Ab Initio Study Of Li2O2 On Noble Metal (Pt, Au, Pd), Pt3M (M=Fe, Co, Ni, Cu) Alloy And Pd3M (M=Fe, Co, Ni, Cu) Alloy Surfaces

Li- O₂ cells has shown great potential to be one of the energy storage systems with energetic capabilities beyond the lithium-ion cell. The early ab-initio theoretical picture of the Oxygen Reduction Reaction (ORR) at the cathode envisioned a Li₂O₂ covered surface after the initial cycle and further ORR as occurring on the deposited Li₂O₂[2]. Density Functional Theory (DFT) studies invoking such mechanisms have predicted enlarged overpotentials in agreement with experimentally observed high overpotentials. But the issue of electron transfer across the poorly conducting peroxide layer in spite of low conductivity has had to be explained using mechanisms such as d-band bending [2] and poloron hopping [3].

This picture of an electrode surface that is covered and does not influence the ORR or influences it by an indirect at-distance mechanism is at odds with experimental evidence that the surface plays a crucial role in the ORR. ORR on Noble metals [1], metal oxide [4], metal alloys [5], and carbon nanotubes [6] 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 [6] and even after multiple cycles a significant portion of the surface is bare [6].

The present study addresses the twin challenges of understanding the binding of Li₂O₂ on active sites and identifying possible candidate materials which would facilitate easy removal of deposited Li₂O₂ during the charge process. The Li₂O₂ binding energy (B.E_Li2O2) was calculated using DFT on periodic models of noble metal (Pt, Au, Pd), Pt₃M (M=Fe, Co, Ni, Cu) alloy and Pd₃M (M=Fe, Co, Ni, Cu) alloy surfaces after geometric optimization and by bonding Li₂O₂ to the surfaces at various orientations and sites. The effect of the various sites on the binding energy and the variation of the binding energy at each site with the change in material were studied. Top and Bridge sites were found to have the lowest B.E_Li2O2 in case of the noble metals.  Systematic permutation of the first two atomic layers of FCC (111) Pt₃M and Pd₃Mskin alloys slabs was carried out to find the most stable geometric surface configuration and the influence of the dopant modification of the lattice constant on the B.E_Li2O2. Possible guidelines for future alloy design to achieve desired B.E_Li2O2 values were formulated and B.E_Li2O2 was shown to be a key parameter for screening future Li-O₂ORR catalyst candidates

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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