Molecular-Level Synthesis of Cobalt Phosphide Nanocrystals Confined in Highly Nitrogen-Doped Mesoporous Carbon Electrocatalyst for Highly Efficient Dye-Sensitized Solar Cells

Developing an efficient synthetic approach for the accurate synthesis of nonprecious metal counter electrode (CE) electrocatalysts with superior catalytic activity and electrochemical stability is critically important for the commercial application of dye-sensitized solar cells (DSSCs). In this study, a new molecular-level synthetic strategy was innovatively developed by copolymerization of melamine and resorcinol with formaldehyde under specific pH conditions and their hydrothermal cooperative assembly with organic–inorganic cobalt phosphate and F127 to construct the high-molecular-weight EDTMPA-Co/melamine-formaldehyde-resorcinol resin/F127 copolymer with good thermal stability and compositional homogeneity. The subsequent carbothermal reduction of EDTMPA-Co using melamine-formaldehyde-resorcinol resin as carbon–nitrogen cosources and F127 as a soft template contributed to the accurate synthesis of uniformly dispersed cobalt phosphide nanoparticles incorporated in a highly nitrogen-doped mesoporous carbon (CoP/NMC) catalyst for the first time. It was found that the abundant mesopores, high-nitrogen-content conductive carbon walls (12.8 atom %), and numerous exposed CoP nanoparticles were creatively formed in the resultant CoP/NMC hybrid, which greatly accelerated the electrolyte/electron transportation and supplied sufficient ion-accessible electrocatalytic nanoactive sites for catalysis. When applied as a CE catalyst, the CoP/NMC hybrid revealed a strong synergistic effect in adsorption and catalytic reduction of triiodide ions, and the CoP/NMC reached a low charge transfer resistance (Rcₜ) of 1.35 Ω. As a consequence, the CoP/NMC CE assembled DSSC delivered a high photo-to-electricity conversion efficiency (η) of 8.53% and exceptional long-term electrochemical stability with a remnant efficiency of 7.80% after 200 h of illumination, outperforming the state-of-the-art Pt. The catalytic mechanism of CoP/NMC CE toward triiodide reduction was studied by density functional theory calculations, which revealed that the relative energy for breaking the I–I bond was significantly improved by chemisorption-induced formation of Co–I covalent bonds between the triiodide intermediate and Coσ⁺ atoms in the exposed CoP nanoactive sites, and the electrocatalytic reduction of I₂* to I–* was substantially facilitated.