Ni)Mn-LDH

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Formation of Hierarchical Structure Composed of (Co/Ni)Mn-LDH Nanosheets on MWCNT Backbones for Efficient Electrocatalytic Water Oxidation Gan Jia, Yingfei Hu, Qinfeng Qian, Yingfang Yao, Shiying Zhang, Zhaosheng Li, and Zhigang Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02733 • Publication Date (Web): 23 May 2016 Downloaded from http://pubs.acs.org on May 28, 2016

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ACS Applied Materials & Interfaces

Formation of Hierarchical Structure Composed of (Co/Ni)Mn-LDH Nanosheets on MWCNT Backbones for Efficient Electrocatalytic Water Oxidation Gan Jia,† Yingfei Hu,† Qinfeng Qian,† Yingfang Yao,† Shiying Zhang,‡ Zhaosheng Li,*,† Zhigang Zou† †

Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid

State Microstructures, College of Engineering and Applied Sciences, 22 Hankou Road, Nanjing 210093, People's Republic of China, E-mail: [email protected] ‡

Hunan Key Laboratory of Applied Environmental Photocatalysis, Changsha University,

Changsha, People’s Republic of China KEYWORDS: Layered Double Hydroxide; Carbon Nanotube; Electrocatalyst; Oxygen Evolution; Water Splitting

ABSTRACT: Active, stable and cost-effective electrocatalysts are attractive alternatives to the noble metal oxides that have been used in water splitting. The direct nucleation and growth of electrochemically active LDH materials on chemically modified MWCNTs exhibit considerable electrocatalytic activity toward oxygen evolution from water oxidation. CoMn-based and NiMnbased hybrids were synthesized using a facile chemical bath deposition method and the assynthesized materials exhibited three-dimensional hierarchical configurations with tunable Co/Mn and Ni/Mn ratio. Benefiting from enhanced electrical conductivity with MWCNT

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backbones and LDH lamellar structure, the Co5Mn-LDH/MWCNT and Ni5Mn-LDH/MWCNT could generated a current density of 10 mA cm-2 at overpotentials of ~ 300 and ~ 350 mV, respectively, in 1 M KOH. In addition, the materials also exhibited outstanding long-term electrocatalytic stability.

INTRODUCTION The ever-increasing demands for energy and environmental concerns have stimulated global efforts to explore clean energy substitutes for fossil fuels. H2, which is a clean and portable fuel source, has been considerably studied as an alternative to of fossil fuels due to its high gravimetric energy density and the production of no unfavorable greenhouse emissions on combustion.1-4 Water is the most abundant source for the production of hydrogen. It is an effective way to split water molecules by using electrochemical method to create pure hydrogen and oxygen. Unfortunately, the sluggish kinetics of the oxygen evolution reaction (OER) substantially limits the efficiency of water splitting devices.5,6 Among the electrocatalysts used for the OER, precious metal oxides such as IrO2 and RuO2 are typically employed. However, the scarcity of noble metals has made it divorced from practical global-scale applications.7-9 Motivated by this challenge, searching for the supersession of noble metal catalysts by earth abundant and inexpensive materials is provided the availability for large-scale applications. While there have been some reports on alternatives,10-17 among other things, the low electrical conductivity intrinsically limits catalytic efficiency of electrocatalyst. To overcome this issue, electroactive transition metal compounds are normally anchored onto conductive substrates or backbones such as carbon nanomaterials, e.g., graphene,18-21 carbon fiber22,23 and carbon


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nanotube.24,25 Therefore, the development of electrocatalysts with favorable electron-transport abilities is needed. Layered materials represent an innovative and eye-catching topic of material systems with special structural characteristics and high specific surface areas that are of great importance for catalysis,11,26-28 sensing,29 and energy storage applications.30-33 Layered double hydroxides (LDH) are a family of inorganic ionic lamellar compounds composed of layers of bivalent and trivalent metallic cations (MII and MIII) with charge compensating anions occupying the interlayer regions.34,35 This structure is conducive to anion insertion and the exchange capacity, which may facilitate a rapid redox reaction. However, the relatively inferior conductivity of LDH is unfavorable for electron transfer and ultimately affects the electrocatalytic performance. Multi-wall carbon nanotubes (MWCNTs) consist of concentric cylindrical layers or shells of several rolled up graphene sheets. MWCNTs have extensively been chosen as one of the most efficient electrochemical functional materials for different purposes due to their peculiar onedimensional structure and competitive prices.36-38 The variety of MWCNT electronic structures results in high carrier mobility, which affords excellent electron accepting and shuttling capabilities. Hybrid materials based on LDH compounds and MWCNTs have been synthesized by various methods.30,39-41 However, the resulting composites have not been extensively studied for utilize in the electrocatalytic oxygen evolution reaction. The direct nucleation and growth of electrochemically active LDH materials on chemically modified MWCNTs afford intimate chemical and electrical coupling between the LDH nanoplates and the surface of nanocarbons, allowing for rapid electron transfer from active sites of the electrocatalyst to the current conductor. Therefore, this approach could provide a path toward higher electrocatalytic activity


