N Doped Carbon Nanosheets as

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Reactive Template Derived CoFe/N Doped Carbon Nanosheets as Highly Efficient Electrocatalysts towards Oxygen Reduction, Oxygen Evolution, and Hydrogen Evolution Shanyong Chen, Feifei Bi, Kun Xiang, Yu Zhang, Panpan Hao, Muhong Li, Bin Zhao, and Xuefeng Guo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b02426 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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ACS Sustainable Chemistry & Engineering

Reactive Template Derived CoFe/N Doped Carbon Nanosheets as Highly Efficient Electrocatalysts towards Oxygen Reduction, Oxygen Evolution, and Hydrogen Evolution Shanyong Chen, Feifei Bi, Kun Xiang, Yu Zhang, Panpan Hao, Muhong Li, Bin Zhao*, Xuefeng Guo * Key Lab of Mesoscopic Chemistry MOE, School of Chemistry and Chemical Engineering, Nanjing University, No.163, Xianlin Road, Nanjing 210023, China. E-mail addresses: [email protected] (X. Guo), [email protected] (B. Zhao).

ABSTRACT: The increasing needs for clean and sustainable energy inspire researchers to explore low-cost non-precious metal electrocatalysts for advanced energy storage and conversion. Herein, we develop a reactive template route to fabricate highefficiency ORR/OER/HER trifunctional electrocatalyst (CoFe/NH-C NS) via pyrolyzing the mixture containing CoFe-layered double hydroxide@Glucosaminoglycan (CoFe-LDH@p-Glu) and urea/dicyandiamide. In this strategy, the CoFe-LDH not only provides the well-defined two-dimensional template to form carbon nanosheets, but also employs as the well-distributed CoFe precursor to form uniform CoFe nanoparticles. Such synthetic strategy has been demonstrated effective to controllably fabricate the special nanostructure with metal nanoparticles embedded in N-doped carbon nanosheets and favorable exposure of active sites, leading to strong synergistic effect between the CoFe and N-doped carbon nanosheets and abundant electrocatalytic active sites for energy electrocatalysis. The CoFe/NH-C NS exhibits superior ORR performances to Pt/C with more positive half-wave potential (844 mV for CoFe/NH-C NS vs 832 mV for Pt/C), longer stability, and better methanol tolerance in alkaline conditions. Furthermore, the CoFe/NH-C NS displays an identical current density to commercial RuO2 at 1.8 V (vs.RHE) towards OER and a remarkable electrocatalytic property towards HER in alkaline conditions. This work presents fresh strategies for the design and fabrication of high-performance carbon-based energy materials. KEYWORDS: CoFe-layered double hydroxide, Reactive multifunctional template, N doped carbon nanosheet, Oxygen reduction; Water splitting

Corresponding authors. E-mail addresses: [email protected] (X. Guo), [email protected] (B. Zhao). ACS Paragon Plus Environment

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INTRODUCTION Energy shortage and environmental pollution are current important issues, which have motivated researchers to explore advanced and eco-friendly energy technologies, like fuel cells 1-3, water splitting systems 4-6, and the metal–air batteries 7-8. Among these new technologies, there are some crucial reactions which need to be deeply explored, such as the oxygen reduction reaction (ORR) 9-10, the oxygen evolution reaction (OER) 11-12 , and the hydrogen evolution reaction (HER) 13-16. Although platinum-based materials present the high activity towards ORR and HER 17-18, and Ir- and Ru-based materials are excellent catalysts for OER 19, considerable drawbacks of these precious metals including the high cost, low storage abundance, and easy aggregation limit their widely actual application in energy-relevant devices 20. On the other hand, the performance of electrocatalysts varies with different pH medium 21 (for example, the reported OER electrocatalysts often exhibit better activity in alkaline medium than in acidic solution 22), which hinders practical application of energy integrated systems. Therefore, it is necessary to develop non-precious-metal based tri-functional electrocatalysts (for ORR/OER/HER) in the same operating environment with high performance 23-25. Recently, some transition-metal/nitrogen-doped carbon materials 26-28 have been reported as highly effective, stable, non-precious metal-based catalysts for catalyzing ORR/OER/HER. In these materials 29-31, nitrogen atoms were doped into the carbon matrix, decreasing electron density of the adjacent C atoms and polarizing the C atoms boost both mass and charge transfer during the ORR process 32-33. Moreover, it is well demonstrated that composition 34-35 (such as the dispersion of the metallic components) and the nanoarchitecture 36-37 (including morphology and meso/micro-porosity of the carbon materials) have extensive effects on the electrocatalytic activity for ORR/OER/HER. For the metal electrocatalyst, the alloys consisting of two different transition metals have also attracted increasing attention due to their enhanced electrochemical performance 38-39. The alloying of different transition metals can tune the working function of the transition metals and adjust their adsorption energy for hydrogen/oxygen, resulting in the improvement of the catalytic properties on ORR/OER/HER. For example, CoFe alloy has recently earned extensive attentions as highly efficient electrocatalyst 40-44. Yan et al. have reported that the DG@FeCo catalyst, composed of FeCo nanoparticle and defective graphene, lead to remarkable ORR performance in terms of the half-wave potential (816 mV vs. RHE) 40. Siraj Sultan et al. have successfully synthesized the CoFe alloy encapsulated in nitrogen-doping graphitic tube towards efficient oxygen reduction reaction 41. Generally, transition-metal/nitrogen-doped carbon materials are fabricated via hardtemplate method to achieve special nanoarchitecture of the carbon materials 45-48. However, the hard-template method, involving extra procedure of using hydrofluoric acid or hot alkaline to remove the templates, is dangerous and easily leads to the loss of active sites. Recently, Meng et al. reported the Fe3O4 mesoporous microspheres were employed as the reactive template to obtain Fe-N-doped mesoporous carbon microspheres catalyst with superior ORR performance 49. During the synthesis process,

