Direct methanol fuel cells (DMFCs) have attracted considerable attention as ideal candidates for automotive and portable applications due to the advantages of high energy conversion efficiency and ability to be easily handled [1]. The major obstacles to DMFC practical application are the low catalytic performance, high cost, and poor durability of the noble metal catalysts [2]. A catalyst support with a large surface area and stable structure will lead to high dispersion, high utilization and stability of Pt nanoparticles and overcome these obstacles [3]. Carbon materials with high specific surface areas (SSAs) are good supports in DMFCs due to their high electrical conductivity, corrosion resistance, and low cost [3, 4]. Apart from commercial activated carbon supports, e.g.,Valcan XC-72 [5], which usually engulf a lot of Pt in their primary micropores and thus cause some catalytic activity loss, several novel carbon nanomaterials such as carbon nanotubes [6], carbon nanofibers [7], carbon nanocoins [8] and graphene [9] have also been explored. Owing to the surface inertness, surface modification are usually used to introduce anchoring sites, e.g., by acid oxidation and covalent grafting [10, 11]. However, these methods usually involve complex processes and inevitably damage the structural, mechanical, and electrical properties. Alternatively, in situ nitrogen doping into the carbon matrix during growth can introduce chemically active sites for anchoring metal and metal oxide nanoparticles [12-18]. Our studies indicated that Pt-based nanoparticles could be easily and uniformly immobilized on N-doped carbon nanotubes (NCNTs), and these showed enhanced electrocatalytic performance for methanol oxidation and oxygen reduction [13-15]. However, the SSA of the NCNTs is usually not large enough, as it is only 200 m2/g. Very recently, by an in situ MgO template method, we reported unique 3D hierarchical carbon-based nanocages that featured high SSAs up to 1400 m2/g and good conductivity [19-23], which exhibited excellent electrochemical performances. By using hierarchical N-doped carbon nanocages (hNCNC) as a new support, here we report the high dispersion of a Pt electrocatalyst on hNCNC made by a modified microwave-assisted polyol method. The constructed Pt/hNCNC demonstrated excellent electrochemical activity and good stability for methanol oxidation, which were superior to those of their counterparts supported on carbon nanocages and commercial Vulcan XC-72. Both the strong interaction between Pt and hNCNC and the excellent dispersion of Pt by nitrogen incorporation were responsible for the enhanced performance in methanol electrocatalytic oxidation. This illuminates the application of hNCNC in the field of DMFCs.
hNCNC was prepared by the in situ MgO template method developed recently by our group [20, 21], which is characterized by co-existing micro-meso-macropores, good conductivity, and a large SSA generally up to 1400 m2/g. Briefly, basic magnesium carbonate (4 g) was placed in a quartz tube and heated to 900 °C. Then 1.8 mL pyridine was introduced into the reactor at the feeding rate of 60 μL/min by a syringe pump. Sequentially, the MgO template was removed by 1 mol/L HCl aqueous solution. Finally, hNCNC with 8 at% nitrogen doping was obtained after washing repeatedly with deionized water and drying at 110 °C. Pt particles were supported on hNCNC by a modified microwave-assisted polyol process [24]. Typically, 25.0 mg hNCNC was ultrasonically dispersed in 50.0 mL ethylene glycol (EG). Then 1.0 mL H2PtCl6/EG solution (7.5 mg Pt/mL EG) and 0.25 mL NaOH/EG solution (0.2 mol/L) were dropped into the suspension under magnetic stirring. After irradiating with a domestic microwave oven (800 W) for 100 s,Pt/hNCNC with a nominal loading of 23 wt% was obtained after filtrating, rinsing, and drying under vacuum for 12 h. For comparison, hierarchical carbon nanocages (hCNC) from a benzene precursor and Valcan XC-72 were also used as the support to prepare Pt/hCNC and Pt/XC-72 in a similar way.
The structure and composition of the catalysts were characterized by high resolution transmission electron microscopy (HRTEM,JEM-2100 operating at 200 kV), scanning electron microscopy (SEM,Hitachi S4800),X-ray diffraction (XRD,Philips X’pert Pro X-ray diffractometer), and X-ray photoelectron spectroscopy (XPS,ULVAC-PHI INC,PHI 5000 VersaProbe,Al Ka). Thermogravimetry (TG,NETZSCH STA449F3) was carried out in air flow at a rate of 10.0 °C/min. N2 adsorption isotherms were measured on Micromeritics ASAP 2020 at -196 °C with the specimens degassed at 300 °C for 4 h.
