Type-II g-C3N4/AgM-CP Enables Tunable Syngas at Low Bias

Abstract Photoelectrochemical (PEC) CO2 reduction is hindered by sluggish kinetics, poor selectivity, and competing hydrogen evolution. Herein, a g-C3N4/Ag-melamine coordination polymer (AgM-CP) heterostructure is developed to enhance CO2 conversion performance. Incorporation of Ag modulates the electronic structure, improves light absorption, and promotes selective syngas production without forming liquid byproducts. The catalyst achieves a total current density of 52 mA cm–2, with ca. 15 mA cm–2 attributed to CO2 reduction at −1.48 V vs RHE. Under illumination, a maximum CO generation rate of (269.0 ± 13.4) μmolcm–2h–1 is achieved, with a higher CO/H2 ratio at −0.88 V, which declines at more negative potentials. The I–V curve for g-AM collected under chopped illumination exhibits a moderate photocurrent density of 0.07 mA cm–2 at −0.88 V vs RHE. Reduction of Ag+ partially consumes electrons, slightly lowering Faradaic efficiency. The g-AM exhibits impressive applied bias photon-to-current efficiency of ca. (1.75 ± 0.1) % at −1.08 V vs RHE. Interfacial wettability and local CO2/H2O concentrations govern activity; Tafel analysis shows first electron transfer limits, while low charge-transfer resistance enhances CO production at lower bias.