Highly efficient Co―N―C electrocatalysts with a porous structure for the oxygen reduction reaction
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摘要: 开发一种低成本、高效率和稳定的燃料电池的氧还原反应（ORR）催化剂具有极大挑战性。作者通过先在纳米聚苯乙烯（PS）球体表面均匀生长ZIFs，然后分解核壳结构的ZIF@PS，开发了具有中空完整球形结构和大表面积的Co-N-C ORR催化剂，并进行了系统的表征。所制催化剂Co-NHCP-2具有分层的多孔结构，超大的比表面积（1817.24 m2 g−1），吡啶-N、吡咯-N、石墨-N含量高，且Co分布均匀。作为一种高效的电催化剂，Co-NHCP-2催化剂具有高起始电位（0.96 V）、半波电位（0.84 V）和极限电流密度（5.50 mA cm−2）。与市场上的Pt/C催化剂相比，该催化剂在碱性溶液中表现出约4e的ORR途径以及更强的甲醇耐受性和更高的稳定性。这些结果表明，该Co-N-C复合材料可以作为一种有前景的ORR电催化剂。Abstract: Developing low-cost, highly-efficient and stable catalysts for the oxygen reduction reaction (ORR) of fuel cells is highly desirable yet challenging. We have developed a Co―N―C ORR catalyst with an intact hollow spherical structure and a large surface area which has been systematically characterized. It was produced by the uniform growth of zeolitic imidazolate frameworks (ZIF s) on the surface of nano-polystyrene (PS) spheres followed by their decomposition. Notably, the as-prepared catalyst Co-NHCP-2 (2 represents a mass ratio of 0.6 between Zn(NO3)2·6H2O and 2-methylimidazole) has a porous structure, a super large specific surface area (1817.24 m2 g−1), high contents of pyridinic-N, pyrrolic-N, and graphitic-N, and a uniform Co distribution. As an efficient electrocatalyst, it shows promise in terms of a high onset potential (Eonset) of 0.96 V, a high half-wave potential (E1/2) of 0.84 V, and a limited current density of 5.50 mA cm−2. The catalyst has a nearly 4e pathway for the ORR in an alkaline solution as well as stronger methanol tolerance and higher long-term durability than commercially available Pt/C catalysts. These results show that the obtained material may be a promising electrocatalyst for the ORR.
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Key words:
- Oxygen reduction reaction /
- Hierarchical porous structure /
- Co―N―C catalyst /
- ZIFs /
- Polystyrene sphere
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Figure 1. SEM images of (a) PS, (b) ZIF@PS-2 and (c) Co-NHCP-2. (d-i) EDS elemental analyses of Co-NHCP-2
Figure 2. (a-c) TEM images of Co-NHCP-2 at different magnifications; (d) SAED pattern of Co-NHCP-2; (e-f) HRTEM images of Co-NHCP-2; (g-j) TEM EDS elemental mappings of Co-NHCP-2
Figure 3. (a) XRD patterns, (b) Raman spectra, (c) N2 adsorption-desorption isotherm curve and (d) the corresponding pore distribution of Co-NHCP-1, Co-NHCP-2 and Co-NHCP-3
Figure 4. High resolution XPS spectra of Co-NHCP-2: (a) survey, (b) C 1s, (c) N 1s and (d) Co 2p
Figure 5. Contact angle of water on the surface of (a) Co-NHCP-1, (b) Co-NHCP-2 and (c) Co-NHCP-3
Figure 6. (a) CV curves of Co-NHCP-2 on glassy carbon electrodes in O2 or N2-saturated 0.1 mol L−1 KOH. (b) LSVs of Co-NHCP-1, Co-NHCP-2, Co-NHCP-3, and 20% commercially available Pt/C catalyst in O2-saturated 0.1 mol L−1 KOH at a sweep rate of 5 mV s−1 at 1600 r min−1. (c) Tafel slopes of Co-NHCP-1, Co-NHCP-2, Co-NHCP-3 and commercially available Pt/C catalyst. (d) Rotating-disk voltammograms of Co-NHCP-2 in O2-saturated 0.1 mol L−1 KOH with a sweep rate of 5 mV s−1 at different rotation rates. (e) K-L plots of Co-NHCP-2 at 0.2-0.5 V. (f) Electron transfer number (n) and H2O2 yield derived from the RRDE results
Figure 7. (a) Chronoamperometric responses of Co-NHCP-2 and commercially available Pt/C catalyst by injecting 3 mL of 2% methanol at 300 s; (b) chronoamperometric responses of Co-NHCP-2 and commercially available Pt/C catalyst obtained under 0.1 mol L−1 O2-saturated KOH electrolyte at 1600 r min−1
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