(TNS) hybrids for Oxygen and Hydrogen Evolution

Nano Research DOI (automatically inserted by the publisher) Research Article

Bi-functional

Catalysts

of

Co3O4@GCN

tubular

nanostructured (TNS) hybrids for Oxygen and Hydrogen Evolution Reactions

Muhammad Tahir

1,2‡

1

3‡

4

1

, Nasir Mahmood , Xiaoxue Zhang , Tariq Mahmood , 1

1

5

4

1

1

Faheem. K. Butt , Imran Aslam , 2

1

M.Tanveer , Faryal Idrees ,Syed Khalid , Imran Shakir , Yi-Ming Yan , Ji-Jun Zou (), ChuanbaoCao 3 (),YanglongHou () 1

Research Centre of Materials Science, Beijing Institute of Technology, Beijing 100081,China E-mail: [email protected] 2 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 E-mail: [email protected] 3 Department of Materials Science and Engineering, Peking University, Beijing 100081, China E-mail: [email protected] 4 Beijing Key Laboratory for Chemical Power Source and Green Catalyst, School of Chemical Engineering and Environment, Beijing Institution of Technology, Beijing, 100081, China 5 Sustainable Energy Technologies (SET) center building No 3, Room 1c23, College of Engineering, King Saud University, PO-BOX 800, Riyadh 11421, Kingdom of Saudi Arabia

‡ These authors contribute equally.

Received: day month year

ABSTRACT

Revised: day month year

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

Accepted: day month year (automatically inserted by the publisher) © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2014

Nano Res.

2

KEYWORDS carbon nitride; cobalt oxide; bi-functional catalyst; oxygen evolution reaction; hydrogen evolution reaction

1

composition of the hybrid bring all redox sites easily available for catalysis and provide faster ionic and electronic conduction. The Co3O4@GCN tubular 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.

Introduction

catalysts (Pt) has only moderate activity for OER [16,

Growing energy demands have stimulated intensive

21]. One possible way to develop bi-functional

research on alternative energy production and

catalyst for HER and OER is by combining these

storage systems with high efficiency at low cost and

noble metals/metal oxides, but higher cost and rarity

environment benignity [1-10]. Hydrogen production

of these metals are big hurdles [17, 22]. Therefore,

from water splitting can play a pivotal role to

development of low-cost and stable bi-functional

overcome the challenges of increasing energy

catalyst with lowest possible over potentials for both

demands [11-13]. Water splitting reaction is a

reactions remains great challenge [23].

combination of two half reactions: first is oxygen

Graphitic carbon nitride (GCN) is one of the most

evolution reaction (OER) and the other one is

attractive

hydrogen evolution reaction (HER)[14, 15]. In

electrochemical properties [18, 24-32]. Further GCN

addition, the demand of green production of H2 is

has the ability for both OER and HER electrocatalysis,

going to be increased to reduce the CO2 emission

but its poor conductivity and unavailability of redox

because H2 is mainly produced from fossil fuels to

sites in pure phase is big stone for the applications of

process

GCN

the

heavier

petroleum

feedstock [16].

based

materials

materials.

that

Thus,

have

to

excellent

improve

the

Furthermore, the existence of large quantity of water

limitations of GCN, several strategies were adopted

in universe makes these reactions very economical

e.g. composite fabrication with highly conductive and

and approximately inexhaustible [17]. However, the

active counterparts, but the results are still far from

concerns related to the stability of electrode and high

the practical utilization of GCN for water splitting.

over potentials of OER and HER catalysts are two

However, the nanostructured hybrid materials that

fundamental constrain for large scale hydrogen and

can bring the redox active sites easily available on

oxygen production [18, 19]. A catalyst that can drive

surface with improved conductivity can make

both

is

possible the practical usage of GCN [33]. Therefore,

fundamental necessities of the most important

pinning of active metal oxides nanoparticles (NPs) at

energy harvesting device i.e. water splitting [20].

the surface of tubular structure can resolve the

However, finding efficient and stable catalysts, which

aforementioned problems by bringing the active sites

can drive both of these reactions simultaneously at

at surface that can be accessed easily by electrolyte

lower over potential to make the water-splitting

and improving the mass and electrons transfer by

reaction more energy-efficient is very difficult [17].

shortening the diffusion path and high conductivity.

Because the best catalysts for OER (RuO2 and IrO2)

By utilizing the advantages of both components, the

have usually poor HER activity while the best HER

hybrid nanostructure can lower the over potential

HER/OER

is

highly

desirable

as

it

| www.editorialmanager.com/nare/default.asp

Nano Res.

3

and enhance the current density for both half reactions. Further GCN contains large amount of

2

RESULTS AND DISCUSSION

nitrogen atoms that can improve the electron donor-accepter ability of GCN and provide the

Morphological characterizations of as-synthesized

anchoring sites to NPs [34]. Thus, the strong coupling

products were done using field emission scanning

of NPs with GCN can make possible the faster and

electron microscope (FESEM) and transmission

reversible transfer of electrons, which bring the

electron microscope (TEM). Figure 1a is presenting

excellent performance as bi-functional catalyst for

the FESEM image of as-synthesized Co3O4@GCNTNS

both OER and HER. To the best of our knowledge,

hybrid (Co3O4@GCN-5-450), from where it is clear

such a unique design to realize the bi-functionality of

that the hybrid shows tubular structure and all the

hybrid composed of metal oxide embedded in

NPs are well-dispersed on the inner and outer walls

tubular nanostructured (TNS) GCN for OER and

of GCN TNS. However, such a unique structure of

HER catalysts is rarely reported. Among various

the hybrid which consists of GCN at backbone and

metals,Co3O4 got tremendous attention but alone it

NPs are completely embedded in the tube walls is

shows very little OER activity, however, when grew

highly favorable for catalysis because it can allows

on carbonaceous materials exhibits surprisingly high

faster ionic and electronic transport. Furthermore,

performance as catalyst [35].

