A Synergistic Catalytic Mechanism for Oxygen Evolution Reaction in

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A Synergistic Catalytic Mechanism for Oxygen Evolution Reaction in Aprotic Li-O2 Battery Senrong Cai, Ming Sen Zheng, Xiaodong Lin, Ming Lei, Ruming Yuan, and Quan Feng Dong ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02236 • Publication Date (Web): 17 Jul 2018 Downloaded from http://pubs.acs.org on July 23, 2018

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A Synergistic Catalytic Mechanism for Oxygen Evolution Reaction in Aprotic Li-O2 Battery ‡Senrong Cai, ‡Mingsen Zheng, Xiaodong Lin, Ming Lei, Ruming Yuan,* and Quanfeng Dong* ‡These authors contributed equally to this work. State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Xiamen University, Xiamen 361005, China

ABSTRACT

The large polarization of a Li-O2 battery is derived from oxygen evolution reaction (OER) processe. To achieve a long-life Li-O2 battery with high round-trip efficiency, various catalysts have been extensively investigated for oxygen cathodes, especially for OER processes. Here, we designed an in-situ growth of α-MnO2/RuO2 composite on graphene nanosheet with carbon embedded structure as the cathode electrode for Li-O2 battery. The synergistic catalytic effect between the α-MnO2 and RuO2 has significantly improved the OER kinetics. The fabricated LiO2 battery can deliver a high reversible capacity of 2895 mAh/gcomposite with a low charge overpotential of 0.25 V (0.34 V lower than bare RuO2 cathode). The results revealed that more LiO2 intermediates formed when α-MnO2 was introduced into RuO2 electrode during the oxidation of Li2O2. The facilitation of initial Li extraction was confirmed by density functional

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theory (DFT) calculations, which shows that the α-MnO2 and RuO2 interfaces can stabilize the primary Li ions and Li2−xO2 intermediates respectively. Subsequently, Li2−xO2 would be easily oxidized to O2 by RuO2 catalyst. With the synergy between α-MnO2 and RuO2, the initial delithiation process and O2 evolution are promoted simultaneously. By combining theoretic and experimental results, we proposed a synergistic catalytic mechanism for the OER processes.

KEYWORDS: Li-O2 Battery, OER catalyst, Synergistic mechanism, density functional theory (DFT), Low charge overpotential

With extremely high theoretical specific energy densities, rechargeable Li-O2 batteries have attracted worldwide attention due to their potential application in electric vehicles.1-6 A typical Li-O2 battery is composed of a porous oxygen cathode, Li+ conducting electrolyte and a metallic Li anode. During the battery operation, O2 obtained from the air atmosphere is reduced and combines with Li+ to form solid Li2O2 (the oxygen reduction reaction, ORR) in the discharge process.7-8 The discharge products are converted back into Li and O2 (the oxygen evolution reaction, OER) at the subsequent charge process.9-10 But, there are still many challenges that should be solved before the practical applications of this energy storage device.11-13 Firstly, owing to the insulating nature of insoluble Li2O2 and sluggish kinetics of oxygen evolution reaction, large overpotentials exist during the charge process.14-18 Besides, the high reactivity of the reduced intermediate species (LiO2) would trigger some undesired side reactions, which results in the corrosion of electrolyte and carbon cathode.19-22 All these obstacles will limit the Li-O2 battery in a low round-trip efficiency, low rate capability as well as poor cycling life.


