Core–shell cobalt oxide mesoporous silica based efficient electro-catalyst for oxygen evolution

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Core–shell cobalt oxide mesoporous silica based efficient electro-catalyst for oxygen evolution ARTICLE in NEW JOURNAL OF CHEMISTRY · JULY 2015 Impact Factor: 3.16 · DOI: 10.1039/C5NJ00521C

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Core–shell cobalt oxide mesoporous silica based efficient electro-catalyst for oxygen evolution Shahid Ali Khan,ab Sher Bahadar Khan*ab and Abdullah M. Asiriab In the last few decades, renewable resources received considerable attention for the production of hydrogen. Herein, we present oxygen evolution from water using cobalt oxide based nanomaterials (Co3O4, Co3O4@SiO2, Co3O4/TiO2, Co3O4/Fe2O3 and ZnO@SiO2). These nanomaterials were grown in a controlled size and were characterized by various spectroscopic techniques. The Co3O4, Co3O4@SiO2, Co3O4/TiO2, Co3O4/Fe2O3, and ZnO@SiO2 were screened for their electro-catalytic properties towards H2O oxidation. All cobalt oxide based nanomaterials showed good oxygen evolution activity and high stability in alkaline conditions. However, Co3O4@SiO2 showed a higher current density at lower overpotentials and a lower Tafel slope (107.7 mV dec1) as compared to Co3O4/TiO2, Co3O4/Fe2O3, ZnO@SiO2, and Co3O4. At 1.0 V

Received (in Montpellier, France) 2nd March 2015, Accepted 6th May 2015 DOI: 10.1039/c5nj00521c

(overpotential 735 V versus Ag/AgCl), Co3O4@SiO2 supplied a current density of 63.0 mA cm2 in 0.3 M KOH solution. This indicated a superior electrocatalytic performance then the other electrocatalyst. The excellent electrocatalytic performance of Co3O4@SiO2 might be due to certain structural features, which elevate its electrical conductivity, its oxidizing aptitude, and the affinity between OH ions and the Co3O4@SiO2 surface and ultimately enhance smooth mass transports, which give superior oxygen

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evolution activity to Co3O4@SiO2.

1. Introduction Hydrogen is the main component of fuel cells and has considerable vital application in numerous important industrial processes. It could perform a vital role in the future economy as an energy carrier. Currently, energy crises, energy production and the environmental threats associated with energy production are a popular topic at the global level. The main challenge for the present and the future is the storage of fossil fuels and climatic change.1 To cope with these challenges, it is necessary to have a clean and benign environmental system for energy production.2 It seems that the need for energy might be doubled by the mid-century, as compared to the present demand, due to social development.3 Some of the energy requirements were met by burning fossil fuels, but this severely deteriorates our climate.4,5 Due to the adverse effects of fossil fuels on the environment, scientists have diverted their attention to convert solar energy to chemical energy using hydrogen as a carrier of energy.6,7 With the advent of the industrial revolution, the CO2 level rise in the past 800 000 years was associated with an increase in the atmospheric temperature, and the process is

a

Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21589, P. O. Box 80203, Saudi Arabia. E-mail: [email protected]; Tel: +966-593709796 b Chemistry Department, Faculty of Science, King Abdulaziz University, P. O. Box 80203, Jeddah 21589, Saudi Arabia

known as the greenhouse effect.8 It was reported that among the renewable resources of energy, including geothermal, hydro power and wind energy, solar energy provides one of the largest quantities of energy to the earth’s surface.3 However, one of the main challenges for scientists is how to capture and convert this solar energy to chemical energy. Electrochemical water oxidation provides one of the most promising and environmentally benign methods for the production of hydrogen.8 A good electrocatalyst must have a low overpotential, which is close to the Nernstian potential.9 A number of cathodic materials were reported in connection with the half reaction of H2O–H2 in water oxidation.10–13 The catalytic activity of metal oxides was reported under different conditions i.e. Mn4OxCa for the H2O–O2 half-reaction and nickel as an anode at high temperatures, perovskite metal oxide in highly alkaline solutions and transition metal oxides such as IrO2, PtO2, RuO2, and Rh2O3, which showed good activity in acidic media towards water oxidation.3 To date, IrO2 and RuO2 catalysts are considered to be the best anode materials for OER production in water oxidation.14 However, their use is prohibited due to their high cost,4 rare availability in the earth’s crust and their lesser applicability on a large scale,14 and this compels researchers to search for alternative benevolent materials. Catalysts for oxygen evolution and reduction are the central theme for fuel cells and in renewable source technology. Although tremendous attempts were made for the development of oxygen evolving catalyst, the development of a low cost material with a high catalytic performance is

