapcing of 0.25 nm corresponding to the (101) planes of the NiFe LDH is observed (Fig. 4g-4i). The detailed elemental distribution studied by STEM-mappings are shown in Fig. 4j-4n, which reveal the uniform distribution of Ni, Co, Fe, and S atoms at the surface of the unique heterostructures. It suggested that the hydroxide truly is NiFe layer double hydroxide. To further investigate the NiCo2S4@NiFe LDH/NF, the XRD patterns of as prepared NiCo2S4/NF, NiFe LDH/NF, NiCo2S4@NiFe LDH/NF are shown in Fig 5a. The XRD patterns confirm the co-existence of NiCo2S4/NF, NiFe LDH/NF. The XRD spectrum of the as-grown NiCo2S4@NiFe LDH/NF shows peaks at 2θ = 12.1°, 24.4°, 33.3° and 59.5° are assigned to the (003), (006), (012) and (110) planes of Ni(OH)2 (JCPDS No. 25-1363), while the other peaks are consistent with NiCo2S4 (JCPDS No. 43-1477). In addition, the peak at 21.7° is due to the small amount of Ni3S2.40 The Ni3S2 might have formed when the Ni foam was immersed in to the sodium sulfide solution, other high peaks at 44.2° and 52° can be observed is the Ni foam (JCPDS No. 03-1051). X-ray photoelectron spectroscopy (XPS) is carried out to study the chemical states of NiCo2S4@NiFe LDH/NF heterostructures. The Ni XPS spectrum shows in Fig. 5b, there is a pair of peaks at 856.1 eV and 873.7 eV, corresponding to Ni 2p3/2 and Ni 2p1/2, which suggested that the valence of Ni is 2+. And a spin-energy separation of


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17.6 eV indicates the existence of the Ni(OH)2 phase, which is also agreement with the XRD analysis as mention above.41 The other two peaks at 861.8 eV and 880.1 eV are related to shakeup satellites. The XPS spectrum of Fe 2p shows in Fig. 5c, a pair of peaks at 713.7 eV and 723.7 eV corresponds to Fe 2p3/2 and 2p1/2, indicating the Fe3+ oxidation states of NiFe LDH nanosheets. For the XPS spectrum of Co 2p in Fig. 5d, the peaks of Co 2p1/2 at 796.8 eV and Co 2p3/2 at 781.4 eV, and the spin-energy separation of 15.4 eV suggests the coexistence of Co2+ and Co3+.42-44 In the S 2p spectrum of NiCo2S4@NiFe LDH/NF (Fig.S3), the peak at 161.2 eV is corresponding to the S2-, and the peak at 169 eV is corresponding to the shakeup satellite. Thus, according to the above XPS analysis, the near-surface of the NiCo2S4@NiFe LDH/NF sample has a composition of Co2+, Co3+, Ni2+, Fe3+cations, and S2- anions. This result confirms that NiFe LDH have successfully grown on the NiCo2S4 nanotube arrays. The catalytic activities of the NiCo2S4@NiFe LDH/NF electrode have been characterized for OER in 1 M KOH using a three-electrode system at a scan rate of 2 mV s-1, and the polarization curve is shown in Fig. 6a. In order to examine the effect of heterostructures on the Ni foam substrate, the NiCo2S4/NF and NiFe LDH/NF were also prepared, and tested their OER activities under the identical conditions. Fig. 6a shows the corresponding polarization data of the as-obtained composite electrodes with 95% iR-compensation. Liner sweep voltammetry (LSV) polarization curves reveal great difference of OER activities for the various electrocatalysts. Fig. 6b shows the NiCo2S4@NiFe LDH/NF with a low overpotential of 201 mV at the current density of 60 mA cm-2, which is extremely lower than that of NiCo2S4/NF (306 mV)



