Enhancing Catalyzed Decomposition of Na2CO3 with Co2MnOx

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Enhancing Catalyzed Decomposition of Na2CO3 with Co2MnOx Nanowires Decorated Carbon Fibers for Advanced Na-CO2 Batteries Cong Fang, Jianmin Luo, Chengbin Jin, Huadong Yuan, Ouwei Sheng, Hui Huang, Yongping Gan, Yang Xia, Chu Liang, Jun Zhang, Wenkui Zhang, and Xinyong Tao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04034 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

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ACS Applied Materials & Interfaces

Enhancing Catalyzed Decomposition of Na2CO3 with Co2MnOx Nanowires

Decorated

Carbon

Fibers

for

Advanced Na-CO2 Batteries Cong Fang, Jianmin Luo, Chengbin Jin, Huadong Yuan, Ouwei Sheng, Hui Huang, Yongping Gan, Yang Xia, Chu Liang, Jun Zhang, Wenkui Zhang, Xinyong Tao* College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, P R China

ABSTRACT: The metal-CO2 batteries, especially Na-CO2 batteries come into sight owing to their high energy density, ability for CO2 capture and the abundance of sodium resource. Besides the sluggish electrochemical reactions at gas cathodes and the instability of electrolyte at high voltage, the final discharge product Na2CO3 is solid and poor conductor of electricity, which may cause the high overpotential and poor cycle performance for Na-CO2 batteries. The promoting decomposition of Na2CO3 should be an efficient strategy to enhance the electrochemical performance. Here, we design a facile Na2CO3 activation experiment to screen the efficient cathode catalyst for Na-CO2 batteries. It is found that Co2MnOx nanowires decorated carbon fibers (CMO@CF) can promote the Na2CO3 decomposition at lowest voltage among all these metal oxides decorated carbon fiber structures. After assembling the Na-CO2 batteries, the electrodes based on CMO@CF show lower overpotential and better cycling performance compared with the electrodes based on pristine carbon fibers and other metal oxides modified carbon fibers. We believe this catalyst screening method and the freestanding structure of CMO@CF electrode may provide important reference for development of advanced Na-CO2 batteries. KEYWORDS: Na-CO2 batteries, Na2CO3 decomposition, Co2MnOx nanowires, catalyst screening, freestanding structure

INTRODUCTION The continuous increasing consumption of fossil-fuel has caused the global

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shortage of fossil-fuel resources and the pollution of the environment. Meanwhile, the atmospheric CO2 concentration is ever-increasing, which has arisen from 278 ppm at the initial stage of the industrial revolution to >400 ppm nowadays,1,2 leading to numerous environment-related issues, such as global warming, melting of the polar ice, ocean acidification, etc. Therefore, the development of advanced energy storage technologies and CO2 utilization and conversion is a global imperative. Confronting with these two unprecedented challenges, recycling CO2 in energy storage system, such as metal-CO2 batteries, represents a promising “green” strategy that can both reduce the fossil-fuel consumption and restrain the environment and climate effect of CO2 emissions. Fortunately, massive efforts have been devoted to this new energy storage system.3-15 Among all these metal-CO2 batteries, Li-CO2 batteries were preferentially researched that was inspired by the influence of CO2 in Li-O2 batteries.16 In the reported works, carbon based nanomaterials, such as Ketjen Black3, graphene,6,13 CNT8,11,16 and super P14 were proved to be the feasible and rechargeable electrodes for Li-CO2 batteries. The comparatively good electrochemical performance of above-mentioned Li-CO2 batteries can be attributed to carbon cathode materials with good electrical conductivity, high surface area and porous structure. Compared with the limited lithium resources in Li-CO2 system, Na-CO2 batteries possess advantages with regard to the low cost and abundance of sodium resources. More recently, Chen group11 reported rechargeable room-temperature Na-CO2 batteries by using electrolyte-treated multi-wall carbon nanotube (MWCNT) as the cathode, Na foil as the anode and ether as the electrolyte. Due to unique 3D porosity electrode with high conductivity and good wettability to electrolyte, the obtained batteries displayed good electrochemical performance. Furthermore, the reaction of 4 Na + 3CO 2 → 2 Na 2CO3 + C was firstly demonstrated by in-situ Raman, ex-situ XRD and XPS analysis by Chen group. Although significant progress has been made in improving the electrochemical performance of metal-CO2 batteries, the problems such as high overpotential and poor cyclability have not be completely solved, which hinder their further development.


