Turning Waste Chemicals into WealthâA New Approach To

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Turning Waste Chemicals into Wealth-A New Approach to Synthesize Efficient Cathode Material for Li-O2 Battery Ying Yao, and Feng Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09483 • Publication Date (Web): 28 Aug 2017 Downloaded from http://pubs.acs.org on August 28, 2017

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ACS Applied Materials & Interfaces

Turning Waste Chemicals into Wealth-A New Approach to Synthesize Efficient Cathode Material for Li-O2 Battery Ying Yao1*, Feng Wu1 1 Beijing Key Laboratory of Environmental Science and Engineering, School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China * Corresponding authors, YY ([email protected])

Abstract Li-O2 battery requires the oxygen breathing cathode to be highly electronically conductive, fastoxygen diffusive and structurally stable, and often time electrocatalytically active. Catalystdecorated porous carbonaceous materials are the choice of air cathode in this regard. On the other hand, biomass-derived carbons possess great ability to remove heavy and toxic metal ions from the waste, forming a metal-adsorbed porous carbon. The similar structure between the air cathode and the metal-adsorbed biomass-derived carbon nicely bridges these two irrelevant areas. In this study, we investigated the electrochemical activity of a biochar material Ag-ESB directly synthesized from ethanol sludge residue in a rechargeable aprotic Li-O2 battery. Ag ions were absorbed from sewage and became Ag nanoparticles with the uniform coverage on the biochar surface. The as-prepared material exhibits good electrochemical behavior in battery testing, especially towards the battery efficiency and cycleability. This study provides the possibility of synthesis efficient cathode material by reusing the “waste” such as biofuel sludge residue. It is an economic and environmental-friendly approach for both energy storage system and waste recycling. Keywords: Li-O2 batteries; biomass-derived carbon; silver; catalyst; oxygen electrodes.

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1. Introduction Aprotic Li-O2 battery, which is based on the conversion reaction of lithium and oxygen via a reversible formation of lithium oxides to store chemical energy into electrical energy, becomes one of the most promising systems for energy storage in the past decade thanks to its extremely high theoretical energy density, about 10 times higher than that of the commercial Liion batteries.1,2 Since Li-O2 battery was firstly reported to be reversible in an aprotic electrolyte in 1996,3 remarkable progress has been achieved in the recent years with significant improvement of electrode materials as well as fundamental understandings. However, poor cycleability and large overpotential are still some of the major hurdles preventing the practical application of such innovative technology.4 There are various reasons for poor cycleability and large overpotential in the Li-O2 cell, including slow kinetics and electrolyte decomposition. It is generally accepted that the nature of the applied electrocatalysts or active cathode materials are responsible for the poor performance of the cell.5-7 Carbonaceous materials, due to their feature of high electronic conductivity, fast oxygen diffusion and stable integrity, has been ubiquitously selected as oxygen-breathing cathode. In addition, the light weight of carbonaceous material premises the high specific capacity for Li-O2 battery. To this end, various porous carbon, such as Super P, Graphitized carbon black, and reduced Graphene oxide (rGO), have been intensively investigated in Li-O2 battery.8-11 However, bare carbon cathodes usually suffers from high overpotential, especially in oxygen evolution reactions (OER) during charge process. Consequently, this could lead to the low efficiency of the battery system, and more seriously, the decomposition of organic electrolytes under high charge potential – one of the main side reactions that degrades the cell performance.11-14 In order to address high overpotential issue, electrocatalysts (nonprecious metals15,16, precious metals17-20, transition-metal oxides21, et al.)


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has been inevitably employed in the cathode materials to facilitate the electrochemical reactions, and thereby improve the battery efficiency. Expect for the high cost, noble metals are effective electrocatalysts for oxygen reduction reaction (ORR) or oxygen evolution reaction (OER) in Li-O2 cells. For instance, electrochemically deposited Ag particles on carbon electrode showed very low charge overpotential of 3.6 V by cycling.22 Further experiments on deposition of size-controllable subnanometer Ag clusters on an Al2O3-passiviated carbon surface demonstrated dramatically different morphologies of the electrochemically grown lithium peroxide in Li-O2 battery dependent on the size of the Ag clusters. This dependence also affects the charge process, suggesting that precise control of subnanometer surface structure on cathodes can be used as an effective approach to improve the performance of Li-O2 cells.18 Other noble metals such as Ir, Pt and Au also showed much improved electrochemical performance in terms of reducing the polarization of Li-O2 cells.19,20,23 All these encouraging results reveal that the realm of the high performance oxygen electrodes has been significantly expanded benefiting from the combination of the suitable electrocatalysts and conductive carbon support. However, how to make cost-effective electrocatalysts/carbon composite is always a concern to enable the Li-O2 battery for future practical application. Although carbon-based materials are generally cheap, it would be more economical and environmentally friendly to use renewable biomass-derived carbons. Biomass is plant materials or animal wastes from natural or agricultural actions, and marine or industrial wastes. Biomass-derived carbonaceous materials, due to their abundance, very low synthetic cost and environmental benignity, have been investigated intensively for energy storage system, showing good electrochemical performance in these devices.24-26 In addition, biomass-derived carbons (often refers to biochar) has also been



