Highly efficient electrocatalyst of N-doped graphene ...

Carbon 137 (2018) 358e367

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Highly efficient electrocatalyst of N-doped graphene-encapsulated cobalt-iron carbides towards oxygen reduction reaction Jagadis Gautam a, Tran Duy Thanh a, Kakali Maiti a, Nam Hoon Kim a, Joong Hee Lee a, b, * a Advanced Materials Institute for BIN Convergence Technology (BK21 Plus Global Program), Department of BIN Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk, 54896, Republic of Korea b Carbon Composite Research Center, Department of Polymer-Nano Science and Technology, Chonbuk National University, Jeonju, Jeonbuk, 54896, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 April 2018 Received in revised form 16 May 2018 Accepted 20 May 2018 Available online 21 May 2018

Development of highly functional and durable catalysts in a cost-effective way is a promising approach for practical energy conversion applications. In this study, a novel catalyst, cobalt iron carbide nanoparticles encapsulated by nitrogen doped graphene nanosheets, is successfully synthesized through a simple refluxing strategy followed by a post annealing process. It is found that the catalyst exhibits excellent catalytic activity for oxygen reduction reaction in alkaline medium. The oxygen reduction reaction kinetics of the catalyst mainly follow a 4-electron transferred pathway along with good diffusion limit current density, highly positive onset potential (0.04 V) and half-wave potential (0.11 V). In addition to catalytic activity, the catalyst demonstrates advanced superior stability, and excellent methanol tolerance in comparison with commercial platinum catalyst. The impressive catalytic performance of the catalyst is attributed to the unique mesoporous metal-core/graphene-shell architecture in which high interactions between two transition metals and transition metal-carbon synergistically provide enhanced catalytically active sites, accelerate interfacial charge transfer, and optimize oxygen adsorption energy. The results demonstrate that such catalyst can be an alternative low-cost and efficient catalyst for oxygen reduction reaction in energy conversion applications. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Cobalt iron carbide Nitrogen doped graphene Encapsulation Catalyst Oxygen reduction reaction

1. Introduction The mushrooming crisis of energy has urgently motivated researchers to search for alternative energy conversion and storage devices with the features of high efficiency, low cost, and environmental friendliness. Correspondingly, fuel cells have recently been considered as benign energy sources for portable electronics and electric vehicles, because they simultaneously meet the requirements of high energy and power density, high efficiency and low or zero emission [1,2]. Although fuel cell technologies have been regarded as sustainable energy sources, fuel cell efficiency critically depends on the sluggish electron transfer rate of the oxygen reduction reaction (ORR) occurring on the surface of cathode [3]. The use of Pt-based catalysts can effectively solve this issue;

* Corresponding author. Advanced Materials Institute for BIN Convergence Technology (BK21 Plus Global Program), Department of BIN Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk, 54896, Republic of Korea. E-mail address: [email protected] (J.H. Lee). https://doi.org/10.1016/j.carbon.2018.05.042 0008-6223/© 2018 Elsevier Ltd. All rights reserved.

however, low susceptibility to methanol crossover, low stability, and high cost have significantly hindered the widespread use of these catalysts in a cost-effective fuel cell [4]. Therefore, the development of novel approaches for discovering potential catalyst with different unique micro-morphologies has become a major focus of ORR research in future fuel cell applications [5e10]. Among different catalyst materials, metal carbides have raised much concern due to their similar electronic configurations to noble metals close to the Fermi level, which can significantly improve intrinsic activity for a particular reaction [11]. Also, metal carbides have high mechanical durability, excellent electric conductivity, chemically stability, poisoning resistance, corrosion resistance, and “platinum”-like behavior for oxygen chemisorption toward ORR [12e14]. In this regard, various metal carbides [15e21] have been investigated and have demonstrated better catalytic activity or selectivity compared to their parent metals. Although satisfactory results have primarily been obtained for metal carbides, there is significant gap for meeting performance required for their practical use in fuel cells. Basically, the ORR behavior of the catalysts mainly depends on the type of the transition-metal precursors used,

