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ordered mesoporous carbons as noble-metal free electrocatalysts for oxygen reduction reaction and the influence of structure of catalyst support on the electrochemical activity of NiCo2O4
Journal of Power Sources 288 (2015) 1e8
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NiCo2O4 spinel/ordered mesoporous carbons as noble-metal free electrocatalysts for oxygen reduction reaction and the influence of structure of catalyst support on the electrochemical activity of NiCo2O4 Xiangjie Bo, Yufan Zhang, Mian Li, Anaclet Nsabimana, Liping Guo* Faculty of Chemistry, Northeast Normal University, Renmin Street 5268, Changchun 130024, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
High electrical conductivity of NiCo2O4 supported on ordered mesoporous carbons. Increase of electrocatalytic activity of NiCo2O4 by ordered mesoporous carbons. Dependence of activity of NiCo2O4 on structure of carbon support.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 January 2015 Received in revised form 24 March 2015 Accepted 16 April 2015 Available online
Three ordered mesoporous carbons (OMCs) with different structures are used as catalyst supports for growth of NiCo2O4 spinel. The high surface area of OMCs provides more active sites to adsorb metal precursors. The porous structure confines the growth of NiCo2O4 and supplies more efficient transport passage for reactant molecules to access the active sites. Due to the structural characteristics of OMCs and catalytic properties of NiCo2O4, NiCo2O4/OMCs composites are highly active, cheap, and selective noble metal-free electrocatalysts for the oxygen reduction reaction (ORR) in alkaline solution. The electrochemical activity of NiCo2O4 supported on three OMCs with different structures, surface areas, pore sizes, pore volumes, and defective sites is studied. NiCo2O4/OMCs composites may be further used as efficient and inexpensive noble metal-free ORR catalysts in alkaline solution. © 2015 Elsevier B.V. All rights reserved.
Keywords: NiCo2O4 spinel Ordered mesoporous carbons Oxygen reduction reaction Noble metal-free electrocatalysts Carbon support
1. Introduction The oxygen reduction reaction (ORR) at the cathode is an important reaction in energy converting systems such as fuel cells. In proton exchange membrane (PEM) fuel cells, the ORR kinetics are very slow at the cathode. In order to accelerate the ORR kinetics to
* Corresponding author. E-mail address: [email protected] (L. Guo). http://dx.doi.org/10.1016/j.jpowsour.2015.04.110 0378-7753/© 2015 Elsevier B.V. All rights reserved.
reach a practical usable level in a fuel cell, a cathodic ORR catalyst is needed. Generally, the ORR activity is effectively promoted by Ptbased electrocatalysts, which can generate high cathodic current densities. However, the rarity and high cost of Pt catalysts hinders the large-scale commercialization of fuel cells. In addition, the poor durability of Pt-based catalysts is another key challenge to widespread application. The loss of electrochemical surface area of Pt electrocatalysts contributes to the instability of Pt-based catalysts and thus a decrease of catalytic activity in the overall fuel cells. With the aim of accelerating the application of fuel cells and
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alleviate the cost, extensive research over the past several decades has focused on developing alternative non-precious metal catalysts [1,2]. These electrocatalysts include heteroatoms-doped carbon materials [3,4], transition metal macrocyclic compounds [5], transition metal oxides [6], transition metal chalcogenides [7], and transition metal carbides [8]. In order to increase the activity and durability of transition metal-based catalysts, carbon nanomaterials, such as carbon black [5,9], carbon nanotubes [10,11] and graphene [12], are widely used as supports for dispersion of transition metal-based catalysts. Although much progress has been achieved by using graphene as a catalyst support, the irreversible aggregation of graphene layers and low surface area of graphene limit the performance of supported catalysts. Due to the irreversible agglomeration, catalysts supported on graphene layers are easily sandwiched between aggregated graphene sheets or secluded inside the stacked layers and therefore not available for ORR catalysis. As for carbon nanotubes and carbon black, the catalysts supported on their external surface easily peel off from the surface of carbon nanotubes or carbon black due to the low interaction between the catalysts and carbon supports. So, dispersion of ORR catalysts on carbon materials with high surface area and porous structure is an effective way to increase the ORR activity of catalysts. Ordered mesoporous carbons (OMCs), different from carbon nanotubes and graphene, not only retain good electronic characteristics and chemical stability, but also show many unique properties, such as ordered porous structure, narrow pore size distribution, and high surface area [13]. The ordered porous structure provides a favorable path for mass transport, while the high surface area and a large number of mesopores of OMCs facilitate the accommodation of catalysts. OMCs doped with nitrogen [14e17], boron [18], sulfur [19], and phosphorus [20] have been used as high-performance catalysts for ORR with long-term stability and high tolerance to methanol. Due to a synergetic effect arising from nitrogen and sulfur co-doping, the sulfur and nitrogen co-doped OMCs showed a superb ORR performance without crossover effect and with a high catalytic activity in alkaline medium [21]. To increase the activity of nitrogen-doped OMCs, some researchers have investigated the addition of transition metal (Fe or Co) into nitrogen-doped OMCs, and a significantly enhanced performance has been obtained [22,23]. These novel OMCs-based ORR catalysts showed high activity for ORR process. The general methods for preparation of heteroatoms-doped carbon involve treatment of carbon with ammonia gas [24], hydrogen sulfide [25], halogen gas [26] or using heteroatom-containing organic compound as precursors during pyrolysis [15,16,20,21,27]. However, due to the evaporation of organic compounds at high temperatures under a flowing atmosphere of inert gas, the toxic nitrogencontaining gas, produced by the decomposition of organic compounds, is emitted into atmosphere and is harmful to environment and human health. Therefore, it is very important to find an environment-friendly method for preparation of OMCs-based noble metal-free catalysts for ORR without the use of toxic heteroatom-containing organic compounds or toxic gases. The spinel is a class of minerals of general formulation AB2O4. The spinel has been widely used in supercapacitors [28], lithiumion batteries [29] and electrocatalysts [30]. As noble metal-free ORR catalysts, the spinel exhibited good ORR performance with better fuel crossover resistance and long-term durability in alkaline medium [9,11,30,31]. The spinel is attractive candidate for low cost, earth-abundant electrocatalysts for ORR catalysis. However, owing to their insulating nature, their widespread application and activity has been limited. Carbon materials, with high electrical conductivity, were used as catalyst to supplement the insufficient conductivity of spinel. The combination of spinel and nanostructured
carbon materials leads to the formation of composites that can take full advantages of each kind of material and increase the ORR activity of spinel [10,11,32e36]. Herein, NiCo2O4 was selected as a model spinel to investigate its ORR activity and the effect of structure of carbon supports on ORR activity. Three OMCs (FDU-15, CMK-8, and CMK-3) with different structural parameters were selected as catalyst supports for growth of NiCo2O4 spinel. As mentioned above, the electrochemical performance of spinel is seriously hindered by its low conductivity. The spinel and OMCs exhibit different physical and chemical properties, which are complementary to each other. Therefore, the combination of spinel and OMCs into hierarchical structure is a promising method to integrate their distinguishing properties together. The accumulation of nickel in the environment may represent a serious hazard to human health [37]. However, compared with heteroatomcontaining compounds, for preparation of ORR catalysts, the toxicity of nickel is lower. So, the synthesis of the NiCo2O4/OMCs is a relatively economical method with little gas emission and little environmental harmfulness in terms of environmental implications. The electrochemical activity of NiCo2O4 supported on three OMCs with different structures, surface areas, pore volumes, pore sizes, and defective sites was studied. Although ordered mesoporous NiCo2O4 prepared by using ordered mesoporous SiO2 as template have been applied as active catalysts for the ORR [38], our methodology is special because: (1) The high electrical conductivity of OMCs can effectively supplement the insufficient conductivity of NiCo2O4 spinel and increase the ORR activity; (2) The mesopores of OMCs act as confined space, greatly restricting the growth of NiCo2O4 spinel. Compared with NiCo2O4 and OMCs, NiCo2O4/OMCs catalysts show high electrochemical performance for ORR in alkaline solution. 