Nano Res

1

Bi-functional catalysts of Co3O4@GCN tubular

nanostructured (TNS) hybrids for oxygen and hydrogen

evolution reactions

Muhammad Tahir1,5, Nasir Mahmood2, Xiaoxue Zhang3, Tariq Mahmood1, Faheem. K. Butt1, Imran Aslam1, M.

Tanveer1, Faryal Idrees1, Syed Khalid1, Imran Shakir4, Yi-Ming Yan3, Ji-Jun Zou5, ChuanbaoCao1 ( ), and

Yanglong Hou2 ( )

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0872-1

http://www.thenanoresearch.com on July 31, 2015

© Tsinghua University Press 2015

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Nano Research

DOI 10.1007/s12274-015-0872-1

1

Bi-functional Catalysts of Co3O4@GCN tubular

nanostructured (TNS) hybrids for Oxygen and Hydrogen

Evolution Reactions

Muhammad Tahira,e,‡, Nasir Mahmoodb,‡, Xiaoxue Zhangc, Tariq Mahmooda, Faheem. K. Butta,

Imran Aslama, M.Tanveera, Faryal Idreesa ,Syed Khalida, Imran Shakird, Yi-Ming Yanc, Ji-Jun

Zoue, ChuanbaoCaoa,*,YanglongHoub,*

a Research Centre of Materials Science, Beijing Institute of Technology, Beijing 100081,China

E-mail: cbcao@bit.edu.cn

b Department of Materials Science and Engineering, Peking University, Beijing 100081, China

E-mail: hou@pku.edu.cn

C Beijing Key Laboratory for Chemical Power Source and Green Catalyst, School of Chemical

Engineering and Environment, Beijing Institution of Technology, Beijing, 100081, China

dSustainable Energy Technologies (SET) center building No 3, Room 1c23, College of Engineeri

ng, King Saud University, PO-BOX 800, Riyadh 11421, Kingdom of Saudi Arabia

e Key Laboratory for Green Chemical Technology of the Ministry of Education, School of

Chemical Engineering and Technology, Tianjin University; Collaborative Innovative Center of

Chemical Science and Engineering (Tianjin), Tianjin 300072, China

Keywords: carbon nitride; cobalt oxide; bi-functional catalyst; oxygen evolution reaction;

hydrogen evolution reaction

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ABSTRACT: Catalysts for oxygen and hydrogen evolution reactions (OER/HER) are the

heart of renewable green energy source like water splitting. Although incredible efforts have

been done to develop catalysts for OER and HER with good efficiency but still great

challenges remain to come up with single bi-functional catalysts. Here, we report a novel

hybrid of Co3O4 embedded in tubular nanostructures of graphitic carbon nitride (GCN)

synthesized through a facile and large scale chemical method at low temperature. Strong

synergistic effect among Co3O4 and GCN results in excellent performance as a bi-functional

catalyst for OER and HER. High surface area, unique tubular nanostructure and composition

of the hybrid bring all redox sites easily available for catalysis and provide faster ionic and

electronic conduction. The Co3O4@GCNtubular nanostructured (TNS) hybrid exhibits the

lowest over potential (0.12 V) and excellent current density (147 mAcm-2) for OER, better

than benchmark IrO2 and RuO2, with superior durability in alkaline media. Furthermore, the

Co3O4@GCN TNS hybrid demonstrates excellent performance for HER with much lower

onset and over potential as well as stable current density. It is expected that the

Co3O4@GCN TNS hybrid developed in the present study is an attractive alternative catalyst

than noble metals for large scale water splitting and fuel cells.

1. INTRODUCTION

Growing energy demands have stimulated intensive research on alternative energy

production and storage systems with high efficiency at low cost and environment benignity. 1-10

Hydrogen production from water splitting can play a pivotal role to overcome the challenges of

increasing energy demands.11-13 Water splitting reaction is a combination of two half reactions:

first is oxygen evolution reaction (OER) and the other one is hydrogen evolution reaction

(HER).14,15 In addition, the demand of green production of H2 is going to be increased to reduce

3

the CO2 emission because H2 is mainly produced from fossil fuels to process the heavier

petroleum feedstock.16 Furthermore, the existence of large quantity of water in universe makes

these reactions very economical and approximately inexhaustible.17 However, the concerns

related to the stability of electrode and high over potentials of OER and HER catalysts are two

fundamental constrain for large scale hydrogen and oxygen production. 18,19 A catalyst that can

drive both HER/OER is highly desirable as it is fundamental necessities of the most important

energy harvesting device i.e. water splitting.20 However, finding efficient and stable catalysts,

which can drive both of these reactions simultaneously at lower over potential to make the water-

splitting reaction more energy-efficient is very difficult.17 Because the best catalysts for OER

(RuO2 and IrO2) have usually poor HER activity while the best HER catalysts (Pt) has only

moderate activity for OER.16,21 One possible way to develop bi-functional catalyst for HER and

OER is by combining these noble metals/metal oxides, but higher cost and rarity of these metals

are big hurdles.17,22 Therefore, development of low-cost and stable bi-functional catalyst with

lowest possible over potentials for both reactions remains great challenge.23

Graphitic carbon nitride (GCN) is one of the most attractive materials that have excellent

electrochemical properties.18,24-32 Further GCN has the ability for both OER and HER

electrocatalysis, but its poor conductivity and unavailability of redox sites in pure phase is big

stone for the applications of GCN based materials. Thus, to improve the limitations of GCN,

several strategies were adopted e.g. composite fabrication with highly conductive and active

counterparts, but the results are still far from the practical utilization of GCN for water splitting.

However, the nanostructured hybrid materials that can bring the redox active sites easily

available on surface with improved conductivity can make possible the practical usage of GCN. 33

Therefore, pinning of active metal oxides nanoparticles (NPs) at the surface of tubular structure

4

can resolve the aforementioned problems by bringing the active sites at surface that can be

accessed easily by electrolyte and improving the mass and electrons transfer by shortening the

diffusion path and high conductivity. By utilizing the advantages of both components, the hybrid

nanostructure can lower the over potential and enhance the current density for both half reactions.

