“Substituents optimization” and “push effect” dual strategy for design of cobalt phthalocyanine catalyst on oxygen reduction reaction

Abstract

Molecular metallocycle electrocatalysts like metalloporphyrins and metallophthalocyanines were found to be effective for oxygen reduction reaction (ORR) due to their M–N4 active sites and large conjugated electronic molecular structures. Herein, the “substituents optimization” strategy combined with “push effect” modification was innovatively employed to target a single Co–N4 active site in three substituted phthalocyaninato cobalt complexes: tetranitrophthalocyaninato cobalt (CoTNPc), tetra(4-nitrophenoxy) phthalocyaninato cobalt (CoTPNPc), and tetraphenoxy phthalocyaninato cobalt (CoTPPc) electrocatalyst, also with 4-phenylpyridine axial coordination on Co–N4 unit. Through substituents screening, the half-wave potential (E1/2) for ORR increases in the order of CoTPNPc (0.75 V) < CoTPPc (0.80 V) < CoTNPc (0.83 V) along with decreased electron-withdrawing ability of their substituents from –OC6H4–NO2, –OC6H5 to –NO2 in the three cobalt phthalocyanine derivatives. CoTNPc with the weakest electron-withdrawing substituent exhibits the best ORR performance among the three compounds. This is attributed to its higher electron delocalization and lifted HOMO energy level with the lower energy barrier in the rate-determining step relative to the other two compounds, which facilitate the electron transfer and reduction of oxygen as evidenced by XPS, UPS, and DRS analysis combined with DFT calculations. Further coordination of 4-phenylpyridine shifts the E1/2 up to 0.78, 0.82, and 0.85 V for CoTPNPc, CoTPPc, and CoTNPc. DFT calculations demonstrate that the introduction of the electron-donating phenylpyridine ligand into the cobalt phthalocyanines breaks the symmetry of the Co–N4 center and also raises the electron density of Co sites, which promotes O2 adsorption and improves ORR performance. After comparing the two strategies, the substituents on metallophthalocyanine are more determined by the electroactivity than the axial group, which directly regulates the coordination environment and then the activation barrier of the ORR process. This work provides theoretical and experimental guidance by two coupling strategies for the design of highly active molecular CoPc-based ORR electrocatalysts in the practical application.