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carbon nanocomposite as efficient bi-functional electrocatalyst for electrochemical water splitting
international journal of hydrogen energy xxx (xxxx) xxx
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Co3O4 nanoparticles decorated Polypyrrole/carbon nanocomposite as efficient bi-functional electrocatalyst for electrochemical water splitting Santhana Sivabalan Jayaseelan, Narayanamoorthy Bhuvanendran, Qian Xu, Huaneng Su* Institute for Energy Research, Jiangsu University, 301 Xuefu Road, Zhenjiang, 212013, PR China
highlights Uniformly
graphical abstract
distributed
Co3O4
nanoparticles on Ppy/C composite was
realized
by
simple
dry
synthesis. The effect of carbon support on the performance of Co3O4/Ppy/C was investigated. Enriched conductivity by Ppy on carbon
supports
results
in
improved electrocatalytic activity. Co3O4/Ppy/MWCNT shows good bifunctional activity and durability for water splitting.
article info
abstract
Article history:
An effective bi-functional electrocatalyst of Co3O4/Polypyrrole/Carbon (Co3O4/Ppy/C)
Received 7 August 2019
nanocomposite was prepared through a simple dry chemical method and used to catalyze
Received in revised form
the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Three types of
30 September 2019
carbon support as Vulcan carbon, reduced graphite oxide (RGO) and multi-walled carbon
Accepted 12 December 2019
nanotubes (MCNTs) were used to study the influence on electrochemical reactions.
Available online xxx
Spherical shaped Co3O4 nanoparticles with 8e10 nm was found uniformly distributed on Ppy/C composite, which were analyzed by X-ray diffraction and transmission electron
Keywords:
microscopy techniques. Amongst, Co3O4/Ppy/MWCNT shows improved bifunctional elec-
Cobalt oxide electrocatalyst
trocatalytic activity towards both OER and HER with relatively low over potential (340 mV
Polypyrrole
vs. 490 mV at 10 mA cm2) and Tafel slope (87 vs. 110 mV dec1). In addition to that,
Bifunctional catalyst
MWCNT supported Co3O4/Ppy nanocomposite exhibits good electronic conductivity and electrochemical stability up to 2000 potential cycles. The results clearly indicate that the
* Corresponding author. E-mail address: [email protected] (H. Su). https://doi.org/10.1016/j.ijhydene.2019.12.085 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Jayaseelan SS et al., Co3O4 nanoparticles decorated Polypyrrole/carbon nanocomposite as efficient bi-functional electrocatalyst for electrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.085
2
international journal of hydrogen energy xxx (xxxx) xxx
Oxygen evolution
Co3O4/Ppy/MWCNT nanocomposite could be the promising bi-functional electrocatalyst
Water splitting
for efficient water electrolysis. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Electrolysis of water, a highly significant way for the generation of hydrogen (H2) and oxygen (O2) to afford sustainable and green energy technology as a better alternative to conventional energy sources, has got great attention in this decade. For electrochemical water splitting, oxygen evolution reaction (OER) is critical due to the sluggish reaction kinetics, thermodynamically unfavorable and higher overpotential. Similarly, hydrogen evolution reaction (HER) is another half reaction of water splitting to produce hydrogen for energy conversion and storage techniques, which also need an efficient catalyst to overcome the energy barrier [1]. Although platinum group metals (PGM) are well known electrocatalyst for water splitting reactions, the high cost and low durability of PGM catalysts are restrictions for its large-scale application. Therefore, developing active, durable and cost-effective nonnoble electrocatalyst is critical to the future commercialization of water electrolysis [2e5]. Many researchers have been studied the influence of controlled morphology of non-noble (Co, Fe, Ni, Mn etc.) nanocatalysts with various carbon free and carbon supports for water electrolysis [6e11]. Among the non-noble catalysts, cobalt based nanocomposite materials has got much attention due to its improved bifunctional electrocatalytic activity for both OER and HER [4,12e15]. In the context of morphology controlled cobalt electrocatalysts, Naseri et al. have reported the electrodeposited cobalt oxide nano flakes and decoration of cobalt oxide nanoparticles over various substrates were used as electrocatalyst for water oxidation reaction [16e18]. Masa et al. reported the preparation of cobalt-cobalt phosphide nanoparticles supported on nickel foam by thermal decomposition of metal precursor under reductive conditions, which showed high activity for water splitting with low overpotential [19]. Interestingly, Prasanth et al. demonstrated that the highly durable helical cobalt borophosphate can be employed as a good bi-functional water splitting electrocatalyst with high energetic efficiency up to 75% [20]. Xu et al. have reported the preparation of cobalt oxide nanoplates by ligand assisted polyol reduction, which also showed a low over potential of 306 mV for OER [21]. In the work conducted by Ranaweera et al., a flower shaped Co3O4 was prepared by binder free synthetic approach and employed as flexible OER electrode material, which demonstrated the good electronic conductivity and stability during operation [22]. Nickel and Cobalt incorporated tungsten sulfide catalyst was also reported by Yang et al., which showed a prolonged stability over 24 h with retention of its initial overpotential for HER [23]. Recently, Li et al. reported Co nanoparticles encapsulated N and S doped carbon nanotubes prepared by thermal reduction
method and it shows higher electrocatalytic activity for HER [24]. Although Cobalt and Co-oxide catalysts shows good electrocatalytic performance towards OER and HER [25e27], but still suffers from poor dispersion, low conductivity and durability. In order to address the above issues carbon materials can be used as catalyst support owing to the following factors; (i) high surface area, particle dispersion and improved accessibility of the metal/metal oxide nanoparticles and (ii) electronic interactions between support and catalyst [28,29]. Various carbon supports have been employed for metal/metal oxide catalysts such as, carbon black (CB), reduced graphene oxide (RGO), single-walled carbon nanotubes (SWNT), and multi-walled carbon nanotubes (MCNT). Furthermore, the conductivity and dispersibility of metal nanoparticles can be enhanced by the surface functionalization of carbon support materials through acid treatment [30], ion exchange [31], insitu polymerization [32] and electrochemical deposition [33]. The surface functionalization of carbon supports with conducting polymers can induce the improved conductivity through p-p conjugation [34] and also providing more accessible active sites for electrochemical surface reaction. This could be increase the electron transfer rate over the active sites of the catalyst, then leading to improved OER performance. The development of tailored carbon supports with conducting polymers as substrate to hold metal/metal oxide nanoparticles could be the promising strategy to attain better performance towards OER and HER [35,36]. Herein, the polypyrrole/carbon (Ppy/C) composite was prepared by in-situ oxidative polymerization method and the cobalt oxide (Co3O4) nanoparticles were deposited onto carbon supports through a simple dry synthetic procedure. Three types of carbon supports (Vulcan carbon (VC), reduced graphite oxide (RGO) and multi-walled carbon nanotubes (MWCNT)) were employed, and the resultant Co3O4-Ppy/C materials were studied to evaluate their electrochemical performances towards OER and HER in KOH medium.