J. Gautam et al. / Carbon 137 (2018) 358e367

morphology of the carbon support material, and synthesis conditions [22e25]. Researchers have explored the synthesis of numerous bimetallic carbides, which displayed good ORR activity compared to its monometallic carbide counterpart, due to synergistic effects arising from charge transfer between individual components thereby maximizing the surface strain and active sites [26e28]. They also discovered that bimetallization strategy is an effective route to stabilize the metal carbide catalyst under ORR conditions. Recently, Co/Fe carbide and bimetallic catalysts based on Co and Fe have been reported as very active catalyst for ORR [29e31]; therefore, it is interesting to combine CoFe carbide as a new type of low-cost catalyst for enhanced ORR activity. Additionally, stability is another important parameter of catalyst due to the catalyst's decomposition, agglomeration, and dissolution during operation. Therefore, hybridization of carbide catalysts with a highly conductive supporting material, along with the formation of unique micromorphology has been expected to be a viable approach to address stability problem [32e34]. The use of graphene nanosheets has long been considered as a highly pragmatic approach to enhance the ORR performance, in terms of both catalytic activity and stability [35e37]. In particular, the graphene material doped with nitrogen heteroatoms demonstrated superior behavior compared to the pristine one, because of the enhanced electrochemical active sites, large surface area, and excellent charge transfer ability [38e41]. It has found that nitrogen containing groups can efficiently anchor and promote the uniform dispersion of the metal catalyst NPs, thereby improving numbers of electrochemical active site of catalyst material [42,43]. Recent studies on N-doped graphene based Catalyst has also showed the positive role of different types of N sites in graphene structure towards ORR, such as pyridinic-N, graphitic-N, pyrrolic-N, and oxide-N [44e47]. It has been proposed that the increased density of the catalytically active sites, including pyridinic-N and graphitic-N, can stimulate the ORR activity. Consequently, the pyridinic-N amount can enhance the onset potential while the graphitic-N content can determine the limiting current density for the ORR. Furthermore, the presence of nitrogen sites can effectively accelerate the interfacial charge transfer abilities for a fast ORR process [48,49]. Currently, core-shell nanostructure, in which metal catalyst is encapsulated in N-doped carbon layers, has shown enhanced catalytic activity and electrochemical stability [50e52]. The outstanding ORR performance of the core-shell nanostructure can

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be due to exceptional electronic interaction between the metal catalyst and N-doped carbon shells [50e52]. Therefore, to amplify the electrochemical surface and increase the ORR active site numbers, it is necessary to stabilize the metal catalyst without aggregations and dissolution, as well as improve the amount of active N components on the carbon shell, such as pyridinic N and graphitic N for better ORR performance [53,54]. In this study, we report a facile synthetic protocol involving refluxing step, followed by annealing step, for the synthesis of CoFe carbide nanostructures effectively encapsulated within nitrogendoped graphene nanosheets (CoFe Carbide/NG). As-synthesized CoFe Carbide/NG catalyst exhibited good electrocatalytic activity with high cycling stability and toxic tolerance for ORR in alkaline solution. Moreover, the catalytic activity of CoFe Carbide/NG was comparable to that of commercial Pt/C catalyst. 2. Experimental 2.1. Materials Graphite powder, potassium permanganate (KMnO4, 99%), hydrogen peroxide solution (H2O2, 30%), cobalt nitrate hexahydrate (Co(NO3)2.6H2O, 99%), iron nitrate monohydrate (Fe(NO3)3. 9H2O, 99.9%), urea (NH2CONH2, 99%), Nafion solution (5 wt%), and Pt/C catalyst (20 wt%) were purchased from Sigma Co. (USA). Methanol (99.9%), ethanol (99.9%), hydrochloric acid (HCl, 35e37%), and potassium hydroxides (KOH, 99.5%) were purchased from Samchun Co. (Korea). Deionized water was prepared by the EYELA Still Ace SA-2100E1 filtering system (Tokyo Rikakikai Co., Japan). 2.2. Synthesis of CoFe Carbide/NG hybrid In a typical procedure, 60 mg graphene oxide (synthesis procedure is shown in the Supporting information), 0.075 g cobalt nitrate, and 0.025 g iron nitrate were dispersed in 100 mL of water by sonication for 1 h. The mixture was then transferred to a 250 mL R.B flask containing 0.48 g urea, and heated at 80 C for 12 h. Subsequently, the solution was freeze-dried to obtain solid product, which was then annealed in Ar atmosphere at 800 C for 3 h, to achieve CoFe Carbide/NG hybrid (Scheme 1). The amount of metal species in the CoFe Carbide/NG hybrid was estimated to be 59.8 wt % by thermogravimetric analysis (TGA) (Fig. S1). For

Scheme 1. Schematic diagram for the synthesis of the CoFe Carbide/NG hybrid.