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still a great challenge.16 It was reported in the literature that cobalt based nanomaterials play an important role in electrochemical water oxidation. An interesting anodic cobalt oxide oxygen evolving catalyst (Co-OEC) was produced in situ in an aqueous phosphate solution containing Co2+.9,15 The Co-OEC functions as a reference for an oxygen evolving complex in photosystem II17 in mild conditions with an overpotential of 420 mV and a self-healing catalyst.18 Among the ORR cathodic materials, Pt and its alloys play an important role in fuel cells. However, due to the high cost of these precious metals, alternative catalysts were investigated, as well as OERs instead of ORRs in energy storage and solar fuel cells.9,10,16 Cobalt oxide grown on reduced and mildly oxidized graphene oxide showed a high performance in alkaline solution with respect to ORR and OER catalytic activity.19 In the current approach, we selected different nanomaterials for water splitting and explored the doping effect on the performance of cobalt oxide by doping with titanium oxide, iron oxide and silica. The silica core–shell over cobalt oxide exhibited an excellent performance in OERs. To further assess the effect of silica on the water oxidation performance, Co3O4@SiO2 was compared with ZnO@SiO2, which suggested that silica is not responsible for a high catalytic performance but that the mesoporous nature of Co3O4@SiO2 is important in water oxidation. The overpotential and the current of the Co3O4@SiO2 were measured at various concentration levels ranging from 0.1–0.3 M with a difference of 0.5, which indicates that the overpotential is decreasing and the current is increasing with increase in concentration of KOH solution.

Cobalt(II) nitrate hexahydrate, Nafion, ethanol, acetone, potassium hydroxide, and all other reagents were purchased from SigmaAldrich and were used as received. All the solutions were prepared in deionized water obtained from the departmental Millipore-Q water purification system (18.2 MO cm @ 25 1C, TOC o 10 ppb).

nanoparticles (0.2 g) were added to a conical flask charged with water (20 mL), CTAB (0.6 g) and concentrated ammonia solution (1 g, 4.0 mL, 28 wt%) and were well dispersed. After dispersion, 0.4 g of tetraethyl orthosilicate (TEOS) was added dropwise and the reaction was allowed to proceed for 12 h under continuous mechanical stirring. The resulting product was washed with distilled water and ethanol (1 : 1) and the core–shell cobalt oxide mesoporous silica microspheres were dried at 50 1C. 2.2.3. Synthesis of Co3O4 co-doped TiO2 (Co3O4/TiO2). Equi-molar aqueous solutions of TiO2 and cobalt nitrate hexahydrate were mixed together and basified with NH4OH solution until the pH was greater than 10.0. The ensuing highly basic solution was stirred at 60.0 1C overnight and the resulting product was washed with a mixture of distilled water and ethanol (1 : 1). The product was then dried at room temperature and further calcined at 400.0 1C for 5 hours. 2.2.4. Synthesis of Co3O4 co-doped Fe2O3 (Co3O4/Fe2O3). Equal amounts of ferric and cobalt salts were accurately weighed and dissolved entirely in 100 mL distilled water at ambient temperature. The pH of the solution was adjusted to 11 by the dropwise addition of freshly prepared 0.2 M NaOH solution under continuous vigorous stirring. After that, the solution was kept at 60–70 1C overnight with continuous stirring. The temperature of the solution was subsequently decreased and the solution was centrifuged at 2000 rpm to separate the precipitate. The supernatant solution was discarded and the precipitate was washed thrice with ethanol. The precipitate was dried in an oven at 50–60 1C, ground and stored in clean, dry and inert plastic vials. 2.2.5. Synthesis of core–shell zinc oxide silica nanoparticles (ZnO@SiO2). ZnO@SiO2 nanoparticles were synthesized by the same sol–gel method as Co3O4@SiO2. The aqueous dispersion of ZnO nanoparticles (0.2 g) was mixed with ethanol (80 mL), water (20 mL), 2 g tetraethyl orthosilicate (TEOS) and concentrated ammonia solution (6 mL, 28 wt%) in a conical flask and the reaction was allowed to proceed for 12 h under continuous stirring. The resulting product was washed with a 1 : 1 mixture of distilled water and ethanol and the core shell of ZnO@SiO2 was dried at 50 1C.

