For its high energy conversion efficiency, high power density, low operating temperature, long life span, and low or zero emission of pollutants, fuel cell technology has attracted attention [1, 2]. The commercial Pt/C catalyst remains the most popular electrocatalyst for the oxygen reduction reaction (ORR) in fuel cells [3-5]. However, the limited reserve, relatively low stability,CO poisoning and deactivation by crossover methanol have prevented Pt-based electrocatalysts from widespread commercialization in fuel cells [6, 7]. Therefore, the development of a high performance non-precious metal electrocatalyst (NPME) for the replacement of Pt-based electrocatalysts would be an ideal solution [8].
In recent years, metal organic framework (MOF) as a precursor has been employed for the preparation of NPMEs. MOFs are crystalline porous materials with a periodic network formed by the self-assembly of transition metal ions and organic ligands [9]. Due to their tunable pores, ultra-low density, and high specific surface area [7],MOFs have drawn tremendous attention as a new class of porous materials with potential applications in hydrogen storage [10, 11],CO2 capture [12], chemical separation [13, 14], catalysis [15], biomedical imaging [16, 17], chemical sensors [18] and magnetic applications [19]. Compared to the commonly used NPME precursors comprising a carbon support, nitrogen-containing ligands and transition metal salts,MOF possesses a periodic 3D-network structure that can provide not only abundant desired moieties but also a uniform and regular porous structure [20]. Hence, many MOFs have already been selected for the preparation of ORR electrocatalysts. Li et al. [21] synthesized a NPME with high electrochemical performance in both acidic and alkaline condition by heat-treating a mixture of a novel Co-containing MOF material with giant polyhedral cages, dicyandiamide (DCDA) and iron species. Zhang et al. [22] reported an alkaline NPME produced by the pyrolysis of triphenylphosphine and highly porous doped carbon nanofibers derived from ZIF-8 nanofibers.
Imidazole zeolite frameworks (ZIFs) are an important class of MOFs. Their thermal and chemical stability, high specific surface area, high nitrogen content, and uniform distribution of transition metals and nitrogen-containing ligands make ZIFs unique NPME precursors [1], in particular,ZIF-67 has a sodalite topology in which every cobalt cation is bridged with four nitrogen atoms of 2-methylimidazolate anions. Wang et al. [23] have made electrocatalysts with ZIF-67,ZIF-8 and Co2(bdc)2(dabco) in both acidic and alkaline solution. Hu et al. [24] and Chen et al. [25] have made S-doped electrocatalysts for ORR and both oxygen reduction and evolution reactions with ZIF-67. Aijaz et al. [26] have made bifunctional electrocatalysts for oxygen electrodes comprising Co@Co3O4 nanoparticles embedded in CNT-grafted N-doped carbon-polyhedra obtained by the pyrolysis of ZIF-67 in a reductive H2 atmosphere and subsequent controlled oxidative calcinations. Xia et al. [20] have used ZIF-67 with different particle sizes as precursors to synthesize ORR electrocatalysts in acidic solution. From the results, we can see that the electrochemical performance of NPME is closely related to the size of the ZIF-67 precursors. The smallest ZIF-67 particles (300 nm) showed the best electrochemical performance. This may be because the smaller ZIF-67 supplied more active and easily accessible sites and thus promoted faster mass and electron transfer. In this study, we report the synthesis of the smaller ZIF-67 (about 44 nm) and subsequent pyrolysis with and without a BP 2000 carbon support. The resultant NPME demonstrated high ORR performance in terms of the half-wave potential and is more stable in alkaline medium than a commercial Pt/C catalyst.
Co(NO3)2·6H2O (> 99%) was purchased from Xilong Chemical Reagent Co. (Guangdong,China) and 2-methylimidazole was purchased from Aladdin Chemical Reagent Co. (Shanghai,China). The chemicals were used as received. Milli-Q UV-plus water (18.2 MΩ·cm) from a Millipore water system (Synergy® UV,France) was used in the experiments.
In a typical synthesis, 921 mg Co(NO3)2·6H2O was placed into a 250 mL round bottom flask, followed by the addition of 125 mL of anhydrous methanol. After mild sonication in a water bath cleaner for 5 min,Co(NO3)2·6H2O was homogeneous dissolved in methanol. Next, 2.079 g 2-methylimidazole was added to the mixture. Another 5 min of mild sonication was applied to ensure the complete dissolution of 2-methylimidazole. The mixture was incubated at 25 °C with stirring for 22 h. Finally,ZIF-67 was obtained as a purple precipitate. The product was collected by centrifugation at 8000 r/min for 8 min, followed by drying under vacuum at 80 °C for 4 h.
