Hierarchical Flowerlike Highly Synergistic Three-Dimensional Iron

Subscriber access provided by Kaohsiung Medical University

Energy, Environmental, and Catalysis Applications

Hierarchical flower-like highly synergistic three-dimensional iron tungsten oxide nanostructure anchored nitrogen-doped graphene as an efficient and durable electrocatalyst for oxygen reduction reaction Kakali Maiti, Jayaraman Balamurugan, Jagadis Gautam, Nam Hoon Kim, and Joong-Hee Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11406 • Publication Date (Web): 03 Sep 2018 Downloaded from http://pubs.acs.org on September 3, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Hierarchical flower-like highly synergistic three-dimensional iron tungsten oxide nanostructure anchored nitrogen-doped graphene as an efficient and durable electrocatalyst for oxygen reduction reaction Kakali Maiti,a Jayaraman Balamurugan,a Jagadis Gautam,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: 3D Fe-WO3 nanoflower, electrocatalyst, nitrogen-doped graphene, fuel cell, oxygen reduction reaction. ABSTRACT: A unique and novel structural morphology with the high specific surface area, highly synergistic, and remarkable porous conductive networks with outstanding catalytic performance and durability of oxygen reduction electrocatalyst are typical promising properties in fuel cell application; however, to exploring and interpreting this fundamental topic still a challenging task in the whole world. Herein, we have demonstrated a simple, and inexpensive synthesis strategy to design three-dimensional (3D) iron tungsten oxide nanoflower anchored nitrogen-doped graphene (3D Fe-WO3 NF/NG) hybrid for a highly efficient synergistic catalyst for ORR. The construction of flowerlike Fe-WO3 nanostructures, based on synthesis parameters and their ORR performances are systematically investigated. While pristine 3D Fe-WO3 NF or reduced graphene oxide holds poor catalytic performance and even their hybrids shows unsatisfactory results. Impressively, the excellent ORR activity and its outstanding durability are

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

further improved by N doping, especially due to pyridinic and graphitic nitrogen moieties into a graphene sheet. Remarkably, 3D Fe-WO3 NF/NG hybrid nanoarchitecture reveals an outstanding electrocatalytic performance with remarkable onset potential value (~0.98 V), a half-wave potential (~0.85 V) vs RHE, significant methanol tolerance and extraordinary durability of ~95% current retention, even after 15,000 potential cycles, which is superior to a commercial Pt/C. The exclusive porous architecture, excellent electrical conductivity, and the strong synergistic interaction between 3D Fe-WO3 NF and NG sheets are the beneficial phenomenon for such admirable catalytic performance. Therefore, this finding endows design of a highly effective non-precious transition metal-based electrocatalyst for high-performance ORR in alkaline media. 1. INTRODUCTION There is an urgent need to develop high-performance energy conversion systems with superior ability of power and energy density because of the increasing demands of renewable energy technologies throughout all over the world. The fuel cell has considered promising green energy conversion and storage systems among the other renewable and clean energy based technologies because of their excellent electric energy conversion properties, negligible loss of toxic substances, tremendous power density, quiet operation, and eco-friendliness.1,2 Electrochemical performances on the cathode for ORR plays an important contribution in the clean energy–based platform of fuel cells, rechargeable batteries, and water splitting.3,4 In general, the ORR process has relatively slow reaction kinetics, an ORR catalyst should exhibit remarkable electrical conductivity and brilliant catalytic performance to simplify the more favorable four-electron pathway, which would enhance fuel cell performances. It is well known that platinum (Pt) or its alloy nanostructures have shown outstanding electrochemical performances towards ORR due to its superior electrical conductivity and fantastic catalytic activity; however, their high cost, lack


Page 2 of 43

Page 3 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of operational stability, and CO poisoning, impedes their activity in fuel cells on an industrial scale.5 Thus, the development of simple, facile, and cost-effective non-precious-transition metal catalysts in a well-designed architecture for fuel cell applications remains a key issue. Under this circumstance, numerous efforts have been dedicated to finding Pt and its alloys substitutes for fuel cell catalysts. Recently, transition metal-based compounds, such metal oxides/hydroxides, metal sulfides, metal nitride, and metal carbide, have been intensively explored as promising, inexpensive metal architecture based electrocatalysts to substitute the existing catalyst employing Pt and its bimetallic alloy nanostructures in fuel cell applications2,6,7 but these prospective replacements have low catalytic activity and poor electrical conductivity compared to Pt and its alloys. Therefore, among the transition metal based nanostructures, cost-effective tungsten-based metal nanostructure would be effective electrocatalyst for ORR due to their unique properties, such as noble metal-like behaviour, excellent electrical conductivity, high corrosion resistance, CO poisoning effects and methanol tolerance, all these are comparable qualities found in Pt and its alloys.6,8,9 Tungsten oxides (WO3) have been widely used in various alternative energy based potential applications, such as photocatalysis, batteries, supercapacitors, photovoltaic solar energy, and fuel cells8,10,11 due to their exclusive properties, which relates (i) enhanced value of surface-to-volume ratio, which offers enlarged active surface area for effective reaction kinetics of chemical as well as physical interaction, and (ii) surface energies alteration, which may promote the enhancement of the materials properties, which affect the bond nature by manipulating electronic band structure as well as charge transport kinetics. Numerous researchers have developed WO3 with different nanostructures, including nanoparticles, nanotubes, nanosheets, nanowires, and flower-like structures, which have been quite better catalytic activity than bulk WO3 crystal. Among them, flower-like architectures of WO3, which