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the reactive template is in-situ converted into final product without further treatment of removal templates, featuring great advantages compared to normal hard template method. Up to date, however, the nanostructured electrocatalysts composed of metal alloy/nitrogen-doped carbon prepared via reactive template method have been still far less investigated. Herein, we report a novel strategy to synthesize CoFe alloy nanoparticles embedded in nitrogen-doped carbon nanosheets (CoFe/NH-C NS) by using flower-like layered double hydroxides (LDH) nanosheets as the reactive multifunctional template. The CoFe LDH nanosheet is chosen as metal source and the structure template. And the glucosaminoglycan coating on the CoFe LDH serves as the carbon precursor to form N-doped carbon nanosheets, inheriting the nanostructure of the CoFe LDH template. Moreover, the nitrogen-doped carbon nanosheets can largely avoid the aggregation and oxidation of the CoFe nanoparticles. The obtained CoFe/NH-C NS with welldesigned structure presents excellent electrocatalytic performance for ORR/OER/HER due to the synergistic effect between the CoFe and N-doped carbon nanosheets, even better than those of noble metal-based electrocatalysts.

Figure 1. Schematic illustration for the synthesis of CoFe/NH-C NS

EXPERIMENTAL SECTION The Preparation of the samples Synthesis of CoFe LDH 0.0011 mol FeCl2·4H2O (0.2187 g) and 0.0022 mol Co(NO3)3·H2O (0.6403 g) were dissolved in an solutions consisting of 5 ml H2O and 35 ml ethylene glycol to form a homogeneous solutions. 1.869 g C6H12N4 was added into the above solutions, after magnetically stirring for 30 minutes, these mixtures were transferred into a 50.0 ml Teflon-lined stainless steel autoclave and stayed in 120 oC oven for 6 hours. Subsequently, the yellow solids were collected after washing by water and alcohol for three times and dried at 80 oC oven. Synthesis of CoFe LDH@p-Glu 0.5 g CoFe LDH and 1 g glucosamine hydrochloride were added into 30 ml H2O leading to a homogeneous mixture after 1 hour continuous stirring. Then, the mixture was hydrothermally treated for 12 hours at 180 oC oven. After washing by water and alcohol for several times and drying, CoFe-LDH@p-Glu can be obtained.



Synthesis of CoFe/NH-C NS 0.1 g CoFe-LDH@p-Glu, 0.6 g dicyandiamide and 1.5 g urea were mixed homogeneous by grinding. Next, these above powders were transferred into a high temperature furnace and annealed at 550 °C for 2 h, followed by heating up to 900 °C for 1 h under Ar with a heating rate of 2 °C/min. The mass ratio of the nitrogen source to CoFe-LDH@p-Glu is 3:1 (including 0.1 g CoFe-LDH@p-Glu, 0.6 g dicyandiamide and 1.5 g urea), which is denoted as CoFe/NH-C NS. The mass ratio of the nitrogen source to CoFe-LDH@p-Glu are 1:1 and 5:1 which are named as CoFe/NH-C NS-1 and CoFe/NH-C NS-5 in Supporting Information. Synthesis of CoFe/NL-C NS 0.1g CoFe-LDH@p-Glu was directly calcined at 550 °C for 2 h, followed by heating up to 900 °C for 1 h under Ar with a heating rate of 2 °C/min. Synthesis of CoFe/N-C NP 0.435 g FeCl2·4H2O, 1.281 g Co(NO3)2·H2O , 2 g Glucosamine Hydrochloride, 1.2 g C6H12N4 and 3 g urea were dissolved in water. Then these powders can be obtained from the above solutions by the way of freeze drying. Finally, a heat-treatment, similar to the way of CoFe/NH-C NS, was performed for these powders. Characterization The TGA analyses were performed on Perkin-Elmer Pyrisis from room temperature to 900 °C under air. Powder X-ray diffraction (XRD) was conducted on the Phillips X’Pro diffractometer using Co K radiation ( = 1.7902Å) and Cu K radiation ( = 1.5418 Å) at 40 kV and 40 mA. The morphology of the samples was characterized on transmission electron microscopy (JEOL JEM-1011 Electron Microscope with an accelerating voltage of 100 kV). High resolution transmission electron microscopy (HRTEM) images were recorded on JEOL JEM-2100 instrument with an accelerating voltage of 200 kV. The specific surface area was conducted on an ASAP 2020 apparatus. The Raman spectra were acquired using Renishaw inVia. X-ray photoelectron spectroscopy (XPS) experiments were carried out on a PHI 5000 Versa Probe using Al radiation. The corresponded binding energies were revised according to the C 1s line at 284.6 eV from carbon. Electrochemical measurement ORR, HER, and OER were tested on CHI760D electrochemical workstation in a two-electrode set-up. Hg/HgO was used as the reference electrode and the graphite rod was employed as the counter electrode for all test. All of the electrochemical experiments were carried out at room temperature of ca. 25 °C. The working electrode for ORR was prepared as follows: 8 mg of catalyst was added into 1 mL of isopropanol containing 10 L of 5 wt% Nafion solutions. To acquire a homogeneous ink, these mixtures were dispersed by ultrasonic treatment for 30 min. Next, 10 L of the ink was uniformly dropped upon a freshly polished glassy carbon electrode (5.61 mm in diameter) and was dried at room temperature (loading density of 0.32 mg cm–2). As for OER and HER, a commercial glassy carbon (GC) electrode (5 mm in diameter, 0.19625 cm2) was used as the working electrode and the loading density of the catalyst is 0.32 mg/cm2. For ORR test, cyclic voltammetry (CV) curves were obtained from a linear potential scan at a scan rate of 20 mV s-1 at the O2 or N2-


