Probing the Stability of Ni2P Nanoparticle Electrocatalysts via Operando Benchtop X-Ray Absorption Spectroscopy
Authors: Rivera-Maldonado Ricardo Andres , Gironda Anthony , Abramson Jared E. , Varughese Abraham , Seidler Gerald , Cossairt Brandi Michelle 
Published: December 2024

The electrification of the petrochemical industry will greatly 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, which can lead 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. Previous reports on Ni-P alloys found passivation regimes through anodic polarization but others saw complete dissolution at more oxidizing potentials. Bulk nickel phosphides have been shown to have a passivated surface, and characterization, often limited to ex-situ spectroscopy after the fact, has shown that the surface is composed of nickel hydroxides, phosphates, and oxyhydroxides. In the present work, operando Ni K-edge X-ray absorption spectroscopy and polarization are combined to understand the electrochemical degradation of Ni2P nanoparticles.
Ni2P nanoparticles were synthesized by heating NiCl2 and tris(diethylamino)phosphine in oleylamine and thermally annealed at 450 °C under 5% H2/95% N2. Transmission-mode Ni K-edge X-ray absorption spectroscopy was conducted on benchtop spectrometers (easyXAFS) using a 3D-printed electrochemical cell designed for operando spectroscopy. Nanoparticles maximize surface to bulk atom ratio, which is beneficial both for electrocatalysis, less atoms are necessary to have the same number of active sites, and for X-ray absorption, spectra are more surface sensitive. The use of a benchtop spectrometer offers the flexibility to iterate on experiments on the fly and the power to acquire synchrotron quality data on a routine basis. Preliminary results in neutral phosphate buffered electrolyte indicate that Ni2P significantly oxidizes and dissolves at and beyond 0.8 V vs RHE which agrees with anodic polarization corrosion experiments. Ongoing work extends anodic polarization experiments to acidic and basic pH and probes the potentials at which oxidation is no longer reversible. We hypothesize that dissolution will be less severe with increased pH and reduction of oxidized species to Ni2P will no longer be possible after the onset of Ni dissolution.