Flexible direct synthesis of phosphorus-rich CoP3 on carbon black and its examination in hydrogen evolution electrocatalysis

Abstract

In contrast to metal-rich cobalt phosphides, phosphorus-rich CoP₃ is less studied as a hydrogen evolution reaction (HER) electrocatalyst, though it has distinct polyphosphide structural features and electronic properties that can be useful in facilitating proton reduction. While bulk crystalline solids are ideal surfaces for electrocatalytic studies, methods to disperse active catalysts on inexpensive high surface area supports can lead to improved HER activity with less catalyst. This paper describes a straightforward two-step procedure to produce a series of crystalline CoP₃ particles supported on conducting carbon black powder. Anhydrous CoCl₂ was deposited onto carbon black in various amounts from a methanol solution. The dried CoCl₂/C products were directly reacted with elemental phosphorus in an ampoule at 500 °C. These reactions produce crystalline CoP₃/C with nominally 5–25 mol% of CoP₃ on carbon black. These CoP₃/C composites possess a ∼3 to 10 times higher surface area (∼11–35 m² g⁻¹) versus similarly synthesized bulk CoP₃. The CoP₃ crystallites grow in aggregated form with carbon black nanoparticles. The HER electrocatalytic behavior of these composites in 0.5 M H₂SO₄ was evaluated and all CoP₃/C composites were HER active and require only applied potentials near −95 mV to achieve 10 mA cm⁻² current densities. The lower content CoP₃ catalyst particles dispersed on carbon black show similar HER activity to bulk CoP₃. These CoP₃/C composites are stable in acid and show a high degree of electrocatalytic stability in extended time HER electrocatalytic experiments.