Non-Precious Bimetallic CoCr Nanostructures Entrapped in Bamboo

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Non-Precious Bimetallic CoCr Nanostructures Entrapped in Bamboo-like Nitrogen-doped Graphene Tube as a Robust Bi-functional Electrocatalyst for Total Water Splitting Bidushi Sarkar, Barun Kumar Barman, and Karuna Kar Nanda ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00233 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 4, 2018

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ACS Applied Energy Materials

Non-Precious

Bimetallic

Entrapped

in

Graphene

Tube

CoCr

Bamboo-like as

a

Robust

Nanostructures Nitrogen-doped Bi-functional

Electrocatalyst for Total Water Splitting Bidushi Sarkar, Barun Kumar Barman, and Karuna Kar Nanda* Materials Research Centre, Indian Institute of Science, Bangalore-560012, India KEYWORDS: Non-precious, bimetallic, N-doped graphene tube, electrocatalyst, total water splitting

ABSTRACT: Developing an efficient and cost-effective electrocatalyst for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) is of paramount importance for designing metal–air batteries and water electrolyzers. Herein, we present an economical approach for the synthesis of a bi-functional electrocatalyst consisting cobalt-chromium nanostructures entrapped in graphene tube doped with nitrogen (CoCr@NGT). The graphene tube is of large size (cross-sectional diameter ~ 100 nm) with wall thickness >10 graphene layers. The Cr alloying with the entrapped Co in the NGT drastically enhanced both the HER and OER performance with the low over potential (η) and better current density along with the long-term durability. Hence, CoCr@NGT can be used a total alkaline water electrolyzer as both

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anode and cathode catalyst delivering a current density of 10 mA/cm2 at around 1.58 V for a long period of time competing with the state-of-the-art combination of Pt/C and RuO2. The electrochemical performance strongly depends on the Cr due to its corrosion resistance capability and improved catalytic sites that leads to long-term stability and very high activity, respectively.

INTRODUCTION Hydrogen is an efficient energy carrier as it has high gravimetric energy density and clean combustion in air. The electrochemical water splitting units with renewable energy input, such as solar or wind, offers a clean, renewable and potentially cost-effective route to the production of hydrogen gas1-3. The water splitting reaction can be classified into two half reactions: the oxygen evolution reaction (OER) at anode and the hydrogen evolution reaction (HER) at cathode.4-6 The sluggish and slow kinetics of water splitting reactions demand the development of robust and economically feasible electrocatalyst. Although precious metals such as Pt7-12 and noble metal oxides such as RuO213 and IrO214-15 exhibits excellent HER and OER catalytic activity respectively, their commercialization is limited due to high cost and low abundance. As a consequence, tremendous research is being pursued to develop cost-effective and earth abundant electrocatalysts. The prevailing approaches desire incompatible combination of two different catalyst for overall water splitting resulting in poor efficiency. Thus, development of a bifunctional electrocatalyst, efficient for both HER and OER in the same electrolyte is desirable but challenging. Co-C composites with N doping have been widely studied as efficient and robust catalysts for total water splitting.16-18 Co species catalyse the formation of N-doped carbon nanostructures, such as N-doped graphene sheets or N-doped carbon nanotubes (N-CNTs) at high temperatures


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which makes the species resistant to harsh conditions. The carbon support prevents the agglomeration of the nanostructures, their corrosion and provides a highly conductive support for electron transport.19 Introduction of various heteroatoms, such as N doping could enhance the catalytic activity of carbon-based nanostructures by modulating the electronic structure and surface polarities to produce more carbon defects which can activate the p electrons of sp2 C, thus, facilitating the chemisorption of intermediates that cumulatively favours the overall electrochemical performance.20 Also, metallic Co has a moderate H-bonding energy and the entrapped Co could decrease local work function on the carbon surface owing to facile electron transfer from Co nanoparticles to the N-CNT, which is crucial for an efficient electrocatalyst.21,22 Several groups have developed various approaches to synthesize metal/metal alloys encapsulated in NCNTs for water splitting. CNT-based Co-N-C and various alloys like FeCo@NCNTs23, CoNi@NC24, NiS2 HMSs (hollow microspheres)25 for HER, CoS2@N,S-codoped graphene oxide26, CoNS-C27 for OER have been reported as good electrocatalysts. Various bimetallic electrocatalysts like CoxMoy@NC (N-doped carbon)28, Ni@graphene29, Co-Pi/Co-P/Ti30, Co– Ni–B@NF31, Co-Fe-P-1.7/NF32, Ni0.7Fe0.3S233, NiMo HNRs/TiM34, FeNi3N/NF (Nickel foam)35, NiCo2O436, Ni3ZnC0.7-550/NF37, NiSe/NF38, NiFe LDH (Layered Double Hydroxide)/NF39 have also been developed for total water splitting application. Adding an electropositive and corrosion resistant metal like Cr into Co can enhance the catalytic activity by enhancing durability and stability of the catalyst. Addition of Cr can stabilise the surface adsorption energy by shifting the metal d-band centre and optimising the ∆GH that can influence the catalytic activity. This is expected due to changes in bond length and strain effects caused by doping of a heteroatom like Cr into Co lattice.40 CoCr LDH 41 and Co3−xCrxO4 spinel42 catalysts has been reported as good