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ACS Catalysis

As we all known, the large polarization of lithium-oxygen batteries comes mainly from charging processes. Substantial efforts have been devoted to reducing the charging overpotentials of Li-O2 batteries by employing electrode catalysts, such as carbon materials,23-24 noble metals/oxides (Au,25-26 Ru,27-28 RuO229), transition metal oxides (MnO2,30-31 CoFe2O4,32-33 Co3O434-35 ) and so on. However, the OER mechanism has not yet been understood well. Nazar et al. revealed that an off-substoichiometric Li2-xO2 intermediate was formed during the charge process based on the Operando X-ray diffraction technique.36 Meanwhile, Liu et al. also used Operando synchrotron X-ray diffraction to understand the oxidation of Li2O2 and found similar results as Nazar reported.37 Zhu et al. suggested that the lowest-energy reaction pathway for the decomposition of Li2O2 are initial delithiation process and oxygen evolution step, and the ratedetermining step was predicted as the O2 evolution step according to the first-principles calculations.38 These reports demonstrated a two-step oxidation of Li2O2: Li2O2 – e- → LiO2 + Li+; LiO2 – e- → O2 + Li+. Although, various materials have been employed as the oxygen electrode catalysts, the role of these catalysts in OER process remains unclear. Extensive studies have proposed that RuO2 was a promising catalyst for oxygen evolution reaction.39-40 And, most researchers attributed the good OER catalytic activity to the formation of thin amorphous Li2O2.41-43 As mentioned above, the decomposition of Li2O2 involves a delithiation process, so the initial Li extraction from Li2O2 during charge should be a critical issue. Therefore, to design a composite catalyst with such a synergistic effect, which can make the initial Li extraction easer, is desirable for the OER process. In this work, we report an in-situ growth of α–MnO2/RuO2 composite on graphene nanosheet with carbon embedded structure as the catalytic cathode for Li-O2 batteries. The experimental results revealed that the Li-O2 batteries with such cathode electrode exhibited extremely high


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catalytic activity toward OER processes with only 0.2 V of charge overpotential, as well as for ORR with only 0.07 V of discharge overpotential, at the current density of 100 mA/g under a limited capacity of 500 mA/g. The UV-vis results indicated that more LiO2 intermediates formed when α-MnO2 was introduced into RuO2 electroded during the oxidation of Li2O2, which means the initial delithiation of Li2O2 to LiO2 intermediates was facilitated. According to density functional theory (DFT) calculations, α-MnO2 and RuO2 interfaces would facilitate the delithiation process in which Li2−xO2 intermediates can be deposited on RuO2 surface, while Li ions could diffuse to the α-MnO2 surfaces or crystal channels where they were adsorbed due to the affinity between α-MnO2 and Li atoms. As the result, the stoichiometric Li2O2 would readily convert into the off-stoichiometric one, which could be facilely decomposed to O2 with the catalysis of RuO2. By the synergistic catalyst, the delithiation and O2 evolution during the OER process are promoted significantly, leading to the decreased overpotential .

RESULTS AND DISCUSSION 1. Characterization of α-MnO2/RuO2@GN composite. GNs were prepared by reduction of graphene oxides. The α-MnO2@GN composite was firstly synthesized via the redox reactions between the prepared GN and KMnO4. Subsequently, RuO2 nanoparticles were anchored on the α-MnO2@GN composite by precipitation (the composite is marked as MRG). The detailed synthetic procedure is provided in the experimental section. The crystal structure and surface composition of the as-prepared MRG were investigated using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). XRD patterns (Figure S2) indicate the formation of tetragonal α-MnO2 (JCPDS 01-044-0141). The broad peak at about 2θ= 26 degree can be indexed to graphene nanosheets. While no typical diffraction peaks of RuO2 were observed, this could be


