Polypyrrole-derived nitrogen and oxygen co-doped mesoporous carbons as efficient metal-free electrocatalyst for hydrazine oxidation.

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Polypyrrole-Derived Nitrogen and Oxygen Co-Doped Mesoporous Carbons as Efficient Metal-Free Electrocatalyst for Hydrazine Oxidation Yuying Meng, Xiaoxin Zou, Xiaoxi Huang, Anandarup Goswami, Zhongwu Liu,* and Tewodros Asefa* Direct liquid fuel cells have drawn much attention in recent years due to their ability to serve as power systems to generate electrical energy from chemical compounds that are liquid at ambient conditions or those that are relatively easy to transport, store or use as fuels in fuel cells (e.g., ethanol and methanol).[1] Among many compounds that can be used as fuels in direct liquid fuel cells, hyrdazine stands out for its multitude of added advantages, namely its: a) high theoretical cell voltage (+1.61 V), b) high energy and power density (especially when compared with hydrogen, which is arguably one of the ‘best’ fuels for fuel cells), c) high hydrogen density (which is equal to that of methanol), and d) electrooxidation leading to no CO2 or harmful byproducts.[2] Hence, although there had always been some research on hydrazine fuel cells since their inception in 1967,[3] recently there has been a huge resurgence of interest on them. In particular, substantial effort has been devoted to finding sustainable catalysts for hydrazine oxidation reaction (HOR)[4] as most of the conventional catalysts for HOR in hydrazine fuel cells involve noble metals such as Pt, which are unsustainable.[5] Some of these efforts have fortunately begun to bear some fruit. For example, Strasser group[6] showed that Ni60Co40 can serve as noble metal-free electrocatalyst for HOR with 6-fold higher catalytic activity over comparable benchmark catalysts. In another work, Srivastava et al.[7] reported that TiO2/

Y. Meng, Prof. T. Asefa Department of Chemical and Biochemical Engineering Rutgers The State University of New Jersey Piscataway, New Jersey 08854, USA E-mail: [emailprotected] X. Huang, Dr. A. Goswami, Prof. T. Asefa Department of Chemistry and Chemical Biology Rutgers The State University of New Jersey Piscataway, New Jersey 08854, USA Y. Meng, Prof. Z. Liu School of Materials Science and Engineering South China University of Technology Guangzhou 510640, China E-mail: [emailprotected] Dr. X. Zou State Key Laboratory of Inorganic Synthesis & Preparative Chemistry College of Chemistry Jilin University Changchun 130012, China

DOI: 10.1002/adma.201401969

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graphene oxide can also electrocatalyze HOR. However, such metal and metal oxide based catalysts can suffer from dissolution, catalyst poisoning, sintering and/or agglomeration— physical and chemical processes that can reduce the activity and shelf-life of catalysts over time during typical fuel cell operations.[8] The above-mentioned issues, together with the recent discoveries that carbon-based systems can electrocatalyze different half reactions in direct liquid fuel cells, have thus inspired us to explore the potential of non-conventional, non-metallic systems as catalyst for HOR. This particularly led us to look into mesoporous carbons because of their high surface area, high pore volume, and potential application as electrocatalysts as well as support materials for electrocatalytic groups in direct fuel cells.[9–13] For instance, Nazar group[9] demonstrated that mesoporous carbon-supported nanocrystalline intermetallic materials can electrocatalyze the oxidation reaction in direct formic acid fuel cells. In another example, Wen et al.[10] showed that mesoporous carbon-supported Pt/C core/shell nanoparticles can serve as methanol-tolerant electrocatalyst in direct methanol fuel cells. Meanwhile, other studies indicated that the electrocatalytic properties of carbon-based systems can easily be improved by doping the carbon with heteroatoms such as N, B, P or S.[11] Herein we show that polypyrrole-derived N and O co-doped mesoporous carbons (PPY-NOMPCs) can serve as efficient metal-free electrocatalyst for HOR. The PPY-NOMPCs are synthesized via polymerization of pyrrole within the channel pores of SBA-15 mesoporous silica (resulting in PPY/SBA-15), followed by carbonization of the PPY in PPY/SBA-15 and subsequent removal of the mesoporous silica framework (Scheme 1). It is worth noting that the pyrrole in our case is polymerized while it is in liquid state as well as in absence of any metal (cf. Jaroniec and Dai groups reported a synthetic method, involving immobilization of Fe ions into SBA-15, followed by chemical vapor deposition of pyrrole, to produce first PPY/SBA-15 and finally ordered Fe2O3-doped mesoporous carbons[14]). The liquid phase polymerization of PPY in our case might, in turn, account for the PPY-NOMPCs’ three dimensional interconnected structure as well as interesting electrocatalytic properties (vide infra). It is also important to note here that restricting the polymerization of PPY only inside the channel pores of SBA-15 was crucial for the successful synthesis of the resulting mesoporous carbons. This was achieved by protecting the silanol groups residing on the outer surfaces of as-synthesized mesostructured silica with TMS (-SiMe3) groups before the

