摘要/Abstract

金属/氮/碳催化剂（M/N/C，M=Fe、Co等）是最有发展前景的非贵金属电催化剂之一，其性能依赖于催化剂表面的活性物种密度.通过常规的热解含氮前驱物与金属盐的方法制得的催化剂往往存在金属活性物种被包埋而不能有效利用的缺点.考虑到石墨相氮化碳（g-C₃N₄）富含类吡啶氮和亚纳米孔腔结构，将g-C₃N₄包覆在高导电性碳纳米笼（hCNC）表面，进而利用表层g-C₃N₄的配位和限域作用锚定大量Co²⁺离子，获得的Co/g-C₃N₄/hCNC复合物经热解后形成了活性位高度暴露、导电性好、孔结构丰富的Co/N/C催化剂.800℃热解得到的最优化催化剂在碱性介质中展现出优异氧还原活性，其起始电位（0.97 V）与商业Pt/C催化剂相当，且抗甲醇干扰性能和稳定性优异.此项研究提供了一种构建具有高度暴露活性位的M/N/C催化剂的有效策略.
关键词: 钴/氮/碳, 氧还原催化剂, 高度暴露活性位, 碳纳米笼, 燃料电池
The ever-growing crises of fossil fuel shortage and environmental pollution urgently call for the exploration of clean and renewable energies. Fuel cells present high power efficiency and emit zero pollutants, showing great potential in the future energy system. The main bottleneck of fuel cell commercialization is the sluggish oxygen reduction reaction (ORR) at the cathode. To date, the most active electrocatalysts for ORR are platinum and its alloys. However, the scarcity, high cost and susceptibility to methanol crossover of precious metals hinder the large-scale application of fuel cells. The development of highly efficient and stable non-precious metal ORR electrocatalysts with high resistance to methanol crossover is of great significance. M/N/C (M=Fe, Co, etc.) catalysts are attractive non-precious metal based ORR electrocatalysts and their performance depends on the density of active sites on the catalyst surface. The common synthesis of M/N/C catalysts is to pyrolyze the mixture of metal salt, nitrogen-containing precursor and carbon support. However, so-synthesized catalysts usually contain large metal-based particles, leading to the shortcomings of low density and partial embedding of active sites. Graphitic carbon nitride (g-C₃N₄) with high concentration of pyridine-like nitrogen in heptazine heterorings can provide abundant and uniform nitrogen coordination sites, which can capture metal ions by the interaction between metal ions and N sites. In addition, g-C₃N₄ would be decomposed largely during pyrolysis, which is beneficial to form highly exposed M/N/C active sites by pyrolyzing the g-C₃N₄ with adsorbed metal ions. Herein, we reported the construction of Co/N/C electrocatalysts with highly exposed active sites. Specifically, the g-C₃N₄ was uniformly supported on the surface of high-conductive hierarchical carbon nanocages (hCNC) by the impregnation and pyrolysis process, leading to the formation of g-C₃N₄/hCNC composite. Co²⁺ ions were then captured by the g-C₃N₄ species on the surface owing to the interaction between the lone pair electrons of nitrogen and the Co²⁺ ions, and the subsequent pyrolysis led to the Co/N/C catalysts with highly exposed active sites, high conductivity and multiscale pore structure. The optimized catalyst obtained at 800℃ exhibits excellent ORR performance in alkaline medium, with a high onset potential (0.97 V) comparable to commercial Pt/C catalyst, while much better stability and high immunity to methanol crossover. This study demonstrates an effective strategy for the construction of high-efficient M/N/C catalysts with highly exposed active sites.
Key words: cobalt/nitrogen/carbon, oxygen reduction electrocatalysts, highly exposed active sites, carbon nanocages, fuel cell


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