Rational Design of NiCo₂S₄ Quantum Dot-Modified Nitrogen-Doped Carbon Nanotube Composites as Robust Pt-Free Electrocatalysts for Dye-Sensitized Solar Cells

Composite materials made of bimetallic sulfide quantum dots and carbon nanotubes are promising electrocatalysts due to the large specific surface area and synergistic effect between sulfides and carbon components. In this work, NiCo2S4 quantum dot-modified nitrogen-doped carbon nanotubes (NiCo2S4 QD@NCNTs) are prepared by a two-step solvothermal method, followed by calcination. The dye-sensitized solar cells containing NiCo2S4 QD@NCNTs as a counter electrode (CE) achieve a photoelectric conversion efficiency of 7.65%, which is superior to that of Pt (7.39%), and exhibit good electrochemical stability in iodine-based electrolytes. The notable performance is attributed to the synergistic effect between NiCo2S4 QD and NCNTs. Hence, polypyrrole-derived NCNTs with high electrical conductivity and many uniformly dispersed N-doped species on the surface tightly anchor NiCo2S4 QD through metal–nitrogen bonds (metal–N–C bonds). As a result, the transfer of electrons from the skeleton to the active site is promoted, the agglomeration of NiCo2S4 QD is hampered, and more active sites are available.