Nowadays, lithium ion battery (LIB) is widely used not only in mobile phones, laptops and other portable electronic products, but also in the electric vehicles and hybrid electric vehicles. LIBs have obtained commercial success in the field of portable electronic devices due to their high energy density, light weight and long lifetime [1-4]. However, LIBs or their packs often have the energy densities of no more than 250 Wh/kg, which cannot meet the requirement of a long-distance journey. Novel energy-storage devices are urgently needed to solve the problem and to meet the demand of high energy density and other electrochemical performance [5]. Rechargeable Li-O2 batteries used metallic lithium as their anode with a high theoretical specific capacity of 3862 mAh/g and the lowest electrochemical potential (-3.04 V vs. standard hydrogen electrode, SHE) [6], which contributes to a high energy density of up to 3500 Wh/kg [7]. Therefore, Li-O2 batteries are considered to be an extremely excellent development prospects for the electric vehicles.
In 1996, Abraham and Jiang [8] firstly reported a novel rechargeable Li-O2 battery in organic polymer electrolyte with lithium metal foil as anode and thin carbon composite on Ni current collector as the cathode, but it maintained a high charge– discharge efficiency and capacity only at the initial stage. In fact, the Li-O2 cells successfully attract world-wide attention in 2006. Ogasawara et al. [9] obtained a good electrochemical performance Li-O2 battery using carbonate-based electrolyte and MnO2 as the catalyst. Currently, there are four major cell systems for Li-O2 batteries. Their classification is based on their different electrolyte employed: aprotic [9, 10], aqueous [11], all-solid-state [12] and hybrid aqueous/aprotic [13] cells as shown in Scheme 1 [14]. "Non-aqueous" is exclusively refers to Li-O2 battery electrolytes that composed of organic solvents and salts. The structure of non-aqueous Li-O2 batteries is relatively simpler than other Li-O2 batteries' structures, which conforms to the classic rocking chair type structure and easily possesses the highest theory energy density and the most stable system.
Non-aqueous Li-O2 batteries are usually made of air cathodes, organic electrolyte and lithium metal anode. The electrochemical reaction principles of Li-O2 batteries are different from LIBs. In the discharge process of Li-O2 batteries, lithium metal anode loses the electrons to become lithium ion, and oxygen in the air cathode obtains electrons and combines with the lithium ion to form Li2O2 or Li2O (Eqs. (1) and (2)). Reverse reaction occurs when Li-O2 battery charge. Overall reaction of Li-O2 battery expresses by the following equations:
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Experiments showed that superoxide radicals generated in the process of discharging in Li-O2 batteries [11, 15]. For example, Peng et al. [15] took 0.1 mol/L LiTFSA/CH3CN as the electrolyte and confirmed that LiO2 is the intermediate production by surface enhanced Raman spectroscopy. Laoire et al. [11] took 0.1 mol/L LiPF6/DMSO as the electrolyte and discovered three oxidation peaks by cyclic voltammetry (CVs). They kept the view that oxygen reduction reaction (ORR) of Li-O2 batteries could use the following formulas (Eqs. (3)–(6)):
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The oxygen evolution reaction (OER) was shown as below (Eqs. (7)–(9)):
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But the mechanism is still in dispute. McCloskey et al. [16] found only two electronic oxygen reductions in the discharge processes of Li-O2 batteries with 0.1 mol/L LiTFSA/DME electrolyte through the differential electrochemical mass spectrometry. There is no doubt that the processes of electrochemical reactions are complex because of active superoxide radical in Li-O2 batteries. Further understanding of the reaction mechanism for Li-O2 batteries plays a vital role in improving their performances.
The electrochemical reaction mechanism of Li-O2 batteries shows a catalyst is needed to accelerate the generation and decomposition of Li2O2. The capacity of Li-O2 batteries is directly associated with the quality of the cathode materials. The discharge products were stored in the porous cathode, which decreased the interface between the cathode catalysts and oxygen. In this case, the reduced electronic conductivity of the cathode made the reaction terminated in the end. The high over-potentials from both ORR (η discharge) and OER (η charge) also primarily caused by the poor electrical conductivity and difficult oxidation of Li2O2. It significantly diminished the capacity and efficiency of Li-O2 batteries [17]. The critical strategy is to construct novel frameworks of high-efficient catalysts, which are very important to the efficiencies of Li-O2 batteries [18].
