Investigation of the electrochemical performance of barium-based catalysts for intermediate temperature ammonia synthesis using zinc oxide doped proton conducting electrolyte

Abstract

Ba and Ce doubly promoted Co catalysts (2BaCoCe0.05, 5BaCoCe0.05 and 5BaCoCe0.20) have been tested for electrochemical ammonia (NH₃) synthesis in proton conducting solid oxide cells between 400 to 500 ⁰C and atmospheric pressure, using 750 µm thick 1 mol% zinc oxide (ZnO) doped BaCe_0.2Zr_0.7Y_0.1O_3 − δ electrolyte and Ba-Ag-based composite electrodes. Faraday efficiency of proton conducting under all testing conditions was ∼ 89%. NH₃ production under open circuit conditions (OCV) was between 1.93 × 10^-11 to 2.50 × 10^-11 mol s^-1cm^-2 that is a purely thermochemical phenomenon. Under bias, NH₃ formation increased with highest value of 5.80 × 10^-11 mol s^-1cm^-2 being recorded at 20 mA current. It was also observed that for all three catalysts upon doubling the current, NH₃ formation increased by ∼ four-fold, indicating electrochemical promotion of catalytic activity (EPOC). Swapping the anode gas from dry to humid H₂ further increased the formation rate, yielding the highest value of 9.05 × 10^-11 mol s^-1cm^-2 at 20 mA current. In general, NH₃ production (thermochemical) rates at OCV and 500⁰C followed the order 2BaCoCe0.05>5BaCoCe0.20>5BaCoCe0.05 whereas electrochemical NH₃ production rates under bias followed the order 5BaCoCe0.05>2BaCoCe0.05>5BaCoCe0.20. According to the literature, NH₃ synthesis mechanisms are different for thermochemical and electrochemical pathways.