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The emergence of above challenges is mainly due to the sluggish electrochemical reactions at gas cathodes, the instability of electrolyte at high voltage and difficult reversibile conversion of the discharge products, carbonate, because of the good thermodynamic stability.17 However, most of the current reported works focused on the pure carbon electrodes with poor catalytic activity. Developing new carbon based cathode materials with high catalytic activity through surface and interface modifying seems to be a promising strategy to solve the above problems. It is well known that the noble metal has excellent catalytic activity in OER/ORR, but theirs widespread applications are limited owing to high cost and scarcity.18-22 Transition metal oxides (TMO), particularly mixed metal oxides were adequately researched as functional electrocatalysts thanks to their high catalytic activity and natural abundance.23-28 Inspired by remarkable catalytic activity of TMO in OER/ORR, we tried to fabricate the TMO decorated carbon fibers (TMO@CF) as the freestanding electrodes of Na-CO2 batteries for the first time. To compare the electrocatalytic performances of all TMO@CF electrodes, a new strategy was designed to screen the catalyst for Na-CO2 battery by loading Na2CO3 on prepared electrodes and testing under the corresponding limited capacity. Moreover, the Na-CO2 batteries based on the freestanding TMO@CF electrodes were assembled and tested in CO2 atmosphere, and the discharge products phase and morphology were also investigated to understand the working mechanism in Na-CO2 batteries.

RESULTS AND DISCUSSION The synthesis process of CMO@CF electrode is displayed in Figure 1a. At first, carbon fibers with clean and smooth surface (Figure 1b) were immersed into solutions containing Co and Mn resources in a Teflon-lined stainless steel autoclave. Then a hydrothermal treatment at 120 °C for 5 hours was performed for in-situ growth the CMO precursor nanowires on carbon fibers surface. During the hydrothermal process, NH4F acts as an accelerant to prompt the formation of the CMO precursor nanowires on carbon fibers and urea serves as the precipitant in this reaction. After the hydrothermal process, the precursor nanowires were uniformly distributed on carbon ACS Paragon Plus Environment


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fibers with sea urchin like structure (Figure 1c). Finally, the precursors were annealed in muffle furnace to obtain the target material, which perfectly maintained the former structure of precursors (Figure 1d). The corresponding digital images of the carbon fibers during the synthesis process of CMO@CF were provided in Figure 1a. It is found that the colors of carbon fibers after hydrothermal and annealing processes are uniform, which indicates the homogeneous distribution of nanowires precursors and CMO nanowires on carbon fibers. XRD (Figure 1e) and XRF (Table S1) results proved that the product after annealing can be defined as Co2MnOx (4≤x≤4.5; Co2MnO4.5,

JCPDS

32-0297;

Co2MnO4,

JCPDS

23-1237).

The

detailed

microstructures of CMO@CF were observed by TEM and shown in Figure 1f,g. As can be seen, the CMO nanowires are composed of numerous nanoparticles with diameter ranging from 20 to 40 nm. The clear lattice fringes in Figure 1g with inter-planar spacing of 0.47 nm belong to the (111) plane of Co2MnO4.5, which further confirms the successful preparation of CMO.


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Figure 1. (a) Schematic illustration and the digital images of synthesis of CMO@CF electrodes; (b,c,d) SEM images of carbon fibers, intermedium product after hydrothermal process and before annealing, and CMO@CF electrode; (e) XRD pattern of CMO powder. (f,g) TEM and HRTEM images of CMO@CF electrode; Inset in (e) represents the structure model of cubic-structure Co2MnO4.

