Improved ORR activity of non-noble metal electrocatalysts by increasing ligand and metal ratio in synthetic complex precursors

In an effort to improve oxygen reduction reaction (ORR) activity by increasing the catalytic active site density in carbon-supported non-noble metal catalysts, several nitrogen-containing catalysts were synthesized through a heat treatment process at 900 °C using precursor complexes of Fe(II) and tripyridyl triazine (TPTZ). Fe to TPTZ mole ratios of 1:2, 1:3, 1:4, 1:5, 1:6, and 1:7 were used to prepare the precursor complexes. X-ray diffraction and surface electrochemical techniques were used to characterize these catalysts (Fe–Nx/C), and revealed that when the amount of TPTZ in the precursor complex was increased, the decomposition of Fe–Nx sites, which are considered active sites for the ORR, was effectively reduced, resulting in higher Fe–Nx site density and thus improving the catalysts’ ORR activity. This beneficial effect was validated through rotating disk electrode tests and analysis of the ORR kinetics catalyzed by these catalysts. The obtained results showed that as the Fe to TPTZ mole ratio in the precursor complex was decreased, the catalytic ORR activity of Fe–Nx/C increased monotonically in the mole ratio range of 1:2–1:6. Therefore, increasing the amount of ligand in the precursor metal complex was demonstrated to be an effective way to reduce the decomposition of ORR active site density and thereby enhance the ORR activity of non-noble metal catalysts.

► Various mole ratios between precursor Fe(II) and nitrogen-containing ligand of tripyridyl triazine (TPTZ) were investigated in order to further improve the ORR activity of Fe–Nx/C catalyst. ► The research results revealed that as the Fe to TPTZ mole ratio in the precursor complex was decreased, the catalytic ORR activity of Fe–Nx/C increased monotonically in the mole ratio range of 1:2–1:6. ► Increasing the amount of ligand in the precursor metal complex was demonstrated to be an effective way to compress the decomposition of ORR active site density and thereby enhance the ORR activity of Fe–Nx/C.