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Electrochemical properties of lithium air batteries with Pt100−xRux (0 ≤ x ≤ 100) electrocatalysts for air electrodes
Journal of Power Sources 340 (2017) 121e125
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Electrochemical properties of lithium air batteries with Pt100xRux (0 x 100) electrocatalysts for air electrodes Yuhki Yui*, Shuhei Sakamoto, Masaya Nohara, Masahiko Hayashi, Jiro Nakamura, Takeshi Komatsu NTT Device Technology Labs., NTT Corporation, Japan
h i g h l i g h t s Pt100xRux/carbon was prepared by the formic acid reduction method. A higher Ru content inhibits the growth of Pt100xRux particles. The lithium air battery incorporating Pt10Ru90 showed superior cycle stability.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 August 2016 Received in revised form 21 October 2016 Accepted 17 November 2016
Electrochemical properties of lithium air secondary battery cells with Pt100xRux (0 x 100) electrocatalysts, prepared by the formic acid reduction method and loaded into air electrodes were examined in 1 mol/l LiTFSA/TEGDME electrolyte solution. Among the cells, the one with the Pt10Ru90 (x ¼ 90)/ carbon sample showed the largest discharge capacity of 1014 mAh/g and the lowest average charge voltage of 3.74 V. In addition, the x ¼ 90 sample showed comparatively good cycle stability with discharge capacity of over 800 mAh/g at the 8th cycle. As a result, x ¼ 90 was confirmed to be the optimized composition as the electrocatalyst for the air electrode. © 2016 Elsevier B.V. All rights reserved.
Keywords: Lithium air battery Electrocatalyst Pt-Ru
1. Introduction In order to meet the great demand for power sources in mobile devices, electric vehicles, and stationary power storage systems, the research and development of various next-generation batteries, such as, the sodium ion [1e4], magnesium [5e8], aluminum ion [9,10], lithium sulfur [11e13], and Na-O2 [14e17] batteries, have been actively conducted. Among the next generation of batteries, the lithium air secondary battery (LAB) has been attracting attention because it has the highest theoretical energy density (~3505 Wh/kg) [18e21]. The discharge reaction in a LAB produces Li2O2 from lithium and oxygen from air on the air electrode: 2Li þ O2 / Li2O2. However, LABs have problems related to the sluggish kinetics for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in Liþ-containing nonaqueous electrolytes [22]. This leads to their poor cyclability and large
* Corresponding author. Tel.: þ81 46 240 2799; fax: þ81 46 240 4047. E-mail address: [email protected] (Y. Yui). http://dx.doi.org/10.1016/j.jpowsour.2016.11.063 0378-7753/© 2016 Elsevier B.V. All rights reserved.
overpotential for ORR and OER. To improve the cyclability and reduce large overpotential, various electrocatalysts, such as noble metals (Pd, Pt, Ru, Au) [22,23], metal oxides (MnO2, RuO2) [24e29], and perovskite-type oxide [30,31], have been studied for use as the electrocatalysts of air electrodes of LABs. In addition, some metal alloys (Pt3Co, Pd3Co) [32,33] have shown good electrochemical properties. In Ref. [33], a LAB cell with Pd3Co/KetjenBlack EC600JD (KB) exhibited a 200 mV lower charging overpotential (4.2 V) than ones with the typical electrocatalyst MnO2/KB. Furthermore, the Pd3Co/KB exhibited superior cyclability with 1000 mAh/g during 35 cycles. According to S. M. Cho et al. [33], metal oxide electrocatalysts show larger overpotential than metal/alloy electrocatalysts due to the former's electrical conductivity. Additionally, weaker adsorption of reaction intermediate LiO2 on the Pd3Co alloy surface would result in the lower overpotential and superior cyclability for LAB cells with Pd3Co/KB. Thus, metal/alloy material is promising as the electrocatalyst for the air electrodes of LABs. Pt-Ru alloy has been studied as the electrocatalyst for direct methanol fuel cells, and it has shown good catalytic activity