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and durability compared to an inorganic material alone or a simple physical mixture with nanocarbons. In this study, we present a strategy for preparing CoMn-LDH/MWCNT and NiMnLDH/MWCNT hybrids in which the bimetallic nanoplates and MWCNT backbone are synthesized using a facile chemical bath deposition (CBD) approach. By virtue of the large amounts of available catalytic sites provided by bimetallic hydroxides and the desirable conductivity of the carbon-based materials, the obtained composite catalysts exhibit enhanced catalytic activity toward the OER that is comparable to benchmark electrocatalysts based on highly active transition metal compounds. EXPERIMENTAL The graphitized MWCNTs were obtained from Nanjing XF NANO Materials Tech Co., Ltd (Nanjing, China). All other chemicals were used without further purification. The obtained MWCNTs were purified by calcination at 773 K for 1 h to remove the amorphous carbonaceous impurities. Then, 100 mL of concentrated HNO3 was added to the MWCNTs (3 g) in a 250 mL round-bottom flask, and the mixture was treated at 383 K heating under reflux for 6 h, whilst stirring to remove the metal impurities involved in the pristine MWCNTs. The resulting mixture was collected by filtration and washed thoroughly with deionized water and ethanol followed by drying at room temperature. The precursor solutions of the hybrid catalysts were prepared by dissolving Ni(NO3)2•6H2O/Co(NO3)2•6H2O, Mn(NO3)2•4H2O (a total concentration of 5 mM) and NH4F (20 mM) in 250 mL of decarbonated water. Next, 8 mg of the pre-oxidized MWCNTs was dispersed in this mixture. Then, the suspension was ultrasonicated in an ice bath for 5 min. A


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second solution (50 mL) containing NaOH (30 mM) and Na2CO3 (20 mM) was added dropwise into the mixure with vigorous stirring over the course of 5 h. To accelerate the oxidation of Mn2+, oxygen (99.99% purity) was bubbled throughout the entire experimental process. Then the resulting suspension was aged at room temperature for 24 h. The final product was isolated by centrifugation, washed with copious amounts of deionized water and dried by lyophilisation. The electrochemical tests involving a rotating disk electrode (RDE) were performed on a RRDE-3A RRDE apparatus (ALS Co., Ltd, Japan) with a typical three-electrode cell including a platinum mesh as the auxiliary electrode. In addition, a Hg/HgO reference electrode with a 1 M KOH filling solution was used throughout the measurements. The measured potentials vs. Hg/HgO were converted to the reversible hydrogen electrode (RHE) scale according to the Nernst equation: ERHE = Emeasured + 0.059pH + Eref, where ERHE is the converted potential vs. RHE, Emeasured is the experimental potential measured against the Hg/HgO reference electrode, and Eref is 0.098 V at 298 K. An RDE with a diameter of 3 mm was used. It was polished successively on a microcloth polishing pad having with a slurry consisting of 1, 0.3 and 0.05 µm alumina powder. After each polish, the electrode was thoroughly rinsed with distilled water and sonicated in ethanol for ~ 5 s. A total of 4 mg of the electrocatalyst was ultrasonically dispersed into a mixture containing 0.73 mL of deionized water and 0.23 mL of ethanol. To increase the binding strength, 40 µL of a Nafion solution (5 wt. %) was added to this suspension. Then 5 µL of catalyst ink was uniformly drop-casted onto the RDE electrode with a loading of ~ 0.283 mg cm−2. All the electrochemical measurements were performed on a CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Co., China). The electrochemical impedance spectroscopy (EIS) measurements were carried out by applying a 5 mV AC perturbation in a


