Synergistic Effect of Cobalt Nanoparticles Embedded in Nitrogen-Doped Carbon As a Pt Alternative Electrocatalyst for Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) are an attractive, innovative energy conversion device because of several distinctive benefits that are hardly obtainable from other next-generation solar cells. A typical configuration of DSSCs includes a nanoporous TiO₂ anode sensitized with an inorganic complex, an electrolyte containing I^-/I^3-couple, and a counter electrode (CE). Platinum (Pt) has been preferred most as an electrocatalyst for the CE, because it possesses excellent properties such as a low charge-transfer resistance, a high electrical conductivity, high transmittance. However, the scarcity of Pt and the resulting cost issue has stimulated research to develop low-cost, earth-abundant materials as Pt alternatives in recent years. A wide variety of materials including metal chalcogenides, oxides, nitrides, and carbides have been explored to date.

In this work, we extend the choice of Pt alternatives up to a metal/carbon composite material and elucidate the hidden mechanism of electrocatalytic activity of this novel catalyst. A fast, large-scalable synthesis route via calcination of zeolitic imidazolate framework-67 (ZIF-67) is developed to obtain N-doped carbon incorporated with Co metals (Co/N-C). 2-methylimidazole coordinating to cobalt ions serves as an excellent carbon source that allows for high incorporation of N atoms within carbon lattice. In addition, in situ reduction of the cobalt ions aids in increasing graphitic nature of Co/N-C. Our in-depth electrochemical investigation including cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and Tafel polarization measurements revealed that the electrocatalytic activity of Co/N-C catalyst toward the reduction of I^3- is superior to that of traditional Pt counterpart. When employed for the CE material, the DSSCs equipped with the Co/N-C catalyst outperform those fabricated with Pt: the power conversion efficiency of 8.06% is obtained as compared to Pt CE (7.52%). Our study also provides a new sight into this metal/carbon composite catalyst: the Co nanoparticles embedded in the carbon significantly promote the electrocatalytic activity of N-doped carbon. The stability of the Co/N-C catalyst was investigated to ensure the long-term use, revealing that Co/N-C is substantially more resistive against corrosion by the I^-/I^3- couple, rendering excellent stability to Co/N-C.