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comparison, rGO, NG, Co Carbide/NG, and Fe Carbide/NG were prepared under identical refluxing and annealing conditions. 2.3. Characterization The surface morphology and microstructure of the assynthesized materials were analyzed by field emission scanning electron microscopy (FE-SEM) on a JSM-6701F instrument (JEOL Co., Japan) installed at the Center for University-Wide Research Facilities (CURF) in Chonbuk National University, and transmission electron microscopy (TEM) on an H-7650 (Hitachi Ltd., Japan) at the Jeonju KBSI center. Powder X-ray diffraction (XRD) was achieved on a Max 2500V/PC (Rigaku Corporation, Tokyo, Japan), which used a Cu-Ka target (l ¼ 0.154 nm) with a 2q range of 5e80 at a scan rate of 2 C,min1. Raman spectra were recorded using a Nanofinder 30 (Tokyo Instruments Co., Osaka, Japan). The chemical composition of materials was analyzed by X-ray photoelectron spectroscopy (XPS) with a Theta Probe system (Thermo Fisher Scientific Inc., USA). Specific surface area of the materials according to BrunauerEmmett-Teller (BET) theory was evaluated on an ASAP 2020 Plus system (Micromeritics Instrument Corp., USA). TGA analysis was performed by a TGA Q50 system (TA Instrument Co., USA) at a heating rate of 10 C$min1 under oxygen flow. 2.4. Electrochemical measurements The oxygen reduction activity of the catalysts was evaluated by rotating ring-disk electrode rotator RRDE-3A (ALS Co., Japan) and electrochemical analyzer CHI 660D (CH Instruments Inc., USA) workstation, where Pt wire, Ag/AgCl, and RDE (0.071 cm2) electrode were used as counter, reference, and working electrodes, respectively. Prior to ORR testing, the working electrode was prepared by sonication of 5 mL of Nafion (5 wt%), 0.5 mL ethanol, and 2.5 mg of catalyst in a small vial for 30 min to form a catalyst ink. Then, 5 mL of the as-prepared ink was coated on the clean surface of

RDE, followed by drying for 3 h at room temperature. For comparison, a similar procedure was used to make commercial Pt/C deposited RDE. The cyclic voltammetry (CV) measurements were performed in the range of 0.8 V to 0.2 V in 0.1 M KOH solution saturated with N2/O2 gas at a scan rate of 0.05 V s1. Linear sweep voltammetry (LSV) measurements were carried out in O2-saturated 0.1 M KOH solution at different rotating speed (400e2000 rpm) at a scan rate of 10 mV s1. Electrochemical impedance spectroscopy (EIS) was conducted in the frequency range of 103 to 105 Hz in the potential amplitude of 5 mV at 0.1 V. The number of electrons transferred was determined from the Koutecky-Levich (K-L) equation, by analyzing K-L plots at various potentials [55].

1 1 ¼ J JL

þ

1 JK

¼

B ¼ 0:62nFC0 D0 v1=6 2=3

1 1 þ JK Bu1=2

(1)

(2)

where J, JK, and JL are the current density (mA,cm2), kineticlimiting current density (mA,cm2), and diffusion-limiting current density (mA,cm2), respectively. u is the rotation speed (rad,s1), n is electron transfer number, F is denotation of the Faraday constant (F ¼ 96,485 C mol1), DO2 is the diffusion coefficient of O2 (DO2 ¼ 1.9 105 cm2 s 1), n is announced to be the kinematic viscosity (n ¼ 0.01 cm2 s1), and CO2 is the concentration of O2 in the solution (CO2 ¼ 1.2 106 mol cm3). The methanol (MeOH) tolerance test was measured in 0.1 M KOH solution saturated with O2 in the presence of 0.5 M MeOH. A current-time (i-t) chronoamperometry was performed at room temperature at 0.3 V at a rotating speed of 1600 rpm. The durability test was conducted by chronoamperometric measurement at 0.3 V in 0.1 M KOH saturated with O2 at 1600 rpm for 12,000 s.

