CoFe Nanoalloys Encapsulated in N-Doped Graphene Layers as a Pt

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CoFe Nanoalloys Encapsulated in N-doped Graphene Layers as Pt-Free Multifunctional Robust Catalyst: Elucidating the Role of Co-Alloying and N-doping Barun Kumar Barman, and Karuna Kar Nanda ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01861 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 19, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Sustainable Chemistry & Engineering

CoFe

Nanoalloys

Encapsulated

in

N-doped

Graphene Layers as Pt-Free Multi-functional Robust Catalyst: Elucidating the Role of Co-Alloying and N-doping Barun Kumar Barman and Karuna Kar Nanda* Materials Research Centre, Indian Institute of Science, Bangalore-560012, India E-mail: [email protected] KEYWORDS: Nanoalloy, N-doped Graphene, Tri-functional robust catalyst, Oxygen reduction reaction (ORR), Hydrogen evolution reaction (HER) and 4-NP reduction

ABSTRACT: Pt is known to be the state-of-the-art catalyst for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) while it can also be used for the hydrogenation of 4nitrophenol (4-NP) to 4-aminophenol (4-AP). Quest is on to find a suitable catalyst to circumvent the problem associated with the precious Pt. Here, we report a facile and green strategy to fabricate CoFe nanoalloy encapsulated in N-doped graphene layers (CoxFe1-x@N-G) by pyrolysis and their catalytic activity towards ORR, HER and hydrogenation of 4-NP. Intensive studies have been carried out to elucidate the role of alloying and N-doping. All the

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catalytic activity is found to improve with the amount of Co in CoFe core and N doping in graphene layers. Similar onset potential with better current density as compared to the state-ofthe-art Pt/C catalyst in alkaline medium, have been achieved for CoxFe1-x@N-G towards ORR activity. They also show efficient and highly stable HER activity and very efficient and magentcially separable catalyst towards hydrogenation of 4-NP to 4-AP. Overall, the nonprecious alloy nanostructures can be exploited as multi-functional catalysts in fuel cells, hydrogen storage systems and waste water-treatment.

INTRODUCTION Fuel cells,

1-8

metal-air batteries

9-13

and water electrolysers

14-22

have attracted considerable

interest due to the demand of the petroleum resources and concern over the global warming. Oxygen reduction reaction (ORR) is a vital cathodic reaction for various electrochemical energy storage devices such as fuel cells and metal-air batteries converting chemical energy into electrical energy.23-33 The oxygen electrode kinetic is sluggish with 15% power loss which is the major problem in achieving highly efficient energy devices. Hydrogen evolution reaction (HER) is another class of electrochemical reaction for the generation of hydrogen that can be used as fuel in fuel cells. Both for ORR and HER, Pt or Pt based metal alloys have been exploited to overcome the sluggish oxygen kinetics as well as facilitate the HER.34-36 However, the cost of Pt based electro-catalysts hinders the commercialization of the energy devices. The conversion of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) has also fascinated researchers because 4-NP is one of the most dominant organic pollutants in waste water generated from industrial sources, while 4-AP is a powerful intermediate in manufacturing analgesic, antipyretic drugs, anticorrosion lubricants, hair dyeing agents, etc. This is also a model reaction to study the rate


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kinetics of the catalyst and mostly noble metals such as Pt, Pd, Au, etc. and their hybrids are very active towards this conversion of process.

37-48

To eliminate the noble metal-based catalysts,

substantial efforts are being made to develop non-precious metal catalysts for ORR, HER and hydrogenation of 4-NP to 4-AP. The recent progress on three-dimensional (3-d) transition metal based oxides, sulfides, carbides, selenides, nitrides, phosphides have been demonstrated as promising catalyst towards the ORR and HER catalysts.46-55 Various research groups have also developed CoFe alloys encapsulated in N-doped carbon/graphene layers for various catalytic applications.56-59 The carbon/graphene layers prevent the metals from oxidation and provide the stability, whereas M-N complex forms active catalytic sites. However, the catalytic activities are still low when compared with Pt based catalysts. It may be noted that the composition of Co, Fe and N doping is fixed due to the single source precursor. For example, thermal decomposition of Co-imidazole framework (ZIFs) or Prussian blue analogue (PBA) in N2 atmosphere yields Fe to Co ratio as ~1:1 and 3:1.57, 58 In order to elucidate the role of N doping in graphene layers and Co in CoFe towards their catalytic activity, it is desirable to fabricate CoFe nanoalloys with different Co:Fe ratio encapsulated in N-doped graphene and carry out systematic studies. It is well known that the performance of N-doped carbon ORR catalysts improves due to the doping-induced charge relocation around the N dopants that weakens the O-O bonding helping the ORR performance by lowering the potential. It has been also predicted that N-doping lowers the H* adsorption energy and favours HER. Furthermore, CoFe core transfers charges to graphene support and enhances the HER activity as well.56 Overall, CoFe encapsulated in N-doped graphene is expected to be an efficient catalyst and can be exploited for ORR as well as 4nitrophenol reduction apart from HER.


