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Ce-Doped NiFe-Layered-double-hydroxide Ultrathin Nanosheets/Nanocarbon Hierarchical Nanocomposite as Efficient Oxygen Evolution Catalyst Hua-Jie Xu, Bingkai Wang, Changfu Shan, Pinxian Xi, Wei-Sheng Liu, and Yu Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17939 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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ACS Applied Materials & Interfaces

Ce-Doped NiFe-Layered-Double-Hydroxide Ultrathin Nanosheets/Nanocarbon Hierarchical Nanocomposite as Efficient Oxygen Evolution Catalyst Huajie Xu,†‡ Bingkai Wang,† Changfu Shan,† Pinxian Xi,*,† Weisheng Liu,† and Yu Tang*,† †

State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous

Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China ‡

School of Chemical and Material Engineering, Fuyang Normal University, Fuyang

236037, People’s Republic of China

Abstract: Developing convenient doping to build highly active oxygen evolution reaction (OER) electrocatalysts are practical processes for solving the energy crisis.. Herein, a facile and low cost in-situ self-assembly strategy for preparing Ce-doped NiFe-LDH Nanosheets/Nanocarbon (denoted as NiFeCe-LDH/CNT, LDH = layered double hydroxide, CNT = carbon nanocube) hierarchical nanocomposite is established for enhanced OER, in which the novel material provides its overall advantageous structural features, including the high intrinsic catalytic activity，the rich redox properties and high, flexible coordination number of Ce3+ and the strongly coupled interface. Further experimental results indicate that doped Ce into NiFe-LDH/CNT 1

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nanoarrays bring about the reinforced specific surface area, electrochemical surface area (ECSA), lattice defects, and the electron transport between the LDH nanolayered structure and the framework of CNTs. The effective synergy prompt the NiFeCe-LDH/CNT nanocomposite possessing superior OER electrocatalytic activity with a low onset potential (227 mV) and Tafel slope (33 mv dec-1), better than most non-noble metal based OER electrocatalysts reported. Therefore, the combination of remarkable catalytic ability and the facile normal temperature synthesis conditions endows the Ce-doped LDH nanocomposite as a promising catalyst to expand the field of lanthanide-doped layered materials for efficient water splitting electrocatalysts with scale-up potential. Keywords:

electrocatalyst,

oxygen

evolution

reaction,

Ce-doping,

in-situ

self-assembly, layered double hydroxides，carbon nanotube 1. Introduction Oxygen evolution reaction (OER) is promising candidate for providing a sustainable supply system of clean energy.1-3 However, the OER process usually suffers from relatively sluggish kinetic reactions because of multiple steps of proton-coupled electron transfer.4 Therefore, OER process is critical component controlling the ultimate performance of the catalysts. Although, iridium, ruthenium and their oxidation (Ir, Ru, IrO2 and RuO2) show the most actively catalytic performance for the OER, their poor abundance and high prices restrict their large-scale application.5-6 Currently, numerous efforts have been devoted to developing transition metal based catalysts,7-10 but there are still a challenge to 2


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synthesize the noble free catalysts with excellent activity and remarkable stability. The transitional metal compounds two-dimensional (2D) nanostructures with low cost and high-performance electrocatalysts represent a promising direction due to their multiple advantages, such as intrinsic anisotropic morphology, high flexibility, and large surface area, making them work more effectively for electrocatalytic activity and durability compared with their 0D counterparts.11-13 Nevertheless, there are still several drawbacks of those 2D transitional metal compounds, such as low electrical conductibility, poor active sites, and poor free carrier density. Recently, more attention is paid to precise control about transitional metal compounds at the nanoscale for regulating their surface structure and composition, so as to enhance their electrocatalytic performance and explore their new functionality.14-19 Herein, we design a hierarchical nanocomposite fabricated by Ce-doped NiFe-LDH (LDH = layered double hydroxide) ultrathin nanosheets with nanocarbon materials through a facile in-situ self-assembly strategy (Scheme 1). The corresponding merits are summarized as follows: i) through a facile in-situ synthetic route, especially at low reaction temperature, Ce-doped LDH nanosheets prefer to hold the (oxy)hydroxide phases which are the active phases present during the OER;20-22 ii) nanocarbon materials, such as carbon nanotube (CNT), composited with Ce-doped NiFe-LDH nanosheets, will provide efficient conductive scaffolds with an interconnected electron-transfer pathway and will significantly improve the utilization rate of LDH-derived catalysts derive from hollow nanostructures with the anisotropic morphology and large specific surface area;23-25 iii) doping Ce3+ into NiFe-LDH may 3



