Graphene Oxide Sheathed ZIF-8 Microcrystals: Engineered

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Graphene Oxide Sheathed ZIF-8 Microcrystals: Engineered Precursors of Nitrogen-doped Porous Carbon for Efficient Oxygen Reduction Reaction (ORR) Electrocatalysis Minju Thomas, Rajith Illathvalappil, Sreekumar Kurungot, Balagopal N Nair, Abdul Azeez Peer Mohamed, Gopinathan M. Anilkumar, Takeo Yamaguchi, and U. S. Hareesh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06979 • Publication Date (Web): 12 Oct 2016 Downloaded from http://pubs.acs.org on October 13, 2016

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ACS Applied Materials & Interfaces

Graphene Oxide Sheathed ZIF-8 Microcrystals: Engineered Precursors of Nitrogen-doped Porous Carbon for Efficient Oxygen Reduction Reaction (ORR) Electrocatalysis Minju Thomas†⊥, Rajith Illathvalappil‡, Sreekumar Kurungot‡⊥, Balagopal N. Nair§$, Abdul Azeez Peer Mohamed†, Gopinathan M. Anilkumar §∗, Takeo Yamaguchi∥, and U. S. Hareesh†⊥∗ †

Materials Science and Technology Division (MSTD), National Institute for Interdisciplinary Science and Technology,

Council of Scientific and Industrial Research (CSIR-NIIST), Pappanamcode, Thiruvananthapuram, Kerala 695019, India ⊥Academy of Scientific and Innovative Research, Delhi−Mathura Road, New Delhi 110 025, India ‡

Physical and Materials Chemistry Division, CSIR−National Chemical Laboratory, Pune, Maharashtra, India 411008

§

R&D Centre, Noritake Company LTD, 300 Higashiyama, Miyoshi, Aichi 470-0293, Japan

$Nanochemistry Research Institute, Department of Chemistry, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia ∥Chemical Resources Laboratory, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8503, Japan

ABSTRACT Nitrogen containing mesoporous carbon obtained by the pyrolysis of graphene oxide (GO) wrapped ZIF-8 (Zeolitic Imidazolate Frameworks-8) micro crystals is demonstrated to be an efficient catalyst for the oxygen reduction reaction (ORR). ZIF-8 synthesis in the presence of GO sheets helped to realise layers of graphene oxide over ZIF-8 microcrystals and the sphere like structures thus obtained, on heat treatment, transformed to highly porous carbon with a nitrogen content of about 6.12 % and surface area of 502 m2/g. These catalysts with a typical micro-meso porous architecture exhibited an onset potential of 0.88Vvs RHE in a four electron pathway and also demonstrated superior durability in alkaline medium compared to that of the commercial Pt/C catalyst. The N-doped porous carbon derived from GO sheathed ZIF-8 core shell structures could therefore be employed as an efficient electrocatalyst for fuel cell applications. Keywords: Graphene oxide, ZIF-8, oxygen reduction reaction, N-doped carbon, porous carbon, alkaline fuel cell

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1. Introduction Fuel cells are believed to be the greener solutions for next generation power sources.1,2 Among the different types of fuel cells, alkaline fuel cells are widely pursued due to their high electrical efficiency, low operational temperatures, increased durability3,4 and the use of a liquid fuel like ammonia. The electrochemical cell reactions are normally catalysed by a noble metal catalyst like platinum due to the high overpotential requirements and the sluggish kinetics of oxygen reduction reaction (ORR).5-7 However, high cost and scarcity of platinum have necessitated the quest for alternate catalysts that are affordable and stable under electrochemical conditions.8-10 This has led to the emergence of affordable materials like porous carbon, transition metal catalysts etc. for ORR catalysis.11-14 The doping of carbon by heteroatoms of N, B, P and S is an effective strategy to promote ORR reactions as it creates catalytically active sites by a modulation of charge and spin densities of carbon near the dopant atoms.15-17 N-doping, by virtue of its size similarity with carbon, provides favourable improvements in electron transport properties of the carbon matrix18-20 and is therefore a preferred dopant for realising highly efficient electrocatalysts for applications in fuel cells.2124

Moreover, the high electronegativity of nitrogen induces increased positive charge density

in carbon, creating active sites for electrochemical reduction of O2. In alkaline fuel cells, the ORR kinetics is relatively faster compared to that of acidic fuel cells and hence N-doped carbon (NDC) is perceived to be a possible replacement for the precious metal (Pt, Pd) electrocatalysts.25,26 A host of synthetic approaches are currently available for the efficient preparation of nitrogen doped carbon. The pyrolysis or chemical vapour deposition of nitrogen and carbon containing precursors such as heterocycles, melamine or aminated sugars resulted in the direct 2 ACS Paragon Plus Environment

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incorporation of the nitrogen atoms in the carbon backbone.9,27 Alternatively, hydrothermal carbonisation of carbohydrate-rich biomass, thermal treatment of nitrogen-rich organic networks and frameworks yield nitrogen-containing carbon materials.28,29The incorporation of nitrogen into the carbon matrices is normally accomplished through the configurations of N atom as pyridinic, pyrrollic and graphitic type nitrogen. Among these three common configurations, pyridinic and graphitic nitrogen find prominence because of the reportedly enhanced catalytic efficiency.30-31 The distribution of nitrogen in the porous carbon network is one of the determining factors that controls the efficiency of catalysts.24 Among the wide variety of precursors employed for the synthesis of NDC catalysts, metal organic frameworks (MOFs) possessing extremely high surface area are proven to form the networked porous carbon materials upon heat treatment in inert atmospheres.32-34 Nitrogen containing MOFs can especially perform the dual functions of being a precursor for carbon and as a source of nitrogen. Zeolitic Imidazolate Frameworks (ZIFs) with high thermal and chemical stability relative to other MOFs are preferred candidates for the preparation of NDC catalyst as they form microporous carbon frameworks during the carbonisation process.33 For example, highly conducting networks of NDC was reported to be derived by the carbonisation of ZIF-8 as a sacrificial template.35 Zong et.al demonstrated the use of 2D sandwich-like ZIF-GO composite structures for processing the NDC sheets by in-situ growth of ZIF-8 on graphene oxide (GO).36 Electro spinning a polymer solution containing ferrous organometallics and ZIF reportedly resulted in the formation of a micro/macro porous nano carbon network with enhanced ORR activity.37 Sreekumar et al., earlier demonstrated the strategy of entrapping N in the carbon matrix through concealing the nitrogen reservoir.38 Graphene wrapped carbon matrix was thus demonstrated to be an efficient method for obtaining 3D nitrogen doped graphene. In the present work, we further supplement the observation by demonstrating that graphene oxide wrapped ZIF-8 microcrystals could be 3 ACS Paragon Plus Environment


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employed as efficient precursors for N-doped interconnected porous network of carbon with appreciable ORR activity and enhanced cyclic stability. The sheathed precursors developed by a simple one pot room temperature synthesis of ZIF-8 in GO dispersions lead to the formation of porous carbons with significant nitrogen doping as the GO sheets wrapped around ZIF-8 crystals prevented larger loss of nitrogen on carbonisation. The heteroporous nature of the carbons thus formed was an added advantage of the synthetic methodology that favourably contributed towards oxygen reduction reaction in providing active sites.