Further GCN contains large amount of nitrogen atoms that can improve the electron donor-

accepter ability of GCN and provide the anchoring sites to NPs.34 Thus, the strong coupling of

NPs with GCN can make possible the faster and reversible transfer of electrons, which bring the

excellent performance as bi-functional catalyst for both OER and HER. To the best of our

knowledge, such a unique design to realize the bi-functionality of hybrid composed of metal

oxide embedded in tubular nanostructured (TNS) GCN for OER and HER catalysts is rarely

reported. Among various metals,Co3O4 got tremendous attention but alone it shows very little

OER activity, however, when grew on carbonaceous materials exhibits surprisingly high

performance as catalyst.35

Here, we present a facile and low cost methodology for large scale synthesis of Co3O4@GCN

TNS hybrid at low temperature. The Co3O4@GCN TNS hybrid possess large active surface area,

unique composition and structure, thus can efficiently accelerate the electrochemical process.

The effectively coupled Co3O4@GCN TNS hybrid is a well suited catalyst for gas-involved

electrochemical reactions due to highly stable and inert nature of GCN while the metal

counterpart deliver exceptional OER activity in alkaline medium. It is worth mentioning that

Co3O4@GCN TNS hybrid exhibits superior OER activity than RuO2 and IrO2 by showing lowest

over potential (0.12 V) and highest current density (147 mAcm-2). The hybrid also displays good

activity for HER comparable with Pt/C. Thus, Co3O4@GCN TNS hybrid is leading towards the

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class of valuable and high performance non-precious metal based bi-functional catalysts for OER

and HER to realize the purposeful water splitting.

2. RESULTS AND DISCUSSION

Morphological characterizations of as-synthesized products were done using field emission

scanning electron microscope (FESEM) and transmission electron microscope (TEM). Figure 1a

is presenting the FESEM image of as-synthesized Co3O4@GCNTNS hybrid (Co3O4@GCN-5-

450), from where it is clear that the hybrid shows tubular structure and all the NPs are well-

dispersed on the inner and outer walls of GCN TNS. However, such a unique structure of the

hybrid which consists of GCN at backbone and NPs are completely embedded in the tube walls

is highly favorable for catalysis because it can allows faster ionic and electronic transport.

Furthermore, FESEM studies show that tubular structures are about 0.6μm in diameter and few

microns in length, which are highly intermingled to build the continues network of GCN that can

accelerate the flow of electron in the electrode as well as offer the highly exposed active surface

area to bring all the redox sites at surface and easily available for catalysis.

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Figure 1. (a) FESEM, (b) TEM and (c) HRTEM images of Co3O4@GCN-5-450 hybrid (d)

SAED pattern of Co3O4@GCN-5-450 hybrid (e) XRD pattern of Co3O4@GCN-5-450 hybrid(the

inset shows crystal structure of Co3O4NPs) (f) TGA curves of Co3O4@GCN-5-400 and

Co3O4@GCN-5-450 hybrids.

Figure 1b shows the TEM image of the Co3O4@GCN TNS hybrid, it is worth noting that the

GCN grew in the form of uniform tubular nanostructures that are interconnected with each other,

providing faster highway to electrons via walls and internal hollow structure facilitate efficient

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mass transfer by shortening the diffusion path. Further the presence of Co3O4 NPs on the both

internal and external surface of GCN TNS, confirmed by the TEM image, can activate the redox

sites for splitting of water molecules. Further high resolution TEM (HRTEM) image of

Co3O4@GCN TNS hybrid is confirming the existence well-attached and dispersed NPs on the

surface of GCN TNS, as shown in Figure 1c. Furthermore, the inter-planner distances of 0.12 nm,

0.16 nm and 0.28 nm are found for various NPs that are well-matched with plans of Co3O4 (622),

(422) and (220) respectively; according to the standard card No JCPDS 78-1969. Thus, HRTEM

analysis shows that Co3O4 NPs grew in well-crystalline form and are strongly pined on the GCN

TNS. Furthermore, the HRTEM studies delineate the amorphous nature of GCN, as indicated by

the arrows in Figure 1c. The structural and morphological features of all other samples are

discussed in the supporting information and presented in Figure S1-6. Scattered area electron

diffraction (SAED) studies were performed to further confirm the structure and crystallinity of

as-synthesized Co3O4 NPs decorated on GCN TNS, interestingly it is found that Co3O4 NPs are

grew in polycrystalline form indicated by the circular fringes with spot pattern, shown in Figure

1d. Thus, the SAED studies further indicate that Co3O4@GCN TNS hybrid bears crystalline

Co3O4 NPs and amorphous GCN, synergistically offering strong electrochemical coupling,

which can make the hybrid highly efficient bi-functional catalyst for both OER and HER as

explained in the respective section below. The inter-planner distances are calculated from SAED

pattern to further verify the structure of Co3O4@GCN TNS hybrid and it is found that the results

are well-matched with HRTEM and XRD studies (Figure 1d). In order to investigate the crystal

structure of as-synthesized samples, x-ray diffraction (XRD) studies were carried out, shown in

Figure 1e (The XRD analysis of all other samples was presented in Figure S7 and discussed in

supporting information). The XRD result of Co3O4@GCN-5-400 (Figure S7c) hybrid exhibits its

8

amorphous nature as no obvious XRD peaks were observed for Co3O4 NPs. However, with the

increasing annealing temperature to 450ºC, the hybrid(Co3O4@GCN-5-450) displays well-

crystalline nature and shows strong X-ray reflection that is well-matched with standard card

JCPDS No. 78-1969 (Figure 1e).Furthermore, to delineate the structure of Co 3O4 NPs, Rietveld

refining of the crystals structures were done and it is found that Co3O4 NPs present in the form of

facet center cubic (FCC)crystal structure (space group Fd-3m and space group number 227),

shown in the inset of Figure 1e. The XRD results further confirmed that the formation of Co3O4

NPs required higher temperature of (450 ºC), which transformed the cobalt precursor to cobalt

oxide, as no formation of Co3O4 NPs occurred at 400 ºC because of transformed reaction energy

barrier. Furthermore, the evaporation of carbon at higher temperature from GCN also facilitates

the formation of Co3O4 NPs at the GCN surface, which was evidenced from the weight loss of

carbon, higher concentration of metallic counterpart and strong reflection of XRD peaks.

