Electron Delocalization Boosting Highly Efficient Electrocatalytic Water

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Electron Delocalization Boosting Highly Efficient Electrocatalytic Water Oxidation in Layered Hydrotalcites Jinkun Liu, Weiren Cheng, Fumin Tang, Hui Su, Yuanyuan Huang, Fengchun Hu, Xu Zhao, Yong Jiang, Qinghua Liu, and Shiqiang Wei J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07567 • Publication Date (Web): 08 Sep 2017 Downloaded from http://pubs.acs.org on September 9, 2017

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The Journal of Physical Chemistry

Electron Delocalization Boosting Highly Efficient Electrocatalytic Water Oxidation in Layered Hydrotalcites †

†

Jinkun Liu , Weiren Cheng , Fumin Tang, Hui Su, Yuanyuan Huang, Fengchun Hu*, Xu Zhao, Yong Jiang, Qinghua Liu*, and Shiqiang Wei National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, Anhui, P. R. China †These authors contributed equally to this work. *E-mail: [email protected]; [email protected]

Abstract Developing high-performance oxygen evolution reaction (OER) electrocatalysts is of great importance for sustainable and renewable energy conversion and storage. Here, via delocalizing the electron population around the active sites of layered hydrotalcites, we significantly facilitate the electron-donation from the active sites in Cr-doped Ni0.75Fe0.25(OH)2.25 and greatly reduce the charge transfer barrier by one order of magnitude for high OER activity. The Cr-doped Ni0.75Fe0.25(OH)2.25 hydrotalcite nanosheets could thus achieve a small overpotential of 235 mV at 10 mA/cm2 with an excellent Tafel slope of ~39 mV/dec. The X-ray absorption fine structure spectroscopy and theoretical calculations reveal that the incorporated Cr ions are substituted for Fe sites in Ni0.75Fe0.25(OH)2.25 and the strong overlap of d orbitals between Cr with unpaired d electrons and Fe ions promotes ~0.3 electron π-donation from Ni2+ to neighboring Cr (Fe) ions, and then evidently decreases the adsorption free energy of water molecule to −1.45 eV for efficient OER performance.

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Introduction Electrochemical water splitting has been regarded as an important approach for sustainable and renewable energy conversion and storage.1-3 However, the efficient water splitting efficiency is greatly limited by the kinetically sluggish multi-electron process of oxygen evolution reaction (OER), which requires highly active electrocatalysts to overcome the large overpotential.4 As is well known, noble metals oxides such as IrOx and RuOx have been widely considered as the active OER catalysts.5-7 Unfortunately, their scarcity and high cost significantly prevent further large-scale applications; therefore, it is highly imperative to seek low-cost and durable alternatives for efficient OER performance.8-14 Discovered from nature, layered double hydroxide (LDH) hydrotalcites, consisting of positively charged host layers and exchangeable interlayer anions, have already intrigued much attention.11,15-17 In particular, Ni0.75Fe0.25(OH)2.25 hydrotalcites with optimized composition has been extensively regarded as a potential candidate for efficient OER catalysts.18-23 However, the pure Ni0.75Fe0.25(OH)2.25 LDH still suffers from high charge transfer barrier in the inert LDH plane, which significantly limits their OER activity. Hence, it is necessary to improve their conductivity and tune the electron structure of active sites in Ni0.75Fe0.25(OH)2.25 LDH for high OER performance.24-26 By supporting LDH on the conductive substrate, the interfacial charge transfer barrier of Ni0.75Fe0.25(OH)2.25/reduced graphene oxide (rGO) has been obviously decreased, leading to a smaller ovperpotential reduced by about 100 mV relative to corresponding pure Ni0.75Fe0.25(OH)2.25 LDH.26 Moreover, via incorporation of Mn4+ in Ni0.75Fe0.25(OH)2.25 LDH, the band structure of Ni0.75Fe0.25(OH)2.25 has been greatly improved, which induced a prominent OER ability.27 However, previous studies have

