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nitrogen-doped reduced graphene as a highly active catalyst for lithium-air batteries
Electrochimica Acta 228 (2017) 36–44
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Facile synthesis of PdSnCo/nitrogen-doped reduced graphene as a highly active catalyst for lithium-air batteries Xiangzhong Ren* , Biyan Liao, Yongliang Li* , Peixin Zhang, Libo Deng, Yuan Gao College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, Guangdong 518060, PR China
A R T I C L E I N F O
Article history: Received 22 December 2016 Accepted 6 January 2017 Available online 7 January 2017 Keywords: electrocatalyst oxygen reduction reaction lithium-air battery nitrogen-doped graphene solvothermal
A B S T R A C T
Well-dispersed trimetallic-alloyed PdSnCo nanocrystals with an average size of 7 nm are supported on nitrogen-doped reduced graphene (NG) nanosheets by a solvothermal method. The PdSnCo/NG nanocatalyst is characterized by scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. As a result, the PdSnCo/NG exhibits high catalytic activity toward oxygen reduction reaction and high electrochemical stability in alkaline medium, compared to commercial 10% Pd black, binary PdSn/NG, and PdCo/NG. Moreover, PdSnCo/NG nanocatalyst shows high discharge capacity of 6750 mAh g1, low charge voltage and predominant cycleability for lithium-air batteries. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction The lithium-air battery has become one of the most promising energy storage and conversion systems due to its high theoretical energy density and significant energy storage capability, and its performance is approximately 10 times better than the performance of lithium-ion batteries. A typical nonaqueous lithium air battery is composed of a lithium negative electrode, a porous cathode material and a nonaqueous electrolyte solution. An important charge and discharge reaction of the lithium air battery þ occurs at the oxygen electrode (2Li þ 2e þ O2 $Li2 O2 ). In the nonaqueous lithium air battery, the electrode reaction includes the oxygen reduction reaction (ORR) in the discharge process and the oxygen evolution reaction (OER) in the charging process [1–4]. However, one of the challenges for lithium-air batteries is the electrochemical polarization of the air electrode, and the effective solution is to apply suitable catalysts, which facilitate the electrode reactions [5–9]. The catalysts increase the energy efficiency and cycling performance of the battery. Pd is a distinct catalyst that exhibits excellent catalytic activity in the process of the oxygen reduction reaction and the oxygen evolution reaction [10–14]. Recently, some studies have demonstrated that Pd-based binary alloy nanocatalysts showed excellent catalytic performance; for instance, the PdNi [15], PdCo [16], PdSn [17], and PdAu nanocatalysts [18] improve the electrochemical performance due to the
* Corresponding authors. E-mail addresses: [email protected] (X. Ren), [email protected] (Y. Li). http://dx.doi.org/10.1016/j.electacta.2017.01.032 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
change of the electronic structure of Pd. To further decrease the content of Pd in the alloys and increase the catalytic performance, it is necessary to investigate the electrochemical performance of Pd-based ternary alloy nanocatalysts. Yang et al. reported that the high electrocatalytic activity of ternary NiAuPt nanoparticles is attributed to the synergetic effect of the three nanostructured metals [19]. However, another way to improve the catalytic activity and reduce the cost is to use supporting materials, such as carbon black, carbon nanotubes and graphene. Graphene exhibits a high surface area, good conductivity, and strong corrosion resistance, and more importantly, it can provide strong adhesion to metal ions and is thus considered as an ideal support material [20,21]. Moreover, the electronic structure of graphene can be modified by doping atoms of other elements, for example, nitrogen doping not only increases the conductivity of graphene but also affects the spin density around the carbon atom, which generates more activated regions on the surface. Therefore, nitrogen-doped graphene not only improves the stability of the Pd-based alloy, it also disperses metallic nanoparticles. Feng et al. reported that Pt-Pd nanospheres supported on reduced graphene oxide nanosheets display enhanced electrocatalytic activity and good stability [22]. Manthiram et al. confirmed that PdCoMo shows high catalytic activity for ORR in PEMFC [23]. Wang et al. prepared PdSn-based catalysts by the NaBH4 reduction method, which presented a good linear correlation with the concentrations of ethanol and formic acid [24]. To the best of our knowledge, there are no reports on lithium storage devices of PdSnCo nanoparticles supported on nitrogendoped graphene (NG). In fact, compared with the Pd-based binary
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alloy, the ternary alloy nanocatalyst is rarely studied for electrocatalytic applications due to the complexity of the ternary systems. Here, ternary PdSnCo nanoparticles were supported on NG by a solvothermal method, and their application as an air-cathode in a nonaqueous lithium air battery was presented. Compared to 10% Pd black, Pd/NG, binary PdSn/NG, and PdCo/NG, the PdSnCo/NG nanocatalyst exhibited high capacity and cycle stability for the lithium-air battery. 2. Experimental 2.1. Preparation of NG Graphene oxide (GO) was synthesized by a modified Hummers method with graphite powder as raw material [25]. The NG was synthesized by a thermal solid-state reaction method. Typically, 100 mg of GO was mixed with 50 mg of melamine, and the mixture was heated in a tube furnace at 800 C for 2 h under N2 atmosphere. 2.2. Preparation of PdSnCo/NG 50 mg of NG was dispersed in 50 mL of alcohol to form a homogeneous dispersion under sonication. Then, PdCl2 (0.034 mmol), SnCl22H2O (0.026 mmol), and C4H6CoO44H2O (0.026 mmol) solutions were added under stirring. After 4 h, the homogeneous dispersion was transferred to a Teflon-lined stainless steel autoclave (100 mL) and heated to 180 C for 12 h. Then, the product was washed three times by centrifugation and dried in a vacuum oven at 100 C for 8 h. Finally, the product was heated in a tube furnace at 500 C for 2 h under N2 atmosphere. PdSn/NG, PdCo/NG, and Pd/NG were synthesized in the same way. 2.3. Physical characterizations X-ray diffraction (XRD, Bruker D8 Advance) was used to analyze the crystal structure of the material. Field emission scanning electron microscopy (FESEM, JEOL-JSM-7800F) was used to observe the microstructure of the material. The composition and content of the prepared materials were obtained by energy dispersive spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS) provided the necessary basis for the catalytic principle of the catalyst. Raman spectroscopy (HORIBA LabRAMHR 800) was used to obtain the structural information of the sample. In addition, the microstructure of the material was observed with a highresolution transmission electron microscope (HRTEM, FEI Tecnai G2). 2.4. Electrochemical characterizations A three-electrode system was used to carry out electrochemical tests. The three electrodes are composed of a platinum wire as the counter electrode, an Ag/AgCl electrode as the reference electrode and a rotating disk electrode (RDE, ALS, Japan, F = 3 mm) as the working electrode. The catalytic activities of the nanocatalysts were evaluated by the CV and RDE techniques. The scan rate of the working electrode was 50 mV s1, and the voltage range was from 0.8 to 0.2 V. The catalyst inks were prepared as follows: 4 mg of the nanocatalysts and 30 mL of Nafion solution (5 wt%) were dispersed in 2 mL of ethanol by ultrasound for 40 min. Then, 5 mL of the inks (approximately 10 mg) was dropped onto the glassy carbon electrode and dried under an infrared lamp. The electrolyte solution (0.1 M KOH) was purged with high purity oxygen for 30 min and then bubbled with an O2 atmosphere during the entire experimental process. All experiments were carried out at room temperature. For comparison, 10% commercial black Pd were purchased from Aladdin for the tests.
