A Bifunctional Air Electrode Catalyzed by Lead Ruthenate for Li-Air Batteries

It has been recognized that the catalyst may have triple functions toward increasing the charge/discharge capacity, decreasing over-voltage and improving cyclability [1]. Many catalysts, such as various carbons, metal oxides, metal nitrides, metal carbides, and precious metals, have been extensively studied and applied to the air electrode and catalysis has become one of the hottest topics for Li-air batteries. The commonly used carbon materials are, basically, good for catalyzing the oxygen reduction reactions (ORRs), however, they are insufficient for the oxygen evolution reactions (OERs). To promote both ORRs and OERs numerous efforts have been made. In particular, transition metal oxides with a perovskite, spinel or pyrochlore structure have been investigated due to their low cost, good catalytic activity and natural abundance. In this study, we report a nano-crystal lead ruthenate as a bifunctional electrocatalyst for non-aqueous Li-O₂ batteries. The performance of the pyrochlore was studied in terms of morphology, polarization and cyclability.

                 The nano-crystal lead ruthenate was synthesized using ruthenium(III) nitrosyl nitrate and lead sub-acetate, which is similar to the method described by Nazar et al. [2]. The precursor solution was prepared by dissolving ruthenium(III) nitrosyl nitrate and lead sub-acetate in deionized water and then 2 M aqueous NaOH solution was added. The resultant solution was stirred for 3 h at 25 °C followed by the addition of sodium hypochlorite solution, and the mixture was stirred for an additional 24 h. The product was then filtered and dried in a vacuum oven at 100 °C to form nano-crystal lead ruthenate. The synthesized catalyst was characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) surface area analysis and Scanning Electron Microscopy (SEM). Air electrodes were fabricated by applying a catalyst ink on Ni foam or carbon paper. The catalyst ink contains catalyst/Ketjen black (KB), polytetrafluoroethylene (PTFE), and solvent. The composition of the KB supported catalyst is 30% catalyst and 70% KB. For comparison, electrodes with KB only were also made. Cell tests were carried out using ECC-AIR cells (EL-Cell, Germany) with an active diameter of 18mm and conducted on a Solartron Analytical 1470E system. All cell performance was measured in tetraethyleneglycol dimethyl ether (TEGDME) containing 1M lithium triflate under pure O₂. All the capacities are calculated based on the weight of catalyst plus carbon support.

               The XRD pattern of the synthesized material indicates that the characteristic diffraction peaks correspond to the literature data [2], which can be readily indexed to the pyrochlore Fd-3m space group with a cubic lattice. SEM images show that the synthesized material is composed of aggregated nanocrystallites.

                  The performance of the lead ruthenate/KB air electrode on carbon paper was carried out at different current densities. To demonstrate its catalytic activity, the performance of KB on carbon paper was also conducted. At a current density of 0.2 mA cm^-2, although the lead ruthenate/KB electrode does not exhibit higher discharge/charge capacities, it has a lower charge plateau and higher columbic efficiency.  As the charge/discharge rate increases to 0.5 mA cm^-2, catalytic effect of the lead ruthenate is apparent as opposed to the KB baseline. Compared to the KB baseline, the lead ruthenate/KB electrode also exhibits impressive cyclabilities with negligible capacity loss after 5 cycles of complete charge/discharge. Results of the prepared nano-crystal lead ruthenate air electrode on Ni foam also shows greatly improved recharge behavior and cyclability.

             In summary, preliminary results have demonstrated that the nano-crystal lead ruthenate has the potential to be used as an efficient bifunctional electrocatalyst for non-aqueous Li-O₂ batteries. Further investigations are ongoing to optimize this catalyst. The comprehensive study will be presented at the conference.

 Acknowledgements

This work is financially supported by the Natural Resources Canada’s ecoENERGY Innovation Initiative (project ETRI-006).

 References

[1] A. Kraytsberg, Y. Ein-Eli, J. Power Sources, 196: 886–893, 2011

[2] S. H. Oh, R. Black, E. Pomerantseva, H. –H. Lee, L. f. Nazar, Nature Chemistry,  4:1004-1010, 2012