Investigation of platinum-based nanoparticles on boron-doped diamond substrate for applications in electrocatalysis

The search of new energy sources has motivated during the last few years numerous research projects due to environmental, economical and political reasons. One of the main subjects of these works is the development of fuel cells. Within this framework the Direct Alcohol Fuel Cell (DAFC), mainly fed with methanol or ethanol, has received great attention. However one of the major limitations of the DAFC is the electrocatalysis of fuel oxidation; the electrocatalyst (dispersed Pt-based particles) is indeed readily poisoned by adsorbed intermediates, hence decreasing the fuel cell efficiency. Consequently the electrocatalysis of methanol and ethanol oxidation on different types of Pt-based nanoparticles, deposited on synthetic boron-doped diamond (BDD) thin film, has been studied during this thesis. This substrate has been chosen due to its outstanding properties of chemical inertness, illustrated by a very low capacitive current. In the first part of this thesis, electrodeposited Pt particles on BDD have been studied. Electrodeposition of Pt particles has been carried out both on bare BDD and on diamond pre-modified with gold nanoparticles deposited via a thermal decomposition technique. In both cases it was established that Pt particles were electrodeposited following a progressive mechanism during which formation of new nuclei and growth of primary nuclei simultaneously occur. The electrocatalytic activity of electrodeposited particles was also studied, and it was shown that the presence of Au particles together with Pt particles does not influence markedly the electrocatalytic behaviour of the later ones. Preliminary calcination of the co-deposit results in a dramatic decrease of its electrocatalytic activity, presumably due to the formation of a core (Pt)–shell (Au) structure less capable of alcohols dehydrogenation. Finally electrodeposition of Pt on BDD leads to particles of diameter in the 150-700 nm domain with very large size distribution. Obviously such particles are not consistent with the definition of nanoparticles in catalysis and electrocatalysis. In a second part of this work, Pt-based nanoparticles synthesized via the microemulsion method have been investigated once deposited on a diamond substrate. Attention has been mainly focused on Pt, Pt/Ru and Pt/Sn nanoparticles. All the synthesized nanoparticles are in the 2-5 nm size range with narrow size distribution. Moreover the compositions of bimetallic particles were very close to those expected. The electrocatalytic behaviour of bimetallic Pt-based nanoparticles toward both methanol and ethanol electrooxidation has been investigated. Bimetallic nanoparticles are more efficient than pure Pt, and it was shown that Pt/Ru nanoparticles were more indicated for electrocatalysis of methanol oxidation whereas Pt/Sn were more efficient in the case of electrocatalysis of ethanol oxidation. Both cooperative and electronic effects are involved in the enhancement of the electrocatalytic activity