Support-induced strain engineering is useful for modulating the properties of two-dimensional materials. However, controlling strain of planar molecules is technically challenging due to their sub-2 nm lateral size. Additionally, the effect of strain on molecular properties remains poorly understood. Here we show that carbon nanotubes (CNTs) are ideal substrates for inducing optimum properties through molecular curvature. In a tandem-flow electrolyser with monodispersed cobalt phthalocyanine (CoPc) on single-walled CNTs (CoPc/SWCNTs) for CO₂ reduction, we achieve a methanol partial current density of >90 mA cm⁻² with >60% selectivity, surpassing wide multiwalled CNTs at 16.6%. We report vibronic and X-ray spectroscopies to unravel the distinct local geometries and electronic structures induced by the strong molecule–support interactions. Grand canonical density functional theory confirms that curved CoPc/SWCNTs improve *CO binding to enable subsequent reduction, whereas wide multiwalled CNTs favour CO desorption. Our results show the important role of SWCNTs beyond catalyst dispersion and electron conduction.

Electrocatalytic CO₂ reduction at near-ambient temperatures requires a complex inventory of protons, hydroxyls, carbonate ions and alkali-metal ions at the cathode and anode to be managed, necessitating the use of ion-selective membranes to regulate pH. Anion-exchange membranes provide an alkaline environment, allowing CO₂ reduction at low cell voltages and suppression of hydrogen evolution while maintaining high conversion efficiencies. However, the local alkaline conditions and the presence of alkali cations lead to problematic carbonate formation and even precipitation. Here we report a pure-water-fed (alkali-cation-free) membrane–electrode–assembly system for CO₂ reduction to ethylene by integrating an anion-exchange membrane and a proton-exchange membrane at the cathode and anode side, respectively, under forward bias. This system effectively suppresses carbonate formation and prevents salt precipitation. A scaled-up electrolyser stack achieved over 1,000 h stability without CO₂ and electrolyte losses and with 50% Faradaic efficiency towards ethylene at a total current of 10 A.