Switching off Competing Hydrogen Formation in CO 2 Electroreduction via Substrate Defect Engineering

Abstract Carbon nanotubes (CNTs) are widely used as supports for immobilizing molecular electrocatalysts, such as for CO 2 reduction (CO 2 R), with π‐π interactions often assumed to govern the catalyst immobilization. However, the nature of catalyst/CNTs interactions remains insufficiently understood. Here, nickel phthalocyanine (NiPc) is investigated, a benchmark CO 2 R catalyst, supported on CNTs. NiPc preferentially anchors at defect‐sites on CNTs rather than adsorbing uniformly via π‐π stacking is found, an observation validated by theoretical simulations. Notably, CNTs with the fewest defects, despite exhibiting non‐uniform NiPc distribution, deliver the highest CO 2 R activity and CO selectivity. Operando X‐ray absorption spectroscopy reveals that high defect densities induce D 4 h symmetry distortion of the NiPc macrocycle under cathodic bias, compromising catalyst integrity and CO 2 R performance. Guided by these insights, CNT defect density is optimized via thermal graphitization, yielding a NiPc/CNT composite with unprecedented selectivity (CO:H 2 > 16 100:1) and a turnover frequency of 1072 s⁻ 1 at −0.60 V versus RHE, switching off the competing hydrogen formation. Integrated into a 100 cm 2 zero‐gap electrolyzer, the optimized catalyst sustains 50 A current with >95% CO selectivity at ≈3.5 V, outperforming state‐of‐the‐art Ag‐based systems. This work establishes CNT defect‐engineering as an effective strategy for advancing molecular catalysts for CO 2 R electrolysis.