Hollow Co₃O₄ dodecahedrons with controlled crystal orientation and oxygen vacancies for the high performance oxygen evolution reaction

Abstract

Rational design and preparation of advanced electrocatalysts for the oxygen evolution reaction (OER) are highly desirable, where various strategies have been applied to enhance the electrocatalytic activities, such as developing nanostructures, facet control, and creating oxygen vacancies. Herein, we have successfully integrated multiple strategies into one catalytic system for further enhancing the OER performance. A series of hollow Co₃O₄ dodecahedrons with controlled crystal orientation and oxygen vacancies were prepared by using ZIF-67 as a precursor and adjusting the atmosphere during calcination. Hollow Co₃O₄ dodecahedrons with both controlled exposure of the (111) facets and high content of oxygen vacancies showed an excellent OER performance with an overpotential of 307 mV at 10 mA cm⁻² and a Tafel slope as low as 55 mV dec⁻¹ and significantly superior to its counterpart with low content of oxygen vacancies or a broken Co₃O₄ dodecahedron being ground and losing the preferred facets of (111). The excellent OER performance should be attributed to the unique hollow structure and the effective control of the (111) facets and oxygen vacancies, which allows for more highly active sites and enhanced conductivity.

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1. Introduction

With the continuous excessive consumption of fossil energy, the related energy and environmental issues are increasingly prominent.^1–3 Electrochemical energy storage, such as electrical catalyzed water splitting,⁴ nitrogen fixation,⁵ and carbon dioxide reduction,⁶ are promising strategies to convert redundant electricity into chemical energy.^7–10 The oxygen evolution reaction (OER) is a half reaction of overall water splitting, which plays a critical role in various next-generation energy storage techniques including the use of fuel cells and metal–air batteries.^11–14 The OER is a four electron–proton coupled reaction but far from practical application due to the lack of proper catalysts to promote the sluggish kinetics.^15,16 Iridium and ruthenium oxides are proved to have excellent OER catalytic performance. However, the vulnerable stability and high cost prevent them from wide application in practical production.^17–21

Cobalt-based compounds, such as oxides and hydroxides, are regarded as active non-precious metal-based OER catalysts with relatively low cost and easy availability.^22–26 Among them, Co₃O₄ has been frequently investigated due to its considerable OER activity and corrosion resistance in alkaline solution,^27–30 although its bulk material is less active for the OER because of the limited surface area and conductivity. Therefore, various strategies have been applied to enhance the OER activity of Co₃O₄, such as developing nanostructured materials for increasing surface area, facet control for generating more highly active sites,^31–33 creating oxygen vacancies for regulating electronic states and improving conductivity.³⁰ For instance, Gupta et al.²⁵ reported a flower-shaped Co₃O₄ nanostructure showing a low overpotential of 356 mV at 10 mA cm⁻² with a Tafel slope of 68 mV dec⁻¹. Liu et al.³⁴ synthesized four types of Co₃O₄ nanocrystals by predominantly exposing (111), (112), (110) and (001) facets, respectively, and illustrated the highest OER activity of the Co₃O₄ nanocrystals with exposed (111) facets due to the highest surface energy, largest dangling bond density and smallest free energy. Xu et al.³⁵ presented a plasma-engraving strategy to create more oxygen vacancies on Co₃O₄ nanosheets and thus resulted in a higher atomic ratio of Co²⁺/Co³⁺(1.2) than that (1.0) of those without plasma-engraving, which substantially improves the electronic conductivity and generates more active defects for the OER. Nevertheless, these strategies are still expected to be combined into one catalytic system for further enhancing the OER performance, which remains a great challenge.