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saturated 0.1 M KOH. LSV curves were recorded on RDE measurements or RRDE measurements at different rotation rates with a scan rate of 10 mV s–1. Polarization curves of OER were obtained at 1600 rmp with a scan rate of 5 mV s–1 in O2-saturated 1 M KOH. While polarization curves of HER were collected at 1600 rmp with a scan rate of 5 mV s–1 in N2-saturated 1 M KOH. All potentials were converted to the reversible hydrogen electrode (RHE) and all polarization curves were corrected with the iR contribution. For overall water splitting system, the catalysts were loaded on the carbon paper (loading density of 1.8 mg cm–2). In addition, Pt/C catalysts and commercial RuO2 (loading density of 0.10 mg cm–2) were tested for comparison. The ORR kinetics were evaluated by using the Koutecky-Levich equation diffusion as follows: =

=

.

+

(1) /

/

/

(2)

j: measured current density; jk: kinetic current density; jd: diffusion limiting current density; n: transferred electron number; F: the Faraday constant (96485 C mol-1); D02: The diffusion coefficient of oxygen (1.9×10-5 cm2 s-1); : the kinetic viscosity of the solution (0.01 cm2 s-1); C0: the saturated concentration of oxygen in 0.1 M KOH (1.2×10-6 mol ml-1); : the rotation rate (rad·s 1) of the electrode. Lastly, the percentage of peroxide species (HO ) with regard to the all collected oxygen reduction products and the electron reduction number (n) were obtained as: H

= 200

n=4

/

/

/

/

(3) (4)

Id: the disk current; Ir: the ring current; N: the current collection efficiency (0.37) of the RRDE. RESULTS AND DISCUSSION In a typical synthesis, the CoFe LDH nanosheets are firstly hydrothermally synthesized with a mixture of the cobalt source, iron source, ethylene glycol and hexamethyleneimine. As shown in Figure 1, the obtained CoFe LDH nanosheets are uniformly coated with glucosaminoglycan via hydrothermal treatment to form CoFe LDH@p-Glu. It is noted that the original sheet-like structures of CoFe LDH could be reserved after the polymerization of glucosaminoglycan. Eventually, after grinding CoFe-LDH@p-Glu, urea and dicyandiamide for 5 minutes, the resulted mixture is carbonized at high temperature in Ar to obtain the final product (CoFe/NH-C NS). For comparison, the CoFe/NL-C NS is directly collected from the carbonization of the CoFe-LDH@p-Glu without adding the extra nitrogen source and the CoFe/N-C NP is prepared by the direct carbonization of the mixture of the same cobalt, iron, carbon and nitrogen sources as those of CoFe/NH-C NS (Details of preparation of samples see EXPERIMENTAL SECTION).



Figure 2. a,e) the typical SEM and TEM images of the CoFe LDH. b,f) SEM and TEM images of the CoFe-LDH@p-Glu. c,d,g,h) the HRTEM images of the CoFe/N H-C NS. Inset in d) is the particle size distribution of CoFe/NH-C NS. Inset in h) is the corresponding fast Fourier transforms of the lattice-resolved images.i-m) EDS elemental mapping images of CoFe/NH-C NS showing the distribution of cobalt, iron, carbon and nitrogen, respectively.

