Co, N co-doped porous carbons as high-performance oxygen reduction electrocatalysts
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摘要: 钴和氮共掺杂炭催化剂(Co-NC),由于成本低廉和资源丰富而备受关注,但其低的氧还原反应(ORR)活性和对氧气的双电子(2e-)还原生成H2O2的高选择性,进一步影响了其在燃料电池中的应用。因此,Co-NC催化剂是通过在650、750和850 ℃下热解CoCl2和壳聚糖的混合物(用ZnCl2预处理),然后用HNO3洗涤并在900 ℃下退火而制备。结果表明,ZnCl2有利于壳聚糖与Co2+的络合,同时也是造孔的化学活化剂。此外,退火会导致通过碳热还原锌离子形成球形Zn金属纳米颗粒蒸发,从而形成Co-NC催化剂独特的多孔结构,其球形孔填充了球形炭纳米颗粒,而球形炭纳米颗粒在Co催化过程中氮掺杂炭生长而形成。在Co催化下,催化剂的石墨化度得到了改善。在750 ℃的热解温度下获得的Co-NC催化剂与商业Pt/C催化剂相比,具有相同的4e-路径,展现出更高的ORR催化活性、长期稳定性和甲醇耐受性,这得益于其大的比表面积、可分散Co物种的高吡啶氮和石墨氮含量及其优异的导电性。Abstract: Although the Co and N co-doped carbon catalyst (Co-NC) has attracted much attention because of its low cost and the natural abundance of the dopants, it has a low oxygen reduction reaction (ORR) activity and a high selectivity for the two-electron (2e-) reduction of oxygen to H2O2, which affects its use in fuel cells. Co-NC catalysts were prepared by the pyrolysis of a mixture of cobalt chloride and chitosan pretreated with zinc chloride at 650, 750 and 850 oC, followed by washing with nitric acid and annealing at 900 oC. The results indicate that zinc chloride helps the complexing of chitosan with Co2+, which is also a chemical activator that generates pores, and annealing caused the evaporation of the spherical Zn metal nanoparticles formed by the carbothermal reduction of Zn ions, leading to a unique porous structure of the catalysts with spherical pores filled with spherical carbon nanoparticles formed by the growth of nitrogen-doped carbon as a result of the Co catalyst. The degree of graphitization is also improved by the Co catalyst. The Co-NC catalyst obtained at a pyrolysis temperature of 750 oC shows the same four-electron (4e-) reduction of oxygen as a commercial Pt/C catalyst, and a significantly higher ORR catalytic activity, longer-term stability and better methanol tolerance than a commercial Pt/C catalyst. These are due to its large specific surface area, high contents of pyridinic nitrogen and graphitic nitrogen that disperse the Co species and its excellent electrical conductivity.
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Key words:
- Oxygen reduction reaction /
- ORR catalyst /
- Porous carbon /
- Carbon spheres /
- N-doped
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Figure 6. (a) CV curves of Co-NC-750 in N2- and O2-saturated 0.1 mol L−1 KOH electrolytes, (b) LSV curves of Co-NC-850, Co-NC-750, Co-NC-650, 20% Pt/C and NC at a rotating rate of 1600 r/rim in the O2-saturated 0.1 mol L−1 KOH electrolyte, (c) E1/2, Jd and (d) Tafel plots of Co-NC-850, Co-NC-750, Co-NC-650, 20% Pt/C and NC, (e) LSV curves of Co-NC-750 at different rotations of 400–2025 r/min, (f) corresponding K-L plots and electron transfer numbers, (g) LSV curves of Co-NC-750 before and after 10000 cycles, (h) chronoamperometric response of Co-NC-750 and 20% Pt/C and (i) methanol tolerance tests of Co-NC-750 and 20% Pt/C.
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