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ascribed to amorphous or very low crystallinity. The formation of RuO2 and α-MnO2 was confirmed by XPS spectra. As shown in Figure 1a, two peaks at 463.6 eV and 485.7 eV, assigned to Ru 3p3/2 and Ru 3p1/2 respectively, were observed. Also two lines at 642.2 eV and 653.8 eV are corresponding to Mn 3p3/2 and Mn 3p1/2, respectively. Scanning electron microscopy (SEM) images, shown in Figure S3, reveals that α-MnO2/RuO2 nanoparticles (NPs) are closely deposited on the surface of graphene sheets, forming a carbonembedded structure. The unique structure can prevent the direct contact between graphene and discharged products (Li2O2), which could suppress the side reactions. Meanwhile the MRG composite with two-dimensional structure and porous properties can provide enough space to accommodate the discharged products. The detailed morphology and microstructure were further confirmed by transmission electron microscopy and high-resolution transmission electron microscopy (HRTEM). A two-dimensional structure was observed in Figure 2a, also tremendous nanoparticles were compactly anchored on graphene, forming a carbon-embedded structure. As shown in Figure 2b, The SAED pattern of MRG demonstrated the existence of RuO2. Also the diffuse ring patterns implied the poor crystallinity of RuO2, which was consistent with XRD result (no diffraction peak of RuO2 was detected). Combining with SEM images of MRG (Figure S3) in which an amorphous thin coat was observed on the surface of MRG, we can speculate that the amorphous thin coat should be assigned to RuO2. According to the Scherrer’s equation based on diffraction peak at 2θ=37.53°, the particles size of α-MnO2 was around 14.6 nm. Additionally, Figure 2c and 2d provided the statistic size distribution of α-MnO2. The size of MnO2 range from 10-15 nm according to six particles in Figure 2c, which was similar to the result of XRD. Moreover, the energy-dispersive X-ray spectroscopy (EDX) elemental mapping (Figure S4) verifies the uniform distribution of C, O, Mn and Ru elements.


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2. Applications of the α-MnO2/RuO2@GN composite in Li-O2 batteries. Cyclic voltammetry (CV) measurements were performed within a voltage widow of 2.2–4.5 V (Figure S5). Compared with RuO2 electrode (2.87 V), MnO2 electrode exhibited a higher ORR onset potential (2.95 V), implying a lower ORR kinetic polarization. Furthermore, MRG electrode showed the same onset potential with MnO2 electrode, which indicates very good ORR activity. During anodic scans, there is only one oxidation peak (3.87 V) in MnO2 electrode, showing poor OER activity. But three anodic peaks (3.37 V, 3.74 V and 4.18 V) appeared in RuO2 electrode. The peak at 3.37 V corresponded to the largest current density, which mean most discharge products were decomposed at this voltage. The peak at 4.18 V could be attributed to the oxidation of byproduct (The following XPS results of these three electrodes after cycling 20 times showed that RuO2 cathodes resulted in much side reaction). However, the MRG electrode show larger current density than RuO2 at the peak of 3.37 V, which demonstrated the best OER activity, implying the synergy between MnO2 and RuO2. The electrocatalytic activity of the as-prepared MRG was investigated by galvanostatic discharge/charge method and compared with α-MnO2@GN composite and RuO2/GN mixture. The details of electrode fabrication and cell assembly are provided in the experimental section. Figure 3a compares the first discharge/charge voltage profile of Li-O2 battery with α-MnO2@GN, RuO2/GN and MRG electrodes between 2.2 V and 4.0 V, respectively. The Li-O2 battery with the RuO2/GN cathode shows a discharge capacity of only 1015 mAh/gcomposite (all capacities are based on the mass of composite) at the current density of 100 mA/g. However, battery with MRG cathode delivers a discharge capacity of 2895 mAh/g at the same current density, which is similar to that with α-MnO2@GN cathode (2776 mAh/g). The higher capacity can be ascribed to the superior ORR catalytic activity of α-MnO2. As shown in Figure 3a, the discharge and charge


