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Tailoring the Active Site for Oxygen Electrocatalysis: Geometric and Electronic Effects

Topic: 

Dr. Maria Escudero-Escribano 

Date: 
Tuesday, May 3, 2016 - 4:00pm
Location: 

200-305, Lane History Corner, Main Quad

 

Abstract

Proton exchange membrane fuel cells (PEMFCs) are potentially zero emission sources of power, which are expected to play a key role in a future societybased on sustainable energy. However, appreciable loadings of Pt-based catalysts are needed to drive the oxygen reduction reaction. In order to improve the reaction kinetics and reduce the Pt loading, we need to develop more efficient catalysts. This can be achieved by modification of the geometric structure (atomic ensembleeffects1) and/or alteration of the electronic properties of the surface atoms (electronic effects2,3).

First, I will address atomic ensemble effects in electrocatalysis. We have used a self-ordered molecular pattern, cyanide-modified Pt(111),[CN-Pt(111)] to study the oxygen reduction1. CN groups form an ordered structurethat blocks all the three-fold hollow sites, effectively blocking the adsorption of spectator anionsin the electrolyte, such as sulphate and phosphate. Nonetheless, the holes in this ordered structure are sufficiently large to allow the adsorption of reaction oxygenmolecules. As a consequence, CN-Pt(111) presents a 25-fold enhancement overPt(111)1.

Secondly, I will focus on electronic effects by alloying Pt. Researchers have intensively studied alloys of Pt with late transition metals, such as Ni and Co. However, these alloys typically degrade under fuel cell conditions, due to dealloying. In contrast, Pt-lanthanide alloys present a very negative alloying energy, which should increase their resistance to degradation.
We have studied novel Pt-lanthanide and Pt-alkaline earth electrodes for the oxygen reduction. These materials are amongst the most active polycrystalline Pt-based catalyst ever reported2,3. A Pt overlayer with a thickness of few Pt layers is formed onto the bulk alloys by acid leaching. The oxygen reduction activity versus the lattice parameter follows a volcano relation3 (Fig. 1). We use the lanthanide contraction to control strain effects and tailor the activity, stability and reactivity of Pt alloys.
 
 
 
Fig. 1. Experimental activity-lattice parameter volcano    
 

References

1  D. Strmcnik, M. Escudero-Escribano, K. Kodama, V. R. Stamenkovic, A. Cuesta, N. Markovic, Nature  Chem.

2010, 2, 880.

2 M. Escudero-Escribano, A. Verdaguer-Casadevall, P. Malacrida, U. Grønbjerg, B.P. Knudsen, A.K. Jepsen, J. Rossmeisl, I.E.L. Stephens, I. Chorkendorff,  J. Am. Chem. Soc. 2012, 130, 16476.

3 M. Escudero-Escribano*, P. Malacrida, M.H. Hansen, U.G. Vej-Hansen, A. Velázquez-Palenzuela, V. Tripkovic, J. Schiøtz,J. Rossmeisl, I.E.L.Stephens*, I. Chorkendorff*, Science 2016, 352, 73.