The surface chemical composition and the bonds configuration in the CMO sample were obtained by X-ray photoelectron spectroscopy (XPS). The binding energies exhibited in the XPS spectra were calibrated using the C 1s photoelectron peak at 284.8 eV as the reference. The full XPS spectra (Figure 2a) evidences the existence of Co, Mn, O and C elements in the CMO sample, and the atomic ratio 1.99:1:4.24 of Co, Mn and O was detected (Table S2), which further verified the definition product Co2MnOx (4≤x≤4.5) in XRD results (Figure 1e). Figure 2b exhibits the high-resolution spectrum of Co 2p region, with two major peaks with binding energy at 780.2 and 795.4 eV, corresponding to the Co 2p3/2 and Co 2p1/2 peaks, respectively. The Co 2p spectrum is fitted with two spin-orbit doublets, characteristic peaks of Co3+ and Co2+, and one shakeup satellite. For Co 2p, the peaks at 779.5 and 794.6 eV are characteristic peaks of Co3+, while the peaks at 781.2 and 796.3 eV are attributed to the Co2+.29-31 Similarly, the spectrum of Mn 2p (Figure 2c) is attributed to Mn 2p1/2 and Mn 2p3/2 at binding energy of 642.1 and 653.5 eV, respectively. The fitting peaks at 641.9 and 653.3 eV confirm the existence of Mn2+, while another two peaks at 643.6 and 654.4 eV are consistent with the characteristic peak of Mn3+.32,33 The O 1s spectrum (Figure 2d) can be divided into two subpeaks at 530.1 and 531.6 eV, which respectively represents the lattice oxygen and the hydroxide ions.34,35 It has been reported that the existence of redox couples and defective structure are favorable for improving the electrocatalytic activity.36,37 Therefore, the as-prepared CMO with coexisting the hybrid Co2+/Co3+ and Mn2+/Mn3+ redox couples is expected to show good catalytic performance in Na-CO2 battery. Considering about wettability issue, we tested water contact angle of CF and CMO@CF electrodes and shown in Figure S1. As can be seen, the CF electrode exhibits a large contact angle of 131.1° at the water/CF electrode surface. Interestingly, the water contact angle decreases to 71.1° ACS Paragon Plus Environment


when water droplet contacts the CMO@CF, demonstrating the improved hydrophilicity after CMO loading which is beneficial for the infiltration of the polar electrolyte. [38]

Figure 2. (a) The XPS spectrum of CMO sample; High-resolution XPS spectra of Co 2p (b), Mn 2p (c) and O 1s (d).

To compare the electrochemical performances of various TMO decorated carbon fibers, pure cobalt oxide (CO) and manganese oxide (MO) grown on carbon fibers as control groups were also fabricated via the similar synthesis process (denoted as CO@CF and MO@CF). The XRD pattern of CO powder is presented in Figure S2a. All the diffraction peaks can be identified as Co3O4 phase according to the standard power diffraction file (JCPDS No. 43-1003). The SEM image of CO@CF in Figure S2b shows that the obtained CO nanowires have uniform morphology with length about 5 µm. However, the CO nanowires seriously agglomerate and turn to be microbundles. TEM image of CO shows the interplanar crystal spacing of 0.47 and 0.25 nm, which can be readily assigned to (111) and (311) planes of Co3O4, respectively (Figure S2c). Similarly, sample MO was also successfully grew on the carbon fibers and displayed the high purity cubic Mn2O3 phase (JCPDS No. 41-1442) in Figure S2d. As shown in Figure S2e, the MO exhibits abundant microcones, which