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towards CO oxidation [34,35]. Increasing the effective catalyst surface area by using high-dispersion nanosized catalyst particles would enhance the catalytic activity. In Ref. [34], particles of Pt-Ru of about 2 nm were found to be thoroughly dispersed in carbon. This paper evaluates the electrochemical properties of Pt-Ru electrocatalysts for air electrodes of LABs. Pt100xRux/carbon was prepared by the soft liquid process to synthesize nanosized particles of Pt100xRux supported with a high-dispersion state on the carbon. The electrochemical properties of LABs incorporating the air electrode loaded with Pt100xRux/carbon were examined to improve the cyclability and reduce the overpotential by optimizing the composition of Pt-Ru. 2. Experimental 2.1. Synthesis and characterization of Pt100xRux/carbon Pt100xRux/carbon was prepared by the formic acid reduction method [36]. KetjenBlack (KB) EC600JD (Lion Co.) was used as the carbon support material. The KB powder was dispersed in formic acid solution by sonication, and a solution of H2PtCl6$6H2O (Kanto Chemical Co.) and/or RuCl3 (Furuya Metal Co., Ltd.) was dropped into the formic acid solution, which was then stirred overnight. Here, the ratio of Pt100xRux and carbon was adjusted to 1:8 by weight. In Ref. [37], the Pt-Ru phase diagram indicates that Pt and Ru can form a Pt phase (0e60 at. % Ru), a mixture of the Pt and Ru phases (60e83 at. % Ru), and a Ru phase (83e100 at. % Ru). To investigate the catalytic activity of all phases and the boundary between them, the Pt100xRux was prepared with Ru content, x, of 0 (Pt alone) 30, 60, 75, 83, 90, or 100 (Ru alone). The mixture was evaporated to dryness, and then dried Pt100xRux/KB powder was obtained by heat-treating the mixture at 300 C for 12 h in Ar. The metal/alloy phases of the resulting powder were identified with a powder X-ray diffractometer (XRD) (Rigaku Co., X-ray diffractometer Ultima IV) using CuKa radiation. The morphologies and dispersion state of metal/alloy over KB powder were observed with a scanning transmission electron microscope (STEM) (JEOL Ltd., JEM-2100F) at accelerating voltage of 200 kV in bright field (BF) or high-angle annular dark field (HAADF) modes. In addition, the Pt/Ru ratio in the alloy phase powder was determined by using an energy dispersive X-ray spectrometer (EDS) coupled to the STEM. 2.2. Electrochemical measurements The air electrodes were prepared by coating the mixture of Pt100xRux/KB powder and PVdF (Kureha Battery Materials Japan Co.) binder in N-methylpyrrolidone solvent (Tomiyama Pure Chemicals Industries Ltd.) on a mesh of titanium as a current collector and drying it at 90 C. An air electrode with a diameter of 5 mm consisted of KB:Pt100xRux:PVdF ¼ 80:10:10 in weight. The LAB cells (ECC-Air, EL-Cell GmbH) were assembled, incorporating the air electrode loaded with Pt100xRux/KB, an electrolyte solution of 1 mol/l lithium bis(trifluoromethanesulfonyl) amide [LiTFSA]/ tetraethylene glycol dimethyl ether [TEGDME] (Tomiyama Pure Chemicals Industries Ltd.), a glass separator (EL-Cell GmbH), and Li metal sheets (600 mm thickness, Honjo Metal Co., Ltd.). Electrochemical measurements were carried out using an automatic galvanostatic discharge-charge system (Hokuto HJ1001SD8) at a constant current density of 0.1 mA/cm2 between 2.0 and 4.2 V in a dry air atmosphere with a dew point of less than 50 C. The discharge and charge capacities were normalized by the weight of Pt100xRux/KB powder and PVdF in the air electrodes. To analyze the discharged/charged electrodes with the XRD, the LAB cells were opened after the charge/discharge process, and the electrodes were
washed with dimethyl carbonate and dried in vacuum atmosphere. 3. Results and discussion Fig. 1 shows HAADF-STEM images of the Pt100xRux/KB precursor before the heat treatment. The particle size of the Pt100xRux precursor is about 1 nm, and there are no differences between the Pt and Pt10Ru90 precursor. This suggests that Pt100xRux precursors are highly dispersed before the heat treatment. Fig. 2 shows BFSTEM images of Pt100xRux/KB samples after the heat treatment. Black points are Pt100xRux particles and the semi-transparent areas are KB, and the particle size of Pt100xRux obviously depends on the Pt/Ru composition. The particle size in the samples with x ¼ 0 and 30 ranges from 7 to 100 and 2e30 nm, respectively. In addition, Pt particles of the sample with x ¼ 0 are agglomerated with low dispersion, and various particle sizes are observed. By comparing Fig. 1(a) with Fig. 2(a), one can see that the heat treatment caused the Pt particles to grow. As shown in Ref. [38], Pt particles migrate toward and coalesce with each other under 200 C. This is the reason the Pt particles became larger and agglomerated. The particle size in the samples with x ¼ 60 and 75 