(Ion Power Poster Award Winner) Electrodeposition of Novel Catalyst Materials for the Oxygen Reduction Reaction

Despite significant improvements over the past decade, the efficiency of fuel cells is still far below its thermodynamic limit. Additionally the amount of Pt catalyst required to deliver the power needed, e.g. for automotive applications, and therefore the costs are still too high for widespread market introduction. Especially the high overpotential at the cathode side (several hundred mV) due to the sluggish kinetics of the oxygen reduction reaction (ORR) is 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 of the fuel cells. Recently it has been discovered that Platinum-rare-earth-metal 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 [1,2]. Because of their high heat of formation they also have a good stability. The cause of the high activity is the formation of a dense Pt skin layer during the initial de-alloying steps that is under compressive strain [2]. Apart from studies on polycrystalline model alloys, an enhanced mass activity of nanoparticles prepared in a cluster source was reported [3].

However, for actual application in fuel cells a method for the production of such nanoparticles is required that can be up-scaled to provide enough material for MEA fabrication. This is challenging due to the very low standard potentials of the rare earth elements.

In this study the electrochemical deposition was selected as the (scalable) method of choice. Because of the low deposition potential of rare-earth-metals the standard aqueous electrolytes could not be used for these experiments. Therefore ionic liquids were used as electrolyte, because they offer a wide electrochemical window. In literature a successful deposition of selected pure rare-earth-metals was claimed, also [4].

Since also deposition processes in ionic liquids can be complicated, and not all reactions are mechanistically understood, in a second 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. In the latter case, removal of water impurities is more difficult than for ionic liquids.

In both cases, the deposition of pure Pt and pure rare-earth-metals as well as the alloy deposition were studied by electrochemical techniques, in part with combination with the electrochemical quartz crystal microbalance (EQCM) technique, ex-situ and in-situ scanning probe microscopy techniques and surface analytical methods. Figure 1 shows the frequency change Δf of a quartz resonator in the deposition region of a cyclic voltammogram in a La³⁺containing electrolyte. In this measurement Δf is consistent with the deposition of metallic La. 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 [5].

Figure 1: Third voltammetric cycle in Glyme + 0.5M La(NO₃)₃ + 0.5M tetrabutylammonium-perchlorate, υ=5 mV/s. Electrochemically active surface area: ~ 0.28 cm².

References:

1.   Stephens, I.E.L., et al., Understanding the electrocatalysis of oxygen reduction on platinum and its alloys. Energy & Environmental Science, 2012. 5(5): p. 6744-6762.

2.   Malacrida, P., et al., Enhanced activity and stability of Pt-La and Pt-Ce alloys for oxygen electroreduction: the elucidation of the active surface phase. Journal of Materials Chemistry A, 2014. 2(12): p. 4234-4243.

3.   Hernandez-Fernandez, P., et al., Mass-selected nanoparticles of Pt_xY as model catalysts for oxygen electroreduction. Nat Chem, 2014. 6(8): p. 732-738.

4.   Legeai, S., et al., Room-temperature ionic liquid for lanthanum electrodeposition. Electrochemistry Communications, 2008. 10(11): p. 1661-1664.

5.   S. Ayata, et al., The electro-oxidation of water and alcohols at BDD in hexafluoroisopropanol, Journal of Electroanalytical Chemistry , 2013. 701: p. 1-6.