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Cite this: Chem. Commun., 2014, 50, 6349
Boron-doped carbon–iron nanocomposites as efficient oxygen reduction electrocatalysts derived from carbon dioxide†
Received 14th March 2014, Accepted 28th April 2014
Junshe Zhang, Ayeong Byeon and Jae W. Lee*
DOI: 10.1039/c4cc01903b www.rsc.org/chemcomm
Developing cost-effective oxygen reduction reaction (ORR) catalysts is pivotal for development of fuel cells. While Fe–N–C catalysts were proposed for ORR, Fe–B–C catalysts have not been explored. This work introduces the B-doped carbon catalysts encapsulating iron cores using CO2 as a carbon source. The Fe–B–C catalysts show enhanced ORR activity and durability due to the iron core within the graphitic layers.
The polymer electrolyte fuel cells (PEFCs) have been demonstrated to be one of the promising devices to convert chemical energy of fuels such as hydrogen directly to electricity with high energy efficiency, high power density, and low environmental impact.1 They have found wide applications in many areas, such as remote sensing, residential power generation, and automotive transportation.2 To realize the large-scale commercialization of PEFCs, the expensive platinum-based electrocatalysts for oxygen reduction reaction (ORR) at the cathode have to be replaced by other efficient, low-cost, and durable electrodes.3–6 Therefore, numerous efforts have been devoted to developing novel catalytic materials, including enzymatic electrocatalysts,7 non-precious metal catalysts,2,3,8–10 and heteroatom-doped carbon catalysts.4,11 For non-precious metal catalysts like iron (Fe), it was demonstrated that the high electrocatalytic performance comes from the metal cations coordinated by nitrogen atoms of aromatic rings which are incorporated into a graphitized carbon matrix.9 On the other hand, without any non-precious metal, both nitrogen(N)- and boron(B)-doping can enhance the electrocatalytic activity of carbon-based materials toward ORR, such as nanotubes.11 To overcome the problems (e.g., dissolution, sintering, and agglomeration) associated with non-precious metal or metal ¨llen and coworkers encapsulated Fe3O4 oxide electrocatalysts, Mu nanoparticles within nitrogen-doped graphene layers.12 Although several studies have focused on Fe–N–C composite electrocatalysts,2,10,12 there are no reports on Fe–B–C composite ones. Department of Chemical and Biomolecular Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon, 305-701, Korea. E-mail: [email protected]; Fax: +82-42-350-3910; Tel: +82-42-350-3940 † Electronic supplementary information (ESI) available: Detailed experimental procedures and analytic data. See DOI: 10.1039/c4cc01903b
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Here, we present the enhanced ORR activity of Fe-containing nanoparticles confined within B-doped graphitic carbon shells, which are formed from the reaction of sodium borohydride (NaBH4) with carbon dioxide (CO2) in the presence of iron hydr(oxides). The B-doped carbon–iron composites were synthesized through the reduction of CO2 by NaBH4 with the Fe precursor at 500 1C and atmospheric pressure. After leaching salts from the solid product, the resulting Fe–B–C composite is further annealed at 850 1C and 1050 1C in an argon atmosphere. The solid products synthesized at 500, 850, and 1050 1C were denoted as FeBC050, FeBC850, and FeBC105. The electrocatalytic performance of the FeBC composites and Pt (20 wt%)/carbon supported on the glass carbon toward ORR was evaluated by cyclic voltammetry (CV) in alkaline solution (1 M NaOH) saturated with 1 atm argon (Ar) or oxygen (O2). As shown in Fig. 1, at a potential range of 0 to 1.0 V (vs. Ag/AgCl), the CV curve of Pt/carbon in Ar-saturated solution exhibits the oxide formation/reduction peaks (between 0.5 and 0.1 V) and H2 adsorption/desorption peaks (between 0.88 and 0.55 V), which is consistent with the one for Pt electrode in alkaline solutions.13,14 The CV curve of FeBC050 in Ar-saturated solution is nearly featureless at a potential range of 0 to 1.2 V, while the CV curves of FeBC085 and FeBC105 reveal a pair of well-developed redox peaks between 1.2 and 0.4 V (Fig. 1). A remarkable observation is that these redox peaks become sharp as the annealing temperature increases from 850 to 1050 1C. For FeBC105, the DEp (peak potential difference) is about 360 mV. The large DEp suggests a slow electron transfer of the redox process involving surface species, possibly related to Fe3+/Fe2+. In O2-saturated solution, a well-defined cathodic peak centered at 0.14, 0.19, 0.21, and 0.31 V appears on CV curves for Pt/carbon, FeBC105, FeBC085, and FeBC050, respectively (Fig. 1 and Fig. S1, ESI†). In contrast, the peak potential for borondoped carbon nanotubes under the same conditions is more negative than 0.31 V.11 Furthermore, the onset potentials for Pt/carbon, FeBC105, FeBC085, and FeBC050 are 0.068, 0.14, 0.15, and 0.22 V, respectively (refer to Fig. S1, ESI†). Both the onset and peak potentials, along with maximum cathodic current density (1.02, 1.02, and 0.51 mA cm 1 for FeBC105, FeBC085, and FeBC050, respectively), demonstrate that thermal annealing
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Fig. 1 Cyclic voltammograms of Pt/carbon and FeBC composites in O2-saturated 1.0 M NaOH solution.