Fig. 1. FE-SEM images of (a) NG and (b and c) CoFe Carbide/NG at different magnifications; (d) particle distribution diagram of CoFe Carbide NPs in CoFe Carbide/NG hybrid. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)


3. Results and discussion 3.1. Morphological studies The surface morphology of NG and CoFe Carbide/NG was studied by Fe-SEM analysis technique. FE-SEM imagery (Fig. 1a) shows the wavy and crumpled structure of NG without any residual impurities, implying the effective doping of nitrogen heteroatoms onto carbon backbone structure in the presence of urea as a nitrogen source at high temperature of 800 C. Meanwhile, FE-SEM imagery of the CoFe Carbide/NG hybrid (Fig. 1b and c) showed high density of CoFe Carbide NPs, which were uniformly impregnated on the surface of NG, without the agglomeration or aggregation of nanoparticles. The particle size of these NPs was found to be ~40e50 nm (Fig. 1d). The presence of elements like Co, Fe, C, and N was demonstrated by EDAX analysis (Fig. S2), which basically verified the successful formation of well anchored CoFe Carbide NPs (along with Co: Fe weight ratio of ~2:1) on nitrogen-doped graphene nanosheets. The detailed morphology of NG and CoFe Carbide/NG hybrid was further examined by TEM and HR-TEM analysis techniques. Wrinkles were observed on the surface of NG nanosheets, mainly originated from the presence of abundant N atoms and defects in the graphene structure which caused edge instabilities, strain in two dimensional crystals, dislocations, and surface anchorage (Fig. 2a). Besides, the solvent trapping effect and thermal contraction may also generate surface corrugations, including wrinkles, ripples, and crumples when the nitrogen doping process was carried out through a wet process followed by an annealing step at high temperature [56e60]. In addition, the thickness of the thin

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graphene nanosheets was found to be around one layer, implying that NG presented exfoliated architecture with high surface area and high defect (Fig. 2b and c). The surface morphology of NG was further evaluated by AFM, which confirmed the monolayerd structure of NG (thickness of ~1.2 nm) along with the presence of wrinkles on its crumpled surface (Fig. S3). The TEM image of CoFe Carbide/NG showed almost similar morphological characteristics to those obtained from FE-SEM analysis. High-density CoFe Carbide NPs were homogeneously dispersed on the graphene surface (Fig. 2d and e). HR-TEM analysis revealed the formation of coreshell nanostructure, in which CoFe Carbide NPs were encapsulated by NG nanosheets (Fig. 2f and Fig. S4). The thickness of shell was determined to be around 7 nm, which accords with a few-layer structure. Such a unique core-shell nanostructure was expected to improve the interaction between metal catalysts with high conductive NG supporting material, and then accelerate the interfacial charge transfer possibility. In addition, such nanostructure may enhance the stability of material during electrochemical operation due to effectively avoiding the dissolution and agglomeration of metal catalyst. The nanostructure of CoFe Carbide/NG hybrid was further characterized by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), along with corresponding elemental color mapping technique (Fig. 2gek). The color mapping indicated the simultaneous presence of Co and Fe element at every particle, and available N and C signal for the entire scanned area, confirming the formation of CoFe Carbide NPs encapsulated into graphene structure, which was effectively doped by certain amount of N heteroatoms in the present synthesis procedure. The elemental mapping of a selected nanoparticle confirmed the uniformly random mixing of Co and Fe

