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PEDOT composite as cathode materials for oxygen reduction reaction
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 16 (2019) 2023–2029
www.materialstoday.com/proceedings
Bio-CAM 2017
Synthesis and characterization of reduced graphene oxide/PEDOT composite as cathode materials for oxygen reduction reaction Farhanini Yusoffa*, Nurul’Ain Basyirah Muhamada a
School of Marine and Environmental Sciences, Universiti Malaysia Terengganu, 21030 Kuala Nerus, Malaysia.
Abstract An electrocatalyst of poly(3,4-ethyleendioxthiophene) (PEDOT) doped on reduced graphene oxide (rGO) denoted as PEDOT/rGO was successfully synthesized through electrodeposition technique. These composites were deposited on glassy carbon electrode by drop casting technique and its electrocatalytic activity toward reduction of oxygen was determined. The composition, structural and morphology properties of the catalyst were characterized by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electrochemical analysis results demonstrated that electrochemical behavior of modified electrode is stable and controlled by electron transfer mechanism. Increase in oxygen reduction current in 0.1 M KOH confirms that PEDOT/rGO modified electrode has great electroreduction of oxygen and is suitable for potential application as an ORR electrocatalyst in fuel cells. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Biomedical and Advanced Materials (Biocam 2017). Keywords: Graphene Oxide; Poly(3,4-Ethyelenedioxythiophene; Electrochemical Stability; Oxygen Reduction Reaction.
1. Introduction Oxygen reduction reaction (ORR) is a most important reaction in electrochemical process, especially in the development of fuel cells (FCs) and lithium-air batteries. FCs is expected to become one of promising green energy sources, particularly for transportation [1]. However, slow kinetics of reduction of oxygen is a key issue for
*
* Corresponding author. Tel.: +60-139434880; fax: +60-96684390. E-mail address: [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Biomedical and Advanced Materials (Biocam 2017).
2024
F. Yusoff et al. / Materials Today: Proceedings 16 (2019) 2023–2029
commercialization of FCs [2]. In FCs, ORR may proceed mainly in two reaction pathways, either by 2 electron transfer to produced H2O2, or by 4 electron transfer produced H2O depending on materials of electrode used [3]. In the case of cathode materials, platinum-based catalyst currently considered as a commercial electrode but it contributed towards instability of electrode and high cost of the FCs. Accordingly, various electrocatalyst for modification of electrode has been studied and applied in FCs to replace platinum in term of stability and low cost [4-8]. Graphene, a single layer of sp2 hybridized carbon atom that tightly packed and arranged in honeycomb lattice have a great interest and wide range in electrochemical application due to the large surface area, excellent structural, thermal and mechanical properties [9]. In addition, graphene has a great potential application as a catalyst for materials of electrode due to it has excellent properties of large surface area and electron transfer property [11]. However, reduced graphene oxide (rGO) is preferred over graphene because it’s relatively more cost effective when produced in bulk quantity which is essential for real-life application. Recently, conducting polymer incorporated with GO have been used as the materials in sensing applications [12]. Among numerous conducting polymer, poly(3,4-ethylenedioxythiophene) (PEDOT) exhibits a good electrochemical behavior due to it has a low redox potential where it has a fast redox reaction, besides PEDOT has a good conductivity and good stability in oxidized state [13]. Moreover, the PEDOT/rGO composite has ability in electrical conductivity, dispensability and energy conversion [14], so it is to be expected that this composite can improve the efficiency of electrode for FCs. This paper presents the fabrication of PEDOT with rGO electrode by the electrochemical method as the potential electrocatalyst for reduction of oxygen. 2. Methodology 2.1. Materials Graphite powder and 3,4-ethylenedioxythiophene (EDOT) were purchased from Sigma Aldrich. Potassium chloride, potassium ferrocyanide and potassium hydroxide were obtained from HmbG chemicals. 