Towards Higher NH3 Faradaic Efficiency: Selective‐Poisoning of HER Active Sites by Co‐Feeding CO in NO Electroreduction**

Abstract Direct electroreduction of nitric oxide offers a promising avenue to produce valuable chemicals, such as ammonia, which is an essential chemical to produce fertilizers. Direct ammonia synthesis from NO in a polymer electrolyte membrane (PEM) electrolyzer is advantageous for its continuous operation and excellent mass transport characteristics. However, at a high current density, the faradaic efficiency of NO electroreduction reaction is limited by the competing hydrogen evolution reaction (HER). Herein, we report a CO‐mediated selective poisoning strategy to enhance the faradaic efficiency (FE) towards ammonia by suppressing the HER. In the presence of only NO at the cathode, Pt/C and Pd/C catalysts showed a lower FE towards NH 3 than to H 2 due to the dominating HER. Cu/C catalyst showed a 78 % FE towards NH 3 at 2.0 V due to the stronger binding affinity to NO* compared to H*. By co‐feeding CO, the FE of Cu/C catalyst towards NH 3 was improved by 12 %. More strikingly, for Pd/C, the FE towards NH 3 was enhanced by 95 % with CO co‐feeding, by effectively suppressing HER. This is attributed to the change of the favorable surface coverage resulting from the selective and competitive binding of CO* to H* binding sites, thereby improving NH 3 selectivity.