Study of magnetic field to promote oxygen transfer and its application in zinc–air fuel cells

Highlights: ► High magnetic strength reduces Rct and increases Cd in oxygen reduction reaction. ► Oxygen diffusion and transfer coefficient become large in high magnetic strength. ► The magnetic ZAFC discharge performance is better than the nonmagnetic ZAFC. ► Increased NdFeB/C load density improves the magnetic ZAFC discharge performance. ► Excess NdFeB/C load density decreases the magnetic ZAFC discharge performance. -- Abstract: This study investigates the effects of magnetic field on oxygen transfer and the correlations of electrochemical parameters in different magnetic strengths. The discharge performance of zinc–air fuel cell (ZAFC) was tested under magnetic and nonmagnetic conditions using neodymium–iron–boron/carbon (NdFeB/C) magnetic particles in ZAFC cathode. The results showed that the oxygen diffusion coefficient (DOi) and transfer coefficient (αi) increased by 102.14% and 52.38% when the magnetic strength increased from 0 mT to 5.0 mT, respectively. In addition, the electric double-layer capacitance (Cd) increased from 8.16 to 22.46 μF cm−2, the charge-transfer resistance (Rct) decreased from 9.43 Ω cm2 to 6.02 Ω cm2, and the oxygen reduction reaction (ORR) current was improved. With the NdFeB/C load density of 2.4 mg cm−2 in ZAFC cathode, the discharge current of magnetic ZAFC increased by 13.86% compared with the nonmagnetic ZAFC at the 0.80 V discharge voltage. These results indicate that magnetic strength has a positive correlation with DOi, αi, and the ORR current. Under magnetic attractions, the oxygen transfer process is easier at the Pt/C catalytic surface, and the discharge performance of magnetic ZAFC is superior to the nonmagnetic ZAFC. At lower NdFeB/C load density, increasing the NdFeB/C load density facilitates oxygen transfer and improves the discharge performance of ZAFC. However, the magnetic ZAFC discharge performance decreases at a higher NdFeB/C load density because of the blocked oxygen transfer channel and the magnetic field disorder caused by the magnetic interactions among different magnetic particles