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2.5. Li-air battery performance Preparation of air cathode: The catalysts and PVDF were mixed with a weight ratio of 9:1. Then, the mixture was coated on the carbon paper with a diameter of 15 mm as the air cathode. The loading of the active material was controlled at 1.0 0.1 mg cm2, and the electrode was placed in a vacuum oven at 80 C for 10 h. Assembly of lithium-air batteries: The entire cell assembly process of the lithium-air battery was carried out in a glove box under argon atmosphere, in which the water and oxygen contents were lower than 0.5 ppm. The battery is a CR2032-type coin cell, and the positive side has uniform small holes (F = 1 mm). The lithium-air battery includes a lithium metal anode, a glass fiber separator, electrolyte solution (1 M LiTFSI/TEGDME) and the prepared cathode. All tests were performed by a battery testing system (Land CT2001A) in a pure oxygen atmosphere. 3. Results and Discussion Fig. 1a shows the SEM image of NG; the doped-graphene exhibits a wrinkled surface and a thin layer structure. In addition, the porosity formation was due to the thermal reduction process. As shown in Fig. 1b, the Pd alloy particles are spherical and uniformly distributed on the surface of NG. Moreover, there is no obvious agglomeration on the surface of NG, which occurs because the nitrogen doping provides more defect locations for the nanoparticles. The EDS spectrum analysis of PdSnCo/NG indicates that the catalyst contains C, O, N, Pd, Sn and Co elements (Fig. 1c). The appearance of the O element is due to the residual oxygencontaining functional groups in the process of thermal reduction. Notably, the atomic ratio of Pd:Sn:Co is approximately 1:1.2:0.8, which is close to the designed ratio. Raman spectroscopy was used to further examine the microstructure of graphene. Fig. 1d shows the Raman spectrum of PdSnCo/NG. There are two peaks present at 1357 and 1598 cm1, which correspond to the D-band and G-band of graphene. The D-band is the disordered vibrational peak of graphene, and the G-band is caused by the internal vibration of the carbon atoms. Moreover, a small wide peak appeared at approximately 2600 cm1, which can be described as graphene with few layers. The intensity ratio of the D- and G-bands is usually used to estimate the graphitization degree. The ID/IG is approximately 1.15 for PdSnCo/NG, which is higher than that of NG (0.91). Therefore, the PdSnCo/NG contains more active sites for ORR [26,27]. Fig. 2a shows the XRD patterns for the ternary PdSnCo/NG, the binary PdCo/NG and PdSn/NG, and the monometallic Pd/NG catalysts. All the catalysts show four sharp peaks, which are attributed to the (111), (200), (220), and (311) planes of the crystallite fcc structure of Pd (JCPDS No.65-2867). No diffraction peaks for Co or Sn appeared in the XRD patterns, which indicates that the Sn and Co elements were incorporated into the Pd lattice [24]. Notably, the (111) peak for PdCo/NG and PdSnCo/NG shifted to a higher degree than that of Pd/NG, while the peak for PdSn/NG shifted to a lower degree. Therefore, the (111) diffraction peak was used to evaluate the lattice parameter of the nanocatalysts. Table 1 shows the change in the lattice parameter of the nanocatalysts, which follows the order: PdSn/NG (3.914 Å) > Pd/NG (3.891 Å) > PdSnCo/NG (3.854 Å) > PdCo/NG (3.751 Å). It is clear that the lattice parameter of PdSnCo/NG and PdCo is slightly smaller than that of Pd/NG, which indicates the occurrence of a lattice contraction. By contrast, the lattice parameter of PdSn/NG is larger, which is because Sn with a large atomic radius increases the lattice parameter of Pd [28]. There are two peaks, as shown in Fig. 2b, which are allocated to the (002) and (101) peaks of NG. The transformation of the (002) peak from 11 to 26 suggests that most of the oxygen-containing functional groups were removed
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Fig. 1. (a) SEM image of NG, (b) SEM image, (c) EDS spectrum analysis and (d) Raman spectrum of PdSnCo/NG.
during the thermal reduction. From the discussion above, it can be concluded that the Pd-based alloys have been successfully loaded on the surface of NG [29]. TEM was an effective way to observe the structure and crystallinity of PdSnCo/NG (Fig. 3). It is suggested that the nanoparticles are spherical with a smooth surface, and the particle size is approximately 6–7 nm. Importantly, the nanoparticles are uniformly dispersed on the surface of NG, which may due to the addition of nitrogen into the graphene framework, and this is conducive to improved catalytic activity [30,31]. The SAED pattern shows clear spots and rings, indicating that the catalyst has a good degree of crystallinity (inset of Fig. 3c). Fig. 3d exhibits the crystal lattice with a d-spacing of 0.222 nm, which corresponds to the (111) planes of Pd. Compared to Pd, the lattice spacing of the Pd alloy has a slight deviation, which is due to the Sn and Co embedded in the lattice of Pd [32]. This matches well with the XRD analysis results. The XPS spectra of PdSnCo/NG are shown in Fig. 4. The survey scan result shows that the sample contains C, N, Pd, Co, and Sn elements. Fig. 4b shows the spectra with two peaks of Pd, which can be attributed to Pd3d5/2 (335.88 eV) and Pd3d3/2 (340.98 eV). As shown in Fig. 4b, the binding energies of Pd for the PdSnCo/NG, PdSn/NG and PdCo/NG were significantly different from Pd/NG. The shift occurs because Sn and Co are embedded in the Pd lattice, and there is a strong coupling effect between them. This type of electron interaction is beneficial to the electrochemical
Table 1 The XRD analysis of as-synthesized Pd/NG, PdSn/NG, PdCo/NG and PdSnCo/NG catalysts.
Fig. 2. XRD patterns of (a) PdSnCo/NG, PdCo/NG, PdSn/NG, and Pd/NG, (b) GO and NG.
Sample
Pd(111) at 2U/( )
d(111)/(Å)
Lattice parameter/(Å)
Pd/NG PdSn/NG PdCo/NG PdSnCo/NG
40.10 39.74 41.68 40.11
2.246 2.260 2.165 2.225
3.89 3.91 3.75 3.85
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Fig. 3. (a, b) TEM and (c, d) HRTEM images of PdSnCo/NG. The inset of c is the selected area electron diffraction (SAED) pattern.
Fig. 4. XPS spectra of (a) survey scan of PdSnCo/NG, (b) Pd3d, (c) C1s and (d) N1s.
performance [33]. Fig. 4c shows that the C C, C O, and C¼O bonds appear at 284.6, 286.6 and 288.1 eV, and the peak at 285 eV is attributed to sp2C bonded to N. Nitrogen in graphene mainly exists in three states: pyrrolic N (399.3 eV), pyridinic N (398.3 eV) and graphitic N (401 eV). The lone pair electrons of the N atoms form a conjugated system with the sp2 hybrid carbon skeleton, which indicates that N is replaced by C and incorporated into the
graphene framework. This can affect the charge distribution around the carbon atom and therefore, improve its electrochemical activity. Fig. 5a displays the CV plots of PdSnCo/NG in N2- and O2saturated 0.1 M KOH solutions. Compared to the absence of noticeable reduction features between 0.8-0.2 V in N2- saturated 0.1 M KOH solution, an obvious and strong reduction current peak
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Fig. 5. (a) CV curves of PdSnCo/NG in (a) O2- and (b) N2-saturated 0.1 M KOH solutions. (b) ORR polarization curves of different electrode materials.
