Assoc. Prof. DENG Dehui and Prof. BAO Xinhe from State Key Laboratory of Catalysis (SKLC), Dalian Institute of Chemical Physics(DICP), in cooperation with Prof. ZHANG Wenhua from China Academy of Engineering Physics (CAEP) had new discoveries based on their previous studies towards two-dimensional catalytic materials and nano confined catalysis.
They found that graphene confined CoN4 structure possesses double-optimum in its activity and stability in catalyzing reduction reaction of I3- to I- in dye-sensitized solar cells (DSSCs). This work has 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”.
It has been known that, it is difficult to achieve high activity and stability at the same time in the field of heterogeneous catalysis. Hence, many researchers are exploring and designing new catalytic materials to achieve both simultaneously. Prof. BAO’s group 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. These work was inspired by their previous studies on the 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). The strong covalent bonds between C and N atoms, N and Metal atoms efficiently anchor the coordinative 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 for 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, the feature of rareness and expensiveness of Pt element impede 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-. Meanwhile, the CoN4/GN possessed the best performance and the electrochemical properities and power conversion effiency (8.40%), which is even superior to that of precious Pt catalyst (7.98%). It shows 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 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. The N atoms acting 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. The desorption of the adsorbed I atom on Co sites was much easier than other transition metal sites. Hence, CoN4/GN possessed the best activity and stability in all MN4/GN catalysts. These findings pave an efficient way for rational design of heterogeneous catalysts with high catalytic activity and stability.
Single-atom metal active sites confined in graphene for iodine reduction reaction in dye-sensitized solar cells (Image by CUI Xiaoju)
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). (Text and Image by JIANG Xiumei Jiang and CUI Xiaoju)
Dr. LU Xinyi
Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
457 Zhongshan Road, Dalian, 116023, China,