In-situ developed carbon spheres function as promising support for enhanced activity of cobalt oxide in oxygen evolution reaction

Journal of Colloid and Interface Science 530 (2018) 264–273

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Regular Article

In-situ developed carbon spheres function as promising support for enhanced activity of cobalt oxide in oxygen evolution reaction Mrinmoyee Basu Department of Chemistry, BITS Pilani, Pilani, Rajasthan 333031, India

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 8 May 2018 Revised 26 June 2018 Accepted 27 June 2018 Available online 28 June 2018 Keywords: Carbon sphere Oxide nanoparticles Conducting support Heterostructure Oxygen evolution reaction

a b s t r a c t Highly active, stable electrocatalyst for oxygen evolution reaction (OER) is sincerely required for the practical application of water splitting to get rid from the sluggish reaction kinetics and the stability issue. Here, Co3O4 is studied as OER catalyst and to improve the electrocatalytic activity, carbon is chosen as the conducting support. A simple and cost-effective synthetic route is developed for the synthesis of Co3O4 on carbon support following hydrothermal route using various hydrolyzing agents. The heterostructure ‘Co3O4/C’ perform well in OER as a non-precious metal catalyst. The best Co3O4/C electrocatalyst can generate 10 and 30 mA/cm2 current densities upon application of 1.623 V and 1.678 V vs. RHE whereas, bare Co3O4 can generate current density of 10 and 30 mA/cm2 upon application of 1.677 and 1.754 V vs. RHE. Carbon in the heterostructure helps to improve the conductivity and at the same time enhances the charge transfer ability which further leads to increase current density and stability to the catalyst. Co3O4/C can generate unaltered current density up to 1000 cycles. Ó 2018 Published by Elsevier Inc.

1. Introduction Continuous and uprising demand for energy inspired to develop an efficient process to generate fuel from renewable sources of energy. Water splitting reaction is an economical and promising process to generate highly pure hydrogen [1]. Electrocatalytic water E-mail address: [email protected] https://doi.org/10.1016/j.jcis.2018.06.087 0021-9797/Ó 2018 Published by Elsevier Inc.

splitting is considered as greener approach compared to other process like coal gasification, partial oxidation etc., because it does not emit any pollutant to environment [2]. Water splitting reaction composed of two half reactions. Hydrogen evolution reaction occurs in cathode. Pt and Pt-based compounds are considered as the best and efficient electrocatalysts for hydrogen evolution reaction [3,4]. On the other hand, in anode oxygen evolution reaction (OER) takes place and the kinetics of OER reaction is sluggish with