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Scheme 1. Schematic representation for the synthesis of CoxFe1-x nano-alloy encapsulated in Ndoped graphene layers. There are several approaches for the fabrication of the encapsulated CoFe nanostructures. However, it is desirous to develop a simple synthesis technique that can produce encapsulated nanoalloys with different Co: Fe ratio along with different N-doping. In this work, we demonstrate a one-step simple pyrolysis for the synthesis of CoxFe1-x nanoalloy encapsulated in N-doped graphene layers (CoxFe1-x@N-G) with tunable N doping in graphene layers and different percentage of Co alloying in the core region. Thereafter, the catalytic activity toward ORR, HER and hydrogenation of 4-NP to 4-AP are investigated in order to elucidate the role of alloying and N-doping. Interestingly, the activity is found to be dependent on the amount of Co in CoFe core and N in graphene layers. Co-alloying and high N-doping in CoxFe1-x@NG show outstanding ORR activity and has the potential to replace precious Pt/C catalyst. CoxFe1-x@NG also shows efficient HER activity and hydrogenation of 4-NP. The encapsulated nanostructures are fabricated by pyrolysing a mixture of Prussian blue (PB: Fe4[Fe(CN)6]3) and cobaltocene (Co(C5H5)2) or Cobalt nitrate hexahydrate (Co(NO3)2.6H2O) as presented in Scheme 1. Choice of different precursors helps to control the N-doping in graphene and Co alloying in Fe. During


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the pyrolysis of PB alone, it forms Fe-Fe3C@N-G (X0). Pyrolysis of Co(C5H5)2 and PB formed CoxFe1-x/NG with 1:10 and 1:5 ratio and named as X1 and X2. With Co(C5H5)2,Co:Fe ratio increases along with the reduction of N-doping. Therefore, PB and (Co(NO3)2.6H2O) is pyrolyzed to obtain CoxFe1-x@N-G (X3) with higher percentage of N-doping. All the encapsulated nanostructures were subjected to various catalytic activities. Interestingly, the electrocatalytic ORR performance is found to dependent on the nanoalloying as well as the Ndoping in the graphene layers. The ORR performance of X3 hybrid shows higher limiting current density (JL) and similar onset potential compared with the benchmark precious Pt/C catalyst as well as the reported values. Similarly, the catalytic activity towards the conversion of 4-NP to 4AP in presence of NaBH4 is demonstrated and shown that the catalyst is magnetically separable, efficient, stable and better as compared with the noble metal based catalysts. The HER activity is also found to be improved with Co alloying and N-doping.

EXPERIMENTAL SECTION Materials Prussian blue, cobaltocene and sodium borohydride were purchased from Sigma-Aldrich. Cobalt nitrate hexahydrate and 4-nitrophenol (4-NP) were purchased from S D Fine-Chem Limited. Commercial Pt/C (20 wt. %) was purchased from Johnson Matthey.

Methods Various encapsulated nanostructures in N-doped graphene layers (CoxFe1-x@N-G) are synthesized by simply pyrolysing the mixture of different ratio of PB and cobaltocene/cobalt nitrate hexahydrate. The precursor mixtures are taken in 1 cm diameter quartz tube in close


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condition and pyrolyzed at 750 0C for 2 h. The synthesis parameters for various CoxFe1-x@N-G are presented in Table 1.

Table 1: Synthesis parameters and the characterization results.