drastically enhances the electrocatalytic activity of the FeBC composites toward ORR. To further assess the ORR activity over the FeBC composites, we carried out the linear sweep voltammetric (LSV) measurements on a rotating disk electrode (RDE). FeBC105 exhibits a two-step process, with a second onset potential of about 0.55 V (Fig. S2a, ESI†). Compared with Pt/carbon, a well-defined limiting current cannot be identified on the LSV curves for FeBC105. This could be attributed to the heterogeneity of electrocatalytic sites and the depth of O2 penetration inside porous electrodes varying with the potential.15 The crystalline form of irons confined within the B-doped carbons can be identified by X-ray diffraction (XRD). The analysis demonstrates that maghemite (g-Fe2O3) is the main crystalline phase in FeBC050 (Fig. S3a, ESI†), which is produced from the thermal decomposition of iron hydr(oxides). After the heat treatment at 850 1C, maghemite is reduced to iron. As the annealing temperature increases to 1050 1C, other crystalline compounds such as carbides form (Fig. S3a, ESI†). Thus, the thermal treatment affects not only the properties of iron compounds, but also the nature of solid carbon. For FeBC050, a weak and broad diffraction peak centered at ca. 261 is observed in the XRD spectra (Fig. S3a, ESI†). However, for both FeBC085 and FeBC105, Bragg peaks related to the metastable crystalline carbon appear (Fig. S3a, ESI†). Also, Raman spectra exhibit the D band that shifts from 1370 to 1350 cm 1 after annealing at 850 and 1050 1C (Fig. S3b, ESI†). Another interesting finding is that the D band becomes sharp, together with an increased intensity (peak heights) with respect to the G band (ca. 1580 cm 1) as the annealing temperature increases. This is because incorporating heteroatoms into the carbon network breaks its hexagonal symmetry and thus induces an increased D band in the Raman spectrum. A further analysis of Raman spectra reveals that the 2D band becomes a pronounced peak in the spectra after the heat treatment (Fig. S3b, ESI†). This is related to the presence of metastable crystalline carbon as in the XRD spectra of FeBC 850 and FeBC105 (Fig. S3a, ESI†).
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The chemical composition and the bonding state of elements of a solid surface can be obtained using X-ray photoelectron spectroscopy (XPS). For the untreated composite (FeBC050), the B 1s spectrum exhibits two envelopes, one between 184 and 190 eV and the other ranging from 190 to 195 eV (Fig. 2a). The former envelope is fitted by two peaks centered at 187.1 and 188.4 eV, corresponding to B atoms connected to B atoms (B clusters) and B atoms bound to C carbons in B4C species,18 respectively. In contrast, the latter one is only fitted by one peak centered at 191.6 eV, corresponding to BCO2 species. The C 1s spectral envelope between 280 and 292 eV can be deconvoluted into three peaks, located at 283.7, 284.5, and 287.6 eV (Fig. 2b), which are assigned to sp2-hydridized carbon, defective carbon, and 4CQO or C–O, respectively.19,20 The O 1s spectral envelope between 526 and 539 eV consists of three peaks centered at 529.9, 531.6, and 533.5 eV (Fig. S4a, ESI†), which are attributed to Fe–O, CQO, and C–O species, respectively.19–21 Neither the C 1s spectrum nor the O 1s spectrum exhibits the characteristic signal of BCO2 species, possibly due to its low surface concentration. The Fe 2p spectrum has two envelopes, one for Fe 2p3/2 (704 to 715.5 eV) and the other for Fe 2p1/2 (721 to 730.5 eV), and a satellite peak for Fe 2p3/2 at 718.9 eV (Fig. S4b, ESI†). In addition, there seems to be another peak at 732.6 eV; this may be a satellite peak for 2p1/2. Fe 2p3/2 has degeneracy of four states but Fe 2p1/2 has only two, thus the former envelope is fitted by multiplets (709.1, 710.1, 711.3, and 712.7 eV) and a high binding energy (BE) surface peak at 714 eV,22 whilst the latter one is deconvoluted into two peaks (724.1 and 726.3 eV).
Fig. 2 XPS B 1s (a) and C 1s (b) spectra of untreated and annealed FeBC composites.