2. Experimental 2.1. Materials 20 wt.% Pt/C commercial electrocatalysts were purchased from Johnson Matthey. Nafion (5 wt.%) was purchased from SigmaeAldrich. Co(NO3)2$6H2O, Ni(NO3)2$6H2O, and hexamethylenetetramine were purchased from Aladdin Reagent Company. 2.2. Preparation of OMCs and NiCo2O4/OMCs Three OMCs (CMK-3, CMK-8, and FDU-15) were synthesized according to literature [39e41]. 1 g of mesoporous silica SBA-15 was added to 5 mL of water containing 1.25 g of sucrose and 0.14 g of H2SO4. The mixture was placed in a drying oven at 100 C and 160 C for 6 h, respectively. The impregnation of 0.8 g sucrose and 0.09 g of H2SO4 and the drying procedure were repeated. The product was pyrolyzed in flowing nitrogen at a temperature of 900 C. CMK-3 was obtained after the removal of SBA-15 with 5 wt.% hydrofluoric acid at room temperature. The preparation of CMK-8 is same as CMK-3 using KIT-6 as silica template. FDU-15 was prepared using triblock copolymer F127 as soft template. 1.0 g of F127 was dissolved in 20.0 g of ethanol. Then, 5.0 g of 20 wt.% resol in ethanol was added into the mixture. After stirring for 10 min, the solution was then poured into a dish to evaporate the ethanol at room temperature. The dish was placed in an oven of 100 C for 24 h to induce further polymerization. The resol/F127 composites were then pyrolyzed in nitrogen at 350 C for 2 h at a heating rate of 1 C min1 to remove the F127, and at 5 C min1 to 900 C for 4 h for further carbonization. For the preparation of NiCo2O4/OMCs, 50.0 mg of CMK-3 were dispersed in 50 mL of water. Then, 5.0 mg of Ni(NO3)2$6H2O and 10.0 mg of Co(NO3)2$6H2O were added to the suspension. After ultrasonication for 2 h, and stirring for 6 h at
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room temperature, the metal ions could diffuse into the pores of OMCs. Then the resultant sample was dried under vacuum to give a fine and completely dry powder. The powder was dispersed in 25 mL of water containing 25 mg of hexamethylenetetramine. The mixture was ultrasonicated for another 30 min and then transferred to a poly(tetrafluoroethylene)-lined autoclave (40 mL). The autoclaves were sealed and hydrothermally treated at 120 C for 6 h. The precipitates were collected by centrifugation and washed several times by double distilled water and then dried at 70 C. The black precipitates were calcined at 300 C for 2 h in the air. For comparison, the NiCo2O4 was synthesized under the same conditions without the addition of carbon supports.
3
and percentage of H2O2 calculated from RRDE can be determined by the following equation (N is RRDE collection efficiency with value of 0.37):
n¼
4 ID
ID þ INR
H2 O2 % ¼
200 IR ðID N þ IR Þ
3. Results and discussion 2.3. Apparatus X-ray diffraction (XRD) patterns were conducted by an X-ray D/ max-2200vpc (Rigaku Corporation, Japan) operating at 20 mA and 40 kV using Cu Ka radiation (k ¼ 0.15406 nm). TEM images were conducted on JEOL JEM-2100F (Japan) operating at 200 kV. SEM measurement was obtained on Philips XL-30 ESEM. Nitrogen adsorptionedesorption isotherms were performed on a Micromeritics instrument ASAP 2020. Raman spectra were obtained using a confocal microprobe Raman system (HR800, Jobin Yvon). The thermogravimetric analysis (TGA) of NiCo2O4/OMCs was performed using a TGA Q5000 V3.15 Build 263 thermogravimetric analyzer. 2.4. Electrochemical test GC electrode was pretreated carefully with 1.0, 0.3, and 0.05 mm polishing powder, and then sonicated in water and absolute ethanol during each polishing step. 3 mg of three NiCo2O4/OMCs or Pt/C was dispersed in 1 mL of Nafion (0.5 wt.%) by sonication. After dropping the catalysts onto GC electrode surface, the electrode was dried at room temperature. In our work, the loading amount of different catalysts was 0.21 mg cm2 (geometric surface area of GC). A three-electrode configuration was used, with a modified glassy carbon (GC) electrode serving as the working electrode, and platinum wire and Ag/AgCl (in saturated KCl solution) serving as the counter and reference electrodes, respectively. Cyclic voltammogram (CV, GC electrode with diameter of 3.0 mm) experiments were conducted in 0.1 M KOH solution saturated with oxygen for ORR at 50 mV s1. Electrochemical impedance spectroscopy (EIS) was carried out on Par 2273 Potentiostats-Electrochemistry Workstation in 0.1 M KOH solution saturated with oxygen from 0.1 Hz to 10.0 KHz. The rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) experiments were performed using a ring-disk electrode (5.61 mm of disk outer diameter, Pine Instrument Co.) using CHI 750 potentiostat. The electron number (n) transferred in ORR process was calculated using the KouteckyeLevich equation:
1 1 1 ¼ þ J Jk Bu0:5 where Jk is the kinetic current density and B is related to the diffusional current density. B is related to the diffusion-limiting current density expressed using the following expression:
B ¼ 0:2nFD2=3 n1=6 CO2 where n represents the kinematic viscosity of the electrolyte (1.13 102 cm2 s1); n is the electron number exchanged during ORR process; the Faraday constant is represented by F (96,485C mol1); CO2 is the concentration of oxygen (1.2 103 mol L1); D is the oxygen diffusion coefficient (1.9 105 cm2 s1). The n values
The morphology and structure of CMK-3, CMK-8, and FDU-15 were examined by TEM and SEM (Fig. 1). TEM image of CMK-3 shows highly ordered carbon nanowires and the hexagonal honeycomb-like arrangement of mesopores (Fig. 1A). The structure of CMK-3 inversely replicates the structure of template SBA-15 [42]. SEM observation indicates that the CMK-3 is made of rod-like particles (Fig. 1D). The highly regular pore structure of CMK-8 is clearly visible in Fig. 1B. The TEM image of FDU-15 also exhibits the ordered structure of carbon wires (Fig. 1C). Unlike the morphology of CMK-3, the SEM image of CMK-8 (Fig. 1E) and FDU-15 (Fig. 1F) exhibits unshaped morphology. CMK-8 has an ordered pore arrangement with cubic Ia3d symmetry [40]. Both CMK-3 and FDU15 exhibit highly ordered 2D hexagonal (p6mm) mesostructure [43]. However, the structure of FDU-15 is isostructural to SBA-15. The structure of CMK-3 is exactly an inverse replica of their mother mold SBA-15. The structure of CMK-3 is reciprocal reverse replica of FDU-15. The mesoporous nature of CMK-3, CMK-8, and FDU-15 was studied by nitrogen adsorption isotherm. Fig. 2 shows the nitrogen adsorptionedesorption isotherms of CMK-3, CMK-8, and FDU-15. The sorption isotherms of CMK-3, CMK-8, and FDU-15 are type IV isotherm with a pronounced capillary condensation step, characteristic of high-quality mesoporous material. The structural parameters of CMK-3, CMK-8, and FDU-15 are summarized in Table S1. The BET specific surface areas of CMK-3, CMK-8, and FDU-15 are 1154.7, 635.5, and 589.1 m2 g1, respectively. The BET surface area of CMK-3 is higher than those of CMK-8 and FDU-15. The pore size distributions of three OMCs are shown in inset of Fig. 2. CMK-3, CMK-8, and FDU-15 have narrow pore size distributions centered at 4.8, 6.7, and 3.7 nm, respectively. The high specific surface area, large pore volume, and porous structure can provide more active sites to adsorb reactant molecules and facilitate accessibility of reactant. Raman spectra were used to study the carbon structure of CMK3, CMK-8, and FDU-15 (Fig. S1). Raman spectra of CMK-3, CMK-8, and FDU-15 show two characteristic peaks located at around 1595 (G-band) and 1412 cm1 (D-band), which correspond to the E2g mode of sp2 carbon atoms and the disordered carbon in the carbon lattice, respectively. The relative peak intensity ratio of the D-band to G-band (ID/IG) for the CMK-3 (2.23) is larger than those of CMK-8 (1.50) and FDU-15 (1.75), indicating the presence of more defects into the framework of CMK-3. The influence of deposition of NiCo2O4 on the structures of OMCs was investigated by small-angle XRD. Here, the sample of NiCo2O4/CMK-3 is taken as an example. Fig. S2A displays the typical small-angle XRD patterns of CMK-3 and NiCo2O4/CMK-3. The synthesized CMK-3, as is evident from the presence of three XRD peaks at 2q of 0.9, 1.5, and 1.8 , is replica of the parent template SBA-15. After the growth of NiCo2O4, the disappearance of three XRD peaks indicates the deterioration of the ordered mesoporous structure. The decreased intensity for NiCo2O4/CMK-3 diffraction
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Fig. 1. TEM images of (A) CMK-3, (B) CMK-8, and (C) FDU-15. SEM images of (D) CMK-3, (E) CMK-8, and (F) FDU-15.
