Porous Iron-Tungsten Carbide Electrocatalyst with High Activity and

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Porous Iron-Tungsten Carbide Electrocatalyst with High Activity and Stability toward Oxygen Reduction Reaction: From the Selfassisted Synthetic Mechanism to Its Active-Species Probing Li Song, Tao Wang, Yilin Wang, Hairong Xue, Xiaoli Fan, Hu Guo, Wei Xia, Hao Gong, and Jianping He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14754 • Publication Date (Web): 09 Jan 2017 Downloaded from http://pubs.acs.org on January 11, 2017

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ACS Applied Materials & Interfaces

Porous Iron-Tungsten Carbide Electrocatalyst with High Activity and Stability toward Oxygen Reduction Reaction: From the Self-assisted Synthetic Mechanism to Its Active-Species Probing Li Song, Tao Wang, Yilin Wang, Hairong Xue, Xiaoli Fan, Hu Guo, Wei Xia, Hao Gong and Jianping He* College of Materials Science and Technology, Jiangsu Key Laboratory of Materials and Technology for Energy Conversion, Nanjing University of Aeronautics and Astronautics, 210016 Nanjing, PR China. Abstract We synthesized a novel non-precious metal electrocatalyst by pyrolysis of a colloid mixture consisting of tungsten source and phenolic resin, with the simultaneous addition of ferric salt. The rationally designed electrocatalyst has a unique structure, with nano-sized WC and Fe3W3C uniformly dispersed in a three-dimensional porous carbon framework. WC, which was thought difficult to produce, is successfully prepared at a relatively low temperature of about 750 oC at inert atmosphere. XRD studies demonstrate the self-assisted effect of Fe, which accelerates the formation of WC, getting around the pathway of direct carbonaceous reduction of tungsten by carbon. The porous iron-tungsten carbide (Fe-W-C) nanocomposite as electrocatalyst shows excellent ORR activity with the onset and half-wave potentials of 0.864 and 0.727 V (vs. RHE), respectively, which are close to those of Pt/C (0.976 and 0.820 V vs. RHE). Electrochemical measurements show that Fe-W-C follows almost the effective four-electron-transfer pathway and would not be disturbed by methanol. The presence of a protective graphite shell outside the active carbide cores substantially improves the durability of the electrocatalyst. Both the removal of Fe species and the absence of W 1

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species would severely degrade the activity, while halide ions Cl- and sulfur-containing species SCNcan significantly suppress the ORR activity by the blocking of Fe species. These facts indicate that the ORR active-species of Fe-W-C should be relevant to both W and Fe species. Keywords: Carbide, mesoporous carbon, oxygen reduction reaction, high durability, methanol tolerant

INTRODUCTION Oxygen electrocatalysis is one of the most frequently researched topics in the fields of electrochemical energy conversion and storage devices.1 The oxygen reduction reaction (ORR) at cathode is a crucial process in fuel cells2 and metal-air batteries,3-5 which is also a major limiting factor of operating efficiency for these devices.6 Platinum-based materials, so far, are still the most effective and commercially available ORR catalysts to lower the overpotential, achieving a high-voltage output under the harsh conditions.7-10 Recently, numerous studies have been applied to enhance the efficiency of platinum-group metals utilization. For examples, platinum-based nano-cages with controllable facets and sub-nanometer-thick walls are fabricated to show prominent ORR catalytic activity.11 Transition metal-doped Pt3Ni octahedra are reported to show predominant ORR performance, with a specific activity and mass activity of 81 and 73 fold enhancements, respectively, compared with the commercial Pt/C catalyst.12 However, the exorbitant cost and low reserves of the noble metals make it not easy to bring about the commercialization and large-scale application of these devices without abandoning the opinion of precious metals.13 To avoid using noble metal catalyst, the research orientation must move to rational design and synthesis of earth-abundant non-precious metal catalysts (NPMCs) as the substitutes of platinum-based materials. A range of alternative NPMCs have been developed to facilitate the ORR 2