2. Experimental Section 2.1 Materials and Methods

Graphite powder (Merck, Germany CAS No.7782425), sodium nitrate (NaNO3,99%, Merck, India), sulphuric acid (H2SO4, ≥98%, Merck, India), potassium permanganate (KMnO4, Merck), potassium hydroxide (KOH, Merck, India), hydrogen peroxide (H2O2, 30%, Merck, India), zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Merck, India),

2-methylimidazole

(C4H6N2, 98%, Sigma-Aldrich), methanol (CH3OH, ≥99.5, Merck), hydrochloric acid (HCl, 98% Merck) and Nafion solution (DuPont) were used “as purchased” without any further purification.

2.2 Characterisation Phase identification of the synthesised samples was performed by powder X-ray diffraction (PXRD) measurements (PW1710 Philips, The Netherlands). Scanning electron microscopy (SEM) was utilised for morphological characterisation (SEM, Carl Zeiss, Germany) and elemental mapping and analysis were carried out by using Energy Dispersive Spectroscopy (EDS). High resolution transmission electron microscope (HRTEM, TecnaiG2, FEI, The Netherlands) operating at 300 kV was employed for high magnification imaging. Raman 4 ACS Paragon Plus Environment

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analysis was performed on alpha 300 R confocal Raman microscopes (WI Tec Germany, 633Laser). Thermo gravimetric analysis (TGA) was performed using a TGA apparatus (Perkin Elmer STA1000 TGA) in the temperature range of 50-900 °C at a heating rate of 5 ºC min-1 in N2 atmosphere. X-ray photo electron spectroscopy was carried out on scanning X-ray microprobe ULAC-PH1, Inc. PHI 4700V with monochromatic Al-Kα X-ray source operating at 14kV and 220W. The surface area and pore size characteristics of synthesised samples were analysed by N2 adsorption and desorption measurements using a Micromeritics (Tristar11, USA) surface area and porosity analyser. Samples were degassed at a temperature of 200oC before the adsorption measurements. The FTIR spectra were recorded on powders using Bruker αE FT-IR spectrometer. All the electrochemical measurements were performed on a Bio-Logic electrochemical workstation (SP-300) under a rotating disk electrode (RDE) configuration. A three-electrode setup has been used to analyse the oxygen reduction activity of the materials. The catalyst slurry for the electrochemical analysis has been made according to the following procedure. 5 mg of the catalyst was dispersed in 1 mL of DI water- Isopropyl alcohol mixture (3:1). Subsequently 40 µL of 5 wt. % Nafion solution was added into it and then sonicated for 1 h in an ultrasonic bath. About 10 µL of the above catalyst slurry was coated on the glassy carbon working electrode (0.196 cm2) and dried. Hg/HgO was used as the reference electrode and a graphite rod was used as the counter electrode. The analysis was carried out in N2 and O2 saturated 0.1M KOH solution. For comparison, the state-of-the-art catalyst 40 wt. % Pt/C (Johnson Matthey) with the same catalyst loading was analysed in the 0.1M KOH solution. The rotating ring disc electrode (RRDE) experiments were performed in an O2 saturated 0.1M KOH solution with glassy carbon disc (0.2826 cm2) having a platinum ring as a working electrode, Hg/HgO as the reference electrode and graphite rod as the counter electrode. Onset potential is measured at the potential corresponding to a point where the



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ORR current initially deviates from the background current by a value of 25 µA cm–2. 39The accelerated durability test (ADT) was performed to analyse the stability of the catalyst by performing the cyclic voltammetry (CV) in the O2 saturated 0.1M KOH solution in the potential window of 0.57 V to 0.97 V with a scan rate of 100 mV sec-1 and the LSV is compared before and after the ADT. All the potentials were converted into RHE by calibrating the Hg/HgO reference electrode in H2 saturated 0.1M KOH solution by running a LSV with a scan rate of 1mVsec-1 and the conversion value is around 0.87 V. 2.3 Synthesis of graphene oxide/ZIF-8 composites (GZx) GZx (where ‘x’ is the weight percentage of ZIF-8 x= 30, 50, 80) compositions were prepared by a one-pot synthesis approach. In a typical synthesis for the composition GZ80, 2.7 g of ZnNO3.6H2O dispersed in 19 mL water was mixed with 0.5 g graphene oxide (GO) in 20 mL water. Similarly, GZ50 and GZ30 were obtained by changing the amount of ZnNO3.6H2O (1.8 g and 0.49 g respectively). The mixture was then ultrasonicated for 30 minutes for the effective dispersion of Zn2+ in between the GO sheets. An aqueous solution of 2methylimidazole (Zn:HmIm -1:16 molar ratio) was then added to the mixture under continuous stirring and kept for 1h. The solid mass thus obtained was further centrifuged, washed with methanol and dried at 60 °C for 24 h. GO was synthesised by modified Hummers method as reported elsewhere.40 ZIF-8 was synthesised by procedures as reported by Kida et. al.41 2.4 N-doped porous carbon (GZXC) N-doped porous carbons (GZxC) were synthesised by the high temperature treatment of GZx in nitrogen atmosphere. For the carbonisation, the dried GZx samples were heated to 900 °C at a heating rate of 5 °C/min and kept for 3h at 900 °C in N2 atmosphere. The products were then washed with 3M H2SO4 solution to remove the remaining metal content and washed further with deionised water several times for acid removal. For a comparison, the ZIF-8 and 6 ACS Paragon Plus Environment

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GO samples were also carbonised as explained above and the samples are named as ZIF-8C and GOC respectively.

3. RESULTS AND DISCUSSION Morphology and microstructural features of the as prepared GO-ZIF-8 composites (GZx) and N-doped porous carbons (GZxC) were investigated via transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The TEM image of the GO sample (Figure S1a) clearly indicated its sheet like morphology having sizes ranging up to micrometers. On the other hand, well defined hexagonal particles of ~400 nm in sizes (Figure S1b) are seen for ZIF-8. The GZ30 composition presented in Figure 1a indicated an agglomerated structure of the GO sheets and ZIF-8 crystals. As the amount of ZIF-8 was increased, GZx composites containing GO sheathed ZIF-8 crystals were realised for GZ50 and GZ80 samples, as shown in Figure1b and 1c respectively.

Figure 1. TEM images of a) GZ30, b) GZ50, c) GZ80, d) GZ30C, e) GZ50C and f) GZ80C.

