Hybridizing NiCo2O4 and amorphous NixCoy layered double

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Hybridizing NiCo2O4 and amorphous NixCoy layered double hydroxides with remarkably improved activity towards the efficient overall water splitting Man Li, Leiming Tao, Xin Xiao, Xingxing Jiang, Mingkui Wang, and Yan Shen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05044 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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ACS Sustainable Chemistry & Engineering

Hybridizing NiCo2O4 and amorphous NixCoy layered double hydroxides with remarkably improved activity towards the efficient overall water splitting Man Li, Leiming Tao, Xin Xiao, Xingxing Jiang, Mingkui Wang, and Yan Shen* Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan 430074, P.R. China E-mail: [email protected] Keywords: amorphous material, bifunctional electrocatalyst, layered double hydroxide,nanowire arrays, water splitting

Abstract: Overall water splitting is an attractive technology to produce clean hydrogen and oxygen. In this study, we constructed amorphous NixCoy layered double hydroxide (LDH) hybridized with threedimensional NiCo2O4 to fabricate core-shell nanowire array on Ni foam (NiCo2O4@NixCoy LDH/NF) as highly efficient electrocatalyst for overall water electrolysis. By tuning the Ni/Co molar ratio in NixCoy LDH, extremely low overpotentials of 193 mV for oxygen evolution reaction (OER) and 115 mV for hydrogen evolution reaction (HER) at a current density of 10 mA cm−2 can be achieved for the [email protected] LDH/NF. Detailed investigations verify that the hybrid structure can increase intrinsic activity of the [email protected] LDH/NF and enhance the charge-transfer rate. Moreover, a

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strong electronic interaction between the heterogeneous elements Ni and Co at the interface of the NiCo2O4 and NixCoy LDH might ultimately influence the catalytic performance.

Introduction Water electrolysis into hydrogen and oxygen is one of the most attractive methods for pursuing clean and continuous energy.1 For the water electrolysis process, both of the cathodic two-electron hydrogen evolution reaction (HER) and anodic four-electron oxygen evolution reaction (OER) require electrocatalysts to lower the overpotential for each process and thus facilitate the system efficiency.2 Therefore, development of high-active and efficient electrocatalysts for HER and OER has become a big challenge for electrochemical-related communities. Currently, noble metal-based electrocatalysts such as Pt-based and Ru-based materials have been regarded as the optimal catalysts for HER and OER respectively, because of their excellent catalytic performance. Whereas, due to the high cost and scarcity of these noble metal-based catalysts, they could not be widely used in industrial applications.3 In the last decades, many efforts and advances have been made in the use of high-performance HER and OER electrocatalysts rich in rare earth materials. At the same time, we must notice that most of those electrocatalysts usually exhibit high activity either the HER or the OER in different electrolytes. When two kinds of electrocatalysts are paired in the same solution, this incompatible integration might lead to reduction in water splitting efficiency.4,

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To meet these challenges, durably bi-functional

electrocatalysts based on three dimensional (3D) transition metal oxides/hydroxides,6, selenides,10,

11

7

sulfides,8,

9

carbonate hydroxides,12 phosphides,13-15 and borides16 have been developed as

bifunctional electrocatalysts for HER and OER simultaneously in the same solution. The intrinsic activity and the number of active sites are considered as two most important factors of catalytic activity.17 Recent studies have shown that the intrinsic activity and the density of active sites can be increased by adjusting the composition, size, morphology and the crystallinity of electro2


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catalysts.17 Therefore, increasing surface area of catalyst and the preparation of 3D nano-structure are efficient way to increase the density of active site and thus improve their activities.18 Another attractive promising strategy is amorphous electrocatalysts.8,

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Because of their unique short range ordered

structure and high density of active site, the amorphous nanomaterials have a broad application prospect in the field of electrocatalysis.19-22 Therefore, it might be desirable to reasonably design and product amorphous electrocatalyst, maximizing electrochemically active sites so as to achieve efficient overall water splitting. Among various amorphous electrocatalysts, NiCo-based (oxy)hydroxides have been considered a promising candidate as high-performance electrocatalyst for the HER and OER. At the same time, there are two major obstacles to the use of NiCo-based catalysts. Firstly, the semiconductor properties based on NiCo catalysts could lead to larger resistance, requiring additional overpotentials to overcome obstacles. The energy barrier to a large extent hindered the NiCo oxides and (oxy)hydroxides application. So far, a lot of efforts have been made to solve or improve the above problem, such as mixing another metal into the electrocatalyst23,

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or combining carbon materials with the

electrocatalyst.25-28 Whereas, some of the active sites of the electrocatalysts are blocked, and improvements in performance are usually limited by methods of combining the carbon materials or doping. Secondly, lots of reported NiCo-based catalysts have a smaller surface area, resulting in less exposure to the active site and low utilization of the active sites. Additionally, another important way to support NiCo-based electrocatalysts by constructing advanced nanostructure can also improve the electrocatalytic properties. The appropriate 3D hierarchical structure could increase the electrocatalytic surface area, maximizing the exposurement of the catalytically active sites.29, 30 NiCo2O4 can be a suitable matrix for hierarchical structures because of its easy preparation and diversity of morphology.31, 32 Additionally, NiCo2O4 is considered a good water splitting electrocatalyst,33, 34 so it is more important for promoting the optimization of the properties of 3