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After thermally annealing the composite at 1050 1C (FeBC105), the most remarkable observation is that the B 1s spectrum has only one high BE envelope (186 to 196 eV) that can be deconvoluted into two peaks centered at 190.3 and 192.1 eV (Fig. 2a); these two peaks are assigned to BCxOy species. Also, the high BE envelope only appears for the composite annealed at 850 1C (Fig. S5, ESI†). The C 1s spectral envelope between 280 and 292 eV consists of three peaks, located at 283.7, 284.5, and 288.7 eV (Fig. 2a); they are ascribed to sp2-hydridized carbon, defective carbon, and –COO, respectively. A notable finding is that the relative concentration of defective carbon decreases after annealing at 1050 1C, consistent with the appearance of metastable crystalline carbon in the XRD spectra (Fig. S3a, ESI†). The O 1s spectral envelope (526 to 539 eV) is fitted by three peaks centered at 530.3, 532.1, and 534.8 eV (Fig. S4a, ESI†), assigned to Fe–O, CQO, and C–O species, respectively. The relative concentration of Fe–O species decreases after the thermal annealing, due to the reduction of iron oxides as shown in the XRD spectra (Fig. S3a, ESI†). Two new peaks (706.4 and 708.0 eV) appear on the low BE side of the Fe 2p3/2 envelope (Fig. S4b, ESI†); the former is assigned to metallic iron and latter is possibly related to iron carbides for FeBC105. Thus, all of XPS and XRD observations indicate that B4C species vanish, together with a decrease of defects in the carbon structure and the reduction of iron oxide to iron after the composite is subjected to the thermal annealing. Moreover, its effect on the microstructure of the composite is revealed by scanning electron microscopy (SEM) and highresolution transmission electron microscopy (HRTEM). It appears that the thermal annealing increases the porosity of boron-doped carbon–iron composites (SEM images in Fig. S6, ESI†); this plausibly comes from the reduction of iron oxide which is surrounded by carbon layers. During the formation of composites, sodium borohydride melts first and then it reacts with CO2 to produce carbon which preferably deposits on the solid iron precursor, resulting in a core–shell structure. This specific structure is attested by HRTEM images in Fig. 3 (for additional images, refer to Fig. S7, ESI†). It shows that iron-containing nanoparticles are encapsulated into well-defined onion-like graphitic carbon shells with an interplanar distance of ca. 0.34 nm, corresponding to the d-spacing of the (002) basal planes in graphite. The thickness of the shell is between 5 and 15 nm, while the size of the encapsulated particles varies from tens to hundreds of nanometers. For the untreated composite (FeBC050), the interplanar distance of cores is
Fig. 3 Representative TEM images of untreated composite (a) and annealed composite (b).
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about 0.24 nm, close to the d-spacing of the (311) planes in g-Fe2O3.23 For the thermally annealed composite (FeBC105), the interplanar distance of cores is around 0.19 nm, approximating to the d-spacing of the (110) planes in a-Fe.23 Upon combining XRD analyses, XPS studies, and SEM and HRTEM observations, the thermal annealing changes the surface state and more active sites are generated by both the reduction of g-Fe2O3 and the decomposition of B4C species. At a potential range of 0.2 to 0.4 V, the average electron transfer number per oxygen molecule is 1.96, but it increases monotonically from 2.4 to 4.7 as the potential changes from 0.5 to 0.8 V (Fig. 4a). Thus, we believe that oxygen reduction proceeds by a two-electron pathway at less negative potentials and a four-electron ORR reaction commences at a potential more negative than 0.7 V. This behaviour is consistent with ORR polyelectrolyte-functionalized graphene but different from borondoped carbon nanotubes and graphene.11,16,17 It should be noted here that other surface reduction reactions (Fig. 1) become important at 0.8 V, which gives rise to an electron transfer number as high as 4.7. At potentials between 0.4 and 0.6 V, the intercept of the slope of I 1 vs. N 1/2 plots weakly depends on the potential (Fig. 4b). Plausibly, this is because the oxygen reduction mechanism is dependent on the potential, as revealed by the electron transfer number increasing with negatively shifted potentials. Besides the high electrocatalytic activity toward ORR, FeBC105 exhibits an excellent tolerance of methanol crossover, another main challenge faced by metal-based cathode catalysts in fuel cells. The chronoamperometric responses at 0.5 V and a rotating rate of 2500 rpm to methanol introduction into O2-saturated 1 M NaOH solution for FeBC105 and Pt/carbon are presented in Fig. 4c. In the absence of methanol, the current maintains around 0.17 and 0.29 mA for FeBC105 and Pt/carbon, respectively. After introducing
Fig. 4 (a) The average electron transfer number per oxygen molecule and the kinetic limiting current at different potentials for the annealed composite (FeBC105) at different potentials; (b) Koutecky–Levich plots (I 1 vs. N 1/2) for the annealed composite (FeBC105) at different potentials; (c) chronoamperometric response at 0.5 V and 2500 rpm in O2-saturated 1 M NaOH solution (80 cm3) on Pt/carbon and annealed composite (FeBC105) electrodes before and after introducing 1 cm3 methanol; (d) CV curves of acid-leached FeBC105 subjected to 10 000 cycles measured at 100 mV s 1.