OER catalyst. But the total water splitting (both HER and OER) catalytic activity of CoCr catalyst with graphene tube like morphology has not been explored so far. Herein, we develop a simple pyrolysis technique (Scheme 1) for the synthesis of bimetallic (Co-Cr) nanostructures which are entrapped in bamboo-like nitrogen-doped graphene tube (NGT) and denote as CoCr@NGT. They are shown to be highly active and stable bifunctional (HER and OER) electrocatalyst enabling overall water splitting in alkaline medium. Cobaltous sulphate monohydrate acts as a source for cobalt, chromium (III) nitrate nonahydrate is used as a source for chromium and dicynadiamide is used as nitrogen and carbon source. Our approach for the synthesis of NGT is more facile and cost-effective as compared to the reported CVD technique to produce GT using Ni nanowire templates.43 The terminology “graphene tube” was first used by Li et al.44,45 The cross-sectional diameter is ~100 nm and the walls consist of ~10 graphene layers. A total alkaline water electrolyser assembled by using CoCr@NGT as catalyst on both anode and cathode delivered a current density of 10 mA/cm2 at around 1.58 V over a long period of time competing with the state-of-art combination of Pt/C and RuO2. Co/Cr ratio has been varied simply by pyrolysing different ratio of cobaltous sulphate monohydrate and chromium (III) nitrate nonahydrate and denoted as CoCr(10-X:X)@NGT. The catalyst synthesized from only Co precursor is denoted as Co@NGT and the one from only Cr precursor is denoted as CrN@G. Only Cr precursor results in the formation of chromium nitride (CrN) nanoparticles over graphitic carbon. A detailed study has been carried out to elucidate the role of Cr on catalytic activity.


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Scheme 1. Synthesis of CoCr nanostructures entrapped in bamboo-like N-doped graphene tube (CoCr@NGT) via pyrolysis.

30

40

50

Intensity (a.u.)

(220)

60

CrN@G (311) (222)

(220)

(111) (200)

Co@NGT

70

80

D band G band

Id/Ig= 1.04 Id/Ig= 1 Id/Ig= 1.1

Co-2p

O-1s C-1s

Cr-2p N-1s Co-3s *

Id/Ig= 1

90

600

2θθ (degree)

1200

Pyrrolic-N

Intensity (a.u.)

200

2400

400

600

Co 2p

(f)

Cr 2p

2p

3/2

1/2

Co (II)

Co (III)

Co (II)

Co (III)

2p

2p

Co (III)

1/2

Co (0)

800

Binding Energy (eV)

CoCr(9:1)@NGT 2p

Pyridinic-N

1800

Raman shift (cm-1)

(e)

(d) Graphitic-N

(c) Intensity (a.u.)

(111)

CoCr(9:1)@NGT

20

CoCr(7:3)@NGT CoCr(9:1)@NGT CrN@G Co@NGT

3/2

Co (III)

Co (0)

2p3/2

2p1/2

Intensity (a.u.)

10

(b)

CoCr(7:3)@NGT (200)

Intensity (a.u.)

(a)

G raphite

RESULTS AND DISCUSSION

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


Co@NGT

404

402

400

398

396

Binding Energy (eV)

394

798

792

786

780

774

Binding Energy (eV)

588

584

580

576

572

Binding Energy (eV)

Figure 1. (a) XRD patterns, (b) Raman spectra of as-prepared samples, and (c-f) survey XPS and HRXPS spectrum corresponding to N 1s, Co 2p, and Cr 2p, respectively of CoCr(9:1)@NGT.



The XRD patterns of different samples are shown in Figure 1(a). A broad peak at 26.1° corresponds to the presence of (002) plane of the graphitic carbon. Additional peaks at 44.21, 51.5 and 75.8° corresponding to (111), (200) and (220) planes, respectively confirm the formation of pristine cobalt (JCPDS 15-0806).46 The formation of CrN over graphene is observed with presence of peaks at 37.7°, 43.8°, 63.5°, 76.2° and 80.3° corresponding to (111), (200), (220), (311) and (222) planes of CrN and a (002) graphitic carbon peak at 26.1° (JCPDS 03-065-2899).47 With increasing Cr content in Co, peaks are shifted towards low angle, i.e. from 44.21° in Co@NGT to 44.12 and 44.03° in CoCr(9:1)@NGT and CoCr(7:3)@NGT, respectively which may be attributed to the bigger atomic size of Cr as compared to Co. This implies an increase in d-spacing as observed from 2.046 to 2.051 to 2.054 Å, respectively. The average crystallite size determined using the Scherrer equation is found to be ~66 nm and ~60 nm for CoCr(9:1)@NGT and Co@NGT, respectively. Figure 1(b) shows the Raman spectra of the obtained nanostructures in 600-2400 cm-1 range. It revealed two peaks located at around 1370 cm-1 and 1595 cm-1, corresponding to the reported D and G bands of carbon materials, respectively.48,49 The D band is assigned to the in-plane vibrations of terminal carbon atoms of disordered graphite with dangling bonds, and G is assigned to the E2g mode related to the vibration of sp2 hybrid carbon atoms. The high ID/IG band intensity ratio is ~1.1 for CrN@G, while it is ~1.0 for Co@NGT and CoCr(9:1)@NGT and ~1.04 for CoCr(7:3)@NGT. This suggests that a large amount of N doping in the graphene layers. This may also be the reason of enhanced electrocatalytic activity on Cr alloying with Co. The X-ray photoelectron spectroscopy (XPS) measurements were carried out to determine the elemental composition and chemical state of the CoCr(9:1)@NGT. The XPS survey spectrum of CoCr(9:1)@NGT as shown in Figure 1(c) reveals the presence of C, Co, Cr, N and O. The sharp peak of C is due to graphitic C and sharp