2.2. Procedure PEDOT/rGO was synthesized by two main steps. Firstly, graphene oxide (GO) was obtained from modified Hummers method [15]. Briefly, graphite powder (3 g) and NaNO3 (1.5 g) were added to 23 ml of concentrated H2SO4 and stirred at room temperature. Then, 3 g of KMnO4 was added under stirring in iced condition. The mixture was diluted with distilled water, and finally 10 ml of H2O2 was added to stop the reaction. The mixture was washed, filtered and dried at 60 °C. The resulting product was reduced by electrochemical procedure to remove oxygen functional group, denoted as reduced graphene oxide (rGO). Secondly, the PEDOT/rGO composite was obtained by adding EDOT monomer to GO solution in 30 ml of acetonitrile by sonication for 10 minutes. The mixture was centrifuged, washed and dried at 50 °C for 24 hours (Fig. 1). Then, 10µL EDOT/GO suspension (1 mg/ml) was dropped on GCE by drop casting technique and left to dry at room temperature. EDOT/GO/GCE was electrochemically reduced and polymerized by cyclic voltammetry technique in 1.0 M of KCl solution. The product of PEDOT/rGO/GCE was rinsed and smoothly scratched out from GCE surface. Cyclic voltammogram experiments of ORR for modified electrode were carried out in 0.1 M KOH solution earlier deaerated by purging N2, then followed by purging with O2 saturated. The cyclic voltammogram was scanned in the potential range of -1.0 V to 0.2 V to obtain stable responses. The Ag/ACl (saturated KCl) was used as reference electrode, glassy carbon electrode with or without composite as working electrode and platinum wire as counter electrode, respectively.
F. Yusoff et al. / Materials Today: Proceedings 16 (2019) 2023–2029
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Fig. 1. Schematic diagram synthesis of PEDOT/rGO
3. Experimental results 3.1. Surface morphology The morphology of composites surface were characterized by SEM, which to observe the surface of structure modification. The SEM images of Fig. 2(a) show a wrinkled morphology of GO structure. After the electrochemical reduction of GO, the morphology of rGO (Fig. 2(b)) become smooth-liked structure. The morphology of PEDOT /rGO in Fig. 2(c) shows the crumpled and roughness surface indicating the PEDOT closely doped on rGO sheets. a
b
c
Fig. 2. SEM image of (a) GO; (b) rGO; (c) PEDOT/rGO.
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3.2. Phase determination Figure 3 shows the X-ray diffraction of structural characteristic of all composites. The obtained XRD pattern of graphite (Fig. 3(a)) shows the very sharp peaks (002) at 2θ = 26.38°. There is also the weak diffraction peak at 2θ = 54.72°, showing the layer of spatial arrangement of microcrystals structure [16]. Fig. 3(b) shows the enlarged view of GO and rGO. The diffraction peak of GO at about 2θ = 10.68° corresponding to the oxygen functional group intercalate in the layer of graphite. After the electrochemical reduction of GO, it was observed that the diffraction peak of GO is weaker indicating the removal of oxygen functional group [17]. The XRD pattern of PEDOT/rGO (Fig. 3(c)) shows a broad peak at 2θ = 28.10°, indicating the amorphous nature of polymeric PEDOT. However, the crystallographic peak is still appeared on PEDOT/rGO due to the incomplete reduction of GO. a
b
c
Fig. 3. XRD patterns of (a) graphite, GO and rGO; (b) enlarged view of GO and rGO; (c) PEDOT/rGO.
3.3. Cyclic voltammetry In order to evaluate the electrochemical characteristic of modified electrode, cyclic voltammogram of 5.0 mM K4[Fe(CN)6] with reference to 1.0 M KCl solution was measured at 100 mV/s of scan rate. In Fig. 4(a), the bare GCE has a low response toward the [Fe(CN)6]-3/-4 redox probe which may be due to the inactive site to the electron transfer. Among them, PEDOT/rGO/GCE has a highest redox peak current. It might be due to the incorporation of PEDOT with rGO providing more electrochemical active site to facilitate electron transfer [18]. Fig. 4(b) shows cyclic voltammogram of the effect of scan rate toward PEDOT/rGO/GCE. The redox peak current is increased as the scan rate is increased. The peak to peak separation for both oxidation and reduction peak are less than 200 mV indicating the electrode has a good reversibility system. Meanwhile, the redox peak current is increase linearly with
F. Yusoff et al. / Materials Today: Proceedings 16 (2019) 2023–2029
2027
the square root of the scan rate. Thus, it can be seen that PEDOT/rGO/GCE has an electron diffusion-control behavior, conforming that the modified electrode is stable. a
b
Fig.4. (a) CV of modified electrode in 5 mM K4[Fe(CN)6] of 1.0 M KCl solution at 100 mV/s scan rate and (b) CV of PEDOT/rGO/GCE recorded at different scan rate (Inset: linear relationship of redox peak with square root of scan rate).
