Effect of annealing temperature on oxygen reduction reaction of reduced graphene oxide incorporated cobalt oxide nanocomposites for fuel cell applications

Accepted Manuscript Title: Effect of annealing temperature on oxygen reduction reaction of reduced graphene oxide incorporated cobalt oxide nanocomposites for fuel cell applications Authors: Gokuladeepan P, Karthigeyan A PII: DOI: Reference:

S0169-4332(17)33741-8 https://doi.org/10.1016/j.apsusc.2017.12.153 APSUSC 38023

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Received date: Revised date: Accepted date:

15-10-2017 9-12-2017 18-12-2017

Please cite this article as: Gokuladeepan P, Karthigeyan A, Effect of annealing temperature on oxygen reduction reaction of reduced graphene oxide incorporated cobalt oxide nanocomposites for fuel cell applications, Applied Surface Science (2010), https://doi.org/10.1016/j.apsusc.2017.12.153 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of annealing temperature on oxygen reduction reaction of reduced graphene oxide incorporated cobalt oxide nanocomposites for fuel cell applications Gokuladeepan.P and Karthigeyan.A*

SRM University, Kattankulathur-603203, Tamilnadu, India [email protected] *[email protected]

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Corresponding author phone no: +91-44- 27417831

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Department of Physics and Nanotechnology, Faculty of Engineering and Technology

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* Corresponding author: Dr.A. Karthigeyan

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Assistant Professor (Sr.G)

Department of Physics and Nanotechnology

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SRM University, Kattankulathur Tamilnadu-603203, India Phone no: +91-44- 27417831 E mail id: [email protected] 1

Highlights Effect of annealing temperature on oxygen reduction reaction

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Reduced graphene oxide incorporation in cobalt oxide

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Porous rGO-Co3O4 nanocomposite

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Abstract

Reduced graphene oxide incorporated cobalt oxide nanocomposites were prepared by a

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simple hydrothermal route followed by annealing the product at 300 °C, 400 °C and 500 °C in air.

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Structural analysis by X-ray diffraction confirmed the formation of rGO presence and Co3O4

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spinel structure. Scanning electron microscopic images of as-synthesized product showed

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layered reduced graphene oxide structure and the annealed samples showed agglomerated nanoparticles to porous structure. Functional group analysis confirmed the formation of Co3O4

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phase and reduced Graphene oxide. The surface area was increased from 13.72 m2/g, to 23.16

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m2/g for the sample annealed at 300 °C to 500 °C. Electrochemical properties of annealed

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products were investigated by cyclic voltametry and Rotating Disc Electrode (RDE) measurements produced Oxygen Reduction Reaction (ORR) catalytic activity with 3.2 electron

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transfer number. The observed onset potential is +0.76 V vs RHE and current density of 3.25

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mA/cm2 in 1600 rpm for the sample annealed at 500 °C.

Keywords: rGO-Co3O4 nanocomposites; Hydrothermal synthesis; Oxygen reduction reaction

1. Introduction 2

Oxygen reduction reaction (ORR) is an important catalytic process in electrochemical devices such as fuel cells and metal air batteries. About 15% of power loss due to sluggish oxygen electrode kinetics is a major drawback in fuel cell applications. To overcome this issue,

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advanced electrocatalyst is important to enhance the efficiency of the fuel cells[1–4]. Recently platinum and platinum based alloys such as PtM/C, M= Co, Ni, Cr, Ru, Ir[5–10] have been investigated to promote the oxygen reduction reaction. However, limited source, high cost and poor durability of platinum based catalysts limit the mass production and commercialization of fuel cells[1,11,12]. Therefore, a lot of research efforts have been directed towards the

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development of new alternative ORR electrocatalyst based on non-precious metal such as

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transition metal chalcogenides[13], nanostructured metal oxides[14], nitrides[15], carbides[16]

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and metal free carbon nanomaterials[17–19].

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Recently, low cost and earth abundant transition metals like Fe, Co, Ni, Mn and their oxides have been investigated owing to high electrocatalytic activity towards ORR[20–26].

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However, these materials suffer due to poor electrical conductivity and low electron transfer rate.