Experimental procedure Synthesis of polypyrrole/carbon (Ppy/C) composites The Ppy/C composites were synthesized by in-situ oxidative polymerization method [37]. In this procedure, acid treated carbon support (1 mg/ml) was dispersed in 1:5 ratio of ethanol and HCl mixture and stirred for 30 min. Equal molar (2 mM) ratio of pyrrole monomer and ammonium persulphate was added into the mixture and continued the stirring for 8 h. After that a little amount of acetone was added to end the
Please cite this article as: Jayaseelan SS et al., Co3O4 nanoparticles decorated Polypyrrole/carbon nanocomposite as efficient bi-functional electrocatalyst for electrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.085
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polymerization reaction and the final yield was washed by copious amount of deionized (DI) water and dried at 60 C for 8 h in a hot air oven.
Synthesis of Co3O4/polypyrrole/carbon composites Co3O4 decorated Ppy/C composites were prepared by simple mechanical mixing followed by heat treatment method [38]. 5 wt % of Cobalt Acetate (Co (Ac)2.4H2O, Sigma-Aldrich) was mixed with 50 mg of Ppy/C composites by agate-mortar to get homogenous mixture and heat treated at 300 C for 4 h under N2 atm. The weight percentage of Co3O4 varied to 5 and 20% over Ppy/MWCNT composites. The detailed synthesis procedure was schematically represented in Scheme. 1.
Physical characterization The crystalline nature of the catalyst was analysed by Powder X-ray diffraction (XRD) patterns using PANalytical X’pert Pro X-ray Diffractometer with Cu-Ka radiation. Surface morphology and elemental composition were studied by Energy dispersive X-ray analysis (EDX) equipped with field emission scanning electron microscopy (FESEM, Ultra plus FE Zeiss) instrument. High resolution transmission electron microscopy (HR-TEM) was performed using Hitachi 300 kV instrument to determine the particle size and distribution of catalyst. The electronic state of the element was determined using X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250Xi system equipped with a monochromatic Al Ka as the X-ray source.
Electrochemical characterization The electrochemical characterization was carried out using the electrochemical workstation (CHI 760e, CHI instruments) with three-electrode electrochemical cell composed of an Ag/ AgCl and a Pt-wire as a reference and counter electrode respectively. The glassy carbon-rotating disk electrode (0.196 cm2, GC-RDE, PINE instruments, USA) was used as a working electrode. To prepare the catalyst ink, 2 mg of catalyst was dispersed in 750 ml of DI water, 250 ml of isopropanol and 20 ml of Nafion (Sigma-Aldrich) and sonicated for 45 min to get homogenous ink. 10 ml of suspension was pipetted and coated onto the GC electrode and dried at 60 C. 0.1 M potassium hydroxide (KOH) was used as electrolyte and all the potentials were converted to a reversible hydrogen electrode (RHE) using the following equation (1), ERHE ¼ EAg=AgCl þ 0:197 þ 0:059* pH
(1)
The electrocatalytic activity of the prepared catalysts was studied by linear scan voltammetry (LSV) measurements
3
conducted in 0.1M KOH solution between 1.2 V and 1.7 V for OER and 0.5 V and 0 V for HER at a scan rate of 10 mV/s. The electrode rotation speed was fixed at 1600 rpm. The onset potential (Eonset) was determined by linearly extrapolating the fast-rising current portion of the peak to the linear extrapolation of the background current [39]. The durability test was performed for OER through potential cycling between 1.2 V and 1.7 V up to 2000 potential cycles under similar conditions. Electrochemical impedance spectroscopy (EIS) was studied in the frequency range of 100 kHz to 0.1 Hz with an amplitude voltage 5 mV.
Results and discussion The FTIR spectra of Ppy/C and Co3O4/Ppy/C are shown in Figs. S1a and S1b. The broad absorption band in the range between 4000 cm1 to 3000 cm1, which is commonly attributed for the absorption band for N-H, O-H groups. The MWCNT have been confirmed by the C-C absorption band at 1583 cm1. Also the predominant bands at 3400 cm1, 1631 cm1 and 1069 cm1 have confirms the functional groups of RGO in the composite. The absorption band around 1667 cm1 indicates the presence of Ppy along with Co3O4 absorption bands were observed in 667 cm1 and 570 cm1. The Figs. S2a and S2b shows Raman spectra of prepared composites. The peaks at ~1350 cm1 and ~1580 cm1 can be ascribed to well-known D and G bands of carbon supports with the second order peak at ~2750 cm1 called 2D band. The encapsulation of Ppy over carbon supports have been confirmed by the additional peaks at ~937 cm1 and ~1021 cm1 [37]. Compared to other carbon supports, the notable change in G band and D band of MWCNT proves the interfacial interactions between MWCNT and Ppy. Also, the suppressed G and D bands of Co3O4/Ppy/MWCNT than Co3O4/MWCNT confirms the Ppy coating over the MWCNT surface shown in Fig. S2b. The typical characteristics peaks at 412 cm1, 515 cm1 and 690 cm1 have describes the decoration of Co3O4 particles over Ppy/C composite layer. The powder XRD patterns of Co3O4/Ppy supported on various carbon supports were shown in Fig. 1a and Fig. S3. The presence of polypyrrole was clearly observed from the low angle crystalline peak appeared at 18 as shown in Fig. 1a. In addition to that, the presence of Ppy suppressed the characteristic peak intensity of carbon support (26 (100) for VC and MWCNT and 11 (002) for RGO) as clearly observed from Figure 1a, S3a and S3b, implying the uniform surface coating of polymer. From the respective XRD patterns of all the three carbon based Co3O4/Ppy nanocomposites, the characteristic peaks for Co3O4 nanoparticles can be observed at 2q values of 31.2 , 36.6 , 44.7 , 59.1 and 65 attributed to (220), (311), (400), (511) and (440) of cubic crystalline planes of cobalt oxide
Scheme. 1 e Schematic representation of synthetic pathway of Co3O4/Ppy/C nanocomposites. Please cite this article as: Jayaseelan SS et al., Co3O4 nanoparticles decorated Polypyrrole/carbon nanocomposite as efficient bi-functional electrocatalyst for electrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.085
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Fig. 1 e (a) X-ray diffraction spectra for Co3O4/Ppy/MWCNT, (b)(c) TEM images of Co3O4/Ppy/MWCNT (inset: SAED pattern), (d) HRTEM image with lattice fringes.
