Abstract
The structural design of nanosized electrocatalysts is extremely important for cathodic oxygen reduction reactions (ORR) and anodic oxidation reactions in small organic compounds in direct fuel cells. While Pt is still the most commonly used electrode material for ORR, the Pd electrocatalyst is a promising alternative to Pt, because it exhibits much higher electrocatalytic activity towards formic acid electrooxidation, and the electrocatalytic activity of ORR on the Pd electrode is the higher than that of all other precious metals, except for Pt. In addition, the mass activity of Pt in a core–shell structure for ORR can be improved significantly by using Pd and Pd-based materials as core materials. Herein, we review various nanoscale Pd-based bimetallic, trimetallic and core–shell electrocatalysts for formic acid oxidation and ORR of polymer electrolyte fuel cells (PEFCs). This review paper is separated into two major topics: the electrocatalytic activity towards formic acid oxidation over various Pd-based electrocatalysts, and the activity of ORR on Pd-based materials and Pd core–Pt shell structures.
Highlights
Owing to their high electrocatalytic activity, Pt-based electrocatalysts have been extensively investigated as electrode materials for oxygen reduction reaction (ORR) and the electrooxidation of small organic compounds in polymer electrolyte fuel cells (PEFCs), which are potential alternatives to fossil fuel-based devices [1]
CO is not formed on the surface of intermetallic PtPb during the electrooxidation of formic acid and methanol. This indicates that intermetallic PtPb electrooxidizes formic acid and methanol via the direct pathway, in which CO is not formed [36]. These results indicate that the electrooxidation of formic acid over Pt and intermetallic PtPb catalysts proceeds via different reaction mechanisms
Electrocatalysts developed over recent years can be classified into three categories: (1) Pd-based materials for formic acid oxidation, (2) Pd-based materials for ORR, and (3) Pd-core–Pt shell-structured catalysts
Summary
Owing to their high electrocatalytic activity, Pt-based electrocatalysts have been extensively investigated as electrode materials for oxygen reduction reaction (ORR) and the electrooxidation of small organic compounds in polymer electrolyte fuel cells (PEFCs), which are potential alternatives to fossil fuel-based devices [1]. In order to prepare Pt-based electrocatalysts with lower d-band centers and higher ORR activity, three-dimensional transition metals such as Fe, Co, Ni and Cu are used as alloying materials to modify the electronic state and crystal structure of Pt [10,11,12]. Effective alloying with intermetallic compounds and a core–shell structure can reduce Pd and Pt loading and increase electrocatalytic activity
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