Highly Active and Durable CoreâShell fct-PdFe@Pd Nanoparticles

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Energy, Environmental, and Catalysis Applications

A Highly Active and Durable Core-Shell fct-PdFe@Pd Nanoparticles Encapsulated NG as an Efficient Catalyst for Oxygen Reduction Reaction Kakali Maiti, Jayaraman Balamurugan, Shaik Gouse Peera, Prof. Nam Hoon Kim, and Joong-Hee Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04060 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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ACS Applied Materials & Interfaces

A Highly Active and Durable Core−Shell fctPdFe@Pd Nanoparticles Encapsulated NG as an Efficient Catalyst for Oxygen Reduction Reaction Kakali Maiti,a Jayaraman Balamurugan,a Shaik Gouse Peera,a Nam Hoon Kim,a and Joong Hee Leea,b* a

Advanced Materials Institute of BIN Convergence (BK21 plus Global), Department of BIN

Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk 54896, Republic of Korea b

Center for Carbon Composite Materials, Department of Polymer & Nano Science and

Technology, Chonbuk National University, Jeonju, Jeonbuk 54896, Republic of Korea KEYWORDS: nitrogen-doped graphene, core-shell, fct-PdFe@Pd@NG, oxygen reduction reaction, fuel cells ABSTRACT: Development of highly active and durable catalysts for oxygen reduction reaction (ORR) alternative to Pt-based catalyst is an essential topic of interest in research community but challenging task. Here, we have developed a new type of face-centered tetragonal (fct) PdFe alloy nanoparticles encapsulated Pd (fct-PdFe@Pd) anchored onto nitrogen-doped graphene (NG). This core-shell fct-PdFe@Pd@NG hybrid is fabricated by facile and cost-effective technique. The effect of temperature on the transformation of face-centered cubic (fcc) to fct structure and their effect on ORR activity are systematically investigated. The core-shell fctPdFe@Pd@NG hybrid exerts high synergistic interaction between fct-PdFe@Pd NPs and NG shell, beneficial to enhance the catalytic ORR activity and excellent durability. Impressively, 1 ACS Paragon Plus Environment

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core-shell fct-PdFe@Pd@NG hybrid exhibits an excellent catalytic activity for ORR with onset potential of ∼0.97 V and half-wave potential of ∼0.83 V vs RHE, ultra-high current density and decent durability after 10000 potential cycles, which is significantly higher than that of marketable Pt/C catalyst. Furthermore, the core-shell fct-PdFe@Pd@NG hybrid also shows excellent tolerance to methanol, unlike commercial Pt/C catalyst. Thus, these finding opens a new protocol for fabricating another core-shell hybrid by facile and cost-effective techniques emphasizing great prospect in next-generation energy conversion and storage applications. 1. INTRODUCTION Impact on the climate change and continuous global warming alert due to increased greenhouse gases released from the usage of fossil fuel, demands for sustainable energy storage and conversion technologies.1 As emerging energy conversion system, fuel cells with their excellent power densities are considered as an alternative source of energy technology which uses hydrogen as fuel and oxygen as a reductant. In fuel cell setup, oxygen reduction reaction (ORR) happens at the cathodic part, which plays a vital role in regulating the fuel cells performance. The perfect cathode should possess high catalytic activity, superior electrical conductivity, outstanding durability and low manufacturing cost.2 Platinum (Pt) based catalyst has been considered as one of the potential cathode catalysts for ORR, but the high cost of Pt hinders its industrial-scale application of fuel cells. Therefore, alternate catalysts to Pt, with high catalytic activity, selectivity, durability and low-cost, are extremely required to develop for highperformance fuel cells, which remain a challenging task. In this view, among the metal-based catalysts, low-cost transition metal-based catalyst such as Ag, Au, and Pd are intensively explored as alternative catalysts for alkaline ORR catalyst.

