However, the fundamental understanding of the cathode reactions in SWBs has not yet been fully elucidated; therefore, an investigation into their mechanisms is essential for future development. In this study, it was observed that carbon fibers of the current collector cathode undergo side reactions such as fiber-breaking carbon corrosion during charging and surface-insulating CaCO3 precipitation via carbon dioxide capture during discharge, both of which lead to cathode performance failure. Additionally, carbon corrosion was determined to be the dominant factor behind performance degradation under normal charge-discharge cycle conditions compared to CaCO3 precipitation, which was found to be a reversible phenomenon during the operation of the SWB.
These results provide insight for future work to improve the longevity of SWBs by identifying carbon corrosion as the main cathode performance degradation mechanism. As a result of these findings, an alternative cathode was found that exceeds the performance and cycle stability of the previously investigated ACC. An activated carbon fiber (ACF) cathode with a greater degree of graphitization than the ACC was found to resist carbon corrosion.
Characterization and comparison of the level of carbon disorder in ACC and ACF was performed to elucidate the origin of the difference in carbon corrosion resistance.
List of Tables
Introduction 1.1 Energy Storage Systems
- Seawater Batteries
- Research Proposal
With the ever-increasing demand for energy consumption, environmental problems such as global warming and pollution from fossil fuels are becoming a constant problem, making the exploitation of renewable energy sources such as wind and solar energy imperative. However, there are disadvantages such as long construction period, high initial investment cost, risk of environmental destruction, limited construction areas due to geographical restrictions, disconnection of ecosystem and high loss of electricity due to long supply distance from power plant to consumer. must be taken into account. Other battery-based ESS (BESS) technologies also exist for their compatibility with the grid in terms of frequency regulation, for their ability to quickly provide small-scale electricity in sudden change scenarios.
Lead-acid batteries have the longest history in terms of actual use, but their use is currently declining due to the disadvantages listed above. NaS batteries are suitable for large-scale energy storage systems due to their large capacity and high efficiency charge/discharge characteristics, but they must maintain high temperatures for charge/discharge, resulting in in an inherent risk of explosion. SWBs have the potential to be used as an energy storage method especially in marine applications, due to their intrinsic ability to use sodium ions in natural seawater as a source of cathode active material.
Furthermore, SWBs have advantages in terms of safety due to the use of natural seawater as a coolant and the use of solid electrolyte, which increases the safety of the SWB, minimizing the need for additional thermal management facilities. Due to the advantages mentioned above, SWB is promising in terms of its application to marine applications such as smart ICT buoy systems, sonar buoys, marine navigation beacons and auxiliary power sources for ships. In more detail, carbon corrosion was found to occur as a side reaction in fuel cells26-33 under start-up/shutdown conditions and vanadium redox flow batteries34-38 under charging conditions, both due to their use of carbonaceous electrodes for oxidation situations.
In this study, we investigated the failure mechanisms of the ACC cathodic current collector in SWB systems, which were found to be caused by cathodic side reactions occurring during charge and discharge processes instead of the desired target reactions, OER and ORR.
Experimental
- Cell assembly
- Material Characterization
- Electrochemical Characterization
Scanning electron microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDS) mapping were performed on an S-4800 electron microscope (Hitachi High-Technologies, 10kV, 7uA) to observe the impact of side reactions on the surface of the ACC- cathode. Specific surface area and pore size distribution of the ACC were determined by N2 adsorption-desorption isotherms at 77K by the BELSORP-Max physisorption analyzer (BEL Japan) by the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) to use. method. Raman spectroscopy was used to compare the degree of graphitization in the ACC and ACF cathode by calculating the Id/Ig ratio by the Alpha300S AFM-Raman analyzer (WITec).
The charge-discharge performance of SWBs using ACC as cathode was tested using the WonATech battery test system (WBCS 3000) with a current of 0.25mA (0.13mA cm-2) for different periods. The biological electrochemical workstation (VSP-300) was used to conduct cyclic voltammetry (CV) and polarization curves through a three-electrode configuration with Ag/AgCl as the reference electrode, platinum coil as the counter electrode, ACC as the working electrode, and seawater as the electrolyte. A modified three-electrode setup composed of two cathodes and one anode was additionally used to identify individual side reactions under normal cycling conditions.
A differential electrochemical mass spectrometer (DEMS) was used to identify CO2 evolution during the charging process, and its detailed setup is described elsewhere and shown in Figure 16.41. 42 An ORION Star A216 pH meter (Thermo Fisher Scientific) was used on site to monitor the pH change during the filling process.
Results and discussion
- Cathode side-reactions during charging
- Cathode side-reactions during discharging
- Side-reactions at actual cycle environments
- Resistance to side-reactions according to graphitization
In order to investigate the physical effects of carbon corrosion on the cathode structure, the morphology and microstructure change of ACC cathode fibers were observed by SEM images. 19 shows low and high magnification SEM images of the ACC cathode after 20 mAh pre-charge. Nitrogen adsorption/desorption isotherms of pristine and prefilled ACC samples are shown in Fig.
XPS surface analysis technique was also performed to analyze the change in the surface functional groups on the surface of the ACC cathode when it undergoes carbon corrosion. The recording spectra of the ACC cathodes before and after charging for 20mAh are shown for comparison in Fig. The electrochemical performances of the ACC cathode before and after pre-charging were compared by assembling SWB cells for galvanostatic charge tests.
This decrease in surface area and electrochemical activity occurs due to the unwanted carbon corrosion reaction that occurs as a side reaction during SWB charging. These results indicate that carbon corrosion led to the deterioration of ACC cathode performance. Identification of CaCO3 precipitation as a side reaction during SWB cell discharge was performed via XRD analysis.
25 shows a digital photograph of an ACC sample taken to identify the precipitates formed on the surface after a 20 mAh discharge. Nitrogen adsorption/desorption isotherms of ACC samples before and after discharge are shown in the figure. The electrochemical properties of the ACC cathode before and after predischarge were evaluated by assembling SWB cells for galvanostatic discharge tests.
Compared to the pristine ACC cathode, the pre-discharged cathode exhibited a loss of ECSA, although not as severe as in the case of the carbon corrosion voltammogram. The CV results are in correspondence with these previous results, elucidating that CaCO3 precipitation has blocked the access of the reactant to the active sites of the ACC cathode. Carbon corrosion and CaCO3 precipitation at the ACC cathode were identified to occur as side reactions during SWB charging and discharging.
SEM and EDS images of the surface fibers after charging the pre-discharged 20 mAh ACC with an identical capacity of 20 mAh are shown in Figure. The carbon corrosion resistance of the ACC and ACF was compared by monitoring the gas evolution with DEMS during galvanostatic charging of the SWB cell at a current of 0.25 mA (0.13 mAcm-2), as shown in Fig.
Conclusion
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Acknowledgements
