Revealing Structural Evolution of Nickel Phosphide-Iron Oxide Core–Shell Nanocatalysts in Alkaline Medium for the Oxygen Evolution Reaction

Metal phosphide-containing materials have emerged as a potential candidate of nonprecious metal-based catalysts for alkaline oxygen evolution reaction (OER). While it is known that metal phosphide undergoes structural evolution, considerable debate persists regarding the effects of dynamics on the surface activation and morphological stability of the catalysts. In this study, we synthesize NiPx-FeOx core–shell nanocatalysts with an amorphous NiPx core designed for enhanced OER activity. Using ex situ X-ray absorption spectroscopy, we elucidate the local structural changes as a function of the cyclic voltammetry cycles. Our studies suggest that the presence of corner-sharing octahedra in the FeOx shell improves structural rigidity through interlayer cross-linking, thereby inhibiting the diffusion of OH–/H2O. Thus, the FeOx shell preserves the amorphous NiPx core from rapid oxidation to Ni3(PO4)2 and Ni(OH)2. On the other hand, the incorporation of Ni from the core into the FeOx shell facilitates absorption of hydroxide ions for OER. As a result, Ni/Fe(OH)x at the surface oxidizes to the active γ-(oxy)hydroxide phase under the applied potentials, promoting OER. This intriguing synergistic behavior holds significance as such a synthetic route involving the FeOx shell can be extended to other systems, enabling manipulation of surface adsorption and diffusion of hydroxide ions. These findings also demonstrate that nanomaterials with core–shell morphologies can be tuned to leverage the strength of each metallic component for improved electrochemical activities.

The stoichiometric relationship of the chemical reaction was solved based on mass conservation of Ni, P, H, and O using linear algebra.A set of stoichiometric coefficient a, b, c, d, e, and f for each substance was expressed in eq.1: To balance the eq. 1, the mass of each substance must be conserved.We then establish the following four equations, eq.2-5: O: With 4 equations, multiple solutions exist for the stoichiometry coefficients that are redundant.We   Fe-M3 3.49±0.013.3±0.90.007±0.002

Figure S2 .
Figure S2.CV profiles of the NiPx-FeOx nanoparticles of 30 cycles at a scan rate of 10 mV/s from

Figure S3 .
Figure S3.(A) XRD of NiPx nanoparticles with a TEM image as the insert; (B) CV profiles of the

Figure S4 .
Figure S4.Histograms of the size distribution of the NiPx-FeOx core-shell nanoparticles before

Figure S5 .
Figure S5.XPS spectra of Ni 2p (A), Fe 2p (B), and P 2p (C) of the core-shell nanoparticles after set d to one.And solve the coefficients a, b, c, and e as a function of the oxygen coefficient f.This allows us to model the variation of product distribution as O2 partial pressure changes., c and e gives the stoichiometries of Ni3(PO4)2, PH3, and Ni(OH)2 at different f values.The result was plotted in Figure 4B.

Figure S7 .
Figure S7.(A-B) The real (brightly colored) and imaginary (faded color) components of the Ni

Figure S8 .
Figure S8.(A-B) The real (brightly colored) and imaginary (faded color) components of the Fe

Figure S9 .
Figure S9.The real (brightly colored) and imaginary (faded color) components of the (A) Ni and

Figure S11 .
Figure S11.(A) Ni and (B) Fe K-edge k 3 -weighted FT EXAFS spectra of the pristine NiPx and

Figure S12 .
Figure S12.(A) Ni path contributions of the Ni K-edge FT EXAFS fit for 12h KOH treated NiPx-

Figure S13 .
Figure S13.Fe path contributions of the Fe K-edge FT EXAFS fits: (A) 12 h KOH treated NiPx-