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in the past few decades, including heteroatom-doped (N, P or S) carbonaceous materials,14-17 non-precious metal oxides,18-22 nitrides,23-26 carbides,27-30 and transition-metal-N/C composites.31-33 Among those several catalysts, transition-metal carbide materials are particularly interesting as they exhibit high stability, excellent conductivity and prominent corrosion resistance.35, 36 Since tungsten carbide was discovered by Levy and Boudart to possess similar electronic structure and catalytic behavior to platinum group metals for some reactions, it has been regarded as a possible ORR catalyst material.37, 38 Shen et al. synthesized a composite with palladium/iron on tungsten carbide nano-crystalline as an ORR electrocatalyst, showing distinguished activity comparable to Pt/C.27 It was reported that Pt clusters can be highly dispersedly supported on the WC surface, which enhanced the catalyst activity significantly.38 Although the blueprint of the using WC as ORR catalyst is promising, many issues remain to be settled. Firstly, WC was discovered to obtain inferior intrinsic electrocatalytic performance for ORR in direct comparison to platinum.39 In most cases, WC acts just as electrocatalyst support or co-catalyst for precious metals Pt or Pd.27, 40 Secondly, WC is very difficult to prepare, which is synthesized either by carbothermic reduction at a high temperature above 1000 oC,38, 40 or at the assistance of reducing ambient.41 Herein, we first develop a facile self-catalyzed synthetic method at the assistance of Fe salt for the preparation of WC, with the resol as carbon source. As a result, we successfully synthesized a highly active and stable 3D porous Fe-W-C electrocatalyst at a relatively low temperature of 750 oC at inert atmosphere, which is ascribed to the catalytic effect of Fe salt for the formation of WC. WC and Fe3W3C are testified as the essential active species, which are uniformly incorporated in the open mesoporous carbon framework and well sheltered under the auspices of the outer graphitic carbon shell. 3



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RESULTS AND DISCUSSION Physicochemical characterization of Fe-W-C composite The in-situ soft-template synthesis strategy for Fe-W-C catalysts is demonstrated in Figure 1. In the precursor solution, Fe(NO3)3·9H2O first reacts with H4SiW12O40 to form the tungstate. The soluble phenol-formaldehyde oligomers can bridge the F127 template and the tungstate via hydrogen bond and electrostatic interactions during evaporation-induced self-assembly (EISA) process, resulting in the formation of rod micelles. During thermosetting and subsequent pyrolysis, the resol molecules provide carbon source for forming “rigid” phenolic resin and consist of a stable 3D carbon framework, thus ensuring the successful synthesis of porous Fe-W-C nanocomposite. Typical TEM (Figure 2a, b) and SEM images (Figure S1) for the Fe-W-C reveal the formation of ordered mesoporous carbon framework, in which the nanoparticles are uniformly embedded. The porous Fe-W-C composite presents typical stripe-like and hexagonally arranged images, viewed from the [110] and [001] directions, respectively. The 3D open porous system protects the active nanoparticles from agglomeration due to the confinement effect, promotes electrolyte infiltration and accelerates charge transfer, gas diffusion.42, 43 The TEM image with higher resolution (Figure 2c) discloses that the nanoparticles are well encapsulated by the thin graphite wall, which has the interlayer spacing of 0.363 nm. The porous feature and BET specific surface area (616.5 m2 g-1) of Fe-W-C can be studied through N2-adsorption and desorption techniques. The sharp adsorption at low relative pressure in the typical type-IV curves proves the existence of rich micropores (Figure 2d). A remarkable H1-type hysteresis loop indicates the typical cylindrical mesopores.44 The corresponding pore size distribution plots demonstrate that the Fe-W-C nanocomposite possesses narrow pore size distribution with a average radius of around 2.3 nm. The textural parameters (BET 4


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surface area, pore size, total pore volume) are summarized in Table S1. All the above textural properties are in favor of anchoring active species and facilitating the mass transfer steps. The small-angle XRD measurements were also performed to confirm the ordered mesoporous structure, which can be seen in Figure S2. The diffraction peak at 2θ = 1.0o can be indexed the (100) reflection, indicating a 2D hexagonal ordered mesoporous structure.45,46

Figure 1. Schematic representation of porous Fe-W-C nanocomposite.

