Lanthanider

Cheaper platinum alloys for fuel cells

Polymer electrolyte membrane fuel cells (PEMFCs) are attractive as zero-emission sources of power, particularly well-suited for automotive vehicles. However, the use of costly and scarce platinum in fuel cells constitute a major obstacle for the widespread utilization of PEMFCs. Now a team of researchers from DTU and the University of Copenhagen have discovered  a group of cheaper and efficient platinum-lanthanide alloys.

María Escudero-Escribano*, Paolo Malacrida, Martin H. Hansen, Ulrik G. Vej-Hansen, Amado Velázquez-Palenzuela, Vladimir Tripkovic, Jakob Schiøtz, Jan Rossmeisl, Ifan E.L. Stephens*, Ib Chorkendorff*
*Correspondence to: ibchork@fysik.dtu.dkifan@fysik.dtu.dkmaria.escudero@fysik.dtu.dk

Science 2016352, 73

A team of researchers from the Technical University of Denmark (DTU) and the University of Copenhagen have discovered Pt-lanthanide and Pt-alkaline earth alloys as highly efficient catalysts for oxygen reduction in fuel cells. They tune the performance of the materials by means of the lanthanide contraction.

3-6 times improved catalytic activity
The research, published April 1st
 in the journal Science, reports new materials that are amongst the most efficient Pt-based catalysts for oxygen reduction in the literature. They present a three- to six-fold enhancement in catalytic activity―or kinetic rates―over pure platinum. A single parameter, the interatomic platinum-platinum distance, controls the catalytic activity and stability for oxygen reduction. Mixing platinum with differently sized lanthanide atoms provides a means of controlling this distance 

Polymer electrolyte membrane fuel cells (PEMFCs) are attractive as zero-emission sources of power, particularly well-suited for automotive vehicles. They are likely to play a key role in the switch from fossil fuels combustion technologies to sustainable energy. However, the slow kinetics of the oxygen reduction reaction causes significant potential losses. Minimizing these losses requires copious amounts of platinum at the cathode. The cost and scarcity of platinum arguably constitute the main obstacles preventing the widespread utilization of PEMFCs.

In order reduce the platinum loading at the cathode, researchers have been very actively developing novel electrocatalysts with enhanced activity and long-term stability. The main strategy of industry and academia alike, has been to reduce the loading of platinum by alloying it with late transition metals such as nickel and cobalt. However, this result in compounds that typically degrade under fuel cell conditions, due to dealloying. The dealloying results in the dissolution of the more reactive solute metal (e.g. nickel or cobalt) in the acidic electrolyte of a PEMFC, leaving behind a platinum overlayer. In contrast, alloys of platinum and lanthanides present exceptionally negative alloying energy, which should increase their resistance to degradation.

Harnessing the lanthanide contraction to tune platinum alloy catalysts
In 2012, a team of researchers from DTU Physics discovered a novel active and stable oxygen reduction catalyst, platinum-gadolinium). 
Their experimental results on platinum gadolinium showed the formation of a thick platinum overlayer on the bulk alloy. They explained the enhanced performance on the basis that the overlayer was under compressive strain, imposed by the bulk.

The results on platinum-gadolinium catalysts led the team to conjecture that other Pt-lanthanide alloys, exhibiting more optimal levels of compressive strain, would reach a maximum value of ORR activity. It turns out that with increased filling of the f-shell, the lanthanide atoms tend to become smaller:  the so called lanthanide contraction. This provides the ideal lever to tune the reactivity of platinum and hence the performance of these materials.

In the current work, they have studied oxygen reduction activity and stability trends of novel, previously undiscovered platinum-lanthanide and platinum-alkaline earth alloys. Platinum-terbium is the most active polycrystalline platinum-based catalyst ever reported. All the materials present a 3 to 6-fold activity enhancement over platinum. The active phase consists of a strained platinum overlayer formed by acid leaching. Notably, the experimental ORR activity versus the bulk lattice parameter follows a “volcano” relation. The stability against corrosion also depends on the bulk lattice parameter. They explain their findings on the basis of a theoretical model based on first principles calculations.

In summary, DTU and University of Copenhagen researchers have harnessed the lanthanide contraction as a means of accelerating oxygen reduction. This provides fundamental insight that helps to guide the design and development of even further improved fuel-cell catalysts.


Contact

Ib Chorkendorff
Professor
DTU Physics
+45 45 25 31 70

Contact

Ifan Stephens
Associate Professor
DTU Physics
+45 45 25 31 74

Contact

Maria Escudero Escribano
Postdoc
DTU Physics
+45 45 25 32 27