Core–Shell NiFe-LDH@NiFe-Bi Nanoarray: In Situ Electrochemical

Letter www.acsami.org

Core−Shell NiFe-LDH@NiFe‑Bi Nanoarray: In Situ Electrochemical Surface Derivation Preparation toward Efficient Water Oxidation Electrocatalysis in near-Neutral Media Lin Yang,†,‡ Lisi Xie,‡ Ruixiang Ge,‡ Rongmei Kong,§ Zhiang Liu,§ Gu Du,⊥ Abdullah M. Asiri,|| Yadong Yao,*,† and Yonglan Luo*,# †

College of Materials Science and Engineering and ‡College of Chemistry, Sichuan University, Chengdu 610064, China § College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, Shandong, China ⊥ Chengdu Institute of Geology and Mineral Resources, Chengdu 610064, China || Chemistry Department, King Abdulaziz University, Jeddah 21589, Saudi Arabia # Chemical Synthesis and Pollution Control, Key Laboratory of Sichuan Province, School of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637002, Sichuan, China S Supporting Information *

ABSTRACT: The corrosion issue with acidic and alkaline water electrolyzers can be avoided by developing water oxidation catalysts performing efficiently under benign conditions. In this Letter, we report that a NiFe-borate layer can be generated on a NiFe-layered double hydroxide nanosheet array hydrothermally grown on carbon cloth via an in situ electrochemical surface derivation process in potassium borate (K− Bi) solution. The resulting 3D NiFe-LDH@NiFe-Bi nanoarray (NiFe-LDH@NiFe-Bi/ CC) demonstrates high activity for water oxidation, demanding overpotentials of 444 and 363 mV to achieve 10 mA cm−2 in 0.1 and 0.5 M K−Bi (pH: 9.2), respectively, rivaling the performances of most reported non-noble-metal catalysts in near-neutral media. Notably, this electrode also shows strong electrochemical durability with a high turnover frequency of 0.54 mol O2 s−1 at overpotential of 600 mV. All these features promise its use as an efficient earth-abundant catalyst material for water oxidation under eco-friendly conditions. KEYWORDS: core−shell NiFe-LDH@NiFe-Bi nanoarray, in situ electrochemical surface derivation, earth-abundant catalyst, water oxidation, near-neutral media

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biomass fuels or chemical products from carbon dioxide require WOCs that operate under benign conditions.10,11 As such, active and stable such WOCs made from earth-abundant elements are also in great need. Electrodeposited Ni-borate (Ni−Bi) particles film is an active WOC at near-neutral pH.12,13 Compared to such particle catalyst film, nanoarray catalyst has obvious advantages of exposing more active sites and facilitating diffusion of electrolyte and gas evolved.14−16 Given that bimetal material shows higher activity for water oxidation than that of single metal catalyst,17,18 we anticipate that bimetallic Ni−Bi nanoarray would offer us superior water oxidation activity at near-neutral pH. In this Letter, NiFe-layered double hydroxide nanosheet array hydrothermally grown on carbon cloth (NiFe-LDH/CC) has been utilized as both template and metal source to develop NiFe-Bi ultrathin layer via in situ electrochemical surface derivation process in potassium borate (K−Bi) solution.19−21

he energy crisis and environment pollution with increased depletion of fossil fuels call for an urgent demand for clean and renewable alternatives as energy carriers.1 Sustainable hydrogen is regarded to be an ideal such candidate with zero emission of greenhouse gas.2,3 Water electrolysis provides an attractive way to produce pure hydrogen. Compared to water reduction at cathode, water oxidation at anode (2H2O → O2 + 4H+ + 4e−), where the formation of a covalent O−O bond requires a four electron transfer process, suffers from sluggish kinetics and still remains a critical bottleneck in the improvement of water-splitting technologies.4 Efficient water oxidation catalysts (WOCs) are thus needed to accelerate the water oxidation rate, achieving a high current density at low overpotential.5 Ru- and Ir-based catalysts with the highest activity however suffer from the scarcity and high cost, hindering their wide uses.6 Water splitting is usually performed in either strongly acidic or alkaline solution to minimize the overpotentials.7,8 The extreme pH conditions however causes severe corrosion issues, limiting the types of electrodes and cell components.9 On the other hand, microbial electrolysis cell and electrosynthesis of © 2017 American Chemical Society

Received: February 5, 2017 Accepted: April 19, 2017 Published: April 19, 2017 19502

