An Ultrafast-Versatile-Domestic-Microwave-Oven Based Graphene

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An Ultrafast-Versatile-Domestic-Microwave-Oven Based Graphene Oxide Reactor for the Synthesis of Highly Efficient Graphene Based Hybrid Electrocatalysts Barun Kumar Barman, and Karuna Kar Nanda ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04398 • Publication Date (Web): 14 Jan 2018 Downloaded from http://pubs.acs.org on January 15, 2018

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ACS Sustainable Chemistry & Engineering

An

Ultrafast-Versatile-Domestic-Microwave-Oven

Based Graphene Oxide Reactor for the Synthesis of Highly

Efficient

Graphene

Based

Hybrid

Electrocatalysts Barun Kumar Barman and Karuna Kar Nanda* Materials Research Centre, Indian Institute of Science, Bangalore-560012, India E-mail:[email protected] KEYWORDS: Graphene oxide (GO) Reactor, Hybrid electro-catalyst, Oxygen evolution reaction (OER), Oxygen reduction reaction (ORR)

ABSTRACT: When solid graphene oxide (GO) is treated with microwave, it generates huge heat followed by reduction and exfoliation. This can be used as a high temperature REACTOR for ultrafast and in-situ synthesis of reduced graphene oxide (rGO) based hybrids within 60 s in open atmosphere. rGO based hybrids such as Fe3C-G@rGO, Co-Fe3C-G@rGO, Fe-Fe3C-NG@rGO, CoO@rGO and Pt@rGO (G represents graphene and NG represents N-doped graphene) have been synthesized by simply mixing appropriate precursors with GO and treating with microwave. The experiments neither require any external high temperature reactors/furnaces or

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any chemical reagents or solvents. Then, rGO based hybrids have been exploited for oxygen evolution reaction (OER), oxygen reduction reaction (ORR) and methanol oxidation activity. CoO@rGO and Co-Fe3C-G@rGO show outstanding OER performances with very low over potential () and a current density of 10 mA/cm2 at 1.52 and 1.56 V with long term stability. FeFe3C-NG@rGO hybrid shows better oxygen reduction performances and the onset potential is comparable with precious Pt/C catalyst. The Pt@rGO is highly stable towards methanol oxidation as compared to the Pt/C catalyst. The high catalytic activity and stability are believed to be due to the better adherence of different inorganic nanostructures onto rGO. We strongly believe that this methodology would pave the way for a new era of synthesis of rGO based various hybrids for various applications.

INTRODUCTION Graphene with unique electronic, mechanical, optical, thermal and chemical properties has the great potential in electronics, photonics, optoelectronics, energy, environmental and biomedical applications.1-14 Parallelly, graphene hybrids have emerged as a new class of exciting materials that hold promise for many applications where graphene serves as the support of various inorganic nanostructures that are known for specific applications.15-21 In this context, it is worthy to note that graphene based hybrids have attracted considerable attention for various applications and one such application is in electrocalysis and the activity is far superior as compared to the bare inorganic counterpart.22-27 Recent days, the fuel cells and metal air batteries are extremely promising electrochemical devices for energy storage and have potential applications in the field of electronics, hybrid electric vehicles, and stationary power generation, etc. The performance of such devices is mainly limited by the sluggish kinetics of the fuel


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oxidation, oxygen reduction and evolution reactions that requires electrocatalysts for the reactions to take place at a practical rate. The cost and durability of the state-of-the-art catalysts based on Pt or its alloys for fuel oxidation and oxygen reduction reaction (ORR) and RuO2/IrO2 for oxygen evolution reaction (OER) hinders the commercialization of the devices. It is of paramount importance to develop non-precious highly active, durable and efficient catalysts for these reactions.28-33 For the large scale production of graphene, graphene oxide (GO) is one of the promising precursor as it can be produced easily in large scale and reduction of GO can yield reduced graphene oxide (rGO) that is equivalent to graphene.34,

35

There are several methods

such as thermal, chemical and electrochemical reduction to obtain rGO and there are various methods in the literature for the synthesis of rGO based hybrids: (i) in-situ electroless deposition method where metal, metal oxide and metal chalcogenide are deposited on graphene surface in presence of reducing/chemical agent,36-38 (ii) hydrothermal or solvothermal

method where

metal, metal oxide and chalcogenide are synthesized by reducing both GO and precursors in these condition,39,

40

(iii) electrochemical and electrophoresis deposition where metal or metal

oxide can be deposited on graphene by applying voltage or current,41-43 and (iv) photochemical reaction whereas metal oxide or metal can be directly grown on graphene by external light source.44, 45 All the above method requires some external chemical reagent or light source for synthesis of graphene based hybrids. Above all, additional chemicals need to be removed by washing several times that requires additional effort as well as time. It has been shown recently that GO generates enormous amount of heat when treated with microwave, reduces by liberating the oxygenated functional groups and exfoliates by itself as graphene layers.46 Here, we have established an ultrafast (within 60 s), unique and universal method for the synthesis of rGO based hybrids in solid state, examine the catalytic activity of


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these hybrids and compare with the benchmark catalyst. Overall, GO acts as a versatile reactor for the fabrication of various graphene based hybrids such as carbide, oxides and bare metallic nanoparticles such as Fe3C-G@rGO, Co doped Fe3C-G@rGO (Co-Fe3C-G@rGO), Fe-Fe3CNG@rGO, CoO@rGO and Pt@rGO (G represents graphene and NG represents N-doped graphene) by simply mixing appropriate precursors with GO and treating them with microwave. The experiments neither require any external high temperature reactors/furnaces or any chemical reagents or solvents. As-synthesized rGO based hybrids have been examined mainly for oxygen evolution reaction (OER), oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR).