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mainly focused on tuning the interfacial electronic structure or density of electronic states to improve the OER performance of Ni0.75Fe0.25(OH)2.25 LDHs. As is well known, the Ni active sites in Ni0.75Fe0.25(OH)2.25 LDHs undergo valence-state-rising process before catalyzing water oxidation reaction. Therefore, fast electron π-donation from Ni active sites to neighboring atoms in the local catalytic plays a pivotal role for the high OER activity.28, 29 In principle, it would be an effective way to delocalize the local electron distribution around active sites of hydrotalcites and then promote the electron donation from Ni active sites in Ni0.75Fe0.25(OH)2.25 LDHs via doping of metal elements with unpaired d orbital electrons.28 In this work, we present a design of electron delocalization via incorporation of transition metal ions to facilitate the electron-donation from the active sites in layered hydrotalcites for high OER activity. We choose Cr as the dopant as Cr3+ ion has unpaired d-electrons (d3) and can easily incorporate into the hydrotalcite host. It is found that these Cr3+ ions favor to substitute for Fe sites and form unique Ni-Fe-Cr triangular local structure in Cr-doped Ni0.75Fe0.25(OH)2.25 LDH basal plane, which can effectively tune the electron density equilibrium among Ni, Cr, and Fe and successfully improve the electronic property of Ni active sites for efficient OER performance. Thereby, the as-synthesized Cr-doped Ni0.75Fe0.25(OH)2.25 LDHs perform prominent OER activity at a quite low overpotential of 235 mV at 10 mA/cm2 with an excellent Tafel slope of 39 mV/dec, better than the benchmarking IrO2 and RuO2. The X-ray absorption spectra and theoretical calculations show that the Cr incorporation can facilitate the electron donation from Ni active sites to neighboring Cr (Fe) ions and then significantly lower the water molecule adsorption barrier to −1.45 eV. These results provide a new insight of designing low-cost and

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efficient catalysts for clean and renewable energy production.

Experimental section Synthesis of Cr-doped Ni0.75Fe0.25(OH)2.25 LDH nanosheets. The Cr-doped Ni0.75Fe0.25(OH)2.25 LDHs were synthesized at room temperature by a “one-pot” co-precipitation method.27,45 Typically, 24 mmol Ni(NO3)2·6H2O, 8 mmol Fe(NO3)3·9H2O, and Cr(NO3)3·9H2O with various molar ratios were dissolved in 100 ml DI water to form a transparent solution, and then 0.24 M NaOH solution was added dropwise under ultrasonic condition. Subsequently, the sludge aged at room temperature for 12 h and then was centrifuged and washed to obtain the final products. The Cr-doped Ni0.75Fe0.25(OH)2.25 LDH nanosheets were dried in an oven under 60 ºC overnight before characterization. The pure Ni0.75Fe0.25(OH)2.25 LDHs were synthesized by a similar procedure without adding Cr(NO3)3·9H2O. Morphology and structure characterizations. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were performed by using a JEOL-2010 TEM with an acceleration voltage of 200 kV. XRD patterns were recorded by using a Philips X’PertPro Super diffractometer with Cu Kα radiation (λ = 1.54178 Å) in a 2θ range from 5° to 70°. X-ray photoelectron spectra (XPS) were acquired on Thermo ESCALAB 250 with Al Kα (hν = 1486.6 eV) as the excitation source and at the photoemission end-station at beamline BL10B in the National Synchrotron Radiation Laboratory, China. The Ni, Fe, and Cr K-edge X-ray absorption near-edge structure (XANES) spectra were collected at the 1W1B station of BSRF. The concentrations of Ni, Fe, and Cr were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES, Jarrel Ash model 955).

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Electrochemical characterization. Electrochemical measurements were performed using an electrochemical workstation (Model CHI760D, CH instruments, Inc., Austin, TX) with a three-electrode system, operated with the modified glassy carbon disk electrode as working electrode, platinum mesh as the counter electrode, and saturated Ag/AgCl as reference electrode in 1M KOH electrolyte. The Linear Sweep Voltammetry (LSV) curves were measured at a rate of 1 mV/s without IR correction after dozens of cyclic voltammetric scans until stable. Electrochemical impedance spectroscopy (EIS) was recorded with frequency range of 0.1–1000 kHz at a bias potential of 1.6 V vs RHE. To avoid Faradaic region, the electrochemical double layer capacitance (Cdl) was measured at a range of 1.0–1.1 V vs RHE at rate ranging from 10 to 100 mV/s with interval 10 mV increment. For electrode preparation, 2 mg of fresh prepared Ni0.75Fe0.25(OH)2.25 LDHs, was dispersed in 1ml of 3:1(v/v) DI-water/ethanol mixture solvent under ultrasonic water bath for about 30 min. After that, an amount of carbon black and 20 µl Nafion solution (5 wt%, Sigma-Aldrich) was added, and the mingled solution was sonicated in an ultrasonic water bath for another 30 min. Subsequently, 5 µl of the dispersion was transferred onto the glassy carbon disk with a diameter of 3 mm, corresponding to the catalyst loading about 0.15 mg/cm2. To accurately measure the conductivity, the samples were filtered and pressed onto a Au/glass substrate with a size of 5 mm × 5 mm × 1 µm to fabricate the sheet-based thin film; after that, they were attached to four copper wires on the surface with silver paste to improve contact performance.

