Fig. 2: Theoretical calculation.

a Adsorption energy of R-TEMPOH on clean NiOOH surface, inset of (a) is the corresponding DFT optimized structural model of NiOOH surface. The color for each element is pale blue for Ni, red for O, and white for H. b Adsorption energy of AcNH-TEMPOH (ACTH) on the Ni active site of M-NiOOH surface (M = Ni, V, Mn, Fe, Co, Cu). c Adsorption energy of R-TEMPOH on the Ni active site of V-NiOOH surface, inset of (c) is the corresponding DFT optimized structural model of R-TEMPOH on the V-NiOOH surface. The color for each element is light gray for V, pale blue for Ni, red for O, dark gray for C, blue for N, and white for H.