can be seem in O2- saturated 0.1 M KOH solution, which indicates that PdSnCo/NG possesses good electrocatalytic activity. The oxygen reduction performance of the catalysts was studied by LSV at a scan rate of 50 mV s1 and a voltage range from 0.2 to 0.8 V. In general, the onset potential, half-wave potential and limiting current density are used to distinguish the ORR performance [34]. As shown in Fig. 5b, the half-wave potentials of the catalysts are PdSnCo/NG (0.20 V), Pd black (0.28 V), PdSn/NG (0.22 V), PdCo/NG (0.22 V), Pd/NG (0.215 V) and NG (0.37 V); the limiting current density values of the catalysts are PdSnCo/NG (3.9 mA cm2), Pd black (3.5 mA cm2), PdSn/NG (3.26 mA cm2), PdCo/NG (3.26 mA cm2), Pd/NG (3.5 mA cm2) and NG (2.25 m Acm2). Both the half-wave potentials for oxygen reduction and the evident limiting current density values follow the order: PdSnCo/NG > Pd/NG > PdSn/NG > PdCo/NG > Pd black > NG, which indicates that PdSnCo/NG had the best ORR electrocatalytic activity compared to the other catalysts. Compared to commercial 10% Pd black, the Pd content in PdSnCo/NG was decreased to 6%. There is a clear indication that the improved catalytic activity of PdSnCo/NG due to the presence of dopant Sn and Co atoms not only increased the utilization efficiency but also decreased the d-band center of Pd and thereby altered the electronic properties of the overall catalyst [35,36]. Another important parameter for characterizing the performance of ORR is the four-electron selectivity of the catalyst because the direct reduction of oxygen to water or OH by the fourelectron process is more efficient than the reduction of hydrogen peroxide via a two-electron process. RDE tests was carried out in O2-saturated 0.1 M KOH solution at a scan rate of 20 mVs1, and the number of electrons involved in the oxygen reduction reaction can be calculated from the Koutecky-Levich (K-L) equation [37]: 1 1 1 ¼ þ 0:5 J JK v
ð1Þ
B = 0.62nF(DO2)2/3v1/6CO2
(2)
where J is the current density, v is the rotating rate of the electrode, Jk is the kinetic current density, and the B value could be obtained from the K-L plots using Eq. (1). In addition, F in Eq. (2) is the Faraday constant (96485C mol1), DO2 is the diffusion coefficient of O2 (1.9 105 mol cm3), v is the kinetic viscosity (0.01 cm2 s1), and CO2 is the bulk concentration of O2 (1.2 106 mol cm3). As shown in Fig. 6a, the K-L plots of J1 vs. v1/2 at potentials of 0.4, 0.5, 0.6, 0.7, and 0.8 V display good linearity, which shows that the ORR on PdSnCo/NG follows first-order kinetics. Fig. 6b shows that the slopes from 0.4 to 0.8 V are 7.79, 7.78, 7.44, 7.03 and 6.95, respectively, and the corresponding n values are 3.62, 3.65, 3.79, 3.95 and 3.99, which suggests a direct reduction of O2 to H2O via the 4-electron pathway. In particular, the catalytic reaction mechanisms of oxygen catalysts for aqueous and nonaqueous electrolytes are different. Nevertheless, a significant number of previously reported results for catalytic performance characteristics of oxygen catalysts exhibit similarities in both types of electrolytes [38,39]. In addition, this association suggests that oxygen catalysts in aqueous systems could be extended to their application in the nonaqueous system of lithium air batteries [40]. The cathode electrodes in electrolyte solution (1 M LiTFSI/TEGDME) were studied. The stability of the ether electrolyte is much better than that of the carbonate electrolyte. And the high stability of the electrolyte (1 M LiTFSI/ TEGDME) led to its use in lithium air batteries [41,42]. Fig. 7a shows the initial discharge curves of the cathode electrodes including PdSnCo/NG, Pd/NG, NG and commercial Pd black catalysts, which were investigated at a current density of 0.3 mA cm2. The current and specific capacities are calculated based on the average weight of the cathode material under the same testing conditions. The
Fig. 6. (a) Rotating rate dependent ORR polarization curves with the scan rate of 20 mV s
1
; (b) K-L plots of J1 vs v1/2 at different potentials obtained from A.
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Fig. 7. (a) The initial discharge profiles of lithium-air battery equipped with PdSnCo/NG, Pd/NG, NG, and commercial Pd black, (b) The initial discharge/charge curves of PdSnCo/NG, Pd/NG, Pd black, NG. All the tests were performed at a current density of 0.3 mA cm2.
discharge capacity of the PdSnCo/NG electrode was 6750 mAhg1, which is better than those of Pd/NG, Pd black and NG. There are two reasons for the increased capacity of PdSnCo/NG: NG as the supporting material increases the surface area, and the emergence of Co and Sn generates more oxygen vacancies in the catalyst. Fig. 7b shows the first discharge/charge voltage profiles of the nanocatalysts at a constant capacity of 500 mAh g1 and a current density of 0.3 mA cm2. The charge/discharge overpotentials were calculated from the discharge/charge average voltages [43]. The charge/discharge overpotentials of the electrodes were approximately 0.85 V for PdSnCo/NG, 1.08 V for Pd/NG, 1.37 V for Pd black and 1.5 V for the NG electrode. Therefore, both in the charge and discharge processes, the PdSnCo/NG showed the lowest electrode polarization. This can be attributed to the ternary alloy and the modified graphene composites. The polarization was decreased by introducing the bifunctional catalysts to improve the catalytic activity and conductivity. Pd displays excellent activity for the ORR and OER, and the addition of transition metals (Sn and Co), which decreased the d-band center of Pd and thereby altered the electronic properties of the overall catalyst, further promoted the OER [44]. It is confirmed that PdSnCo/NG exhibits the best redox activity, which is consistent with the results of the ORR analysis and some recent related results, as observed in Table 2. As shown in Fig. 8, the cycling performance was determined under the condition of a constant capacity of 500 mAh g1, a voltage range from 2 to 4.5 V and a current density of 0.3 mA cm2 [49]. As the number of cycles increased, the discharge platform gradually reduced, the charging platform gradually increased, and therefore, the overpotential also increased. When the voltage was greater than 4.5 V, the battery with the NG electrode only operated for 20 cycles. This number increased to 30 cycles after the addition of Pd, which reduced the electrochemical polarization phenomenon caused by the charge-discharge process. The incorporation of Co and Sn into Pd resulted in an increase of the cycle number of PdSnCo/NG to 60, which is 2.4 times larger than that of commercial black Pd. Therefore, NG not only acts as a supporting material for
nanoparticles but also increases the specific surface area and provides active sites. Sn and Co have a coordinated effect with Pd, which exhibits a synergetic effect on the electrochemical reactions. Therefore, it greatly reduces the electrochemical polarization phenomenon and improves the stability of the lithium-air battery [50]. The electrode materials were also investigated under the condition of a constant capacity of 1000 mAh g1, a voltage range from 2 to 4.5 V and a current density of 0.3 mA cm2 (Fig. S1). The batteries was cycled 5 times at 0.3 mA cm2 for comparison. PdSnCo/NG also exhibits the highest cycle performance (Fig. S2), and its discharge/charge voltage platform gap is the lowest. The charge voltages of the PdSnCo/NG and Pd/NG cathodes were lower than 4 V, and their discharge voltages were 2.75 V. This clearly shows that for a constant capacity of either 500 or 1000 mAh g1, the sample exhibited good electrochemical performance. Fig. 9 displays the continuously cycled discharge/charge curves of the nanoparticles cathode at a current density of 0.3 mA cm2, including the average discharge/charge voltages for 10 cycles. In addition, the discharge/charge profiles of all the catalyst cathodes are shown in Fig. 8. It is observed that the charge voltages of the PdSnCo/NG and Pd/NG cathodes were lower than 4 V, and their discharge voltages were 2.75 V. The charge voltages of the NG and Pd black electrode were close to 4 V, and the discharge voltages were 2.65 V and 2.63 V, respectively. It can be determined that nitrogen-doped graphene has catalytic properties, and the existence of Pd is beneficial to the oxygen reduction reaction and the oxygen release reaction of the electrode, thereby reducing the electrochemical polarization phenomenon during the charging and discharging processes [51,52]. Furthermore, after the formation of the composite material of the Pd nanocatalysts, the OER/ ORR activity of the PdSnCo/NG is remarkably increased, which includes a high capacity and good cycle stability. The improvement in the specific capacity and the reduction of the discharge/charge voltage platform gap were attributed to the synergistic effects when PdSnCo and NG were combined to form a composite catalyst. Based on the analysis above, the origin of the excellent
Table 2 Comparison of the lithium-air battery performance of PdSnCo/NG cathode with those of Pd-based cathodes reported in the literature. Materials
Pd content
Current Density
1st Discharge Capacity/mAh g1
Cycles/Fixed Capacity
Discharge/charge plateau
Ref.