The incorporation of ZIF-8 in sufficient quantities was presumed to facilitate the de-stacking of the GO sheets. This mechanism in turn should have lead to the wrapping of the ZIF-8 7 ACS Paragon Plus Environment


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crystals by the GO sheets resulting in the formation of a GO sheathed ZIF-8 architecture. In the composite formation, the dispersed Zn2+ ions are weakly coordinated to the epoxy oxygen and hydroxyl groups of GO.35,42 The 2-methylimidazole co-ordinated to these Zn2+, then

Figure 2. Schematic of the mechanism of formation of GO sheathed ZIF-8 structures.

facilitated the casing of ZIF-8 microcrystals within the GO sheets. A schematic representation of the mechanism of formation of the GO sheathed ZIF-8 structures is shown in Figure 2. In composite derived carbons, the microstructures are characterised by a mixed morphology of both sheet and hexagons representing porous carbons from GO and ZIF-8 respectively. The hexagonal morphologies became prominent as the amount of ZIF-8 increased in the composites (Figures 1d-f).The carbonisation of GO and ZIF-8 retained their sheet like and hexagonal morphology respectively without any considerable changes (Figure S1c-d). The carbonisation of ZIF-8 and the composite structures have been traced using TGA analysis under N2 flow atmosphere; results are presented in Figure S2. It is clear that carbonisation proceeded well with weight loss values in line with the ZIF-8 content. It is expected that during the carbonisation process, Zn+2 ions are reduced by carbon to Zn metal which gets subsequently vapourised at high temperature. Acid washing after carbonisation removed any residual Zn metal.43 HRTEM images of the GZ80C and the diffraction pattern evidenced the graphitic nature of the porous carbons obtained (Figure 3). The d-value of 0.33 nm was consistent with the 8 ACS Paragon Plus Environment

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graphitic structure and could favourably influence the conductivity of the sample. The TEM observations were further validated by the SEM images presented in Figure S3 and S4. ZIF-8

Figure 3 a) HRTEM images GZ80C (FFT is given in the inset), b) HRTEM showing d spacing in GZ80C and c) SAED of GZ80C.

crystals had a hexagonal morphology which was retained even after carbonisation in the absence of GO (Figure S3 a and b). The composite samples before carbonisation appeared as spherical shaped GO sheathed ZIF-8 particles of approximately 1 micron size (Figure S4 a-c). The homogeneity of the preparation process adapted is substantiated by the size and morphological uniformity of these GO sheathed ZIF-8 crystals. As the GO sheets were wrapped around the ZIF-8 crystals, the separation and stacking of the same during drying were found limited. The closely wrapped architecture of the composite presumably limit the nitrogen loss from ZIF-8 during the high temperature carbonisation process.38 The carbonised samples shown in Figure S4 (d-f), appeared as highly porous materials. Agglomerates were observed for the carbons derived from the lower ZIF-8 composition (Figure S4d) whereas porosity was found enhanced in the composites derived carbons having a higher amount of ZIF-8 (Figure S4 e and f). Surface area measurements and pore structure analysis performed by nitrogen adsorption confirmed these results. As shown in Figure 4a, isotherms for all the GZx samples were Type 1 and adsorption/desorption branches were largely reversible, confirming that all the composites were predominantly microporous materials. Adsorption results of the composites were in line



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with the intrinsic microporosity of ZIF-8, where in significant adsorption usually occurs at low relative pressure values (P/P0) less than 0.1 (also shown in Figure 4a).

Figure 4.a) Nitrogen adsorption isotherms of the a) ZIF-8, composites (GZx) and b) the respective carbonised samples (ZIF-8C and GZxC).

The composites exhibited specific surface area of 67, 790 and 917 m2/g for GZ30, GZ50 and GZ80 respectively compared to the parent precursor surface area of 19 m2/g of GO (isotherm shown separately in Figure S5a, because of the small adsorption values) and 1380 m2/g of ZIF-8. The low surface area obtained for GO sample may be due to the re-stacking of layers, during drying in the absence of ZIF-8 crystals. ZIF-8 retained its microporous nature even after carbonisation and is confirmed by the Type I isotherm of ZIF-8C in Figure 4b. Unlike ZIF-8C, the high temperature treatment altered the pore characteristics of GZx samples to slit shaped mesopores as evidenced by the hysteresis loop of the Type IIb isotherms shown in Figure 4b. In all cases, the adsorption extended towards the high pressure regime, indicating the presence of larger pores in the aggregated macrostructures. Surface area of 65, 256 and 502 m2/g was calculated for the GZ30C, GZ50C, GZ80C samples based on the N2 adsorption values. These values were lower than the surface area obtained for the composite samples prior to carbonisation. Under similar conditions GOC showed an increase in surface area from19 m2/g to180 m2/g. On the contrary, the ZIF-8C samples showed a decrease in surface area from 1300 to 851 m2/g. 10 ACS Paragon Plus Environment

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Changes in surface area values as discussed above revealed only a part of the transformation in porosity and pore structure of the carbonised samples. Pore structural changes in these materials are due to the evaporation of zinc from ZIF-8 as well as the increased destacking of the GO layers during carbonisation. ZIF-8C sample showed a decrease in porosity on carbonisation, but the pore sizes remained mostly in microporous range (Figure 4b). As a result, the percentage of micropore volume in total porosity (pore volume) in these materials hardly changed on carbonisation. However, carbonisation of the GO sheets resulted in its destacking leading to a large increase in pore volume, mostly in mesoporous and macroporous range (Figure S5 and Table S1).

Figure 5. a) Cumulative pore volume of ZIF-8C and GZxC samples and b) pore size distribution of ZIF-8C and GZ80C samples calculated based on DFT.

The pore structural (pore volume and pore size) changes in the carbonised samples of the composites were more remarkable and are explained below based on the pore volume results in Table S1 and pore size results shown in Figure 5. As mentioned earlier, all the samples prior to carbonisation showed Type 1 adsorption isotherms and therefore their pore-sizes were obviously in the micropore range. The cumulative pore volume and the pore size distribution shown in Figure 5 (a and b) revealed the presence of both mesopores and micropores in the composite derived carbons, unlike the ZIF-8C which had predominantly



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micropores. GZ30C sample, which had the minimum ZIF-8 content during synthesis, showed almost no micropores (pore size < 20 Å). GZ50C sample showed substantial microporosity along with some amount of mesoporosity. In this case, we expect the microporosity formation as a result of carbonisation of the ZIF-8 microstructure and mesoporosity from the destacking of the GO part of the mixture. Surprisingly, further increase of ZIF-8 content in the processing mixture showed more mesoporosity on carbonisation than expected from its GO content (GZ80C). In fact, from the cumulative pore size distribution curves shown in Figure 5(a), it is clear that more than 70% of the pore volume was in the mesoporous range for this sample. We expect that the GO sheets around the ZIF-8 crystals transformed the carbonisation of ZIF-8 leading to enlargement of the micropores. To understand this point more clearly, we have included the dV/dD pore size values calculated based on Density Functional Theory (DFT) as provided in the measurement set-up. This comparison of the pore size values of ZIF-8C and GZ80C, as shown in Figure 5b, is noteworthy. ZIF-8C samples showed microporous nature with most of the pores in the size range of < 20 Å while pore sizes of GZ80C sample were mostly in the 20-50 Å range. The importance of having pores in this size range and its impact on ORR of the catalysts will be discussed further in the following sections.