However, the concentration of cobalt precursor also plays critical role in defining the

composition and crystallinity of the hybrid as it is found that the XRD peaks of Co3O4@GCN-5-

450 are more intense and less broad compared to Co3O4@GCN-1-450, indicating the improved

crystallinity of the acquired sample at higher concentration, shown in Figure S7. Thermal

gravimetric analysis (TGA) was performed in order to determine the stability and composition of

Co3O4@GCN-5-400 and Co3O4@GCN-5-450 hybrids. Figure 1f is presenting the TGA curves of

Co3O4@GCN-5-400 and Co3O4@GCN-5-450 hybrids, from where it is obvious that there are

two weight losses at the same temperature range for both hybrids. The initial weight loss starts

around 100 ºC and continues to 400 ºC, assigned to the loss of trapped water molecules and

attached functional groups on the surface of GCN in both samples. The second major weight loss

occurred at 400 ºC where Co3O4@GCN-5-400and Co3O4@GCN-5-450 hybrids decomposed and

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the weight loss of about 64% and 40% was observed, respectively, because of the removal of

GCN as TGA studies were performed in air. Thus, with increasing Co precursor concentration,

higher Co3O4 NPs were loaded with the removal of more GCN during the synthesis procedure, as

Co was involved in the catalysis of GCN to produce carbon, thus higher Co concentration

catalyze more GCN.17

To determine the chemical composition and nature of chemical bonding of constituent

elements in as-synthesized hybrids, x-ray photoelectron spectroscopy (XPS) was carried out, as

shown in Figure 2.The full scan spectra of XPS reveal the existence of core levels of C, N, O and

Co in all the samples, as indicated in Figure 2a, further approve the high purity of as-synthesized

products. XPS studies also support the HRTEM and XRD results that an increase in Co precursor

concentration increased the amount of Co3O4 NPs in the product, as 3.05, 5.03 and 17.02 wt.%

are obtained for Co3O4@GCN-1-400, Co3O4@GCN-5-400 and Co3O4@GCN-5-450,

respectively. Slightly lower concentration values of metallic counterparts are observed than the

values calculated from TGA studies based on surface analysis of XPS. However, lower

concentration values further confirm that NPs are well-embedded in the GCN matrix which can

bring better synergistic effect to improve the overall conductivity and catalytic properties of

hybrid structure. The de-convoluted C 1s spectrum of Co3O4@GCN-5-450 shows three distinct

peaks at 284.47, 285.60 and 288.03 eV that correspond to graphitic carbon, C-OH and the sp2

bonded carbon in the hetero-cycles (N-C=N), respectively.18,22,25 The high resolution C 1s

spectrum of Co3O4@GCN-1-400 also shows similar behavior as there are three analogous peaks

present at same binding energy values (Figure S9a&b). To explore the nature of existing

nitrogen,de-convolution of N 1s is carried out, shown in Figure 2c and four different kinds of

nitrogen centers are present in the Co3O4@GCN-5-450 at 400.6, 399.56, 398.5 and 397.80 eV

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which corresponds to graphitic, pyrrolic, amino and pyridinic, respectively.24,36,37 It was proved

that presence of various nitrogen centers can change the density of state and accelrate the

electronic cloud of graphetic carbon which can enhance the conductivity andelectrochemical

properties of GCN.34,38 Similarly, high resolution N 1s spectra of Co3O4@GCN-1-400 and

Co3O4@GCN-5-400 indicate the presence of pyrrolic, amino and pyridinic nitrogen centers in

both samples (Figure S9c&d). Figure 2d and S10a&b show the high resolution spectra for O1s of

Co3O4@GCN-5-450,Co3O4@GCN-1-400 and Co3O4@GCN-5-400 hybrids, respectively. The

O1s exhibit three peaks at 531, 530.1 and 529 eV. These peaks are associated with oxygen ions

in low coordination states at the surface and metal-oxygen bonds for Co3O4@GCN-5-450.13 The

existence of metal-oxygen bond confirms the bridging of NPs with carbon through the oxygen,

which makes them stable during the catalysis of water. Further the existence of carboxyl and

hydroxyl groups on the surface of GCN act as active sites to catalyze the splitting of water

molecules.13 XPS spectrum of Co 2p (Figure 2e) shows two spin-orbit doublets of Co 2p1/2 at

780.5 and 796.5 eV that attributed to Co2+, while two spin-orbit doublets of Co 2p3/2 at 779.1 and

794.8 eV are belongs to Co3+.39 As the water splitting is a surface reaction thus exposed surface

of the catalyst is very important factor to enhance the catalytic process. To determine the

exposed surface, BET measurement was carried out and it is found thatCo3O4@GCN-5-450

hybrid brings highest surface area (62.50 m2g-1) among all the samples, shown in Figure 2f.

WhileCo3O4@GCN-5-400 hybrid shows surface area (50.54 m2g-1) which further decrease by

decreasing the concentration of Co precursor, while Co3O4@GCN-1-400 hybrid shows only

surface area of 29.06 m2g-1 (Figure 2f). Thus, it was identified that the hypothesis of GCN

evaporation with higher concentration of Co precursor and prolonged annealing temperature

brings better porosity in the hybrid and improve its catalytic properties in better way. So, it is

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worth mentioning that Co3O4@GCN-5-450 hybrid with higher surface area and larger contents

of NPs will provide the better results for OER and HER. Moreover, the pores in the products act

as tunnels for the deep penetration of electrolyte inside the electrode and can improve the mass

transport, thus are highly important for better catalytic properties.40 Here, pore size distribution is

also calculated to evaluate their effect on catalysis, presented in Figure S8b, the major pore size

distribution fall in the range of 2-4 nm for Co3O4@GCN-5-450 that is very helpful for efficient

transfer of ions.

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Figure 2. (a) Full scan XPS spectra of Co3O4@GCN-1-400, Co3O4@GCN-5-400 and

Co3O4@GCN-5-450 hybrids (b) High resolution C 1s (c) N 1s (d) O 1s (e) Co 2p spectra of

Co3O4@GCN-5-450 hybrid (f) N2 absorption curves of Co3O4@GCN-1-400, Co3O4@GCN-5-

400 and Co3O4@GCN-5-450 hybrids.