The synthesis of ZIF-67/C was identical to that described above for ZIF-67, except that 75 mg BP 2000 was added after the addition of 2-methylimidazole.
The ZIF-67 was pyrolyzed under an argon atmosphere for 2 h at different temperatures ranging from 500 to 900 °C with a heating ramp of 5 °C/min and cooled down naturally to ambient temperature. The eletrocatalysts were marked as ZIF-67-Tc, where Tc stands for the thermal treatment temperature. The heat treatment procedure for ZIF-67/C was the same as that of ZIF-67 and the corresponding electrocatalyst was marked as ZIF-67/C-600. Finally, all the electrocatalysts were ground to a fine powder.
Transmission electron microscopy (TEM) was conducted on JEM-2000EX operated at 120 kV and JEM-2100 operating at 200 kV. Powder X-ray diffraction (XRD) patterns were collected on a D/MAX2500VB2/PC with a 2θ range of 5°-90° and a scan speed of 5°/min at ambient temperature. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Scientific ESCA Lab250 Xi spectrometer. The metal content of the electrocatalysts was determined by a PerkinELmer 7300DV inductively coupled plasma atomic emission spectrometry (ICP-OES).
Nitrogen sorption measurements at -196 °C were carried out using a Quantachrome Autosorb-IQ gas adsorption analyzer. The samples were degassed at 90 °C for 2 h and 200 °C for 5 h under vacuum first. The resulting BET surface area was calculated from the adsorption branch. The pore size distribution was fitted using the Quenched Solid Density Functional Theory (QSDFT) method.
A CHI electrochemical station (Model 760D) and a conventional three-electrode electrochemical cell were used for electrochemical measurements at room temperature. A platinum plate and mercuric oxide electrode (Hg/HgO) were used as the counter and reference electrode, respectively. The electrolyte was 0.1 mol/L KOH aqueous solution. For the preparation of the catalyst ink, some Nafion solution (5 wt% ), ultra-pure water and ethanol (VNafion:Vwater:Vethanol = 0.1:1:9) were mixed with an electrocatalyst and sonicated in a water bath for 10 min to get the catalyst ink (2 mg/mL). An amount of catalyst ink was loaded on the glassy carbon rotating disk electrode (RDE, diameter 5 mm,Pine Instruments,USA) for the electrochemical measurements.
Cyclic voltammetry (CV) experiments were performed in O2 or N2 purged aqueous KOH (0.1 mol/L) with the potential ranging from 0.097 to 1.097 V. The scan rate was 100 mV/s. Linear sweep voltammograms (LSVs) were performed on the catalyst-coated RDE in O2 saturated KOH (0.1 mol/L) between 0.097 and 1.097 V at 1600 r/min and a scan rate of 5 mV/s.
In the rotating ring-disk electrode tests (RRDE,PINE AFE7R9GCPT), the ring potential was set at 1.197 V versus RHE. The number of electron transferred (n) during the ORR and the H2O2 yield (%H2O2) were calculated using the following equations:
where N is the collection efficiency (0.37),Ir and Id are the ring current and disk current, respectively.
Koutecky-Levich plots were obtained on the basis of the RDE tests at different rotating rates. The electron transfer number during the ORR process was determined from the slopes of Koutecky-Levich plots by the following equation:
where jlim is the limiting current density,n is the number of electrons transferred per oxygen molecule,F (96485 C/mol) is the Faraday constant,D (1.9×10-5 cm2/s) is the diffusion coefficient of O2 in 0.1 mol/L KOH and C0 (1.2×10-6 mol/L) is the concentration of O2 in the electrolyte,ν is the kinetic viscosity of the solution (0.01 cm2/s), and ω is the electrode rotation rate (rad/s).
For the accelerated stability test (AST), the electrodes were cycled between 0.497 and 1.097 V for a total number of 2000 cycles in O2 saturated KOH (0.1 mol/L) with a scan rate of 100 mV/s. Meanwhile, the CV curves and ORR polarization curves were collected in aqueous KOH solution at specific cycles to track the degradation of ZIF-67/C-600 and commercial Pt/C.