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1 cm3 methanol into 80 cm3 electrolyte solution, the current for FeBC105 remains unchanged at around 0.17 mA. In contrast, the current for Pt/carbon immediately decreases to 0.37 mA, and then it instantaneously increases back to 0.03 mA, followed by maintaining around this value for the rest of the run. The FeBC105 was leached by 2 M H2SO4 at 80 1C for 3 h to identify the acid durability of residual Fe particles in oxygen reduction reaction, showing a slight decrease of ORR peak potential from 0.19 V to 0.20 V at a scan rate of 50 mV s 1 (Fig. S8, ESI†). In addition, the acid-leached FeBC105 showed high long-term stability for 10 000 cycles with an insignificant peak potential change from 0.223 V to 0.225 V at a scan rate of 100 mV s 1 (Fig. 4d). The preceding results demonstrate that the annealed B-doped carbon–iron composites can act as electrocatalysts for ORR with excellent activity, high tolerance to the crossover effect, relatively small generation of peroxide (Fig. S10), and good long-term stability. In summary, we have demonstrated a facile and reproducible approach to synthesize boron-doped carbon–iron composites from CO2 at 500 1C and 1 atm. The resulting composite has a core@shell structure, in which the iron-containing nanoparticles are confined within onion-like graphitic carbon shells. After thermally annealing the composite at temperatures higher than 850 1C in argon, the electrocatalytic activity toward ORR in alkaline solutions drastically enhances, and it is much higher than that of boron-doped carbon nanotubes. Along with the high activity, the thermally treated composite also exhibits excellent tolerance of methanol crossover and long-term stability. The high performance is attributed to the change in the surface state, which stems from the decomposition of B4C species and reduction of iron oxide by carbon. The changed surface state facilitates ORR by acting as an electron transfer mediator, an adsorption site for intermediate species, or both, resulting in favorable reduction kinetics. Currently, efforts are being made to tune the microstructure of graphitic carbon shells, which will increase the exposed area of iron-containing cores to oxygen and subsequently a direct 4-electron pathway could become favorable.
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The authors are grateful for the financial support from the Korea CCS R & D Center funded by the Ministry of Science, ICT, and Future Planning (no. NRF-2013M1A8A1040703).
Notes and references 1 R. Borup, J. Meyers, B. Pivovar, Y. S. Kim, R. Mukundan, N. Garland, D. Myers, M. Wilson, F. Garzon, D. Wood, P. Zelenay, K. More, K. Stroh, T. Zawodzinski, J. Boncella, J. E. McGrath, M. Inaba, K. Miyatake, M. Hori, K. Ota, Z. Ogumi, S. Miyata, A. Nishikata, Z. Siroma, Y. Uchimoto, K. Yasuda, K. I. Kimijima and N. Iwashita, Chem. Rev., 2007, 107, 3904. 2 G. Wu, K. L. More, C. M. Johnston and P. Zelenay, Science, 2011, 332, 443. 3 B. Bashyam and P. Zelenay, Nature, 2006, 443, 63. 4 K. Gong, F. Du, Z. Xia, M. Durstock and L. Dai, Science, 2009, 323, 760. ¨llen, Angew. Chem., Int. Ed., 2010, 5 R. Liu, D. Wu, X. Feng and K. Mu 49, 2565. ¨llen, Angew. Chem., Int. Ed., 6 S. Yang, X. Feng, X. Wang and K. Mu 2011, 50, 5339. ´au, Y. Yang, Y. Yan, W. Ebina, 7 J. P. Collman, N. K. Devaraj, R. A. Decre T. A. Eberspacher and C. E. D. Chidsey, Science, 2007, 315, 1565. `vre, E. Proietti, F. Jaouen and J. Dodelet, Science, 2009, 8 M. Lefe 324, 71. 9 D. S. Su and G. Sun, Angew. Chem., Int. Ed., 2011, 50, 11570. 10 C. Zhang, R. Huo, H. Yin, F. Liu and Y. Hou, Nanoscale, 2012, 4, 7362. 11 L. Yang, S. Jiang, Y. Zhao, L. Zhu, S. Chen, X. Wang, Q. Wu, J. Ma, Y. Ma and Z. Hu, Angew. Chem., Int. Ed., 2011, 50, 7132. ¨llen, J. Am. 12 Z. S. Wu, S. Yang, Y. Sun, K. Parvez, X. Feng and K. Mu Chem. Soc., 2012, 134, 9082. 13 C. Zhang, F. F. Fan and A. J. Bard, J. Am. Chem. Soc., 2009, 131, 177. 14 W. Jin, H. Du, S. Zheng, H. Xu and Y. Zhang, J. Phys. Chem. B, 2010, 114, 6542. ´, S. Gupta and R. F. Savinell, J. Electroanal. Chem., 15 S. L. Gojkovic 1999, 462, 63. 16 S. Wang, D. Yu, L. Dai, D. W. Chang and J. B. Baek, ACS Nano, 2011, 8, 6202. 17 Z. Sheng, H. Gao, W. Bao, F. Wang and X. Xia, J. Mater. Chem., 2012, 22, 390. ´, M. Me ´ne ´trier, A. Tressaud and S. Flanrois, 18 T. Shirasaki, A. Derre Carbon, 2000, 38, 1461. 19 H. Estrade-Szwarchopf, Carbon, 2004, 42, 1713. 20 T. I. T. Okpalugo, P. Papakonstantinu, H. Murphy, J. McLaughlin and N. M. D. Brown, Carbon, 2005, 43, 153. 21 A. P. Grosvenor, B. A. Kobe, M. C. Biesinger and N. S. Mclntyre, Surf. Interface Anal., 2004, 36, 1564. 22 T. Yamashit and P. Hayes, Appl. Surf. Sci., 2008, 254, 2441. 23 E. D. Smit and B. M. Weckhuysen, Chem. Soc. Rev., 2008, 37, 2758.