Fig. 2 e X-ray photoelectron spectroscopy for Co3O4/Ppy/MWCNT, (a) survey of presence of elements, high resolution spectra of (b) Co2p3/2 (c) N1s and (d) O 1s.
Please cite this article as: Jayaseelan SS et al., Co3O4 nanoparticles decorated Polypyrrole/carbon nanocomposite as efficient bi-functional electrocatalyst for electrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.085
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Fig. 3 e LSV of (a) Co3O4/Ppy/carbon composites, (b) lower and higher Co3O4 loaded Ppy/MWCNT, (c) Tafel plots of different Co3O4/Ppy/carbon composites and (d) LSV for long term durability up to 2000 potential cycles for Co3O4/Ppy/MWCNT catalysts for OER activity in 0.1M KOH at scan rate of 10 mV/s.
respectively (JCPDS card no.43-1003) [40], indicating that the crystallinity of cobalt oxide was not affected by the nature of carbon supports in Ppy/C composites [18]. The comparison of Co3O4 with different wt. % supported over Ppy/MWCNT was shown in Fig. 1a. It clearly reveals that the intensity of Co3O4 crystalline peaks got increased when increasing the wt. % of cobalt precursor on Ppy/MWCNT. The crystallite size of the Co3O4 nanoparticles supported on Ppy/C was calculated using Scherer equation and found to be 8e10 nm. As shown in Figure 1b and c, Co3O4 nanoparticles were uniformly distributed on Ppy/MWCNT composite with particle size between 8 and 10 nm. The selected-area electron diffraction (SAED) pattern of Co3O4 supported on Ppy/MWCNT (inset of Fig. 1c) shows the concentric rings composed of tiny bright spots, bringing out the polycrystalline nature of the catalyst. The acquired interplanar spacing from SAED pattern can be attributed to (311), (400), (511) and (440) crystalline planes, which are consistent with XRD results. The HRTEM image (Fig. 1d) shows crystalline lattice fringes corresponding to (311) plane of Co3O4 nanoparticles and the d-spacing value (0.245 nm). FESEM images of Co3O4/Ppy over RGO, VC and MWCNT nanocomposites show clustered morphology of metal oxide nanoparticles dispersed over Ppy/C composite (Figs. S4aec). Also, the morphology was confirmed by TEM
studies, showing the larger particle size and nonuniform distribution of particles over RGO and VC supported catalysts comparing to MWCNT supported catalyst (Fig. S5). The elemental mapping of Co3O4/Ppy/MWCNT clearly exhibits the homogenous dispersion of metal oxides over Ppy/MWCNT and confirms the presence of Co, O, N and C with appropriate percentages, as shown in Fig. S6. Surface and electronic properties of Co3O4/Ppy/MWCNT was determined by XPS spectra, as shown in Fig. 2. The XPS survey spectra (Fig. 2a) clearly indicates the presence of the Co, N, O and the deconvoluted spectra of Co 2p, N 1s and O 1s are presented in Fig. 2bed respectively. For Co3O4, the deconvoluted Co 2p peaks were fitted into 779.6 eV and 794.9 eV for 2P3/2 and 2P1/2 electronic states referring to Co3þ and Co2þ oxidation states respectively. The weak satellite peaks appeared at around 785.3 eV and 803.2 eV from the spinorbit component of Co-OH interaction with surface hydroxyl groups [41]. The N 1s spectrum has broad and intense peaks appeared at 399.1 eV and 397.8 eV, which is attributed to encapsulation of Ppy over carbon supports. Fig. 2d shows the peaks for O 1s spectrum with three different oxygen states denoted as O1, O2 and O3 at 531.7 eV, 530.3 eV 528.8 eV respectively. The characteristic oxygen peaks are typically attributed to surface adsorbed oxygen (O1), hydroxyl group oxygen (O2) and metal-oxide oxygen (lattice oxygen, O3)
Please cite this article as: Jayaseelan SS et al., Co3O4 nanoparticles decorated Polypyrrole/carbon nanocomposite as efficient bi-functional electrocatalyst for electrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.085
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[42,43]. Therefore, the presence of Co3þ as major species in Co 2P is highly favorable for improved OER [40]. The electrochemical performance of Co3O4/Ppy nanocomposite with three different carbon supports for OER was studied by LSV measurements and shown in Fig. 3a. The over potential at 10 mA cm2 was found to be 340, 330 and 300 mV for Co3O4/Ppy/MWCNT, Co3O4/Ppy/VC and Co3O4/Ppy/RGO respectively, which indicating better OER activity and comparable with reported OER catalysts (Table S1) [44e53]. The exchange current density was found to be 1.13 mA/cm2 for Co3O4/Ppy/MWCNT, which was nearly two times higher than that of VC (0.55 mA/cm2) and RGO (0.62 mA/cm2) supported nanocomposites. This improved OER activity of Co3O4/Ppy/ MWCNT nanocomposite could be due to uniform distribution of Co3O4 nanoparticles decorated over Ppy/MWCNT substrate, consequently more active sites were exposed for surface reactions. This phenomenon could be clearly confirmed from the TEM images of Co3O4/Ppy/MWCNT (Figure 1b and c). Fig. 3b shows the LSV profiles recorded for 5 and 20 wt % of Co3O4 supported on Ppy/MWCNT at 1600 rpm. The overpotential of higher metal loading (20 wt %) was found to be slightly increased compared to 5 wt %, might be due to the aggregation of metal oxide nanoparticles on Ppy/ MWCNT. The catalytic activity of Co3O4 based catalysts were examined by Tafel slope values, which were obtained from respective Tafel plots, as shown in Fig. 3c. The lower Tafel slope values of 87 mV/dec indicates that the Co3O4/Ppy/ MWCNT possesses more active sites than Co3O4/Ppy/VC (95 mV/dec) and Co3O4/Ppy/RGO (105 mV/dec), which was considered as to be advantageous to OER. The calculated overpotential, Tafel slope and exchange current density of Co3O4/ Ppy/C nanocomposites were summarized in Table 1. The calculated electrochemical surface areas (ECSA) are 0.00156 cm2, 0.00116 cm2 and 0.00071 cm2 for MWCNT, RGO and VC supported Co3O4/Ppy composites respectively. Obviously, Co3O4/Ppy/MWCNT exhibits higher ECSA, which could lead to improved catalytic activity towards OER. The stability of Co3O4/Ppy/MWCNT catalyst was investigated by continuous potential cycling method. As presented in Fig. 3d, the overpotential of Co3O4/Ppy/MWCNT was only slightly increased (~18 mV) after 2000 potential cycles, which clearly demonstrated the good stability of the catalyst. After long term durability test, the morphology of catalyst showed increased particle size with random accumulation over composite layer, as shown in Fig. S7. This is probably caused by the formation of oxide/hydroxide from metallic/oxide in basic