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and NiFe LDH/NF (260 mV). Fig. 6c displays the Tafel plots obtained with the NiCo2S4@NiFe LDH/NF, NiCo2S4/NF and NiFe LDH/NF electrodes derived from the OER polarization curves. The NiCo2S4@NiFe LDH heterostructures show lower Tafel slope of 46.3 mV dec-1. In contrast, the Tafel slope of NiCo2S4/NF and NiFe LDH/NF is calculated as 56.8 and 48.7 mV dec-1, respectively. The electrocatalytic durability of the NiCo2S4@NiFe LDH/NF heterostructures is tested in 1 M KOH by the chronopotentiometry method at the current density of 10 mA cm-2 as shown in Fig. 6d. The operating potential is very stable for 10 h, which exhibit the excellent durability of the heterostructures. The polarization curves without iR-correction are shown in Fig. S4a. And the stability of the NiCo2S4 is shown in the Fig. S5a, the overpotential of the NiCo2S4 with small increased. It would be the irreversible reaction between the electrode materials and the electrolyte.45 The HER electrocatalystic performance of the NiCo2S4@NiFe LDH/NF heterostructures was conducted using the linear sweep voltammetry (LSV) at a scan rate of 5 mV s-1 in 1 M KOH aqueous solution. For comparison, the corresponding properties of NiCo2S4/NF and NiFe LDH/NF were also measured in 1 M KOH solution. As shown in Fig. 7a the NiCo2S4@NiFe LDH/NF requires a very low overpotential of 200 mV to deliver the current density of 10 mA cm-2. The overpotential for 20% Pt/C and NiCo2S4/NF and NiFe LDH/NF are 40 mV, 248 mV and 225 mV, respectively. Furthermore, the Tafel slope of the 20% Pt/C, NiCo2S4/NF, NiFe LDH/NF and NiCo2S4@NiFe LDH/NF is 48 mV/dec, 113.7 mV/dec, 85.8 mV/dec, and 101.1 mV/dec, respectively (Fig. 7b). The polarization curves without


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iR-correction are shown in Fig. S4b. And the error bars is shown in the Fig. S6 to indicate this experiment is repeatable. In order to obtain the optimal HER and OER activity, the relative mass ratio of NiCo2S4 and NiFe LDH is varied through changing the reaction time (Fig. S7). If any component was too much, for both of which the catalytic performance would be degraded.32 All of the studies described above indicate that the NiCo2S4@NiFe LDH/NF would be an active and stable bifunctional electrocatalysts for both OER and HER in alkaline solution. Therefore, the two electrode system was conducted by employing NiCo2S4@NiFe LDH/NF as both anode and cathode electrocatalysts for overall water splitting. This system was measured in 1 M KOH solution at a scan rate of 2 mV s-1. As shown in Fig. 7c, the voltage of NiCo2S4@NiFe LDH as a bifunctional overall water splitting catalysts is 1.6 V at the current density of 10 mA cm-2, which is comparable with other bifunctional electrocatalysts reported in the literature (Table.1). And a comparative summary of the performance of state-of-the art catalysts for OER and HER in recent literature is provided in Table S1, S2. The durability of NiCo2S4@NiFe LDH/NF as catalyst is investigated by multiple current tests at the continuous current density of 10 mA cm-2. As seen from Fig. 7d, the voltage of overall water splitting remains to be stable after 12h. The oxidation state, crystal structure and the morphology of the catalyst after overall water splitting are shown in the Fig. S8 and Fig. S9. X-ray photoelectron spectroscopy (XPS) is carried out to study the chemical states of NiCo2S4@NiFe LDH/NF heterostructures after overall water splitting in Fig. S8. the XPS spectrum of