2.2.

2.3.

2. Experimental 2.1.

Materials

Synthesis of nanomaterials

2.2.1. Synthesis of Co3O4. Salt of Co(NO3)26H2O (0.1 M) was dissolved in 100 mL deionized water and 0.5 M of NaOH was added to the reaction mixture to raise the pH of the solution above 10; the solution was stirred thoroughly overnight at 60 1C before centrifugation. The supernatant liquid was withdrawn and the remaining solid part was washed thrice with deionized water and collected by centrifugation. At room temperature, the surface of the nanomaterials was polished by removing the adsorbed unwanted materials and then drying the material by exposing in an oven at 50 1C. 2.2.2. Synthesis of core–shell cobalt oxide mesoporous silica microspheres (Co3O4@SiO2). Co3O4@SiO2 microspheres were synthesized using a versatile solution sol–gel method20 as follows. The above mentioned synthesized cobalt oxide

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Characterization of nanomaterials

The morphology and the average size of the as-grown Co3O4, Co3O4@SiO2, Co3O4/TiO2, Co3O4/Fe2O3, and ZnO@SiO2 were analyzed by a field-emission scanning electron microscope (FESEM), JEOL (JSM-7600F, Japan). Energy dispersive X-ray spectrometry (EDS) of nanomaterials was carried out for the elemental analysis using an Oxford-EDS system. The structure of nanomaterials was analyzed by an ARL Service powder diffractometer, while FT-IR analysis was carried out using a Bruker (ALPHA, USA) in the range 4000–400 cm1. 2.4. Preparation of modified gold electrodes (AuE) for H2O splitting The surface of AuE was polished with alumina slurry (0.05 mM), thoroughly rinsed with deionized water, ultra-sonicated with

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pure deionized water and air dried. The slurry was made by properly mixing the nanomaterials (5 mg) with 5 mL of 0.01% Nafion solution. The slurry was pasted on the AuE and dried at room temperature. The current generated by the catalyst was measured in aqueous KOH electrolytes using three electrodes: AuE coated with nanomaterial operating as the working electrode, Pt metal wire as the counter electrode, and Ag/AgCl as a reference electrode. All three electrodes were dipped in a glass cell containing KOH solution and separated from each other at certain distance. To avoid disruption during the experiment, nitrogen gas was bubbled thoroughly to evacuate the glass cell. The current versus potential was measured at a sweep rate of 50 mV s1. An Epsilon electrochemical workstation coupled with a BASi Cell Stand C3 was used for electrochemical studies. The measurements for water oxidation were carried out in aqueous KOH electrolytes at pH 13, 13.2, 13.3, 13.4, and 13.6.