EXPERIMENTAL SECTION METHODs: GO is first synthesized by modified Hummer’s method using graphite flakes and then sonicated in 20 ml of de-ionized water for 1 h before each of the experiment. The precursor solution is then added with 10 ml ethanol solution and sonicated for another 10 minutes in 100 ml beaker. We keep the sonicated solution under the lamp overnight for complete drying. Then the mixture of GO was treated in domestic microwave for 60 s with the 1450 W of the microwave power in open atmosphere. The precursors used for the synthesis of a variety of hybrid nanostructures are provided in Table 1.


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Table 1. Synthesis of various kind of rGO based hybrid from the mixture of GO and the corresponding precursor. GO (mg)

Precursor (mg)

Hybrid

75 mg

0

rGO

75 mg

300 mg ferrocene

Fe3C/G@rGO

75 mg

250 mg ferrocene +

Co-Fe3C/G@rGO

50 mg cobaltocene 75 mg

300 mg Prussian blue

Fe-Fe3C/NG@rGO

75 mg

300 mg Cobalt acetylacetonate

CoO-rGO

30 mg

60 mg Platinum acetylacetonate

Pt@rGO

CHERECTERIZATIONS: All the hybrids are characterized by X-ray diffraction (XRD) with PAN analytical instrument using Cu K (= 1.54 Å) radiation source. The morphology of the hybrids is characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). The SEM images and Energy-dispersive X-ray spectroscopy (EDS) are taken on a FEI-INSPECTF50 instrument. TEM images, high resolution TEM (HRTEM) images and selected area electron diffraction (SAED) pattern are acquired using JEOL- JEM-2100F. All the TEM samples are prepared by dispersing the sample in ethanol solution using ultrasonic bath for 15 min., dropcasted on carbon coated copper grid, and then dried at 60 0C. X-ray photoelectron spectroscopy (XPS) is performed for the elemental analysis on an ESCALAB 250 (Thermo Electron) with a


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monochromatic Al K (1486.6 eV) source. The binding energy of all the elements is calibrated by placing the principal C1s peak at 284.6 eV.

ELECTROCHEMICAL MEASURMENTS: Cyclic voltammetry (CV) studies are performed with glassy carbon rotating disk electrode (RDE) using CH Instruments electrochemical work station. A conventional three-electrode cell is used where catalyst film coated RDE is the working electrode, Ag/AgCl is the reference electrode, and Pt wire is the counter electrode. For ORR performances, the hybrid catalyst ink is prepared by dispersing 5 mg catalyst in 0.9 ml of ethanol + 0.1 ml of 5 % of Nafion solution ultrasonically. The loadings on RDE is 0.57 mg/cm2 for hybrids and 0.10 mg/cm2 of 20 % (20g 2 (Pt)/cm )

for the Pt/C catalyst. The ORR performances are performed via CV measurements in

N2- and O2-saturated 0.1 M KOH solution. The liner sweep voltammetry ((LSV) measurements are carried out at different rotation speeds between 600 and 2200 rpm in the O2-saturated 0.1 M KOH aqueous solution to evaluate the kinetic performances of ORR catalytic activity. The stability is evaluated by LSV up-to 1000nd cycles with a scan rate of 50 mV/s and a rotation speed of 600 rpm. The RDE data are analyzed using Koutecky-Levich (K-L) plots (J-1 vs. -1/2) at different applied potentials. The slopes of the linear fit are used to calculate the electron transfer (ET) number (n) based on the K-L equation: 1/J = 1/JL + 1/Jk= 1/B1/2 + 1/JK; B= 0.62 n F C0 (D0)2/3ν-1/6, where J= measured current density, Jk = Kinetic current, electrode rotation rate, F= Faraday constant (96485 C/mol), C0 = saturated concentration of O2 in 0.1 M KOH solution at room temperature (1.2 × 10-6mol/cm3), D0 = Diffusion coefficient of O2 in KOH (1.9 × 10-5 cm2/s), = kinetic viscosity of electrolyte (0.01 cm2/s). According to the KL plot, the slope (1/B) can be used to calculate n from n = B/0.62FC0 (D0)2/3ν-1/6. The OER performances are


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recorded in N2 saturated 0.1 and 1 M KOH solution via LSV measurments with a scan rate of 10 mV/s and a rotation speed of 600 rpm from 0 to 0.8 V vs. AgCl/Ag as referance electrode where as catalyst loading is 0.22 mg/cm2. The stability test was performed by using LSV measurements at a scan rate of 100 mV/s in 1 M KOH solution. The MOR is performed in N2-saturated 1 M KOH solution mixed with 1 M MeOH solution with a scan rate 50 mV/s followed by stability upto 500th cycle (scan rate 100 mV/s) with the catalyst loading of 0.257 mg/cm2. All the experiments are repeated at least three times to check the reproducibility of the electrocatalytic performances.