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large over potential. RuO2 and IrO2 have established themselves as efficient electrocatalysts in OER reaction [5,6]. However, availability, cost, and stability of RuO2 and IrO2 electrocatalysts restrict them for their practical applications. So, it is highly demanding to introduce an efficient and stable OER catalyst for the improvement of water splitting efficiency. Considerable attention has already been devoted for the development of efficient electrocatalyst as alternative of RuO2, IrO2 for OER with high durability. A large number of catalysts are already discovered which include metal, metal and carbon based materials [7]. Oxides, alloys, hydroxides, layer double hydroxides, sulfides, selenides, phosphides of Fe, Co, Ni and Mo have already established themselves as efficient catalysts for OER [8–18]. Scientists are dedicatedly trying to improve the catalytic activity of these developed materials. To improve the electrocatalytic activity, doping with hetero atom is also an important approach [19–21]. Different carbon based materials like graphene, CNT, conductive carbon is also developed to function as a support for the synthesis of a wide variety of metal based materials [22–25]. In the last few years it is observed that CoOx received a potential consideration in OER due to the presence of mixed valence state (Co2+/3+/4+) [7,26,27]. Co3O4 undergoes phase transformation in the OER condition to generate hydroxides and oxy-hydroxides which play a crucial role. Chen et al., have reported activity of Co3O4 largely dependent on the crystal structure and they have synthesized single crystal Co3O4 nanocube underlay with a CoO layer which exhibits stable, continuous OER for 1000 h. In situ generated oxyhydroxide layer protects the underlying electrocatalyst [28]. The role of oxygen vacancy in the catalytic activity has been discussed by Xu et al. Following plasma engraving strategy Co3O4 was synthesized with high surface area, more oxygen vacancy, and certainly more amount of Co2+. Specific activity of this plasma engraved Co3O4 is 10 times higher than that of pristine Co3O4 [29]. Activity of Co3O4 is largely dependent on the surface area, morphology, oxidation state and also on active charge transportation. Wang et al., discussed that Co3O4 nanowire helps in higher charge transportation leads to enhanced electrocatalytic activity towards OER [30]. Therefore, charge transportation is an important parameter to regulate the electrocatalytic activity. Different carbon based materials like graphene, graphene oxide, CNT, C3N4, carbon are getting used as conducting supporting material to develop efficient and stable electrocatalyst. Pan et al., have reported that Mo2C stabilized by a carbon layer on reduced graphene oxide shows improved electrocatalytic activity towards HER reaction due to the close contact of Mo2C and graphene and the presence of carbon layer on Mo2C [31]. Yu et al., have synthesized carbon coated porous NiP, where the amorphous carbon improves conductivity by enhancing the charge transfer ability [32]. Yang et al., have generated an amorphous layer of carbon on Co3O4 at the time of synthesis on carbon cloth. In situ formed carbon layer helps to enhance the stability in OER catalysis. It can work up to 86.8 h at a constant current density of 100 mA/cm2 [33]. Zhang et al., reported the efficient electrocatalytic activity of Co3O4 particles on highly conductive heteroatom doped carbon support. The heterojunction of Co3O4 and carbon support facilitates the electron transfer by avoiding the charge accumulation on Co3O4 particles [34]. Therefore, literature reports suggest that the electronic conductivity and the structural stability can be improved when the desired materials are anchored on carbon [35]. Singh et al., have observed that when Co3O4 is grafted on N-doped graphene it can function as an efficient catalyst for OER reaction. It can generate current density of 10 mA/cm2 with a very low over potential of 280 mV [36]. Being inspired from the previous studies, in the present report amorphous carbon supported Co3O4 nanoparticles are synthesized following a simple hydrothermal route. Here, Co3O4 is synthesized on carbon support, where, the synthesized metal oxide is anchored

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in a single step. The structural information has been observed from XRD analysis. Raman, Field Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM), and X-ray Photoelectron Spectroscopy (XPS) analyses have also been performed to know the synthesized materials in detail. In the hydrothermal process, hydrolyzing agent was varied to change size, shape, and morphology of Co3O4. Co3O4/C can function as an efficient catalyst for OER reaction in alkaline condition. The most efficient Co3O4/C can generate 10 mA/cm2 current density upon application of 1.62 V vs. RHE. From the point of activity and stability, the catalytic efficiency of Co3O4/C is comparable with commercial RuO2. Co3O4/ C shows a very small Tafel value of 70 mV/decade and unaltered current density up to 1000 cycles. 2. Experimental section 2.1. Synthesis For the synthesis of Co3O4 on carbon support, CoCl2
6H2O was used as the precursor of cobalt. Ammonium hydroxide, sodium hydroxide and triethylene amine (TEA) were used as hydrolyzing agent in different sets. Glucose was used as the carbon source. Following a simple hydrothermal technique Co3O4 was decorated on C. The overall synthesis procedure is shown in Scheme 1. In the following hydrothermal condition glucose undergoes dehydration to generate ‘C’. Controlled hydrothermal reaction was carried out using glucose as the starting material at 180 °C for 16 h and the developed material was washed and collected for characterization. 2.2. Co3O4/C in TEA method First, 1.0 g glucose was dissolved in 20 mL of DI water and to this 5 mL of 0.1 M aqueous solution of CoCl2
6H2O was slowly added and stirred for 15 min. Second, 20 mL of TEA was directly added along with 5 mL of DI water and stirred well to get a clear solution. Finally, hydrothermal reaction was carried out at 180 °C for 16 h. After completion of the reaction, it was cooled down and the black product was thoroughly washed with DI water followed by ethanol for five to six times. Product was dried at 60 °C for 10 h in air oven and kept for further characterization and application. Co3O4/C synthesized from TEA method was denoted as Co3O4/C-T throughout the MS. 2.3. Co3O4/C in NaOH method Similar methodology was followed for the synthesis of Co3O4/C only sodium hydroxide was used as hydrolyzing agent instead of TEA. Throughout the MS, Co3O4/C synthesized using NaOH as hydrolyzing method was denoted as Co3O4/C-N. 2.4. Co3O4/C in NH3 method For the synthesis of Co3O4/C, ammonium hydroxide was used as hydrolyzing agent keeping unchanged the other conditions. Co3O4/ C synthesized by using NH3 was denoted as Co3O4/C-A. Different amounts of glucose were used in the reaction to vary the amount of ‘C’ in Co3O4/C. 2.0 g, 0.5 g and 1.0 g of glucose were used in three different sets of reaction keeping other parameters unaltered and the samples were denoted as Co3O4/C-A-C1, Co3O4/C-A-C2 and Co3O4/C-A-C3. 2.5. Pristine Co3O4 nanostructure Bare Co3O4 was synthesized using the similar hydrothermal procedure with unaltered reaction parameters, except glucose was avoided.