Table 1 e Comparison table for OER activity of different Co3O4 catalysts. Catalysts
Co3O4/Ppy/VC Co3O4/Ppy/RGO Co3O4/Ppy/ MWCNT
Over potential (V) at 10 mA cm2
Tafel slope (mV dec1)
Exchange current density (mA cm2)
1.56 1.53 1.57
105 95 87
0.55 0.62 1.13
medium, consequently resulting in the loss of surface active sites during long term durability [16]. The electrocatalytic activity of Co3O4/Ppy/MWCNT for HER was also examined, as shown in Fig. 4. The polarization curves (LSV profiles) for HER was recorded in 0.1M KOH medium between 0.5 V and 0 V vs. RHE. Fig. 4a shows the HER-LSV profiles for 5 and 20 wt % of Co3O4/Ppy/MWCNT nanocomposites. The earlier onset potential of 0.287 V for 5 wt % Co3O4/Ppy/MWCNT shows lower overpotential (490 mV) at 10 mA/cm2, indicating the appreciable electrocatalytic performance. Tafel slope values were found to be 110 and 139 mV/dec for 5 and 20 wt % Co3O4/Ppy/MWCNT obtained from Fig. 4b. The lower Tafel slope value of 5 wt % catalyst suggests fast HER kinetics than that for 20 wt % Co3O4/Ppy/MWCNT. As previously mentioned, the catalyst with higher cobalt loading possessed less active sites because random growth of cobalt oxide could lead to particle aggregation. It is well supported from the electronic conductivity of the nanocomposites obtained from EIS studies, as presented in Fig. 4c. The small semi-circle of 5 wt % Co3O4/Ppy/MWCNT shows very low charge transfer resistance (47 U), which is almost lower than the 20 wt % catalyst (51 U). The good conductivity of 5 wt % Co3O4/Ppy/MWCNT can be attributed to the low content of the oxide and the uniform distribution of the nanoparticles over Ppy/MWCNT, which enable the MWCNTs to remain its good electron conductivity. The stabilities of the two Co3O4/Ppy/ MWCNT nanocomposites for HER were tested using chronoamperometry technique, as shown in Fig. 4d. The gradual current decay was observed for both the catalysts over the period (3600 s) at a fixed potential of 0.25 V vs. RHE. The percentage of current loss was found to be nearly same (~27%) for both nanocomposites, implying the good stabilities. From these results, it can be concluded that the Co3O4/Ppy/MWCNT nanocomposite is a promising bifunctional catalyst for both OER and HER in water electrolysis. It is considered that the high electron conductive MWCNT was encapsulated by proton conducting polypyrrole, then providing hybrid nature of the catalyst support, which was beneficial to the immobilization and distribution of Co3O4 nanoparticles, consequently providing more surface-active sites for electrochemical reactions. In addition, the presence of N within Co3O4/Ppy composite could further enhance its electrochemical activity due to the electronic synergy effect [41,54]. It should be noticed here the performance of the composite needs to be further improved to reach the level of benchmark catalysts (such as IrO2/RuO2 and Pt/Pd) for water splitting. However, it should be mentioned that developing costeffective composites with satisfactory performance is critical to future water splitting application, which has attracted many attentions in recent years. Furthermore, the Co3O4/Ppy/ MWCNT developed in this works demonstrated appreciable competing performance to previously reported cobalt and cobalt oxide based electrocatalysts for OER as shown in Table S1. Therefore, this work actually proposed a new and facile strategy to develop cost-effective and efficient non-noble electrocatalyst with decent performance for water splitting, which was considered to be meaningful for the future water splitting researches.
Please cite this article as: Jayaseelan SS et al., Co3O4 nanoparticles decorated Polypyrrole/carbon nanocomposite as efficient bi-functional electrocatalyst for electrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.085
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Fig. 4 e LSV of (a) Co3O4/Ppy/MWCNT, (b) Tafel plots of different Co3O4/Ppy/MWCNT and (c) EIS of Co3O4/Ppy/MWCNT in 0.1M KOH at scan rate of 10 mV/s.
Conclusion In this present work, we have successfully prepared Ppy/C composites by in-situ polymerization and uniform distribution of Co3O4 nanoparticles have been successfully achieved by simple dry synthetic approach. Surface characterization results confirmed the uniform distribution of Co3O4 nanoparticles with crystallite size of 8e10 nm over Ppy encapsulated carbon supports. Among the various carbon supported catalysts, Co3O4/Ppy/MWCNT catalyst exhibited improved activity for OER and HER with lower overpotential (340 vs. 490 mV) at 10 mA cm2, which are comparable to the results previously reported in literatures. The surface functionalization of MWCNT with polypyrrole plays a vital role to attain homogenous distribution of Co3O4 nanoparticles results improved electrocatalytic activity and stability. Furthermore, the Co3O4/Ppy/MWCNT showed better performance for OER and HER, implying that it is a promising bi-functional catalyst for electrochemical water splitting reactions.
Acknowledgements We thank the financial support from National Natural Science Foundation of China (Nos. 21676126, 51676092), Natural Science Foundation of Jiangsu Province (Nos. BK20171296, BK20180877), Key R&D project (No. GY2018024) and High-tech
Research Key Laboratory (No. SS2018002) of Zhenjiang City, the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions and the Research Fund Program of Key Laboratory of Fuel Cell Technology of Guangdong Province.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.12.085.