Fig. 1

3. Results and discussion 3.1.

Structural characterization of nanoparticles

In this study, we prepared five different nanomaterials, Co3O4, Co3O4@SiO2, Co3O4/TiO2, Co3O4/Fe2O3, and ZnO@SiO2. FESEM was used to examine the morphology of the as grown nanomaterials as shown in Fig. 1. The FESEM images show that the prepared nanomaterials have grown in the form of particles, except for Co3O4/Fe2O3, which is a mixture of nanoparticles and fibers. The highly magnified FESEM image of the nanomaterials shows that the particles were o50 nm in diameter. The FESEM images clearly indicated that Co3O4@SiO2 is mesoporous in nature and therefore might have a larger surface area compared to other nanomaterials. The surface area (SA) of Co3O4@SiO2 was measured using the N2-sorption technique and was found to be 14 m2 g1. We investigated all the nanomaterials discussed above with respect to the oxygen evolution reaction (OER), in

Typical low and high resolution FESEM images of Co3O4, Co3O4@SiO2, Co3O4/TiO2, Co3O4/Fe2O3, and ZnO@SiO2.

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which Co3O4@SiO2, TiO2/Co3O4, and Co3O4 showed good catalytic activity. However, it was found that among all the nanomaterials, Co3O4@SiO2 showed excellent catalytic activity in water oxidation and this might be due to the large surface area of Co3O4@SiO2, which is generally responsible for its high catalytic activity in water oxidation. By viewing the efficient catalytic properties of nanomaterials, especially Co3O4@SiO2, we further characterized the nanomaterials by EDS, XRD and FTIR and selected it for further detailed electrochemical analysis. The composition of the synthesized Co3O4@SiO2 was determined by EDS, as shown in Fig. 2. The EDS spectrum of Co3O4@SiO2 shows peaks for O, Co, Si and C. These peaks confirm the formation of Co3O4@SiO2, whereas the peak for carbon is due to the surfactant used during the formation of Co3O4@SiO2. Thus, EDS confirmed that the as-grown nanoparticles were composed of cobalt, silicon, carbon and oxygen. It was further confirmed from the weight% compositions determined by EDS that Co and Si exist in 5 and 3 wt%, respectively. The EDS spectrum of Co3O4 has shown peaks for O and Co, whereas peaks for O, Co and Ti were observed for Co3O4/TiO2. Similarly, Co3O4/Fe2O3 and ZnO@SiO2 samples displayed peaks related to O, Co, Fe and O, Zn and Si, respectively. The structure of Co3O4@SiO2 was characterized by XRD, as shown in Fig. 3(a). The XRD spectrum exhibits a hallow peak along with a well-defined crystalline peak. The hallow peak is attributed to the silica core shell, whereas the sharp crystalline

Fig. 2

peaks appeared for Co3O4 in the XRD spectrum. The Co3O4 peaks exactly matched with JCPDS # 80-1536. The XRD patterns confirmed that the synthesized Co3O4@SiO2 is composed of Co3O4.21 XRD of Co3O4, Co3O4/TiO2, Co3O4/Fe2O3, and ZnO@ SiO2 is shown in Fig. 3(c). Co3O4 shows peaks at 2y equal to 20.41, 29.11, 37.31, 39.11, 50.91, 56.01, 59.81, 65.71 and 69.31, which reveal that cobalt oxide exists in the tetragonal arrangement of Co3O4.21 Co3O4/TiO2 exhibits peaks for Co3O4 and TiO2, which suggests that cobalt oxide and titanium oxide exist in the doped material. The XRD data closely match the literature.22 Similarly, Co3O4/Fe2O3 shows peaks for both Co3O4 and Fe2O3 and the results are in good agreement with the literature.21 ZnO@SiO2 presents a peak sequence consistent with wurtzite hexagonal ZnO nanoparticles.23 The FT-IR spectrum of Co3O4@SiO2 shows absorptions for various functional groups at 567 (MQO), 667 (M–O–M, stretching), 1605 (O–H bending vibration) and 3227 cm1 (O–H stretching) and the other peaks in the range of 1086– 1320 cm1 are due to the presence of SiO2. The CO2 or CO32 were absorbed due to the mesoporous nature of the nanomaterials and so appeared at 1330 cm1, as indicated in Fig. 3(b). These data suggest that the synthesized nanomaterial is a metal oxide-based nanostructure.24 The FTIR spectrum of the Co3O4 showed absorption at 567 cm1 (MQO) and 655 cm1 (M–O–M, stretching), whereas Co3O4/TiO2 displayed absorptions at 1374 (CO2 or CO32), 558 (MQO), and 655 cm1, as shown in Fig. 3(d).