The XRD pattern of Fe-W-C-750 (Figure 3a) depicts the presence of WC and Fe3W3C unambiguously, indicating that WC and Fe3W3C make up the main reaction products. There are three major intensive diffraction peaks exhibited with the 2θ values of 31.6°, 35.8°, and 48.5°, which are indexed to (001), (100), and (101) planes of hexagonal WC (JCPDS No. 51-0939) phase, respectively. The diffraction peaks with 2θ values of 64.3°, 73.6°, 75.5°, 77.3° and 84.2° can be assigned as the planes of WC (110), (111), (200), (102), and (201), respectively. The well-defined 5



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characteristic peaks for Fe3W3C are also in good agreement with the cubic structure of iron tungsten carbide (JCPDS No.41-1351), revealing that the strong interaction exists between Fe and W. The large broad peak at 2θ of around 23° is corresponded to (002) plane diffraction for the amorphous carbon. In addition, the peaks with 2θ values of around 44.6° and 64.7° are assigned to crystalline facets Fe (110) and Fe (200). In combination with the images of TEM, we can conclude that the hexagonal lattice of WC is a stable configuration in ultrasmall nanoparticles, which is in agreement with the calculations results of density functional theory (DFT) carried out by Zavodinsky.47

Figure 2. (a), (b), (c) TEM images and (d) N2 adsorption-desorption isotherms of Fe-W-C. The inset of (d) shows the corresponding pore size distribution.

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Figure 3. (a) XRD pattern of Fe-W-C and XPS spectra of the Fe-W-C nanocomposite: survey (b), W (c), Fe 2p (d) and C 1s (e).

Figure 4a~d is the HRTEM images of the nanostructure for the porous Fe-W-C nanocomposite. Figure 4a clearly displays the structure of crystalline WC surrounded by the amorphous carbon, which would inhibit the active particles from aggregation and erosion. Higher resolution of image in Figure 4b clearly presents an interplanar spacing of 0.284 nm, which corresponds to (001) planes of hexagonal WC. Figure 4c and d show both the lattice distance of 0.251 nm for (100) crystal planes of WC and the lattice spacing of 0.212 nm for Fe3W3C (511) planes. This result unveils that WC and 7



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Fe3W3C coexist in the porous carbon framework. Moreover, the STEM image and EDX spectrometry mapping (Figure 4e) confirm that the WC and Fe3W3C nanoparticles homogeneously cover the carbon framework.

Figure 4. (a)~(d) High-resolution transmission electron microscopy (HRTEM) images, with the corresponding Fourier transform patterns in the inset images and (e) EDX mapping of the of the porous Fe-W-C nanocomposite.

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Figure 5. XRD patterns of (a) Fe-W-C and (b) W-C without the addition of Fe at various heat-treatment temperatures.

In order to make sense of the synthetic mechanism of WC, we prepared a series of Fe-W-C nanocomposites at different calcination temperatures (Figure 5a). It can be seen that the major phase at 500 oC is ferberite FeWO4 (JCPDS no.46-1446), while its characteristic peaks decrease at 650 oC and almost disappear at the higher temperatures. Conversely, the characteristic peaks of Fe3W3C emerge at 650 oC, demonstrating the existence of reaction between FeWO4 and the carbon. With the heating temperature and holding time increasing, WC comes into being, while bits of metallic Fe are reduced along with the declining of Fe3W3C. At the temperature of 850 oC, WC transforms into the main crystal phase. Comprehensively considering the results of XRD, we argue that the synthesis mechanism of Fe-W-C can be described with the following equations: Fe3+ → Fe2+

(1)

WO42+ + Fe2+ →FeWO4

(2)

FeWO4 + C → Fe3W3C

(3)

Fe3W3C + C → WC + Fe

(4)

The Fe3+ in carbonaceous chemical condition is easy to be reduced to Fe2+ and then readily react with WO42+ to form FeWO4. It is well known that the addition of Fe and W would catalyze the 9



