Urea-Assisted Synthesis and Tailoring Cobalt Cores for Synergetic Promotion of Hydrogen Evolution Reaction in Acid and Alkaline Media

Interconnected N-doped carbon nanotubes enclosed with Co nanoparticles have been synthesized by a strategy of urea-assisted pyrolysis of bimetallic metal–organic framework (MOF) precursor. The modulation of Co/Zn ratio in bimetallic MOF precursors can not only regulate spatial dispersion and local structural disorder of Co cores, but also lead to the alterable proportion of the component nitrogen types in catalysts, which impacts the charge distribution of the carbon sheath and further ameliorates the charge transfer. Moreover, urea, an assisted nitrogen/carbon source, plays an active role in the formation and growth of N-doped carbon nanotubes. Such Co–N–C system results in more provided active sites (e.g., Co and Co–N_x moieties), noncorrosive metal cores protected by carbon materials as well as short diffusion pathway of ions due to the synergetic effect of core–sheath structure. Benefiting from these controllable strategies, C1Z1 (pyrolysis product of Co_0.5Zn_0.5-ZIF and urea precursor) embraces the current density of 10 mA cm⁻² at the overpotential of 251 and 279 mV for hydrogen evolution reaction (HER) in acid and alkaline media, respectively, which enables it to be an efficient electrocatalyst for HER under both extreme pH conditions.