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

Carbon nanotubes (CNTs) have emerged as effective substrates for immobilizing molecular catalysts towards many electrocatalytic reactions, such as CO2 reduction (CO2R). However, despite the prevailing notion of strong π-π stackings between the molecular catalyst and CNTs, our understanding of their interactions remains inadequate. Here, we employ functionalized nickel phthalocyanines (NiPc), established CO2R catalysts, immobilized on CNTs as a model system to investigate the catalyst/substrate interactions. Firstly, we find that NiPc-catalysts preferentially anchor on the defects on CNTs rather than adhering via π-π interaction with the ideal graphene-like CNT surface, a finding further validated by theoretical simulations. Consequently, we observe the least uniform NiPc-catalysts distributions on CNTs when the defect-content is the lowest. Notably, this combination exhibits the highest CO2R selectivity and activity despite the non-uniform catalyst distributions. Through operando X-ray adsorption spectroscopy and theoretical simulations, we reveal that high CNT defect-contents tend to induce substantial D4h symmetry breaking of the NiPc plane under cathodic potential, consequently resulting in reduced CO2R selectivity and activity. Therefore, maintaining a low to moderate defect level on CNTs is critical. Guided by this understanding, we fine-tune the defect-level of CNTs through graphitization, achieving an unprecedently high selectivity for CO2 to CO conversion (CO to H2 molar ratio exceeding 16100:1, a remarkable suppression of hydrogen evolution by three orders of magnitude) and improved intrinsic-activity (turnover frequency of 1072 s−1 at −0.60 V vs. reversible hydrogen electrode) on an optimized Ni-Pc/CNTs composite. Furthermore, we achieved practical relevant CO production in a zero-gap electrolyzer (electrode size of 100 cm^-2), reaching high current (up to 50 A), with high CO selectivity (> 95%) and reasonably low cell voltage (approximately 3.5 V), substantially outperforming the state-of-the-art silver catalyst. Moreover, we extend this knowledge to a Co-based molecular catalyst, achieving a high Faradaic efficiency (over 50%) towards methanol production with a high partial current density over 150 mA cm−2. Overall, our findings underscore the significance of tuning defect levels on CNT substrates for achieving desired performance for immobilized molecular catalysts.