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graphitization of the amorphous carbon,48, 49 which can explain the formation of graphitic carbon wall outside the carbide cores. The active carbon can interact with FeWO4 and generate Fe3W3C. Finally, the Fe3W3C would further get carbonized to WC, leaving Fe behind. However, as for the method of carbonaceous thermal reduction, only tungsten oxides were formed at the temperature of up to 900 oC (Figure 5b). The fact that WC accompanied by metallic W appears at the high temperature of 1300 oC comes as no surprise, which is in accordance to the results of the relative researches.38, 40 We argue that the self-assistance of Fe salt in the multi-component system gets around the pathway of direct carbonaceous reduction of tungsten by carbon. The intermediate state tungstate FeWO4 acts as a role of assist-catalyst, which accelerates the preparation of WC at a relatively low temperature without any reducing gases. The surface chemical nature, including the local chemical environment and coordinate states of the metals in Fe-W-C nanocomposite was studied through XPS experiments. As exhibited in the survey scan spectra (Figure 3b), the weak peaks of W and Fe, compared with that of carbon and oxygen are ascribed to the fact that WC and Fe3W3C are encased by the carbon in the outer layer, also indicated by the TEM images. The enlarged curve of W species in Figure 3c is deconvoluted into five major components located at 31.7, 34.1, 37.6 and 35.5, 38.2 eV, corresponding to W 4f7/2, W 4f5/2, W 5p3/2 peaks of zero-valence W (carbide) and W 4f7/2, W 4f5/2 orbits of W6+ (oxide), respectively.30, 50, 51 It is indicated that oxidation state of W in Fe-W-C has been carbonized into WC, which is consistent with the XRD patterns. It's worth noting that there are still two peaks of W oxide, which can be attributed to the subsequent unavoidable oxidation at the surface of the obtained Fe-W-C in air atmosphere. Fe-W-C sample pyrolyzed at 500 oC presents mainly two evident peaks at 35.9 and 38.2 eV, with the binding energies assigned to W 4f orbits of W6+ in the form of FeWO4.52, 53 In the Fe 2p 10


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spectrum (Figure 3d) of Fe-W-C pyrolyzed at 750 oC, the two peaks at 707.7 (Fe 2p3/2) and 720.7 eV (Fe 2p1/2) can be assigned to zero-valence Fe (carbide). We found that the characteristic peaks of Fe 2p show a little positive shift compared with those of pure Fe3C without W species, reported in relative articles.51, 54, 55 This should be explained by the formation of Fe3W3C and attributed to the electron transfer from Fe to WC, which is similar to the results of related literature.56, 57 This strong interaction between Fe and WC will be helpful to improve the ORR performance of the electrocatalyst.58 Fe-W-C pyrolyzed at 500 oC exhibits only obvious binding energies for Fe 2p3/2 and 2p1/2 orbits. The peak at around 711 eV can be further deconvoluted into two components of Fe2+ (FeWO4) and bits of Fe3+ species, respectively.53 As shown in Figure 3e, an asymmetric C 1s spectrum can be fitted into three peaks positioned at 284.6, 285.3 and 288.6 eV, attributed to C=C, C–O, and O=C–O, respectively.59 The dominating peak at 284.6 eV describes the sp2 hybridized graphitic structure, which can confirm the graphitization of carbon. The formation of graphitic structure can improve the electroconductivity of the matrix, which would accelerate electron transfer throughout the carbon frameworks.60, 61 By combining the data of XPS spectra and XRD patterns, we can further confirm the self-assisted synthetic mechanism of Fe-W-C nanocomposite.

Electrochemical activity of porous Fe-W-C nanocomposite In order to comprehensively explore the ORR activity of Fe-W-C nanocomposite, we first performed CV measurements in N2- or O2- saturated electrolyte, separately. As a benchmark, Fe-W-C was compared with a commercial 20 wt% Pt/C under the same testing environment. As shown in Figure 6a, in the N2-saturated 0.1 M KOH solution, quasi-rectangular graphs without any voltammetric peaks can be found for both of the samples, demonstrating a typical double layer capacity current characteristic. We can observe a well-defined cathodic peak at 0.666 V for Fe-W-C 11