peaks is related to the growth of NiCo2O4 in the framework of CMK3. Two broad diffraction peaks at 2q of 23.4 and 44.5 are observed at CMK-3, corresponding to the (002) and (101) diffractions of graphite (Fig. S2B). The wide-angle XRD pattern of NiCo2O4/CMK-3 exhibits the characteristic peaks for NiCo2O4 (JCPDS: 20-0781). These peaks correspond to the planes (111), (220), (311), (222), (400), (422), (511), and (440) at 2q of around 18.9, 31.1, 36.7, 38.4, 44.6, 55.4, 59.1, and 65.0 , respectively. This result confirms the formation of NiCo2O4/CMK-3 composites. TEM analyses were conducted to determine the microstructure of NiCo2O4 supported on CMK-3, CMK-8, and FDU-15. Fig. 3A exhibits that well-dispersed NiCo2O4 particles are facilely supported on the framework of the CMK-3. Compared with the pristine CMK3, the pore channels are obscure to a certain degree after the growth of NiCo2O4 spinel, which reveals that NiCo2O4 has been assembled on the framework of CMK-3. However, the ordered structure of CMK-3 can also be observed from the TEM images of NiCo2O4/CMK-3. The SEM image of NiCo2O4 without the use of OMCs (Fig. S3) shows the NiCo2O4 was mostly irregular aggregates
Fig. 2. Nitrogen adsorptionedesorption isotherms of CMK-3, CMK-8, and FDU-15. Inset: the pore size distributions of CMK-3, CMK-8, and FDU-15.
of particles. From the TEM image of NiCo2O4/CMK-3, less aggregation of NiCo2O4 is observed on the surface of CMK-3. On counting 100 particles, the average diameter is found to be 5.7 nm, as shown in the histogram in inset of Fig. 3A. The large surface area of CMK-3 provides many active sites for the adsorption of metal precursors. The mesopores of CMK-3 act as confined space for the growth of NiCo2O4 spinel. The mesoporous structure restricts the particle growth during hydrothermal, resulting in less aggregation of NiCo2O4. From the TEM images of NiCo2O4/CMK-8 (Fig. 3B) and NiCo2O4/FDU-15 (Fig. 3C), the NiCo2O4 with the mean size of 7.9 and 8.6 nm are dispersed on the framework of CMK-8 and FDU-15, respectively. Compared with CMK-8 and FDU-15, the large surface area and pore volume of CMK-3 facilitate the formation of smaller NiCo2O4 particles. The content of NiCo2O4 supported on CMK-3, CMK-8, and FDU-15 was determined by TGA (Fig. S4). The content of NiCo2O4 on CMK-3, CMK-8, and FDU-15 was calculated to be 25.2%, 27.2%, and 26.1%, respectively. EIS was used to study the electron-transfer kinetics of the NiCo2O4/OMCs. Here, the sample of NiCo2O4/CMK-3 is also taken as an example. Fig. S5 shows the EIS results of NiCo2O4/CMK-3 and NiCo2O4 in oxygen-saturated 0.1 M KOH. Obviously, NiCo2O4 catalysts exhibit a semicircle portion with a large diameter, reflecting large electron-transfer resistance at the interface. In contrast, NiCo2O4/CMK-3 catalysts show a very low electron-transfer resistance, suggesting that the electrical conductivity of NiCo2O4 was increased by CMK-3 support. The electrocatalytic activity of NiCo2O4/OMCs towards oxygen was investigated by CVs in oxygen-saturated 0.1 M KOH. Fig. 4A shows the voltammetric response corresponding to the electrocatalytic reduction of oxygen at CMK-3 (green), NiCo2O4 (blue), NiCo2O4/CMK-3 (black), and Pt/C (red) catalysts. The Pt/C catalysts show a reduction peak at 0.17 V for oxygen. A cathodic peak for ORR around 0.28 V is observed at CMK-3 (green). NiCo2O4 catalysts exhibit a substantial reduction process in the presence of oxygen with the cathodic reduction peak at around 0.29 V. NiCo2O4/CMK-3 catalysts show an efficient reduction process in the presence of oxygen with a reduction peak at around 0.20 V. The comparison between NiCo2O4/CMK-3 and CMK-3 indicates that an enhancement of peak current density and a significant positive shift of peak potential are visible for NiCo2O4/CMK-3. The enhancement in response and positive reduction potential obtained at NiCo2O4/CMK-3 reflects that NiCo2O4/CMK-3 catalysts
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Fig. 3. TEM images of (A) NiCo2O4/CMK-3, (B) NiCo2O4/CMK-8, and (C) NiCo2O4/FDU-15.
Fig. 4. (A) CVs and (B) LSV curves of Pt/C (red), NiCo2O4/CMK-3 (black), CMK-3 (green), and NiCo2O4 (blue) in oxygen-saturated 0.1 M KOH. Scan rate of CVs: 50 mV s1. Scan rate of LSVs: 5 mV s1. Rotation rate: 1600 rpm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
show better electrocatalytic activity towards ORR than CMK-3. Although deterioration of the ordered mesoporous structure was observed after incorporation of the NiCo2O4, the NiCo2O4/CMK-3 catalysts show improved activity compared to CMK-3. The comparison of electrochemical reduction of oxygen between NiCo2O4/ CMK-3 and NiCo2O4 reveals the use of CMK-3 support can promote ORR activity. The high electrical conductivity of CMK-3 can effectively supplement the insufficient conductivity of NiCo2O4 spinel and increase the ORR activity. The combination of catalytic properties of NiCo2O4 and mesoporous structure of CMK-3 contributes to the high activity of NiCo2O4/CMK-3. The comparison of ORR activity for different catalysts by linear sweep voltammetry (LSV) at RDE is shown in Fig. 4B. In agreement with the voltammetric result, NiCo2O4 and CMK-3 catalysts show low activity for ORR. As shown in Fig. 4B, the onset potential of ORR at CMK-3 catalysts commences at around 0.20 V, whereas the onset potential at NiCo2O4/CMK-3 catalysts significantly shifts positively to 0.09 V. The limiting diffusion current density for NiCo2O4/CMK-3 catalysts is larger than those of CMK-3 and NiCo2O4 catalysts. The electrochemical activity of NiCo2O4 supported on three OMCs with different surface areas, pore sizes, defective sites, and pore volumes was studied. Fig. 5A compares the effect of different catalyst supports on activity for ORR in 0.1 M KOH. Compared with NiCo2O4/CMK-8 and NiCo2O4/FDU-15, an obvious increase of catalytic activity is found at NiCo2O4/CMK-3. The cathodic current is about 0.93 mA cm2 at NiCo2O4/CMK-3, compared to 0.67 mA cm2 at NiCo2O4/CMK-8 and 0.61 mA cm2 at NiCo2O4/FDU-15.