% Co Sample

Precursors (mg) PB

% N doping

Co(C5H5)2

Co(NO3)2.6H2O 0

X0

500

0

0

8.3 8

X1

500

50

0

4.02 12

X2

500

100

0

1.01 10

X3

500

0

40

8.3

Characterizations Crystal structure of the as-synthesized materials were characterized by X-ray diffraction (XRD) technique with PANalytical instrument using a Cu K= 1.54 Å) radiation source. The Raman measurements were performed with a WITec Raman system at room temperature using a Nd:YAG laser (532 nm) as an excitation source. Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDS) date were taken on a FEI-INSPECTF50 instrument. Transmission electron microscope (TEM), high resolution TEM (HRTEM) images


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and selected area electron diffraction (SAED) pattern were acquired using JEOL- JEM-2100F. All the TEM samples were prepared by dispersing the sample in ethanol solution using ultrasonic bath for 15 min. and drop-casted on carbon coated copper grid, and then dried at 70 0C. X-ray photoelectron spectroscopy (XPS) was carried out for the elemental investigation on an ESCALAB 250 (Thermo Electron) with a monochromatic Al K (1486.6 eV) radiation source. Electrochemical measurements Cyclic voltammetry (CV) studies were performed using electrochemical work station with rotating disk electrode (RDE) from CH Instruments. Each time, the catalyst ink was prepared by ultrasonically dispersing 5 mg catalyst in a solution of 0.1 ml of 5 % Nafion and 0.9 ml of ethanol solution. The loading on RDE was 0.64 mg/cm2 for CoxFe1-x@N-G and 0.360 mg/cm2 of 20 % (72 g (Pt)/cm2) of the Pt/C catalyst. A conventional three-electrode cell was used where Ag/AgCl is the reference electrode, a Pt wire is the counter electrode and the catalyst film coated RDE is the working electrode. The RDE measurements were carried out at different rotation speeds between 600 and 2400 rpm in the O2-saturated 0.1 M KOH aqueous solution to evaluate the kinetic of ORR performance. The stability was performed by linear sweep voltammetry (LSV) up-to 1000nd cycles with a scan rate of 50 mV/s and a rotation speed of 600 rpm. The RDE data were analyzed using Koutecky-Levich (K-L) plots (J-1 vs. -1/2) at different applied potentials. The slopes of the linear fit were 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 nFC0 (D0)2/3ν1/6

, where JL= 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 × 10-6 mol/cm3), D0 = Diffusion coefficient of O2 in KOH (1.9 × 10-5


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cm2/s), = kinetic viscosity of electrolyte (0.01 cm2/s). According to the K-L plot, the slope (1/B) can be used to calculate n from n = B/0.62FC0 (D0)2/3ν-1/6. The HER activity were recorded in N2 saturated 0.1 and 1 M KOH solution with a scan rate of 10 mV/s and 100 mV/s with a rotation speed of 1400 rpm. The stability test was performed by using LSV measurements at a scan rate of 100 mV/s in 1 M KOH solution. The HER performance was also studied with carbon paper as both working and counter electrodes.

Catalytic reduction of 4-NP to 4- AP The conversion of 4-NP to 4-AP is an important reaction that also requires catalyst. The catalytic conversion of 4-NP is monitored using Hitachi UV-vis spectrophotometer in the range of 200500 nm at room temperature. An aqueous solution of 10 ml of 1 mM of 4-NP solution is added to 10 ml of 0.125 M of freshly prepared NaBH4 solution. Then, 2 mg of solid catalyst is added and mixed thoroughly. Catalytic performance with different NaBH4 concentration is also carried out. Each catalytic experiment is performed at least 3 times (from each batch) to ensure the reproducibility.


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RESULTS AND DISCUSSION

(a)

(b)

(c)

(d)

(e)

(f)

Figure 1. (a and b) low and high magnified SEM images of CoxFe1-x/N-G (X3). (c and d) TEM and HRTEM images of CoxFe1-x/N-G (X3). (e-f) TEM and HRTEM images of Fe-Fe3C/N-G (X0).


