Zn-Mediated Synthesis of Sub-5 nm PtIr─Zn Nanocubes for Direct Ammonia Fuel Cells

Direct ammonia fuel cells (DAFCs) are promising for decarbonized electricity generation. However, the sluggish ammonia oxidation reaction (AOR) has long been the major roadblock. Although Pt(100) facet features high AOR activity, fabricating ultrasmall (<5 nm) Pt-based nanocubes (NCs) with (100) facet orientation remains a challenge. Here, we develop a Zn-mediated strategy for the synthesis of PtIr─Zn NCs with an ultrasmall size of 4.1 ± 0.5 nm. Systematic investigations reveal that surface-inserted Zn atoms serve as anchors to enhance the specific adsorption of dibenzyl ether, which directs the exclusive (100)-oriented growth into well-defined NCs. The initial nucleation process is not affected, so there are abundant PtIr seeds, which are the key to yielding the sub-5 nm size. Oxophilic Ir atoms serve as the *OH adsorption sites and promote the *NH₃ dehydrogenation. Moreover, Zn-induced surface compression lowers the barrier of rate-determining dehydrogenation step by shortening the hydrogen bonds between *NH₃ and *OH. Our PtIr─Zn NCs catalyst exhibits both high intrinsic activity and mass activity, delivering an ultralow onset potential of 0.355 V and a peak mass activity of 238.3 mA mg_Pt+Ir ^-1. The DAFC employing PtIr─Zn NCs achieves an open-circuit voltage of 0.60 V and a peak power density of 76.0 mW cm^-2.