The electrochemical data in Fig. 5B and Table S2 also reflect the superior performance of CMK-3 over CMK-8 and FDU-15 as a catalyst support for NiCo2O4. There is a minor discrepancy in the content of NiCo2O4 supported on CMK-3, CMK-8, and FDU-15. The influence of the content of NiCo2O4 supported on different supports can be omitted. The influence of the structures and properties of carbon supports on the ORR catalytic activities of NiCo2O4 was studied in Fig. 6. The current density of cathodic peak at 0.50 V in LSVs for three catalysts is plotted against the surface area, pore size, pore volume, mean size of NiCo2O4 particle, and the value of ID/IG. These plots indicate that the there is no dependence of current density on the pore size and the value of ID/IG. However, the current density of catalysts is dependent on the surface area, pore volume, and mean size of NiCo2O4 particle. From the nitrogen adsorption isotherms in Fig. 2, the BET surface area of CMK-3 is larger than those of CMK-8 and FDU-15. The low surface area of CMK-8 and FDU-15 is not favorable for the adsorption of metal precursors. The high surface area of CMK-3 facilitates the adsorption of metal precursors. The large pore volume provides accommodation for NiCo2O4 deposition inside the pore channels. The activity of NiCo2O4/OMCs increases with the decrease of average particle size. The large surface area and pore volume favor the formation of smaller NiCo2O4 particles. Smaller NiCo2O4 particles provide more surface active sites for ORR catalysis. The combination of higher surface area, larger pore volume, and smaller NiCo2O4 particle enable the high activity of NiCo2O4/CMK-3. On the basis of above optimized data, NiCo2O4/CMK-3 was selected for subsequent
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Fig. 5. (A) CVs and (B) LSVs of NiCo2O4/CMK-3 (solid), NiCo2O4/CMK-8 (short dot), and NiCo2O4/FDU-15 (dash) in oxygen-saturated 0.1 M KOH. Scan rate of CVs: 50 mV s1. Scan rate of LSVs: 5 mV s1. Rotation rate: 1600 rpm.
Fig. 6. The effect of BET surface area of three OMCs (black), pore size of three OMCs (green), pore volume of three OMCs (blue), mean size of NiCo2O4 particle (red), and the value of ID/IG for three OMCs (magenta) on the current density of NiCo2O4 catalysts at 0.50 V of LSV curves. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. (A) LSV curves at different rotation rates recorded for ORR at NiCo2O4/CMK-3 in oxygen-saturated 0.1 M KOH. Scan rate: 5 mV s1. Rotation rate: 100, 400, 900, and 1600 rpm. (B) KouteckyeLevich plots of NiCo2O4/CMK-3 at different potentials. (C) The dependence of electron transfer number n on potential for NiCo2O4/CMK-3 (black) and Pt/C (red). (D) RRDE curves of NiCo2O4/CMK-3 (black) and Pt/C (red) in oxygen-saturated 0.1 M KOH. Scan rate: 5 mV s1. Rotation rate: 1600 rpm. Ring potential: 0.5 V. (E) The dependence of electron transfer number n on potential for NiCo2O4/CMK-3 (black) and Pt/C (red) estimated from RRDE data. (F) The H2O2 generation for NiCo2O4/CMK-3 (black) and Pt/C (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 8. CVs of (A) NiCo2O4/CMK-3 and (B) Pt/C in oxygen-saturated 0.1 M KOH (solid line) and in the presence of 3 M methanol (short dot line). LSV curves of (C) NiCo2O4/CMK-3 and (D) Pt/C in oxygen-saturated 0.1 M KOH (solid line) and in the presence of 3 M methanol (short dot line). Scan rate: 5 mV s1. Rotation rate: 1600 rpm.
experiment. Fig. 7A shows LSVs on NiCo2O4/CMK-3 catalysts in oxygensaturated 0.1 M KOH at different rotation rates. The increase of rotation rate is accompanied with the increase of the diffusion current density for NiCo2O4/CMK-3. The KouteckyeLevich plots (J1 vs. u1/2) at different potentials show good linearity (Fig. 7B). The data in Fig. 7C show the dependence of electron transfer number n on potential at NiCo2O4/CMK-3. The value of n was calculated to be about 3.80e3.85 from 0.25 to 0.60 V according to the slopes of KouteckyeLevich plots in Fig. 7B, reflecting that the ORR follows mainly 4 electron process. Fig. S6 shows LSVs and KouteckyeLevich plots of Pt/C catalysts. The value of n estimated from RRDE also indicates that the value of n at NiCo2O4/CMK-3 is around 3.80e3.90 from 0.20 to 0.80 V (Fig. 7E). From the data in Fig. 7F, hydrogen peroxide yield (H2O2%) during ORR is lower than 10%. The low generation of hydrogen peroxide and approximately four-electron transfer reflect that oxygen is directly reduced to OH ions through four-electron transfer process in alkaline solution. Fig. 8 shows CVs of NiCo2O4/CMK-3 in oxygen-saturated 0.1 M KOH in the presence (short dot line) or absence (solid line) of 3 M methanol. The similarity between the voltammetric responses for NiCo2O4/CMK-3 with 3 M methanol suggests that NiCo2O4/CMK-3 catalysts show a good selectivity for ORR (Fig. 8A). In contrast, the disappearance of ORR current and obvious oxidation of methanol at Pt/C indicates that ORR process is seriously retarded by methanol (Fig. 8B). Compared with the commercial Pt/C catalysts, the better methanol tolerance of NiCo2O4/CMK-3 towards ORR indicates that NiCo2O4/CMK-3 may be developed as methanol-tolerant ORR catalysts in fuel cells. The comparison between Fig. 8C and D also indicates the better tolerance to methanol at NiCo2O4/CMK-3 than at Pt/C. The long-term stability of NiCo2O4/CMK-3 catalysts for ORR was investigated in 0.1 M KOH solution. Pt/C catalysts show a rapid decay of the current depression (~37%) after 30,000 s, reflecting a low stability for ORR (Fig. S7). In contrast, the response of NiCo2O4/
CMK-3 catalysts retains acceptable stability during the ORR catalysis, with only 25% current diminutions after 30,000 s. This comparative result indicates the better durability of NiCo2O4/CMK3 compared with Pt/C. This result indicates that NiCo2O4/CMK-3 is a good alternative to Pt as cathode catalysts. 4. Conclusion OMCs with large surface areas and pore volumes were used as a catalyst support for NiCo2O4 spinel in ORR. The NiCo2O4/OMCs catalysts bring together the structural property of OMCs and catalytic property of NiCo2O4. NiCo2O4/OMCs composites are highly active, cheap, and selective noble metal-free electrocatalysts for ORR in alkaline solution. From the comparative study of different catalyst supports, the ORR activity of NiCo2O4 is dependent on the surface area and pore volume of catalyst supports. Acknowledgments The authors gratefully acknowledge China Postdoctoral Science Foundation funded project (2014M550164), the Science and Technology Development Planning of Jilin Province (20150520014JH) and the Fundamental Research Funds for the Central Universities (No. 14QNJJ011). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.04.110. References [1] D.-W. Wang, D. Su, Energy Environ. Sci. 7 (2014) 576e591. [2] A. Morozan, B. Jousselme, S. Palacin, Energy Environ. Sci. 4 (2011) 1238e1254. [3] Y. Zheng, Y. Jiao, M. Jaroniec, Y. Jin, S.Z. Qiao, Small 8 (2012) 3550e3566.