ALD Pd/C Pd/5c-ZnO/C Pd/Co3O4/NF MGP-20 PdCu/C Pd/PNCNF PdNDS-GNP/GO This work
– 11.5% 20% 20% 21% 25% 40% 6%
100 mA g1
6500 – 1842.7 10000 13000 9405 – 6750
– 20/1000 mAh g1 70/300 mAh g1 163/1000 mAhg1 – 50/1000 mAh g1 30/500 mAh g1 60/500 or 1000 mAh g1
2.6 V/3.7 V 2.7 V/3.2 V 2.8 V/3.8 V 2.0 V/4.0 V 2.4 V/4.4 V 2.6 V/4.2 V 2.98 V/3.6 V 2.75 V/3.6 V
[45] [40] [46] [47] [44] [48] [49] –
2
0.05 mA cm 70 mA g1 0.12 mA cm2 100 mA g1 200 mA g1 0.3 mA cm2
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Fig. 8. The representative discharge and charge curves (at a current density of 0.3 mA cm2 and the limited specific capacity of 500 mAh g1) of (a) PdSnCo/NG, (b) Pd/NG, (c) Pd black, and (d) NG.
Fig. 9. Cycling performance of the lithium-air batteries with (a) PdSnCo/NG, (b) Pd/NG, (c) Pd black, and (d) NG electrodes.
electrochemical performance should be attributed to the following: (1) the incorporation of Co and Sn into Pd with a proper ratio; (2) more dispersed and small PdSnCo nanoparticles; and (3) the strong synergistic or coupling effect of the PdSnCo nanoparticles and NG. In addition, the rate performances of three electrodes were obtained (Fig. S3). The cycling performance of PdSnCo/NG was investigated under the condition of a constant capacity of 500 mAh g1 and at various current densities of 0.15 mA cm2, 0.3 mA cm2 and 0.6 mA cm2. When the test was carried out at 0.15 mA cm2 and 0.3 mA cm2, PdSnCo/NG delivered constant charge and discharge voltages of 3.75 V and 2.75 V. The charge voltage remained at 3.85 V at a current density of 0.6 mA cm2. This
suggests that PdSnCo/NG exhibited a good rate capability and stability. The good electrochemical performance is due to the synergistic effects between the PdSnCo nanoparticles and NG, which prevented the agglomeration of electrode materials [43]. Here, the PdSnCo/NG composites demonstrated not only better cycling stability but also higher reversible specific capacity and higher rate performance, resulting from the NG conductive network and the PdSnCo composite advantages [24]. 4. Conclusions In summary, PdSnCo/NG was successfully synthesized by a onepot solvothermal method. Compared to the binary PdCo/NG and
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PdSn/NG and commercial Pd black, the PdSnCo/NG nanocatalyst exhibits a high oxygen reduction activity and stability in alkaline medium. In addition, the PdSnCo/NG nanocatalyst shows a high discharge capacity and cycling performance for the lithium-air battery, which demonstrates tis potential as a catalyst in the air battery system. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 21671136 and 51502177), the Natural Science Foundation of Guangdong (No. 2015A030313542, 2014A030310323 and 2016A030313057). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2017.01.032. References [1] M. Sevim, T. Sener, O. Metin, Monodisperse MPd (M: Co, Ni, Cu) alloy nanoparticles supported on reduced graphene oxide as cathode catalysts for the lithium-air battery (vol 40, pg 10876, 2015), Int. J. Hydrogen Energy 41 (2016) 3318. [2] C.H. Ahn, R.S. Kalubarme, Y.H. Kim, K.N. Jung, K.H. Shin, C.J. Park, Graphene/ doped ceria nano-blend for catalytic oxygen reduction in non-aqueous lithium-oxygen batteries, Electrochim. Acta 117 (2014) 18–25. [3] W. Yan, W. Bian, C. Jin, J.H. Tian, R. Yang, An efficient bifunctional electrocatalyst based on strongly coupled CoFe2O4/Carbon nanotubes hybrid for oxygen reduction and oxygen evolution, Electrochim. Acta 177 (2015) 65–72. [4] W. Zhou, J. Li, H. Nie, Y. Zhang, X. Xi, H. Zhang, Carbon electrode for nonaqueous Li-O2 battery: the influence of surface oxygen species, Electrochim. Acta 138 (2014) 410–416. [5] M. He, P. Zhang, L. Liu, B. Liu, S. Xu, Hierarchical porous nitrogen doped threedimensional graphene as a free-standing cathode for rechargeable lithiumoxygen batteries, Electrochim. Acta 191 (2016) 90–97. [6] X. Lin, Y. Cao, S. Cai, J. Fan, Y. Li, Q.H. Wu, M. Zheng, Q. Dong, Ruthenium@mesoporous graphene-like carbon: a novel three-dimensional cathode catalyst for lithium-oxygen batteries, J. Mater. Chem. A 4 (2016) 7788–7794. [7] J. Liu, Z. Wang, J. Zhu, Binder-free nitrogen-doped carbon paper electrodes derived from polypyrrole/cellulose composite for Li-O2 batteries, J. Power Sources 306 (2016) 559–566. [8] J. Wang, Y. Li, X. Sun, Challenges and opportunities of nanostructured materials for aprotic rechargeable lithium-air batteries, Nano Energy 2 (2013) 443–467. [9] H. Lin, Z. Liu, Y. Mao, X. Liu, Y. Fang, Y. Liu, D. Wang, J. Xie, Effect of nitrogendoped carbon/Ketjenblack composite on the morphology of Li2O2 for highenergy-density Li-air batteries, Carbon 96 (2016) 965–971. [10] C. Kim, O. Gwon, I.Y. Jeon, Y. Kim, J. Shin, Y.W. Ju, J.B. Baek, G. Kim, Cloud-like graphene nanoplatelets on Nd0.5Sr0.5CoO3-delta nanorods as an efficient bifunctional electrocatalyst for hybrid Li-air batteries, J. Mater. Chem. A 4 (2016) 2122–2127. [11] Y.S. Jeong, J.B. Park, H.G. Jung, J. Kirn, X. Luo, J. Lu, L. Curtiss, K. Amine, Y.-K. Sun, B. Scrosati, Y.J. Lee, Study on the catalytic activity of noble metal nanoparticles on reduced graphene oxide for oxygen evolution reactions in lithium-air batteries, Nano Lett. 15 (2015) 4261–4268. [12] G. Jiang, H. Zhu, X. Zhang, B. Shen, L. Wu, S. Zhang, G. Lu, Z. Wu, S. Sun, Core/ shell face-centered tetragonal FePd/Pd nanoparticles as an efficient non-Pt catalyst for the oxygen reduction reaction, ACS Nano 9 (2015) 11014–11022. [13] Z.T. Liu, K.L. Huang, Y.S. Wu, Y.P. Lyu, C.L. Lee, A comparison of physically and chemically defective graphene nanosheets as catalyst supports for cubic Pd nanoparticles in an alkaline oxygen reduction reaction, Electrochim. Acta 186 (2015) 552–561. [14] Y. Li, W. Li, T. Ke, P. Zhang, X. Ren, L. Deng, Microwave-assisted synthesis of sulfur-doped graphene supported PdW nanoparticles as a high performance electrocatalyst for the oxygen reduction reaction, Electrochem. Commun. 69 (2016) 68–71. [15] Q. Zhen, G. Haoran, W. Xiaoguang, Z. Changchun, J. Hong, Z. Chi, X. Junling, Z. Zhonghua, Novel nanocrystalline PdNi alloy catalyst for methanol and ethanol electro-oxidation in alkaline media, J. Power Sources 196 (2011) 5823–5828. [16] Y. Wang, X. Wang, C.M. Li, Electrocatalysis of Pd-Co supported on carbon black or ball-milled carbon nanotubes towards methanol oxidation in alkaline media, Appl. Catal. B 99 (2010) 229–234. [17] J. Kim, T. Momma, T. Osaka, Synthesis of carbon-supported Pd-Sn catalyst by ultrasonic irradiation for oxygen reduction reaction, J. Power Sources 189 (2009) 909–915.