are composed of low-dimension nanoscale blocks,12 could be ideal candidates for ORR because of their hierarchical nature and large active surface areas with unique porous architectures.10,13,14 Furthermore, the hierarchical flower-like WO3 exhibit numerous interfaces between each nanosheet array to boost the specific capacity as well as power and energy density of the energyrelated device systems, and this may enhance electrolyte accessibility to active materials. During the synthesis process, the flower-like WO3 morphology loses its structural integrity and creates dams on the edges of flowers, which may hinder the catalytic activity of the fuel cells.15 Therefore, incorporating/doping of another transition metal into the WO3 nanostructure is one of the best methods for retaining the flower-like morphology and increasing crystallinity, which enhances the electrocatalytic performances, cyclic stability, and durability of the fuel cells.16,17 For example, gold (Au) or palladium (Pd) doped WO3 has superior sensing performance than that of pure WO3. Moreover, Fe-doped WO3 exhibited hierarchical nanostructures with exclusive porous networks. Among the transition metals used for doping, Fe showed better catalytic activity and higher durability due to its high number of electroactive site.18,19,20 According to the previous study, Fe-doped WO3 could efficiently enhance the catalytic activities through nanostructure crystallinity modification with adjustable oxidation states. Therefore, it is reasonable to anticipate the hierarchical flower-like, Fe-doped WO3 nanostructures as an advanced cathode material for highly active ORR electrocatalysts,21 but it suffers from poor durability in an aqueous KOH electrolyte because of its continuous stacking of Fe-doped WO3 nanostructures. To overcome these issues, construction of Fe-WO3 nanostructures with suitable conductive support might be a promising route for facilitating electron-transport kinetics by protecting the self-agglomeration and restacking of the active nanostructures, resulting in high catalytic activity and outstanding durability.


Page 4 of 43

Page 5 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A conductive support material with high surface area is tremendously suitable as a cathode catalyst by making good contact with active catalyst material and effectively facilitates both transportations of oxygen and electrons.3 Graphene have been extensively used as an effective conductive support due to its excellent conductivity, ultra-high specific surface area, extraordinary flexibility, chemically inert nature, and high corrosion resistance properties.22,23 Interestingly, the doping of heteroatoms in the graphene sheet not only increases the chemical reactivity of carbon architecture but facilitates high electron transport kinetics.22,24 Compared to graphene, N-doped graphene (NG) acts as a most promising conducting support to boost the chemical reactivity of the graphene, resulting in enhancing the synergistic interaction among the gust flower-like Fe-WO3 nanostructures and conducting NG sheets. It is expected that the overall integrated criteria of the hybrid catalyst may enhance the entire electrochemical performances and long-term durability of the flower-like Fe-WO3 catalyst. Till, now there are no reports of highly active ORR electrocatalyst, based on hierarchical, NG supported flower-like Fe-WO3 nanostructures for fuel cell applications. Therefore, this hierarchical, flower-like, Fe-WO3anchored NG hybrid nanoarchitecture has been introduced first time as an efficient and remarkable electrocatalyst for high-performance ORR. Herein, with the aim of designing an ORR catalyst with a high catalytic activity, excellent methanol tolerance behaviour, superior durability, introducing a new type electrocatalyst of hierarchical NG anchored 3D Fe-WO3 nanoflowers (3D Fe-WO3 NF/NG) hybrid nanostructure. Graphene oxide, metal precursors, and melamine (as N source) are used to fabricate this hybrid catalyst by a simple, cost-effective, and one-step thermal treatment at 700 ºC. Fe doping into a WO3 crystal may alter the oxidation states, which can further enhance the ORR activity through hierarchical, flower-like nanostructure formation and prevent degradation of the Fe-WO3 crystals



during the durability test. Interestingly, the hierarchical 3D Fe-WO3 NF/NG hybrid exhibits outstanding electrocatalytic performance with a redox potential of ~0.83 V vs RHE, and excellent durability (95%), even after 15,000 potential cycles. The hierarchical flower-like nanostructure, extraordinary porous network with the high surface area, and highly synergistic interaction of Fe-WO3 species and NG matrices are thought to be responsible towards superior electrocatalytic performances for ORR. Furthermore, the NG not only acts as a support and also boosts electrical conductivity by enhancing the electron transport kinetics of the active Fe-WO3 flower in the hybrid architecture. 2. RESULTS AND DISCUSSION 2.1 Fabrication and Structural Analysis of 3D Fe-WO3 NF/NG Hybrid. The fabrication of 3D hierarchical flower-like Fe-WO3-nanostructure-anchored NG hybrid (FeWO3 NF/NG) is schematically illustrated in Figure 1. A facile, low-cost technique was used to fabricate the hierarchical 3D Fe-WO3 NF/NG hybrid. The GO and melamine mixture was pyrolyzed at ~900 ºC to synthesize the NG sheets. The as-synthesized NG sheets and mixed metal precursors were dispersed in an ethanol: H2O (3:1) solvent and heat treated under continuous stirring at ~80 ºC until the solution becomes evaporated. The as-obtained powder was ground well and subjected to pyrolysis at ~700 ºC in H2/Ar gas flow and after that, it was treated with 0.5 M H2SO4 solution at ~80 ºC for 6 h to obtain hierarchical Fe-WO3 NF/NG hybrid nanostructure (see the experimental section for more details).


Page 6 of 43

Page 7 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Schematic diagram for the synthesis of hierarchical 3D Fe-WO3 NF/NG hybrid nanostructure and its catalytic performance in a fuel cell application. SEM analysis was used to examine the structural features of the as-prepared Fe-WO3 NF/NG hybrid (Figure 2a). The SEM picture clearly depicts the flower-like, Fe-doped; WO3 nanostructure with an average diameter of 300–400 nm is uniformly anchored into NG matrices (Figure 2b). A close view of SEM image (Figure 2c) further indicates that several tinny nanosheets of varying thickness of ~10–12 nm with thin adjacent gaps were constructed to make a flower-like nanostructure and then anchored into the NG network, indicating the presence of high synergistic interaction between Fe-doped WO3 nanocrystals and NG matrices. A part of the