RESULTS AND DISCUSSION Figure 1 illustrates the microwave-assisted synthesis of rGO based hybrids whereas GO acts as a reactor. By exploiting this very simple idea and the appropriate precursors, we have synthesized various hybrids. When we take the mixture (I) of GO and Prussian blue, the final product is Fe-Fe3C-NG@rGO. The N doping on the graphitic shell is due to the presence of cyano (CN) group in the Prussian blue. The product is Fe3C-G@rGO when GO and ferrocene mixture (II) is treated. When the mixture (III) of cobaltocene, ferrocene and GO is treated, CoFe3C-C@rGO is obtained. Similarly, mixture (IV and V) of cobalt acetylacetonate (Co(acac)2) and platinum acetylacetonate (Pt(acac)2) yield CoO@rGO and Pt@rGO, respectively.


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Figure 1. Illustration of the formation of rGO based hybrids by microwave treatment:(I) Iron hexacyanoferrate (Prussian blue) for Fe-Fe3C-NG@rGO, (II) ferrocene for Fe3C-G@rGO, (III) 5:1 ratio of ferrocene and cobaltocene for Co-Fe3C-G@rGO, (IV) Cobalt acetylacetonate for CoO@rGO, and (V) Platinum acetylacetonate for Pt@rGO. The nomenclature of the hybrid is depicted in the inset. where X: Core, Y: coated graphitic layer and X: rGO substrate.


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Figure 2 (a, b) show the SEM and TEM images of rGO consisting of curled, randomly aggregated, crumpled solid sheets. The inset of Figure 2 (b) shows the 7-12 graphitic layers with the interlayer distance of 0.336 nm. Figure 2 (c) shows the TEM image of the hybrids synthesized from the mixture of ferrocence and GO. HRTEM image of the hybrids as shown in Figure 2. (d) clearly reveals the core-shell type (Fe3C-G) structure consisting of Fe3C core with the d-spacing 0.201 nm corresponding to (031) crystal plane encapsulated by the 15-22 graphitic layers with a spacing of 0.336 nm. The size of core-shell structure is in the range of diameter 4060 nm.

(a)

(b)

(c)

(e)

(f)

(g)

(h)

(i)

(j)

(k)

(l)

(d)

Figure 2. (a and b) SEM and TEM images of rGO. Inset of (b) shows the 7-12 graphitic layers. (c and d) TEM and HRTEM images of Fe3C-G@rGO. (e and f) TEM and HRTEM images of Co-Fe3C-G@rGO. (g and h) TEM and HRTEM images of Fe-Fe3C-NG@rGO. (i and j) TEM and HRTEM images of CoO@rGO, (k and l) TEM and HRTEM images of Pt@rGO


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Figure 2 (e and f) show the typical TEM and HRTEM images of Co-Fe3C-G@rGO indicating core-shell nanostructures on rGO surface. The average diameter of the Co-Fe3C is in the range 7-10 nm in the core which is covered by the 18-24 graphitic layers and the core-shell nanostructures were uniformly coated on the graphene surface (Figure S1). The d-spacing of the Fe3C remains unchanged even with Co doping. When the experiment was performed with the mixture of Prussian blue and GO, the product is Fe-Fe3C-NG@rGO and the size of the core is in the range of 30 to 50 nm as shown in Figure 2 (g). HRTEM image as shown in Figure 2 (h) reveals that the d spacing of 0.203 nm corresponding to  phase of Fe which is covered by the few graphene layers. The Fe-Fe3C-NG core-shell NPs are uniformly anchored on the graphene surface (Figure S2). Similarly, the ultra-small CoO@rGO is obtained when the mixture of GO and cobalt acetylacetonate (Co(acac)2), is treated with microwave. On the other hand, Pt@rGO is obtained from the mixture of Platinum acetylacetonate (Pt(acac)2) and GO. Figure 2 (i and j) display the TEM and HRTEM images of as-synthesized CoO@rGO. The CoO nanoparticles (NPs) are in the size range of 3-6 nm with a d-spacing of 0.213 nm that corresponds to (200) plane of CoO. Figure 2 (k and l) display the TEM and HRTEM images of as-synthesized Pt@rGO. The size of Pt NPs is in the range of 4-10 nm with a d-spacing of 0.223 nm that corresponds to (111) crystal plane of Pt. The low magnified TEM images also reveal that the CoO and Pt NPs are homogeneously decorated on the graphene surface along with few bigger NPs (Figure S3).


10

(b)

C-O C=O

292

294 292 290 288 286 284 282 280

290

Binding Energy (eV)

288

286

284

282

Binding Energy (eV)

286

284

282

C=O

292

290

288

286

284

Intensity (a.u.)

C-C

288

4f Pt-O

76

74

72

284

(i)

4f7/2

78

286

282

Binding Energy (eV)

4f5/2

80

282

282

C-O

290

282

C=O

284

284

C=O

Binding Energy (eV)

C-O

286

(f)

(h)

286

288

Binding Energy (eV)

C-C

C-C

288

290

C-O

(g)

290

C-O

Intensity (a.u.)

C-O C=O

290

288

(e)

C-C

C-C

C=O

Binding Energy (eV)

Intensity (a.u.)

Intensity (a.u.)

(d)

(c)

C-C

Intensity (a.u.)

Intensity (a.u.)