The electrochemical performances were characterized by a CHI 760C workstation (CH Instrument,Inc.) with a three-electrode system at 25 °C. Ag/AgCl and a platinum thread served as the reference and counter electrodes, respectively. The work electrode was a thin film electrode with the catalyst casted on a glassy carbon disk of 3.0 mm in diameter. Typically, a suspension of catalyst (4.0 mg/mL) was prepared by ultrasonically dispersing 2.0 mg catalyst in 400 μL ethanol/water solution (1/2,V/V) and 100 μL Nafion solution (5 wt% Nafion,Alfa Aesar). Then the 10 μL suspension was dropped onto the glassy carbon disk to construct a thin film electrode. Cyclic voltammetry (CV) of hydrogen electrochemical adsorption was measured in N2-saturated 0.5 mol/L H2SO4 solution between -0.23 and 1.0 V at a scan rate of 50 mV/s. CO stripping CV was measured in 0.5 mol/L H2SO4 solution after CO bubbling for 15 min between 0 and 1.25 V at a scan rate of 10 mV/s. The electrocatalytic CV for methanol oxidation was performed in 1.0 mol/L methanol and 0.5 mol/L H2SO4 solution in the potential range of 0-1.0 V at a scan rate of 50 mV/s. The stability of the catalysts was evaluated by chronoamperometry at 0.60 V for 2000s.
Fig. 1 displays SEM and TEM images of Pt/hNCNC,Pt/hCNC and Pt/XC-72. Pt particles were homogenously immobilized on hNCNC with the average size of 3.3 nm, which was slightly smaller than 4.3 nm for Pt/hCNC and 4.9 nm for Pt/XC-72, which indicated the better dispersion on hNCNC than on hCNC and XC-72 due to the nitrogen participation as expected [13-15]. The interplanar spacing of 0.226 nm corresponded to d111 of (fcc) Pt (PDF 88-2343). The three catalysts were further characterized by TG and XRD. The results are presented in Fig. 2. The Pt loadings were obtained to be 21.6, 21.2, and 19.0 wt%, respectively, close to the 23.1 wt% Pt nominal value (Fig. 2(a)). The Pt particles were assigned to (fcc) Pt (PDF 88-2343). The d111 spacing of the peaks at 39.8° (Fig. 2(b)) was in agreement with the TEM fringes (Fig. 1).
XPS spectra of the catalysts are shown in Fig. 3. The Pt 4f peaks can be deconvoluted into three Pt species, i.e.,Pt(0) (71.2 eV),Pt(Ⅱ) (72.4 eV) and Pt(Ⅳ) (74.4 eV) for Pt 4f7/2 [25], with the corresponding ratio of 60.3:30.8:8.9 for Pt/hNCNC. Thus, the Pt species exist predominantly as metallic Pt(0) in the three catalysts (Fig. 3(b)-(d)). The total Pt contents were characterized to be 23.0, 22.3 and 20.1 wt% in Pt/NCNC,Pt/hCNC and Pt/XC-72, respectively, which were close to the TG analysis results (Fig. 2(a)).
The electrochemical surface area (ECSA) of Pt in Pt/hNCNC,Pt/hCNC and Pt/XC-72 were measured by CV tests of hydrogen adsorption and CO stripping, as illustrated in Fig. 4. ECSAH were obtained to be 45.6, 34.2 and 24.2 m2/g, and ECSACO to be 50.7, 34.3 and 21.8 m2/g, respectively. Both results showed the ECSA order of Pt/hNCNC < Pt/hCNC < Pt/XC-72, which was consistent with the size order of the Pt particles from TEM and XRD characterization (Fig. 1,Fig. 2(b)). These results clearly indicated the Pt dispersion was in the order of Pt/hNCNC < Pt/hCNC < Pt/XC-72.
