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FeNC catalyst modified graphene sponge as a cathode material for lithium-oxygen battery
Journal of Alloys and Compounds 595 (2014) 185–191
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
FeANAC catalyst modified graphene sponge as a cathode material for lithium-oxygen battery Ling Yu, Yue Shen ⇑, Yunhui Huang ⇑ State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
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Article history: Received 28 October 2013 Received in revised form 18 January 2014 Accepted 20 January 2014 Available online 28 January 2014 Keywords: Lithium-oxygen battery Graphene sponge Oxygen reduction reaction Oxygen evolution reaction
a b s t r a c t The cathode of a lithium-oxygen battery needs the synergism of a porous conducting material and a catalyst to facilitate the formation and decomposition of lithium peroxide. Here we introduce a graphene sponge (GS) modified with FeANAC catalyst for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). The porous, 3-dimensional conductive and free standing nature of the graphene sponge makes it become excellent skeleton of cathode for lithium-oxygen battery. The FeANAC catalyst nanoparticles dispersed uniformly on the graphene sheets show excellent catalytic reactivity in both discharge and charge processes. This kind of composite material greatly improves the capacity and cyclability of the lithium-oxygen battery. With dimethyl sulphoxide as electrolyte, the capacity reaches 6762 mAh g1 which is twice of the pure graphene sponge. In addition, the cell containing FeANAGS air electrode exhibits stable cyclic performance and effective reduction of charge potential plateau, indicating that FeANAGS is promising as an OER catalyst in rechargeable lithium-air batteries. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The last two decades have seen intense attention paid to lithium-ion batteries, which have been widely used in electric vehicles and plug-in hybrid vehicles. However, the power density and energy density of state-of-the-art lithium-ion batteries (LIBs) cannot meet the strict requirements of these high energy consumption devices even though comprehensive research has been conducted [1–5]. Therefore, in the last decade, several new kinds of energy storage devices have been extensively explored. Among them, the lithium-oxygen battery (LOB) has attracted much more attention owing to its extremely high theoretical specific energy density, far exceeding that of the highest performing conventional LIBs [6–8]. A LOB is composed of a lithium metal anode and a porous air cathode which provides channels for oxygen to pass through. The reversible discharge and charge processes occur based on the reaction of 2Li þ O2 Li2 O2 for non-aqueous LOBs. However, LOBs with organic electrolytes still face many formidable scientific and technical problems that limit the practical applications of now-established LOBs [9]. These technological hurdles are mainly derived from deficient kinetics of both the oxygen ⇑ Corresponding authors. Address: School of Materials Science and Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, Hubei, China. Tel./fax: +86 27 87558241. E-mail addresses: [email protected] (L. Yu), [email protected] (Y. Shen), [email protected] (Y. Huang). http://dx.doi.org/10.1016/j.jallcom.2014.01.148 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
reduction reaction (ORR) and oxygen evolution reaction (OER) [10–12]. Unlike proton-exchange membrane fuel cells (PEMFCs), in which the discharge product is dissolved in the electrolyte [13], a LOB involves catalyst–Li2O2–electrolyte–oxygen multiphase reactions during the discharge and charge processes. Considering the discharge product accommodation, oxygen diffusion and catalyst–reactant contact problems, the porous conducting framework and catalyst need to be carefully designed so that they work synergistically. An ideal LOB cathode catalyst should have many active catalytic sites intensively distributed over the carbon supports, with minimum separation between the sites, to attain maximum contact with the deposition of solids, such as Li2O2. The pore size of the carbon framework should be large enough to guarantee oxygen diffusion and Li2O2 accommodation, but small enough to make sure electrons can pass through the insulating Li2O2. Previously, advanced electrocatalysts such as MnO2, Co3O4, and perovskite La0.75Sr0.25MnO3 were studied [14–16]. However, mixing catalytic particles directly with the catalysts cannot guarantee close contact between the catalysts and the carbon support, which results in a capacity decrease after several cycles. Recently, a transition metalnitrogencarbon composite, which has been extensively researched in the area of PEMFCs [17–18] began to attract attention as a lithium-oxygen battery catalyst. Shui et al. [19] studied an FeANAC composite supported on carbon black as a cathode catalyst for nonaqueous LOBs. The research showed that Li-O2 batteries that use FeANAC as a catalyst exhibited excellent cyclic
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Fig. 1. Photo of the graphene sponge after being moistened with DMSO solvent.
performance (more than 50 cycles with excellent capacity retention). However, an appropriate carbon material is also crucial for the improvement of battery performance. A graphene sponge with unique three-dimensional (3D) structure, large surface area, high conductivity, and favorable flexibility is a good candidate [20– 23]. Additionally, the process to synthesize the graphene sponge materials has proven to be quite effective. Wei et al. [24] prepared a graphene sponge with pre-encapsulated Fe3O4 nanospheres through a simple hydrothermal process. The as-prepared material showed high conductivity, flexibility, and low density. In this work, we introduce a lithium-air cathode material, which consists of a 3D conductive graphene sponge framework and uniformly dispersed FeANAC catalyst nanoparticles. The synergetic effect between the porous structure of the graphene sponge and the ORR/OER
Fig. 3. FTIR spectrum of GS. No obvious signal characteristic of a carboxylic group can be observed at 1730 cm1 (indicated by the red ring), indicating considerable deoxygenation of GS. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
catalytic activity of the FeANAC catalyst greatly improves the capacity and cyclability of the lithium-oxygen battery.
