A methanol-tolerant Pt/CoSe2 nanobelt cathode catalyst for direct methanol fuel cells.

Direct methanol fuel cells (DMFCs) have received considerable and persistent attention, because methanol is an abundant, inexpensive liquid fuel that is easier to store and transport than hydrogen. Despite the great advances made in this field, two main issues affecting efficiency and power density must still be considered, that is, sluggish kinetics of the fuel-cell anode reaction and so-called methanol crossover. The small methanol molecule can easily cross over from the anode to the cathode side through the polymer membranes of DMFCs, and then reacts directly with the cathode catalyst and O2 to decrease the cathode potential and thus reduce fuel efficiency. One approach to addressing this problem is the development of methanol-tolerant cathode catalysts for the oxygen reduction reaction (ORR). Recent research on methanol-tolerant catalysts has shown that transition metal macrocycles, Ru-based chalcogenides, and some platinumbased alloys all show methanol tolerance while retaining catalytic activity for the ORR. Nevertheless, disadvantages still exist. For instance, Pt-free electrocatalysts often show much lower activity and inferior long-term stability under fuel-cell conditions; Pt-based alloy cathode electrocatalysts are available only with low metal loading and thus are not quite suitable for DMFCs. Therefore, development of novel methanol-tolerant electrocatalysts with considerable stability and high ORR activity is important. Currently, cobalt chalcogenides are attracting enormous interest as new ORR electrocatalysts. Cobalt sulfides such as Co3S4 and Co9S8 are rather active for four-electron ORR in acidic electrolytes. In addition, cobalt selenides and tellurides show electrocatalytic ORR activity in nanocrystal form. In particular, the CoSe2/C nanoparticles fabricated by Alonso-Vante et al. exhibit good methanol tolerance. However, the ORR activity of these materials is still low, and they are far from DMFC application. Recently, we described a synthetic strategy that allows large-scale fabrication of ultrathin lamellar mesostructured CoSe2/diethylenetriamine (DETA) nanobelts in a binary solution. The lamellar nanobelts have several advantages over previous cobalt chalcogenides: homogeneously distributed, copious surface amino groups that allow loading of highly dispersed metal nanoparticles, and exceptional stability under strongly acidic conditions. With these merits, we expect that methanoltolerant electrocatalysts with high performance can be designed on the basis of this material. Here we report that a new methanol-tolerant Pt/CoSe2 nanobelt electrocatalyst for DMFC applications can be synthesized by in situ loading of Pt nanoparticles on CoSe2/ DETA nanobelts through a polyol reduction approach. The Pt/CoSe2 electrocatalysts display relatively high ORR catalytic activity in acidic medium. More importantly, the nanohybrid structures are highly resistant to methanol, even at concentrations of up to 5m. Mesostructured CoSe2/DETA nanobelts were first synthesized in high yield by a simple solvothermal strategy reported previously. Then, Pt NPs were synthesized in situ on the surface of CoSe2/DETA nanobelts through a facile polyol reduction approach. The multilayered CoSe2/DETA nanobelts are highly acid resistant, although selenides are generally vulnerable to attack by acids. The H2SO4 treatment process is followed the recent report by Kanatzidis et al. on treatment of mesostructured c-C20PyPtSnSe materials with strong acids. The H2SO4-treated sample retains the singlecrystalline nature and growth direction of the original CoSe2/ DETA nanobelts (see Supporting Information Figure S1). High-resolution (HR) TEM studies along the lateral thickness direction of H2SO4-treated CoSe2/DETA nanobelts showed that interlayer distance decreased from 1.08 to 0.67 nm (see Supporting Information Figure S1d). In fact, acid treatment is a simple ion-exchange process. Only protonated DETA molecules between two neighboring CoSe2 slabs are replaced by protons, and then the flexible inorganic skeleton contracts accordingly. The results suggest that the nanobelts have exceptional stability under strongly acidic conditions and retain their shape, composition, structural integrity, and single-crystalline nature. Moreover, they have a high BET surface area of 77 m g 1 (see Supporting Information Figure S2). Loading of Pt NPs on the surface of CoSe2/DETA nanobelts was confirmed by XRD patterns (see Supporting Information Figure S3c, left). The TEM images in Figure 1a and b show that Pt NPs are homogeneously decorated on the backbone of CoSe2/DETA nanobelts. The average size of the Pt NPs is about 8.3 nm (inset in Figure 1a), which corresponds [*] Dr. M.-R. Gao, Q. Gao, J. Jiang, C.-H. Cui, W.-T. Yao, Prof. Dr. S. H. Yu Division of Nanomaterials & Chemistry Hefei National Laboratory for Physical Sciences at Microscale Department of Chemistry, University of Science and Technology of China Hefei 230026 (P. R. China) Fax: (+ 86)551-360-3040 E-mail: shyu@ustc.edu.cn Homepage: http://staff.ustc.edu.cn/~ yulab/