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voltage platform of the α-MnO2@GN cathode are 2.79 V and 3.85 V, respectively, showing poor OER catalysis; the RuO2/GN cathode showed a slight improvement OER activity with a charge voltage platform of 3.55 V. Then RuO2 and MnO2 were mechanically mixed, and marked as MnO2@GN/RuO2 hybrid. As shown in Figure 3a, the overpotential of Li-O2 batteries with MnO2@GN/RuO2 hybrid was 0.42 V, which lower than RuO2/GN hybrid (0.59 V) and MnO2@GN (0.89 V), indicating the synergy between MnO2 and RuO2. The MRG cathode, however, exhibited excellent catalytic activity for both ORR and OER processes, with a discharge/charge voltage platform of 2.82 V/3.21 V, respectively. Apparently, whether RuO2 or α-MnO2 alone cannot reduce the charge plateau, only can the composite of α-MnO2/RuO2@GN achieve a low charge plateau. The total overpotential is only 0.39 V, which resulted in an extremely high round-trip efficiency of about 88%. Importantly, most of discharge products can be decomposed below 3.5 V, implying that side reactions, including the decomposition of electrolyte and the oxidation of carbon, would significantly suppressed. α-MnO2/RuO2 composite (MR) without graphene was also prepared. The synthesis of MR was provided in Experiment Section. From the SEM image (Figure S6), RuO2 nanoparticles were anchored on MnO2 nanorods, showing an 1D structure. Obviously, the specific surface area of MR (68.52 m2/g) was significantly lower than that of MRG composite (121.88 m2/g, Figure S7), resulting in a reduced capacity (1355 mAh/g compared to 2895 mAh/g of discharge capacity, Figure S8), which indicates the 2D structure of MRG could accommodate much more Li2O2. To further investigate the rate performance of the MRG cathode, it was tested at various current densities from 100 mA/g to 500 mA/g with a cut-off capacity of 500 mAh/g. As shown in Figure 3b, the average discharge/charge voltages of the cathode are 2.89/3.16 V, 2.84/3.21 V and 2.78/3.61 V at the current densities of 100 mA/g, 200 mA/g, and 500 mA/g, respectively. The


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charge/discharge potential gap is only 0.27 V at a low current density, corresponding to the round-trip efficiency of 91.4%. With increasing current density, the overpotantial increases due to the increased internal resistance of the battery. However, even at a high current density of 500 mA/g, the battery can still yield a round-trip efficiency of 77.0%, demonstrating moderate rate capability. The cycle stability of the MRG hybrid was evaluated at the current density of 100 mA/g with a cut-off capacity of 1000 mAh/g. As depicted in Figure 3d, the specific capacity presented no decay for over 45 cycles, exhibiting good cycle stability. Figure 3c displays the selected discharge/charge voltage profiles of the battery, no obvious variation is observed before 35th cycle. Additionally, most of the charge capacity was carried out below 3.5 V, far below the decomposition potential of carbon, thus decreasing the undesirable side reactions. The high cycling stability could be ascribed to the excellent catalytic activity of α-MnO2/RuO2 nanocomposite with a synergistic catalytic mechanism, which will be discussed late. Quasi in-situ XPS analysis was employed to identify the component of discharge product and the reversibility of the cell. The batteries containing MRG cathode were discharged or recharged after 10 cycles at 100 mA/g with a cut-off capacity of 1000 mAh/g. Figure 4a shows the Li 1s XPS spectra of the air electrode at different states. Only a Li2O2 peak (55.4 eV) was observed for the discharged electrode, suggesting that Li2O2 is the main discharge product for Li-O2 batteries with MRG cathode. After recharged, the Li 1s peak disappears, meaning the full decomposition of Li2O2. The morphologies of oxygen electrodes after discharging and charging were also observed by SEM to reveal the change of electrode surface. As shown in Figure 4b, there was a formation of amorphous and toroid-shaped Li2O2 in the surface of electrode, which becomes fresh again after charging. The SEM results confirm that MRG cathode can reversibly catalyze the formation and decomposition of Li2O2. In order to quantitatively analyze the by-product, all