Hollow nanostructures possess some unique advantages for practical application in wide fields,^28,36–50 such as high specific surface area, abundant active sites, and multiple mass-transferring channels. Among all the methods for fabricating hollow structured materials, the self-templating methods based on metal–organic frameworks (MOFs) are receiving more and more attention.^51–57 For example, Yao and coworkers⁵⁸ prepared a hollow Co₃O₄/C OER electrocatalyst through a simple two-step thermal cracking process by using the cobalt ion enriched MOF as a precursor, which exhibited a low overpotential of 349 mV at 10 mA cm⁻² and a low Tafel slope of 60 mV dec⁻¹, showing a better performance of hollow structures compared to its solid counterpart. Recently, Wang and coworkers⁵⁹ first reported a hollow multi-shelled Co₃O₄ dodecahedron templated from zeolitic imidazolate framework-67 (ZIF-67), which showed a dominant exposure of (111) facets and 3 times higher photocatalytic activity toward the reduction of CO₂ than that of its counterpart without facet control. Additionally, our previous work showed that such a hollow Co₃O₄ dodecahedron with dominant exposure of (111) facets could be also used as a promising electrocatalyst for glucose oxidation.²⁸ However, its direct use for the OER is rarely reported, which should deserve more attention.

Herein, we have prepared a series of hollow Co₃O₄ dodecahedrons with controlled crystal orientation and oxygen vacancies using ZIF-67 as a precursor and attempted to combine multiple strategies into one catalytic system for further enhancing the OER performance, as shown in Scheme 1. By finely adjusting the atmosphere (the volume ratio of argon to oxygen, V_Argon/V_Oxygen) during calcination, hollow Co₃O₄ dodecahedrons with controlled exposure of the (111) facets and high content of oxygen vacancies could be obtained, which substantively resulted in both an increased electrochemical active surface area (ECSA) and enhanced conductivity. As a result, a better OER performance with an overpotential of 307 mV at 10 mA cm⁻² and a Tafel slope as low as 55 mV dec⁻¹ were realized by using hollow Co₃O₄ dodecahedrons obtained at a relatively low oxygen partial pressure (V_Argon/V_Oxygen = 9 : 1), which is significantly superior to those for its counterpart with low content of oxygen vacancies obtained at a relatively high oxygen partial pressure (V_Argon/V_Oxygen = 7 : 2) or broken Co₃O₄ dodecahedrons being ground and losing the preferred facets of (111).

Scheme 1 Schematic of the preparation of hollow Co₃O₄ dodecahedrons and oxygen content controlled dominant exposure of the (111) facet and oxygen vacancies for the high performance OER.

2. Experimental section

2.1. Materials

Cobalt(II) nitrate hexahydrate (Co(NO₃)₂·6H₂O, >99.0%) and 2-dimethyl imidazole and potassium hydroxide (KOH, AR) were purchased from Aladdin Reagent. Methanol, anhydrous ethanol and other solvents were obtained from Sinopharm chemical reagent Co., Ltd (Shanghai, China). All reagents were used without further purification. High purity argon and oxygen were purchased from Yinchuan Jinfeng Ningfeng Oxygen Co., Ltd (Yinchuan, China). Ultrapure water with a resistivity of 18 MΩ was used in all experiments.

2.2. Synthesis of ZIF-67

In a typical procedure, 22.5 mmol of Co(NO₃)₂·6H₂O and 96.9 mmol of 2-dimethyl imidazole were dissolved in a mixture of 60 mL of methanol and 60 mL of ethanol, respectively. Then the solution of 2-dimethyl imidazole was poured into the solution of Co(NO₃)₂, and the mixed solution was aged for 24 hours at 40 °C. After centrifugation, the purple precipitate was obtained and dried under vacuum for 24 hours.

2.3. Synthesis of hollow Co₃O₄ dodecahedrons

The hollow Co₃O₄ dodecahedrons were obtained by the calcination of ZIF-67 in a tube furnace under different atmospheres. In a typical method, the tube furnace was purged by mixed gases (the V_Argon/V_Oxygen values were 7 : 2, 4 : 1 and 9 : 1, respectively) for 30 min before calcination. Then the temperature was raised to 425 °C with a heating rate of 0.5 °C and annealed for 1 hour.