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Facile synthesis of PdSnCo/nitrogen-doped reduced graphene as a highly active catalyst for lithium-air batteries Xiangzhong Ren* , Biyan Liao, Yongliang Li* , Peixin Zhang, Libo Deng, Yuan Gao College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, Guangdong 518060, PR China
A R T I C L E I N F O
Article history: Received 22 December 2016 Accepted 6 January 2017 Available online 7 January 2017 Keywords: electrocatalyst oxygen reduction reaction lithium-air battery nitrogen-doped graphene solvothermal
A B S T R A C T
Well-dispersed trimetallic-alloyed PdSnCo nanocrystals with an average size of 7 nm are supported on nitrogen-doped reduced graphene (NG) nanosheets by a solvothermal method. The PdSnCo/NG nanocatalyst is characterized by scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. As a result, the PdSnCo/NG exhibits high catalytic activity toward oxygen reduction reaction and high electrochemical stability in alkaline medium, compared to commercial 10% Pd black, binary PdSn/NG, and PdCo/NG. Moreover, PdSnCo/NG nanocatalyst shows high discharge capacity of 6750 mAh g1, low charge voltage and predominant cycleability for lithium-air batteries. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction The lithium-air battery has become one of the most promising energy storage and conversion systems due to its high theoretical energy density and significant energy storage capability, and its performance is approximately 10 times better than the performance of lithium-ion batteries. A typical nonaqueous lithium air battery is composed of a lithium negative electrode, a porous cathode material and a nonaqueous electrolyte solution. An important charge and discharge reaction of the lithium air battery þ occurs at the oxygen electrode (2Li þ 2e þ O2 $Li2 O2 ). In the nonaqueous lithium air battery, the electrode reaction includes the oxygen reduction reaction (ORR) in the discharge process and the oxygen evolution reaction (OER) in the charging process [1–4]. However, one of the challenges for lithium-air batteries is the electrochemical polarization of the air electrode, and the effective solution is to apply suitable catalysts, which facilitate the electrode reactions [5–9]. The catalysts increase the energy efficiency and cycling performance of the battery. Pd is a distinct catalyst that exhibits excellent catalytic activity in the process of the oxygen reduction reaction and the oxygen evolution reaction [10–14]. Recently, some studies have demonstrated that Pd-based binary alloy nanocatalysts showed excellent catalytic performance; for instance, the PdNi [15], PdCo [16], PdSn [17], and PdAu nanocatalysts [18] improve the electrochemical performance due to the
* Corresponding authors. E-mail addresses: [email protected] (X. Ren), [email protected] (Y. Li). http://dx.doi.org/10.1016/j.electacta.2017.01.032 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
change of the electronic structure of Pd. To further decrease the content of Pd in the alloys and increase the catalytic performance, it is necessary to investigate the electrochemical performance of Pd-based ternary alloy nanocatalysts. Yang et al. reported that the high electrocatalytic activity of ternary NiAuPt nanoparticles is attributed to the synergetic effect of the three nanostructured metals [19]. However, another way to improve the catalytic activity and reduce the cost is to use supporting materials, such as carbon black, carbon nanotubes and graphene. Graphene exhibits a high surface area, good conductivity, and strong corrosion resistance, and more importantly, it can provide strong adhesion to metal ions and is thus considered as an ideal support material [20,21]. Moreover, the electronic structure of graphene can be modified by doping atoms of other elements, for example, nitrogen doping not only increases the conductivity of graphene but also affects the spin density around the carbon atom, which generates more activated regions on the surface. Therefore, nitrogen-doped graphene not only improves the stability of the Pd-based alloy, it also disperses metallic nanoparticles. Feng et al. reported that Pt-Pd nanospheres supported on reduced graphene oxide nanosheets display enhanced electrocatalytic activity and good stability [22]. Manthiram et al. confirmed that PdCoMo shows high catalytic activity for ORR in PEMFC [23]. Wang et al. prepared PdSn-based catalysts by the NaBH4 reduction method, which presented a good linear correlation with the concentrations of ethanol and formic acid [24]. To the best of our knowledge, there are no reports on lithium storage devices of PdSnCo nanoparticles supported on nitrogendoped graphene (NG). In fact, compared with the Pd-based binary
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alloy, the ternary alloy nanocatalyst is rarely studied for electrocatalytic applications due to the complexity of the ternary systems. Here, ternary PdSnCo nanoparticles were supported on NG by a solvothermal method, and their application as an air-cathode in a nonaqueous lithium air battery was presented. Compared to 10% Pd black, Pd/NG, binary PdSn/NG, and PdCo/NG, the PdSnCo/NG nanocatalyst exhibited high capacity and cycle stability for the lithium-air battery. 2. Experimental 2.1. Preparation of NG Graphene oxide (GO) was synthesized by a modified Hummers method with graphite powder as raw material [25]. The NG was synthesized by a thermal solid-state reaction method. Typically, 100 mg of GO was mixed with 50 mg of melamine, and the mixture was heated in a tube furnace at 800 C for 2 h under N2 atmosphere. 2.2. Preparation of PdSnCo/NG 50 mg of NG was dispersed in 50 mL of alcohol to form a homogeneous dispersion under sonication. Then, PdCl2 (0.034 mmol), SnCl22H2O (0.026 mmol), and C4H6CoO44H2O (0.026 mmol) solutions were added under stirring. After 4 h, the homogeneous dispersion was transferred to a Teflon-lined stainless steel autoclave (100 mL) and heated to 180 C for 12 h. Then, the product was washed three times by centrifugation and dried in a vacuum oven at 100 C for 8 h. Finally, the product was heated in a tube furnace at 500 C for 2 h under N2 atmosphere. PdSn/NG, PdCo/NG, and Pd/NG were synthesized in the same way. 2.3. Physical characterizations X-ray diffraction (XRD, Bruker D8 Advance) was used to analyze the crystal structure of the material. Field emission scanning electron microscopy (FESEM, JEOL-JSM-7800F) was used to observe the microstructure of the material. The composition and content of the prepared materials were obtained by energy dispersive spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS) provided the necessary basis for the catalytic principle of the catalyst. Raman spectroscopy (HORIBA LabRAMHR 800) was used to obtain the structural information of the sample. In addition, the microstructure of the material was observed with a highresolution transmission electron microscope (HRTEM, FEI Tecnai G2). 2.4. Electrochemical characterizations A three-electrode system was used to carry out electrochemical tests. The three electrodes are composed of a platinum wire as the counter electrode, an Ag/AgCl electrode as the reference electrode and a rotating disk electrode (RDE, ALS, Japan, F = 3 mm) as the working electrode. The catalytic activities of the nanocatalysts were evaluated by the CV and RDE techniques. The scan rate of the working electrode was 50 mV s1, and the voltage range was from 0.8 to 0.2 V. The catalyst inks were prepared as follows: 4 mg of the nanocatalysts and 30 mL of Nafion solution (5 wt%) were dispersed in 2 mL of ethanol by ultrasound for 40 min. Then, 5 mL of the inks (approximately 10 mg) was dropped onto the glassy carbon electrode and dried under an infrared lamp. The electrolyte solution (0.1 M KOH) was purged with high purity oxygen for 30 min and then bubbled with an O2 atmosphere during the entire experimental process. All experiments were carried out at room temperature. For comparison, 10% commercial black Pd were purchased from Aladdin for the tests.