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Figure 1 (a and b) shows the typical SEM images of X3 which clearly reveal the metallic part encapsulated by graphene layers. Figure 1 (c and d) shows TEM and HRTEM images of X3. The TEM image reveals that the black portion corresponding to the metallic nanostructures covered with the crumpled graphene layers. The size of the overall nanostructures is between 30 and 50 nm. The HRTEM image reveals the nanoalloy of CoxFe1-x with a d-spacing of 0.20 nm corresponding to the (110) crystal plane of Fe and is wrapped with few layers (6-12 layer) of graphene layers. The interlayer distance is 0.336 nm that corresponds to the van der Waals bond distance of two graphene layers. High-angle annular dark-field scanning TEM (HAADF-STEM) image (Figure S1) reveals bright spots corresponding to the alloy (Fe and Co) core of the nanostructures. Elemental distributions ensure the incorporation of Co into the Fe nanoparticles inside the core region as well as N-doping in the graphene. Fe (red) and Co (green) mappings indicate that CoxFe1-x nanoparticles are in the core, while C (yellow) and N (pink) are distributed all over (Figure S2). Overall, it is confirmed that CoxFe1-x nanoalloy is encapsulated in N-doped graphene to form core-shell nano-architectures. Figure (e and f) display the TEM and HRTEM images of the Fe-Fe3C encapsulated N-doped graphitic nanostructure (X0). It also clearly reveals that the metallic nanostructures encapsulated by graphitic layers. The d-spacing of 0.202 nm corresponding to the (110) crystal plane of Fe is apparent from Figure 1(f) and is wrapped with 32-40 layer of graphene layers with an interlayer distance of 0.336 nm. SEM, TEM and HRTEM images of other hybrids (X0, X1 and X2) also confirm core-shell like structures (Figure S3 and Figure S4). HRTEM images reveal that Co alloying of the X3 hybrid reduces the number of graphene layers from 32-40 to 8-15 layers (Figure S5) which can also influence the catalytic activity.


10

X1

30

40

* 50

X3

60

Fe-2p

70

800

80

1200

1600

(e)

2p3/2

Fe(0)

715

710

705

Binding Energy (eV)

Intensity (a.u.)

Fe(III) Fe(0)

720

2000

2400

200

2p3/2

Co-2p

300

400

500

600

700

Binding Energy (eV)

(f)

Co(0)

Pyrolic-N

Fe(II)

725

X1

Raman Shift (cm-1)

Fe(II)

2p1/2 Fe(III)

X0

X2

2 (degree)

(d)

X3

X0

2p1/2 Co(0)

Sattelite

804

Co(III)

Co(III)

798

Sattelite

792

786

780

Binding Energy (eV)

Intensity (a.u.)

*

*

O-1s N-1s

Intensity (a.u.)

D

X2

C-1s

G

Intensity (a.u.)

X3

(c)

(b)

(211)

(200)

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


(110)

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Pyridinic-N Graphitic-N

402

400

398

Binding Energy (eV)

Figure 2. (a) XRD patterns of X0, X1, X2 and X3. (b) Typical Raman spectrum of X3. (c) XPS survey spectra of X0, X1, X2 and X3. (d-f) de-convoluted XPS spectra of Fe-2p, Co-2p and N-1s of the X3 hybrid, respectively. Figure 2(a) shows the XRD patterns of all the samples. When simply PB is pyrolyzed, core-shell structures of Fe-Fe3C/N-doped graphene (Fe3C is assigned by * of X0) is obtained.26, 60

However, inclusion of Co seems to resist the formation of Fe3C. Three foremost peaks centred

around 44.74, 65.2 and 82.500 for (110), (200) and (211) planes that correspond to phase of the Fe (JCPDS # 870722) are observed. No appreciable change in the XRD patterns is observed after the incorporation of Co and is believed to be due to low amount of Co alloying with the Fe. There are no XRD peaks corresponding to metallic Co which indicate that Co substitutes Fe in the lattice. Figure 2(b) represents the Raman spectrum of X3 which reveals two most prominent peaks at 1344 and 1579 cm-1 corresponding to D and G bands, respectively. The presence of


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prominent D peak with ID/IG ratio of 1.1 signifies the defective nature of the graphene layer and is due to the N-doping.61-62 Figure 2(c) display the XPS survey spectra of all the hybrids. The survey spectra clearly show strong peak of C-1s at 284.6 eV due to graphene layers, a small peak around 400 eV due to N doping and a peak around 533 eV due to the adsorbed atmospheric oxygen. The full survey spectrum of X3 displays the presence of C, N, Co and Fe (Figure S6). Figure 2(d and e) displays the de-convoluted XPS spectra of Fe-2p and Co-2p spectra of X3. Figure 2 (d) shows a peak centered on 706.6 eV corresponding Fe (0). Two broad peaks at 710.1 and 713.3 eV are due to the 2p3/2 orbitals of Fe2+ and Fe3+ oxidation state, respectively. Interestingly, the peak at higher binding energy (713.3 eV) indicates the presence of Fe 2+-N bonding .63 Similarly, the de-convoluted XPS spectrum of Co 2p in Figure 2 (e) displays two major peaks centered at 778.4 eV and 780.5 eV corresponding to Co(0) and Co(III) oxidation state of the 2p3/2 valence state. The de-convoluted N-1s spectrum as shown in Figure 2(d) clearly reveals different types of N environment: pyridine-type (398.4 eV), pyrrolic-type (399.4 eV) and graphite-type (401.1 eV) with 26.08%, 44.67% and 29.92%, respectively. The XPS results of Fe and Co and N Fe further confirm the presence of chemical metallic and metal-nitron type bonding the hybrid nanostructure, respectively. In this context, it may be noted that the % of N doping decreases from 8.3, 4.02 and 1.01 for X0, X1 and X2, respectively due to the absence of N in the cyclopentadiene ring in cobaltocence. When the PB and cobaltocene ratio is 10:1, 8% Co is incorporated (FigureS7 (a)) in Fe (X1), while 11 % Co is incorporated (FigureS7 (b)) when the ratio is 5:1 (X2). When PB is pyrolysed with Co(NO3)2.6H2O in the 12:1 ratio (X3), 10 % Co in CoFe core region (FigureS7 (c)) and ~ 8.3 % of N-doping in graphene as the case of X0, is realised. The overall elemental composition of all the encapsulated nanostructures is presented in Table 1.


