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.

Manipulating the interaction of Pt NPs with N-hollow carbon spheres by F-doping for boosting oxygen reduction/methanol oxidation reactions

“…04-0802), respectively, further confirming the precise loading of the small-sized Pt NPs with the heteroatomic doping of the hollow carbon sphere support. 39 The lattice defects and disordered structure of carbon materials were effectively characterized by Raman spectroscopy. As displayed in Fig.…”

Construction of Pt/Powder Charcoal Electrocatalyst Utilizing MnO2 as an Additive to Improve the Stability for Oxygen Reduction Reaction

“…And we also calculated the electron transfer number ( n ) at different voltages as shown in Figure b. The n values of PC and MnO 2 -PC are significantly lower than 4, indicating that the products in the oxygen reduction process contain not only water but also hydrogen peroxide. ,, For Pt/PC and Pt/M-PC, the n values are 4.04 and 4.01, respectively. This implied the typical first-order reaction kinetics with a direct 4-electron transfer mechanism under alkaline condition and makes them highly suitable as a cathode catalyst for PEMFCs. , …”

Catalyst Stability in Aqueous Electrochemistry

“…The controlled addition of specifically binding adsorbates alters the selectivity, usually at the expense of activity. For the electrochemical production of H 2 O 2 , the addition of chlorides, − protecting carbon layers, and mineral acids or salts − has been found to increase the selectivity, even though they are not always stable . Similarly, in an alkaline medium, the presence of iron impurities increases the OER activity of Ni­(OH) 2 /NiOOH. − Even a trace amount of Fe, which are typically present in commercially available high-purity KOH, makes it difficult to accurately assess the intrinsic activity of Ni-based catalysts.…”