High performance Al-air batteries (AABs) with reduced corrosion

• Main Author: Chen, Juntong
• Other Authors: Lee Pooi See
• Format: Thesis-Master by Research
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Materials::Energy materials
• Online Access: https://hdl.handle.net/10356/163094
• Institution: Nanyang Technological University

Al-air batteries are candidates for next-generation energy storage devices due to advantages including theoretical high energy density, low cost, eco-friendliness, etc. However, the self-corrosion reaction of the Al anode in the alkaline electrolyte is a fatal drawback that prevents the practical usage of Al-air batteries. The traditional methods for solving this problem are alloying and adding corrosion inhibitors into electrolytes. The inhibitors will form a protection layer (PL) on surface of the Al anode. This PL can change the electrochemical properties of Al, which in turn reduces the corrosion rate. In this thesis, stabilizing layer (SL) was added to the Al/PL interface to maintain the integrity of PL so that the PL will be densely formed and present a better inhibition effect. Different PVDF membranes and hydrogel electrolyte were selected as stabilizing layers. SEM, EDS, and XRD tests were used to analyze the Al anode in different conditions, and the results proved that PL was preserved by SL. Besides, linear sweep voltammetry (LSV) showed the Al anodes have lower corrosion potential and reduced corrosion when SL was applied. These data supported the hypothesis that SL can preserve PL and the PL with better integrity demonstrated outstanding inhibition performance. The hydrophilic PVDF stabilizing layer improved the specific capacities of AABs to 1833.3 mAh g-1 and 2503 mAh g-1 at 3 mA cm-2 and 15 mA cm-2, respectively. A comparison was made using agarose hydrogel as the stabilizing layer, and the AABs reach a 1750 mAh g-1 specific capacity at 15 mA cm-2 which is lower than AABs with the PVDF membrane. The better performance of AABs with PVDF membrane at high discharge current density proved that the high ion diffusion across the SL could reduce the anode polarization and the accumulation of irreversible deposits. Moreover, the Al showed a lower corrosion current in organic electrolytes such as methanol and ethanol in previous reported works. Dual/tri-electrolyte systems based on organic anolytes and aqueous catholytes were developed to mitigate the corrosion problem of the Al anode. This thesis introduced ethyl acetate (EA), which takes the advantage of the high electrochemical stability of EA molecules, and this high electrochemical stability prevents the EA anolyte from being decomposed by high energy electrons from the Al anode. Al anodes were treated with different methods for activating the surface, and two types of battery configurations were adopted to find out whether the Nafion membrane can separate the anolyte and catholyte. Comparing Tafel polarization curves, the Al anode in the dual-electrolyte system demonstrated a much lower corrosion current (0.03 mAh g-1) than the anode in the single NaOH electrolyte (18 mAh g-1) verifying the low self-corrosion of Al anode in EA anolyte. The full battery polarization tests demonstrate that the Nafion membrane and the longer electrodes-distance in type 2 battery configuration lead to large concentration polarization and lower discharge voltage. Galvanostatic discharge tests were performed at 0.1 mA cm-2 to evaluate the Al utility. At this low current density, the type 1 configuration AAB attained 2912 mAh gAl-1 specific capacity, and the type 2 configuration AAB with Nafion membrane attained 2873 mAh gAl-1 specific capacity (based on consumed Al). However, the discharge times of AABs in the dual-electrolyte system are very short that can only last 1.2 h. The EIS test of discharged Al anodes demonstrated high anodes surface impedances giving evidence to the formation of a passive layer, and the SEM analyses proved this speculation. XRD spectra show the passive layer mainly consists of Al(OH)3. Therefore, the short discharge time of the dual-electrolyte AABs is ascribed to a passive layer on the Al anode. In summary, this work provides new strategies by introducing SL in the alkaline electrolyte and applying EA as an anolyte in a dual-electrolyte system to AABs for reducing corrosion and enhancing battery performance.