Figure 6. a) XRD patterns of GO, ZIF-8 and the composite samples and b) samples after carbonisation.


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The powder x-ray diffraction (PXRD) patterns of the synthesised composite samples are compared with that of GO and ZIF-8 in Figure 6a. The XRD pattern of GO was characterised by a broad peak at a 2θ value of 10°. The ZIF-8 has a well crystallized XRD pattern depicting the characteristic peaks while the ZIF-GO composites of GZ30, GZ50 and GZ80 were dominated by characteristic peaks of ZIF-8. A careful examination of the patterns revealed that as the GO amount increased, peak widening is observed due to the amorphous nature of GO. After the carbonisation process at 900 °C, the peaks corresponding to ZIF-8 vanished and new broad peaks corresponding to the characteristic peaks of graphitic carbon [(002) and (100)] appeared at 2θ values of 25° and 42.2° respectively evidencing the complete conversion of ZIF-8 to carbon at 900 °C in all the samples (Figure 6b). The observations are further confirmed through FT-IR analysis presented in Figure S6a. The precursor GO is characterised by a broad peak at 3500cm-1 representing –OH groups. The peaks at 1230 and 1628 cm-1 signified C=C, C=O, C-O (epoxy) groups.

44

ZIF-8 provided sharp peaks in the

spectral regions of 500-1350 cm-1 and 1350-1500 cm-1 due to the respective plane bending and stretching of the imidazole ring. The peaks for the aromatic and aliphatic C-H stretches were at 3140 cm-1 and 2929 cm-1 respectively.45

Figure 7. Raman spectra of GZ80C, GZ50C and GZ30C.



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FTIR and Raman's spectra were employed to investigate the chemical nature of the carbonised samples (Figure S6b and Figure 7). FT-IR spectroscopy results (Figure S6) confirmed the formation of N-doped carbon upon high temperature carbonisation. The spectra of the carbonised composite samples indicated the presence of C–N bonds together with N–H functional groups. The presence of such functional groups is believed to impart hydrophilicity on the carbon surfaces.35 Raman spectra as in Figure 7 revealed the graphitic nature of carbon present in the samples. The two peaks at 1330 cm-1 and 1590cm-1 indicated the D and G peak of the graphitic carbon respectively. The D band arising from the A1g vibration mode of carbon is mainly due to the in-plane imperfections (defects and heteroatoms) of the graphitic lattice of the disordered sp2hybridized carbon. The E2g vibration mode of C-C stretching results in the G band and signifies highly ordered crystalline carbon.46The chemical modifications that occurred in the sample during the high temperature carbonisation process could be elucidated from the ID/IG values. The ID/IG ratios of the carbon derived from the GZ samples are in the order GZ30C>GZ50C > GZ80C with values of 1.25, 1.03, 0.77 respectively. This is presumably a reflection of the amount of the defects in the carbon due to varying levels of nitrogen doping in the carbon matrices. XPS analysis is therefore pursued to confirm the nitrogen analysis. XPS analysis of the GZxC and ZIF-8C samples was carried out to estimate the extent of nitrogen doping and its chemical environments, a parameter that signifies the ORR activity in the carbon matrix. The survey spectrum showed (Figure S7) major peaks for C and N at 284.6 eV, ~400.eV, respectively. Furthermore, the presence of surface adsorbed oxygen was indicated by the O1s peak at 540.0 eV.47 The atomic percentages of C and N present and the N/C ratio of each sample are given in Table 1. The ZIF-8 derived carbon has a N/C ratio of 0.059. With increasing amounts of GO


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in the precursor, the N/C ratio of the composite derived carbons are also enhanced from 0.069 to 0.11. In GZ80C, the weight percentage of carbon and nitrogen was 88.62 and 6.12%,

Table1. Atomic % values of C and N of GZxC and ZIF-8C measured by XPS analysis.

Sample

C (%)

N (%)

N/C

ZIF-8C

87.10

5.72

0.059

GZ80C

88.62

6.12

0.069

GZ50C

84.40

8.80

0.10

GZ30C

86.67

9.80

0.11

respectively, giving a N/C value of 0.069 compared to the value of 0.059 measured for the ZIF-8C sample. The N/C values of the other two GZxC samples also showed a similar trend. Even in GZ50C, with a lower ZIF-8 content of 50%, the effective enfolding of the ZIF-8 crystal by the GO sheets allowed the enhancement of the N/C ratio to 0.10 upon carbonisation. GZ30C sample showed a further increase in the N/C ratio to 0.11, signifying the importance of the sheathed architecture of the composites for carbonisation. This is primarily due to the increased trapping of nitrogen in the bulk graphene envelope resulting from the sheathed architectures. The relative increase in the percentage of nitrogen in composite derived carbons, compared to ZIF-8C substantiates further the nitrogen trapping in graphene layers. The imidazolic nitrogen released during the high temperature pyrolysis of GZ80C gets accommodated into the graphitic structure of the GO derived carbon, enhancing thereby the total nitrogen content in the porous carbon thus formed. On the contrary, the pyrolysis of ZIF-8 C leads to the escape of imidazole nitrogen due to the absence of a layered carbon matrix as a shielding layer.



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The chemical nature of the N atoms bonded in the graphene matrix was further analysed using the deconvoluted XPS spectra as in Figure 8. Four types of nitrogen species, namely the pyridinic nitrogen (N1, 398.3eV), pyrrolic nitrogen (N2 400.1 eV) graphitic nitrogen (N3 401.0 eV) and nitrate species (N4, 405.4eV) were identified from the N1s spectra.

Figure 8.The deconvoluted N1 spectra of a) ZIF-8C and b) GZ80C.

In ZIF-8C (Figure 8a), the amounts of pyrrolic and graphitic nitrogen were estimated to be 50 and 38% respectively while GZ80C (Figure 8b) contained high graphitic (56%) and pyrridinic (33%) nitrogen. The increased amounts of nitrogen in GZ80C as pyridinic and graphitic forms were presumably the contribution from the sheathed structure of GO. In higher GO compositions (GZ50 and GZ30), there was an increase in the graphitic layers and subsequently, the defect levels also enhanced (Figure S8). The C1s peak (Figure S9) was resolved into three peaks where the major peak at 284.6 eV corresponded to the graphitic sp2 carbon and the other two peaks at 258.8 eV and 288.2 eV corresponded to C-N and C-O respectively. The percentage compositions of carbon and nitrogen in the carbonised samples are also quantified by the EDS elemental analysis (Table S2) and the results are in line with XPS analysis which supported the hypothesis of enhanced nitrogen trapping in the carbon matrix due to the wrapping of GO sheets around ZIF-8 crystals. The distribution of the


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nitrogen in the carbon matrix was also characterized by the elemental mapping (Figure S10S12). A uniform distribution of nitrogen is clear in all composite derived carbons.