FESEM studies show that tubular structures are

Here, we present a facile and low cost methodology

about 0.6μm in diameter and few microns in length,

for large scale synthesis of Co3O4@GCN TNS hybrid

which are highly intermingled to build the continues

at low temperature. The Co3O4@GCN TNS hybrid

network of GCN that can accelerate the flow of

possess

unique

electron in the electrode as well as offer the highly

composition and structure, thus can efficiently

exposed active surface area to bring all the redox

accelerate

sites at surface and easily available for catalysis.

large the

active

surface

electrochemical

area,

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 class of valuable and high performance non-precious metal based bi-functional catalysts for OER and HER to realize the purposeful water splitting.

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

hybrid

(d)

SAED

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano

pattern

of

Research

Nano Res.

4

of

inter-planner distances are calculated from SAED

Co3O4@GCN-5-450 hybrid (the inset shows crystal structure of

pattern to further verify the structure of Co3O4@GCN

Co3O4NPs) (f) TGA curves of Co3O4@GCN-5-400 and

TNS hybrid and it is found that the results are

Co3O4@GCN-5-450 hybrids.

well-matched with HRTEM and XRD studies (Figure

Figure 1b shows the TEM image of the Co3O4@GCN

1d). In order to investigate the crystal structure of

TNS hybrid, it is worth noting that the GCN grew in

as-synthesized samples, x-ray diffraction (XRD)

the form of uniform tubular nanostructures that are

studies were carried out, shown in Figure 1e (The

interconnected with each other, providing faster

XRD analysis of all other samples was presented in

highway to electrons via walls and internal hollow

Figure S7 and discussed in supporting information).

structure

by

The XRD result of Co3O4@GCN-5-400 (Figure S7c)

shortening the diffusion path. Further the presence of

hybrid exhibits its amorphous nature as no obvious

Co3O4 NPs on the both internal and external surface

XRD peaks were observed for Co3O4 NPs. However,

of GCN TNS, confirmed by the TEM image, can

with the increasing annealing temperature to 450ºC,

activate the redox sites for splitting of water

the

molecules. Further high resolution TEM (HRTEM)

well-crystalline nature and shows strong X-ray

image of Co3O4@GCN TNS hybrid is confirming the

reflection that is well-matched with standard card

existence well-attached and dispersed NPs on the

JCPDS No. 78-1969 (Figure 1e). Furthermore, to

surface of GCN TNS, as shown in Figure 1c.

delineate the structure of Co3O4 NPs, Rietveld

Furthermore, the inter-planner distances of 0.12 nm,

refining of the crystals structures were done and it is

0.16 nm and 0.28 nm are found for various NPs that

found that Co3O4 NPs present in the form of facet

are well-matched with plans of Co3O4 (622), (422) and

center cubic (FCC)crystal structure (space group

(220) respectively; according to the standard card No

Fd-3m and space group number 227), shown in the

JCPDS 78-1969. Thus, HRTEM analysis shows that

inset of Figure 1e. The XRD results further confirmed

Co3O4 NPs grew in well-crystalline form and are

that the formation of Co3O4 NPs required higher

strongly pined on the GCN TNS. Furthermore, the

temperature of (450 ºC), which transformed the

HRTEM studies delineate the amorphous nature of

cobalt precursor to cobalt oxide, as no formation of

GCN, as indicated by the arrows in Figure 1c. The

Co3O4 NPs occurred at 400 ºC because of transformed

structural and morphological features of all other

reaction energy barrier. Furthermore, the evaporation

samples are discussed in the supporting information

of carbon at higher temperature from GCN also

and presented in Figure S1-6. Scattered area electron

facilitates the formation of Co3O4 NPs at the GCN

diffraction (SAED) studies were performed to further

surface, which was evidenced from the weight loss of

confirm

of

carbon, higher concentration of metallic counterpart

as-synthesized Co3O4 NPs decorated on GCN TNS,

and strong reflection of XRD peaks. However, the

interestingly it is found that Co3O4 NPs are grew in

concentration of cobalt precursor also plays critical

polycrystalline form indicated by the circular fringes

role in defining the composition and crystallinity of

with spot pattern, shown in Figure 1d. Thus, the

the hybrid as it is found that the XRD peaks of

SAED studies further indicate that Co3O4@GCN TNS

Co3O4@GCN-5-450 are more intense and less broad

hybrid bears crystalline Co3O4 NPs and amorphous

compared

GCN, synergistically offering strong electrochemical

improved crystallinity of the acquired sample at

coupling, which can make the hybrid highly efficient

higher concentration, shown in Figure S7. Thermal

bi-functional catalyst for both OER and HER as

gravimetric analysis (TGA) was performed in order

explained in the respective section below. The

to determine the stability and composition of

Co3O4@GCN-5-450

hybrid

facilitate

the

(e)

efficient

structure

XRD

mass

and

pattern

transfer

crystallinity


hybrid

to

(Co3O4@GCN-5-450)

displays

Co3O4@GCN-1-450, indicating the

Nano Res.

5

Co3O4@GCN-5-400 and Co3O4@GCN-5-450 hybrids.