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NH 2

N H

diamine-grafting

N H2

NH 2

Me-SBA-15

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(EtO) 3Si

SBA-15/diamine

Na 2S2O8, H +

H N

under Ar) Pyrolysis

N H

H N

Polymerization N H

PPY-NOMPCs

H N

N H3

S 2O 8

NH 2

NH 3

N H2 2-

O8 S2

PPY/SBA-15 (for clarity, supported ligands and counter-ions are removed)

Scheme 1. Synthesis of PPY-derived N and O co-doped mesoprous carbons (PPY-NOMPCs) by in situ polymerization of pyrrole within the channel pores of SBA-15, followed by pyrolysis of the resulting PPY/SBA-15 and etching the silica framework.

surfactant templates were removed from the latter (see Supporting Information, SI, for details). The final silica material after removal of surfactant (hereafter denoted as Me-SBA-15), was used to introduce PPY via in-situ polymerization. After a low temperature heat treatment step, the resulting PPY/SBA-15 was pyrolyzed at different high temperatures (600, 700, 800, 900 or 1000 °C) to let the carbon undergo graphitization; this resulted in different PPY-NOMPC-x materials, where x represents the final pyrolysis temperature. In the process, SBA-15 served as hard template aiding PPY to form N and O co-doped mesoporous carbon structures. For comparison, PPY-derived N and O co-doped carbon black (PPY-NOCB-900) was synthesized by pyrolysis of bulk PPY at 900 °C, without using SBA-15 as template (see SI). The structures of the parent materials leading to PPYNOMPCs were characterized by various techniques (see SI and Figures S1-S3 for some of the results). N2 gas adsorption/desorption measurements (Figure S1a) revealed that the surface areas of Me-SBA-15, SBA-15/diamine and PPY/SBA-15 are 501, 235 and 218 m2/g, respectively (Table S1). Their BJH pore size distributions (Figure S1b) show that all the three materials have mesopores, whose pore sizes decrease as the pores are filled with grafted organic groups and then PPY. By comparing the weight loss associated with the organic groups of PPY/SBA-15 vis-á-vis with that of its precursor on thermogravimetric analysis (TGA) (Figure S1c), the wt% of PPY in PPY/SBA-15 is calculated to be ∼6.3%. The TEM image of PPY/SBA-15 shows that the material has mesoscale channel pores without any visible polymer aggregates, both inside its channel pores as well as on its outer surfaces (Figure S1d). The PPY/SBA-15-derived PPY-NOMPCs were also characterized. As the material obtained after pyrolysis at 900 °C