Fe-WO3 nanoflower is built up by individual nanosheet arrays interconnecting to the center part with proper edges to develop a hierarchical nanostructure with the exclusive porous network (Figure 2c). This study confirms that the current synthesis protocol for developed hierarchical Fe-WO3 nanoflower anchored NG is due to the neighbouring atom of C and N in the NG matrices, and this may improve the catalytic performance and long-term durability of the ORR electrocatalyst.18,25 SEM-EDS colour mapping was used to examine the elemental composition of the hierarchical Fe-WO3 NF/NG hybrid (Figure S1). The presence of Fe, W, O, N and C, elements in the hierarchical Fe-WO3 NF/NG hybrid is clearly revealed by the SEM-EDS analysis. Therefore, it is further confirmed that the Fe-WO3 nanostructure is successfully anchored into the NG matrices and fully covers the entire area of the NG sheets. The doping effect of Fe atom into the Fe-WO3 nanostructure may retain the flower-like morphology from structural disorder, and it also enhances the synergistic interaction between the Fe-WO3 species and NG sheets. In contrast, the SEM picture of the pristine Fe-WO3 NF reveals that the thick nanosheets ranging from ~80–200 nm are grown in a random order (Figure S2a). The fact that the crumpled and wavy nature of the NG matrices stabilizes the Fe-WO3 NF (Figure S2b) and prevents aggregation due to close contact with the effective conductive support of N incorporated graphene network, and these results in the super-structure formation of MOxN-C. Besides, NG provides fast electron transport kinetics with numerous electrochemically active interface sites. However, in case of WO3 NF/NG hybrid, the SEM image displays that thicker and more disordered flower-like morphology of WO3 is randomly distributed on the surface of the NG network (Figure S2c). This study further confirms that the doping strategy by the Fe atom is one of the most effective ways for tuning the morphology with a hierarchical flower shape, reducing the thickness of the nanosheet arrays, and enhancing the effective surface


Page 8 of 43

Page 9 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


area. The growth mechanism plays an imperative factor in controlling the morphology of the FeWO3 NF/NG hybrid nanostructure. To investigate the nanoflower formation mechanism, the acid-treated hybrids at various time intervals was examined by SEM analysis and the results are illustrated in Figure S3.

Figure 2.

(a, b) SEM images of hierarchical Fe-WO3 NF/NG hybrid with a different

magnification, (c) High-magnification SEM image of hierarchical Fe-WO3 NF/NG hybrid, and (d) EDAX spectrum of Fe-WO3 NF/NG hybrid (inset shows atomic weight percentage of the elemental composition). The metal oxide nanoparticles (NPs)-decorated NG sheets could be observed in the initial stage (Figure S3a). After 60 min, the metal NPs converted into sheet-like morphology due to exposure of the acid-leaching process. This was due to the possible agglomeration of metal NPs



(Figure S3b).26 Extending the time to 3h resulted in the randomly distributed Fe-WO3 nanosheets exhibiting a tendency to interconnect with each other and start to grow nanoflowers structures (Figure S3c). When the acid-leaching time was increased to 6h, the uniform nanosheets were assembled to construct a hierarchical uniform flower-like nanostructure (Figure S3d). Reports in previous literature can explain the growth mechanism.27,28 The proposed reaction mechanisms are given below under acidic (H2SO4) medium:

During the H2SO4 treatment, the effective protonation starts with tungstate anions (WO42−) to make a polymerization of polytungstate intermediate anions.29 Further increasing the acidleaching reaction time at the reaction temperature of ~80 ºC (optimized by this present work), the nucleation process of negative charge bearing WO3 crystal nucleus has been enhanced due to the attachment of Fe2+ positive ions over the surface of the WO3 crystal.30 As a result, an effect of ultra-fast nucleation with fundamental anisotropic may cause the constructing of flower-shaped Fe-WO3 crystals on the NG matrices. However, the concentration of the precursors is another factor that can affect the morphology of the 3D Fe-WO3 NF/NG hybrid nanostructure, which is further studied by the different molar ratio of Fe:W (1:0.5 and 1:3), and the resulting hybrids are designated as Fe-WO3 NF/NG-1 and Fe-WO3 NF/NG-2, respectively. The SEM images of FeWO3 NF/NG-1 and Fe-WO3 NF/NG-2 hybrids are displayed in Figure S4a,b. Therefore, it is concluded that the optimum molar ratio for Fe: W is ~1:1, which is appropriate for the formation of the hierarchical flower-like morphology of 3D Fe-WO3 NF on NG sheets. Therefore, this


Page 10 of 43

Page 11 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


exposed porous architecture could supply sufficient catalytic active sites and supply an ion pathway for the electrolyte ions inserted into the integrated nanosheets, resulting in a lowering of the charge transfer resistance value (Rct) and an enhancement in the utilization for active materials. Figure 2d provides the EDAX analysis of the Fe-WO3 NF/NG hybrid, which further confirms the existence of Fe, W, O, C, and N demonstrating the fabrication of a hierarchical FeWO3 NF/NG hybrid in a highly pure form. TEM and HR-TEM analyses were used to further investigate the intrinsic morphology of the hierarchical 3D Fe-WO3 NF/NG hybrid. The flower-like morphology of Fe-WO3 NFs is uniformly anchored into the NG matrices, as the shown in the TEM image (Figure 3a) of the FeWO3 NF/NG hybrid. This study clearly reveals that the nanosheets or petals are assembled into the central part of the flower with ragged edges, indicating variations of sheet density inside the nanostructures (Figure S5a,b). The HR-TEM image shows that the there was no interlayer space between the Fe-WO3 petals and the NG matrices (Figure 3b and Figure S5c), representing that the high synergistic interaction between Fe-WO3 NG and the NG sheets in the hybrid architecture. Therefore, the Fe-WO3 NF-anchored NG hybrid developed an exclusive porous architecture. Moreover, the sharp contrast between the dark edges and the faded area of these petals, which clearly confirms that the porous architectures are generated by Fe-WO3 NFs. These results are also supported by SEM observations (Figure 2). The morphology of the pristine NG exhibit highly transparent, ultrathin and wrinkled structure is analyzed by TEM image, as represented in Figure S6. As depicted in Figure S6, there were no impurities, demonstrating that our synthesis protocol delivered highly pure NG sheets. The high synergistic interaction of the nanosheet-based ordered, flowered structures allow high access of the electrolyte to the integrated nanosheets, which is beneficial to the decrease of the ion pathway among the



electrolyte and the electrode materials, which may enhance the long-term durability of electrocatalyst in fuel cell devices.31