DOI: 10.1021/acsami.7b01637 ACS Appl. Mater. Interfaces 2017, 9, 19502−19506

Letter

ACS Applied Materials & Interfaces

derivation (Figure 1h and 1i). The corresponding selected area electron diffraction (SAED) patterns (Figure S2a) show discrete spots indexed to (012) and (102) planes of crystalline NiFe-LDH, and the analysis result for NiFe-LDH@NiFe-Bi remains unchanged except for weak diffuse rings from the amorphous NiFe-Bi (Figure S2b). The EDX elemental mapping images (Figure S3) indicate the uniform distribution of Ni, Fe, B, and O elements on the surface of NiFe-LDH@NiFe-Bi nanosheet. Figure 2 shows the X-ray photoelectron spectroscopy (XPS) spectra in the Ni 2p, Fe 2p, B 1s, and O 1s regions for NiFe-

As a 3D WOC, such core−shell NiFe-LDH@NiFe-Bi nanoarray (NiFe-LDH@NiFe-Bi/CC) shows high catalytic activity with the need of overpotentials of 444 and 363 mV to drive a geometrical catalytic current density of 10 mA cm−2 in 0.1 and 0.5 M K−Bi (pH 9.2), respectively, outperforming most reported non-noble-metal catalysts in benign media. This catalyst electrode is also durable with a high turnover frequency (TOF) of 0.54 mol O2 s−1 at overpotential of 600 mV. Figure 1a shows the X-ray diffraction (XRD) patterns of bare CC, NiFe-LDH/CC, and NiFe-LDH@NiFe-Bi/CC. NiFe-

Figure 1. (a) XRD patterns of bare CC, NiFe-LDH/CC, and NiFeLDH@NiFe-Bi/CC. SEM images for (b) NiFe-LDH/CC and (c) NiFe-LDH@NiFe-Bi/CC. (d) Cross-section SEM image for NiFeLDH@NiFe-Bi/CC. (e) EDX elemental mapping images of Ni, Fe, B, and O elements in NiFe-LDH@NiFe-Bi/CC. TEM and HRTEM images of (f, g) NiFe-LDH and (h, i) NiFe-LDH@NiFe-Bi.

Figure 2. XPS spectra for NiFe-LDH@NiFe-Bi in the (a) Ni 2p, (b) Fe 2p, (c) B 1s, and (d) B 1s regions.

LDH@NiFe-Bi. In Figure 2a, the Ni 2p spectrum shows two peaks at 856.2 and 873.8 eV assigned to Ni 2p3/2 and Ni 2p1/2, respectively, suggesting the existence of Ni2+ or Ni3+ bound to oxygen.23,24 Another two shakeup satellites (identified as “Sat.”) at 862.0 and 880.0 eV also correspond to the Ni2+ or Ni3+.25,26 The peaks at 713.0 and 724.0 eV in the Fe 2p region (Figure 2b) are consistent with Fe 2p3/2 and Fe 2p1/2, respectively, indicating that the Fe exists as Fe3+.27 In the B 1s region (Figure 2c) and O 1s region (Figure 2d), the binding energies at 191.0 and 530.6 eV can be assigned to the core levels of central boron and oxygen atoms in borate species,24 respectively. Above-stated analyses verify that the amorphous layer is NiFe-Bi in nature. The in situ formation of core−shell structured NiFe-LDH@NiFe-Bi nanoarray can be rationally explained as follows. First, Ni and Fe at NiFe-LDH surface are electrochemically oxidized into higher oxidation valence states. And then borate anions interact with such Ni and Fe cations for charge compensation to form NiFe-Bi layer on NiFe-LDH surface. Such deposition process can proceed repeatedly until NiFe-LDH was fully covered with amorphous NiFe-Bi layer, obtaining a core−shell structured NiFe-LDH@NiFe-Bi consequently. We further investigated the water oxidation activity of NiFeLDH@NiFe-Bi/CC (NiFe-LDH@NiFe-Bi loading: 2.3 mg cm−2) using a typical three-electrode system with a scan rate of 5 mV s−1 in 0.1 M K−Bi. All experimental data were corrected with ohmic potential drop (iR) losses arising from solution resistance and potentials were reported on a reversible

LDH/CC shows diffraction peaks at 22.7, 33.4, 34.4, 38.6, 46.0, 60.0, and 61.3° indexed to (006), (101), (012), (015), (018), (110), and (113) planes of α-Ni(OH)2 phase, respectively (JCPDS No. 38−0715), and the peak at 52.1° is indexed to the (102) plane of β-Ni(OH)2 phase. These results are consistent with reported NiFe-LDH.22 After electrochemical surface derivation, the obtained NiFe-LDH@NiFeBi/CC still presents characteristic peaks of NiFe-LDH but with decreased intensities. Figure 1b shows the scanning electron microscopy (SEM) images of NiFe-LDH/CC, indicating that the entire surface of CC is fully covered by NiFe-LDH nanosheet array. Interestingly, the resulting NiFe-LDH@NiFeBi/CC still maintains its nanoarray feature (Figure 1c). Crosssection analysis for NiFe-LDH@NiFe-Bi/CC indicates that this nanoarray is about 3.8 μm in thickness (Figure 1d). The energy-dispersive X-ray (EDX) spectrum (Figure S1) confirms the existence of Ni, Fe, B, and O elements in the product, and the corresponding EDX elemental mapping images of NiFeLDH@NiFe-Bi/CC (Figure 1e) indicate the uniform distribution of Ni, Fe, B and O elements. The high-resolution transmission electron microscopy (HRTEM) image taken from NiFe-LDH nanosheet (Figure 1f) reveals well-resolved lattice fringes with an interplanar distance of 0.25 nm indexed to the (012) plane of NiFe-LDH phase,22 as shown in Figure 1g. It is seen that a thin amorphous layer (4−5 nm in thickness) was developed on NiFe-LDH surface after electrochemical surface 19503

