Development of ultrafine crystalline Co particles embedded in 3D flower-like highly conductive carbon matrix for water splitting

Non-noble transition metals at the nanoscale offer a cost-effective and efficient solution for water splitting due to their abundance active sites and highly catalytic activity. However, nanoparticle aggregation during fabrication and electrochemical processes often leads to reduced stability and poor activity, particularly in the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). In this study, we directly prepared ultrafine cobalt nanograins embedded in a three-dimensional (3D) flower-like N-doped carbon matrix (Co/N-C). These ultrafine Co nanoparticles provide abundant active sites, while the N-C matrix ensures high electrical conductivity. The Co/N-C composite demonstrates outstanding catalytic performance, with low overpotentials of 361 mV for OER and 80 mV for HER at 10 mA cm^-2. The Tafel slopes are also favorable, at 49.28 mV dec^-1 for OER and 63.09 mV dec^-1 for HER. Moreover, the Co/N-C catalyst outperforms commercial IrO₂, exhibiting a lower overpotential of 398 mV at 50 mA cm^-2 for OER compared to 464 mV for IrO₂. A cell constructed using the Co/N-C||Co/N-C couple achieves a voltage of 1.676 V at 10 mA cm^-2 and maintains a stable current for an impressive 35 h. This novel synthesis approach and material provide a valuable reference for integrating N-doped carbon with other ultrafine metal nanoparticles for various energy-related applications. © 2025 Elsevier Ltd