Electrochemical Nitrogen Reduction to Ammonia with Mesoporous Molybdenum-Carbon Electrocatalysts in a Gas Diffusion Flow Cell

Abstract Commercialization of electrochemical nitrogen reduction (eNRR) is hindered by limited catalyst performance, the lack of scalable electrochemical cell architectures, and an incomplete understanding of performance-limiting processes. This work aims to address these challenges through a systematic investigation of catalyst composition and microstructure, device configuration, and reaction mechanisms. Molybdenum (Mo) metal single atoms and clusters supported on mesoporous nitrogen-doped carbon were synthesized with tunable pore sizes via a five-second photothermal pyrolysis of a self-assembled bottlebrush block copolymer (BBCP) template. For eNRR, improved Mo dispersion enhanced electrochemical accessibility, while the coexistence of atomically dispersed Mo species and Mo clusters produced a structurally heterogeneous catalytic environment that benefits eNRR. A gas-diffusion electrode (GDE) flow cell was constructed for continuous operation testing; the catalyst–device system achieved a Faradaic efficiency of 21.6% and an ammonia yield rate of 34.7 μg h–1 mg–1. Intermediate pore sizes and balanced electrode wettability promoted effective triple-phase boundary formation and active-site utilization. Electrochemical characterization combined with machine-learning analysis of flow-cell parameters indicated that activation polarization is the dominant loss mechanism, governed primarily by intrinsic active-site kinetics and electrochemically accessible surface area. In contrast, pore size, metal dispersion, and electrode wettability primarily regulate site accessibility rather than mass-transport limitations. Together, these results provide a framework linking catalyst structure, electrode architecture, and kinetic limitations in eNRR flow-cell systems and offer practical design guidelines for advancing scalable eNRR technology.