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Scheme 1. Schematic representation of the synthesis of Co3O4/C.

Detailed of the electrochemical measurements, characterization of the materials, calculation methods, experiments for checking stability are given. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jcis. 2018.06.087. 3. Results and discussion 3.1. Morphology and structure Using X-ray diffraction (XRD) analysis, crystal structure, purity and composition of the as synthesizes samples were determined. Glucose was treated hydrothermally for 16 h and the product was collected for characterization by PXRD analysis. PXRD pattern is shown as Fig. S1 a which confirms the synthesis of amorphous ‘C’. Fig. 1a shows the XRD pattern of Co3O4/C-T, Co3O4/C-N and Co3O4/C-A. In the XRD pattern of Co3O4/C-T, clearly three peaks are observed at 2h value of 16.09, 31.9 and 39.2 correspond to (1 1 1), (2 2 0) and (3 1 1) crystal planes, respectively and well matched with the JCPDS no 43–1003 [37]. XRD pattern of all these three Co3O4/C-T, Co3O4/C-N, Co3O4/C-A show the presence of characteristic broad peak of C centered at 2h = 25°, which confirm that both CCT and CCN sample composed of C. Although in case of Co3O4/C-A no peak of Co3O4 is observed from XRD. Therefore, from the XRD analysis it is clear that in the glucose mediated method Co3O4 is synthesized on carbon support without having any impurity or any side product. Bare Co3O4 nanostructure was also synthesized following a similar hydrothermal route without using glucose. XRD pattern of the as-synthesized bare Co3O4 is shown in Fig. S2. Fig. S2 shows the characteristic reflections of (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) for the formation of Co3O4. Raman spectroscopy was used to determine the composition and crystallinity of the material. Raman spectra of Co3O4/C-T and Co3O4/C-A is shown in Fig. 1b measured in the range of 200– 2000 cm 1. In both cases, two strong peaks centered at 1346 and 1590 cm 1 which are due to the D and G bands of carbon. In case of Co3O4/C-T and Co3O4/C-A, the peaks at 673 cm 1 and 677 cm 1 can be assigned to the vibrational mode of A1g of cubic Co3O4

[38]. Raman analysis confirms the phase purity of the synthesized material. The morphology of the as-synthesized Co3O4 on carbon support was characterized initially with the help of FESEM analysis and shown in Fig. 1c–e. Fig. S1b shows the FESEM image of the bare ‘C’ synthesized from hydrothermal reaction of glucose only. It exhibits spherical ‘C’ particles are synthesized following the developed method. Fig. 1c, Fig. S3 show the FESEM images of Co3O4 synthesized by TEA hydrolysis process in different magnifications. Low magnification image shows some spherical particles are distributed throughout the sample. From the medium and high magnification images it is clear that the spherical particles are decorated with small particles, which presumably due to Co3O4. From FESEM images it is clear that there is an intimate contact between Co3O4 particles and the carbon spheres. EDS analysis exhibits the presence of ‘C’, ‘Co’ and ‘O’ as element and shown in Fig. S4. EDS mapping shows the uniform distribution of carbon, cobalt and oxygen throughout the sample which confirms that the synthesized Co3O4 is anchored on carbon substrate (Fig. S5). Fig. 1d, Fig. S6 show FESEM image of Co3O4 by NH3 hydrolysis in different magnifications. Low magnification image shows that the 3D spherical particles are anchored on a sheet. With higher magnification, it is clear that the 3D spherical particles are decorated with very small particles. Particle size of the small particles are not very clear from the FESEM images. Fig. 1e, Fig. S7 show the FESEM image of Co3O4/C using NaOH as the hydrolyzing agent. These also confirm the attachment of Co3O4 nanoparticles on the surface of carbon spheres. EDS analysis (Fig. S8) shows the signals of ‘Co’, ‘O’ and ‘C’ as element. EDS mapping shows the uniform distribution of ‘Co’, ‘O’ and ‘C’ throughout the sample (Fig. S9). FESEM images of bare Co3O4 in different magnifications are shown in Fig. S10. From the FESEM image it is clear that aggregated particles of Co3O4 with high density is synthesized following our method. High magnification FESEM images shows that the aggregated form of Co3O4 is made of very small particles having the diameter 100– 150 nm. EDS analysis shows the presence of Co and O which confirms the synthesis of Co3O4 and shown in Fig. S11. EDS mapping exhibits the uniform distribution of both Co and O throughout the sample (Fig. S12).