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2.5. Li-air battery performance Preparation of air cathode: The catalysts and PVDF were mixed with a weight ratio of 9:1. Then, the mixture was coated on the carbon paper with a diameter of 15 mm as the air cathode. The loading of the active material was controlled at 1.0 0.1 mg cm2, and the electrode was placed in a vacuum oven at 80 C for 10 h. Assembly of lithium-air batteries: The entire cell assembly process of the lithium-air battery was carried out in a glove box under argon atmosphere, in which the water and oxygen contents were lower than 0.5 ppm. The battery is a CR2032-type coin cell, and the positive side has uniform small holes (F = 1 mm). The lithium-air battery includes a lithium metal anode, a glass fiber separator, electrolyte solution (1 M LiTFSI/TEGDME) and the prepared cathode. All tests were performed by a battery testing system (Land CT2001A) in a pure oxygen atmosphere. 3. Results and Discussion Fig. 1a shows the SEM image of NG; the doped-graphene exhibits a wrinkled surface and a thin layer structure. In addition, the porosity formation was due to the thermal reduction process. As shown in Fig. 1b, the Pd alloy particles are spherical and uniformly distributed on the surface of NG. Moreover, there is no obvious agglomeration on the surface of NG, which occurs because the nitrogen doping provides more defect locations for the nanoparticles. The EDS spectrum analysis of PdSnCo/NG indicates that the catalyst contains C, O, N, Pd, Sn and Co elements (Fig. 1c). The appearance of the O element is due to the residual oxygencontaining functional groups in the process of thermal reduction. Notably, the atomic ratio of Pd:Sn:Co is approximately 1:1.2:0.8, which is close to the designed ratio. Raman spectroscopy was used to further examine the microstructure of graphene. Fig. 1d shows the Raman spectrum of PdSnCo/NG. There are two peaks present at 1357 and 1598 cm1, which correspond to the D-band and G-band of graphene. The D-band is the disordered vibrational peak of graphene, and the G-band is caused by the internal vibration of the carbon atoms. Moreover, a small wide peak appeared at approximately 2600 cm1, which can be described as graphene with few layers. The intensity ratio of the D- and G-bands is usually used to estimate the graphitization degree. The ID/IG is approximately 1.15 for PdSnCo/NG, which is higher than that of NG (0.91). Therefore, the PdSnCo/NG contains more active sites for ORR [26,27]. Fig. 2a shows the XRD patterns for the ternary PdSnCo/NG, the binary PdCo/NG and PdSn/NG, and the monometallic Pd/NG catalysts. All the catalysts show four sharp peaks, which are attributed to the (111), (200), (220), and (311) planes of the crystallite fcc structure of Pd (JCPDS No.65-2867). No diffraction peaks for Co or Sn appeared in the XRD patterns, which indicates that the Sn and Co elements were incorporated into the Pd lattice [24]. Notably, the (111) peak for PdCo/NG and PdSnCo/NG shifted to a higher degree than that of Pd/NG, while the peak for PdSn/NG shifted to a lower degree. Therefore, the (111) diffraction peak was used to evaluate the lattice parameter of the nanocatalysts. Table 1 shows the change in the lattice parameter of the nanocatalysts, which follows the order: PdSn/NG (3.914 Å) > Pd/NG (3.891 Å) > PdSnCo/NG (3.854 Å) > PdCo/NG (3.751 Å). It is clear that the lattice parameter of PdSnCo/NG and PdCo is slightly smaller than that of Pd/NG, which indicates the occurrence of a lattice contraction. By contrast, the lattice parameter of PdSn/NG is larger, which is because Sn with a large atomic radius increases the lattice parameter of Pd [28]. There are two peaks, as shown in Fig. 2b, which are allocated to the (002) and (101) peaks of NG. The transformation of the (002) peak from 11 to 26 suggests that most of the oxygen-containing functional groups were removed
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Fig. 1. (a) SEM image of NG, (b) SEM image, (c) EDS spectrum analysis and (d) Raman spectrum of PdSnCo/NG.
during the thermal reduction. From the discussion above, it can be concluded that the Pd-based alloys have been successfully loaded on the surface of NG [29]. TEM was an effective way to observe the structure and crystallinity of PdSnCo/NG (Fig. 3). It is suggested that the nanoparticles are spherical with a smooth surface, and the particle size is approximately 6–7 nm. Importantly, the nanoparticles are uniformly dispersed on the surface of NG, which may due to the addition of nitrogen into the graphene framework, and this is conducive to improved catalytic activity [30,31]. The SAED pattern shows clear spots and rings, indicating that the catalyst has a good degree of crystallinity (inset of Fig. 3c). Fig. 3d exhibits the crystal lattice with a d-spacing of 0.222 nm, which corresponds to the (111) planes of Pd. Compared to Pd, the lattice spacing of the Pd alloy has a slight deviation, which is due to the Sn and Co embedded in the lattice of Pd [32]. This matches well with the XRD analysis results. The XPS spectra of PdSnCo/NG are shown in Fig. 4. The survey scan result shows that the sample contains C, N, Pd, Co, and Sn elements. Fig. 4b shows the spectra with two peaks of Pd, which can be attributed to Pd3d5/2 (335.88 eV) and Pd3d3/2 (340.98 eV). As shown in Fig. 4b, the binding energies of Pd for the PdSnCo/NG, PdSn/NG and PdCo/NG were significantly different from Pd/NG. The shift occurs because Sn and Co are embedded in the Pd lattice, and there is a strong coupling effect between them. This type of electron interaction is beneficial to the electrochemical
Table 1 The XRD analysis of as-synthesized Pd/NG, PdSn/NG, PdCo/NG and PdSnCo/NG catalysts.
Fig. 2. XRD patterns of (a) PdSnCo/NG, PdCo/NG, PdSn/NG, and Pd/NG, (b) GO and NG.
Sample
Pd(111) at 2U/( )
d(111)/(Å)
Lattice parameter/(Å)
Pd/NG PdSn/NG PdCo/NG PdSnCo/NG
40.10 39.74 41.68 40.11
2.246 2.260 2.165 2.225
3.89 3.91 3.75 3.85
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Fig. 3. (a, b) TEM and (c, d) HRTEM images of PdSnCo/NG. The inset of c is the selected area electron diffraction (SAED) pattern.
Fig. 4. XPS spectra of (a) survey scan of PdSnCo/NG, (b) Pd3d, (c) C1s and (d) N1s.
performance [33]. Fig. 4c shows that the C C, C O, and C¼O bonds appear at 284.6, 286.6 and 288.1 eV, and the peak at 285 eV is attributed to sp2C bonded to N. Nitrogen in graphene mainly exists in three states: pyrrolic N (399.3 eV), pyridinic N (398.3 eV) and graphitic N (401 eV). The lone pair electrons of the N atoms form a conjugated system with the sp2 hybrid carbon skeleton, which indicates that N is replaced by C and incorporated into the
graphene framework. This can affect the charge distribution around the carbon atom and therefore, improve its electrochemical activity. Fig. 5a displays the CV plots of PdSnCo/NG in N2- and O2saturated 0.1 M KOH solutions. Compared to the absence of noticeable reduction features between 0.8-0.2 V in N2- saturated 0.1 M KOH solution, an obvious and strong reduction current peak
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Fig. 5. (a) CV curves of PdSnCo/NG in (a) O2- and (b) N2-saturated 0.1 M KOH solutions. (b) ORR polarization curves of different electrode materials.