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Among them, cobalt oxide with little ORR activity combined with carbon support such as

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graphene, graphene oxide, reduced graphene oxide and carbon nanotubes show improved ORR activity. Graphene is known to posses high electrical conductivity, high surface area and high

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mechanical stability and has been studied for various applications such as hybrid catalytic

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material for enhanced ORR activity[27–29], conducting switching[30] and bio imaging[31]. Mixed vacancies (Co2+ and Co3+) in cobalt oxide play a crucial role for ORR activity in

alkaline electrolyte[32–34]. As Co3+ ions occupy half of the octahedral B site in the normal spinel structure and play a key role for the ORR performance, it is important to enhance the exposed sites of Co3+ ions for better catalytic activity of Co3O4 nanostructures[33–35]. Dai et al. 3

reported improved ORR activity of Co3O4 with N doped graphene[14]. Chen et al. reported Co3O4/rGO with high ORR activity and durability[34]. Hong Yu et al. reported NG/CNT/ Co3O4 composite paper exhibited remarkably enhanced ORR activity[29]. Several methods have been nanocomposites such as co-reduction method[36], microwave

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used to prepare rGO-Co3O4

assisted synthesis[37], surfactant assisted synthesis[38] and solvothermal/hydrothermal[34,39] method. Among these, hydrothermal method is a preferred route to produce rGO-Co3O4 nanocomposites due to low cost, high yield and controllability. Thermal treatment of the catalyst can change particle size, surface structure, active site and stability[40–42]. However, there are

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scant reports of the effect of thermal treatment of rGO-Co3O4 nanocomposites on ORR activity.

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In this work, we present the preparation of rGO incorporated Co3O4 nanocomposites by simple

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hydrothermal route followed by annealing at 300 °C, 400 °C and 500 °C in air and reported the

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ORR performance of the catalysts in alkaline electrolyte. 2. Materials and methods

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2.1. Materials

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Graphite powder (99.99%) was purchased from Alfa-Aesar. Potassium permanganate,

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Sulphuric acid, Hydrochloric acid, and Urea were purchased from Rankem chemicals. Hydrogen peroxide (30% w/v) was procured from Fisher scientific, potassium hydroxide from Merk and

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Cobalt nitrate hexahydrate from Sisco Research Laboratory. In all the experiments, the chemicals were used as received and DI water was used in all the experiments and electrochemical studies.

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2.2. Synthesis of rGO- Co3O4 nanocomposites First, Graphene oxide was synthesized following the Hummers method[43]. Then, 48 mg

of the synthesized graphene oxide was dispersed in 40 ml of DI water and adding 600 mg of Co(NO3)2.6H2O and 4 g of urea in the solution followed by stirring for two and half hours. The 4

pH was maintained by adding 1.5 ml of ammonia solution. The above solution was transferred to a Teflon lined stainless steel autoclave and heated to 180 °C in a laboratory oven for 12 hours. The product containing autoclave was cooled to room temperature naturally and washed several

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times by DI water and ethanol and kept in oven for drying overnight. The dried as-synthesized sample were then annealed at 300 °C, 400 °C and 500 °C for 4 hours in air and named as S1, S2 and S3. 2.3. Electrochemical measurements

Electrochemical measurements were carried out using CHI760E electrochemical

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workstation (CH Instruments Inc. USA) with glassy carbon rotating disk working electrode

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(0.0707 cm2 geometric area), a platinum wire counter electrode and a saturated Ag/AgCl

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reference electrode.

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The catalyst ink was prepared by ultrasonicating 5 mg of catalyst in 5ml of Isopropyl alcohol (IPA) and 25 µL Nafion (5wt% Aldrich) for 30 minutes in ice bath. Then 4 μL of above

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catalyst ink was transferred on the surface of the glassy carbon and dried overnight under ethanol

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atmosphere at room temperature.

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0.1 M aqueous KOH solution was used as electrolyte purged with N2 and O2 for 30 minutes prior to the electrochemical testing. The Catalyst loaded working electrode cleaned

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electrochemically by cycling from -1.0 V to 0 V vs Ag/AgCl in 0.1 M KOH solution saturated with N2. Recording of linear sweep voltammograms (LSVs) was done by sweeping the potential

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from -1 V to 0 V with the scan rate of 10mV/s and rotation speed from 400 to 2500 rpm[44]. 2.4. Characterization techniques X-ray diffraction was recorded using PANalytical ‘X’pert PRO powder X-ray diffractometer (Netherland) with Cu Kα radiation (λ=1.542 Å). Morphological and composition 5

analysis was performed

by Field Emission Scanning Electron Microscope (FESEM) and

Energy-Dispersive X-ray using FEI Quanta FEG 200. FTIR measurements were performed by using Agilent Cary 660. Raman spectra were recorded by LABRAM-HR micro Raman system

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using 514 nm laser source. BET surface area was analyzed by Micromeritics TriStar II- 3020, Micromeritics Instrument Corporation, USA. 3. Results and discussion 3.1.