Therefore, we summarized the research progress of cathode electrocatalysts in non-aqueous Li-O2 battery including catalysts category, architectures and electrochemical performance in recent years. Moreover, this review also pointed out the challenges of the cathode materials and the main research direction in the future.
Electrode catalysts are often considered in several aspects: catalytic active sites, the transport channels for oxygen diffusion and electrons transport. Enough space is also required for discharge product storage. Currently, non-aqueous Li-O2 battery of the catalyst can be roughly divided to noble metals, transition metal oxides, nano-carbon materials, nano-carbon hybrid materials and other catalyst materials.
Noble metals have been used as catalysts for many reactions due to their high selectivity, oxidation resistance and corrosion resistance. Lu's group used noble metals catalysts in non-aqueous Li-O2 batteries [19]. Their results proved that the bifunctional Pt–Au nano-alloy have superior catalytic properties for rechargeable Li-O2batteries. They also showed that the order of catalytic activity for ORRs is palladium > platinum > ruthenium = gold > glassy carbon [20]. Su et al. [21] synthesized polyhedral Au nanocrystals (NCs) as effective catalysts for Li-O2 batteries with LiNO3/DMSO electrolyte. The Au NCs could reduce the charge potential and improve reversible capacities. A possible mechanism was proposed for the Au NCs ORR/OER activity in Scheme 2. Noble metals are promising materials because of their excellent ORR performance compared with carbon materials.
Sun et al. [22] synthesized 2.3 nm Ru nanocrystals supported on carbon black with the aid of surfactant method. The experimental results show that their synthesized Ru-CB catalysts have much higher catalyst activity comparing to CB catalysts and exhibit a very stable cycling performance and excellent catalytic activity with a high reversible capacity of 9800 mAh/g. This excellent performance should be contributed to the high catalytic activity of noble metal catalysts.
Two or more noble metals alloys also act as efficient catalysts with their synergistic effect [23]. One-dimensional (1D) AgPd–Pd porous nanotubes (NTs), which were synthesized by galvanic replacement reaction, were used as electrocatalysts for Li-O2 battery with LiCF3SO3/TEGDME electrolyte. The 1D electrocatalyst shows an excellent performance, superior to the single Pt/C and Ag nanowires (NWs). Their charge plateaus could be reduced to 3.69 V at a current density of 200 mA/g. Moreover, NTs could facilitate the diffusion of oxygen.
Noble metals oxides were also widely applied as the bifunctional catalyst for non-aqueous Li-O2 battery. Compared with transition metal oxides (such as MnO2 [24], Co3O4 [25], NiO [26] and Fe2O3 [27]), noble metals oxides have the highest electronic conductivity and catalytic performance [28]. RuO2 nanosheets [29] were obtained by a two-step process including heat treatment and exfoliation. The result shows low discharge– charge over-potentials and high specific capacity. The morphology changes of the Li2O2 formation-decomposition process were also successfully collected by SEM analysis. RuO2 nanosheets exhibited reversible capacities of more than 900 mAh/g and over-potentials as low as 0.15–0.59 V in the 50 cycles. We also summarized the various noble metals catalysts and their electrochemical performances in the non-aqueous Li-O2 battery in Table 1.
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A series of noble metals were widely applied in the nonaqueous Li-O2 battery for their superior activities and suppression of the coproducts. However, they are so expensive that they are greatly restricted to the economic benefits. Transition metal oxides are expected to become the substitutes for noble metal catalysts due to low cost, abundance and good catalytic activity.
Liu et al. [34] synthesized hierarchical Co3O4 porous nanowires using a hydrothermal method and following calcination. The Co3O4 porous nanowires could effectively improve the cycling stability and efficiency of batteries because of their high surface area and tailored structure. It exhibited stable 70 cycles with limited discharge capacity of 1000 mAh/g at the current density of 100 mA/ g. Hierarchical porous δ-MnO2 nanoboxes were also fabricated by Zhang et al. [35]. δ-MnO2 nanoboxes and carbon electrode could increase the catalytic activity and reduced the over-potentials by 1.36 V at a rate of 0.08 mA/cm2. The cycling performance was improved up to 112 cycles with the limited discharge capacity of 1000 mAh/g at a rate of 0.16 mA/cm2. Liu et al. [36] obtained TiO2 nanowire arrays on carbon textiles (NAs/CT) (Fig. 1) as a free-standing cathode for flexible Li-O2 batteries with LiCF3SO3/TEGDME electrolyte. A superior catalytic performance and long cycling performance could be achieved.
