Fabrication of Ni-Pt-Ir Ternary Alloy Nanoparticles Throught Ionic Liquid/Metal Sputtering and Their Electrocatalytic Activity for Urea Oxidation
Authors: Hiraide Kento , Liu Yixiong , Akiyoshi Kazutaka , Kuwabata Susumu , Torimoto Tsukasa 
Published: December 2024

Hydrogen is widely used as a fuel for fuel cells but presents safety concerns and challenges in transportation. In contrast, urea has emerged as a promising alternative due to its high energy density and stability under atmospheric conditions.[1] It has been reported that nickel (Ni) exhibited high electrocatalytic activity for urea oxidation.[2,3] While Ni-based alloy catalysts have shown electrocatalytic activity for urea oxidation (UOR), there has been limited research on improving the activity through alloying or combining Ni with noble metals. Recently we reported the preparation of noble metal nanoparticles (NPs) by metal sputtering deposition onto ionic liquid (IL) under reduced pressure (IL/metal sputtering technique). Thus-obtained NPs were uniformly and stably dispersed without additional stabilizing agents.[4] This technique enabled the preparation of alloy metal NPs, such as AuPd and AuRh, through simultaneous sputtering of corresponding pure metals.[5,6] In this study, we focus on the preparation of ternary Ni-Pt-Ir alloy NPs via IL/metal sputtering and evaluate their electrocatalytic activity for urea oxidation (UOR) was evaluated.

The simultaneous deposition of Ni, Pt, and Ir onto an IL of N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate (DEME-BF4) was achieved by sputtering multi-component target composed of pure metal plates of Ni, Pt, and Ir under an argon pressure of 10 Pa. The resulting IL containing NPs was mixed with carbon black (CB) powders and subjected to heat treatment at 300°C under a nitrogen atmosphere for 30 minutes to load NiPtIr NPs onto CB particles (NiPtIr/CB). The activity of NiPtIr/CB for urea oxidation was evaluated in a 1 M KOH aqueous solution containing 0.33 M urea.

TEM measurements and EDX analysis revealed that spherical NPs with the particle size of 1-2 nm, consisting of Ni, Pt, and Ir, were deposited in the IL. Loading these NPs on CB enlarged the particle size to 3-5 nm. XRD patterns of NiPtIr/CB indicated the shift of diffraction peaks to higher angles with increasing Ni content, confirming the formation of NiPtIr alloy. The electrochemically active surface area (ECSA) of Ni was determined by cyclic voltammetry. The UOR reaction current was observed at the potential more positive than 0.37 V vs. Ag/AgCl. The UOR activity was evaluated from the current at 0.5 V vs. Ag/AgCl normalized by ECSA. The UOR activities of NiPtIr/CB were smaller than that of Ni/CB when the Ni fraction exceeded 30%. However, the NiPtIr/CB catalysts with the Ni fraction of < ca.20% exhibited higher activity for UOR compared to the Ni/CB. The activity was optimal for Ni-Pt-Ir/CB with a 9% Ni fraction. XPS spectra revealed the change in the Ni species on the catalyst surface, contributing to the improvement of UOR activity with the change in the catalyst composition. In conclusion, we successfully prepared NiPtIr ternary alloy NPs using the IL/metal sputtering technique and demonstrated their UOR activity depending on the composition of the catalyst particles.

References

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