2. Experimental section 2.1. Synthesis of graphene oxide (GGO) Graphite flakes with average diameter of 500 lm (Qingdao Henglide Graphite Co., Ltd.) were used to synthesize GGO sheets based on a modified process reported by Xu et al. [25]. Firstly, 5 g natural graphite flakes, 150 mL 98% sulfuric acid and
Fig. 2. SEM images of (a) as-prepared GS and (b) FeANAGS composite, (c) nitrogen adsorption–desorption isotherms and BJH pore distribution (inset) of pure GS, and (d) nitrogen adsorption–desorption isotherms and BJH pore distribution (inset) of FeANAGS.
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Fig. 4. Raman spectra for (a) graphene sponge without heat treatment at 1000 °C and (b) FeANAGS composites.
50 mL fuming nitric acid were successively added into a 500 mL flask, followed by being stirred for 24 h at room temperature (RT). The mixture was then poured slowly into 1 L distilled water. The solid was collected by filtration after the product temperature decreased to RT. Then the solid was washed with distilled water for several times. After drying at 60 °C for 24 h, the graphite intercalated compound (GIC) was attained. Then the dry GIC powder was thermally expanded at 1000 °C for 10 s to get expanded graphite (EG). Secondly, 5 g EG powder and 300 mL sulfuric acid were added into a 500 mL flake and maintained at 60 °C for 18 h under vigorous stir. Then 4.2 g K2S2O8 and 6.2 g P2O5 were added slowly into the mixture and kept at 80 °C for 12 h until it changed into starchiness. After cooling down to room temperature, the mixture was diluted with 2 L water and washed with hydrochloric acid (2 mol/L) and distilled water for several times until the product became a neutral slurry. The GGO sheets were obtained. 2.2. Preparation of graphene sponge (GS) sheet
Fig. 5. XRD patterns of FePc, graphene, graphene/FePc and FeANAC/graphene composite.
To prepare GS, homogeneous GGO (4–5 mg/mL) suspension and pyrrole in volume ratio of 95:5 were premixed by vigorous stir. Then the mixture was transferred into a Teflon-lined autoclave that contains a mould to produce 250 lm thick slide. After that, a hydrothermal treatment was conducted at 180 °C for 12 h to obtain the graphene hydrogel slides. The hydrogel was heated in ammonia solution (14 vol%)
Fig. 6. (a) TEM image of graphene without catalyst, (b) TEM image of GSAFePc composite, (c) TEM image of FeANAGS composite obtained by heat treatment of GSAFePc at 450 °C for 3 h in Ar/H2 atmosphere, and (d and e) HR-TEM image of FePc and FeANAGS composite obtained by the same heat treatment process.
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Fig. 7. (a) XPS of GS and FeANAGS recorded between 270 and 730 eV and (b) XPS N 1s binding energy region of FeANAGS sample.
Fig. 8. (a) SEM image of the discharge products, (b) XRD pattern of the cathode materials after discharge, (c) the cycle performance of the battery using pure graphene as cathode with fully discharge–charge and (d) the cycle performance of the battery using FeANAGS as cathode with fully discharge–charge. at 90 °C for 1 h in a sealed vessel to relieve structural damage during the following freeze drying process [26]. Then the previous product was heated at 1000 °C for 3 h in an inert atmosphere.
previous mixture. After that, the wet GS was dried in a vacuum oven at 90 °C for 12 h. The FeANAGS with a typical loading of 0.4–0.5 mg/cm2 was obtained by thermolysis of dry FePcAGS at 450 °C for 3 h in an Ar/H2 atmosphere.
2.3. Preparation of FeANAC catalyst and GS composite
2.4. Materials characterization
FeANAC catalyst supported on GS was synthesized via a rapid pyrolysis process of iron phthalocyanine (FePc) over the graphene sheets. In detail, 0.2273 g FePc and 0.0632 g pyridine was dissolved into 10 mL N,N-dimethylformamide to form a homogeneous solution. Then the graphene sponge sheets were soaked into the
The general morphologies of the products were characterized by field-emission scanning electron microscopy (FE-SEM, FEI, Sirion 200) coupled with an EDAX spectrometer. Transmission electron microscopy (TEM, Tecnai G2 20) was utilized to observe morphology and structure of the cathode materials. High-resolution TEM
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(HR-TEM) was conducted to study the crystallinity of the FeANAC composites. Nitrogen adsorption and desorption isotherms were determined by nitrogen physisorption at 77 K on a Micrometritics ASAP 2020 analyzer. X-ray diffraction (XRD) was carried out on a X’Pert PRO (PANalytical B.V.) diffractometer using Cu Ka radiation. Fourier transform infrared spectroscopy (FTIR, VERTEX 70) measurement was applied to determine the functional group and chemical bonds of GS and FeANAGS composites. Raman spectra were recorded at ambient temperature on a LabRAM HR800 spectroscopy with a 532 nm wavelength laser. X-ray photoelectron spectroscopy (XPS) data was obtained using a VG Multilab 2000 XPS system with a monochromatic Al X-ray source.
2.5. Li-air cell construction and electrochemical characterization The cell consisted of a lithium metal anode, a Celgard-3501 membrane with a thickness of 20 lm as a separator, and a 250 lm thick graphene sponge slide as a binder-free air electrode. 0.1 mol/L LiTFSI in DMSO solvent was used as electrolyte. The cell construction was a button-like design with some pores on the cathode shell for oxygen transmission. The cell was assembled in an argon-filled glove box with <1 ppm O2 and moisture content. Discharge–charge capacity measurement of the cell assembled was conducted in pure oxygen atmosphere at room temperature using a LAND CT2001A battery test system with a cut-off voltage range of 2.0–4.5 V. Cycling performance was tested with a controlled discharge–charge depth at a current density of 0.1 mA/ cm2. Both the current densities and capacities of electrodes were calculated based on the total mass of each electrode material.