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[24] X. Wang, J.S. Lee, Q. Zhu, J. Liu, Y. Wang, S. Dai, Chem. Mater. 22 (2010) 2178e2180. [25] S. Yang, L. Zhi, K. Tang, X. Feng, J. Maier, K. Müllen, Adv. Funct. Mater. 22 (2012) 3634e3640. [26] I.Y. Jeon, H.J. Choi, M. Choi, J.M. Seo, S.M. Jung, M.J. Kim, S. Zhang, L. Zhang, Z. Xia, L. Dai, N. Park, J.B. Baek, Sci. Rep. 3 (2013) 1810. [27] Y. Zheng, Y. Jiao, J. Chen, J. Liu, J. Liang, A. Du, W. Zhang, Z. Zhu, S.C. Smith, M. Jaroniec, G.Q. Lu, S.Z. Qiao, J. Am. Chem. Soc. 133 (2011) 20116e20119. [28] Y. Lei, J. Li, Y. Wang, L. Gu, Y. Chang, H. Yuan, D. Xiao, ACS Appl. Mater. Interfaces 6 (2014) 1773e1780. [29] G. Zhang, B.Y. Xia, C. Xiao, L. Yu, X. Wang, Y. Xie, X.W. Lou, Angew. Chem. Int. Ed. 52 (2013) 8643e8647. [30] Y. Xiao, C. Hu, L. Qu, C. Hu, M. Cao, Chem. Eur. J. 19 (2013) 14271e14278. [31] C. Jin, F. Lu, X. Cao, Z. Yang, R. Yang, J. Mater. Chem. A 1 (2013) 12170e12177. [32] D.U. Lee, B.J. Kim, Z. Chen, J. Mater. Chem. A 1 (2013) 4754e4762. [33] W. Bian, Z. Yang, P. Strasser, R. Yang, J. Power Sources 250 (2014) 196e203. [34] Z. Pu, Q. Liu, C. Tang, A.M. Asiri, A.H. Qusti, A.O. Al-Youbi, X. Sun, J. Power Sources 257 (2014) 170e173. [35] R. Ning, J. Tian, A.M. Asiri, A.H. Qusti, A.O. Al-Youbi, X. Sun, Langmuir 29 (2013) 13146e13151. [36] X. Ge, Y. Liu, F.W.T. Goh, T.S.A. Hor, Y. Zong, P. Xiao, Z. Zhang, S.H. Lim, B. Li, X. Wang, Z. Liu, ACS Appl. Mater. Interfaces 6 (2014) 12684e12691. [37] E. Denkhaus, K. Salnikow, Crit. Rev. Oncol. Hematol. 42 (2002) 35e56. [38] Y. Li, L. Zou, J. Li, K. Guo, X. Dong, X. Li, X. Xue, H. Zhang, H. Yang, Electrochim. Acta 129 (2014) 14e20. [39] S. Jun, S.H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc. 122 (2000) 10712e10713. [40] F. Kleitz, S. Hei Choi, R. Ryoo, Chem. Commun. (2003) 2136e2137. [41] Z. Wu, P.A. Webley, D. Zhao, Langmuir 26 (2010) 10277e10286. [42] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548e552. [43] C. He, Y. Liang, R. Fu, D. Wu, S. Song, R. Cai, J. Mater. Chem. 21 (2011) 16357e16364.
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NiCo2O4 spinel/ordered mesoporous carbons as noble-metal free electrocatalysts for oxygen reduction reaction and the influence of structure of catalyst support on the electrochemical activity of NiCo2O4 Xiangjie Bo, Yufan Zhang, Mian Li, Anaclet Nsabimana, Liping Guo* Faculty of Chemistry, Northeast Normal University, Renmin Street 5268, Changchun 130024, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
High electrical conductivity of NiCo2O4 supported on ordered mesoporous carbons. Increase of electrocatalytic activity of NiCo2O4 by ordered mesoporous carbons. Dependence of activity of NiCo2O4 on structure of carbon support.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 January 2015 Received in revised form 24 March 2015 Accepted 16 April 2015 Available online
Three ordered mesoporous carbons (OMCs) with different structures are used as catalyst supports for growth of NiCo2O4 spinel. The high surface area of OMCs provides more active sites to adsorb metal precursors. The porous structure confines the growth of NiCo2O4 and supplies more efficient transport passage for reactant molecules to access the active sites. Due to the structural characteristics of OMCs and catalytic properties of NiCo2O4, NiCo2O4/OMCs composites are highly active, cheap, and selective noble metal-free electrocatalysts for the oxygen reduction reaction (ORR) in alkaline solution. The electrochemical activity of NiCo2O4 supported on three OMCs with different structures, surface areas, pore sizes, pore volumes, and defective sites is studied. NiCo2O4/OMCs composites may be further used as efficient and inexpensive noble metal-free ORR catalysts in alkaline solution. © 2015 Elsevier B.V. All rights reserved.
Keywords: NiCo2O4 spinel Ordered mesoporous carbons Oxygen reduction reaction Noble metal-free electrocatalysts Carbon support
1. Introduction The oxygen reduction reaction (ORR) at the cathode is an important reaction in energy converting systems such as fuel cells. In proton exchange membrane (PEM) fuel cells, the ORR kinetics are very slow at the cathode. In order to accelerate the ORR kinetics to
* Corresponding author. E-mail address: [email protected] (L. Guo). http://dx.doi.org/10.1016/j.jpowsour.2015.04.110 0378-7753/© 2015 Elsevier B.V. All rights reserved.
reach a practical usable level in a fuel cell, a cathodic ORR catalyst is needed. Generally, the ORR activity is effectively promoted by Ptbased electrocatalysts, which can generate high cathodic current densities. However, the rarity and high cost of Pt catalysts hinders the large-scale commercialization of fuel cells. In addition, the poor durability of Pt-based catalysts is another key challenge to widespread application. The loss of electrochemical surface area of Pt electrocatalysts contributes to the instability of Pt-based catalysts and thus a decrease of catalytic activity in the overall fuel cells. With the aim of accelerating the application of fuel cells and
2
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alleviate the cost, extensive research over the past several decades has focused on developing alternative non-precious metal catalysts [1,2]. These electrocatalysts include heteroatoms-doped carbon materials [3,4], transition metal macrocyclic compounds [5], transition metal oxides [6], transition metal chalcogenides [7], and transition metal carbides [8]. In order to increase the activity and durability of transition metal-based catalysts, carbon nanomaterials, such as carbon black [5,9], carbon nanotubes [10,11] and graphene [12], are widely used as supports for dispersion of transition metal-based catalysts. Although much progress has been achieved by using graphene as a catalyst support, the irreversible aggregation of graphene layers and low surface area of graphene limit the performance of supported catalysts. Due to the irreversible agglomeration, catalysts supported on graphene layers are easily sandwiched between aggregated graphene sheets or secluded inside the stacked layers and therefore not available for ORR catalysis. As for carbon nanotubes and carbon black, the catalysts supported on their external surface easily peel off from the surface of carbon nanotubes or carbon black due to the low interaction between the catalysts and carbon supports. So, dispersion of ORR catalysts on carbon materials with high surface area and porous structure is an effective way to increase the ORR activity of catalysts. Ordered mesoporous carbons (OMCs), different from carbon nanotubes and graphene, not only retain good electronic characteristics and chemical stability, but also show many unique properties, such as ordered porous structure, narrow pore size distribution, and high surface area [13]. The ordered porous structure provides a favorable path for mass transport, while the high surface area and a large number of mesopores of OMCs facilitate the accommodation of catalysts. OMCs doped with nitrogen [14e17], boron [18], sulfur [19], and phosphorus [20] have been used as high-performance catalysts for ORR with long-term stability and high tolerance to methanol. Due to a synergetic effect arising from nitrogen and sulfur co-doping, the sulfur and nitrogen co-doped OMCs showed a superb ORR performance without crossover effect and with a high catalytic activity in alkaline medium [21]. To increase the activity of nitrogen-doped OMCs, some researchers have investigated the addition of transition metal (Fe or Co) into nitrogen-doped OMCs, and a significantly enhanced performance has been obtained [22,23]. These novel OMCs-based ORR catalysts showed high activity for ORR process. The general methods for preparation of heteroatoms-doped carbon involve treatment of carbon with ammonia gas [24], hydrogen sulfide [25], halogen gas [26] or using heteroatom-containing organic compound as precursors during pyrolysis [15,16,20,21,27]. However, due to the evaporation of organic compounds at high temperatures under a flowing atmosphere of inert gas, the toxic nitrogencontaining gas, produced by the decomposition of organic compounds, is emitted into atmosphere and is harmful to environment and human health. Therefore, it is very important to find an environment-friendly method for preparation of OMCs-based noble metal-free catalysts for ORR without the use of toxic heteroatom-containing organic compounds or toxic gases. The spinel is a class of minerals of general formulation AB2O4. The spinel has been widely used in supercapacitors [28], lithiumion batteries [29] and electrocatalysts [30]. As noble metal-free ORR catalysts, the spinel exhibited good ORR performance with better fuel crossover resistance and long-term durability in alkaline medium [9,11,30,31]. The spinel is attractive candidate for low cost, earth-abundant electrocatalysts for ORR catalysis. However, owing to their insulating nature, their widespread application and activity has been limited. Carbon materials, with high electrical conductivity, were used as catalyst to supplement the insufficient conductivity of spinel. The combination of spinel and nanostructured