The CV measurements of all the carbonised samples (GOC, ZIF-8C and GZxC) were carried out in both N2/O2 saturated aqueous solutions of 0.1M KOH (Figure S13). The CV curve of GZ80C in N2 saturated aqueous solution is rather different from that of the O2 saturated solution. The N2 saturated solution curve is devoid of any characteristics peak while the peak at 0.88V observed for the O2 saturated aqueous solution represents the oxygen reduction process (Figure 9a).

Figure 9 a) Cyclic voltammograms of GZ80C in N2 and O2 saturated 0.1M KOH solution measured at a scan rate of 50 mV sec-1 at an electrode rotation rate of 900 rpm, b) Comparison of the rotating ring disc electrode (RRDE) voltammograms obtained for the different samples in O2 saturated 0.1M KOH solution. The measurement was performed at a scan rate of 5 mV sec-1 and an electrode rotation speed of 1600 rpm and c) LSV plots of GZ80C in O2 saturated 0.1M KOH solution measured at different rotation speeds, at a scan rate of 10 mV sec-1.



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Figure 9b illustrates the RRDE voltammograms of the different samples in oxygen saturated 0.1 M KOH at a rotation speed of 1600 rpm. The GOC sample had an onset potential of 0.706V with mixed diffusion kinetics. On the other hand, the composite derived carbons outperformed the GOC in terms of onset potential, half-wave potential and limiting current, as evidenced from the RRDE voltammograms presented in Figure 9b. GZ80C showed 0.88V of onset potential and was positively shifted to 180 mV compared to that of GOC. The shift was 80 and 70 mV respectively from GZ30C and GZ50C. The increased nitrogen content in the GZ30C and GZ50C samples helped to improve the onset potential but the activity was limited by lower surface area. Since the onset potential is a direct measure of the intrinsic activity of the system towards ORR, the present results confirmed the generation of activity modulated sites especially in the case of GZ80C, which can facilitate ORR at a substantially reduced over potential. Benefitted from this activity modulation, the over potential for GZ80C compared to the state-of-the-art Pt/C is only 0.12V, which is a remarkably good improvement, compared to the many heteroatom doped materials discussed in the literature.48,32,49 The GZ80C surpassed the ZIF-8C due to the synergistic effects of pore characteristics, surface area and nitrogen content. The benefit from the controlled interplay of these parameters was evident from the half-wave potential (E1/2) value of GZ80C (0.75V), which prevented a major drop due to iR and mass transfer issues. Along with the onset potential, E1/2 is another important parameter that decides how well the system survives under the current dragging conditions. Rotating disk electrode (RDE) experiments at varying rotating speeds were carried out and using the Koutecky–Levich (K–L) equation the kinetic parameters have been calculated. By calculating the kinetic current density and the number of electrons transferred based on the ORR polarization curves, the effectiveness of a catalyst can be estimated using the Koutecky–Levich (K–L) equation. 18 ACS Paragon Plus Environment

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The K–L equation is 1 1

1

0.62

Figure10. a) Koutecky-Levich (K-L) plots of GZ80C, b) the number of electrons transferred during ORR measured from the RRDE experiment as a function of the different applied potentials to GZ80C, c) Tafel plot comparison of GZ80C with Pt/C and d) LSVs before and after the durability analysis of GZ80C.

where the kinetic current density, jk= ¼ nFAkCO2. Here, ‘n’ is the number of electrons, ‘F’ is the Faraday constant (96485.5 C), ‘A’ is the area of the electrode (0.196 cm2), ‘CO2’ is the bulk O2 concentration (1.2106 mol-1 cm3), ‘DO2’ is the diffusion coefficient of O2 in the electrolyte (1.9105 cm2s1), ‘ υ’ is the kinematic viscosity of the electrolyte (0.01009 cm2s1), ‘ω’ is the rotation rate of the electrode in radians per second (2π rpm/60). The plot of 1/j vs. ω-1/2 gives a linear relationship, with the y-intercept leading to the inverse of the kinetic 19 ACS Paragon Plus Environment


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current. The number of electrons (n) transferred during ORR can be calculated from the slope of the plot. The linear relationship between j-1versus ω-1/2 at different potentials of GZ80C sample is represented in Figure 10a.The linearity of K–L plots for GZ80C indicated the first order kinetics during the ORR. RRDE technique was employed for quantifying the electron transfer number (n) involved in the ORR process. The following equation was employed to calculate ‘n’ from the RRDE data:

4 − − − − − − /" where, ‘Ir’ is the Faradaic ring current, ‘Id’ is the Faradaic disc current, ‘N’ is the collection efficiency (0.37) and ‘n’ is the number of transferred electrons. The number of electron transfer (n) value at 0.6 V for GZ80C was calculated to be 3.2, which represents the more favourable four electrons ORR pathway. The electron transfer number is almost constant in all the potential indicating the consistency during ORR (Figure 10b). Also, a comparative plot representing the ‘n’ value as a function of V (vs RHE) for all the samples is given in (Figure S14 f). The ‘n’ value for ZIF-8C was above 3.3, which also indicates the conversion of O2 into OH- during the ORR.

Further, the current-voltage relationship (Tafel plot) in the kinetically controlled region has been investigated by plotting the log jk values obtained from the RDE measurements against the corresponding potentials. The plotting has been done by applying 65% iR compensation in order to eliminate the possible contributions from the ohmic polarization. Thus obtained Tafel plots in the case of GZ80C and Pt/C, as presented in Figure 10c, are found to be nearly parallel and straight, indicating similar electrode kinetics, especially the rate determining step. The calculated Tafel slope values in the case of GZ80C and Pt/C are 81.73 and 68.5