Results and discussion The successful synthesis of Cr-doped Ni0.75Fe0.25(OH)2.25 hydrotalcite nanosheets

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can be demonstrated by the transmission electron microscopy (TEM) image, energy dispersive X-ray spectroscopy (EDX) image, X-ray diffraction (XRD) pattern, and X-ray absorption fine structure (XAFS) spectroscopy.30,31 Figure 1a shows the uniform sheet-like Cr-doped Ni0.75Fe0.25(OH)2.25 LDH with smooth surface and average lateral size of ~20 nm. The corresponding lattice fringe distance of 0.25 nm in inset of Figure 1a could be attributed to (012) face of Ni0.75Fe0.25(OH)2.25 LDH.18,32 The EDX images in Figure 1b display the random distribution of Ni, Fe, Cr, and O elements throughout the samples, preliminarily suggesting that the Cr ions are incorporated into the basal plane of Ni0.75Fe0.25(OH)2.25 LDH. Furthermore, the XRD patterns in Figure 1c exhibit that the as-synthesized pure sample could be readily indexed to the hexagonal Ni0.75Fe0.25(OH)2.25 LDH (JCPDF:51-0463) and the Cr-doped Ni0.75Fe0.25(OH)2.25 nanosheets maintain the lattice of Ni0.75Fe0.25(OH)2.25 LDH under 3% and 7% of Cr doping.27 It can be seen that no peaks corresponding to Cr, Fe, and Ni oxides/hydroxides are observed for Cr-doped Ni0.75Fe0.25(OH)2.25 LDH nanosheets. No significant peak shifts are observed from the XRD results, which indicates that Cr-doped LDHs maintain the crystal lattice of NiFe LDH. Moreover, from the extended-XAFS (EXAFS) Cr K-edge Fourier transform (FT) curves in Figure 1d, the Cr-related oxides and hydroxides can be easily excluded by the evidently different peak positions and intensities for Cr-doped Ni0.75Fe0.25(OH)2.25 compared with those of Cr2O3 and Cr(OH)3 reference compounds. From the Ni K-edge FT curves, it can be seen that the Cr-doping does almost not influence the shape of the FT curves of Ni0.75Fe0.25(OH)2.25. In contrast, in the Fe K-edge FT curves, the intensities of both the first and second peaks located at 1.5 and 2.6 Å, respectively, are decreased after Cr-doping. More importantly, from the Cr K-edge curves, the peak

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positions and intensities of Cr-doped Ni0.75Fe0.25(OH)2.25 are quite similar to those of Fe K-edge FT curves. These results demonstrate that the Cr ions are incorporated into the lattice of Ni0.75Fe0.25(OH)2.25 LDH nanosheets and substituted for Fe sites. Furthermore, the inductively coupled plasma atomic emission spectroscopy (ICP-AES) results reveal that the Fe contents in Cr-doped Ni0.75Fe0.25(OH)2.25 LDHs are scaled down from 28.3% to 22.1% with the increase of Cr ratio; whereas, the Ni contents are almost constant. From the respects of charge equilibrium and structure stability, it would be energetically favorable for Fe3+ ions in NiFe LDH replaced by Cr3+ ions. All these above experimental results confirm the substitution of Fe by Cr ions in Ni0.75Fe0.25(OH)2.25 hydrotalcites.

Figure 1. (a) TEM image of the Cr-doped NiFe hydrotalcites, (b) EDX mapping of the Cr-doped hydrotalcites, (c) XRD patterns of pure and Cr-doped NiFe hydrotalcites, and (d) EXAFS Fourier transform (FT) spectra of k3χ(k) for Cr doped NiFe hydrotalcites, Cr(OH)3, and Cr2O3. 7



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To evaluate the OER activity of the Cr-doped Ni0.75Fe0.25(OH)2.25 LDH nanosheets, the water oxidation performances of the electrocatalysts were carefully investigated by steady-state electrochemistry measurements in 1 M KOH solution using a typical three-electrode setup. The polarization curves of Cr-doped Ni0.75Fe0.25(OH)2.25 LDH nanosheets with different Cr incorporation concentrations (Figure 2a) demonstrate a notably small onset overpotential of about 200 mV for the OER, beyond which the anodic current raises quickly by applying a smaller overpotential.