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(PPY-NOMPC-900) exhibited the best electrocatalytic activity for HOR (see below), it was chosen for detailed discussion here. Its small angle X-ray diffraction (XRD) pattern (Figure S4) shows a sharp peak at 0.65° indicating the presence of some degree of ordered mesostructure in it. In contrast, the control sample, PPY-NOCB-900, which was synthesized without using SBA-15 as template, showed no such low angle Bragg reflection or ordered structure (Figure S4). These results confirm the importance of SBA-15 as a hard template in aiding PPY to form mesoporous carbons. The PPY-NOMPC-900 was further investigated by N2 gas adsoption/desorption porosimetry (Figure 1a). Its BET surface area and pore volume are found to be 398 m2/g and 0.16 cm3/g, respectively, confirming the material’s high porosity. Its BJH pore size distribution plot (Figure 1b) shows that PPY-NOMPC-900 has monodisperse pores, with average pore diameter of 4.0 nm. The TEM images of PPY-NOMPC-900 (Figures 1c and S5) reveal that the material has well-connected, worm-hole like, mesoporous structure. In addition, the TEM images in Figure S5 show that the material has the original SBA-15-like morphology. The Raman spectra of PPY-NOMPC-900 and NOCB-900 (Figure 1d) show two distinct peaks at 1349 cm−1 and 1586 cm−1, corresponding to the characteristic D and G bands, respectively, of graphitic carbon materials.[15] The D band is commonly associated with structural defects, which in our case is presumably due to the presence of N and O dopant atoms and/or concomitant absence of some of C atoms in the structure of the mesoporous carbon. The G band, which is due to first-order scattering of the E2g modes of graphitic materials,[16] indicates the presence of graphitic structure in PPY-NOMPC-900 and NOCB-900. The ID/IG ratio for PPY-NOMPCs decreases from

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Figure 1. a) N2 adsorption/desorption isotherm, b) BJH pore size distribution, and c) TEM image of PPY-NOMPC-900. d) FT-Raman spectra of PPYNOMPC-900 and PPY-NOCB-900.

1.00 to 0.93 when the pyrolysis temperature used to make the materials is raised from 600 °C to 1000 °C (Figure S6); this means the relative amount of ordered, graphitic structure in the PPY-NOMPCs increases as higher pyrolysis temperature is used for the synthesis of the materials.[17] Moreover, the ID/IG ratio of PPY-NOMPC-900 (0.97) is higher than that of PPY-NOCB-900 (0.90); this indicates that the former possesses relatively more defect sites than the latter. X-ray photoelectron spectroscopy (XPS) was performed to obtain the composition of PPY-NOMPCs as well as the nature of defect sites in them.[18] As can be seen in Figure 2a, the XPS survey spectra for the PPY-NOMPCs show a narrow peak at ca. 284.5 eV associated with graphitic C 1s electrons and a peak at ca. 400 eV due to N 1s electrons. In addition, a peak at ca. 533 eV associated with O 1s electrons is observed. These oxygen species could be due to physisorbed oxygen, moisture, CO2,[19] or oxygen dopants introduced into the materials from the silica framework upon pyrolysis of PPY/SBA-15.[13] With increasing pyrolysis temperature, the N/C atomic ratio for PPY-NOMPCs decreases; e.g., from ca. 8.1% for PPY-NOMPC-600 to ca. 2.1% for PPY-NOMPC-1000 (Figure 2b). By acquiring in high-resolution the N 1s and C 1s peaks of the PPY-NOMPC materials and control samples (Figures S7-S8), the chemical state of the nitrogen and carbon moieties in the materials were determined. The N1s peak was deconvoluted into two peaks with binding energies at 398.5 eV and 401.1 eV, corresponding to pyridinic and graphitic-like nitrogen moieties, respectively.[20] As illustrated in Figure 2c, despite their

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different pyrolysis temperatures, all PPY-NOMPCs possess relatively higher amount of graphitic N than pyridinc N moieties. Moreover, the content of graphite N increases with increasing pyrolysis temperature, especially between 900 °C and 1000 °C. However, the pyridinic N follows an opposite trend, as its content in the materials decreases at higher pyrolysis temperatures. This is in agreement with previous studies on N-doped graphene,[21] in which increasing pyrolysis temperature was reported to favor the formation of graphitic N species at the expense of the less stable pyridinic N species. The high resolution C 1s spectra of PPY-NOMPCs (Figure S7) shows the presence of C=C (284.8 eV) and C=N (285.8 eV) species and O-attached carbons, possibly due to C=O (287.4 eV) and C-O (285.8 eV) moieties.[22] The peaks corresponding to C=O and C-O carbons are indicative of the presence of O dopant atoms in the materials, which could only come from the silica framework, as reported earlier.[23] As can be seen in Figure 2d, the percentage of C=O is fairly similar in all the PPY-NOMPC materials. However, the percentage of C=C is noticeably low in PPY-NOMPC-900. Additionally, the ratio of graphitic to pyridinic N in PPY-NOCB-900 is lower than that in PPY-NOMPC-900. Moreover, in the high resolution C 1s spectra, a peak at 284.4 eV and a minor lower band peak at 282.5 eV, both of which correspond to C=C carbons,[24] are observed. Elemental analysis revealed that PPY-NOCB-900 has much lower amount of N (1.63 at%) than PPY-NOMPC-900 does (3.29 at%). Besides being consistent with the results obtained

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N/C Atomic Ratio / %

Intensity / a.u.