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250

a

Co3O4/C-T

Co3O4/C-A

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Co3O4/C-N

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Fig. 1. (a) XRD pattern of Co3O4/C-T, Co3O4/C-N and Co3O4/C-A, (b) Raman spectra of Co3O4/C-T and Co3O4/C-A showing the presence of Co3O4 and carbon, FESEM image of (c) Co3O4/C-T, (d) Co3O4/C-A, (e) Co3O4/C-N.

Morphology, crystallinity were rechecked with the help of transmission electron microscopy (TEM). TEM images with different magnifications of Co3O4/C from TEA method is shown in Fig. 2a–d. Small particles of Co3O4 are anchored on the spheres of carbon. From the high magnification image, the attachment of these two materials are clearly observed. It can be seen from Fig. 2c that small particles of Co3O4 having diameter 20–30 nm is synthesized following our method. Fig. 2d shows the HRTEM image of Co3O4/C-T which confirms the lattice spacing of 0.245 nm, corresponds to the spacing of two (3 1 1) crystal plane of Co3O4. EDS line mapping shows the relative presence of cobalt, carbon and oxygen (Fig. S13) and the EDS analysis illustrates the presence of all these three elements which again clearly confirms that the prepared sample is Co3O4 and decorated on carbon only. Similarly, in case of Co3O4/C in ammonia hydrolysis method, TEM images in different magnifications show the decoration of nanoparticles of Co3O4 on carbon sphere (Fig. 3). To observe a clear view on the oxidation state, composition of the synthesized material, XPS analysis was carried out and shown in Fig. 4a–d. The binding energies are calibrated with respect to C1s peak at 284.6 eV. Survey spectrum shows the presence of only Co, carbon and oxygen (Fig. 4a). Survey spectrum also confirms the absence of any impurity or side product. High resolution spectrum of Co2p is shown as Fig. 4b which consists of two main peaks of Co 2p1/2 and 2p3/2 centered at 797.7 eV and 781.6 eV which are separated by 16 eV. This result is as expected as supported from the literature also [39]. There are two satellite peaks which are also noted at binding energy of 785.6 and 802.8 eV. These are approximately 4 eV above the binding energy of 2p1/2 and 2p3/2. High resolution spectrum of O1s shows two contributions which are centered at 532 and 533.9 eV (Fig. 4c). Synthesis of Co3O4 is also confirmed from the O1s peak. These Co3O4/C was synthesized in

presence of hydroxyl environment. So, the small peak of O1s centered at 533.9 eV may be due to the oxygen of hydroxyl group. The high resolution peak of C1s is shown in Fig. 4d. The strong peak at 284.6 eV is due to the amorphous carbon generated from the thermal dehydration of glucose. The main peak at 284.6 eV is accompanied with two other peaks centered at 286.4 and 288.4 eV which are denoted as ‘B’ and ‘C’ are due to the oxidative forms of hydrocarbon. So, from the XPS spectra it is clear that pure phase carbon supported Co3O4 is synthesized following our methodology. 3.2. Electrocatalytic activity Electrocatalytic activity for OER reaction of Co3O4/C was evaluated following linear sweep voltamogramme (LSV) technique. Electrochemical measurement was carried out in 1.0 M NaOH solution in a three electrode system where graphite was used as the counter electrode, synthesized sample decorated on glassy carbon electrode as working electrode and Ag/AgCl was used as reference electrode. Fig. 5a and b shows the polarization curve of Co3O4/C-T, Co3O4/C-N, Co3O4/C-A, Co3O4, RuO2 and blank GC electrode measured at a scan rate of 5 mV/s. From the observed polarization curve it is pretty sure that the blank GC does not show any catalytic activity in the measured potential window. Co3O4/C-T shows most efficient catalytic activity compared to others. Co3O4/C-T can generate 10 and 30 mA/cm2 current densities upon application of 1.623 V and 1.67 V vs. RHE. Co3O4/C-N requires 1.626 and 1.678 V vs. RHE for the generation of 10 and 30 mA/cm2 current density. However, Co3O4/C-A can generate current density 10 and 30 mA/cm2 upon applied potential of 1.656 and 1.704 V vs. RHE. Electrocatalytic activity of Co3O4/C is compared with a standard reference i.e., commercial RuO2. RuO2 can generate 10 and 30 mA/cm2 current density upon application of 1.589 and

