Synthesis and Evaluation of Core-Shell Electrocatalysts for Oxygen Reduction Reaction

Abstract

The slow kinetics of the oxygen reduction reaction (ORR) at the cathode and high cost of electrocatalysts (commonly Pt and Pt-based alloys) for this reaction are major hurdles that hinder the large scale production of proton exchange membrane fuel cells (PEMFCs).[1] A core-shell structure consisting of an atomically thin layer (up to several atoms thick) of Pt deposited on Pd has been recognized as a promising ORR electrocatalysts to replace bulk Pt-based materials.[2] Ideally, all of the Pt atoms will be fully utilized since they are all on the surface in the case of an atomically thin layer of Pt on a core material. The large scale synthesis of Pd@Pt core-shell catalysts, however, has not been fully realized. This study reported two synthesis methods to prepare gram scale batch of Pd@Pt/C electrocatalysts with the assistance of citric acid. The first is Cu-mediated-Pt-displacement method involving the displacement of an underpotentially deposited (UPD) Cu monolayer by Pt.[3] In this method, the Cu monolayer deposited on Pd then is displaced by Pt via a surface limited redox replacement (SLRR) reaction: Pd@Cu + PtCl4 2- → Pd@Pt + Cu2+ + 4Cl-. Recent studies have shown that the SLRR reaction is rather complicated and poorly-controlled resulting in formation of 3D Pt islands instead of a uniform overlayer, especially in the large batch size synthesis. By adding citric acid in the displacement reaction, the activity of Pd@Pt/C can be enhanced by 2 times compared with that synthesized without citric acid. The second method is direct chemcial reduction of PtCl4 2- with the assistance of citric acid without preformation of a Cu UPD layer. Comparing with other reducing agents, the reducing power of citric acid is neither too strong nor too week and allows the reduction reaction to occur at the room temperature, which significantly simplifies the reaction process. In addition, the strong interaction of citric acid with metals may reduce the Pd dissolution and introduce charge transfer from adsorbed citric acid molecules to Pd surface, which can also be used to reduce Pt cations. Both methods resulted good quality of core-shell catalyts in terms of activity and durability. The Pt activities of Pd@Pt/C preapred by the Cu-mediated-Pt-displacement and chemcial reduction methods were 0.95 and 0.78 A mg-1 at 0.9 V, respectively. Fuel cell performance will be also reported and the role of citric acid will be discussed in the presentation.