C 1s

O 1s 1000 °C

N 1s

900 °C 800 °C 700 °C 600 °C

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500 400 300 Binding Energy / eV

6 4 2

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Pyrolysis Temp. / °C

60 40

C=C C=N/C-O C=O

20 0

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700 800 900 1000 Pyrolysis Temp. / °C

Figure 2. XPS results for different PPY-NOMPCs: a) survey spectra, and percentage of b) N/C atomic ratios, c) atomic content of different types of N species, and d) atomic content of different types of C moieties as a function of pyrolysis temperature.

from Raman spectra (Figure 2d), this suggests that the SBA-15 framework inhibits the loss of N from the materials during pyrolysis of PPY/SBA-15, making the N atoms to remain as dopant in the mesoporous carbons. The electrocatalytic activities of all the PPY-NOMPCs prepared at different temperatures were then investigated. This was performed with cyclic voltammetry (CV) and rotating disc electrode (RDE)-based experiments, where a typical three electrode system consisting of a glassy carbon electrode (GCE) modified with PPY-NOMPCs was used as the working electrode (see Experimental section in SI). The results are presented in Figures 3 and S9–18. As mentioned above, finding good catalysts for electrooxidation of hydrazine is appealing because hydrazine can deliver high hydrogen density, high theoretical cell voltage, and no CO2 emission. Figure 3a shows the cyclic voltammograms (CVs) at a scan rate of 10 mV/s for different concentrations of hydrazine (ranging from 10 mM to 100 mM) in 0.1 M phosphate buffer saline (PBS) at pH 7.4 in presence of PPY-NOMPC-900. The result shows that PPY-NOMPC-900 displays remarkable electrocatalytic activity toward HOR, with an onset potential of -0.36 V and peak potential of −0.06 V (vs. saturated calomel electrode, SCE); these values are comparable with those of commercial Pt/C (20 wt% Pt) (Figure S11). When the hydrazine concentration is increased, a linear relationship between peak current and hydrazine concentration in the range of 10 mM to 100 mM hydrazine is observed (Figure S12h). On the other hand, the kinetic studies show that increase in scan

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rate induces an increase in the electrocatalytic peak current and a shift to a more positive peak potential for the catalytic HOR (Figure 3b). The oxidation current is found to vary linearly with the square root of scan rate (Figure S13h), suggesting that the electrocatalytic HOR is diffusion-controlled. As there is no corresponding cathodic peaks during the reverse scans, the oxidation of hydrazine in PBS in the presence of the PPYNOMPC-900 can be said to be completely irreversible. This is further verified by the polarization curves obtained with the RDE-based experiments (Figure S14). Furthermore, the total electron transfer number involved in HOR in presence of PPY-NOPMC-900 is determined to be ca. 4 (See Table S2 and Figure S16 and S17). Thus, the PPY-NOPMC-900-catalyzed HOR can be proposed to proceed as: N2H4 + OH− → N2 + H2O + 4e−. Additionally, stability measurement of PPYNOMPC-900 shows that after 500 cycles there is only 4% loss of the initial peak current density (Figure S15), indicating PPYNOMPC-900’s relatively higher stability during hydrazine oxidation. Besides PPY-NOMPC-900, the materials synthesized under pyrolysis temperatures other than 900 °C also show good electrocatalytic activity toward HOR;. But the results also show that their electocatalytic activities towards HOR depend on their pyrolysis temperatures, with the one obtained at 900 °C still being the most efficient (Figures S12–13). Figure 3c displays the current density at different potentials of 50 mM hydrazine in 0.1 M PBS with scan rate of 10 mV/s on the different PPY-NOMPC materials synthesized. As can be seen from the

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600

700 800 900 1000 Pyrolysis Temp. ( C)

