Materials Chemistry of Iron Phosphosulfide Nanoparticles: Synthesis, Solid State Chemistry, Surface Structure, and Electrocatalysis for the Hydrogen Evolution Reaction

Transition-metal phosphosulfides represent an emerging category of earth-abundant electrocatalyst materials, and some metal phosphosulfides have been shown to outperform the corresponding sulfides and phosphides. To fully realize the potential and benefit energy storage and conversion, it is necessary to study the chemistry of unknown phosphosulfide materials. In this article, we report on the materials chemistry of iron phosphosulfides. We systematically investigate the materials synthesis, solid state chemistry, surface structures, and electrocatalytic properties of iron phosphosulfide nanoparticles supported on carbon nanotubes. Two types of iron phosphosulfide nanomaterials, adopting either the FeS or the FeP crystal structure, have been successfully synthesized by two distinct synthetic routes designed in accordance with the different thermodynamic properties of the two structures. The compositions (i.e., P/S ratios) of the phosphosulfides can be adjusted within certain ranges without phase separation occurring. We discover that all the phosphosulfide nanoparticles exhibit higher P/S ratios on the surface than in the,bulk and that the presence of P atoms suppresses the oiddation of Fe and S atoms on the surface. We further find that there is a positive correlation between the P content of the iron phosphosulfide nanomaterials and their electrocatalytic activity for the hydrogen evolution reaction, which renders high-performance electrocatalysts for hydrogen production and the understanding that the Fe atoms coordinated by P atoms are the most active catalytic sites in the materials.