Integrated production-transportation strategy to regulate active hydrogen behavior on single-atom catalyst surface for boosted electroreduction of nitrate to ammonia.

Controlling the behavior of active hydrogen (H*) on the surface of single-atom catalysts is an effective strategy for enhancing the electrochemical nitrate (NO3-) reduction reaction (NO3RR) to ammonia (NH3). However, achieving a balance between H* supply and demand remains a fundamental challenge. Herein, we propose an integrated production-transportation strategy to regulate H* behavior by constructing a hierarchical porous carbon (HC)-supported catalyst containing Fe single-atom sites (FeSA) and Fe nanoparticles (FeNP) (denoted as FeSA-FeNP/HC). Theoretical modeling and operando characterization revealed that FeNP promotes water dissociation to produce abundant H*, which can transport to the H*-deficient FeSA sites via the HC substrate for NO3- reduction, instead of diffusing into the electrolyte or recombining to evolve H2. FeSA-FeNP/HC exhibits the highest NH3 faradaic efficiency of 96.5% and an NH3 yield rate of 24.2 mg h-1 mgcat-1, which significantly outperforms the FeNP-lacking single-atom catalyst FeSA/HC and microporous carbon-supported catalyst FeSA/C. Moreover, FeSA-FeNP/HC was adopted as a bifunctional catalyst to couple NO3RR with a thermodynamically favorable hydrazine oxidation reaction to circumvent the energy-intensive anodic oxygen evolution reaction, resulting in a 1.0 V drop in operating voltage at 30 mA cm-2, and theoretically saving 47.6% of the energy consumption.