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Fig. 2. TEM images of Co3O4/C synthesized in TEA hydrolysis method shown in different magnifications (a) Low, (b) Medium and (c) High, (d) HRTEM image.

Fig. 3. TEM images of Co3O4/C synthesized in NH3 hydrolysis method shown in different magnifications (a) Low, (b) High.

1.683 V vs. RHE. So, it is clear that the electrocatalytic activity of Co3O4/C-T is comparable with the commercial RuO2. To have clear understanding about the role of the carbon support, bare Co3O4 is synthesized and the electrocatalytic activity for OER is checked. It is observed that Co3O4 can generate current density of 10 and 30 mA/cm2 upon application of 1.677 and 1.754 V vs. RHE (Fig. 5a). So, from this result it can be concluded that the carbon support helps to achieve increased current density with clear shift in the

onset potentials (Fig. S14). Co3O4/C-T, Co3O4/C-N, Co3O4/C-A, Co3O4 can generate unaltered current density even up to 1000 cycles (Fig. S15a–d). Result shows that retention of 100% current density only observed in presence of carbon support. So, it can be concluded that the carbon support helps in faster charge transportation which results in increased current density, cathodic shift in the onset potential and at the same time more stability. To investigate the underlying mechanism for OER and role of ‘C’ on

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Co 2p

a

Intensity (a.u.)

Intensity (a.u.)

C1s

O1s

2p1/2

Sat

2p3/2

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Sat

Co2p 16 eV

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Binding Energy (eV)

O1s

C1s

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Intensity (a.u.)

c

B C

535

530

290

525

288

286

284

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Binding Energy (eV)

Binding Energy (eV)

Fig. 4. XPS spectra of the synthesized Co3O4/C-T (a) wide scan, (b) Co2p, (c) O1s and (d) C1s.

40

160 Co3O4/C-A

120

Co3O4/C-T

b

Co3O4/C-A Co3O4/C-T

GC electrode RuO2

Current Density (mA/cm2)

Current Density (mA/cm2)

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Fig. 5. Comparative polarization curves of Co3O4/C-T, Co3O4/C-N, Co3O4/C-A, RuO2, blank GC electrode and bare Co3O4 for OER in 1 M NaOH solution.

Co3O4, pseudocapacitive behavior of Co3O4/C-T, Co3O4/C-N and Co3O4 was determined under applied potential of 1.0 to 1.6 V vs. RHE. The applied potential is certainly lower than that of OER

starting potential. Literature confirms that the pseudocapacitance before OER is proportional to the OER activity [40]. In our present study, plot for pseudocapacitive behavior is shown as Fig. 6. Area