3. Results and discussion 3.1. Photograph and structure of the FeANAGS Fig. 1 is the macroscopic photograph of the graphene sponge slice wet with DMSO solvent. It shows a remarkable wettability and an available flexibility which are of great importance as a binder-free cathode. The morphologies of the as-prepared GS and FeANAGS were observed by SEM (shown in Fig. 2a and b). The as-prepared GS and FeANAGS consist of almost transparent wrinkle-like thin nanosheets, which intersect with each other and form the flexible graphene sponge. For comparison, the FeANAGS composite does not show obvious difference from GS on morphologies. To examine the surface area of GS, nitrogen adsorption–desorption isotherms were measured (Fig. 2c and d). The specific Brunauer– Emmett–Teller (BET) surface areas of GS and FeANAGS are 60.662 m2/g and 50.243 m2/g, respectively. The results show that both of the two surface areas are not very high due to the fact that the graphene nanosheets may adhere to each other during the process of hydrothermal treatment. The The BET surface area of FeANAGS is comparable to that of pure GS, indicating the structure preservation after the combination of FeANAC catalyst with GS. To estimate the quality of the GS obtained, we carried out FTIR measurements (Fig. 3). Peaks appeared at 1212, 1547 and 3428 cm1 correspond to CAO, C@C, OAH bond accordingly, revealing that residual oxygen-containing functional groups still exist on the graphene sheets. However, no obvious band arising from a carboxylic group at 1730 cm1 can be observed, indicating the effective deoxygenation and partial restoration of the carbon network of during the graphitization process [27]. The representative Raman spectra (Fig. 4) for the freezing-dried graphene and FeANAGS give two strong major bands, i.e., the Dband at around 1350 cm1 and G-band at around 1580 cm1, to determine the structural confinement. The D band is ascribed to edges, other defects and disordered carbon, while the G-band corresponds to ordered sp2-bonded carbon atoms [28]. The ratio of the intensity of the D-band to that of the G-band indicates the degree of disorder from graphitic structure. It is obvious that the heat treated graphene sponge has a relatively decreased ID/IG of 1.03 relative to 1.16 in the graphene sponge before heat treatment at 1000 °C. The obvious decrease in ID/IG confirms that the FeANAGS composites show a better crystallinity compared to the untreated graphene sponge.
Fig. 9. The cycle performance of the lithium-oxygen batteries at controlled discharge–charge depth. (a) Cycle performance of the battery with pure GS as cathode, (b) cycle performance of the battery with FeANAGS as cathode and (c) the discharge and charge end potential at different cycles.
Fig. 5 displays the X-ray diffraction patterns of GS, FePc, FePc/GS hybrid and FeANAC modified GS which was obtained by heat treatment of GSAFePc at 450 °C in Ar/H2 atmosphere. Peaks for FePc are proved to be in accordance with the reference code (JCPDS No. 014-0926) of FePc. For GS, a broad peak appears at 26.3°, implying the break of the interplanar carbon bonds of the pristine graphite and the formation of graphene nanosheets [29]. XRD pattern for GSAFePc hybrid is the combination of the previous two XRD patterns, demonstrating that FePc has been integrated into
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the graphene sheets. After heat treatment, FePc was pyrolyzed into FeANAC, meanwhile GS-FePc converted to FeANAGS. The XRD pattern of FeANAGS shows a single peak located at 26.3°, demonstrating the total conversion of FePc into FeANAC catalyst. In addition, the inexistence of other peaks implies the amorphous structure of FeANAC catalyst. TEM and HRTEM images provide more insights into the composition and microstructure of the products. Fig. 6a presents the typical bright-field TEM image for the GS. It shows very smooth surface and resembled crumpled silk veil waves. Fig. 6b shows TEM image of the GSAFePc composite. On the graphene nanosheets, the FePc flakes are overlapped and aggregated to form big clusters with size of 10–40 nm. These clusters distribute uniformly onto the graphene nanosheets. The TEM image of FeANAGS composite (Fig. 6c) shows an even distribution of FeANAC originated from pyrolysis of FePc over graphene nanosheets at 450 °C for 3 h; In addition, the FeANAC clusters are slightly smaller compared with FePc clusters. Additional high-resolution TEM images of the FeANAC from heat treatment of pure FePc and GSAFePc are presented in Fig. 6d and e. They clearly demonstrate that these iron-rich particles are amorphous. From the morphology of FeANAGS composites, it can be concluded that most FeANAC clusters are closely anchored on the graphene nanosheets and contact intimately with graphene nanosheet matrix. Compared with conventional air electrode obtained by simple mechanically mixing of carbon sources with catalysts, the FeANAGS electrode possesses a much larger contact area between carbon sources and catalysts, which can decrease the inner resistance of the electrode and promote electron transportation to/from active sites during discharge/charge process. XPS of the FeANAGS sample can provide further evidence for the incorporation of Fe and N into the graphene sheets upon annealing. Fig. 7a shows the XPS of as-prepared GS and FeANAGS recorded between 270 and 730 eV, which reveals the presence of carbon (C 1s, 284.6 eV), oxygen (O 1s, 531 eV), nitrogen (N 1s, 401 eV) and ion (Fe 2p, 711 eV). Based on quantitative analysis of the XPS data of FeANAGS, the atomic percentages of O, N and Fe relative to C of the FeANAGS sample were determined to be 5.8%, 3.0% and 0.8%, respectively. The XPS N 1s spectrum reveals the presence of pyridinic N (398.4 eV), iron-associated pyridinic N (399.3 eV), quaternary N (401 eV) and graphitic N (403.3 eV). The N 1s XPS (Fig. 7b) analysis shows that 21.9% of the N atoms are incorporated with Fe atoms (calculated from the peak area of the FeApyr N). The ratio of Fe incorporated nitrogen is comparable to the FeANAC composite reported by Byon et al. [30]. Besides, the LOBs using FeANAGS as cathode show a high specific capacity,