Considering unique structure and composition of Co3O4@GCNTNS hybrids, here we explore

their OER and HER property using rotating ring disk electrode (RRDE) for future applications of

fuel cells, lithium-air battery and water splitting. Initially, electrocatalytic properties of the

Co3O4@GCNTNS hybrids were investigated as catalyst for OER by charging them uniformly on

a glassy carbon electrode and OER polarization curves were recorded at slow scan rate of 5mVs-

1 to minimize the capacitive current. The bare GCN shows very poor performance both at the

onset potential and current density compared to the hybrids because of the poor access to redox

sites and lower conductivity (Figure 3a). In contrast to bare GCN, the onset potential of

Co3O4@GCN-5-450 hybrid is 1.40 V along with excellent current density of 147 mAcm-2

(Figure 3a), which confirm that incorporation of NPs to GCN tubular structure brings all the

redox sites available at surface and catalyze maximum water molecules to produce large amount

of oxygen. Furthermore, it is interesting that at lower and higher concentration of NPs, the hybrid

shows poor performance, confirmed from the lower onset potentials values of 1.42 and 1.45 V

for Co3O4@GCN-1-450 and Co3O4@GCN-10-450 hybrids, respectively, along with poor current

densities (110 to 130 mAcm-2) as shown in Figure 3a (a close view is presented in Figure S11).

Thus, comparative studies have proved that to improve the electrocatalytic properties of GCN, a

specific concentration of NPs are required as presented above that can activate the redox sites

and brings high active surface area to exposed the maximum redox sites to electrolyte.

Furthermore, to compare the electrocatalytic property of hybrid with noble metal catalysts, the

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linear sweep voltammetry (LSV) curves of Co3O4@GCN-5-450 hybrid (1.40 V) along with

RuO2 (1.30 V) and IrO2 (1.45 V) were obtained (Figure 3b), from where it is worth noting that

the hybrid has much better performance than both noble metals catalysts not only in terms of

onset potential but also in case of current density (65 and 87 mAcm-2 for RuO2 and IrO2,

respectively). Since the potential reached at a current density of 10 mAcm-2 is significant

performance index for OER catalyst, because it is about the current density for a 10% efficient

solar-to-fuel conversion device.22 Figure 3c shows the onset potentials, over potentials (the

difference between the theoretical and onset potential) and potentials at current density reaching

to 10 mAcm-2. The higher onset potential values of 1.58 and 1.6 V are found for RuO2 and IrO2,

respectively, at the current density of 10 mAcm-2 compared to the excellent value of 1.5 V for

Co3O4@GCN-5-450. The superior activity of Co3O4@GCN-5-450 TNS hybrid can also be seen

from lower over potential, Co3O4@GCN-5-450 exhibits over potential of 0.12 V compared to

0.14 and 0.16 V for RuO2 and IrO2, respectively. To the best of our knowledge, the over

potential value found here is the best reported value yet,23,25,41-45 not only Co3O4@GCN-5-450

TNS hybrid outperformed the RuO2 and IrO2, in the case of over potential other hybrids

(Co3O4@GCN-5-400 and Co3O4@GCN-10-400)also show lower value of over potential 0.13 V.

The extraordinary performance of the hybrid at over potential, current density and potential value

at current density of 10 mAcm-2 are confirming the advantages of unique structure and

composition of as-synthesized hybrid and proved that hybrid has ability to replace the expensive

and rare traditional noble metal catalysts. Furthermore, to explore the effect of NPs concentration

and synthesis temperature, potential values of different hybrids at current density of 10 mAcm-2

are calculated, shown in Table S2. Interestingly, it is found that as the concentration of Co

precursor is increased from 0.01g to 0.05g (Figure S12b-d), an improved onset potential and

14

current density is found, because the maximum redox sites are available on the surface with an

appropriate amount of Co3O4 NPs which are uniformly distributed on the both sides of tube walls.

However, when the concentration is further increased to 0.1g (Figure S12e&f), it reduces the

performance by increasing non-reactive sites and destroying the synergism among the GCN and

NPs. To further investigate the effect of synthesis temperature on the OER activity of

Co3O4@GCN hybrid, Co3O4@GCN-5 hybrid was prepared at 400 and 450 ºC (Figure S12d &

Figure3d, respectively). As explained above in XRD results, increasing temperature improved

the crystalline nature of the Co3O4 NPs and brought larger concentration of NPs on surface with

more active sites, resulting in better OER performance both in case of onset potential and current

density. Hence, all the results described above confirmed that to bring better onset potential and

current density as well as excellent performance at cut off current density, it is highly desirable

that GCN tubular structure should be decorated with highly active metal counterpart (Co3O4 NPs)

with appropriate loading and crystal quality. Figure 3d shows the LSV curves of Co3O4@GCN-

5-450 hybrid at different rotation rates, it is noted from the graph that with the increase of

rotation speed, the current density is also improved because the penetration of electrolyte

increased inside the electrode at higher rotation. In order to investigate the catalytic kinetics of

OER, Tafel plot is obtained to represent the relationship of over potential and current density and

compare the performance of various samples, shown in Figure 3e. The smaller Tafel slope (76

mV/dec) is observed for Co3O4@GCN-5-450 TNS hybrid which indicates that it is highly

favorable for OER by offering low energy barrier for the evolution of oxygen as presented in

Figure 3e. In one word, the advantage of Co3O4@GCN-5-450 hybrid over noble metals oxides

can be observed in every aspect (low onset potential, low over-potential and high current density

along with excellent Tafel slope), thus assures that these catalysts can be replaced with cheap and

15

earth abundant catalyst. To further verify the outstanding performance of Co 3O4@GCN-5-450

hybrid, its stability was measured by charging it at 0.5 V for 10 h, shown in Figure3f. The better

stability of the hybrid comes up because of the structural stability contributed by GCN matrix

and strong pinning of NPs to the GCN matrix. So, the excellent stability further highlights that

the hybrid structure efficiently took the benefits from each part; as a result, better synergism

provides excellent performance and stability as OER catalyst. Thus, it is expected that the

Co3O4@GCNTNS hybrid developed in the present study is a potential candidate to catalyze the

chemical reactions in air batteries, fuel cell and water splitting.