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widely used in sewage treatment to remove heavy and toxic metal ions, which otherwise cannot degrade into harmless end-products by biological degradation.27,28

For example, biochar

converted from bioethanol stillage residue showed great potential to selectively remove silver ions, which is classified as hazardous substances, from sewage systems. It is interesting to note that the silver-adsorbed biomass-derived carbon share the same features as carbon cathode materials in Li-O2 battery (i.e., same components (porous carbon) with uniformly distributed metal ions). Inspired by such similarity, we attempt to synthesize a cathode material directly from ethanol sludge, which could make both scientific and economic significance.29,30 In this work, we investigated the electrochemical activity of a silver-adsorbed ethanol sludge biochar (Ag-ESB). Silver ion is chosen as an example of the heavy metal ions from sewage. It is one of the most common heavy metal ions in wastewater from dentistry, clothing, food industry and jewelry manufacturing; and its catalytic activity has been reported in the study of Li-O2 battery. The as-prepared Ag-ESB was tested as the cathode material in an aprotic Li-O2 battery and exhibited much improved catalytic performance toward the battery efficiency and cycleability. The cathodes after electrochemical testing were characterized by scanning electron microscopy (SEM), Raman spectroscopy, and synchrotron X-ray diffraction. The results of this study have proved that biofuel sludge biochar has the potential to uniformly adsorb silver and thus form an efficient cathode material in Li-O2 battery due to its porosity and the uniformly distribution of metal ions. In addition, this work can be generalized to guide the design of economic and environmental-friendly materials used in electric energy storage system.

2. Experimental


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Ethanol sludge (stillage) residue collected from the Biofuel Pilot Plant at University of Florida was used as raw materials for biochar production. The pilot plant produces ethanol from sugarcane bagasse and after the distillation process in which ethanol was separated from fermentation broth the stillage residue was collected and dried in an oven at 60 °C. A tube furnace (MTI, Richmond, CA) was used to pyrolyze them into carbon materials in a N2 environment at temperature of 600 °C for 1 h. The resulting biochar were immersed into 100 mg/l silver solutions for 2 h and separated and dried at 60 ˚C for further tests. The prepared sample was labeled as Ag-ESB. The Electrochemical characterization was carried out using a Swagelok-type cell composed of a lithium metal anode, a glass fiber separator impregnated with ether electrolyte (TEGDME 1M LiCF3SO3) and a porous cathode (7/16 inch diameter). The cathode was fabricated by laminating the as-prepared slurry of cathode material and binder in a molar ratio of 80:20 on to a porous carbon substrate. The cells were sealed in 1 bar pure O2 atmosphere, to avoid any negative effects of humidity and CO2. The electrochemical investigations were carried out on MACCOR cycler, under a constant current density of 100 mA/g. The observed capacity was normalized with the weight of the cathode material. Field-emission scanning electron microscopy (SEM, Hitachi S-4700) coupled with EDAX, Field-emission transmission electron microscope (FEI Titan 80-300ST) with a spherical and chromatic aberration imaging corrector were employed to determine the morphology and elements of the pristine material and discharge products. The spherical and chromatic aberration correction enables the microscope to attain resolution better than 0.1 nm (measured by Young’s fringes) at 80 kV.



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High-energy synchrotron XRD was used to characterize the crystal structure of discharge products. The characterization was carried out at the 11-ID-D beamline of the Advanced Photon Source (APS), Argonne National Laboratory. The employed X-ray wavelengths were 0.6702 Å and 0.7999 Å. The samples were completely protected with Kapton tape to against any side reactions or contaminations from the air moisture. The XRD patterns were collected in the transmission mode and then integrated into conventional one-dimensional patterns for final data analysis using the Fit2d software. Raman spectra of the discharged cathode were obtained using a Renishaw 2000 or inVia microscope spectrometer with a HeNe laser at an exciting wavelength of 633 nm. The sample was loaded inside a glove box into a gas-tight Raman cell with a glass window. Raman spectrum collection was set up in a 180° reflective mode. Roughly 10% of the maximum 13 mW laser intensity was applied.