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frequency range from 105 Hz to 10-1 Hz. The powder X-ray diffraction (XRD) measurements were performed at room temperature in a 2theta range of 5°-70° using Cu Ka radiation (1.54178 Å) at 40 kV and 40 mA with a Rigaku Ultima-III diffractometer. The Brunauer–Emmett–Teller (BET) nitrogen adsorption data were obtained at 77 K using a TriStar 3000 surface area analyzer (Micromeritics Instrument Corp., Norcross, GA, USA) to measure the surface areas and pore distribution of the samples. The X-ray photoelectron spectroscopy (XPS) measurements were obtained using a PHI 5000 VersaProbe instrument (Ulvac-PHI, Inc., Japan) with Al Kα monochromatic X-ray radiation. Scanning electron micrographs were recorded using a Zeiss Ultra-plus thermal field emission scanning electron microscope (Carl Zeiss SMT AG, Germany). The SEM samples were prepared by dropping an ethanol suspension of the as-prepared samples on a silicon wafer and which was allowed to dry in air. An energy dispersive X-ray (EDX) detector was used to analyze the elemental composition of the samples. The transmission electron microscope (TEM) images were recorded on a JEOL-JEM 200CX, using an accelerating voltage of 200 kV. RESULTS AND DISCUSSION The compositional and structural analysis on CoMn-LDH/MWCNT and NiMnLDH/MWCNT may shed light on tracing the origin of the improved OER activity. The underperformed electrocatalytic efficiencies of the transition metal compounds are primarily due to the fully occupied bonding t2g orbitals of the MO6 center, which gives rise to inferior electron transfer conductivity that impedes the reaction of the hydroxide anion with the adsorbed oxygen atom on the catalytic active sites to form adsorbed -OOH species. Moreover, the insufficient contact area between the electrolytes and the catalytic active sites also decreases the catalytic activity during the water oxidation reaction. To solve these problems, a CBD method was


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employed to fabricate CoMn-based and NiMn-based hybrids, and the integrated hierarchical structures were constructed by self-assembling electroactive bimetallic compounds onto MWCNTs backbones (Figure 1). The graphited-MWCNTs, which acts as an electronic transmission channel, improved the kinetics of electron transfer from the electrocatalysts to the solution and served as backbones of the binary transition metal electrocatalysts. The representative FESEM images indicated that the CoMn-based (Figure 2A) and NiMn-based (Figure 2B) bimetallic hybrids were uniformly coated on the nanotube structure. Due to the presence of a large amount of oxygen-containing functional groups on the surface, these preoxidized MWCNTs are very attractive as nucleation centers for the deposition of bimetallic precursors as well as stable backbones for the growth of LDH nanoplates. Furthermore, the strong interactions between the bimetallic hybrids and pre-oxidized MWCNTs may impede the aggregation of plate-like nanostructure into larger aggregates via Ostwald ripening. These results suggest that CoMn-LDH and NiMn-LDH are formed on MWCNTs, resulting in a hierarchically structure. This type of configuration is beneficial for electrocatalytic oxygen evolution. To gain additional insight into the structure of the hybrid catalysts, selected area EDS mapping was performed and revealed the homogeneous distribution of Co, Mn, and O (Figure 2C) and Ni, Mn, and O (Figure 2D) in the catalysts. The SEM images of hybrids made with different Co/Mn and Ni/Mn molar ratios are shown in Figure S1 and S2 (Supporting Information), respectively. The atomic ratios of Co/Mn and Ni/Mn in the hybrids were further determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and EDS (Tables S1 and S2). Typical TEM images of CoMn-LDH/MWCNT (Figure 2E) and NiMn-LDH/MWCNT (Figures 2F and S3) displayed folded, sheet-like morphologies that were nearly transparent to the electron beams due to their ultra-thin nature. It is important to note that the dark wrinkled strips