In order to obtain better catalytic activity, MnOx loaded CeO2 nanorods (MnOx@CeO2) had been synthesized by Zhang et al. [35] through an in-situ redox reaction. MnO2 as a traditional catalyst has good mechanical resistances and high oxygen ion conductivities [36, 37]. The results show that MnOx@CeO2 has a high catalytic efficiency to reduce the over-potential and improve poor cycling performance and rate capability. Moreover, alloy-metal oxides also got an exciting progress. 3D foam-like NiCo2O4 had been synthesized as a high-efficient catalyst [38]. The spinel oxide has a higher electronic conductivity than that of NiO and Co3O4 catalysts, and has perfect bifunctional catalytic activity for ORR and OER. There are a high discharge capacity of 10, 137 mAh/g and a stable cycling behavior with no capacity loss in 80 cycles under specific capacity cut of 1000 mAh/g. Mohamed et al. [39] comprehensively studied that Co2+ in the tetrahedral spinel A site of Co3O4 was replaced by different metal ions (Mn2+, Fe2+, Ni2+, and Zn2+) to format the spinel MCo2O4 (M = Mn, Fe, Ni, and Zn) porous structures. The results indicated that FeCo2O4 cathodes show a higher discharge plateau voltage of 2.67 V vs. Li/Li+ and a lower charge plateau voltage of 3.96 V. FeCo2O4 had a high O2 absorption on the interface, which contributed to a high catalytic efficiency.
Transition metal oxides also involved perovskite-type (ABO3) [40] and pyrochlore-type oxides (A2B2O7-δ) [41]. Perovskite-type oxides show relative high OER activities, originating from their high spin electron configuration and molecular level oxygen vacancies. Thus perovskite-type metal oxides are a new subject for high-performance electrocatalysts without noble metals [42]. Pyrochlore-type oxides are also a new topic for effective catalyst due to their high surface area affording high concentration of surface active sites, which provided excellent catalytic performance for oxygen evolution due to their perfect electron transport and intrinsically variable oxidation state [41]. The transition metal oxides catalysts and their performances are also provided in Table 2.
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Recently, nano-carbon materials have been widely explored. It is worth noting that the carbon materials always exhibited a poor catalytic activity in aqueous systems and may be suitable for the non-aqueous Li-O2 battery. Firstly, carbon with large surface area could provide enough active sites. Secondly, the porous structure may provide storage space for discharge product. Moreover, the carbon materials also have a certain catalytic activity. The carbon materials include 0D carbon black and Super P, 1D nanofiber and nanotubes, 2D carbon nanosheets and graphene, and 3D porous carbon materials.
Carbon materials were widely used in the non-aqueous Li-O2 battery as well as lithium–sulfur system. The surface areas and structures were also considered. For example, Wu et al. [47] chose activated carbon with large surface area and high porous structure in which sulfur particles could be embedded. The relationship between the non-aqueous Li-O2 battery discharge capacity and microchannel structure is derived from the different interactions in the process of battery discharge. For example, Ding et al. [48] synthetically studied the effect of different carbon pore sizes on the discharge processes. In Fig. 2, they studied eight kinds of carbon materials which exhibit different initial charge–discharge capacities at a rate of 50 mA/g. There was a direct correlation between pore size and discharge capacity. Thus there are increasing interests in preparing corresponding carbon materials with rational pore structures.
Mesoporous carbon nanocubes were obtained through CVD process [49]. Compared with the traditional CB, this architecture of carbon materials has excellent catalytic activity for Li-O2 battery. The assembled battery has demonstrated high discharge capacities of 26, 100 and 12, 100 mAh/g at rates of 200 and 2000 mA/g respectively. The high activity was attributed to its unique structure including its mesoporous channel and macropores. Wang et al. [50] fabricated a high efficient graphene material which exhibited a perfect performance. Wang et al. [51] synthesized a GO material by in-situ sol–gel method and a high discharge capacity of 11, 060 mAh/g was obtained at the rate of 0.2 mA/cm2. Guo et al. [52] synthesized a leaf-like carbon material which had a discharge capacity of 6000 mAh/g and long cycling performance of 150 cycles with a limited discharge capacity of 1000 mAh/g. This good performance was attributed to the high electronic conductivity of CNT and the efficient active sites of GO. Zhang et al. [53] used the mixture of carbon nanotube and carbon nanofiber as air electrode materials, which demonstrated a high specific capacity up to 2540 mAh/g at the rate of 0.1 mA/cm2. Table 3 summarizes different nano-structured carbon materials and their performances for the non-aqueous Li-O2 battery.