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Among them, Pd based catalyst have employed as alternate ORR catalysts to Pt-based catalyst.3-5 However, monometallic Pd based catalysts still suffer from low catalytic activity and especially durability compared to commercial Pt-based catalyst. In this context, alloying of Pd with other transition metal such as Ni, Cu, Co, Fe, etc., are widely explored as an effective catalyst to enhance the ORR activity in relation to the monometallic Pd based catalysts.6 Among them, the PdFe-alloy system has established significant attention because of their exclusive properties such as high electrical conductivity, superior catalytic activity and outstanding durability, which is favorable for ORR.7 Nevertheless, PdFe alloy NPs is employed as a cathode in fuel cell applications. However, it has poor catalytic activity and short cycling life due to their deprived charge-transport properties across the alloy NPs. In this context, the imperative approach is to synthesis alloy nanostructure with core-shell architecture due to their high catalytic activity and durability than that of mixed alloy nanostructure.8 The synergistic interaction between core and shell brings out the strain-stress surface effect and modifies the electronic structure of nanoparticles which favors oxygen adsorption and its subsequent reduction.9 On the other hand, crystalline phase of the alloy cathodes plays a key role in improving the catalytic activity and durability of the catalyst.10,11 At the nanoscale level, numerous studies have been explored in synthesizing NPs with distinct crystalline phases such as face-centered cubic (fcc), and face-centered tetragonal (fct).11,12 Among them, fct crystalline phase is shown to have high catalytic activity and durability than that of most commonly considered fcc.13-15 For instance, Zhang et al. reported that the core-shell FePt/Pt NPs displayed superior ORR activity due to their crystalline phase transformation from fcc structure to fct structure.16 However, not much attention is paid in the context of the effect of phase transformation from fcc to fct on the electrocatalytic ORR. In general, the high-temperature treatment is necessary to bring out this 3 ACS Paragon Plus Environment


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crystalline phase transformation from fcc to fct structure. Nevertheless, when subjecting the NPs to high-temperature treatment, NPs agglomerates leading to the increased NPs size and hence reduces the active surface area and its catalytic activity. Recently, several types of research have been conducted to discover efficient non-Pt based catalysts to replace the traditional Pt-based catalysts (Pt/C). Heteroatoms (N, S, P, B and F) doped carbon-based materials such carbon nanotubes, carbon nanofibers, fullerenes,17,18 and graphene19 as conducting polymers derived catalysts20,21 have been extensively considered as promising metal free ORR catalyst or as a conductive support materials in fuel cell applications. Among them, nitrogen-doped graphene (NG) has attracted numerous interest because of its ability of metal-free catalytic activity, high electrical conductivity, extraordinary specific surface area, excellent catalytic activity, and high chemical stability under fuel cell atmospheric conditions.22 However, the ORR catalytic activity of NG is typically lower than that of Pt-based catalysts, especially in a real fuel cell atmospheric condition and hence metal-based catalyst in the combination of doped carbons still represents the best choice in terms of deliverable power densities. In such circumstances, one of the most gifted approaches to avoid this agglomeration problem of catalyst through introduction of conductive NG support into the metal NPs by restrain them inside the specific NG shell and then to allocate the core metal NPs within NG, which can successively alleviate the agglomeration of the metal NPs and hence boost the catalytic activity and long-term cycling stability.23 In this work, we endeavored to explore the effect of phase transformation of fct-PdFe@Pd from its fcc phase, together with confining the PdFe@Pd inside an NG shell. Herein, we demonstrate a new synthesis route for the preparation of fct-PdFe@Pd NPs encapsulated NG hybrid as a highly active, low-cost and durable ORR catalyst alternative to