Experimental Fitted C-C C-O C=O

Intensity (a.u.)

(a)

Intensity (a.u.)

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


Intensity (a.u.)

Page 11 of 31

70

68

Binding Energy (eV)

Binding Energy (eV)

2p3/2

Co-2p Satellite peak

2p1/2 Satellite peak

810 805 800 795 790 785 780 775

Binding Energy (eV)

Figure 3. XPS C-1s spectra of (a) GO, (b) rGO, (C) Fe3C-G@rGO, (d) Co-Fe3C-G@rGO, (e) Fe-Fe3C-NG@rGO, (f) Pt@rGO, (g) CoO@rGO. (h) XPS Pt 4f spectrum of Pt@rGO, and (i) XPS Co 2p spectrum of CoO@rGO. The as-synthesized hybrids and the reduction of GO have been examined by XPS spectroscopy. Figure 3 (a-g) show the high resolution XPS (HRXPS) of C 1s spectra of GO, rGO and all the hybrids along with Pt 4f and Co 2p spectra (Figure 3 (h and i)). Figure 3 (a) shows the XPS C1s spectrum of GO deconvoluted into three major peaks which can be assigned to C-C (sp2 bonding in the G), C-O (epoxy, alcohol, ester and acid groups) and C=O (carbonyl


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groups) bonds, respectively.47 The GO contains ~30, ~51 and ~19 % of C-C , C-O and C=O bonds, while rGO obtained by microwave treatment, the % of C-C bond increases to 88.1%, whereas the C-O and C=O bonds reduce to 9.1 % and 2.8 % (Figure 3. (b)), respectively due to the release of oxygen containing functional groups from the GO and restoring the sp2 carbon centres of graphene. Similarly, de-convoluted C 1s spectra of other hybrids also suggest almost the same degree of reduction as summarized in Table S1. The XPS characteristic peaks of Fe in Fe3C@rGO/ Fe-Fe3C@rGO and Co doping in Fe3C are very weak due to presence of graphitic or few layer of graphene over Fe or Fe3C (Figure S4(a)). However, the presence of Co is confirmed by the EDS measurements (Figure S4 (b and c)). XPS survey spectrum of Fe-Fe3C-NG@rGO also reveals the presence of N and Fe (Figure S5 (a)). The deconvoluted XPS spectra of N 1s spectrum ((Figure S5 (b))) clearly reveals three different types of N: pyridinic (398.4 eV), pyrrolic (399.4 eV) and graphitic (401.1 eV) in hexagonal carbon layer.48 The XPS Fe-2p and EDS spectra in Figure S5 (c and d) reveal the presence of Fe (~ 29 wt %). The Pt 4f5/2 peak appears at a binding energy of 74.4 eV and the Pt 4f7/2 peak appears at a binding energy of 71.1 eV with the spin-orbit splitting of 3.3 eV (energy difference between Pt 4f7/2 and 4f3/2) as shown in Figure 3 (h). The Pt 4f XPS spectrum also reveals the formation of slightly surface oxidation Pt (Pt-O: 72 eV ) NPs on the rGO surface believed to be due to the high temperature generation by GO. The Pt loading in the Pt@rGO is ~ 16.76 wt % as evaluated from the EDS spectrum (Figure S6). Figure 3 (i) displays the Co 2p spectrum for CoO@rGO. The presence two distinct peaks at binding energies of 780.6 and 796.2 eV correspond to Co 2p3/2 and 2p1/2 indicating the Co (II) oxidation state.49 The spin-orbit splitting (energy difference between Co 2p3/2 and 2p1/2) of 15.6 eV are observed in the Co 2p core level spectrum. It also contains satellite peaks of Co


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2p3/2 and 2p1/2 at 785.8 eV and 803.1 eV, respectively and the estimated CoO loading is ~ 84 wt % as determined from the TGA curves (Figure S7).

20

40

60

Graphite (002)

&

80

20

20

* 40

* 60

2 (degree)

80

&

40

&#

60

#

80

(111)

Pt@rGO (200)

Intensity (a.u.)

(200) (111)

Graphite (002)

&

2 (degree)

(e) CoO@rGO

#=Fe &= Fe3C Fe-Fe3C-NG@rGO

2(degree)

(d) Intensity (a.u.)

50

#

Graphite (002)

20

40

60

(311) (222)

40

(311) (222)

30

2 (degree)

(002)

(220)

20

Graphite

Intensity (a.u.)

Intensity (a.u.)

(002)

(001)

GO rGO

10

(c)

(b)

(220)

(a) Intensity (a.u.)

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


80

2 (degree)

Figure 4. XRD patterns of (a) GO and rGO, (b) Fe3C-G@rGO and Co-Fe3C-G@rGO, (c) FeFe3C-NG@rGO, (d) CoO@rGO (* represent the XRD peaks corresponding to metallic Co) and (e) Pt@rGONG@rGO.