Figure 3. (a) TEM image, (b) HR-TEM image of Fe-WO3 NF/NG hybrid (c) The SAED pattern of Fe-WO3 NF/NG hybrid, and (d) Dark-field STEM image of the Fe-WO3 NF/NG hybrid, and STEM-EDS mapping of (e) W, (f) Fe, (g) O, ( h) N, and ( i) C, respectively. In the present study, the hierarchical Fe-WO3 NF/NG hybrid is constructed through successful doping of Fe into the WO3 nanostructures. Thus, Fe doping is one of the imperative steps to tuning the structural morphology of WO3 crystals and assembling them in a perfect flower shape


Page 12 of 43

Page 13 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


rather than the pure WO3 nanostructures (Figure S7a), which is also previously explained by SEM study (Figure 2). The corresponding selected area electron diffraction (SAED) pattern (Figure 3c) of the hierarchical Fe-WO3 NF/NG hybrid implies the polycrystalline nature of FeWO3 NF that confirms the existence of strong coupling interaction among the Fe-WO3 NF and the NG sheets. In the HR-TEM image (Figure 3b) of Fe-WO3 NF/NG hybrid, the presence of lattice fringe with inter-planar d-spacing of ~0.38 nm, corresponds to the (110) plane for WO3 crystal structure, which confirmed the Fe atom doping inside the pure WO3 crystal by its larger d-spacing (Figure S7b). Furthermore, the comparative structural analysis of as-prepared Fe-WO3 NF/NG-1 and Fe-WO3 NF/NG-2 hybrids are investigated by TEM (Figure S8). The electrontransport kinetics and durability of fuel cells may be enhanced by the strong coupling interaction between the Fe-WO3 NF and the NG sheets. According to a previous report, mixed transition metals could provide donor–acceptor chemisorption sites, which validate the catalytic activity by electron bonding with different valences carrying ions.32 STEM-EDS mapping analysis was used to further examine the elemental composition and uniform distribution of these elements (Fe, W, O, N, and C) is confirmed in the hierarchical Fe-WO3 NF/NG hybrid (Figure 3d-i). The darkfield STEM images clearly reveal that the Fe atom was homogeneously incorporated into the flower-like WO3 nanostructures, and also strongly authorized by the NG support. The STEMEDS line mapping further exposed that Fe-WO3 NF/NG hybrid nanostructure consists of W, Fe, O, N and C elements, which demonstrates that flower-like Fe-WO3 nanostructures are successfully anchored into the NG sheets (Figure S9). For comparison, STEM-EDS line mapping of WO3 NF/NG hybrid also showed the existence of W, O, N, and C (Figure S10). To investigate the specific surface area (SSA) and pore size distribution of hierarchical FeWO3 NF/NG hybrid was examined through N2 sorption isotherms (Figure 4a). The Fe-WO3



NF/NG hybrid consists of type IV isotherms in the region of ~0.41 to ~1.0 (P/P0), revealing that the hybrid consists of plentiful mesoporous features. The calculated SSA of the Fe-WO3 NF/NG hybrid is ~143 m2.g−1, which is almost three times higher than that of WO3 NF/NG hybrid (SA~47 m2.g−1) (Figure S11) and earlier reported tungsten oxide-based material.28 Thus, the higher SSA of Fe-WO3 NF/NG hybrid demonstrated that the Fe-WO3 NFs are strongly anchored into the NG sheets, which keep away from the relative agglomeration and stacking of the NG sheets. Therefore, the effect of Fe doping into the WO3 nanostructure plays an imperative role in enhancing the SSA by forming ordered flower-like nanostructure. It is earlier well investigated and clearly explained by SEM and TEM analysis of the Fe-WO3 NF/NG hybrid (Figure 2 and 3). Barret-Joyner-Halenda (BJH) study is further conducted to examine the pore size distribution of the Fe-WO3 NF/NG hybrid (Figure 4b), which reveals that the pore size distribution of Fe-WO3 NF/NG hybrid is ~3–5 nm, which corresponds to its mesoporous structure. This clearly indicates that the Fe-WO3 NF/NG hybrid holds remarkable SSA with exclusive mesoporous nanostructure due to its hierarchical arrangement of Fe-WO3 NF supported on NG sheets, which could reduce the ion/electron diffusion path, and this will further boost the ORR activity of the Fe-WO3 NF/NG hybrid.18,33 The powder X-ray diffraction (XRD) patterns were carried out to investigate the crystalline properties of the NG, WO3 NF/NG, and Fe-WO3 NF/NG hybrids, as presented in Figure 4c. In the case of Fe-WO3 NF/NG hybrid, a series of diffraction peaks correspond to crystal planes of WO3, which reveals that the tungsten precursors are fully converted into WO3.29 In contrast, no additional peaks of iron have been observed, which indicates the effective integration of iron into the crystalline lattice of WO3.16


Page 14 of 43

Page 15 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) N2 sorption isotherms, (b) BJH pore size distribution of hierarchical Fe-WO3 NF/NG hybrid, (c) XRD pattern, and (d) Raman spectra of NG, WO3 NF/NG, and Fe-WO3 NF/NG hybrids. In the diffraction peaks of the Fe-WO3 NF/NG hybrid, all 2θ values were a little shifting toward higher angles relative to the WO3 NF/NG hybrid. Therefore, it can be concluded that smaller Fe ions are incorporated into larger W ions, and more vacancies are generated in the crystalline WO3 lattices (Figure 4c and Figure S12).34,35 In addition, (002) plane appeared at ~2θ = 26.3°, indicating the existence of a carbon plane on the NG sheets. This further confirms that