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commercial Pt/C. We propose the effect of temperature on the alloy transformation from fcc to fct and its effect on the ORR activity. To the best of our knowledge, no research reports in the literature have been centered on the catalytic activity of ordered fct crystalline phase of the core−shell PdFe@Pd@NG hybrids for used as ORR catalyst in fuel cells. The core-shell PdFe@Pd NPs of the fct crystalline phase and average particles size ∼10−15 nm, thus expressively enhanced the surface area of the PdFe@Pd@NG hybrid are encouraged by the porous architecture of conductive NG support. The core-shell fct-PdFe@Pd@NG hybrid is synthesized via a facile, two-step method, at the heat treatment temperature of 500 °C. The resulting core−shell fct-PdFe@Pd@NG hybrids was gifted to entirely combine the benefits of both NG nanonetwork and core−shell fct-PdFe@Pd NPs, accomplishing more positive onset potential than that of fcc crystalline phase of the catalyst. This synergistic coupling effect between core−shell fct-PdFe@Pd NPs and NG enhances the ORR activity and durability of the core−shell fct-PdFe@Pd@NG hybrids superior to that of commercial Pt/C. The core-shell hybrid meets the morals of superior catalytic activity, outstanding stability, and cost-effectiveness, and thus has the countless potential to replace commercial Pt/C for ORR activity in fuel cell applications. 2. RESULTS AND DISCUSSION 2.1 Structural analysis of fct-PdFe@Pd@NG hybrid. Figure 1 presents the scheme for the fabrication of core-shell fct-PdFe@Pd@NG hybrids. First, NG was synthesized with melamine as the source of N at the pyrolysis temperature of 900 °C. To this, 3 mmol PdCl2 and 1 mmol FeCl3.6H2O are dissolved in ethanol, was then added to the ethanolic NG suspension. The obtained mixture was stirred at 80 ºC for 12 h to remove the solvent from this mixture.



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Afterward, the resultant powder was heat treated at 300 ºC for 2 h in H2/Ar atmosphere to generate fcc crystalline phase of PdFe/NG hybrid. Further, the resultant fcc-PdFe/NG hybrid was heat treated at 500 ºC for 2 h in H2/Ar atmosphere by generating fct crystalline phase of the coreshell fct-PdFe@Pd@NG hybrid. This obtained hybrid called as “fct-PdFe@Pd@NG hybrid” for the whole manuscript. Details synthesis method is well described in Experimental Section.

Figure 1 Schematic illustration for the synthesis of core-shell fct-PdFe@Pd@NG hybrid catalyst and its fuel cell application. The morphology of the as-synthesized NG and fct-PdFe@Pd@NG hybrid was examined by SEM and TEM analysis. The as-synthesized NG displayed crumpled graphene nanostructure, representing that N atom are well-incorporated in the graphene network (Figure S1). Also, there were no any other impurities were observed, which demonstrates that the melamine precursors fully decomposed and uniformly decorated on graphene matrices without affecting the 6 ACS Paragon Plus Environment

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morphology of graphitic nanostructures as reported in previous literature.24 The SEM images of core-shell fct-PdFe@Pd@NG hybrids reveal that the large number of the fct crystalline phase of core-shell PdFe@Pd NPs are uniformly anchored and encapsulated by NG matrices (Figure S2a and S2b), emerging a high-class hierarchical nanostructure. This investigation clearly reveals that the crumpled NG matrices act as an effective bridging medium, which can encapsulate the coreshell PdFe@Pd NPs to resist their agglomeration. The high-magnification SEM image of coreshell fct-PdFe@Pd@NG hybrid (Figure S2b) indicates that fct-PdFe@Pd NPs with a uniform diameter of ∼10–15 nm (Figure S2c) is encapsulated by NG network designating a high coupling effect between fct-PdFe@Pd NPs and NG layers. This high synergistic interaction may enhance the electron transport kinetics between catalytic fct-PdFe@Pd NPs and NG, resulting in high ORR catalytic activity and tremendous durability towards fuel cell applications.25 The elemental distribution (Figure S2d) and EDS color mapping (Figure S3) of fctPdFe@Pd@NG hybrid is examined by SEM-energy dispersive X-ray spectroscopy, demonstrating that the existence of the elements such as Pd, Fe, N, C and O in the core-shell hybrid, further proves that fct-PdFe@Pd NPs are successfully encapsulated by NG matrices. This result confirms that the fabrication protocol proposed here can effectively simplify the development of strong interaction between the core-shell structure of fct-PdFe@Pd@NG hybrid. To study the temperature effect, we evaluate the structural morphology of fcc-PdFe/NG hybrid further heat-treated at ~700 °C (PdFe/NG_700) for 2h (the molar ratio of the Fe: Pd is (~1: 3) is retained,) but, it displayed irregular distribution, as well as different size of PdFe NPs, are anchored on NG nanosheets (Figure S4). Also, PdFe/NG hybrids were synthesized using different Pd/Fe molar ratios of 5:1 and 1:1, resulting hybrids denoted as PdFe/NG-A and PdFe/NG-B, respectively. The as-obtained PdFe/NG-A and PdFe/NG-B exhibited that PdFe



