Probing the Stability of Nickel Phosphide Nanoparticle Electrocatalysts via Advanced Benchtop X-ray Spectroscopy

Abstract

The electrification of the petrochemical industry will significantly reduce greenhouse gas emissions in the manufacturing of commodity chemicals; however, electrification on a global scale is only achievable using earth-abundant materials for electrocatalysis in place of state-of-the-art catalysts made from precious metals. A promising catalyst that has been used for industrial hydrodesulfurization and electrocatalytic hydrogen evolution, nitrate reduction, carbon dioxide reduction, and oxygen evolution is Ni2P. Ni2P and other transition metal phosphides benefit from active site ensembles that moderate the binding of reaction intermediates, making them promising electrocatalysts for reactions beyond the ones listed, especially other hydrogenation reactions. However, Ni2P and other TMPs are susceptible to oxidation, leading to catalyst degradation and loss in certain conditions. Stability is an important part of catalysis; therefore, it is important to understand the degradation mechanism of Ni2P in order to mitigate corrosion, accurately predict surface reactivity, and prepare Ni2P for industrial electrocatalysis. In the present work, operando Ni K-edge X-ray absorption spectroscopy (XAS) and ex situ P Kα and Kβ X-ray emission spectroscopy (XES) couple together to understand the degradation of Ni2P and show the effects of electrolyte and air on Ni2P nanoparticles. Upon exposure to air, as-synthesized Ni2P nanoparticles were studied via P Kα and Kβ XES in an inert atmosphere glovebox. Results showed that the phosphide species converted to phosphate with analysis of intermediates well fit by a simple two-component mixture. Electrochemical corrosion results demonstrate that Ni2P nanoparticles corrode significantly before achieving a passive surface. The corrosion follows a phosphate-first pathway, followed by the rapid oxidation of excess Ni to form a passive surface. Approximately 80-90% of Ni dissolves before passivation is achieved. Future work with Ni2P should consider the chemical structure of its surface in aqueous conditions and take advantage of its anodic passivation.