Co3O4@GCN-5-450 shows three distinct peaks at

Figure

of

284.47, 285.60 and 288.03 eV that correspond to

Co3O4@GCN-5-400 and Co3O4@GCN-5-450 hybrids,

graphitic carbon, C-OH and the sp2 bonded carbon

from where it is obvious that there are two weight

in the hetero-cycles (N-C=N), respectively [18, 22,

losses at the same temperature range for both

25]. The high resolution C 1s spectrum of

hybrids. The initial weight loss starts around 100 ºC

Co3O4@GCN-1-400 also shows similar behavior as

and continues to 400 ºC, assigned to the loss of

there are three analogous peaks present at same

trapped water molecules and attached functional

binding energy values (Figure S9a&b). To explore

groups on the surface of GCN in both samples. The

the nature of existing nitrogen, de-convolution of N

second major weight loss occurred at 400 ºC where

1s is carried out, shown in Figure 2c and four

Co3O4@GCN-5-400and

hybrids

different kinds of nitrogen centers are present in the

decomposed and the weight loss of about 64% and

Co3O4@GCN-5-450 at 400.6, 399.56, 398.5 and 397.80

40% was observed, respectively, because of the

eV which corresponds to graphitic, pyrrolic, amino

removal of GCN as TGA studies were performed in

and pyridinic, respectively [24, 36, 37]. It was

air. Thus, with increasing Co precursor concentration,

proved that presence of various nitrogen centers

higher Co3O4 NPs were loaded with the removal of

can change the density of state and accelrate the

more GCN during the synthesis procedure, as Co

electronic cloud of graphetic carbon which can

was involved in the catalysis of GCN to produce

enhance

carbon, thus higher Co concentration catalyze more

properties

GCN [17].

resolution N 1s spectra of Co3O4@GCN-1-400 and

To determine the chemical composition and nature

Co3O4@GCN-5-400 indicate the presence of pyrrolic,

of chemical bonding of constituent elements in

amino and pyridinic nitrogen centers in both

as-synthesized

photoelectron

samples (Figure S9c&d). Figure 2d and S10a&b

spectroscopy (XPS) was carried out, as shown in

show the high resolution spectra for O1s of

Figure 2.The full scan spectra of XPS reveal the

Co3O4@GCN-5-450,

existence of core levels of C, N, O and Co in all the

Co3O4@GCN-5-400 hybrids, respectively. The O1s

samples, as indicated in Figure 2a, further approve

exhibit three peaks at 531, 530.1 and 529 eV. These

the high purity of as-synthesized products. XPS

peaks are associated with oxygen ions in low

studies also support the HRTEM and XRD results

coordination states at the surface and metal-oxygen

that an increase in Co precursor concentration

bonds for Co3O4@GCN-5-450 [13]. The existence of

increased the amount of Co3O4 NPs in the product,

metal-oxygen bond confirms the bridging of NPs

as 3.05, 5.03 and 17.02 wt.% are obtained for

with carbon through the oxygen, which makes

Co3O4@GCN-1-400,

and

them stable during the catalysis of water. Further

lower

the existence of carboxyl and hydroxyl groups on

concentration values of metallic counterparts are

the surface of GCN act as active sites to catalyze the

observed than the values calculated from TGA

splitting of water molecules [13]. XPS spectrum of

studies based on surface analysis of XPS. However,

Co 2p (Figure 2e) shows two spin-orbit doublets of

lower concentration values further confirm that NPs

Co 2p1/2 at 780.5 and 796.5 eV that attributed to Co2+,

are well-embedded in the GCN matrix which can

while two spin-orbit doublets of Co 2p3/2 at 779.1

bring better synergistic effect to improve the overall

and 794.8 eV are belongs to Co3+ [39]. As the water

conductivity and catalytic properties of hybrid

splitting is a surface reaction thus exposed surface

structure. The de-convoluted C 1s spectrum of

of the catalyst is very important factor to enhance

1f

is

presenting

TGA curves

Co3O4@GCN-5-450

hybrids,

Co3O4@GCN-5-450,

the

x-ray

Co3O4@GCN-5-400 respectively.

Slightly

the of

conductivity GCN

[34,

andelectrochemical 38].

Similarly,

Co3O4@GCN-1-400


high

and

Research

Nano Res.

6

the catalytic process. To determine the exposed surface, BET measurement was carried out and it is found that Co3O4@GCN-5-450 hybrid brings highest surface area (62.50 m2g-1) among all the samples, shown in Figure 2f. While Co3O4@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 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

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

for the deep penetration of electrolyte inside the

Co3O4@GCN-5-400 and Co3O4@GCN-5-450 hybrids (b) High

electrode and can improve the mass transport, thus

resolution C 1s (c) N 1s (d) O 1s (e) Co 2p spectra of

are highly important for better catalytic properties

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

[40]. Here, pore size distribution is also calculated

Co3O4@GCN-1-400,

to evaluate their effect on catalysis, presented in

Co3O4@GCN-5-450 hybrids.

Figure S8b, the major pore size distribution fall in

Considering unique structure and composition of

the range of 2-4 nm for Co3O4@GCN-5-450 that is

Co3O4@GCNTNS hybrids, here we explore their OER

very helpful for efficient transfer of ions.

and HER property using rotating ring disk electrode (RRDE)

for

future

Co3O4@GCN-5-400

applications

of

and

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 | www.editorialmanager.com/nare/default.asp

Nano Res.