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Co3O4/C-A-C3 shows the best electrocatalytic activity where 1 g of glucose was used as ‘C’ source. On the other two cases 2 g and 0.5 g glucose are used. The probable reason for not getting good response towards OER may be that in case of Co3O4/C-A-C1, excess carbon may have hindered the charge transportation and in case of Co3O4/C-A-C2, very less carbon support also is not sufficient for proper charge transportation. Non-faradic capacitive current allied with electrochemical double layer charging current is calculated following some literature report [42–44]. Electrochemically active surface area (ECSA) is determined for Co3O4/C-T, Co3O4/C-N, Co3O4/C-A and bare Co3O4. First CV curve was recorded upon applied potential of 1.025 V vs. RHE to 1.225 V vs. RHE under application of different scan rates from 10 mV/s to 100 mV/s in 1 M NaOH solution and shown in Fig. 7a–d. Double layer charging current was determined from the CV curves at a fixed potential of 1.125 V vs. RHE. Double layer charging current is plotted against the scan rates which gives a straight line. From the slope of the straight line double layer capacitance Cdl can be calculated (Fig. 7e). Cdl values of Co3O4/C-T, Co3O4/C-N, Co3O4/C-A and Co3O4 are 1.96  10 3 F, 1.43  10 3 F, 4.10  10 4 F and 4.66  10 5 F respectively. ECSA and the roughness factor can be calculated and the values are 32.66 cm2 and 460 for Co3O4/C-T, 23.83 cm2 and 335 for Co3O4/ C-N, 6.83 cm2 and 96 for Co3O4/C-A, 0.776 cm2 and 11 for Co3O4. Co3O4/C-T shows the highest ECSA and the roughness factor which also supports the observed current density data. Co3O4 decorated on carbon support increases the ECSA and roughness factor which finally increases the current density. Like ECSA, mass activity is also an important parameter to determine the effectiveness of the catalyst [45,46]. Mass activity of Co3O4/C-T, Co3O4/C-N, Co3O4/C-A and bare Co3O4 was determined at a fixed potential of 1.7 V vs. RHE and the values are 76.73, 65.45, 43.87 and 38.9 A/g. Mass activity of Co3O4/C-T, Co3O4/C-N, Co3O4/C-A is comparatively higher than Co3O4 which reflects the higher electrocatalytic activity of Co3O4/C-T, Co3O4/C-N, Co3O4/C-A. All the comparative OER results are shown in Table 1. A detailed comparison with

0.7 Co3O4 0.6

Co3O4/C-T Co3O4/C-N

Current (mA)

0.5 0.4 0.3 0.2 0.1 0.0 -0.1

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Potential (V) vs. RHE Fig. 6. Pseudocapacitive behavior of Co3O4/C-T, Co3O4/C-N and Co3O4 electrocatalysts in 1 M NaOH at a scan rate of 50 mV/s.

under the curve directly dictates the specific capacitance value [41]. Fig. 6 clearly indicates that carbon supported Co3O4 have higher capacitance compared to bare Co3O4. To increase the crystallinity of the synthesized Co3O4 on C, Co3O4/C-T was calcined at 250 °C for 30 min. Electrocatalytic activity for OER is checked and it is observed that there is certainly cathodic shift in onset potential. ‘Co3O4/C-T calcined’ can generate 10 and 30 mA/cm2 current density upon application of 1.6 and 1.65 V vs. RHE (Fig. S16a). From impedance spectroscopy it is clear that the calcined Co3O4/C-T shows decreased charge transfer resistance (Fig. S16b). To check the role of carbon as a support towards OER, Co3O4/C is synthesized with different Co:C ratio and the comparative electrocatalytic activity is shown as S17. 0.3

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Fig. 7. Cyclic voltammetry curves of (a) Co3O4/C-T, (b) Co3O4/C-N, (c) Co3O4/C-A and (d) Co3O4 recorded in 1 M NaOH at scan rates of 10, 20, 40, 60, 80 and 100 mV/s, (e) plot of capacitive current at 1.125 V vs. RHE with scan rate.

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M. Basu / Journal of Colloid and Interface Science 530 (2018) 264–273 Table 1 Comparative result for overall OER activity for different catalyst. Anode

Potential required to generate 10 mA/cm2

Mass Activity (A/g) at 1.7 V vs. RHE

Tafel slope (mV/decade)

ECSA (cm2)

Co3O4/C-T Co3O4/C-N Co3O4/C-A Co3O4 RuO2

1.623 V 1.626 V 1.656 V 1.677 V 1.58 V

76.73 65.45 43.87 38.9 –

70 74 73 144 –

32.66 23.83 6.83 0.776 –

-100

1.70 Co3O4/C-T

Co3O4/C-T

Co3O4/C-A

Co3O4/C-A -80

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Co3O4

Co3O4

1.65

Potential (V) vs. RHE

Co3O4/C-N

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Z' (ohm)

Fig. 8. (a) Nyquist plots, (b) Tafel plots of Co3O4/C-T, Co3O4/C-N, Co3O4/C-A bare Co3O4.

