Tandem Electrocatalytic Nitrate Reduction to Ammonia on MBenes.

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 -1  cm -2 at -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 - 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 - -to-NH 3 conversion.