Typical EDS spectrum of (a) Co3O4@SiO2 and (b) Co3O4, Co3O4/TiO2, Co3O4/Fe2O3, and ZnO@SiO2.

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Fig. 3 Typical (a) XRD of Co3O4@SiO2, (b) FTIR of Co3O4@SiO2, (c) XRD of Co3O4, Co3O4/TiO2, Co3O4/Fe2O3, ZnO@SiO2 and (d) FTIR of Co3O4, Co3O4/ TiO2, Co3O4/Fe2O3, and ZnO@SiO2.

The absorption peaks at 1374 cm1 (CO2 or CO32) and 558 cm1 (MQO) appeared in the FTIR spectrum of Co3O4/ Fe2O3. Similarly, the FTIR spectrum of ZnO@SiO2 showed a prominent peak at 1084 cm1, confirming the presence of SiO2 in ZnO@SiO2 along with a peak at 558 cm1 (MQO), as shown in Fig. 3(d).21–24 3.2. Electrocatalytic performance of nanomaterials for H2O oxidation Cobalt oxide is an efficient catalyst for oxygen evolution reactions in alkaline conditions. Therefore, we determined the electrocatalytic properties of cobalt oxide based nanoparticles in water splitting under alkaline conditions. Cobalt-based nanomaterials, such as Co3O4, Co3O4@SiO2, Co3O4/TiO2, Co3O4/Fe2O3, and ZnO@SiO2 were coated on the surface of AuE and evaluated for their water oxidation properties in 0.1 M KOH aqueous solution (pH 13) by observing linear sweep voltammograms (LSVs), as shown in Fig. 4(a and b). Among all nanomaterials, Co3O4@SiO2 was found to be the most active electrocatalyst, playing an important role in the process of oxygen evolution in alkaline conditions. The linear sweep voltammogram of Co3O4@ SiO2 was also compared with bared AuE in Fig. 4(c and d) and it was largely found that Co3O4@SiO2 displayed a higher catalytic activity towards the OER in water oxidation compared to bared AuE.

By comparing the OER catalytic performance of Co3O4@SiO2 with pure Co3O4 and other doped cobalt oxides such as Co3O4/ TiO2 and Co3O4/Fe2O3, it was found that Co3O4@SiO2 exhibits an excellent performance in the OER. At 1.0 V (overpotential 735 V vs. Ag/AgCl), Co3O4@SiO2 exhibited a current density of 63.0 mA cm2 in 0.1 M KOH solution. Under the same conditions, other nanomaterials such as Co3O4 and Co3O4/TiO2 displayed current densities (at 1.0 V) of 26.7 and 6.7 mA cm2, respectively, whereas Co3O4/Fe2O3 exhibited a negligible current density. Co3O4@SiO2 showed a low overpotential value of 529 mV at a current density of 10 mA cm2, whereas the other nanomaterials exhibited higher overpotentials at the same current density. Furthermore, to assess the effect of silica on the water oxidation performance, Co3O4@SiO2 was compared with ZnO@SiO2. ZnO@ SiO2 shows a low current even at a high overpotential, which suggests that silica is not responsible for the high catalytic performance, but the mesoporous nature of Co3O4@SiO2 might be important in water oxidation. At pH 13, the standard potential for the anode was  EOH  =O ¼ 0:463 V vs. the standard hydrogen electrode (SHE), 2 which corresponds to 0.265 V vs. the Ag/AgCl reference electrode, and is used for the all overpotential measurements.17,25 The overpotentials of the bared electrode and the nanomaterials (Co3O4, Co3O4@SiO2, Co3O4/TiO2, Co3O4/Fe2O3 and ZnO@SiO2) were observed at specified current densities (5 mA cm2 and

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Fig. 4 Linear sweep voltammograms of (a, b) Co3O4, Co3O4@SiO2, Co3O4/TiO2, Co3O4/Fe2O3, and ZnO@SiO2 and (c, d) comparison of Co3O4@SiO2 linear sweep voltammogram with bared AuE in 0.3 M KOH solution.