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in the electrolyte saturated with O2, illustrating the effective electrocatalytic reduction for O2 in alkaline medium. Although the ORR peak potential of Fe-W-C is around 0.17 V more negative than that of Pt/C (0.833V), the ORR peak current of Fe-W-C is 1.98 mA cm-2, which is higher than that of Pt/C (1.56 mA cm-2). The higher capacitance currents in CV suggest that Fe-W-C promotes O2 diffusion and facilitates electrolyte infiltration within the catalyst layer due to the large specific area and 3D porous system. Similar ORR performances were observed by RDE measurements. The collected polarization curves of Fe-W-C and Pt/C catalysts were demonstrated in Figure 6b. Fe-W-C exhibits an ORR onset potential (Eonset) and a half-wave potential (E1/2) of 0.864 and 0.727 V, which has the poorer ORR electrocatalytic performance, compared with Pt/C, whose Eonset and E1/2 are 0.976 and 0.821 V. Note that the ORR performance of Fe-W-C outperforms that of previously reported carbide materials without subsequent supporting of noble metals as shown in Table S2.30, 51, 62

Besides, the Tafel plots (the inset of Figure 6b) of Fe-W-C and Pt/C can be constructed from the

RDE datum at the rotation speed of 1600 rpm and be used to glean the mechanism of ORR. The linear part of the Tafel plots can be fitted to the Tafel equation (η=b log j + a, where j is the kinetic current density and b represents the Tafel slope), obtaining the corresponding Tafel slopes of about -92 mV dec-1 for Fe-W-C, which is similar to -74mV dec-1 for Pt/C, making it clear that the ORR mechanisms of Fe-W-C and Pt/C are analogous. Heat treatment temperature, which influences to a great extent the phase composition, dispersity of functional species and graphitized degree of carbon matrix, is a crucial factor for acquiring an excellent ORR activity.43, 63 Therefore, the polarization curves of Fe-W-C composites at various carbonization temperatures were exhibited in Figure S3, indicating that 750 oC is the optimal heat treatment temperature. LSV measurements were performed by RDE apparatus at a series of rotation speeds in 12


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O2-saturated electrolyte. As shown in Figure 6c and d, the current densities of Fe-W-C and Pt/C increase with increasing rotation rate from 400 to 2025 rpm, which is ascribed to the shortened diffusion distance, causing larger limiting current density at high speed.64 Koutecky–Levich (K–L) plots (Figure S4) were also performed to investigate the kinetic features. The good linearity and near parallelism of the fitting lines at various potentials indicate first-order reaction kinetics to the concentration of dissolved oxygen and the similar values of transferred electron number n over the potential range.65 Calculated from K−L equations, n value of Pt/C and Fe-W-C at 0.5 V can be determined to be about 4 and 3.6, respectively, demonstrating that the designed electrocatalyst almost favors the preferred 4-electron ORR pathway. In order to further investigate the electrochemical behavior of Fe-W-C, we performed rotating ring-disc electrode (RRDE) measurement (Figure 6e and f). The RRDE result also exhibits a decent transferred electron number n above 3.58 and a H2O2 yield below 20% for Fe-W-C at the scan range. The transferred electron number n derived from the RRDE test is in keeping with the K− L method. The methanol-tolerant ability of Fe-W-C was also investigated. Fe-W-C is tested for the possible methanol crossover effect by carrying out CVs upon addition of 1 M methanol into O2-saturated electrolyte. As seen in Figure 7a, at the existence of 1.0 M methanol, the ORR peak of Fe-W-C is scarcely changed, indicating little impact of methanol on ORR process of Fe-W-C. However, Pt/C shows significant methanol oxidation peaks without any cathode current because the electrochemical oxidation of methanol obstructs the ORR process. Moreover, the chronoamperometric measurements (Figure S5) of Fe-W-C and Pt/C catalysts show that Pt/C suffered serious ORR activity depression, with an obvious cathodic current fading caused by the methanol oxidation after injection of 3 M methanol, while Fe-W-C displayed slight current decay. Those results suggest that Fe-W-C shows 13



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high selectivity to ORR, with strong tolerance for methanol.