can be seem in O2- saturated 0.1 M KOH solution, which indicates that PdSnCo/NG possesses good electrocatalytic activity. The oxygen reduction performance of the catalysts was studied by LSV at a scan rate of 50 mV s1 and a voltage range from 0.2 to 0.8 V. In general, the onset potential, half-wave potential and limiting current density are used to distinguish the ORR performance [34]. As shown in Fig. 5b, the half-wave potentials of the catalysts are PdSnCo/NG (0.20 V), Pd black (0.28 V), PdSn/NG (0.22 V), PdCo/NG (0.22 V), Pd/NG (0.215 V) and NG (0.37 V); the limiting current density values of the catalysts are PdSnCo/NG (3.9 mA cm2), Pd black (3.5 mA cm2), PdSn/NG (3.26 mA cm2), PdCo/NG (3.26 mA cm2), Pd/NG (3.5 mA cm2) and NG (2.25 m Acm2). Both the half-wave potentials for oxygen reduction and the evident limiting current density values follow the order: PdSnCo/NG > Pd/NG > PdSn/NG > PdCo/NG > Pd black > NG, which indicates that PdSnCo/NG had the best ORR electrocatalytic activity compared to the other catalysts. Compared to commercial 10% Pd black, the Pd content in PdSnCo/NG was decreased to 6%. There is a clear indication that the improved catalytic activity of PdSnCo/NG due to the presence of dopant Sn and Co atoms not only increased the utilization efficiency but also decreased the d-band center of Pd and thereby altered the electronic properties of the overall catalyst [35,36]. Another important parameter for characterizing the performance of ORR is the four-electron selectivity of the catalyst because the direct reduction of oxygen to water or OH by the fourelectron process is more efficient than the reduction of hydrogen peroxide via a two-electron process. RDE tests was carried out in O2-saturated 0.1 M KOH solution at a scan rate of 20 mVs1, and the number of electrons involved in the oxygen reduction reaction can be calculated from the Koutecky-Levich (K-L) equation [37]: 1 1 1 ¼ þ 0:5 J JK v
ð1Þ
B = 0.62nF(DO2)2/3v1/6CO2
(2)
where J is the current density, v is the rotating rate of the electrode, Jk is the kinetic current density, and the B value could be obtained from the K-L plots using Eq. (1). In addition, F in Eq. (2) is the Faraday constant (96485C mol1), DO2 is the diffusion coefficient of O2 (1.9 105 mol cm3), v is the kinetic viscosity (0.01 cm2 s1), and CO2 is the bulk concentration of O2 (1.2 106 mol cm3). As shown in Fig. 6a, the K-L plots of J1 vs. v1/2 at potentials of 0.4, 0.5, 0.6, 0.7, and 0.8 V display good linearity, which shows that the ORR on PdSnCo/NG follows first-order kinetics. Fig. 6b shows that the slopes from 0.4 to 0.8 V are 7.79, 7.78, 7.44, 7.03 and 6.95, respectively, and the corresponding n values are 3.62, 3.65, 3.79, 3.95 and 3.99, which suggests a direct reduction of O2 to H2O via the 4-electron pathway. In particular, the catalytic reaction mechanisms of oxygen catalysts for aqueous and nonaqueous electrolytes are different. Nevertheless, a significant number of previously reported results for catalytic performance characteristics of oxygen catalysts exhibit similarities in both types of electrolytes [38,39]. In addition, this association suggests that oxygen catalysts in aqueous systems could be extended to their application in the nonaqueous system of lithium air batteries [40]. The cathode electrodes in electrolyte solution (1 M LiTFSI/TEGDME) were studied. The stability of the ether electrolyte is much better than that of the carbonate electrolyte. And the high stability of the electrolyte (1 M LiTFSI/ TEGDME) led to its use in lithium air batteries [41,42]. Fig. 7a shows the initial discharge curves of the cathode electrodes including PdSnCo/NG, Pd/NG, NG and commercial Pd black catalysts, which were investigated at a current density of 0.3 mA cm2. The current and specific capacities are calculated based on the average weight of the cathode material under the same testing conditions. The
Fig. 6. (a) Rotating rate dependent ORR polarization curves with the scan rate of 20 mV s
1
; (b) K-L plots of J1 vs v1/2 at different potentials obtained from A.
X. Ren et al. / Electrochimica Acta 228 (2017) 36–44
41
Fig. 7. (a) The initial discharge profiles of lithium-air battery equipped with PdSnCo/NG, Pd/NG, NG, and commercial Pd black, (b) The initial discharge/charge curves of PdSnCo/NG, Pd/NG, Pd black, NG. All the tests were performed at a current density of 0.3 mA cm2.
discharge capacity of the PdSnCo/NG electrode was 6750 mAhg1, which is better than those of Pd/NG, Pd black and NG. There are two reasons for the increased capacity of PdSnCo/NG: NG as the supporting material increases the surface area, and the emergence of Co and Sn generates more oxygen vacancies in the catalyst. Fig. 7b shows the first discharge/charge voltage profiles of the nanocatalysts at a constant capacity of 500 mAh g1 and a current density of 0.3 mA cm2. The charge/discharge overpotentials were calculated from the discharge/charge average voltages [43]. The charge/discharge overpotentials of the electrodes were approximately 0.85 V for PdSnCo/NG, 1.08 V for Pd/NG, 1.37 V for Pd black and 1.5 V for the NG electrode. Therefore, both in the charge and discharge processes, the PdSnCo/NG showed the lowest electrode polarization. This can be attributed to the ternary alloy and the modified graphene composites. The polarization was decreased by introducing the bifunctional catalysts to improve the catalytic activity and conductivity. Pd displays excellent activity for the ORR and OER, and the addition of transition metals (Sn and Co), which decreased the d-band center of Pd and thereby altered the electronic properties of the overall catalyst, further promoted the OER [44]. It is confirmed that PdSnCo/NG exhibits the best redox activity, which is consistent with the results of the ORR analysis and some recent related results, as observed in Table 2. As shown in Fig. 8, the cycling performance was determined under the condition of a constant capacity of 500 mAh g1, a voltage range from 2 to 4.5 V and a current density of 0.3 mA cm2 [49]. As the number of cycles increased, the discharge platform gradually reduced, the charging platform gradually increased, and therefore, the overpotential also increased. When the voltage was greater than 4.5 V, the battery with the NG electrode only operated for 20 cycles. This number increased to 30 cycles after the addition of Pd, which reduced the electrochemical polarization phenomenon caused by the charge-discharge process. The incorporation of Co and Sn into Pd resulted in an increase of the cycle number of PdSnCo/NG to 60, which is 2.4 times larger than that of commercial black Pd. Therefore, NG not only acts as a supporting material for
nanoparticles but also increases the specific surface area and provides active sites. Sn and Co have a coordinated effect with Pd, which exhibits a synergetic effect on the electrochemical reactions. Therefore, it greatly reduces the electrochemical polarization phenomenon and improves the stability of the lithium-air battery [50]. The electrode materials were also investigated under the condition of a constant capacity of 1000 mAh g1, a voltage range from 2 to 4.5 V and a current density of 0.3 mA cm2 (Fig. S1). The batteries was cycled 5 times at 0.3 mA cm2 for comparison. PdSnCo/NG also exhibits the highest cycle performance (Fig. S2), and its discharge/charge voltage platform gap is the lowest. The charge voltages of the PdSnCo/NG and Pd/NG cathodes were lower than 4 V, and their discharge voltages were 2.75 V. This clearly shows that for a constant capacity of either 500 or 1000 mAh g1, the sample exhibited good electrochemical performance. Fig. 9 displays the continuously cycled discharge/charge curves of the nanoparticles cathode at a current density of 0.3 mA cm2, including the average discharge/charge voltages for 10 cycles. In addition, the discharge/charge profiles of all the catalyst cathodes are shown in Fig. 8. It is observed that the charge voltages of the PdSnCo/NG and Pd/NG cathodes were lower than 4 V, and their discharge voltages were 2.75 V. The charge voltages of the NG and Pd black electrode were close to 4 V, and the discharge voltages were 2.65 V and 2.63 V, respectively. It can be determined that nitrogen-doped graphene has catalytic properties, and the existence of Pd is beneficial to the oxygen reduction reaction and the oxygen release reaction of the electrode, thereby reducing the electrochemical polarization phenomenon during the charging and discharging processes [51,52]. Furthermore, after the formation of the composite material of the Pd nanocatalysts, the OER/ ORR activity of the PdSnCo/NG is remarkably increased, which includes a high capacity and good cycle stability. The improvement in the specific capacity and the reduction of the discharge/charge voltage platform gap were attributed to the synergistic effects when PdSnCo and NG were combined to form a composite catalyst. Based on the analysis above, the origin of the excellent
Table 2 Comparison of the lithium-air battery performance of PdSnCo/NG cathode with those of Pd-based cathodes reported in the literature. Materials
Pd content
Current Density
1st Discharge Capacity/mAh g1
Cycles/Fixed Capacity
Discharge/charge plateau
Ref.
