Electrocatalytic performance of CNTs/graphene composited rare earth phthalocyanines (M=La, Y, Yb, Sc)

Abstract

In this study, a class of rare earth composite sandwich phthalocyanines (MPcs, M = La, Y, Yb, Sc) were prepared and compounded with graphene and carbon nanotubes to obtain MPc/Gr and MPc/CNTs composites. The electrocatalytic behaviors of MPc/Gr and MPc/CNTs electrodes were further investigated. The results show that the central rare earth metal has a large influence on the electrocatalytic performance. For the MPcs/Gr samples, ScPc with the smallest ionic radius and molecular size can be more uniformly dispersed in graphene, and the hydrogen precipitation overpotential of ScPc/Gr electrode is 514 mV, corresponding to a Tafel slope of 148 mV/dec, with better electrocatalytic performance than other rare earth metal phthalocyanines. As for the MPc/CNTs composites, LaPc, which has the largest ionic radius and molecular size, is more uniformly dispersed on the surface of CNTs, so that the LaPc/CNT electrode exhibits the best LSV performance with the smallest corresponding Tafel slope (176 mV/dec). The main reason is the different binding modes of MPcs molecules in Gr and CNTs. When rare earth phthalocyanine is combined with layered graphene, the smallest size of rare earth phthalocyanine (ScPc) is more easily embedded in the graphene layer, resulting in better homogeneity of the composite, larger effective contact area and better electrocatalytic performance. In contrast, when rare earth phthalocyanine is bound to carbon nanotubes in a tubular structure, it is mainly bound by attaching to the surface or being entangled by the carbon nanotubes. In this case, the rare-earth phthalocyanine molecules (LaPc) with larger layer spacing can provide more contact area with CNTs, forming a more uniform and effective composite, which eventually provides more active sites and better electrocatalytic performance.