Facile Solvent-Free Synthesis of Metal Thiophosphates and Their Examination as Hydrogen Evolution Electrocatalysts

Abstract

The facile solvent-free synthesis of several known metal thiophosphates was accomplished by a chemical exchange reaction between anhydrous metal chlorides and elemental phosphorus with sulfur, or combinations of phosphorus with molecular P2S5 at moderate 500 °C temperatures. The crystalline products obtained from this synthetic approach include MPS3 (M = Fe, Co, Ni) and Cu3PS4. The successful reactions benefit from thermochemically favorable PCl3 elimination. This solvent-free route performed at moderate temperatures leads to mixed anion products with complex heteroatomic anions, such as P2S64−. The MPS3 phases are thermally metastable relative to the thermodynamically preferred separate MPx/ MSy and more metal-rich MPxSy phases. The micrometer-sized M-P-S products exhibit room-temperature optical and magnetic properties consistent with isolated metal ion structural arrangements and semiconducting band gaps. The MPS3 materials were examined as electrocatalysts in hydrogen evolution reactions (HER) under acidic conditions. In terms of HER activity at lower applied potentials, the MPS3 materials show the trend of Co > Ni >> Fe. Extended time constant potential HER experiments show reasonable HER stability of ionic and semiconducting MPS3 (M = Co, Ni) structures under acidic reducing conditions.

Tunable Synthesis of Metal-Rich and Phosphorus-Rich Nickel Phosphides and Their Comparative Evaluation as Hydrogen Evolution Electrocatalysts

“…The relative electrocatalytic HER performance of crystalline nickel-rich to phosphorus-rich phases synthesized in this work was evaluated in a three-electrode system in H 2 -saturated 0.5 M H 2 SO 4 with powders adhered to sticky conducting carbon-wax working electrodes (50% graphite: 50% paraffin wax by mass) analogous to our recent work (Figure S1). ,, Representative negative applied potential linear sweep voltammetry (LSV) curves for each Ni–P sample are shown in Figure . LSV data were obtained without iR compensation, and higher relative HER activity was indicated by increased current flow at lower negative applied potentials.…”

“…The nickel phosphides and Pt/C powder reference catalyst were evaluated using a graphite powder/paraffin wax composite electrode similar to that described in our recent work. ,, Solid conducting graphite/carbon wax electrodes have been used in electrochemical analysis and are stable and inert with excellent reproducibility and negligible background current. − Relative to our prior work, slight modifications were made to the working electrode’s graphite powder/paraffin wax composition, tip connections, and process of catalyst particle deposition (Figure S1). A 50% graphite/50% paraffin wax composition (by mass) for working electrode tips was identified as having better conductivity and retaining good adhesion for catalyst particles.…”

“…The 50% C wax electrode with the catalyst sample embedded on its surface leads to ∼40–50 Ω cell resistance values. Details of the experimental setup are similar to those described in our recent work. ,, …”

“…Details of the experimental setup are similar to those described in our recent work. 38,59,60 Hydrogen evolution reaction (HER) polarization curves were obtained using linear sweep voltammetry (LSV) at 5 mV s −1 in acidic 0.5 M H 2 SO 4 (pH = 0.3) aqueous solutions. The electrolyte solutions were continually purged with H 2 (ultrahigh purity 99.999%, Praxair) to maintain a reversible (H + /H 2 ) Nernstian potential in the solution for HER reactions.…”

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

“…S1, ESI †). 34,51 The working electrode tips were formed by placing melted graphite/carbon wax into threaded and coned PTFE tubing (1.4 cm length, 3.2 mm ID, 6.4 mm OD) which has a 0.080 cm 2 geometric internal surface area. The blank 50% graphite/carbon wax (C wax ) electrode tips were connected to 3.2 mm diameter brass current collectors, placed in a polyethylene bag, and submerged in a preheated (55 1C) water bath for B10 minutes to soften the wax for catalyst embedment.…”