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peak of O is due to the adsorbed oxygen on the graphene surface. The small peaks of N indicate the doping in hexagonal carbon matrix. The high resolution XPS (HRXPS) spectrum of C-1s (Figure S1) depicts a sharp peak at 284.6 eV corresponding to the sp2 carbon. The N 1s peak as shown in Figure 1(d) can be fitted into three peaks with binding energy of 401.4, 400.5 and 398.5 eV that correspond to graphitic, pyrrolic and pyridinic N, respectively. The atomic % of N doping is ~ 5-6% in both CoCr(9:1)@NGT and CoCr(7:3)@NGT. Figure 1(e) displays the deconvoluted XPS spectra of Co 2p of both Co@NGT and CoCr(9:1)@NGT. The two distinct XPS energy bands correspond to the Co 2p3/2 (lower binding energy) and Co 2p1/2 (higher binding energy). The de-convoluted spectrum of Co@NGT reveals two bands at binding energies of 778.2 and 793.1 eV corresponding to Co 2p3/2 and Co 2p1/2, respectively for metallic Co (0) and two bands at binding energy of 781.1 and 796.7 eV due to the formation of CoOx with the Co (III) oxidation state.50,51 In the case of CoCr(9:1)@NGT, the Co 2p3/2 and Co 2p1/2 bands at binding energies of 781.1 and 796.7 eV, respectively correspond to Co (III) as is the case with Co@NGT, while Co 2p3/2 and Co 2p1/2 bands at binding energies of 784.3 and 799.6 eV, respectively correspond to Co (II).52 Figure 1(f) shows the HRXPS spectra of Cr 2p which contains a spin-orbit doublet at 576.7 and 586.0 eV for Cr 2p3/2 and Cr 2p1/2, respectively which corresponds to Cr (III) oxidation state. This is believed to be due to formation of chromium oxide layer.53,54 The overall elemental analysis is also performed by the energy dispersive X-ray spectroscopy (EDS) and shown in Figure S2. The CoCr(9:1)@NGT contains only 2.52 wt% of Cr, whereas CoCr(7:3)@NGT contains 4.85 wt% of Cr. The thermogravimetric analysis (TGA) was carried out in air atmosphere. It suggests that amount of the bimetallic moiety, i.e., CoCr is 57% and that of carbon and nitrogen in CoCr(9:1)@NGT is 43% (Figure S3).



The morphology was characterised by scanning electron microscopy (SEM) and the SEM image of the as-prepared CoCr(9:1)@NGT as shown in Figure 2(a). The bamboo-like NGT can easily be depicted. Bimetallic Co-Cr nanostructures can be seen at the tip of the NGT entrapped in graphene layers. The transmission electron microscopy (TEM) image as shown in Figure 2(b) clearly depicts NGT morphology and the dark particles encapsulated in graphitic shells can be attributed to the presence of bimetallic Co-Cr nanostructure. A large size NGT with variable tube size (cross-sectional diameter ~ 100 nm) is observed. high resolution TEM (HRTEM) image as shown in Figure 2(c) reveals the wall thickness to be 11-13 layers graphene for CoCr(9:1)@NGT. The bamboo-like structure is a typical morphological feature of N doping as seen in inset Figure 2(c).45,50 The interlayer spacing of 0.334 nm as shown in Figure 2(d), is in good agreement with the (100) plane of graphitic carbon. Figure 2(d) also depicts the d-spacing of the tip in case of 0.205 nm for CoCr(9:1)@NGT corresponding to (111) crystal plane corresponding to the Co-Cr which is in accordance with the value predicted by XRD analysis. The TEM and HRTEM images of the Co@NGT hybrid nanostructures also reveal the formation of tip growth of graphene tube. The d-spacing of the Co is 0.204 nm corresponding to the (111) crystal plane of the metallic Co (Figure S4). Similarly, SEM morphology of the Co@NGT and CoCr(7:3)@NGT also depicted metal nanostructures encapsulated in bamboo like graphene tube hybrid nanostructures (Figure S5). The average crystallite size on the basis of TEM image is found to be 65 nm and 82 nm for Co@NGT and CoCr(9:1)@NGT, respectively. This is in good accordance with that estimated from XRD data using Scherrer equation. The SEM image of CrN@G is shown in Figure S6. The TEM and HRTEM image clearly depicts the formation of CrN nanoparticles over graphitic carbon (Figure S6). Figure 2(e) displays the high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image of the hybrid