7

at surface and catalyze maximum water molecules to

hybrid outperformed the RuO2 and IrO2, in the case

produce large amount of oxygen. Furthermore, it is

of over potential other hybrids (Co3O4@GCN-5-400

interesting that at lower and higher concentration of

and Co3O4@GCN-10-400)also show lower value of

NPs, the hybrid shows poor performance, confirmed

over potential 0.13 V. The extraordinary performance

from the lower onset potentials values of 1.42 and

of the hybrid at over potential, current density and

1.45

and

potential value at current density of 10 mAcm-2 are

Co3O4@GCN-10-450 hybrids, respectively, along with

confirming the advantages of unique structure and

poor current densities (110 to 130 mAcm-2) as shown

composition of as-synthesized hybrid and proved

in Figure 3a (a close view is presented in Figure S11).

that hybrid has ability to replace the expensive and

Thus, comparative studies have proved that to

rare traditional noble metal catalysts. Furthermore, to

improve the electrocatalytic properties of GCN, a

explore the effect of NPs concentration and synthesis

specific concentration of NPs are required as

temperature, potential values of different hybrids at

presented above that can activate the redox sites and

current density of 10 mAcm-2 are calculated, shown

brings high active surface area to exposed the

in Table S2. Interestingly, it is found that as the

V

for

Co3O4@GCN-1-450

maximum redox sites to electrolyte. Furthermore, to

concentration of Co precursor is increased from 0.01g

compare the electrocatalytic property of hybrid with

to 0.05g (Figure S12b-d), an improved onset potential

noble metal catalysts, the linear sweep voltammetry

and current density is found, because the maximum

(LSV) curves of Co3O4@GCN-5-450 hybrid (1.40 V)

redox sites are available on the surface with an

along with RuO2 (1.30 V) and IrO2 (1.45 V) were

appropriate amount of Co3O4 NPs which are

obtained (Figure 3b), from where it is worth noting

uniformly distributed on the both sides of tube walls.

that the hybrid has much better performance than

However,

both noble metals catalysts not only in terms of onset

increased to 0.1g (Figure S12e & f), it reduces the

potential but also in case of current density (65 and

performance by increasing non-reactive sites and

87 mAcm for RuO2 and IrO2, respectively). Since the

destroying the synergism among the GCN and NPs.

potential reached at a current density of 10 mAcm is

To

significant performance index for OER catalyst,

temperature on the OER activity of Co3O4@GCN

because it is about the current density for a 10%

hybrid, Co3O4@GCN-5 hybrid was prepared at 400

efficient solar-to-fuel conversion device [22]. Figure

and 450 ºC (Figure S12d & Figure3d, respectively). As

3c shows the onset potentials, over potentials (the

explained

difference between the theoretical and onset potential)

temperature improved the crystalline nature of the

and potentials at current density reaching to 10

Co3O4 NPs and brought larger concentration of NPs

mAcm . The higher onset potential values of 1.58

on surface with more active sites, resulting in better

and 1.6 V are found for RuO2 and IrO2, respectively,

OER performance both in case of onset potential and

at the current density of 10 mAcm compared to the

current density. Hence, all the results described

excellent value of 1.5 V for Co3O4@GCN-5-450. The

above confirmed that to bring better onset potential

superior activity of Co3O4@GCN-5-450 TNS hybrid

and current density as well as excellent performance

can also be seen from lower over potential,

at cut off current density, it is highly desirable that

Co3O4@GCN-5-450 exhibits over potential of 0.12 V

GCN tubular structure should be decorated with

compared to 0.14 and 0.16 V for RuO2 and IrO2,

highly active metal counterpart (Co3O4 NPs) with

respectively. To the best of our knowledge, the over

appropriate loading and crystal quality. Figure 3d

potential value found here is the best reported value

shows the LSV curves of Co3O4@GCN-5-450 hybrid at

yet [23, 25, 41-45], not only Co3O4@GCN-5-450 TNS

different rotation rates, it is noted from the graph

-2

-2

-2

-2

further

when

the

investigate

above

in

concentration

the

XRD

effect of

results,


is

further

synthesis

increasing

Research

Nano Res.

8

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 earth abundant catalyst. To further verify the

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

outstanding

Co3O4@GCN-5-450

KOH for OER(b) LSV curves of Co3O4@GCN-5-450 hybrid,

hybrid, its stability was measured by charging it at

IrO2 and RuO2 at 1600 rpm in 1 M KOH for OER (c)onset

0.5 V for 10 h, shown in Figure3f. The better stability

potentials, over potentials and potentials required to reach 10

of the hybrid comes up because of the structural

mAcm-2 current density of the OER catalyzed by all

stability contributed by GCN matrix and strong

samples(here sample 1,2,3,4,5,6,7,8 and 9 represents GCN,

pinning of NPs to the GCN matrix. So, the excellent

Co3O4@GCN-1-400,

Co3O4@GCN-1-450,

stability further highlights that the hybrid structure

Co3O4@GCN-5-400,

Co3O4@GCN-5-450,

efficiently took the benefits from each part; as a result,

Co3O4@GCN-10-400, Co3O4@GCN-10-450, IrO2 and RuO2

better synergism provides excellent performance and

respectively) (d) LSV curves ofCo3O4@GCN-5-450 hybrid at

stability as OER catalyst. Thus, it is expected that the

different rpm in 1 M KOH for OER(e) Tafel plots of

Co3O4@GCNTNS hybrid developed in the present

Co3O4@GCN-5-450 hybrid, IrO2 and RuO2 (f) Stability test of

study is a potential candidate to catalyze the

Co3O4@GCN-5-450 for 10 h in 1 M KOH for OER.

chemical reactions in air batteries, fuel cell and water

To make the hybrid more practical potential, we

splitting.

explore its bi-functionality as a catalyst for HER to

performance

of

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 CO2 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 | www.editorialmanager.com/nare/default.asp

Nano Res.

