Ultrathin CoFe-Borate Layer Coated CoFe-Layered Double Hydroxide

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Letter

Ultrathin CoFe-Borate Layer Coated CoFe-LDH Nanosheets Array: A Non-Noble-Metal 3D Catalyst Electrode for Efficient and Durable Water Oxidation in Potassium Borate Chao You, Yuyao Ji, Zhiang Liu, Xiaoli Xiong, and Xuping Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03780 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

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ACS Sustainable Chemistry & Engineering

Ultrathin CoFe-Borate Layer Coated CoFe-LDH Nanosheets Array: A NonNoble-Metal 3D Catalyst Electrode for Efficient and Durable Water Oxidation in Potassium Borate Chao You,1,2 Yuyao Ji,1,2 Zhiang Liu,3 Xiaoli Xiong,1,* and Xuping Sun2,* 1

College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610068, Sichuan, China, 2College of Chemistry, Sichuan University, Chengdu 610064, China, and 3College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, China. Corresponding Author: [email protected] [email protected]

ABSTRACT: It is highly desired to develop earth-abundant catalyst materials for efficient and durable water oxidation under benign conditions. In this Letter, we report on the development of ultrathin CoFe-borate layer coated CoFe-LDH nanosheets array supported on Ti mesh (CoFe-Bi@CoFe-LDH NA/TM) as a high-active 3D catalyst electrode at near-neutral pH for the oxygen evolution reaction (OER). In 0.1 M K2B4O7 solution (K-Bi, bulk pH: 9.2), such CoFe-Bi@CoFe-LDH NA/TM displays superior catalytic activity with small overpotential of 418 mV for 10 mA cm-2. In addition, such catalyst electrode also exhibits superior long-period electrochemical activity and durability for OER and a relatively high turnover frequency (TOF) of 0.482 mol O2 s-1 achieved at overpotential of 600 mV. KEYWORDS: CoFe layered double hydroxide, CoFe borate layer, OER, Potassium borate, Near-neutral electrolyte

INTRODUCTION Seeking new energy carriers to deal with the fast fossil energy depletion and ever-worsening environmental pollution problems has been becoming major international endeavours.13 Hydrogen, as a clean and portable energy carrier, is regarded as an ideal candidate to replace fossil fuels and attracted extensive attention. Electrochemical water splitting offers a promising solution to produce pure hydrogen because of its clean without any other useless byproduct and renewable like solar and wind power.4 Compared with the cathodic hydrogen evolution reaction (HER), the oxygen evolution reaction (OER) at anode proceeding through multistep proton-coupled electron transfer process is kinetically sluggish and it initially demands 1.23 V vs. RHE.5-7 High-efficiency electrocatalytic materials are demanded to promote the reaction for high current density at a low overpotential.8-13 So far, RuO2 and IrO2 catalysts are considered as most active OER catalysts, but the high cost limits their widespread use.14,15 It is thus highly desirable and imperative to develop efficient and earth-abundant alternatives. Given that well-studied water splitting under strongly acidic16 or alkaline17 conditions can bring about severe corrosion and such problems restrict the materials type of electrodes and cell assemblies.18 So, the development of OER catalysts operating efficiently at near-neutral or neutral pH is also highly needed. The formation of amorphous cobalt-phosphate (Co-Pi) film has been widely reported as a reliable method to improve catalytic activity towards water oxidation at neutral media.19-22

Whereas, cobalt-borate (Co-Bi), as amorphous film electrodeposited from Co2+in 0.1 M K2B4O7 solution (K-Bi, bulk pH: 9.2) is also proved as a highly effective water splitting catalyst at near-neutral electrolytes,23-26 received relatively less attention. Recent work shows that bimetal material generally has a superior proformance to single metal counterpart towards OER.27-30 As bimetallic materials, layered double hydroxides (LDHs) are typical class of anionic clays materials, representing one of the most technologically promising materials because of its low cost, relative ease of preparation, and the large number of composition.31-37 Li et al. reported that Co-Fe LDHs is an efficient OER catalyst in alkaline media,38 but its application toward OER at near-neutralpH conditions remains unexplored. So we anticipate that CoFe-LDH transformed bimetallic CoFe-Bi would offer us superior water oxidation activity and good long-term stability at near-neutral pH. In this Letter, we report the CoFe-layered double hydroxide nanosheets array supported on Ti mesh (CoFe-LDH NA/TM)38 as both template and metal source to develop ultrathin CoFeBi layer on CoFe-LDH via in-situ electrochemical surface derivation process25,26 in 0.1M K-Bi. The resulting CoFeBi@CoFe-LDH NA/TM behaves as a high-performance 3D catalytic material towards OER in 0.1 M K-Bi, which exhibits superior catalytic activity with small overpotential of 418 mV for 10 mA cm-2. In addition, such catalyst also has a considerable long-period electrochemical durability and relatively high turnover frequencies (TOFs) of 0.07 and 0.482 mol O2 s-1 at overpotentials of 400 and 600 mV, respectively.