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nanostructure along with the line scan. The elemental mapping shown in Figure 2(f) reveals the presence of Co, Cr on the tip and the uniform coverage of C and N. There is a significant difference in the distribution of Co and Cr in the elemental mapping because the amount of Cr is very less as compared to that of Co. On the basis of these experimental results, we propose a possible growth mechanism for CoCr(9:1)@NGT catalyst. Cr3+/Co2+ ions are reduced to form nano-sized bimetallic crystallites by the reducing environment. Simultaneously, the cyanide group in dicyanadiamide forms the polymeric carbon nitride via poly-addition/condensation process. At temperature above 7000C, the polymeric carbon nitride decomposes with concurrent release of large amount of cyano fragments (e.g., C2N2+, C3N2+, C3N3+) which provides both C and N source for the formation of NGT. The growth of NGT is catalysed by the as formed bimetallic (Co-Cr) nanocrystal by the widely known tip-growth mechanism.55-57 In this context, it is worthy to note that Cr does not promote graphitization, while Co promotes graphitization and hence, the formation of NGTs.



(a)

(b)

(c)

(d)

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0.205 nm

11-13 layers

0.334 nm

(f)

(e)

400 nm

C

Co

Cr

N

Figure 2. (a and b) show the SEM and TEM image, (c) HRTEM images of the layer structure of graphene in the side wall of NGT and inset shows the NGT structure. (d) HRTEM image of the graphene encapsulated CoCr nanostructure, (e) HAADF-STEM image along with the line scan, and (f) elemental mapping for CoCr(9:1)@NGT.


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The electrocatalytic activity of the as-prepared samples for OER was carried out in 1M KOH solution. For comparison, the commercial RuO2 catalyst is also tested under the same conditions. The polarisation curve was obtained from linear sweep voltammetry (LSV) measurement as shown in Figure. 3(a) at a scan rate of 10 mV/s at 1600 rpm for the as-prepared samples and state-of-the-art RuO2 catalyst with a mass loading of 0.714 mg/cm2. The onset overpotential is 150 mV for CoCr(9:1)@NGT, while that for RuO2 is 179 mV as shown in inset Figure 3(a). To reach a current density of 10 mA/cm2 (a parameter of utmost importance in solar cells), CoCr(9:1)@NGT requires the least overpotential of 330 mV, whereas CoCr(7:3)@NGT requires 362 mV and RuO2 requires 360 mV. On the other hand, Co@NGT shows a current density of 10 mA/cm2 at an overpotential of 520 mV and CrN@G samples shows a maximum current density of 8.2 mA/cm2 at an overpotential of 620 mV. This clearly shows the enhancement in catalytic activity by the addition of Cr into Co. At a fixed overpotential of 500 mV, the current density is 67, 38, and 34 mA/cm2 for CoCr(9:1)@NGT, CoCr(7:3)@NGT and RuO2, respectively. This signifies that addition of Cr in Co should not exceed an optimum of 10% for the best catalytic activity. Apart from the current density at a given overpotential, the rate of change of this current density with overpotential is important.58 Tafel slope is a measure for the rate of change of overpotential with log of current density and was evaluated from a plot of overpotential (η) against log (J) as shown in Figure 3(b). Tafel slope was found to be 98, 95, and 115 mV/dec for RuO2, CoCr(9:1)@NGT, and CoCr(7:3)@NGT, respectively. It is interesting to note that Tafel slope for CoCr(9:1)@NGT is comparable to RuO2 and is lower as compared to CoCr(7:3)@NGT. Overall, the as-prepared CoCr(9:1)@NGT catalyst shows better current density as well as overpotential as compared to many reported Co-based catalyst (Table S1).



90

9 6 3 0 1.3

60

(b)

(a )

12

Overpotential /V

J (mA/cm2)

J (mA/cm2)

0.45

15

120

1.4

1.5

1.6

Potential (V vs. RHE) CoCr(9:1)@NGT CoCr(7:3)@NGT Co@NGT CrN@G RuO2

30 0 1.2

1.4

1.6

1.8

0.40 98 mV/dec 115 mV/dec

0.35

0.30

0.25 -0.9

2.0

-0.6

-60 0

-80

-100

-10

-20

O verpotential /V

-40

J (mA/cm2)

J (mA/cm2)

CoCr(9:1)@NGT CoCr(7:3)@NGT Co@NGT CrN@G Pt-C

-30

-0.2

-0.1

118 mV/dec

0.2 113 mV/dec

0.1

CoCr(9:1)@NGT CoCr(7:3)@NGT Pt-C

0.0

Potential (V vs. RHE)

-0.4

-0.2

0.0

0.0 0.3

0.2

0.6

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15

i10k/i0 = 80.5 %

12

0

After 2000 cycles Initial

-60

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-18

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1.2

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(f)