good cyclic performance and reduction of charge potential, demonstrating that FeANAC catalyst using FePc as precursor has effective OER catalytic performance. Other than that, the pyrolysis of FePc seems to be a rapid and facile route to synthesize the FeANAC catalyst. 3.2. Electrochemical performance The actual catalytic effect of FeANAC in promoting cathodic reactions in Li-O2 battery is somewhat controversial at present. For the electrochemical performance of LOBs, the selectivity of electrolyte is a significant factor to affect the charge and discharge process. In our work, we applied dimethyl sulphoxide (DMSO) with 0.1 mol/L LiTFSI dissolved in as electrolyte since it was proved to have low viscosity and good chemical stability. The catalytic activity of FeANAGS composite was investigated through comparison with pure GS without FeANAC catalyst inserted in. The cells were tested in pure oxygen and the discharge and charge current were set to be 0.04 mA so that the cells were discharged with a constant current density of 0.1 mA/cm2. The discharge product on the cathode was investigated with SEM (Fig. 8a) and XRD (Fig. 8b). The SEM image indicates that the discharge products are uniformly grown on the surface of the graphene nanosheets. The diffraction pattern of the discharge product after first discharge shows peaks located at 32.8°, 34.8°, 41°, 47°, 48.7°, 58.5° and 63.7°, demonstrating a pure phase of Li2O2. However, LiOHH2O also exists after 25 cycles, indicating the participation of H2O molecules during the discharge and charge process. Fig. 8c and d gives a detailed charge and discharge performance of both GS and FeANAGS using as cathode materials for LOBs. For the pure GS electrode, the initial average discharge voltage plateau is 2.68 V along with the charge voltage plateau to be 4.35 V, and the discharge capacity is 3474 mAh/g. This value is much higher than that of Super P [31]. A possible reason is that GS has numerous defects on the graphene sheets which can act as active sites for charge/discharge reaction. In contrast, with FeANAC catalyst on the graphene sheets, the discharge voltage plateau remained to be 2.66 V. However, the initial charge voltage plateau converts to 3.9 V and at the same time the discharge capacity increases up to 6762 mAh/g, showing a non-negligible improvement in electrochemical performance. The cyclic stability of the batteries were tested with a discharge–charge depth controlled to 500 mAh/g. Fig. 9 shows that the cyclability of LOBs is much improved with FeANAC as the electrocatalyst. The charge end potential of the cell without the catalyst increases to 4.52 V after only 20 cycles. In contrast, the cell with the FeANAC catalyst has a charge end potential plateau of
Fig. 10. The possible mechanism of the catalyzed OER process.
L. Yu et al. / Journal of Alloys and Compounds 595 (2014) 185–191
4.37 V after 50 cycles, indicating the significant catalytic properties of FeANAC catalyst for the oxygen evolution reaction. There are two charge potential plateaus after 25 cycles, corresponding to the oxidation of Li2O2 and LiOHH2O, respectively. The rise in the charge potential plateau after 20 cycles can be attributed to the accumulation of solid-state discharge products, which can lead to poor contact between discharge products and catalyst. The exact mechanism of the catalyzed OER is difficult to determine, but it is generally believed that the [FeA4N] planar coordination structure in the FePc molecules would somehow be maintained after the carbonization. The Fe atom in the [FeA4N] structure has two empty orbitals, which may coordinate with the oxygen atom in the LiOOLi molecule. This kind of coordination may help the oxidation of the LiOOLi without breaking the OAO bond (shown in Fig. 10). Regarding the discharge processes, the discharge end potential of the cell with the FeANAGS cathode remained at 2.65 V even after 50 cycles while the cell without the catalyst delivers a discharge end potential of 2.03 V at the end of the 20th cycle, in a way demonstrating the synchronous ORR catalytic property of the FeANAC catalyst. 4. Conclusions The graphene sponge obtained from the hydrothermal process is an excellent cathode skeleton for LIBs due to its highly porous, ultra-light, conductive, and freestanding nature. Simple pyrolysis of FePc absorbed in the sponge can result in a FeANAGS composite cathode with FeANAC catalyst nanoparticles uniformly dispersed on the graphene sheets. The synergy of the FeANAC catalyst and the graphene sponge skeleton gives the air cathode good electrochemical performance. A high discharge capacity of 6762 mAh/g based on the total mass of electrode and low charge voltage of 3.8 V was observed. In addition, an enhanced battery lifespan under controlled cycling, with 50 discharge–charge cycles was also achieved. Our results indicate that optimizing the structure of the carbon cathode skeleton and the catalytic activity of the catalyst is a promising strategy to improve the capacity and cyclability of Li-oxygen batteries. Acknowledgments This work was supported by the China Postdoctoral Science Foundation (2012M510178 and 2013T60716) and Natural Science Foundation of China (51202076 and 20825520). The authors thank Analytical and Testing Center of HUST for XRD and SEM measurements.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
FeANAC catalyst modified graphene sponge as a cathode material for lithium-oxygen battery Ling Yu, Yue Shen ⇑, Yunhui Huang ⇑ State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
a r t i c l e
i n f o
