Effect of Morphology of Co3O4 for Oxygen Evolution Reaction in Alkaline Water Electrolysis

Abstract

In this paper, three different morphological Co3O4 electrodes for oxygen evolution reaction (OER) have been synthesized. By comparing the three morphologies of Co3O4, the electrocatalytic properties show that the urchin-like spheres of Co3O4 electrode has relative low overpotential and good electrocatalysis stability, indicating that the structure of urchin-like Co3O4 spheres exhibit an easy OER for water splitting.

1. INTRODUCTION

As a potential solution to provide a carbon-free source of hydrogen for the global energy crisis and environmental pollution, the electrochemical splitting of water has attracted great attention [1-3]. One of the critical drawbacks to making the electrochemical process viable on an industrial scale is the unavailability of cheap and highly efficient OER electrocatalysts. Currently, the commonly used catalysts for OER in electrolysis cells are still RuO₂ and IrO₂, which are among the rarest elements on the earth [4-5]. Therefore, searching for alternative electrocatalyst which can reduce the overpotential and enhance the energy efficiency are critical to viable water electrolytic systems.

Recently, numerous efforts have been made to explore cheap and high efficient OER catalysts that have sufficient stability in prolonged exposure to oxidizing conditions [6-12]. Amongst these non-precious catalysts, cobalt is earth-abundant and both Co₃O₄ and the substituted cobaltites M_xCo_3-xO₄ (M = Ni, Fe, Cu or Zn) exhibit excellent OER activities [13-20].Upon electrochemical oxidation, metallic Co undergoes gradually oxidation to form Co(OH)₂, CoO, Co₃O₄ and CoO(OH), etc [21-24]. In particular, as one of the well-known spinel electrocatalysts, Co₃O₄ has been studied as a highly efficient corrosion-stable OER catalyst in alkaline media. Worth to be mentioned, to enhance the electrochemical activity of Co₃O₄, increasing the effective surface area of catalysts is needed. Therefore, it was found that the morphologies of Co₃O₄ electrocatalysts play a key role in OER. In general, Co₃O₄ electrocatalysts for the OER are thin film or particle agglomerates bound together by polymers. [25-27]. Recently, one-dimensional Co₃O₄ nanowires as electrocatalyst for the OER have garnered significant attention [28, 29]. Compared with Co₃O₄ particles and thin films, the one-dimensional Co₃O₄ nanowires possess a large interfacial area, high efficient active sites, and superior mass transport property.

In this work, the urchin-like Co₃O₄ spheres have been successfully synthesized for OER via a simple and effective solution method. Simultaneously, other two morphologies of Co₃O₄ were obtained by hydrothermal process and sol-gel autocombustion method. By comparing the different morphologies of Co₃O₄, We examined their electrochemical OER activities and investigated the relationship among the catalyst activity and morphology, and surface area of Co₃O₄.

2. EXPERIMENTAL

Materials Synthesis. All the solvents and chemicals used were analytical grade and used directly without any purification. In a typical solution method：4 mmol of Co(NO₃)₂·6H₂O was dissolved into a 80 mL mixed solution (50 volume % ethanol: 50 volume % H₂O) at room temperature to form a clear pink solution, followed by the addition of 24 mmol of urea. And then the mixed solution was transferred to a 100 mL bottle with a cap. Firstly, the Ti foil (2 cm × 1 cm) was immersed in a 3 M HCl solution for 15 min to get rid of the possible surface oxide layer, then put into the bottle for reaction. The bottle was capped and heated at 85°C for 8 h in an electric oven. Finally, the Ti foil with grown urchin-like Co₃O₄ spheres (denoted as S₁) was obtained by annealed at 400°C in atmosphere for 2 h.

In a typical hydrothermal method：1 mmol of Co(NO₃)₂·6H₂O was dissolved in 20 mL deionized water under constant stirring, followed by the addition of 10 mmol of urea. The mixture was kept stirring for 30 min, and transferred to a Teflonlined autoclave (25 mL capacity). The hydrothermal reaction was conducted at 120°C for 6 h. The precipitates were collected and rinsed with deionized water and ethanol, and finally the layered Co₃O₄ (denoted as S₂) was obtained by annealed at 400°C in atmosphere for 2 h.

In a typical sol-gel autocombustion method: 1 mmol of Co(NO₃)₂·6H₂O was dissolved in 20 mL deionized water under constant stirring, followed by the addition of 2 mmol of citric acid. And the mixed solution (sol) was kept stirring and heated at 80°C till the sol was converted into gel. Ammonia solution was used to maintain the pH value of the sol at about 7-8 until the gel was formed. Then the gel was ignited at 120°C by the self-combustion process and thus obtained loose fluffy powder. Finally, the agglomerate Co₃O₄ (denoted as S₃) was obtained by sintered at 400°C in atmosphere for 2 h.