2.4. Characterizations

The surface morphology of ZIF-67 was observed using a ZEISS EVO 18 scanning electron microscope (SEM) with an acceleration voltage of 15 kV. Transmission electron microscopy (TEM) images of hollow Co₃O₄ dodecahedrons were obtained using a Hitachi HT-7700 electron microscope with an operating voltage of 120 kV. High-resolution TEM images, selected area electron diffraction (SAED), high-angle annular dark field scanning TEM (HAADF-STEM) images and elemental mappings were recorded by an FEI Talos 200S electron microscope operated at 200 kV. The powder X-ray diffraction (XRD) patterns were recorded on an AXS D8 ADVANCE A25 diffractometer with an operating voltage of 40 kV and a scanning speed of 5° min⁻¹. The nitrogen adsorption–desorption isotherms under liquid nitrogen (−196 °C) were measured on the Autosorb iQ adsorption analyzer. Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectroscopy (XPS) was used to observe the chemical environment of the samples.

2.5. Electrochemical measurements

The electrochemical properties of the samples were determined by a PINE electrochemical workstation (Pine Research Instrumentation, USA) using a conventional three-electrode system. The rotation speed of the rotating-disk electrode (RDE) was set at 1600 rpm, and the disk electrode (RDE 5 mm) deposited with the samples was used as the working electrode, the Ag/AgCl electrode as the reference electrode and the Pt wire electrode as the counter electrode. Before testing, the disk electrode was polished by Al₂O₃ particles with different meshes, and the electrolyte (1 M KOH aqueous solution) was purged by argon for 30 min. The samples (3 mg) were dispersed in 1 mL of anhydrous ethanol under ultrasound treatment for 30 min. Then, 5 μL of the dispersions were dropped on the surface of the disk electrode and dried under ambient temperature conditions. This process was repeated 5 times, so 25 μL of the dispersions were deposited on the surface of the working electrode. Finally, 5 μL of 5 wt% Nafion solution was dropped as a binder on the surface of the electrodes.

The turnover frequency (TOF) calculation:²⁹ the values of TOF were calculated by using the following equation: TOF = (j × S)/(4 × F × n), where j (mA cm⁻²) is the measured current density at an overpotential of 300 mV, S is the working surface area of the glassy carbon electrode, the number 4 means a four-electron OER, F is Faraday's constant (96485.3 C mol⁻¹), and n is the moles of the coated Co atom on the electrode calculated from the loading weight and the molecular weight of the coated catalysts (Co₃O₄).

3. Results and discussion

Hollow Co₃O₄ dodecahedrons were synthesized using ZIF-67 as a precursor, which was prepared using a typical procedure.⁵⁹ As shown in Fig. 1a, the SEM image indicates the dodecahedron morphology of ZIF-67 with a uniform size of 2.5 μm. The XRD pattern in Fig. S1 (ESI†) further confirms the crystal structure of ZIF-67. After calcination in a mixture of argon and oxygen, the dodecahedron morphology of ZIF-67 was well-preserved to give samples of Co₃O₄-7 : 2, Co₃O₄-4 : 1 and Co₃O₄-9 : 1 based on the value of V_Argon/V_Oxygen. The TEM images in Fig. 1b–d demonstrate the hollow structure of Co₃O₄-7 : 2, Co₃O₄-4 : 1 and Co₃O₄-9 : 1 with a similar size of about 1.2 μm. Instead of the smooth surface of ZIF-67, the shell of the Co₃O₄ dodecahedrons was composed of small Co₃O₄ nanoparticles, resulting in a rough surface and high specific areas of about 36 m² g⁻¹, as illustrated in Fig. S2, (ESI†).

Fig. 1 (a) SEM image of ZIF-67 and (b–d) TEM images of Co₃O₄-7 : 2, Co₃O₄-4 : 1 and Co₃O₄-9 : 1, respectively.