Figure 6. (a) CV curves of Fe-W-C and 20 wt% Pt/C on GC electrodes in O2-saturated (solid line) or N2-saturated (dash line) 0.1 M KOH solution; (b) RDE polarization curves at 1600 rpm with a sweep rate of 5 mV s-1 in O2-saturated 0.1 M KOH solution. (Inset: Tafel plots derived from the RDE polarization curves ); Rotating-disk voltammograms recorded for Fe-W-C (c) and Pt/C (d) in O2-saturated 0.1 M KOH with a sweep rate of 5 mV s-1 at various rotating rates; (e) RRDE voltammograms for Fe-W-C; (f) Hydrogen peroxide yield and corresponding electron transfer number during ORR of Fe-W-C.

The I–t plots for Fe-W-C and Pt/C were investigated at 0.2 V for 10 h in 0.1 mol L-1 KOH to evaluate the durability. As shown in Figure 7c, about 82.7% of relative current for Fe-W-C still persists after 10 h, implying a small degradation of surface active sites under the experimental 14


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conditions. However, Pt/C suffers from a rapid current decline with approximately 54.7% retention at 10 h. The main reason of evident inactivation behavior of Pt/C is thought to be the dissolution of Pt nanoparticles at high overpotentials.13 Cycling durability tests were also carried out in O2-saturated electrolytes to further verify the stability of Fe-W-C. The polarization curve (Figure 7d) of Fe-W-C recorded after 5000 CV cycles shows a little negative shift of 16 mV for E1/2, indicating the superior durability, compared with Pt/C (Figure 7e), which is in accordance with the I–t plots and confirms that the Fe-W-C electrocatalyst affords stable active sites and restrains the degradation of active species during ORR process. Figure 7f described the ORR process in the porous Fe-W-C system, in which the active sites are stably immobilized and well protected. The rigid 3D porous system and overcoating of carbon shells (Figure 1) would prevent the corrosion and oxidation of active sites during ORR process.66

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Figure 7. CVs on (a) Fe-W-C and (b) 20 wt.% Pt/C in O2-saturated 0.1 M KOH without and with 1.0 M CH3OH at the scan rate of 20 mV s-1; (c) I−t plots at 0.2 V in O2-saturated 0.1 M KOH solution for Fe-W-C and 20 wt.% Pt/C; Endurance tests of (d) Fe-W-C and (e) 20 wt.% Pt/C catalyst for 5 000 cycles in O2-saturated 0.1 M KOH. For LSV measurements, the sweep rate is 5 mV s−1 at 1600 rpm; (f) Schematic illustrating the ORR process in the 3 D porous system.

To be designed as an efficient electrocatalyst, the carbide-base materials should be fabricated to have high concentration of active sites without inactive species.67 Therefore it is significant to probe the active species of Fe-W-C for ORR. Ferruginous species are usually vulnerable to be dissolved in acid,13 so we employed the method of acid soaking to probe the roles of ferruginous species on ORR in Fe-W-C nanocomposite. As shown in Figure S6a, after acid soaking, Fe-W-C only consists of WC. The related W XPS spectrum of Fe-W-C after acid soaking is exhibited in Figure S6b, in which no

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noticeable changes are observed compared with Figure 3c, thus confirming the chemical stability of WC. Figure 8a compares the polarization curves of Fe-W-C before and after acid soaking. We can observe that the catalyst after acid soaking shows inferior activity with well-marked negative shift of the E1/2, compared with that of Fe-W-C before acid soaking, indicating that Fe species are necessary for Fe-W-C to keep a high ORR activity. The dissolution of Fe species in Fe-W-C after acid soaked resulted in the collapse of the integrated porous carbon framework with many vacancies in the carbon matrix (Figure S7). Moreover, we test the role of Fe species by adding halide or thiocyanate ions into the electrolyte. It was observed that trace amount of Cl- and SCN- ions can suppress the ORR activity of Fe-W-C. According to Figure 8a, the E1/2 values of Fe-W-C in 5 mM Cl- and SCNions show great negative shift of 52 and 43 mV, respectively. The current density at 0.70 V has decreased by around 1.6 times for Fe-W-C in 5 mM Cl . The inhibitory effect of Cl- and SCN- ions to −

