不同氮掺杂石墨烯氧还原反应活性的密度泛函理论研究

Density Functional Theory Study of Oxygen Reduction Reaction on Different Types of N-Doped Graphene

N掺杂石墨烯作为一种具有较高活性和稳定性的氧还原反应(ORR)催化剂,受到人们的广泛关注。然而不同的N掺杂类型对氧还原活性的影响一直存在争议。本文通过密度泛函理论分别对石墨型和吡啶型两种N掺杂石墨烯的ORR活性进行比较研究。能带结构分析表明,石墨氮掺杂石墨烯(GNG)的导电性随掺N量的增加而降低;吡啶氮掺杂石墨烯(PNG)的导电性则随掺N量的增加先提高后降低。当N掺杂浓度达到4.2%(原子分数)时,PNG具有最优导电性。且当N掺杂浓度大于1.4%时,PNG的导电率总是高于GNG。氧还原自由能阶梯曲线发现O_2的质子化是整个氧还原过程的潜在控制步骤。在同等氮掺杂浓度下,O_2的质子化自由能能变在GNG上低于在PNG上,意味着若在同等电子传输能力的情况下,GNG具有比PNG更优异的催化活性。进一步分析发现:当N掺杂浓度在低于2.8%时,GNG和PNG导电性差异小,其催化ORR活性由O_2质子化反应难易程度决定,GNG的催化活性优于PNG;当N掺杂浓度高于2.8%时,氮掺杂石墨烯的电子传输性能(导电性)成为决定催化剂ORR活性的主要因素,因此PNG表现出较GNG更高的活性。

N-doped graphene has aroused much interest owing to its high activity and stability in oxygen reduction reaction （ORR） catalysis. However, the contribution of different types of N-doped graphene to ORR activity remains in dispute. Based on this issue, this paper conducts a comparative study of the ORR on graphitic N-doped graphene （GNG） and pyridinic N-doped grapheme （PNG）. Band structure calculations show that the conductivity of GNG decreases as the nitrogen content increases; while that of PNG first increases to the highest at nitrogen content of 4.2% （atomic fraction）, and then decreases. The conductivity of PNG is always higher than GNG when the doped nitrogen content is greater than 1.4%. Additionally, the free energy diagram of ORR shows that protonation of 02 is the potential-determining step among the whole ORR process, and the free energy change of this step on GNG is lower than on PNG, suggesting that GNG has higher ORR activity than PNG if their electron transport ability are the same. When the N content is lower than 2.8%, the conductivity difference between GNG and PNG is almost negligible, thus GNG with a higher capacity of O2 protonation exhibits better ORR activity than PNG. When the N content is greater than 2.8%, in this case conductivity rather than free energy change will dominate, therefore the ORR on PNG will occur faster than on GNG because of its higher conductivity.