3.4. Electrochemical impedance spectroscopy Impedance methods allow evaluating mass transfer of electron in between electrolyte the double layer interface of electrode surface as shown in Fig. 5. The Nyquist plot presents information of semicircle portion in the high frequency region corresponds to the charge transfer process (Rct) and electrochemical double layer (Cdl), representing the electron transfer kinetic of faradaic reaction between the electrode and [Fe(CN)6]-3/-4 redox probe. In Fig. 5(a), the semicircle portion is only appeared on bare GCE. As the composite attached on the surface of electrode, the charge transfer resistant is decreased (Table 1). From all modified electrode, PEDOT/rGO/GCE has a lower charge transfer resistance (6.39 Ωcm2) at high frequency and almost vertical line at lower frequency range, thus PEDOT/rGO/GCE has ideal capacitive behavior and better electron transfer kinetic [19]. Fig. 5(b) shows fitted equivalent circuit for the EIS. The best fit circuit is R(C(RW)) for bare GCE and R([RW]C)Q for modified electrode, respectively. From the Nyquist plot (Fig. 5 (b)), the Bare GCE has a higher Rct value compare to modified electrode respectively, indicating a better electron transfer process. On the contrary, straight line in the low frequency correlated to the diffusion-limiting process. PEDOT/rGO/GCE exhibits more closely to the capacitor behavior possibly due to the fast charge transfer of doping rates during redox reaction. As shown in Fig. 5(c), the impedance results from Bode phase showed only bare GCE has a resistive behavior at lower frequency, while rGO/GCE and PEDOT/rGO/GCE has a capacitive behavior at lower frequency. However, PEDOT/rGO/GCE has a highest capacitive behavior due to the active site of PEDOT/rGO/GCE for the diffusion process of [Fe(CN)6]-3/-4. Meanwhile, for bode magnitude plot (Fig. 4(d)), the impedance value decreased after the addition of composites, suggesting a fast electron charge-transfer at the PEDOT/rGO/GCE electrode. The values for the components of the related the equivalent circuit elements are listed in Table 1. It can be seen that the apparent electron transfer rate (Kapp) increased as the Rct decreases, indicating that the PEDOT/rGO/GCE has a faster electron transfer rate as compared to the rGO/GCE and bare GCE.
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a
b
c
d
Fig.5. (a) Modified Randles equivalent circuit; (b) Nyquist plot; (c) Bode phase and (d) Bode magnitude for the bare GCE, rGO/GCE and PEDOT/rGO/GCE in 5 mM K4[Fe(CN)6] containing 1.0 M KCl solution. Table 1. Parameter of equivalent circuit obtained from EIS for modified electrode. Modified electrode
Rs/Ωcm2
Rct/Ωcm2
CPE/Ω-1cm-1Sn
Zw/mΩcm2
n
10-3 Kapp/cms-1
Bare GCE
24.40
35.62
4.40x 10-6
rGO/GCE PEDOT/rGO/GCE
29.50 6.39
11.19 6.39
1.1x 10-3
1.1
1.49
-6
3.7x 10-3
0.99
4.76
-5
-3
0.93
8.33
8.80 x 10 2.31 x 10
5.3x 10
3.5. Oxygen reduction reaction Fig. 6 shows the comparison of the modified electrode with bare GCE and rGO/GCE respectively toward the ORR in 0.1 M KOH solution. The onset potential of PEDOT/rGO/GCE shifted positively about -0.18 V indicating the incorporation of PEDOT with rGO has created a larger electrochemically active surface area for enhance ORR activity. Meanwhile, the cathodic peak current was two times higher compared to bare GCE. It was revealed that PEDOT/rGO/GCE has a good response toward reduction of oxygen reaction due to the largest surface area to catalyze O2.
