Single-atom　catalysts　(SACs)　are　emerging　as　promising　candidates　for　electrochemical　nitrogen　reduction　reaction　(NRR).　Previous　studies　have　shown　that　the　single-atom　centers　of　SACs　can　not　only　serve　as　active　sites,　but　also　act　as　promoters　to　affect　the　catalytic　properties.　However,　the　use　of　single　metal　atoms　as　promoters　in　electrocatalysis　has　rarely　been　studied.　In　this　work,　the　defective　Keggin-type　phosphomolybdic　acid　(PMA)　is　used　as　a　substrate　to　support　the　single　metal　atoms.　We　aim　to　tune　the　electronic　structures　of　the　exposed　molybdenum　active　sites　on　defective　PMA　by　using　these　supported　single　atoms　as　promoters　for　efficient　NRR.　Firstly,　the　stability　and　N2　adsorption　capacity　were　studied　to　screen　for　an　effective　catalyst　capable　of　activating　N2.　Most　of　the　SACs　were　found　to　have　good　stability　and　N2　adsorption　capacity.　Then,　we　compared　the　selectivity　and　NRR　activity　of　the　catalysts　and　found　that　catalysts　with　metal　atom　promoters　have　improved　NRR　selectivity　and　activity.　Finally,　electronic　structure　analysis　was　carried　out　to　understand　the　promoting　effect　of　the　promoter　on　N2　activation　and　the　activity　of　the　NRR　process.　This　work　provides　a　new　strategy　for　designing　efficient　catalysts　for　electrocatalytic　reactions　by　introducing　promoters.　©　2024　The　Royal　Society　of　Chemistry.