Article history: Received 28 October 2013 Received in revised form 18 January 2014 Accepted 20 January 2014 Available online 28 January 2014 Keywords: Lithium-oxygen battery Graphene sponge Oxygen reduction reaction Oxygen evolution reaction
a b s t r a c t The cathode of a lithium-oxygen battery needs the synergism of a porous conducting material and a catalyst to facilitate the formation and decomposition of lithium peroxide. Here we introduce a graphene sponge (GS) modified with FeANAC catalyst for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). The porous, 3-dimensional conductive and free standing nature of the graphene sponge makes it become excellent skeleton of cathode for lithium-oxygen battery. The FeANAC catalyst nanoparticles dispersed uniformly on the graphene sheets show excellent catalytic reactivity in both discharge and charge processes. This kind of composite material greatly improves the capacity and cyclability of the lithium-oxygen battery. With dimethyl sulphoxide as electrolyte, the capacity reaches 6762 mAh g1 which is twice of the pure graphene sponge. In addition, the cell containing FeANAGS air electrode exhibits stable cyclic performance and effective reduction of charge potential plateau, indicating that FeANAGS is promising as an OER catalyst in rechargeable lithium-air batteries. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The last two decades have seen intense attention paid to lithium-ion batteries, which have been widely used in electric vehicles and plug-in hybrid vehicles. However, the power density and energy density of state-of-the-art lithium-ion batteries (LIBs) cannot meet the strict requirements of these high energy consumption devices even though comprehensive research has been conducted [1–5]. Therefore, in the last decade, several new kinds of energy storage devices have been extensively explored. Among them, the lithium-oxygen battery (LOB) has attracted much more attention owing to its extremely high theoretical specific energy density, far exceeding that of the highest performing conventional LIBs [6–8]. A LOB is composed of a lithium metal anode and a porous air cathode which provides channels for oxygen to pass through. The reversible discharge and charge processes occur based on the reaction of 2Li þ O2 Li2 O2 for non-aqueous LOBs. However, LOBs with organic electrolytes still face many formidable scientific and technical problems that limit the practical applications of now-established LOBs [9]. These technological hurdles are mainly derived from deficient kinetics of both the oxygen ⇑ Corresponding authors. Address: School of Materials Science and Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, Hubei, China. Tel./fax: +86 27 87558241. E-mail addresses: [email protected] (L. Yu), [email protected] (Y. Shen), [email protected] (Y. Huang). http://dx.doi.org/10.1016/j.jallcom.2014.01.148 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
reduction reaction (ORR) and oxygen evolution reaction (OER) [10–12]. Unlike proton-exchange membrane fuel cells (PEMFCs), in which the discharge product is dissolved in the electrolyte [13], a LOB involves catalyst–Li2O2–electrolyte–oxygen multiphase reactions during the discharge and charge processes. Considering the discharge product accommodation, oxygen diffusion and catalyst–reactant contact problems, the porous conducting framework and catalyst need to be carefully designed so that they work synergistically. An ideal LOB cathode catalyst should have many active catalytic sites intensively distributed over the carbon supports, with minimum separation between the sites, to attain maximum contact with the deposition of solids, such as Li2O2. The pore size of the carbon framework should be large enough to guarantee oxygen diffusion and Li2O2 accommodation, but small enough to make sure electrons can pass through the insulating Li2O2. Previously, advanced electrocatalysts such as MnO2, Co3O4, and perovskite La0.75Sr0.25MnO3 were studied [14–16]. However, mixing catalytic particles directly with the catalysts cannot guarantee close contact between the catalysts and the carbon support, which results in a capacity decrease after several cycles. Recently, a transition metalnitrogencarbon composite, which has been extensively researched in the area of PEMFCs [17–18] began to attract attention as a lithium-oxygen battery catalyst. Shui et al. [19] studied an FeANAC composite supported on carbon black as a cathode catalyst for nonaqueous LOBs. The research showed that Li-O2 batteries that use FeANAC as a catalyst exhibited excellent cyclic
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Fig. 1. Photo of the graphene sponge after being moistened with DMSO solvent.
performance (more than 50 cycles with excellent capacity retention). However, an appropriate carbon material is also crucial for the improvement of battery performance. A graphene sponge with unique three-dimensional (3D) structure, large surface area, high conductivity, and favorable flexibility is a good candidate [20– 23]. Additionally, the process to synthesize the graphene sponge materials has proven to be quite effective. Wei et al. [24] prepared a graphene sponge with pre-encapsulated Fe3O4 nanospheres through a simple hydrothermal process. The as-prepared material showed high conductivity, flexibility, and low density. In this work, we introduce a lithium-air cathode material, which consists of a 3D conductive graphene sponge framework and uniformly dispersed FeANAC catalyst nanoparticles. The synergetic effect between the porous structure of the graphene sponge and the ORR/OER
Fig. 3. FTIR spectrum of GS. No obvious signal characteristic of a carboxylic group can be observed at 1730 cm1 (indicated by the red ring), indicating considerable deoxygenation of GS. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
catalytic activity of the FeANAC catalyst greatly improves the capacity and cyclability of the lithium-oxygen battery.