9

potential of commercial Pt/C (-0.01V), but slightly

stability

lower

for

galvanostatic discharge for 10 h, presented in Figure

Co3O4@GCN-5-450. Furthermore, to explore the role

4d. Such a high stability of the hybrid is contributed

of Co3O4 NPs on the catalytic ability of GCN, bare

from the strongly interconnected network of GCN to

GCN was also employed as catalyst and it is found

accelerate the electronic conduction and catalysis of

that the GCN alone is not good catalyst as onset

water molecule at surface by highly active redox sites.

potential of GCN is very poor -0.27V. Thus, the onset

However,

potential values of GCN and the hybrid confirms that

performance of the Co3O4@GCN hybrid both for OER

to attain better performance, an appropriate loading

and HER is still not fully understood. It is expected

of Co3O4 NPs is highly required. It is worth noting

that the rich state provided by cobalt, nitrogen and

that only small loading of NPs brings a big difference

carboxyl/hydroxyl groups in the Co3O4@GCNtubular

in the performance of hybrid that based on the

structure electrode play important roles in its

unique design of catalyst presented here, like hybrid

enhanced OER and HER performance with a low

offers short diffusion path to ions, highly conductive

over potential. The existence of carboxyl/hydroxyl

highway for electrons, extremely exposed active

groups along with partially negative nitrogen centers

surface area and easy access to redox sites. Similar to

helps to absorb the water molecules on their surfaces,

OER, the effect of different concentration of Co

which is a significant initial step in OER and HER.

precursor and synthesis temperature were also

Moreover, the strong synergistic relationship among

explored as shown in Figure 4a. It is found that as the

the GCN and Co3O4 in Co3O4@GCNTNS hybrid along

concentration increased from 0.01g to 0.05g, better

with unique tubular structure, high active surface

onset potential was observed, but with further

area and easily available redox sites are most likely

increase to 0.1g, again poor value is attained. Once

other important factors for the excellent OER/HER

again verifies our hypothesis that appropriate

performances

required concentration of Co precursor is 0.05g

Co3O4@GCNTNS hybrid is a novel catalyst for energy

which is necessary to activate the redox sites in the

conversion technologies based on non-precious

hybrid and enhanced its conductivity. Furthermore,

earth-abundant metallic catalysts.

current

density

was

observed

when

the

tested

at

mechanism

of

the

constant

for

the

hybrid.

current

excellent

Thus,

the

the hybrids prepared at lower temperature (400ºC) shows poor onset 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

Figure 4. (a) LSV curves of all the samples and Pt/C at 1600

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

rpm

Co3O4@GCN-5-450 hybrid also bears very good

ofCo3O4@GCN-5-450 at different rpm in 0.5 M H2SO4 for

in

0.5

M

H2SO4

for

HER

(b)

LSV


curves

Research

Nano Res.

10

HER (c) Onset potentials and over potentials for HER of all

were dispersed in 20mL of ethanol in separate glass

samples (here sample 1,2,3,4,5,6,7 and 8 represents GCN,

beakers and sonicated for 1 h. Then the dispersed

Co3O4@GCN-1-400,

Co3O4@GCN-1-450,

solutions were mixed and magnetically stirred for 1

Co3O4@GCN-5-400,

Co3O4@GCN-5-450,

hour and dried at 60ºC for 12h, finally this mixture

Pt/C,

was annealed at 450ºC for 2h at heating rate of

respectively) (d) Stability test of Co3O4@GCN-5-450 for 10 h

10ºC/min. Different samples were prepared with

in 0.5 M H2SO4 for HER.

different masses ofCoCl2⋅6H2O(10, 50 and 100 mg)

Co3O4@GCN-10-400,

3

Co3O4@GCN-10-450

and

and these samples were annealed at two different

CONCLUSIONS

temperatures (400 and 450ºC). The samples are given

In summary, we have synthesized Co3O4@GCNTNS

name according to temperature and concentration of

hybrid through simple chemical method at low

CoCl2⋅6H2O,

temperature.

represents initial mass in % of the CoCl2⋅6H2Oand

As-synthesized

hybrid

exhibited

like

Co3O4@GCN-x-y,

here

“x”

excellent bi-functional catalytic activity for both OER

“y”

and HER. Co3O4@GCNTNS hybrid demonstrates low

Co3O4@GCN-1-400 corresponds to the sample with

onset potential and high current density for both

1% mass content of CoCl2⋅6H2O and at 400ºC.

electrode reactions due to fully disperse Co3O4NPs in

represents

4.2

the

temperature

Characterizations:

X-Ray

in

ºC.

diffraction

GCN, special structures, unique composition and

pattern of prepared samples was recorded by XRD;

high active surface area which bring maximum redox

Philips X'Pert Pro MPD, using Cu-Ka radiation

sites

source, x-ray Photoelectron Spectra was done by

at

the

surface.