In order to investigate the detailed microstructure of the synthesized Co₃O₄ dodecahedrons, HR-TEM was performed to observe the crystal structure and chemical composition of Co₃O₄-9 : 1. As illustrated in Fig. 2a, the shell of the Co₃O₄-9 : 1 dodecahedron was formed of small Co₃O₄ nanoparticles with various morphologies. The average diameter of the Co₃O₄ nanoparticles is measured to be 22.3 ± 5.4 nm. The crystal facet of the spinel Co₃O₄ was determined by calculating the crystal plane spacing based on the HR-TEM image in Fig. 2b. The d-spacings of 0.462, 0.246 and 0.285 nm are ascribed to the (111), (311) and (220) planes, respectively. Besides, the SAED pattern in Fig. 2c also indicates the polycrystalline structure of the Co₃O₄ nanoparticles. The content of all elements of Co₃O₄-9 : 1 can be determined by EDS (Fig. S3, ESI†), and the atomic fraction of Co and O is 38.4% and 55.0%, in accordance with the stoichiometric ratio of Co₃O₄. Moreover, there is also residual carbon with an atomic fraction of 4.7%, which might increase the conductivity of the samples. The HAADF-STEM image and elemental mappings in Fig. 2d–f reveal the homogeneous distribution of Co and O elements.

Fig. 2 (a) TEM, (b) HR-TEM image and (c) SAED of Co₃O₄-9 : 1; (d) HAADF-STEM image and (e and f) elemental mappings of Co₃O₄-9 : 1.

It has been reported that the (111) plane dominant Co₃O₄ hollow multi-shell structures could be obtained by the calcination of ZIF-67 due to the topological arrangement of metal atoms in ZIF-67, which exhibited significantly enhanced catalytic activity for CO₂ photoreduction.⁵⁹ Besides, the (111) plane also showed excellent catalytic performance toward the OER.^34,60 Therefore, we changed the calcination conditions to control the exposure of the (111) plane by changing the oxygen partial pressure during calcination. As shown in Fig. 3, XRD analysis was carried out to probe into the crystal structure of the Co₃O₄-7 : 2, Co₃O₄-4 : 1 and Co₃O₄-9 : 1. The main diffraction peaks of (111), (220), (311), (222), (400), (511) and (440) planes are observed, which is in good accordance with the standard card and previous report.⁶¹ However, the intensity of the (111) plane of Co₃O₄-4 : 1 and Co₃O₄-9 : 1 is abnormally strong (I₍₁₁₁₎/I₍₃₁₁₎ = 0.61, 0.42, respectively) compared to that of the (311) plane (curve (b and c) in Fig. 3). When the oxygen partial pressure increased to 7 : 2, the selective exposure of the (111) facet disappeared, as shown in Fig. 3a. Besides, the orientation of the (111) facet also disappeared after grinding (Fig. 3d), demonstrating the importance of the hollow dodecahedron structure to maintain the selective exposure of the (111) facet. A previous study has also demonstrated that the dominant exposure of the (111) facet of hollow Co₃O₄ dodecahedrons was the result of the orientated growth and alignment of Co₃O₄ nanocrystals during the shell formation induced by the precursor ZIF-67.⁵⁹ The calcination atmosphere might influence the oriented growth and arrangement of the Co₃O₄ nanocrystals.

Fig. 3 XRD patterns of (a) Co₃O₄-7 : 2, (b) Co₃O₄-4 : 1, (c) Co₃O₄-9 : 1 and (d) Co₃O₄-9 : 1 after grinding.