Fig. 2. (a) TEM and (b) HR-TEM images of NG; (c) SAED pattern of NG; (d and e) TEM images of CoFe Carbide/NG at different magnifications; (f) HR-TEM image of a CoFe Carbide NP encapsulated by NG; (g) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of CoFe Carbide/NG and the corresponding elemental color maps of (h) C, (i) N, (j) Fe, and (k) Co element. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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at atomic scale to form a unique alloy nanostructure attaching on NG nanosheets (Figs. S5aee). Furthermore, SAED pattern showed the single crystal nature of the nanoparticle (Fig. S5f). For comparison, TEM images of Fe carbide/NG and Co Carbide/NG were also shown in Fig. S6, which indicated the high dense Fe carbide NPs and Co Carbide NPs with average particle size around 50 nm well dispersed on the surface of NG nanosheets. 3.2. XRD, Raman, BET, and XPS analyses The crystallinity of the as-synthesized materials, such as GO, NG, and CoFe Carbide/NG hybrid, was verified by XRD technique. Fig. 3a shows a sharp peak at 2q of ~11.2 that is consistent with its d(002) interspacing, indicating a highly oxidized structure of GO [61]. However, in the case of NG and hybrid material, this peak was shifted to the 2q value of ~25.9 , which is due to the efficiency of simultaneous reduction and nitrogen doping effect on graphene structure during a high-temperature annealing process [62]. In particular, the CoFe Carbide/NG hybrid exhibits the presence of specific peaks relevant to features of Fe Carbide (PDF-#35e0772) [63] and Co Carbide (PDF-#26e0450) [64]. However, due to the predominant Co amount in hybrid, the diffraction peaks of the proposed hybrid look close to feature of Co Carbide material. A list of characteristic diffraction peaks at 2q values of 43.9, 44.9, 51.2, 65.3, and 75.4 were attributed to crystal planes of d(111), d(210), d (400), d(002), and d(131), respectively [65,66]. The XRD and TEM evidenced the highly crystalline structure of CoFe Carbide NPs on the NG nanosheets. The physical properties of materials were studied by Raman technique. Fig. 3b shows the Raman spectra of GO, NG, and CoFe Carbide/NG, in which all the samples exhibited two obvious peaks

at 1350 and 1590 cm1, respectively, indicating the D and G bands of graphene material. The value of intensity ratio (ID/IG) was increased for NG (1.34) and CoFe Carbide/NG (1.27) as compared to GO (1.02), suggesting that the disordering of graphene increases upon the insertion of nitrogen atom in graphitic plane [67]. The 2D band was almost invisible for NG and CoFe Carbide/NG, implying highly disordered sp2 structure of graphene due to the efficient nitrogen doping effect and the strong coupling of high dense metal NPs with graphene surface, as similarly achieved in some previous reports [68e70]. For a different perspective, nitrogen adsorptiondesorption isotherm and Brunauer-Emmett-Teller (BET) analysis were carried out to evaluate the porosity and specific surface area of NG and hybrid materials (Fig. 3c and Fig. S7). The BET analysis showed that the specific surface area for CoFe Carbide/NG hybrid, Fe Carbide/NG hybrid, and Co Carbide/NG hybrid was around 74.7 m2 g1, 72.4 m2 g1, and 65.3 m2 g1, respectively, which was higher than that of the pure NG (61.7 m2,g1) under the same measured conditions, due to the effective intercalation of Carbide NPs into the interspacing of graphene nanosheets. In addition, the mesoporous feature of the hybrids was observed with the porous thickness of 3e5 nm. The improved specific surface area together with the mesoporous nature of the CoFe Carbide/NG catalyst were advantageous for providing enhanced catalytically active sites and O2 moving channels, as well as decreasing the ion/electron conduction pathway, thereby increasing the electrocatalytic performance of CoFe Carbide/NG catalyst [71]. XPS technique was used to investigate the chemical composition and expected interactions between different components of CoFe Carbide/NG hybrid (Fig. 3d). The reduction of O1s/C1s intensity ratios compared to that of GO and the appearance of N1s for NG and hybrid indicated the efficient reduction and doping effect on the