Figure 2a and 2e show the typical SEM and TEM image of the as-prepared CoFe LDH nanosheet, respectively, which indicates the CoFe LDH nanosheet possesses the flower-like structure assembled by smooth thin nanosheets. The XRD pattern of the as-prepared CoFe LDH precursors (Figure S1 in Supporting Information) reveals all peaks assigned to LDH phase 50-51, confirming the LDH structure is fabricated successfully. As illustrated by Figure 2b and 2f, the CoFe-LDH@p-Glu inherits the flower-like structure of the CoFe LDH template and consists of rough thin nanosheets. The difference between the surface roughness of CoFe LDH and CoFe LDH@p-Glu may be attributed to the coating of glucosaminoglycan on the surface of LDH nanosheets. Furthermore, the similar XRD patterns (Figure S1, Supporting Information) of the CoFe LDH and CoFe LDH@p-Glu indicate that the coating of glucosaminoglycan does not change the crystalline phase of CoFe LDH. After carbonization of the mixture of CoFe LDH@p-Glu and urea/dicyandiamide (Figure 2c), the resulted CoFe/NH-C NS shows the well-defined nanosheet structure inheriting the structure of the pristine CoFe LDH nanosheets, which can be further confirmed by the nanosheet morphology of CN obtained after the acid treatment of the CoFe/NH-C NS (Figure S2 in Supporting Information). As shown in the HRTEM images of CoFe/NH-C NS (Figure 2d-h and Figure S3 in Supporting Information), the CoFe nanoparticles about 21 nm are uniformly embedded in the carbon nanosheet matrix, partly surrounded by carbon layers. The lattice spacing of the nanoparticle is measured to be 0.200 nm corresponding to the (110) facet of the CoFe alloy phase, which is consistent with the results of fast Fourier transforms of the lattice-resolved image. Energy-dispersive X-ray spectroscopy (EDS) elemental-mapping images of CoFe/NH-


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C NS in Fig 2j-m show the distributions of Co, Fe ,C and N elements, respectively, the well overlapping images of Co (Figure 2j) and Fe (Figure 2k) in relevant areas further verify that Co and Fe are dispersed in the form of CoFe alloy. Meanwhile, elemental distributions of C and N indicate that N atoms are homogeneously doped in the carbon nanosheet matrix. In contrast, the size of large nanoparticles ranging from 200 to 1000 nm is uneven in the CoFe/N-C NP prepared by the direct high-temperature treatment of the homogeneously mixed precursor without CoFe LDH nanosheets as templates (Figure S4 in Supporting Information). The subtle element distribution of CoFe nanoparticles can be verified by EDS of High-Angle Annular Dark Field (HAADF) image. As shown in Figure 3b-e and Table S1 (Supporting Information), the Co:Fe ratios (molar ratio) of different nanoparticles in CoFe/NH-C NS are similar and is determined to be 40.0 ( ±3) :60.0 ( ±3) ,indicating the homogeneous element distribution of uniform CoFe nanoparticles. In sharp contrast, the Co:Fe ratios in CoFe/N-C NP ranges from 45.2:54.8 to 63.7:36.3 (Figure3g-j and Table S2 in Supporting Information), suggesting that the elemental distribution of CoFe nanoparticles is not homogeneous, the components of CoFe alloy nanoparticles varies to each other. The CoFe LDH with homogeneous Co/Fe distribution is selected as reactive multifunctional template, leading to the uniform CoFe alloy nanoparticles with same components in CoFe/NH-C NS. These analysis results confirm that the wellfabricated lamellar sandwich structure of the CoFe LDH@p-Glu precursor leads to the special structure of CoFe/NH-C NS in which the CoFe alloy nanoparticles with uniform size and components are embedded in the nitrogen-doped carbon nanosheets.

Figure 3. a) HAADF image of CoFe/NH-C NS.b-e) EDS of selected area for CoFe/NH-C NS. f) HAADF image of CoFe/N-C NP. g-j) EDS of selected area for CoFe/N-C NP.

The X-ray powder diffraction (Co K radiation with = 1.7902 Å) patterns of the obtained samples are shown in Figure 4a. It is obvious that the characteristic peaks of CoFe alloy phase (JCPDS, No.48-1818) can be found in the three as-prepared samples,



consistent with the HRTEM results in Figure 2h. A wide peak at 30.7°assigned to the (002) facet of the graphitic carbon could be seen clearly in the CoFe/N-C NP and CoFe/NH-C NS. Interestingly, the peaks belonging to the Fe2O3 (JCPDS, No.25-1402) obviously emerge in the XRD pattern of the CoFe/N-C NP, but barely exist in the CoFe/NL-C NS and CoFe/NH-C NS. This suggests that CoFe nanoparticles in the CoFe/N-C NP are easy to be oxidized. In the case of CoFe/NL-C NS or CoFe/NH-C NS, the carbon layer coating on the CoFe nanoparticles prohibits the oxidization. Further, the metal contents of the three samples, based on the analysis of the final content of metal oxides, are measured to be 29.4, 28.3 and 48.4 wt. % for CoFe/NH-C NS, CoFe/NC NP and CoFe/NL-C NS, respectively (Table S3 in Supporting Information), which indicates that CoFe/NH-C NS possesses more N-doped carbon species than CoFe/NL-C NS. The surface properties of the three samples are studied by the nitrogen adsorptiondesorption analysis and electrochemistry cyclic voltammetry (CV) measurements. The Brunaue–Emmett–Teller (BET) specific surface area of the CoFe/NH-C NS is measured to be 271 m2 g 1, much higher than that of the CoFe/NL-C NS (131 m2 g 1), which may be due to CoFe/NH-C NS possessing less metal nanoparticles and more N-doped carbon species than CoFe/NL-C NS (Figure S5 in Supporting Information). The electrochemical active surface areas (ECSAs), calculated from the CV curves measured at different scanning rates, are usually represented by their electrochemical double layer capacitances (Cdl). As shown in Figure 4b, the Cdl of the CoFe/NH-C NS is 2.37 times higher than that of CoFe/N-C NP, and 2.96 times higher than that of CoFe/NL-C NS, suggesting the CoFe/NH-C NS possesses the highest electrochemical active surface areas. Benefiting from the unique 2D structure inheriting from the LDH nanosheet template, the CoFe/NH-C NS has the higher active surface area in the electrochemical test conditions compared to the CoFe/N-C NP. Comparing with CoFe/NL-C NS, the addition of extra nitrogen source generating more N-doped carbon species in CoFe/NHC NS leads to the enhancement of ECSAs. In a word, the CoFe/NH-C NS with the highest ECSAs will offer more active sites for the electro-catalytic reaction.