J (m A /cm 2 )

J (mA/cm 2 )

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(e)

Initial After 2000 cycles

150

0.9

log(J(mA/cm-2))

Potential (V vs. RHE) 24

0.3

116 mV/dec

0.3

-120 -140

0.0

(d)

(c)

-20

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log(J(mA/cm-2)) 0.4

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CoCr(9:1)@NGT CoCr(7:3)@NGT RuO2

95 mV/dec


J (m A/cm 2 )

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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-90 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1

-15


-12

i10k/i0 = 85.7 %

-9

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Time (x 103 s)

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Time (x 103 s)

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10

Figure 3. (a) OER LSV curves in 1 M KOH at a scan rate of 10 mV/s, (inset: onset overpotential for OER) (b) corresponding Tafel plots, (c) HER LSV curves in 1 M KOH at a scan rate of 10 mV/s, (inset: onset overpotential for HER) and (d) corresponding Tafel plots, (e) chronoamperometric curve at η=370 mV for OER catalyzed by CoCr(9:1)@NGT (inset: polarization curve of CoCr(9:1)@NGT after 2000nd continuous cycles at 100 mV/s). (f)


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chronoamperometric curve at η=200 mV for HER catalyzed by CoCr(9:1)@NGT

(inset:

polarization curve of CoCr(9:1)@NGT after 2000nd continuous cycles at 100 mV/s). The electrocatalytic activity for HER was also evaluated in N2-saturated 1.0 M KOH solution at a scan rate of 10 mV/s. The polarization curve obtained from LSV as shown in Figure 3(c) of the samples reveals a low onset potential of 64 mV for CoCr(9:1)@NGT, 115 mV for CoCr(7:3)@NGT and 174 mV for Co@NGT as shown in inset Figure 3(c). The overpotential required to reach a current density of 10 mA/cm2 was found to be 55 mV for Pt-C, followed by 179 mV for CoCr(9:1)@NGT and 237 mV for CoCr(7:3)@NGT. The maximum current density shown by CrN@G is 11 mA/cm2 at an overpotential of 390 mV. This signifies that similar to OER, adding Cr in Co also improves the HER activity. Pt-C exhibits the best activity till a potential of 320 mV and beyond, CoCr(9:1)@NGT takes over and shows the best catalytic activity at an overpotential of 350 mV with a current density of 75 mA/cm2 as compared to the current density of 66 mA/cm2 shown by Pt-C. Tafel slope (Figure 3(d)) for the CoCr(9:1)@NGT was found out to be 113 mV/dec. This suggests that the reaction follows a Volmer-Heyrovsky pathway. Tafel slope for Pt-C and CoCr(7:3)@NGT was found as 118 and 116 mV/dec, respectively. Though the Tafel slopes are comparable, CoCr(9:1)@NGT catalyst shows better HER activity as compared to many reported non-precious electrocatalysts (Table S2). For a catalyst to be used in water electrolyser, the stability and durability in strong alkaline medium is very important. The chronoamperometric response of CoCr(9:1)@NGT is recorded over a period of 10,000 s with continuous operation at an overpotential of 370 mV and presented in Figure 3(e). It shows a retention of 80.5% of the initial current density which implies good durability of the catalyst. The retention of 78.7% is shown by Co@NGT at an overpotential of 520 mV (Figure S7). Accelerated degradation test (ADT) also was carried out at a scan rate of



100 mV/s and the results are presented in the inset of Figure 3(e). It shows a slight positive shift of 10 mV to achieve a current density of 10 mA/cm2 after 2000nd cycles. The activity which is retained can be attributed to large wall thickness (11-13 layers of graphene) of NGT and durability enhanced by corrosion resistant nature of Cr. The HER chronoamperometric response of CoCr(9:1)@NGT at an overpotential of 200 mV in 1 M KOH and the ADT were carried out at a scan rate of 100 mV/s for 2000nd cycles shown in Figure 3(f). Slight decrease of cathodic current density at higher overpotential is observed. It can be noted from Figure 3(f) that 85.7% of the initial current density was retained by the catalyst over a period of 10,000s of continuous operation pointing towards the excellent stability. The chronoamperometric response of Co@NGT at an overpotential of 330 mV showed a retention of 82.8% for the same duration of operation (Figure S7). This suggests that addition of Cr has a significant impact on the stability towards both OER and HER. This is believed to be due to the synergistic effect of anti-corrosive nature of Cr, doping of nitrogen and large wall thickness of NGT.


