A study of the suitability of different methods for preparing highly loaded, well dispersed carbon nanofiber (CNF) supported Pt catalysts intended for application in fuel cells is reported. Preparation routes that are successfully applied on conventional carbon supports are hampered by the lower surface area and number of surface groups on CNFs. Ion exchange, homogeneous deposition precipitation and impregnation are all techniques that are limited to low metal loading on this CNF support. The most promising methods are the colloidal methods. By the modified polyol method, a Pt-content of 24 wt% with a particle size of 2-4 nm was achieved. CNFs could also be completely covered by 2-3 nm Pt oxide particles by using the metal-oxide colloid route, reaching a Pt-content of 17 wt%. The merits that make these methods more suitable than the other methods and the mechanism for deposition of Pt particles on CNFs are discussed.

In this work synthesis of Pt/C catalysts by reduction of H 2 PtCl 6 with sodium citrate has been investigated. The strong pHdependence of citrate as a reducing and stabilizing agent has been explored, and an optimum pH range for production of well dispersed catalysts is proposed. To achieve stabilizing and reducing conditions, the presence of both citrate anions and protonated citrates are required. This is achieved in an intermediate pH range between pK a2 and pK a3 (4.76 and 6.4) of citric acid, where both C 6 H 5 O 3-7 (denoted CA 3-) and C 6 H 7 O -6 (denoted H 2 CA -) are present. At pH 5.3-5.4 a catalyst with particles around 3 nm was thus successfully prepared. At high pH (∼12) the reduction of Pt is limited, whereas at low pH reduction is fast, but the stabilizing ability of the citrate in solution is poor resulting in large cubic Pt particles. CO-stripping voltammetry indicate that Pt(111) faces are the dominating crystal plane in the nanoparticles formed when citrate anions are used as stabilizing agent. This effect is presumably caused by the distance between oxygen groups in citrate correlating well with the Pt-Pt distance on (111) faces.

The effect of chloride on the stability of platinum electrocatalysts was studied by rotating disk measurements in sulfuric acid electrolyte with a continuously increasing concentration of chloride anions. The activity towards oxygen reduction was found to be reduced by a factor of seven when 140 ppm chloride was present. Platinum corrosion was severe at high potentials, presumably accelerated by potential cycling, and greatly enhanced by mass transport. A five-fold increase in corrosion rate was found when the electrode was rotated at 1600 rpm with respect to stagnant conditions. At potentials where oxygen reduction occurs, dissolved Pt can be redeposited on the electrode. The Pt dissolution rate increased with increasing Cl -concentration up to 20 ppm. Above this threshold the corrosion rate was unaffected by increases in Cl -content.