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Cite this: Chem. Commun., 2014, 50, 6349
Boron-doped carbon–iron nanocomposites as efficient oxygen reduction electrocatalysts derived from carbon dioxide†
Received 14th March 2014, Accepted 28th April 2014
Junshe Zhang, Ayeong Byeon and Jae W. Lee*
DOI: 10.1039/c4cc01903b www.rsc.org/chemcomm
Developing cost-effective oxygen reduction reaction (ORR) catalysts is pivotal for development of fuel cells. While Fe–N–C catalysts were proposed for ORR, Fe–B–C catalysts have not been explored. This work introduces the B-doped carbon catalysts encapsulating iron cores using CO2 as a carbon source. The Fe–B–C catalysts show enhanced ORR activity and durability due to the iron core within the graphitic layers.
The polymer electrolyte fuel cells (PEFCs) have been demonstrated to be one of the promising devices to convert chemical energy of fuels such as hydrogen directly to electricity with high energy efficiency, high power density, and low environmental impact.1 They have found wide applications in many areas, such as remote sensing, residential power generation, and automotive transportation.2 To realize the large-scale commercialization of PEFCs, the expensive platinum-based electrocatalysts for oxygen reduction reaction (ORR) at the cathode have to be replaced by other efficient, low-cost, and durable electrodes.3–6 Therefore, numerous efforts have been devoted to developing novel catalytic materials, including enzymatic electrocatalysts,7 non-precious metal catalysts,2,3,8–10 and heteroatom-doped carbon catalysts.4,11 For non-precious metal catalysts like iron (Fe), it was demonstrated that the high electrocatalytic performance comes from the metal cations coordinated by nitrogen atoms of aromatic rings which are incorporated into a graphitized carbon matrix.9 On the other hand, without any non-precious metal, both nitrogen(N)- and boron(B)-doping can enhance the electrocatalytic activity of carbon-based materials toward ORR, such as nanotubes.11 To overcome the problems (e.g., dissolution, sintering, and agglomeration) associated with non-precious metal or metal ¨llen and coworkers encapsulated Fe3O4 oxide electrocatalysts, Mu nanoparticles within nitrogen-doped graphene layers.12 Although several studies have focused on Fe–N–C composite electrocatalysts,2,10,12 there are no reports on Fe–B–C composite ones. Department of Chemical and Biomolecular Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon, 305-701, Korea. E-mail: [email protected]; Fax: +82-42-350-3910; Tel: +82-42-350-3940 † Electronic supplementary information (ESI) available: Detailed experimental procedures and analytic data. See DOI: 10.1039/c4cc01903b
This journal is © The Royal Society of Chemistry 2014
Here, we present the enhanced ORR activity of Fe-containing nanoparticles confined within B-doped graphitic carbon shells, which are formed from the reaction of sodium borohydride (NaBH4) with carbon dioxide (CO2) in the presence of iron hydr(oxides). The B-doped carbon–iron composites were synthesized through the reduction of CO2 by NaBH4 with the Fe precursor at 500 1C and atmospheric pressure. After leaching salts from the solid product, the resulting Fe–B–C composite is further annealed at 850 1C and 1050 1C in an argon atmosphere. The solid products synthesized at 500, 850, and 1050 1C were denoted as FeBC050, FeBC850, and FeBC105. The electrocatalytic performance of the FeBC composites and Pt (20 wt%)/carbon supported on the glass carbon toward ORR was evaluated by cyclic voltammetry (CV) in alkaline solution (1 M NaOH) saturated with 1 atm argon (Ar) or oxygen (O2). As shown in Fig. 1, at a potential range of 0 to 1.0 V (vs. Ag/AgCl), the CV curve of Pt/carbon in Ar-saturated solution exhibits the oxide formation/reduction peaks (between 0.5 and 0.1 V) and H2 adsorption/desorption peaks (between 0.88 and 0.55 V), which is consistent with the one for Pt electrode in alkaline solutions.13,14 The CV curve of FeBC050 in Ar-saturated solution is nearly featureless at a potential range of 0 to 1.2 V, while the CV curves of FeBC085 and FeBC105 reveal a pair of well-developed redox peaks between 1.2 and 0.4 V (Fig. 1). A remarkable observation is that these redox peaks become sharp as the annealing temperature increases from 850 to 1050 1C. For FeBC105, the DEp (peak potential difference) is about 360 mV. The large DEp suggests a slow electron transfer of the redox process involving surface species, possibly related to Fe3+/Fe2+. In O2-saturated solution, a well-defined cathodic peak centered at 0.14, 0.19, 0.21, and 0.31 V appears on CV curves for Pt/carbon, FeBC105, FeBC085, and FeBC050, respectively (Fig. 1 and Fig. S1, ESI†). In contrast, the peak potential for borondoped carbon nanotubes under the same conditions is more negative than 0.31 V.11 Furthermore, the onset potentials for Pt/carbon, FeBC105, FeBC085, and FeBC050 are 0.068, 0.14, 0.15, and 0.22 V, respectively (refer to Fig. S1, ESI†). Both the onset and peak potentials, along with maximum cathodic current density (1.02, 1.02, and 0.51 mA cm 1 for FeBC105, FeBC085, and FeBC050, respectively), demonstrate that thermal annealing
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Fig. 1 Cyclic voltammograms of Pt/carbon and FeBC composites in O2-saturated 1.0 M NaOH solution.