the existing literature is done where metal chalcogenides are synthesized on any kind of carbon support and used as catalyst for HER and OER are shown in Table S1. From that table the role of the support can be clearly understood. Electrocatalytic activity of Co3O4/C is comparable with the existing literature. In order to judge superior electrocatalytic activity of carbon supported Co3O4 compared to bare Co3O4, Tafel slope values were determined from the LSV curve of each and shown in Fig. 8b. From the plot of potential vs. log |j|, Tafel slope can be determined and the values are 70, 74, 73 and 110 mV/decade for Co3O4/C-T, Co3O4/C-N, Co3O4/C-A and bare Co3O4. Lower the Tafel slope value reflects the higher electrocatalytic activity of Co3O4/C-T, Co3O4/C-N, Co3O4/C-A. Remarkably enhanced activity observed in case of Co3O4/C-T, Co3O4/C-N, Co3O4/C-A only because of the carbon support. From the FESEM images it is clear that the Co3O4 nanoparticles are highly attached with the spherical C support which provides ready charge transportation. Electrochemical impedance spectroscopy was also carried out to investigate the role of carbon support by knowing the feasibility of charge transfer on the electrode (Fig. 8a). An equivalent circuit is drawn to fit the Nyquist plot. The equivalent circuit is composed of RS and RCT and a constant phase element (CPE). Rs is the solution resistance and RCT is the charge transfer resistance. Lower RCT value reflects the faster charge transportation on the electrode. RCT of Co3O4/C-T, Co3O4/C-N, Co3O4/C-A and bare Co3O4

Table 2 Values of Charge transfer resistance and solution resistance for all samples. Anode

Rs (X)

RCT (X)

Co3O4/C-T Co3O4/C-N Co3O4/C-A Co3O4

11.45 12.22 13.44 14.53

33.05 50.59 111.18 142.82

are 33.05 O, 50.59 O, 111.18 O and 142.82 O which suggests that Co3O4/C-T, Co3O4/C-N, Co3O4/C-A are catalytically more active compared to bare Co3O4. All the RS and RCT values are summarized in Table 2. This lower RS and RCT value of Co3O4/C-T, Co3O4/C-N, Co3O4/C-A reflects the higher electrocatalytic activity compared to bare Co3O4. Carbon present in Co3O4/C-T, Co3O4/C-N, Co3O4/C-A helps in faster charge transportation compared to bare Co3O4. 4. Conclusion In conclusion, a new methodology is demonstrated for the fabrication of carbon supported Co3O4 nanoparticles as an efficient and stable electrochemical catalyst for OER in alkaline condition. For the synthesis of Co3O4 on carbon support different hydrolyzing agents like ammonia, NaOH, TEA were utilized. FESEM images confirms the intimate contact between carbon and Co3O4 nanoparticles which helps in faster charge transportation from the catalyst surface to electrolyte. Carbon supported Co3O4 shows enhanced electrocatalytic activity towards OER with excellent durability. Co3O4/C-T shows the best performance in OER elecrtocatalysis. It can generate 10 and 30 mA/cm2 current density upon application of 1.623 V and 1.678 V vs. RHE. All the carbon supported Co3O4 shows enhanced activity compared to bare Co3O4. Psudocapacitive behavior of Co3O4/C-T, Co3O4/C-N, and bare Co3O4 were checked to confirm the superior activity of carbon supported Co3O4 compared to bare Co3O4. It is obvious that the OER activity is proportional to the pseudocapacitance recorded before OER. It is observed that specific capacitance of Co3O4/C-T > Co3O4/C-N > Co3O4, which confirms that Co3O4/C-T is the most active for OER. Co3O4/C can generate unaltered current density up to 1000 cycles. So, carbon sphere present in the heterostructure helps to enhance the charge transportation which results in higher current density as well as cathodic shift of the onset potential. Enhancement in electrocatalytic