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Carbon based hybrid materials not only serve as the catalyst support, but also were used as catalysts. Hybrid electrocatalysts are conducive to batteries' cycling performance by providing enough spaces for solid products (Li2O2) and employing effective catalysts. The hybrid materials may have superior performance to each part due to their synergy. Hybrid electrocatalysts are classified into the following three categories: (1) noble metal/carbon composites; (2) transition metal/carbon composites; (3) N-doped hybrid materials.
Noble metal/carbon hybrid materials have been applied in the non-aqueous Li-O2 battery. The noble metal hybrid materials can be divided into the following kinds: Noble metal/carbon hybrid, noble metal oxides/carbon hybrid and noble metal/transition metal oxide/carbon hybrid materials. Huang et al. [56] wrapped Pd, Pt, Ru and Au on the carbon nanotubes to form the noble metals/ carbon hybrid materials. The charge over-potential and stability have been clearly improved. They thought that surface electron density of carbon nanotubes increased owing to noble metal existed, leading to discharge products nanocrystals uniformly covering on the electrode interface. Jung et al. [57] synthesized the hybrid materials of Ru and RuO2·0.64H2O nanoparticles on reduced graphene oxide with an average size of 2.5 nm, they were tested as catalysts for Li-O2 battery in TEGDME-LiCF3SO3 electrolyte. Compared with rGO and Ru/rGO hybrid, the RuO2·0.64H2O/ rGO hybrid electrodes exhibited the best catalytic performance with the stable and the lowest charge voltage platform in 30 cycles (Fig. 3). Noble metal/transition metal oxide/carbon hybrid materials have been synthesized and applied for the non-aqueous Li-O2 battery. For example, FeOx/Pd/3D ordered mesoporous carbon hybrid materials also exhibited a perfect catalytic performance [58]. The results indicated that the Pd nanoparticles, 3D ordered mesoporous carbon and FeOx contribute to a superior catalyst in their synergistic effect.
Compared with the noble metals, the economic benefit is also one of the important factors that we need to consider. So nanocarbon hybrid materials, transition metal hybrids were widely investigated for non-aqueous Li-O2 battery. These materials could be divided into the following four kinds: Transition metal/carbon hybrids, transition metal oxides/carbon hybrids, transition metal nitride or carbon hybrids and transition metal carbide/carbon hybrids. The Co-CNFs could decompose the discharge product (Li2O2) easily and reduce over-potential in the discharge/charge process. There were also bimetallic cathode materials used in nonaqueous Li-O2 battery. Xie et al. [58] synthesized graphene supported Co–Cu bimetallic yolk–shell nanoparticles as cathode catalysts for Li-O2 battery. They compared four different kinds of the cathode materials: graphene, Co/graphene, Cu/graphene, and Co–Cu/graphene and found that Co–Cu/graphene cathode materials had the lowest charge platform (at -4.0 V) and the highest discharge capacity of 14, 821 mAh/g at a current density of 200 mA/g. The perfect performance may be attributed to the large conductive network and effective Co and Cu catalytic sites. Cobalt nanoparticles dispersed carbon nanofibers had also been reported [59–61].
Transition metal oxides/carbon hybrids were synthesized and applied as cathode catalytic materials, such as δ-MnO2 on 3D-graphene [62], NiO nanoplates/graphene matrix [63], Fe2O3/carbon nanotubes [64], Co3O4@CNTs composite [65], TiO2 nanowire arrays/carbon textiles [36] and spinel ZnCo2O4/SWCNTs [55]. Transition metal oxides/carbon nanostructured electrodes are effective strategies to exploit new and high-efficient catalysts, because conductive carbon had a large surface area which could provide plenty of sites for discharge–charge reaction to obtain high activity and capacity [66–68]. Furthermore, Ryu et al. [69] designed a kind of high-efficient catalyst used Co3O4 nanofibers/graphene nanoflakes. The synthetic strategy is also provided in Scheme 3. The high capacity of 10, 500 mAh/g and stable cycle performance of 80 cycles were achieved at 200 mA/g with a limit capacity of 1000 mAh/g.
