Platinum Metals Review - Volume 46, Issue 3, 2002

Platinum and Palladium behave in an unexpected manner when cathodically polarised in the presence of electrolytes dissolved in carefully dried polar organic solvents, such as N,N-dimethylformamide. A reductive layer is formed on the metal the thickness of which depends on the amount of electricity consumed during the course of the electrolyses. Although this reaction seems to be of a general character with most of the common electrolytes, in this paper we will focus on results obtained with a large palette of tetraalkylammonium salts and alkali metal iodides.

The effect of adding small amounts of rare earth elements to Palladium is to strengthen the Palladium. These strengthening effects are discussed here, based on known phase diagrams of Palladium-rare earths, Palladium-rare earth alloying behaviour and atomic (or ionic) size effects. The Solid solubilities of the rare earths in Palladium, transition temperatures of various intermediate phases and eutectic temperature in these systems are influenced by the ionic (or atomic) size of the rare earth elements. A parameter, Hs, the product of the relative difference in atomic weights and the relative difference in atomic radii, between a rare earth and Palladium is used to examine the Solid solution strengthening effects caused by dilute rare earths. The alloying behaviours of Palladium with the rare earths are very analogous, and could perhaps be used to predict alloying behaviour in some unexamined Palladium systems.

In the first part of this three part paper, published in January 2002, we dealt with enhancing the performance of low temperature proton exchange membrane fuel Cells by improvements to the platinum-based cathode materials and to the cathode design. In this second part of the paper we shall discuss the improvements in the reformate (CO and CO2) tolerance at the anode and in extending the MEA durability. Improvements have been achieved by advances in platinum/ruthenium electrode design, in the application of bilayer anodes for durable air bleed operation, and in the addition of a water electrolysis electrocatalyst to the anode for Cell reversal tolerance. In the third part of the paper the particular challenges presented by the direct methanol fuel Cell will be discussed.