We have also characterized all the hybrids by XRD. Figure 4 (a) displays the XRD pattern of GO that exhibits a strong peak at 2 = 11.6. This corresponds to an interlayer spacing of about 0.76 nm indicating the presence of oxygen functionalities. The peak centered at 2 = 26.6 for rGO indicates the removal of a large number of oxygen-containing groups resulting in the extensive conjugated sp2 carbon network (i.e., the ordered crystal structure). The d-value is estimated to be 0.336 nm which represents the interlayer spacing corresponding to the van deer


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Waals bond distance between two layers of the graphite and is in accordance with the d spacing obtained from HRTEM. Figure 4 (b) shows the XRD patterns of Fe3C-G@rGO and Co-Fe3CG@rGO. The peaks centered at 2values are 35.35, 37.50, 40.08, 42.87, 43.65, 44.5, 44.9, 45.70, 48.78, 51.85, 54.2, 64.9, 77.8, 83.10 that correspond to (200), (121), (201), (211), (102), (220), (031), (112), (131), (122), (040), (321), (401), (332) crystal plane of Fe3C. In this context, it is worthy to note that Co doping does not influence the lattice constant. Figure 4 (c) displays the XRD pattern of [email protected] major peaks centred around 44.74, 65.20 and 82.50 are attributed to (110), (200) and (211) planes of the -phase of the Fe (JCPDS # 870722). The small diffraction peak at 26.4 corresponds to the (002) planes of graphitic carbon while very small peaks centred around 37.75, 43.7, 51.6 and 74.70 correspond to the Fe3C (JCPDS # 851317). Similarly, XRD patterns of the CoO@rGO and Pt@rGO are shown in Figure 4 (d and e), respectively. The major XRD peaks centered at the 2values are 36.56, 42.5, 61.58, 73.77, 77.60 corresponding to the (111), (200), (220), (311), (222) crystal plane of face centered cubic (fcc) CoO NPs (JCSDS # 78-0431) respectively and very small peaks at 44.3 and 51.60 are assigned as * signify the presence of metallic Co (Figure 4 (d)). The XRD peaks centered at the 2values are 39.5, 46.2, 67.3, 81.2 and 85.750 corresponding to the (111), (200), (220), (311), (222) crystal plane of fcc Pt NPs (JCSDS # 87-0642) respectively (Figure 4 (e)). Overall, the graphitic peak is present in all the cases due to rGO.

We evaluate the electrochemical OER (4OH- =2H2O + O2 + 4e-) catalytic activity of all the

non-precious

hybrids

(Fe3C-G@rGO,

Co-Fe3C-G@rGO,

Fe-Fe3C-NG@rGO,

and

CoO@rGO) in alkaline medium and compared with the commercial RuO2. The catalyst loading in all the cases is 0.22 mg/cm2. Figure 5 (a) displays the OER activity of all the hybrids and


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RuO2 in 0.1 M KOH solution. The bare rGO shows very poor OER activity and excellent activity is observed for the hybrids. The onset potential for CoO@rGO, Fe3C-G@rGO, Co-Fe3CG@rGO and RuO2 is 1.39, 1.58, 1.45 and 1.42 V, respectively. It is interesting to note that the OER activity that improves drastically with Co doping (~ 7.7 %). The onset potential reduces from 1.58 to 1.45 V indicating the Co doing in the Fe3C increases the active catalytic sites for OH- adsorption. In the electrochemical reaction, the additional energy required to overcome the reaction which is known as over potential () and the onset potential stems from the extent of the barrier in energy conversion process. Lower the onset potential/over potential, higher is the catalytic activity and lower is the energy consumption in the catalytic process. The over potential (0) of OER catalytic activity is 190, 220, 160 mV for RuO2, Co-Fe3C-G@rGO and CoO@rGO, respectively. We also performed the OER activity of these catalysts in 1 M KOH solution as shown in Figure 5 (b) and realized that the current density is enhanced manifold without any change in the onset potential. A current density of 10 mA/cm2 is achieved at 1.53, 1.56 and 1.60 V for CoO@rGO, Co-Fe3C-G@rGO and RuO2, respectively. Comparison of the OER performances of the all the catalysts is presented in Table 2. CoO@rGO outperforms other hybrids as well as that reported Co based OER catalyst (Table S2). To gain additional understanding into the OER process, Tafel plots are shown in Figure 5 (c) with its linear region fitted with the Tafel equation (=blog J + a, where is over potential, J is the current density, and b is the Tafel slope). Tafel slope is found to be 58, 66 and 82 mV/dec for Fe3C-G@rGO, CoO@rGO and RuO2, respectively. Figure 5 (d) shows the stability polarization curves of the CoO@rGO hybrid indicating a 94 % retention of current density after the 5000nd cycle, respectively and inset plot also confirms the higher stability up-to 8 h at constant potential of 1.53 V, whereas the RuO2 losses 45 % current density (Figure S8). Overall, Co-Fe3C-G@rGO


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and CoO@rGO outperform other hybrids as well as the benchmark RuO2 catalyst. An alkaline electrolyzer (1.0 M KOH) employing carbon paper as the cathode and CoO@rGO (0.75 mg/cm 2) or RuO2 (0.5 mg/cm2) loaded carbon paper as the anode has been fabricated and tested for water oxidation. The onset potential of CoO@rGO hybrid is 1.38 V which is lower compared to the RuO2 and the current density is 2.5 times as shown in Figure 5(e). Furthermore, CoO@rGO exhibits excellent stability upto 2500th cycles and stable upto 6 h at 1.54 V (Figure 5 (f)). The inset of Figure 5 (e) shows the optical photograph of generation of H2 and O2 via water oxidation electrolysis. The Faradaic efficiencies of water oxidation is very close to 100% (Figure S9) indicating a high efficiency with negligible side reactions. Post characterization of CoO@rGO hybrid was performed by the TEM/HRTEM. It may be noted that the size and distribution of CoO do not change even after 2500th LSV of OER performances in 1 M KOH solution (Figure S10 (a and b)).


