Heterogenized phthalocyanine-based molecular catalysts are the ideal electrocatalytic platforms for CO₂ reduction reaction (CO₂RR) because of their well-defined structures and potential properties. In addition to the pursuit of catalytic performances at industrial potentials, it is equally important to explore experimental rules and design considerations behind activity and selectivity. Herein, we successfully developed a series of nickel phthalocyanines (NiPcs) with different alkyl chains immobilized on multi-walled carbon nanotubes (CNT) to unveil the structure–performance relationship for electrocatalytic CO₂RR in neutral electrolyte. Interestingly, a volcano-type trend was found between the activity for CO₂-to-CO conversion and alkyl chain lengths of NiPcs on CNT. Experimental results further indicate that their electrocatalytic CO₂RR activities are highly related to the molecular dispersion and the heterointerfacial charge transfer capability adjusted by the alkyl chains. Particularly, the optimized electrocatalyst via accurate clipping at the molecular level exhibits an ultrahigh activity with Faradaic efficiency of CO up to 99.52%.