1. Introduction

Due to their high power density and energy-conversion efficiency, direct methanol fuel cells (DMFCs) are promising candidates for portable, transportation and mobile applications ^[1-4]. Pt nanoparticles supported on carbon materials have been recognized among the most active electrocatalysts for methanol oxidation ^[5-7]. However, the use of carbon material supported Pt catalyst usually leads to a thicker catalyst layer in the membrane- electrode assembly (MEA) of the DMFC compared with the use of the same mass of unsupported Pt catalyst, resulting in the increased inner electrical resistance and diffusion length in the catalyst layer. Due to this issue, the design of catalysts with higher loading of Pt nanoparticles on the carbon supports may be a promising strategy for the fabrication of thin-layered MEA.

Since high Pt loading can unavoidably lead to the increased size and agglomeration of Pt particles, great efforts have been focused on developing various carbon materials with particular structures to improve the distribution and restrict the growth of the Pt nanoparticles. In this regard, a variety of nanostructured carbon materials, such as graphene ^{[8, 9]}, carbon black ^[10-12], carbon nanotubes ^[13-16], and mesoporous carbons ^[17-19], have been applied to achieve a high Pt loading (>50 wt%) catalyst while maintaining a high dispersion of small nanoparticles. Among them, orderedmesoporous carbon materials ^{[17, 20]} have been considered as promising high loading catalyst supports, because they provide a high surface area for highly dispersed Pt nanoparticles and ordered uniform mesopores for ion diffusion. In our previous study, a hierarchical nanostructured carbon (OMCS)with a mesopore size of ca. 8 nm had been used as a catalyst support to disperse Pt with low Pt loading (20 wt%) ^[21]. The preliminary results showed that the OMCS-supported Pt (20 wt%) catalyst has significantly enhanced electrocatalytic performance compared with the carbon blacksupported Pt. Inspired by the results, hereinwe employ theOMCS to support Pt nanoparticles with high Pt loading (60 wt%). The higher loading of Pt nanoparticles supported onOMCS is very important for reducing the diffusion layer thicknessandmass transport resistance. In addition, the ordered bicontinuous macropores (between the close-packed mesoporous carbon spheres) facilitate the mass transport of fluids, allowing for enhanced access of reactant molecules to the active sites, while the mesopores provide the large surface area for high dispersion of Pt nanoparticles. In this work, we first demonstrate the preparation of high loading (60 wt%) Pt nanoparticles supported on OMCS and investigate their electrocatalytic properties for the methanol oxidation reaction (MOR).

2. Experimental 2.1. Synthesis of the Pt/OMCS catalyst

Scheme 1 illustrates the procedure for the fabrication of the Pt/OMCS. The detailed preparation is described below.

Synthesis of 3D ordered mesoporous carbon sphere arrays (OMCS): Monodisperse PMMA microspheres with ca. 460 nm in diameter were synthesized using a previously reported method ^[22]. The resulting PMMA microsphere suspension was then transferred to a glass bottle and allowed to stand for several weeks at room temperature to allow the spheres to precipitate completely. After the water was evaporated, PMMA colloidal crystal monoliths were formed. The prepared PMMA colloidal crystal monoliths were heated at 120 ℃ for 15 min to induce a stronger contact between each of the spheres. Then, the PMMA colloidal crystal monolith was immersed in the silica precursor solution (TEOS:0.1 mol L^-1 HCl:ethanol = 1:1:1.5 in weight ratio) for 1 h and then removed carefully from the solution and dried in air at room temperature for 24 h. The freestanding silica inverse opal was obtained after removal of the PMMA spheres by heating at 450 ℃ for 5 h. For the formation of the ordered mesoporous carbon sphere arrays, 1.0 g silica inverse opal was immersed in 4.0 g of a homogeneous ethanol solution containing 1.0 g of resol (Mw ＜ 500) and 0.5 g of amphiphilic triblock copolymer Pluronic F127 for 1 h. The impregnated composites were maintained at 25 ℃ for 6 h to evaporate the ethanol, followed by heating at 100 ℃ for 24 h. The resulting composites were then heated in N₂ at 350 ℃ for 2 h at a heating rate of 1 ℃ min^-1 to remove the F127, and at 5 ℃ min^-1 rising to 900 ℃, followed by a 2 h soak for further carbonation ^{[23, 24]}. Finally, the OMCS was obtained by etching the silica inverse opal with a 5 wt% HF solution for 48 h.