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are adhered on carbon fibers with particle size of about several microns. The TEM result of samples MO shows the inter-planar spacing of 0.24 and 0.47 nm, which are correlated to (200) and (400) planes of Mn2O3, respectively (Figure S2f). These TEM results are in consistent with XRD results. In addition, the N2 adsorption-desorption isotherm of TMO@CF electrodes is shown in Figure S3. As can be seen, the specific surface area of CO@CF, MO@CF and CMO@CF electrodes are 22.1 m2/g, 20.8 m2/g and 29.3 m2/g, respectively. There is no big difference between the surface area of CMO@CF, CO@CF and MO@CF. The specific conductance of CO@CF, MO@CF, CMO@CF and pure CF is showed in Table S3. It can be obviously observed that the specific conductance of CF electrodes decrease with TMO decorating. Nevertheless, the specific conductance of CMO@CF, CO@CF and MO@CF electrodes are very close, while the CO@CF electrode exhibits the highest specific conductance. To evaluate and compare the electrocatalytic performances of the CO@CF, MO@CF and CMO@CF electrodes, we designed a facile Na2CO3 activation experiment. The electrodes were prepared by loading Na2CO3 (the final discharge products of Na-CO2 battery) on synthesized electrodes and sodium metal was selected as the reference electrode. Then the batteries were tested with limited charging capacity in argon atmosphere. Based on this experiment, the energy barrier for the transformation from Na2CO3 to CO2, can be clearly and directly obtained from the typical voltage-capacity profiles. Thus this strategy can be developed to effectively screen the appropriate catalyst for Na-CO2 battery. The voltage-capacity profiles of Na2CO3-based batteries are shown in Figure 3a. It can be seen that all cells can charge to the limited capacity of 1.0 mAh cm-2 at the current density of 0.1 mA cm-2, corresponding to the theoretical capacity for complete decomposition of loaded Na2CO3 (Supporting Information, [Eq. (1)] and [Eq. (2)]). The Na2CO3 battery based on CMO@CF electrode (CMO-NCO) delivers a charge voltage (3.85 V), much lower than those of CO@CF (CO-NCO, 4.02 V) and MO@CF (MO-NCO, 3.96 V) electrodes, which indicates the better electrocatalytic performances of CMO@CF. Furthermore, it is interesting to notice that Na2CO3-based cells can be discharged to the limited capacity with the terminal voltage around 1.9 V and the CMO-NCO ACS Paragon Plus Environment


battery also exhibits higher discharge voltage plateau (Figure 3a). According to our knowledge, this phenomenon is firstly reported and indicates that the carbonate could acts as the electrode material for future energy storage (Figure S4). The XRD patterns of CO-NCO, MO-NCO and CMO-NCO electrodes before and after charging were tested to confirm the decomposition of the loaded Na2CO3 and shown in Figure 3b, c. The Na2CO3 diffraction peaks can be observed obviously in all electrodes before charging process. After charging to the limited capacity, there is no signal of Na2CO3 peaks found in the XRD results, revealing that the loaded Na2CO3 has been decomposed. The surface morphology and sodium element distribution of the CO-NCO, MO-NCO and CMO-NCO electrodes before and after charging are shown in Figure 3d-o. After dropwise adding Na2CO3 solution on prepared electrodes and drying, the Na2CO3 particles can be easily observed in all electrodes (Figure 3d-f). In CMO-NCO and CO-NCO electrodes, the Na2CO3 shows sheet structure and serious agglomeration, and the corresponding EDS sodium mapping images obviously exhibit the Na2CO3 distribution (Figure 3g,h). While in MO-NCO electrode, some Na2CO3 whiskers can be found and the corresponding sodium element mapping also demonstrates the asymmetrical distribution and serious agglomeration of Na2CO3 (Figure 3i). After charging, the electrodes structures were remained, but the loaded Na2CO3 can be hardly found except the MO-NCO electrode. It can be easily concluded that the MO has poor catalytic activity in the Na2CO3 decomposition. The more uniform distribution of sodium element after charging also indicates that most of the loaded Na2CO3 has been decomposed during charging process, while the residual of sodium for the all electrodes may be influenced by NaClO4 in the electrolyte (Figure 3m-o). In general, the lower charge voltage and higher discharge plateau demonstrate that the CMO nanowires have superior electro-catalytic activity in both decomposing and producing of Na2CO3 compared to the CO and MO samples. Considering the similar crystal structure and morphology of CO and CMO, the superior activity of CMO may benefit from its hybrid redox couples. As Na2CO3 is the final discharge product of Na-CO2 battery, thus the activation process of Na2CO3


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is of great significance, which is related with the reversible capacity and cycling performance.

Figure 3. Characterization of the Na2CO3 based cells before and after charge process: (a) voltage-capcaity profiles; (b,c) XRD patterns of Na2CO3-based electrodes before and after charge; SEM images of CMO-NCO electrode, CO-NCO electrode and MO-NCO electrode (d,e,f) before and (j,k,l) after charging; EDS sodium mapping images captured from the regions shown in the corresponding SEM images for CMO-NCO electrode, CO-NCO electrode and MO-NCO electrode (g,h,i) before and (m,n,o) after charging.