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three cathodes (MnO2@GN, RuO2/GN and MRG) after cycling 20 times were investigated by XPS. As shown in Figure S9, both MnO2@GN and RuO2/GN cathodes were detected with the formation of Li2CO3, indicating undesired side reactions. The MRG cathode, however, has no any signal of Li2CO3, implying the negligible side reactions. 3. The processes of Li2O2 oxidation. To understand Li2O2 decomposition further, α-MnO2@GN composite, RuO2/GN mixture, and α-MnO2/RuO2@GN composite were all loaded with commercial Li2O2 to form so-called artificial cathode in advance. Then these three cathodes were galvanostatic charged to 4.0 V at a current density of 0.05 mA. As shown in Figure S10, the catalytic activity order of these three artificial cathodes for commercial Li2O2 oxidation is αMnO2/RuO2@GN (3.41 V) > RuO2/GN (3.65 V) > α-MnO2@GN (3.80 V), which is consistent with results in 2.2, also indicating the synergetic catalysis between α–MnO2 and RuO2 for the oxidation of commercial Li2O2. To confirm the decomposition of Li2O2, the electrodes after charge were investigated by SEM and X-ray diffraction. Figure S11 illustrates the surface morphology changes in α–MnO2, RuO2, and α–MnO2/RuO2 electrodes, Li2O2 particles were completely removed after charge in all three cathodes. The oxidation of Li2O2 is also supported by the XRD results (Figure S12). Previous studies have experimentally demonstrated that the electrochemical oxidation of Li2O2 would proceed in two steps: (1) Li2O2 → Li+ + e- + LiO2 and (2) LiO2 → Li+ + e- + O2. During charge, Li2O2 firstly undergoes a Li atom extraction process, with the formation of LiO2 intermediate. Then LiO2 sequentially decomposes into oxygen with the help of catalyst. For Li2O2 oxidation on α-MnO2/RuO2@GN catalyst, the UV-vis measurements were carried out to quantitatively analyze the LiO2 intermediates. Based on the spectra obtained from DMSO solution dissolved with 1 mM KO2 (Figure S13), the absorption peak at 262 nm should be


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assigned to superoxide species. As displayed in Figure 5, the UV-vis spectra of DMSO-extracted superoxide products from different charging cathodes also present a similar absorption peak at around 262 nm. The existent of the superoxide species confirms the two-step decomposition of Li2O2. Noteworthy is that the UV-vis absorbance measured from these three artificial cathodes at the same charge status are different. When charged to 0.25 mAh (state a in inset map), the absorbance of DMSO-extracted superoxide products from RuO2 cathode is 0.10. However, the value is 0.14 and 0.13 for α–MnO2 and MRG, respectively, which means more LiO2 intermediates formed during the oxidation of Li2O2. This result demonstrates that the reaction (1) is promoted by α–MnO2. When charged to 0.5 mAh (state b in inset map), the tendency of UVvis results was consistent with charged to 0.25 mAh: Abs (α–MnO2-b) > Abs (α–MnO2/RuO2-b) > Abs (RuO2-b). It has been well documented that the formation of the off-stoichiometric Li2−xO2 compounds would boost the OER process. In theory, periodic density functional theory (DFT) calculations were performed. For crystalline Li2O2, the extraction of a Li atom (Li2O2(s)=Li2-xO2(s)+Li(bulk)) was predicted to be as high as 2.89 eV~3.34 eV (∆Eb (g)). Such a situation could be alleviated by considering the formation of nanoscale Li2O2 on the surface. Here we chose Li16O16 cluster as a model of surface Li2O2 and found that the Li atom extraction (∆Ec (g)) of Li16O16 (g) = Li16O16 (g) + Li(bulk) could be reduced to 1.68 eV. Here we used RuO2 (110) surface as instructive example and explored the extraction process over it. According to our calculations, the adsorption energies (∆Eads) of Li16O16, Li15O16 and Li on RuO2 (110) were predicted to be -6.20 eV, -6.58 eV, and -2.38 eV, respectively. We also preformed the calculations on the off-stoichiometric Li16-xO16 (x =2~4), as shown in Figure S15. And we found that further extraction of Li atoms (x≥4) from the Li16-xO16 cluster would break down the host structure and generate adsorbed O2.


