Enhancement of Oxygen Transfer by Design Nickel Foam Electrode for Zinc−Air Battery

Abstract

To develop a long-lifetime metal-air battery, oxygen reduction electrodes with improved mass-transfer routes are designed by adjusting the mass ratio of the hydrophobic polytetrafluoroethylene (PTFE) to carbon nanotubes (CNTs) in nickel foam. The oxygen reduction catalyst MnO 2 is grown on the nickel foam using a hydrothermal method. Scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Brunauer-Emmett-Teller analysis are employed to characterize the morphology, crystal structure, chemical composition, and pore structure of the electrodes, respectively. The air electrodes are evaluated using constant-current tests and electrochemical impedance spectroscopy. A PTFE:CNT mass ratio of 1:4-2:1 with 3-mm-thick nickel foam yields the optimal performance due to the balance of hydrophilicity and hydrophobicity. When the electrodes are applied in primary zinc-air batteries, the electrode with a PTFE:CNT mass ratio of 1:4 achieves the maximum power density of 95.7 mW cm −2 with a discharge voltage of 0.8 V at 100 mA cm −2 , and completes stable discharge for over 14400 s at 20 mA cm Growing global interest in the development of a smart grid and electric vehicles requires long-lifetime, cost-effective, and environmentally friendly batteries, such as zinc-air and lithium-air batteries. Metal-air batteries offer beneficial properties, such as high theoretical energy and power densities, low operating temperature, low cost, and material recyclability.1 In particular, metal-air batteries offer an advantage over other batteries in that the cathode electroactive species (oxygen) is not stored in the battery system but supplied from the surrounding environment during the discharge process. This unique nature simplifies the metal-air battery structure, which leads to a lighter and more compact battery, thereby increasing the specific energy, which approaches 470 and 1700 Wh kg −1 for zinc−air and lithium−air batteries, respectively. 2The oxygen reaction electrode is the core component of metalair batteries, where the oxygen is reduced through multistep electron transfer processes, involving complicated oxygen-containing species such as O, OH, O 2 2− , HO 2 − . [3][4][5][6] It is generally accepted that oxygen reduction may proceed via a four-electron pathway or two-electron pathway. The specific reactions of oxygen reduction reaction (ORR) in alkaline media are as followings:1. In a four-electron pathway, O 2 is reduced to OH − ;2. In a two-electron pathway, O 2 is reduced to peroxide ion followed by either further reduction or disproportionation.The oxygen reduction reaction (ORR) is usually kinetically sluggish relative to the negative metal anode in these batteries, which results in great voltage loss in the ORR cathode and limits battery performance. This behavior can be partially attributed to the low solubility of O 2 of 1.25 mM in aqueous solutions, 7 and 10 −4 mM in 30 wt% KOH at 25• C, 8 which makes it difficult for oxygen to adsorb on the surface of catalysts in the cathode. 9 Oxygen has ...

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