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Ni appears two new peaks at 875.5 eV and 857.8 eV, compared to that of the original catalyst, which indicates the generation of Ni3+. It means partial Ni would change from Ni2+ to Ni3+ after a long time chronoamperometry test. Meanwhile the XPS spectrum of Co, Fe do not show any obvious change. To further investigate the change of the NiCo2S4@NiFe LDH/NF heterostructures, the XRD pattern is shown in Fig. S9a. The peaks of NiFe LDH are disappeared for NiCo2S4@NiFe LDH/NF after overall water splitting. The reason is that the NiFe LDH might change into amorphous Fe-NiOOH after long time overall water splitting test.46, 47 And it is consistent with the XPS results of new peaks of Ni3+. The morphology of NiCo2S4@NiFe LDH/NF after overall water splitting is shown in Fig. S9b, indicates almost no change of the nano structure. All the results indicate that the heterostructures of NiCo2S4@NiFe LDH/NF could be a promising candidate catalyst for alkaline water splitting. In order to carefully investigate the mechanism from experimental data, the electrochemically active surface area (ECSA) and electrochemical impedance spectroscopy (EIS) are supplemented to compare the electrochemically active surface area and the reaction kinetics of the designed NiCo2S4@NiFe LDH heterostructures and NiCo2S4 on Ni foam. The electrochemical double-layer capacitance (Cdl) is estimated by measuring voltammograms in a non-faradic region from 0 to 0.05 V vs. Ag/AgCl (Fig. S10). The Cdl is linearly proportional to the ECSA, which can be derived from the slope of the linear relationship between current density and scan rate (Fig. 8a). The Cdl of NiCo2S4/NF (43.6 mF cm-2) is nearly equal with that of NiCo2S4@NiFe LDH/NF (42.1 mF cm-2), which means that they have the same active


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site number at the solid-liquid interface. Therefore, the good OER performance for the NiCo2S4@NiFe LDH/NF heterostructures is not mainly attributed to the increasing of the specific surface area and the number of catalytic site, but enhanced the activation of electrocatalytic site. To further investigate the kinetics of the as-obtained catalysts, electrochemical impedance spectroscopy (EIS) measurements were carried out in 1.0 M KOH solution (Fig. 8b.). The Nyquist plots based on the equivalent circuits are exhibited in the frequency range from 100 kHz to 0.1 Hz. Clearly, both NiCo2S4 and NiCo2S4@NiFe LDH consist of semicircles. The diameter of the semicircle indicates the charge transfer resistance (Rct), while the Rct is smaller, means the faster charge transfer ability. From Fig. 8b, the Rct value of NiCo2S4@NiFe LDH/NF is 1 Ω，much smaller than the value of 2 Ω for NiCo2S4/NF. The enhanced charge transfer ability is attributed to the band structure changes for the NiCo2S4 after composite with NiFe LDH, which is also match with our calculated results. According to the experimental and calculation results, interface engineering is an efficient strategy to change the activated of catalytic site, rather than increase the number of catalytic site. The overall operational voltage (Vop) for water splitting can be described as:48 Vop = 1.23 V + Ƞa + |Ƞc| + ȠΩ Where ȠΩ represents the excess potential applied for compensating the system internal resistance. Ƞa and Ƞc are the overpotentials for the anode and cathode, respectively. In consideration of this, the water splitting efficiency is improved by



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using highly active OER and HER catalysts. In this work, the OER performance has noticeable improvement. And the HER performance has a little improvement. Therefore, it would be a high-efficient approach to enhance the performance of overall water splitting.

4. Conclusion In summary, a novel hierarchical NiCo2S4@NiFe LDH/NF heterostructures has been successfully prepared through a facile three-step hydrothermal method for overall water splitting. The DFT calculation results of the interface are consistent with the experimental data. The NiCo2S4@NiFe LDH/NF heterostructures as a bifunctional electrocatalysts is exactly in favor of overall water splitting, especially for OER process. This improved OER performance with a low overpotential 201 mV at current density of 60 mA cm-2 also shows a low Tafel slope 46.3 mV/dec. The overall water splitting exhibits a voltage of 1.6 V at the current density of 10 mA cm-2. Based on these results, the interface engineering is suggested to be an effective strategy for enhancing the electrochemical catalytic performance, and the hierarchical NiCo2S4@NiFe LDH/NF heterostructures could be a promising candidate for highefficiency overall water splitting.

Acknowledgements This research work is supported by National Natural Science Foundation of China (Grant No. 51302097 and No. 51571096). The authors thank the Analytical and Testing Center of Huazhong University of Science and Technology for their support.


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Supporting Information The Schematic structure of NiFe LDH with the top view and side view, SEM images for the bare Ni foam, and XPS spectrum for the S. The polarization curves of the NiFe LDH/NF, NiCo2S4/NF and NiCo2S4@NiFe LDH/NF without iR correction. The detail for estimation of electrochemically active surface area, table comparing OER and HER behavior of the novel electrode with recently documented materials. Chronopotentiometry curves of NiCo2S4 /NF at the current density of 10 mA cm-2. The morphology, XRD and XPS data of the catalysts after stability test.