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Please cite this article as: Jayaseelan SS et al., Co3O4 nanoparticles decorated Polypyrrole/carbon nanocomposite as efficient bi-functional electrocatalyst for electrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.085
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Co3O4 nanoparticles decorated Polypyrrole/carbon nanocomposite as efficient bi-functional electrocatalyst for electrochemical water splitting Santhana Sivabalan Jayaseelan, Narayanamoorthy Bhuvanendran, Qian Xu, Huaneng Su* Institute for Energy Research, Jiangsu University, 301 Xuefu Road, Zhenjiang, 212013, PR China
highlights Uniformly
graphical abstract
distributed
Co3O4
nanoparticles on Ppy/C composite was
realized
by
simple
dry
synthesis. The effect of carbon support on the performance of Co3O4/Ppy/C was investigated. Enriched conductivity by Ppy on carbon
supports
results
in
improved electrocatalytic activity. Co3O4/Ppy/MWCNT shows good bifunctional activity and durability for water splitting.
article info
abstract
Article history:
An effective bi-functional electrocatalyst of Co3O4/Polypyrrole/Carbon (Co3O4/Ppy/C)
Received 7 August 2019
nanocomposite was prepared through a simple dry chemical method and used to catalyze
Received in revised form
the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Three types of
30 September 2019
carbon support as Vulcan carbon, reduced graphite oxide (RGO) and multi-walled carbon
Accepted 12 December 2019
nanotubes (MCNTs) were used to study the influence on electrochemical reactions.
Available online xxx
Spherical shaped Co3O4 nanoparticles with 8e10 nm was found uniformly distributed on Ppy/C composite, which were analyzed by X-ray diffraction and transmission electron
Keywords:
microscopy techniques. Amongst, Co3O4/Ppy/MWCNT shows improved bifunctional elec-
Cobalt oxide electrocatalyst
trocatalytic activity towards both OER and HER with relatively low over potential (340 mV
Polypyrrole
vs. 490 mV at 10 mA cm2) and Tafel slope (87 vs. 110 mV dec1). In addition to that,
Bifunctional catalyst
MWCNT supported Co3O4/Ppy nanocomposite exhibits good electronic conductivity and electrochemical stability up to 2000 potential cycles. The results clearly indicate that the
* Corresponding author. E-mail address: [email protected] (H. Su). https://doi.org/10.1016/j.ijhydene.2019.12.085 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Jayaseelan SS et al., Co3O4 nanoparticles decorated Polypyrrole/carbon nanocomposite as efficient bi-functional electrocatalyst for electrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.085
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international journal of hydrogen energy xxx (xxxx) xxx
Oxygen evolution
Co3O4/Ppy/MWCNT nanocomposite could be the promising bi-functional electrocatalyst
Water splitting
for efficient water electrolysis. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Electrolysis of water, a highly significant way for the generation of hydrogen (H2) and oxygen (O2) to afford sustainable and green energy technology as a better alternative to conventional energy sources, has got great attention in this decade. For electrochemical water splitting, oxygen evolution reaction (OER) is critical due to the sluggish reaction kinetics, thermodynamically unfavorable and higher overpotential. Similarly, hydrogen evolution reaction (HER) is another half reaction of water splitting to produce hydrogen for energy conversion and storage techniques, which also need an efficient catalyst to overcome the energy barrier [1]. Although platinum group metals (PGM) are well known electrocatalyst for water splitting reactions, the high cost and low durability of PGM catalysts are restrictions for its large-scale application. Therefore, developing active, durable and cost-effective nonnoble electrocatalyst is critical to the future commercialization of water electrolysis [2e5]. Many researchers have been studied the influence of controlled morphology of non-noble (Co, Fe, Ni, Mn etc.) nanocatalysts with various carbon free and carbon supports for water electrolysis [6e11]. Among the non-noble catalysts, cobalt based nanocomposite materials has got much attention due to its improved bifunctional electrocatalytic activity for both OER and HER [4,12e15]. In the context of morphology controlled cobalt electrocatalysts, Naseri et al. have reported the electrodeposited cobalt oxide nano flakes and decoration of cobalt oxide nanoparticles over various substrates were used as electrocatalyst for water oxidation reaction [16e18]. Masa et al. reported the preparation of cobalt-cobalt phosphide nanoparticles supported on nickel foam by thermal decomposition of metal precursor under reductive conditions, which showed high activity for water splitting with low overpotential [19]. Interestingly, Prasanth et al. demonstrated that the highly durable helical cobalt borophosphate can be employed as a good bi-functional water splitting electrocatalyst with high energetic efficiency up to 75% [20]. Xu et al. have reported the preparation of cobalt oxide nanoplates by ligand assisted polyol reduction, which also showed a low over potential of 306 mV for OER [21]. In the work conducted by Ranaweera et al., a flower shaped Co3O4 was prepared by binder free synthetic approach and employed as flexible OER electrode material, which demonstrated the good electronic conductivity and stability during operation [22]. Nickel and Cobalt incorporated tungsten sulfide catalyst was also reported by Yang et al., which showed a prolonged stability over 24 h with retention of its initial overpotential for HER [23]. Recently, Li et al. reported Co nanoparticles encapsulated N and S doped carbon nanotubes prepared by thermal reduction
method and it shows higher electrocatalytic activity for HER [24]. Although Cobalt and Co-oxide catalysts shows good electrocatalytic performance towards OER and HER [25e27], but still suffers from poor dispersion, low conductivity and durability. In order to address the above issues carbon materials can be used as catalyst support owing to the following factors; (i) high surface area, particle dispersion and improved accessibility of the metal/metal oxide nanoparticles and (ii) electronic interactions between support and catalyst [28,29]. Various carbon supports have been employed for metal/metal oxide catalysts such as, carbon black (CB), reduced graphene oxide (RGO), single-walled carbon nanotubes (SWNT), and multi-walled carbon nanotubes (MCNT). Furthermore, the conductivity and dispersibility of metal nanoparticles can be enhanced by the surface functionalization of carbon support materials through acid treatment [30], ion exchange [31], insitu polymerization [32] and electrochemical deposition [33]. The surface functionalization of carbon supports with conducting polymers can induce the improved conductivity through p-p conjugation [34] and also providing more accessible active sites for electrochemical surface reaction. This could be increase the electron transfer rate over the active sites of the catalyst, then leading to improved OER performance. The development of tailored carbon supports with conducting polymers as substrate to hold metal/metal oxide nanoparticles could be the promising strategy to attain better performance towards OER and HER [35,36]. Herein, the polypyrrole/carbon (Ppy/C) composite was prepared by in-situ oxidative polymerization method and the cobalt oxide (Co3O4) nanoparticles were deposited onto carbon supports through a simple dry synthetic procedure. Three types of carbon supports (Vulcan carbon (VC), reduced graphite oxide (RGO) and multi-walled carbon nanotubes (MWCNT)) were employed, and the resultant Co3O4-Ppy/C materials were studied to evaluate their electrochemical performances towards OER and HER in KOH medium.
