Two-dimensional layered SnO2 nanosheets for ambient ammonia synthesis

Abstract

Industrial production of ammonia remains dependent on the energy-intensive Haber-Bosch process, causing huge environmental burden. Electrochemical ammonia synthesis is a promising alternative approach capable of producing ammonia under ambient conditions via nitrogen reduction reaction on electrocatalysts. Although promising, this approach is still challenged by poor selectivity and low yield rate, requiring advanced electrocatalysts with rational designs for efficient nitrogen reduction reaction. Here, we design and synthesize SnO2 nanosheets with a unique two-dimensional layered architecture, potentially as an efficient electrocatalyst for ambient nitrogen reduction to ammonia. First, SnO2 nanosheets with a thickness of few nanometers expose oriented crystalline (101) facets as a majority, preferably for nitrogen adsorption and reduction. Second, the presence of oxygen vacancies in SnO2 nanosheets creates defective Sn nanostructures with dangling bonds, which can significantly enhance the nitrogen adsorption via strengthening the local binding effect between Sn and N atoms. Last, the two-dimensional architecture enables the presence of abundant uncoordinated surface atoms that are thermodynamically unstable, in which nitrogen adsorption and dissociation mostly take place, thus greatly enhancing the catalytic reactivity. Density functional theory calculations found that the adsorption energy of N2 molecules on the (101) facet with a two-coordinate oxygen vacancy is 10.45 kJ mol-1, larger than that on the (110) facet (4.60 kJ mol-1). The strong binding between N2 and abundantly exposed (101) facets facilitates the nitrogen adsorption and dissociation. Hence, two-dimensional layered SnO2 is a promising electrocatalyst candidate for electrochemical ammonia synthesis. The ammonia production performance has been experimentally evaluated via construction of a three-electrode electrochemical cell, in which the as-prepared SnO2 nanosheets grown on a carbon cloth serves as the working electrode, displaying an ammonia yield rate of 38.18 μg h-1 mg-1 and a Faradaic efficiency of 11.33%, both of which are higher than those achieved by most recently reported metal-based electrocatalysts. The ammonia production capability is also found to be stable, with a 93% retention of ammonia yield rate after six cycles. Such a substantial improvement opens a window of opportunity for breakthroughs in the development of ambient ammonia synthesis technologies.