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0.6

3

(a)

0.2 0.0 -0.2 10 mV 25mV 50 mV 75 mV 100 mV

-0.4 -0.6 -0.8

(b)

2

Co@NGT

0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80

J (mA/cm2)

J (mA/cm2)

0.4

1 0

-2

CoCr(7:3)@NGT

-3

0.45

0.55

0.60

0.65

0.70

2 0 -2 10 mV 25 mV 50 mV 75 mV 100 mV

-4 -6

CoCr(9:1)@NGT

-8 0.45

0.50

0.55

0.60

0.65


0.70

∆ J = J a - J c (mA/cm 2)

8

(c)

4

-10

0.50


8 6

10 mV 25 mV 50 mV 75 mV 100 mV

-1


J (mA/cm 2 )

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


(d) 6

38 mF/cm2

CoCr(9:1)@NGT CoCr(7:3)@NGT Co@NGT

4 12.5 mF/cm2

2 2

2.5 mF/cm 0 0

20

40

60

80

100

Scan Rate (mV/s)

Figure 4. CV at different scan rates in faradaic silent region for (a) Co@NGT, (b) CoCr(7:3)@NGT, (c) CoCr(9:1)@NGT, and (d) charging current-density difference plotted against scan rate to yield Cdl, double-layer capacitance of the catalysts. The electrochemically active surface area (ECSA) of the catalyst is calculated from the doublelayer capacitance (Cdl) in non-faradaic region.59 Cyclic voltammetry (CV) was carried out in faradaic silent region for different scan rates for the samples as presented in Figure. 4(a-c) and Figure S8. Cdl is a half of the slope of plots for the difference of anodic and cathodic current density (∆J = Ja - Jc) against scan rates in the non-faradaic region. Figure 4(d) displays the variation of ∆J as a function of the scan rate which yields Cdl of 38, 12.5, and 2.5 mF/cm2 for CoCr(9:1)@NGT, CoCr(7:3)@NGT and Co@NGT, respectively which correlates well with the observed activity trends. The Cdl for CrN@G is found to be 8 mF/cm2 as shown in Figure S8.



Though the Cdl for CrN@G is higher as compared to Co@NGT, the HER/OER activity of the former is inferior to the latter. This indicates that activity of Co@NGT is far better as compared to CrN@G. The possible mechanism based on the experimental study is that Co (II) acts as the catalytically active site and Cr (III) act as the charge transfer site. As seen from the HER and OER LSV curves, the catalytic activity of Co@NGT is slightly better even though Cdl is higher as compared to Co@NGT The mechanism for the HER is governed by Volmer Heyrovsky pathway, while the mechanism of OER can be understood as a combination of three steps, i.e., adsorption of water on the active site, formation of intermediates like *OH, *O, *OOH and finally evolution of O2. The incorporation of Cr stabilizes the surface adsorption energy of the intermediates formed by shifting the d-band center of Co.40 Also, Cr (III) withdraws electron density from Co making it more electrophilic and thus facilitating the nucleophilic addition of water.41,42 This increases the catalytic activity towards both HER and OER. In addition, XRD analysis reveals that the introduction of Cr increases the lattice parameter, i.e., distance between two adjacent Co atoms. This means that with increasing Cr content, there is a decrease in surface density of catalytically active site. Thus, the Co to Cr ratio needs to be optimized for the best activity and it is 10% as realized from our experimental studies. To depict the role of Co-Cr nanostructures in electrocatalysis, the as-prepared CoCr(9:1)@NGT catalyst was kept in 1 M H2SO4 solution overnight. The SEM images shown in Figure 5 (a and b) show that majority of the Co-Cr moiety present at the tip of the tube was leached out after acid-treatment. The decrease of major XRD peaks as evident from Figure 5 (c) depicts the removal of Co-Cr moiety to a large extent. The OER activity observed in 1 M KOH solution at 10 mV/s scan rate of LSV as shown in Figure 5(d) depicts an increase in overpotential


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by 24 mV to reach the current density of 10 mA/cm2 after acid-treatment. For an overpotential of 470 mV, the current density decreases by 28 mA/cm2 after acid-treatment. As shown in Figure 5(e), HER activity also decreases and the overpotential has to increase by 19 mV to obtain a current density of 10 mA/cm2. To reach a fixed current density of 40 mA/cm2, an increase in overpotential by 100 mV is observed after acid-treatment of the CoCr(9:1)@NGT. The ECSA is also evaluated and found to decrease after the acid-treatment. The Cdl value calculated as shown in Figure 5(f) was 22 mF/cm2, whereas it was 38 mF/cm2 before. The decrease observed in ECSA corresponds well with the decrease in the electrochemical activity. This signifies that the Co-Cr nanostructures act as catalytically active sites and provide increased surface area for electron transfer. The chronoamperometric response shown in Figure 5(g and h) depicts a retention of 86% for OER and 81.6% for HER.



(111)

(c)

(200)

Graphitic

CoCr(9:1)@NGT

(220)

(b) Intensity (a.u.)