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alter the topology of the layer, which will bring about lattice defect and promote electron interactions between the LDH-derived catalysts and nanocarbon materials, and accordingly will possibly promote the electrocatalytic performance.26-29 As expect, the obtained NiFeCe-LDH/CNT hierarchical structure catalyst (doping 5% Ce3+ replace of Fe3+) could effectively accelerate water oxidation with low overpotential of 227 mV for delivering 10 mA cm-2 and low Tafel slope of 33 mv dec-1 and possess considerable durability in 1 M KOH, which is competitive with most reported NiFe-based electrocatalysts and Ir/C catalyst. To the best of our knowledge, this is the first time Ce3+ ion is doped into a functional NiFe-based electrocatalyst, which offers an inspiring OER performance. Furthermore, the catalyst is expected to expand the field of lanthanide-based LDHs materials and result in further applications in energy conversion and storage, next-generation regenerative fuel cells, etc.

Scheme 1. Assembly of the NiFeCe-LDH/CNT nanocomposite. 4


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2. Experimental section 2.1. Preparation of NiFeCe-LDH/CNT nanocomposites. All reagents used were of analytical grade without further purication. The surface of multi-walled carbon nanotubes (MWCNT) and single-walled carbon nanotubes (SWCNT) used in this work was modified by acid oxidation reported elsewhere,30-31 and preparation of NiFe-LDH/CNT, NiCe-LDH/CNT and FeCe-LDH/CNT nanocomposites for the detailed processes, please refer to the Supporting Information. 20 mg of CNT was dispersed in 50 mL of ultrapure water by ultra-sonication for 10 min, then metal salts containing Ni(NO3)2·6H2O, Fe(NO3)3·9H2O, Ce(NO3)3·6H2O (a total concentration of 6 mM, the concentration of Ni(NO3)2·6H2O was fixed as 3.6 mM in the synthesis of different ratios of Ce-incorporated LDHs) and NH4F (0.018 M) were added with vigorous stirring for 30 min. A second solution (60 mL) containing NaOH (0.012 M) and Na2CO3 (0.03 M) was added dropwise into the first solution with vigorous stirring, and the whole process lasts more than five hours. Mixed suspension was aged at room temperature for 24 h, then centrifuged and washed three times with water and ethanol before dried in vacuum oven. 2.2. Materials characterization. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and elemental mapping were performed on a JEOL JEM 2100 TEM (200kV). The chemical compositions were investigated by energy dispersive X-ray spectroscopy (EDX). X-ray diffraction (XRD) patterns were collected on an X'Pert ProX-ray diffractometer with Cu Kα radiation (λ = 0.1542 nm) (40 kV and 40 mA). X-ray photoelectron spectroscopy 5



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(XPS) spectra were recorded on a VG ESCALAB 220I-XL device and corrected using C1s line at 284.6 eV. The Brunauer–Emmett–Teller (BET) specific surface area was determined using N2 adsorption–desorption on an Autosorb-IQ2-MPC system, and the pore size distribution was computed based on quenched solid density functional theory using the adsorption branch. The Raman spectra were obtained with a Renishaw Raman spectrometer model in via using a 532 nm line of Ar+ ion laser as the excitation source at room temperature. 2.3. Electrochemical measurement. Electrochemical measurements were carried out in a typical three-electrode configuration connected to a CHI 760E Electrochemical Workstation, comprising a platinum foil counter electrode, a saturated Hg/HgO reference electrode, and a glassy carbon working electrode coated with electrocatalysts. The catalysts were uniformly loaded on a glassy carbon electrode with a total loading of 200 µg cm-2. Before the electrochemical measurements, the working electrode was fabricated just follow the steps. 5.0 mg catalyst powder were first dispersed in 0.95 mL of ethanol and 0.05 mL of Nafion solution (5.0 wt%) by sonication for 1.0 h. Then 10.0 µL catalyst ink was pipetted onto a glassy carbon electrode, which was mechanically polished and ultrasonically washed before use. Finally, the working electrode was dried at room temperature. All potentials were calculated with respect to reversible hydrogen electrode (RHE) based on: E (RHE) = E (Hg/HgO) + 0.059 × pH + 0.098 V. The overpotential (η) was calculated by η (V) = E (RHE) - 1.23V. The scan rate of Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) measurements were recorded 1 mV s-1. 6