Characterizations: The as-prepared samples were characterized by using field emission scanning electron microscopy (FE-SEM, JSM-7001F) and transmission electron microscopy (TEM, 300 kV, Tecnai™ G2 F30) to analyze the surface morphology. The crystal structure of the samples were analyzed using powder X-ray diffraction (XRD, Bruker, D8 ADVANCE) with Kα radiation (λ = 1.5418 Å). The chemical state and composition of the samples were carried out by X-ray Photoelectron Spectroscopy (XPS, ESCALab250). Nitrogen adsorption/desorption measurements were conducted on an ASAP 2020M instrument. All samples were outgassed at 100°C for 300 min under flowing nitrogen before adsorption/desorption measurements.

Electrochemical measurements: All the electrochemical measurements were performed on PARSTAT 4000 electrochemical workstation in a 1.0 M KOH electrolyte with a conventional three-electrode glass cell at the room temperature. A Pt plate and a standard calomel electrode (SCE) were served as the counter electrode and the reference electrode, respectively. The samples prepared on carbon rods as working electrodes for the catalysis of the OER in 1.0 M KOH solution. The mass loading of Co₃O₄ on carbon rod is around 0.1 mg cm^-2.

3. RESULT AND DISCUSSION

To identify the crystal structure of the as-prepared samples, the X-ray diffraction (XRD) patterns were collected and shown in Fig. (1). All of the diffraction peaks can be perfectly indexed and assigned to the cubic phase of Co₃O₄ (JCPDF: 43-1003, Space group: Fd3m(227)) with lattice constants a = b = c = 8.084 Å. No other peaks could be observed from the patterns. Moreover, the intense and sharp peaks indicate that the samples are highly pure and well crystallized. Fig. (2a) shows a typical SEM image of the as-prepared S₁. It is observed that numerous spherical urchin shape structures of Co₃O₄ were obtained with uniform diameters of 6 μm. Fig. (2b) shows TEM image of the sample, clearly revealing each urchin-like Co₃O₄ sphere has uniform diameters of 6 μm with numerous small nanowires radially-grown from the center. Furthermore, the high-resolution TEM (HRTEM) image in Fig. (2c), which is observed with a distinct lattice spacing of 0.286 nm, corresponding to be (220) plane of Co₃O₄. Fig. (2d-f) shows the SEM, TEM and HRTEM images of the S₂ sample with nanosheet structure. Accordingly, as shown in Fig. (2f), the crystalline nature of S2 is also displayed in the HRTEM, the uniform 0.286 nm interplanar distance corresponds to the (220) plane of Co₃O₄. The typical thickness of these nanosheets are 150-200 nm. The typical SEM images of the S₃ sample are shown in Fig. (2g-I), which reveals the bulk structures of Co₃O₄.

Fig. (1).

XRD patterns of Co3O4 samples.

Fig. (2).

SEM, TEM and HRTEM images of Co3O4 samples: (a-c) S1, (d-f) S2, (g-i) S3.

The more detailed chemical composition and the oxidation state of the Co₃O₄ samples were further analyzed by XPS measurements. Fig. (3a) compares the XPS survey spectrum of three different morphological Co₃O₄ electrodes. Fig. (3b-c) shows the Co 2p and O 1s XPS spectrum of urchin-like Co₃O₄ spheres. As shown in Fig. (3b), two major peaks at binding energies of 780.3 and 795.4 eV corresponding to the Co 2p_3/2 and Co 2p_1/2 of Co₃O₄ phase are observed with a spin energy separation of 15.1 eV [30]. The high-resolution spectrum for the O 1s region in Fig. (3c) exhibits two oxygen contributions, which have been denoted as O_Ⅰ, and O_Ⅱ, respectively. The fitting peak of O_Ⅰ at 530.0 eV is typical of metal-oxygen bonds [31]. Furthermore, the component O_Ⅱ at 531.6 eV can be attributed to a number of oxygen defect sites in the material with small particle size [32, 33]. On the basis of the XPS results, it can be concluded that the urchin-like spheres consist of Co₃O₄. The Co₃O₄ samples were also investigated by nitrogen adsorption and desorption measurement at 77 K. As shown in Fig. (3d), the nitrogen adsorption-desorption curves demonstrate a Type IV isotherm [33],and the specific surface areas of Co₃O₄ samples S₁ to S₃ have been 40.45, 19.85, and 22.25 m²g^-1, respectively. The BET result is consistent with the SEM observations discussed above, showing urchin-like structure with relatively high interfacial area. The structural feature gives implications of the applications in electrocatalytic field, which comes as following below.

