Nitrous oxide abatement and valorization in an ammonia-fueled SOFC stack - Nitric oxide and electricity production

Abstract

Nitrous oxide (N2O) is a harmful gas that strongly impacts climate change. Several strategies for denitrification have been addressed, where the use of solid oxide fuel cells (SOFCs) has been demonstrated to be attractive for N2O reduction and simultaneous electricity production. The reduction of N2O to N2 is studied for the first time at the cathode of an SOFC, where ammonia is supplied to the anode as fuel. A diluted ammonia stream is particularly interesting since it is normally available in HNO3 plants - the most important concentrated source of N2O emission - and is carbon-free. The combined reduction of N2O with the oxidation of NH3 allows the production of nitric oxide (NO) at the anode, a valuable precursor for HNO3 production. This work uses a commercial SOFC stack comprehending six cells, equipped with membrane electrode assemblies of LSM-CGO yttria-stabilized zirconia (YSZ) YSZ-NiO. Electrochemical Impedance Spectroscopy (EIS) was successfully employed to characterize the stack performance. Data were analyzed by combining a Distribution Function of Relaxation Times (DFRT) and Equivalent Circuit Models (ECMs), for different operating conditions, namely in terms of temperature and fuel feed composition. Three major processes were identified at the anode: one related to the adsorption/desorption of reactants on the surface of the electrocatalyst, a diffusion process of charge carriers/intermediate species to the triple phase boundary (TPB), and a charge-transfer process at the TPB. The electrochemical production of nitric oxide (NO) was investigated, and a 20% selectivity towards NO production and an energy stack efficiency of 88 % was obtained. Overall, the current work provides a critical first step toward using SOFC to perform efficient N2O electroreduction using diluted ammonia as fuel.