Phosphorus-Rich Metal Phosphides: Direct and Tin Flux-Assisted Synthesis and Evaluation as Hydrogen Evolution Electrocatalysts

Abstract

Metal phosphides from the 3d period exhibit a range of structures and compositions. Many metal-rich phosphides and monophosphides function as heterogeneous electrocatalysts in the hydrogen evolution reaction. This paper describes the direct and tin flux-assisted synthesis of phosphorus-rich metal phosphides with MP2 or MP3 compositions. The facile synthesis of FeP2, CoP3, NiP2, and CuP2 is thermochemically driven by PCl3 formation from reactions of anhydrous metal halides and P4 vapor at 500 °C. Well-crystallized micrometer-sized particles result from these solvent-free reactions. A tin flux leads to more complete reactions at lower temperature for FeP2 and enables synthesis of a monoclinic polymorph of NiP2 rather than the kinetic cubic product formed by direct reaction. These crystalline metal phosphides are investigated as electrocatalyts for hydrogen evolution in acidic and buffered aqueous solutions. All phosphorus-rich products show very good stability in strongly acidic media. The catalytic activity for hydrogen evolution ordered by higher current at a fixed electrode geometric area and low onset potential is CoP3 > NiP2 (cubic and monoclinic) > FeP2 ≫ CuP2. At high applied potentials, CuP2 undergoes surface reactions and roughening that improve its electrocatalytic activity. Correlations of the observed electrocatalytic activity with electrochemically active surface area, particle size, metallic versus semiconducting properties, and local metal coordination environment are noted for these phosphorus-rich 3d metal phosphides.

Recent Progresses in Engineering of Ni and Co based Phosphides for Effective Electrocatalytic Water Splitting

“…In addition, the partial electron transfer between the metal ions and phosphides increases the stability and sustainability of the catalyst both in acidic and alkaline medium. In case of phosphorous rich metal phosphides, the rich P−P source encourage to accommodate M n+ cations results increased electrochemical stability in both acidic and alkaline medium [79,80] . In case of rich metal content, the metal rich phosphides (with extended M−M bonding structures) showing moderate stability in the acidic medium, whereas phosphorous rich TMPs are electrochemical stable/more reactive towards the water electrolysis [80] .…”

“…In case of phosphorous rich metal phosphides, the rich P−P source encourage to accommodate M n+ cations results increased electrochemical stability in both acidic and alkaline medium [79,80] . In case of rich metal content, the metal rich phosphides (with extended M−M bonding structures) showing moderate stability in the acidic medium, whereas phosphorous rich TMPs are electrochemical stable/more reactive towards the water electrolysis [80] . The interesting report with studying the electrocatalytic performance of various NiP was reported by Laursen et al [84] .…”

General Approach for the Synthesis of Nitrogen-Doped Carbon Encapsulated Mo and W Phosphide Nanostructures for Electrocatalytic Hydrogen Evolution

“…The synthesis of phosphorus-rich metal phosphide is challenging and it involves a tedious procedure and toxic reagent/gas. 18 Various approaches have been established for the shape controlled synthesis of MoP and WP, and it is demonstrated that the ultrasmall spherical nanoparticles and one-dimensional nanostructure have improved catalytic activity. 19−23 The lack of long-term durability is one of the major concerns with MoP and WP; surface oxidation and agglomeration of the active catalyst have been observed.…”

Flux Growth of Phosphide and Arsenide Crystals

“…The volatility of P and As has resulted in poor exploration of many metal-phosphorus (arsenic) phase diagrams in the areas of high pnictogen content. Nevertheless, compounds with substantial P(As) content often exhibit interesting chemistry and useful practical properties, with potential applications as heat conductors, thermoelectrics, catalysts, and non-linear optical materials (Soheilnia et al, 2007;Lindsay et al, 2013;Dolyniuk et al, 2017;Nuss et al, 2017;Pöhls et al, 2017;Li et al, 2018;Owens-Baird et al, 2018, 2019Woo et al, 2018;Coleman et al, 2019;Mark et al, 2019;Yu et al, 2019).…”