2. Experimental section 2.1. Synthesis of graphene oxide (GGO) Graphite flakes with average diameter of 500 lm (Qingdao Henglide Graphite Co., Ltd.) were used to synthesize GGO sheets based on a modified process reported by Xu et al. [25]. Firstly, 5 g natural graphite flakes, 150 mL 98% sulfuric acid and
Fig. 2. SEM images of (a) as-prepared GS and (b) FeANAGS composite, (c) nitrogen adsorption–desorption isotherms and BJH pore distribution (inset) of pure GS, and (d) nitrogen adsorption–desorption isotherms and BJH pore distribution (inset) of FeANAGS.
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Fig. 4. Raman spectra for (a) graphene sponge without heat treatment at 1000 °C and (b) FeANAGS composites.
50 mL fuming nitric acid were successively added into a 500 mL flask, followed by being stirred for 24 h at room temperature (RT). The mixture was then poured slowly into 1 L distilled water. The solid was collected by filtration after the product temperature decreased to RT. Then the solid was washed with distilled water for several times. After drying at 60 °C for 24 h, the graphite intercalated compound (GIC) was attained. Then the dry GIC powder was thermally expanded at 1000 °C for 10 s to get expanded graphite (EG). Secondly, 5 g EG powder and 300 mL sulfuric acid were added into a 500 mL flake and maintained at 60 °C for 18 h under vigorous stir. Then 4.2 g K2S2O8 and 6.2 g P2O5 were added slowly into the mixture and kept at 80 °C for 12 h until it changed into starchiness. After cooling down to room temperature, the mixture was diluted with 2 L water and washed with hydrochloric acid (2 mol/L) and distilled water for several times until the product became a neutral slurry. The GGO sheets were obtained. 2.2. Preparation of graphene sponge (GS) sheet
Fig. 5. XRD patterns of FePc, graphene, graphene/FePc and FeANAC/graphene composite.
To prepare GS, homogeneous GGO (4–5 mg/mL) suspension and pyrrole in volume ratio of 95:5 were premixed by vigorous stir. Then the mixture was transferred into a Teflon-lined autoclave that contains a mould to produce 250 lm thick slide. After that, a hydrothermal treatment was conducted at 180 °C for 12 h to obtain the graphene hydrogel slides. The hydrogel was heated in ammonia solution (14 vol%)
Fig. 6. (a) TEM image of graphene without catalyst, (b) TEM image of GSAFePc composite, (c) TEM image of FeANAGS composite obtained by heat treatment of GSAFePc at 450 °C for 3 h in Ar/H2 atmosphere, and (d and e) HR-TEM image of FePc and FeANAGS composite obtained by the same heat treatment process.
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Fig. 7. (a) XPS of GS and FeANAGS recorded between 270 and 730 eV and (b) XPS N 1s binding energy region of FeANAGS sample.
Fig. 8. (a) SEM image of the discharge products, (b) XRD pattern of the cathode materials after discharge, (c) the cycle performance of the battery using pure graphene as cathode with fully discharge–charge and (d) the cycle performance of the battery using FeANAGS as cathode with fully discharge–charge. at 90 °C for 1 h in a sealed vessel to relieve structural damage during the following freeze drying process [26]. Then the previous product was heated at 1000 °C for 3 h in an inert atmosphere.
previous mixture. After that, the wet GS was dried in a vacuum oven at 90 °C for 12 h. The FeANAGS with a typical loading of 0.4–0.5 mg/cm2 was obtained by thermolysis of dry FePcAGS at 450 °C for 3 h in an Ar/H2 atmosphere.
2.3. Preparation of FeANAC catalyst and GS composite
2.4. Materials characterization
FeANAC catalyst supported on GS was synthesized via a rapid pyrolysis process of iron phthalocyanine (FePc) over the graphene sheets. In detail, 0.2273 g FePc and 0.0632 g pyridine was dissolved into 10 mL N,N-dimethylformamide to form a homogeneous solution. Then the graphene sponge sheets were soaked into the
The general morphologies of the products were characterized by field-emission scanning electron microscopy (FE-SEM, FEI, Sirion 200) coupled with an EDAX spectrometer. Transmission electron microscopy (TEM, Tecnai G2 20) was utilized to observe morphology and structure of the cathode materials. High-resolution TEM
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(HR-TEM) was conducted to study the crystallinity of the FeANAC composites. Nitrogen adsorption and desorption isotherms were determined by nitrogen physisorption at 77 K on a Micrometritics ASAP 2020 analyzer. X-ray diffraction (XRD) was carried out on a X’Pert PRO (PANalytical B.V.) diffractometer using Cu Ka radiation. Fourier transform infrared spectroscopy (FTIR, VERTEX 70) measurement was applied to determine the functional group and chemical bonds of GS and FeANAGS composites. Raman spectra were recorded at ambient temperature on a LabRAM HR800 spectroscopy with a 532 nm wavelength laser. X-ray photoelectron spectroscopy (XPS) data was obtained using a VG Multilab 2000 XPS system with a monochromatic Al X-ray source.
2.5. Li-air cell construction and electrochemical characterization The cell consisted of a lithium metal anode, a Celgard-3501 membrane with a thickness of 20 lm as a separator, and a 250 lm thick graphene sponge slide as a binder-free air electrode. 0.1 mol/L LiTFSI in DMSO solvent was used as electrolyte. The cell construction was a button-like design with some pores on the cathode shell for oxygen transmission. The cell was assembled in an argon-filled glove box with <1 ppm O2 and moisture content. Discharge–charge capacity measurement of the cell assembled was conducted in pure oxygen atmosphere at room temperature using a LAND CT2001A battery test system with a cut-off voltage range of 2.0–4.5 V. Cycling performance was tested with a controlled discharge–charge depth at a current density of 0.1 mA/ cm2. Both the current densities and capacities of electrodes were calculated based on the total mass of each electrode material.