Transition metal nitride/carbon hybrids and transition metal carbide/carbon hybrids were also investigated in recent years, which demonstrated efficient ORR and OER catalysts as cathode materials, such as TiN/carbon microfibers [69], TiN nanoparticles/ carbon [70], Mo2C nanoparticles/CNTs [71] and g-C3N4@carbon paper [72]. Because the metal nitride has a higher electrochemical conductivity than that of metal oxides, it is popular to be applied in the electrocatalysts, for example, mesoporous TiN/carbon microfibers hybrid materials [69], which exhibited higher capacity than that of Super-P and Pt/C cathode materials. The composites provided efficient channel and enough space for transfer of the electrolyte and storage of Li2O2. Fe/Fe3C-carbon nanofibers hybrid materials also presented a stable cycle performance of 20 cycles at a rate of 300 mA/g with a limit capacity of 1200 mAh/g and 40 cycles at a same current density with a limit capacity of 600 mAh/g [73]. The perfect performance could be ascribed to the high catalytic activity of Fe/Fe3C and the fast electron transport in carbon nanofibers.
In this section, we mainly address N-doped carbon hybrids to form a metal/N-carbon complex, which could improve the inherent activity for ORR, recombining the transitional metal to form the superior catalyst materials [74, 75]. There are one more electrons in N atoms than C atoms in the N-doping structures, which could be in favor of the electron transfer [76, 77]. Simple architectures of N-doped carbon cathode material had been synthesized by Zhao et al. [78] and Shui et al. [79] etc. 3D porous N-doped graphene frameworks and N-doped coral-like carbon fiber arrays demonstrated good catalytic activity for non-aqueous Li-O2 battery. Mi et al. [80] further studied different levels of N-doped CNTs used as the cathode materials. There is a lower charge platform and higher discharge platform than CNTs cathodes materials. The excellent performance could be ascribed to the N doping into the carbon materials which could improve the oxygen adsorption and fast electron transfer on the CNTs surface, resulting in a low voltage for O2 dissociation.
In general, the N-doped structures are used as the catalysts to improve their catalytic activity. Transition metals were also introduced in the hybrids [81]. Co/N-doped carbon [82], which had been synthesized by a facile sol–gel method, exhibited a higher discharge capacity of 5000 mAh/g at a current density of 300 mA/g. The excellent performance could be attributed to highly dispersed cobalt nanoparticles on N-doped carbon substrates. The similar works were the graphene composite Co/N-MWCNTs catalyst (Scheme 4) reported by Wu et al. [83]. The catalyst hybrid exhibited a higher capacity and much more stable cycling performance than the Co–N–KJ, Pt/C and N–C, KJ cathode materials. LaTi0.65Fe0.35O3-δ nanoparticle bedecked N-doped carbon nanorods showed high OER and ORR activities [84]. We also summarized the various nano-carbon hybrid catalysts and their performances applied in the non-aqueous Li-O2 battery in Table 4.
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In addition to the common materials, there are also some unusual materials or additives may satisfy the requirement, they had been synthesized and utilized as effective catalyst for highperformance rechargeable non-aqueous Li-O2 battery [87–90], such as transition metal hydroxide [91], transition metal sulfides [92], electrolyte additive of iodine ions [93] and incorporating photo catalyst [94]. It should be noticed that there was a method to reduce the charge voltage by incorporating a photo catalyst [94] in electro-catalyst. To solve the problem of high over-potential they integrated the g-C3N4 photocatalyst and obtained a very low charge voltage at 1.9 V (Fig. 4), the g-C3N4 acted the dual roles of ORR catalyst and photocatalyst.
In conclusion, Li-O2 battery gets more and more attention around the world with the advantages of high energy density, low cost and friendly environment, and showed a broad application prospect in the fields of electric vehicles, hybrid electric vehicle etc. In recent years, variety of electrode catalysts has made important progress in various aspects. Researchers have made great strides in electrocatalysts for non-aqueous Li-O2 battery and developed many methods to fabricate a variety of novel materials. However, the reactional principles of catalyst are unclear and in disputes. In this review, the electrocatalysts for non-aqueous Li-O2 battery are summarized including catalysts category, architectures and electrochemical performance in recent years. The five kinds of catalysts were divided based on their features, that is, noble metals, transition metal oxides, nano-carbon materials, nanocarbon hybrid materials and other catalyst materials. Noble metals have high catalytic activity, selectivity, oxidation resistance and corrosion resistance except high cost. Transition metals based catalysts are desired substitutes for noble metal catalysts due to low cost, abundance and good catalytic activity. In addition to the common materials, there are also other materials or additives to meet the requirement of high-performance rechargeable nonaqueous Li-O2 battery. Efficient bifunctional catalysts can not only increase the battery capacity but also improve energy utilization by reducing overvoltage at charging process. On the other hand, the charging product, Li2O2, would deposit into the pores of catalysts and cover gradually on the active sites of catalyst when discharging, which leaded to the inactivation of electrocatalysts. Therefore, Li-O2 batteries' catalysts also need to consider the effective surface area, nanoarchitectures and pore size & volume of the cathode materials, which are helpful for researchers to develop an efficient bifunctional catalyst and investigate their working mechanism.
