Ab Initio Study Of Surface Segregation Effects And Li- O2 Cell Oxygen Reduction Reaction Activity On Pd3M (M=Fe, Co, Ni, Cu) Alloys

To achieve practical application as a replacement for Li-ion cells, current Li- O2 cells must overcome problems of (i) poor cycle life, (ii) high charge and discharge overpotentials resulting in low coloumbic efficiency and low power capability [1]. The poor cycle life reported in many articles can be traced to the oxidative instability of the electrolyte [2] in conjunction with electrode passivation due to irreversible product deposits and difficulty in decomposing Li2O2. The high overpotentials are also a consequence of the stability of the peroxide.

ORR on Noble metals [1], metal oxide [3], metal alloys [4], and carbon nanotubes[5] has been reported to occur with reduced overpotential and longer cycle life. Also studies of the cathode surface have shown that the Li2O2 nucleates in various 3-D morphologies [5] and even after multiple cycles a significant portion of the surface is bare [5]. Thus the pristine surface retention is expected to have a significant role in the ORR kinetics. Previous DFT studies on fuel cell ORR have predicted that the Pd skin alloys have better catalytic activity compared to pure Pd. This has been also found to agree very well with experiment. The present study is examining these alloys as potential catalysts for the ORR.

Periodic models of the close-packed (111) surfaces of these alloys of (2x2x4) L1₂ –type unit cell configuration were constructed and systematic permutation of the first two atomic layers of FCC (111) Pt3M and Pd3Mskin alloys slabs was carried out to model surface segregation phenomena and to find the most stable geometric surface configuration. This was repeated with 0.25ML of adsorbed O, O₂, LiO, LiO₂, Li₂O₂, and Li₂O to understand the effects of adsorbate induced surface activity on the most stable surface configurations. The Oxygen Binding energy (BE_O) an important parameter for ORR catalytic activity on transition metal alloys, was used to probe the ORR activity of  Pd₃M/C (M = Fe,Ni,Cu) as shown in figure 1.The reaction free energies were extrapolated to the rate constants of the rate determining step using molecular kinetics theory. The reaction free energy on these surfaces was analyzed to identify surfaces and sites on those surfaces that (i) favor LiO₂ desorption (favorable to 1e- transfer reaction) (ii) favor LiO₂ adsorption (favorable to 2e- transfer reaction)(iii) show least energy barrier for O₂ dissociation (favorable to 4e- transfer reaction). After screening the various surfaces for the most likely reaction route, the activity of the various surfaces for each of the routes was calculated. The insight gained from this exercise can be used to guide the choice of catalyst for test in Li- O₂ cells.

Acknowledgement: Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.  

References:

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