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Calibration of the potential was done by a reversible hydrogen electrode (RHE) in 1 M KOH solution. The ECSA was measured by cyclic voltammetry (CV) using the same working electrodes at a potential range of 0.3-0.4 V vs. RHE. The Cdl was estimated by plotting current density differences (∆J = Ja - Jc at the potential of 0.3 V) vs scan rates, the slope is used to represent the ECSA. The stability of catalysts was performed by galvanostatic measurement at 10 mA cm-2. AC impedance spectroscopy was acquired in a frequency range from 100 kHz to 0.1 Hz at an amplitude of 10 mV. 3. Results and Discussion 3.1. Fabrication of NiFeCe-LDH/CNT Hierarchical Nanocomposites The fabrication process of the nanocomposite NiFeCe-LDH/CNT is displayed in Scheme 1. MWCNTs were first oxidized by a modified Hummers method, and the hydroxyl, carbonyl, and carboxyl functional groups were all introduced onto the surface of CNT (Scheme 1: Step I). Then during the coprecipitation process at room temperature, the metal ions prefer to be adsorbed and anchored on the oxygen-containing groups of CNT (Scheme 1: Step II), which can be served as nucleation centers for in-situ growth of NiFeCe-LDH, thus led to uniform NiFeCe-LDH nanosheets vertical decorated on the skeleton of CNT (Scheme 1: Step III). By simply varying the doping amounts of Ce3+ as a Fe3+ substituent, the ultrathin nanosheets on the skeleton of CNT were created with Ce molar ratio of 1.0%, 2.5% and 5.0%, which were denoted as 1.0%Ce-NiFe-LDH/CNT, 2.5%Ce-NiFe-LDH/CNT and 5.0%Ce-NiFe-LDH/CNT, respectively. The structural information of synthetic precursors NiFe-LDH/CNT and 7



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NiFeCe-LDH/CNT nanocomposites were obtained by X-ray diffraction (XRD) patterns. As shown in Figure 1a, for the NiFe-LDH/CNT, the peaks at 10.8°, 22.7°, 34.5° and 60.5° are present, which can be indexed as (003), (006), (012) and (110) characteristic diffraction peaks of the hydrotalcite-like structure of NiFe-LDH.32-35 Meanwhile, the diffraction characteristic peaks of NiFeCe-LDH/CNT could match well with NiFe-LDH/CNT, demonstrating that the LDHs phase was well crystallized in these composites when doping Ce3+ less than 5%.36-38 However, an isomorphic substitution of Fe3+ by Ce3+ may induce a lattice distortion and lead to imperfections in the layers due to much larger ionic radius Ce3+ (102 pm) than Fe3+ (55 pm). Meanwhile, the basal spacing of the NiFe-LDH (d003) is ~0.75 nm, assigned to carbonate-intercalated LDHs, but the interlayer spacing (d003) increased to ~0.78 nm after doping 5% Ce3+ into the brucite layer of the NiFe-LDH. The formation of NiFeCe-LDH crystal was further confirmed by Fourier translation infrared (FT-IR) spectroscopy (Figure S1). The absorption bands observed at ~670 cm-1 and ~500 cm-1 are assigned to vibrations of metal-O and metal-O-metal, and these absorption peaks show a slight shift in NiFeCe-LDH/CNT, indicating that the new metal-O and metal-O-metal bonds are formed in LDH after Ce3+ doping. However, the doping of more Ce3+ will obviously destroy the main structure of LDH, which can be deduced from Figure S2. Moreover, Ce3+ doped composites exhibited much broader reflection peaks compare with the NiFe-LDH/CNT, which may be attributed to the lattice defects formed by the Ce doping. To further analyze the size, morphology, and structure of the as-obtained 8