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represent the folded edges or wrinkles of the nanoplates. Based on TEM, the thickness of one single LDH platelet was estimated to be on the order of several nanometers (Figure S4). The TEM image of a representative nanoplate indicates that numerous pores with a diameter less than 5 nm were present in the ultrathin lamellas. The advantageous combination of conducting and flexible MWCNT backbones with porous and ultrathin bimetallic nanoplates endows the asprepared hybrid with remarkable electrocatalytic performance because this configuration is beneficial to fast mass transport of the electrolytes within the catalyst for rapid redox reactions. Discernible lattice fringes of 0.155 nm and 0.153 nm, which correspond to the (110) and (110) crystal planes of the CoMn-LDH (Figure S5) and NiMn-LDH (Figure S6) phases, respectively, were observed in the high-resolution TEM (HRTEM) image. To gain additional information on the chemical composition of the samples, we employed Xray photoelectron spectroscopy (XPS) measurements to characterize the samples. For Co5MnLDH/MWCNT, the Co 2p core lines split into Co 2p3/2 (780.3 eV) and Co 2p1/2 (796.7 eV) peaks accompanied by two satellite bands (Figure 3A). Intense satellite structures were observed for the Co 2p spectra with binding energy values that were approximately 5~6 eV above the Co 2p1/2 and Co 2p3/2 transitions for the high-spin Co 2p-containing compounds. The peaks corresponding to Mn 2p3/2 and Mn 2p1/2 were located at 642.2 and 653.9 eV (Figure 3B), respectively. This results indicate that the Co and Mn ions primarily exist as Co(II) and Mn(IV), respectively.31,42 For Ni5Mn-LDH/MWCNT, as shown in the Ni 2p XPS spectra in Figure 3C, two major peaks located at 855.7 eV (Ni 2p3/2) and 873.2 eV (Ni 2p1/2), which were accompanied by two satellite bands, suggested the existence of Ni(II). In Figure 3D, the Mn 2p3/2 and Mn 2p1/2 peaks were located at 642.6 and 653.9 eV, respectively, suggesting the presence of Mn(IV) in the sample.43 Mn(III) (d4) ions in bimetallic LDHs are present in the high spin state when hydroxyl


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ions act as weak field ligands. The high spin d4 systems are susceptible to Jahn-Teller distortions which leading to lattice instability in bimetallic LDHs. In this case, Mn(III) is easily oxidized to Mn(IV), and that is the reason why the characteristic peak of Mn(IV) can be observed in Mn 2p3/2 XPS spectrum. The X-ray diffraction (XRD) pattern (Figures 4A and 4B) confirmed the LDH structure of the hybrids. The peaks at ~ 26.5° were assigned to the (002) plane of a hexagonal graphite structure, which indicates that the MWCNTs were present in both the CoMn-based and NiMnbased hybrids. According to Bragg’s law, the interlayer distance of the LDH materials can be estimated from the angles of the XRD peaks. Based on the XRD patterns for Co5MnLDH/MWCNT and Ni5Mn-LDH/MWCNT, the reflections were indexed to a rhombohedral LDH phase with an inter-brucite-like sheet space (d003) of ~ 7.66 Å and ~ 7.79 Å, respectively. The XRD patterns of the CoMn-based and NiMn-based hybrids with various Co/Mn and Ni/Mn ratios are shown in Figures S7 and S8 (Supporting Information), respectively. The angular positions of the 003 reflections were slightly different for the CoMn- and NiMn-based composites, which is primarily due to the difference in the interlayer distances. The order of electrochemical activity can be speculated based on the differences in the BET surface area because the more efficient Co5Mn-based (194.2 m2 g-1) and Ni5Mn-based (166.9 m2 g-1) hybrids exhibited a relatively larger surface area than those of a corresponding series of hybrids (Figures 4C and 4D). The specific surface areas of (Co2Mn, Co3Mn, Co7Mn)-LDH/MWCNT and (Ni2Mn, Ni3Mn, Ni7Mn)-LDH/MWCNT were 114.7, 121.1, 159.7 m2 g-1 and 62.9, 91.0, 97.7 m2 g-1, respectively. The adsorption–desorption curves exhibit a typical type IV isotherm with an H3type hysteresis loop, indicating that CoMn-LDH/MWCNT and NiMn-LDH/MWCNT possess a well-defined mesoporous structure. Based on the pore size distributions, both the CoMn-based