The morphologies of the ZIF-67 precursor,ZIF-67-600, and ZIF-67/C-600 were analyzed by TEM. In this MOF, each cobalt cation is bridged with four nitrogen atoms from the 2-methylimidazolate anions into tetrahedral frameworks and further assembled into a sodalite (SOD) topology. An average particle size of 44 nm ZIF-67 precursor was synthesized at room temperature in methanol solvent as shown in Fig. 1(a).
Compared with ZIF-67, both ZIF-67-600 and ZIF-67/C-600 samples did not preserve their original shape (Fig. 1(b) and (c)). Instead, they were composed of graphite layer-wrapped Co nanoparticles embedded in the carbon matrix. The Co nanoparticles exhibited (111) lattice fringes with a typical spacing of 0.204 nm (Fig. 1(d), inset). The Co nanoparticles were formed during the heat-treatment process of ZIF-67 at a high temperature. Multi-layered graphite was observed to envelope the Co nanoparticles as shown in Fig. 1(d). The formation of the graphite layer around Co likely resulted from the catalytic effect of Co for carbonization during the pyrolysis process. The graphite layer is beneficial for the stabilization of the Co nanoparticles in the carbon matrix.
The ZIF-67,ZIF-67-Tc and ZIF-67/C-600 structures were characterized by XRD. It showed that the synthesized precursor was pure ZIF-67 crystal without any detectable byproducts. The heat-treated products were composed of the pure metallic phase of face-centered cubic Co as shown in Fig. 2(a).
Based on the ICP-OES result,ZIF-67/C-600 contained up to 23 wt% Co. Indeed,Co has long been recognized to promote the four-electron ORR process in alkaline solution [27].
Nitrogen is viewed as a n-type carbon dopant that can facilitate the ORR [28]. Thus the nitrogen content and species are of great importance for the electrochemical performance of NPME. In this study,ZIF-67-600 has a relatively large nitrogen content (5.8 at%,Table 1) [20], which is beneficial for the ORR. As shown in Fig. 2(b) and (c), the spectra can be decomposed into four peaks, namely pyridinic-N (398.8 eV), pyrrolic-N (400.4 eV), graphitic-N (401.3 eV), and oxidized-N (403.8 eV) [29]. From Table 1, we can see that pyridinic-N was the dominant species with smaller amounts of pyrrolic-N, graphitic-N, and oxidized-N for both electrocatalysts. The amount of pyrrolic-N slightly increased for ZIF-67/C-600. In particular, pyridinic-N has been proposed as the active site to promote the four-electron process in the ORR which reduces the adsorption energy of O2 [30, 31], so it is good for the ORR activity.
The surface area of the electrocatalyst also affects its electrochemical performance. Fig. 2(d) shows the N2 adsorption-desorption isotherms of ZIF-67,ZIF-67-600, and ZIF-67/C-600. ZIF-67 showed a typical Type I isotherm, indicating its microporous nature with a BET surface area of ABET 1146 m2/g. After heat-treatment at 600 °C, the isotherms of ZIF-67-600 and ZIF-67/C-600 also showed the presence of pores. The slight hysteresis loop and vertical tail of ZIF-67/C-600 revealed the presence of some meso- and macropores. Since the particle size of the ZIF-67 precursor was only 44 nm, the macropores are not derived from the heat treatment of ZIF-67. On the other hand, they may come from the space between the pyrolyzed precursor and BP 2000. The surface area of ZIF-67/C-600 was 296 m2/g, which was a little higher than ZIF-67-600 (268 m2/g). The relatively large surface area contributed to the enhanced ORR performance of ZIF-67/C-600 as compared to that of ZIF-67-600.
