Co3O4/CeO2異質結制備及其在堿性介質中電催化析氧性能

朱賽楠 王偉 陶友榮 張心羽 吳興才 張墩明

(南京大學化學化工學院，教育部介觀化學重點實驗室，配位化學國家重點實驗室，南京 210093)

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 H2owing 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,RuO2and IrO2are 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 CeO2to 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 CeO2needs to be modified by controlling morphology or modulating electronic structure (other components doping or loading)[18-20]. Kang et al. developed the Co-doped CeO2nanosheet arrays,which presented a better hydrogen evolutin reaction (HER) performance than bare CeO2with 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/CeO2heterolayered nanotube as an efficient OER catalyst with an overpotential of 300 mV at 80.2 mA·cm-2in 1.0 mol·L-1NaOH owing to the strong electron interactions between the CeO2and 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/CeO2heterojunction composite oxide was prepared by a facile two-step method for alkaline OER, and the supported Co3O4nanoparticles 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 CeO2can be also increased due to the strong surface electron interaction. The synthesized 58.5%Co3O4/CeO2composite exhibited a superior OER performance with a low overpotential of 347 mV at 10 mA·cm-2and small Tafel slope of 72.7 mV·dec-1as 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/CeO2composite 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/CeO2catalysts 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 asX%Co3O4/CeO2whereX% represented the mass ratio between Co3O4and CeO2in composite oxide(X=14.6,29.3,43.9,58.5,73.2,87.8).

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

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 (CuKαradiation,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, AlKαradiation,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-1KOH at room temperature. The reference,counter, and working electrodes were an Hg/HgO electrode (1.0 mol·L-1KOH), 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 (ERHE=EHg/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 (Cdl), as previously described in the literature[26-27]. Additionally, the electrochemical impedance spectra (EIS) were gained at 1 600 r·min-1from 10 000 to 0.01 Hz in O2-saturated 1.0 mol·L-1KOH.

2 Results and discussion

2.1 Structure and morphology of Co3O4/CeO2 catalysts

Fig.2 shows the XRD patterns of CeO2, Co3O4andX%Co3O4/CeO2catalysts. 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 allX%Co3O4/CeO2samples, which illustrates that Co3O4nanoparticles have been successfully decorated on the CeO2surfaces.Meanwhile,the characteristic peak intensity of Co3O4(311)gradually increased with the increase of Co3O4loading, indicating the augment of average crystallite size for supported Co3O4nanoparticles.

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

The detailed morphologies and structures of Cebased catalysts were analyzed by SEM and TEM techniques. Obviously, the pure CeO2with 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/CeO2composite, it is clearly observed that many Co3O4nanoparticles with a small size of about 10～30 nm were attached onto the surfaces of CeO2(Fig. 3b, 3d), and the nanocrystallines of supported Co3O4nanoparticles 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 CeO2and Co3O4and two lattice fringe spacings of 0.192 and 0.232 nm, corresponding to the(220)plane of CeO2and 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/CeO2composite(Fig.3f).

Fig.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

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/CeO2composite. The Co2pXPS spectra verify the existence of both Co3+and Co2+in pure Co3O4and 58.5%Co3O4/CeO2composite(Fig.4b), and the binding energy of the Co2porbit in the 58.5%Co3O4/CeO2composite 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 Co3O4and 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+(Tdcoordination structure)changes into Co3+(Ohstructure),it is helpful to coordination of oxygen species.

The Ce3dXPS spectra of pure CeO2and 58.5%Co3O4/CeO2are 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 CeO2surfaces[34]. Ce3d5/2peaks in 58.5%Co3O4/CeO2composite shows significant positive shift than CeO2. Moreover,the 58.5%Co3O4/CeO2composite had a smaller ratio of Ce4+/Ce3+(4.19)than that of pure CeO2(4.52)due to the partial electron transfer from Co3O4nanoparticles to CeO2surfaces. The existence of Ce3+can introduce unsaturated chemical bonds and oxygen vacancies in the CeO2crystal 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].

Fig.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

The O1sXPS spectra for CeO2and 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/CeO2exhibits 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 Co3O4nanopartides.

2.3 OER performance of Ce-based catalysts

The electrochemical performance of Co3O4, CeO2,commercial RuO2, and Co3O4/CeO2samples were tested by RDE techniques in an O2-saturated 1.0 mol·L-1KOH solution. The LSV curves of different catalysts were displayed in Fig.5a. Distinctly, the 58.5%Co3O4/CeO2sample 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/CeO2composite 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 ofX%Co3O4/CeO2were constantly increasing with the ever-reduced values of Tafel slope(Fig.5c, 5d). But excess Co3O4loading amounts (73.2%～87.8%) can decrease the OER catalytic activity,which may be related to the superfluous Co3O4nanoparticles with low intrinsic activity occupying the active sites on interface ofX%Co3O4/CeO2composites. The OER activity shows a volcanic trend with the increase of Co3O4loading amounts,as shown in Fig.5e.

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

Fig.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)Cdl of CeO2,Co3O4 and 58.5%Co3O4/CeO2

The specific activities were calculated byj/ECSA(wherejis current density), as shown in Fig.6b. The turnover frequency (TOF) was calculated by formula TOF=jA/(4Fn), wherejis current density at overpotentials of 0.36～0.40 V, andAis the geometry area of the glassy carbon electrode, andFis the Faraday constant(96 485 C·mol-1), andnis 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/CeO2possesses 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 Co3O4provide many active sites, and the excess loading amounts of Co3O4could 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, allX% Co3O4/CeO2composites exhibit smaller arc radii than those of pure Co3O4and CeO2due to the electron transfer on the interface of 58.5%Co3O4/CeO2(XPS results),and the 58.5%Co3O4/CeO2possesses 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/CeO2composites 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/CeO2catalyst performed an outstanding stability with almost unchanged potential at a current density of 10 mA·cm-2for 18 h in an alkaline electrolyte. After the stability was measured, XRD patterns of the samples were still coexistence of Co3O4and CeO2(as shown in Fig.S2 from ESI).Moreover, the XPS spectra of 58.5%Co3O4/CeO2obtained after the stability test showed almost no change compared with the sample before the test, indicating that 58.5%Co3O4/CeO2possesses a good stability (as shown in Fig.S3 from ESI).

Fig.6 (a)Cdl 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

Fig.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

3 Conclusions

Co3O4/CeO2heterojunction 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/CeO2composite oxides displayed excellent properties which possesses an overpotential of 347 mV and a Tafel slope of 72.7 mV·dec-1at the current density of 10 mA·cm-2and had a good stability. The effect that electrons transfer from Co3O4to CeO2results in the increase of oxygen vacancies of CeO2may be the key factors to promote the OER.

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