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

Abstract

Selective ammonia production through the electrochemical nitrate reduction reaction (NO3RR) often faces significant challenges, primarily due to the irreversible desorption of catalytic intermediates and competing hydrogen evolution reaction. Improving the catalytic activity via single-atom doping in a material lattice is one of the most fascinating approaches. Herein, a dilute quantity of iron atoms is impregnated into a 2-dimensional SnS lattice via precisely controlling the Fe to Sn ratio during the synthesis of Fe0.2Sn0.98S. 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 SnII and FeII state in Fe0.02Sn0.98S, while a significant amount of lattice defect is induced due to the replacement of tin with the 3d-metal, resulting in some available unoccupied electronic state near the Fermi level. The redox study with the SnS and Fe0.02Sn0.98S in aqueous acetate buffer of pH 3.7 revealed a diffused redox feature within 0.2 to -0.34 V (vs RHE) with a peak maxima at -0.2 V (vs RHE) for the reduction of SnII sites dominantly, which concomitantly facilitates the nitrate reduction. Rotating disk electrochemistry with the Fe0.02Sn0.98S in 0.5 M KNO3 containing electrolyte showed a pseudo-limiting current at different rotating speeds, emphasizing multistep electron transfer. while from the kinetic current analyses, ~6 e- transfer and a kobs value of 2.62 x 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 Fe0.02Sn0.98S modified electrode produces exclusively ammonia, confirmed through 15N-isotope labeling, with ~85% Faradaic efficiency and yield rate of 345.4 μg h-1cm-2; those are at least two times larger than that observed for SnS. In-situ Raman spectroscopic analyses infer the binding of nitrate on the surface of the Fe0.02Sn0.98S even at open circuit potential and identify adsorbed [NO2]- as an active intermediate formed in the rate-limiting step. Post-NO3RR characterization of the electrode confirms the electrochemical stability of Fe0.02Sn0.98S and/or any metallic dissolution. In this study, atomic-level iron doping modifies the electronic-microenvironment of tin sites to boost the nitrate reduction.