New Insights into Pt Active Sites for Anion Exchange Membrane Direct Ammonia Fuel Cells

Ammonia, consisting of a nitrogen atom bonded with three hydrogen atoms, has been considered a prominent hydrogen carrier candidate to realize the carbon-free hydrogen economy due to its carbon-free properties, ease of liquefaction, and already well-equipped infrastructure and market. Beyond its role as a carrier, some strategies to directly use the ammonia itself as a fuel have been widely studied [1]. Direct ammonia fuel cells (DAFCs) generate electricity via electrochemical redox reactions involving ammonia oxidation reactions (AOR) and oxygen reduction reactions (ORR) at the anode and the cathode, respectively. Like other fuel cells that directly use liquid fuel, the DAFCs face significant challenges related to fuel crossover, resulting in mixed potential, electrocatalyst poisoning, and low kinetics. In particular, platinum-group metal (PGM) electrocatalysts, known for their excellent activity and durability for the ORR, have not been widely used in the cathode of DAFCs. This is because the ammonia that crosses over from the anode to the cathode can easily be oxidized or poisoned on the Pt surface, degrading cell performance.<br/>In this study, we propose new insights into Pt atomic active sites with high ORR activity and ammonia oxidation resistance. Ultralow Pt-loaded catalysts (&lt; 0.5 wt.% of Pt) derived from three kinds of metal-organic frameworks (MOFs) were prepared with different active sites: M-N_x atomic sites, PtFe alloy, and a combination of both by simply varying pyrolysis conditions. Carbon quantum dot additives with abundant N composition were coated on the MOF substrate to maximize metal atomic structure without metal agglomeration. Pt-Fe-NC catalyst_900°C catalyst primarily created Fe and Pt atomic active sites, where a metal atom coordinates with nitrogen atoms in the carbon matrix. In contrast, PtFe metal alloys were formed in Pt-Fe-NC_1000°C catalyst. These catalysts demonstrated high ORR activity in alkaline electrolytes, outperforming commercial Pt/C catalysts. Surprisingly, compared to commercial Pt/C catalysts, the Pt-Fe-NC_950°C catalyst had a mass activity value of 4.266 A/mg_Pt(@ 0.9 V vs. RHE), nearly 142 times higher than Pt/C (0.03 A/mg_Pt). To investigate the ammonia oxidation resistance of the catalysts, ORR activity was compared in an ammonia-added alkaline environment. In the presence of ammonia, the ultralow Pt-loaded Pt-Fe-NC catalysts showed a much lower decrease ratio of half-wave potential than that of Pt/C (7.4%). In particular, the Pt-Fe-NC catalyst_900°C catalyst with multiple metal atomic active sites exhibited only a 0.6% decrease ratio. DFT calculations confirmed that Pt atomic active sites had high ammonia oxidation resistance, as ammonia molecules rarely adsorbed to the Pt surface in Pt-N_x sites. As a result, we suggest that an ultralow Pt-loaded catalyst with atomic-scale Pt active sites could be a viable cathode catalyst for DAFCs.<br/>[1] Y. Guo et al., J. Power Sources., 476 (2020) 228454