To evaluate the difference of the surface components and the chemical environment of the elements of hollow Co₃O₄ dodecahedrons with different dominant exposure of the (111) plane, XPS was carried out. The survey XPS spectra in Fig. S4 (ESI†) confirmed the existence of Co, O and C in the samples, while the similar C 1s high resolution XPS spectra of Co₃O₄-7 : 2, Co₃O₄-4 : 1 and Co₃O₄-9 : 1 could be divided into three peaks, belonging to C=C/C–C (284.8 eV), C–O (286.3 eV) and C=O (288.8 eV), as shown in Fig. S5 (ESI†). Fig. 4a–c show the high resolution XPS spectra of Co 2p of Co₃O₄-7 : 2, Co₃O₄-4 : 1 and Co₃O₄-9 : 1, respectively, which present two prominent peaks at 780.2 and 795.1 eV, assigned to the Co 2p_3/2 and Co 2p_1/2 peaks, respectively. The Co 2p_3/2 peak could be subdivided into two peaks centred at 779.6 and 780.4 eV, which were the characteristic peaks of Co³⁺ 2p_3/2 and Co²⁺ 2p_3/2, respectively. Similarly, the Co 2p_1/2 peak could also be subdivided into two peaks that belonged to Co³⁺ 2p_1/2 (794.7 eV) and Co²⁺ 2p_1/2 (796.1 eV), respectively.^62–64 Calculating from the peak area of Co³⁺ and Co²⁺, the relative content of Co²⁺ of Co₃O₄-9 : 1 was as high as 75.9%, while the content of Co²⁺ of Co₃O₄-4 : 1 and Co₃O₄-7 : 2 was slightly reduced to 72.1% and 71.9%, respectively, which was consistent with the previous study that Co²⁺ was the dominant species in the (111) plane.⁶⁵ Besides, a high content of Co²⁺ facilitates the formation of active sites for the OER.⁶⁰

Fig. 4 High resolution (a–c) Co 2p and (d–f) O 1s XPS spectra of (a and d) Co₃O₄-7 : 2, (b and e) Co₃O₄-4 : 1 and (c and f) Co₃O₄-9 : 1.

The high resolution XPS spectra of O 1s of Co₃O₄-7 : 2, Co₃O₄-4 : 1 and Co₃O₄-9 : 1 were also obtained, which clearly displayed three oxygen contributions, as illustrated in Fig. 4d–f. The peak at around 529.9 eV is assigned to the metal oxygen bond, while the other two peaks centred at 531.7 and 533.4 eV belong to the chemosorbed oxygen species.⁶¹ The chemosorbed species centred at 533.4 eV is ascribed to the high binding energy peak from surface oxygen defect species, implying the existence of oxygen vacancies on the surface of the samples.⁶⁶ Based on the peak areas of different oxygen species in Fig. 4f, we calculated that the relative content of oxygen vacancies of Co₃O₄-9 : 1 was 13%, which was much higher than that of Co₃O₄-4 : 1 (4.0%) and Co₃O₄-7 : 2 (4.2%). The high content oxygen vacancy of Co₃O₄-9 : 1 might be due to the high content of Co²⁺ resulting from the lack of oxygen during calcination.⁶⁰ Because the oxygen vacancy is the active site for the OER,^24,35 electrochemical measurements were conducted to evaluate the catalytic activities of Co₃O₄-7 : 2, Co₃O₄-4 : 1 and Co₃O₄-9 : 1.

The electrochemical measurements for the OER were carried out using a standard three-electrode system in oxygen-saturated 1.0 M KOH aqueous solution with the Ag/AgCl electrode as a reference electrode and the Pt wire electrode as a counter electrode. The sample deposited Pt rotating-disk electrode (RDE) with an area of 0.196 cm² was used as a working electrode. Linear sweep voltammetry (LSV) was performed at a scan speed of 0.1 mV s⁻¹ and a rotating speed of 1600 rpm to remove the generated oxygen. First of all, the optimal mass loading was determined by depositing different amounts of Co₃O₄-9 : 1 on the surface of the RDE, as shown in Fig. S6 (ESI†). The LSV curves and Tafel plots indicated the optimal mass loading of 0.38 mg cm⁻² (25 μL), showing a small overpotential of 307 mV and a Tafel slope of 55 mV dec⁻¹ (Fig. S6, ESI†). So the following test for Co₃O₄-7 : 2, Co₃O₄-4 : 1 and Co₃O₄-9 : 1 was carried out at a mass loading of 0.38 mg cm⁻².

