Study Of Magnetic Field To Promote Oxygen Transfer And The Reduction Kinetics In Fuel Cells

The low cost and long service life are the bottleneck that restricts the fuel cells commercialization. The Pt/C catalyst modification is the key procedure to reduce the fuel cell cost and improve the service life in fuel cells. To improve the reaction gas concentration can reduce fuel cells activation and the concentration polarization in the operation process. The traditional method to improve the reaction gas concentration is the increase of the gas velocity and gas pressure in fuel cells. While the oxygen is paramagnetic, the nitrogen is diamagnetic, if Pt/C catalyst and the ferromagnetic magnetic powder are loaded in fuel cell cathode together, the magnetic field that generated from the magnetic powder can promote the oxygen transfer to the catalyst layer. This is another effective method to improve the oxygen concentration and decrease the activation and the concentration polarization in the fuel cells cathode.Through the electrochemical three-electrode system, rotating disk glassy carbon electrode, and the zinc air fuel cells (ZAFC) and the proton exchange membrane fuel cells (PEMFC). this paper studies how the oxygen diffusion coefficient, charge transfer coefficient, and the electric double layer capacitance and the charge-transfer resistance are changing in different magnetic strength, different air velocity and different magnetization directions. The Nd2Fe14B/PANI, Fe3O4/PANI and Nd2Fe14B/C materials were synthesized through the ball milling, sol-gel, in-situ polymerization and the high-temperature sintering, respectively. The materials were characterized by XRD, FT-IR, SEM, VSM and the thermal analyzer. The effect of oxygen mass transfer was studied with the above materials and the commercially available50%Pt-5%Co catalysts in the electrochemical svstems.The results show that:(1) There is a positive correlation between the magnetic strength and the oxygen diffusion coefficient, charge transfer coefficient and the oxygen electrochemical reduction. In the polarization state, with the increase of magnetic strength, the oxygen diffusion coefficient, charge transfer coefficient and the electric double layer capacitance are increased, the charge transfer resistance is decreased, and the oxygen reduction reaction (ORR) current is increased.(2) The molecular diffusion and turbulence impair the magnetic attractions to oxygen molecules in the micro magnetic field, which leads to the decrease of oxygen diffusion coefficient, charge transfer coefficient and the ORR rate.(3) When the magnetic field direction is perpendicular to the working electrode surface, it is conducive to the oxygen molecules orderly transfer to the Pt/C catalyst surface, and the diamagnetic H2O molecules also can be removed easily from the electrode surface.(4) In comparison of the three prepared magnetic powder, according to the samples" magnetic properties, the sequence of the magnetic property is:Nd2Fe14B/C≈Nd2Fe14B/PANI> Fe3O4/PANI. (5) Increasing the magnetic material load density, the magnetic particles can provide more magnetic field sources, which enhance the magnetic ZAFC and the PEMFC discharge performance. If the magnetic particles load density is high, the magnetic particles can block the oxygen transfer channel, and the increasing magnetic disorder may decrease the magnetic fuel cell discharge performance.(6) At Nd2Fe14B/C0.40mg cm’2load density in PEMFC cathode, the discharge current of the magnetic PEMFC is increased by39.87%than the nonmagnetic PEMFC at0.20V discharge voltage.(7) As the catalyst is loaded on the electrode surface in an external magnetic field, the50%Pt-5%Co/C catalyst is oriented and fixed in the electrode carriers, the easy magnetization axis of different magnetic particles is consistent with each other, the magnetic orientation of50%Pt-5%Co/C catalyst may make the superior catalytic crystal face more expose in the ORR process, thus the ORR catalytic activity is improved.