Phosphorus-doped onion-like carbon for CO2 electrochemical reduction: the decisive role of the bonding configuration of phosphorus

Doping with heteroatoms, such as nitrogen and boron, is crucial for the good performance of metal-free carbon cathode catalysts in the CO2 electrochemical reduction reaction (CERR). In this report, for the first time phosphorus is used to fabricate carbon catalysts and it is found that the bonding configuration of phosphorus has a key effect on the performance of the CERR. Two different phosphorus doped onion-like carbon (P-OLC) catalysts are synthesized with P–O bonding or P–C bonding as the major phosphorus bonding configuration, respectively. Compared to P-OLC with P–O bonding, P-OLC with P–C bonding exhibits a much better CERR performance, with a partial current density of 4.9 mA cm−2, 81% faradaic efficiency, and excellent durability (27 h) at low potential (−0.90 V vs. SHE). Density functional theory calculations show that the binding energy of the key intermediate COOH* in P–C bonding is much higher than that of its counterpart in P–O bonding, which is beneficial for lowering the energy barrier of the rate-limiting step. The better reactivity of the P–C bonding site is also verified by aromaticity analysis. Moreover, ultraviolet photoelectron spectroscopy (UPS) and partial density of state (PDOS) analysis suggest that the electron transfer capability of P–C bonding is stronger than that of P–O bonding. The current study demonstrates that phosphorus doping is an effective strategy for fabricating carbon CERR catalysts and reveals the pivotal role of the phosphorus bonding configuration.