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mV/dec respectively. These relatively matching values between the systems stand out as a valid evidence of the high intrinsic activity of the active sites generated on the homemade catalyst. The accelerated durability test (ADT) was carried out to investigate the stability of the catalysts in 0.1M KOH saturated with O2 at a scan rate of 50 mVsec-1 and the LSV was analysed before and after ADT (Figure 10d and Figure S15). GZ80C showed a shift in E1/2 by only18mV after 5000 ADT cycles, without changing its onset potential (0.88mV)(Figure 10d). However, the E1/2of the Pt/C was shifted negatively by 32 mV after 5000 ADT cycles along with a 20 mV negative shift in the onset potential as well (Figure S15). Thus, the LSV comparison plots before and after ADT revealed that GZ80C sample was electrochemically more stable compared to the commercial Pt/C catalyst in the alkaline medium. To investigate the long term stability, chronoamperometric analysis of GZ80C and Pt/C was done at 0.6 V in O2 saturated 0.1 M KOH solution for 12 h (Figure S17). It can be readily seen from the figure that the 12 h stability test displays excellent stability for GZ80C sample under the testing condition (0.03 mA cm-2 drop during 12 h). However, the corresponding data for Pt/C is showing a slightly higher degradation especially after 9h (0.06 mA cm-2 drop during 12 h). We also performed the methanol poisoning test for GZ80C and Pt/C by chronoamperometric method in O2 saturated 0.1M KOH solution with 1600 rpm of the working electrode for 2000 s and adding 3M methanol at 300 s during the analysis (Figure S18). The addition of 3M methanol at 300 s during chronoamperometry in Pt/C resulted in the reduction of the oxygen reduction current, indicating the oxidation of methanol over Pt surface, leading to the poisoning of the surface. However, the ORR current in the case of GZ80C is found to be intact subsequent to the addition of 3M methanol in the system, which unravels the exceptionally good methanol tolerance of the system.



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The increase in onset potential and limiting current observed in GZxC samples are ascribed mainly to the presence of more active sites. Owing to the electronegativity difference between nitrogen and carbon, nitrogen doping in the carbon matrix enhances the ORR because of the changes in charge density and spin density arising out of the asymmetrical charge distribution.50,51 Thus oxygen adsorption gets facilitated as the first step of the ORR. Moreover, the type of nitrogen in the carbon matrix has an effect on the catalytic activity as has been observed in the case of GZ80C where 56% of the doped nitrogen is graphitic while 33% is pyrridinic. It has been reported that a proper combination of pyrridinic and graphitic type nitrogen could enhance the onset potential and limiting current density. However, the compositions containing lower amounts of ZIF-8 are characterised by low onset potential and limiting current, even though the N/C ratio was higher in those samples. It can therefore be deduced that, the nitrogen doping is not the only factor that determines the high catalytic activity. Evidently, high N/C ratio in combination with large surface area and a micro-meso porous architecture, as in GZ80C sample, helped to realise the very high ORR activity comparable to platinum catalysts. The mesopores mostly in the 2-5 nm size range provided the perfect platform facilitating uninhibited oxygen transport without having any diffusion resistance encountered normally with micropores. At the same time, the pores are small enough to have frequent gas-pore wall interactions allowing the efficient utilisation of nitrogen doped active centres built into them. The accessibility of oxygen at the catalyst surface is further enhanced by the hydrophilic character induced by the presence of the C–N bonds together with N–H functional groups in the carbonised sample.35 However, this hypothesis requires further investigations as a recent study indicated that hydrophobicity is a factor influencing the catalytic activity of the samples. 52 In short, the unique pore structural properties, favourable surface chemistry as well as excellent catalytic activity due to nitrogen


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doping allowed the carbonised ZIF-8/GO sample (GZ80C) to display ORR performance at par with the commercial Pt/C fuel cell catalysts.

4. CONCLUSIONS In summary, we have developed graphene oxide sheets wrapped ZIF-8 micro crystals as precursors for high surface area nitrogen doped carbon catalysts through a simple in-situ synthetic approach. The catalysts derived from the composition of GZ80C exhibited an onset potential of 0.88V along with negligible penalty in E1/2 after 5000 cycles. The characteristic pore structure of the sample having both micro and mesopores aided the catalytic activity. The micropores served as active centres and mesopores acted as channels for oxygen transport increasing the diffusion current. The presence of nitrogen, both graphitic and pyridinic types, contributed to the increased oxygen reduction reaction. The present study thus provides synthetic pathways for the development of porous carbons through precursor engineering.

■AUTHOR INFORMATION Corresponding Author * Email: [email protected] Fax: +0091 471 2491712; Tel: +0091 471 2535504

* Email: [email protected] Fax: +81-561-34-4997 Tel: +81-561-34-6215

■ACKNOWLEDGMENTS Authors acknowledge Council of Scientific and Industrial Research (CSIR), Government of India and Noritake Company Ltd. Japan for the joint collaborative project (CLP 218739). Mr. Kiran Mohan and Mr. Prithviraj are gratefully acknowledged for the TEM and XRD analysis. Dr. K.K Maiti and Mrs.Soumya are acknowledged for Raman spectroscopy and SEM 23 ACS Paragon Plus Environment


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analysis respectively. Minju Thomas acknowledges University Grant Commission (UGC), Government of India for research fellowship.

■ ASSOCIATED CONTENT Supporting Information TEM, SEM, FT-IR, N2 adsorption analysis, TGA, and XPS analysis, Accelerated Durability Test of Pt/C, methanol tolerance and chronoamperometry and LSV plot of some of the samples are included in the file.

REFERENCES (1) Dunn, B.; Kamath, H.; Tarascon, J.M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928-935. (2) Nie, Y.; Li, L.; Wei, Z. Recent Advancements in Pt and Pt-free Catalysts for Oxygen Reduction Reaction. Chem. Soc. Rev. 2015, 44, 2168-2201. (3) Ge, X.; Sumboja, A.; Wuu, D.; An, T.; Li, B.; Goh, F. W. T.; Hor, T. S. A.; Zong, Y.; Liu, Z. Oxygen Reduction in Alkaline Media: From Mechanisms to Recent Advances of Catalysts.

ACS Catal. 2015, 5, 4643-4667. (4) Balgis, R.; Arif, A. F.; Mori, T.; Ogi, T.; Okuyama, K.; Anilkumar, G. M. MorphologyDependent Electrocatalytic Activity of Nanostructured Pt/C Particles from Hybrid Aerosol– Colloid Process. AIChE Journal 2016, 62, 440-450. (5) Illathvalappil, R.; Unni, S. M.; Kurungot, S. Layer-Separated MoS2 Bearing Reduced Graphene Oxide Formed by an Insitu Intercalation-cum-Anchoring Route Mediated by Co(OH)2 as a Pt-Free Electrocatalyst for Oxygen reduction. Nanoscale 2015, 7, 1672916736. (6) Steele, B. C. H.; Heinzel, A. Materials for Fuel-cell Technologies. Nature 2001, 414, 345352. (7) Zhou, L. H.; Fu, P.; Wen, D. H.; Yuan, Y.; Zhou, S. G. Self-Constructed Carbon Nanoparticles-Coated Porous Biocarbon from Plant Moss as Advanced Oxygen Reduction Catalysts. Appl.Catal., B 2016, 181, 635-643. 24 ACS Paragon Plus Environment

Page 24 of 29

Page 25 of 29

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


(8) Liang, J.; Jiao, Y.; Jaroniec, M.; Qiao, S. Z. Sulfur and Nitrogen Dual-Doped Mesoporous Graphene Electrocatalyst for Oxygen Reduction with Synergistically Enhanced Performance.