Pyrolysis Temp. (°C)

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800

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1000

Pyrolysis Temp. ( °C)

Figure 3. Electrochemical and electrocataltytic properties of PPY-NOMPCs: a) CVs with a scan rate of 10 mV/s at pH 7.4 in 0.1 M PBS for different concentrations of hydrazine (ranging from 10 mM to 100 mM) in presence of PPY-NOMPC-900; b) CVs at different scan rates (ranging from 10 to 100 mV/s) for 50 mM hydrazine at pH 7.4 in 0.1 M PBS solution in presence of PPY-NOMPC-900; c) current density in 0.1 M PBS for 50 mM hydrazine at three different potentials in presence of PPY-NOMPCs; and d) the peak potential (Ep) for electrooxidation of 50 mM hydrazine in 0.1 mM hydrazine in 0.1 M PBS (pH = 7.4) in presence of PPY-NOMPCs.

results, among all the PPY-NOMPC materials PPY-NOMPC-900 gives the best electrocatalytic activity, higher current density and lower onset /peak potential (Figure 3c and Figure S12). As the pyrolysis temperature to make the materials is increased from 600 °C to 900 °C, the catalytic activity of the PPY-NOMPCs also increases, as seen from the gradual shift of the peak potentials of the materials to higher negative values when going from PPY-NOMPC-600 to PPY-NOMPC-900 (Figure 3d). The increase in the electrocatalytic activity of the materials appears to correlate well with the increase of the proportion of graphitic structure in the materials, which is evident from the decrease of ID/IG ratio, as discussed above. However, the peak potential (Ep) becomes more positive when the pyrolysis temperature is increased from 900 °C to 1000 °C, which may be ascribed to the low content of N in PPY-NOMPC-1000. Interestingly, the lowest peak potential is obtained for the material possessing the highest content of C=N (C-O) species, i.e., for PPY-NOMPC-900. However, the exact reason to this or the roles by which the different types of N species play towards the materials’ electrocatalytic activity is not well-understood and warrants further future studies. For comparison, the CV at a scan rate of 10 mV/s for the reference material PPY-NOCB-900 with 50 mM hydrazine in 0.1 M PBS solution at pH = 7.4 was also obtained. 6514

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PPY-NOCB-900 gives more positive onset potential (E = −0.1 V) and peak potential (E = 0.4 V) as well as much lower peak current density (1.84 mA/cm2) than PPY-NOMPC-900, whose peak current density is 2.63 mA/cm2 (Figure S18). This difference could partly be due to PPY-NOCB-900’s smaller specific surface area and lower content of N dopant compared with PPYNOMPC-900’s (Table 1). It is also worth noting here that PPYNOMPC-900 shows the highest electrocatalytic activity compared with other polymer-derived mesoporous carbons that can be synthesized under otherwise identical synthetic conditions, e.g., polyaniline (PANI)-derived N and O co-doped mesoporous carbons (Figure S18). This appears to be mainly due to the different density of various dopant species that these different polymers generate when converted into carbon materials via carbonization (Figure S19), and the different roles the various dopant species formed in the materials play towards the electrocatalytic reactions. All the above electrochemical results clearly demonstrate that PPY-NOMPCs are efficient metal-free electrocatalysts for hydrazine oxidation. Thus, these materials can be appealing for their possible use as electrocatalysts in the anode side of direct hydrazine fuel cells, especially given the fact that their electrocatalytic activities are better than those of many other catalysts recently reported for HOR (Table S2). Despite the fact that

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Sample PPY-NOMPC-900 PPY-NOCB-900 a)Eo

Eo/Ep (vs. SCE)a)

BET Surface Area (m2/g)b)

BJH Average Pore Size (nm)b)

Atomic % of Nc)

−0.36 V/−0.06 V

398

3.7

3.29

−0.1 V/0.4 V

–

1.63

= Onset potential, Ep = peak potential obtained from CV of 50 mM hydrazine at pH 7.4 in 0.1 M PBS; from XPS results.