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According to our calculations, the adsorption energies of Li16-xO16 (x =0~4) clusters were all in the range of -6.14 eV to -6.58 eV. Thus, from the thermodynamic cycle shown in Scheme 1, the feasibility of the Li atom extraction critically depends on the adsorption energy of Li atom because there exists no significant difference in the adsorption energies of stoichiometric and off-stoichiometric Li2O2. The adsorption energy of Li on RuO2 (110) is in between -∆Eb (g) and ∆Ec (g), indicating that the stoichiometric Li2O2 species could be formed when nanoscale Li2O2 deposited on RuO2. We also considered the embedding of Li into bulky RuO2. However, the embedding energy was predicted to be far less exothermic than that adsorbed energy (-1.44 eV vs. -2.38 eV), suggesting that the capacity of Li extraction was limited when only using RuO2 as cathode material. When α-MnO2/RuO2 was employed, Li atoms could diffuse to the α-MnO2 surfaces or crystal channels. Computationally, it was predicted that Li atoms strongly adsorbed on not only α-MnO2(211) but also the 8h position of channel in bulky α-MnO2 (-3.75 eV vs. 3.15 eV). Clearly, the addition of α-MnO2 would facilitate the extraction of Li atoms from Li2O2 due to its highly adsorption energy as well as storage capacity of Li atoms. Moreover, the Li atoms embedding in the α-MnO2 would significantly increase the conductivity, which would further enhance the electrochemical reactions. 4. Synergistic catalytic mechanism of Li2O2 oxidation. On the basis of the above analysis, both the CV results and electrochemical performances of Li-O2 batteries with all three catalysts (α-MnO2, RuO2 and their composite) directly demonstrated the synergy between α-MnO2 and RuO2 for oxygen evolution reaction in Li-O2 batteries. When these three electrodes were all loaded with commercial Li2O2 to form so-called artificial cathode to conduct the same property and morphology of Li2O2, the decomposition voltage of Li2O2 catalyzed by MRG still exhibited the smallest overpotential, which confirmed the synergy furtherly. After quantitatively measuring


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the superoxide-like intermediates by UV-vis spectra, we found that the superoxide-like intermediates increased when α-MnO2 was used. Combining with DFT calculation, the improved extraction of Li atoms from Li2O2 was confirmed as the highly adsorption energy as well as storage capacity of Li atoms on the surface of α-MnO2. Simultaneously, DFT results revealed superoxide-like intermediates could be stabilized by RuO2, then further be oxidize and release oxygen. Therefore, we propose that the decomposition of Li2O2 proceeds through two singleelectron steps oxidation, and these two reactions could be facilitated by α-MnO2 and RuO2, respectively. As showed in Figure 6, the Li ion firstly extraction from Li2O2, which could be promoted with the assist of α-MnO2. Then, the Li2-xO2 intermediate would be adsorbed on the surface of RuO2 compactly. And Li2-xO2 would be easily oxidized and release oxygen due to its high catalytic activity for oxygen evolution reaction. With the synergy of α-MnO2 and RuO2, the overpotential of Li2O2 oxidation would be significantly reduced.

CONCLUSION In summary, we have designed and synthesized a α-MnO2/RuO2 composite by in-situ growing on graphene nanosheet as a cathode electrode for Li-O2 battery. This Li-O2 battery delivers a high reversible capacity of 2895 mAh/ghybrid with a low charge overpotential of 0.25 V. Combining theoretic and experimental researches, we have proposed a synergistic catalytic mechanism for the OER process in lithium-air batteries. During charging, the initial Li extraction from Li2O2 to Li2-xO2 could be promoted when α-MnO2 was introduced. Subsequently, the Li2-xO2 intermediate would tend to be adsorbed on the surface of RuO2 and then proceed via a further single-electron oxidation. With the synergy between α-MnO2 and RuO2, both the initial delithiation of Li2O2 and


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O2 evolution step during OER process are facilitated. This synergistic catalytic mechanism could guide the design effective catalysts for high-performance Li-O2 batteries.