WO3 NF, as well as Fe-WO3 NF, is effectively anchored into NG sheets in the as-synthesized hybrid architectures.36 Raman spectroscopy is considered another imperative technique to examine the structural criteria of carbon-based materials. Raman spectra of the as-synthesized NG and WO3 NF/NG and Fe-WO3 NF/NG hybrids are displayed in Figure 4d. Two characteristic peaks are observed at ~1355 cm–1 (D band) and ~1583 cm–1 (G band) in the graphitic structure of NG, that are assigned to the out-of-plane vibrations attributed to the existence of structural defects, and the in-plane vibrations of sp2 bonded carbon atoms, respectively.18 Generally, the ID/IG ratio (intensity ratio) for D band to G bands is a structural disorder measurement of graphitic nanostructures. The ID/IG of the hierarchical Fe-WO3 NF/NG hybrid is ~1.16, significantly higher than that of WO3 NF/NG (ID/IG ~1.09) and pristine NG (ID/IG ~1.01). This is due to a more structural disorder of the graphene sheets due to effective nitrogen doping and besides simultaneous incorporation of the Fe atom into WO3 NF, which can also be beneficial for curing the structural imperfections of the NG sheets.37 Thus, Fe doped WO3 NF nanostructures (Fe-WO3 NF) movement along with defects sites are considered according to entropy, as the less energy of mobility at the defect sites is when compared to those through defect-free sites.37 Thus, adequate defect sites on the graphene sheets surface of Fe-WO3 NF/NG hybrid nanostructure not only improve the morphology of the metal nanostructures also improve the ORR performance. The lower G band shifting of the WO3 NF/NG and 3D Fe-WO3 NF/NG hybrids are observed compared to the pristine NG, representing the great coupling interaction among the NF and NG sheets.38 Subsequently, the fact that some peaks appear in the range of ~700–900 cm–1 is because of the existence of O–W–O bonds, which confirms the presence of a tungsten oxide species in the hybrid.21.39.40 In case of the 3D Fe-WO3 NF/NG hybrid, these peaks are shifted and appeared at lower wavenumber (smaller bond energy), due to


Page 16 of 43

Page 17 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the incorporation of iron (Fe) into the WO3 nanostructure. Thus, high synergistic interaction is developed among Fe and WO3 in the 3D Fe-WO3 NF/NG hybrids.41,42 This result is well supported by XRD analysis, which might influence the enhancement of the durability of electrocatalysts during the long-term cycling process. A profound powerful tool of X-ray photoelectron spectroscopy (XPS) was utilized to investigate the elemental and chemical composition of the as-synthesized NG, WO3 NF/NG, and 3D Fe-WO3 NF/NG hybrids, for detecting the structural properties of graphene-based materials (Figure 5). Figure 5a shows the XPS survey spectra of NG, WO3 NF/NG, and 3D Fe-WO3 NF/NG hybrid. The elemental and chemical composition of the 3D Fe-WO3 NF/NG hybrid nanostructure was strongly supported by the presence of W 4f, W 4d, W 4p, Fe 2p, C 1s, O 1s, and N 1s and the corresponding atomic wt% of the W, Fe, O, N, and C are 1.01%, 0.96%, 13.31%, 6.8%, and 77.90% (Table S1), respectively. The Fe 2p high-resolution XPS spectrum (Figure 5b) deconvoluted spin-orbital located at ~711.4 eV and ~725.2 eV, respectively corresponds to Fe 2p3/2 and Fe 2p1/2.



Figure 5. (a) XPS surveys of the NG, WO3 NF/NG, and Fe-WO3 NF/NG hybrids and the highresolution (b) Fe 2p spectrum, (c) W 4f spectrum, (d) O 1s spectrum, (e) N 1s spectrum, and (f) C 1s spectrum of the Fe-WO3 NF/NG hybrid. Meanwhile, the additional peaks are seen at ~717.9 eV and ~729.5 eV of the Fe 2p spectrum are due to the spin-orbit interaction between Fe2p3/2 and Fe2p1/2, which corresponds to the different chemical valence state of the Fe atom. This result is well-matched with those reported in previous literature.18 In the W 4f high-resolution spectrum (Figure 5c) of the 3D Fe-WO3 NF/NG hybrid, deconvoluted two spin-orbital peaks at ~35 eV and ~37.2 eV, respectively, corresponding to W4f7/2 and W4f5/2, which is due to the predominant oxidation state of W6+. An effective spin-orbit splitting between the W4f7/2 and W4f5/2 spectrum of the 3D Fe-WO3 NF/NG hybrid was assigned for the presence of variable valence states of W (Figure 5c and Figure S14), which is attributed to the O vacancies in the crystalline lattice.8,43,44 The remarkable synergistic interactions of W and Fe in the 3D Fe-WO3 NF/NG hybrid delivering in a negative shift (~0.20 eV) towards lower binding energy rather than in the WO3 NF/NG hybrid (Figure S15). The emission of photoelectrons from the low oxidized W ions (sub-stoichiometric WO3-x) caused the negative shift of binding energies, which is well fitted with the previous literature.45 Thus, the surface electronic state might be changed by the possible intermolecular electron interaction between W and Fe in the 3D Fe-WO3 NF/NG hybrid nanostructures, which could be beneficial to accelerate the ORR catalytic activity. The characteristic peaks (Figure 5d) not only designated the lattice O2– peak (~530–530.8 eV) in the metallic crystalline lattice of the 3D Fe-WO3 NF/NG hybrid but the extra peaks at the higher binding energy consistent to the chemically adsorbed OH–/oxygen species.29,43,44 The deconvolution of high-resolution N1s XPS spectrum of the hybrid exhibited four different peaks at ~403.7 eV, ~401.3 eV, ~399.9 eV, and ~398.5 eV,


