Synthesis and X-ray Characterization of Nickel Phosphide Nanostructures for Water Oxidation

Author

David Thompson, University of Arkansas, FayettevilleFollow

Date of Graduation

5-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Chemistry (PhD)

Degree Level

Graduate

Department

Chemistry & Biochemistry

Advisor/Mentor

Chen, Jingyi

Committee Member

Bare, Simon

Second Committee Member

Heyes, Colin D.

Third Committee Member

Kilyanek, Stefan M.

Fourth Committee Member

Wang, Feng

Keywords

Electrochemistry; Nanoparticles; Nickel Phosphide; X-ray Absorption Spectroscopy

Abstract

The pursuit of earth-abundant 3d transition metal catalysts for alkaline water electrolysis remains an active area of research. While NiFe layered double hydroxides (LDH) are currently the most active catalysts for the oxygen evolution reaction (OER), doping Ni-based catalysts with phosphorus to form Ni metal phosphides has emerged as a promising alternative. However, the complex crystalline and amorphous phases of NiP_x have hindered their characterization, and the OER mechanism and origin of activity remain unclear. This dissertation aims to elucidate the synthesis, OER performance, and in situ reconstruction of amorphous NiP_x-based nanomaterials, with a focus on in situ and ex situ X-ray characterization. Through a systematic investigation, we developed a synthetic route for uniform amorphous Ni₇₀P₃₀ nanoparticles, which served as a template for the formation of phase-pure Ni₁₂P₅, Ni₂P, and Ni₅P₄ via temperature modulation. Using in situ X-ray absorption spectroscopy (XAS), we monitored the transformation of α-Ni(OH)₂ and β-Ni(OH)₂ during OER, revealing that β-Ni(OH)₂ undergoes a potential-induced intercalation of anions to form a mixed α/β Ni(OH)₂ phase. We also investigated amorphous NiP_x with an oxidized α-Ni(OH)₂ shell during OER, finding that the α-Ni(OH)₂ shell protects the conductive NiPx core from structural reconstruction defects, promoting high OER performance. Furthermore, our study demonstrated that the NiPx-α-Ni(OH)₂ core-shell nanoparticle promotes shorter Ni-O bonds at high anodic potentials, enhancing OER activity. We also developed a facile method for incorporating Fe and Co into amorphous Ni₇₀P₃₀ nanoparticles, resulting in uniform alloy and core-shell structures with improved OER activity. Notably, the addition of Co altered the OER mechanism, promoting lattice oxygen activation over the adsorbate evolution mechanism, although it was limited by irreversible inactivation of Co(IV) activation sites. This dissertation provides a fundamental understanding of synthesizing and monitoring OER activities of amorphous Ni-based phosphides, shedding light on their complex structure-activity relationships. The findings of this study have significant implications for the development of efficient and earth-abundant catalysts for alkaline water electrolysis.

Citation

Thompson, D. (2025). Synthesis and X-ray Characterization of Nickel Phosphide Nanostructures for Water Oxidation. Graduate Theses and Dissertations Retrieved from https://scholarworks.uark.edu/etd/5699