b)obtained

from N2 adsorption/desorption isotherms;

c)Obtained

direct comparison is not precisely possible for different results reported in the literature by different research groups (because of the different conditions employed in the experiments), based on onset and/or peak potential values for HOR compiled in Table S3, PPY-NOMPC-900 can still be clearly seen as better electrocatalyst than many other materials. The origin of PPY-NOMPC-900‘s high electrocatalytic activity is most likely to do with its high density of nitrogen and oxygen dopants and concomittant dopant-related structural defect sites, besides obviously its large surface area, which allows it to interact well with the solution/reactant(s) during the electrocatalytic reactions. We hypothesize that the electron-rich mesoporous carbons aid the dissociation of hydrazine molecules to hydrogen atoms[25] under the influence of applied potential. Nevertheless, despite at this stage the exact mechanism by which the PPY-NOMPCs facilitate the catalytic HOR is

unclear, on the basis of the results we have obtained, we illustrate a hypothetical but fairly reasonable explanation as shown in Figure 4 that can shed some lights into it. As evident from the catalytic activities of the materials synthesized at different pyrolysis temperatures, a fine balance between the defects and the degree of graphitization is essential in order to achieve the highest electrocatalytic activity for such types of electrocatalytic reactions. In fact, based on some recent theoretical and experimental studies for related materials,[26] combined with previous reports on hydrazine oxidation using other materials,[27] it can be hypothesized that presence of dopants and defects aid the effective adsorption of hydrazine onto the catalytic surfaces, which can then be effectively dissociated/oxidized by the flow of electrons from or to the electrodes. In case of HOR, during this second step, it is speculated that the carbon atoms in the graphitic network (bearing relatively positive charge owing

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Table 1. Comparison of structures and electrocataytic activity of PPY-NOMPC-900 and PPY-NOCB-900.

Figure 4. Schematic illustration showing the proposed electrocatalytic processes over PPY-NOMPC.

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to the presence of neighboring electronegative nitrogen and oxygen dopant atoms) facilitate the flow of electrons through these materials, allowing the oxidation of N2H4 to form N2 and H+ which can then react with OH− to form water as shown in Figure 4. Subsequent desorption of product (water and nitrogen), followed by adsorption of fresh hydrazine molecules on the surface help to keep the catalytic cycle running under the applied potential. In conclusion, we have shown that polypyrrole-derived N and O co-doped mesoporous carbons (PPY-NOMPCs) can serve as efficient metal-free electrocatalyst for HOR, with low onset/ peak potential and high current density. The materials’s electrocatalytic activity for HOR is found to be better than many other HOR catalysts as well as comparable with noble metal catalytsts such as Pt/C (20 wt%), which is commonly used for this reaction. The nature and type of nitrogen dopants in the PPY-NOMPCs, which include pyridinic and graphitic nitrogens, are found to depend on the pyrolysis temperatures and affect the materials‘ electrocatalytic activity. We believe that the reported synthetic method can be further extended to other types of polymer precursors to produce other types of heteroatom-doped mesoporous carbon catalysts capable of efficiently electrocatalyzing HOR and other reactions.

Supporting Information For detailed synthetic procedures for the materials and additional characterization results, Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements T. A. acknowledges the financial support of the NSF (DMR-0968937 and NanoEHS-1134289). Y. M. acknowledges the Guangzhou Elite Program for her graduate scholarships. Received: May 1, 2014 Revised: July 5, 2014 Published online: August 14, 2014

[1] a) C. Zhu, S. Guo, S. Dong, Adv. Mater. 2012, 24, 2326; b) H. Wu, H. Li, Y. Zhai, X. Xu, Y. Jin, Adv. Mater. 2012, 24, 1594. [2] a) S. Forster, M. Antonietti, Adv. Mater. 1998, 10, 195; b) M. M. E. Ibele, Y. Wang, T. R. Kline, T. E. Mallouk, A. Sen, J. Am. Chem. Soc. 2007, 129, 7762; c) H. Gao, Y. X. Wang, C. B. Ching, H. W. Duan, J. Phys. Chem. C 2012, 116, 7719. [3] G. Evans, K. Kordesch, Science 1967, 158, 1148.

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Adv. Mater. 2014, 26, 6510–6516

Polypyrrole-derived nitrogen and oxygen co-doped mesoporous carbons as efficient metal-free electrocatalyst for hydrazine oxidation. - PDF Download Free (2024)

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