Ni2P nanoflakes for the high-performing urea oxidation reaction: linking active sites to a UOR mechanism

Abstract

Urea electrolysis is regarded as an effective method for addressing both energy and environment issues. Herein, we successfully synthesized Ni₂P nanoflakes for catalyzing the urea oxidation reaction (UOR). Due to the higher electrical conductivity as well as the prevailing tendency in triggering the UOR via a direct electro-oxidation mechanism, Ni₂P nanoflakes exhibit comparable UOR activity (1.33 V vs. RHE for onset-potential, and 95.47 mA·cm⁻² at 1.6 V vs. RHE) to the most active state-of-the-art catalysts, rendering them an effective alternative to precious metals such as Pt and Rh. The accelerated proton-coupled electron transfer (PCET) process caused by PO₄³⁻ facilitates the in situ generation of NiOOH; thus, the UOR process is initiated at a lower onset-potential on Ni₂P nanoflakes than on β-Ni(OH)₂ nanoflakes. The in situ generated NiOOH instead of the Ni₂P phase in Ni₂P nanoflakes functions as an active site during the UOR process, while both NiOOH and the Ni₂P phase serve as active sites in the OER process. This work provides insights into the understanding of the UOR mechanism and opens a new avenue to design low-cost Ni-based phosphide UOR catalysts.