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RESULTS AND DISCUSSION Figure 1a shows the Powder X-ray diffraction (XRD) patterns for CoFe-LDH and CoFeBi@CoFe-LDH. CoFe-LDH presents three diffraction peaks at 11.6°, 23.4° and 33.8° indexed to the (003), (006) and (009) plans, respectively, for typical LDH phase.38 The resulting CoFe-Bi@CoFe-LDH still presents characteristic peaks of CoFe-LDH but with decreased intensities. Scanning electron microscope (SEM) images of CoFe-LDH NA/TM (Figure 1b) showed that CoFe-LDH nanosheets array were full covered on TM. Interestingly, the CoFe-Bi@CoFe-LDH NA/TM still retains its nanoarray feature (Figure 1c). Figure S1 presents the cross-section SEM image of CoFe-Bi@CoFe-LDH NA/TM and the thickness of CoFe-Bi@CoFe-LDH nanosheets array is close to 3.2 µm. Figure 1d and 1e show the transmission electron microscopy (TEM) images for CoFe-LDH and CoFe-Bi@CoFe-LDH, respectively. Figure 1f reveals high-resolution TEM (HRTEM) image recorded from CoFe-LDH, confirmed all serial lattice fringes with an interplanar distance of 0.260 nm matching well with the (012) plane of CoFe-LDH. HRTEM image taken from CoFe-Bi@CoFe-LDH further confirms the formation of a ultrathin amorphous layer (CoFe-Bi) and the thickness of this amorphous layer is about 5-8 nm (Figure 1g). Meanwhile, the thickness of the amorphous layer still retains original width after the water oxidation tests (Figure S2). The energydispersive X-ray (EDX) spectrum for CoFe-Bi@CoFe-LDH NA/TM is shown in Figure S3, indicating the Co and Fe elements exist in the material and atomic ratio is near 4.2:3.7 for Co:Fe. We also collected the selected area electron diffraction (SAED) patterns of CoFe-LDH (Figure S4a) and CoFeBi@CoFe-LDH (Figure S4b). CoFe-LDH shows discrete spots indexed to the (012) and (110) planes of crystalline CoFeLDH, while CoFe-Bi@CoFe-LDH shows extra weak diffraction rings assigned to the amorphous CoFe-Bi. The EDX elemental mapping for elemental analysis further reveals the Fe, Co, O, and B elements distribute uniformly throughout CoFeBi@CoFe-LDH (Figure 1h).

Figure 1. (a) XRD patterns for CoFe-LDH and CoFe-Bi@CoFeLDH. SEM images for (b) CoFe-LDH NA/TM and (c) CoFeBi@CoFe-LDH NA/TM. TEM images for (d) CoFe-LDH and (f) CoFe-Bi@CoFe-LDH. HRTEM images taken from (f) CoFeLDH and (g) CoFe-Bi@CoFe-LDH. (h) Elemental mapping imagesfor CoFe-Bi@CoFe-LDH.

The X-ray photoelectron spectroscopy (XPS) survey spectrum of CoFe-Bi@CoFe-LDH (Figure S5), also confirming the existence of Co, Fe, B and O elements. As shown in Figure 2a, two peaks at 780.3 and 795.7 eV assigned to the binding energies (BEs) of Co 2p3/2 and Co 2p1/2, respectively,33,39 along with two satellite peaks (identified as “Sat.”) located at 786.5 and 802.6 eV.39-41 The value of Co 2p3/2 is so far away from that of Co0 (777.6 ± 0.8 eV)41 but close to that of Co2+ (779.9 ± 0.4 eV),42 indicating the valence state of Co is Co2+. The XPS spectrum of the Fe 2p energy region (Figure 2b), could be resolved with two strong peaks at 711.6 and 724.9 eV, consistent with the BEs of Fe 2p3/2 and Fe 2p1/2, respectively, suggested the valence state of Fe is high oxidation state (Fe3+).42 At the same time, Two Sat. located at 716.6 and 733.8 eV also correspond to the Fe3+.43 Figure 2c shows the O 1s region, the BE at 531.4 eV is correspond tooxygen atoms in center of borate.44 In B 1s spectrum (Figure 2d), the BE at 192.4 eV shows boron element can be assigned to the borate.44

Figure 2. XPS spectra of CoFe-Bi@CoFe-LDH (a) Co 2p, (b) Fe 2p, (c) O 1s and (d) B 1s regions.

