Semicrystalline Conjugated Polymers with Well-Defined Active Sites for Nitrogen Fixation in a Seawater Electrolyte

Faradaic efficiency for the nitrogen reduction reaction (NRR) is often limited by low N₂ solubility in the electrolyte, while a large number of intimate contacts between the electrolyte and solid catalyst can also inevitably sacrifice many active sites for the NRR. Here, it is reported that a "quasi-gas-solid" interface formed in donor-acceptor-based conjugated polymers (CPs) is beneficial to boosting the NRR process and at the same time suppressing the competing hydrogen evolution reaction. Of particular interest, it is found that a semicrystalline CP catalyst, SC-PBDT-TT, exhibits a high Faradaic efficiency of up to 60.5% with a maximum NH₃ production rate of 16.8 µg h^-1 mg^-1 in a neutral-buffered seawater electrolyte. Molecular dynamics and COMSOL Multiphysics simulations reveal the origin of the observed high NRR performance arising from the presence of desirable crystal regions to resist the penetration of H₂ O molecules, leading to the formation of a "quasi-gas-solid" interface inside the catalyst for a favorable direct-contact between the catalyst and N₂ molecules. Furthermore, high-throughput computations, based on density functional theory, reveal the actual real active site for N₂ adsorption and reduction in SC-PBDT-TT. This work provides a new framework for optimizing NRR performance of metal-free catalysts by controlling their crystallinities.