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Figure 3. (a) LSV curves of all the samples at 1600 rpm in 1 M KOH for OER(b) LSV curves of

Co3O4@GCN-5-450 hybrid, IrO2 and RuO2 at 1600 rpm in 1 M KOH for OER (c)onset

potentials, over potentials and potentials required to reach 10 mAcm-2 current density of the OER

catalyzed by all samples(here sample 1,2,3,4,5,6,7,8 and 9 represents GCN, Co3O4@GCN-1-400,

Co3O4@GCN-1-450, Co3O4@GCN-5-400, Co3O4@GCN-5-450, Co3O4@GCN-10-400,

Co3O4@GCN-10-450, IrO2 and RuO2respectively) (d) LSV curves ofCo3O4@GCN-5-450 hybrid

17

at different rpm in 1 M KOH for OER(e) Tafel plots of Co3O4@GCN-5-450 hybrid, IrO2 and

RuO2 (f) Stability test of Co3O4@GCN-5-450 for 10 h in 1 M KOH for OER.

To make the hybrid more practical potential, we explore its bi-functionality as a catalyst for

HER to produce the hydrogen from water because hydrogen is highly required for various

purposes e.g. green energy and to process the heavier petroleum feedstock to lower the CO 2

emission. In order to find out the HER abilities of Co3O4@GCN hybrids, RRDE configuration is

used in 0.5 M H2SO4 against Ag/AgCl and compared with commercially used catalyst (Pt/C),

shown in Figure 4a. The Co3O4@GCN-5-450 hybrid reveals a small onset potential of -0.03 V

toward HER, very close to onset potential of commercial Pt/C (-0.01V), but slightly lower

current density was observed for Co3O4@GCN-5-450. Furthermore, to explore the role of Co3O4

NPs on the catalytic ability of GCN, bare GCN was also employed as catalyst and it is found that

the GCN alone is not good catalyst as onset potential of GCN is very poor-0.27V.Thus, the onset

potential values of GCN and the hybrid confirms that to attain better performance, an appropriate

loading of Co3O4 NPs is highly required. It is worth noting that only small loading of NPs brings

a big difference in the performance of hybrid that based on the unique design of catalyst

presented here, like hybrid offers short diffusion path to ions, highly conductive highway for

electrons, extremely exposed active surface area and easy access to redox sites. Similar to OER,

the effect of different concentration of Co precursor and synthesis temperature were also

explored as shown in Figure 4a. It is found that as the concentration increased from 0.01g to

0.05g, better onset potential was observed, but with further increase to 0.1g, again poor value is

attained. Once again verifies our hypothesis that appropriate required concentration of Co

precursor is 0.05g which is necessary to activate the redox sites in the hybrid and enhanced its

conductivity. Furthermore, the hybrids prepared at lower temperature (400ºC) shows poor onset

18

values due to their amorphous nature, while the hybrids synthesized at 450ºC brings much

improved results both with better onset potential and current density (Figure 4a). Figure 4b

shows the LSV curves of Co3O4@GCN-5-450 hybrid at different rotation speeds, an improved

current density was found with increasing rotation speed due to faster diffusion of electrolyte in

the electrode. Figure 4c shows the onset and over potentials for all the samples along with

commercial Pt/C. It is clear from the Figure 4c that the over potential (0.09V) of Co3O4@GCN-

5-450 hybrid is very close to that of Pt/C (0.06V), which confirm the excellent HER catalytic

activity of Co3O4@GCN-5-450 hybrid. Furthermore, Co3O4@GCN-5-450 hybrid also bears very

good stability when tested at constant current galvanostatic discharge for 10 h, presented in

Figure 4d. Such a high stability of the hybrid is contributed from the strongly interconnected

network of GCN to accelerate the electronic conduction and catalysis of water molecule at

surface by highly active redox sites. However, the mechanism for the excellent performance of

the Co3O4@GCN hybrid both for OER and HER is still not fully understood. It is expected that

the rich state provided by cobalt, nitrogen and carboxyl/hydroxyl groups in the

Co3O4@GCNtubular structure electrode play important roles in its enhanced OER and HER

performance with a low over potential. The existence of carboxyl/hydroxyl groups along with

partially negative nitrogen centers helps to absorb the water molecules on their surfaces, which is

a significant initial step in OER and HER. Moreover, the strong synergistic relationship among

the GCN and Co3O4 in Co3O4@GCNTNS hybrid along with unique tubular structure, high active

surface area and easily available redox sites are most likely other important factors for the

excellent OER/HER performances of the hybrid. Thus, the Co3O4@GCNTNS hybrid is a novel

catalyst for energy conversion technologies based on non-precious earth-abundant metallic

catalysts.

19

Figure 4. (a) LSV curves of all the samples and Pt/C at 1600 rpm in 0.5 M H 2SO4 for HER (b)

LSV curves ofCo3O4@GCN-5-450 at different rpm in 0.5 M H2SO4 for HER (c) Onset potentials

and over potentials for HER of all samples (here sample 1,2,3,4,5,6,7 and 8 represents GCN,

Co3O4@GCN-1-400, Co3O4@GCN-1-450, Co3O4@GCN-5-400, Co3O4@GCN-5-450,

Co3O4@GCN-10-400, Co3O4@GCN-10-450 and Pt/C, respectively) (d) Stability test of

Co3O4@GCN-5-450 for 10 h in 0.5 M H2SO4 for HER.

20

3. CONCLUSIONS

In summary, we have synthesized Co3O4@GCNTNS hybrid through simple chemical method

at low temperature. As-synthesized hybrid exhibited excellent bi-functional catalytic activity for

both OER and HER. Co3O4@GCNTNS hybrid demonstrates low onset potential and high current

density for both electrode reactions due to fully disperse Co3O4NPs in GCN, special structures,

unique composition and high active surface area which bring maximum redox sites at the surface.