ORR could be attributed to the blocking of active sites by the strong complexation between Fe species and those ions, which would increase the electrochemical polarization and result in higher overpotential during the ORR process. This is the typical feature of ferruginous ORR sites, which is similar to the results obtained by Jiang and Wang.55,68 All the above results corroborate the indispensable role of Fe element in the active sites. We prepared Fe-C (750 oC) and W-C (1300 oC) composite, which has no W and Fe source in the precursor solution, respectively, to further confirm the ORR role of W and Fe in Fe-W-C. As seen in Figure S8, the main product of Fe-C and W-C are Hagg carbide and tungsten carbide. There are also traces of metallic Fe in Fe-C and WO3 in W-C, respectively. The phase composition is similar to the result of Zhang’s article.69 We measure the ORR activity of Fe-C and W-C composites and found that the lack of any one of the two components W or Fe would cause poorer performance, compared with that of Fe-W-C (Figure 8b). We argue that 17



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active sites of Fe-W-C should contain both Fe and W species, which can form the synergistic catalytic effect to ORR.

Figure 8. (a) Effects of acid soaking, KCl and KSCN on ORR activity of Fe-W-C; (b) ORR activity comparison of Fe-W-C, W-C and Fe-C. All the polarization curves were recorded at 1600 rpm with a sweep rate of 5 mV s-1.

CONCLUSIONS In summary, we have prepared a novel and promising porous iron-tungsten carbide (Fe-W-C) electrocatalyst with high ORR performance, superior endurance and excellent methanol crossover immunity via a facile multi-component co-assemble method. Notably, the porous Fe-W-C nanocomposite provides a large surface area and abundant carbide active sites for ORR, which are uniformly incorporated in the open 3D porous system and protected from detachment and agglomeration by the “ rigid ” carbon framework. In the process of synthesizing Fe-W-C nanocomposite, the transition metal Fe plays a vital role of self-assisted catalysis in facilitating the formation of WC at relative temperature of about 750 oC. Based on the structural, compositional and electrochemical characterizations of the Fe-W-C nanocomposite, both WC and Fe3W3C are proved to be the main active phases, making up the synergistic catalytic effect to ORR.

EXPERIMENTAL SECTION Chemical materials Triblock copolymer Pluronic F127 was purchased from Sigma-Aldrich Corporation. Silicotungstic 18


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acid (H4SiW12O40), ferric nitrate (Fe(NO3)3·9H2O) and other chemicals were obtained from Nanjing Chemical Reagent Corporation. All the reagents were of analytical grade and used as received without any further purification. Preparation of porous Fe-W-C The phenol-formaldehyde resol (20 wt% in ethanol) was prepared with a base-catalyzed method according to the strategy of Zhao group.70 The porous Fe-W-C nanocomposite was synthesized through a multi-component co-assembly pathway. The detailed process is shown as follows: 1.0 g F127 was dissolved in 10 mL absolute ethanol to form a clear solution A. Meanwhile, 0.05 g H4SiW12O40 was dissolved in 2 mL absolute ethanol and 0.0848 g Fe(NO3)3·9H2O was dissolved in 3 mL absolute ethanol, assigned as solution B and C, respectively. Then 5.0 g resol precursor was added into A. Afterwards, B and C were dropped into the above mixture. The obtained mixture was stirred for 1 h and cast onto Petri dishes, followed with evaporation of solvent for 12 h in air atmosphere. The obtained sticky membrane was subjected to thermo-polymerization in an oven at 100 oC for 24 h. At last, the as-prepared membrane was scraped off and pyrolyzed in tube furnace under nitrogen gas protection. The membrane was first heated to 350 oC with a heating rate of 1 oC min-1 and kept for 3 h, decomposing F127 copolymer templates. Then the temperature was elevated to 750 oC and maintained for 3 h to carbonize the resol and to in-situ generate the carbide nanocomposite. A series of samples with the same metal-contents were synthesized and assigned as Fe-W-C-x, wherein, x refers to the final carbonization temperature. For convenience, unless otherwise stated, Fe-W-C represents the samples obtained at 750 oC. Physical Characterization The bulk phase composition of Fe-W-C nanocomposite was confirmed by Powder X-ray diffraction 19



