Efficient Acidic CO₂ Reduction via Local Strain Engineering of Nickel Single-Atom Catalysts

Acidic CO₂ reduction reaction (CO₂RR) enhances carbon efficiency and electrolyzer stability. Although nickel single-atom catalysts (Ni-SACs) effectively convert CO₂ into CO in neutral/alkaline conditions, their performance in acid is hindered by the competing hydrogen evolution reaction (HER). Here we show that tailoring the local strain of Ni-SACs can enhance HER suppression across a broad potential range. Density functional theory calculations indicate that Ni-SACs with steeper local curvature reduce *COOH adsorption by 0.23 eV while increasing *H adsorption by 0.55 eV. To validate our predictions, we leveraged carbon nanotubes (CNTs) with different diameters to impose controlled local strain on Ni-SACs. In Ar-saturated 0.05 M H₂SO₄, Ni-SACs on 5-nm CNTs (Ni-CNT5) demonstrate the lowest hydronium and water reduction current density among all types of CNT support. In a flow cell with pH 1 catholyte, Ni-CNT5 maintains >95% CO Faradaic efficiency (FE) from -1.0 to -2.4 V, in contrast to Ni-CNT50 with ∼70% FE(H₂) at -2.4 V. Owing to its effective HER inhibition, Ni-CNT5 achieves 80% single-pass CO₂ conversion efficiency and operates stably in acidic electrolyte with negligible loss in current or selectivity. Our findings expand the toolbox for SACs engineering, highlighting the critical role of local stress for controlled activity.