16

CoO@rGO rGO

1.23 V

10 0 1.0

50 mA/cm2

40 20

10 mA/cm2

0

1.2

1.4

1.6

1.8

1.2

2.0

60

14

90

1.4

1.6

1.8

2.0

Potential (V vs. RHE)


(d)

0

1

2

3

4

5

6

7

8

Time (h)

30

st

1 cycle 2500th cycle 5000nd cycle

15 0 1.2

1.4

1.6


1.8

30 2

10 mA/cm

RuO2

Co

O@

O rG

(6

0.26 -0.4

-0.2

0.0

0.2

0.4

0.6

0.8

(f)

9

30

11 12 13 14

1

2

3

4

5

1.4

1.6

1.8

2.0


1.2

6

Time (h)

15

1st cycle 1000th cycle 2000th cycle

0

0 1.2

0.28

45 CoO@rGO

15

)

ec

d V/ 6m

10

J (mA/cm2)

8

45

J (mA/cm2)

J (mA/cm2)

60

Co

60

(e)

45 10

0.30

GO @r

ec)

LogJ(mA/cm2)

12

75

C-G -Fe 3

/d mV

V/ de c)

CoO/rGO

(58

m

Fe-Fe3C-NG@rGO

20

Co-Fe3C@C/rGO

60

(c) 0.32

O 2 (8 2

J (mA/cm2)

30

J (mA/cm2)

RuO2

80 RuO2

Fe3C-G@rGO Co-Fe3C-G@rGO

0.34

(b)

Ru

(a)

J (mA/cm2)

100

40

J (mA/cm2)

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


Over potential ( vs. RHE)

Page 17 of 31

1.4

1.6

1.8


Figure 5. (a) OER activity of various hybrids and RuO2 in 0.1 M KOH solution with a scan rate of 10 mV/s and a rotation speed of 1200 rpm. (b) OER activity of CoFe-Fe3C-NG@rGO, CoO@rGO and RuO2 in 1 M KOH solution. (d) OER stability of CoO@rGO hybrid in 1 M KOH solution (inset plot shows the chronoamporometric curve upto 8 h). (e) photograph of water oxidation of CoO@rGO hybrid structures in 1 M KOH solution. (f) LSV polarization of water oxidation up-to 2000nd cycle and the inset plot display the chronoamporometric curve upto 6 h, respectively.


17


Page 18 of 31

Table 2: sumurizes the OER performances. Catalyst

KOH

Over potential

Potential

(V)

for 10 mA/cm2

Potential (V) for 50 mA/cm2

 (mV) Fe3C-G@rGO

0.1 M

340

1.79

Fe-Fe3C-

0.1 M

270

1.73

0.1 M

220

1.66

1M

220

1.56

CoO@rGO

0.1 M

160

1.63

CoO@rGO

1M

170

1.52

RuO2

0.1 M

190

1.73

NG@rGO Co-Fe3CG@rGO Co-Fe3C-

1.68

G@rGO

1.67

We have also studied the oxygen reduction reaction (ORR) performances in alkaline medium. As N-doped carbon nanostructures due to charge imbalance in hexagonal C-ring, are better suited for ORR reactions, we carried out the cyclic voltammograms (CVs) and linear sweep voltammetry (LSV) studies of Fe-Fe3C-NG@rGO and compare the catalytic activity with the benchmark catalyst of Pt/C.


18

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(a)

4

(b)

-2

N2 sat O2 sat

-3

1 M MeOH

-4 -0.8

-0.6

-0.4

-0.2

0.0

0.2

J (mA/cm2)

J (mA/cm2)

-1

-5

(c) 1400 rpm

O2 sat.

2

1M MeOH

0

-1

Fe-Fe3C@NG-rGO 20 % Pt/C

-2

-2 -3

-4

Potential ( V vs. Ag/AgCl) 0.6

0 N2 sat.

0

J (mA/cm2)

1

-0.8

-0.6

-0.4

-0.2

0.0

-0.8

0.2

Potential ( V vs. Ag/AgCl)

(d)

-0.6

-0.4

-0.2

0.0

Potential (V vs. Ag/AgCl)

(f)

(e)

0.4

0

0.3

J (mA/cm2)

-1

-2 600 rpm 1000 rpm 1400 rpm 1800 rpm

-3 -0.8

0.2

-0.6

-0.4

-0.2

0.0


0.075

0.090

0.105

 (s ) -1/2

-1/2

-0.27 V -0.30 V -0.33 V -0.35 V -0.37 V -0.40 V

0.120

0.135

-0.25 V -0.28 V -0.30 V -0.33 V -0.36 V -0.40 V

0.5 0.0

0.4

-0.5

J (mA/cm2)

0.5

J-1(mA-1cm2)

0.6

J-1(mA-1cm2)

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


600 rpm 1000 rpm 1400 rpm 1800 rpm

-1.0 -1.5 -2.0 -2.5

0.3

-0.8

-0.6

-0.4

-0.2

0.0


0.075

0.090

0.105

0.120

0.135

-1/2(s-1/2)