DOI: 10.1021/acsami.7b01637 ACS Appl. Mater. Interfaces 2017, 9, 19502−19506

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Figure 3c presents the multistep chronopotentiometric curve for NiFe-LDH@NiFe-Bi/CC with the anodic current density increased from 4 to 40 mA cm−2 with an increment of 4 mA cm−2. The potential immediately levels off at 1.70 V for the start current value, and remains constant for the rest 500 s. Moreover, all other steps also show similar results tested up to 40 mA cm−2, demonstrating the good conductivity, mass transportation, and mechanical robustness of NiFe-LDH@ NiFe-Bi/CC electrode.28 Stability is also critical for practical applications of catalysts, so we probed the stability of the NiFeLDH@NiFe-Bi/CC electrode by continuous cyclic voltammetry. The LSV curve after 1000 cycles shows negligible current loss compared with the initial one (Figure 3d), and the SEM images for tested sample after 1000 cycles are shown in Figure S5. Electrolysis result at fixed current density of 10 mA cm−2 further demonstrates that this catalyst is superior in long-term electrochemical durability, maintaining its catalytic activity for 20 h (Figure 3e). Also note that increased K−Bi concentration more favors such catalysis process and a much smaller overpotential of 363 mV is required for NiFe-LDH@NiFeBi/CC to afford 10 mA cm−2 in 0.5 M K−Bi (Figure 3f). Figure S6 presents the LSV curves of NiFe-LDH@NiFe-Bi/CC in 0.1 M K−Bi at varied pH for water oxidation. It shows that increased pH value more benefits such catalysis process and demands overpotentials of 444, 406, and 376 mV to drive a current density of 10 mV cm−2 at pH 9.2, 9.7, and 10.2, respectively. We also estimated the electrochemically active surface area (ECSA) of bare CC, NiFe-LDH/CC, and NiFe-LDH@NiFeBi/CC by determining the double-layer capacitance (CDL) of the systems via cyclic voltammetry. The cyclic voltammograms (CVs) were all collected in the region of 0.59−0.69 V, where the current responses should only be owing to the charging of the double layer (Figure S7a−c). Then, we measured the CDL at the solid/liquid interface for bare CC, NiFe-LDH/CC, and NiFe-LDH@NiFe-Bi/CC electrodes. The CDLs were determined by measuring the nonfaradaic capacitive current densities (jc) associated with double-layer charging from the scan rate (v) dependence of CVs. CDL is usually calculated by the as given equation: jc = vCDL, thus a plot of jc as a function of v yields a straight line with a slope equal to CDL. The CDL for bare CC, NiFe-LDH/CC, and NiFe-LDH@NiFe-Bi/CC are calculated as 0.196, 0.701, and 1.002 mF cm−2, respectively (Figure S7d), suggesting a larger surface area and more exposed active sites for NiFe-LDH@NiFe-Bi/CC.29 TOF is another quantitative parameter to evaluate the activity of catalysts at one fixed overpotential value, corresponding to the amount of oxygen gas produced per unit time. To achieve TOF calculations, we need to quantify the surface concentration of active sites by electrochemistry. Because the amount of Fe atoms is much less than that of Ni atoms in NiFe-LDH@NiFe-Bi/CC, we neglected the effect of Fe atoms. We observe a linear dependence between the plot of the oxidation current for redox species and scan rates from CVs (Figure 4a), and the slope can be derived from the linear relationship. The number of the active Ni species (m) is calculated based on the formula: slope = n2F2m/4RT, where n representing the number of electrons transfer is 1 assuming a one-electron process for oxidation of metal centers in NiFeLDH@NiFe-Bi,13 F is Faradaic constant (96485 C mol−1), m is the number of active species, and R and T are the ideal gas constant and the absolute temperature, respectively. Then TOF value can be calculated as 0.54 mol O2 s−1 at η = 600 mV using

hydrogen electrode (RHE) scale except specifically explained. Figure 3a shows the linear sweep voltammetry (LSV) curves of