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its composites. Moreover, rich resources of the earth and environmental-friendly feature make it suitable for large-scale industry.35 Therefore, we reported for the first time a feasible strategy to prepare 3D NiCo2O4@NixCoy LDH core-shell composite nanowire (NW) array on Ni foam (NF) for efficient water splitting in alkaline solution, where the slim mesoporous NiCo2O4 NW acts the core and amorphous NixCoy LDH being the shell. The smartly designed NiCo2O4@NixCoy LDH hybrid NW arrays growing on NF can increase surface area and overcome the limited electrical conductivity of amorphous NixCoy LDH itself. Because of the good structure design and inherent electrocatalytic properties of [email protected] LDH/NF, the catalyst can deliver a merely cell voltage of 1.6 V with 10 mA cm-2 current density in full water splitting cell of two electrodes, which makes the material incontrovertibly one of the bifunctional catalysts for effective overall water splitting.

Experimental Section Preparation of NiCo2O4@NixCoy LDH core-shell nanowire arrays on NF Firstly, NF (4.0 cm × 3.5 cm2 in rectangular shape) was cleaned to remove impurities and surface oxides. Through hydrothermal reaction and post-calcining, a layer of vertically arranged NiCo2O4 NW array is firstly grown on NF.36 0.582 g of Co(NO3)2·6H2O, 0.291 g of Ni(NO3)2·6H2O, 0.222 g NH4F and 0.9 g of urea were dissolved in 50 mL deionized water to form homogeneous pink solution. Then solution was transferred into a 100 mL Teflon-lined stainless steel autoclave with the pretreated Ni foam immersed into the reaction solution. The autoclave was sealed and maintained at 120 oC for 6 hours, and then cooled down to room temperature. The as synthesized samples were taken out, ultrasonically cleaned at 40 Hz for 5 minutes in the distilled water and rinsed with ethanol for several times, dried at 60 oC for 10 hours, annealed at 450 oC in Ar for 2 hours.

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The NixCoy LDH were electrodeposited onto NiCo2O4/NF with a typical three-electrode system, NiCo2O4/NF as the working electrode, a parallel positioned platinum plate as the counter electrode and an Ag/AgCl (saturated with KCl) as the reference electrode. The total moles of nickel nitrate and Cobalt nitrate were controlled at 6 mM by adjusting molar ratio of Ni/Coto 1:0, 2:1, 1:1, 1:2 and 0:1. The electrodeposition potential was set to -1.0 V (vs. Ag/AgCl). Materials Characterization The scanning electron microscope (SEM) and the transmission electron microscope (TEM) were used to present the morphologies and the lattice fringes of the above samples. At the same time, by Xray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements, we have characterized the composition and crystal structure of the obtained samples. The CHI 750D potentiostat (Chenghua Co. Shanghai) was used for electrochemical measurements, which employed a conventional one-component three-electrode cell. The obtained NiCo2O4/NF,NiCo LDH/NF and NiCo2O4@NixCoy LDH/NF served as the working electrodes, a Hg/HgO electrode as the reference electrode, and a graphite as the counter electrode. Before measuring the polarization curve, the catalysts were scanned several times of potential sweeps until the data was stable, and then corrected with IR- compensation. All of the potentials in this work are relative to the reversible hydrogen electrode (RHE) by following calculation of E(vs. RHE) =E(vs. Hg/HgO) + 0.0985 + 0.059 pH. According to the composition results of XPS (more detailed values shown in Table S1), the asprepared products would hereafter be denoted as [email protected] LDH/NF, [email protected] LDH/NF, [email protected] LDH/NF, [email protected] LDH/NF and [email protected] LDH/NF, corresponding to the Ni/Co molar ratio of 1:0, 2:1, 1:1, 1:2 and 0:1 in the electrodeposition reaction system. Results and Discussion

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Scheme 1 presents the preparation of NiCo2O4@NixCoy LDH hybrid nanostructure on NF. The macroporous NF has excellent conductivity and mechanical strength, which is a good choice as substrate material. A layer of vertically arranged NiCo2O4 NW array was firstly grown on NF via hydrothermal reaction and post-calcination. Then NixCoy LDH layers were further electrodeposited on the surface of NiCo2O4 NWs, and accordingly the NiCo2O4@NixCoy LDH/NF (final product) was fabricated. The SEM images of NiCo2O4/NF and NiCo LDH/NF are shown in the Figure S1. Uniform nanoarray structure of NiCo2O4 grows on NF, while NiCo LDH shows two-dimensional structure. Figure S2a-2e shows SEM images of NiCo2O4@NixCoy LDH/NF with different molar ratio of Ni/Co, confirming the maintenance of brush-like structure after integration of two components. Porous openspace structures formed by NiCo2O4@NixCoy LDH hybrid NWs could provide convenient diffusion channels in full contact with electrolytes to accelerate electrocatalytic process.37 Figure 1a and 1b show typical TEM images of the nanowire (precursor) before calcination treatment. The nanowire surface of the precursor is smooth and the lattice fringes are united in one direction. After calcination treatment, due to the decomposition of H2O and CO2 from the precursor, these

NiCo2O4

NWs

are

highly

porous

(Figure

1c).