Most importantly, Co3O4@GCNTNS hybrid have surpassed the best noble metals oxides

catalysts for OER catalytic activity with excellent over potential (0.12 V) and s uperior current

density (147 mAcm-2), as well as approaches to the onset potential of Pt/C as HER catalyst. This

work presents a novel approach to design low cost OER and HER bi-functional catalysts through

facile method at large scale which can outperform the noble metal-based electrocatalysts and will

motivate the development of renewable energy sources.

4. EXPERIMETAL METHODS

4.1 Fabrication of Co3O4@GCNTNS Hybrid: To synthesize the Co3O4@GCN TNS hybrid,

1g of melamine was dissolved in 30mLof ethylene glycol and a saturated solution was made.

Then 60mLof 0.1M HNO3 was added to the previously prepared solution with continuous

stirring of 10mins, afterward washed with ethanol and dried at 60 ºC for 12h. In result, white

color powder was obtained. Later on50mg CoCl2 6H2O and 1 g of white powder were dispersed

in 20mL of ethanol in separate glass beakers and sonicated for 1 h. The n the dispersed solutions

were mixed and magnetically stirred for 1 hour and dried at 60ºC for 12h, finally this mixture

was annealed at 450ºC for 2h at heating rate of 10ºC/min. Different samples were prepared with

different masses ofCoCl26H2O(10, 50 and 100 mg) and these samples were annealed at two

different temperatures (400 and 450ºC). The samples are given name according to temperature

21

and concentration of CoCl26H2O, like Co3O4@GCN-x-y, here “x” represents initial mass in %

of the CoCl26H2Oand “y” represents the temperature in ºC. Co3O4@GCN-1-400 corresponds to

the sample with 1% mass content of CoCl26H2O and at 400ºC.

4.2 Characterizations: X-Ray diffraction pattern of prepared samples was recorded by XRD;

Philips X'Pert Pro MPD, using Cu-Ka radiation source, x-ray Photoelectron Spectra was done by

using (Thermo Scientific, Escalab250Xi). Morphological characterization was done by Field

emission scanning electron microscopy (FESEM, Hitachi S-4800). Energy dispersive x-ray

spectroscopy (EDS, Hitachi S-4800) was used to determine the composition. The transmission

electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and

selected area electron diffraction (SAED) pattern were measured by (JEOL-JEM-2100F).The

surface area and porosity was measured using Beishide Instrument-ST 3H-2000PS2 through

Brunauer-Emmett-Teller (BET) method. The thermogravimetric analysis (TGA) and differential

scanning calorimetric (DSC) were determined by a SDT Q600 (USA) in air at a heating rate of 10 °C/min

from 25 to 600 °C.

4.3 Electrochemical Characterization: Rotating ring-disk electrode (RRDE) measurements

were carried out by using a CHI 760C electrochemical workstation with a three-electrode system.

Working electrode consisted of glassy-carbon (GC) (5 mm in diameter and 0.25 cm2 thick); Pt

wire electrode is used for counter and Ag/AgCl as reference electrode. Electrode was prepared

by making the suspension of 1mg active materials in ethanol (0.85mL) and Nafion (0.15mL)

under sonication. After sonication 10µL of this solution was incorporated on the GC. Electrolyte

consists 1M KOH aqueous solution for OER and 0.5M H2SO4 for HER.

22

ACKNOWLEDGMENT

Work at Beijing Institute of Technology was supported by NSFC (23171023，50972017)

and Doctoral Program of the Ministry of Education of China (20101101110026); Work at Peking

University was supported by the NSFC-RGC Joint Research Scheme (51361165201), NSFC

(51125001, 51172005), Beijing Natural Science Foundation (2122022) and Doctoral Program of

the Ministry of Education of China (20120001110078). Deanship of Scientific Research at King

Saud University through Prolific Research Group Project no: PRG-1436-25.

ASSOCIATED CONTENT

Supporting Information: Supporting Information contains, detail morphological, structural and

compositional analysis of all the samples along with their electrochemical characterization, it

also contains the information about the samples names and their synthesis conditions as well as

the comparative study with noble metals catalysts. “This material is available free of charge via

the Internet at http://dx.doi.org/10.1007/***********************).”

AUTHORS INFORMATION

Corresponding Authors

*Email: (Prof. C. Cao) cbcao@bit.edu.cn

*Email: (Prof. Y. Hou) hou@pku.edu.cn

‡These authors contributed equally.

REFERENCES

(1) Ali, Z.; Cao, C.; Li, J.; Wang, Y.; Cao, T.; Tanveer, M.; Tahir, M.; Idrees, F.; Butt,

F. K.: Effect of synthesis technique on electrochemical performance of bismuth selenide. Journal

of Power Sources 2013, 229, 216-222.

(2) Butt, F. K.; Tahir, M.; Cao, C.; Idrees, F.; Ahmed, R.; Khan, W. S.; Ali, Z.;

Mahmood, N.; Tanveer, M.; Mahmood, A.; Aslam, I.: Synthesis of Novel ZnV2O4 Hierarchical

23

Nanospheres and Their Applications as Electrochemical Supercapacitor and Hydrogen Storage

Material. ACS Applied Materials & Interfaces 2014, 6, 13635-13641.

(3) Li, J.; Wang, G.; Wang, J.; Miao, S.; Wei, M.; Yang, F.; Yu, L.; Bao, X.:

Architecture of PtFe/C catalyst with high activity and durability for oxygen reduction reaction.

Nano Res. 2014, 7, 1519-1527.

(4) Kim, W.-S.; Hwa, Y.; Kim, H.-C.; Choi, J.-H.; Sohn, H.-J.; Hong, S.-H.:

SnO2@Co3O4 hollow nano-spheres for a Li-ion battery anode with extraordinary performance.

Nano Res. 2014, 7, 1128-1136.

(5) Mahmood, N.; Zhang, C.; Liu, F.; Zhu, J.; Hou, Y.: Hybrid of Co3Sn2@Co

Nanoparticles and Nitrogen-Doped Graphene as a Lithium Ion Battery Anode. ACS Nano 2013,

7, 10307-10318.

