Résumé
Conformal Pt extended surfaces such as nanostructured thin films on whiskers
1
have largely demonstrated their effectiveness in the electrocatalysis of the oxygen reduction reaction (ORR). Their specific morphology allows for the maximum exploitation of platinum as well as improved stability compared to nanoparticulate electrocatalysts, leading to ultra-low loaded electrodes with high activity and durability.
Our approach is based on the preparation by electrochemical and chemical routes of Pt thin films onto carbon or metal nanofibres obtained by electrospinning
2
. The nanometre size and the high aspect ratio of the nanofibrous supports and their highly porous structure are expected to bring associated advanced properties, in particular with regard to mass transport properties, with beneficial effects on the performance and lifetime of the resulting membrane-electrode assemblies. Here we present two different strategies towards nanofibrous core@shell electrocatalyst design for PEMFC.
In the first approach, Pt films are deposited onto self-standing carbon fibre webs by a high overpotential electrodeposition method
3
. In order to tune the fibre density and lacunarity of the nanofibrous electrodes (NFE) carbon nanotubes were also grown on the fibre surface
4
. These structured electrocatalyst layers presented high electrical conductivity for fast charge transport, and engineered density for efficient reactant mass transportation. This method allowed to obtain self-standing NFEs with ultra-low Pt loading (0.05 mg/cm
2
) with very high activity towards the ORR and electrochemical surface areas as high as 150 m
2
/g.
In another approach, Pt films were prepared onto Ni nanofibres by galvanic displacement
5
, a widely adopted route for the preparation of highly active electrocatalysts based on core@shell morphologies.
6
The reaction time for Pt-Ni galvanic displacement was significantly reduced by using a microwave-assisted procedure on Ni nanofibres, which led to high mass activity catalysts (>0.5 A/mg Pt) in only 2 minutes. The shell thickness and density were controlled by extent of Pt-Ni displacement (Pt loading), itself controlled by the concentration of Pt ions in solution. By tuning the Pt content, Pt@Ni core@shell electrocatalysts were prepared that do not require electrochemical etching of surface Ni or de-alloying before utilisation. In addition, the nickel core can be leached leaving chemically stable Pt
4
Ni thin tubular structure.
A detailed microscopic, structural and electrochemical characterisation of the Pt@C and Pt@Ni NFEs will be presented.
References
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