Ultrathin nickel phosphide nanosheet aerogel electrocatalysts derived from Ni-alginate for hydrogen evolution reaction

Abstract Transition metal phosphides (TMPs) have been regarded as an alternative to Pt-based catalysts for hydrogen evolution reaction (HER) on account of their high performance and low cost. The ultrathin two-dimensional (2D) nanosheets of TMPs can expose abundant active sites and facilitate the transfer of mass/charge to enhance HER performance. However, the synthesis of ultrathin 2D nanosheets of TMPs is still a great challenge. Herein, we reported a novel and scalable strategy to prepare 2D ultrathin nickel phosphide nanosheets (∼2.5 nm) by using alginate as precursor through ice-templating process. The role of ice-templating is that the growth of the ice crystals would squeeze Ni-alginate into 2D nanosheets, and then forming Ni-alginate aerogels. The three-dimensional (3D) interconnected porous network of aerogels was advantageous to reduce the lattice strains of subsequent oxidation and phosphorization process. It is the key step to prepare 2D ultrathin nickel phosphides (Ni2P and Ni5P4) nanosheets. As expected, all the prepared Ni5P4 nanosheet aerogels displayed remarkable HER performance in 1 M KOH electrolyte. Especially, due to their ultrathin 2D nanosheets and high P percentage, the Ni5P4-500-350 sample with excellent long-term stability exhibits a low overpotential of 147 mV at current density of 10 mA cm−2 and a low Tafel slope of 57 mV dec−1.