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2

(a)

12

1

(b)

(c)

9

-2

X3

-4

N2 sat. O2 sat. 1M MeOH

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Potential (V vs. Ag/AgCl)

3 0

-3 -0.6

-0.4

-0.2

X2 X0

-3 Pt/C

X3

-5

0.0

0.2

X1

-4

-0.8

-0.6


-0.4

-0.2

0.0


0.4

(d)

(e)

-1 X3 -2 -3

600 rpm 1000 rpm 1400 rpm 1800 rpm 2400 rpm

-4 -5 -6 -0.8

-0.6

-0.4

-0.2

0.0

J-1 (mA-1/cm2)

J (mA/cm2)

O2 sat.

-0.8

1 0

N2 sat.

-2

X3 hybrid

0.3

-0.30 V -0.35 V -0.40 v

-0.32 V -0.37 V

0.2 0.06 0.07 0.08 0.09 0.10 0.11 0.12 0.13

Current density (mA/cm2)

-6

-1

1M MeOH

6

J (mA/cm2)

0

J (mA/cm2)

J (mA/cm2)

0

-1/2


Pt/C

-0.25 V -0.35 V -0.45 V

0.3

0.07

0.08

0.09

0.10

0.11

-0.30 V -0.40 V

0.12

No. of electron (n)

0.4

4

0.13

-1/2

Pt/C

-2 -3

600 rpm 1000 rpm 1400 rpm 1800 rpm

-4 -5 -0.8

-0.6

-0.4

-0.2

0.0

2 1

-0.30

-0.36

0.2

(i)

X3 hybrid

3

0 -0.24

(f)

-1

100

(h)

Normalized current

(g)

0


0.5

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


-0.42

CoxFe1-x/N-G (X3)

80 60 Pt/C

40 20 0

200


400

600

800

1000

No. of Cycle

Figure 3. (a) CV of X3 and 20% Pt/C in N2- and O2-saturated solution with 0.1 M KOH at a scan rate of 50 mV/s with 600 rpm and (b) CV of X3 in the presence of 1 M MeOH solution. (c) Comparison of the polarisation curves of X0, X1, X2 and X3 with Pt/C in O2-saturated solution at scan rate of 10 mV/s with 1400 rpm. (d and e) the polarization curves with different rotation rate at 10 mV/s and K-L plots (J-1 versus-1/2) at different potentials for X3. (f and g) the polarization curves with different rotation rate at 10 mV/s and K-L plots (J-1 versus-1/2) at different potentials for the Pt/C.

(h) Number of electron transferred for X3 and Pt/C. (i)

comparisons of ORR stability plot of X3 with the Pt/C up-to 1000nd cycle.


