An alkali-modified biochar-supported single-atom catalyst with enhanced mass transfer for efficient nitrate electrolysis

The electrochemical nitrate reduction reaction (NitRR) to ammonia (NH3) offers a sustainable route for simultaneous wastewater treatment and green NH3 synthesis, yet it demands highly active and selective catalysts. Single-atom Fe–N–C catalysts show promise but often suffer from aggregation and limited mass transport. Herein, we construct a composite electrocatalyst by confining Fe single atoms within an alkali-modified biochar support (Fe–N–C + OHBC). The OHBC framework features a hierarchical porous structure that enhances mass transfer, increases the electrochemically active surface area, and creates a confined microenvironment around the Fe–Nx sites. This synergistic integration significantly boosts the NitRR performance. The Fe–N–C + OHBC catalyst achieves a remarkable NH3 yield rate of 11.46 mg h–1 cm–2 at –0.71 V vs. RHE and a high Faradaic efficiency of 93.03% at –0.45 V vs. RHE, substantially outperforming its individual components. Mechanistic studies reveal that the Fe single atoms are the primary active centers, while OHBC facilitates proton-coupled electron transfer and enriches local reactant concentration. Furthermore, the catalyst demonstrates excellent stability over 80 hours and maintains high performance under various pH levels and in the presence of common interfering ions, showcasing its potential for practical nitrate remediation and decentralized NH3 production.