ranges from 3 to 10 nm. The samples with x ¼ 83, 90, and 100 have a very small particle size of 2e10 nm. In contrast to the Pt particles, Ru particles would not likely migrate, judging from these results. In addition, the particles with the higher content of Ru alloy were confirmed to be distributed uniformly on the KB support and to inhibit the growth of particles. This would be attributed to the higher melting point of Ru (2334 C) than that of Pt (1768 C). Clarifying the origin of this mechanism requires further investigation. The Pt/Ru composition ratios of Pt100xRux/KB obtained from EDS measured in the range of 1-mm square in the BF-STEM images shown in Fig. 2 are listed in Table 1. The Pt/Ru ratio determined from EDS analysis was in good agreement with the mixing ratio of the raw materials of H2PtCl6 and RuCl3, and it was confirmed that Pt100xRux/KB does not contain any impurities of remaining precursors. The Pt/Ru composition ratio was found to be well adjusted by the formic acid reduction method. XRD patterns of Pt100xRux/KB samples are shown in Fig. 3. The patterns for x ¼ 0 and 30 corresponded to a single phase of Pt (PDF #00-004-0802), and the ones for x ¼ 83, 90, and 100 corresponded to a single phase of Ru (PDF #00-006-0663). The samples with x ¼ 60 and 75 were identified as a mixture of the Pt and Ru phases. These crystal structures of Pt100xRux are consistent with the phase diagram for binary alloy [37]. In addition, the XRD peaks became broader with increasing Ru content. This suggests that adding Ru is effective in reducing the particle size of Pt100xRux alloy, which should enhance the electrocatalytic activities. In fact, it was confirmed that the higher Ru content prevented particle growth of Pt100xRux alloy, as shown in the BF-STEM images in Fig. 2. 1. Fig. 4 shows the first discharge-charge curves of the LAB cells incorporating the air electrodes loaded with Pt100xRux/KB electrocatalysts in the 2.0e4.2 V range. Fig. 5 shows the Ru
Fig. 1. HAADF-STEM images of Pt100xRux/KB: x ¼ (a) 0 and (b) 90.
Y. Yui et al. / Journal of Power Sources 340 (2017) 121e125
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Fig. 2. BF-STEM images of Pt100xRux/KB: x ¼ (a) 0, (b) 30, (c) 60, (d) 75, (e) 83, (f) 90, and (g) 100.
Table 1 Pt/Ru ratios of Pt100xRux/KB obtained from EDS. x in Pt100xRux
x¼0
30
60
75
83
90
100
Pt:Ru
100:0
74.5:25.5
40.4:59.6
23.0:77.0
14.6:85.4
9.2:90.8
0:100
Fig. 3. XRD patterns of Pt100xRux/KB:x ¼ (a) 0, (b) 30, (c) 60, (d) 75, (e) 83, (f) 90, and (g) 100.
content dependence of DVave and discharge/charge capacity obtained from Fig. 4. Here, DVave was defined as the difference between average charge voltage and the average discharge voltage. The discharge/charge capacity increases and DVave decreases as the Ru content increases. The largest discharge/ charge capacity and the lowest DVave were obtained for the x ¼ 90 sample. The cell with x ¼ 90 shows the lowest DVave of 1.16 V, while the one with x ¼ 0 shows rather high DVave of 1.43 V. The discharge/charge capacities and DVave clearly decrease with increasing Ru content, indicating the maximum capacity and the lowest DVave in the x ¼ 90 sample. This effect of
Fig. 4. First discharge-charge curves of LAB cells incorporating air electrodes loaded with Pt100xRux/KB electrocatalysts in the 2.0e4.2 V range: x ¼ (a) 0, (b) 30, (c) 60, (d) 75, (e) 83, (f) 90, and (g) 100.
nanosized particles is supported in the high dispersion on KB since the catalytic reaction occurs at the surface. The Ru phase (x ¼ 83, 90 and 100) of DVave is more than 0.1 V lower than the Pt phase as shown in Fig. 5. Although the high dispersion of nanosized particles is clearly one of the factors in the highactivity of electrocatalysts, there is a possibility that the crystal structure might be affecting electrocatalytic activity as well. In addition, the reason the sample with x ¼ 90 has lower average charging voltage and larger capacity than the one with x ¼ 100 is not yet clear. However, one possibility remains: Pt10Ru90 has high catalytic activity for the hydrogen oxidation reaction in alkaline environments [39]. In Ref. [39], it was claimed that Ru
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Fig. 5. DVave and discharge/charge capacity for LAB cells incorporating air electrodes loaded with various composition ratios of Pt100xRux/KB: (i) DVave, (ii) discharge capacity, and (iii) charge capacity.