Angew. Chem., Int. Ed. 2012, 51, 11496-500. (9) Sheng, Z. H.; Shao, L.; Chen, J. J.; Bao, W. J.; Wang, F. B.; Xia, X. H. Catalyst-Free Synthesis of Nitrogen-Doped Graphene via Thermal Annealing Graphite Oxide with Melamine and its Excellent Electrocatalysis. Acs Nano 2011, 5, 4350-4358. (10) Debe, M. K. Electrocatalyst Approaches and Challenges for Automotive Fuel Cells.

Nature 2012, 486, 43-51. (11) Zhou, R.; Qiao, S. Z. An Fe/N Co-Doped Graphitic Carbon Bulb for High-Performance Oxygen Reduction Reaction. Chem. Commun. 2015, 51, 7516-7519. (12) Unni, S. M.; Illathvalappil, R.; Bhange, S. N.; Puthenpediakkal, H.; Kurungot, S. Carbon Nanohorn-Derived Graphene Nanotubes as a Platinum-Free Fuel Cell Cathode. Acs Appl.

Mater. Interfaces 2015, 7, 24256-24264. (13) Palaniselvam, T.; Kashyap, V.; Bhange, S. N.; Baek, J.-B.; Kurungot, S. Nanoporous Graphene Enriched with Fe/Co-N Active Sites as a Promising Oxygen Reduction Electrocatalyst for Anion Exchange Membrane Fuel Cells. Adv. Funct. Mater. 2016, 26, 2150-2162. (14) Qiu, K.; Chai, G.; Jiang, C.; Ling, M.; Tang, J.; Guo, Z. Highly Efficient Oxygen Reduction Catalysts by Rational Synthesis of Nanoconfined Maghemite in a Nitrogen-Doped Graphene Framework. Acs Catal. 2016, 6, 3558-3568. (15) Duan, J.; Chen, S.; Jaroniec, M.; Qiao, S. Z. Heteroatom-Doped Graphene-Based Materials for Energy-Relevant Electrocatalytic Processes. ACS Catal. 2015, 5, 5207-5234. (16) Qu, K.; Zheng, Y.; Dai, S.; Qiao, S. Z. Polydopamine-Graphene Oxide Derived Mesoporous Carbon Nanosheets for Enhanced Oxygen Reduction. Nanoscale 2015, 7, 12598-12605. (17) Zheng, Y.; Jiao, Y.; Chen, J.; Liu, J.; Liang, J.; Du, A.; Zhang, W.; Zhu, Z.; Smith, S. C.; Jaroniec, M.; Lu, G. Q.; Qiao, S. Z. Nanoporous Graphitic-C3N4@Carbon Metal-Free Electrocatalysts for Highly Efficient Oxygen Reduction. J. Am. Chem. Soc. 2011, 133, 2011620119. (18) Dai, L.; Xue, Y.; Qu, L.; Choi, H.J.; Baek, J.B. Metal-Free Catalysts for Oxygen Reduction Reaction. Chem. Rev. 2015, 115, 4823-4892. (19) Zhan, Y. F.; Huang, J. L.; Lin, Z. P.; Yu, X.; Zeng, D. R.; Zhang, X. X.; Xie, F. Y.; Zhang, W. H.; Chen, J.; Meng, H. Iodine/Nitrogen Co-Doped Graphene as Metal Free Catalyst for Oxygen Reduction Reaction. Carbon 2015, 95, 930-939. 25 ACS Paragon Plus Environment


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

(20) Balamurugan, J.; Thanh, T. D.; Kim, N. H.; Lee, J. H. Nitrogen-Doped Graphene Nanosheets with FeN Core-Shell Nanoparticles as High-Performance Counter Electrode Materials for Dye-Sensitized Solar Cells. Adv. Mater. Interfaces 2016, 3,1500348. (21) Geng, D.; Yang, S.; Zhang, Y.; Yang, J.; Liu, J.; Li, R.; Sham, T.K.; Sun, X.; Ye, S.; Knights, S. Nitrogen Doping Effects on the Structure of Graphene. Appl. Surf. Sci. 2011, 257, 9193-9198. (22) Zhang, J.; Dai, L. Heteroatom-Doped Graphitic Carbon Catalysts for Efficient Electrocatalysis of Oxygen Reduction Reaction. ACS Catal. 2015, 5, 7244-7253. (23) Reddy, A. L. M.; Srivastava, A.; Gowda, S. R.; Gullapalli, H.; Dubey, M.; Ajayan, P. M. Synthesis of Nitrogen-Doped Graphene Films for Lithium Battery Application. ACS Nano 2010, 4, 6337-6342. (24) Guo, D.; Shibuya, R.; Akiba, C.; Saji, S.; Kondo, T.; Nakamura, J. Active Sites of Nitrogen-doped Carbon Materials for Oxygen Reduction Reaction Clarified Using Model Catalysts. Science 2016, 351, 361-365. (25) Chen, S.; Bi, J.; Zhao, Y.; Yang, L.; Zhang, C.; Ma, Y.; Wu, Q.; Wang, X.; Hu, Z. Nitrogen-Doped Carbon Nanocages as Efficient Metal-Free Electrocatalysts for Oxygen Reduction Reaction. Adv. Mater. 2012, 24, 5593-5597. (26) Chen, Z.; Higgins, D.; Tao, H.; Hsu, R. S.; Chen, Z. Highly Active Nitrogen-Doped Carbon Nanotubes for Oxygen Reduction Reaction in Fuel Cell Applications. J. Phys. Chem.

C 2009, 113, 21008-21013. (27) Zhang, H.; Zhou, Y.; Li, C. G.; Chen, S. L.; Liu, L.; Liu, S. W.; Yao, H. M.; Hou, H. Q. Porous Nitrogen Doped Carbon Foam with Excellent Resilience for Self-Supported Oxygen Reduction Catalyst. Carbon 2015, 95, 388-395. (28) Liu, X.; Li, L.; Zhou, W.; Zhou, Y.; Niu, W.; Chen, S. High-Performance Electrocatalysts for Oxygen Reduction Based on Nitrogen-Doped Porous Carbon from Hydrothermal Treatment of Glucose and Dicyandiamide. Chemelectrochem 2015, 2, 803-810. (29) Shen, W.; Fan, W. Nitrogen-Containing Porous Carbons: Synthesis and Application. J.