There are still several important scientific problems in the use of oxygen electro-catalysts. First of all, the carbon-based materials are not stable under high voltage, which will react with Li2O2 and generate Li2CO3. A stable coating on carbon materials is required to prevent the action of Li2O2 with electrolyte and carbon. Carbides and nitrides are expected to become the candidate electrocatalysts for non-aqueous Li-O2 batteries. Secondly, high-efficient catalysts with key features are still welcome in non-aqueous Li-O2 batteries. Electrode materials should have the following characteristics: excellent catalytic activity, high electrical conductivity, high porosity, large specific surface area and low cost. The noble metals show good catalytic activity at present, but the high-cost catalysts are not used in the practical application. Except for looking for cheap substitutes, it is also the development directions to reduce the size of the noble metal catalysts by improving the uniform distribution and developing novel nano-alloys as substitutes for noble metals in the future.
At last but not least, the possible catalytic mechanisms are still in the primary stage. The main problems are waiting for researchers to solve by advanced in-situ technologies including SEM, TEM, Raman and XPS. It should be believed that Li-O2 batteries will be able to be developed from theory to practice, from the research stage to the application level certainly by the joint efforts and sincere cooperation of scientific researchers in different fields. We hope that advanced Li-O2 batteries are able to be widely used in the near future.
This study was supported by the Natural Science Foundation of China (No. 21303038), Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry One Hundred, Talents Program of Anhui Province and Open Funds of the State Key Laboratory of Rare Earth Resource Utilization (No. RERU2016004).
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Scheme 1 The four systems of Li-O2 batteries ((a) aprotic; (b) aqueous; (c) hybrid; (d) solid-state). Reprinted from Ref. [14]. Copyright 2014 American Chemical Society.
Scheme 2 Schematic diagram of the proposed Au NC catalyst mechanismin the Li– O2 cell. Reprinted from Ref. [21]. Copyright 2015 Nature Publishing Group.
Figure 1 (a) Schematic representations for the design and preparation of the TiO2 NAs/CT; (b) Scanning electron microscope image and photograph (inset) of pristine—CT (scale bar, 10 μm); (c) SEM image and photograph (inset) of the obtained TiO2 NAs/CT cathode (scale bar, 10 μm); (d) enlarged image of (c) with 500-nm scale bars. Reprinted from Ref. [36]. Copyright 2015 Nature Publishing Group.
Figure 2 Initial charge–discharge voltage profiles of Li-O2 batteries with various carbons cycled at a current of 0.05 mA (50 mA/gc). Reprinted from Ref. [48]. Copyright 2014 Royal Society of Chemistry.
Figure 3 Discharge–charge cycles of Li–air cells using rGO, Ru–rGO hybrid, and RuO2·0.64H2O–rGO hybrid under various specific capacity limits. (a–c) Current = 200 mA/g; time = 10 h; cycling capacity = 2000 mAh/g; voltage profiles of (a) fifth cycle and following cycles of (b) Ru–rGO hybrid and (c) RuO2·0.64H2O–rGO hybrid. (d–f) Current = 500 mA/g; time = 10 h; cycling capacity = 5000 mAh/g; voltage profiles of (d) fifth cycle and following cycles of (e) Ru–rGO hybrid and (f) RuO20.64H2O–rGO hybrid. The capacity was normalized by the total weight of oxygen electrodes (rGO or rGO + catalyst). Reprinted from Ref. [57]. Copyright 2013 American Chemical Society.
Scheme 3 Schematic illustration of the synthetic strategy of the Co3O4 NF/GNF composite. Reprinted from Ref. [69]. Copyright 2013 American Chemical Society.
Scheme 4 The formation for nitrogen-doped graphene sheets derived from polyaniline and Co precursors using MWNTs as a template. Reprinted from Ref. [83]. Copyright 2012 American Chemical Society.
Figure 4 The formation for nitrogen-doped graphene sheets derived from polyaniline and Co precursors using MWNTs as a template. Reprinted from Ref. [83]. Copyright 2012 American Chemical Society.
Table 3. Li-O2 battery performance with various nano-structured carbon materials catalysts.
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