Most

importantly,

Co3O4@GCNTNS hybrid have surpassed the best

using

noble metals oxides catalysts for OER catalytic

Morphological characterization was done by Field

activity with excellent over potential (0.12 V) and

emission scanning electron microscopy (FESEM,

superior current density (147 mAcm ), as well as

Hitachi

approaches to the onset potential of Pt/C as HER

spectroscopy (EDS, Hitachi S-4800) was used to

catalyst. This work presents a novel approach to

determine

design low cost OER and HER bi-functional catalysts

electron

through facile method at large scale which can

transmission electron microscopy (HRTEM) and

outperform the noble metal-based electrocatalysts

selected area electron diffraction (SAED) pattern

and will motivate the development of renewable

were measured by (JEOL-JEM-2100F).The surface

energy sources.

area and porosity was measured using Beishide

-2

(Thermo

S-4800). the

Energy

composition.

microscopy

Instrument-ST

4 EXPERIMETAL METHODS

Scientific,

Escalab250Xi).

dispersive The

(TEM),

transmission

high

3H-2000PS2

Brunauer-Emmett-Teller

(BET)

x-ray

resolution

through method.

The

thermogravimetric analysis (TGA) and differential 4.1

Fabrication of Co3O4@GCNTNS Hybrid:

scanning calorimetric (DSC) were determined by a

To synthesize the Co3O4@GCN TNS hybrid, 1g of

SDT Q600 (USA) in air at a heating rate of

melamine was dissolved in 30mLof ethylene glycol

10 °C/min from 25 to 600 °C.

and a saturated solution was made. Then 60mLof

4.3

Electrochemical

Characterization:

0.1M HNO3 was added to the previously prepared

Rotating ring-disk electrode (RRDE) measurements

solution

were

with

continuous

stirring

of

10mins,

carried

out

by

using

a

CHI

760C

afterward washed with ethanol and dried at 60 ºC for

electrochemical workstation with a three-electrode

12h. In result, white color powder was obtained.

system. Working electrode consisted of glassy-carbon

Later on50mg CoCl2 6H2O and 1 g of white powder

(GC) (5 mm in diameter and 0.25 cm2 thick); Pt wire


Nano Res.

11

electrode is used for counter and Ag/AgCl as

selenide. Journal of Power Sources 2013, 229,

reference electrode. Electrode was prepared by

216-222.

making the suspension of 1mg active materials in under

[2] Butt, F. K.; Tahir, M.; Cao, C.; Idrees, F.; Ahmed, R.;

sonication. After sonication 10μL of this solution was

Khan, W. S.; Ali, Z.; Mahmood, N.; Tanveer, M.;

incorporated on the GC. Electrolyte consists 1M KOH

Mahmood, A.; Aslam, I.: Synthesis of Novel ZnV2O4

aqueous solution for OER and 0.5M H2SO4 for HER.

Hierarchical Nanospheres and Their Applications as

ethanol

(0.85mL)

and

Nafion

(0.15mL)

Electrochemical Supercapacitor and Hydrogen Storage

Acknowledgements

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

Work at Beijing Institute of Technology was

[3] Li, J.; Wang, G.; Wang, J.; Miao, S.; Wei, M.; Yang, F.;

supported by NSFC (23171023 ， 50972017) and

Yu, L.; Bao, X.: Architecture of PtFe/C catalyst with

Doctoral Program of the Ministry of Education of

high activity and durability for oxygen reduction

China (20101101110026). Work at Tianjin University

reaction. Nano Res. 2014, 7, 1519-1527.

is supported by NSFC (21222607) and Tianjin

[4] Kim, W.-S.; Hwa, Y.; Kim, H.-C.; Choi, J.-H.; Sohn,

Municipal Natural Science Foundation (15JCZDJC37300). Work at Peking

H.-J.;

University was supported by the NSFC-RGC Joint

extraordinary performance. Nano Res. 2014, 7,

Research Scheme (51361165201), NSFC (51125001,

1128-1136.

51172005), Beijing Natural Science Foundation

Hong,

S.-H.:

SnO2@Co3O4

hollow

nano-spheres for a Li-ion battery anode with

[5] Mahmood, N.; Zhang, C.; Liu, F.; Zhu, J.; Hou, Y.:

(2122022) and Doctoral Program of the Ministry of

Hybrid

Education of China (20120001110078). Work at King

Nitrogen-Doped Graphene as a Lithium Ion Battery

Saud University was supported by Deanship of

Anode. ACS Nano 2013, 7, 10307-10318.

Scientific Research at King Saud University through

of

Nanoparticles

and

[6] Mahmood, N.; Hou, Y.: Electrode Nanostructures in Lithium-Based

Prolific Research Group Project no: PRG-1436-25.

Co3Sn2@Co

Batteries.

Adv.

2014,

Sci.

1,

doi:10.1002/advs.201400012.

Electronic Supplementary Material: Supporting Information

contains,

detail

morphological,

[7] Mahmood,

N.;

Zhang,

Sulfide/Nitrogen-Doped

C.;

Hou,

Y.:

Graphene

Nickel

Composites:

structural and compositional analysis of all the

Phase-Controlled Synthesis and High Performance

samples

Anode Materials for Lithium Ion Batteries. Small

along

with

their

electrochemical

characterization, it also contains the information

2013, 9, 1321-1328.

about the samples names and their synthesis

[8] Gong, M.; Dai, H.: A mini review of NiFe-based

conditions as well as the comparative study with

materials as highly active oxygen evolution reaction

noble metals catalysts. “This material is available

electrocatalysts. Nano Res. 2015, 8, 23-39.

free

of

charge

via

the

Internet

at

[9] Fu, G.; Liu, Z.; Chen, Y.; Lin, J.; Tang, Y.; Lu, T.: Synthesis and electrocatalytic activity of Au@Pd

http://dx.doi.org/10.1007/s12274-***-****-*

core-shell nanothorns for the oxygen reduction

References

reaction. Nano Res. 2014, 7, 1205-1214. [10] Hou, J.; Cao, C.; Idrees, F.; Ma, X.: Hierarchical

[1] Ali, Z.; Cao, C.; Li, J.; Wang, Y.; Cao, T.; Tanveer, M.;

Porous Nitrogen-Doped Carbon Nanosheets Derived

Tahir, M.; Idrees, F.; Butt, F. K.: Effect of synthesis

from Silk for Ultrahigh-Capacity Battery Anodes and

technique on electrochemical performance of bismuth

Supercapacitors. ACS Nano 2015, 9, 2556-2564.


