New Progress in Single-Atom Metal Active Sites confined in Two Dimensional Nanomaterials
Recently, on the basis of their previous studies towards two-dimensional catalytic materials and nano confined catalysis, Assoc. Prof. Dehui Deng and Prof. Xinhe Bao etc. from SKLC, DICP, in cooperation with Prof. Wen-Hua Zhang from Academy of Engineering Physics, found that graphene confined CoN4 structure possessed double-optimum in its activity and stability in catalyzing reduction reaction of I3- toI- in dye-sensitized solar cells (DSSCs).This work has recently been published as a communication in Angew. Chem. Int. Ed. (Angew. Chem. Int. Ed. 2016, 55, 6708), and has been selected as “Inside Back Cover”.
As we all know, in the field of heterogeneous catalysis, the activity and stability of the catalysts often like the two ends of the “seesaw”. So, it is difficult to achieve high activity and high stability at the same time. Hence, many researchers are exploring and designing new catalytic materials to achieve the high activity and high stability simultaneously. Inspired by their previous studies on the successful synthesis of single-atom transition metal catalysts confined in two dimensional nanomaterials, such as graphene (Sci. Adv. 2015, 1(11): e1500462) and MoS2(Energy Environ. Sci. 2015, 8, 1594), they synthesized a series of innovative composite materials with confined MN4(M = Mn, Fe, Co, Ni, and Cu) structures in the basal plane of graphene nanosheets (MN4/GN) via high-energy ball milling of transition metal phthalocyanine and graphene nanosheets under controllable conditions. Thestrong covalent bonds between C atoms and N atoms, also between N atoms and Metal atoms can efficiently anchor the coordinatively unsaturated transition metal centers, achieving planar metal N4 central structures. In addition, the N atoms as “anchors” could significantly improve the structural stability of the transition metal centers in the crystal lattice of graphene.
DSSCs have attracted great attention due to its high power conversion efficiency, in which platinum (Pt) has been widely utilized as a standard counter electrode for reduction of I3- to I-. However, Pt element is extremely rare and expensive, which impede badly its use in the large-scale commercialization of DSSCs. On the basis of their previous study about the alternative Pt catalysts(Angew. Chem. Int. Ed. 2014, 53, 7023), they found that the MN4/GN showed excellent catalytic performance for the reduction of I3- to I-, in which CoN4/GN possessed the best performance and the electrochemical properities and power conversion effiency (8.40%) are even superior to that of precious Pt catalyst (7.98%), showing great potential to replace Pt counter electrode in DSSCs.In addition, the catalytic activity and stability of CoN4/GN are located in the peak of the volcano curve in all MN4/GN catalysts, achieving high catalytic activity and high stability simultaneously. Density functional theory calculations found that the high stability can be attributed to the highest stability of C-N-Co bonds in comparison with other metals, in which N atoms as “anchors” can bond with C atoms and Co atoms strongly. And the high activity is tightly correlated with the appropriate adsorption energies of iodine on the counter electrode. Because in the reduction of I3- to I-, the desorption of the adsorbed I atom was recognized as the rate determining step, and the desorption of the adsorbed I atom on Co sites was much easier than other transtion metal sites. Hence, CoN4/GN possessed the best activity and stability in alll MN4/GN catalysts. These findings in the present work pave an efficient way for rational design of heterogeneous catalysts with highly catalytic activity and stability.
These works are supported by National Natural Science Foundation of China, Strategic Priority Research Program of the Chinese Academy of Sciences, and Collaborative Innovation Center of Chemistry for Energy Materials (2011.iChEM). (by Xiumei Jiang and Xiaoju Cui)