Fig. 7. Cycle properties of LAB cells incorporating air electrodes loaded with Pt100xRux/KB: x ¼ (a) 0, (b) 30, (c) 60, (d) 75, (e) 83, (f) 90, and (g) 100.
amorphous like. Then, these peaks disappeared completely after the first charge. It was found that the deposition/decomposition reaction of Li2O2 for the LAB indeed occurred. Fig. 7 shows the cycle properties of LAB cells incorporating the air electrodes loaded with Pt100xRux/KB electrocatalysts. The discharge capacities of all cells gradually decreased. In particular, the cell for x ¼ 0 shows very small capacity of less than 80 mAh/g at the 8th cycle. The eighth discharge capacity of the electrodes with the x ¼ 30, 60, 75, 83, and 100 samples became about half of the first discharge capacity. Among the cells tested, the one with the x ¼ 90 sample showed comparatively better cycle stability with discharge capacity of over 800 mAh/g at the 8th cycle. Its capacity retention ratio was 80% after the 8th cycle. This result indicates the effectiveness of Ru addition for improving the cyclability due to the decrease in overpotential. 4. Conclusion Fig. 6. XRD patterns of air electrodes loaded with Pt10Ru90/KB as prepared and after first discharge and charge.
atoms have adsorptive capability of hydroxyl species (OHad), which leads to an effective reaction with the hydrogen intermediates (Had) on nearby Pt sites. The mechanisms of the higher electrocatalytic activity of Pt10Ru90/KB for LABs might be very similar to the above phenomena. That is, Ru and Pt atoms would have adsorptive capacity for O2 and Liþ, respectively, which would lead to highly electrocatalytic activity for the air electrode. Clarifying the origin of this mechanism requires further investigation. Fig. 6 shows XRD patterns of air electrodes loaded with Pt10Ru90/ KB electrocatalysts as prepared and after the first discharge/charge. With the exception of those from the titanium used as a current collector, there is only a broad peak at around 44.0 deg, which are identified as the Ru phase in the pristine electrode. After the first discharge, new peaks appeared at around 32.9, 35.0, and 58.7 deg, which are assigned to Li2O2. The patterns of the ones with x ¼ 0, 30, 60, 75, 83, and 100 had peaks similar to those with x ¼ 90, though these patterns are not shown here. The peaks are very small and broad, indicating that the crystallite would be very small or
We focused on Pt100xRux/KB electrocatalysts for air electrodes in nonaqueous solutions and investigated suitable composition ratios of Pt/Ru. The sample with x ¼ 0 has particle size of 7e100 nm and low dispersion. In contrast, the one with x ¼ 90 has small particle size of 2e10 nm, and it was found to be more thoroughly dispersed. The small particles in the latter lead to large discharge capacity of 1014 mAh/g, low average charge voltage of 3.74 V, and good cyclability (800 mAh/g up to the 8th cycles). Using the air electrodes loaded with Pt10Ru90/KB electrocatalysts, XRD results revealed the formation of Li2O2 after discharge to 2.0 V and that it was decomposed by the charging process. As a result, there is a significant correlation between the Pt/Ru composition ratio and electrochemical properties. The higher Ru content leads to a higher dispersion state of Pt100xRux electrocatalyst, accompanied by inhibition of particle growth, which results in good electrochemical performance of cells incorporating Pt10Ru90/KB. References [1] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Adv. Funct. Mater. 23 (2013) 947e958. [2] N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Chem. Rev. 114 (2014) 11636e11682. [3] Y. Yui, Y. Ono, M. Hayashi, Y. Nemoto, K. Hayashi, K. Asakura, H. Kitabayashi, J. Electrochem. Soc. 162 (2015) A3098eA3102. [4] Y. Yui, M. Hayashi, J. Nakamura, Sci. Rep. 6 (2016) 22406.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Electrochemical properties of lithium air batteries with Pt100xRux (0 x 100) electrocatalysts for air electrodes Yuhki Yui*, Shuhei Sakamoto, Masaya Nohara, Masahiko Hayashi, Jiro Nakamura, Takeshi Komatsu NTT Device Technology Labs., NTT Corporation, Japan