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composites, the transmission electron microscopy (TEM) and atomic force microscopy (AFM) was utilized. The TEM image (Figure 1b,c and Figure S3) clearly reveals that the abundant ultrathin nanosheets are vertically grown on the CNT with the thickness of about 1.45 nm, and the CNT in the hierarchical structure can efficiently prevent the ultrathin nanosheets from aggregation. The AFM images and height profile of 5.0%Ce-NiFe-LDH/CNT in Supplementary Figure S4 exhibit that the thickness of nanosheets is around 2 nm. These data are in accord with data from the TEM analyses, and slightly thinner than some NiFe-LDH nanosheets.[7a, 17a] Furthermore, from the high-resolution transmission electron microscopy (HRTEM) images and the selected area electron diffraction (SAED) of the as-made 5.0%Ce-NiFe-LDH/CNT (Figure 1d and the inset), the lattice fringe with a spacing of 0.258 nm is consistent with the (012) plane of the LDHs, further indicative of the formation of the LDHs phase, while the interplanar spacing of ~0.356 nm can also be observed, corresponding well to the (002) crystallographic planes of CNT, so that means that NiFeCe-LDH/CNT structure is actually formed. The SAED analysis revealed the formation of polycrystalline NiFeCe-LDH nanosheets by the diffraction rings with hexagonally arranged spots. The faint diffused halo of SAED pattern is observed for NiFeCe-LDH/CNT due to relatively poor crystallinity, implying that the abundant defects are present in the as-made NiFeCe-LDH/CNT.

9



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Figure 1. (a) XRD patterns of 1.0%Ce-NiFe-LDH/CNT, 2.5%Ce-NiFe-LDH/CNT, 5.0%Ce-NiFe-LDH/CNT and NiFe-LDH/CNT; (b and c) TEM images and (d) HRTEM image of 5.0%Ce-NiFe-LDH/CNT, and the corresponding SAED patterns of 5.0%Ce-NiFe-LDH/CNT;

(e)

Elemental

mapping

images

of

5.0%Ce-NiFe-LDH/CNT.

10


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Figure 1e presents a high-angle annular dark-field scanning TEM (HAADFSTEM) and its corresponding elemental mapping images of the 5.0%Ce-NiFe-LDH/CNT. The results clearly demonstrate uniform Ni, Fe and Ce dispersions. From the energy dispersive X-ray (EDX) spectrum in Figure S5, not only signals for Ni and Fe but also the signal for Ce can be clearly observed, further confirming the successful introduction of Ce element in the nanocomposite. We used inductively coupled plasma atomic emission spectroscopy (ICP-AES; see Table S1 in the Supporting Information) to quantify the Ni, Fe, and Ce content and their ratio. And the concentration of Ce in the sample was determined as 5.06%, being close to that in the reactant mixture of the precursor. 3.2. Oxygen Evolution Catalytic Performance and Durability The OER activity of the nanohybrids was evaluated in 1 M KOH aqueous solution by a standard three electrode system. As shown in Figure 2a, relative to NiFe-LDH/CNT, all of the Ce-doped NiFe-LDH/CNT catalysts exhibited lower overpotential and higher current densities. Among all of the tested catalysts, 5.0%Ce-NiFe-LDH/CNT exhibited the lowest overpotential of ~227 mV to deliver 10 mA cm-2 for the OER process, smaller compare with pure NiFe-LDH/CNT (299 mV), 1.0%Ce-NiFe-LDH/CNT (270 mV), and 2.5%Ce-NiFe-LDH/CNT (236 mV) (Figure 2b). Further to compare the performance of all Ce-doped NiFe-LDH/CNT catalysts, the current densities were summarized in Figure 2b at a fixed overpotential of 0.3 V. The 5.0%Ce-NiFe-LDH/CNT showed the highest current density of 48.86 mA cm-2, which was 4.8, 3.3, and 1.5 times as high as that of NiFe-LDH/CNT, 11



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1.0%Ce-NiFe-LDH/CNT, and 2.5%Ce-NiFe-LDH/CNT, respectively. Since the current density is proportional to the yield of oxygen, 5.0%Ce-NiFe-LDH/CNT exhibited the highest OER activity.

Figure 2. (a) LSV curves of NiFe-LDH/CNT and NiFeCe-LDH/CNT for the OER. (b) Overpotential requited for J = 10 mA cm-2 and current density at η = 0.3 V. I, II, III and

IV

represent

NiFe-LDH/CNT,

1.0%Ce-NiFe-LDH/CNT,

2.5%Ce-NiFe-LDH/CNT, and 5%Ce-NiFe-LDH/CNT. For comparison, similar measurements for pure NiFe-LDH/CNT, NiCe-LDH/CNT and Ir/C were also performed by the linear sweeps are shown in Figure 3a. The 5.0%Ce-NiFe-LDH/CNT nanocomposite shows a very small onset overpotential of ~227 mV for the OER process as compared with pure NiFe-LDH/CNT (299 mV) and NiCe-LDH/CNT (417 mV) reference as shown in Figure 3b. Furthermore, 5.0%Ce-NiFe-LDH/CNT indeed outperformed the precious Ir/C catalyst (280 mV), and similar changes in catalytic performance have occurred on the SWCNT (Figure S6). The OER current density of 5.0%Ce-NiFe-LDH/CNT is also much higher than that of NiFe-LDH/CNT, NiCe-LDH/CNT, and Ir/C catalyst at a certain applied voltage (Figure S7). For instance, at a given overpotential of 0.3 V, the current density 12