Structural analysis

Figure 1.presented the powder x-ray diffraction patterns of GO and rGO-Co3O4 (300 °C,

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400 °C and 500 °C annealed products) nanocomposites. The strong GO peak at 10.8° is due to (0

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0 2) reflection. The intense peaks at 19.09o, 31.33o, 36.91o, 38.62o, 44.86o, 55.81o, 59.38o, 65.26o

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and 77.37o are indexed to (1 1 1), (2 2 0), ( 3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0) and (5

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3 3) reflections of Co3O4 spinal structure. The peaks are well matched with JCPDS number 741656 (face centered cubic structure with a=8.065Ǻ). The broad low intense bump around 24°

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indicates the presence of rGO in the sample (S3)[45–47]. The average crystallize size of the

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samples was estimated using Scherrer’s formula[48] and found to be 34 nm(S1), 44 nm(S2), 46

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nm(S3). It was found that the increased annealing temperature produced improved crystallinity

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and increased crystallite size.

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3.2.

Surface morphology Surface morphology of the rGO-Co3O4 nanocomposites is shown in the figure 2. As-

synthesized sample without annealing shows nanoparticles grown on the rGO sheets as seen figure 2(a). Figures 2(b), 2(c) and 2(d) present the SEM images of the annealed samples at 300 °

C, 400 °C and 500 °C. It was found that the samples S1 and S2 with a size of nanoparticles less 6

than 40 nm. As the annealing temperature increased from 300 °C to 500 °C, the particles got agglomerated and formed a dense porous nanostructure. Energy-Dispersive Spectra (EDS) confirmed the presence of Co, O, C and N in the products. The nitrogen element present in the

3.3.

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sample is due to urea used in the sample preparation as reducing agent. Functional group analysis

Figure 3 shows the Fourier transform infrared spectroscopy (FTIR) of GO and rGOCo3O4 (composites produced at 300 °C, 400 °C and 500 °C). The GO curve shows broad absorption peak at 3420 cm-1 corresponds to stretching vibration of OH group. The peak at 1626

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cm-1 attributed to OH bending vibration of absorbed water molecules and aromatic C=C. The

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peak at 1736 cm-1, 1383 cm-1 and 1106 cm-1 correspond to C=O (carboxyl), C-OH (hydroxyl)

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and C-O (epoxy) respectively. The above mentioned oxygen containing functional groups are

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significantly reduced in the annealed samples, which is a clear evidence for the GO reduction into rGO[49–53]. Additionally, two peaks observed at 568 cm-1 and 665 cm-1 of rGO-Co3O4

Raman analysis

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3.4.

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samples correspond to Co2+-O and Co3+-O vibrations of Co3O4 respectively[54–56].

Raman spectroscopy is very efficient tool to analyze the graphene based samples[57–60].

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The figure 4 shows the Raman spectrum of GO and rGO-Co3O4 nanocomposite produced at 500 °C. In the spectrum, GO exhibits the D band at 1337 cm-1, which is associated with structural

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defects and disordered carbon and G band at 1585 cm-1 corresponds to sp2 hybridized carbon. As compared to GO peaks, Co3O4-rGO (S3) exhibits reduced intensity of D and G band at 1357 cm1

and 1579 cm-1 respectively. The intensity ratio of D to G band (ID/IG) is 0.98 for rGO-Co3O4,

which is lower than ID/IG ratio of GO (1.07) possibly due to decreased sp2 domain size of carbon 7

atoms and the reduction of sp3 to sp2 carbon[61–63]. In addition, Raman shift at 444 cm-1, 623 cm-1 and 689 cm-1 correspond to Eg, F2g and A1g modes of crystalline Co3O4 phase respectively[64–66]. Surface area analysis

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3.5.

Figure 5 shows N2 adsorption-desorption curves of S1, S2 and S3 obtained from BET measurements. The BET surface area of the samples increases with increasing annealing temperature. The calculated surface area values are 13.72 m2/g, 14.38 m2/g & 23.16 m2/g for the samples S1, S2 and S3. Electrochemical studies

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3.6.

