Freestanding Millimeter-Scale Porphyrin-Based Monoatomic Layers with 0.28 nm Thickness for CO₂ Electrocatalysis

Developing two-dimensional (2D) and single atomic layered materials is a fascinating challenge. Here we successfully synthesize porphyrin-based monoatomic layer (PML), a freestanding 2D porphyrin-based material of monomer-unit thickness (2.8 Å). The solvothermal method provides a bottom-up approach for tailoring the monoatomic layer from the nanoscale to the milliscale. PMLs containing accurately tailorable M-N₄ units (M=Cu and Au) were synthesized, which present metal center-dependent performance for CO₂ electrocatalysis. PML with Cu-N₄ centers performs high faradaic efficiencies of HCOO^- and CH₄ (80.86 % and 11.51 % at -0.7 V, respectively) while PML with Au-N₄ centers generates HCOO^- and CO as major products (40.90 % and 34.40 % at -0.8 V, respectively). Irreversible restructuring behavior of Cu sites is also observed. Based on the graphene-like properties and metal center-selectivity relationships, we believe that PML will play a distinct role in various applications.