Research

Nano Res.

12

[11] Qing, L.; Mahmood, N.; Zhu, J.; Hou, Y.; Sun, S.:

[20] Suntivich, J.; May, K. J.; Gasteiger, H.

A.;

Graphene and Its Composites with Nanoparticles for

Goodenough, J. B.; Shao-Horn, Y.: A perovskite oxide

Electrochemical Energy Applications. Nano Today

optimized for oxygen evolution catalysis from

2014, 9, 668-683.

molecular orbital principles. Science 2011, 334,

[12] Zhang, C.; Mahmood, N.; Yin, H.; Liu, F.; Hou, Y.:

1383-5.

Synthesis of Phosphorus-Doped Graphene and its

[21] Kanan, M. W.; Nocera, D. G.: In situ formation of an

Multifunctional Applications for Oxygen Reduction

oxygen-evolving catalyst in neutral water containing

Reaction and Lithium Ion Batteries. Adv Mater 2013,

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

25, 4932.

[22] Long, X.; Li, J.; Xiao, S.; Yan, K.; Wang, Z.; Chen,

[13] Mahmood, N.; Zhang, C.; Jiang, J.; Liu, F.; Hou, Y.:

H.; Yang, S.: A strongly coupled graphene and FeNi

Multifunctional co3 s4 /graphene composites for

double hydroxide hybrid as an excellent electrocatalyst

lithium ion batteries and oxygen reduction reaction.

for the oxygen evolution reaction. Angew. Chem. Int.

Chemistry A Eurpeon Juornal 2013, 19, 5183-90.

Ed. 2014, 53, 7584-8.

[14] Cook, T. R.; Dogutan, D. K.; Reece, S. Y.;

[23] Zhou, W.; Wu, X.-J.; Cao, X.; Huang, X.; Tan, C.;

Surendranath, Y.; Teets, T. S.; Nocera, D. G.: Solar

Tian, J.; Liu, H.; Wang, J.; Zhang, H.: Ni3S2

Energy Supply and Storage for the Legacy and

nanorods/Ni foam composite electrode with low

Nonlegacy Worlds. Chem. Rev. 2010, 110, 6474-6502.

overpotential for electrocatalytic oxygen evolution.

[15] Subbaraman, R.; Tripkovic, D.; Chang, K.-C.;

Energy & Environmental Science 2013, 6, 2921-2924.

Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.;

[24] Tahir, M.; Cao, C.; Butt, F. K.; Idrees, F.; Mahmood,

Greeley, J.; Stamenkovic, V.; Markovic, N. M.: Trends

N.; Ali, Z.; Aslam, I.; Tanveer, M.; Rizwan, M.;

in activity for the water electrolyser reactions on 3d

Mahmood, T.: Tubular graphitic-C3N4: a prospective

M(Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts. Nat Mater

material for energy storage and green photocatalysis.

2012, 11, 550-557.

Journal

[16] Kibsgaard,

J.;

Jaramillo,

T.

F.:

Molybdenum

phosphosulfide: an active, acid-stable, earth-abundant

of

Materials

Chemistry

A

2013,

1,

13949-13955. [25] Ma, T. Y.; Dai, S.; Jaroniec, M.; Qiao, S. Z.: Graphitic

catalyst for the hydrogen evolution reaction. Angew.

carbon

Chem. Int. Ed. 2014, 53, 14433-7.

three-dimensional

[17] Zou, X.; Huang, X.; Goswami, A.; Silva, R.; Sathe, B.

nitride

nanosheet-carbon porous

nanotube

composites

as

high-performance oxygen evolution electrocatalysts.

R.; Mikmekova, E.; Asefa, T.: Cobalt-embedded

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

nitrogen-rich carbon nanotubes efficiently catalyze

[26] Martin, D. J.; Qiu, K.; Shevlin, S. A.; Handoko, A. D.;

hydrogen evolution reaction at all pH values. Angew.

Chen, X.; Guo, Z.; Tang, J.: Highly Efficient

Chem. Int. Ed. 2014, 53, 4372-6.

Photocatalytic H2 Evolution from Water using Visible

[18] Zhao, Y.; Zhao, F.; Wang, X.; Xu, C.; Zhang, Z.; Shi,

Light and Structure-Controlled Graphitic Carbon

G.; Qu, L.: Graphitic carbon nitride nanoribbons:

Nitride. Angew. Chem. Int. Ed. 2014, 53, 9240-9245.

graphene-assisted formation and synergic function for

[27] Li, Q.; Yang, J.; Feng, D.; Wu, Z.; Wu, Q.; Park, S.;

highly efficient hydrogen evolution. Angew. Chem. Int.

