Unravelling degradation mechanisms of anion exchange membrane direct ammonia fuel cells via distribution of relaxation times

Abstract

Anion-exchange membrane direct ammonia fuel cells (AEM-DAFCs) have garnered increasing attention due to their carbon-free properties. However, they are not yet ready for widespread application because of their poor stability (< 80 h) and low power density (< 420 mW cm−2). Additionally, the mechanisms behind cell performance degradation remain unclear. This study is the first to utilize electrochemical impedance spectroscopy (EIS) combined with distribution of relaxation times (DRT) analysis to thoroughly investigate the degradation mechanisms of AEM-DAFCs. DRT is a non-parametric model that effectively interprets EIS spectra by translating them from the frequency domain to the time domain, allowing for the distinction of overlapping polarization processes. Operating conditions assist in identifying the characteristic peaks obtained by DRT, which correspond to the kinetics of the ammonia oxidation reaction (AOR), the kinetics of the oxygen reduction reaction (ORR), and ionic transport. During a 50-hour stability test, in addition to interpreting EIS spectra with DRT, a reference electrode is used to separate the overpotential losses of the anode and cathode. Post-characterization tests are also conducted to examine changes in the electrode microstructure and composition before and after the stability test. The results indicate that cell performance decay primarily stems from the deterioration of AOR kinetics, specifically due to the dissolution (particularly of iridium), migration, and agglomeration of the PtIr/C catalyst, as well as poisoning by adsorbed species such as *N and *NOx. These findings elucidate the mechanisms of cell performance degradation and provide guidance for the development of highly stable AEM-DAFCs.