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(XRD) patterns with a Bruker D8 advance diffractometer using Cu Kα radiation (λ= 0.154056 nm), operated at 40 kV and 40 mA. Small-angle X-ray diffractions (0.6-5°) measurements were conducted at a scan rate of 1° min-1. The porous morphology and microstructure of the Fe-W-C nanocomposite were investigated by transmission electron microscopy (TEM) measurements. TEM experiments were performed by a FEI Tecnai G2 system, operated at 200 kV. Microzone composition (STEM-EDS mapping) of the samples was analyzed through the energy dispersive X-ray spectrum (EDS) attached to the FEI Tecnai G2 system. Nitrogen adsorption-desorption isotherms were measured at 77 K using a Micromeritics ASAP 2020 analyzer. Before measurements, the samples were degassed in a vacuum at 200 °C for 6 h. The specific surface areas were calculated by utilizing the Brunauer-Emmett-Teller (BET) method based on the adsorption data. The pore size distributions derived

from

the

adsorption

branche

of

isotherms

were

estimated

based

on

the

Barrett-Joyner-Halenda (BJH) model and the total pore volumes (Vtotal) were estimated from the adsorbed amount at a relative pressure P/P0 of 0.993. X-ray photoelectron spectroscopy (XPS) was performed with a PHI-5000 Versaprobe surface analysis system (ULVAC-PH Japan) to study the surface composition of the Fe-W-C nanocomposite. Electrochemical measurements The ORR electrocatalytic activity, endurance and resistance to menthol of the Fe-W-C nanocomposite were evaluated by cyclic voltammetry (CV) measurements, rotating disk electrode (RDE) techniques for linear sweep voltammetry (LSV) and chronoamperometry measurements. All electrochemical measurements were carried out on a CHI660A electrochemical workstation in a conventional three-electrode system. A glassy carbon (GC) electrode (Pine, 5 mm), coated with the catalyst ink, was used as a working electrode. A saturated calomel electrode (SCE) and a Pt foil were 20


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used as the reference and counter electrodes, respectively. All the potentials are reported in terms of reversible hydrogen electrode (RHE) in the main body of the article, unless otherwise stated. The catalyst ink was prepared as follows: 5 mg Fe-W-C powder accompanied with a certain amount of Vulcan XC-72 was mixed with 1 mL ethanol, 50 µL of 5 wt% Nafion solution and then sonicated to obtain uniform inky slurry. A certain volume of catalyst slurry was dropped onto the GC electrode surface to get a catalysts loading of 625 µg cm-2 and dried at 50 oC to form a thin layer. For comparison, commercial 20 wt.% Vulcan XC-72-supported Pt was also measured with Pt loading of 25µg cm-2. The CVs were scanned between 0.2 and -0.6 V (vs. SCE) with a scan rate of 20 mV s-1 in 0.1 mol L-1 KOH aqueous electrolyte and at least 10 CV cycles were performed before collecting the data. The LSVs were recorded using the RDE technique at a scan rate of 5 mV s-1 from 0.2 to -0.9 V (vs. SCE) at various rotating speeds from 400 to 2025 rpm in O2-saturated 0.1 mol L-1 KOH aqueous electrolyte. For RRDE tests, the disk electrode (Pine, 5.6 mm) was scanned at a rate of 5 mV s-1 and the ring electrode potential was set to 1.4 V. Chronoamperometric responses (I−t plots) were performed in O2-saturated 0.1 mol L-1 KOH aqueous electrolyte at -0.8 V (vs. SCE). Cycling durability tests in O2-saturated electrolytes were carried out for 5000 cycles and the LSVs before and after 5000 CV cycles were recorded to test the durability of Fe-W-C catalyst. In addition, methanol was injected into the testing system to confirm methanol tolerance.

ASSOCIATED CONTENT Supporting Information Supporting figures of XRD patterns, XPS spectra and TEM images, supporting tables about structural parameters and comparison of electrocatalytic performance between this study and some other previous literatures. 21



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AUTHOR INFORMATION Corresponding Author *Tel: +86 25 52112906. Fax: +86 25 52112626. E-mail: [email protected].

Notes The authors declare no competing financial interest

ACKNOWLEDGMENTS This research was funded by National Natural Science Foundation of China (51372115, 11575084), Natural Science Foundation of Jiangsu Province (BK20130737) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Porous Iron-Tungsten Carbide Electrocatalyst with High Activity and

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