Deposition of Pt on OMCS: ～0.05 g of the OMCS was dispersed in a 2 mL solution of 0.2 g of H₂PtCl₆.6H₂O in ethanol and ultrasonicated for 20 min. After drying at room temperature, the resulting powder was placed in a ceramic boat and reduced at 150 ℃ for 2 h in a H₂/Ar flow.

2.2. Characterization

Scanning electron microscopy (SEM) images were obtained on a Hitachi S-4700 FEG scanning electron microscope. Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) were carried out on a Hitachi H800 and a JEOL JEM-3010 transmission electron microscope, respectively (both operating at 200 kV). Powder X-ray diffraction (XRD) data were collected on a Shimadzu XRD-6000 diffractometer with Cu Ka radiation (λ = 1.5418 Å). The actual Pt contents of the catalysts were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES, IRIS Intrepid Ⅱ XSP, Thermo Elemental).

2.3. Electrochemical experiments

The electrochemical measurements were performed in a potentiostat/galvanostat (Reference 600, Gamry Instruments) using a conventional three-electrode cell with a double junction Ag/AgCl (saturated KCl) electrode and a Pt wire as the reference and counter electrodes, respectively. To prepare the working electrode, 1 mg of the catalyst was ultrasonically dispersed in 1 mL of ethanol. Subsequently, 5 mL of the suspension was deposited onto a 4 mm glassy carbon (GC) electrode. After the electrode was dried at room temperature, 5 mL of a Nafion solution (5 wt%) was placed on the top of the GC substrate and dried to completion at room temperature. The total Pt loadings were controlled at ~0.02 mg cm^-2 . The cyclic voltammetry (CV) tests were performed in an Ar-saturated 0.5 mol L^-1 H₂SO₄ solution either with or without 1 mol L^-1 CH₃OH at room temperature at a scan rate of 50 mV s^-1. All the potentials were converted to the normal hydrogen electrode (NHE) scale using the equation E_NHE = E_{Ag/AgCl;sat. KCl} ⁺ 0.197 V.

3. Results and discussion

Fig. 1A shows a typical SEM image of the 3D ordered PMMA sphere arrays (PMMA opal) prepared from highly monodisperse PMMA spheres (ca. 460 nm). The SEM image of the silica inverse opal with uniform macroporous structure fabricated from PMMA opal is shown in Fig. 1B. The size of the macropores is ca. 410 nm, ~11% smaller than the original PMMA sphere diameter due to the volume shrinkage during the calcination process. Fig. 1C displays an SEM image of the OMCS and it retains the ordered face-centered cubic (fcc) structure in the PMMA opal after the two-step replication procedure. However, due to the shrinkage of carbon precursor during the carbonization process, the size of the carbon spheres reduced to~380 nm. Fig. 1D reveals a typical TEM image of the OMCS, and the inset of Fig. 1D shows the higher magnification image of one mesoporous carbon sphere, in which an ordered cubic mesostructure with 7-9 nm diameter mesopores can be seen. Fig. 1E shows the TEM image of high loading (60 wt%, the actual Pt contents of the catalyst is determined to be 58.4 wt% by ICP analysis) Pt nanoparticles on OMCS (Pt/OMCS). The higher magnification TEM image (inset of Fig. 1E) of the Pt/OMCS demonstrates a dense and homogeneous dispersion of the Pt nanoparticles on the surface of the mesoporous walls. From the corresponding high-resolution TEM (HR-TEM) image (Fig. 1F), the average Pt particle size is about 1.8 nm based on 50 Pt particles randomly selected.