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and NiMn-based hybrids prossess a similar mesoporous structure, and the plots indicate a single feature characteristic of a relatively narrow size distribution of pores (inset of Figures 4C and 4D). To evaluate the OER activity, the linear sweep voltammetry curves (LSV) of NiMnLDH/MWCNT and CoMn-LDH/MWCNT modified RDEs were recorded in a 1 M O2-saturated KOH solution at a rotation speed of 1600 rpm. Figure 5A shows the LSV curves with iR compensation over a voltage range from 1 to 1.7 V for different CoMn-based catalysts measured at a scan rate of 5 mV s-1. Co5Mn-LDH/MWCNT exhibited the lowest onset potential for the OER current among the four CoMn-based catalysts, which resulted in the highest activity. The overpotential required to achieve current density of 10 mA cm−2 was measured to be only ~ 300 mV in 1 M KOH. The inset shows the Tafel plots of CoMn-LDH/MWCNT in the OER derived from Figure 5A. Co5Mn-LDH/MWCNT resulted in a Tafel slope of ~ 73.6 mV dec-1 in 1 M KOH, which may be associate to the rate determining step being preceded by a reversible electrochemical step at equilibrium. Similar results were obtained for a series of NiMn-based hybrids. The results in Figure 5C indicate that Ni5Mn-LDH achieved anodic current densities of 10 mA cm−2 at an overpotential of ~ 350 mV, with a Tafel slope of ~ 83.5 mV dec-1. Table S3 systematically summarizes the overpotentials required to deliver a current density of 10 mA cm-2 with similar materials, and Table S4 summarizes the data for the turnover frequency (TOF) calculated based on the loading amount of bimetallic components on Co5Mn-LDH/MWCNT (~0.472 s-1) and Ni5Mn-LDH/MWCNT (~ 0.173 s-1) (see Supporting Information for detailed calculations). Stability tests of Co5Mn-LDH/MWCNT (Figure 5B) and Ni5Mn-MWCNT (Figure 5D) were carried out using chronopotentiometry measurements without iR compensation. The diffusion


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overpotential was caused by an insufficiently fast removal of the product (O2) from the surface of the electrocatalysts. The inset in Figure 5B and 5D indicates that none of the bubble overpotential of the Co5Mn-LDH/MWCNT and Ni5Mn-LDH/MWCNT exceeded 5 mV. This may indicate that lamellar structure of LDH is favorable for gaseous product desorption to some extent. The improved electrocatalytic performance of Co5Mn-LDH/MWCNT and Ni5MnLDH/MWCNT may be due to the reduced amount of stable manganese ions in the corresponding hybrids as the ratio of manganese decreased. Thermodynamically, the excess manganese ions that exist in the crystal structure increase the bond strength between Mn and O, which leads to an increase in the onset potential. In order to make it break more easily on formation of the MnOOH intermediate, weakening the bond strength between Mn and O is necessary. Nevertheless, an excessive amount of Co or Ni could result in local structural distortion of the ordered graphite sheets of MWCNTs.44 In addition, a small amount of Co(OH)2 or Ni(OH)2 may segregates from the mixed hydroxide, which could also lead to deviation from the optimal oxygen binding energies and increase the OER overpotential. The electrochemically active surface area (ECSA) of the hybrid catalysts on the glassy carbon RDE in 1 M KOH was estimated from measurements of the double-layer capacitance according to a previously published protocol.11 The electrochemical capacitance (Cdl) was determined from the cyclic voltammograms measured in a non-Faradaic region with scan rates ranging from 20 to 200 mV s-1. The double layer capacitance (Cdl) was estimated by plotting ∆J = (Ja - Jc) at 0.30 V vs. Hg/HgO as a function of the scan rate. The double layer capacitance (Cdl) is equal to half of the linear slope. The results in Figure S9 indicate that Co5Mn-LDH/MWCNT has a Cdl that is at least approximately six times higher than those of the corresponding series of


