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Turning Waste Chemicals into WealthA New Approach To Synthesize Efficient Cathode Material for an Li−O2 Battery Ying Yao* and Feng Wu Beijing Key Laboratory of Environmental Science and Engineering, School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China ABSTRACT: An Li−O2 battery requires the oxygen-breathing cathode to be highly electronically conductive, rapidly oxygen diffusive, structurally stable, and often times electrocatalytically active. Catalyst-decorated porous carbonaceous materials are the chosen air cathode in this regard. Alternatively, biomass-derived carbonaceous materials possess great ability to remove heavy and toxic metal ions from waste, forming a metaladsorbed porous carbonaceous material. 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 AgESB directly synthesized from ethanol sludge residue in a rechargeable aprotic Li−O2 battery. Ag ions were adsorbed from sewage and became Ag nanoparticles with uniform coverage on the biochar surface. The as-prepared material exhibits good electrochemical behavior in battery testing, especially toward the battery efficiency and cyclability. This study provides the possibility of synthetically efficient cathode material by reusing “waste” such as biofuel sludge residue. It is an economically and environmentally friendly approach both for an energy-storage system and for waste recycling. KEYWORDS: Li−O2 batteries, biomass-derived carbon, silver, catalyst, oxygen electrodes
1. INTRODUCTION The 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, became 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 Li-ion batteries.1,2 Since an Li−O2 battery was first reported to be reversible in an aprotic electrolytic fashion in 1996,3 remarkable progress has been achieved in recent years with significant improvement of electrode materials as well as fundamental understanding. However, poor cyclability 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 cyclability 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 features of high electronic conductivity, fast oxygen diffusion, and stable integrity, have been ubiquitously selected as the oxygenbreathing cathode. In addition, the lightweight carbonaceous material premises the high specific capacity for an Li−O2 battery. To this end, various porous carbonaceous materials, such as Super P, graphitized carbon black, and reduced graphene oxide (rGO), have been intensively investigated in an Li−O2 battery.8−11 However, bare carbon cathodes usually suffer from high overpotential, especially in oxygen-evolution © 2017 American Chemical Society
reactions (OER) during the charging process. Consequently, this could lead to the low efficiency of the battery system, and more seriously, the decomposition of organic electrolytes under high charge potentialone of the main side reactions that degrades the cell performance.11−14 In order to address high overpotential issue, electrocatalysts (nonprecious metals,15,16 precious metals,17−20 transition-metal oxides,21 etc.) 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 an oxygen-reduction reaction (ORR) or oxygen-evolution reaction (OER) in Li−O2 cells. For instance, electrochemically deposited Ag particles on a carbon electrode showed a very low charge overpotential of 3.6 V by cycling.22 Further experiments on the deposition of size-controlled subnanometer Ag clusters on an Al2O3-passiviated carbon surface demonstrated that dramatically different morphologies of the electrochemically grown lithium peroxide in an Li−O2 battery were dependent on the size of the Ag clusters. This dependence also affects the charge process, suggesting that precise control of the 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 performReceived: July 1, 2017 Accepted: August 28, 2017 Published: August 28, 2017 31907
DOI: 10.1021/acsami.7b09483 ACS Appl. Mater. Interfaces 2017, 9, 31907−31912
Research Article
ACS Applied Materials & Interfaces ance in terms of reducing the polarization of Li−O2 cells.19,20,23 All of 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 composites to enable the Li−O2 battery for future practical applications is always a concern. Although carbon-based materials are generally cheap, it would be more economical and environmentally friendly to use renewable biomass-derived carbon-based materials. Biomass is plant material or animal waste 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 systems, showing good electrochemical performance in these devices.24−26 In addition, biomass-derived carbonaceous materials (often refers to biochar) have also been 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 are classified as a hazardous substance, from sewage systems. It is interesting to note that the silver-adsorbed biomass-derived carbonaceous materials share the same features as carbon-cathode materials in an 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). The 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 as well as the clothing, food, and jewelry manufacturing industries; its catalytic activity has been reported in the study of the 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 cyclability. The cathodes after electrochemical testing were characterized by scanning electron microscopy (SEM), Raman spectroscopy, and synchrotron Xray diffraction. The results of this study have proven that biofuel sludge biochar has the potential to uniformly adsorb silver and thus form an efficient cathode material in an Li−O2 battery due to its porosity and its uniform distribution of metal ions. In addition, this work can be generalized to guide the design of economically and environmentally friendly materials used in electric energy-storage systems.