More

importantly,

the

anodic

current

of

7%

Cr-doped

Ni0.75Fe0.25(OH)2.25 LDH nanosheets rapidly reaches to 10 mA/cm2 at quite low overpotential of only 235 mV, superior to the reported values of benchmarking IrO2 (330–400 mV at 10 mA·cm-2) OER catalysts23,33,34 and the single layer Ni0.75Fe0.25(OH)2.25 LDH nanosheets (~300 mV at 10 mA/cm2).23 Notably, relative to pure Ni0.75Fe0.25(OH)2.25 LDH, there is an obvious anodic peak located at ~1.43 V vs. RHE, which corresponds to Ni2+→Ni3+ oxidation before water oxidation for Cr-doped Ni0.75Fe0.25(OH)2.25 LDH nanosheets,35-37 suggesting an easy electron donation from Ni sites after Cr incorporation. The enhanced catalytic activity of the Cr-doped Ni0.75Fe0.25(OH)2.25 LDH nanosheets is also reflected by its low Tafel slope of 39 mV/decade as shown in Figure 2b. This value is obviously lower than that of pure Ni0.75Fe0.25(OH)2.25 LDH nanosheets (~49 mV/decade), leading to an even higher enhancement in OER activity at η > 235 mV. Furthermore, the electrochemical double-layer capacitance (Cdl) results show that the Cr-doped Ni0.75Fe0.25(OH)2.25 LDH nanosheets possess larger Cdl relative to the pure NiFe LDH nanosheets (Figure 2c). Especially, the Cdl of Cr-doped Ni0.75Fe0.25(OH)2.25 LDH nanosheets is three times higher than the pure NiFe LDH nanosheets, indicating much more effective surface catalytic active sites for OER after Cr incorporation. 8


(b)

pure IrO2 3% Cr 7% Cr

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J (mA/cm2)

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0

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Z' ( ohm)

Figure 2. LSV patterns (a), Tafel slopes (b), Cdl curves (c), and electrochemical impedance spectra (d) of pure and Cr-doped NiFe LDH hydrotalcites. The insets in (d) shows the electrical conductivity (left) and electrochemical durability (right) measurement results of Cr-doped NiFe LDH (7% Cr) hydrotalcites.

Figure 2d presents the electrochemical impedance spectroscopy (EIS) results of Cr-doped and pure Ni0.75Fe0.25(OH)2.25 LDH electrodes under OER process. It is readily seen that the charge transfer resistance (Rct) of Cr-doped Ni0.75Fe0.25(OH)2.25 LDH nanosheets at electrode-electrolyte interface is as low as about 50 ohm and only one seventh of Ni0.75Fe0.25(OH)2.25 LDH nanosheets, revealing an ultra-fast charge transfer and surface reaction kinetics at the electrode-electrolyte interface after Cr incorporation.38,39 From the inset of Figure 2d, it can be seen that the electrical conductivity of pure NiFe hydrotalcites is about 6.1 × 10−3 S·m-1. In contrast, the electrical conductivity of Cr doped NiFe hydrotalcites increases to about 5.6 × 10−2 S·m−1, ~10 times that of pure NiFe hydrotalcites. This result provides a direct 9



evidence to confirm the enhanced charge transfer. The expedient electron transfer and accelerated surface kinetics of Cr-doped Ni0.75Fe0.25(OH)2.25 LDH nanosheets means that lower overpotential is required for driving the OER process. To assess the operation durability of the electrocatalysts, the chronopotentiometric measurements were performed and the results in the inset of Figure 2d show that the overpotential at 10 mA/cm2 remains almost constant (235 mV) after 12 h operation. Moreover, the morphology and structure for the samples after 12 h operation are also similar to those of the electrocatalysts before reaction. Therefore, these results undoubtedly reveal that the Cr-doped Ni0.75Fe0.25(OH)2.25 LDH nanosheets are promising electrocatalysts with ultra-high OER activity and excellent durability.

(b)

pure 3% Cr 7% Cr

pure 3% Cr 7% Cr

3% Cr 7% Cr

pure 3% Cr 7% Cr

Ni 2P3/2

2.0

Intensity (a.u.)