Figure 4. a) XRD patterns , b) Charge current density differences ( j = janodic – jcathodic) plotted against scan rates of CoFe/NL-C NS, CoFe/N-C NP and CoFe/NH-C NS.( corresponding Cdl values are calculated from the average of the absolute value of the slope of the linear fitting to the data).

The Raman spectra of all as-obtained samples are displayed in Figure S6


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(Supporting Information), it is apparent that all samples show the characteristic D-band and G-band located around in 1340 cm-1 and 1580 cm-1 corresponding to the typical graphite-based materials 29, 49, 52, in accordance with the XRD results. The D-band to G-band intensity ratios (ID/IG) of the CoFe/NH-C NS, CoFe/N-C NP and CoFe/NL-C NS are 1.18, 0.96 and 0.95, respectively, suggesting the most defects in the carbon nanosheet for CoFe/NH-C NS. The accurate chemical states of samples can be obtained from the X-ray photoelectron spectroscopy (XPS) analyses (Figure 5a-c and Figure S7 in Supporting Information). The atomic percentages of C, N, Co, and Fe of the CoFe/NH-C NS are determined to be 91.68, 3.25, 2.65 and 2.41 at.%. While CoFe/N-C NP and CoFe/NL-C NS exhibit nitrogen contents of 2.99 and 2.25 at. %, in accordance with the Raman results. Due to the addition of the extra nitrogen source, the CoFe/NHC NS possesses higher nitrogen content than CoFe/NL-C NS. The high-resolution N1s spectra (Figure 5a) can be deconvoluted into five peaks consisting of pyridinic-N (398.1±0.3 eV), M-N (399.3±0.3 eV), pyrrolic-N (400.3±0.3 eV), graphitic-N (401.3±0.3 eV), and oxidized-N (402.6±0.3) 53-54. As shown in the Figure 5b and the Table S5 (Supporting Information), the contents of five kinds of nitrogen species are normalized from the peak area of the XPS curves. As it has been proved 55-58 that the high content of the pyridinic-N is beneficial to improving the onset potential of the ORR and the metal-N (M-N) species, in which N atoms bond with the metals to form metal-N4 or metal-N3 macrocycles, have been considered as the highly active species for ORR/OER/HER. Due to the special nanostructure of metal embedded in carbon nanosheet, the CoFe/NH-C NS possesses not only higher content of pyridinic-N (23 % vs. 20 %), but also the much higher content of M-N (21 % vs. 9 %) than those of CoFe/N-C NP, suggesting the strong interaction between metal nanoparticles and Ndoped carbon nanosheets in CoFe/NH-C NS. Consequently, as the following ORR activity order reveals that the CoFe/NH-C NS, possessing the highest contents of pyridinic-N and M-N species, show the highest E1/2 than that of CoFe/N-C NP and CoFe/NL-C NS, indicating the pyridinic-N and M-N species make crucial contributions to the enhanced ORR performance. These peaks (Figure S7a in Supporting Information), derived from the high-resolution XPS spectra of the Co 2p for CoFe/NHC NS, can be fitted with three components, assigned to Co 0 (778.5 eV and 794.0 eV), Co2+ (780.5 and 795.9 eV), Co-N (781.8 eV) and the satellite (786.0 eV), respectively 59-60 . Deconvolution of the Fe 2p spectra (Figure S7b in Supporting Information) shows the existence of metallic iron, oxidized iron and Fe-N species for the three samples 29, 49 . Furthermore, the CoFe/NH-C NS possesses higher contents of metal-N species (CoN or Fe-N) than those of CoFe/N-C NP and CoFe/NL-C NS (Figure S7 and Table S5 in Supporting Information), which is accordance with the N1s results and confirms the strong interaction between metal nanoparticles and N-doped carbon nanosheets in CoFe/NH-C NS. As many studies have reported 27, 29, 60-61 that the transition metal nanoparticles encapsulated or confined in the carbon layers show obvious peaks of oxidized metal species, suggesting that the transition metal nanoparticles are sensitive to aerobic atmosphere and can be oxidized on surface 62-64. According to the XRD results, the three samples mainly show the characteristic patterns of alloy phase, indicating a partly oxidized shell formed on the metallic core. Compared to the CoFe/N-



C NP, the much higher content of metallic species in the CoFe/NH-C NS should be attributed to the protection from the carbon layers (Figure S7 and Table S5 in Supporting Information).

