Switching off Competing Hydrogen Formation in CO₂ Electroreduction via Substrate Defect Engineering

Carbon nanotubes (CNTs) are widely used as supports for immobilizing molecular electrocatalysts, such as for CO₂ reduction (CO₂R), 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 CO₂R 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 CO₂R 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 CO₂R performance. Guided by these insights, CNT defect density is optimized via thermal graphitization, yielding a NiPc/CNT composite with unprecedented selectivity (CO:H₂ > 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 cm² 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 CO₂R electrolysis.