Selective Dissolution of Surface Nickel Close to Platinum in PtNi Nanocatalyst toward Oxygen Reduction Reaction

We report new insights in dissolution mechanisms of nickel in PtNi bimetallic nanoparticles (NPs) to develop active and durable oxygen reduction catalysts for fuel cells. Leaching out nickel by using acidic aqueous solution has been regarded as one of the most efficient chemical treatments to obtain a platinum-rich surface, which has shown both increased activity and stability during oxygen reduction reaction. In this work, we introduce a new approach using hydroquinone dissolved in ethanol to leach out nickel from PtNi NPs. The degree of alloying level is followed by X-ray photoelectron and absorption spectroscopies. Electrochemical measurements including potential cycling under oxygen reduction conditions allow us to investigate the dissolution behavior of nickel, depending on the chemical systems, and assess the relationship with electrochemical activity and stability. From comparative studies regarding the traditional acid treatment and the hydroquinone method introduced in this article, it is revealed that, while acid treatment preferentially removes oxidized Ni clusters, hydroquinone dissolves Ni atoms close to surface platinum. Electrochemical measurements help with the understanding of the different leaching mechanisms and highlight the influence of alloyed nickel on the activity of platinum and durability of the catalyst in the oxygen reduction reaction.