Electrocatalytic　CO2　reduction　(eCO(2)R)　in　acid　holds　promise　in　renewable　electricity-powered　CO2　utilization　with　high　efficiency,　but　the　hydrogen　evolution　reaction　(HER)　often　prevails　and　results　in　a　low　eCO(2)R　selectivity.　Here,　using　cobalt　phthalocyanine/Ketjen　black　(CoPc/KB)　as　the　model　catalysts,　we　systematically　study　the　effect　of　active　site　density,　operational　current　density,　and　hydrated　cations　on　the　acidic　eCO(2)R　selectivity　and　decipher　it　through　the　componential　dynamics　of　electric　double　layer　(EDL).　The　optimal　CoPc-4/KB　demonstrates　a　near-unity　CO　Faradaic　efficiency　from　50　to　400　mA　cm(-2)　and　superb　operational　stability　(＞120　h)　at　100　mA　cm(-2).　Aided　by　in　situ　Raman　and　infrared　spectroscopies,　we　reveal　that　the　proper　cations　establish　an　electrostatic　shield　for　mitigating　bulk　H+　penetration　and　mediate　the　interfacial　water　structure　for　suppressing　HER.　This　study　should　elicit　further　profound　thinking　on　robust　eCO(2)R　system　design　from　the　perspective　of　multiphasic　and　dynamic　EDL.