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Figures captions Fig. 1. Adsorption geometries of the intermediates *H and *OH on the surfaces of (a) NiFe LDH (b) NiCo2S4@NiFe LDH heterostructures, respectively. (c) interfacial electron transfers between NiCo2S4 and NiFe LDH, yellow and light blue represent the charge accumulation and depletion regions, respectively. Fig. 2. The density of states (a) and partial density of states of Fe (b) Ni (c) O (d) for NiFe LDH and NiCo2S4@NiFe LDH heterostructures. Fig. 3. (a) Schematic illustration of the synthesis process of NiCo2S4@NiFe LDH/NF heterostructures. Fig. 4. (a-b) SEM images of NiCo2S4 nanotube arrays; (c) SEM image of NiCo2S4@NiFe LDH/NF heterostructures; (d-f) TEM images of NiCo2S4 nanotube arrays and (g-j) TEM images of NiCo2S4@NiFe LDH/NF heterostructures; (k-n) the corresponding TEM elemental mapping images of (k) Ni, (l) Co, (m) Fe, and (n) S. Fig. 5. (a) XRD pattern of NiCo2S4@NiFe LDH/NF heterostructures; (b-d) XPS spectra of Ni 2p, Fe 2p, and Co 2p in NiCo2S4@NiFe LDH/NF heterostructures. Fig. 6. (a) Polarization curves and (b) the overpotential of NiFe LDH/NF, NiCo2S4/NF, and NiCo2S4@NiFe LDH/NF at 60 mA cm-2; (c) the Tafel slope of NiFe LDH/NF, NiCo2S4/NF, and NiCo2S4@NiFe LDH/NF; (d) chronopotentiometry curves of NiCo2S4@NiFe LDH/NF at the current density of 10 mA cm-2. Fig. 7. (a) Polarization curves and (b) the corresponding Tafel curves of NiCo2S4/NF, NiFe LDH/NF, and NiCo2S4@NiFe LDH/NF; (c) Ploarization curves of the overall water splitting using NiCo2S4@NiFe LDH/NF in a two-electrode system at the scan


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rate of 2 mV s-1; (d) durability of the NiCo2S4@NiFe LDH/NF electrode at the current density of 10 mA cm-2. Fig. 8. (a) the capacitive currents for 0.025 V versus Ag/AgCl with various scan rates, 2, 4, 6 and 8 mV s-1; (b) Nquist plot of NiCo2S4/NF and NiCo2S4@NiFe LDH/NF. Table 1. Comparison of two electrode water splitting voltage of NiCo2S4@NiFe LDH/NF electrocatalysts with other bifunctional electrocatalysts.



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Fig. 1.


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Fig. 2.



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Fig. 3.


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Fig. 4.



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Fig. 5.


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Fig. 6.



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Fig. 7.


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Fig. 8.



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Table 1 Overall voltage Catalyst

Electrolyte

(V)@10 mA cm-2

Ref.

NiCo2S4@NiFe

1 M KOH

1.6 V

This work

MoS2/Ni3S2/NF

1M KOH

1.56 V

31

EG/Co0.85Se/NiFe-

1 M KOH

1.67 V

33

NiCoFe LTH/CC

1 M KOH

1.55 V

49

NiCo2S4/NF

1 M KOH

1.63 V

40

NiFe/NiCo2O4/NF

1 M KOH

1.67 V

30

NiFe LDHs/NF

1 M KOH

1.7 V

50

Ni2P

1 M KOH

1.63 V

51

NiS/NF

1 M KOH

1.64 V

52

Ni3Se2

1 M KOH

1.65 V

53

CoP films

1 M KOH

1.63 V

54

Ni2.5Co0.5Fe/NF

1 M KOH

1.62 V

55

CoFe LDH-F

1 M KOH

1.63 V

56

LDH/NF

LDH


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Graphical Abstract 79x82mm (300 x 300 DPI)


Hierarchical NiCo2S4@NiFe LDH ... - ACS Publications

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