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Figure 3(a and b) displays the CV curves of X3 and commercial Pt/C in N2- and O2saturated 0.1 M KOH solution. Both X3 and Pt/C clearly shows sharp O2 reduction peak at -0.3 V vs. Ag/AgCl in O2-saturated solution which is absent in N2-saturated solution. X3 shows a limiting current density (JL) of 4.8 mA/cm2 that is superior as compared to Pt/C (4.2 mA/cm2) and other catalysts reported in the literature for N-doped graphene based nanostructures/hybrids (Table-S1). Another advantage associated with our CoxFe1-x@N-G is the methanol tolerance. The catalytic activity does not change in the presence of 1 M methanol while the ORR activity is completely suppressed in the case of Pt/C and a sharp methanol oxidation peak at -0.177 V with high current density (12 mA/cm2) is observed. Overall, the higher ORR current density, similar onset potential with the Pt/C and methanol tolerance suggest that nanoalloy encapsulated nanostructures indicate the replacement for the precious Pt/C catalyst. Figure 3 (c) display the comparison of the LSV curves of X0, X1, X2, X3 and 20 % Pt/C. JL is found to be 3.2, 3.4, 2.9 and 4.1 mA/cm2 for X0, X1, X2 and X3, while it is 3.6 mA/cm2 for Pt/C. It is evident from Figure 3 (c) that X3 shows better JL and similar onset potential as compared to Pt/C. The kinetic study of the ORR is performed by the RDE measurements. Figure 3 (d and e) represents the polarization curves with different rotation speed from 600 rpm to 2400 rpm at 10 mV/s and K-L plot that display the inverse current density (J-1) versus inverse of the square root of the rotation speed (-1/2) at different potentials of X3. Similarly, Figure 3 (f and g) represents the polarization curves with different rotation speed from 600 rpm to 2400 rpm at 10 mV/s and K-L plot of the benchmark 20 % Pt/C catalyst. The linearity with parallelism in K-L plots of X3 and Pt/C (Figure S8) suggests that first order rate kinetics of O2 reduction.64 Using the K-L equation, the number of electron transfer (n) is evaluated to be 3.98 and 3.94 for X3 and Pt/C, respectively and shown in Figure 3 (h) which clearly suggests 4 e- transfer process. Figure 3 (i) displays the


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comparison of ORR stability of X3 and Pt/C up-to 1000nd cycle (Figure S9). When the activity of X0 and X1 is compared, it is realized that X1 is superior as compared to X0 and can be attributed to the 8% Co alloying even though N-doping (8.3 to 4.02%) is less. The activity of X2 is inferior as compared to X1 and is attributed to the drastic decrease in N-doping (4.02 to 1.01%) despite Co content increases from 8 to 11%. However, the ORR activity of X3 is far superior as compared to other hybrids (X0, X1 and X2) as both Co in CoFe core and N-doping in graphene is optimum for all the CoxFe1-x@N-G investigated. Overall, incorporation of Co and N-doping in graphene is ideal for ORR activity. It is well established that N-doping in hexagonal carbon matrix enhances the ORR performance. N is more electronegative than C that induces positive charge on C, favors the adsorption of O2 molecule and the ORR performance.65-66 Similarly, Co is more electronegative as compared to Fe and the further enhancement in the ORR activity is attributed to partial electron transfer from the Fe to Co creating charge imbalance. Over all, the charge imbalance both in CoFe core and N-doped graphene layers lead to drastic enhancement in the catalytic site towards ORR performances. In an attempt to increase the percentage of Co in CoFe, the ratio of Co(NO3)2.6H2O to PB is doubled as compared to X3. Bigger and irregular shaped particles without any graphene layer on the surface are obtained due to the insufficient availability of carbon. As expected, the ORR performance (Figure S10) is very low as compared to other hybrids.


15


-5

J (mA/cm2)

0

(a) 0.1 M KOH

-10

X0 X2

-15

X1 X3

-20

(b)

-7

J (mA/cm2)

0

Pt/C

-25

-14 1 M KOH

-21 -28

-0.8

-0.6

-0.4

-0.2

0.0

-0.8

Potential (V vs. RHE) 0

(c)

0.12

-G

0.09

e/N CoF

) (96 (X3

mV

0.06

Pt/C

0.03 0.4

(92

-0.4

-0.2

0.0

(d)

-10

) /dec

ec) mV/d

0.6

-0.6

Potential (V vs. RHE)

J (mA/cm2)

0.15

Pt/C

X3

-35

-1.0


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

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-20 1st cycle

-30 -40

1000nd cycle

-50

0.8

-0.8

Log(J(mA/cm2))

-0.6

-0.4

-0.2

0.0


Figure 4. (a) HER performances of all the encapsulated nanostructures in 0.1 M KOH solution. (b and c) HER performances in 1 M KOH solution and the Tafel slope of X3 and Pt/C. (d) Stability of X3 towards HER activity in 1 M KOH solution.