3. Results and discussion 3.1. Photograph and structure of the FeANAGS Fig. 1 is the macroscopic photograph of the graphene sponge slice wet with DMSO solvent. It shows a remarkable wettability and an available flexibility which are of great importance as a binder-free cathode. The morphologies of the as-prepared GS and FeANAGS were observed by SEM (shown in Fig. 2a and b). The as-prepared GS and FeANAGS consist of almost transparent wrinkle-like thin nanosheets, which intersect with each other and form the flexible graphene sponge. For comparison, the FeANAGS composite does not show obvious difference from GS on morphologies. To examine the surface area of GS, nitrogen adsorption–desorption isotherms were measured (Fig. 2c and d). The specific Brunauer– Emmett–Teller (BET) surface areas of GS and FeANAGS are 60.662 m2/g and 50.243 m2/g, respectively. The results show that both of the two surface areas are not very high due to the fact that the graphene nanosheets may adhere to each other during the process of hydrothermal treatment. The The BET surface area of FeANAGS is comparable to that of pure GS, indicating the structure preservation after the combination of FeANAC catalyst with GS. To estimate the quality of the GS obtained, we carried out FTIR measurements (Fig. 3). Peaks appeared at 1212, 1547 and 3428 cm1 correspond to CAO, C@C, OAH bond accordingly, revealing that residual oxygen-containing functional groups still exist on the graphene sheets. However, no obvious band arising from a carboxylic group at 1730 cm1 can be observed, indicating the effective deoxygenation and partial restoration of the carbon network of during the graphitization process [27]. The representative Raman spectra (Fig. 4) for the freezing-dried graphene and FeANAGS give two strong major bands, i.e., the Dband at around 1350 cm1 and G-band at around 1580 cm1, to determine the structural confinement. The D band is ascribed to edges, other defects and disordered carbon, while the G-band corresponds to ordered sp2-bonded carbon atoms [28]. The ratio of the intensity of the D-band to that of the G-band indicates the degree of disorder from graphitic structure. It is obvious that the heat treated graphene sponge has a relatively decreased ID/IG of 1.03 relative to 1.16 in the graphene sponge before heat treatment at 1000 °C. The obvious decrease in ID/IG confirms that the FeANAGS composites show a better crystallinity compared to the untreated graphene sponge.
Fig. 9. The cycle performance of the lithium-oxygen batteries at controlled discharge–charge depth. (a) Cycle performance of the battery with pure GS as cathode, (b) cycle performance of the battery with FeANAGS as cathode and (c) the discharge and charge end potential at different cycles.
Fig. 5 displays the X-ray diffraction patterns of GS, FePc, FePc/GS hybrid and FeANAC modified GS which was obtained by heat treatment of GSAFePc at 450 °C in Ar/H2 atmosphere. Peaks for FePc are proved to be in accordance with the reference code (JCPDS No. 014-0926) of FePc. For GS, a broad peak appears at 26.3°, implying the break of the interplanar carbon bonds of the pristine graphite and the formation of graphene nanosheets [29]. XRD pattern for GSAFePc hybrid is the combination of the previous two XRD patterns, demonstrating that FePc has been integrated into
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the graphene sheets. After heat treatment, FePc was pyrolyzed into FeANAC, meanwhile GS-FePc converted to FeANAGS. The XRD pattern of FeANAGS shows a single peak located at 26.3°, demonstrating the total conversion of FePc into FeANAC catalyst. In addition, the inexistence of other peaks implies the amorphous structure of FeANAC catalyst. TEM and HRTEM images provide more insights into the composition and microstructure of the products. Fig. 6a presents the typical bright-field TEM image for the GS. It shows very smooth surface and resembled crumpled silk veil waves. Fig. 6b shows TEM image of the GSAFePc composite. On the graphene nanosheets, the FePc flakes are overlapped and aggregated to form big clusters with size of 10–40 nm. These clusters distribute uniformly onto the graphene nanosheets. The TEM image of FeANAGS composite (Fig. 6c) shows an even distribution of FeANAC originated from pyrolysis of FePc over graphene nanosheets at 450 °C for 3 h; In addition, the FeANAC clusters are slightly smaller compared with FePc clusters. Additional high-resolution TEM images of the FeANAC from heat treatment of pure FePc and GSAFePc are presented in Fig. 6d and e. They clearly demonstrate that these iron-rich particles are amorphous. From the morphology of FeANAGS composites, it can be concluded that most FeANAC clusters are closely anchored on the graphene nanosheets and contact intimately with graphene nanosheet matrix. Compared with conventional air electrode obtained by simple mechanically mixing of carbon sources with catalysts, the FeANAGS electrode possesses a much larger contact area between carbon sources and catalysts, which can decrease the inner resistance of the electrode and promote electron transportation to/from active sites during discharge/charge process. XPS of the FeANAGS sample can provide further evidence for the incorporation of Fe and N into the graphene sheets upon annealing. Fig. 7a shows the XPS of as-prepared GS and FeANAGS recorded between 270 and 730 eV, which reveals the presence of carbon (C 1s, 284.6 eV), oxygen (O 1s, 531 eV), nitrogen (N 1s, 401 eV) and ion (Fe 2p, 711 eV). Based on quantitative analysis of the XPS data of FeANAGS, the atomic percentages of O, N and Fe relative to C of the FeANAGS sample were determined to be 5.8%, 3.0% and 0.8%, respectively. The XPS N 1s spectrum reveals the presence of pyridinic N (398.4 eV), iron-associated pyridinic N (399.3 eV), quaternary N (401 eV) and graphitic N (403.3 eV). The N 1s XPS (Fig. 7b) analysis shows that 21.9% of the N atoms are incorporated with Fe atoms (calculated from the peak area of the FeApyr N). The ratio of Fe incorporated nitrogen is comparable to the FeANAC composite reported by Byon et al. [30]. Besides, the LOBs using FeANAGS as cathode show a high specific capacity,
