Nickel incorporated carbon nanotube/nanofiber composites as counter electrodes for dye-sensitized solar cells

A nickel incorporated carbon nanotube/nanofiber composite (Ni-CNT-CNF) was used as a low cost alternative to Pt as counter electrode (CE) for dye-sensitized solar cells (DSCs). Measurements based on energy dispersive X-rays spectroscopy (EDX) showed that the majority of the composite CE was carbon at 88.49 wt%, while the amount of Ni nanoparticles was about 11.51 wt%. Measurements based on electrochemical impedance spectroscopy (EIS) showed that the charge transfer resistance (R_ct) of the Ni-CNT-CNF composite electrode was 0.71 Ω cm², much lower than that of the Pt electrode (1.81 Ω cm²). Such a low value of R_ct indicated that the Ni-CNT-CNF composite carried a higher catalytic activity than the traditional Pt CE. By mixing with CNTs and Ni nanoparticles, series resistance (R_s) of the Ni-CNT-CNF electrode was measured as 5.96 Ω cm², which was close to the R_s of 5.77 Ω cm² of the Pt electrode, despite the significant difference in their thicknesses: ∼22 μm for Ni-CNT-CNF composite, while ∼40 nm for Pt film. This indicated that use of a thick layer (tens of microns) of Ni-CNT-CNF counter electrode does not add a significant amount of resistance to the total series resistance (R_s-tot) in DSCs. The DSCs based on the Ni-CNT-CNF composite CEs yielded an efficiency of 7.96% with a short circuit current density (J_sc) of 15.83 mA cm^-2, open circuit voltage (V_oc) of 0.80 V, and fill factor (FF) of 0.63, which was comparable to the device based on Pt, that exhibited an efficiency of 8.32% with J _sc of 15.01 mA cm^-2, V_oc of 0.83, and FF of 0.67.