Normalilzation

(a)2.5

1.5 1.0

Ni 2P1/2

0.5 Ni k-edge 8340

8360

E(eV)

Fe k-edge 7120 7140

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(c)

720

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3/2

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735

Cr 2P1/2

575

580

585

590

Binding Energy (eV)

Binding Energy (eV)

Figure 3. (a) The XANES curves of pure and Cr-doped NiFe LDH hydrotalcites, the XPS Spectra of Ni (b) and Fe (c) in pure and Cr-doped NiFe LDH hydrotalcites, and (d) the XPS Spectra of Cr in Cr-doped NiFe LDH hydrotalcites.

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To clarify the influence of Cr incorporation on electronic structure of Cr-doped Ni0.75Fe0.25(OH)2.25 LDH nanosheets, the X-ray adsorption near edge structure (XANES) spectroscopy and X-ray photoelectron spectroscopy (XPS), which are highly sensitive to orbital electron distribution in material,40-43 were performed. In the XANES results of Figure 3a, the white line peaks, located at 8351, 7133, and 6011 eV for Ni, Fe, and Cr K-edge, respectively, are associated with the transition of 1s to outer unoccupied d orbitals, and the corresponding peak intensity is proportional to the density of unoccupied state. It can be seen that with Cr concentration increasing, the peak intensity of Ni K-edge is gradually growing up while those for both Fe and Cr K-edge are regularly decreased, suggesting an effective electron donation from Ni to Fe and Cr after Cr incorporation. This deduction can be further supported by the XPS results of Cr-doped Ni0.75Fe0.25(OH)2.25 LDH nanosheets. As seen from Figure 3b, two main peaks of Ni0.75Fe0.25(OH)2.25 LDH located at 855.7 and 873.6 eV are attributed to the Ni 2p3/2 and 2p1/2, respectively, indicating oxidation states of Ni2+ in Ni0.75Fe0.25(OH)2.25 LDH and in consistence with the previous reports.36 Importantly, the binding energies of Ni 2p3/2 and 2p1/2 peaks are shifted positively about 0.7 and 0.5 eV, respectively, with the increasing of Cr content, confirming that the local electrons tends to move away from Ni in Cr-doped Ni0.75Fe0.25(OH)2.25 LDH nanosheets. In sharp contrast, the binding energies of both Fe 2p3/2 (712.9 eV) and 2p1/2 (725.9

eV)

peaks

(Figure

3c),

testifying

Fe3+

oxidation

state

in

Ni0.75Fe0.25(OH)2.25 LDH,19 are shifted to the low energy side by about 0.5 and 0.3 eV, respectively, inferring that the electron density favors to approach Fe after Cr incorporation. Similarly, the binding energies of Cr3+ 2p3/2 (577.3 eV) and 2p1/2 (587.1 eV) peaks also show a negative shift trend along with enhancing Cr ratio (Figure

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3d).44 The above results demonstrate that electrons are delocalized and favor to donate from Ni to Fe (Cr) after Cr incorporation, which is beneficial for high OER activity.

Figure 4. (a) The valence states of Ni, Fe, Cr before and after incorporated of Cr in Ni0.75Fe0.25(OH)2.25, (b) the surface adsorption energy of H2O of Ni(OH)2, Ni0.75Fe0.25(OH)2.25, Ni0.75Cr0.25(OH)2.25, and Cr-doped Ni0.75Fe0.25(OH)2.25, and (c) schematic representation of the electronic coupling among Ni, Fe and Cr in Cr-doped Ni0.75Fe0.25(OH)2.25.

For an in-depth insight into the OER origin of Cr-doped Ni0.75Fe0.25(OH)2.25 LDH nanosheets, the theoretical formation energy and surface adsorption energy calculations were carried out. It is revealed that the formation energy of Cr-doped Ni0.75Fe0.25(OH)2.25 LDH with a surrounding Fe-Ni-Cr triangular local structure was 1.89 eV, much lower than those of Ni0.75Fe0.25(OH)2.25 LDHs (2.89 eV) and