To further confirm the priority of CMO as the candidate catalyst for Na-CO2 batteries, CMO@CF, CO@CF, MO@CF as well as bare CF electrodes based Na-CO2 ACS Paragon Plus Environment


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batteries were assembled using CO2 as gas cathodes. Cyclic voltammetry (CV) was used to identify the electrochemical reactions in Na-CO2 batteries. As exhibited in the cyclic voltammetry curves (Figure S5), the CMO@CF shows more distinct anodic and cathodic peaks with higher peak current, which indicates higher catalytic activity in Na-CO2 batteries, compared with the CO@CF, MO@CF and CF electrodes.[4] The charge-discharge curves of Na-CO2 batteries with TMO@CF electrodes at a current density of 200 mA g-1 are shown in Figure 4a. The CMO@CF cathode delivers a capacity of 8448 mAh g-1 at the cut-off voltage of 1.8 V, while the CO@CF, MO@CF and pure CF cathodes exhibit much lower capacity of 7427, 6634 and 1452 mAh g-1, respectively. Furthermore, the CMO@CF electrode shows a stable discharge plateau of at around 2.02 V, higher than those of CO@CF (1.96 V), MO@CF (1.91 V) and CF (1.81 V) electrodes. In charge process with the cut-off voltage 4.2 V, the CMO@CF cathode exhibits lower charge plateau. The initial Coulombic efficiency is 80.2%, which is higher than that of CO@CF (73.1%), MO@CF (68.4%) and pure CF (2.9%). Figure 4b shows the typical charge-discharge curves of Na-CO2 batteries at the first cycle with the limited capacity of 500 mAh g-1, which is consistent with the previous work.34 The CMO@CF exhibits higher discharge plateau at around 2.10 V and lower overpotential around 1.77 V, compared with the CO@CF (1.94 V for discharge plateau, 1.90 V for overpotential), the MO@CF (1.94 V for discharge plateau, 1.85 V for overpotential) and CF (1.80 V for discharge plateau, 2.88 V for overpotential). All these composite electrodes decorated by TMO have much better electrochemical performance than pure carbon fibers electrodes, indicating the enhanced catalytic performance of CF electrodes by TMO decoration. The rate performance of CMO@CF is shown in Figure 4c. The overpotential of the Na-CO2 batteries using the synthesized CMO@CF electrodes are 1.47, 1.77 and 2.19 V with the limited capacity at 500 mAh g-1 when current density are 100, 200 and 500 mA g-1, respectively.

The

stability

of

CMO@CF

electrodes

were

evaluated

by

charge-discharge cycling of Na-CO2 batteries and shown in Figure 4d, e. As can be seen, the CMO@CF electrode still shows better cycling stability over 75 and 48


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cycles under the limited capacity of 500 and 1000 mAh g-1, while the CO@CF, MO@CF and CF electrodes exhibit poor cycle stability.

Figure 4. Electrochemical performance of Na-CO2 batteries based on TMO@CF and pure CF electrodes; (a) Galvanostatic charge-discharge curves of the TMO@CF and CF cathodes with the cut-off voltage of 1.8-4.2 V, at 200 mA g-1; (b) Galvanostatic discharge-charge curves of the TMO@CF and CF electrodes at first cycles with a capacity limitation of 500 mAh g-1, at 200 mA g-1; (c) The rate performance of the Na-CO2 battery with the CMO@CF electrode; (d) The terminal discharge potential vs. the cycling number of the TMO@CF and CF at 500 mAh g-1; (e) The terminal discharge potential vs. the cycling number of the TMO@CF and CF at 1000 mAh g-1.

In order to reveal the further working mechanism of CMO@CF electrodes with remarkable electrochemical performance, the ex-situ XRD and SEM combined with EDS mapping were employed to investigate the CMO@CF electrode. Figure 5a