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Figure 6(a) shows cyclic voltommogram curves of S3 catalyst in N2 and O2 saturated

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aqueous 0.1 M KOH solution recorded at the scan rate of 20 mV/s. There is no feature found in

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N2 saturated aqueous KOH solution, where as a sharp cathodic peak is observed around 0.6 V in O2 saturated aqueous KOH solution. It indicates the significant electroreduction of oxygen

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during the reaction. ORR polarization curves given in figure 6(b) for the catalyst S3 with

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different rotation speeds, represent an onset potential 0.76 V vs RHE and current density 3.25

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mA/cm2 at 1600 rpm. The electron transfer number of the overall catalytic reaction is calculated

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using Koutecky-Levich (K-L) equation[67].

Where, I is the measured current, Ik is kinetic current, Id is diffusion limiting current and ω is electrode rotating speed and ‘B’ is the K-L slope determined from the following expression, 8

Where ‘n’ is the electron transfer number, ‘F’ is the Faraday constant (96485 C/mol), ‘υ’ is the kinematic viscosity of the electrolyte (0.01 cm2/s), ‘Do2’ is the diffusion coefficient of

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oxygen molecule in 0.1M KOH (1.9x10-5 cm2/s), Co2 is the bulk concentration of oxygen (1.2x10-6 mol/lit)[17] and ‘A’ is area of the working electrode (0.0707 cm2). The K-L plot (figure 6(b)) shows a linear relationship between I-1 and ω-1/2. From the equation (3), the calculated number of electrons transferred (n) is 3.2 for the catalyst S3.

Figure 7(a) shows the ORR polarization curves for the catalysts S1, S2 & S3 recorded at

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1600 rpm at the sweep rate of 10 mV/s. It can be seen that S3 catalyst shows better performance

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of ORR with a positive onset potential 0.76V vs RHE and current density 3.25 mA/cm2 as

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compared to the catalysts S1 & S2. The ORR polarization curves of S1 and S2 catalysts with

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various rotation speed are given in figure S1 (Electronic Supplementary Information). The mass

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activity of the catalysts also supports that the S3 catalyst exhibits better performance as

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compared to other catalysts shown in figure 6(b). The improved performance of the catalyst S3 may be attributed to the improved crystallinity, porosity, increase in surface area and rGO

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incorporation facilitating pathways of ion transporting and conduction. Hence, the annealing temperature of rGO-Co3O4 nanocomposites plays a key role to enhance the catalytic activity

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towards ORR.

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4. Conclusion

In summary, rGO incorporated Co3O4 nanocomposites were prepared by hydrothermal route

followed by annealing at different temperatures. The rGO-Co3O4 nanocomposites were characterized with XRD, RAMAN, SEM, FTIR and BET. Heat treatment affected the morphology, crystallinity and surface area of the catalysts. The surface area of the samples was 9

increased with increasing annealing temperature. The electrochemical studies yielded enhanced ORR activity for the sample annealed at 500 °C exhibiting improved performance of 0.76V vs RHE and current density of 3.25 mA/cm2 in 1600 rpm with 3.2 electron transfer number in

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alkaline electrolyte medium.

Acknowledgement

The authors thank department of Physics and Nanotechnology,SRM University to carry out the research work. The authors are grateful to Dr. Bhalchandra A. Kakade for his valuable suggestions during the course of this work. The authors thank DST-FIST (DST-FIST-

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SR/FST/PSI- 155/2010) and Nanotechnology Research Center, SRM University, Kattankulathur,

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Chennai-603 203 for extending the characterization facilities.

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Figure Captions

Figure 1. Powder XRD patterns of the samples S1, S2, S3 and GO

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Figure 2. SEM images and EDS spectra of as-synthesized sample (a & e), S1 (b & f), S2 (c & g)

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and S3 (d & h)

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Figure 4. Raman spectra of GO and S3.

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Figure 3. FTIR spectra of S1, S2, S3 and GO.

Figure 5. N2 adsorption-desorption isotherms of S1, S2 and S3.

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Figure 6.(a) Comparative CVs of catalyst S3, recorded in N2 and O2 saturated 0.1 M KOH at

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sweep rate of 20 mV/s; (b) ORR polarization curves of S3 catalyst under O2-saturated 0.1 M

catalyst S3.

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KOH recorded at different rotation speeds at sweep rate 10 mV/s. (c) Koutecky-Levich plot of

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Figure 7. (a) Comparative ORR polarization performance under O2-saturated 0.1 M KOH recorded at 1600 rpm at sweep rate of 10 mV/s. (b) Mass activity S1, S2 and S3 at 500 mV vs

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