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Ni0.75Cr0.25(OH)2.25 LDHs (2.41 eV), accounting for why Cr ions are easily incorporated into the basal plane of Ni0.75Fe0.25(OH)2.25 LDHs and favor to replace the Fe sites close to Ni thus forming a unique Ni-Fe-Cr triangular structure. Furthermore, the calculated valence states of Ni in Figure 4a are raised from +2 to +2.3 along with increasing Cr ratio, while the valence states of Fe and Cr are decreased to +2.9 and +2.8, respectively, inferring ~0.3 electron donation from Ni to Fe and Cr in Cr-doped Ni0.75Fe0.25(OH)2.25 LDHs nanosheets. This conclusion is also demonstrated by the XANES and XPS results. The upgraded Ni valence states and better electron donation would extremely facilitate surface species adsorptions and then accelerate OER kinetics. Indeed, our first-principles energy calculations show that the water molecule surface adsorption energy of Cr-doped Ni0.75Fe0.25(OH)2.25 LDHs nanosheets is significantly decreased to −1.45 eV compared with −0.55 eV of Ni0.75Fe0.25(OH)2.25 LDHs nanosheets (Figure 4b). The reduced adsorption barrier indicates that the stronger coupling between Ni and Fe or Cr strengthens the electron delocalization and thus improves the catalytic activity of Ni sites in Cr-doped NiFe LDHs nanosheets. For pure Ni0.75Fe0.25(OH)2.25 LDHs, the π-symmetry (t2g) d-orbitals of Ni2+ 3d8 and Fe3+ 3d5 are fully or almost occupied, hence the electron π-donation from Ni2+ to Fe3+ via bridging O2- is weak.25 As a comparison, the valence electronic configuration of Cr3+ is 3d3 with high spin state, so Cr3+ has enough unpaired electrons in the π-symmetry (t2g) d-orbitals, which will strongly interact with the bridging O2- via π-donation. After Cr3+ incorporation, the π-donation in local triangular Ni-Fe-Cr structure could be strengthened by the electron-electron repulsion between bridging O2- and Ni2+, which triggers more charge transfer from Ni2+ to Cr3+ and Fe3+ as shown in Figure 4c. The DFT results show that the valence states of Ni and Fe in pure NiFe

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hydrotalcites are +2 and +3, respectively. After doping Cr into NiFe hydrotalcites, the valence states of Ni, Fe, and Cr are +2.3, +2.9, and +2.8, respectively. This means that ~0.3 valance electrons of Ni are transferred to Fe and Cr after Cr doping, which could be a quantitative supplement of XPS results. Furthermore, the easy electron-donation of active sites could effectively lower the surface adsorption energy barrier and promote the electron transfer between intermediates and active sites, accelerating OER kinetics under a small overpotential. Via electron delocalization by Cr incorporation, the charge transfer barrier of Cr-doped the NiFe hydrotalcite nanosheets has been reduced by about one order of magnitude and the corresponding surface adsorption energy barrier was decreased by 0.9 eV relative to pure NiFe hydrotalcite nanosheets, contributing to enhanced OER activity. Moreover, the specific areas of pure and Cr-doped Ni0.75Fe0.25(OH)2.25 LDH nanosheets are also basically identical. Therefore, this result further indicates that the enhanced OER activity of the Cr-doped Ni0.75Fe0.25(OH)2.25 LDH nanosheets is attributed to the improved electrical properties and lower adsorption energy barrier induced by the Cr incorporation. Therefore, a better π-donation realized by electron delocalization is helpful to optimize the electronic structure of Ni active sites in Cr-doped Ni0.75Fe0.25(OH)2.25 LDHs nanosheets for higher OER performance.

Conclusions In summary, we have designed for the first time an electron delocalization in Cr-doped Ni0.75Fe0.25(OH)2.25 LDHs nanosheets served as one of the most efficient and durable OER electrocatalysts. The as-obtained Cr-doped Ni0.75Fe0.25(OH)2.25 LDHs nanosheets could effectively boost OER at an onset potential of ~200 mV and achieve

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a small overpotential of ~235 mV at 10 mA/cm2 with Tafel slope of ~39 mV/dec. Electrochemical impedance spectroscopy results confirm that the charge transfer barrier of Cr-doped Ni0.75Fe0.25(OH)2.25 LDHs at electrode/electrolyte interface has been decreased by about one order of magnitude relative to Ni0.75Fe0.25(OH)2.25 LDH. Moreover, the X-ray absorption spectra and theoretical calculations analyses reveal that Cr ions in Cr-doped Ni0.75Fe0.25(OH)2.25 LDHs trend to substitute for the Fe sites near Ni atom to form a Ni-Fe-Cr triangular local structure, which strengthens the electron π-donation from Ni2+ to Cr3+ and Fe3+ for efficient OER performance.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grants No. 21533007, 21603207, 11621063, U1532265, 11435012, and 11422547), and the Fundamental Research Funds for the Central Universities (WK2310000054), and the China Postdoctoral Science Foundation (2016M590581).

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Electron Delocalization Boosting Highly Efficient Electrocatalytic Water

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