Cation-π interactions at the Cu-HHTP/MXene hybrid interface for efficient electrocatalytic CO₂ reduction

The strategic integration of two-dimensional (2D) conductive substrates into electrocatalytic systems provides a promising strategy to enhance structural stability, as the formation of intimate interfacial contacts facilitates electron transfer kinetics and thereby improves catalytic performance. Herein, a highly conductive metal-organic framework (Cu-HHTP, HHTP = 2,3,6,7,10,11-triphenylhexanol) is combined with three functionalized MXene nanosheets (MXene-F/-OH/-NH₂) via ion-π interactions to form Cu-HHTP/MXene hybrids C/MXF, C/MXN, and C/MXO, which serve as cathodes for CO₂ electroreduction. FT-IR, Raman, BET, microcalorimetry, and electrochemical analyses show that C/MXN featuring strong cation-π interactions demonstrates improved interfacial contact and faster electron transfer compared to C/MXO (weak cation-π interaction) and C/MXF (anion-π interaction). These interactions enhance electron density redistribution at Cu sites, increasing the electrochemically active surface area to 2.42 mF·cm^-2. Additionally, the NH₂-functionalized C/MXN exhibits a proton conductivity of 1.59 × 10^-2 S·cm^-1 at 65 °C and a low proton transport activation energy (E_a = 0.144 eV), attributed to the extensive hydrogen bond donor of -NH₂ groups. As a result, C/MXN achieves a significantly higher Faradaic efficiency (FE) for CO₂-to-CH₄ conversion (56 % at -1.0 V vs. RHE) than C/MXO (42 %) and C/MXF (13 %), representing a nearly 7-fold improvement over pure Cu-HHTP. Furthermore, C/MXN retains high crystallinity after 70 h of continuous operation, outperforming most composite electrocatalysts. This study offers insights into developing structurally stable and efficient electron-transporting electrocatalysts through molecular interface engineering.