Mater.Chem.A 2013, 1, 999-1013. (30) Xing, T.; Zheng, Y.; Li, L. H.; Cowie, B. C. C.; Gunzelmann, D.; Qiao, S. Z.; Huang, S.; Chen, Y. Observation of Active Sites for Oxygen Reduction Reaction on Nitrogen-Doped Multilayer Graphene. ACS Nano. 2014, 8, 6856-6862. (31) Shao, Y.; Zhang, S.; Engelhard, M. H.; Li, G.; Shao, G.; Wang, Y.; Liu, J.; Aksay, I. A.; Lin, Y. Nitrogen-Doped Graphene and its Electrochemical Applications. J.Mater. Chem. 2010, 20, 7491-7496. 26 ACS Paragon Plus Environment

Page 26 of 29

Page 27 of 29

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


(32) Pachfule, P.; Dhavale, V. M.; Kandambeth, S.; Kurungot, S.; Banerjee, R. PorousOrganic-Framework-Templated Nitrogen-Rich Porous Carbon as a More Proficient Electrocatalyst than Pt/C for the Electrochemical Reduction of Oxygen. Chem.Eur. J. 2013,

19, 974-980. (33) Palaniselvam, T.; Biswal, B. P.; Banerjee, R.; Kurungot, S. Zeolitic Imidazolate Framework (ZIF)-Derived, Hollow-Core, Nitrogen-Doped Carbon Nanostructures for Oxygen-Reduction Reactions in PEFCs. Chem.Eur. J. 2013, 19, 9335-9342. (34) Chaikittisilp, W.; Ariga, K.; Yamauchi, Y. A New Family of Carbon Materials: Synthesis of MOF-Derived Nanoporous Carbons and their Promising Applications. J.Mater. Chem. A 2013, 1, 14-19. (35) Zhang, L.; Su, Z.; Jiang, F.; Yang, L.; Qian, J.; Zhou, Y.; Li, W.; Hong, M. Highly Graphitized Nitrogen-Doped

Porous

Carbon

Nanopolyhedra Derived

from ZIF-8

Nanocrystals as Efficient Electrocatalysts for Oxygen Reduction Reactions. Nanoscale 2014,

6, 6590-602. (36) Zhong, X.; Wang, J.; Zhang, Y.; Xu, W.; Xing, W.; Xu, D.; Zhang, Y.; Zhang, X. ZIF-8 Derived Graphene-Based Nitrogen-Doped Porous Carbon Sheets as Highly Efficient and Durable Oxygen Reduction Electrocatalysts. Angew. Chem., Int. Ed. 2014, 53, 14235-14239. (37) Shui, J.; Chen, C.; Grabstanowicz, L.; Zhao, D.; Liu, D.J. Highly Efficient Nonprecious Metal Catalyst Prepared with Metal–Organic Framework in a Continuous Carbon Nanofibrous Network. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 10629-10634. (38) Unni, S. M.; Illathvalappil, R.; Gangadharan, P. K.; Bhange, S. N.; Kurungot, S. LayerSeparated Distribution of Nitrogen Doped Graphene by Wrapping on Carbon Nitride Tetrapods for Enhanced Oxygen Reduction Reactions in Acidic Medium. Chem.Commun. 2014, 50, 13769-13772. (39) Rowley Neale, S. J.; Fearn, J. M.; Brownson, D. A. C.; Smith, G. C.; Ji, X.; Banks, C. E. 2D Molybdenum Disulphide (2D-MoS2) Modified Electrodes Explored Towards the Oxygen Reduction Reaction. Nanoscale 2016, 8, 14767-14777. (40) Tang, L.; Chang, H.; Liu, Y.; Li, J. Duplex DNA/Graphene Oxide Biointerface: From Fundamental Understanding to Specific Enzymatic Effects. Adv. Funct. Mater. 2012, 22, 3083-3088. (41) Kida, K.; Okita, M.; Fujita, K.; Tanaka, S.; Miyake, Y. Formation of High Crystalline ZIF-8 in an Aqueous Solution. Crys.tEng.Comm. 2013, 15, 1794-1801. (42) Petit, C.; Bandosz, T. J. MOF–Graphite Oxide Composites: Combining the Uniqueness of Graphene Layers and Metal–Organic Frameworks. Adv. Mater. 2009, 21, 4753-4757. 27 ACS Paragon Plus Environment


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

(43) Aiyappa, H. B.; Pachfule, P.; Banerjee, R.; Kurungot, S. Porous Carbons from Nonporous MOFs: Influence of Ligand Characteristics on Intrinsic Properties of End Carbon.

Cryst.Growth.Des. 2013, 13, 4195-4199. (44) Rattana; Chaiyakun, S.; Witit-anun, N.; Nuntawong, N.; Chindaudom, P.; Oaew, S.; Kedkeaw, C.; Limsuwan, P. Preparation and Characterization of Graphene Oxide Nanosheets.

Procedia Eng. 2012, 32, 759-764. (45) Tran, U. P. N.; Le, K. K. A.; Phan, N. T. S. Expanding Applications of Metal−Organic Frameworks: Zeolite Imidazolate Framework ZIF-8 as an Efficient Heterogeneous Catalyst for the Knoevenagel Reaction. ACS Catal. 2011, 1, 120-127. (46) Unni, S. M.; Devulapally, S.; Karjule, N.; Kurungot, S. Graphene Enriched with Pyrrolic Coordination of the Doped Nitrogen as an Efficient Metal-Free Electrocatalyst for Oxygen Reduction. J. Mater. Chem. 2012, 22, 23506-23513. (47) Lv, R.; Li, Q.; Botello Méndez, A. R.; Hayashi, T.; Wang, B.; Berkdemir, A.; Hao, Q.; Elías, A. L.; Cruz-Silva, R.; Gutiérrez, H. R.; Kim, Y. A.; Muramatsu, H.; Zhu, J.; Endo, M.; Terrones, H.; Charlier, J.C.; Pan, M.; Terrones, M. Nitrogen-Doped Graphene: Beyond Single Substitution and Enhanced Molecular Sensing. Sci. Rep. 2012, 2, 586. (48) Li, R.; Wei, Z.; Gou, X. Nitrogen and Phosphorus Dual-Doped Graphene/Carbon Nanosheets as Bifunctional Electrocatalysts for Oxygen Reduction and Evolution. ACS Catal. 2015, 5, 4133-4142. (49) Silva, R.; Voiry, D.; Chhowalla, M.; Asefa, T. Efficient Metal-Free Electrocatalysts for Oxygen Reduction: Polyaniline-Derived N- and O-Doped Mesoporous Carbons. J.Am. Chem.

Soc. 2013, 135, 7823-7826. (50) Zhang, L.; Xia, Z. Mechanisms of Oxygen Reduction Reaction on Nitrogen-Doped Graphene for Fuel Cells. J. Phys. Chem. C 2011, 115, 11170-11176. (51) dos Santos, R. B.; Rivelino, R.; Mota, F. d. B.; Gueorguiev, G. K. Effects of N Doping on the Electronic Properties of a Small Carbon Atomic Chain with Distinct sp2 Terminations: A First-Principles Study. Phys. Rev. B. 2011, 84, 075417. (52) Seredych, M.; Szczurek, A.; Fierro, V.; Celzard, A.; Bandosz, T. J. Electrochemical Reduction of Oxygen on Hydrophobic Ultramicroporous PolyHIPE Carbon. ACS Catal. 2016, 6, 5618-5628.


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