Experimental procedure Synthesis of polypyrrole/carbon (Ppy/C) composites The Ppy/C composites were synthesized by in-situ oxidative polymerization method [37]. In this procedure, acid treated carbon support (1 mg/ml) was dispersed in 1:5 ratio of ethanol and HCl mixture and stirred for 30 min. Equal molar (2 mM) ratio of pyrrole monomer and ammonium persulphate was added into the mixture and continued the stirring for 8 h. After that a little amount of acetone was added to end the
Please cite this article as: Jayaseelan SS et al., Co3O4 nanoparticles decorated Polypyrrole/carbon nanocomposite as efficient bi-functional electrocatalyst for electrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.085
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polymerization reaction and the final yield was washed by copious amount of deionized (DI) water and dried at 60 C for 8 h in a hot air oven.
Synthesis of Co3O4/polypyrrole/carbon composites Co3O4 decorated Ppy/C composites were prepared by simple mechanical mixing followed by heat treatment method [38]. 5 wt % of Cobalt Acetate (Co (Ac)2.4H2O, Sigma-Aldrich) was mixed with 50 mg of Ppy/C composites by agate-mortar to get homogenous mixture and heat treated at 300 C for 4 h under N2 atm. The weight percentage of Co3O4 varied to 5 and 20% over Ppy/MWCNT composites. The detailed synthesis procedure was schematically represented in Scheme. 1.
Physical characterization The crystalline nature of the catalyst was analysed by Powder X-ray diffraction (XRD) patterns using PANalytical X’pert Pro X-ray Diffractometer with Cu-Ka radiation. Surface morphology and elemental composition were studied by Energy dispersive X-ray analysis (EDX) equipped with field emission scanning electron microscopy (FESEM, Ultra plus FE Zeiss) instrument. High resolution transmission electron microscopy (HR-TEM) was performed using Hitachi 300 kV instrument to determine the particle size and distribution of catalyst. The electronic state of the element was determined using X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250Xi system equipped with a monochromatic Al Ka as the X-ray source.
Electrochemical characterization The electrochemical characterization was carried out using the electrochemical workstation (CHI 760e, CHI instruments) with three-electrode electrochemical cell composed of an Ag/ AgCl and a Pt-wire as a reference and counter electrode respectively. The glassy carbon-rotating disk electrode (0.196 cm2, GC-RDE, PINE instruments, USA) was used as a working electrode. To prepare the catalyst ink, 2 mg of catalyst was dispersed in 750 ml of DI water, 250 ml of isopropanol and 20 ml of Nafion (Sigma-Aldrich) and sonicated for 45 min to get homogenous ink. 10 ml of suspension was pipetted and coated onto the GC electrode and dried at 60 C. 0.1 M potassium hydroxide (KOH) was used as electrolyte and all the potentials were converted to a reversible hydrogen electrode (RHE) using the following equation (1), ERHE ¼ EAg=AgCl þ 0:197 þ 0:059* pH
(1)
The electrocatalytic activity of the prepared catalysts was studied by linear scan voltammetry (LSV) measurements
3
conducted in 0.1M KOH solution between 1.2 V and 1.7 V for OER and 0.5 V and 0 V for HER at a scan rate of 10 mV/s. The electrode rotation speed was fixed at 1600 rpm. The onset potential (Eonset) was determined by linearly extrapolating the fast-rising current portion of the peak to the linear extrapolation of the background current [39]. The durability test was performed for OER through potential cycling between 1.2 V and 1.7 V up to 2000 potential cycles under similar conditions. Electrochemical impedance spectroscopy (EIS) was studied in the frequency range of 100 kHz to 0.1 Hz with an amplitude voltage 5 mV.
Results and discussion The FTIR spectra of Ppy/C and Co3O4/Ppy/C are shown in Figs. S1a and S1b. The broad absorption band in the range between 4000 cm1 to 3000 cm1, which is commonly attributed for the absorption band for N-H, O-H groups. The MWCNT have been confirmed by the C-C absorption band at 1583 cm1. Also the predominant bands at 3400 cm1, 1631 cm1 and 1069 cm1 have confirms the functional groups of RGO in the composite. The absorption band around 1667 cm1 indicates the presence of Ppy along with Co3O4 absorption bands were observed in 667 cm1 and 570 cm1. The Figs. S2a and S2b shows Raman spectra of prepared composites. The peaks at ~1350 cm1 and ~1580 cm1 can be ascribed to well-known D and G bands of carbon supports with the second order peak at ~2750 cm1 called 2D band. The encapsulation of Ppy over carbon supports have been confirmed by the additional peaks at ~937 cm1 and ~1021 cm1 [37]. Compared to other carbon supports, the notable change in G band and D band of MWCNT proves the interfacial interactions between MWCNT and Ppy. Also, the suppressed G and D bands of Co3O4/Ppy/MWCNT than Co3O4/MWCNT confirms the Ppy coating over the MWCNT surface shown in Fig. S2b. The typical characteristics peaks at 412 cm1, 515 cm1 and 690 cm1 have describes the decoration of Co3O4 particles over Ppy/C composite layer. The powder XRD patterns of Co3O4/Ppy supported on various carbon supports were shown in Fig. 1a and Fig. S3. The presence of polypyrrole was clearly observed from the low angle crystalline peak appeared at 18 as shown in Fig. 1a. In addition to that, the presence of Ppy suppressed the characteristic peak intensity of carbon support (26 (100) for VC and MWCNT and 11 (002) for RGO) as clearly observed from Figure 1a, S3a and S3b, implying the uniform surface coating of polymer. From the respective XRD patterns of all the three carbon based Co3O4/Ppy nanocomposites, the characteristic peaks for Co3O4 nanoparticles can be observed at 2q values of 31.2 , 36.6 , 44.7 , 59.1 and 65 attributed to (220), (311), (400), (511) and (440) of cubic crystalline planes of cobalt oxide
Scheme. 1 e Schematic representation of synthetic pathway of Co3O4/Ppy/C nanocomposites. Please cite this article as: Jayaseelan SS et al., Co3O4 nanoparticles decorated Polypyrrole/carbon nanocomposite as efficient bi-functional electrocatalyst for electrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.085
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Fig. 1 e (a) X-ray diffraction spectra for Co3O4/Ppy/MWCNT, (b)(c) TEM images of Co3O4/Ppy/MWCNT (inset: SAED pattern), (d) HRTEM image with lattice fringes.
