AbstractRenewable electricity‐powered nitrate (NO3−) reduction reaction (NO3RR) offers a net‐zero carbon route to the realization of high ammonia (NH3) productivity. However, this route suffers from low energy efficiency (EE, with a half‐cell EE commonly <36%), since high overpotentials are required to overcome the weak NO3− binding affinity and sluggish NO3RR kinetics. To alleviate this, a rational catalyst design strategy that involves the linear assembly of sub‐5 nm Cu/Co nanophases into sub‐20 nm thick nanoribbons is suggested. The theoretical and experimental studies show that the Cu‐Co nanoribbons, similar to enzymes, enable strong NO3− adsorption and rapid tandem catalysis of NO3− to NH3, owing to their richly exposed binary phase boundaries and adjacent Cu‐Co sites at sub‐5 nm distance. In situ Raman spectroscopy further reveals that at low applied overpotentials, the Cu/Co nanophases are rapidly activated and subsequently stabilized by a specifically designed redox polymer that in situ scavenges intermediately formed highly oxidative nitrogen dioxide (NO2). As a result, a stable NO3RR with a current density of ≈450 mA cm−2 is achieved, a Faradaic efficiency of >97% for the formation of NH3, and an unprecedented half‐cell EE of ≈42%.