Table 1 Overpotentials (at 10, 20 and 40 mA cm2) and current densities (at 1.0 V vs. Ag/AgCl) of Co3O4, Co3O4@SiO2, Co3O4/TiO2, Co3O4/Fe2O3, and ZnO@SiO2 (0.3 M KOH)

Catalyst

Overpotential (mV) at 10 mA cm2

Overpotential (mV) at 20 mA cm2

Overpotential (mV) at 40 mA cm2

Current density (mA cm2) at 1 V (vs. Ag/AgCl)

Co3O4 Co3O4@SiO2 Co3O4/TiO2 Co3O4/Fe2O3 ZnO@SiO2

560 529 812 — 1036

678 573 1026 — 1092

910 647 1185 — 1169

26.7 63.2 6.7 0.4 0.7

10 mA cm2), whereas current densities of these nanoparticles vs. the reference Ag/AgCl were observed at 1 V, as shown in Table 1. We observed that at a given potential Co3O4@SiO2 showed the highest current density compared to the bared electrode as well as all other nanomaterials. Co3O4@SiO2 exhibited a current density of 10 mA cm2 at 794 mV (vs. Ag/AgCl), corresponding to an overpotential of 529 mV. Similarly, current densities of 20 and 40 mA cm2 were obtained at 838 and 912 mV (vs. Ag/AgCl) for Co3O4@SiO2, which corresponded to overpotentials of 573 and 647 mV, respectively, displaying an excellent OER activity for Co3O4@SiO2 nanoparticles. To obtain a current density of 10, 20 and 40 mA cm2, it was found that when using an alkaline KOH solution of pH 13, Co3O4@SiO2 needed overpotentials of 529, 573 and 647 mV, respectively.

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For electrochemical reactions, the type of electrolyte, pH and concentration of electrolyte are the key parameters in water splitting.17 Generally, in the case of basic metal oxides such as Co3O4, a concentrated alkali solution is used as an electrolyte for oxygen evolution. A series of different concentrations of KOH solutions was prepared to check the electrocatalytic activity of the Co3O4@SiO2, as shown in Fig. 5(a and b). The effect of different concentrations of alkaline KOH solutions is indicated in Table 2, which shows the direct proportionality of current densities with the concentration of alkaline solution. The current density for Co3O4@SiO2 at 1 V (vs. Ag/AgCl) is 11.5 mA cm2 using pH 13. By increasing the pH of the solution from 13 to 13.6, the current density was also increased from 11.5 mA cm2 to 63.2 mA cm2 at 1 V (vs. Ag/AgCl). In order to compare the catalytic activities

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Fig. 5

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Linear sweep voltammograms and cyclic voltammograms of Co3O4@SiO2 in KOH solutions with different pH values.

Table 2 Overpotentials (at 10, 20 and 40 mA cm2) and current densities (at 1.0 V vs. Ag/AgCl) of Co3O4@SiO2 in KOH electrolytes with different pH values

Catalyst Co3O4@SiO2

Overpotential (mV) at 10 mA cm2

Overpotential (mV) at 20 mA cm2

Overpotential (mV) at 40 mA cm2

Current density (mA cm2) at 1 V (vs. Ag/AgCl)