Figure 5. a) N1s XPS spectra of CoFe/NH-C NS, CoFe/NL-C NS and CoFe/N-C NP. b) N contents of CoFe/NH-C NS, CoFe/NL-C NS and CoFe/N-C NP.

To evaluate the ORR electrocatalytic performance of all as-obtained samples, cyclic voltammetry (CV) experiments are carried out in N2 and O2-saturated 0.1 M KOH solution at a scan rate of 20 mV/s. According to Figure 6a, there are no obvious redox peaks of all the samples in N2-saturated electrolyte (dashed curves), but a characteristic peak of oxygen reduction can be clearly observed in O2-saturated solutions. In addition, the oxygen reduction peak potential of the CoFe/NH-C NS is more positive than those of CoFe/N-C NP and CoFe/NL-C NS, suggesting that the CoFe/NH-C NS possess the highest ORR electrocatalytic property. The polarization curves of these samples, displayed in Figure 6b, are collected from a rotating disk electrode (RDE) at a scan rate of 10 mV/s in O2-saturated 0.1 M KOH. Comparing with the polarization curves of CoFe/NL-C NS, the CoFe/NH-C NS shows a 82 mV more positive half-wave potential (E1/2) and bigger diffusion-limited current density than those of CoFe/NL-C NS. The E1/2 of CoFe/NH-C NS is 45 mV higher than that of CoFe/N-C NP, which indicates that the special structure of the CoFe/NH-C NS makes crucial contributions to the enhanced ORR activity. To acquire the optimized property for ORR, the temperature of calcination treatment ranging from 800-1000 ºC and the different mass ratios of the nitrogen source to CoFe-LDH@p-Glu (1:1, 3:1 and 5:1) are studied (Figure S8 in Supporting Information). The ORR performances of the CoFe/N H-C NS, obtained from the optimal preparation condition (the temperature of calcination treatment: 900 ºC, the mass ratio of the nitrogen source to CoFe LDH@p-Glu: 3:1), surpass the performances of the commercial Pt/C catalysts, such as the more positive half-wave potential (844 mV for CoFe/NH-C NS vs 832 mV for Pt/C) and equivalent diffusion-limited current density. The rotating ring-disk electrode (RRDE) is used to monitor and detect the generation of the hydrogen peroxide during the ORR process. As revealed in Figure 6c, the hydrogen peroxide yields (H2O2%) of the CoFe/NH-C NS are about 0.890-10.6%


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and the corresponding electron-transfer numbers (N) are about 3.78-3.98 within the potentials ranging from 0.8 to 0.2 V, which are higher than those of CoFe/N-C NP ( H2O2%:0.870-22.8%, N: 3.54-3.98) and CoFe/NL-C NS (H2O2%:31.3-41.6%, N: 3.15-3.36). According to these results, the CoFe/NH-C NS with low H2O2% and high N exhibit a dominant 4e- pathway during the oxygen reduction process, which are similar to that of the commercial Pt/C catalyst ( H2O2%:1.1-2.6%, N: 3.94-3.98). In contrast, CoFe/N-C NP and CoFe/NL-C NS show relatively more 2e- pathways for ORR. Linear sweep voltammetry (LSV) curves of CoFe/NH-C NS at different rotation speeds are shown in Figure 6d, the Koutecky-Levich (K-L) curves present linearity at different potentials. Based on the curves of K-L equations, the N is calculated to be a value of 3.97, in good agreement with the results from the RRDE curves, confirming the primary 4e- pathway of the CoFe/NH-C NS for ORR process.

Figure 6. ORR performances of CoFe/N H-C NS and the control samples. a) CV curves at a scan rate of 20 mV/s (solid curves are collected from O 2-saturated 0.1 M KOH solution and dashed curves from N2-saturated 0.1 M KOH solution). b) LSV curves at a scan rate of 10 mV/s. c) electrontransfer numbers (bottom) and hydrogen peroxide yields (top). d) LSV curves of CoFe/NH-C NS at



different rotation speeds; inset is the corresponding K-L plots at different potentials. e) The Currenttime (i-t) chronoamperometric response of CoFe/N H-C NS and commercial 20% Pt/C for 25,000s at 0.515V. e) chronoamperometric response of CoFe/NH-C NS and commercial 20% Pt/C for test of resisting methanol.

