Epitaxially Grown Pt and Pt-Ir Thin Films: Effect of the Deposition Conditions on the Electro-Oxidation of Ammonia

Abstract

Ammonia (NH3) is a highly toxic gas, a major environmental pollutant, and a potential fuel for fuel cells. Thus, the electrooxidation of NH3 has important applications in NH3 safety sensors, NH3 decontamination in wastewater and power production. However, on most electrode materials, NH3 oxidation reaction proceeds with a high overpotential, low current density, and rapid poisoning rate. Better electrocatalysts for this reaction would lower the NH3 detection threshold in sensors, increase the rate of NH3 removal for decontamination, and boost power of NH3 fuel cells. Platinum is the electrocatalyst that shows the highest current density at the lowest overpotential for catalyzing the oxidation of NH3 to N2. The electrooxidation of NH3 on Pt is further complicated by the fact that the reaction rate strongly depends on the metal surface structure. In alkaline medium, the electrooxidation of ammonia on Pt occurs almost exclusively on surface sites with (100) symmetry, especially on wide terraces. Accordingly, the preferred way to study the effect of alloying Pt with other elements would be to rely on a series of mixed Pt-M alloy single crystals with the appropriate (100) surface termination. However, the preparation of single-crystals is difficult. In the case of Pt-Ir, the situation is even more complicated since the Pt-Ir binary phase diagram shows that Pt and Ir are barely miscible at temperature below 1000°C, which could translate into insurmountable technical difficulty in the preparation of Pt-M alloy single crystals. In recent years, Pulsed Laser Deposition (PLD) has emerged as an effective method for preparing thin films with preferential orientation and kinetically stable alloys. In this work, we will study the influence of different deposition parameters (composition, temperature and thickness) on the electro-oxidation of ammonia. To achieve this, a combination of X-ray diffraction (ϴ - 2ϴ scans, rocking curves and pole figures) and atomic force microscopy will be used to get information about the surface organisation and the length of the terraces. Then, electrochemical measurements will be performed to confirm the preferential (100) surface orientation and evaluate the electrochemical performances of the various films for the oxidation of NH3. It will be shown that the current for the electro-oxidation of NH3 varies with the Pt content and the thickness of the film (see Fig. 1). For the thinnest films, the role of Ir atoms in helping the formation of large (100) terraces will be emphasized. Figure 1 Variation of the current density for the electro-oxidation of NH3 with respect to the Pt content. The data were collected from the current at 0.71 V vs RHE during cyclic voltammetry (20 mV.s-1) in 0.1M NaOH + 0.1M NH3. Figure 1