XRD (Fig. 2) was employed to further evaluate the size of the Pt particles on the OMCS. The Pt particle sizes of the Pt/OMCS and commercial 60 wt% Pt/C (HiSpec 9100, Johnson Matthey) catalysts estimated by the Scherrer equation are 2.0 and 2.6 nm, respectively. The Pt particle size of the Pt/OMCS is smaller than that of the commercial 60 wt% Pt/C and the reported high Pt loading (60 wt%) catalysts on carbon black (-3.6 nm) ^[25], ordered mesoporous carbon (-3.0 nm) ^[20], graphene aerogel (3.2 nm) ^[9], shell-core nanostructured carbon (3.2 nm) ^[26], and functionalized multiwall carbon nanotubes (-3.0 nm) ^[13], which is mainly attributed to the mesopore confinement effect that restricts the growth of Pt nanoparticles. Furthermore, the OMCS is more likely to adsorb precursor solution containing Pt ion than bulk mesoporous carbon powders due to its unique hierarchical nanostructure and large mesopores, which favours Pt nanoparticles evenly distributing on the mesoporous walls without large agglomerations.

Fig. 3A shows the CV curves of Pt/OMCS and commercial Pt/C in Ar-saturated 0.5 mol L^-1 H₂SO₄ solution at a scan rate of 50 mV s^-1 at room temperature. The specific electrochemically active surface area (ECSA) of Pt was determined by the area of H desorption between 0 and 0.35 observed on CV curves, and can be calculated by the relation ECSA = QH/(m.c) ^{[27, 28]} where QH is the total charge (mC cm^-2) , m is the actual Pt loading (mg cm^-2) on the GC substrate, and c represents the charge required to oxidize a monolayer of hydrogen on a Pt surface (0.21 mC cm^-2) . The Pt/OMCS catalyst possesses larger ECSA (73.5 m² g^-1) compared with the commercial Pt/C (43.5 m² g^-1) , which is attributed to the smaller and more densely dispersed Pt nanoparticles loaded on the OMCS. And the ECSA value of the Pt/OMCS is higher than or comparable to that of the high loading Pt (60 wt%) supported on surface-functionalized graphene nanosheets (51.0 m² g^-1) ^[8], graphene (42.0 m² g^-1) ^[29], N-doped carbon (68.0 m² g^-1) ^[26], ordered mesoporous carbons (66.2 m² g^-1) ^[30], hollow spherical carbon with mesoporous shell (62.0 m² g^-1) ^[18], functionalized multiwall carbon nanotubes (83.0 m² g^-1) ^[13], and graphene aerogel (84.1 m² g^-1) ^[9]. The ECSA value of the Pt/OMCS is also comparable to or larger than that of the lower loading (20 wt%) Pt nanoparticles supported on the reported mesoporous carbon materials such as the Pt/mesoporous carbon catalyst (82.5 m² g^-1) ^[31], the Pt/ordered mesoporous carbon sphere array catalyst (85.7 m² g^-1) ^[21], and the Pt/highly ordered mesoporous carbons catalyst (40.0 m² g^-1) ^[32].