shows the XRD result of CMO@CF after discharging. No obvious change of the peaks corresponding to CMO can be found. It should be mentioned that extra peaks appeared after discharging, which belong to crystalline Na2CO3 (JCPDS card no. 18-1208). After charging, the Na2CO3 characteristic peaks disappear, but the CMO peaks exist. In addition, FTIR were used to further confirm the CMO@CF electrode state after 1st and 10th cycle. After each discharge, Na2CO3 is identified by peaks at 881 and 1447 cm-1 (Figure S6). While after charge, Na2CO3 peaks disappear. However, the CMO peaks have no change. Both results demonstrate that the Na2CO3 can be thoroughly decomposed and CMO@CF has excellent chemical stability during batteries cycling. In addition, the Raman spectrum was also employed to verify the final product after discharge reaction. To eliminate the effect of carbon from CF electrode, Na-CO2 batteries using CMO decorated nickel foam (CMO@Ni) electrodes (Figure S7) were discharged to the limited capacity of 0.6 mAh cm-2 (Figure S8). As shown by the Raman spectrum in Figure 5b, the peak at 1080 cm-1 can be assigned to the Na2CO3,11,39 while the peaks at 1350 cm-1 (D band) and 1580 cm-1 (G band) are the characteristic peaks of carbon.40,41 These results well confirm the formation of Na2CO3 and carbon during the discharge process of Na-CO2 batteries. The SEM image of CMO@CF electrode after discharging is shown in Figure 5c. It can be seen that the CMO@CF electrode is covered by discharge products, which are agglomerated to form particles. Specially, the stable structure of CMO nanowires has no obvious change. The well coincided Na, O and C mapping combined with XRD, FTIR and Raman results successfully prove that these heterogeneous products are Na2CO3. Combining all characterization test results, the possible working mechanism for CMO@CF electrode in Na-CO2 batteries is illustrated in Figure 5d. During discharge process, when CO2 is adsorbed on the surface of CMO nanowires, and direct surface reaction can be occurred on its polar surface. Na+ ions and electrons transfer to the electrode combining with CO2 to form Na2CO3 and carbon on the surface. While during charge process, Na2CO3 will be decomposed back into Na+ and CO2 under the promoting of CMO@CF.


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Figure 5. (a) XRD pattern of the CMO@CF electrode after charge and discharge; (b) Raman spectra of pristine CMO@Ni electrode and after the initial discharge in Na-CO2 battery; (c) SEM image of CMO@CF after discharge and the corresponding elemental mapping results; (d) Schematic diagram of Na-CO2 battery with CMO@CF and possible reaction mechanism.

CONCLUSIONS In summary, the freestanding CMO@CF composite has been successfully fabricated via a facile hydrothermal method and subsequent annealing process. The designed electrode with metal oxides in-situ grown on carbon fibers improves catalytic performance as well as overcomes the poor conductivity of the metal oxides. Among all these TMO@CF freestanding structure, the Na2CO3 activation experiment demonstrates that CMO@CF electrode shows superior catalytic performance for Na2CO3 decomposition than CO@CF and MO@CF. Compared with CF, CO@CF



and MO@CF electrodes, the Na-CO2 battery using CMO@CF electrode demonstrates larger reversible capacity of 8448 mAh g-1 and less overpotential of 1.77 V, and it can maintain for over 75 and 48 cycles when the capacity is limited at 500 and 1000 mAh g-1, respectively. Furthermore, ex-situ XRD, XPS, Raman and SEM characterization prove that the high catalytic activity of CMO@CF electrode is mainly due to homogeneous morphology, chemical and structural stability and the hybrid Co2+/Co3+ and Mn2+/Mn3+ redox couples. We believe that this novel freestanding electrode design and new facile catalyst screening strategy will be helpful to fabricate and screen other suitable materials for advanced Na-CO2 batteries.

EXPERIMENTAL SECTION Preparation of CMO@CF and CMO@Ni. The CMO nanowires were grew on carbon fibers substrate according to hydrothermal approach. Briefly, 0.388g Co(NO3)2·6H2O and 0.163g Mn(CH3COO)2·4H2O were dissolved in 40 ml deionized (DI) water under stirring for 30 minutes. Then 0.148g NH4F and 0.6005g urea were added. After stirred for 30 minutes, the whole solution was transferred into a 50-ml Teflon-lined stainless steel autoclave. A piece of clean carbon fibers (2 cm x 1 cm) was immersed into the obtained pink solution, which was firstly treated in the nitric acid for 2 h and then washed with DI water for several times. After keeping the autoclave at 120 °C for 5 h, the obtained product was washed with DI water and ethanol for several times and dried at 60 °C in oven overnight. The as-obtained products was further annealed in air at 450 °C for 2h. The Co3O4 nanowires on carbon fibers (CO@CF) were obtained by removing the Mn resources, and the Mn2O3 microcones on carbon fibers (MO@CF) were obtained by removing the Co resources. The mass loading of all active substance are 1.3-1.5 mg cm-2. The current density and specific capacity are based on metal oxide catalysis. Furthermore, the CMO@Ni electrodes were synthesized at same hydrothermal condition except for using nickel foam as substrate. Materials Characterization. The powder XRD (X'Pert Pro diffractometer, Cu Kα, λ = 0.15418nm) was used for characterization of purity and crystalline structure