carbon materials leads to the formation of composites that can take full advantages of each kind of material and increase the ORR activity of spinel [10,11,32e36]. Herein, NiCo2O4 was selected as a model spinel to investigate its ORR activity and the effect of structure of carbon supports on ORR activity. Three OMCs (FDU-15, CMK-8, and CMK-3) with different structural parameters were selected as catalyst supports for growth of NiCo2O4 spinel. As mentioned above, the electrochemical performance of spinel is seriously hindered by its low conductivity. The spinel and OMCs exhibit different physical and chemical properties, which are complementary to each other. Therefore, the combination of spinel and OMCs into hierarchical structure is a promising method to integrate their distinguishing properties together. The accumulation of nickel in the environment may represent a serious hazard to human health [37]. However, compared with heteroatomcontaining compounds, for preparation of ORR catalysts, the toxicity of nickel is lower. So, the synthesis of the NiCo2O4/OMCs is a relatively economical method with little gas emission and little environmental harmfulness in terms of environmental implications. The electrochemical activity of NiCo2O4 supported on three OMCs with different structures, surface areas, pore volumes, pore sizes, and defective sites was studied. Although ordered mesoporous NiCo2O4 prepared by using ordered mesoporous SiO2 as template have been applied as active catalysts for the ORR [38], our methodology is special because: (1) The high electrical conductivity of OMCs can effectively supplement the insufficient conductivity of NiCo2O4 spinel and increase the ORR activity; (2) The mesopores of OMCs act as confined space, greatly restricting the growth of NiCo2O4 spinel. Compared with NiCo2O4 and OMCs, NiCo2O4/OMCs catalysts show high electrochemical performance for ORR in alkaline solution. 2. Experimental 2.1. Materials 20 wt.% Pt/C commercial electrocatalysts were purchased from Johnson Matthey. Nafion (5 wt.%) was purchased from SigmaeAldrich. Co(NO3)2$6H2O, Ni(NO3)2$6H2O, and hexamethylenetetramine were purchased from Aladdin Reagent Company. 2.2. Preparation of OMCs and NiCo2O4/OMCs Three OMCs (CMK-3, CMK-8, and FDU-15) were synthesized according to literature [39e41]. 1 g of mesoporous silica SBA-15 was added to 5 mL of water containing 1.25 g of sucrose and 0.14 g of H2SO4. The mixture was placed in a drying oven at 100 C and 160 C for 6 h, respectively. The impregnation of 0.8 g sucrose and 0.09 g of H2SO4 and the drying procedure were repeated. The product was pyrolyzed in flowing nitrogen at a temperature of 900 C. CMK-3 was obtained after the removal of SBA-15 with 5 wt.% hydrofluoric acid at room temperature. The preparation of CMK-8 is same as CMK-3 using KIT-6 as silica template. FDU-15 was prepared using triblock copolymer F127 as soft template. 1.0 g of F127 was dissolved in 20.0 g of ethanol. Then, 5.0 g of 20 wt.% resol in ethanol was added into the mixture. After stirring for 10 min, the solution was then poured into a dish to evaporate the ethanol at room temperature. The dish was placed in an oven of 100 C for 24 h to induce further polymerization. The resol/F127 composites were then pyrolyzed in nitrogen at 350 C for 2 h at a heating rate of 1 C min1 to remove the F127, and at 5 C min1 to 900 C for 4 h for further carbonization. For the preparation of NiCo2O4/OMCs, 50.0 mg of CMK-3 were dispersed in 50 mL of water. Then, 5.0 mg of Ni(NO3)2$6H2O and 10.0 mg of Co(NO3)2$6H2O were added to the suspension. After ultrasonication for 2 h, and stirring for 6 h at
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room temperature, the metal ions could diffuse into the pores of OMCs. Then the resultant sample was dried under vacuum to give a fine and completely dry powder. The powder was dispersed in 25 mL of water containing 25 mg of hexamethylenetetramine. The mixture was ultrasonicated for another 30 min and then transferred to a poly(tetrafluoroethylene)-lined autoclave (40 mL). The autoclaves were sealed and hydrothermally treated at 120 C for 6 h. The precipitates were collected by centrifugation and washed several times by double distilled water and then dried at 70 C. The black precipitates were calcined at 300 C for 2 h in the air. For comparison, the NiCo2O4 was synthesized under the same conditions without the addition of carbon supports.
3
and percentage of H2O2 calculated from RRDE can be determined by the following equation (N is RRDE collection efficiency with value of 0.37):
n¼
4 ID
ID þ INR
H2 O2 % ¼
200 IR ðID N þ IR Þ
3. Results and discussion 2.3. Apparatus X-ray diffraction (XRD) patterns were conducted by an X-ray D/ max-2200vpc (Rigaku Corporation, Japan) operating at 20 mA and 40 kV using Cu Ka radiation (k ¼ 0.15406 nm). TEM images were conducted on JEOL JEM-2100F (Japan) operating at 200 kV. SEM measurement was obtained on Philips XL-30 ESEM. Nitrogen adsorptionedesorption isotherms were performed on a Micromeritics instrument ASAP 2020. Raman spectra were obtained using a confocal microprobe Raman system (HR800, Jobin Yvon). The thermogravimetric analysis (TGA) of NiCo2O4/OMCs was performed using a TGA Q5000 V3.15 Build 263 thermogravimetric analyzer. 2.4. Electrochemical test GC electrode was pretreated carefully with 1.0, 0.3, and 0.05 mm polishing powder, and then sonicated in water and absolute ethanol during each polishing step. 3 mg of three NiCo2O4/OMCs or Pt/C was dispersed in 1 mL of Nafion (0.5 wt.%) by sonication. After dropping the catalysts onto GC electrode surface, the electrode was dried at room temperature. In our work, the loading amount of different catalysts was 0.21 mg cm2 (geometric surface area of GC). A three-electrode configuration was used, with a modified glassy carbon (GC) electrode serving as the working electrode, and platinum wire and Ag/AgCl (in saturated KCl solution) serving as the counter and reference electrodes, respectively. Cyclic voltammogram (CV, GC electrode with diameter of 3.0 mm) experiments were conducted in 0.1 M KOH solution saturated with oxygen for ORR at 50 mV s1. Electrochemical impedance spectroscopy (EIS) was carried out on Par 2273 Potentiostats-Electrochemistry Workstation in 0.1 M KOH solution saturated with oxygen from 0.1 Hz to 10.0 KHz. The rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) experiments were performed using a ring-disk electrode (5.61 mm of disk outer diameter, Pine Instrument Co.) using CHI 750 potentiostat. The electron number (n) transferred in ORR process was calculated using the KouteckyeLevich equation:
1 1 1 ¼ þ J Jk Bu0:5 where Jk is the kinetic current density and B is related to the diffusional current density. B is related to the diffusion-limiting current density expressed using the following expression:
B ¼ 0:2nFD2=3 n1=6 CO2 where n represents the kinematic viscosity of the electrolyte (1.13 102 cm2 s1); n is the electron number exchanged during ORR process; the Faraday constant is represented by F (96,485C mol1); CO2 is the concentration of oxygen (1.2 103 mol L1); D is the oxygen diffusion coefficient (1.9 105 cm2 s1). The n values
The morphology and structure of CMK-3, CMK-8, and FDU-15 were examined by TEM and SEM (Fig. 1). TEM image of CMK-3 shows highly ordered carbon nanowires and the hexagonal honeycomb-like arrangement of mesopores (Fig. 1A). The structure of CMK-3 inversely replicates the structure of template SBA-15 [42]. SEM observation indicates that the CMK-3 is made of rod-like particles (Fig. 1D). The highly regular pore structure of CMK-8 is clearly visible in Fig. 1B. The TEM image of FDU-15 also exhibits the ordered structure of carbon wires (Fig. 1C). Unlike the morphology of CMK-3, the SEM image of CMK-8 (Fig. 1E) and FDU-15 (Fig. 1F) exhibits unshaped morphology. CMK-8 has an ordered pore arrangement with cubic Ia3d symmetry [40]. Both CMK-3 and FDU15 exhibit highly ordered 2D hexagonal (p6mm) mesostructure [43]. However, the structure of FDU-15 is isostructural to SBA-15. The structure of CMK-3 is exactly an inverse replica of their mother mold SBA-15. The structure of CMK-3 is reciprocal reverse replica of FDU-15. The mesoporous nature of CMK-3, CMK-8, and FDU-15 was studied by nitrogen adsorption isotherm. Fig. 2 shows the nitrogen adsorptionedesorption isotherms of CMK-3, CMK-8, and FDU-15. The sorption isotherms of CMK-3, CMK-8, and FDU-15 are type IV isotherm with a pronounced capillary condensation step, characteristic of high-quality mesoporous material. The structural parameters of CMK-3, CMK-8, and FDU-15 are summarized in Table S1. The BET specific surface areas of CMK-3, CMK-8, and FDU-15 are 1154.7, 635.5, and 589.1 m2 g1, respectively. The BET surface area of CMK-3 is higher than those of CMK-8 and FDU-15. The pore size distributions of three OMCs are shown in inset of Fig. 2. CMK-3, CMK-8, and FDU-15 have narrow pore size distributions centered at 4.8, 6.7, and 3.7 nm, respectively. The high specific surface area, large pore volume, and porous structure can provide more active sites to adsorb reactant molecules and facilitate accessibility of reactant. Raman spectra were used to study the carbon structure of CMK3, CMK-8, and FDU-15 (Fig. S1). Raman spectra of CMK-3, CMK-8, and FDU-15 show two characteristic peaks located at around 1595 (G-band) and 1412 cm1 (D-band), which correspond to the E2g mode of sp2 carbon atoms and the disordered carbon in the carbon lattice, respectively. The relative peak intensity ratio of the D-band to G-band (ID/IG) for the CMK-3 (2.23) is larger than those of CMK-8 (1.50) and FDU-15 (1.75), indicating the presence of more defects into the framework of CMK-3. The influence of deposition of NiCo2O4 on the structures of OMCs was investigated by small-angle XRD. Here, the sample of NiCo2O4/CMK-3 is taken as an example. Fig. S2A displays the typical small-angle XRD patterns of CMK-3 and NiCo2O4/CMK-3. The synthesized CMK-3, as is evident from the presence of three XRD peaks at 2q of 0.9, 1.5, and 1.8 , is replica of the parent template SBA-15. After the growth of NiCo2O4, the disappearance of three XRD peaks indicates the deterioration of the ordered mesoporous structure. The decreased intensity for NiCo2O4/CMK-3 diffraction
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Fig. 1. TEM images of (A) CMK-3, (B) CMK-8, and (C) FDU-15. SEM images of (D) CMK-3, (E) CMK-8, and (F) FDU-15.