ALD Pd/C Pd/5c-ZnO/C Pd/Co3O4/NF MGP-20 PdCu/C Pd/PNCNF PdNDS-GNP/GO This work
– 11.5% 20% 20% 21% 25% 40% 6%
100 mA g1
6500 – 1842.7 10000 13000 9405 – 6750
– 20/1000 mAh g1 70/300 mAh g1 163/1000 mAhg1 – 50/1000 mAh g1 30/500 mAh g1 60/500 or 1000 mAh g1
2.6 V/3.7 V 2.7 V/3.2 V 2.8 V/3.8 V 2.0 V/4.0 V 2.4 V/4.4 V 2.6 V/4.2 V 2.98 V/3.6 V 2.75 V/3.6 V
[45] [40] [46] [47] [44] [48] [49] –
2
0.05 mA cm 70 mA g1 0.12 mA cm2 100 mA g1 200 mA g1 0.3 mA cm2
42
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Fig. 8. The representative discharge and charge curves (at a current density of 0.3 mA cm2 and the limited specific capacity of 500 mAh g1) of (a) PdSnCo/NG, (b) Pd/NG, (c) Pd black, and (d) NG.
Fig. 9. Cycling performance of the lithium-air batteries with (a) PdSnCo/NG, (b) Pd/NG, (c) Pd black, and (d) NG electrodes.
electrochemical performance should be attributed to the following: (1) the incorporation of Co and Sn into Pd with a proper ratio; (2) more dispersed and small PdSnCo nanoparticles; and (3) the strong synergistic or coupling effect of the PdSnCo nanoparticles and NG. In addition, the rate performances of three electrodes were obtained (Fig. S3). The cycling performance of PdSnCo/NG was investigated under the condition of a constant capacity of 500 mAh g1 and at various current densities of 0.15 mA cm2, 0.3 mA cm2 and 0.6 mA cm2. When the test was carried out at 0.15 mA cm2 and 0.3 mA cm2, PdSnCo/NG delivered constant charge and discharge voltages of 3.75 V and 2.75 V. The charge voltage remained at 3.85 V at a current density of 0.6 mA cm2. This
suggests that PdSnCo/NG exhibited a good rate capability and stability. The good electrochemical performance is due to the synergistic effects between the PdSnCo nanoparticles and NG, which prevented the agglomeration of electrode materials [43]. Here, the PdSnCo/NG composites demonstrated not only better cycling stability but also higher reversible specific capacity and higher rate performance, resulting from the NG conductive network and the PdSnCo composite advantages [24]. 4. Conclusions In summary, PdSnCo/NG was successfully synthesized by a onepot solvothermal method. Compared to the binary PdCo/NG and
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PdSn/NG and commercial Pd black, the PdSnCo/NG nanocatalyst exhibits a high oxygen reduction activity and stability in alkaline medium. In addition, the PdSnCo/NG nanocatalyst shows a high discharge capacity and cycling performance for the lithium-air battery, which demonstrates tis potential as a catalyst in the air battery system. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 21671136 and 51502177), the Natural Science Foundation of Guangdong (No. 2015A030313542, 2014A030310323 and 2016A030313057). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2017.01.032. References [1] M. Sevim, T. Sener, O. Metin, Monodisperse MPd (M: Co, Ni, Cu) alloy nanoparticles supported on reduced graphene oxide as cathode catalysts for the lithium-air battery (vol 40, pg 10876, 2015), Int. J. Hydrogen Energy 41 (2016) 3318. [2] C.H. Ahn, R.S. Kalubarme, Y.H. Kim, K.N. Jung, K.H. Shin, C.J. Park, Graphene/ doped ceria nano-blend for catalytic oxygen reduction in non-aqueous lithium-oxygen batteries, Electrochim. Acta 117 (2014) 18–25. [3] W. Yan, W. Bian, C. Jin, J.H. Tian, R. Yang, An efficient bifunctional electrocatalyst based on strongly coupled CoFe2O4/Carbon nanotubes hybrid for oxygen reduction and oxygen evolution, Electrochim. Acta 177 (2015) 65–72. [4] W. Zhou, J. Li, H. Nie, Y. Zhang, X. Xi, H. Zhang, Carbon electrode for nonaqueous Li-O2 battery: the influence of surface oxygen species, Electrochim. Acta 138 (2014) 410–416. [5] M. He, P. Zhang, L. Liu, B. Liu, S. Xu, Hierarchical porous nitrogen doped threedimensional graphene as a free-standing cathode for rechargeable lithiumoxygen batteries, Electrochim. Acta 191 (2016) 90–97. [6] X. Lin, Y. Cao, S. Cai, J. Fan, Y. Li, Q.H. Wu, M. Zheng, Q. Dong, Ruthenium@mesoporous graphene-like carbon: a novel three-dimensional cathode catalyst for lithium-oxygen batteries, J. Mater. Chem. A 4 (2016) 7788–7794. [7] J. Liu, Z. Wang, J. Zhu, Binder-free nitrogen-doped carbon paper electrodes derived from polypyrrole/cellulose composite for Li-O2 batteries, J. Power Sources 306 (2016) 559–566. [8] J. Wang, Y. Li, X. Sun, Challenges and opportunities of nanostructured materials for aprotic rechargeable lithium-air batteries, Nano Energy 2 (2013) 443–467. [9] H. Lin, Z. Liu, Y. Mao, X. Liu, Y. Fang, Y. Liu, D. Wang, J. Xie, Effect of nitrogendoped carbon/Ketjenblack composite on the morphology of Li2O2 for highenergy-density Li-air batteries, Carbon 96 (2016) 965–971. [10] C. Kim, O. Gwon, I.Y. Jeon, Y. Kim, J. Shin, Y.W. Ju, J.B. Baek, G. Kim, Cloud-like graphene nanoplatelets on Nd0.5Sr0.5CoO3-delta nanorods as an efficient bifunctional electrocatalyst for hybrid Li-air batteries, J. Mater. Chem. A 4 (2016) 2122–2127. [11] Y.S. Jeong, J.B. Park, H.G. Jung, J. Kirn, X. Luo, J. Lu, L. Curtiss, K. Amine, Y.-K. Sun, B. Scrosati, Y.J. Lee, Study on the catalytic activity of noble metal nanoparticles on reduced graphene oxide for oxygen evolution reactions in lithium-air batteries, Nano Lett. 15 (2015) 4261–4268. [12] G. Jiang, H. Zhu, X. Zhang, B. Shen, L. Wu, S. Zhang, G. Lu, Z. Wu, S. Sun, Core/ shell face-centered tetragonal FePd/Pd nanoparticles as an efficient non-Pt catalyst for the oxygen reduction reaction, ACS Nano 9 (2015) 11014–11022. [13] Z.T. Liu, K.L. Huang, Y.S. Wu, Y.P. Lyu, C.L. Lee, A comparison of physically and chemically defective graphene nanosheets as catalyst supports for cubic Pd nanoparticles in an alkaline oxygen reduction reaction, Electrochim. Acta 186 (2015) 552–561. [14] Y. Li, W. Li, T. Ke, P. Zhang, X. Ren, L. Deng, Microwave-assisted synthesis of sulfur-doped graphene supported PdW nanoparticles as a high performance electrocatalyst for the oxygen reduction reaction, Electrochem. Commun. 69 (2016) 68–71. [15] Q. Zhen, G. Haoran, W. Xiaoguang, Z. Changchun, J. Hong, Z. Chi, X. Junling, Z. Zhonghua, Novel nanocrystalline PdNi alloy catalyst for methanol and ethanol electro-oxidation in alkaline media, J. Power Sources 196 (2011) 5823–5828. [16] Y. Wang, X. Wang, C.M. Li, Electrocatalysis of Pd-Co supported on carbon black or ball-milled carbon nanotubes towards methanol oxidation in alkaline media, Appl. Catal. B 99 (2010) 229–234. [17] J. Kim, T. Momma, T. Osaka, Synthesis of carbon-supported Pd-Sn catalyst by ultrasonic irradiation for oxygen reduction reaction, J. Power Sources 189 (2009) 909–915.
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