Electrocatalytic NO 2 - reduction to NH 3 (NO 2 RR) is an appealing approach for mitigating NO 2 - pollution and for the synthesis of valuable NH 3 , and so the exploration for high-performance NO 2 RR catalysts is pivotal yet remains challenging. Herein, amorphous MoO 3 nanosheets (am-MoO 3 ) were designed as a high-performance NO 2 RR electrocatalyst, delivering a maximum NO 2 - -to-NH 3 faradaic efficiency of 94.8% and NH 3 yield rate of 480.4 μmol h -1 cm -2 at -0.6 V vs. RHE. Theoretical computations revealed that the largely enhanced NO 2 RR activity of am-MoO 3 originated from the amorphization-induced O-vacancies, which could enhance the NO 2 - -to-NH 3 reaction energetics and hamper the competitive hydrogen evolution.