drastically enhances the electrocatalytic activity of the FeBC composites toward ORR. To further assess the ORR activity over the FeBC composites, we carried out the linear sweep voltammetric (LSV) measurements on a rotating disk electrode (RDE). FeBC105 exhibits a two-step process, with a second onset potential of about 0.55 V (Fig. S2a, ESI†). Compared with Pt/carbon, a well-defined limiting current cannot be identified on the LSV curves for FeBC105. This could be attributed to the heterogeneity of electrocatalytic sites and the depth of O2 penetration inside porous electrodes varying with the potential.15 The crystalline form of irons confined within the B-doped carbons can be identified by X-ray diffraction (XRD). The analysis demonstrates that maghemite (g-Fe2O3) is the main crystalline phase in FeBC050 (Fig. S3a, ESI†), which is produced from the thermal decomposition of iron hydr(oxides). After the heat treatment at 850 1C, maghemite is reduced to iron. As the annealing temperature increases to 1050 1C, other crystalline compounds such as carbides form (Fig. S3a, ESI†). Thus, the thermal treatment affects not only the properties of iron compounds, but also the nature of solid carbon. For FeBC050, a weak and broad diffraction peak centered at ca. 261 is observed in the XRD spectra (Fig. S3a, ESI†). However, for both FeBC085 and FeBC105, Bragg peaks related to the metastable crystalline carbon appear (Fig. S3a, ESI†). Also, Raman spectra exhibit the D band that shifts from 1370 to 1350 cm 1 after annealing at 850 and 1050 1C (Fig. S3b, ESI†). Another interesting finding is that the D band becomes sharp, together with an increased intensity (peak heights) with respect to the G band (ca. 1580 cm 1) as the annealing temperature increases. This is because incorporating heteroatoms into the carbon network breaks its hexagonal symmetry and thus induces an increased D band in the Raman spectrum. A further analysis of Raman spectra reveals that the 2D band becomes a pronounced peak in the spectra after the heat treatment (Fig. S3b, ESI†). This is related to the presence of metastable crystalline carbon as in the XRD spectra of FeBC 850 and FeBC105 (Fig. S3a, ESI†).
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The chemical composition and the bonding state of elements of a solid surface can be obtained using X-ray photoelectron spectroscopy (XPS). For the untreated composite (FeBC050), the B 1s spectrum exhibits two envelopes, one between 184 and 190 eV and the other ranging from 190 to 195 eV (Fig. 2a). The former envelope is fitted by two peaks centered at 187.1 and 188.4 eV, corresponding to B atoms connected to B atoms (B clusters) and B atoms bound to C carbons in B4C species,18 respectively. In contrast, the latter one is only fitted by one peak centered at 191.6 eV, corresponding to BCO2 species. The C 1s spectral envelope between 280 and 292 eV can be deconvoluted into three peaks, located at 283.7, 284.5, and 287.6 eV (Fig. 2b), which are assigned to sp2-hydridized carbon, defective carbon, and 4CQO or C–O, respectively.19,20 The O 1s spectral envelope between 526 and 539 eV consists of three peaks centered at 529.9, 531.6, and 533.5 eV (Fig. S4a, ESI†), which are attributed to Fe–O, CQO, and C–O species, respectively.19–21 Neither the C 1s spectrum nor the O 1s spectrum exhibits the characteristic signal of BCO2 species, possibly due to its low surface concentration. The Fe 2p spectrum has two envelopes, one for Fe 2p3/2 (704 to 715.5 eV) and the other for Fe 2p1/2 (721 to 730.5 eV), and a satellite peak for Fe 2p3/2 at 718.9 eV (Fig. S4b, ESI†). In addition, there seems to be another peak at 732.6 eV; this may be a satellite peak for 2p1/2. Fe 2p3/2 has degeneracy of four states but Fe 2p1/2 has only two, thus the former envelope is fitted by multiplets (709.1, 710.1, 711.3, and 712.7 eV) and a high binding energy (BE) surface peak at 714 eV,22 whilst the latter one is deconvoluted into two peaks (724.1 and 726.3 eV).
Fig. 2 XPS B 1s (a) and C 1s (b) spectra of untreated and annealed FeBC composites.
