Electrochemical Oxidation of Ammonia over Rare Earth Oxide Modified Platinum Catalysts

Hydrogen is now considered as the main fuel source for fuel cells, but its low volumetric energy density and difficulty in handling are main obstacles for the application in transportable devices and automobiles. To overcome this problem, hydrogen careers such as methanol and NaBH₄ have been proposed as alternative fuels for fuel cell systems [1]. Among them, ammonia is one of the promising candidates due to its low production cost, ease in liquefaction at ambient temperatures, and high energy density [2]. Moreover, ammonia is a carbon-free fuel which is expected to work as an incentive for the realization of a low carbon society.

However, the performance of most commonly used fuel cell, polymer electrolyte fuel cells (PEFCs) employing an acidic electrolytes such as Nafion^®, significantly deteriorated even with a trace amount of ammonia in hydrogen fuel [3]. In contrast, ammonia can be electrochemically oxidized in an alkaline electrolyte: The conventional alkaline fuel cell using potassium hydroxide (KOH) as an electrolyte was operated successfully with supplying ammonia directly [4]. In these circumstances, anion exchange membranes (AEMs) have been attracted much attention for electrolytes. The recent development of AEMs has increased the potential and the importance of ammonia as a fuel.

Many studies on the electrochemical ammonia oxidation over Pt electrode in an alkaline aqueous electrolyte have been reported [5], [6]. Throughout these studies, it was suggested that the amount of poisonous N_ad species and reactive OH_ad species have a great effect on the ammonia oxidation reaction over Pt electrode. The negative influence of N_ad species on ammonia oxidation over the platinum group metals (Pt, Ru, Pd, Rh, and Ir) have been reported by de Vooys et al [7]. However, the contribution of OH_ad species to ammonia oxidation has not been discussed yet in detail.  

In this study, then, the Pt-based catalysts with high electrocatalytic activity for ammonia oxidation were developed by focusing on the enhancement of adsorption property of OH_ad species. Many preceding studies have reported that the rare earth oxide such as CeO₂ changes the OH adsorption capacity of catalyst [8], [9]. In addition, rare earth oxides have been used as supporting materials of electrocatalysts for polymer electrolyte fuel cells (PEMFCs), and their stability under electrochemical environments has also been confirmed [10]. Accordingly, a series of rare earth oxides (CeO₂, Y₂O₃, La₂O₃, and Sm₂O₃) modified Pt catalysts was prepared and its electrocatalytic activity for ammonia oxidation reaction was investigated in alkali aqueous solutions. The ammonia oxidation activity was enhanced in accordance with the amount of OH_ad species over electrode surface. From various electrochemical measurements, it was revealed that the rare earth oxide additive improved the supply capacity of OH_ad to the reactive Pt sites. Among the catalysts studied, CeO₂ modified Pt electrocatalyst exhibited the highest activity; the peak current density for ammonia oxidation was 3.5 times higher than that of Pt catalyst. Furthermore, this activity enhancement was also observed at 60°C. These results indicate that the addition of rare earth oxide is one of the promising ways to design the high performance anode for direct ammonia anion exchange membrane fuel cells (AEMFCs).  

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