F. Yusoff et al. / Materials Today: Proceedings 16 (2019) 2023–2029
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Fig.6. CV of ORR of modified electrode in 0.1 M of KOH solution.
4. Conclusion Electrochemical fabrications of rGO and PEDOT/rGO modified electrode have been prepared for ORR catalyst in 0.1 M KOH solution. The entire composite was successfully synthesized by electropolymerization performed by CV. PEDOT/rGO/GCE behave like a capacitor to catalyze ORR. The morphology of composites also proved that PEDOT was successfully doped on rGO surface. Acknowledgements This work was financially supported by the Ministry of Education Research Acculturation Grant Scheme (RAGS) 1/2015/STO/UMT/03/2, Fundamental Research Grant Scheme (FRGS) FRGS/1/2017/STG01/UMT/02/2 and Universiti Malaysia Terengganu for the providing facilities for undertaking this research. References [1] J. Zhang, K. Sasaki, E. Sutter, R.R. Adzic, Sciences 315 (2007) 220-222. [2] K. Shimizu, L. Sepunaru, R. C. Compton, Chemical Science 7 (2016) 3364-3369. [3] C. Song, J. Zhang. Fundan. App. 1 (2008) 89-129. [4] S. G. Peera, K.K. Tintula, A.K. Sahu, S. Shanmugam, P. Sridhar, S. Pitchumani, Electrochim. Acta. 108 (2013) 95–103. [5] F. Yusoff, A. Aziz, N. Mohamed, S. Ab Ghani, Int. J. Electrochem. Sci. 8 (2013) 10672–10687. [6] W. Li, C. Liang, W. Zhou, J. Qiu, Z. Zhou, G. Sun, Q. Xin, J. Phy. Chem. 107 (2003) 6292-6299. [7] H.-W. Ha, I. Y. Kim, S.-J. Hwang, R. S. Ruoff, Electrochem. Solid-State Lett. 14 (2011) B70-B73. [8] R. Carrera-Cerritos, V. Baglio, A. S. Arico, J.-L. Garcia, M. F. Sagrio, D. Pullini, A. J. Pruna, D. B. Mataix, R. F. Ramirez, L. G. Arriaga, Appl. Catal., B: environ. 144 (2014) 554-560. [9] L. Qu, Y. Liu, J. B. Baek, L. Dai, ASC Nano, 4 (2010) 1321-1326. [10] M. Pumera, Chem. Soc. Rev. 39 (2010) 4146-4157. [11] Y. Li, W. Gao, L. Ci, C. Wang, P. M. Ajayan, Carbon, 48 (2010) 1124-1130. [12] F. Alvi, M. K. Ram, P. A. Basnayaka, E. Stefanakos, Y. Goswami, A. Kumar, Electrochim. Acta. 56 (2011) 9406-9412. [13] Y. Wu, K. Zhang, J. Xu, L. Zhang, L. Lu, L. Wu, T. Nie, X. Zhu, Y. Gao, Y. Wen, Int. J. Electrochem. Sci. 9 (2014) 6594-6607. [14] F. Jiang, Z. Yao, R. Yue, Y. Du, J. Xu, Int. J. Hydrogen Energy. 37 (2012) 14085-14093. [15] W. S. Hummers, R. E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339-1339. [16] N. Cao, Y. Zhang, J. Nanomater. 2015 (2015) 2. [17] X. Zhang, D. Zhang, Y. Chen, X. Sun, Y. W. Ma, Chin. Sci. Bull. 57 (2012) 3045-3050. [18] F. Yusoff, N. Mohamed, A. Aziz, S. A. Ghani, Mater Sci Appl. 5 (2014) 199-211. [19] X. Mao, W. Yang, X. He, Y. Chen, Y. Zhao, Y. Zhou, Y. Yang, J. Xu, Mater. Sci. Eng. B, 216 (2017) 16-22.
