In this work, we designed Ru/oxygen-doped-Ru core/shell nanoclusters to electrocatalyze nitrate reduction reaction (NITRR) to ammonia. Oxygen dopants expand the Ru unit-cell to trigger tensile strains.The strains suppress HER but favor •H formation by lifting the barrier of hydrogen-hydrogen coupling. The resulting •H promotes the hydrogenation of reaction intermediates to ammonia. Benefiting from the HER prevention and NITRR promotion, our roomtemperature electrocatalysis exhibits an ammonia-synthesizing rate (5.56 mol gcat-1 h-1 or 1.17 ± 0.04 mmol h-1 cm-2) exceeding that of the state-of-the-art NITRR electrocatalysis. Moreover, our strained nanoclusters maintain a high selectivity across a wide range of applied potentials, and they show superior durability against strong currents because of the firm bonding of Ru with subsurface oxygen dopants.