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of samples. The valence state of elements were performed by X-ray photoelectron spectroscopy (XPS) measurements on a Thermo ESCALAB 250 system with a monochromatic Al-Kα (1486.6 eV) X-ray source. Scanning electron microscopy (SEM; Hitachi S4700) and transmission electron microscopy (TEM; FEI Tecnai G2 F30) equipped with an energy-dispersive spectroscopy (EDS) detector were employed to observe the morphology and component of the samples. Raman Spectrometer ( Renishaw inVia) was used for characterization of electrodes state after discharge. The specific surface area of samples was determined by using an ASAP 2020 physical adsorption analyzer and calculated using the BET theory. Electric conductivity was tested by using a four-point probe (RTS-8, Four probe technology Co., Ltd, Guangzhou, China) with a probe distance of 1 mm. XRF was conducted using Arl Advant’X IntelliPowerTM 4200. FTIR measurements were used to test the electrodes state after charge and discharge. Electrochemical Measurements. The Na-CO2 battery was tested by a type CR2032 coin cell. It was assembled using sodium metal as anode and glass fiber as separator in a glove box filled with argon. Freestanding CMO@CF was cut into a 0.5 cm x 0.5 cm slice and directly used as the porous cathode, while the cathode shell was drilled a hole with 5 mm in diameter to let the CO2 access the electrode. The liquid electrolyte was 1 M NaClO4 in TEGDME. Then assembled batteries were put in sealed glass vessel which was filled with the pure CO2 through the double-rowed pipe method.[7] At the beginning, the batteries rested for 6 h. The galvanostatic test was performed on a battery system (Shenzhen Neware Battery, China) with limited capacity and voltage for Na-CO2 batteries at room temperature. As a contrast, CF, CO@CF and MO@CF were also used in Na-CO2 batteries as the cathode. The assembling of Na2CO3-based batteries were different with that of Na-CO2 battery in three aspects. Firstly, the 20 µL 1.0 M Na2CO3 solution was drop-wise added onto the prepared electrodes by a pipette and then dried at 200 °C in vacuum oven for 24 h to remove the crystal water. The batteries denoted as CO-NCO, MO-NCO and CMO-NCO, respectively. Secondly, the assembled Na2CO3-based batteries were tested in the argon atmosphere, eliminate the interference of CO2. ACS Paragon Plus Environment


Thirdly, it was a charge process with the limited capacity depended on the loading amounts of Na2CO3 at the beginning.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: SEM and TEM images, N2 adsorption-desorption isotherm, FTIR spectroscopy data, water contact angle test, electrochemical performance data and additional equations and table as described in the text .

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT We thank Prof. Zhen Zhou from Nankai University for providing valuable suggestion and information. We acknowledge financial support by the National Natural Science Foundation of China (Grant no. 51722210, 21403196, 51572240, 51677170 and 51777194), the Natural Science Foundation of Zhejiang Province (Grant no. LY16E070004, LY17E020010, LY18B030008 and LD18E020003), the Science and Technology Department of Zhejiang Province (Grant no. 2016C31012, 2016C33009 and 2017C01035) and Ford University Research Program.

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The Na2CO3 activation experiment is designed to compare and screen the catalyst for Na-CO2 batteries by loading Na2CO3 on prepared electrodes and testing under the limited capacity. The selected CMO@CF electrode exhibits the best electrochemical performance, as expected. Such rationally designed strategy may provide important reference for development of advanced Na-CO2 batteries.


Enhancing Catalyzed Decomposition of Na2CO3 with Co2MnOx

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