The long-term stability of Pt/C catalyst and CoFe/NH-C NS are tested by the chronoamperometric methods in 0.1 M KOH O2-saturated solutions. From Figure 6e, one can find that the current density of Pt/C catalyst suffers from a 26% loss after 25,000s test, indicative of the deactivation or aggregation of the Pt nanoparticle 20. The similar results also can be found in the previous reports 40, 48. In striking contrast, 90% current density is maintained for CoFe/NH-C NS sample, implying the high stability for ORR, which may be attributed to the special structure in which the CoFe nanoparticles are embedded in nitrogen-doped carbon nanosheets. As revealed in Figure 6f, after the addition of methanol, the anodic current derived from the electro-oxidation of methanol immediately appears for the Pt/C catalyst. Subsequently, the current density decreases to 61%. In sharp contrast, the current density of CoFe/NH-C NS has no obvious changes after the injection of methanol, indicative of the strong tolerance to methanol. The longer stability and better methanol tolerance than commercial Pt/C catalyst enable the CoFe/NH-C NS as a promising cathode catalyst in alkaline fuel cells.


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Figure 7. OER and HER performances of CoFe/N H-C NS and the control samples. a) The LSVs (1600 rpm) for OER at a scan rate of 5mV/s in 1M KOH solutions. b) The OER potential at 1.8V versus current density for various catalysts. c) Tafel curves for OER. d) The long-terms stability test of CoFe/NH-C NS for OER and HER at a constant potential. e) The LSVs (1600 rpm) for HER at a scan rate of 5mV/s in 1M KOH solutions. f) Tafel curves for HER. g) Polarization curves of the



CoFe/NH-C NS before and after durability test for overall water splitting in 1M KOH solutions. h) The durability test of CoFe/NH-C NS for overall water splitting at 1.75 V (Inset: Overpotential of the CoFe/NH-C NS at 10 mA cm-2 before and after durability test).

To explore the potential application of CoFe/NH-C NS as non-precious-metal based tri-functional electrocatalysts (for ORR/OER/HER) in the same operating environment, the OER performances are also explored. The OER LSV curves of all samples, obtained from RDE at a scan rate of 5 mV/s 1 M KOH solutions, are displayed in Figure 7a. Generally, the overpotential to deliver a 10 mA cm-2 current density is an evaluation criterion for OER activity. The CoFe/NH-C NS exhibits the smaller overpotential (321 mV) at 10 mA cm 2 than that of CoFe/N-C NP (383 mV) and CoFe/NL-C NS (440 mV), which is in accordance with the activity order of ORR. Very interestingly, when the potential increases to 1.8 V ( Figure 7b), the current density of the CoFe/NH-C NS (143.5 mA cm-2) is identical to that of the commercial RuO2 catalyst (143.9 mA cm-2), and far higher than that of CoFe/N-C NP (49.6 mA cm-2) and CoFe/NL-C NS (31.6 mA cm-2). When the potential continues to increase, the current density of the CoFe/NH-C NS surpasses that of the commercial RuO2 catalyst, showing an excellent OER activity. The Tafel slope reflects the kinetic performance of the catalyst in OER process. CoFe/NH-C NS presents a much lower Tafel slope (Figure 7c) value of 61 mV/dec than that of CoFe/N-C NP (102 mV/dec) and CoFe/NL-C NS (132 mV/dec), close to that of RuO2 (46 mV/dec). The long-term stability test of the CoFe/NH-C NS is performed on O2-saturated 1M KOH solutions at a constant potential acquiring a 20 mA cm–2 current density (Figure 7d), and the current density slightly drops to 16.2 mA cm–2 after 25,000s test, indicative of the high durability of the CoFe/NH-C NS for OER. As many documents have reported 35, 61, 65, the transition metals encapsulated by nitrogen-doped carbons (with less than 3 carbon layers), accelerating the electron transfer from the encapsulated metals to the carbon surface, are beneficial for improving catalytic OER activity and stability. The CoFe/NH-C NS, possessing the unique geometrical configuration in which the CoFe alloys are embedded in nitrogen doped carbon nanosheets, may facilitate the electron transfer process and effective OER activity. As for CoFe/N-C NP, the CoFe nanoparticles is easily oxidized due to the absence of the protection from carbon layers, which impede this electron transfer process from the metal nanoparticles to the carbon surface. It is important to note that the carbon layers may suffer from self-oxidation in intense oxidation environment, but as many documents reported 35, 61, 65 the catalysts, in which metal nanoparticles are encapsulated by carbon layers, show improved stability. The CoFe/NH-C NS, showing excellent performances both for ORR and OER, are expected to be a strong candidate to replace noble metal-based catalysts for electrocatalytic application. The HER electrocatalytic performances of the three samples are also investigated in N2-satuared 1 M KOH solutions and the LSV curves recorded with a 5 mV/s scan rate are displayed in Figure 7e. For delivering a current density of 10 mA cm-2, the overpotential of the CoFe/NH-C NS only requires 230 mV, much smaller than that of the CoFe/N-C NP (318 mV) and CoFe/NL-C NS (381 mV), suggesting a much high activity for HER. Although the HER activity of the CoFe/N H-C NS is lower than that of Pt/C catalyst (60 mV), it is still compelling compared the reported values of the Co-