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of 5.0%Ce-NiFe-LDH/CNT is 48.86 mA cm-2, which is 4.8 times, 25.9 times, and 2.9 times higher than that of NiFe-LDH/CNT, NiCe-LDH/CNT, and Ir/C catalyst, respectively. The improved OER activity of 5.0%Ce-NiFe-LDH/CNT catalytic may originated from its unique structural features and a synergetic effect of the NiFe-LDH/CNT with Ce ions, which further facilitated charge transport to enhanced catalytic activity accompany with a low overpotential of ~227 mV for our constructed catalyst. The Faradaic efficiency for water oxidation was determined to be 98.8% by comparing the theoretical and experimental amounts of evolved O2 during electrolysis (Figure S8).

Figure

3.

(a)

LSV

curves

of


NiFe-LDH/CNT,

NiCe-LDH/CNT, and Ir/C catalysts for the OER. (b) The overpotential for J=10 mA cm-2. (c) Tafel plots for the OER. (d) Nyquist plots obtained by EIS at 1.55 V (vs. RHE)

for

the

OER

of


NiFe-LDH/CNT

and

NiCe-LDH/CNT (inset: equivalent RC circuit model). 13



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Tafel slope is also an important parameter to evaluate OER kinetics derived from LSV curves.39 The 5.0%Ce-NiFe-LDH/CNT exhibited the lowest Tafel slope of 33 mV dec-1, much lower than those of NiFe-LDH/CNT (92 mV dec-1), NiCe-LDH/CNT (206 mV dec-1), and Ir/C (51 mV dec-1) (Figure 3c). Therefore, doping Ce into NiFe-LDH/CNT significantly boosted OER kinetics result in a very small Tafel slope. Compared with those of previously reported NiFe-based electrocatalysts, the 5.0%Ce-NiFe-LDH/CNT exhibited both low onset potential and comparable kinetics for OER (Table S2). The electrochemical impedance spectroscopy (EIS) was tested to further insight into the OER kinetics. The Nyquist plots and the equivalent electrical circuit as shown in Figure 3d, the 5.0%Ce-NiFe-LDH/CNT (48.5 Ω) exhibit a smaller charge-transfer resistance (Rct) than NiFe-LDH/CNT (62.7 Ω), suggesting the much faster charge transfer rate and higher charge transfer kinetics induced by the introduction of Ce, so facilitate the charge transfer process during the OER, which is highly accordant with the result of the Tafel slope. The stability of test of the 5.0%Ce-NiFe-LDH/CNT catalyst was performed by means of a chronopotentiometry measurement maintaining at 10 mA cm-2 (Figure 4a). The operating overpotential for the 5.0%Ce-NiFe-LDH/CNT is nearly constant and only a slight rise of 1-2% after 30000 s of testing. Meanwhile, after the stability test, XRD, TEM and XPS analyses were done. There is no obvious structure change on the XRD pattern after OER (Figure S9). Furthermore, the TEM micrographs of 5.0%Ce-NiFe-LDH/CNT after OER studies are shown as Figure S10. The image clearly shows the complete morphological retention of NiFeCe-LDH crystalline 14


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sheets after OER reaction. XPS spectra after OER indicate that Ni 2p, Fe 2p and O 1s still show their characteristic peaks in 5.0%Ce-NiFe-LDH/CNT precursor (Figure 4b-d). All these tests indicate the 5.0%Ce-NiFe-LDH/CNT catalyst possess a good durability in alkaline solution, and hence the optimization of NiFe-based system is promising for practical applications.

