English

• 0.   Introduction

  With the gradual depletion of fossil fuels and deterioration of ecological environment, it is necessary to pursue renewable and sustainable energy sources^[1-3]. Hydrogen (H₂) as a clean and new energy, has received widespread attention due to its high energy density and negligible pollution of combustion products^[4]. Currently, electrolysis of water has been considered as a promising strategy to produce H₂ owing to its giant advantage of abundance in resources and carbon-free emissions^[5-7]. However, the practical application of water splitting has been largely impeded due to the relatively slower kinetics and higher overpotentials of oxygen evolution reaction (OER) at the anode^[8-10]. Up to now, RuO₂ and IrO₂ are regarded as the optimal commercial catalysts to accelerate OER kinetics, but the high cost and low abundance of noble metal oxides greatly prevent their practical application in industrial electrolysis^[11-13]. Therefore, the development of efficient and stable non-noble metal catalysts has become a research hotspot.

  In recent years, cerium oxide (CeO₂) with the simple structure, rich elemental abundance and good catalytic performance^[14], has attracted much research attention and is mainly used in thermocatalytic reactions due to its redox cycle of Ce³⁺/Ce⁴⁺ and lattice oxygen mobility^[15-17]. But at room temperature, it is very difficult for single CeO₂ to be an excellent electrocatalyst due to its rare number of oxygen vacancy and relatively poor electron conductivity. Therefore, to compete with other metal oxide-based materials and significantly improving the electrocatalytic performance, the CeO₂ needs to be modified by controlling morphology or modulating electronic structure (other components doping or loading) ^[18-20]. Kang et al. developed the Co-doped CeO₂ nanosheet arrays, which presented a better hydrogen evolutin reaction (HER) performance than bare CeO₂ with the relatively low overpotential of 132 mV at 100 mA·cm^-2, because the additional cobalt dopant promotes formation of oxygen vacancies^[21]. Li et al. prepared the FeOOH/CeO₂ heterolayered nanotube as an efficient OER catalyst with an overpotential of 300 mV at 80.2 mA·cm^-2 in 1.0 mol·L^-1 NaOH owing to the strong electron interactions between the CeO₂ and FeOOH^[22]. However, most of the reported Ce-based materials need to be grown on the foamed nickel substrate^[23-25], which not only increases the complexity of catalyst preparation, but also cannot directly characterize the intrinsic activity of Ce-based electrocatalysts, because the foamed nickel itself has better performance for the OER/HER reactions. Therefore, it remains a great challenge to design the highly efficient Ce-based electrocatalysts without support substrate.

  Herein, a novel Co₃O₄/CeO₂ heterojunction composite oxide was prepared by a facile two-step method for alkaline OER, and the supported Co₃O₄ nanoparticles can effectively enhance the electron conductivity of composite (decrease the charge transfer resistance). Meanwhile, the concentration of surface oxygen vacancies and active oxygen species of CeO₂ can be also increased due to the strong surface electron interaction. The synthesized 58.5%Co₃O₄/CeO₂ composite exhibited a superior OER performance with a low overpotential of 347 mV at 10 mA·cm^-2 and small Tafel slope of 72.7 mV·dec^-1 as well as an excellent stability relative to commercial RuO₂.

  1.   Experimental

  1.1   Chemicals and materials

  Co(NO₃)₂·6H₂O (AR), Ce(NO₃)₃·6H₂O (AR), and ethylene glycol (EG, AR) were purchased from China Reagent Co., Ltd. Anhydrous ethanol (AR) was purchased from Wuxi Yasheng Chemical Co., Ltd. All chemicals were used as received without further purification.

  1.2   Catalysts preparation and characterization

  The Co₃O₄/CeO₂ composite was prepared by a facile two-step method, the fabrication process is exhibited in Fig. 1. First, 6.079 1 g Ce(NO₃)₃·6H₂O was completely dissolved in 50 mL EG to form a homogeneous solution, and then the solution was transferred to a 60 mL autoclave and maintained at 180 ℃ for 28 h. After the autoclave was cooled to room temperature, the products were separated by filtration, and washed by water and alcohol, and finally dried in vacuum 60 ℃ for 12 h. Second, Co₃O₄/CeO₂ catalysts were developed by wet impregnation of the above as-synthesized Ce-containing precursors with the aqueous solution of Co(NO₃)₂·6H₂O. The impregnated solids were dried at 90 ℃ for 1 h and then calcined at 450 ℃ in air for 3 h. The catalysts were denoted as X%Co₃O₄/CeO₂ where X% represented the mass ratio between Co₃O₄ and CeO₂ in composite oxide (X=14.6, 29.3, 43.9, 58.5, 73.2, 87.8).