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CoMn-based hybrids. The apparent increase in the effective surface area at nearly the identical catalyst loading may be due to the higher electrocatalytic activity of Co5Mn-LDH/MWCNT relative to that of the three other CoMn-based catalysts. Similarly, Ni5Mn-LDH/MWCNT exhibited the highest Cdl among the four samples and three times as many catalytically active sites compared to the corresponding NiMn-based catalysts (Figure S10). The enhanced electrochemical performances of CoMn-based and NiMn-based hybrids were further confirmed based on electrochemical impedance spectroscopy (EIS) measurements. To compare the electrocatalytic activity of the studied CoMn-based hybrids, Nyquist plots at fixed overpotential (η = 300 mV) are shown in Figure 6A. For Co5Mn- LDH/MWCNT, the smallest semicircle in the Nyquist plots represents the impedance response at an overpotential of 300 mV. Similarly, for the NiMn-based hybrids, the results from the Nyquist plot indicate that the lowest charge transfer resistance was obtained for the Ni5Mn-LDH/MWCNT at an overpotential of 350 mV. The impedance results obtained by fitting the experimental data to the equivalent electrical circuit are shown in Figure 6C. The Cdl element models the double-layer capacitance, and Rs represents the electrolyte resistance. The resistive elements Rp and R3 are closely associated with the kinetics of the interfacial charge transfer process. In particular, the polarization resistance Rp can be an indication of the catalytic performance of the OER. The C3R3 may be associated with adsorbed hydroxy and hydrous groups on the LDH surfaces. The inner layer of hybrids (C2R2) is due to the dielectric properties and a non-negligible electronic resistance of the multi-layer feature of the LDH. The time constant (C1R1) is most likely related to the contact between the glassy carbon of RDE and the electrocatalyst. CONCLUSIONS


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In summary, CoMn-LDH/MWCNT and NiMn-LDH/MWCNT were prepared using a chemical bath deposition approach. The as-prepared electrocatalysts with sufficiently exposed electrochemically active sites and hierarchical structures exhibit high catalytic activity toward water oxidation in alkaline solutions. The outstanding electrocatalytic performances were primarily due to the synergistic effect between the active bimetallic layered structure and the desirable conductivity of the carbon-based materials. These properties, as well as the durability and cost effectiveness, of the hybrids make them promising materials for replacing noble metalbased catalysts for water splitting.

FIGURES


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Figure 1. Schematic illustration of the morphologies of the CoMn-LDH/MWCNT and NiMnLDH/MWCNT nanostructures


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Figure 2. FESEM images, EDS-mapping images and EDS spectrum of (A, C) Co5MnLDH/MWCNT and (B, D) Ni5Mn-LDH/MWCNT (red circle: MWCNT; blue circle: LDH nanoplates), TEM images of (E) Co5Mn-LDH/MWCNT and (F) Ni5Mn-LDH/MWCNT nanoplates.


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Figure 3. XPS spectra of Co 2p (A) and Mn 2p (B) for Co5Mn-LDH/MWCNT and Ni 2p (C) and Mn 2p (D) for Ni5Mn-LDH/MWCNT


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Figure 4. XRD patterns of the as-prepared CoMn-LDH/MWCNT (A) and NiMn-LDH/MWCNT (B); Nitrogen-sorption isotherms and (inset) pore size distribution of CoMn-LDH/MWCNT (C) and NiMn-LDH/MWCNT (D)


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Figure 5. Linear sweep voltammetric curves for CoMn-LDH/MWCNT (A) and NiMnLDH/MWCNT (C) using a 1 M KOH solution as the electrolyte with a scan rate of 5 mV s-1. Scan direction was from lower to higher potentials. The inset shows the corresponding Tafel plots. Chronopotentiometric curves obtained for the CoMn-LDH/MWCNT (B) and NiMnLDH/MWCNT (D) in 1 M KOH with a constant current density of 10 mA cm-2. Inset: enlargement of the area denoted by the small box.


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Figure 6. Nyquist plots from EIS of (A) CoMn-LDH/MWCNT measured at a fixed overpotential of 300 mV and (B) NiMn-LDH/MWCNT measured at a fixed overpotential of 350 mV (the solid sphere represent the experimental data, and the empty circle represents the model fitted data). (C) Equivalent circuit for modeling impedance results of the hybrids modified RDEs.


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ASSOCIATED CONTENT Supporting Information. FESEM images, HRTEM images, XRD patterns and the ECSA of different samples. Comparison of electrocatalytic performance and TOF value of Co5MnLDH/MWCNT and Ni5Mn-LDH/MWCNT with some similar previously reported materials. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by National Basic Research Program of China (973 Program, 2013CB632404), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, New Century Excellent Talents in University (NCET-12-0268), and the National Natural Science Foundation of China (No. 21473090 and 51272102).


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Table of Contents Graphic


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Ni)Mn-LDH

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