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alloy NPs with random size supported on NG nanosheets (Figure S5a and S5b). Similar results were also drawn with the monometallic Fe/NG and Pd/NG hybrids, which display the large size of the Fe as well as Pd NPs is supported on the NG support (Figure S6a and S6b). As a control experiment, we have also synthesized support less pure PdFe alloy and the FE-SEM image of pure PdFe alloy NPs shows excessive particle agglomeration, with the particle size > 80 nm (Figure S6c). Therefore, this current investigation reveals that NG matrices not only prevent the aggregation of NPs but also stabilize them in a confined NG matrix and serve as an excellent electron transport medium for metal NPs.26,27 Therefore, we conclude that the present synthesized core-shell fct-PdFe@Pd@NG hybrid is optimized by the following synthesis parameters such as reaction temperature, the composition of the precursors, and support materials, which may play a crucial role in tuning the morphology and ORR activity towards fuel cell applications. The intrinsic morphology of the as-synthesized NG and core-shell fct-PdFe@Pd@NG hybrid is examined by (TEM) and dark-field STEM-EDS mapping analysis. The as-synthesized NG (Figure S7a) displays that the wrinkled and crumpled surface, representing that the synthesis protocol followed in this study can effectively doping the N into the graphene framework. The SAED pattern of NG (inset of Figure S7a) displays amorphous in nature due to the defects induced by N doping, which is consistent with SEM study. Figure 2a,b shows that core-shell PdFe@Pd NPs are uniformly encapsulated by NG nanosheets, representing the highly crystalline behavior (inset of Figure 2b) of PdFe@Pd NPs. It is further confirmed that there was no noticeable interlayer spacing between fct-PdFe@Pd NP and NG matrices (Figure 2b-c and Figure S7b-f), indicating that high coupling effect between them, this is beneficial to enhance the catalytic activity and durability during ORR activity which will improve the catalytic


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performance of the catalysts. Figure 2e-i shows the TEM elemental mapping of the fctPdFe@Pd@NG electrocatalyst, which shows the presence of elements Pd, Fe, N and C and their uniform distribution.

Figure 2 (a) TEM image of core-shell fct-PdFe@Pd@NG hybrid, (b–d) HR-TEM image of coreshell fct-PdFe@Pd@NG hybrid, inset of (b) shows corresponding SAED pattern, and dark-field STEM elemental mapping analysis of fct-PdFe@Pd@NG hybrid: (e) selected area, and corresponding elemental mapping of (f) Pd, (g) Fe, (h) N, and (i) C; and (j) EDS line-mapping of the core-shell fct-PdFe@Pd@NG hybrid. Figure 2j shows the line spectrum and corresponding elemental mapping of Pd and Fe. The metallic Fe peak at the center of nanoparticle indicates that metallic Fe occupies the core and the high-intensity signals for Pd, confirms the shell of the nanostructure. Besides, the direct investigational authorization for the high synergistic interaction between fct-PdFe@Pd NPs and



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NG sheets is hard to accomplish, an indirect evidence can be delivered by the broadly used ultrasonication method.28,29 No obvious changes in core-shell fct-PdFe@Pd@NG hybrid structural intrinsic morphology was noticed after 4 h strong ultrasonic vibration and half day stirring in pure ethanol (Figure S8), which further confirms the strong coupling effect between fct-PdFe@Pd NPs and NG matrices.30 The TEM images of fcc-PdFe/NG hybrid treated at the temperature of ∼300 °C and further heat-treated at 700 °C are shown in Figure S9, clearly indicates that the irregular alloy NPs dispersed on NG sheets in the as-synthesized hybrids. Therefore, this study confirms that uniform size of the fct-PdFe@Pd NPs encapsulated by NG matrices was achieved at ∼500 °C, which is well in-consistent with the FE-SEM. Besides, the HR-TEM image (Figure 2d) of the fct crystalline phase of core-shell fct-PdFe@Pd NPs shows greater lattice compression of (∼0.21 nm) than the (111) plane of fcc Pd structure (Figure S10), due to the lattice strain induced by the doping of Fe into Pd lattice, which is beneficial to enhance the ORR activity and durability.31,32 The elemental distribution of fct-PdFe@Pd@NG hybrid that was investigated by STEM-EDS color mapping analysis, which reveals that the PdFe alloy NPs encapsulated by Pd shell, also the core-shell fct-PdFe@Pd NPs well embedded by NG sheets (Figure 2e-i). This STEM-EDS mapping further reveals the double shell formation around the PdFe alloy NPs, which is beneficial to reduce the aggregation of PdFe alloy NPs and enhance the catalytic activity and durability of the catalyst which is well-matched with the Figure S7b-f. During the synthesis of highly ordered fct-PdFe@Pd@NG hybrid at a temperature of 500 °C, due to the surface segregation effect could migrate the Pd around the surface of the PdFe alloy NPs, resulting in perfect core-shell formation with PdFe core and Pd shell in the core-shell hybrid.33,34 This migration of Pd is also due to the larger hydrogen enthalpy of adsorption as reported in the literature.13,15,,35,36 To further authorize the Pd shell around the PdFe alloy NPs


