Core-shell catalysts leaching

The long-term stability of Pd-based electrocatalysts is one of the unavoidable issues for PEM fuel cell applications. Pd-Pt-based ORR catalysts are more stable than Pd-transition metal alloys under harsh fuel cell conditions, but may still not meet the long-term fuel cell operation requirement due to the Pd leaching out. Future research may focus on improving the durability of Pd-based catalysts by surface modification and composition optimization. Core-shell type of catalyst with Pd-based materials as the core and Pt as the shell may be one of the most promising candidates to be used in the automotive fuel cell due to its low Pt content and high activity and stability. 

Since only the Pt atoms on the Pt particle surface can participate in the catalysis process, one way to maximize Pt atom utilization is to deposit an extremely thin layer of Pt on a non-Pt particle to form a core-shell structure with the Pt layer as the shell and the non-Pt particle as the core. The electron conductivity of the core particles is not important because they are covered by a highly electrical conducting Pt shell. In addition, when a submonolayer to several monolayers of Pt are made on certain core metal particles, the catalytic ability of Pt improves due to electronic interactions between the shell Pt atoms and the interior core metal atoms that increase the Pt 5d orbital vacancies and thus increases the n electron donation from O2 to Pt atoms (electronic effect), and due to the decrease in the Pt-Pt atomic distance (geometric effect). This mechanism is similar to the improved catalytic ability of PtM and PtM Ny alloys, where M and N represent different metals and X and y their atomic contents in the alloy. For the Pt/core, PtM or PtM Ny, if there is some leaching out of the non-Pt metals, their corresponding cations can replace the protons of the PFSA either in the catalyst layer or in the membrane to reduce its proton conductivity as well as the catalyst-PFSA-reactant three-phase boimdaries, and thus decrease the fuel cell performance. The shape and crystalline facet of the Pt nanoparticles can also affect the catalytic activity. 

Electrochemical leaching of Pt alloy catalysts has been deliberately applied to form core-shell materials with enhanced activities, either in half-cell tests [24, 25] or even... 

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