Direct conversion of coordination compounds into Ni₂P nanoparticles entrapped in 3D mesoporous graphene for an efficient hydrogen evolution reaction

This paper reports a simple preparation route to a composite of small Ni₂P nanoparticles (NPs) entrapped in 3D mesoporous graphene by the thermal conversion of a coordination compound followed by phosphidation. Recently, transition metal phosphides (TMPs) have gained increasing attention owing to their promising potential as non-precious metal catalysts in the hydrogen evolution reaction (HER). In order to enhance the catalytic activity of TMPs, researchers have sought to synthesize small TMP NPs to increase the catalytically active surface area. Although surfactant-mediated syntheses can produce small TMP NPs, a cumbersome surfactant removal step is necessary to generate catalytically active clean surfaces. Interfacing TMP NPs with carbon nanomaterials is another promising approach to boost the catalytic performance by providing high electrical conductivity and durability. However, the synthesis of composites of TMP NPs and carbon demands multiple synthetic steps, including the preparation of TMP NPs, synthesis of carbon nanomaterials, and dispersion of TMP NPs onto the carbon support. The essence of our approach toward the 3D graphene encapsulating Ni₂P NPs (Ni₂P@mesoG) lies in the utilization of the conversion phenomenon of [Ni₂(EDTA)] (EDTA = ethylenediaminetetraacetate). The thermolysis of [Ni₂(EDTA)] at 600 °C produces a composite of single-crystalline 5 nm-sized Ni NPs individually entrapped in 3D mesoG (Ni@mesoG), and the following phosphidation completely converts the Ni NPs to single-crystalline Ni₂P NPs in mesoG (Ni₂P@mesoG) without agglomeration. This solvent-free thermal conversion route to the Ni₂P@mesoG composite is simple and scalable. Notably, graphitic shell layers in Ni₂P@mesoG stabilize small Ni₂P NPs possessing a large active surface area, and facilitate the electron transfer due to the intimate contact between them. Consequently, the use of Ni₂P@mosoG exhibits superior electrocatalytic HER activity and durability in both strong acidic and basic media.