Fig. 2 e X-ray photoelectron spectroscopy for Co3O4/Ppy/MWCNT, (a) survey of presence of elements, high resolution spectra of (b) Co2p3/2 (c) N1s and (d) O 1s.
Please cite this article as: Jayaseelan SS et al., Co3O4 nanoparticles decorated Polypyrrole/carbon nanocomposite as efficient bi-functional electrocatalyst for electrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.085
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Fig. 3 e LSV of (a) Co3O4/Ppy/carbon composites, (b) lower and higher Co3O4 loaded Ppy/MWCNT, (c) Tafel plots of different Co3O4/Ppy/carbon composites and (d) LSV for long term durability up to 2000 potential cycles for Co3O4/Ppy/MWCNT catalysts for OER activity in 0.1M KOH at scan rate of 10 mV/s.
respectively (JCPDS card no.43-1003) [40], indicating that the crystallinity of cobalt oxide was not affected by the nature of carbon supports in Ppy/C composites [18]. The comparison of Co3O4 with different wt. % supported over Ppy/MWCNT was shown in Fig. 1a. It clearly reveals that the intensity of Co3O4 crystalline peaks got increased when increasing the wt. % of cobalt precursor on Ppy/MWCNT. The crystallite size of the Co3O4 nanoparticles supported on Ppy/C was calculated using Scherer equation and found to be 8e10 nm. As shown in Figure 1b and c, Co3O4 nanoparticles were uniformly distributed on Ppy/MWCNT composite with particle size between 8 and 10 nm. The selected-area electron diffraction (SAED) pattern of Co3O4 supported on Ppy/MWCNT (inset of Fig. 1c) shows the concentric rings composed of tiny bright spots, bringing out the polycrystalline nature of the catalyst. The acquired interplanar spacing from SAED pattern can be attributed to (311), (400), (511) and (440) crystalline planes, which are consistent with XRD results. The HRTEM image (Fig. 1d) shows crystalline lattice fringes corresponding to (311) plane of Co3O4 nanoparticles and the d-spacing value (0.245 nm). FESEM images of Co3O4/Ppy over RGO, VC and MWCNT nanocomposites show clustered morphology of metal oxide nanoparticles dispersed over Ppy/C composite (Figs. S4aec). Also, the morphology was confirmed by TEM
studies, showing the larger particle size and nonuniform distribution of particles over RGO and VC supported catalysts comparing to MWCNT supported catalyst (Fig. S5). The elemental mapping of Co3O4/Ppy/MWCNT clearly exhibits the homogenous dispersion of metal oxides over Ppy/MWCNT and confirms the presence of Co, O, N and C with appropriate percentages, as shown in Fig. S6. Surface and electronic properties of Co3O4/Ppy/MWCNT was determined by XPS spectra, as shown in Fig. 2. The XPS survey spectra (Fig. 2a) clearly indicates the presence of the Co, N, O and the deconvoluted spectra of Co 2p, N 1s and O 1s are presented in Fig. 2bed respectively. For Co3O4, the deconvoluted Co 2p peaks were fitted into 779.6 eV and 794.9 eV for 2P3/2 and 2P1/2 electronic states referring to Co3þ and Co2þ oxidation states respectively. The weak satellite peaks appeared at around 785.3 eV and 803.2 eV from the spinorbit component of Co-OH interaction with surface hydroxyl groups [41]. The N 1s spectrum has broad and intense peaks appeared at 399.1 eV and 397.8 eV, which is attributed to encapsulation of Ppy over carbon supports. Fig. 2d shows the peaks for O 1s spectrum with three different oxygen states denoted as O1, O2 and O3 at 531.7 eV, 530.3 eV 528.8 eV respectively. The characteristic oxygen peaks are typically attributed to surface adsorbed oxygen (O1), hydroxyl group oxygen (O2) and metal-oxide oxygen (lattice oxygen, O3)
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[42,43]. Therefore, the presence of Co3þ as major species in Co 2P is highly favorable for improved OER [40]. The electrochemical performance of Co3O4/Ppy nanocomposite with three different carbon supports for OER was studied by LSV measurements and shown in Fig. 3a. The over potential at 10 mA cm2 was found to be 340, 330 and 300 mV for Co3O4/Ppy/MWCNT, Co3O4/Ppy/VC and Co3O4/Ppy/RGO respectively, which indicating better OER activity and comparable with reported OER catalysts (Table S1) [44e53]. The exchange current density was found to be 1.13 mA/cm2 for Co3O4/Ppy/MWCNT, which was nearly two times higher than that of VC (0.55 mA/cm2) and RGO (0.62 mA/cm2) supported nanocomposites. This improved OER activity of Co3O4/Ppy/ MWCNT nanocomposite could be due to uniform distribution of Co3O4 nanoparticles decorated over Ppy/MWCNT substrate, consequently more active sites were exposed for surface reactions. This phenomenon could be clearly confirmed from the TEM images of Co3O4/Ppy/MWCNT (Figure 1b and c). Fig. 3b shows the LSV profiles recorded for 5 and 20 wt % of Co3O4 supported on Ppy/MWCNT at 1600 rpm. The overpotential of higher metal loading (20 wt %) was found to be slightly increased compared to 5 wt %, might be due to the aggregation of metal oxide nanoparticles on Ppy/ MWCNT. The catalytic activity of Co3O4 based catalysts were examined by Tafel slope values, which were obtained from respective Tafel plots, as shown in Fig. 3c. The lower Tafel slope values of 87 mV/dec indicates that the Co3O4/Ppy/ MWCNT possesses more active sites than Co3O4/Ppy/VC (95 mV/dec) and Co3O4/Ppy/RGO (105 mV/dec), which was considered as to be advantageous to OER. The calculated overpotential, Tafel slope and exchange current density of Co3O4/ Ppy/C nanocomposites were summarized in Table 1. The calculated electrochemical surface areas (ECSA) are 0.00156 cm2, 0.00116 cm2 and 0.00071 cm2 for MWCNT, RGO and VC supported Co3O4/Ppy composites respectively. Obviously, Co3O4/Ppy/MWCNT exhibits higher ECSA, which could lead to improved catalytic activity towards OER. The stability of Co3O4/Ppy/MWCNT catalyst was investigated by continuous potential cycling method. As presented in Fig. 3d, the overpotential of Co3O4/Ppy/MWCNT was only slightly increased (~18 mV) after 2000 potential cycles, which clearly demonstrated the good stability of the catalyst. After long term durability test, the morphology of catalyst showed increased particle size with random accumulation over composite layer, as shown in Fig. S7. This is probably caused by the formation of oxide/hydroxide from metallic/oxide in basic