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based HER catalysts in alkaline conditions, such as Co3N 66, CoOx@CN 67, and Co/CoOx nanoshoots on perovskite mesoporous nanofibers 25. The Tafel slope for HER is calculated from these LSV curve results (Figure 7f). In particular, the CoFe/NHC NS exhibit a Tafel slope value of 97.5 mV/dec, indicating that the Volmer-Heyrovsky mechanism plays a dominant role in HER process for this catalyst. In sharp contrast, the Tafel slopes of the CoFe/N-C NP (136.3 mV/dec) and CoFe/NL-C NS (144.5 mV/dec) are much larger than that of the CoFe/NH-C NS, suggesting their poor kinetic performances in HER process. As shown in Figure 7d, the current density only drops 0.9 mA cm-2 from original 20 mA cm-2 after 25,000 s chronoamperometric measurement at a constant potential, indicating the CoFe/NH-C NS also possesses excellent stability for HER. Due to the excellent OER and HER performances of CoFe/NH-C NS, the overall electrochemical water splitting system has been assembled by using CoFe/NHC NS as the bi-functional electrocatalyst on carbon paper. As shown in Figure 7g, the overpotential is a low value of 520 mV to achieve the current density of 10 mA cm-2, demonstrating the potential application of CoFe/NH-C NS for the electrocatalytic water splitting system. Furthermore, after 25,000 s durability test at 1.75 V, the current density falls by 2.3 mA cm-2 and the overpotential only positively shift 26 mV (Figure 7h), indicating the high stability of the bi-functional electrocatalyst in the overall water splitting system. The used CoFe/NH-C NS, as a cathode catalyst for H2 generation, has further been characterized by TEM (Figure S10 in Supporting Information) and XRD (Figure S11 in Supporting Information). According to the TEM and XRD results, the physical structure and chemical state of CoFe/NH-C NS remain high stability before and after long-term test. Compared with many recently reported transition-metal/nitrogen-doped carbon materials (Table S6 in Supporting Information), the CoFe/NH-C NS shows top ORR performance and excellent OER/HER properties, demonstrating its promising application as an effective tri-functional eletrocatalyst. Taken together, the present reactive multifunctional template strategy fabricates the unique geometric construction, which results in the enhanced electrocatalytic performance. Specifically, (1) the uniform CoFe LDH nanosheet precursor generates the CoFe nanoparticles with the homogeneous element distributions and the uniform size serving as the highly active site for the ORR/OER/HER. (2) The 2D N-doped carbon nanosheet structures provide a large active surface area for facilitating the diffusion of reactants toward accessible active centers and accelerate the transform of both mass and electron from active site. (3) The strong synergistic effect between the CoFe nanoparticles and the N-doped carbon nanosheets providing the high ECSAs and more highly active sites such as pyridinic-N and M-N species, are beneficial for improving electrocatalytic activity. (4) The protective effect of N-doped carbon layer coating on the CoFe nanoparticle is responsible for the excellent stability. CONCLUSION In summary, we have proposed a reactive multifunctional template strategy to synthesize transition metal/carbon composites, without any template which needs to be removed or active agent. In details, through elaborately selecting CoFe LDH as the reactive multifunctional template, the 2D porous N-doped carbon nanosheet structure,



confining homogeneous CoFe alloy nanoparticles, can be obtained. Especially, comparing the direct heat treatment method, our strategy prepares the catalyst exhibiting stronger synergistic effect between the CoFe nanoparticles and the N-doped carbon nanosheets and more active sites. Benefiting from these advantages, CoFe/NHC NS presents a superior ORR performance surpassing the commercial Pt/C catalyst (such as more positive half-potential, better long-term durability and methanol tolerance), comparable OER activity to commercial RuO2 and remarkable HER property. This work presents a fresh strategy for the design and fabrication of advanced metal embedded in carbon substrate heterostructures and then opens up new approach for the development of high-performance carbon-based energy materials. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (X. Guo); [email protected] (B. Zhao). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Key Technology R&D Program of China (2017YFB0310704), the National Science Foundation of China (21773112, 21173119, and 21273109), the Fundamental Research Funds for the Central Universities and the Hubei Key Laboratory for Processing and Application of Catalytic Materials (CH201401).

ASSOCIATED CONTENT Supporting Information: XRD patterns of CoFe LDH and CoFe LDH@Glucosamine; TEM image of the CN sheets; HRTEM images of CoFe/NH-C NS; TEM images of the CoFe/N-C NP; Nitrogen adsorption-desorption isotherms and pore size distribution curves of the three samples (CoFe/NH-C NS, CoFe/N-C NP and CoFe/NL-C NS); EDX element ratio of Co to Fe in selected area; Metal contents of the three samples; Raman spectra of the three samples; N contents and species of the three samples; Co and Fe 2p XPS spectra of the three samples; The contents of different metal state species; ORR performances of the contrast samples prepared at different conditions; OER and HER performances of the contrast samples prepared at different conditions; TEM images and XRD patterns of the used CoFe/NH-C NS; Comparison of the electrocatalytic performances of CoFe/NH-C NS with the reported materials.

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Table of Contents

CoFe/NH-C Nanosheet prepared by reactive template strategy, exhibits excellent ORR/OER/HER performance, showing the great potential application toward sustainable energy conversion.


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N Doped Carbon Nanosheets as

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