good cyclic performance and reduction of charge potential, demonstrating that FeANAC catalyst using FePc as precursor has effective OER catalytic performance. Other than that, the pyrolysis of FePc seems to be a rapid and facile route to synthesize the FeANAC catalyst. 3.2. Electrochemical performance The actual catalytic effect of FeANAC in promoting cathodic reactions in Li-O2 battery is somewhat controversial at present. For the electrochemical performance of LOBs, the selectivity of electrolyte is a significant factor to affect the charge and discharge process. In our work, we applied dimethyl sulphoxide (DMSO) with 0.1 mol/L LiTFSI dissolved in as electrolyte since it was proved to have low viscosity and good chemical stability. The catalytic activity of FeANAGS composite was investigated through comparison with pure GS without FeANAC catalyst inserted in. The cells were tested in pure oxygen and the discharge and charge current were set to be 0.04 mA so that the cells were discharged with a constant current density of 0.1 mA/cm2. The discharge product on the cathode was investigated with SEM (Fig. 8a) and XRD (Fig. 8b). The SEM image indicates that the discharge products are uniformly grown on the surface of the graphene nanosheets. The diffraction pattern of the discharge product after first discharge shows peaks located at 32.8°, 34.8°, 41°, 47°, 48.7°, 58.5° and 63.7°, demonstrating a pure phase of Li2O2. However, LiOHH2O also exists after 25 cycles, indicating the participation of H2O molecules during the discharge and charge process. Fig. 8c and d gives a detailed charge and discharge performance of both GS and FeANAGS using as cathode materials for LOBs. For the pure GS electrode, the initial average discharge voltage plateau is 2.68 V along with the charge voltage plateau to be 4.35 V, and the discharge capacity is 3474 mAh/g. This value is much higher than that of Super P [31]. A possible reason is that GS has numerous defects on the graphene sheets which can act as active sites for charge/discharge reaction. In contrast, with FeANAC catalyst on the graphene sheets, the discharge voltage plateau remained to be 2.66 V. However, the initial charge voltage plateau converts to 3.9 V and at the same time the discharge capacity increases up to 6762 mAh/g, showing a non-negligible improvement in electrochemical performance. The cyclic stability of the batteries were tested with a discharge–charge depth controlled to 500 mAh/g. Fig. 9 shows that the cyclability of LOBs is much improved with FeANAC as the electrocatalyst. The charge end potential of the cell without the catalyst increases to 4.52 V after only 20 cycles. In contrast, the cell with the FeANAC catalyst has a charge end potential plateau of
Fig. 10. The possible mechanism of the catalyzed OER process.
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4.37 V after 50 cycles, indicating the significant catalytic properties of FeANAC catalyst for the oxygen evolution reaction. There are two charge potential plateaus after 25 cycles, corresponding to the oxidation of Li2O2 and LiOHH2O, respectively. The rise in the charge potential plateau after 20 cycles can be attributed to the accumulation of solid-state discharge products, which can lead to poor contact between discharge products and catalyst. The exact mechanism of the catalyzed OER is difficult to determine, but it is generally believed that the [FeA4N] planar coordination structure in the FePc molecules would somehow be maintained after the carbonization. The Fe atom in the [FeA4N] structure has two empty orbitals, which may coordinate with the oxygen atom in the LiOOLi molecule. This kind of coordination may help the oxidation of the LiOOLi without breaking the OAO bond (shown in Fig. 10). Regarding the discharge processes, the discharge end potential of the cell with the FeANAGS cathode remained at 2.65 V even after 50 cycles while the cell without the catalyst delivers a discharge end potential of 2.03 V at the end of the 20th cycle, in a way demonstrating the synchronous ORR catalytic property of the FeANAC catalyst. 4. Conclusions The graphene sponge obtained from the hydrothermal process is an excellent cathode skeleton for LIBs due to its highly porous, ultra-light, conductive, and freestanding nature. Simple pyrolysis of FePc absorbed in the sponge can result in a FeANAGS composite cathode with FeANAC catalyst nanoparticles uniformly dispersed on the graphene sheets. The synergy of the FeANAC catalyst and the graphene sponge skeleton gives the air cathode good electrochemical performance. A high discharge capacity of 6762 mAh/g based on the total mass of electrode and low charge voltage of 3.8 V was observed. In addition, an enhanced battery lifespan under controlled cycling, with 50 discharge–charge cycles was also achieved. Our results indicate that optimizing the structure of the carbon cathode skeleton and the catalytic activity of the catalyst is a promising strategy to improve the capacity and cyclability of Li-oxygen batteries. Acknowledgments This work was supported by the China Postdoctoral Science Foundation (2012M510178 and 2013T60716) and Natural Science Foundation of China (51202076 and 20825520). The authors thank Analytical and Testing Center of HUST for XRD and SEM measurements.
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