  Figure 1

  

  Figure 1.  Schematic illustration of the synthesis process of Co₃O₄/CeO₂ composite

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  X-ray diffraction (XRD) patterns of all the samples were collected in the range from 10° to 80° (2θ) using a Bruker D8ADVANCE powder diffractometer (Cu Kα radiation, λ=0.154 05 nm), operated at 40 kV and 40 mA. Scanning electron microscopy (SEM, S-4800, Japan), transmission electron microscopy (TEM, JEM-2100, Japan) and high-resolution transmission electron microscopy (HRTEM, JEM-2100, Japan) were employed to analyse the morphology and structure of samples. The working voltages of SEM and TEM were 5 and 200 kV, respectively. The elemental composition and valence state of the sample were analyzed on X-ray photoelectron spectroscopy (XPS, Phi-5000 Versaprobe, Al Kα radiation, hν =1 486.6 eV). All electrochemical measurements including rotating-disk electrode (RDE) polarization curves (linear sweep voltammetry (LSV), 5 mV·s^-1, the positively scanned branch), cyclic voltammetry (CV, 50 mV·s^-1, positive scanning), and chronoamperometry measurements were carried out by a three-electrode cell with an electrochemical workstation (CHI 760E, Chenhua, China) in O₂-saturated 1.0 mol·L^-1 KOH at room temperature. The reference, counter, and working electrodes were an Hg/HgO electrode (1.0 mol·L^-1 KOH), a graphite-rod electrode, and a RDE with a glassy-carbon electrode (GCE, area 0.125 6 cm²), respectively. All potentials (iR corrections) were calibrated to the reversible hydrogen electrode (RHE) by the following conversion formula (E_RHE=E_Hg/HgO+0.059 1pH+0.095). For electrode preparation, typically, 8 mg of catalyst with 2 mg of carbon black (Vulcan XC-72 carbon) were dispersed in 500 μL deionized water to form a homogeneous ink by ultrasound for 5 min. Afterwards, 5 μL of catalyst ink was dropped onto a freshly polished glassy carbon electrode and dried naturally. The loading of all catalysts, including commercial RuO₂, on the disk electrode was 0.79 mg·cm^-2. The electrochemically active surface areas (ECSAs) of various catalysts were acquired by doublelayer capacitance (C_dl), as previously described in the literature^[26-27]. Additionally, the electrochemical impedance spectra (EIS) were gained at 1 600 r·min^-1 from 10 000 to 0.01 Hz in O₂-saturated 1.0 mol·L^-1 KOH.

  2.   Results and discussion

  2.1   Structure and morphology of Co₃O₄/CeO₂ catalysts

  Fig. 2 shows the XRD patterns of CeO₂, Co₃O₄ and X% Co₃O₄/CeO₂ catalysts. The evident peak at 28.5° ascribed to the reflection plane of cubic fluorite CeO₂ (111) (PDF No. 78-0694, as indicated in blue dashed box) ^[28] and the characteristic diffraction peak at 36.9° belonging to the (311) facet of cubic spinel Co₃O₄ (PDF No.71-0816, as indicated in green dashed box) ^[29] can be detected in all X%Co₃O₄/CeO₂ samples, which illustrates that Co 3O4 nanoparticles have been successfully decorated on the CeO₂ surfaces. Meanwhile, the characteristic peak intensity of Co₃O₄ (311) gradually increased with the increase of Co 3O4 loading, indicating the augment of average crystallite size for supported Co₃O₄ nanoparticles.