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After thermally annealing the composite at 1050 1C (FeBC105), the most remarkable observation is that the B 1s spectrum has only one high BE envelope (186 to 196 eV) that can be deconvoluted into two peaks centered at 190.3 and 192.1 eV (Fig. 2a); these two peaks are assigned to BCxOy species. Also, the high BE envelope only appears for the composite annealed at 850 1C (Fig. S5, ESI†). The C 1s spectral envelope between 280 and 292 eV consists of three peaks, located at 283.7, 284.5, and 288.7 eV (Fig. 2a); they are ascribed to sp2-hydridized carbon, defective carbon, and –COO, respectively. A notable finding is that the relative concentration of defective carbon decreases after annealing at 1050 1C, consistent with the appearance of metastable crystalline carbon in the XRD spectra (Fig. S3a, ESI†). The O 1s spectral envelope (526 to 539 eV) is fitted by three peaks centered at 530.3, 532.1, and 534.8 eV (Fig. S4a, ESI†), assigned to Fe–O, CQO, and C–O species, respectively. The relative concentration of Fe–O species decreases after the thermal annealing, due to the reduction of iron oxides as shown in the XRD spectra (Fig. S3a, ESI†). Two new peaks (706.4 and 708.0 eV) appear on the low BE side of the Fe 2p3/2 envelope (Fig. S4b, ESI†); the former is assigned to metallic iron and latter is possibly related to iron carbides for FeBC105. Thus, all of XPS and XRD observations indicate that B4C species vanish, together with a decrease of defects in the carbon structure and the reduction of iron oxide to iron after the composite is subjected to the thermal annealing. Moreover, its effect on the microstructure of the composite is revealed by scanning electron microscopy (SEM) and highresolution transmission electron microscopy (HRTEM). It appears that the thermal annealing increases the porosity of boron-doped carbon–iron composites (SEM images in Fig. S6, ESI†); this plausibly comes from the reduction of iron oxide which is surrounded by carbon layers. During the formation of composites, sodium borohydride melts first and then it reacts with CO2 to produce carbon which preferably deposits on the solid iron precursor, resulting in a core–shell structure. This specific structure is attested by HRTEM images in Fig. 3 (for additional images, refer to Fig. S7, ESI†). It shows that iron-containing nanoparticles are encapsulated into well-defined onion-like graphitic carbon shells with an interplanar distance of ca. 0.34 nm, corresponding to the d-spacing of the (002) basal planes in graphite. The thickness of the shell is between 5 and 15 nm, while the size of the encapsulated particles varies from tens to hundreds of nanometers. For the untreated composite (FeBC050), the interplanar distance of cores is
Fig. 3 Representative TEM images of untreated composite (a) and annealed composite (b).
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about 0.24 nm, close to the d-spacing of the (311) planes in g-Fe2O3.23 For the thermally annealed composite (FeBC105), the interplanar distance of cores is around 0.19 nm, approximating to the d-spacing of the (110) planes in a-Fe.23 Upon combining XRD analyses, XPS studies, and SEM and HRTEM observations, the thermal annealing changes the surface state and more active sites are generated by both the reduction of g-Fe2O3 and the decomposition of B4C species. At a potential range of 0.2 to 0.4 V, the average electron transfer number per oxygen molecule is 1.96, but it increases monotonically from 2.4 to 4.7 as the potential changes from 0.5 to 0.8 V (Fig. 4a). Thus, we believe that oxygen reduction proceeds by a two-electron pathway at less negative potentials and a four-electron ORR reaction commences at a potential more negative than 0.7 V. This behaviour is consistent with ORR polyelectrolyte-functionalized graphene but different from borondoped carbon nanotubes and graphene.11,16,17 It should be noted here that other surface reduction reactions (Fig. 1) become important at 0.8 V, which gives rise to an electron transfer number as high as 4.7. At potentials between 0.4 and 0.6 V, the intercept of the slope of I 1 vs. N 1/2 plots weakly depends on the potential (Fig. 4b). Plausibly, this is because the oxygen reduction mechanism is dependent on the potential, as revealed by the electron transfer number increasing with negatively shifted potentials. Besides the high electrocatalytic activity toward ORR, FeBC105 exhibits an excellent tolerance of methanol crossover, another main challenge faced by metal-based cathode catalysts in fuel cells. The chronoamperometric responses at 0.5 V and a rotating rate of 2500 rpm to methanol introduction into O2-saturated 1 M NaOH solution for FeBC105 and Pt/carbon are presented in Fig. 4c. In the absence of methanol, the current maintains around 0.17 and 0.29 mA for FeBC105 and Pt/carbon, respectively. After introducing
Fig. 4 (a) The average electron transfer number per oxygen molecule and the kinetic limiting current at different potentials for the annealed composite (FeBC105) at different potentials; (b) Koutecky–Levich plots (I 1 vs. N 1/2) for the annealed composite (FeBC105) at different potentials; (c) chronoamperometric response at 0.5 V and 2500 rpm in O2-saturated 1 M NaOH solution (80 cm3) on Pt/carbon and annealed composite (FeBC105) electrodes before and after introducing 1 cm3 methanol; (d) CV curves of acid-leached FeBC105 subjected to 10 000 cycles measured at 100 mV s 1.