The electrocatalytic activity and stability of the catalysts for methanol oxidation were evaluated by CV and chronoamperometry measurements, with the current densities normalized by Pt loading, as shown in Fig. 5. The typical methanol oxidation peak appeared in the forward sweep at 0.64 V. The oxidation peak in the backward sweep at ca. 0.42 V represented the removal of incompletely oxidized carbonaceous species formed in the forward sweep [26, 27]. The peak current density of Pt/hNCNC reached 342 mA/mgPt@0.64V, which was significantly larger than 242 mA/mgPt@0.64V of Pt/hCNC and 213 mA/mgPt@0.65V of Pt/XC-72, in accordance with the relative order of their ECSAs (Fig. 5(a)). Generally, the ratio of the forward to backward sweeping peak current (If/Ib) is used as an index to evaluate the CO tolerance or the accumulation of carbonaceous species for Pt-based catalysts. A higher If/Ib indicates less accumulation of the carbonaceous species and better CO tolerance [9, 28]. The If/Ib values were 1.09, 1.10 and 1.05 for Pt/hNCNC,Pt/hCNC and Pt/XC-72, respectively, indicating a similar CO tolerance of Pt/hNCNC and Pt/hCNC, which was superior to that of Pt/XC-72. In addition, the chronoamperometry curves of Pt/hNCNC and Pt/hCNC presented a relatively flat changing trend, reflecting a higher stability than that of Pt/XC-72 (Fig. 5(b)).
These results suggested the hierarchical carbon-based nanocages were a good new support due to the unique network mesostructure. They give a high SSA, good conductivity and rapid diffusion. Nitrogen doping can further regulate the extended and local electronic structure of the carbon support, which favors the high dispersion of the catalytically active species to achieve a high activity (Figs. 1,4, and 5). Based on our previous study, from quarternary N-doping, the extra electrons occupy the π* orbital and activate the neighboring carbon atoms, while from pyridinic N-doping, the lone pair electrons fill into the p-like nonbonding orbits [12]. Both cases would enhance the interaction between hNCNC and d orbital of Pt, thus facilitating the dispersion of Pt nanoparticles and the electrocatalytic oxidation of methanol [13, 14, 15].
By making use of the unique advantages of hNCNC, a series of catalysts with high Pt loadings of 30, 40 and 60 wt% were prepared and characterized as shown in Fig. 6. Even for the high loading up to 60 wt%, the Pt nanoparticles can still be highly and homogenously dispersed on hNCNC without serious agglomeration, with the average size of 5.5 nm, which was in the optimum particle size range of 3-10 nm for efficient methanol electrooxidation [29] (Fig. 6(a)). Due to the high affinity of nitrogen, nitrogen doping converts the nonpolar covalent bonds of the carbon matrix into polar bonds, which generates the functional anchoring sites beneficial for the heavy loading of Pt. With increasing Pt loading from 21.6 to 60 wt%, the peak-current density per unit mass of catalyst gradually increased from 73.7 to 95.6 mA/mg by 30%, which is beneficial to achieving a high power density (Fig. 6(b) and (d)). However, the current density per unit mass of Pt species decreased from 342 to 159 mA/mgPt due to the increased average size (Fig. 6(c) and (d)). Hence, from the viewpoint of increasing power density and decreasing cost, a suitable Pt loading should be optimized for practical applications.
hNCNC was explored as a new support for Pt nanoparticles. The resulting Pt/hNCNC catalyst exhibited high electrocatalytic performance and stability for methanol oxidation. The hierarchical mesostructure gave a high accessible surface area, good conductivity, and rapid diffusion. Nitrogen doping further increased the functional anchoring sites for Pt, regulated the electronic structure of the carbon matrix, and enhanced the interaction between the support and metal catalyst. At 20 wt% loading,Pt nanoparticles with the sizes of 3.3 nm were evenly dispersed on hNCNC without aggregation, which was better than with the control supports of XC-72 and hCNC. The ECSAH and ECSACO of Pt/hNCNC were 45.6 and 51.8 m2/gPt, which were much larger than those of Pt/hCNC and Pt/XC-72, leading to the best electrocatalytic performance of Pt/hNCNC. In addition, hNCNC exhibited a large loading capacity due to the unique structure and abundant nitrogen sites. Small Pt nanoparticles can be loaded on hNCNC up to 60 wt% without serious agglomeration, and gave higher catalytic currents and higher power density. The hierarchical mesostructure and nitrogen participation of hNCNC can give a general electrode material for energy conversion and storage, e.g., for heterogeneous catalysis, supercapacitors, and lithium air batteries.
2016,
Vol. 37 Issue (7): 1149-1155 
PDF (1137 KB)
2016,
Vol. 37 Issue (7): 1149-1155 PDF (1137 KB)