Electrodeposition of Pt - Rare Earth Alloys as ORR Catalysts for Fuel Cells

Abstract

Hydrogen fuel cell driven electric vehicles are on the verge of market introduction, but the efficiency of the fuel cells is still far below the thermodynamic limit. The high amount of Pt catalyst therefore required to deliver the power needed leads to costs too high for widespread market introduction. Especially overpotentials of several hundred mV at the cathode side due to the sluggish kinetics of the oxygen reduction reaction (ORR) are responsible for these drawbacks. Catalytic materials with higher activity for the ORR and enhanced durability as well as reduced Pt loading can both bring down the costs and enhance the efficiency and therefore the competitiveness of fuel cell driven vehicles. Recently it has been shown that polycrystalline Platinum - rare-earth metal (RE) alloys show very promising catalytic properties for ORR, e.g. an increase in the kinetic current density by a factor of 3-5 compared to pure Pt [1-3]. They were reported to show a good stability due to their high heat of formation and the formation of a dense Pt overlayer during the initial de-alloying steps [2]. The overlayer is under compressive strain, causing the high activity [2]. Also for Pt-RE nanoparticles prepared in a cluster source, an enhanced mass activity was found [4]. For applying these alloys in actual fuel cells, the major challenge is to synthesize such nanoparticles with a method, which can be up-scaled to provide enough material for MEA fabrication and subsequently for widespread production for practical applications. This is challenging due to the very low standard potentials of the rare earth elements. In this study electrochemical deposition was selected as a scalable method. However, due to the low deposition potential of rare-earth metals, aqueous electrolytes could not be used for these experiments. Therefore other electrolytes, especially ionic liquids, were chosen as electrolyte, because they offer a wide electrochemical window and in literature a successful deposition of selected pure rare-earth metals had been claimed [5]. However, there are also reports showing fundamental obstacles for deposition of RE metals from some ionic liquids [6]. Therefore in an additional approach organic solvents with an added supporting electrolyte were evaluated, which have also a wide potential window and where high amounts of precursors can be dissolved. For every electrolyte system, the electrochemical processes in solutions containing Pt precursors, rare earth metal ions and mixtures of both for alloy deposition were studied by the use of electrochemical techniques, partially in combination with the electrochemical quartz crystal microbalance (EQCM) technique, and ex-situ and in-situ scanning probe microscopy techniques as well as electrocatalytic measurements for the ORR. Aside from the gold electrode of the EQCM, Boron-doped Diamond (BDD) was used as working electrode, as it is a very inert electrode and allows even wider potential windows in non-aqueous media [7]. Deposited layers were characterized by EDS and surface science methods.