(a)

Acid treated CoCr(9:1)@NGT

10

100

Acid treated CoCr(9:1)@NGT CoCr(9:1)@NGT

80 60 40

8

(e)

-20 -40 -60 -80

Acid treated CoCr(9:1)@NGT CoCr(9:1)@NGT

20

1.2

1.4

1.6

1.8

50

60

2θθ (degree)

70

80

90

(f) Acid treated CoCr(9:1)@NGT CoCr(9:1)@NGT

6

38 mF/cm2

5 4

22 mF/cm2

3 2

-0.3 -0.2 -0.1

0.0

0.1

0.2

0

20

(g)

-20

18

40

60

80

100

Scan Rate (mV/s)


Potential (V vs. RHE) 20

40

0

-0.5 -0.4

2.0

30

1

-100

0

20

7

∆ J (mA/cm 2)

0

(d) J (mA/cm 2 )

120

J (m A/cm 2 )

(h)

16

J (mA/cm 2)

-18

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

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i10k/i0 = 86%

14 12 10

-16

i10k/i0 = 81.6%

-14 -12 -10

0

2

4

6

Time (x 103 s)

8

10

0

2

4

6

8

10

Time (x 103 s)

Figure 5. SEM images of CoCr(9:1)@NGT (a) before and (b) after the acid-treatment, (c) XRD plot, (d) OER LSV curves in 1 M KOH at a scan rate of 10 mV/s, (e) HER LSV curves in 1 M KOH at a scan rate of 10 mV/s, (f) charging current-density difference plotted against scan rate to yield Cdl, double-layer capacitance of the catalysts, (g) chronoamperometric curve at η=390 mV for OER, and (h) chronoamperometric curve at η=230 mV for HER catalyzed by acidtreated CoCr(9:1)@NGT.


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80

(b)

C paper-RuO2

(a)

Pt-RuO2

J (mA/cm 2)

60

CoCr(9:1)@NGTCoCr(9:1)@NGT

40 20 0 1.2

1.4

1.6

1.8

2.0

Potential (V vs. RHE) 13.0

(c)

(d)

12.5

J (mA/cm 2)

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


12.0

i20k/i0 = 92.3 %

11.5

OER

11.0

O2

H2

10.5 HER

10.0

0

5

10

15

20

Time (x103 s)

Figure 6. (a) Scheme depicting total water splitting, (b) linear polarization curves of RuO2carbon paper, RuO2-Pt-C and CoCr(9:1)@NGT- CoCr(9:1)@NGT for overall water splitting in 1 M KOH at a scan rate of 10 mV/s, (c) stability test of the electrolyser at 11.5 mA/cm2, and (d) photographs of total water splitting. From the above mentioned results of HER and OER, we expected that CoCr(9:1)@NGT could act as a bi-functional (HER and OER) water splitting electrocatalyst (Figure 6(a)). The polarization curves obtained from LSV of CoCr(9:1)@NGT/ CoCr(9:1)@NGT, carbon paper/RuO2 and Pt/RuO2 for overall water splitting in 1 M KOH at a scan rate of 10 mV/s (with a mass loading of 2.5 mg/cm2 onto C paper) is shown in Figure. 6(b). The onset overpotential observed for this catalyst is 150 mV. It achieves a current density of 10 mA/cm2 at an overpotential of 350 mV as compared to 280 mV observed for state-of-the-art RuO2 -Pt catalyst.



Page 20 of 29

The potential required to reach a current density of 10 mA/cm2 was the least for this catalyst as compared to many other reported catalysts as shown in Table 1. The stability test is done at 11.5

2 µm

mA/cm2 for 20,000 s is shown in Figure 6 (c) depicts a net retention of 92.3%. The image shown in Figure 6(d) depicts the generation of O2 and H2 bubbles at the respective carbon paper electrodes. The bubbles generated at cathode are twice as that of anode. This is because during water splitting reaction H2 evolved is twice of O2. It is interesting to note that CoCr(9:1)@NGT / CoCr(9:1)@NGT electrodes surpasses the catalytic activity of Pt/RuO2 at an overpotential of 570 mV and is stable over 20,000 s. Thus, the addition of ~2 weight % of Cr into Co@NGT increases the electrocatalytic activity to a large extent and the as synthesised catalyst is seen to be a good electrocatalyst towards total water splitting. The addition of Cr provides stability to the structure due to its anti-corrosion property and also shifts the metal d-band center of Co which helps in optimising the ∆GH that influences the catalytic activity towards both HER and OER. Table 1. Comparison of total water splitting voltage with some reported electrocatalysts.

