Versatile and tunable solvent-free synthesis of transition metal phosphides and their investigation as effective and stable hydrogen evolution electrocatalysts
Creators: name: Liyanage, Ishanka Arunali
Publisher: name: University of Iowa
Year: 2025
Subjects: Electrocatalysis
Hydrogen evolution
Mixed metal phosphides
Solvent-free synthesis
Supported materials
Transition metal phosphides

description: There is an urgent need to explore new forms of sustainable energy and energy conversion technologies to reduce fossil-fuel dependency and carbon footprint in the near future. Molecular hydrogen as a carbon-free and green energy carrier with high energy density shows a great promise in future energy demands. Large scale hydrogen production is mainly via steam-methane reforming and often relies on fossil fuels, suffers from harsh process conditions, and emit large amounts of carbon dioxide to the atmosphere. With the merits of water as an abundant and ecofriendly resource of hydrogen, electrochemical water splitting has gained enormous attention as a feasible and sustainable method towards hydrogen production in the last decade. The hydrogen evolution reaction (HER) is the cathodic half-cell reaction of electrochemical water splitting and its sluggish reaction kinetics at ambient conditions has attracted a wide range of HER electrocatalysts for efficient generation of hydrogen. Precious metal-based catalysts such as platinum are the most efficient HER electrocatalyst identified but have limited applications due to expensive price and low terrestrial abundance. Therefore, it is highly desirable to develop novel and cost-effective non-precious metal based HER electrocatalysts with high abundance and of high performance. Transition metal compounds such as metal phosphides, sulfides, carbides, oxides, and borides have been widely investigated so far, but has more to be studied in further improving their HER electrocatalytic performances.
description: Metal phosphides are a prominent class of non-oxide materials that are widely explored as potentially useful HER electrocatalysts and show a wide range of metal-rich (x &gt; y, MxPy) to phosphorus-rich (x &lt; y, MxPy) compositions with distinct structural and electronic properties effective towards HER electrocatalysis. Facile, cost-effective, and tunable synthesis methods that can provide access to a wide range of metal phosphide compositions are highly desired on a commercial scale and enable comparable HER electrocatalytic studies and fundamental studies of relationships between structure, surface chemical reactivity and surface hydrogen boding that have direct relevance to HER activity occurring on the surface. Typical metal phosphide preparations use toxic PH3 gas, non-ecofriendly organic phosphorus sources, and surfactants and organic solvents that often resulting in poorly crystalline nanostructured materials and in most cases producing metal-rich and mono phosphide compositions. Solid-state reactions have many advantages over liquid phase and gas phase reactions such as ease in reaction handling, production of crystalline materials, and easy access to phosphorus-rich compositions. However, such reactions are inherently slow and require high temperature annealing processes that involve high production costs. Effective modifications to reaction thermodynamics via stable byproduct elimination can speed up solid-state reactions and lower the required temperatures.
description: This dissertation details facile, solvent-free synthesis of several crystalline metal phosphides containing earth-abundant late transition metals using direct and tin-flux assisted reactions of metal halides (MClx) and red phosphorus (P) at moderate temperatures (500 °C). Both reactions are thermodynamically driven reactions via PCl3 or SnCl2 formation and sufficiently exothermic via stable byproduct formation. Also, these reactions are tunable to provide access to a range of metal phosphide compositions via simple and rational modifications in reaction stoichiometry and the amount of tin-flux in the reaction medium. Direct reactions are tuned through reaction stoichiometry to produce crystalline micrometer-sized M-P materials from metal-rich (Ni2P, Ni5P4) to phosphorus-rich (cubic-NiP2, CoP3, FeP4) compositions. A tin flux in MClx/P reactions allows access to crystalline micrometer-sized faceted particles of mono phosphides (FeP, CoP), monoclinic NiP2 and phosphorus-rich (FeP2, CoP2, NiP3) compositions of late-transition metal phosphides via SnCl2 formation. Intermediates in tin flux reactions (M-Sn and Sn-P phases) were isolated to help identify phosphorus-rich MPy formation mechanisms. Crystalline metal phosphides embedded on carbon wax electrodes demonstrate variably moderate electrocatalytic HER activity and stability in aqueous acidic conditions while mostly retaining bulk crystallinity as well as chemical composition after extended periods of surface reactions in corrosive reducing environments. The observed electrocatalytic HER behaviors of different metal phosphides correspond to the differences in particle size, surface area, surface structures, M-P bonding, and electronic properties.
description: This dissertation also describes the further development and versatility of solvent-free direct and tin-flux assisted MClx/P reactions to facile synthesis of more advanced and complex structures such as mixed metal phosphides and metal phosphide/carbon black or titanium dioxide composites at 500 °C. Tin-flux assisted one-step and two-step sequential MCl2/M´Cl2/P reactions are inexpensive, flexible, and effective methods to prepare mixed metal phosphides, Co1-x NixP3 (x= 0.25, 0.5, 0.75) using simply variable metal compositions. Mixed metal phosphides are moderately electrocatalytically active for HER, and they exhibit systematic improvements in electrocatalytic activity behavior from parent NiP3 to CoP3. Mixed metal structures show excellent HER stability, retention of bulk crystallinity, and chemical composition after extended time electrochemistry experiments. This dissertation also demonstrates the adoption and extension of direct MCl2/P reactions to facile and flexible growth of different amounts of crystalline metal phosphides on high surface area conducting carbon black (C) and semiconducting titanium dioxide (TiO2) supports for more complex supported composites formation. Depositions of anhydrous CoCl2 onto C or TiO2 in various amounts from a methanol solution followed by solvent-free, direct solid-gas (CoCl2+C/ or TiO2)/P exchange reactions at moderate temperatures (500 °C) produce conducting CoP3/C or semi-conducting CoP3/TiO2 composite materials. The HER electrocatalytic behaviors of these composites in aqueous acidic conditions was evaluated and compared to the support and these new cost-effective methods to deposit dispersed active catalysts on inexpensive high surface area supports led to improve HER activity with less catalyst. Conducting CoP3/C composites are stable in acid and show a high degree of electrocatalytic stability in extended time HER electrocatalytic experiments.In summary, facile, solvent-free MCl2/P reactions are very tunable and versatile towards a large range of compositions (metal-rich to phosphorus-rich) and complex materials (binary, ternary, free-standing, composites) of metal phosphides. The HER electrocatalytic performance of metal phosphides across compositions show complex trends that may influenced by several factors such as particle size, electrochemically active surface area, surface charge, composition, polyphosphide anionic arrays, metal redox properties, electrical and electronic properties of metal phosphide catalysts. In general, introduction of phosphorus into metal phosphide structures are effective towards improving their HER activity and stability in corrosive acidic HER conditions. The HER activity of metal phosphides can be further improved by different metal mixing into a phosphide structure and uniform dispersions of less catalyst on to high surface area conducting supports.