Sacrificial Dopant to Enhance the Activity and Durability of Electrochemical N₂ Reduction Catalysis

Abstract

Electrochemical nitrogen reduction reaction (eNRR) is a promising alternative to the Haber–Bosch process for environmentally sustainable ammonia synthesis. However, the reduction of the dinitrogen molecule to ammonia is known for its extremely sluggish kinetics, and the catalytic activity and selectivity of eNRR catalysts remain significantly low for practical deployment of the technology. Herein, a sacrificial dopant for eNRR catalysts is introduced in order to improve the activity and durability of the electrochemical N2 reduction catalysis. Specifically, iridium-doped mesoporous copper sulfide hexagonal prism nanoparticles provide an ammonia production yield of 18.2 ± 0.8 μg/h cm2 at −0.6 VRHE and a Faradaic efficiency of 2.4 ± 0.1% in neutral aqueous electrolytes. The dopant modifies the electronic states of the eNRR active site to achieve appropriate *N2H and *H adsorption energies. The doping process also results in an increased active surface area of catalyst particles along with a 3-fold increase in durability compared to the undoped copper sulfide, thus further enhancing the eNRR activity. Last, the developed eNRR catalyst is employed in a practical ammonia production device displaying a production yield of 1.1 μgNH3 /h cm2 at 1.8 Vcell. The present results suggest a design factor to enhance the catalyst durability, that is, by the introduction of a sacrificial dopant, in the development of efficient eNRR catalysts for electrochemical ammonia production.

Directing the Surface Atomic Geometry on Copper Sulfide for Enhanced Electrochemical Nitrogen Reduction

“…CS-60 was active in the eNRR for 30 min and degraded to Cu oxide in accordance with our previous work (Figure S14). , Note that we focused on the morphology-driven eNRR performance of CS- X and scrutinized the catalysts to determine their surface geometry-dependent electrocatalytic behaviors.…”

“…The NRR activity was investigated using CA performed from −0.4 to −0.8 V RHE for 1 h each at different potentials. For NMR analysis, purging was performed with 14 N 2 (99.999%, Sinyang Gas Inc.) or 15 N 2 for 30 min before the measurements. The cathodic compartment was continuously purged during the experiments at flow rates of 250 and 5 cc/min, respectively, for both 14 N 2 and 15 N 2 .…”

“…TPD was performed using a Micromeritics AutoChem II 2920 TPR/TPD instrument. In addition, 1 H NMR spectra for 14 N and 15 N were acquired using a Bruker Avance III HD 400 MHz spectrometer to confirm NH 3 production. Overall, 64 scans were performed for all samples.…”

“…Bilgaard et al calculated N 2 chemisorption energies on catalyst surfaces and revealed that the eNRRs on metals such as Pt, Pd, Au, and Cu are limited by weak N 2 adsorption in the initial step . However, it was recently reported that chalcogenization of Cu to Cu x S can promote the eNRR by modifying the N 2 and proton surface-binding energies. − …”

“…As heterogeneous catalysis is a surface-limited process, facet-dependent catalytic performance and surface feature optimization have been consistent foci in developing nanocatalysts for electrochemical catalysis applications. − In addition, facet engineering has proven to be an effective design strategy for catalysts that operate through associative processes, such as oxygen (*O 2 + *H → *OOH + *) and carbon dioxide (*CO 2 + *H → *COOH + *) reductions. − In our previous study, we developed hexagonal Cu 1.81 S (roxbyite) nanoprisms with two types of exposed facets ((100) and (010)). ,− Note that the hexagonal Cu 1.81 S nanoprism constitutes an ideal model for investigating facet-specific catalysis as the crystal exposes (100)- and (010)-type facets only. These facets present notably different surface features, i.e., flat and smooth (100) facets and (010) facets with a corrugated surface called “zigzag facets.” As the surface geometry significantly influences N 2 adsorption at the adsorption site, we hypothesize considerable variation of the eNRR kinetics with the different facets of the hexagonal Cu 1.81 S nanoprisms.…”