Composition effects of platinum alloys for ammonia oxidation reaction

Abstract

Ammonia is a carbon-free chemical energy source with hydrogen storage capacity of 17.7 wt% and energy density of 3 kWh/kg, which is much higher than other small organic molecular fuels such as methanol. As a promising fuel candidate, ammonia has already been successfully used in an alkaline fuel cell by directly feeding ammonia; however, the current technology of catalysts for ammonia electro-oxidation reaction (AOR) with respect to both cost and performance is inadequate to ensure large scale commercial application of direct ammonia fuel cells. Recent studies found that alloying Pt with other transition metals may improve the AOR activity to address the cost issue. In this study, Pt-3d transition metal (Fe, Co, Ni, Zn) nanocubes are synthesized and the effects of types of metals on the AOR activities are studied. It is found that the addition of Zn, Fe, Co and Ni elements can enhance the AOR activity due to the decrease of oxophilicity and the bifunctional mechanism. The mass activity of Pt-Zn, Pt-Fe, Pt-Co and Pt-Ni nanocubes are 0.41, 0.32, 0.31and 0.26 A/mg_pt which are 1.6, 1.3, 1.2 and 1.1 times higher than Pt nanocubes respectively. As for specific activity, Pt-Zn, Pt-Fe, Pt-Co and Pt-Ni nanocubes are 1.5, 1.2, 1.2 and 1.1 times higher than Pt nanocubes with a value of 2.02, 1.62, 1.60 and 1.47 mA/cm² respectively. Furthermore, the effects of size of Pt nanoparticles and catalyst supports for Pt nanoparticles on AOR were also investigated. It is found that the AOR mass activity increased with growth of Pt particles initially and then decreased with the maximum activity observed at a size of 3.43 nm. The increase of AOR activity with size is attributed to decrease of oxophilicity and increase the ratio of Pt (100) sites to total surface atoms. With regards to support effect, using CeO₂ as the support for Pt nanoparticles may suppresses the oxidation of ammonia on Pt.

Date of Award	2019
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology