Ruthenium-Based Nonaparticles Supported on Ketjenblack/Reduced Graphene Oxide As Air Electrode for Lithium-O2 Batteries

Recently, substantial efforts have been made in exploiting Lithium-O₂ battery owing to its high theoretical capacity. As we all known that, large overpotential and poor cyclic performance are two of the most significant challenges which prevent the implementation of Li-O₂ batteries. Oxygen reduction reaction (ORR) corresponding to discharge process is generally considered insensitive to catalyst. The carbon materials alone are regarded to satisfactorily promote ORR process. However, they are not satisfied with the subsequent oxygen evolution reaction (OER) corresponding to the recharge process. [1] Ru-based materials have recently attracted wide attention due to its catalytic performance on the OER process. Reports have demonstrated that Ru-based materials effectively lower the charge potential of Li-O₂batteries, which mitigate the decomposition of electrolyte thus extend the life of the battery. [2, 3] Nevertheless, more details on how the Ru-based materials can influence the growth and distribution of the discharge products are still needed to be further discussed.

  In this study, we choose the reduced graphene oxide (RGO) with 2-dimensional sheet-like structure and Ketjen Black (KB) with particle-like structure to support a series of Ru-based materials. The electrochemical performances of these Ru-based hybrid materials as air electrodes for Li-O₂battery were evaluated. Growth and distribution of the discharge products have also been discussed.

  Figure 1 presents the first curves for galvanostatic discharge/charge of Li-O2 cells using RGO, RGO/RuO2, RGO/Ru, KB, KB/RuO2 and KB/Ru as oxygen electrode under capacity limits of 500 μAh cm-2 at 50 μA cm-2. All the cells except for RGO/RuO2 present a stable voltage plateau between 2.7 V and 2.75 V vs Li indicating the ORR process is insensitive to the presence of Ru-based catalysts. The relatively low discharge voltage of RGO/RuO2 can be attributed to the low heat treatment temperature, which guaranteed the formation of pure-phase of RuO2 but degrade the electrical conductivity of RGO. Compared with pristine RGO and KB, the charge overpotential is effectively lowered by the hybrid of RGO/RuO2, RGO/Ru, KB/RuO2 and KB/Ru. Figure 2 shows the morphology of RGO, RGO/RuO₂ and RGO/Ru after discharge. As can be seen from Fig. 2a, there are many toroidal discharge products with size of several hundred nanometers inhomogeneously distributed on the surface of RGO. For the RGO/RuO₂, the particle size of discharge products seems larger, but they covered the whole surface of RGO without naked RGO exposed. In contrast, the discharge products with very small particle size homogeneously covered the whole RGO/Ru surface. The particles size is so small that it is difficult to identify the boundary of discharge particles. The increased thickness suggesting that the RGO/Ru sheets are densely decorated with Li₂O₂ on both sides of their basal planes. The continuous covering of the Li₂O₂ on the surface of RGO may ensure a good electrical contact during the following recharge process. The cycling stability was tested following the capacity-limited cycle method. Fig.2d presents the cyclic performance of RGO/Ru at a current density of 50 μA cm^-2 and a fixed capacity of 500 μAh cm^-2. Although the discharge voltage decreased and charge voltage increased with the process of cycling, the RGO/Ru still maintained more than 100 cycles, which is much better than KB, RGO, KB/Ru, KB/RuO₂ and RGO/RuO₂with the cycling performance of 20, 9, 49, 21 and 88 cycles, respectively. The improved cycling performance of RGO/Ru may be attributed to catalytic performance of Ru and the special two-dimensional sheet-like structure, lowering the charge overpotential and maintaining the structural stability during the discharge/charge process.

Acknowledgement

This work was partly supported by the ALCA-SPRING of JST. 

References

[1] G. Girishkumar, B. McCloskey, A. C. Luntz, S. Swanson and W. Wilcke, J Phys Chem Lett., 1, 2193 (2010).

[2] H. G. Jung, Y. S. Jeong, J. B. Park, Y. K. Sun, B. Scrosati and Y. J. Lee, Acs Nano, 7, 3532 (2013).

[3] E. Yilmaz, C. Yogi, K.Yamanaka, T. Ohta and H. R. Byon, Nano letters, 13, 4679 (2013)