Fig. 3: Reaction pathways and barriers for CO₂ to cis-COOH conversion.

a The minimum energy path calculation using the CI-NEB method with implicit solvation. We consider total six images (00–05) for this process. The initial linear CO₂ interacts with neighboring water molecule to form a slightly bent structure (image 02). Later this bent CO₂ takes a proton from water to produce cis-COOH product (05 image) via the transition state (TS) at image 04. The arrow shows the forward reaction. b Reaction energies and TS barrier for the protonation step for CO₂ reduction on the Ni–N₄ and Ni–N₂C₂ sites at −0.8 V vs RHE applied potential. The proton transfer occurs most easily on Ni–N₂C₂, followed by Ni–N₃C₁ and Ni–N₄ site. Note that in these calculations the charge of the system changes continuously as the H moves from water to OCO to form OCOH* and OH⁻. There is not a separate electron transfer step as in the proton coupled electron transfer (PCET) surface hopping picture. (Gray color: entire surface represents implicit solvation along the whole reaction pathways, brown: carbon, blue: nitrogen, green: nickel, red: oxygen, off white: hydrogen atom).