  Figure 2

  

  Figure 2.  XRD patterns of CeO₂, Co₃O₄ and X%Co₃O₄/CeO₂ catalysts

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  The detailed morphologies and structures of Ce-based catalysts were analyzed by SEM and TEM techniques. Obviously, the pure CeO₂ with a smooth surface displays the mixed shapes of nanorods and nanoparticles with sizes of above 3 nm×15 nm~17 nm× 150 nm and 50 nm, respectively (Fig. 3a). For 58.5%Co₃O₄/CeO₂ composite, it is clearly observed that many Co₃O₄ nanoparticles with a small size of about 10~30 nm were attached onto the surfaces of CeO₂ (Fig. 3b, 3d), and the nanocrystallines of supported Co₃O₄ nanoparticles was smaller than that of CeO₂ (Fig. 3d). The energy dispersive X-ray spectra (EDX) mappings (Fig. 3c) suggest that Co, Ce and O elements were homogeneously distributed on the surfaces of the composite. The HRTEM image in Fig. 3e shows a clear boundary between CeO₂ and Co₃O₄ and two lattice fringe spacings of 0.192 and 0.232 nm, corresponding to the (220) plane of CeO₂ and the (222) plane of Co₃O₄, respectively^[30-31]. The corresponding selected area electron diffraction (SAED) patterns display a few bright rings with discrete spots, which demonstrates the polycrystalline structure of the Co₃O₄/CeO₂ composite (Fig. 3f).

  Figure 3

  

  Figure 3.  (a) SEM image of CeO₂; (b) SEM image, (c) elemental mappings, (d) TEM image, (e) HRTEM image and (f) SAED pattern of 58.5%Co₃O₄/CeO₂

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  2.2   XPS spectra of X%Co₃O₄/CeO₂ catalysts

  The elemental composition and valence states of the catalysts were analyzed by XPS. The XPS survey spectrum (Fig. 4a) clearly shows the presence of Co, Ce, and O elements in 58.5%Co₃O₄/CeO₂ composite. The Co2p XPS spectra verify the existence of both Co3+ and Co²⁺ in pure Co₃O₄ and 58.5%Co₃O₄/CeO₂ composite (Fig. 4b), and the binding energy of the Co2p orbit in the 58.5%Co₃O₄/CeO₂ composite shifts to lower value (780.11 eV) than that (780.35 eV) of pure Co₃O₄. In addition, the ratio of Co³⁺/Co²⁺ in 58.5%Co₃O₄/CeO₂ (1.45) was higher than that of pure Co₃O₄ (1.11). These results demonstrate the presence of electron transfer between Co₃O₄ and CeO₂. The increased amount of surface Co³⁺ (high valence state Co) can effectively promote the adsorption of OH^- and improve the OER performance^[32-33]. Here because Co²⁺ (T_d coordination structure) changes into Co³⁺ (O_h structure), it is helpful to coordination of oxygen species.

  Figure 4

  

  Figure 4.  (a) XPS survey spectra of 58.5%Co₃O₄/CeO₂; (b) High-resolution Co2p XPS spectra of Co₃O₄ and 58.5%Co₃O₄/CeO₂; High-resolution (c) Ce3d and (d) O1s XPS spectra of CeO₂ and 58.5%Co₃O₄/CeO₂

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  The Ce3d XPS spectra of pure CeO₂ and 58.5%Co₃O₄/CeO₂ are shown in Fig. 4c. Specifically, the peaks at 884.16 and 902.76 eV belong to Ce³⁺, and the peaks at 881.85, 888.27, 897.85, 900.45, 906.87 and 916.45 eV are assigned to Ce⁴⁺, respectively, indicating the coexistence of Ce³⁺ and Ce⁴⁺ in the CeO₂ surfaces^[34]. Ce3d_5/2 peaks in 58.5%Co₃O₄/CeO₂ composite shows significant positive shift than CeO₂. Moreover, the 58.5%Co₃O₄/CeO₂ composite had a smaller ratio of Ce⁴⁺/Ce³⁺ (4.19) than that of pure CeO₂ (4.52) due to the partial electron transfer from Co₃O₄ nanoparticles to CeO₂ surfaces. The existence of Ce³⁺ can introduce unsaturated chemical bonds and oxygen vacancies in the CeO₂ crystal to create a charge imbalance, and these oxygen vacancies can effectively realize the adsorption and activation of oxygen species, which is beneficial to the enhanced OER activity^[35-36].