Ha, C.-S.; Zhao, D.: Facile synthesis of porous carbon

Ed. 2014, 53, 13934-9.

nitride spheres with hierarchical three-dimensional

[19] Jahan, M.; Liu, Z.; Loh, K. P.: A Graphene Oxide and Copper-Centered

Metal

Organic

mesostructures for CO2 capture. Nano Res. 2010, 3,

Framework

Composite as a Tri-Functional Catalyst for HER, OER,

632-642. [28] Han, C.; Wang, Y.; Lei, Y.; Wang, B.; Wu, N.; Shi,

and ORR. Adv. Funct. Mater. 2013, 23, 5363-5372.

Q.; Li, Q.: In situ synthesis of graphitic-C3N4 nanosheet hybridized N-doped TiO2 nanofibers for


Nano Res.

13

efficient

photocatalytic

H2

production

and

[38] Xing, T.; Zheng, Y.; Li, L. H.; Cowie, B. C.; Gunzelmann, D.; Qiao, S. Z.; Huang, S.; Chen, Y.:

degradation. Nano Res. 2015, 8, 1199-1209. [29] Han, Q.; Zhao, F.; Hu, C.; Lv, L.; Zhang, Z.; Chen, N.;

Observation of Active Sites for Oxygen Reduction

Qu, L.: Facile production of ultrathin graphitic carbon

Reaction on Nitrogen-Doped Multilayer Graphene.

nitride nanoplatelets for efficient visible-light water

ACS Nano 2014, 8, 6856–6862. [39] Mahmood, N.; Tahir, M.; Mahmood, A.; Zhu, J.; Cao,

splitting. Nano Res. 2015, 8, 1718-1728. [30] Tahir, M.; Cao, C.; Mahmood, N.; Butt, F. K.;

C.; Hou, Y.: Chlorine-doped carbonated cobalt

Mahmood, A.; Idrees, F.; Hussain, S.; Tanveer, M.;

hydroxide for supercapacitors with enormously high

Ali, Z.; Aslam, I.: Multifunctional g-C3N4 Nanofibers:

pseudocapacitive performance and energy density.

A Template-Free Fabrication and Enhanced Optical,

Nano Energy 2015, 11, 267-276.

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

[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

photodegradation ability. CrystEngComm 2014, 16,

nanocrystalline spinels as oxygen reduction and

1825-1830.

evolution electrocatalysts. Nat Chem 2011, 3, 79-84.

[32] Tahir, M.; Mahmood, N.; Zhu, J.; Mahmood, A.; Butt,

[42] Masa, J.; Xia, W.; Sinev, I.; Zhao, A.; Sun, Z.;

F. K.; Rizwan, S.; Aslam, I.; Tanveer, M.; Idrees, F.;

Grutzke, S.; Weide, P.; Muhler, M.; Schuhmann, W.:

Shakir, I.; Cao, C.; Hou, Y.: One Dimensional

Mn(x)O(y)/NC

Graphitic Carbon Nitrides as Effective Metal-Free

embedded in a nitrogen-doped carbon matrix for

Oxygen Reduction Catalysts. Sci. Rep. 2015, 5.

high-performance bifunctional oxygen electrodes.

[33] Gogotsi, Y.: What Nano Can Do for Energy Storage.

and

Co(x)O(y)/NC

nanoparticles

Angew. Chem. Int. Ed. 2014, 53, 8508-12. [43] Gong, M.; Li, Y.; Wang, H.; Liang, Y.; Wu, J. Z.;

ACS Nano 2014, 8, 5369-5371. [34] Mahmood, N.; Zhang, C.; Yin, H.; Hou, Y.:

Zhou, J.; Wang, J.; Regier, T.; Wei, F.; Dai, H.: An

Graphene-based nanocomposites for energy storage

advanced

and conversion in lithium batteries, supercapacitors

electrocatalyst for water oxidation. J Am Chem Soc

and fuel cells. J. Mater. Chem. A 2014, 2, 15.

2013, 135, 8452-5.

Ni-Fe

layered

double

hydroxide

[35] Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.;

[44] Ma, T. Y.; Ran, J.; Dai, S.; Jaroniec, M.; Qiao, S. Z.:

Regier, T.; Dai, H.: Co3O4 nanocrystals on graphene

Phosphorus-Doped Graphitic Carbon Nitrides Grown

as a synergistic catalyst for oxygen reduction reaction.

in Situ on Carbon-Fiber Paper: Flexible and Reversible

Nat Mater 2011, 10, 780-786.

Oxygen Electrodes. Angew. Chem. Int. Ed. 2014,

[36] Zhang, C.; Hao, R.; Liao, H.; Hou, Y.: Synthesis of

10.1002/anie.201411125.

amino-functionalized graphene as metal-free catalyst and exploration of the roles of various nitrogen states

[45] Peng, S.; Li, L.; Han, X.; Sun, W.; Srinivasan, M.;

in oxygen reduction reaction. Nano Energy 2013, 2,

Mhaisalkar, S. G.; Cheng, F.; Yan, Q.; Chen, J.;

88-97.

Ramakrishna,

[37] Choi, C. H.; Park, S. H.; Woo, S. I.: Binary and

S.:

Cobalt

Nanosheet/Graphene/Carbon

Sulfide Nanotube

Ternary Doping of Nitrogen, Boron, and Phosphorus

Nanocomposites as Flexible Electrodes for Hydrogen

into Carbon for Enhancing Electrochemical Oxygen

Evolution. Angewandte Chemie International Edition

Reduction Activity. ACS Nano 2012, 6, 7084-7091.

2014, 53, 12594-12599


Research