Figure 4. (a) Chronopotentiometric curve for 5.0%Ce-NiFe-LDH/CNT at 10 mA cm-2. XPS spectra: (b) Ni 2p, (c) Fe 2p and (d) O 1s of 5.0%Ce-NiFe-LDH/CNT before and after the OER reaction. 3.3. Further Discussion on Electrocatalytic Activity and Vital Role of Ce Surface area and pore structures are important factors in catalysis, and the enlarged surface area is the best approach for enhanced catalytic activity by regulating nanostructuring.40-41 The N2 adsorption-desorption isotherm and pore size distribution 15



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of the obtained hierarchical materials are shown in Figures 5a,b. Both of the composites show type IV isotherms because of the obvious hysteresis loop ranging from 0.4 to 1.0 in the relative pressure (Figure 5a), indicating the existence of a mesoporous structure (mainly centered at 3-5 nm and 10-50 nm) in these composite materials

(Figure

5b).

The

Brunauer-Emmett-Teller

(BET)

area

for

the

NiFe-LDH/CNT is 221.54 m2/g, when doping concentration of 5% Ce3+ into NiFe-LDH/CNT, which induces BET area rapidly increase to 320.43 m2/g. We believe that the influence of Ce-doping effects may be attributed to the following two aspects. First, the need of high and flexible coordination number of cerium results in the formation of an 8-fold dodecahedron polyhedron or 9-fold monocapped square antiprism coordination layers.42-46 So after the incorporation of Ce3+ into NiFe-LDH/CNT, the layer composition and coordination structure of LDH may be changed to contain eight and nine coordinate Ce sites with bridging hydroxide groups and coordinated water molecules, which alters the topology of the layer, accompanying buckled layers that could also be observed from previous TEM in this material. Second, for such layered hydroxides, Ce-doping (especially the heavier Ce3+ ions with larger ionic radii) can bring about lattice distortion and induce imperfections in the layers. Such lattice distortion hampers the crystallite growth of the layers, thus induces small and dense approximately hexagonal platelike morphology (see TEM images in Figure S11). Analogously to the layered hydroxides, particle size could be also controlled by doping lanthanides reported previously.18, 47-48 The existence of all above factors may be responsible for the BET area increased. 16


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Figure 5. (a) N2 absorption/desorption isotherms and (b) pore size distribution of 5.0%Ce-NiFe-LDH/CNT and NiFe-LDH/CNT composite respectively. (c) Typical cyclic voltammetry curves of 5.0%Ce-NiFe-LDH/CNT electrode in 1M KOH with different scan rates. (d) Capacitive J versus scan rate for 5.0%Ce-NiFe-LDH/CNT and NiFe-LDH/CNT. We

further

compared

the

electrochemical

surface

area

(ECSA)

of

5.0%Ce-NiFe-LDH/CNT and NiFe-LDH/CNT, which were obtained from cyclic voltammetry (CV) curves with different scan rates in 1M KOH (Figure 5c and Figure S12). The Cdl can be obtained by plotting the ∆J(= Ja - Jc) at 0.3V versus the scan rate, and the linear slope is normally used to represent the corresponding ECSA (Figure 5d).34, 40 The Cdl for 5.0%Ce-NiFe-LDH/CNT is 4.2 mF cm-2, which is higher than that

for

NiFe-LDH/CNT

of

2.8

mF

cm-2.

The

higher

ECSA

for

5.0%Ce-NiFe-LDH/CNT relative to NiFe-LDH/CNT is probably associated with 17



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buckled smaller platelike structure and the increase of lattice defects as mentioned above, and all this can prompt an effective improvement on its OER activity. Nevertheless, It is of particular concern that comparing to NiFe-LDH/CNT, 5.0%Ce-NiFe-LDH/CNT has only 1.5-fold increased ECSA from 2.8 to 4.2 mF cm-2, however, the corresponding current density shows significantly 4.8 times improvement from 10.16 to 48.86 mA cm-2 at η = 0.3 V (Figure S13). This clue strongly indicates that doping Ce into NiFe-LDH can increase the active surface area as well as the intrinsic catalytic property. In order to further investigate the influence of Ce-doping on the electrochemical performance toward OER, XPS spectra were performed to investigate the interaction and charge transfer in these hybrid materials. XPS spectra of Ce 3d, Ni 2p, Fe 2p and O1s are shown in Figure 6a-d, respectively. As shown in Figure 6a, for

the

high-resolution XPS spectrum of Ce 3d, the peaks at 920-911 eV and 909-895 eV assigned to Ce 3d3/2, and the peaks at 889-878 eV assigned to Ce 3d5/2, which revealed the coexistence of Ce3+ and Ce4+ in NiFeCe-LDH/CNT. Such cerium exhibits its rich redox properties for oxygen activation. Figure 6b-d show the fine XPS