Figure 6. (a) CV of Fe-Fe3C-NG@rGO in N2- and O2-saturated solution with 0.1 M KOH at a scan rate of 50 mV/s with 600 rpm, (b) CV of 20% Pt/C in N2- and O2-saturated and 1 M MeOH solution at scan rate of 50 mV/s with 1600 rpm, (c) Comparison ORR polarisation curves for hybrid nanostructure with the Pt/C catalyst (scan rate of 10 mV/s and a rotation speed of 1400 rpm). (e) and (f) K-L plot of inverse current density (J-1) versus1/2 at different potentials of hybrid nanostructure and Pt/C (inset plots are the ORR polarization curves different rotation rate at 10 mV/s in 0.1 m KOH solution). (f) Schematic of the ORR processes using Fe-Fe3CNG@rGO. Oxygen gets reduced by Fe-Fe3C-NG and the electrons are transported through rGO.

Figure 6. (a) displays the ORR activityof Fe-Fe3C-NG@rGO in N2- saturated and O2saturated 0.1M KOH solution. It shows high cathodic current with the sharp O2 reduction peak at


19


Page 20 of 31

-0.29 V vs. Ag/AgCl which is absent in the case of N2-saturated solution. We also performed the ORR activity in presence of 1 M MeOH solution and no change in the ORR current density is observed. Figure 6. (b) displays the ORR activity of commercial Pt/C in 0.1 M KOH solution both in N2- and O2- saturated 0.1M KOH solution. It shows cathodic O2 reduction peak at 0.30 V vs. Ag/AgCl which is absent in the case of N2-saturated solution. However, the ORR peak completely disappears and an oxidation peak appears in presence of 1 M MeOH solution. In this context, it may be noted that the performance of a fuel cell is limited by the methanol crossover from anode to cathode when Pt/C is used as cathode catalyst. Figure 6. (c) shows the comparison of ORR activity of Fe-Fe3C-NG@rGO and Pt/C. It is interesting to note that the hybrid shows better ORR performances with higher current density and comparable onset potential. Both the catalysts show onset potential 0.03 V vs. Ag/AgCl, while the current density at -0.4 V is 2.1 and 2.4 mA/cm2 for Pt/C and the hybrid, respectively. Figure 6. (d and e) present Koutecky-Levich (K-L) plots that establish the inverse current density (J-1) as a function of the inverse of the square root of the rotation speed (-1/2) at different potential and the inset of Figure 6. (d and e) present the polarisation ORR curve with different rotation speed of hybrid and Pt/C. The linearity with parallelism in K-L plots suggests that first order rate kinetics of O2 reduction. According to the K-L equation, the number of electron transfer is 3.90 and 3.94 for ORR in hybrid and Pt/C, respectively. We study the ORR stability of hybrids up to 1000nd cycles and compare with Pt/C (Figure S11 (a and b)). The hybrid exhibits better stability as compared to the Pt/C. We also study the ORR performances of the other catalyst (Fe 3C@C-rGO and CoFe3C@C-rGO). Both the hybrid structures shows low activity compared to Fe-Fe3C@NG-rGO but ORR activity in term of the onset as well as the current density improves (Figure S12) with Co doping. The high ORR activity of Fe-Fe3C-NG@rGO is due to N-doping in the hybrids and


20

Page 21 of 31

the rGO acts as a conducting substrate to transfer the electron from catalyst surface to electrode as depicted in Figure 6. (f).

12

(a)

Pt/C Pt@rGO

12

10

Pt@rGO (pyrolysis)

8 4 0 -4

J (mA/cm2)

16

J (mA/cm2)

(b)

8 6 4 2 0

-1.0 -0.8 -0.6 -0.4 -0.2

0.0

0.2

0.4

-0.6


-0.4

-0.2

0.0

0.2

Potential (V vs. Ag/AgCl) Normalised current (%)

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


100

(d) Pt/C Pt@rGO Pt@rGO (Pyrolysis)

80 60 40 0

100

200

300

400

500

No. of Cycle

Figure 7. (a) CVs of Pt/C and Pt@rGO in 1 M KOH +1 M MeOH solution at a scan rate of 50 mV s-1. (b and c) Electrocatalytic stability of Pt/C and Pt@rGO upto 500th cycle. (d) Comparison of MeOH oxidation stability of Pt@rGO hybrid with the commercial Pt/C catalyst (cycle stability data was normalized to the peak current of first cycle).

It is well known that Pt/C is the best anode catalyst in direct alcohol fuel cell (DAFCs) though carbon support is not a good choice due to corrosion in alkaline medium. In this regard, rGO is known to be a very good catalytic support and therefore, we examine the methanol oxidation performances of Pt@rGO. Figure 7. (a) Comparison of the methanol oxidation activity


