Sulfur vacancies in ultrathin cobalt sulfide nanoflowers enable boosted electrocatalytic activity of nitrogen reduction reaction

Abstract

Large-scale electrosynthesis of ammonia from nitrogen reduction reaction (NRR) relies on the efficient electrocatalyst with restrained competitory reaction, hydrogen evolution reaction (HER), and more active sites for nitrogen chemisorption and relative protonation. In this work, sulfur vacancies in cobalt sulfide (CoS) nanoflowers are created by Ar-plasma for NRR catalysis. In comparison with untreated CoS, a boosted NRR catalytic activity with Faradaic efficiency of 16.5 ± 1.5% and ammonia yield rate of 12.1 ± 1.4 μgNH3 h−1 mgcat−1 is achieved for CoS1-x electrocatalyst at a low applied potential of −0.15 V vs. RHE in acidic media. The superior NRR catalytic activity of CoS1-x is also confirmed by NMR test. The robust NRR performance is related to the sulfur vacancies in ultrathin CoS nanoflowers effectively suppressing HER catalysis due to the selective chemisorption of N2 molecules on low coordinated Co atoms resulting in higher favorability for nitrogen adsorption since the created Lewis acid sites are more preferable to form stable Co-N bond; moreover, a lower Gibbs free energy for the initial hydrogenation of nitrogen molecules, limiting step of NRR, is required for CoS1-x electrocatalyst because more Co sites with low coordination are created and the adjacent two Co sites are involved for the splitting of N2 molecules synergistically; thereby, an exceptional NRR performance is achieved for CoS1-x electrocatalyst.