spectra

of

Ni,

Fe

and

O elements of the

NiFe-LDH/CNT and

5.0%Ce-NiFe-LDH/CNT composite respectively. For Ni 2p and Fe 2p XPS spectra (Figure 6b,c), the peaks exhibit the Ni2+ and Fe3+ oxidation states of a typical LDH phase for NiFe-LDH/CNT and 5.0%Ce-NiFe-LDH/CNT.36, 49 When the substitution of 5%Ce onto Fe sites in NiFe-LDH/CNT, the XPS spectra display clearly a positive shift in the Ni 2p and Fe 2p peaks, which indicates a strong electronic interaction 18


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between Ce and NiFe-LDH/CNT. Moreover, compared to NiFe-LDH/CNT, the O 1s XPS spectra of 5.0%Ce-NiFe-LDH/CNT in Figure 6d shift positively by ~0.4 eV, again revealing the electron transfers between Ce and NiFe-LDH/CNT in 5.0%Ce-NiFe-LDH/CNT sample. Obviously, the binding energy of Ni 2p, Fe 2p and O 1s for the 5.0%Ce-NiFe-LDH/CNT composite shift to a higher value, and the phenomenon is similar to those LDHs reported.50-52 The charge potential model is responsible for explaining the binding energy shift, which is also related to the space structure, valence and the surrounding chemical environment. On this issue, doping Ce into NiFe-LDH lamella can promote the positive charges on the LDH nanosheets shifting to the CNT, and lead to reduce the electron density on the surface of lamella. Therefore, the strong coupling between NiFeCe-LDH and the framework of CNTs can tune the ion/charge transport behavior as well as improve the conductivity and process stability of the electrocatalyst with better catalytic activities.

19



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Figure 6. XPS spectra: (a) Ce 3d of 5.0%Ce-NiFe-LDH/CNT; (b) Ni 2p, (c) Fe 2p and (d) O 1s of NiFe-LDH/CNT and 5.0%Ce-NiFe-LDH/CNT. Meanwhile, a considerable amount of defective sites resulted in the CNTs skeleton layer, as reflected by the increasing value of ID/IG in Raman spectra (Figure 7a).53 The more defects should be more beneficial for NiFeCe-LDH nanosheets dispersion and anchoring, and cerium-induced effects may give rise to topological defect of LDHs nanosheets, which result in more strongly coupled interfacial junctions and exposed active sites derived from NiFeCe-LDH nucleation and growth stage on the skeleton of CNTs. Additionally, the strong interfacial bonding facilitates a rapid charge transfer, which is consistent with the lower charge-transfer resistance and Tafel slopes.

20


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Figure 7. (a) Raman spectra of 5.0%Ce-NiFe-LDH/CNT and NiFe-LDH/CNT; (b) ESR spectra of 5.0%Ce-NiFe-LDH/CNT and NiFe-LDH/CNT; (c) O 1s XPS spectra of the samples; (d) Ce 3d XPS spectra of 5.0%Ce-NiFe-LDH/CNT after the OER reaction. To further investigate the intrinsic defects of the 5.0%Ce-NiFe-LDH/CNT catalyst, we analyzed electron spin resonance (ESR) spectra to probe the existence of oxygen vacancy defects for the catalyst materials. ESR analysis is highly sensitive to evidence the electrons trapped on oxygen vacancies and has been widely used to characterize the spin state of electron and surface structure of nanomaterials. As shown in Figure 7b,

the

enhanced

signal

intensity

at

g=2.0106

illustrates

that

the

5.0%Ce-NiFe-LDH/CNT possess more oxygen vacancies when doping Ce3+ into NiFe-LDH/CNT. Interestingly, after OER the oxygen vacancy concentrations are 21