ScienceDirect Materials Today: Proceedings 16 (2019) 2023–2029
www.materialstoday.com/proceedings
Bio-CAM 2017
Synthesis and characterization of reduced graphene oxide/PEDOT composite as cathode materials for oxygen reduction reaction Farhanini Yusoffa*, Nurul’Ain Basyirah Muhamada a
School of Marine and Environmental Sciences, Universiti Malaysia Terengganu, 21030 Kuala Nerus, Malaysia.
Abstract An electrocatalyst of poly(3,4-ethyleendioxthiophene) (PEDOT) doped on reduced graphene oxide (rGO) denoted as PEDOT/rGO was successfully synthesized through electrodeposition technique. These composites were deposited on glassy carbon electrode by drop casting technique and its electrocatalytic activity toward reduction of oxygen was determined. The composition, structural and morphology properties of the catalyst were characterized by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electrochemical analysis results demonstrated that electrochemical behavior of modified electrode is stable and controlled by electron transfer mechanism. Increase in oxygen reduction current in 0.1 M KOH confirms that PEDOT/rGO modified electrode has great electroreduction of oxygen and is suitable for potential application as an ORR electrocatalyst in fuel cells. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Biomedical and Advanced Materials (Biocam 2017). Keywords: Graphene Oxide; Poly(3,4-Ethyelenedioxythiophene; Electrochemical Stability; Oxygen Reduction Reaction.
1. Introduction Oxygen reduction reaction (ORR) is a most important reaction in electrochemical process, especially in the development of fuel cells (FCs) and lithium-air batteries. FCs is expected to become one of promising green energy sources, particularly for transportation [1]. However, slow kinetics of reduction of oxygen is a key issue for
*
* Corresponding author. Tel.: +60-139434880; fax: +60-96684390. E-mail address: [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Biomedical and Advanced Materials (Biocam 2017).
2024
F. Yusoff et al. / Materials Today: Proceedings 16 (2019) 2023–2029
commercialization of FCs [2]. In FCs, ORR may proceed mainly in two reaction pathways, either by 2 electron transfer to produced H2O2, or by 4 electron transfer produced H2O depending on materials of electrode used [3]. In the case of cathode materials, platinum-based catalyst currently considered as a commercial electrode but it contributed towards instability of electrode and high cost of the FCs. Accordingly, various electrocatalyst for modification of electrode has been studied and applied in FCs to replace platinum in term of stability and low cost [4-8]. Graphene, a single layer of sp2 hybridized carbon atom that tightly packed and arranged in honeycomb lattice have a great interest and wide range in electrochemical application due to the large surface area, excellent structural, thermal and mechanical properties [9]. In addition, graphene has a great potential application as a catalyst for materials of electrode due to it has excellent properties of large surface area and electron transfer property [11]. However, reduced graphene oxide (rGO) is preferred over graphene because it’s relatively more cost effective when produced in bulk quantity which is essential for real-life application. Recently, conducting polymer incorporated with GO have been used as the materials in sensing applications [12]. Among numerous conducting polymer, poly(3,4-ethylenedioxythiophene) (PEDOT) exhibits a good electrochemical behavior due to it has a low redox potential where it has a fast redox reaction, besides PEDOT has a good conductivity and good stability in oxidized state [13]. Moreover, the PEDOT/rGO composite has ability in electrical conductivity, dispensability and energy conversion [14], so it is to be expected that this composite can improve the efficiency of electrode for FCs. This paper presents the fabrication of PEDOT with rGO electrode by the electrochemical method as the potential electrocatalyst for reduction of oxygen. 2. Methodology 2.1. Materials Graphite powder and 3,4-ethylenedioxythiophene (EDOT) were purchased from Sigma Aldrich. Potassium chloride, potassium ferrocyanide and potassium hydroxide were obtained from HmbG chemicals. 