Figure 3. (a) LSV curves of NiFe-LDH@NiFe-Bi/CC, RuO2/CC, and bare CC for water oxidation. (b) Corresponding Tafel plots for NiFeLDH@NiFe-Bi/CC and RuO2/CC. (c) Multicurrent process of NiFeLDH@NiFe-Bi/CC. The current density started at 4 mA cm−2 at 1.7 V and ended at 40 mA cm−2, with an increment of 4 mA cm−2 per 500 s without iR correction. (d) LSV curves for NiFe-LDH@NiFe-Bi/CC before and after 1000 CV cycles for water oxidation. (e) Chronopotentiometric curve of NiFe-LDH@NiFe-Bi/CC at 10 mA cm−2. (f) LSV curves of NiFe-LDH@NiFe-Bi/CC in 0.1, 0.3, and 0.5 M K−Bi for water oxidation. All experiments were tested in 0.1 M K− Bi unless specially stated.

bare CC, NiFe-LDH@NiFe-Bi/CC, and RuO2 on CC (RuO2/ CC with the same loading) in 0.1 M K−Bi. As observed, bare CC has poor water oxidation activity, whereas RuO2/CC is highly active for water oxidation with the need of overpotential of 280 mV to drive 10 mA cm−2. Our NiFe-LDH@NiFe-Bi/CC is also efficient for water oxidation and it demands overpotentials of 415 and 444 mV to afford geometrical catalytic current densities of 5 and 10 mA cm−2, respectively. It is observed that NiFe-LDH/CC (Figure S4) needs the overpotential of 670 mV to drive 50 mA cm−2, larger 100 mV than NiFe-LDH@NiFe-Bi/CC at the same current density. Note that NiFe-LDH@NiFe-Bi/CC compares favorably to the behaviors of most reported noble-metal-free water oxidation catalysts at near-neutral pH (Table S1). Figure 3b presents the Tafel plots for RuO2/CC and NiFe-LDH@NiFe-Bi/CC. These Tafel plots were fitted to the Tafel equation: η = b log j + a, where j is the current density and b is the Tafel slope, yielding a Tafel slope of 50 mV dec−1 for NiFe-LDH@NiFe-Bi/CC, smaller than that of NiFe-LDH/CC in Figure S4 (75 mV dec−1), which implies more favorable catalytic kinetics on NiFeLDH@NiFe-Bi/CC. 19504

DOI: 10.1021/acsami.7b01637 ACS Appl. Mater. Interfaces 2017, 9, 19502−19506

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the formula: TOF = jA/4Fm, herein, j is the current density, A is the geometrical electrode area, and 4 expresses the moles of electron consumption for one mole O2 evolution, higher than that of other reported non-noble-metal water oxidation catalysts, like Ni−Bi film/ITO (0.01 mol O2 s−1, η = 600 mV),12 Ni−Bi/CC (0.45 mol O2 s−1, η = 600 mV),17 and NiOxen/FTO (0.015 mol O2 s−1, η = 610 mV).30 In summary, ultrathin amorphous NiFe-Bi layer has been successfully developed on NiFe-LDH nanoarray via a simple in situ electrochemical surface derivation process in K−Bi. Such core−shell NiFe-LDH@NiFe-Bi nanoarray behaves as a highactive and durable catalyst for water oxidation, with the need of overpotentials of 444 and 363 mV to drive 10 mA cm−2 in 0.1 and 0.5 M K−Bi (pH: 9.2), respectively. The whole fabrication process is cost-effective and easy to scale-up. All these remarkable features, together with the flexible nature of NiFeLDH@NiFe-Bi/CC, promise its practical use as an attractive catalyst electrode in water-splitting technological devices for production of hydrogen fuels at near-neutral pH. This study also opens up an exciting new avenue to explore the use of LDH nanoarrays as interesting starting materials for in situ electrochemical conversion preparation of other nanoarrays for applications.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01637. Experimental section, EDX spectrum, SAED patterns, EDX elemental mapping images, LSV curves, Tafel plot, SEM images, cyclic voltammograms, jc−v curves, Table S1 (PDF)

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Figure 4. (a) CVs for NiFe-LDH@NiFe-Bi/CC in the faradic capacitance current range at scan rates from 5 to 30 mV s−1 (inset: the corresponding plot of oxidation peak current versus the scan rate from CVs) in 0.1 M K−Bi. (b) Plot of TOF for NiFe-LDH@NiFe-Bi/ CC as a function of overpotential.

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Letter

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.Y.). *E-mail: [email protected] (Y.L.). ORCID

Yonglan Luo: 0000-0003-4299-8449 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21575137) and the Key Technologies Research and Development Program of Sichuan Province (2017GZ0419). 19505

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DOI: 10.1021/acsami.7b01637 ACS Appl. Mater. Interfaces 2017, 9, 19502−19506