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Molecular complexes are an important class of catalysts for the electrochemical carbon dioxide reduction reaction (CO2RR). However, selective CO2RR in strong acids remains challenging due to competition with the hydrogen evolution reaction. Peripheral functionalization is effective for tailoring the intrinsic activity of molecular catalysts, mostly attributed to the inductive effect or to stabilization of reaction intermediates. Here we report that peripheral functionalization of immobilized molecular complexes with quaternary ammonium groups can regulate the catalytic activity by tuning the mass distribution surrounding the active sites, enabling high-performance CO2RR in strong acids. The positively charged and hydrophobic alkylammonium groups affect the migration of water and hydronium in the double layer, while their immobilized configuration enables a stable cationic layer, inhibiting the hydrogen evolution reaction over extended potential windows. Dodecyl ammonium-functionalized cobalt phthalocyanine and tin porphyrin suppress the hydrogen Faradaic efficiency to <10% in pH ~0.5 media, while providing a single-pass conversion efficiency up to ~85%. The selectivity can be maintained at 90% even in Li+ solutions, which often exhibit poor proton shielding. Our study underscores the role of second-sphere structure for selective molecular electrochemistry. (Figure presented.) © The Author(s), under exclusive licence to Springer Nature Limited 2024.

Metalloporphyrins and metallophthalocyanines emerge as popular building blocks to develop covalent organic nanosheets (CONs) for CO2 reduction reaction (CO2RR). However, existing CONs predominantly yield CO, posing a challenge in achieving efficient methanol production through multielectron reduction. Here, ultrathin, cationic, and cobalt-phthalocyanine-based CONs (iminium-CONs) are reported for electrochemical CO2-to-CH3OH conversion. The integration of quaternary iminium groups enables the formation of ultrathin morphology with uniformly anchored cobalt active sites, which are pivotal for facilitating rapid multielectron transfer. Moreover, the cationic iminium-CONs exhibit a lower activity for hydrogen evolution side reaction. Consequently, iminium-CONs manifest significantly enhanced selectivity for methanol production, as evidenced by a remarkable 711% and 270% improvement in methanol partial current density (j(CH3OH)) compared to pristine CoTAPc and neutral imine-CONs, respectively. Under optimized conditions, iminium-CONs deliver a high j(CH3OH) of 91.7 mA cm(-2) at -0.78 V in a flow cell. Further, iminium-CONs achieve a global methanol Faradaic efficiency (FECH3OH) of 54% in a tandem device. Thanks to the single-site feature, the methanol is produced without the concurrent generation of other liquid byproducts. This work underscores the potential of cationic covalent organic nanosheets as a compelling platform for electrochemical six-electron reduction of CO2 to methanol.

Support-induced strain engineering is a powerful strategy to modulate the electronic structure of two-dimensional materials. However, controlling strain of planar molecules such as metallophthalocyanines and metalloporphyrins is technically challenging due to their sub–2 nm lateral size. In addition, the effect of strain on molecular properties remains poorly understood. Starting with cobalt phthalocyanine (CoPc), a model CO₂ reduction reaction (CO₂RR) catalyst, we show that carbon nanotubes (CNTs) are ideal substrates for inducing optimum properties through molecular curvature. Using a tandem-flow electrolyzer with monodispersed CoPc on single-walled CNTs (CoPc/SWCNT) as the catalyst, we achieve a methanol partial current density of >90 mA cm⁻² with a selectivity of >60%. CoPc on wide multi-walled CNTs (MWCNTs) leads to only 16.6% selectivity. We report X-ray spectroscopic characterizations to unravel the distinct local coordinations and electronic structures induced by the strong molecule-support interactions. These results agree with our Grand Canonical Density Functional Theory that calculates the energetics as a function of applied potential. We find that SWCNTs induce curvature in CoPc, which improves *CO binding to enable subsequent formation of methanol, while wide MWCNTs favor CO desorption. Thus, we demonstrate that the SWCNT-induced molecular strain increases methanol formation. We also show that induced strain can accelerate the oxygen reduction reaction and CO2RR for other catalysts. Our results show the important role of SWCNTs beyond catalyst dispersion and electron conduction.