Nano-Structured Transition Metal Phosphides As an Efficient Electrocatalysts for Oxygen Evolution Reaction

Although the catalytic activity of transition metal phosphide compounds, such as dinickel phosphide (Ni2P), have been known for over half a century. they have only recently been investigated for heterogeneous (electro)catalysis. In 1995, it was shown that the performance of electrodeposited amorphous Ni–P electrodes (ca. 3 wt% phosphorous content) toward the HER in alkaline conditions can be orders of magnitude higher than that of Ni electrodes [1]. Detailed structural analysis of these transition metal phosphide materials had not been discussed in such early works. it was only very recently realized that the transition metal phosphides have reemerged as one of the most promising classes of earth-abundant inorganic electrocatalysts. Depending on the P:Ni molar ratio and the reaction temperature, different phases have been reported for Ni-phosphide compounds. Among all NixPy phases, Ni12P5 nanoparticles shows high electrocatalytical activity toward OER due to its unique crystallite structure and the higher concentration of nickel in this phase. which enables the formation of an amorphous NiOOH/Ni(OH)2 shell on a modified multiphase with a disordered phosphide/phosphite core [2]. However, the synthesis of Ni12P5 is quite challenging and a small deviation in the P:Ni molar ratio will result in Ni2P phase. In this study, pure phase of Ni12P5 and Ni0.9Fe0.1P colloidal nanoparticles have been synthesized by the reaction of [Ni(acac)2] with oleylamine (OA) and trioctylphosphine (TOP) reactants. OA is the reducing agent and thus controls the nucleation rate and TOP acts a source of phosphorus and also provides surface stabilization for tuning particles size. This synthesis results in monodisperse nanoparticles of diameter approximately 12nm [3]. These Ni12P5 and Ni0.9Fe0.1P nanoparticles show enhanced electrocatalytical activity toward OER in 0.1M KOH electrolyte, resulting in a current density of 10 mA cm−2 with overpotentials at 0.304 V and 0.20 V respectively. References: [1] Yu, F., Zhou, H., Huang, Y., Sun, J., Qin, F., Bao, J., ... Ren, Z. (n.d.). metal phosphide catalyst for overall water splitting, (2018), 1–9. [2] Menezes, P. W., Indra, A., Das, C., Walter, C., Göbel, C., Gutkin, V., ... Driess, M. (2017). Uncovering the Nature of Active Species of Nickel Phosphide Catalysts in High-Performance Electrochemical Overall Water Splitting. ACS Catalysis, 7(1), 103–109. [3] Muthuswamy, E., Savithra, G. H. L., & Brock, S. L. (2011). Control of Phase, Size, and Morphology in Nickel Phosphide Nanoparticles, (3), 2402–2411.