Page 18 of 43

Page 19 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


respectively (Figure 5e), which confirm the presence of pyridine N-oxide, graphitic N, pyrrolic N, and pyridinic N, respectively.19 These pyridinic, graphitic, pyrrolic, and pyridine N-oxide type nitrogen sites in an NG matrix acts as effective active sites in the hybrid nanostructure and strongly influenced the catalytic activity for ORR. The deconvolution of C1s XPS spectrum (Figure 5f) displays five different peaks at ~288.7 eV, ~287.4, ~286.3, ~285.4, and ~284.5, respectively corresponding to the COO, C=O, C–O, C=N, and C=C bonds.18 This is a direct indication of the N doping in the graphene nanosheets, which is again supported by the presence of C–N bonds in the C1s spectrum (Figure 5f). Thus, high N doping is around ~6.8 wt% in the Fe-WO3 NF/NG hybrid, which may strongly enhance the ORR activity and long-term durability for fuel cells. 2.2 ORR Activity of the 3D Fe-WO3 NF/NG Hybrid. To investigate the electrocatalytic performance and long-term durability of the as-synthesized different hybrid ORR catalysts have been performed through RDE (rotating disk electrode) method.



Figure 6. ORR activity of the Fe-WO3 NF/NG hybrid: (a) CV curves of 3D Fe-WO3 NF/NG hybrid in the N2 and O2 saturated 0.1 M KOH electrolyte, (b) LSV curves of Fe-WO3 NF/NG hybrid, (c) LSV curves of NG, WO3 NF/NG, Fe-WO3 NF/NG–2, Fe-WO3 NF/NG–1, and FeWO3 NF/NG hybrids in O2 saturated 0.1 M KOH (Rotation speed: 1600 rpm), and (d) LSV curves of Fe-WO3 NF/NG hybrid and Pt/C at a rotation speed of 1600 rpm in O2 saturated 0.1 M KOH solution. The cyclic voltammograms (CV) of the 3D Fe-WO3 NF/NG catalyst have been carried out in N2 and O2 saturated aqueous 0.1 M KOH electrolyte at a constant sweep rate of 50 mV s−1 in ambient condition, as presented in Figure 6a. The CV curves exhibit a clear O2 reduction peak (Eredox) at ~0.83 V that is significantly higher than that in other ORR catalysts of NG, WO3 NF/NG, Fe-WO3 NF/NG-1, and Fe-WO3 NF/NG-2 hybrids (Figure S16). This enhanced ORR activity of porous carbon-based catalysts has already been reported; due to the reduction of O2 on


Page 20 of 43

Page 21 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


their high surface area nanostructure.46 In contrast, no noticeable peak is seen for all ORR catalysts in N2 saturated 0.1 M KOH solution. The electrochemical performances of all the catalysts were strongly proved by their typical ORR activity with gradual shifting to higher redox potentials in the following orders: NG < WO3 NF/NG < Fe-WO3 NF/NG–2 < Fe-WO3 NF/NG–1 < Fe-WO3 NF/NG. This result demonstrates that the enhanced electrocatalytic performance of the Fe-WO3 NF/NG catalyst is because of the fact that the distinct hierarchical flower-like FeWO3 nanostructures are anchored into the NG sheets and the enhanced surface area could greatly affect its improved catalytic activity, according to BET result (Figure 4a). Meanwhile, there is a high synergistic interaction between Fe-doped WO3 crystals and the NG support, which could be beneficial for enhancing the durability of fuel cells.18,38 Additionally, incorporation of N-atoms into the graphene sheets, which could also influence strongly the favourable catalytic efficiency. The earlier reported literature and quantum mechanical calculations of the carbon-based material gives the evidence of improved ORR activity,47 which essentially originates from the different active sites of N atoms (pyridinic, graphitic, and pyrrolic) and the coupling effect of Fe-WO3 and NG in the hybrid catalyst,19,48,49,50 which effectively increase electron transport kinetics of active Fe-WO3 NF towards ORR,51 that is consistently well supported due to low activity of RGO and its hybrid catalyst with Fe-WO3 (Figure S17). Furthermore, the support less Fe-WO3 NF also demonstrated its reduced activity (Figure S17), which further confirmed the presence NG support leads to improvement of electrocatalytic activity by providing highly conductive charge transfer pathway through good dispersion of metal nanostructures and hinders the coarsening possibility. 48 To investigate the as-synthesized catalyst’s kinetic behaviour for ORR reaction in fuel cell applications, generally, 4 electron reduction pathways is more preferable for the ORR catalysts



compared to 2 electron reduction pathways. Regarding this, the reaction kinetics of the assynthesized Fe-WO3 NF/NG hybrid was measured by ORR polarization curves at a various rotation (from 400 rpm to 2400 rpm) of RDE (Figure 6b). When the rotational speeds were increased, the LSV curves of Fe-WO3 NF/NG hybrid and other catalysts exhibited dramatic increases in current density (Figure S18 and S19), demonstrating enhanced O2 absorption and activation on the surface of the catalysts, which was caused by the reduced distance between the electrolyte and electrode. To know the potential capabilities of the electrocatalysts towards ORR, a half-wave potential (E1/2) are a suitable indicator which relates the active sites and surface reaction efficiencies of the hybrid catalysts. Therefore, ORR activities have been further examined by LSV analysis of the catalysts in the O2-saturated 0.1 M KOH electrolyte (Figure 6c). This study clearly demonstrates that the 3D Fe-WO3 NF/NG hybrid catalyst provided higher half-wave potential (E1/2 ~0.85 V) with enhanced value of diffusion controlled kinetic and limiting current, which is superior to the performance of the WO3 NF/NG electrocatalyst (E1/2~0.80 V), confirming its improved ORR activity due to the effective incorporation of Fe in the WO3 nanostructure16,52 and suitable flowerlike morphology might be the possible reasons.53 More interestingly, the comparison of the Fe-WO3 NF /NG hybrid electrocatalyst with Pt/C catalyst, possesses a higher positive shift (~10 mV) of E1/2 than that of commercial Pt/C (E1/2 ~0.84 V) catalyst (Figure 6d), expresses its inexpensive and significantly higher catalytic activity compared to Pt/C. The probable electron transfer mechanism of the highly porous Fe-doped WO3 catalyst stabilized in NG toward enhanced ORR activity (Figure S20), regulating the internal structural properties, which is well matched with previous reports.54,55,56 The electron transfer between Fe-WO3 NF and NG matrices in the 3D Fe-WO3 NF/NG hybrid nanostructure, may diminish the NG surface work function and further could alter the electronic structural properties


