Does the Encapsulation Strategy of Pt Nanoparticles with Carbon Layers Really Ensure Both Highly Active and Durable Electrocatalysis in Fuel Cells?

Abstract

Platinum is a key component of commercialized proton exchange membrane fuel cells (PEMFCs) to lower the energy cost of the sluggish oxygen reduction reaction (ORR) at the cathode. Beyond the significant advances in improving its initial activity, securing catalytic durability is the next challenge for the successful implementation of PEMFCs. Encapsulation of Pt nanoparticles (NPs) with thin carbon or silica layers has recently been highlighted as a promising strategy for alleviating Pt degradation. However, unexpectedly similar or occasionally even better catalytic activity on site-blocked Pt NPs has raised fundamental interest about the nature of their catalytic sites and the origin of the prolonged durability. Herein, to answer these questions, we investigate the ORR and methanol oxidation reaction activities of carbon-encapsulated Pt NP (C@Pt/C) catalysts. By controlling the robustness of the carbon shells synthetically and electrochemically, directly exposed Pt sites, at which facile transport of reactant/product molecules occurs via the loosely packed or highly defective carbon layers, are identified as the main catalytic sites. Interestingly, online differential electrochemical mass spectroscopy and inductively coupled plasma-mass spectrometry coupled to electrochemical flow cells verify a trade-off relationship between the activity and stability of the catalysts. In addition to their role as a physical barrier for prohibiting the dissolution and agglomeration of the Pt NPs, the carbon shell acts as a sacrificial agent, questioning the practical legitimacy of the strategy for achieving both high activity and durability.