Pt-Based Ternary Catalysts for Electrochemical Ammonia Oxidation

Abstract

Ammonia (NH3) has proven to be a cost-effective alternative to hydrogen for powering fuel cells via its direct oxidation to nitrogen and water. However, sluggish kinetics of the ammonia oxidation reaction (AOR) have greatly hindered the commercial application of direct ammonia fuel cells (DAFCs). Here we report an efficient AOR catalyst that consists of PtIrNi alloy nanoparticles anchored on a binary support of porous silicon dioxide and carboxyl-functionalized carbon nanotubes (PtIrNi/SiO2-CNT-COOH) through a sonochemical-assisted synthesis strategy. The PtIrNi alloy nanoparticles, with the aid of abundant OHad provided by porous SiO2, and the improved electronic conductivity contributed by CNT, exhibit remarkable catalytic activity for the AOR in alkaline media, with a lower onset potential (~0.40 V vs. RHE) at room temperature, than that of commercial PtIr/C (~0.43 V vs. RHE). Constant-potential density functional theory (DFT) calculations showed that the Pt-Ir ensembles on {100}-terminated surfaces serve as the active site, the introduction of Ni to which raises the center energy of the density of states projected onto the group d-orbitals and thus lowers the theoretical onset potential for *NH2 dehydrogenation to *NH when compared to Pt and Pt3Ir alloy. Varying the NH3 concentration and operation temperature shows a significant influence on the AOR performance of this catalyst. Specifically, AOR activity of the optimal PtIrNi nanoparticle catalyst can be significantly enhanced by elevating the temperature to 80ºC, with a much lower onset potential (~0.32 V vs. RHE), indicating that DAFC can be operated at higher temperature for increased performance.