Effect of an oxygen plasma treatment on the specific surface of platinum electrodeposits for fuel cells

Fuel cells involve electrochemical reactions often catalysed by platinum whose surface has to be maximized. In this paper, platinum nano-particles are deposited onto graphite by a potentiostatic reduction of a dilute 1.0 mM solution of hexachloroplatinate acid. Some samples are pre-treated by highly dissociated oxygen plasma and exhibit an increase of their specific surface compared to the untreated ones. The gain factor on specific surface reaches 3.6 and even 4.4 when the plasma treatment is coupled with the impregnation technique. Surface functionalization made by the plasma treatment lead to denser deposits thanks to an organized nucleation and growth of platinum nuclei. XPS analyses suggest the existence of C–O bonds in the platinum clusters. Although the testing conditions were not optimized, we have measured the performances of a microfuel cell made with the optimized catalyst. The Scharifker and Hills electrocrystallisation model was used to fit the current transients. Untreated samples transients were correctly fitted by the model whereas plasma treated samples transients did not have the requested shape for this model. Hence, nucleation mechanism was determined and confirmed by observation on untreated samples only. The calculated value of the diffusion coefficient of the Pt(IV) anion PtCl₆²⁻ determined by the Cottrell theory was close to the literature, i.e. 2.6 ± 0.6 × 10⁻⁶ cm² s⁻¹.