Notably, the nanoparticles are interconnected without clear cracks. Such a structure ensures a strong interaction among nanoparticles and provides a large number of exposed area. The high-power TEM (HR-TEM) image in Figure 1d implies that the NiCo2O4 is highly crystalline and has good lattice fringes. The inter-planar spacing of 0.24 nm and 0.20 nm can represent the (311) and (400) crystal surface of the NiCo2O4 phase, respectively. As shown in Figure 1e and 1f, the diameter of a typical hybrid [email protected] LDH NW is ~70 nm. The outer shell is uniform and symmetrical, and the thickness is ~ 10 nm. The NiCo2O4 nanowires are completely wrapped with the shell as evidenced in Figure 1c and 1e. Moreover, the HR-TEM image in Figure 1f verifies LDH layer is amorphous. It should be noted that the neighboring parts are inter-connected by the amorphous part, no obvious gap. 6


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The amorphous structure facilitates the electron transport and close inter-contact with the underlying NiCo2O4/NF.38 The XRD spectrum of NiCo2O4 scraped from NF, as shown in Figure 2, confirm that NiCo2O4 NWs are pure spinel structure. Meanwhile, after electrodeposition, the XRD result of [email protected] LDH scraped from NF only verifies the spinel NiCo2O4 phase. No discernible diffraction signal of NixCoy LDH can be found from the corresponding XRD spectrum, which is consistent with the previous absence of NixCoy LDH typical lattice fringes in Figure 1f. We further performed XPS measurements to probe the mechanism of enhanced activity by hybridizing NiCo2O4 and NixCoy LDH. Figure 3a-3c compare the detailed XPS spectrum of relevant elements before (the thick and black line at the bottom) and after (the thin lines at the top) hybridizing NiCo2O4 and NixCoy LDH. The O 1s region (Figure 3a) can be decomposed into two types of oxygen, ranging from low to high energy, which are respectively attributed to metal-bond oxygen and surface materials including hydroxyl groups, chemisorbed oxygen, or incompatible lattice oxygen.39 After hybridizing the two components, the corresponding fraction of surface hydroxyls became dominant, consistent with the deposition of NixCoy LDH on the NiCo2O4 NWs. Furthermore, as shown in Figure 3b and 3c, the peaks of Ni 2p3/2 and Co 2p3/2 shift to higher binding energies after hybridization of the NixCoy LDH, indicating that Ni and Co were transformed into higher oxidation states. It is important to note that higher oxidation states facilitate rapid charge transfer at the interface between the electrode and electrolyte, which may be beneficial for electrocatalytic reactions.40, 41 Figure 3d summarizes the actual molar ratio of Ni/Co and the binding energy shifts of Ni 2p3/2 and Co 2p3/2 tested by XPS with different Ni/Co molar ratio in the electrodeposition system (Table S1). When only Ni exists in the electrodeposition system without Co, the [email protected] LDH/NF shows the smallest Ni 2p3/2 binding energy shift of 0.09 eV. Similarly, when only Co exists in the electrodeposition system without Ni, the [email protected] LDH/NF shows the smallest Co 2p3/2 binding energy shift of 0.15 eV. 7



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Moreover, the slope of actual Ni/Co molar ratio curve tested by XPS shows a turning point at 0.796. Meanwhile, the Ni 2p3/2 and Co 2p3/2 peaks of [email protected] LDH hybrid show the largest shift of 0.45 eV and 0.52 eV respectively. These results confirm a strong electronic interaction between the heterogeneous elements Ni and Co at the interface of the NiCo2O4 and NixCoy LDH. The synergistic effect of surface NixCoy LDH and inner NiCo2O4 will directly affect the performance of catalysts. The electrocatalytic activities of prepared NiCo2O4@NixCoy LDH/NF for the OER were first assessed in 1.0 M KOH. Figure 4a compares linear sweep voltammograms (LSVs) of the NiCo2O4@NixCoy LDH/NF with different Ni/Co mole ratio tested by XPS. We should emphasize that the [email protected] LDH/NF electrode shows the lowest overpotential when the current density is 10 mA cm−2, [email protected] LDH/NF (247 mV) > [email protected] LDH/NF (239 mV) > [email protected]/NF

(193

mV)

Hybridizing NiCo2O4 and amorphous NixCoy layered double

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