Oxygen Reduction Reaction by Copper Complex based Electrocatalysts

My research focuses on catalysis of oxygen reduction reaction (ORR) by a series of Cu(II) [copper with positive two valence] -1,2,4-triazole complex-based electrocatalysts at the cathode of PEMFC (polymer electrolyte membrane fuel cell), an efficient and environmental friendly energy conversion system compared to internal combustion engines in use today. The sluggish kinetics of ORR considerably limited the performance of PEMFCs. Understanding of ORR mechanism is important for developing affordable, active and durable ORR catalysts for such devices.

The first part of my work focused on improving the ORR performance of Cu(II)-1,2,4-triazole complex-based catalysts in an acidic environment by exploring synthesis conditions including carbon support pretreatment, anions of Cu(II) compounds and reactants stoichiometry. Further mechanistic study of ORR by Cu(II)- DATZ (3,5-amino-1,2,4-triazole) complex-based catalyst provided information about the Tafel slope, and reaction orders of possible reactants involved in RDS (rate-determining step). An ORR mechanism for this catalyst was proposed based on these experimental results. Electronic properties of the Cu(II)-triazole complex-based catalysts were investigated by varying the substituents on the triazole ring. Results from calculated electron densities of N in the triazole ring and ORR performance of substituted catalysts showed that electron-donating groups are more favorable for ORR catalysis in general.

The last part addresses the problem of nature of Cu(II) complexes which are regarded as catalytic centers of ORR for synthesized electrocatalysts. Solid state EPR (electron paramagnetic resonance) spectra shows that the Cu(II) centers of the five substituted catalysts bear a tetragonal coordination feature with an elongated or compressed coordination at axial positions. The design of an in situ electrochemical cell in the EPR spectrometer enables direct observation of Cu [copper] species coupling with O₂ [oxygen] species in Cu-DATZ-based electrocatalyst. This technique was applied to a synthesized Cu catalyst and its pyrolyzed sample, which demonstrates the highest ORR activity among the pyrolyzed Cu catalysts reported so far. This in situ study found that the Cu(II) sites decompose rapidly in the presence of electrolyte for the pyrolyzed sample while Cu(II) acts as major active sites in the non-pyrolyzed sample during ORR catalysis.