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core in the core-shell fct-PdFe@Pd@NG hybrid, the core-shell fct-PdFe@Pd single NP was performed by atomic-scale chemical mapping (Figure S11a), which clearly display the distinct contrast between PdFe core and Pd shell in the hybrid. This further confirms that the individual distribution of the Pd and Fe maps (Figure S11b and S11c), and an overlay of PdFe alloy core within the Pd-enriched cell (Figure S11d). The STEM-EDS line mapping further confirms that the PdFe alloy NPs encapsulated by Pd shell (Figure 2j) and also the core-shell fct-PdFe@Pd encapsulated by NG shell. Therefore, this result clearly demonstrates that the ~3 nm of selforganized Pd around the PdFe alloy NPs and also another NG shell around the core-shell fctPdFe@Pd NPs, which further confirms that the double-shell formed around the PdFe alloy NPs. This is due to the creation of effective lattice strain on the surfaces core-shell fct-PdFe@Pd NPs, multisource synergistic effect, which can provide a conductive path for fast charge transfer on the surface of catalyst.16,25,26 The crystal nature and lattice phase of core-shell fct-PdFe@Pd@NG hybrid investigated by XRD pattern, as shown in Figure 3a. During the reaction, the sharp peak at ~2θ ~11.8° completely vanished and the new broad peak appeared at 2θ (~26.3 and ~43.1°) (Figure 3a), which corresponds to the (002) and (100) planes of NG, respectively.37,38 The interlayer spacing of the NG ∼0.365 nm, which is superior to graphite flakes (∼0.34 nm). In case of the core-shell fct-PdFe@Pd@NG hybrid, the peak positions shifted towards positive direction when compared to fcc–Pd/NG hybrid (Figure 3a), indicating the lattice contraction because of the Fe doping into the Pd lattice in the hybrids (as also described in HR-TEM). This result reveals that the formation of the intermetallic heterostructure of fct-PdFe@Pd NPs into the NG. Remarkably, the construction of highly ordered fct-crystal phase of PdFe@Pd@NG hybrid (Figure 3a) is confirmed by the presence of superlattice peaks series (marked with “#”) at 2θ of ~32.6, ~41.4,



