Boron-Doped Ti₃C₂T_x MXene for Effective and Durable High-Current-Density Ammonia Synthesis

Ammonia (NH₃) synthesis via the nitrate reduction reaction (NO₃RR) offers a competitive strategy for nitrogen cycling and carbon neutrality; however, this is hindered by the poor NO₃RR performance under high current density. Herein, it is shown that boron-doped Ti₃C₂T_x MXene nanosheets can highly efficiently catalyze the conversion of NO₃RR-to-NH₃ at ambient conditions, showing a maximal NH₃ Faradic efficiency of 91% with a peak yield rate of 26.2 mgh^-1 mg_cat. ^-1, and robust durability over ten consecutive cycles, all of them are comparable to the best-reported results and exceed those of pristine Ti₃C₂T_x MXene. More importantly, when tested in a flow cell, the designed catalyst delivers a current density of ‒1000 mA cm^-2 at a low potential of ‒1.18 V versus the reversible hydrogen electrode and maintains a high NH₃ selectivity over a wide current density range. Besides, a Zn-nitrate battery with the catalyst as the cathode is assembled, which achieves a power density of 5.24 mW cm^-2 and a yield rate of 1.15 mgh^-1 mg_cat. ^-1. Theoretical simulations further demonstrate that the boron dopants can optimize the adsorption and activation of NO₃RR intermediates, and reduce the potential-determining step barrier, thus leading to an enhanced NH₃ selectivity.