Fig. 3. (a) XRD pattern, and (b) Raman spectra of GO, NG, and CoFe Carbide/NG; (c) N2 adsorption-desorption isotherms of NG and CoFe Carbide/NG (inset: pore distribution of NG and CoFe Carbide/NG); (d) XPS of GO, NG, and CoFe Carbide/NG hybrid. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)


carbon backbone of graphene. In addition, the XPS spectrum of CoFe Carbide/NG showed two additional peaks corresponding to binding energy values of Co 2p and Fe 2p at 781 and 714 eV, respectively, suggesting the attachment of Co and Fe atom on graphene structure. The high-resolution C1s spectra of NG and CoFe Carbide/NG can be deconvoluted into various peaks relating to binding energy values of C¼C, C¼N, C-O-C, C¼O, O-C¼O, and p-p* bonding [70,72]. The significant reduction of oxygen-containing functional groups of NG and CoFe Carbide/NG was clearly observed, as compared to GO (Fig. 4a and Figs. S8a and S8b). Interestingly, the appearance of a binding energy value for C-metal bonding at 283.5 eV for CoFe Carbide/NG clearly demonstrated the formation of metal carbide structure, along with high interactions between metal carbide catalysts with carbon material [73]. The efficient doping of nitrogen atom onto graphene structure was further revealed by the C-N bonding at 285.4 eV in the high-resolution N1s analysis (Fig. 4b and Fig. S8c). In this regard, the high-resolution N1s spectra of NG and CoFe Carbide/NG showed the dominant pyridinic N þ graphitic N sites of 63.9% and 66.4%, respectively, in the total doped N content on graphene (Table S1), which was expected to promote ORR kinetic, due to the strong activity of the Nsites for oxygen adsorption [53,54]. The high-resolution Co2p and Fe2p spectra of the CoFe Carbide/NG indicated the presence of different bonding states of Co and Fe atoms (Fig. 4c and d). The high-resolution Co2p and Fe2p spectra was deconvoluted into doublets that were consistent with Co2p3/2, Co2p1/2, Fe2p3/2, Fe2p1/2, and satellite components, due to the presence of Co and Fe

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[74]. In particular, the presence of Co-C at 778.5 eV and Fe-C at 707.5 and 718.5 eV, respectively [73,75,76], was attributed to the formation of stoichiometric cobalt iron carbides during high-temperature annealing process. 3.3. Electrocatalytic activity towards ORR The electrocatalytic behavior of the as-synthesized catalyst towards ORR was initially evaluated by CV measurements in N2 and O2 saturated 0.1 M KOH solution. Fig. 5a shows that CoFe Carbide/ NG does not exhibit any cathodic peak in N2-saturated 0.1 M KOH solution. Conversely, there is a well-defined cathodic peak at 0.163 V in O2-saturated 0.1 M KOH, suggesting high oxygen reduction activity of the CoFe Carbide/NG catalyst. CV Comparison of different catalyst in O2 environment, Fig. 5b shows higher peak current density and positive peak potential of NG compared to rGO, implying the superior catalytic activity of NG, attributed to the abundant electroactive sites from pyridinic N and graphic N, and enhanced charge transfer ability of NG [53,54]. Impressively, the hybridization of CoFe Carbide with NG resulted in the highest current density among the surveyed materials. In addition, its peak potential (Epeak ¼ 0.163 V) was similar to Fe carbide/NG (Epeak ¼ 0.163 V), but much better than those of rGO (Epeak ¼ 0.25 V), NG (Epeak ¼ 0.23 V), and Co Carbide/NG (Epeak ¼ 0.186 V) (Fig. 5b), suggesting the synergistic effects produced from N active sites of NG and bimetallic carbide towards ORR activity. The insight into electrocatalytic activity of as synthesized catalyst was further evaluated by LSV measurements (Fig. 5c). A

