Enhancing electrocatalytic nitrate reduction to ammonia via atomically dilute iron doping in SnS

Abstract

Selective ammonia production through the electrochemical nitrate reduction reaction (NO₃RR) under aqueous conditions faces significant challenges like poor selectivity and Faradaic efficiency, solely due to the competing hydrogen evolution reaction and irreversible loss of catalytic intermediates. Doping of a reactive single atom in an inactive material lattice is the most fascinating approach to overcome these potential issues of the NO₃RR. Herein, a dilute concentration of iron atoms is impregnated into a two-dimensional SnS lattice via precisely controlling the Fe to Sn ratio during the hydrothermal synthesis of 1–2% Fe-doped SnS. Near single-atom doping has been confirmed through high-angle annular dark-field STEM elemental mapping along with other microscopic techniques. X-ray absorption (Sn-K edge and Fe-K edge XANES/EXAFS) and photoelectron spectroscopic analyses revealed the Sn^II and Fe^II states in Fe_xSn_1−xS, while a significant number of lattice defects are induced due to the replacement of tin with the 3d-metal, resulting in some available unoccupied electronic states near the Fermi level. A redox study with SnS and Fe_xSn_1−xS in aqueous acetate buffer of pH 3.7 revealed a diffuse redox feature between 0.2 and −0.34 V (vs. RHE) with a peak maximum at −0.2 V (vs. RHE) for the reduction of dominantly Sn^II sites, which concomitantly facilitates nitrate reduction. Rotating disk electrochemistry with the Fe_xSn_1−xS in 0.5 M KNO₃ containing electrolyte showed a pseudo-limiting current at different rotating speeds, emphasizing multistep electron transfer. Meanwhile, from kinetic current analyses, ∼6 e⁻ transfer and a k_obs value of 2.62 × 10⁻⁴ cm s⁻¹ are obtained. Electrolysis of 0.5 M KNO₃ solution in acetate buffer of pH 3.7 at −0.3 V (vs. RHE) with the Fe_xSn_1−xS modified electrode produces exclusively ammonia, confirmed through ¹⁵N-isotope labeling, with ∼85% Faradaic efficiency and a yield rate of 354.4 µg h⁻¹ cm⁻²; these are at least two times larger than those observed for SnS. In situ Raman spectroscopic analyses infer the binding of nitrate on the surface of the Fe_xSn_1−xS even at open circuit potential and identify adsorbed [NO₂]⁻ as an active intermediate formed in the rate-limiting step. However, raising the applied potential towards a more cathodic direction (>−0.35 V) results in the formation of H₂ and NO_x species through the non-productive pathway. Post-NO₃RR characterization of the electrode confirms the electrochemical stability of Fe_xSn_1−xS and/or any metallic dissolution. In this study, atomic-level iron doping modifies the electronic microenvironment of tin sites to boost nitrate reduction.