Influence of preparation process on non-noble metal-based composite electrocatalysts for oxygen reduction reaction

Composite electrocatalysts based on the transition metal cobalt are synthesized for the oxygen reduction reaction (ORR) by mechanically mixing three basic precursors containing carbon black, cobalt acetate and tetramethoxy-phenylporphyrin (TMPP). The influence of various mixing processes, solvents, pyrolysis temperature and precursor ratios on the ORR activity of the electrocatalysts is investigated by means of their electrochemical characteristics. Levich–Koutecky plots show that the number of transferred electrons during ORR on these catalysts varies between 2 and 4. A pyrolyzed mixture synthesized by ultrasonication exhibits better activity for ORR than that prepared by ball milling. The solvent is found to have a significant effect on the performance of the catalysts in acidic media. The catalyst synthesized in water, a poor solvent of TMPP, has better activity than that synthesized in a good solvent of TMPP, such as N,N-dimethylformamide and acetic acid. The optimum pyrolysis temperature is 600 °C. Various mole ratios and Co/carbon weight ratios are examined. Maximum activity is found at a 1:1 TMPP/Co mole ratio and a 5% Co/carbon weight ratio.