This research deals with the elaboration and study of solid state nanocomposite structures built using pre-synthesized objects handled in liquids (bottom-up approach). This approach affords a way to study the impact of nanoscale modifications on the macroscopic properties of nanostructures which can be therefore optimized. It is developed on model systems based on platinum nanoparticles chemically functionalized with 4-mercaptoaniline (mother particles) from which chemical modification performed on this organic crown affords new nano-objets with controlled features and composition. Thanks to the organic crown grafted on the platinum core (2nm diameter), the different nanoparticles can be handled in solution like molecules.
This point allows the nanoparticles to be organized in solid state structures in particular on conducting substrates. Then, their electrical and electrochemical properties can be studied. We used the Langmuir-Blodgett (L.B.) method to build up ultra-thin films with a controlled number of nanoparticle monolayers. The L.B. films are mixed films which contain the capped platinum nanoparticles and amphiphilic molecules. They allow the Langmuir-film formed at the air-water interface to be transferred onto solid substrate with a good yield. The amphiphilic molecules can be removed from the solid state structure by the so-called skeletonization procedure.
We have also developed a process by which porous hetero-nanostructures combine two kinds of nano-objects: capped platinum nanoparticles and carbon nanotubes. The combination of both nano components is performed in liquid media and produce nanocomposite liquid dispersions in which the coverage of the nanotubes by the nanoparticles is well controlled.
The liquid dispersion can be deposited on carbon felts to give model electrodes for electrochemical studies.
a) Oxygen reduction activity as a function of platinum loading, b) Fuel cell test for commercial electrode (500 µg Pt/cm2) and laboratoty electrodes (115µgPt/cm2)
The main property studied in the different solid state structures is the electrocatalytical behaviour towards oxygen reduction. The related application concerns the electrocatalyst, platinum being the reference material, used in fuel cell electrodes. This device combines oxygen and hydrogen into water and deliver electrical power without emitting greenhouse gases.
In collaboration with UMR 8180 at Versailles Saint-Quentin University France ( A. Etcheberry group) the study of the electrochemical properties of our platinum based solid state nanostructure allowed a significant activity towards oxygen reduction to be evidenced. This quite high activity is surprising for two reasons. First because it is recorded while molecules are grafted at the platinum core surface and oxygen reduction is supposed to involve platinum surface. Secondly, because the nanostructures do not show significant electroactive surface areas classically observed on uncapped platinum nanoparticles. This intriguing behaviour raises the question of the sites at which oxygen reduction occurs. We currently attempt to address this fundamental question using our solid state structures. The study of the effect of a controlled alteration of the organic crown while keeping the platinum core intact offers an interesting approach. Langmuir-Blodgett. film structures allowed to show that, in planar electrodes, the oxygen response is optimized with 1 to 2 monolayers of nanoparticules. Porous electrodes offer a particular interest because the platinum loading can be controlled over a wide range (1µg/cm2 to a few hundreds). In addition, they can be characterized towards aqueous oxygen or under gaseous oxygen feeding in a three electrode cell as well as in real fuel cell devices (collaboration with CEA-DRT-LITEN laboratory).
We showed that oxygen reduction can be demonstrated in electrodes with ultra-low platinum loading. Therefore, these electrodes are model systems which offer an interesting tool to optimize platinum loadings and electrode structure. Fuel cell tests were performed using our electrodes loaded with capped platinum electrocatalyst on which no conditioning treatment was performed. The results show that our electrodes with platinum loading much lower that commercial electrodes exhibit similar performances.