Fig. 4. High resolution XPS spectra of (a) C1s, (b) N1s, (c) Co2p, and (d) Fe2p for CoFe Carbide/NG hybrid. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 5. (a) The CV curves of the CoFe Carbide/NG catalyst in 0.1 M KOH solution saturated with N2 and O2 at a scan rate of 0.05 V s1; (b) The CV curves of rGO, NG, Co Carbide/NG, Fe Carbide/NG, and CoFe Carbide/NG catalyst in 0.1 M KOH solution saturated with O2 a scan rate of 0.05 V s1; (c) The LSV curves of rGO, NG, Co Carbide/NG, Fe Carbide/NG, and CoFe Carbide/NG catalyst in 0.1 M KOH solution saturated with O2 at the electrode rotation speed of 1600 rpm and potential scan rate of 0.01 V s1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

sharper slope in the kinetic zone and a better diffusion-limiting current density of LSV at negative potentials is observed for CoFe Carbide/NG, compared to other catalyst. The Tafel slope of CoFe Carbide/NG (61 mV$dec1) showed a smaller overpotential value toward ORR than those of NG (79 mV$dec1), Co Carbide/NG (64 mV$dec1), and Fe Carbide/NG (72 mV$dec1), indicating its enhanced activity, a highly appropriate feature for electrochemical applications (Fig. S9). In addition, the onset potential and half-wave potential values were evaluated as specific indicators for the measurement of the catalytic performance of catalyst towards ORR. In this regard, the CoFe carbide/NG hybrid displayed most positive onset potential (0.04 V) and half wave potential (0.11 V), compared to that of rGO, NG, Co Carbide/ NG, and Fe Carbide/NG, further suggesting the superior catalytic activity of the CoFe carbide/NG hybrid (Fig. 6a). The high activity of catalyst is mainly attributed to maximized active sites and decreased strain introduced by the metal carbide subsurface compared with the parent metals, and the synergistic effects arising from two metals. Lower strain behavior can produce a more moderate oxygen binding energy, which is highly beneficial for the ORR process. From another perspective, EIS measurements were performed to evaluate the charge transfer resistance (Rct), which is considered as an important parameter to evaluate the catalytic activity of the catalyst. EIS results showed the smallest Rct value for CoFe Carbide/ NG catalyst (~88.8 U) compared with other catalyst, such as rGO (~241.2 U), NG (~117.4 U), Fe Carbide/NG (~115.7 U), and CoFe Carbide/NG (~105.7 U), suggesting the faster interfacial charge transfer ability of CoFe Carbide/NG catalyst (Fig. 6b). To further understand

the mechanism of ORR process, the LSV curves of the rGO (Fig. S10a), NG (Fig. S10b), Co Carbide/NG (Fig. S10c), Fe Carbide/NG (Fig. S9d), and CoFe Carbide/NG (Fig. 6c) in O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s1 and different rotation rates of RDE were measured. The K-L plots at different potentials from LSV results at different rotation speeds of electrodes were shown in Figs. S11(aed) and Fig. 6d. The linearity and parallelism in the K-L plots indicated firstorder kinetics toward the dissolved oxygen concentration and the same number of electron transferred at different potential values [76,77]. Equations (1) and (2) were used to calculate the electron transfer number from 0.2 to 0.45 V. The value of electron transfer number for the CoFe Carbide/NG catalyst was found to be 3.65e3.83 at the surveyed potential range, which range was much higher than that of rGO, NG, Co Carbide/NG, and Fe Carbide/NG catalyst (Fig. 6e). This result indicates that the CoFe Carbide/NG catalyzed ORR occurred through the dominating four-electron transfer pathway following faster reaction kinetics for highly efficient ORR process. Stability is another important parameter to evaluate the feasibility of catalyst for practical application. In this regard, LSV measurements of the CoFe Carbide/NG were recorded at 1st cycle and after 2500 consecutive cycles (Fig. 6f) and compared with each other. The LSV comparison shows that there is no any significant loss in catalytic activity, even after 2500 continuous cycles, demonstrating the high stability of the CoFe Carbide/NG for ORR. To the best of our knowledge, as synthesized CoFe Carbide/NG hybrid is able to maintain high stability along with the good achieved activity towards ORR in comparison with the previously reported


