Cobalt nanoparticles-embedded porous carbon nanocages uniformly dispersed hollow carbon fibers as the accelerated electrocatalysts toward water splitting

The fast, low-cost, and efficient design of zeolite imidazolate frameworks (ZIF) based derivatives with perfect hollow channels is an effective method for enhancing their full water-splitting catalytic activities and achieving their commercial use. In this study, the novel cobalt nanoparticles embedded in nitrogen-doped porous carbon nanocages uniformly dispersed over two curved surfaces of biomass-derived hollow carbon fibers (Co@NC nanocage/HCF) were synthesized via the carbonization of the ZIF-67/hollow biomass fiber precursors. After the usage amounts of biomass fibers were optimized, the optimal Co@NC nanocage/HCF ₂₀₀ exhibited a larger surface area and higher dispersion states of Co@NC nanocages than the tightly packed pure Co@NC nanocages. Particularly, owing to the hollow and porous structures of Co@NC nanocages, more electrocatalytic active centers ( i.e. the pyridinic-N, Co-N _x-C, pyrrolic-N, graphitic-N, high-valence Co, and other oxygen-containing functional groups) were exposed along surfaces of the Co@NC nanocage/HCF ₂₀₀ catalyst. In an alkaline solution, the hydrogen evolution reaction potential at 10 mA cm ⁻² for Co@NC nanocage/HCF ₂₀₀ was 165 mV more negative than that of 20 wt% Pt/C. Particularly, the oxygen evolution reaction potential at 10 mA cm ⁻² for Co@NC nanocage/HCF ₂₀₀ was 17 mV more negative than that of RuO ₂. For overall water splitting, the bifunctional Co@NC nanocage/HCF ₂₀₀ catalyst-based electrolysis cell achieved a current density of 10 mA cm ⁻² at a cell voltage of 1.618 V, which is 4 mV smaller than that of the state-of-the-art 20 wt% Pt/C||RuO ₂ cell (1.614 V). The perfect hollow and porous structures, uniformly dispersed Co@NC nanocages (locked by vast in situ formed carbon nanotubes), and abundant electrocatalytic active sites along the inner and outer surfaces of Co@NC nanocage/HCF ₂₀₀ contributed to the excellent water splitting catalysis. Particularly, owing to the excellent stabilities of hollow structures and uniformly dispersed porous Co@NC nanocages, the Co@NC nanocage/HCF ₂₀₀ exhibited higher stability toward water electrolysis.