h i g h l i g h t s Pt100xRux/carbon was prepared by the formic acid reduction method. A higher Ru content inhibits the growth of Pt100xRux particles. The lithium air battery incorporating Pt10Ru90 showed superior cycle stability.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 August 2016 Received in revised form 21 October 2016 Accepted 17 November 2016
Electrochemical properties of lithium air secondary battery cells with Pt100xRux (0 x 100) electrocatalysts, prepared by the formic acid reduction method and loaded into air electrodes were examined in 1 mol/l LiTFSA/TEGDME electrolyte solution. Among the cells, the one with the Pt10Ru90 (x ¼ 90)/ carbon sample showed the largest discharge capacity of 1014 mAh/g and the lowest average charge voltage of 3.74 V. In addition, the x ¼ 90 sample showed comparatively good cycle stability with discharge capacity of over 800 mAh/g at the 8th cycle. As a result, x ¼ 90 was confirmed to be the optimized composition as the electrocatalyst for the air electrode. © 2016 Elsevier B.V. All rights reserved.
Keywords: Lithium air battery Electrocatalyst Pt-Ru
1. Introduction In order to meet the great demand for power sources in mobile devices, electric vehicles, and stationary power storage systems, the research and development of various next-generation batteries, such as, the sodium ion [1e4], magnesium [5e8], aluminum ion [9,10], lithium sulfur [11e13], and Na-O2 [14e17] batteries, have been actively conducted. Among the next generation of batteries, the lithium air secondary battery (LAB) has been attracting attention because it has the highest theoretical energy density (~3505 Wh/kg) [18e21]. The discharge reaction in a LAB produces Li2O2 from lithium and oxygen from air on the air electrode: 2Li þ O2 / Li2O2. However, LABs have problems related to the sluggish kinetics for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in Liþ-containing nonaqueous electrolytes [22]. This leads to their poor cyclability and large
* Corresponding author. Tel.: þ81 46 240 2799; fax: þ81 46 240 4047. E-mail address: [email protected] (Y. Yui). http://dx.doi.org/10.1016/j.jpowsour.2016.11.063 0378-7753/© 2016 Elsevier B.V. All rights reserved.
overpotential for ORR and OER. To improve the cyclability and reduce large overpotential, various electrocatalysts, such as noble metals (Pd, Pt, Ru, Au) [22,23], metal oxides (MnO2, RuO2) [24e29], and perovskite-type oxide [30,31], have been studied for use as the electrocatalysts of air electrodes of LABs. In addition, some metal alloys (Pt3Co, Pd3Co) [32,33] have shown good electrochemical properties. In Ref. [33], a LAB cell with Pd3Co/KetjenBlack EC600JD (KB) exhibited a 200 mV lower charging overpotential (4.2 V) than ones with the typical electrocatalyst MnO2/KB. Furthermore, the Pd3Co/KB exhibited superior cyclability with 1000 mAh/g during 35 cycles. According to S. M. Cho et al. [33], metal oxide electrocatalysts show larger overpotential than metal/alloy electrocatalysts due to the former's electrical conductivity. Additionally, weaker adsorption of reaction intermediate LiO2 on the Pd3Co alloy surface would result in the lower overpotential and superior cyclability for LAB cells with Pd3Co/KB. Thus, metal/alloy material is promising as the electrocatalyst for the air electrodes of LABs. Pt-Ru alloy has been studied as the electrocatalyst for direct methanol fuel cells, and it has shown good catalytic activity
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towards CO oxidation [34,35]. Increasing the effective catalyst surface area by using high-dispersion nanosized catalyst particles would enhance the catalytic activity. In Ref. [34], particles of Pt-Ru of about 2 nm were found to be thoroughly dispersed in carbon. This paper evaluates the electrochemical properties of Pt-Ru electrocatalysts for air electrodes of LABs. Pt100xRux/carbon was prepared by the soft liquid process to synthesize nanosized particles of Pt100xRux supported with a high-dispersion state on the carbon. The electrochemical properties of LABs incorporating the air electrode loaded with Pt100xRux/carbon were examined to improve the cyclability and reduce the overpotential by optimizing the composition of Pt-Ru. 2. Experimental 2.1. Synthesis and characterization of Pt100xRux/carbon