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denser deduced from the significantly enhanced ESR signal intensity. The same trend can be confirmed by O 1s XPS spectra in Figure 7c, three peaks were clearly identified from the O 1s core level spectra. The peak at ~529.8 eV and ~532.7 eV corresponding to oxygen atoms bound to metals and hydroxyl species, but the presence of defect sites in low oxygen coordination is responsible for ~531.2 eV. The number of defect sites can be evaluated by peak area of O2 region, which is the larger for the 5.0%Ce-NiFe-LDH/CNT than NiFe-LDH/CNT, indicating that doping Ce3+ effectively promotes oxygen vacancy formation. Such rich in oxygen vacancies can be attributed to high and flexible coordination number of cerium ions alters the topology of the layer and brings about lattice distortion and laminar imperfections. Meanwhile, the ultrathin nanosheets further increase the number of active sites and facilitate

reaction

process,

leading

to

the

as-obtained

Ce-doped

nanosheets-nanocarbon hybrids perform excellent OER activity. Notably, after OER state-of-the-art 5.0%Ce-NiFe-LDH/CNT catalyst possess more oxygen vacancies consistent with the large peak area of the shaded part in Figure 7c. This finding could be further confirmed by Ce 3d XPS spectra after OER (Figure 7d), the Ce 3d exhibits the enganced signal indexed to Ce4+ at 881.1 and 914.8 eV, and reduced signal at 897.5 eV comparing with Figure 6a, suggesting the Ce-doped nanocatalyst possess Ce3+ ↔ Ce4+ redox transformation, and this can prompt a rapid diffusion of oxygen through short ion diffusion paths, and the dynamics of released oxygen in the lattice through oxygen vacancy formation, thus ensuring good electron transport.54-57

22


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4. Conclusion In summary, we have doped Ce into NiFe-LDH/CNT nanoarrays successfully by a simple and easily scalable in-situ self-assembly method at room temperature. The obtained NiFeCe-LDH/CNT electrocatalyst displays highly efficient OER activity with a substantially decreased overpotential (227 mV required for 10 mA cm-2) and Tafel slope (33 mV dec-1) in basic medium, showing compete favorably against reported NiFe-based LDHs and outperform commercial Ir/C catalysts. OER activity of the nanocomposite was significantly enhanced mainly due to the reinforced specific surface area, electrochemical surface area, lattice defects, electron transport and the synergetic effect by virtue of the rich redox properties and high, flexible coordination number of Ce3+ for the nanolayered structure. Therefore, this study successfully demonstrates an accessible strategy through designing the cheap lanthanide-doped LDH based electrocatalysts towards global scale clean energy production. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental details; additional TEM and TFM images; EDX spectra; OER polarization curves; cyclic voltammetry curves and supplementary tables (PDF). Corresponding Authors *E-mail: [email protected]. (Y.T.). Phone: 86 931 8912552. Fax: 86 931 8912582. *E-mail: [email protected]. 23



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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Projects 21471071, 21431002, 21571089) and Fundamental Research Funds for the Central Universities Grants (Project lzujbky-2017-Ot05). References (1) Zhang, J.; Xiao, W.; Xi, P.; Xi, S.; Du, Y.; Gao, D.; Ding, J. Activating and Optimizing Activity of CoS2 for Hydrogen Evolution Reaction through the Synergic Effect of N Dopants and S Vacancies. ACS Energy Lett. 2017, 2, 1022-1028. (2) Zhang, Y.; Xia, X.; Cao, X.; Zhang, B.; Tiep, N. H.; He, H.; Chen, S.; Huang, Y.; Fan, H. J. Ultrafine Metal Nanoparticles/N-Doped Porous Carbon Hybrids Coated on Carbon Fibers as Flexible and Binder-Free Water Splitting Catalysts. Adv. Energy Mater. 2017, 7, 1700220. (3) Yin, J.; Li, Y.; Lv, F.; Fan, Q.; Zhao, Y.-Q.; Zhang, Q.; Wang, W.; Cheng, F.; Xi, P.; Guo, S. NiO/CoN Porous Nanowires as Efficient Bifunctional Catalysts for Zn-Air Batteries. ACS Nano 2017, 11, 2275-2283. (4) Suen, N.-T.; Hung, S.-F.; Quan, Q.; Zhang, N.; Xu, Y.-J.; Chen, H. M. Electrocatalysis for the Oxygen Evolution Reaction: Recent Development and Future Perspectives. Chem. Soc. Rev. 2017, 46, 337-365. (5) Panda, C.; Menezes, P. W.; Walter, C.; Yao, S.; Miehlich, M. E.; Gutkin, V.; Meyer, K.; Driess, M. From a Molecular 2Fe-2Se Precursor to a Highly Efficient Iron Diselenide Electrocatalyst for Overall Water Splitting. Angew. Chem. Int. Ed. 2017, 56, 10506-10510.

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for Enhanced Electrocatalytic Water Oxidation. Angew. Chem. Int. Ed. 2015, 54, 7399-7404.

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