Page 22 of 43

Page 23 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of the nanoarchitecture hybrid. Thus, the interaction between electrolyte and electrode may be increased owing to the being of a large number of electrocatalytic active centers in the hierarchical hybrid. Accordingly, improvement of the catalytic activity of Fe-WO3 NF/NG hybrid is more superior rather than that of the conventional Pt counter electrode.57,58 The numbers of transferred electrons (n) for ORR reaction are calculated by the KouteckyLevich (K–L) plots, which are estimated from the plot of the inverse of the square root of the rotation speeds (ω–1/2) versus the inverse of the current density (ij–1). The K–L plots show a linear parallelism in the first-order ORR reaction kinetics of the Fe-WO3 NF/NG, WO3 NF/NG, hybrids and pristine NG catalysts (Figure 7a and Figure S21). The calculated overall transferred electron numbers (‘n’) for the Fe-WO3 NF/NG hybrid catalyst was found to be ~3.97, which is significantly closer to 4e– pathway than the WO3 NF/NG hybrid (n ~3.75) and NG (n ~3.50) respectively (Figure S22). Accordingly, the preferred ORR 4e− pathway is found in the present electrocatalyst of Fe-WO3 NF/NG hybrid through the ORR reaction, because of the further enhancement of oxygen vacancies in the WO3 crystal lattice by Fe doping along with NG support.8,52,59,60 These observed results were further confirmed by rotating ring-disk electrode (RRDE) test, which conveys the measurement of ORR current density and also the yield of intermediate H2O2 product on the surface of the disk electrode.50,61 The evaluation of transferred electrons number (n) and the % of hydrogen peroxide anion (HO2−) yield of the Fe-WO3 NF/NG hybrid surface during the process of ORR were shown in Figure S23. The calculated ‘n’ value was ranging from ~3.80-3.90 besides with the 7~11% yield of HO2−, which again confirm its four-electron pathway of oxygen reduction ability rather than those of other hybrid materials (Figure S22). Furthermore, EIS spectra of the Fe-WO3 NF/NG hybrid with a smaller Rct value (~54.1 Ω) supports its high electrocatalytic activity is achieved by lowering its electrical



resistance, compared to the WO3 NF/NG hybrid (~61.5Ω) and NG (~68.4Ω) catalysts, respectively (Figure 7b).

Figure 7. (a) Curve of J−1 versus ω−1/2 according to the K–L equation at various electrode potentials of Fe-WO3 NF/NG hybrid, (b) The EIS of the as-prepared NG, WO3 NF/NG, and FeWO3 NF/NG hybrid catalysts (inset: the EIS at higher magnification, and a well fitted kinetics model consistent with the EIS), (c) Tefel plots of as-prepared NG, WO3 NF/NG, and Fe-WO3 NF/NG hybrid catalysts, (d) Methanol tolerance test, (e) durability test for the Fe-WO3 NF/NG hybrid, and (f) cycling stability of Fe-WO3 NF/NG hybrid at the first cycle and 2000th cycles in O2 saturated 0.1 M KOH (inset shows the corresponding CV curves). Moreover, the lowest Tafel slope (~32 mV) of the Fe-WO3 NF/NG hybrid catalyst (Figure 7c) again supports its enhanced catalytic performances by providing enriched ORR active sites through a small transport path between electrode and electrolyte ions, and fast transfer of electrons, leading to a considerable improvement of ORR performance than that of the other catalysts, which well consistent with the CV and LSV studies, appreciates its cost-effective and


Page 24 of 43

Page 25 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


high efficiency compared to Pt/C catalyst. The above studies conclude the ORR activity of the Fe-WO3 NF/NG hybrid electrocatalyst is quite greater than that of the earlier reported electrocatalysts (Table S2). The methanol (MeOH) tolerance of the Fe-WO3 NF/NG hybrid catalyst towards ORR activity was investigated in order to explore the practical applicability of an active electrocatalyst in the fuel cells laterally with Pt/C. The methanol crossover can poison the ORR catalysts through equilibrium loss in the electrode materials, and oxidation takes place at the cathodic part, which strongly deactivates the ORR activities in a direct methanol fuel cell (DMFC).62 Therefore, it would be highly desirable as a selective cathode catalyst that should be inert for methanol sensitivity during ORR activity. The chronoamperometric analysis is one of the best ways to investigate the methanol sensitivity of the active electro-catalysts, and this analysis is presented in Figure 7d. For commercial Pt/C, the significant current density decrease is found during the addition of methanol to the KOH electrolyte solution. This was caused by the poor selectivity and sensitivity of the commercial Pt/C in presence of MeOH because of a competitive reaction between ORR and methanol oxidation. Impressively, even after methanol injection under the similar experimental condition of the commercial Pt/C catalyst, the hierarchical Fe-WO3 NF/NG hybrid catalyst is almost stable, without showing any obvious changes in the current density. Consequently, it is accomplished that present 3D Fe-WO3 NF/NG hybrid catalyst holds extraordinary methanol tolerance and ultra-high sensitivity towards ORR. This superior catalytic activity of the Fe-WO3 NF/NG hybrid catalyst means it has the potential for use in MeOH based fuel cells industrial sectors. The durability test was further examined at the potential of ~0.66 V for the Fe-WO3/NG hybrid catalyst through chronoamperometric analysis. Figure 7e shows the durability comparison of the as-synthesized Fe-WO3 NF/NG hybrid nanoarchitecture and Pt/C. This is also a critical issue for high-performance ORR



