Temperature-controlled shape transformation of PtCo alloy catalysts for enhanced ammonia oxidation in anion-exchange membrane direct ammonia fuel cells

Abstract

Low-temperature anion-exchange membrane direct ammonia fuel cells (AEM-DAFCs) are emerging as attractive and environmentally friendly energy devices for small-scale transportation. However, challenges arise in the ammonia oxidation reaction (AOR) owing to the high activation energy required for direct ammonia modification and the issue of decreased activity in the AOR caused by the strong adsorption of ammonia and nitrogen species, necessitating solutions. In this study, we aimed to address the challenges related to the ammonia activation energy and ammonia adsorption capacity by alloying Pt and Co. Furthermore, during the fabrication of the PtCo catalysts, the synthesis process was controlled at different reaction temperatures to produce core–shell and hollow structures. In particular, the PtCo catalyst synthesized at 60 °C (PtCo-60), with a particle size of 7.65 nm in the hollow structure, exhibited the highest electrochemically active surface area among the PtCo catalysts produced (∼43.0 m² g⁻¹). This increase in the active surface area owing to structural changes was attributed to the increase in AOR activity. The onset potential of PtCo-60, adjusted for particle size and structure, was determined to be 0.454 V vs. RHE, with a mass activity of 128 A g_Pt⁻¹, in cyclic voltammetry experiments using 1 M KOH +1 M NH₄OH. Ultimately, in the AEM-DAFC unit cell test conducted at 80 °C, an output density of 11.33 mW cm⁻² was achieved.