pH pH pH pH pH

715 593 552 529 529

— 680 616 585 575

— — 758 670 647

11.5 26.2 38.8 52.1 63.2

13 13.2 13.3 13.4 13.6

at different pH values, the thermodynamic potentials and over potentials of the catalyst were calculated at particular current densities. At different concentrations (pH 13, 13.2, 13.3, 13.4 and 13.6), overpotentials of 715, 593, 552, 529, and 529 mV were measured at 10 mA cm2 current density, as shown in Table 2. It was concluded that the oxidative catalytic activity of Co3O4@ SiO2 was increased by increasing the OH ion concentration in the electrolyte. The catalysis activity of Co3O4@SiO2 was scrutinized by Tafel slope values. Tafel plots showed the results of cobalt based composite nanoparticles at different concentrations of alkaline KOH, as shown in Fig. 6. The linear portions of the Tafel plots are fitted to the Tafel equation i.e. Z = b log( j/j0) where Z represents overpotential, j denotes the current density, j0 is the exchange current density, and b is the Tafel slope.6 The Tafel slope of Co3O4@SiO2 was found to be 151.3 mV dec1 at pH 13, whereas Tafel slopes of 127.5, 111.3, 107.9 and 107.7 were observed at pH 13.2, 13.3, 13.4, and 13.6 mV dec1, as shown in Fig. 6. This suggests the apparently similar kinetics of the OER in different concentrations of KOH solution. These results indicate that cobalt based composite nanoparticles have higher OER catalytic activity at higher pH values. As anticipated, the cobalt based composite nanoparticles catalyst gives an insignificant Tafel slope of 107.7 mV dec1 in 0.3 M KOH solution. The stability of a catalyst is very important for the long term use of a catalyst in the OER process.26 Therefore, the stability and durability of Co3O4@SiO2 was studied by measuring OER activities many times at one minute intervals using LSV (Fig. 7). After many scans, the OER activity of Co3O4@SiO2 did not show

Fig. 6

Tafel plots of Co3O4@SiO2 at different pH values.

any deactivation in the activity of Co3O4@SiO2. The superior stability and durability of Co3O4@SiO2 might be due to SiO2, which maintains the original structure even after a long stability and durability test.26 Nowadays, Co-based OER catalysts are of interest in water oxidation (Fig. 8), yet continuous attempts are being made to find better cobalt-based catalysts for electrochemical OER studies, both in basic and in neutral electrolyte solutions, due to their low overpotential values.16 It is difficult to directly compare the OER activity presented in this study with previous results, because of different experimental conditions. However, under some similar experimental conditions,13–19 comparisons of some recent OER results with the data presented in this study

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Co3O4/TiO2, Co3O4/Fe2O3, ZnO@SiO2). Among these nanomaterials, the super catalytic performance in electrochemical water splitting was shown by Co3O4@SiO2. This superior catalytic performance was attributed to the mesoporous nature of Co3O4@SiO2, which increases the surface area and could possibly expose the active sites of the catalyst. They further elevated the electrical conductivity, the oxidizing capacity, and the affinity between OH ions and the catalyst. The mesoporous nature of Co3O4@SiO2 also increases surface and smooth mass transports, which ultimately reduces the overpotential required for the oxygen evolution reaction and leads to superior oxygen evolution activity and a higher catalytic performance of Co3O4@SiO2. Fig. 7 Different scans of linear sweep voltammograms of Co3O4@SiO2.

Acknowledgements This project was funded by the Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, Jeddah, under grant no. (CEAMR-SG-3-436).

References

Fig. 8 Schematic view of water splitting mechanism using Co3O4@SiO2 as electrocatalyst.

Table 3

Tafel slope comparison of Co3O4@SiO2 with the literature

Catalyst

Electrolyte

Tafel slope (mV dec1)

Ref.

Co3O4/SWNTs G–Mn–NiCo PNG–NiCo Ni–NG Ni3S2/Ni Co3O4@SiO2 Co3O4@SiO2

1.0 0.1 0.1 0.1 0.1 0.1 0.3

104 371.3 156 188.6 159.3 151.3 107.7

27 28 29 30 31 This work This work

M M M M M M M

KOH KOH KOH KOH KOH KOH KOH

were made, indicating the superior catalytic activity of the Co3O4@SiO2 catalyst. Furthermore, the Tafel slope and overpotential values of Co3O4@SiO2 catalysts at 10 mA cm2 were considerably less than in the literature, using the same 0.1 M KOH electrolyte, as summarized in Table 3.27–31

4. Conclusions We have evaluated the potential application of electrocatalytic activity of various cobalt based nanomaterials (Co3O4, Co3O4@SiO2,

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