Lithium air secondary batteries (LABs) exhibit much higher theoretical energy density than lithium ion batteries and are anticipated as one of the most promising candidates for the next generation of batteries.  However, problems with LABs include their electrochemical properties, such as cyclability, overpotential, and round-trip efficiency need further improvement[1-2].  Various kinds of electrocatalysts have been studied to overcome these drawbacks [1-2].  We have reported that a battery incorporationg Pt₃₃Ru₆₇ electrocatalyst shows discharge capacities of more than 400 mAh/g for over 30 cycles in TEGDME-based electrolyte solution [2].  In this study, to further improve the cyclability, we prepared Pt_100-xRu_x/carbon electrocatalysts with wide composition range of 0 ≤ x ≤ 100 by the soft liquid process and examined the electrochemical properties of the cells loaded with the electrocatalysts into air electrodes.

Experimental

Pt_100-xRu_x/carbon was prepared by the formic acid reduction method [3].  KetjenBlack EC600JD (KB) was used as the carbon support material.  The KB powder was dispersed in formic acid solution by sonication, and H₂PtCl₆¥6H₂O and/or RuCl₃ solution was dropped into the solution, which was then stirred overnight.  The Ru content, x, in Pt_100-xRu_x was 0 (Pt alone) 30, 60, 75, 83, 90, or 100 (Ru alone).  Then, dried Pt_100-xRu_x/KB powder was obtained by heat-treating the mixture at 300°C for 12 h in Ar.  The air electrodes were prepared by coating the mixture of Pt-Ru/KB powder, and PVdF in N-methylpyrrolidone solvent, and drying it at 90°C.  The composition ratio of an air electrode with a diameter of 5 mm was KB/Pt_100-xRu_x/PVdF = 80: 10: 10.  The LAB cell (ECC-Air, EL-Cell) was assembled, incorporating the air electrode loaded with Pt_100-xRu_x/KB, an electrolyte solution (1 mol/l LiTFSA/TEGDME), a glass separator, and Li metal sheets.  Electrochemical measurements were carried out under a galvanostatic condition of 0.1 mA/cm² in a dry air atmosphere.  The discharge and charge capacities were normalized by the weight of Pt_100-xRu_x/KB powder and PVdF in the air electrodes.

Results and discussion

Figure 1 shows XRD patterns of samples with various composition ratios of Pt_100-xRu_x/KB.  The patterns of x = 0 and 30 corresponded to a single phase of Pt (PDF #00-004-0802), and the ones of x = 83, 90, 100 corresponded to a single phase of Ru (PDF#00-006-0663).  The sample with x = 60 and 75 was identified as a mixture of the Pt and Ru phase.  In addition, the XRD peaks became broad by increasing the content of Ru.  This suggests that the added Ru is effective in reducing the particle size of Pt_100-xRu_xalloy, and then it should enhance the electrocatalytic activities.

Figure 2 shows the typical first discharge-charge curves of the cells incorporating the air electrodes loaded with Pt_100-xRu_x/KB catalysts.  First discharge capacities are 496, 671, 905, 1014, and 909 mAh/g in the cells with samples with x = 0, 30, 75, 90, and 100, respectively.  The discharge capacities clearly increased with increasing Ru content, indicating the maximum capacity in the x = 90 sample.  Moreover, the cell for the x = 90 shows the highest average voltage of 2.58 V, while the one for x = 0 shows rather low voltage of 2.43 V.  In the charging process, the average charge voltages decreased with increasing Ru content and, accordingly, the charge capacities increased as a result of the smaller charge overpotentals, indicating the maximum capacity of 867 mAh/g in the x = 90 sample.

Figure 3 shows the cycle properties of cells incorporating the air electrodes loaded with Pt_100-xRu_x/KB catalysts.  The discharge capacities of all cells gradually decreased.  In particular, the cell for x = 0 shows very small capacity of less than 80 mAh/g at the 8th cycle. This result indicates the effectiveness of Ru addition for improving the cyclability.   Among the cells tested, the one with the x = 90 sample showed comparatively better cycle stability with discharge capacity of over 800 mAh/g at the 8th cycle.  These behaviors are very similar to the above discharge/charge properties.  As a result, the x= 90 sample was confirmed to be the optimized composition as the electrocatalyst for the air electrode.

From the above results, it was found that the higher Ru content leads to a higher surface area and higher dispersion state of Pt_100-xRu_x electrocatalyst accompanied by inhibition of particle growth, which result in good electrochemical performance of cells incorporating Pt₁₀Ru₉₀/KB.

[1] A. Debart et al., Angew.Chem., 120, 4597 (2008).

[2] M. Hayashi et al., The 56^th Battery Symposium in Japan, 3G02, (2015).

[3] J. Prabhuram et al., J. Power Sources, 134, 1 (2006).