2.2. Procedure PEDOT/rGO was synthesized by two main steps. Firstly, graphene oxide (GO) was obtained from modified Hummers method [15]. Briefly, graphite powder (3 g) and NaNO3 (1.5 g) were added to 23 ml of concentrated H2SO4 and stirred at room temperature. Then, 3 g of KMnO4 was added under stirring in iced condition. The mixture was diluted with distilled water, and finally 10 ml of H2O2 was added to stop the reaction. The mixture was washed, filtered and dried at 60 °C. The resulting product was reduced by electrochemical procedure to remove oxygen functional group, denoted as reduced graphene oxide (rGO). Secondly, the PEDOT/rGO composite was obtained by adding EDOT monomer to GO solution in 30 ml of acetonitrile by sonication for 10 minutes. The mixture was centrifuged, washed and dried at 50 °C for 24 hours (Fig. 1). Then, 10µL EDOT/GO suspension (1 mg/ml) was dropped on GCE by drop casting technique and left to dry at room temperature. EDOT/GO/GCE was electrochemically reduced and polymerized by cyclic voltammetry technique in 1.0 M of KCl solution. The product of PEDOT/rGO/GCE was rinsed and smoothly scratched out from GCE surface. Cyclic voltammogram experiments of ORR for modified electrode were carried out in 0.1 M KOH solution earlier deaerated by purging N2, then followed by purging with O2 saturated. The cyclic voltammogram was scanned in the potential range of -1.0 V to 0.2 V to obtain stable responses. The Ag/ACl (saturated KCl) was used as reference electrode, glassy carbon electrode with or without composite as working electrode and platinum wire as counter electrode, respectively.
F. Yusoff et al. / Materials Today: Proceedings 16 (2019) 2023–2029
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Fig. 1. Schematic diagram synthesis of PEDOT/rGO
3. Experimental results 3.1. Surface morphology The morphology of composites surface were characterized by SEM, which to observe the surface of structure modification. The SEM images of Fig. 2(a) show a wrinkled morphology of GO structure. After the electrochemical reduction of GO, the morphology of rGO (Fig. 2(b)) become smooth-liked structure. The morphology of PEDOT /rGO in Fig. 2(c) shows the crumpled and roughness surface indicating the PEDOT closely doped on rGO sheets. a
b
c
Fig. 2. SEM image of (a) GO; (b) rGO; (c) PEDOT/rGO.
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3.2. Phase determination Figure 3 shows the X-ray diffraction of structural characteristic of all composites. The obtained XRD pattern of graphite (Fig. 3(a)) shows the very sharp peaks (002) at 2θ = 26.38°. There is also the weak diffraction peak at 2θ = 54.72°, showing the layer of spatial arrangement of microcrystals structure [16]. Fig. 3(b) shows the enlarged view of GO and rGO. The diffraction peak of GO at about 2θ = 10.68° corresponding to the oxygen functional group intercalate in the layer of graphite. After the electrochemical reduction of GO, it was observed that the diffraction peak of GO is weaker indicating the removal of oxygen functional group [17]. The XRD pattern of PEDOT/rGO (Fig. 3(c)) shows a broad peak at 2θ = 28.10°, indicating the amorphous nature of polymeric PEDOT. However, the crystallographic peak is still appeared on PEDOT/rGO due to the incomplete reduction of GO. a
b
c
Fig. 3. XRD patterns of (a) graphite, GO and rGO; (b) enlarged view of GO and rGO; (c) PEDOT/rGO.
3.3. Cyclic voltammetry In order to evaluate the electrochemical characteristic of modified electrode, cyclic voltammogram of 5.0 mM K4[Fe(CN)6] with reference to 1.0 M KCl solution was measured at 100 mV/s of scan rate. In Fig. 4(a), the bare GCE has a low response toward the [Fe(CN)6]-3/-4 redox probe which may be due to the inactive site to the electron transfer. Among them, PEDOT/rGO/GCE has a highest redox peak current. It might be due to the incorporation of PEDOT with rGO providing more electrochemical active site to facilitate electron transfer [18]. Fig. 4(b) shows cyclic voltammogram of the effect of scan rate toward PEDOT/rGO/GCE. The redox peak current is increased as the scan rate is increased. The peak to peak separation for both oxidation and reduction peak are less than 200 mV indicating the electrode has a good reversibility system. Meanwhile, the redox peak current is increase linearly with
F. Yusoff et al. / Materials Today: Proceedings 16 (2019) 2023–2029
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the square root of the scan rate. Thus, it can be seen that PEDOT/rGO/GCE has an electron diffusion-control behavior, conforming that the modified electrode is stable. a
b
Fig.4. (a) CV of modified electrode in 5 mM K4[Fe(CN)6] of 1.0 M KCl solution at 100 mV/s scan rate and (b) CV of PEDOT/rGO/GCE recorded at different scan rate (Inset: linear relationship of redox peak with square root of scan rate).
