Influence of Synthesis Temperature of ORR Catalysts Based on CNTs Doped with Cobalt and Copper Phthalocyanines and Modified with Palladium on Their Activity

Abstract

The effect of synthesis temperature on catalytic activity of oxygen electroreduction reaction (ORR) catalysts based on multi-walled carbon nanotubes (MWCNT) doped with cobalt and copper phthalocyanines and modified with palladium was studied. Pyrolysis of phthalocyanines was carried out in an inert atmosphere at different temperatures—750, 850, and 1000°C. SEM study showed that the particle size increases with increasing temperature of synthesis. Raman spectroscopy showed that the catalyst synthesized at 1000°C is characterized by a hierarchical structure due to the formation of a structured graphite layer on the surface. XPS method showed that increasing of the synthesis temperature from 850 to 1000°C does not lead to the disappearance of nitrogen in the material, but inactive types of nitrogen are transformed to more active ones: pyridine and pyrrole. The XRD method showed that the MWCNT_CoPc_CuPc_Pd_1000 catalyst contains copper-palladium intermetallics (Cu₃Pd and Cu_2.85Pd_1.15) and metallic cobalt in its structure, which contributes to an increase in the efficiency of the oxygen electroreduction reaction in an alkaline electrolyte. In the electrokinetic region, the most effective catalyst is MWCNT_CoPc_CuPc_Pd_1000. The E_1/2 value for the MWCNT_CoPc_CuPc_Pd_1000 catalyst is slightly inferior to the commercial platinum catalyst with a metal content of 40 wt %; the difference in E_1/2 values is about 0.02 V. Corrosion tests lead to a slight increase in the efficiency of the MWCNT_CoPc_CuPc_Pd_1000 catalyst: the E_1/2 and E_onset values increase by 0.005 and 0.001 V, respectively. This effect may be associated with self-activation of the catalyst surface.