MOF衍生的单原子催化剂中γN原子在ORR反应中的远程作用及其调制机制

Insights into the critical roles ofγN atoms in understanding the oxygen reduction on MOF-derived single-atom catalysts

燃料电池的发展有助于促进“双碳”目标的早日实现,但其阴极氧还原反应(ORR)的多步质电耦合传输机制导致其动力学过程缓慢.目前,金属有机框架材料衍生的单原子催化剂(SACs)因具有较高的ORR催化活性而被认为是一种优势燃料电池阴极材料.然而,由于SACs活性位点的外层结构难以精确表征,因此相关催化反应机制尚不明确.鉴于此,本研究着眼第三配位层γN原子对其邻近βC原子位点和活性金属中心的双重影响开展了深入研究.结果表明,不同位置和数量的γN掺杂会改变Fe–N_(4)/C–γN_(x)结构中被取代C位点的电子结构,从而影响邻近金属中心的βC原子电荷分布.同时,这些变化还将通过长程效应间接影响金属中心的电子分布及自旋,从而增强4e−反应路径的ORR性能.更重要的是,电负性增强的βC原子更容易吸附OH基团,因此使得γN掺杂的Fe–N_(4)/C–γN_(x)具备更加优异的碱性环境ORR理论性能.基于以上结果,我们还发现γN调制的βC辅助ORR路径具有比常规4e−反应路径更低的过电势.本研究针对决速步的波函数分析促进了从电子层次对该反应作用机制的理解,揭示了该类单原子催化剂中γN位点精确设计和表征的重要意义.

The development of fuel cells is conducive to achieving carbon peaking and carbon neutrality goals,but the oxygen reduction reaction(ORR)of its cathodes is still kinetically slow due to multi-step proton-coupled electron transfer.Currently,metal-organic frameworks-derived single-atom catalysts(SACs)are emerging as efficient ORR catalysts.However,the outer layer structure of the active site is difficult to be determined,resulting in the uncertainty of the reaction mechanism.Here,the dual effect of the outerγN atom on its neighboringβC atom and the distant metal center was investigated.The results show that the doping ofγN with different locations and amounts changes the electronic configuration of the replaced C sites in Fe–N_(4)/C–γNx structures,which in turn affects the charge distribution of theβC atom closer to the metal center.And these changes indirectly affect the electron distribution and spin of the metal center through long-range effects,significantly enhancing the ORR performance of the 4-e−paths.More importantly,the electronegativity-enhancedβC atoms are thus more likely to adsorb the OH moiety in alkaline electrolyte typically used in experiments.Thus,aγN-modulatedβC-assisted pathway was predicted and a higher thermodynamic limiting potential of ORR than that of the common 4-e−pathway was achieved.Further wavefunction analysis,based on the potential limiting steps,provided an understanding on the electronic level and revealed the significance of precise design and characteristics ofγN site in analogous SACs.