Proton exchange membrane fuel cell (PEMFC) as the promising clean energy conversion technology has gained considerable attention and been envisioned for large-scale applications. However, the high cost and low durability of Pt-based electrocatalysts supported on high surface area carbon hinder the wide adoption of this technology. Developing carbon-based non-precious metal catalysts with comparable oxygen reduction reaction (ORR) activity to Pt is a feasible approach to addressing the high cost issue. Despite the significant progress that has been achieved on transition metal and nitrogen doped carbon (Me-N-C), the activity and durability are still far from the targets for commercialization.

In this thesis, I try to address the low durability issue of Fe-N-C electrocatalysts by developi...[ Read more ]

Proton exchange membrane fuel cell (PEMFC) as the promising clean energy conversion technology has gained considerable attention and been envisioned for large-scale applications. However, the high cost and low durability of Pt-based electrocatalysts supported on high surface area carbon hinder the wide adoption of this technology. Developing carbon-based non-precious metal catalysts with comparable oxygen reduction reaction (ORR) activity to Pt is a feasible approach to addressing the high cost issue. Despite the significant progress that has been achieved on transition metal and nitrogen doped carbon (Me-N-C), the activity and durability are still far from the targets for commercialization.

In this thesis, I try to address the low durability issue of Fe-N-C electrocatalysts by developing hybrid Pt-Fe-N-C materials with unprecedented durability. They consist of abundant Pt and Fe single atoms homogeneously dispersed on the nitrogen-doped carbon support and a small amount of Pt-Fe alloy nanoparticles. The PEMFC with Pt-Fe-N-C as the cathode shows a larger peak power density than that with Fe-N-C as the cathode. The remarkable durability of Pt-Fe-N-C is reflected from no noticeable drop in the half-wave potential after 70000 potential cycles between 0.6 and 1.0 V in the liquid cell, and 80% current retention after 85 h of potential hold at 0.4 V in the fuel cell.

Although the durability of Pt-Fe-N-C is improved from ultra-low Pt doping, the unsatisfied current density at kinetic region, i.e., the intrinsic activity, needs further improvement. Thus, new Pt-Fe-N-C catalyst with multi-scale active sites was developed after an additional ammonia treatment. It includes of abundant Pt and Fe single atoms and small intermetallic PtFe alloy nanoparticles homogeneously dispersed on the nitrogen-doped carbon support. The synergistic work among different active sites in Pt-Fe-N-C cathode contributes to a high Pt MA of 0.94 A mg^-1 at 0.9 V and peak power density of 0.95 W cm^-2 in fuel cell. The remarkable durability of Pt-Fe-N-C is also maintained, indicating from the 93.6% power retention after 100000 potential cycles and no noticeable drop in the current density after 206 h at 0.6 V holding in the fuel cell tests. The work demonstrates the activity and durability improvement by bridging the non-precious metal-based and precious metal-based electrocatalyst.

In addition, ORR active Fe-N-C electrocatalysts with improved Fe doping and fuel cell performance are also developed in this thesis. The conventional synthesis protocols of Fe-N-C require large quantities of solvents and are time-consuming. For the first time, Fe₂O₃ is adopted as the Fe precursor to derive abundant single Fe atoms dispersed electrocatalysts. The Fe-N-C catalyst showed excellent ORR activity with half-wave potentials of 0.82 and 0.90 V in acidic and alkaline solutions, respectively. A PEMFC with optimized Fe-N-C cathode shows the outstanding peak power density of 0.84 W cm^-2.