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Fig. 6. (a) The onset potential and half wave potential values of different materials; (b) EIS results of rGO, NG, Co Carbide/NG, Fe Carbide/NG, and CoFe Carbide/NG catalyst; (c) The LSV curves of CoFe Carbide/NG catalyst in 0.1 M KOH solution saturated with O2 at a scan rate of 10 mV s1 and different rotation rates of RDE; (d) Plot of J1 versus u1/2 according to the Koutecky-Levich equation at different electrode potentials for CoFe Carbide/NG catalyst; (e) The number of electron transfer for rGO, NG, Co Carbide/NG, Fe Carbide/NG, and CoFe Carbide/NG catalyst towards ORR; (f) The LSV stability of CoFe Carbide/NG catalyst towards ORR in 0.1 M KOH solution saturated with O2 at a scan rate of 10 mV s1 and rotation rate of 1600 rpm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. (a) LSV curves of the CoFe Carbide/NG catalyst and commercial Pt/C product in O2-saturated 0.1 M KOH solution at electrode rotation speed of 1600 rpm and potential scan rate of 0.01 V s1; (b) MeOH tolerance, and (c) working stability of Pt/C and CoFe Carbide/NG catalyst; (d) EIS results of the CoFe Carbide/NG catalyst at initial, and after 12, 000 s of stability test. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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metal carbide-based catalysts (Table S2). To gain further insight into the suitability of the practical use of CoFe Carbide/NG, the catalytic performance of the CoFe Carbide/NG catalyst was compared with a commercial Pt/C catalyst under the same working conditions. Fig. 7a shows that there is a minute difference between the onset potential, half wave potential, and limit current density of the two catalyst materials. The CoFe Carbide only showed 30 mV and 4 mV difference for onset potential and half wave potential, respectively, as compared with the Pt/C product. This indicated that the CoFe Carbide/NG has almost similar catalytic behavior with commercial Pt/C. On the other hand, the methanol tolerance ability of CoFe Carbide/NG and Pt/C was also investigated by chronoamperometry at 0.25 V in 0.1 M KOH solution by adding 0.5 M methanol (Fig. 7b). The Pt/C catalyst immediately exhibited a sharp fluctuation in current density immediately after injecting the methanol, indicating the low methanol tolerance ability of Pt/C. Meanwhile, no noticeable change was observed in the current density for CoFe Carbide/NG electrode at the same time, suggesting the superior methanol tolerance ability of the catalyst. In addition, the chronoamperometric technique was also applied to evaluate the stability of the catalysts at a constant voltage of 0.25 V. Chronoamperometric results showed that the current density of the CoFe Carbide/NG and Pt/C catalysts retained 96.2% and 85.5%, respectively, after running for 12,000 s (Fig. 7c). Furthermore, the Nyquist plots based on EIS measurements at initial time and after 12,000 s exhibited nearly similar semicircles, suggesting the excellent retention of conductivity for CoFe Carbide/ NG catalyst (Fig. 7d). These results demonstrated that high stability of CoFe Carbide/NG catalyst, which could be due to the unique morphology and strong coupling of CoFe NPs with C atoms of graphene nanosheets. The high integrations between C and CoFe atoms can lock metal catalyst into stable positions, avoiding its segregation or diffusion toward the surface. As a result, metal catalysts are prevented from their dissolution into KOH solution, thus leading to respectable long-term stability during operation. 4. Conclusion For the first time, a novel catalyst based on CoFe Carbide NPs encapsulated by nitrogen-doped graphene nanosheets was successfully synthesized by a simple refluxing step followed by annealing process. Upon testing as electrocatalyst for ORR, CoFe Carbide/NG showed high onset potential, half-wave potential, cycling stability and methanol tolerance ability. In addition, CoFe Carbide/NG demonstrated performance close to commercial Pt/C catalyst towards ORR in alkaline medium. These extraordinary features of CoFe Carbide/NG are attributed to the mesoporous structure, large surface area, and numerous catalytically active conducting channels due to the synergistic effect of the two metals, and high interactions between metal and carbon shell in the catalyst. The above result suggests that this hybrid could be employed as a new type of cost-effective and efficient catalyst for future applications in energy storage and conversion devices. Acknowledgements The authors acknowledge support from the Basic Research Laboratory Program (2014R1A4A1008140), Nano-Material Technology Development Program (2016M3A7B4900117), and the Leading Human Resource Training Program of Regional Neo Industry (2016H1D5A1909049) through the National Research Foundation (NRF) funded by the Ministry of Science and ICT of Republic of Korea.

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