separator impregnated with ether electrolyte (TEGDME 1 M LiCF3SO3), and a porous cathode (7/16 in. diameter). The cathode was fabricated by laminating the as-prepared slurry of cathode material and binder in a molar ratio of 80:20 onto a porous carbon substrate. The cells were sealed under 1 bar of pure O2 atmosphere to avoid any negative effects of humidity and CO 2. The electrochemical investigations were carried out on a 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 S4700) coupled with energy-dispersive X-ray analysis (EDAX) and a 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 chromaticaberration correction enables the microscope to attain resolution better than 0.1 nm (measured by Young’s fringes) at 80 kV. 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 and 0.7999 Å. The samples were completely protected with Kapton tape to against any side reactions or contamination 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 glovebox into a gastight 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 DISCUSSION The porous carbon-based material 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 (Figure 1a). The wall of the tube was covered by the discrete spherical nanoparticles without agglomeration (Figure 1b). The particles are Ag clusters smaller than 15 nm (Figure 2a) as determined by EDAX and electron diffraction (Figure 2b and c). The dispersion of the nano-Ag is clearly shown in the TEM image (Figure 2a). The average diameters of the Ag nanoparticles is ∼5.8 nm, with a maximum of 12.1 nm and a minimum of 1.5 nm. Smaller particles are difficult to identify due to the contrast limitations of the image. The electron-diffraction pattern (Figure 2b) shows the Ag diffraction rings corresponding to (111), (200), (220), (311), and (222) planes, indicating the Ag fcc crystalline structure. Under ultrahigh magnification (Figure 2c), the Ag (111) plane is observed with a lattice diameter d = 2.36 Å. The specific surface area of the Ag-ESB is about 25 m2/ g, which was obtained from nitrogen adsorption−desorption isotherm measurements. In our previous study,18 the particle size of the electrocatalyst behaved as a key influence on the electrochemical reactions and the products. By tuning the character and density of active site on the electrocatalyst 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 (13 mm diameter) with a specific amount of Ag-ESB loading, a Li-metal disk anode, and a glass-fiber separator wetted with aprotic electrolyte (1 M LiCF3SO3 in tetraethylene glycol dimethyl ether, TEGDME). The cell was filled with a constant high-purity O2 flow for 30
2. EXPERIMENTAL SECTION Ethanol sludge (stillage) residue collected from the Biofuel Pilot Plant at University of Florida was used as raw material for biochar production. The pilot plant produces ethanol from sugar cane 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 an N2 environment at temperature of 600 °C for 1 h. The resulting biochar was 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 31908
DOI: 10.1021/acsami.7b09483 ACS Appl. Mater. Interfaces 2017, 9, 31907−31912
Research Article
ACS Applied Materials & Interfaces
100 mA cm−1. With a discharge plateau at the potential of 2.65 V (Figure 3), the cell delivers a discharge specific capacity over
Figure 3. Voltage profile and SEM image (inset) of the Ag-ESB cathode discharged to 2.2 V.
6000 mAh/g. In comparison with published records, this cathode provides a relatively high specific capacity and low overpotential for an oxygen reduction reaction (ORR), i.e. during the discharge process. The discharged cathode resulting from the discharge reaction was collected and examined by SEM (Figure 3 inset), high-energy XRD (Figure 4a), and Raman spectroscopy (Figure 4b), in order to determine the discharge products of an ORR. The SEM image reveals that toroid-like product with the diameter of 200−500 nm flourished on the discharged-cathode surface. Such a toroid shape is accepted as the morphology of one typical discharged product, Li2O2 in Li−O2 batteries.1 The determination of the Li2O2 discharge product was also proven by the synchrotron high-energy XRD scan at the Advanced Photon Source, Argonne National Laboratory. The resulted pattern in Figure 4a is consistent with hexagonally structured Li2O2 according to the peaks corresponding to the (100), (101), (102), (110), and (105) Li2O2 lattice planes. It is clearly demonstrating that Li2O2 is a nanocrystalline material. The nucleation and formation of the Li2O2 toroid product at a specific state of discharge are observed to be linked to the nature of electrocatalyst and cathode. Moreover, the Raman spectrum of the discharged AgESB cathode in Figure 4b presents peaks at 1126 and 1508 cm−1, attributed to the O−O bond and the strong interaction between LiO2 and the graphitic carbon surface, respectively.20 LiO2 has been reported as one of the major discharge products of an Li−O2 battery and has been proven to be able to reduce the OER overpotential and improve the cyclability.20 Although it is not the major product and is only located 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 an Ag-ESB cathode was cycled at constant current density with controlled cutoff capacity to investigate the discharge/charge behavior. The operated voltage window is 2.2−4.5 V. The charge-cutoff capacity was aligned with the determined discharge capacity of 500 mAh/g, in order to avoid electrolyte decomposition caused by overcharge (Figure 5). The cell is cycled at room temperature right after being filled with O2 gas. The discharge voltage stayed at a steady plateau of ∼2.7 V upon the increase of the capacity. Such discharge voltage is very common for a carbon-based cathode
Figure 1. SEM images of the pristine Ag-ESB sample at low (a) and high (b) magnifications.
Figure 2. TEM image (a), electron-diffraction pattern of metallic particles (b), and EDAX spectrum of the pristine Ag-ESB sample (c). Inset of (c) is the high-resolution TEM showing the Ag nanoparticle lattice.
min followed by testing on a MACCOR cycler. The charge and discharge were operated at a relatively low current density of 31909
DOI: 10.1021/acsami.7b09483 ACS Appl. Mater. Interfaces 2017, 9, 31907−31912
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a) Synchrotron XRD pattern and (b) Raman spectrum of the Ag-ESB cathode discharged to 2.2 V.
catalyst-loaded cathode and a stable electrolyte became one of the main topics in lithium−oxygen research, in order to lower the overpotential and eliminate the side reactions.36 To this end, a step-by-step understanding of inside mechanisms is the key to estimate the level of side reactions and to ultimately point out the reason for these deficiencies. Therefore, the cycled cathodes were detailed and examined by high-energy XRD and Raman spectroscopy to confirm the electrolyte decomposition and byproducts. The XRD pattern (Figure 6a)
Figure 5. Voltage profile of the Ag-ESB cathode cycled under capacitycontrolled mode.
due to the catalytic function of carbon for ORR.8 During the first charge, a short charge plateau at 3.2−3.3 V initially appeared, followed by other ones at ∼3.95 V and 4.0−4.2 V. Such a 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