catalysts for fuel cell applications. Interestingly, the as-synthesized Fe-WO3 NF/NG hybrid holds an outstanding durability of ~95%, even after 15,000 seconds of continuous ORR activities, where Pt/C catalyst shows lower durability (~86%) under the similar experimental condition. Further, the durability of Fe-WO3 NF/NG hybrid nanostructure is greater than that of earlier ORR catalyst reports.63,64 Examinations of the potential cycling of the Fe-WO3/NG hybrid nanostructure and the commercial Pt/C through LSV measurement have been carried out to check their long-term cycling applications for ORR activities (Figure 7f and Figure S24). Remarkably, the commercial Pt/C catalyst clearly displays a negative shift in E1/2 (half-wave potential) of ~50 mV after 2000 cycles (Figure S24), whereas the Fe-WO3 NF/NG electrocatalyst display just ~10 mV (E1/2) negative shift (Figure 7f) under the same experimental conditions, demonstrating that it is an outstanding durable and low-cost catalyst of Fe-WO3 NF/NG hybrid. The poor durability properties of the Pt/C catalyst are because of the agglomeration of Pt nanoparticles on an amorphous C support and its highly corrosive nature, which is well fitted with the findings of a prior report.65 The improved cycling stability of Fe-WO3 NF/NG electrocatalyst is attributed because of its strong synergistic interaction among W and Fe orbital states by their spin‐orbit coupling and the strong attachment of conducting NG surface with FeWO3 NF active materials, makes more stable and active towards ORR electrocatalytic performance. The morphology of the Fe-WO3 NF/NG material was further checked after the long-term ORR test, which showed its dense flower-like nanostructures and anchored on the surface of the NG sheets (Figure S25). The effective N-doping on the graphene sheets might be the possible reasons for protecting the flower-like Fe-WO3 nanostructure by NG shell, prevent from aggregation and the possibilities of dissolution during the ORR measurement.


Page 26 of 43

Page 27 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Therefore, the above all electrochemical data suggests the higher ORR activity Fe-WO3 NF/NG hybrid than the WO3 NF/NG hybrid and more acceptable with Pt/C catalyst. The improved catalytic efficiency of the Fe-WO3 NF/NG is ascribed to the ordered 3D flower-like Fe-WO3 morphology in the bimetallic alloy structures,53 which provide strong synergistic interaction that could alter the electronic structures of the hybrid rather than pure randomly distributed WO3 crystal. This phenomenon also proved from the structural analysis. The higher durability and enhanced ORR activity of Fe-WO3 NF/NG hybrid is reasonably related due to (i) effective lattice compression by incorporation of Fe into the WO3 crystal, that possibly weakens the binding energy of the oxygenated species.66 (ii) 3D Fe-WO3 architecture improves the accessibility of the electrolyte through different interfaces of each nanosheet arrays, which is one of main reasons for high ORR activity.67 (iii) Furthermore, highly conductive NG support plays a vital role to decorate the Fe-WO3 NF uniformly, by preventing the agglomeration of both support and active materials.49,68 (iv) porous NG network facilitate more capable charge transfer kinetics and also provides the active regions for efficient tri-phase (solid–liquid–gas) electrochemical reactions at the cathode.63,68 (v) Meanwhile, the nitrogen doping induce sufficient changes of both electronic and structural properties of graphene, that could be beneficial for accelerating the fast charge transfer between the Fe-WO3 NF and NG support during ORR study.68,69,70 3. CONCLUSIONS In summary, a hierarchical Fe-WO3 NF-anchored NG has been successfully synthesized the first time and employed as an inexpensive, long-term durable, and highly efficient ORR electrocatalyst. The hierarchical Fe-WO3 NF/NG possesses remarkable specific surface area, excellent conductivity, and unique porous architectures. Fe doping into the WO3 crystal is a key factor to tuning the morphology with flower-like shapes of the Fe-WO3 nanostructure.



Meanwhile, N incorporation in the graphene network plays an energetic role in fabricating the Fe-WO3 NF/NG hybrid through establishing extensive required defect sites and regulating the growth of the Fe-WO3 nanostructure. The high and different type N moieties of NG network cause the flower-like Fe-WO3 nanostructure to be well anchored on the NG sheet. Thus, the 3D architecture of the Fe-WO3 NF/NG hybrid architecture leads to enhance in a numerous number of active sites by reducing the ion/electron diffusion path with a hierarchical nanostructure for superior ORR activity. Therefore, the hierarchical Fe-WO3 NF/NG exhibits ORR activity, MeOH tolerance, outstanding durability, and long-term cyclic stability, which would be more preferable than Pt/C catalyst and the recently reported mixed metal oxide-graphene based electrocatalysts. Therefore, the present investigation has been carried out into a synthesis protocol for the hierarchical 3D Fe-WO3 NF/NG catalyst to explore the possibility of expending to an industrial scale for commercial energy storage and conversion systems. 4. EXPERIMENTAL DETAILS 4.1 Materials Ferric (III) chloride hexahydrate (FeCl3. 6H2O > 99.9 %), sodium tungstate dehydrate (Na2WO4·2H2O ≥ 99.9%), nafion solution (~0.05 wt%), potassium permanganate (KMNO4 ≥ ~99%), hydrogen peroxide solution (H2O2 ~ 30%), hydrochloric acid (HCl, ~35%), graphite powder (

Hierarchical Flowerlike Highly Synergistic Three-Dimensional Iron

Recommend Documents