peaks is related to the growth of NiCo2O4 in the framework of CMK3. Two broad diffraction peaks at 2q of 23.4 and 44.5 are observed at CMK-3, corresponding to the (002) and (101) diffractions of graphite (Fig. S2B). The wide-angle XRD pattern of NiCo2O4/CMK-3 exhibits the characteristic peaks for NiCo2O4 (JCPDS: 20-0781). These peaks correspond to the planes (111), (220), (311), (222), (400), (422), (511), and (440) at 2q of around 18.9, 31.1, 36.7, 38.4, 44.6, 55.4, 59.1, and 65.0 , respectively. This result confirms the formation of NiCo2O4/CMK-3 composites. TEM analyses were conducted to determine the microstructure of NiCo2O4 supported on CMK-3, CMK-8, and FDU-15. Fig. 3A exhibits that well-dispersed NiCo2O4 particles are facilely supported on the framework of the CMK-3. Compared with the pristine CMK3, the pore channels are obscure to a certain degree after the growth of NiCo2O4 spinel, which reveals that NiCo2O4 has been assembled on the framework of CMK-3. However, the ordered structure of CMK-3 can also be observed from the TEM images of NiCo2O4/CMK-3. The SEM image of NiCo2O4 without the use of OMCs (Fig. S3) shows the NiCo2O4 was mostly irregular aggregates
Fig. 2. Nitrogen adsorptionedesorption isotherms of CMK-3, CMK-8, and FDU-15. Inset: the pore size distributions of CMK-3, CMK-8, and FDU-15.
of particles. From the TEM image of NiCo2O4/CMK-3, less aggregation of NiCo2O4 is observed on the surface of CMK-3. On counting 100 particles, the average diameter is found to be 5.7 nm, as shown in the histogram in inset of Fig. 3A. The large surface area of CMK-3 provides many active sites for the adsorption of metal precursors. The mesopores of CMK-3 act as confined space for the growth of NiCo2O4 spinel. The mesoporous structure restricts the particle growth during hydrothermal, resulting in less aggregation of NiCo2O4. From the TEM images of NiCo2O4/CMK-8 (Fig. 3B) and NiCo2O4/FDU-15 (Fig. 3C), the NiCo2O4 with the mean size of 7.9 and 8.6 nm are dispersed on the framework of CMK-8 and FDU-15, respectively. Compared with CMK-8 and FDU-15, the large surface area and pore volume of CMK-3 facilitate the formation of smaller NiCo2O4 particles. The content of NiCo2O4 supported on CMK-3, CMK-8, and FDU-15 was determined by TGA (Fig. S4). The content of NiCo2O4 on CMK-3, CMK-8, and FDU-15 was calculated to be 25.2%, 27.2%, and 26.1%, respectively. EIS was used to study the electron-transfer kinetics of the NiCo2O4/OMCs. Here, the sample of NiCo2O4/CMK-3 is also taken as an example. Fig. S5 shows the EIS results of NiCo2O4/CMK-3 and NiCo2O4 in oxygen-saturated 0.1 M KOH. Obviously, NiCo2O4 catalysts exhibit a semicircle portion with a large diameter, reflecting large electron-transfer resistance at the interface. In contrast, NiCo2O4/CMK-3 catalysts show a very low electron-transfer resistance, suggesting that the electrical conductivity of NiCo2O4 was increased by CMK-3 support. The electrocatalytic activity of NiCo2O4/OMCs towards oxygen was investigated by CVs in oxygen-saturated 0.1 M KOH. Fig. 4A shows the voltammetric response corresponding to the electrocatalytic reduction of oxygen at CMK-3 (green), NiCo2O4 (blue), NiCo2O4/CMK-3 (black), and Pt/C (red) catalysts. The Pt/C catalysts show a reduction peak at 0.17 V for oxygen. A cathodic peak for ORR around 0.28 V is observed at CMK-3 (green). NiCo2O4 catalysts exhibit a substantial reduction process in the presence of oxygen with the cathodic reduction peak at around 0.29 V. NiCo2O4/CMK-3 catalysts show an efficient reduction process in the presence of oxygen with a reduction peak at around 0.20 V. The comparison between NiCo2O4/CMK-3 and CMK-3 indicates that an enhancement of peak current density and a significant positive shift of peak potential are visible for NiCo2O4/CMK-3. The enhancement in response and positive reduction potential obtained at NiCo2O4/CMK-3 reflects that NiCo2O4/CMK-3 catalysts
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Fig. 3. TEM images of (A) NiCo2O4/CMK-3, (B) NiCo2O4/CMK-8, and (C) NiCo2O4/FDU-15.
Fig. 4. (A) CVs and (B) LSV curves of Pt/C (red), NiCo2O4/CMK-3 (black), CMK-3 (green), and NiCo2O4 (blue) in oxygen-saturated 0.1 M KOH. Scan rate of CVs: 50 mV s1. Scan rate of LSVs: 5 mV s1. Rotation rate: 1600 rpm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
show better electrocatalytic activity towards ORR than CMK-3. Although deterioration of the ordered mesoporous structure was observed after incorporation of the NiCo2O4, the NiCo2O4/CMK-3 catalysts show improved activity compared to CMK-3. The comparison of electrochemical reduction of oxygen between NiCo2O4/ CMK-3 and NiCo2O4 reveals the use of CMK-3 support can promote ORR activity. The high electrical conductivity of CMK-3 can effectively supplement the insufficient conductivity of NiCo2O4 spinel and increase the ORR activity. The combination of catalytic properties of NiCo2O4 and mesoporous structure of CMK-3 contributes to the high activity of NiCo2O4/CMK-3. The comparison of ORR activity for different catalysts by linear sweep voltammetry (LSV) at RDE is shown in Fig. 4B. In agreement with the voltammetric result, NiCo2O4 and CMK-3 catalysts show low activity for ORR. As shown in Fig. 4B, the onset potential of ORR at CMK-3 catalysts commences at around 0.20 V, whereas the onset potential at NiCo2O4/CMK-3 catalysts significantly shifts positively to 0.09 V. The limiting diffusion current density for NiCo2O4/CMK-3 catalysts is larger than those of CMK-3 and NiCo2O4 catalysts. The electrochemical activity of NiCo2O4 supported on three OMCs with different surface areas, pore sizes, defective sites, and pore volumes was studied. Fig. 5A compares the effect of different catalyst supports on activity for ORR in 0.1 M KOH. Compared with NiCo2O4/CMK-8 and NiCo2O4/FDU-15, an obvious increase of catalytic activity is found at NiCo2O4/CMK-3. The cathodic current is about 0.93 mA cm2 at NiCo2O4/CMK-3, compared to 0.67 mA cm2 at NiCo2O4/CMK-8 and 0.61 mA cm2 at NiCo2O4/FDU-15.
