Transition-Metal Based Oxygen Reduction Reaction Catalysts and Their Application in Zinc-Air Battery
Student thesis: Doctoral Thesis
Related Research Unit(s)
Oxygen reduction reaction (ORR)catalysts play an essential role in the large-scale implementation of various applications (e.g.fuel cells and metal–air batteries. Noble metal (e.g. Pt, Ir or Ru)-based catalysts have been intensively studied due to their low over-potential and high current density,but their high cost, scarcity and aggregation in alkaline electrolytes, and susceptibility to methanol and carbon monoxide (CO) poisoning remain an outstanding obstacle and refrain them from being used in large-scale commercial applications. It is therefore highly desirable to shift away from noble metal-based catalysts to cost-effective transition metal-based catalysts with competitive ORR performance. This thesis examines three kinds of transition metal-based catalysts: iron nitrate-polyoxometalates with two-dimensional wrinkle structure derived Mo12Fe22C10,Mo2C, and C composites (FeMoC@Mo2C/C); Fe, N,S tri-doped carbon nanotube (FeNSCNT), and Zr-based metal–organic framework (UiO-66-NO2),carbon nanotube and cobalt phthalocyanine composite (CoCNT@UiO-NO2). For the FeMoC@Mo2C/Cand FeNSCNT, catalytic activity was significantly enhanced by homogeneous doping, while CoCNT@UiO-NO2 shows superior ORR performance due to its exceptional oxygen adsorption ability. Moreover,when evaluated in real battery conditions, all three catalysts demonstrated great potential for replacing Pt/C and boosting the commercialization of zinc–air batteries.
The FeMoC@Mo2C/C catalysts were derived from FePOM wrinkled nanosheet,which is facilely fabricated in room temperature without organic additives or solvent, and no harsh fabricate methods like solventhermal methods or hydrothermal methods are required. The derived FeMoC@Mo2C/C exhibited superior electrochemical catalytic properties in alkaline aqueous solutions with a half-wave potential of 0.752V at 1,600 rpm, good stability (no significant degradation in 25,000 s), and high methanol tolerance. In real battery operating conditions, FeMoC@Mo2C/C has higher power density than commercial 20% Pt/C (FeMoC@Mo2C/C: 87 mWcm-2; 20% Pt/C: 75 mW cm-2).
However,the electron transfer number of FeMoC@Mo2C/C is only 3.5, indicating the mix of 2-electron (2e-) and 4-electron (4e-) oxygen reduction pathway. A more efficient ORR catalyst (FeNSCNT) with 4e-oxygen reduction pathway was fabricated. The fabrication process of Fe-NSCNT catalysts is facile and ready for mass production. More importantly, the Fe-NSCNT showed excellent ORR performance, with a four-electron selectivity, high methanol tolerance,enhanced stability (no significant loss after 6 h, cf. 19% loss for 20% Pt/C), and high diffusion-limited current density (6.01 mA cm-2 higher than 5.79 mA cm-2 of the commercial Pt/C), comparable to that of the state-of-the art Pt/C catalyst in alkaline media.Furthermore, when used as Zn–air battery cathode materials, the Fe-NSCNT catalyst enabled the same voltage (1.17 V at 20 mA cm-2) and specific capacity comparable (720 mA h gZn-1 at 10 mA cm-2) to that of the commercial Pt/C (735mA h gZn-1 at 10 mA cm-2), indicating its great potential in replacing Pt/C for the practical applications in noble metal-free Zn–air batteries.Besides fabricating highly efficient ORR catalysts with close to 4e- oxygen reduction pathways, another approach to improve ORR performance is to enhance the oxygen diffusion rate of the catalyst. UiO-66-NO2 serves as an oxygen “pump” in CoCNT@UiO-NO2. As the first UiO based ORR catalyst reported, CoCNT@UiO-NO2 exhibited superior electrochemical catalytic properties exceeding those of state-of-the-art commercial 20% Pt/C catalyst, with 15 mV more positive half-wave potential, better stability, and higher methanol tolerance. When investigated in real battery operation conditions, the CoCNT@UiO-NO2 remains a competitive alternative to commercial 20%Pt/C catalyst due to its significantly higher specific capacitance and power density (improvements of 44% and 113%, respectively).