To verify the electrochemical OER performance, such CoFe-Bi@CoFe-LDH NA/TM electrode was tested, using a common three lectrodes configurationin 0.1 M K-Bi. Forcomparison, bare TM and RuO2 on TM (RuO2/TM) were also measured under the same conditions. All experimental data were corrected with ohmic potential drop (iR) losses arising from solution resistance.45 The LSV curves in Figure 3a exhibit that RuO2/TM has excellent OER activity with overpotential of 211 mV for 10 mVcm-2, while bare TM exhibits almost no OER activity, furthermore, CoFeBi@CoFe-LDH NA/TM is also highly active, capable of achieving geometrical catalytic current densities of 10 mA cm-2 and 20 mA cm-2 at overpotentials of 418 and 470 mV, respectively, which critical standing compares with the behaviors of recently published work about transition-metalbased water oxidation catalytic materials in near-neutral and neutral electrolyte (Table S1). The water oxidation kinetics was further studied by Tafel plots derived from LSV data (Figure 3b). The Tafel plots are fit to the Tafel equation: η = b log j + a, where j is the current density and b is the Tafel slope. Tafel slope of CoFe-Bi@CoFe-LDH NA/TM is 131


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mV dec-1, which is just 23 mV dec-1 higher than the compared Tafel of RuO2/TM, indicating CoFe-Bi@CoFe-LDH NA/TM catalyst electrode have fast OER kinetics. Figure 3c presents the multi-step chronopotentiometric curve of CoFeBi@CoFe-LDH NA/TM. The potential immediately levels off at 1.64 Vat the start of 10 mA cm-2 and maintains unchanged for the rest 500s. This CoFe-Bi@CoFe-LDH NA/TM electrode materials occur similar be haviors for other potentials measured up to 40 mA cm-2, indicating such material have superior performance on electron conductivity, mass transport properties, and mechanical robustness.42,46

explored. Figure 3f shows the catalytic current density increases significantly with increasing the concentrations of K-Bi (0.1 M, 0.3 M and 0.5 M). This catalyst electrode demands overpotentials of only 404 and 370 mV in 0.3 and 0.5 M K-Bi, respectively to drive 10 mA cm-2. The catalytic performances of CoFe-Bi@CoFe-LDH NA/TM, CoFeBiNA/TM and CoFe-LDH NA/TM were compared in Figure S7a, indicating CoFe-Bi@CoFe-LDH NA/TM has higher activity. In addition, we also collected the LSVs of CoFeBi@CoFe-LDH NA/TM at different pH, and the results show that CoFe-Bi@CoFe-LDH NA/TM demands overpotentials of only 228 mV and 460 mV in 1.0 M KOH and 0.1 M phosphate buffer solution (PBS, pH=7), respectively, to afford current density of 10 mA cm-2 (Figure S7b). Electrochemically active surface area (ECSA) was estimated by determining the double-layer capacitance (CDL) of the system from CVs (Figure S8a and b).47,48 We tested the CDL of CoFe-Bi@CoFe-LDH NA/TM electrode and bare TM. The slope for the linear plot of non-faradaic capacitance currents as a function of scan rates is equal to CDL. Thus the CDL for

CoFe-Bi@CoFe-LDH NA/TM material and bare TM are obtained as 0.40 and 0.097 mF cm-2(Figure S8c and d), respectively. The CDL of CoFe-Bi@CoFe-LDH NA/TM is much larger than that of bare TM, reflecting CoFe-Bi@CoFeLDH NA/TMprovides more active sites for OER catalysis and a higher surface area than bare TM.

Figure 3. (a) LSV curves of CoFe-Bi@CoFe-LDH NA/TM, bare TM and RuO2/TM at a sweep rate of 5 mV s-1. (b) Tafel plots for CoFe-Bi@CoFe-LDH NA/TM and RuO2/TM. (c) Mmulti-current process curve of CoFe-Bi@CoFe-LDH NA/TM. (d) LSV curves for CoFe-Bi@CoFe-LDH NA/TM fresh and after 1000 CV cycles. (e) Time dependence of the potential curve of CoFe-Bi@CoFe-LDH NA/TM at 10 mA cm2 . (f) LSV curves of CoFe-Bi@CoFe-LDH in K-Bi with different concentrations.