Table 1 e Comparison table for OER activity of different Co3O4 catalysts. Catalysts
Co3O4/Ppy/VC Co3O4/Ppy/RGO Co3O4/Ppy/ MWCNT
Over potential (V) at 10 mA cm2
Tafel slope (mV dec1)
Exchange current density (mA cm2)
1.56 1.53 1.57
105 95 87
0.55 0.62 1.13
medium, consequently resulting in the loss of surface active sites during long term durability [16]. The electrocatalytic activity of Co3O4/Ppy/MWCNT for HER was also examined, as shown in Fig. 4. The polarization curves (LSV profiles) for HER was recorded in 0.1M KOH medium between 0.5 V and 0 V vs. RHE. Fig. 4a shows the HER-LSV profiles for 5 and 20 wt % of Co3O4/Ppy/MWCNT nanocomposites. The earlier onset potential of 0.287 V for 5 wt % Co3O4/Ppy/MWCNT shows lower overpotential (490 mV) at 10 mA/cm2, indicating the appreciable electrocatalytic performance. Tafel slope values were found to be 110 and 139 mV/dec for 5 and 20 wt % Co3O4/Ppy/MWCNT obtained from Fig. 4b. The lower Tafel slope value of 5 wt % catalyst suggests fast HER kinetics than that for 20 wt % Co3O4/Ppy/MWCNT. As previously mentioned, the catalyst with higher cobalt loading possessed less active sites because random growth of cobalt oxide could lead to particle aggregation. It is well supported from the electronic conductivity of the nanocomposites obtained from EIS studies, as presented in Fig. 4c. The small semi-circle of 5 wt % Co3O4/Ppy/MWCNT shows very low charge transfer resistance (47 U), which is almost lower than the 20 wt % catalyst (51 U). The good conductivity of 5 wt % Co3O4/Ppy/MWCNT can be attributed to the low content of the oxide and the uniform distribution of the nanoparticles over Ppy/MWCNT, which enable the MWCNTs to remain its good electron conductivity. The stabilities of the two Co3O4/Ppy/ MWCNT nanocomposites for HER were tested using chronoamperometry technique, as shown in Fig. 4d. The gradual current decay was observed for both the catalysts over the period (3600 s) at a fixed potential of 0.25 V vs. RHE. The percentage of current loss was found to be nearly same (~27%) for both nanocomposites, implying the good stabilities. From these results, it can be concluded that the Co3O4/Ppy/MWCNT nanocomposite is a promising bifunctional catalyst for both OER and HER in water electrolysis. It is considered that the high electron conductive MWCNT was encapsulated by proton conducting polypyrrole, then providing hybrid nature of the catalyst support, which was beneficial to the immobilization and distribution of Co3O4 nanoparticles, consequently providing more surface-active sites for electrochemical reactions. In addition, the presence of N within Co3O4/Ppy composite could further enhance its electrochemical activity due to the electronic synergy effect [41,54]. It should be noticed here the performance of the composite needs to be further improved to reach the level of benchmark catalysts (such as IrO2/RuO2 and Pt/Pd) for water splitting. However, it should be mentioned that developing costeffective composites with satisfactory performance is critical to future water splitting application, which has attracted many attentions in recent years. Furthermore, the Co3O4/Ppy/ MWCNT developed in this works demonstrated appreciable competing performance to previously reported cobalt and cobalt oxide based electrocatalysts for OER as shown in Table S1. Therefore, this work actually proposed a new and facile strategy to develop cost-effective and efficient non-noble electrocatalyst with decent performance for water splitting, which was considered to be meaningful for the future water splitting researches.
Please cite this article as: Jayaseelan SS et al., Co3O4 nanoparticles decorated Polypyrrole/carbon nanocomposite as efficient bi-functional electrocatalyst for electrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.085
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Fig. 4 e LSV of (a) Co3O4/Ppy/MWCNT, (b) Tafel plots of different Co3O4/Ppy/MWCNT and (c) EIS of Co3O4/Ppy/MWCNT in 0.1M KOH at scan rate of 10 mV/s.
Conclusion In this present work, we have successfully prepared Ppy/C composites by in-situ polymerization and uniform distribution of Co3O4 nanoparticles have been successfully achieved by simple dry synthetic approach. Surface characterization results confirmed the uniform distribution of Co3O4 nanoparticles with crystallite size of 8e10 nm over Ppy encapsulated carbon supports. Among the various carbon supported catalysts, Co3O4/Ppy/MWCNT catalyst exhibited improved activity for OER and HER with lower overpotential (340 vs. 490 mV) at 10 mA cm2, which are comparable to the results previously reported in literatures. The surface functionalization of MWCNT with polypyrrole plays a vital role to attain homogenous distribution of Co3O4 nanoparticles results improved electrocatalytic activity and stability. Furthermore, the Co3O4/Ppy/MWCNT showed better performance for OER and HER, implying that it is a promising bi-functional catalyst for electrochemical water splitting reactions.
Acknowledgements We thank the financial support from National Natural Science Foundation of China (Nos. 21676126, 51676092), Natural Science Foundation of Jiangsu Province (Nos. BK20171296, BK20180877), Key R&D project (No. GY2018024) and High-tech
Research Key Laboratory (No. SS2018002) of Zhenjiang City, the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions and the Research Fund Program of Key Laboratory of Fuel Cell Technology of Guangdong Province.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.12.085.
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Please cite this article as: Jayaseelan SS et al., Co3O4 nanoparticles decorated Polypyrrole/carbon nanocomposite as efficient bi-functional electrocatalyst for electrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.085