Synthesis of an Ultrafine CoP Nanocrystal/Graphene Sandwiched Structure for Efficient Overall Water Splitting

A CoP/graphene composite was synthesized through a coprecipitation and in situ phosphorization protocol using α-Co(OH)₂ and graphene oxide as precursors. The similar two-dimensional layered structures ensured evenly attached α-Co(OH)₂ nanosheets on the graphene oxide support and the formation of a sandwich-like structure. The sequential in situ phosphorization strategy not only generated a high density of ultrafine CoP nanocrystals but also simultaneously reduced the graphene oxide support. The enough exposed active sites combined with a highly conductive matrix resulted in an excellent electrochemical catalyst for overall water splitting. The overpotential is only 125 mV at 10 mA·cm² in 0.5 M H₂SO₄. Good electrocatalytic performance was also exhibited in alkaline conditions for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The overpotential is 119 mV for HER and 374 mV for OER at 10 mA·cm² in 1 M KOH. More importantly, the composite exhibited much higher exchange current densities during HER processes (1.64 × 10^-4 A·cm^-2 in 0.5 M H₂SO₄ and 2.93 × 10^-4 A·cm^-2 in 1 M KOH) when compared with similar materials reported before. This low-cost, simple, and efficient approach is suitable for mass production and practical applications.