Self-Supported Nickel Phosphide Nanoparticles with High Activity for Hyderogen Evolution Reacition

Abstract

Hydrogen (H2) production have received considerable attention resulting from the urgent need of clean and renewable energy. Water splitting is a typical and eco-friendly strategy for H2 production method through efficient acidic electrolyzers with expensive platinum group metal catalysts (Pt). Therefore, researchers have developed alternative, cheap and robust catalysts made from earth-abundant elements. Very recently, nickel phosphides (NixPy) have been proven to be promising catalysts with high activity and durability in strong acidic media (0.5M or 1M H2SO4) for hydrogen evolution reaction (HER). The crystalline phase of NixPy is an important factor for determining the activity for HER (better HER activity as higher P contents in NixPy). Herein, highly active and durable NixPy catalysts for HER are prepared using modified thermal decomposition method. In this process, nickel oxide (NiO) powder reduced to NixPy by phosphine gas (PH3) produced from sodium hypophosphite (NaH2PO2). Unlike conventional method, NiO is phosphidized from bi-located P source (NaH2PO2). One is placed upstream of furnace (traditional method) and the other is mechanically mixed with NiO powder. Based on X-ray diffraction analysis, synthesized NixPy catalysts had multi-phase crystalline structure (Ni2P, Ni5P4 and NiP2) and the NixPy with higher P content exhibited the smaller crystallite size. (Ni2P < 50 nm, Ni5P4 ~ 10 nm, NiP2 ~ 2nm). Consequently, Ni5P4 and NiP2 nanoparticles with higher HER activity are supported on micro-scaled Ni2P with better conductivity (self-supported NixPy nanoparticles), which result in improvement of electrochemical properties for HER. The NixPy catalysts showed exceptionally higher HER activity and durability in acidic media than single-phased Ni2P or Ni5P4 and this performance of the NixPy is comparable with that of Pt. Therefore, modified phosphidation method and self-supported NixPy catalyst provide new instruction for designing excellent HER catalysts.