The electrocatalytic HER performance of different catalysts is investigated in N2-purged 0.1 and 1 KOH. Figure 4(a) displays the HER polarization curves for different catalysts in 0.1 M KOH solution whereas the X3 shows better HER performance compared to others. It requires only 85 mV of overpotential () to overcome the HER activity in alkaline medium. Figure 4 (b) displays the comparison of HER activity of X3 and commercial benchmark Pt/C catalyst. A current density of 10 mA/cm2 is achieved at 272 mV. Figure 4 (c) shows the Tafel plot of X3 and


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Pt/C. The Tafel slope of X3 is 96 mV/dec and that of Pt/C is 92 mV/dec. Figure 4 (d) displays the HER stability of X3 up-to 1000nd cycle which proves the stability of the catalyst up-to 1000nd cycle with almost no change in the current density (97.6% retention). Overall, the HER performance of X3 is comparable to that of Pt/C with very high stability. Recent studies have shown that the use of Pt based counter electrodes for HER studies without using any ion exchange membrane to separate the working electrode from the counter electrode can influence the HER performance substantially due to the dissolution-deposition process of Pt on the working electrode. Therefore, HER performance has been examined using the carbon paper as both the working electrode (catalyst loaded) as well as counter electrode (Figure S11). It also shows good HER performance with an onset potential of ~ 82 mV and long term stability.67


17


1 0 -1 -2

10 mV/s 50 mV/s 100 mV/s

-3 0.45

0.50

0.55

25 mV/s 75 mV/s

0.60

J=Ja-Jc (mA/cm2)

(a)

(b)

2

2 0 -2 10 mV/s 50 mV/s 100 mV

-4

0.65

0.5


0.6

25 mV/s 75 mV/s

J=Ja-Jc (mA/cm2)

4 2

J=Ja-Jc (mA/cm2)

(c)

1 0 -1 -2 -3

10 mV/s 50 mV/s 100 mV

-4 -5 0.45

0.7

0.50

(d)

4

4 2 0 -2 -4 10 mV/s 50 mV/s 100 mV

-6 -8

0.5

0.6

25 mV/s 75 mV/s

J= Ja-Jc (mA/cm2)

6

0.8

(e) X3

3

(38

/c mF

2

F/c 9m

1

X0 (17

Potetial (V vs. RHE)

0.65

2) m

24 m X2 (

20

0.60

2) m

(2 X1

0

0.7

0.55

25 mV/s 75 mV/s



J=Ja-Jc (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

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40

60

2) F/cm

2) mF/cm

80

100

Scan rate (mV/s)

Figure 5. CV curves of (a) X0, (b) X1 and (c) X2 and (d) X3, respectively. (e) Difference in current density variation (J= Ja-Jc) at 0.55 V plotted against scan rate. Half of these values from the linear regression enable the estimation of Cdl. In order to find the rationality of the Co alloying with the Fe in core region and N doping in graphene towards the electro-catalytic surface area, electrochemical double layers capacitance (Cdl) which is directly proportional to the electrochemical active surface area (ECSA), is evaluated for all CoxFe1-x@N-G. The ECSA has been determined by CV studies in non-redox regime in 1 M KOH solution with different scan rates (Figure 5) followed by the plot of difference current density (difference between cathodic and anodic current) with the scan rate. Figure 5 (a-d) display the CV of X0, X1, X2 and X3 respectively. Figure 5 (d) display the Cdl of


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all the hybrid nanostructures. It can be seen that Cdl of X3 is 19 mF/cm2, whereas that of X0, X1 and X2 is 17, 14.5 and 12 mF/cm2, respectively. This result indicates that the electrochemically active sites towards HER of X3 is better due to high N doping as well as Co alloying with the Fe that possesses high ECSA as compared to other hybrids leading to better ORR and HER performance.

4-NP

1.0

0 min 2 min 4 min 6 min 7 min

0.5 4-AP 0.0

(b)

1.5

4-NP

(c) 0 min 2 min 4 min 5 min

1.0

0.5

4-AP

Absorbance (a.u.)

1.5

(a)

Absorbance (a.u.)

Absorbance (a.u.)

1.5

0.0

250

300

350

400

450

500

0.0

250

300

350

400

450

0 min 1.5 min 3 min

X3 4-AP

0.5

0.0 250

500

4-NP

1.0

300

350

400

450

500

Wavelength (nm)

Wavelength (nm)

Wavelength (nm)

(d)

200

(f)

-0.5 -1.0

Time (sec)

ln (At/A0)

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


-1.5 X0 X1 X2 X3

-2.0 -2.5 -3.0

150 100 50 0

0

1

2

3

4

5

6

7

8

1

2

3

4

5

Cycles

Time (min)

Figure 6. Successive reduction of 4-NP catalyzed by (a) X0, (b) X1 and (c) X3 in presence of NaBH4 (10 ml of 1 mM 4-NP and 10 ml of 0.125 M NaBH4 solution with 2 mg of catalyst). (d) Pseudo-first order rate plot: ln (At/A0) at 400 nm vs. time for the reduction of 4-NP. (e) Optical photographs of 4-NP (left) to 4-AP (right) conversion followed by magnetic separation of the X3 catalyst. (f) Recyclability performances of X3 during 5 successive cycles.