Pt100xRux/carbon was prepared by the formic acid reduction method [36]. KetjenBlack (KB) EC600JD (Lion Co.) was used as the carbon support material. The KB powder was dispersed in formic acid solution by sonication, and a solution of H2PtCl6$6H2O (Kanto Chemical Co.) and/or RuCl3 (Furuya Metal Co., Ltd.) was dropped into the formic acid solution, which was then stirred overnight. Here, the ratio of Pt100xRux and carbon was adjusted to 1:8 by weight. In Ref. [37], the Pt-Ru phase diagram indicates that Pt and Ru can form a Pt phase (0e60 at. % Ru), a mixture of the Pt and Ru phases (60e83 at. % Ru), and a Ru phase (83e100 at. % Ru). To investigate the catalytic activity of all phases and the boundary between them, the Pt100xRux was prepared with Ru content, x, of 0 (Pt alone) 30, 60, 75, 83, 90, or 100 (Ru alone). The mixture was evaporated to dryness, and then dried Pt100xRux/KB powder was obtained by heat-treating the mixture at 300 C for 12 h in Ar. The metal/alloy phases of the resulting powder were identified with a powder X-ray diffractometer (XRD) (Rigaku Co., X-ray diffractometer Ultima IV) using CuKa radiation. The morphologies and dispersion state of metal/alloy over KB powder were observed with a scanning transmission electron microscope (STEM) (JEOL Ltd., JEM-2100F) at accelerating voltage of 200 kV in bright field (BF) or high-angle annular dark field (HAADF) modes. In addition, the Pt/Ru ratio in the alloy phase powder was determined by using an energy dispersive X-ray spectrometer (EDS) coupled to the STEM. 2.2. Electrochemical measurements The air electrodes were prepared by coating the mixture of Pt100xRux/KB powder and PVdF (Kureha Battery Materials Japan Co.) binder in N-methylpyrrolidone solvent (Tomiyama Pure Chemicals Industries Ltd.) on a mesh of titanium as a current collector and drying it at 90 C. An air electrode with a diameter of 5 mm consisted of KB:Pt100xRux:PVdF ¼ 80:10:10 in weight. The LAB cells (ECC-Air, EL-Cell GmbH) were assembled, incorporating the air electrode loaded with Pt100xRux/KB, an electrolyte solution of 1 mol/l lithium bis(trifluoromethanesulfonyl) amide [LiTFSA]/ tetraethylene glycol dimethyl ether [TEGDME] (Tomiyama Pure Chemicals Industries Ltd.), a glass separator (EL-Cell GmbH), and Li metal sheets (600 mm thickness, Honjo Metal Co., Ltd.). Electrochemical measurements were carried out using an automatic galvanostatic discharge-charge system (Hokuto HJ1001SD8) at a constant current density of 0.1 mA/cm2 between 2.0 and 4.2 V in a dry air atmosphere with a dew point of less than 50 C. The discharge and charge capacities were normalized by the weight of Pt100xRux/KB powder and PVdF in the air electrodes. To analyze the discharged/charged electrodes with the XRD, the LAB cells were opened after the charge/discharge process, and the electrodes were
washed with dimethyl carbonate and dried in vacuum atmosphere. 3. Results and discussion Fig. 1 shows HAADF-STEM images of the Pt100xRux/KB precursor before the heat treatment. The particle size of the Pt100xRux precursor is about 1 nm, and there are no differences between the Pt and Pt10Ru90 precursor. This suggests that Pt100xRux precursors are highly dispersed before the heat treatment. Fig. 2 shows BFSTEM images of Pt100xRux/KB samples after the heat treatment. Black points are Pt100xRux particles and the semi-transparent areas are KB, and the particle size of Pt100xRux obviously depends on the Pt/Ru composition. The particle size in the samples with x ¼ 0 and 30 ranges from 7 to 100 and 2e30 nm, respectively. In addition, Pt particles of the sample with x ¼ 0 are agglomerated with low dispersion, and various particle sizes are observed. By comparing Fig. 1(a) with Fig. 2(a), one can see that the heat treatment caused the Pt particles to grow. As shown in Ref. [38], Pt particles migrate toward and coalesce with each other under 200 C. This is the reason the Pt particles became larger and agglomerated. The particle size in the samples with x ¼ 60 and 75 ranges from 3 to 10 nm. The samples with x ¼ 83, 90, and 100 have a very small particle size of 2e10 nm. In contrast to the Pt particles, Ru particles would not likely migrate, judging from these results. In