3. Results and Discussions The porous carbon was selected as the substrate hosting the Ag-ESB cathode material. The as-prepared Ag-ESB material has a tube-bundle structure, which is 5-10 µm in diameter and around 0.5 µm thick (Fig. 1a). The wall of the tube was covered by the discrete spherical nano particles without agglomeration (Fig. 1b). The particles are less than 15 nm Ag clusters (Fig. 2a) determined by energy dispersive X-ray analysis (EDAX) and electron diffraction (Fig. 2b and c). The dispersion of the nano-Ag is clearly shown in the TEM image (Fig. 2a). The average diameters of Ag nanoparticles is ~5.8 nm, with the maximum of 12.1 nm and minimum of 1.5 nm. Smaller particles are difficult to identify due to the contrast limitation of the image. The electron diffraction pattern (Fig. 2b) shows the Ag diffraction rings corresponding to (111), (200),


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(220), (311), and (222) planes, indicating the Ag fcc crystalline structure. Under ultra-high magnification (Fig. 2c), Ag (111) plane is observed with lattice diameter d=2.36 Å. The specific surface area of the Ag-ESB is about 25 m2/g obtained from nitrogen adsorption–desorption isotherm measurements. In our previous study,18 particle size of the electrocatalyst behaved key influence on the electrochemical reactions and the products. By tuning the character and density of active site on the electrocatalysts surface, the formation mechanism and charge process were dramatically different.31,32 The electrochemical activity of the Ag-ESB cathode was investigated in a Swagelok-type aprotic Li-O2 cell. A typical cell contains a porous carbon cathode (13mm diameter) with specific amount of Ag-ESB loading, a Li-metal disk anode, and a glass fiber separator wetting with aprotic electrolyte (1M LiCF3SO3 in tetraethylene glycol dimethyl ether, TEGDME). The cell was full filled with a constant high purity O2 flow for 30 min followed by testing on a MACCOR cycler. The charge and discharge were operated at relatively low current density of 100 mA cm-1. With a discharge plateau at the potential of 2.65 V (Fig. 3), the cell delivers a discharge specific capacity over 6000 mAh/g. Comparing with published records, this cathode provide a relatively high specific capacity and low overpotential for oxygen reduction reaction (ORR), i.e. during discharge process. The discharged cathode resulting from discharge reaction were collected and examined by SEM (Fig. 3 inset), high-energy XRD (Fig. 4a), and Raman spectroscopy (Fig. 4b), in order to determine the discharge products at ORR. The SEM image reveals that toroid-like product with the diameter of 200-500 nm flourished on the discharged cathode surface. Such toroid shape is accepted as the morphology of one typical discharged product Li2O2 in Li-O2 batteries.1 The determination of Li2O2 discharge product was also proven by the synchrotron high-energy XRD scan at advanced photon source, Argonne National



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Laboratory. The resulted pattern in Fig. 4a is consistent with hexagonal structured Li2O2 according to the peaks corresponding to the (100), (101), (102), (110), and (105) Li2O2 lattice planes. It is clearly demonstrating that the Li2O2 are nanocrystallines. The nucleation and formation of Li2O2 toroid produced at a specific state of discharge are observed linked to the nature of electrocatalyst and cathode. Moreover, the Raman spectrum of the discharged Ag-ESB cathode in Fig. 4b presents peaks at 1126 cm-1 and 1508 cm-1, attributed to the O-O bond and the strong interaction between LiO2 and graphitic carbon surface, respectively.20 LiO2 has been reported as one of the major discharge products of Li-O2 battery and been proved to be able to reduce the OER overpotential and improve the cycleability.20 Although it is not the major product and only locates on the surface of the discharge products in this study, the presence of LiO2 makes it possible to obtain an enhanced battery performance. The aprotic cell with Ag-ESB cathode was cycled at constant current density with controlled cut-off capacity to investigate the discharge/charge behavior. The operated voltage window is 2.2-4.5 V. The charge cut-off capacity aligned with the determined discharge capacity of 500 mAh/g, in order to avoid electrolyte decomposition caused by overcharge (Fig. 5). The cell is cycled at room temperature right after full filled with O2 gas. The discharge voltage stayed at a steady plateau of ~2.7 V upon increasing of the capacity. Such discharge voltage is very common for carbon-based cathode due to the catalytic function of carbon for ORR.8 During the first charge, a short charge plateau at 3.2-3.3 V were initially appeared, followed by other ones at ~ 3.95 V and 4.0-4.2 V. Such huge fluctuation was only observed at the first charge. After that, the charge voltage exhibited only one smooth plateau for each cycle. Also, the charge potential decreased to

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