The electrochemical data in Fig. 5B and Table S2 also reflect the superior performance of CMK-3 over CMK-8 and FDU-15 as a catalyst support for NiCo2O4. There is a minor discrepancy in the content of NiCo2O4 supported on CMK-3, CMK-8, and FDU-15. The influence of the content of NiCo2O4 supported on different supports can be omitted. The influence of the structures and properties of carbon supports on the ORR catalytic activities of NiCo2O4 was studied in Fig. 6. The current density of cathodic peak at 0.50 V in LSVs for three catalysts is plotted against the surface area, pore size, pore volume, mean size of NiCo2O4 particle, and the value of ID/IG. These plots indicate that the there is no dependence of current density on the pore size and the value of ID/IG. However, the current density of catalysts is dependent on the surface area, pore volume, and mean size of NiCo2O4 particle. From the nitrogen adsorption isotherms in Fig. 2, the BET surface area of CMK-3 is larger than those of CMK-8 and FDU-15. The low surface area of CMK-8 and FDU-15 is not favorable for the adsorption of metal precursors. The high surface area of CMK-3 facilitates the adsorption of metal precursors. The large pore volume provides accommodation for NiCo2O4 deposition inside the pore channels. The activity of NiCo2O4/OMCs increases with the decrease of average particle size. The large surface area and pore volume favor the formation of smaller NiCo2O4 particles. Smaller NiCo2O4 particles provide more surface active sites for ORR catalysis. The combination of higher surface area, larger pore volume, and smaller NiCo2O4 particle enable the high activity of NiCo2O4/CMK-3. On the basis of above optimized data, NiCo2O4/CMK-3 was selected for subsequent
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Fig. 5. (A) CVs and (B) LSVs of NiCo2O4/CMK-3 (solid), NiCo2O4/CMK-8 (short dot), and NiCo2O4/FDU-15 (dash) in oxygen-saturated 0.1 M KOH. Scan rate of CVs: 50 mV s1. Scan rate of LSVs: 5 mV s1. Rotation rate: 1600 rpm.
Fig. 6. The effect of BET surface area of three OMCs (black), pore size of three OMCs (green), pore volume of three OMCs (blue), mean size of NiCo2O4 particle (red), and the value of ID/IG for three OMCs (magenta) on the current density of NiCo2O4 catalysts at 0.50 V of LSV curves. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. (A) LSV curves at different rotation rates recorded for ORR at NiCo2O4/CMK-3 in oxygen-saturated 0.1 M KOH. Scan rate: 5 mV s1. Rotation rate: 100, 400, 900, and 1600 rpm. (B) KouteckyeLevich plots of NiCo2O4/CMK-3 at different potentials. (C) The dependence of electron transfer number n on potential for NiCo2O4/CMK-3 (black) and Pt/C (red). (D) RRDE curves of NiCo2O4/CMK-3 (black) and Pt/C (red) in oxygen-saturated 0.1 M KOH. Scan rate: 5 mV s1. Rotation rate: 1600 rpm. Ring potential: 0.5 V. (E) The dependence of electron transfer number n on potential for NiCo2O4/CMK-3 (black) and Pt/C (red) estimated from RRDE data. (F) The H2O2 generation for NiCo2O4/CMK-3 (black) and Pt/C (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 8. CVs of (A) NiCo2O4/CMK-3 and (B) Pt/C in oxygen-saturated 0.1 M KOH (solid line) and in the presence of 3 M methanol (short dot line). LSV curves of (C) NiCo2O4/CMK-3 and (D) Pt/C in oxygen-saturated 0.1 M KOH (solid line) and in the presence of 3 M methanol (short dot line). Scan rate: 5 mV s1. Rotation rate: 1600 rpm.
experiment. Fig. 7A shows LSVs on NiCo2O4/CMK-3 catalysts in oxygensaturated 0.1 M KOH at different rotation rates. The increase of rotation rate is accompanied with the increase of the diffusion current density for NiCo2O4/CMK-3. The KouteckyeLevich plots (J1 vs. u1/2) at different potentials show good linearity (Fig. 7B). The data in Fig. 7C show the dependence of electron transfer number n on potential at NiCo2O4/CMK-3. The value of n was calculated to be about 3.80e3.85 from 0.25 to 0.60 V according to the slopes of KouteckyeLevich plots in Fig. 7B, reflecting that the ORR follows mainly 4 electron process. Fig. S6 shows LSVs and KouteckyeLevich plots of Pt/C catalysts. The value of n estimated from RRDE also indicates that the value of n at NiCo2O4/CMK-3 is around 3.80e3.90 from 0.20 to 0.80 V (Fig. 7E). From the data in Fig. 7F, hydrogen peroxide yield (H2O2%) during ORR is lower than 10%. The low generation of hydrogen peroxide and approximately four-electron transfer reflect that oxygen is directly reduced to OH ions through four-electron transfer process in alkaline solution. Fig. 8 shows CVs of NiCo2O4/CMK-3 in oxygen-saturated 0.1 M KOH in the presence (short dot line) or absence (solid line) of 3 M methanol. The similarity between the voltammetric responses for NiCo2O4/CMK-3 with 3 M methanol suggests that NiCo2O4/CMK-3 catalysts show a good selectivity for ORR (Fig. 8A). In contrast, the disappearance of ORR current and obvious oxidation of methanol at Pt/C indicates that ORR process is seriously retarded by methanol (Fig. 8B). Compared with the commercial Pt/C catalysts, the better methanol tolerance of NiCo2O4/CMK-3 towards ORR indicates that NiCo2O4/CMK-3 may be developed as methanol-tolerant ORR catalysts in fuel cells. The comparison between Fig. 8C and D also indicates the better tolerance to methanol at NiCo2O4/CMK-3 than at Pt/C. The long-term stability of NiCo2O4/CMK-3 catalysts for ORR was investigated in 0.1 M KOH solution. Pt/C catalysts show a rapid decay of the current depression (~37%) after 30,000 s, reflecting a low stability for ORR (Fig. S7). In contrast, the response of NiCo2O4/
CMK-3 catalysts retains acceptable stability during the ORR catalysis, with only 25% current diminutions after 30,000 s. This comparative result indicates the better durability of NiCo2O4/CMK3 compared with Pt/C. This result indicates that NiCo2O4/CMK-3 is a good alternative to Pt as cathode catalysts. 4. Conclusion OMCs with large surface areas and pore volumes were used as a catalyst support for NiCo2O4 spinel in ORR. The NiCo2O4/OMCs catalysts bring together the structural property of OMCs and catalytic property of NiCo2O4. NiCo2O4/OMCs composites are highly active, cheap, and selective noble metal-free electrocatalysts for ORR in alkaline solution. From the comparative study of different catalyst supports, the ORR activity of NiCo2O4 is dependent on the surface area and pore volume of catalyst supports. Acknowledgments The authors gratefully acknowledge China Postdoctoral Science Foundation funded project (2014M550164), the Science and Technology Development Planning of Jilin Province (20150520014JH) and the Fundamental Research Funds for the Central Universities (No. 14QNJJ011). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.04.110. References [1] D.-W. Wang, D. Su, Energy Environ. Sci. 7 (2014) 576e591. [2] A. Morozan, B. Jousselme, S. Palacin, Energy Environ. Sci. 4 (2011) 1238e1254. [3] Y. Zheng, Y. Jiao, M. Jaroniec, Y. Jin, S.Z. Qiao, Small 8 (2012) 3550e3566.
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