addition, the particles with the higher content of Ru alloy were confirmed to be distributed uniformly on the KB support and to inhibit the growth of particles. This would be attributed to the higher melting point of Ru (2334 C) than that of Pt (1768 C). Clarifying the origin of this mechanism requires further investigation. The Pt/Ru composition ratios of Pt100xRux/KB obtained from EDS measured in the range of 1-mm square in the BF-STEM images shown in Fig. 2 are listed in Table 1. The Pt/Ru ratio determined from EDS analysis was in good agreement with the mixing ratio of the raw materials of H2PtCl6 and RuCl3, and it was confirmed that Pt100xRux/KB does not contain any impurities of remaining precursors. The Pt/Ru composition ratio was found to be well adjusted by the formic acid reduction method. XRD patterns of Pt100xRux/KB samples are shown in Fig. 3. The patterns for x ¼ 0 and 30 corresponded to a single phase of Pt (PDF #00-004-0802), and the ones for x ¼ 83, 90, and 100 corresponded to a single phase of Ru (PDF #00-006-0663). The samples with x ¼ 60 and 75 were identified as a mixture of the Pt and Ru phases. These crystal structures of Pt100xRux are consistent with the phase diagram for binary alloy [37]. In addition, the XRD peaks became broader with increasing Ru content. This suggests that adding Ru is effective in reducing the particle size of Pt100xRux alloy, which should enhance the electrocatalytic activities. In fact, it was confirmed that the higher Ru content prevented particle growth of Pt100xRux alloy, as shown in the BF-STEM images in Fig. 2. 1. Fig. 4 shows the first discharge-charge curves of the LAB cells incorporating the air electrodes loaded with Pt100xRux/KB electrocatalysts in the 2.0e4.2 V range. Fig. 5 shows the Ru
Fig. 1. HAADF-STEM images of Pt100xRux/KB: x ¼ (a) 0 and (b) 90.
Y. Yui et al. / Journal of Power Sources 340 (2017) 121e125
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Fig. 2. BF-STEM images of Pt100xRux/KB: x ¼ (a) 0, (b) 30, (c) 60, (d) 75, (e) 83, (f) 90, and (g) 100.
Table 1 Pt/Ru ratios of Pt100xRux/KB obtained from EDS. x in Pt100xRux
x¼0
30
60
75
83
90
100
Pt:Ru
100:0
74.5:25.5
40.4:59.6
23.0:77.0
14.6:85.4
9.2:90.8
0:100
Fig. 3. XRD patterns of Pt100xRux/KB:x ¼ (a) 0, (b) 30, (c) 60, (d) 75, (e) 83, (f) 90, and (g) 100.
content dependence of DVave and discharge/charge capacity obtained from Fig. 4. Here, DVave was defined as the difference between average charge voltage and the average discharge voltage. The discharge/charge capacity increases and DVave decreases as the Ru content increases. The largest discharge/ charge capacity and the lowest DVave were obtained for the x ¼ 90 sample. The cell with x ¼ 90 shows the lowest DVave of 1.16 V, while the one with x ¼ 0 shows rather high DVave of 1.43 V. The discharge/charge capacities and DVave clearly decrease with increasing Ru content, indicating the maximum capacity and the lowest DVave in the x ¼ 90 sample. This effect of
Fig. 4. First discharge-charge curves of LAB cells incorporating air electrodes loaded with Pt100xRux/KB electrocatalysts in the 2.0e4.2 V range: x ¼ (a) 0, (b) 30, (c) 60, (d) 75, (e) 83, (f) 90, and (g) 100.
nanosized particles is supported in the high dispersion on KB since the catalytic reaction occurs at the surface. The Ru phase (x ¼ 83, 90 and 100) of DVave is more than 0.1 V lower than the Pt phase as shown in Fig. 5. Although the high dispersion of nanosized particles is clearly one of the factors in the highactivity of electrocatalysts, there is a possibility that the crystal structure might be affecting electrocatalytic activity as well. In addition, the reason the sample with x ¼ 90 has lower average charging voltage and larger capacity than the one with x ¼ 100 is not yet clear. However, one possibility remains: Pt10Ru90 has high catalytic activity for the hydrogen oxidation reaction in alkaline environments [39]. In Ref. [39], it was claimed that Ru
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Fig. 5. DVave and discharge/charge capacity for LAB cells incorporating air electrodes loaded with various composition ratios of Pt100xRux/KB: (i) DVave, (ii) discharge capacity, and (iii) charge capacity.
