Dilute Interstitial Carbon Stabilization of Highly Strained Metastable HCP Nickel for Enhanced Electrocatalysis

Lattice strain engineering can regulate the electronic structure of electrocatalysts, but stabilizing distorted metastable phases remains challenging. Here, we report an N,N-dimethylformamide-assisted strategy for synthesizing metastable hexagonal close-packed (HCP) Ni nanoparticles with local lattice-spacing deviations reaching approximately 5.2%. Structural and spectroscopic analyses support a dilute interstitial-carbon-stabilized HCP Ni framework dominated by expanded metallic Ni─Ni coordination rather than ordered Ni₃C. Carbon-associated perturbations inhibit structural relaxation and retain the distorted HCP framework. Operando x-ray absorption and infrared spectroscopies indicate the formation of a NiOOH-like working surface coupled to an HCP-derived subsurface framework, accompanied by potential-dependent changes in the interfacial hydrogen-bonding environment and urea-derived species. Calculations using idealized metallic models show that HCP lattice strain induces a Ni d-band upshift, anisotropic Ni─Ni bonding, and localized charge redistribution, modifying adsorption-energy trends for the urea oxidation reaction. The optimized HCP-Ni delivers an apparent steady-state Tafel slope of 26.95 mV dec^-1, over twice the electrochemically active surface area-normalized activity of the face-centered cubic-containing controls, and approximately 98% apparent urea conversion after 22 h. It sustains current for over 120 h in three-electrode testing and enables over 400 h of urea-assisted zinc-air battery cycling. This work demonstrates dilute interstitial-carbon stabilization as a route to strained metastable electrocatalysts.