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~49.7, ~70.46, and ~83.51°, corresponding to the (110), and splitting of (111), (002), (202), and (113) planes, respectively.33,39 This superlattice peak serious further authorize (Figure S12a-c) the formation of highly ordered core-shell fct-PdFe@Pd@NG hybrid (heat-treated at ∼500 °C), instead of chemically disordered fcc-PdFe/NG hybrid (heat treated at ∼300 °C).39,40 The diffraction peak of Fe (Figure S12a) at 2θ ~44.6° and ~64.9°, corresponding to the (110) and (200) planes, respectively, confirming that the formation of core-shell fct-PdFe@Pd@NG hybrid, rather than that of fcc-PdFe/NG hybrid.41 When fcc-PdFe/NG nanostructure is further annealed to ∼700 °C (Figure S12d), the superlattice peaks fully disappears. Meanwhile, there was no any clear fct crystal facets observed for PdFe/NG-A, PdFe/NG-B hybrids and pure PdFe alloy (Figure S12e). Therefore, this result clearly indicates that reaction temperature plays a vital role in the synthesis of chemically-ordered core-shell fct-PdFe@Pd@NG hybrid. Furthermore, the occurrence of (002) plane of NG confirms a duel role both as a shell to the fct-PdFe@Pd NPs and as a good conductive support material that can effectively reduce the aggregation of the PdFe@Pd NPs and enhance the durability of the ORR catalyst due to protective NG coating over the PdFe@Pd NPs through strong interaction among them. This core-shell fct-PdFe@Pd@NG hybrid accelerates the O2 mass transfer by offering enhanced the active sites into the matrices of the catalysts and improving the ORR activity in fuel cells. To further investigate the structural ordered of the core-shell fct-PdFe@Pd@NG hybrid is investigated by Raman spectroscopy, as presented in Figure S13a of the supporting information. Two characteristic peaks of NG at ~1364 cm–1 and ~1591 cm–1, corresponding to the D and G bands, respectively.37,42 The ID/IG ratio of the GO, NG, and core-shell fct-PdFe@Pd@NG hybrid is found to be ~0.87, ~1.07, and ~1.17, respectively. The ID/IG ratio of the core-shell fctPdFe@Pd@NG hybrid slightly is higher than that of GO and NG, which is due to the presence


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of numerous sp2 graphitic domains created on the reduced GO and makes more disorder graphitic structure. Meanwhile, the encapsulation of core-shell fct-PdFe@Pd NPs may create the defects on NG sheets in a hybrid architecture.43 The lower wave number (~1579 cm−1) shift of G bands (inset of Figure S13a), further confirming the strong coupling interaction between coreshell fct-PdFe@Pd NPs and NG nanosheets.44 Moreover, the NG with appropriate defect sites may play a vital role in promoting the nucleation of core-shell fct-PdFe@Pd NPs without aggregation of core-shell fct-PdFe@Pd NPs, which is beneficial to enhance their catalytic activity and durability of the hybrid during the ORR activity. Furthermore, the 2D band at ~2701 cm–1 and D+G band at ~2913 cm−1 designate that the GO fully converted into NG and a higher density of defects, respectively.45 Therefore, the more intensive 2D band for core-shell fctPdFe@Pd@NG hybrid should relate to NG defects, as well as the strong synergistic interaction between core-shell fct-PdFe@Pd NPs and NG nanosheet facilitate the electron transfer, which hinders the self-restacking of the NG nanosheets. Therefore, this highly chemically ordered coreshell fct-PdFe@Pd@NG hybrid may enhance the catalytic activity and durability during the ORR activity.26,44,45 To measure the porous nature of core-shell fct-PdFe@Pd@NG hybrid, surface area and pore size distribution were investigated by N2 adsorption-desorption isotherms (Figure S13b). This N2 adsorption-desorption isotherm exhibits type–IV hysteresis loop at relative pressure (P/P0 ~0.45– 1.0), which implies the mesoporous nature of core-shell fct-PdFe@Pd@NG hybrid. The BET surface area of core-shell fct-PdFe@Pd@NG hybrid is about ~90 m2. g–1, which is superior to pure PdFe alloy NPs (~10 m2. g–1) (Figure S13b). The pore size distribution of (inset of Figure S13b) the core-shell fct-PdFe@Pd@NG hybrid is around ~2–10 nm, calculated based on the Barret–Joyner–Halenda (BJH) model. The core-shell fct-PdFe@Pd@NG hybrid has superior



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specific surface area and pore size distribution to that of the recently reported core-shell hybridbased nanostructure.46 Therefore, this study strongly proves that NG matrices provide an enhanced specific surface area, rather than aggregated pure PdFe alloy NPs. Therefore, the provision of the extraordinary surface area and exclusive porous architecture of core-shell fctPdFe@Pd@NG hybrid boost catalytic active sites that may facilitate the oxygen mass transfer on the surface of the catalyst and provide more conductive paths to improve the catalytic activity.45,47