Fig. 7. Cycle properties of LAB cells incorporating air electrodes loaded with Pt100xRux/KB: x ¼ (a) 0, (b) 30, (c) 60, (d) 75, (e) 83, (f) 90, and (g) 100.
amorphous like. Then, these peaks disappeared completely after the first charge. It was found that the deposition/decomposition reaction of Li2O2 for the LAB indeed occurred. Fig. 7 shows the cycle properties of LAB cells incorporating the air electrodes loaded with Pt100xRux/KB electrocatalysts. The discharge capacities of all cells gradually decreased. In particular, the cell for x ¼ 0 shows very small capacity of less than 80 mAh/g at the 8th cycle. The eighth discharge capacity of the electrodes with the x ¼ 30, 60, 75, 83, and 100 samples became about half of the first discharge capacity. Among the cells tested, the one with the x ¼ 90 sample showed comparatively better cycle stability with discharge capacity of over 800 mAh/g at the 8th cycle. Its capacity retention ratio was 80% after the 8th cycle. This result indicates the effectiveness of Ru addition for improving the cyclability due to the decrease in overpotential. 4. Conclusion Fig. 6. XRD patterns of air electrodes loaded with Pt10Ru90/KB as prepared and after first discharge and charge.
atoms have adsorptive capability of hydroxyl species (OHad), which leads to an effective reaction with the hydrogen intermediates (Had) on nearby Pt sites. The mechanisms of the higher electrocatalytic activity of Pt10Ru90/KB for LABs might be very similar to the above phenomena. That is, Ru and Pt atoms would have adsorptive capacity for O2 and Liþ, respectively, which would lead to highly electrocatalytic activity for the air electrode. Clarifying the origin of this mechanism requires further investigation. Fig. 6 shows XRD patterns of air electrodes loaded with Pt10Ru90/ KB electrocatalysts as prepared and after the first discharge/charge. With the exception of those from the titanium used as a current collector, there is only a broad peak at around 44.0 deg, which are identified as the Ru phase in the pristine electrode. After the first discharge, new peaks appeared at around 32.9, 35.0, and 58.7 deg, which are assigned to Li2O2. The patterns of the ones with x ¼ 0, 30, 60, 75, 83, and 100 had peaks similar to those with x ¼ 90, though these patterns are not shown here. The peaks are very small and broad, indicating that the crystallite would be very small or
We focused on Pt100xRux/KB electrocatalysts for air electrodes in nonaqueous solutions and investigated suitable composition ratios of Pt/Ru. The sample with x ¼ 0 has particle size of 7e100 nm and low dispersion. In contrast, the one with x ¼ 90 has small particle size of 2e10 nm, and it was found to be more thoroughly dispersed. The small particles in the latter lead to large discharge capacity of 1014 mAh/g, low average charge voltage of 3.74 V, and good cyclability (800 mAh/g up to the 8th cycles). Using the air electrodes loaded with Pt10Ru90/KB electrocatalysts, XRD results revealed the formation of Li2O2 after discharge to 2.0 V and that it was decomposed by the charging process. As a result, there is a significant correlation between the Pt/Ru composition ratio and electrochemical properties. The higher Ru content leads to a higher dispersion state of Pt100xRux electrocatalyst, accompanied by inhibition of particle growth, which results in good electrochemical performance of cells incorporating Pt10Ru90/KB. References [1] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Adv. Funct. Mater. 23 (2013) 947e958. [2] N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Chem. Rev. 114 (2014) 11636e11682. [3] Y. Yui, Y. Ono, M. Hayashi, Y. Nemoto, K. Hayashi, K. Asakura, H. Kitabayashi, J. Electrochem. Soc. 162 (2015) A3098eA3102. [4] Y. Yui, M. Hayashi, J. Nakamura, Sci. Rep. 6 (2016) 22406.
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