Controlled Synthesis of Nickel Encapsulated into Nitrogen-Doped Carbon Nanotubes with Covalent Bonded Interfaces: The Structural and Electronic Modulation Strategy for an Efficient Electrocatalyst in Dye-Sensitized Solar Cells

Developing an efficient and low-cost synthetic approach to controllably synthesize non-precious-metal counter electrode (CE) electrocatalysts with superior catalytic activity and electrochemical stability is critically important for the mass production of dye-sensitized solar cells (DSSCs). Herein, we proposed a simple, economical, and easily scalable synthetic route for copyrolysis of melamine and nickel acetate precursors to access the well-defined Ni-encapsulated and nitrogen-doped carbon nanotubes (Ni-NCNTs). The synthetic mechanism was comprehensively investigated by creatively analyzing the phase structure evolution and dynamical decomposition behaviors, and revealed the construction of Ni-NCNTs based on the Ni-catalyzed tip-growth mechanism. Furthermore, the meticulous structural design of Ni nanoparticles intercalated in N-doped CNTs endows Ni-NCNTs with homogeneously distributed Ni-C interfaces, abundant structural defects, and a porous architecture, as well as good electrical conductivity and corrosion-resistance properties. When used as counter electrode for DSSCs, the device delivers a high power conversion efficiency of 8.94% under simulated sunlight (AM 1.5, 100 mW cm(-2)) and long-term stability with a remnant efficiency of 8.34% after 100 h of illumination, superior to those of conventional Pt. The outstanding catalytic performance of Ni-NCNTs was mainly attributed to the synergetic effect of intercalated Ni with N-doped CNTs at the unique Ni-C interfaces, and the concomitant electronic interaction of Ni and N with C atoms in the interfacial nanoregime. The systematic studies on the synthetic mechanism and structure activity relationship provide a new insight into the rational design of structural and electronic properties for high-performance Ni-NCNT CEs, as well as into the fundamental understanding of their catalytic mechanism for triiodide reduction.