Chem. Commun., 2014, 50, 6349--6352 | 6351
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1 cm3 methanol into 80 cm3 electrolyte solution, the current for FeBC105 remains unchanged at around 0.17 mA. In contrast, the current for Pt/carbon immediately decreases to 0.37 mA, and then it instantaneously increases back to 0.03 mA, followed by maintaining around this value for the rest of the run. The FeBC105 was leached by 2 M H2SO4 at 80 1C for 3 h to identify the acid durability of residual Fe particles in oxygen reduction reaction, showing a slight decrease of ORR peak potential from 0.19 V to 0.20 V at a scan rate of 50 mV s 1 (Fig. S8, ESI†). In addition, the acid-leached FeBC105 showed high long-term stability for 10 000 cycles with an insignificant peak potential change from 0.223 V to 0.225 V at a scan rate of 100 mV s 1 (Fig. 4d). The preceding results demonstrate that the annealed B-doped carbon–iron composites can act as electrocatalysts for ORR with excellent activity, high tolerance to the crossover effect, relatively small generation of peroxide (Fig. S10), and good long-term stability. In summary, we have demonstrated a facile and reproducible approach to synthesize boron-doped carbon–iron composites from CO2 at 500 1C and 1 atm. The resulting composite has a core@shell structure, in which the iron-containing nanoparticles are confined within onion-like graphitic carbon shells. After thermally annealing the composite at temperatures higher than 850 1C in argon, the electrocatalytic activity toward ORR in alkaline solutions drastically enhances, and it is much higher than that of boron-doped carbon nanotubes. Along with the high activity, the thermally treated composite also exhibits excellent tolerance of methanol crossover and long-term stability. The high performance is attributed to the change in the surface state, which stems from the decomposition of B4C species and reduction of iron oxide by carbon. The changed surface state facilitates ORR by acting as an electron transfer mediator, an adsorption site for intermediate species, or both, resulting in favorable reduction kinetics. Currently, efforts are being made to tune the microstructure of graphitic carbon shells, which will increase the exposed area of iron-containing cores to oxygen and subsequently a direct 4-electron pathway could become favorable.
6352 | Chem. Commun., 2014, 50, 6349--6352
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The authors are grateful for the financial support from the Korea CCS R & D Center funded by the Ministry of Science, ICT, and Future Planning (no. NRF-2013M1A8A1040703).
Notes and references 1 R. Borup, J. Meyers, B. Pivovar, Y. S. Kim, R. Mukundan, N. Garland, D. Myers, M. Wilson, F. Garzon, D. Wood, P. Zelenay, K. More, K. Stroh, T. Zawodzinski, J. Boncella, J. E. McGrath, M. Inaba, K. Miyatake, M. Hori, K. Ota, Z. Ogumi, S. Miyata, A. Nishikata, Z. Siroma, Y. Uchimoto, K. Yasuda, K. I. Kimijima and N. Iwashita, Chem. Rev., 2007, 107, 3904. 2 G. Wu, K. L. More, C. M. Johnston and P. Zelenay, Science, 2011, 332, 443. 3 B. Bashyam and P. Zelenay, Nature, 2006, 443, 63. 4 K. Gong, F. Du, Z. Xia, M. Durstock and L. Dai, Science, 2009, 323, 760. ¨llen, Angew. Chem., Int. Ed., 2010, 5 R. Liu, D. Wu, X. Feng and K. Mu 49, 2565. ¨llen, Angew. Chem., Int. Ed., 6 S. Yang, X. Feng, X. Wang and K. Mu 2011, 50, 5339. ´au, Y. Yang, Y. Yan, W. Ebina, 7 J. P. Collman, N. K. Devaraj, R. A. Decre T. A. Eberspacher and C. E. D. Chidsey, Science, 2007, 315, 1565. `vre, E. Proietti, F. Jaouen and J. Dodelet, Science, 2009, 8 M. Lefe 324, 71. 9 D. S. Su and G. Sun, Angew. Chem., Int. Ed., 2011, 50, 11570. 10 C. Zhang, R. Huo, H. Yin, F. Liu and Y. Hou, Nanoscale, 2012, 4, 7362. 11 L. Yang, S. Jiang, Y. Zhao, L. Zhu, S. Chen, X. Wang, Q. Wu, J. Ma, Y. Ma and Z. Hu, Angew. Chem., Int. Ed., 2011, 50, 7132. ¨llen, J. Am. 12 Z. S. Wu, S. Yang, Y. Sun, K. Parvez, X. Feng and K. Mu Chem. Soc., 2012, 134, 9082. 13 C. Zhang, F. F. Fan and A. J. Bard, J. Am. Chem. Soc., 2009, 131, 177. 14 W. Jin, H. Du, S. Zheng, H. Xu and Y. Zhang, J. Phys. Chem. B, 2010, 114, 6542. ´, S. Gupta and R. F. Savinell, J. Electroanal. Chem., 15 S. L. Gojkovic 1999, 462, 63. 16 S. Wang, D. Yu, L. Dai, D. W. Chang and J. B. Baek, ACS Nano, 2011, 8, 6202. 17 Z. Sheng, H. Gao, W. Bao, F. Wang and X. Xia, J. Mater. Chem., 2012, 22, 390. ´, M. Me ´ne ´trier, A. Tressaud and S. Flanrois, 18 T. Shirasaki, A. Derre Carbon, 2000, 38, 1461. 19 H. Estrade-Szwarchopf, Carbon, 2004, 42, 1713. 20 T. I. T. Okpalugo, P. Papakonstantinu, H. Murphy, J. McLaughlin and N. M. D. Brown, Carbon, 2005, 43, 153. 21 A. P. Grosvenor, B. A. Kobe, M. C. Biesinger and N. S. Mclntyre, Surf. Interface Anal., 2004, 36, 1564. 22 T. Yamashit and P. Hayes, Appl. Surf. Sci., 2008, 254, 2441. 23 E. D. Smit and B. M. Weckhuysen, Chem. Soc. Rev., 2008, 37, 2758.
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