3.4. Electrochemical impedance spectroscopy Impedance methods allow evaluating mass transfer of electron in between electrolyte the double layer interface of electrode surface as shown in Fig. 5. The Nyquist plot presents information of semicircle portion in the high frequency region corresponds to the charge transfer process (Rct) and electrochemical double layer (Cdl), representing the electron transfer kinetic of faradaic reaction between the electrode and [Fe(CN)6]-3/-4 redox probe. In Fig. 5(a), the semicircle portion is only appeared on bare GCE. As the composite attached on the surface of electrode, the charge transfer resistant is decreased (Table 1). From all modified electrode, PEDOT/rGO/GCE has a lower charge transfer resistance (6.39 Ωcm2) at high frequency and almost vertical line at lower frequency range, thus PEDOT/rGO/GCE has ideal capacitive behavior and better electron transfer kinetic [19]. Fig. 5(b) shows fitted equivalent circuit for the EIS. The best fit circuit is R(C(RW)) for bare GCE and R([RW]C)Q for modified electrode, respectively. From the Nyquist plot (Fig. 5 (b)), the Bare GCE has a higher Rct value compare to modified electrode respectively, indicating a better electron transfer process. On the contrary, straight line in the low frequency correlated to the diffusion-limiting process. PEDOT/rGO/GCE exhibits more closely to the capacitor behavior possibly due to the fast charge transfer of doping rates during redox reaction. As shown in Fig. 5(c), the impedance results from Bode phase showed only bare GCE has a resistive behavior at lower frequency, while rGO/GCE and PEDOT/rGO/GCE has a capacitive behavior at lower frequency. However, PEDOT/rGO/GCE has a highest capacitive behavior due to the active site of PEDOT/rGO/GCE for the diffusion process of [Fe(CN)6]-3/-4. Meanwhile, for bode magnitude plot (Fig. 4(d)), the impedance value decreased after the addition of composites, suggesting a fast electron charge-transfer at the PEDOT/rGO/GCE electrode. The values for the components of the related the equivalent circuit elements are listed in Table 1. It can be seen that the apparent electron transfer rate (Kapp) increased as the Rct decreases, indicating that the PEDOT/rGO/GCE has a faster electron transfer rate as compared to the rGO/GCE and bare GCE.
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a
b
c
d
Fig.5. (a) Modified Randles equivalent circuit; (b) Nyquist plot; (c) Bode phase and (d) Bode magnitude for the bare GCE, rGO/GCE and PEDOT/rGO/GCE in 5 mM K4[Fe(CN)6] containing 1.0 M KCl solution. Table 1. Parameter of equivalent circuit obtained from EIS for modified electrode. Modified electrode
Rs/Ωcm2
Rct/Ωcm2
CPE/Ω-1cm-1Sn
Zw/mΩcm2
n
10-3 Kapp/cms-1
Bare GCE
24.40
35.62
4.40x 10-6
rGO/GCE PEDOT/rGO/GCE
29.50 6.39
11.19 6.39
1.1x 10-3
1.1
1.49
-6
3.7x 10-3
0.99
4.76
-5
-3
0.93
8.33
8.80 x 10 2.31 x 10
5.3x 10
3.5. Oxygen reduction reaction Fig. 6 shows the comparison of the modified electrode with bare GCE and rGO/GCE respectively toward the ORR in 0.1 M KOH solution. The onset potential of PEDOT/rGO/GCE shifted positively about -0.18 V indicating the incorporation of PEDOT with rGO has created a larger electrochemically active surface area for enhance ORR activity. Meanwhile, the cathodic peak current was two times higher compared to bare GCE. It was revealed that PEDOT/rGO/GCE has a good response toward reduction of oxygen reaction due to the largest surface area to catalyze O2.
