(Invited) Partially Reduced Metal Oxide Supported Ni-Fe Electrocatalyst for N2 Reduction to NH3 at Ambient Conditions

Abstract

Haber-Bosch ammonia synthesis is a mature industrial process, via which >150 million tons ammonia per year was produced for the fertilizer industry to support the large global population growth over a century. However, this process consumes about 1~2% of all global energy and emits 2~3% of all global greenhouse gas CO2 emissions. Synthesis of ammonia directly from air and water, driven by renewable electricity, has been envisioned as an alternative and green process, as the global energy supply is shifting from fossil fuel based sources to renewable energy sources. The major technical challenges associated with nitrogen reduction reaction (NRR) using water as proton source are the low rate and very low selectivity (e.g. current efficiency or faradaic efficiency), in which most protons and current are used to produce H2. Intensive research efforts on the development of efficient and robust electrocatalysts and electrocatalytic process are highly desirable. Guided by the qualitative model laid out by Norskov group (ACS Catal. 2017, 7, 706−709), our rational design of NRR catalytic materials is based on the combination of the transition metals (e.g. Ni and Fe) that were predicted to be optimal for N adsorption (Phys. Chem. Chem. Phys., 2013, 15, 7785-7795) and to support them on different composite metal oxide substrate. Despite of the low surface area of the metal oxide support (ca. 35 m2/g) versus carbon black (XC-72, ca. 254 m2/g), the strong interaction between Ni-Fe and metal oxide after partial reduction in the flow of forming gas significantly improves the NRR activity tested in an H-type cell filled with 0.1 M KOH at ambient conditions (25°C and 100kPa). Comparing the NRR selectivity (in term of faradaic efficiency to ammonia production) and activity (in term of NRR rate), it clearly shows that at an optimized potential, the faradic efficiency of NRR over NiFeOx/C, partially reduced-NiFeOx/C and partially reduced NiFeOx/metal oxide is 0.54%, 3.97%, 15.27%, respectively, and the NH3 production rate also follows the order of NiFeOx/C << partially reduced-NiFeOx/C < partially reduced NiFeOx/metal oxide. The fundamental study of the NRR mechanisms on these composite catalysts based on the advanced physical characterizations and primary theoretical calculations, as well as our latest technology development of efficient alkaline electrolyzers for ammonia synthesis will be presented.