Catalyst

Voltage/V at Reference 10 mA/cm2

Co–Ni–B@NF

1.72

31

Co-Fe-P-1.7/ NF

1.60

32

Ni0.7Fe0.3S2

1.625

33

NiMo HNRs/TiM

1.64

34

FeNi3N/NF

1.62

35

NiCo2O4

1.65

36

Ni3ZnC0.7-550/NF

1.65

37

NiSe/NF

1.63

38


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Ni2P/NF

1.63

60

NiFe LDH/NF

1.70

39

NiS–Ni2P2S6/NF

1.64

61

CoCr(9:1)@NGT 1.58

This work

CONCLUSION Co-Cr nanostructures entrapped in N-doped graphene tube are successfully synthesized and their applications as bifunctional HER and OER electrocatalyst for total water splitting (in alkaline medium) have been reported for the first time. Among the different samples prepared by varying the Co-Cr ratio, CoCr(9:1)@NGT showed the best catalytic activity competing with the state-ofthe-art catalysts for both HER as well as OER. A total alkaline water electrolyzer made by employing CoCr(9:1)@NGT as catalyst on both anode and cathode delivered a current density of 10 mA/cm2 at around 1.58 V for a long period of time competing with the commercially used combination of Pt/C and RuO2. The electrochemical performance strongly depends on the Cr due to its corrosion resistance activity that leads to long-term stability. METHODS Material synthesis: All chemicals used in this work were of analytical reagent grade and commercially available, and used without further purification. In a typical procedure, Cobaltous sulphate monohydrate (CoSO4.H2O) and Chromium (III) nitrate nonahydrate (Cr(NO3)3.9H2O) were taken in different ratio by weight (in mg, 100:0, 90:10, 70:30, 0:100) and added dicyandiamide (C2H4N4, 0.5g) in a beaker. Added 10 mL DI water and 10 mL ethanol. The



solution was dried at 80°C. The powder obtained was scratched and pyrolyzed at 900 °C for 1 h in a closed quartz tube. The samples are duly named as Co@NGT, CoCr(9:1)@NGT, CoCr(7:3)@NGT and CrN@G. The as synthesized product is collected for characterization and is subsequently tested for catalytic activity. Material Characterizations: X-ray diffraction (XRD) data was collected using a wide-angle Xray diffractometer (XRD, PANalytical) equipped with a Cu Kα radiation (1.54 Å). The scanning electron microscopy (SEM) images were obtained using a FESEM FEI Inspect 50. The EDX spectra was recorded on a FE-SEM and used to determine the elemental ingredients of as prepared samples. TEM characterizations were carried out using JEOL JEM-2100F at an accelerating voltage of 200 kV. Raman spectra of the samples were recorded on a WITec system with a 532 nm excitation wavelength. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250 (Thermo Electron) with a monochromatic Al Kα (1486.6 eV) source. Thermo gravimetric analysis (TGA) of the samples were carried out on a Perkin Elmer model TGA Q50 V20.13 Build 39. Electrochemical measurements: The electrochemical tests were carried out on a conventional three electrode cell by using a CHI750E workstation coupled with a rotating disc electrode (RDE) system. In a typical three electrode electrochemical system, a rotating disc electrode i.e., commercial glassy carbon electrode (GCE, 3 mm diameter, 0.07 cm2 area) served as the working electrode while a silver/silver chloride electrode and Pt wire worked as the reference and counter electrode, respectively. The electrode preparation was carried out as follows: 0.5 mg of catalyst was added to 85 µL ethanol followed by 15 µL of 5 wt% Nafion© 117 solution. The as-prepared mixture was sonicated for 30 min. 10 µL of the catalyst ink so obtained was drop-casted onto the


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GCE and air-dried which resulted in a catalyst loading of 0.714 mg/cm2. For total-water splitting studies, 2.5 mg of the catalyst was dispersed in 100 µL ethanol and 15 µL Nafion© solution and the mixture was sonicated for 30 min followed by drop-casting the complete solution on carbon paper. The potential measured against Ag/AgCl electrode was converted to potential versus reversible hydrogen electrode (RHE) according to the equation, EvsRHE = EvsAg/AgCl + E°Ag/AgCl + 0.059 pH. EvsAg/AgCl is the experimental potential measured against the Ag/AgCl reference electrode, and E0Ag/AgCl is the standard potential of Ag/AgCl (0.222 V). All the measurements were carried out in 1 M KOH solution (pH=14).

ASSOCIATED CONTENT: Supporting Information available Characterisations of the catalyst: Additional XPS spectra, EDS, TGA, TEM, HRTEM, SEM, chronoamperometeric curve for Co@NGT catalyst and comparison of catalytic activities with other reported OER and HER catalysts.

AUTHOR INFORMATION Corresponding Author * Phone: +91-080-2293 2996. Fax: +91-80-2360 7316. E-mail: [email protected] Author Contributions BS, BKB and KKN designed the experiment. BS and BKB performed the experiment. BS, BKB and KKN analyzed the data and wrote the manuscript.



ORCID id: Bidushi Sarkar: 0000-0003-4783-3870, Barun Kumar Barman: 0000-0002-98941890, Karuna Kar Nanda: 0000-0001-9496-1408 Funding Sources The authors acknowledge Department of Science and Technology (DST), India for supporting the proposal. The idea of this work is an outcome of the proposal. 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 Monalisa Ghosh for XPS characterization and Jaschin for TEM imaging in the Chemical Science Division in IISc.

SYNOPSIS: An economical approach for the synthesis of a bi-functional electrocatalyst consisting cobalt-chromium nanostructures entrapped in N-doped graphene tube (CoCr@NGT). Cr alloying with the entrapped Co in the NGT drastically enhanced both the HER and OER performance with the low over potential (η) and better current density along with the long-term durability.


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61. Zhang, X.; Zhang, S.; Li, J.; and Wang, E., One-step synthesis of well-structured NiS– Ni2P2S6 nanosheets on nickel foam for efficient overall water splitting, J. Mater. Chem. A, 2017, 5 (42), 22131-22136.


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