Fig. 5a shows the polarization curves of Co₃O₄-7 : 2, Co₃O₄-4 : 1 and Co₃O₄-9 : 1. The peaks at around 1.45 V vs. RHE can be ascribed to the oxidation of Co²⁺ to Co³⁺ or Co⁴⁺.⁶⁷ The Co₃O₄-9 : 1 shows a much lower onset potential (1.49 V vs. RHE) than that of Co₃O₄-4 : 1 (1.51 V vs. RHE) and Co₃O₄-7 : 2 (1.57 V vs. RHE). The overpotential of Co₃O₄-9 : 1 is 307 mV to reach a current density of 10 mA cm⁻², which is lower than that of Co₃O₄-4 : 1 (333 mV) and Co₃O₄-7 : 2 (371 mV), as well as most of the previous reports, as shown in Table S1 (ESI†). The OER kinetics of electrocatalysis was further analysed by Tafel plots. As shown in Fig. 5b, the Tafel slopes of Co₃O₄-9 : 1, Co₃O₄-4 : 1 and Co₃O₄-7 : 2 are 55, 55 and 60 mV dec⁻¹, demonstrating the fast OER catalytic reaction kinetics. The high content of oxygen vacancies of Co₃O₄-9 : 1 was believed to contribute to the enhanced catalytic activity.

Fig. 5 Electrochemical performance toward the OER. (a) LSV curves, (b) Tafel plots, (c) EIS spectra and (d) capacitive currents as a function of the scan rate to give the double-layer capacitance (C_dl) of Co₃O₄-9 : 1, Co₃O₄-4 : 1 and Co₃O₄-7 : 2.

The resistances of the samples were obtained by electrochemical impedance spectroscopic (EIS) measurement at a potential of 1.548 V vs. RHE, as shown in Fig. 5c. R_ct is the charge transfer resistance, which can be estimated from the diameter of the semicircles in the Nyquist plots. The R_ct value of Co₃O₄-9 : 1 is the smallest (271 Ω), much lower than that of Co₃O₄-4 : 1 (386 Ω) and Co₃O₄-7 : 2 (584 Ω), respectively, indicating that the OER is easier at the interface of the electrolyte/electrode of Co₃O₄-9 : 1. We believed that the best conductivity of Co₃O₄-9 : 1 was contributed by the high content of oxygen vacancies, which supplies more charge carriers compared to the counterparts.³⁵

ECSA is a key parameter to determine the catalytic activity of an electrocatalyst, which can be roughly evaluated by the double-layer charging current (C_dl).^68,69 As illustrated in Fig. 5d, the linear plots were obtained by the fitting of the current densities and scan rates, presenting a straight line. The current densities were measured at 1.3 V vs. RHE from the cyclic voltammetry (CV) curves at different scan rates (10–100 mV s⁻¹) in the region of 1.148–1.348 V vs. RHE (no faradaic current is observed). The C_dl value of the samples could be estimated from the slope of the fitted lines,^15,70–72 which are calculated to be 19.71, 12.70, and 10.82 mF cm⁻² for Co₃O₄-9 : 1, Co₃O₄-4 : 1 and Co₃O₄-7 : 2. According to the calculation equation (ECSA = C_dl/C_s, C_s is equal to 40 μF cm⁻² under alkaline conditions),^68,69 the ECSAs of Co₃O₄-9 : 1, Co₃O₄-4 : 1 and Co₃O₄-7 : 2 are calculated to be 129.7, 83.7 and 71.3 m² g⁻¹, respectively, considering that the mass loading of the samples is 0.38 mg cm⁻². Compared to Co₃O₄-4 : 1 and Co₃O₄-7 : 2, the ECSA of Co₃O₄-9 : 1 was significantly improved, indicating the more exposure of the catalytically active site contributed by oxygen vacancies, which was consistent with the results of XPS. The TOFs of Co₃O₄-9 : 1, Co₃O₄-4 : 1 and Co₃O₄-7 : 2 were calculated to be 4.17, 1.56 and 0.56 s⁻¹, respectively, further confirming the excellent OER performance of Co₃O₄-9 : 1.