To evaluate the electrochemical performance of ZIF-67-Tc and ZIF-67/C-600 for the ORR,CV and RDE experiments were conducted in 0.1 mol/L KOH aqueous solution. Fig. 3(a) shows the RDE polarization curves of the electrocatalysts prepared by the pyrolysis of the ZIF-67 precursor at 500 to 900 °C and ZIF-67/C at 600 °C. With the increase of pyrolysis temperature, the electrochemical performance increased from 500 to 600 °C in terms of the onset potential and half-wave potential for the ORR. ZIF-67-600 exhibited the highest performance among the ZIF-67-Tc electrocatalysts with a ORR half-wave potential of 0.833 V and limiting current density of 5 mA/cm2. This may be due to the dominant presence of pyridinic-N in ZIF-67-600 for the promotion of the four-electron process in ORR, which gave a better performance than the N/Co-doped PCP electrocatalyst of Hou et al. [32] who directly pyrolyzed the as-synthesized 500 nm ZIF-67. This may be because the smaller ZIF-67 can supply more active and easily accessible sites and thus promote a faster mass and electron transfer. Further increase of the pyrolysis temperature led to a decline of ORR performance. The different heat treatment temperatures can result in a different pore structure and different relative contents of carbon, nitrogen and metal species on the electrocatalysts, which affect the ORR performance of the electrocatalysts. When loaded on BP 2000, the resulting ZIF-67/C-600 showed an even better electrocatalytic performance than ZIF-67-600 with the ORR onset and half-wave potential as high as 0.982 and 0.842 V, respectively, relative to RHE. The dominant pyridinic-N and slightly increased pyrrolic-N were the active sites for the four-electron process in ORR. In addition, the relatively large surface area contributed to the enhanced ORR performance of ZIF-67/C-600 compared to ZIF-67-600. Furthermore, the addition of BP 2000 improved the dispersion and electroconductivity of ZIF-67/C-600, which would also enhance the ORR performance of ZIF-67/C-600.
The loading of ZIF-67/C-600 on RDE was optimized as shown in Fig. 3(b). The optimal loading was reached at 1 mg/cm2 (1.017 V for onset and 0.857 V for half wave potential vs. RHE). It is worth pointing out that the half-wave potential was even superior to that of commercial Pt/C (1.017 V for onset and 0.853 V for half wave potential vs. RHE). Fig. 3(c) shows the ORR polarization curve of ZIF-67/C-600 at different rotating rates. The diffusion limited current density of ZIF-67/C-600 became larger with increased rotation rate. As shown in Fig. 3(d), the corresponding Koutecky-Levich (K-L) plots showed good linearity and were nearly parallel at different potentials, which revealed first ordered reaction kinetics with respect to the concentration of dissolved O2.
Fig. 4(a) illustrates the electron transfer number (n) in the ORR process. Fig. 4(b) shows the hydrogen peroxide yield (%H2O2) calculated from the RRDE results for ZIF-67/C-600 and commercial Pt/C in alkaline solution. A value of n of 4 represents 100% of water formation, while a value of n of 2 represents 100% peroxide formation. The n number for both electrocatalysts was between 3.9 and 4.0, indicating that the ORR on both ZIF-67/C-600 and Pt/C was dominated by a 4e process and %H2O2 (or %HO2−) was below 1% for Pt/C and 3.5% for ZIF-67/C-600, respectively. This confirmed that the 4e reduction to water dominated for both electrocatalysts in alkaline medium.
During the accelerated stability test (AST),CV curves and ORR polarization curves were collected in aqueous KOH solution at specific cycles to track the degradation of ZIF-67/C-600 and commercial Pt/C. Fig. 4(c) shows the current density degradation of ZIF-67/C-600 and commercial Pt/C after being cycled between 0.497 and 1.097 V for a total number of 2000 cycles in O2 saturated KOH (0.1 mol/L) with a scan rate of 100 mV/s. It is clear that 96% of the initial current density of ZIF-67/C-600 toward ORR was maintained, which is much better than that of the commercial Pt/C (59%). The deactivation of Pt/C was mainly due to the dissolution of Pt nanoparticles active sites at high potentials. This was not the case for NPME. Also, the chemically and mechanically stable BP 2000 contributed to the stability improvement [33]. More importantly,Fig. 4(d) showed that ZIF-67/C-600 also exhibited better methanol tolerance than commercial Pt/C. A sharp increase in the current density was observed for commercial Pt/C after the introduction of 10 vol% methanol into the alkaline solution, while the ZIF-67/C-600 was only slightly affected.
Smaller Co-based ZIF-67 nanoparticles were synthesized for the first time. A NPME was made by the pyrolysis of ZIF-67 with and without a BP 2000 support. The ZIF-67/C-600 electrocatalyst exhibited high ORR performance in alkaline solution, and it was superior to the commercial Pt/C catalyst in terms of the ORR half-wave potential. ZIF-67/C-600 also has a better long term stability and resistance to methanol crossover. The high electrochemical performance was attributed to the abundant pyridinic nitrogen species and the high surface area of ZIF-67/C-600. These advantages make the ZIF-67/C-600 catalyst a possible candidate for substituting the conventional Pt-based electrocatalysts in alkaline media.
2016,
Vol. 37 Issue (7): 1127-1133 
PDF (999 KB)
2016,
Vol. 37 Issue (7): 1127-1133 PDF (999 KB)