(6) Mahmood, N.; Hou, Y.: Electrode Nanostructures in Lithium-Based Batteries.

Adv. Sci. 2014, 1, doi:10.1002/advs.201400012.

(7) Mahmood, N.; Zhang, C.; Hou, Y.: Nickel Sulfide/Nitrogen-Doped Graphene

Composites: Phase-Controlled Synthesis and High Performance Anode Materials for Lithium Ion

Batteries. Small 2013, 9, 1321-1328.

(8) Gong, M.; Dai, H.: A mini review of NiFe-based materials as highly active

oxygen evolution reaction electrocatalysts. Nano Res. 2015, 8, 23-39.

(9) Fu, G.; Liu, Z.; Chen, Y.; Lin, J.; Tang, Y.; Lu, T.: Synthesis and electrocatalytic

activity of Au@Pd core-shell nanothorns for the oxygen reduction reaction. Nano Res. 2014, 7,

1205-1214.

(10) Hou, J.; Cao, C.; Idrees, F.; Ma, X.: Hierarchical Porous Nitrogen-Doped Carbon

Nanosheets Derived from Silk for Ultrahigh-Capacity Battery Anodes and Supercapacitors. ACS

Nano 2015, 9, 2556-2564.

(11) Qing, L.; Mahmood, N.; Zhu, J.; Hou, Y.; Sun, S.: Graphene and Its Composites

with Nanoparticles for Electrochemical Energy Applications. Nano Today 2014, 9, 668-683. (12) Zhang, C.; Mahmood, N.; Yin, H.; Liu, F.; Hou, Y.: Synthesis of Phosphorus-

Doped Graphene and its Multifunctional Applications for Oxygen Reduction Reaction and

Lithium Ion Batteries. Adv Mater 2013, 25, 4932.

(13) Mahmood, N.; Zhang, C.; Jiang, J.; Liu, F.; Hou, Y.: Multifunctional co3 s4

/graphene composites for lithium ion batteries and oxygen reduction reaction. Chemistry A

Eurpeon Juornal 2013, 19, 5183-90.

(14) Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.; Nocera,

D. G.: Solar Energy Supply and Storage for the Legacy and Nonlegacy Worlds. Chem. Rev. 2010,

110, 6474-6502.

(15) Subbaraman, R.; Tripkovic, D.; Chang, K.-C.; Strmcnik, D.; Paulikas, A. P.;

Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M.: Trends in activity for the

water electrolyser reactions on 3d M(Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts. Nat Mater 2012,

11, 550-557.

(16) Kibsgaard, J.; Jaramillo, T. F.: Molybdenum phosphosulfide: an active, acid-

stable, earth-abundant catalyst for the hydrogen evolution reaction. Angew. Chem. Int. Ed. 2014,

53, 14433-7.

(17) Zou, X.; Huang, X.; Goswami, A.; Silva, R.; Sathe, B. R.; Mikmekova, E.; Asefa,

T.: Cobalt-embedded nitrogen-rich carbon nanotubes efficiently catalyze hydrogen evolution

reaction at all pH values. Angew. Chem. Int. Ed. 2014, 53, 4372-6.

24

(18) Zhao, Y.; Zhao, F.; Wang, X.; Xu, C.; Zhang, Z.; Shi, G.; Qu, L.: Graphitic

carbon nitride nanoribbons: graphene-assisted formation and synergic function for highly

efficient hydrogen evolution. Angew. Chem. Int. Ed. 2014, 53, 13934-9.

(19) Jahan, M.; Liu, Z.; Loh, K. P.: A Graphene Oxide and Copper-Centered Metal

Organic Framework Composite as a Tri-Functional Catalyst for HER, OER, and ORR. Adv.

Funct. Mater. 2013, 23, 5363-5372.

(20) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y.: A

perovskite oxide optimized for oxygen evolution catalysis from molecular orbital principles.

Science 2011, 334, 1383-5.

(21) Kanan, M. W.; Nocera, D. G.: In situ formation of an oxygen-evolving catalyst in

neutral water containing phosphate and Co2+. Science 2008, 321, 1072-5.

(22) Long, X.; Li, J.; Xiao, S.; Yan, K.; Wang, Z.; Chen, H.; Yang, S.: A strongly

coupled graphene and FeNi double hydroxide hybrid as an excellent electrocatalyst for the

oxygen evolution reaction. Angew. Chem. Int. Ed. 2014, 53, 7584-8.

(23) Zhou, W.; Wu, X.-J.; Cao, X.; Huang, X.; Tan, C.; Tian, J.; Liu, H.; Wang, J.;

Zhang, H.: Ni3S2 nanorods/Ni foam composite electrode with low overpotential for

electrocatalytic oxygen evolution. Energy & Environmental Science 2013, 6, 2921-2924.

(24) Tahir, M.; Cao, C.; Butt, F. K.; Idrees, F.; Mahmood, N.; Ali, Z.; Aslam, I.;

Tanveer, M.; Rizwan, M.; Mahmood, T.: Tubular graphitic-C3N4: a prospective material for

energy storage and green photocatalysis. Journal of Materials Chemistry A 2013, 1, 13949-

13955.

(25) Ma, T. Y.; Dai, S.; Jaroniec, M.; Qiao, S. Z.: Graphitic carbon nitride nanosheet-

carbon nanotube three-dimensional porous composites as high-performance oxygen evolution

electrocatalysts. Angew. Chem. Int. Ed. 2014, 53, 7281-5.

(26) Martin, D. J.; Qiu, K.; Shevlin, S. A.; Handoko, A. D.; Chen, X.; Guo, Z.; Tang,

J.: Highly Efficient Photocatalytic H2 Evolution from Water using Visible Light and Structure-Controlled Graphitic Carbon Nitride. Angew. Chem. Int. Ed. 2014, 53, 9240-9245.

(27) Li, Q.; Yang, J.; Feng, D.; Wu, Z.; Wu, Q.; Park, S.; Ha, C.-S.; Zhao, D.: Facile

synthesis of porous carbon nitride spheres with hierarchical three-dimensional mesostructures for

CO2 capture. Nano Res. 2010, 3, 632-642.

