Density-Functional-Theory Calculation Analysis of Active Sites for Four-Electron Reduction of O2 on Fe/N-Doped Graphene
Density-Functional-Theory Calculation Analysis of Active Sites for Four-Electron Reduction of O2 on Fe/N-Doped Graphene
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DOI：
10.1021/cs501170a
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发表时间：
2014-11-01
期刊：
影响因子：
12.9
通讯作者：
Chen, Shengli
中科院分区：
文献类型：
--
作者：
Liang, Wei;Chen, Junxiang;Chen, Shengli
掺杂有氮（N）和/或金属-N配位结构的纳米碳在替代Pt用于催化燃料电池中的氧还原反应（ORR）方面具有很大的前景。对这些材料中ORR活性位点的性质缺乏明确的看法，阻碍了通过合理的掺杂结构来减少它们与Pt的活性间隙的进展。使用14种类型的N和FeN的掺杂结构在石墨烯作为模型系统，系统的密度泛函理论（DFT）计算进行统一的电化学热力学框架内和相同的反应机制，以获得洞察掺杂纳米碳的ORR活性位点。在与各种石墨烯掺杂结构相关的表面位点处的关键ORR中间体的计算的吸附自由能之间获得标度关系。反应自由能分析表明，质子电子转移耦合的O-2吸附和/或吸附的羟基基团（*OH）的还原是大多数掺杂石墨烯上的ORR中的活性决定步骤，并且各种石墨烯掺杂结构的ORR活性可以用单个热力学描述符描述，即 *OH的吸附自由能（三角形G*OH）。建立了掺杂石墨烯中ORR活性随三角形G*OH变化的模型火山图，表明与扶手椅型石墨型N、锯齿型吡啶型N和锯齿型吡啶型N氧化物等边缘N掺杂结构相关的表面活性位为催化ORR提供了最佳的含氧物种结合强度。一些其他结构，如面内石墨N和FeN 4络合物和氢化锯齿形吡啶N，也有望形成ORR活性位点。根据活性碳原子费米能级附近的pz态密度，分析了不同石墨烯掺杂结构上含氧物种不同结合强度的可能电子结构起源。这些结果可作为设计掺杂纳米碳的ORR电催化剂的指导。特别是，它表明，仅仅N掺杂确实可以产生高活性的电催化位点的ORR在纳米碳。
Nanocarbons doped with nitrogen (N) and/or metal-N coordination structures hold great promise in replacing Pt for catalyzing the oxygen reduction reaction (ORR) in fuel cells. The lack of clear views on the natures of ORR active sites in these materials has hindered the progress in reducing their activity gap to Pt through a rational desire of doping structures. Using 14 types of N and FeN doping structures in graphene as model systems, systematic density-functional-theory (DFT) calculations are performed within a unified electrochemical thermodynamic framework and the same reaction mechanism to gain insights into ORR active sites in doped nanocarbons. Scaling relations are obtained between the calculated adsorption free energy of key ORR intermediates at surface sites associated with various graphene doping structures. Reaction free energy analysis indicates that the protonelectron transfer coupled O-2 adsorption and/or reduction of adsorbed hydroxyl group (*OH) are the activity-determining steps in the ORR on most doped graphenes and that the ORR activity of various graphene doping structures can be described with a single thermodynamic descriptor, namely, the adsorption free energy of *OH (triangle G*OH). A model volcano plot of ORR activity as a function of triangle G*OH is established for active sites in doped graphenes, which indicates that the surface sites associated with a few edge N-doping structures, such as armchair graphitic N, zigzag pyridinic N, and zigzag pyridinic N oxide, offer optimized binding strength of oxygenated species for catalyzing the ORR. Some other structures, such as in-plane graphitic N and the FeN4 complex and hydrogenated zigzag pyridinic N, are also expected to form ORR activity sites. The possible electronic structure origin of the differing binding strength of oxygenated species on various graphene doping structures is analyzed in terms of the density of pz states near the Fermi level of active carbon atoms. These results may serve as guidance for designing ORR electrocatalysts of doped nanocarbons. Especially, it is revealed that merely N doping indeed can produce highly active electrocatalytic sites for the ORR in nanocarbons.