CVs of the catalysts toward methanol oxidation obtained in 0.5 mol L^-1 H₂SO₄ and 1 mol L^-1 CH₃OH solution at a scan rate of 50 mV s^-1 at room temperature are shown in Fig. 3B and C. The peak current densities of the Pt/OMCS and commercial Pt/C catalysts during the forward scan reach 510 mA mg^-1 (0.69 mA cm^-2) and 92 mA mg^-1 (0.22 mA cm^-2) , respectively. The methanol oxidation mass current density of the Pt/OMC is 5.5-fold higher than that of the commercial Pt/C, and this change is greater than those reported for the Pt/mesoporous carbon materials with lower Pt loadings, such as the Pt/ordered mesoporous carbon spheres (671 mA mg^-1) ^[21], the Pt/nitrogen/ sulfur dual-doped mesoporous carbon (505 mA mg^-1) ^[33], and the Pt/boron-doped ordered mesoporous carbon (511 mA mg^-1) ^[34] (4.6-, 1.7-, and 1.7-fold increases in current density, respectively, compared with the commercial Pt/C). In addition, the ratio of If (forward anodic current peak) to Ib (reverse anodic current peak) can be used to infer the poisoning tolerance of the catalyst. The Pt/OMCS shows a higher value of If/Ib (1.38) than the commercial Pt/C (0.72) , indicating an improved poison tolerance performance of the Pt/OMCS catalyst. The If/Ib value of the Pt/OMCS is comparable to or higher than that of the reported low metal-loading Pt/mesoporous carbon materials, such as the Pt/ordered mesoporous carbon spheres (1.56) ^[21], the Pt/ mesoporous carbon nanofibers (1.23) ^[35], the Pt/highly ordered mesoporous carbon nanofiber arrays (1.20) ^[36], and the Pt/ ordered mesoporous carbon FDU-15 (0.98) ^[37].

The chronoamperomograms (Fig. 3D) recorded at 0.8 V indicate that the current density of the Pt/OMCS is higher than that of the commercial Pt/C catalyst over the entire time range, also exhibiting the improved electrocatalytic performance of the Pt/OMCS catalyst for MOR. In addition, the long-term poisoning rate (d) is calculated by measuring the linear decay of the current for a period of more than 500 s from Fig. 3D by using the following equation: ^[38-40]

$\delta {\rm{ = }}{{100} \over {{I_0}}} \times {\left( {{{{\rm{d}}I} \over {{\rm{d}}t}}} \right)_{t > 500s}}\left( {\% {s^{ - 1}}} \right)$

where (dI/dt)_t>500s is the slope of the linear portion of current decay and I0 is the current at the start of polarization back extrapolated from the linear current decay. The poisoning rates are calculated to be 0.0019 and 0.0104% s^-1 for Pt/OMCS and commercial Pt/C catalysts, respectively. The result further reveals the better poisoning tolerance of the Pt/OMCS catalyst.

Compared with the commercial Pt/C, the significantly enhanced electrocatalytic performance of the Pt/OMCS is probably related to the better dispersion of the Pt nanoparticles and the unique ordered interconnected marcopores and mesopores of the OMCS which allow the facile diffusion of reactants during the reactions. The MOR performance of the Pt/OMCS is higher than or comparable to that of the reported Pt/mesoporous carbon catalysts with low Pt loadings, suggesting that the Pt/OMCS, even with a high Pt loading, would be one of the most active MOR catalysts.

4. Conclusion

In summary, OMCS has been explored to support high loading Pt nanoparticles (60 wt%) as highly efficient electrocatalysts for methanol oxidation reaction (MOR). The prepared Pt/OMCS exhibits uniformly dispersed Pt nanoparticles with an average particle size of ~2.0 nm on the mesoporous walls of the OMCS. The Pt/OMCS catalyst shows larger ECSA and higher MOR performance in comparison to the commercial Pt/C, which can be attributed to the improved dispersion of Pt nanoparticles and the fantastic hierarchical nanostructure with interconnected ordered macropores and mesopores that ensure fast mass transport during the reactions. The high metal-loading Pt/OMCS catalyst with great MOR performance is very important for reducing the diffusion layer thickness and mass transport resistance in electrodes; therefore, this system has potential as an electrocatalyst in the fabrication of thin-layered MEAs.

Acknowledgment

The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 51172014) , the National 973 Program of China (No. 2009CB219903) , and the Scientific Innovation Grant for Excellent Young Scientists of Hebei University of Technology (No. 2015001) .