In order to further confirm the contribution of the hollow structure and exposure of the (111) facet of hollow Co₃O₄ dodecahedrons to the OER catalytic activity, the electrochemical properties of Co₃O₄-9 : 1 were tested after grinding, which destroyed the hollow structure and the dominant exposure of the (111) facet (Fig. 3d).⁵⁹ As shown in Fig. S7 (ESI†), the overpotential of Co₃O₄-9 : 1 increased to 344 mV at 10 mA cm⁻² after grinding. Besides, the Tafel slope also increased to 68 mV dec⁻¹. The EIS and double layer capacitance were also tested, showing an increased charge transfer resistance (851 Ω) and a decreased ECSA of 29.3 m² g⁻¹, much lower than that before grinding (129.7 m² g⁻¹). The results demonstrated that the hollow structure and exposure of the (111) facet of Co₃O₄-9 : 1 supplied more active sites for the OER.

The ECSA-normalized LSV curves are presented in Fig. S8 (ESI†), showing similar results as presented in Fig. 5a. The potential of Co₃O₄-9 : 1 is 1.544 V at 0.1 mA cm_ECSA⁻², which is 14 and 47 mV lower than that of Co₃O₄-4 : 1 and Co₃O₄-7 : 2, respectively. The results implied that the high content of oxygen vacancies might optimize the electron structure of Co₃O₄-9 : 1, which accelerated the charge transfer and the OER process.⁷³ Overall, the highest catalytic activity of Co₃O₄-9 : 1 to the OER was determined by several factors including the exposure of the (111) facet, the content of oxygen vacancies, charge transfer resistance and the ECSA. Moreover, Co₃O₄-9 : 1 also showed reasonable durability, as illustrated in Fig. S9 (ESI†). The overpotential increased 56 mV at a current density of 10 mA cm⁻² after 1000 cycles compared to the initial overpotential. However, the stability is still far from meeting the requirements of real-world applications. We speculate that the “soft” attached structure of the hollow Co₃O₄ dodecahedrons is not stable enough to guarantee the long term catalytic stability.⁵⁹

4. Conclusions

In summary, we have successfully prepared a series of hollow Co₃O₄ dodecahedrons with controlled exposure of the (111) facet and oxygen vacancies by using ZIF-67 as a precursor and adjusting the atmosphere during calcination, and realized the combination of multiple strategies into one catalytic system for further enhancing the OER performance. Hollow Co₃O₄ dodecahedrons with both controlled exposure of the (111) facets and high content of oxygen vacancies were obtained at a relatively low oxygen partial pressure, which showed an excellent OER performance with an overpotential of 307 mV at 10 mA cm⁻² and a Tafel slope as low as 55 mV dec⁻¹ which is significantly superior to its counterpart with low content of oxygen vacancies obtained at a relatively high oxygen partial pressure or a broken Co₃O₄ dodecahedron being grinded and losing the preferred facets of (111). The excellent OER performance should be attributed to the hollow structure and the effective control of the (111) facets and oxygen vacancies, which allow for more highly active sites and enhanced conductivity.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was partly supported by the National Natural Science Foundation of China (No. 51672138), the National First-Rate Discipline Construction Project of Ningxia (NXYLXK2017A04), the Talent-introducing Special Foundation for Key Research Program of Ningxia (2018BEB04032) and the Natural Science Foundation of Ningxia (2020AAC03003). X. Lai gratefully acknowledges the Ningxia Fostering Program for Innovative Leading Talents in Science and Technology (KJT2017003), the Program for Youth Excellent Scholars of State Key Laboratory of High-Efficiency Utilization of Coal and Green Chemical Engineering and the Natural Science Foundation of Ningxia University (ZR1719). We thank Mr Dekang Li for his help on schematic images.

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Footnotes

† Electronic supplementary information (ESI) available: XRD patterns, nitrogen adsorption–desorption isotherms, EDS spectra, XPS spectra and electrochemical measurement results. See DOI: 10.1039/d0qm00671h

‡ These authors contributed equally to this work.

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