Development of coin-type cell and engineering of its compartments for rechargeable seawater batteries
Introduction
Excessive emissions of CO2 from the combustion of fossil fuels are the main causes of climate change and environmental pollution, which are the global issues to be solved for a sustainable society. These issues require the development of new types of systems and technologies for energy conversion and storage based on renewable energy [1], [2]. The increasing demand for the efficient utilization of renewable energy has led to the development of various types of energy storage systems (ESSs) [3], [4]. The main purpose of ESSs is to store renewable energy and use it with high energy efficiency on demand. Currently, lithium-ion batteries (LIBs) technology is a mature battery technology that holds a major position in the ESS market because of their high energy density (∼300 Wh kg−1), long cyclability (≥2000 times), and competitive energy cost (250–400 $ kWh−1) [4], [5], [6]. However, their long-term sustainable use may be limited because of the steeply rising prices of Li-containing raw materials due to the increasing demand for LIBs in electric vehicles and large-scale ESSs and due to their finite geographical distribution [7].
Our group has recently introduced a novel, low-cost, and eco-friendly rechargeable seawater battery using earth-abundant, natural seawater as the active material [8], [9], [10], [11], [12], [13], [14], [15]. Fig. 1a illustrates the schematic of the basic structure and the components of a seawater battery cell. The cell consists of two compartments, an anode and a cathode, which are separated by a NASICON ceramic electrolyte (Na3Zr2Si2PO12). The anode compartment is composed of a sodium metal anode attached to a current collector and a non-aqueous liquid electrolyte. The cathode part consists of a cathode current collector and seawater catholyte. Utilizing seawater that contains Na+ and Cl− ions, the batteries operate based on the redox reactions of Na+ ions at the anode side and the simultaneous evolution/reduction reactions of gaseous O2 and Cl2 in seawater at the cathode side during charge and discharge processes. In principle, the oxygen evolution/reduction reaction (OER/ORR) is thermodynamically preferred over the chlorine evolution/reduction reaction (ClER/ClRR), according to the Pourbaix diagram of a water electrolyte containing Cl− ions; however, the proportion of the two reactions depends on the operating conditions near the cathode current collector. Considering the pH of seawater (∼8), Na+ ion content in seawater (∼0.47 M) and oxygen partial pressure at 100% saturation from ambient air (∼0.2 atm), the half-cell and full-cell reactions during the charge/discharge processes and the theoretical cell voltage (Ecell) can be described as follows [11]:
During discharge, the Na metal anode is oxidized to Na+ ions and transported into the seawater catholyte through the NASICON membrane. At the same time, the ORR occurs, forming water-soluble NaOH at the cathode side. The cell is charged by the reduction of Na+ ions from seawater onto the anode in the opposite manner, while seawater oxidation (OER) occurs at the cathode side.
The seawater battery system employs multilayer electrolytes consisting of non-aqueous (anolyte) and aqueous (seawater catholyte) electrolytes and a ceramic electrolyte (NASICON) between them. Such a structural feature of the cell requires a new type of cell platform and a testing environment other than a typical 2032 coin-type cell. Previously, we have reported a comparative study between sodium, beta-alumina (Na, β”-Al2O3), and Na3Zr2Si2PO12 as the Na+ ion conducting solid electrolyte for seawater batteries [16]. We also investigated non-aqueous electrolytes at the anode side such as ether-based or ionic liquid electrolytes [10], [17], as well as negative electrodes to replace the Na metal, and hence to achieve Na metal-free seawater batteries [8], [10], [14]. It was found that the cell design, the choice of the component material, and its engineering could affect the cell performance of the materials being investigated as potential electrodes and electrolytes. Hence, it is essential to have the normalized cell and its standard testing condition so that the potential chemicals can be easily tested and their results can be compared to those obtained in the other labs. This will greatly contribute to the further development of seawater batteries by providing many choices of key materials. For example, the great success of the Li-ion battery technology also started with developing a coin-type cell design that allowed researchers to investigate and discover its key electrode and electrolyte materials.
In this work, we show the importance of cell components in seawater batteries and the optimized cell performance by engineering them and highlighting the significance of the cell design and component engineering. The effect of wettability of the cathode current collector was investigated. As a low-cost, highly conductive cathode current collector, a commercially available carbon felt was selected and the surface wettability was examined to improve the charge-discharge behaviors. In addition, the flow effect of the seawater catholyte was studied by conducting a comparison test of flow ON/OFF states. Furthermore, we proved the sluggish kinetics of the OER/ORR on the cathode current collector in seawater, which induces major kinetic limitations during the charge and discharge processes of seawater batteries, by comparing a seawater cell with a fast Na-ion-intercalating electrode material. To improve the cathode reaction kinetics, we employed several electrocatalysts facilitating the OER/ORR, highlighting significance of the role of the cathode current collector in achieving low-cost, high-performance seawater batteries.
Section snippets
Preparation of solid electrolyte
NASICON (Na3Zr2Si2PO12) was used as a solid electrolyte to separate the seawater cathode from the anode. The NASICON was fabricated by a solid-state reaction based on our previous study [9], [10], [18]. Briefly, the precursor powders of Na3PO4·12H2O, SiO2, and ZrO2 (Aldrich) were uniformly mixed using a mechanical ball mill and calcined at 400 °C and 1100 °C in ambient air. The calcined powder was ground and uniaxially pressed into pellets at 7 MPa. The pellets were sintered at 1230 °C for 10 h
Design of coin-type cell
The basic structure and components of a seawater battery cell are schematically depicted in the bottom portion of Fig. 1a. A photograph of a coin-type cell assembled into a flow-cell tester is shown in Fig. 1b. The key point of the cell is to completely separate the highly reactive Na metal anode part from the seawater catholyte, with only the NASICON exposed for the selective transport of Na+ ions between the two compartments. The NASICON ceramic was designed to be round in shape with a
Conclusions
In order to collect reliable data from the materials that are the key elements in seawater batteries, a coin-type cell, a flow-cell tester, and their key compartments were designed and fabricated by trial and error and the testing environment was also determined. We examined the wettability of seawater on the carbon felt cathode current collector and its effect on the charge-discharge cycling performance. The air-heating process of carbon felts made the surface hydrophilic, resulting in a
Acknowledgements
This study was supported by the 2017 Research Fund (1.170012.01) of UNIST (Ulsan National Institute of Science and Technology) and the Human Resources Program in Energy Technology of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), which was granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (20174010201240).
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2021, DesalinationCitation Excerpt :SWB (SWB2465, 4TOONE, Republic of Korea) was assembled using a previously described procedure [35]. The coin cell compartments consisted of seven components in the following order: 1) the coin bottom equipped with a NASICON (Na3Zr2Si2PO12; CE&Chem Co. Ltd., Republic of Korea) membrane with a diameter of 16 mm and a thickness of 1 mm [38], 2) polyethylene (PE; Celgard, Republic of Korea) separator with a 19 mm diameter, 3) 200 μL of a mixed organic electrolyte (1 M NaCF3SO3) dissolved in tetraethylene glycol dimethyl ether (TEGDME; Sigma-Aldrich, USA), 4) an electrode comprised of activated carbon fiber (ϕ = 16 mm, CNF Co. Ltd., Republic of Korea) coated with sodium metal (119 mg; Sigma-Aldrich), 5) metal spacer, 6) metal spring, and 7) coin top (Fig. S1). After the components were stacked, the coin cell was perfectly sealed using a coin cell sealing machine (4TOONE) to prevent electrolyte leakage.