Ultrafine Co nanodots embedded in N-doped carbon nanotubes grafted on hexagonal VN for highly efficient overall water splitting

Nanoscale non-noble transition metals are crucial to the development of efficient electrocatalysts for water splitting due to the lower cost compared to Pt-based catalysts, natural abundance, excellent activity, and high utilization per nanoparticle. However, inevitable aggregation during the fabrication and electrochemical processes result in unfavorable oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) activities as well as poor stability and durability. Herein, novel ultrafine metallic Co nanodots 2.2 nm in diameter are embedded in the wall of N-doped carbon nanotubes (N-CNTs) grafted on VN to form the hybrid structure of Co/N-CNT/VN by a simple process. Owing to the abundant active sites on the Co nanodots, high electron/mass transfer ability, and good structural and electrochemical durability, Co/N-CNT/VN has outstanding OER and HER properties such as low overpotentials of 240 mV and 63.4 mV at 10 mV cm⁻², respectively. For demonstration, the overall water splitting cell composed of the bifunctional Co/N-CNT/VN catalyst as both electrodes can be driven by a standard 1.5 V AAA battery. The novel concept and materials can be extended to the preparation of CNTs embedded with other ultrafine metal nanoparticles such as Fe, Co, and Ni for energy applications.