Pt Atoms on the Surfaces of Ordered and Disordered Aucu/C for Oxygen Reduction Reaction: Comparison of Electrochemical Performance and Mechanism

Abstract

To mitigate the effects of dwindling fossil fuel and the detrimental climate problems originating from the excessive CO2 emission, significant efforts are required to exploit alternative clean energy resources and advanced energy utilization strategies. Owing to its high energy density, high energy efficiency and low environmental impact, proton exchange membrane fuel cells (PEMFCs) are widely considered as the ideal alternatives to replace the traditional internal combustion engines. However, the slow rate of oxygen reduction reaction (ORR) in PEMFCs is the main limitation for automotive applications. This limitation could be eliminated if stable and active cathode catalysts could be developed. Nowadays, carbon supported platinum nanoparticles (Pt/C) still serve as the state-of-the-art ORR catalyst. The high cost and limited availability of Pt seriously impede its large-scale commercialization. Though tremendous efforts have been devoted to exploiting new Pt-free catalysts, there is still a large gap in the electrochemical performance between Pt/C and Pt-free catalysts in fuel cell. One strategy is to alloy Pt with other transition metals to form as PtM (M represents other transition metal) catalysts. The introduction of a secondary transition metal could change the environment of Pt atoms including: chemical composition, catalytic surface and coordination environment. What’s more, to maximize Pt utilization for fuel cell, various methods are explored to synthesize various structure of PtM catalysts, including hollow, cage, wire, dendrites and core-shell. Due to the so-called ligand effect and strain effect, the core-shell structure where the Pt atoms are usually arranged on the surface of the as-prepared alloy, is the most promising alternative to replace the traditional commercial Pt/C. However, few literature have studied if the structure ordering of the core materials has any effects on the catalytic performance of the outmost Pt atoms. The structure ordering refers to the different atomic arrangement in the unit cell. Taking the AuCu alloy as an example, in an ordered structure, the atoms only occupy the specific site of the unit cell, while the atoms of the disordered one will randomly distribute in the cubic of lattice. It means that in any nodes of the disordered unit cell, the atoms may exist statistically according to their mole ratios. Recently, there are several reports on the ordered alloys that they could have better ORR performance than their corresponding disordered samples. The enhanced activity and durability may be attributed to their different heteroatomic bonding, surface configuration and distribution of active sites. In our previous work, we obtained the ordered and disordered AuCu/C by a post-heating treatment at different temperatures. After a systematic comparison, we demonstrated the ordered AuCu/C catalysts could exhibit superior ORR performance, which is similar to that of Pt/C, than disordered ones in 0.1 M KOH solution. However, the Cu atoms are unstable in acid electrolyte which limited its further application in PEMFCs. In present work, Cu atoms on the surface of ordered AuCu/C nanoparticles were selectively replaced by Pt atoms by means of acid leaching and wet chemical deposition (denoted as O-Pts–AuCu/C) as shown in Figure 1. For comparison, the disordered AuCu/C was also used as core material to prepare D-Pts–AuCu/C. The high resolution transmission electron microscopy （HR-TEM）and high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) coupled with line scan demonstrated the formation of as O-Pts–AuCu/C and D-Pts–AuCu/C. The electrochemical characterization displayed the O-Pts–AuCu/C have a superior activity and remarkable stability in 0.1 M HClO4 solution with respect to commercial Pt/C and D-Pts–AuCu/C. To better understand their different electrochemical performance, X-ray photoelectron spectroscopy and X-ray absorption spectroscopic measurements were performed to study the electronic structure of O-Pts–AuCu/C and D-Pt s–AuCu/C. It was found that the ordered AuCu core could exert a compressive strain on the outmost Pt atoms. Compared to D-Pts–AuCu/C, lower d-band vacancy could be observed in O-Pts–AuCu/C which could account for its superior ORR performance. Figure 1