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Generally, the catalytic conversion of 4-NP to 4-AP is carried out in presence of NaBH4. During the addition of the NaBH4 with the 4-NP, the colour of the solution changes from pale yellow to dark greenish yellow due to the formation of 4-nitrophenolate ions and when the catalyst is added the solution changes the color from greenish yellow to colorless one. The conversion process of 4-NP ions to 4-AP is monitored by UV-visible spectroscopy. Figure 6 (a-c) display the successive reduction of 4-NP to 4-AP by X0, X1 and X3, respectively. The catalytic performance of 4-NP reduction of X2 is provided Figure S12. The absorbance peak at ~400 nm corresponding to 4-NP decreases, while the peak at ~296 nm corresponding to 4-AP increases. It can be noted that X3 exhibits better activity as compared to others and it take only 3 minutes to complete the conversion. This is believed to be due to the high N at% doping in graphene as well as the presence of Co as is the case with ORR activity. The apparent rate constant (kapp) of the reaction is estimated by the pseudo-first order rate kinetic relation:-ln(At/A0)=kappt, where At and A0 are the absorbances at a time interval of t and 0 (initial value) shown in Figure 6 (d). The kapp of the catalysts is 0.406, 0.52, 0.46 and 0.71 min-1 for X0, X1, X2 and X3, respectively. Interestingly, the rate constant is high for X3 even as compared with the noble metal (Pt, Ag, Au and its hybrids) assisted reduction of 4-NP.35-45 One of the advantages of the present catalysts is that they can be recovered by magnetic separation (Figure 6 (e)) followed by the recycling test of X3 indicates that the time of complete conversion of 4-NP to 4-AP increased from 3 to 3.5 minutes for 1st to 5th cycle suggesting the robustness of the catalyst (Figure 6 (f)). Overall, the developed catalyst exhibits far superior activity as compared to Pt based materials (typical reduction time is 10-15 min) toward the conversion of 4-NP into 4-AP.


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CONCLUSION In conclusion, we have demonstrated a facile approach to synthesize non-precious nanoalloys encapsulated in N-doped graphene (CoxFe1-x/N-G) by pyrolysing the mixture of Prussian blue and cobaltocene/cobalt nitrate. For the first time, the role of both Co in the CoFe core and Ndoping in graphene is elucidated. Interestingly, the presence of Co alloying in the core region with the high % of N doping in the graphene improves the catalytic activity towards ORR, HER and the conversion of organic pollutant 4-NP to 4-AP. In addition, the ORR activity is found to be superior as compared to the traditional electro-catalysts in terms of current density. Overall, the nanoalloy encapsulated in N-doped graphene enhances the stability of the catalysts. We strongly believe that the methodology can be applied to synthesize a variety of nanoalloys (with the other 3-d transition metals) encapsulated in N-doped graphene layers which can be tuned further for a variety of other applications.

ASSOCIATED CONTENT Supporting Information SEM images, EDX and XPS spectra, Tabled S1 and S2, cyclic voltammograms, LSV. AUTHOR INFORMATION Corresponding Author *Phone: +91-080-2293 2996. Fax: +91-80-2360 7316. E-mail:[email protected] ORCID id: Barun Kumar Barman: 0000-0002-9894-1890, Karuna Kar Nanda: 0000-0001-94961408


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Author Contributions BKB and KKN designed the experiments and BBK performed the experiments. Both the BKB and KKN analyzed the data and wrote the paper. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors acknowledge Council of Scientific and Industrial Research (CSIR), India for the financial support. The authors also acknowledge Chemical Science Division of IISc for providing access to FETEM facility. Authors also acknowledge Dr. Sanjoy Mukherjee for helping with the schematic preparation.


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Table of Content:

SYNOPSIS: Attempt towards optimization of alloying and N-doping in CoFe encapsulated by N-doped graphene layers for various endurable catalytic reactions.


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CoFe Nanoalloys Encapsulated in N-Doped Graphene Layers as a Pt

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