Figure 3 (a) Comparative XRD spectra, and (b) XPS surveys of the fct-PdFe@Pd@NG hybrid with higher resolution, (c) Pd 3d spectrum, (d) Fe 2p spectrum, (e) C 1s spectrum, (f) N 1s spectrum of the fct-PdFe@Pd@NG hybrid. The chemical environment and elemental composition of the core-shell fct-PdFe@Pd@NG hybrid were performed by XPS analysis (Figure 3b). The survey spectra of NG, displays C1s, 14 ACS Paragon Plus Environment

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N1s, and O1s spectra, which corresponds to the binding energies of ∼284.3 eV, 399.8 eV, and 532.2 eV, respectively (Figure S14a-c). The smaller width of the C 1s peak of NG at hightemperature pyrolysis (~900 °C) suggesting that the graphitization was enhanced at elevated temperature. The surface composition of core-shell fct-PdFe@Pd@NG hybrid was observed by Pd 3d, Fe 2p, C 1s, and N 1s (Table S1). The atomic weight percentage of the Pd, Fe, C, N, and O in this core-shell fct-PdFe@Pd@NG hybrid are 1.12%, 0.41%, 81.94%, 6.4%, and 10.13%, respectively. The Pd 3d spectrum (Figure 3c) showed the zero-oxidation state of metallic Pd comprises of two doublets at ~335.4 and ~340.7 eV, corresponding to the Pd 3d5/2 and Pd 3d3/2, respectively.48 This two-peaks of Pd is due to the spin-orbital interface of the hybrid. In addition, two distinguished peaks of Fe 2p spectrum are observed at ~711.3 and ~725.7 eV, which corresponds to Fe 2p3/2 and Fe 2p1/2, respectively (Figure 3d), which is consistent with the recent report.26,38 The high synergistic interactions between Pd and Fe, and/or the formation of metal oxidized species (Pd and Fe) in core-shell fct-PdFe@Pd@NG hybrid resulting in a positive shift (~0.36 eV) to higher binding energy, which corresponds to the Pd 3d peaks of Pd/NG hybrid (Figure S15a).49 Thus, the electron transfer between Fe and Pd of fct-PdFe@Pd NPs might modify the Pd surface electronic state, which might enhance the ORR activity and durability of the core-shell hybrid catalyst. The C 1s XPS spectrum (Figure 3e) showed the five deconvoluted peaks at ~284.5, ~285.6, ~286.7, ~288, and ~288.9 eV, which are associated with C=C, C=N, C– O, C=O, and COO bonds, respectively.48 The high-resolution N1s spectrum (Figure 3f) can be deconvoluted into four peaks at ~398.5, ~399.9, ~401.1, and ~402.7 eV, which corresponds to the pyridinic N, pyrrolic N, graphitic N, and pyridine N-oxide, respectively.25 These results strongly authorize N are effectively incorporated into the graphene network, which is further confirmed by the existence of C–N bonds in the spectrum of C1s. The atomic weight percentage



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of N is around 6.4 wt. % in the core-shell fct-PdFe@Pd@NG hybrid, which is superior to that of recently reported NG based hybrid architecture.50 The high doping of N into the core-shell fctPdFe@Pd@NG hybrid reveals the excellent interaction between fct-PdFe@Pd and N species, which enhance the electrical conductivity, catalytic activity, and durability for ORR activity. In the high-resolution O1s spectrum (Figure S15b), the peak at ~529.3 eV appears from oxidized metal originating from the core-shell fct-PdFe@Pd NPs. Furthermore, the additional peaks at the higher binding energy correspond to chemically adsorbed OH−/oxygen species in the core-shell fct-PdFe@Pd NPs.27,45,51 2.2. ORR activity of the core-shell fct-PdFe@Pd@NG hybrid. The catalytic performances of the as-prepared catalysts were assessed for their ORR activities in 0.1 M KOH electrolyte. Figure 4a shows the cyclic voltammograms of the catalysts recorded in N2 as well as O2 saturated aqueous 0.1 M KOH electrolytes. It is observed that all the catalysts presented a definite redox peak in O2 saturated electrolytes, which was not realized in N2 saturated electrolytes, indicating the characteristics of their typical O2 reduction peaks (Eredox). It is also observed that the Eredox peaks shifts to higher potentials in the order of NG

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