21


Page 22 of 31

of the Pt@rGO, commercial Pt/C catalyst also Pt anchored rGO synthesized via pyrolysis method (Pt@rGO_pyrolysis) measured in 1 M KOH + 1 M MeOH solution. Interestingly, two well separated anodic peaks related to the MeOH oxidation (forward scan) and due to the oxidation of carbonate (reverse scan) are observed. The Pt@rGO shows a forward peak current density (If) of 14.5 mA/cm2 with an onset and forward current peak potential of -0.63V and 0.223 V vs. Ag/AgCl which is better compared to commercial 20 % Pt/C (onset and forward current peak potential of -0.6 V and -0.176 V vs. Ag/AgCl) with equivalent of mass loading. Figure 7. (b and c) present the CVs towards the methanol oxidation stability up-to 500th cycle and Figure 7. (d) shows the electro-catalytic cycling stability of Pt/C and Pt@rGO. It may be noted that Pt@rGO shows outstanding stability compared to Pt/C indicating a very good adherence of Pt NPs onto rGO. We also synthesized Pt NPs (size ~ 4 -12 nm) decorated on rGO by pyrolysing the mixture of Pt(acac)2 and GO at 600 0C for 1 h in inert condition (FigureS13) and compared the methanol oxidation activity and stability with the microwave-assisted synthesis of Pt@rGO as shown in Fig.4 (c and d). Pt@rGO (pyrolysis) also shows excellent stability as is the case of Pt@rGO (microwave) but less active in term of current density and potentials (onset potential of -0.6 V, peak potential of -0.176 V and current density of 12.31 mA/cm2). In this context, we would like to note that one of the byproducts in MeOH oxidation is CO that poisons the catalyst surface and deteriorates the catalytic activity. The ratio between the forward (If) and backward (Ib) current density is a measure of CO poisoning.50 From Fig. 4(c), the ratio If/Ib is found to be 5.2, 4.27 and 1.96 for Pt@rGO (microwave), Pt@rGO (pyrolysis)) and commercial Pt/C, respectively. The size and distribution of Pt NPs remain the same on the rGO substrate even after the 500th cycles (Figure S13 (a and b)) that leads to no change in the current density after the long term stability of the catalyst. As the ratio is highest for Pt@rGO


22

Page 23 of 31 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


(microwave), it is believed that CO poisoning is low in this case.

Over all, Pt@rGO

(microwave) prepared in just 60 s is superior as compared to Pt@rGO (Figure S14 (a and b)) which was synthesized via pyrolysis method from the mixture of Pt(acac)2 and GO in 1 h (experiment takes few hours for completion).

CONCLUSION Here, we demonstrated that GO can act as versatile high temperature reactor for the synthesis of various rGO based hybrids when treated with a domestic microwave oven. Overall, the synthesis is complete within 60 s. To prove the versatility of this method, different types of rGO based hybrids such as Fe3C-G@rGO, Co-Fe3C-G@rGO, Fe-Fe3C-NG@rGO, CoO@rGO and Pt@rGO are synthesized from the mixture GO and corresponding precursors and studied their catalytic activity. CoO@rGO is found to be the best catalyst for OER activity and outperforms the benchmark RuO2. Similarly, Fe-Fe3C-NG@rGO shows superior ORR activity as compared to benchmark Pt/C catalyst in alkaline medium. Furthermore, Pt@rGO exhibit excellent stability and activity towards methanol oxidation with very high resistance to CO poisoning as compared to commercial Pt/C catalyst. Another interesting observation is that Co doping in Fe3C-G@rGO enhances the ORR and OER performances. The high catalytic activity is believed to be due to the better adherence of inorganic nanostructures onto rGO. Overall, the methodology adopted here, can open a new era for the synthesis of graphene based hybrids.

ASSOCIATED CONTENT Supporting Information SEM images, EDX and XPS spectra, Tabled S1 and S2, cyclic voltammograms, LSV.


23


Page 24 of 31

AUTHOR INFORMATION Corresponding Author *Phone: +91-080-2293 2996. Fax: +91-80-2360 7316. E-mail:[email protected] Author Contributions BKB and KKN design the experiments and BBK performed the experiments. Both the BKB and KKN analyzes the data and wrote the paper. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors acknowledge the Department of Science and Technology (DST) and the Council of Scientific and Industrial Research (CSIR), India for the financial support. The authors also acknowledge the Chemical Science Division, IISc for the TEM facility and Debanjan Das for Faradic efficiency measurement. REFERENCES (1) Geim, A. K. Graphene: Status and Prospects. Science, 2009, 324, 1530-1534. (2) Allen, M. J.; Tung, V. C.; Kaner, R. B. Honeycomb Carbon: A Review of Graphene. Chem. Rev., 2010, 110, 132-145. (3) Zhu, Y.; Murali, S.; Cai, W.; Li, X.; Suk, J. W.; Potts, J. R.; Ruoff, R. S. Graphene and Graphene Oxide: Synthesis, Properties, and Applications. Adv. Mater. 2010, 22, 3906–3924


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SYNOPSIS: Graphene oxide (GO) itself can act as a high temperature REACTOR for synthesis of graphene based hybrids within 60s. When solid GO treated with microwave, it generates huge heat followed by reduction and exfoliation within 60s. By exploiting this simple GO reactor, graphene based hybrids such as Fe3C-G@rGO, Co-Fe3C-G@rGO, Fe-Fe3C-NG@rGO, CoO@rGO and Pt@rGO are synthesized. These rGO based hybrids show very efficient oxygen evolution, oxygen reduction reactions, and methanol oxidation activity compared to their traditional counterparts. The high catalytic activity and stability are believed to be due to the better adherence of inorganic nanostructures onto rGO. Table of Content:


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An Ultrafast-Versatile-Domestic-Microwave-Oven Based Graphene

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