Transition Metal Phosphides Fabricated through a Codeposition-Annealing Technique As Electrocatalysts for Hydrogen Evolution Reaction
Authors: Bernasconi Roberto , Khalil Md Ibrahim , Nobili Luca , Magagnin Luca 
Published: February 2020

In the present work, transition metal phosphides were fabricated through a facile codeposition-annealing process. Pure red phosphorous particles were electrolytically codeposited with amorphous phosphorus containing alloys of general structure Me-P (with Me being a transition metal). A subsequent annealing step was applied to promote the formation of intermetallics and the interdiffusion between pure phosphorus particles and the metallic matrix. The main advantage of this approach is the possibility to easily obtain metal phosphides characterized by a very high P content, well beyond the limit typical of normal electrochemical processes involving induced codeposition of P [1, 2]. Another interesting benefit of the technique presented is the confinement of elemental P inside the coating during the annealing step. This strongly limits the presence of gaseous phosphorus, which is typical of different phosphorization processes used in literature to obtain metal phosphides [3]. Avoiding gaseous P is advantageous, since in specific conditions the formation of the highly dangerous white allotrope can take place. The use of P particles confined inside the coating is preferable also to phosphorization methods involving phosphine, an extremely poisonous gas commonly employed at lab level.

The codeposition-annealing route described in the present work can be in principle applied to a number of transition metals for many different applications. One of the most attractive application for metal phosphides, however, is electrocatalysis for water splitting [4]. In this context, the codeposition-annealing technique was applied to the fabrication of Ni and Co phosphides, which can be used as catalytic substrates for Hydrogen Evolution Reaction (HER). These two materials present remarkable advantages over commonly used Pt based catalysts: lower cost, comparable performances and ease of manufacturing.

Operatively, amorphous Ni-P and Co-P alloys were codeposited with P particles and subsequently annealed. As a result, different phase pure metal phosphides like Ni2P, Ni12P5, Co2P, Cu3P etc. were obtained after annealing. Characterization techniques like XRD, SEM (EDS), cross-sectional analysis and XPS were used to determine phase composition of the materials obtained and to characterize their morphology. Effect of annealing temperature and duration on phase composition were investigated as well. The electrocatalytic HER activity and stability of the different transition metal phosphides were tested in 0.5 M H2SO4. In all the cases, remarkable results were obtained, which matched up well with present existing literature where similar electrocatalysts have been fabricated through different methods [5]. For example, a Co2P electrocatalyst plated on copper and annealed at 290 °C exhibited excellent electrocatalytic activity for HER, showing a low overpotential of 62 mV at a current density of 10 mA/cm2 in 0.5M H2SO4 solution.

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[2] A. Brenner; Electrodeposition of Alloys: Principles and Practice, Elsevier (2013)

[3] X. Wang et al.; Angew. Chem. 54 (28), 8188-8192 (2015)

[4] P. Xiao et al.; Adv. Energy Mater. 5 (24), 1500985 (2015)

[5] Z.Pu et al.; Chem.Mater. 26 (15), 4326-4329 (2014)