Fig. (3).

(a) XPS survey spectrum of Co3O4 samples; (b) Co 2p and (c) O 1s XPS spectrum of S1; (d) N2 adsorption/desorption isotherms of Co3O4 samples.

The cyclic voltammetric (CV) curves obtained from the different morphological Co₃O₄ electrodes in the potential range of 0.0-0.7 V are shown in Fig. (4). Fig. (4a-c) shows CV behavior of the as-prepared Co₃O₄ electrodes in 1.0 M KOH at different scan rates. The current was normalized to the amount of Co₃O₄ in 1 cm² electrode. As shown in Fig. (4d), the CVs of different morphological Co₃O₄ electrodes exhibit an anodic peak at about 0.52 V and a corresponding cathodic peak assigned to the Co (Ⅲ)/Co (Ⅳ), which result in the emergence of the catalytic phase for the OER [14, 28]. Furthermore, the different sites Co (Ⅲ) and Co (Ⅳ) derived from the oxidative process, which may form active sites to adsorb the OH^- during the OER in alkaline solution. The corresponding linear sweep voltammetry (LSV) curves of the above electrodes for the OER are shown in Fig. (5).

Fig. (4).

(a-c) CVs of Co3O4 electrodes recorded in 1.0 M KOH at different scan rates; (d) CVs of Co3O4 electrodes in 1.0 M KOH at a scan rate of 2 mv/s.

Fig. (5).

LSV curves for the OER on electrodes of Co3O4 recorded in 1.0 M KOH with a sweep rate of 1 mv/s at room temperature.

The sweep rate is 1 mV/s in the potential range from 0 V to 0.9 V at the potential scanning rate of 1 mV/s in 1.0 M KOH solution. It is well known that the low value of onset potential is beneficial to OER for water splitting. The onset potentialof S₁ is 0.64 V. Before 0.64 V, no OER peak happens on the S₁ electrode. In addition, the onset potential of S₁ is lower 14.8 mV than that of S₂, and lower 40 mV than that of S₃, indicating that the urchin-like sphere of Co₃O₄ has a high activity for OER.

The stability of OER on the Co₃O₄ electrodes was investigated with chronoamperometry. As shown in Fig. (6a), the current decay exhibits a deactivation of the electrodes. However, at the end of the test, the oxidation current on the Co₃O₄ (S₁)electrode is larger than that of the other two electrodes (S₂ and S₃). The current densityof S₁ decays quickly in a short time after starting OER. Furthermore, the current density is still 0.82 mA cm^-2 after 180 min. The current density on the S₂ and S₃ electrodes decay quickly and stabilize at 0.51 mA cm^-2 and 0.21 mA cm^-2 after 180 min, respectively. The current densityof S₁ is higher 0.31 mA cm^-2 than that of S₂, and higher 0.61 mA cm^-2 than that of S₃, indicating that the S₁ electrode has higher activity and better stability than S₂ and S₃ electrodes for OER. Furthermore, EIS is complementary to cyclic voltammetry (CV) and charge-discharge measurements. The more vertical the slope of the straight line in the low frequency, the better performance in oxygen evolution reaction. Obviously, the slope of S₁ larger than S₂ and S₃ in Fig. (6b), indicating that the structure of Co₃O₄ with the morphology of urchin-like spheres has better performance in oxygen evolution reaction. Therefore, the improved OER performances of S₁ electrode can be attributed to following reasons: (1) the nanowires consisted of urchin-like sphere structure can facilitate the diffusion of active species and electronic transport. (2) S₁ provide a larger electrode surface areas than S₂ and S₃.

Fig. (6).

(a) Chronoamperometric curves of Co3O4 electrodes recorded in 1.0 M KOH at room temperature; (b) Nyquist plots of Co3O4 electrodes in 1.0 M KOH.

CONCLUSIONS

In summary, on the basis of the OER activity on Co₃O₄ electrodes with various surface morphologis, it can be concluded that the onset potential of OER on the S₁ electrode is lower than that on the S₂ and S₃, indicating that the structure of Co₃O₄ has an easier OER for water splitting. The Co₃O₄ electrode with urchin-like spheres is beneficial for the water splitting due to their high efficient activity and the ability to detach bubbles from their interfacial.

CONFLICT OF INTEREST

The authors confirm that this article content has no conflict of interest.