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Exfoliation of amorphous phthalocyanine conjugated polymers into ultrathin nanosheets for highly efficient oxygen reduction

Liu, Wenping, Wang, Chiming, Zhang, Lijie, Pan, Houhe, Liu, Wenbo, Chen, Jun, Yang, Dongjiang, Xiang, Yanjuan, Wang, Kang, Jiang, Jianzhuang, Yao, Xiangdong
Journal of materials chemistry A 2019 v.7 no.7 pp. 3112-3119
atomic force microscopy, batteries, catalysts, cobalt, electric potential difference, electrochemistry, electronics, graphene, iron, nanosheets, oxygen, polymers, positional isomers, potassium hydroxide, transmission electron microscopy
It is a significant challenge to develop a high-efficiency synthetic methodology to access fully conjugated 2D conjugated polymer (CP)/covalent organic framework (COF) nanosheets (NSs) that have great application potential for electronics and energy. Herein, we report the exfoliation of a series of amorphous ethynyl-linked phthalocyanine (Pc) CPs (MPc-CPs, M = Fe, Co, Fe₀.₅Co₀.₅) into ultrathin MPc-CP NSs. Random coupling between the four regioisomers (with D₄ₕ, D₂ₕ, C₂ᵥ and Cₛ symmetry) of the two tetra-β-substituted phthalocyanine precursors endows the resulting phthalocyanine conjugated polymers MPc-CPs with intrinsic structural defects and a disordered framework on individual layers. This in turn induces a diminished interlayer overlapping and a weakened interlayer π–π stacking interaction, facilitating the possible exfoliation of MPc-CPs into ultrathin 2D NSs with a yield of over 50%. The direction observation by transmission electron microscopy (TEM) and atomic force microscopy (AFM) demonstrates that the ultrathin MPc-CP NSs possess a smooth surface with a uniform thickness of 1–3 nm and a lateral size of hundreds of nanometers. The as-prepared bimetallic Fe₀.₅Co₀.₅Pc-CP NSs were further used to fabricate a heterostructure Fe₀.₅Co₀.₅Pc-CP NS@G with graphene NSs as an oxygen reduction reaction (ORR) catalyst, which exhibits an onset potential of 1006 mV and a half-wave potential of 927 mV in 0.1 M KOH, representing one of the best values in an alkaline medium. Moreover, the excellent ORR activity of the exfoliated tetrapyrrole-based conjugated NSs hybridized with graphene has also been demonstrated by a Zn–air battery device, showing an open circuit voltage of 1.34 V and a peak power density of ca. 180 mW cm⁻².