  The O1s XPS spectra for CeO₂ and 58.5%Co₃O₄/CeO₂, as shown in Fig. 4d, are fitted into three peaks corresponding to the lattice oxygen (O1, 529.00 eV), chemisorbed oxygen or surface oxygen vacancies (O2, 531.06 eV), and surface hydroxyl groups or adsorbed water species (O3, 532.98 eV), respectively^[37]. The 58.5%Co₃O₄/CeO₂ exhibits the higher ratio (34.5%, atomic fraction) of O2 to (O1+O2+O3) than that of pure CeO₂ (25.1%), indicating that the generation of more surface oxygen vacancies after the supported Co₃O₄ nanopartides.

  2.3   OER performance of Ce-based catalysts

  The electrochemical performance of Co₃O₄, CeO₂, commercial RuO₂, and Co₃O₄/CeO₂ samples were tested by RDE techniques in an O₂-saturated 1.0 mol·L^-1 KOH solution. The LSV curves of different catalysts were displayed in Fig. 5a. Distinctly, the 58.5%Co₃O₄/CeO₂ sample shows the best performance with the lower overpotential of 347 mV at 10 mA·cm^-2, lower than Co₃O₄ (440 mV), commercial RuO₂ (359 mV) and CeO₂ (570 mV). The smallest value of Tafel slope for 58.5%Co₃O₄/CeO₂ composite indicates the relatively fast OER kinetics on surfaces (Fig. 5b). Additionally, with the increase of loading amounts from 14.6% to 58.5%, the activities of X%Co₃O₄/CeO₂ were constantly increasing with the ever-reduced values of Tafel slope (Fig. 5c, 5d). But excess Co₃O₄ loading amounts (73.2%~87.8%) can decrease the OER catalytic activity, which may be related to the superfluous Co₃O₄ nanoparticles with low intrinsic activity occupying the active sites on interface of X% Co₃O₄/CeO₂ composites. The OER activity shows a volcanic trend with the increase of Co₃O₄ loading amounts, as shown in Fig. 5e.

  Figure 5

  

  Figure 5.  (a) LSV and (b) Tafel curves of CeO₂, Co₃O₄, RuO2 and 58.5%Co₃O₄/CeO₂; (c) LSV and (d) Tafel curves of X%Co₃O₄/CeO₂ with different loads; (e) Dependence of overpotential and Tafel slope on X%Co₃O₄/CeO₂; (f) C_dl of CeO₂, Co₃O₄ and 58.5%Co₃O₄/CeO₂

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  ECSA is also an important parameter to evaluate the OER performance of electrocatalysts. ECSAs can be calculated by ECSA=C_dl/C_s, where C_dl is the double layer capacitances and C_s is the specific capacitance of planar surface with an atomically smooth under identical electrolyte conditions (C_s=40 μF·cm^-2 in our case)^[38]. C_dl was determined from the CV curves (Fig.S1 in electronic supporting information (ESI)) measured in a potential range according to the following equation: C_dl= (j_a-j_c)/(2v) = Δj/(2v), where j_a and j_c are the anodic and cathodic voltammetric current densities, respectively, and v is the scan rate. The C_dl values of all the samples are displayed in Fig. 5f and 6a (the ECSAs were included in Table S1 from ESI).

  Figure 6

  

  Figure 6.  (a) C_dl of X%Co₃O₄/CeO₂; (b) Relativity of specific activity to overpotentials; (c) Relativity of the TOF to overpotentials; (d) Nyquist curves of CeO₂, Co₃O₄ and X%Co₃O₄/CeO₂

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  The specific activities were calculated by j/ECSA (where j is current density), as shown in Fig. 6b. The turnover frequency (TOF) was calculated by formula TOF=jA/(4Fn), where j is current density at overpotentials of 0.36~0.40 V, and A is the geometry area of the glassy carbon electrode, and F is the Faraday constant (96 485 C·mol^-1), and n is the number of moles of the active materials for OER, assuming that all metal ions in the samples are active^[39-40]. The relativity of TOF to overpotentials are shown in Fig. 6c. So 58.5%Co₃O₄/CeO₂ possesses the strongest activity and its OER activity is also superior to the majority of other types of materials (Table S2 from ESI). It is attributed that low or appropriate loading amounts of Co₃O₄ provide many active sites, and the excess loading amounts of Co₃O₄ could occupy the surface active sites.