METHODS Synthesis of α–MnO2@GN composite. Graphene oxide (GO) was synthesized via the modified Hummers’ method. With the thermal reduction of GO, including an initial hydrothermal treatment and a subsequent calcination in N2 atmosphere at 1500 ℃ for 3h, graphene nanosheets (GN) were obtained. Then, 50 mg GN was dispersed in 3 M sulfuric acid solution by magnetic stirring for 0.5 h. Then 400 mg KMnO4 was added to the solution, and the mixed solution was immediately transferred to a water bath at 90 ℃ with continually stirring for 0.5 h. After cooling to room temperature naturally, the α–MnO2@GN composite was obtained by filtration and was washed with de-ionized water and ethanol repeatedly before drying in vacuum at 80 ℃ overnight. Synthesis of α–MnO2/RuO2@GN composite. The RuO2 nanoparticles were anchored on the as-prepared α–MnO2@GN by precipitation. The α–MnO2@GN (0.2 g) was primarily dispersed in 3 mmol/L H2SO4 solution (15 mL) under ultrasonication for 10 min. Then 0.15 g RuCl3 (anhydrous) was added to the mixed solution with continuous ultrasonication for 10 min. 1 M NaHCO3 aqueous solution (90 mL) was then dropwise added to the above mixture under stirring at room temperature. Finally, the precipitation was filtered and washed with de-ionized water and ethanol repeatedly and then dried at 85 ℃ overnight. The black powder was α–MnO2 /RuO2@GN composite.


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Synthesis of RuO2/GN mixture. RuO2/GN mixture was prepared by mechanical ball- milling, the mass percent (17%) of graphene nanosheet is equal to that of α–MnO2@GN composite and α–MnO2/RuO2@GN composite. Synthesis of α–MnO2/RuO2 composite. The preparation of α–MnO2/RuO2 composite was the same as α–MnO2/RuO2@GN composite , excepting no graphenen was added. The as-synthesis α–MnO2/RuO2 composite was marked as MR. Materials Characterization. The Ultraviolet-Visible absorption spectrometry was performed using a Thermo Evolution 300 spectrophotometer. The morphology of α–MnO2/RuO2@GN nanocomposite was investigated by scanning electron microscopy (SEM, HITACHI S-4800). Transmission electron microscope and energy dispersive X-ray spectroscopy (EDX) elemental mapping were carried out on a Tecnai F30. The crystallographic information was characterized by power X-ray diffraction (XRD, Philips X’Pert Pro Super X-ray diffractometer) with Cu Kα (λ=1.5418 Å) radiation at a 2 range of 10-80°. TG curve of the α–MnO2/RuO2@GN sample was obtained using a Diamond TG thermoanalyzer (Perkin Elmer) under air flow at a heating rate of 8 ℃ min-1. XPS was performed on an Omicron photoelectron spectrometer (Al Kα with 1486.6 eV), and the data were fitted by XPSPEAK41 after calibrated by setting the hydrocarbon peak to 284.5 eV. All samples about discharge and charge products were washed three times by anhydrous dimethyl sulfoxide (DMSO, Alfa Aesar) in the glove box before testing. Electrochemical Measurements. For the measurement in Figure S5, air electrodes were prepared by mixing catalysts (α–MnO2@GN composite, RuO2/GN mixture and α– MnO2/RuO2@GN composite, 45 wt.%), commercial Li2O2 (45 wt.%) and poly (vinylidene fluoride) (PVDF) (10 wt.%) in 1-methyl-2pyrrolidinone (NMP). Cathodes tested in 2.2 (Figure 3) were prepared in a 90:10 weight percent of active material: PVDF (MR cathodes were


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ACS Catalysis

prepared in a 80:10:10 weight percent of MR: Super-P: PVDF). The slurries were then coated onto the carbon paper (10.5 mm in diameter) and dried at 120 ℃ in a vacuum oven for 12 h. The typical loading of the oxygen catalysts on the electrode is about 0.7 mg/cm2. A Swagelok type cell was assembled inside a glove box using Li foil as anode, 0.5 M LiClO4/DMSO as electrolyte (the water content in DMSO electrolyte is

A Synergistic Catalytic Mechanism for Oxygen Evolution Reaction in

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