In order to investigate the stability of the electrode, we collected the LSV curves of CoFe-Bi@CoFe-LDH NA/TM before and after 1000 cyclic voltammetry (CV) cycles with the potential range from +1.78 to +2.18 Vwith the scan rate of 50 mV s-1. Obviously, this material reminds OER activity before and after 1000 CV cycles (Figure 3d), revealing such CoFeBi@CoFe-LDH NA/TM electrode materials with a superiorstability. OER electrolysis experiment (Figure 3e) further confirms the highly long-period electrochemical activity and durability of this catalyst electrode at constant density of 10 mAcm-2, maintaining the OER activity for 50 h, demonstrating that such catalyst electrode is excellent longterm electrochemical active and durable.The SEM images for CoFe-Bi@CoFe-LDH NA/TM after stability test (Figure S6), indicates that such electrode still retains its nanoarray feature. The effection of K-Bi concentration towards OER activity was

Figure 4. (a) CVs at various scan rates (2, 4, 6, 8, 10, 12, 14, 16, 18 and 20 mV s-1) for CoFe-Bi@CoFe-LDH NA/TM. (b) Scan rateversus oxidation peak current plot for CoFe-Bi@CoFe-LDH NA/TM.

The evaluation of TOF at different overpotentials gives us assistance to evaluate the intrinsic catalytic activities of CoFeBi@CoFe-LDH NA/TM for comparison with other catalytic electrodes.50 Determinating the surface concentration of active sites by electrochemical method, and we make a rough estimation of TOF for each active site using nextformula:49 TOF = JA/4F m, where J is current density (A cm-2) measured at the specified overpotentials during the test of LSV; A is the geometric area of the electrode; 4 is the mole of electrons transfer for evolving one mole of O2 during the water oxidation; F is the Faradic constant; m is the active sites (mol), and calculation of m is according to the equation: slope = n2F2m/4RT (n: the number of electrons transferred, which is denoted as 1 to achieve the upper limit in concentration of active sites; R: the ideal gas constant of 8.314 J mol-1 k-1; T: the absolute temperature of 298 K). The linear relationship between the scan rates and oxidation peak current (Figure 4) make it possible to calculate the concentration of active siteson surface of electrode. Based on the above formula, we obtained the slope is 0.070and the relationship between TOFs and potentials (Figure S9). It is clear that the TOF for CoFe-



Bi@CoFe-LDH NA/TM is 0.482 mol O2 s-1 at 600 mV, which is larger than that many reported neutral or near-neutral nonnoble-metal OER catalytic materials, such as Ni-Bi (0.01 mol O2 s-1, η= 600 mV),23Fe based film/GC (0.21 mol O2 s-1, η = 530 mV),43Fe-Bi (0.17 mol O2 s-1, η= 600 mV),49 and NiOx-en (0.02 mol O2 s-1, η= 610 mV).50

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CONCLUSION In summary, we successfully developed a CoFe-Bi layer which transformed from CoFe-LDH and coated on CoFe-LDH nanoarray. The CoFe-borate layer coated CoFe-LDH nanosheets array is high-active and durable in the process of water oxidation, requiring small overpotential of 418 mV for 10 mA cm-2. This work not only provides us an excellent watersplitting catalytic material under the benign conditions,51 but would offer a new idea to the rational design of LDH-derived catalysts for electrocatalysis and sensing applications.52-54

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/xxxxxxxxxxxx.xxxxxxx. Experimental section; Cross–section SEM image; HRTEM images after the water oxidation tests; SEM images after stability test; EDX and XPS spectra; SAED patterns; LSV; CVs; Plots capacitive currents vs. scan rate; Plot of TOF vs. potential; Table S1.

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AUTHOR INFORMATION Corresponding Author

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*

E-mail: [email protected] (X.X.); [email protected] (X. S.)

Notes The authors declare no competing financial interest. (14)

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 21575137). (15)

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CoFe-LDH@CoFe-Bi nanoarray on Ti mesh (CoFe-LDH@CoFe-Bi /TM) acts as a activity and durable OER catalyst needing overpotential of 418 mV for 10 mA cm-2 in 0.1 M K-Bi.

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Ultrathin CoFe-Borate Layer Coated CoFe-Layered Double Hydroxide

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