  EIS measurements (Fig. 6d) were performed at a potential of 0.6 V (vs Hg/HgO) to investigate the interfacial properties between the electrolyte and electrode. The semicircle of Nyquist plots in the high-frequency region represents a charge transfer process. Clearly, all X% Co₃O₄/CeO₂ composites exhibit smaller arc radii than those of pure Co₃O₄ and CeO₂ due to the electron transfer on the interface of 58.5%Co₃O₄/CeO₂ (XPS results), and the 58.5%Co₃O₄/CeO₂ possesses the smallest arc radius, illustrating the lowest charge transfer resistance, which raises the electron transfer efficiency in OER process.

  Stability is an indispensable metrics to evaluate OER catalysts. The LSV curve of 58.5%Co₃O₄/CeO₂ composites remained almost the same as the initial curve after 1 000 cycle scans (Fig. 7a). Besides, the electrochemical chronoamperometry measurement (Fig. 7b) shows that the 58.5%Co₃O₄/CeO₂ catalyst performed an outstanding stability with almost unchanged potential at a current density of 10 mA·cm^-2 for 18 h in an alkaline electrolyte. After the stability was measured, XRD patterns of the samples were still coexistence of Co 3O4 and CeO₂ (as shown in Fig.S2 from ESI). Moreover, the XPS spectra of 58.5%Co₃O₄/CeO₂ obtained after the stability test showed almost no change compared with the sample before the test, indicating that 58.5%Co₃O₄/CeO₂ possesses a good stability (as shown in Fig.S3 from ESI).

  Figure 7

  

  Figure 7.  (a) LSV curves of 58.5%Co₃O₄/CeO₂ before and after 1 000 cycles of CV; (b) Chronoamperometry curve of 58.5%Co₃O₄/CeO₂ at 10 mA·cm^-2

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  3.   Conclusions

  Co₃O₄/CeO₂ heterojunction oxides with different mass ratios were prepared by a simple two-step method (solvothermal and impregnation method). Their electrocatalytical properties for OER were measured in alkaline media. 58.5%Co₃O₄/CeO₂ composite oxides displayed excellent properties which possesses an overpotential of 347 mV and a Tafel slope of 72.7 mV·dec^-1 at the current density of 10 mA·cm^-2 and had a good stability. The effect that electrons transfer from Co₃O₄ to CeO₂ results in the increase of oxygen vacancies of CeO₂ may be the key factors to promote the OER.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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  Figure 1  Schematic illustration of the synthesis process of Co₃O₄/CeO₂ composite

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  Figure 2  XRD patterns of CeO₂, Co₃O₄ and X%Co₃O₄/CeO₂ catalysts

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  Figure 3  (a) SEM image of CeO₂; (b) SEM image, (c) elemental mappings, (d) TEM image, (e) HRTEM image and (f) SAED pattern of 58.5%Co₃O₄/CeO₂

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  Figure 4  (a) XPS survey spectra of 58.5%Co₃O₄/CeO₂; (b) High-resolution Co2p XPS spectra of Co₃O₄ and 58.5%Co₃O₄/CeO₂; High-resolution (c) Ce3d and (d) O1s XPS spectra of CeO₂ and 58.5%Co₃O₄/CeO₂

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  Figure 5  (a) LSV and (b) Tafel curves of CeO₂, Co₃O₄, RuO2 and 58.5%Co₃O₄/CeO₂; (c) LSV and (d) Tafel curves of X%Co₃O₄/CeO₂ with different loads; (e) Dependence of overpotential and Tafel slope on X%Co₃O₄/CeO₂; (f) C_dl of CeO₂, Co₃O₄ and 58.5%Co₃O₄/CeO₂

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  Figure 6  (a) C_dl of X%Co₃O₄/CeO₂; (b) Relativity of specific activity to overpotentials; (c) Relativity of the TOF to overpotentials; (d) Nyquist curves of CeO₂, Co₃O₄ and X%Co₃O₄/CeO₂

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  Figure 7  (a) LSV curves of 58.5%Co₃O₄/CeO₂ before and after 1 000 cycles of CV; (b) Chronoamperometry curve of 58.5%Co₃O₄/CeO₂ at 10 mA·cm^-2

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