The electrochemical N2 reduction reaction (NRR) offers a promising approach for sustainable NH3 production, and modulating the structural/electronic configurations of the catalyst materials with optimized electrocatalytic properties is pivotal for achieving high‐efficiency NRR electrocatalysis. Herein, vacancy and heterostructure engineering are rationally integrated to explore O‐vacancy‐rich MoO3‐x anchored on Ti3C2Tx‐MXene (MoO3‐x/MXene) as a highly active and selective NRR electrocatalyst, achieving an exceptional NRR activity with an NH3 yield of 95.8 µg h−1 mg−1 (−0.4 V) and a Faradaic efficiency of 22.3% (−0.3 V). A combination of in situ spectroscopy, molecular dynamics simulations and density functional theory computations is employed to unveil the synergistic effect of O‐vacancies and heterostructures for the NRR, which demonstrates that O‐vacancies on MoO3‐x serve as the active sites for N2 chemisorption and activation, while the MXene substrate can further regulate the O‐vacancy sites to break the scaling relation to effectively stabilize *N2/*N2H while destabilizing *NH2/*NH3, resulting in more optimized binding affinity of NRR intermediates toward reduced energy barriers and an enhanced NRR activity for MoO3‐x/MXene.

Fe-doping induced synergetic effects, including the morphological change of crystalline CeO2 to partial-amorphous nanosheets, enriched O-vacancies and active Ce3+–Ce3+ pairs, were all responsible for the significantly enhanced NRR activity of Fe-CeO2.

We demonstrate the great feasibility of MBenes as a new class of tandem catalysts for electrocatalytic nitrate reduction to ammonia (NO 3 RR). As a proof of concept, FeB 2 is first employed as a model MBene catalyst for the NO 3 RR, showing a maximum NH 3 -Faradaic efficiency of 96.8 % with a corresponding NH 3 yield of 25.5 mg h À 1 cm À 2 at À 0.6 V vs. RHE. Mechanistic studies reveal that the exceptional NO 3 RR activity of FeB 2 arises from the tandem catalysis mechanism, that is, B sites activate NO 3 À to form intermediates, while Fe sites dissociate H 2 O and increase *H supply on B sites to promote the intermediate hydrogenation and enhance the NO 3 À -to-NH 3 conversion.

The synergistic effect of B-dopants and B-dopant-induced O-vacancies led to the significantly enhanced NRR activity of MnO2 nanosheets with an NH3 yield of 54.2 μg h−1 mg−1 (−0.4 V) and a faradaic efficiency of 16.8% (−0.2 V).