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activity is significant, it is because the support ‘C’ sphere generates in an in-situ process which provides intimate contact with the support and Co3O4. Therefore, this approach leads to in-situ synthesis of support and catalyst, which can serve as a better electrocatalyst. Author contributions Experimental work and MS writing are done by M Basu. Notes The authors declare no competing financial interest. Acknowledgements MB thankfully acknowledges financial support from Department of Science and Technology (DST) Inspire (DST/ INSPIRE/04/2015/000239) program, and DST Science and Engineering Research Board (SERB) (YSS/2015/000100), Govt. of India. I am thankful to BITS Pilani. I am thankful to Dr. Surojit Pande and Chavi Mahala. The instrumental support for TEM, FESEM, and XPS measurements from the Material Research Centre (MRC), MNIT Jaipur is highly acknowledged. I also thank to the Department of Physics, BITS Pilani for assistance with powder x-ray diffraction studies (DST-FIST sponsored). References [1] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [2] D. Kang, T.W. Kim, S.R. Kubota, A.C. Cardiel, H.G. Cha, K.-S. Choi, Electrochemical synthesis of photoelectrodes and catalysts for use in solar water splitting, Chem. Rev. 115 (2015) 12839–12887. [3] N.M. Markovic´, B.N. Grgur, P.N. Ross, Temperature-dependent hydrogen electrochemistry on platinum low-index single-crystal surfaces in acid solutions, J. Phys. Chem. B 101 (1997) 5405–5413. [4] E. Sku´lason, G.S. Karlberg, J. Rossmeisl, T. Bligaard, J. Greeley, H. Jo´nsson, J.K. Nørskov, Density functional theory calculations for the hydrogen evolution reaction in an electrochemical double layer on the Pt(111) electrode, Phys. Chem. Chem. Phys. 9 (2007) 3241–3250. [5] G. Li, S. Li, J. Ge, C. Liu, W. Xing, Discontinuously covered IrO2–RuO2@Ru electrocatalysts for the oxygen evolution reaction: how high activity and longterm durability can be simultaneously realized in the synergistic and hybrid nano-structure, J. Mater. Chem. A 5 (2017) 17221–17229. [6] T. Audichon, T.W. Napporn, C. Canaff, C. Morais, C. Comminges, K.B. Kokoh, IrO2 coated on RuO2 as efficient and stable electroactive nanocatalysts for electrochemical water splitting, J. Phys. Chem. C 120 (2016) 2562–2573. [7] M. Tahir, L. Pan, F. Idrees, X. Zhang, L. Wang, J.-J. Zou, Z.L. Wang, Electrocatalytic oxygen evolution reaction for energy conversion and storage: a comprehensive review, Nano Energy 37 (2017) 136–157. [8] X. Zhao, Y. Fu, J. Wang, Y. Xu, J.-H. Tian, R. Yang, Ni-doped CoFe2O4 hollow nanospheres as efficient Bi-functional catalysts, Electrochim. Acta 201 (2016) 172–178. [9] C. Mahala, M. Basu, Nanosheets of NiCo2O4/NiO as efficient and stable electrocatalyst for oxygen evolution reaction, ACS Omega 2 (2017) 7559–7567. [10] Y. Fang, X. Li, S. Zhao, J. Wu, F. Li, M. Tian, X. Long, J. Ji, J. Ma, Coaxial ultrathin Co1-yFeyOx nanosheet coating on carbon nanotubes for water oxidation with excellent activity, RSC Adv. 6 (2016) 80613–80620. [11] F. Malara, S. Carallo, E. Rotunno, L. Lazzarini, E. Piperopoulos, C. Milone, A. Naldoni, A Flexible Electrode Based on Al-Doped Nickel Hydroxide Wrapped around a Carbon Nanotube Forest for Efficient Oxygen Evolution, ACS Catal. 7 (2017) 4786–4795. [12] F. Song, X. Hu, Exfoliation of layered double hydroxides for enhanced oxygen evolution catalysis, Nat. Commun. 5 (2014) 4477 (1–9). [13] J. Jiang, C. Yan, X. Zhao, H. Luo, Z. Xue, T. Mu, A PEGylated deep eutectic solvent for controllable solvothermal synthesis of porous NiCo2S4 for efficient oxygen evolution reaction, Green Chem. 19 (2017) 3023–3031. [14] C. Tang, N. Cheng, Z. Pu, W. Xing, X. Sun, NiSe nanowire film supported on nickel foam: an efficient and stable 3D bifunctional electrode for full water splitting, Angew. Chem. Int. Ed. 54 (2015) 9351–9355. [15] T. Liu, L. Xie, J. Yang, R. Kong, G. Du, A.M. Asiri, X. Sun, L. Chen, Self-Standing CoP Nanosheets Array: A Three-Dimensional Bifunctional Catalyst Electrode for Overall Water Splitting in both Neutral and Alkaline Media, ChemElectroChem 4 (2017) 1840–1845.

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