Rational　design　of　efficient　catalysts　for　ammonia　(NH3)　synthesis　at　mild　conditions　is　of　paramount　importance　for　the　development　of　electrolysis　driven　Haber-Bosch　(eHB)　process.　Ammonia　synthesis　is　a　structure　sensitive　reaction,　and　any　small　change　of　electronic　structure　could　result　in　dramatic　change　of　catalytic　activity,　especially　for　the　Ru-based　NH3　synthesis　catalysts.　Electronic　structure　modulation　of　the　Ru-based　catalysts　could　provide　desirable　catalytic　activity.　Herein,　we　explore　a　class　of　Ru-Co　single-atom　alloy　catalysts　(marked　as　RuxCo1　SAA,　x　stands　for　Ru/Co　molar　ratio),　in　which　Co　single　atoms　are　located　on　Ru　nanoclusters　to　form　Co-Ru　coordination,　and　then　electronic　structure　of　RuxCo1　SAA　could　be　effectively　tuned　by　regulating　the　Ru/Co　molar　ratio.　Our　studies　show　that　a　moderate　ratio　of　Ru/Co　(1.7:1)　favors　superior　NH3　synthesis　rate　and　low　activation　energy.　Using　a　suite　of　elaborate　characterizations,　we　have　observed　that　the　outstanding　performance　over　Ru1.7Co1　SAA　can　be　attributed　to　the　electron　redistribution　between　Ru　and　Co　atom,　resulting　in　an　upshift　of　d　band　center　and　lowering　of　work　function.　However,　there　is　decrease　of　NH3　synthesis　rate　when　Ru/Co　molar　ratio　is　2,　due　to　the　partial　detachment　of　Ru　entities　from　SAA　structure,　and　the　formed　individual　Ru　nanoparticles　(NPs)　overlay　the　surface　of　catalyst.　In　such　a　case,　N-2　activation　behavior　over　Ru2Co1　is　alike　that　of　Ru　NPs　catalysts.　The　present　study　demonstrates　that　the　Ru/Co　ratio　in　SAA　structure　can　effectively　regulate　the　electronic　structure　of　catalyst　for　NH3　synthesis　rate　enhancement.　(C)&　nbsp;2022　Elsevier　Inc.　All　rights　reserved.