Hierarchical Nickel Clusters Encapsulated in Ultrathin N-doped Graphitic Nanocarbon Hybrids for Effective Hydrogen Evolution Reaction

Three-dimensional (3D) nanospheres with a hollow interior derived from self-sacrificing templates have triggered great enthusiasm due to their structure-related performance for splitting water into hydrogen. Herein, a nickel-organic compound constituted of elemental nickel and glycine was first synthesized through a solvothermal process in an ethyl alcohol solution. Then, novel 3D hierarchical nanocatalysts consisting of ultrathin N-doped graphitic-nanocarbon-coated nickel clusters with a hollow interior structure were facilely fabricated via calcining the prepared nickel-organic compound. Outstanding electrocatalytic activity with a small overpotential of 70 mV and a low Tafel slope of 119 mV dec(-1) for the hydrogen evolution reaction process can be readily achieved in a 1 M KOH aqueous solution through a controllable synthesis method. The investigation on electrocatalytic activity certifies that the thickness of the graphitic nanocarbon shells has a great influence on water splitting efficiency for hydrogen; the thinner the graphitic nanocarbon shells, the more excellent the electrocatalytic efficiency. Additionally, the detailed electrochemically active surface area suggests that the 3D hollow structured hybridized electrocatalysts with defect-rich ultrathin graphitic nanocarbon shells could limit the aggregation of nickel clusters, and small electrochemical impedance accelerates the penetration of electrons, inducing a high efficiency for electrochemical water splitting. Therefore, thoughtful design using the self-sacrificing template method provides a promising strategy for the fabrication of other hybridized composites with hierarchical architectures consisting of nanoclusters and nanocarbon for more efficient water splitting.