Curvature-induced geometric strain engineering of Ni@S4CNTs for bifunctional oxygen electrocatalysis.

Curvature effects are crucial in regulating catalyst activity, yet their structure-activity relationship remains unclear. Here, spin-polarized density functional theory calculations were performed on a series of Ni@S4CNTs-(m,0) models with systematically varied nanotube curvature. Increasing curvature induces a gradual transition from sp2-dominated to more sp3-like hybridization in CNTs, mainly manifested as enhanced SNiS angular distortion and increased Ni out-of-plane displacement. Electronic-structure analysis shows that curvature does not monotonically change the net Ni charge but induces charge redistribution around the local NiS4 coordination unit. Thermodynamic analysis further demonstrates that curvature can precisely tune the catalytic activity of Ni@S4CNTs, with the most favorable bifunctional thermodynamic performance achieved at Ni@S4CNTs-(7,0) (ηORR/OER = 0.33/0.31 V). COHP/ICOHP bonding analysis of the ∗OH and ∗O adsorption states further reveals that curvature does not uniformly regulate the corresponding NiOads interactions. In particular, ∗O -related NiOads bonding exhibits a substantially stronger curvature dependence than ∗OH-related bonding. Meanwhile, the intrinsic ∗OH / ∗OOH scaling relation remains largely preserved. Curvature therefore primarily changes the relative thermodynamic position of ∗O between ∗OH and ∗OOH, redistributing the free-energy burden between the ∗OH → ∗O and ∗O → ∗OOH steps. This behavior is attributed to the differential binding of catalytic intermediates induced by spin-polarized redistribution of Ni 3d orbitals under curvature. Such orbital differentiation, quantified by Δεdorb, elucidates the theoretical mechanism of curvature-regulated oxygen electrocatalysis.