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Fig.6. CV of ORR of modified electrode in 0.1 M of KOH solution.
4. Conclusion Electrochemical fabrications of rGO and PEDOT/rGO modified electrode have been prepared for ORR catalyst in 0.1 M KOH solution. The entire composite was successfully synthesized by electropolymerization performed by CV. PEDOT/rGO/GCE behave like a capacitor to catalyze ORR. The morphology of composites also proved that PEDOT was successfully doped on rGO surface. Acknowledgements This work was financially supported by the Ministry of Education Research Acculturation Grant Scheme (RAGS) 1/2015/STO/UMT/03/2, Fundamental Research Grant Scheme (FRGS) FRGS/1/2017/STG01/UMT/02/2 and Universiti Malaysia Terengganu for the providing facilities for undertaking this research. References [1] J. Zhang, K. Sasaki, E. Sutter, R.R. Adzic, Sciences 315 (2007) 220-222. [2] K. Shimizu, L. Sepunaru, R. C. Compton, Chemical Science 7 (2016) 3364-3369. [3] C. Song, J. Zhang. Fundan. App. 1 (2008) 89-129. [4] S. G. Peera, K.K. Tintula, A.K. Sahu, S. Shanmugam, P. Sridhar, S. Pitchumani, Electrochim. Acta. 108 (2013) 95–103. [5] F. Yusoff, A. Aziz, N. Mohamed, S. Ab Ghani, Int. J. Electrochem. Sci. 8 (2013) 10672–10687. [6] W. Li, C. Liang, W. Zhou, J. Qiu, Z. Zhou, G. Sun, Q. Xin, J. Phy. Chem. 107 (2003) 6292-6299. [7] H.-W. Ha, I. Y. Kim, S.-J. Hwang, R. S. Ruoff, Electrochem. Solid-State Lett. 14 (2011) B70-B73. [8] R. Carrera-Cerritos, V. Baglio, A. S. Arico, J.-L. Garcia, M. F. Sagrio, D. Pullini, A. J. Pruna, D. B. Mataix, R. F. Ramirez, L. G. Arriaga, Appl. Catal., B: environ. 144 (2014) 554-560. [9] L. Qu, Y. Liu, J. B. Baek, L. Dai, ASC Nano, 4 (2010) 1321-1326. [10] M. Pumera, Chem. Soc. Rev. 39 (2010) 4146-4157. [11] Y. Li, W. Gao, L. Ci, C. Wang, P. M. Ajayan, Carbon, 48 (2010) 1124-1130. [12] F. Alvi, M. K. Ram, P. A. Basnayaka, E. Stefanakos, Y. Goswami, A. Kumar, Electrochim. Acta. 56 (2011) 9406-9412. [13] Y. Wu, K. Zhang, J. Xu, L. Zhang, L. Lu, L. Wu, T. Nie, X. Zhu, Y. Gao, Y. Wen, Int. J. Electrochem. Sci. 9 (2014) 6594-6607. [14] F. Jiang, Z. Yao, R. Yue, Y. Du, J. Xu, Int. J. Hydrogen Energy. 37 (2012) 14085-14093. [15] W. S. Hummers, R. E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339-1339. [16] N. Cao, Y. Zhang, J. Nanomater. 2015 (2015) 2. [17] X. Zhang, D. Zhang, Y. Chen, X. Sun, Y. W. Ma, Chin. Sci. Bull. 57 (2012) 3045-3050. [18] F. Yusoff, N. Mohamed, A. Aziz, S. A. Ghani, Mater Sci Appl. 5 (2014) 199-211. [19] X. Mao, W. Yang, X. He, Y. Chen, Y. Zhao, Y. Zhou, Y. Yang, J. Xu, Mater. Sci. Eng. B, 216 (2017) 16-22.