(28) Han, C.; Wang, Y.; Lei, Y.; Wang, B.; Wu, N.; Shi, Q.; Li, Q.: In situ synthesis of

graphitic-C3N4 nanosheet hybridized N-doped TiO2 nanofibers for efficient photocatalytic H2

production and degradation. Nano Res. 2015, 8, 1199-1209.

(29) Han, Q.; Zhao, F.; Hu, C.; Lv, L.; Zhang, Z.; Chen, N.; Qu, L.: Facile production

of ultrathin graphitic carbon nitride nanoplatelets for efficient visible-light water splitting. Nano

Res. 2015, 8, 1718-1728.

(30) Tahir, M.; Cao, C.; Mahmood, N.; Butt, F. K.; Mahmood, A.; Idrees, F.; Hussain,

S.; Tanveer, M.; Ali, Z.; Aslam, I.: Multifunctional g-C3N4 Nanofibers: A Template-Free

Fabrication and Enhanced Optical, Electrochemical, and Photocatalyst Properties. ACS Applied

Materials & Interfaces 2014, 6, 1258-1265.

(31) Tahir, M.; Cao, C.; Butt, F. K.; Butt, S.; Idrees, F.; Ali, Z.; Aslam, I.; Tanveer, M.;

Mahmood, A.; Mahmood, N.: Large scale production of novel g-C3N4 micro strings with high

surface area and versatile photodegradation ability. CrystEngComm 2014, 16, 1825-1830.

(32) Tahir, M.; Mahmood, N.; Zhu, J.; Mahmood, A.; Butt, F. K.; Rizwan, S.; Aslam,

I.; Tanveer, M.; Idrees, F.; Shakir, I.; Cao, C.; Hou, Y.: One Dimensional Graphitic Carbon

Nitrides as Effective Metal-Free Oxygen Reduction Catalysts. Sci. Rep. 2015, 5.

25

(33) Gogotsi, Y.: What Nano Can Do for Energy Storage. ACS Nano 2014, 8, 5369-

5371.

(34) Mahmood, N.; Zhang, C.; Yin, H.; Hou, Y.: Graphene-based nanocomposites for

energy storage and conversion in lithium batteries, supercapacitors and fuel cells. J. Mater.

Chem. A 2014, 2, 15.

(35) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H.: Co3O4

nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction. Nat Mater 2011,

10, 780-786.

(36) Zhang, C.; Hao, R.; Liao, H.; Hou, Y.: Synthesis of amino-functionalized

graphene as metal-free catalyst and exploration of the roles of various nitrogen states in oxygen

reduction reaction. Nano Energy 2013, 2, 88-97.

(37) Choi, C. H.; Park, S. H.; Woo, S. I.: Binary and Ternary Doping of Nitrogen,

Boron, and Phosphorus into Carbon for Enhancing Electrochemical Oxygen Reduction Activity.

ACS Nano 2012, 6, 7084-7091.

(38) Xing, T.; Zheng, Y.; Li, L. H.; Cowie, B. C.; Gunzelmann, D.; Qiao, S. Z.; Huang,

S.; Chen, Y.: Observation of Active Sites for Oxygen Reduction Reaction on Nitrogen-Doped

Multilayer Graphene. ACS Nano 2014, 8, 6856–6862.

(39) Mahmood, N.; Tahir, M.; Mahmood, A.; Zhu, J.; Cao, C.; Hou, Y.: Chlorine-

doped carbonated cobalt hydroxide for supercapacitors with enormously high pseudocapacitive

performance and energy density. Nano Energy 2015, 11, 267-276.

(40) Yin, H.; Zhang, C.; Liu, F.; Hou, Y.: Hybrid of Iron Nitride and Nitrogen-Doped

Graphene Aerogel as Synergistic Catalyst for Oxygen Reduction Reaction. Adv. Funct. Mater.

2014, 10.1002/adfm.201303902.

(41) Cheng, F.; Shen, J.; Peng, B.; Pan, Y.; Tao, Z.; Chen, J.: Rapid room-temperature

synthesis of nanocrystalline spinels as oxygen reduction and evolution electrocatalysts. Nat

Chem 2011, 3, 79-84. (42) Masa, J.; Xia, W.; Sinev, I.; Zhao, A.; Sun, Z.; Grutzke, S.; Weide, P.; Muhler, M.;

Schuhmann, W.: Mn(x)O(y)/NC and Co(x)O(y)/NC nanoparticles embedded in a nitrogen-doped

carbon matrix for high-performance bifunctional oxygen electrodes. Angew. Chem. Int. Ed. 2014,

53, 8508-12.

(43) Gong, M.; Li, Y.; Wang, H.; Liang, Y.; Wu, J. Z.; Zhou, J.; Wang, J.; Regier, T.;

Wei, F.; Dai, H.: An advanced Ni-Fe layered double hydroxide electrocatalyst for water

oxidation. J Am Chem Soc 2013, 135, 8452-5.

(44) Ma, T. Y.; Ran, J.; Dai, S.; Jaroniec, M.; Qiao, S. Z.: Phosphorus-Doped

Graphitic Carbon Nitrides Grown in Situ on Carbon-Fiber Paper: Flexible and Reversible

Oxygen Electrodes. Angew. Chem. Int. Ed. 2014, 10.1002/anie.201411125.

(45) Peng, S.; Li, L.; Han, X.; Sun, W.; Srinivasan, M.; Mhaisalkar, S. G.; Cheng, F.;

Yan, Q.; Chen, J.; Ramakrishna, S.: Cobalt Sulfide Nanosheet/Graphene/Carbon Nanotube

Nanocomposites as Flexible Electrodes for Hydrogen Evolution. Angewandte Chemie

International Edition 2014, 53, 12594-12599.

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Entry for the Table of Contents (TOC)

Utilizing a facile and large scale chemical method at low temperature, a strong synergism was

built among GCN and Co3O4that results excellent performance as a bi-functional catalyst for

OER and HER. The Hybrid exhibits lowest over potential of 0.12 V and current density of 147

mAcm-2 for OER better than benchmark IrO2 and RuO2. Furthermore, the hybrid demonstrates

excellent performance for HER with much lower onset, over potential and a stable current

density.