The impact of transition metals in diphenylporphyrinato (DPP) on the capabilities for the electrocatalysis of oxygen reduction reaction (ORR)

Abstract

The transition metal complexed with different porphyrin-based ligands can influence the oxygen reduction reaction (ORR) capabilities in distinct ways, depending on the structure of the ligands. Therefore, the ORR capabilities of 5,15-diphenylporphyrinato (DPP) complexes with different transition metals (Fe, Co, Ni, Cu, Zn) were systematically investigated and compared, including an analysis of the kinetics mechanism in alkaline media. In this study, the electrocatalytic performance varied significantly depending on the metal center within the DPP cavity. Unlike typical porphyrin ligands, where Fe is often superior to other metals, CoDPP exhibited the highest reduction peak potential and the best current density. Using the Koutecky-Levich approach based on linear sweep voltammetry (LSV) measurements, the CoDPP complex supported on Vulcan XC 72R (CoDPP/C) emerged as the most promising catalyst, with an electron transfer number (n) of 3.37 at 0.7 V, a Tafel slope of 103 mV/dec, and an onset potential of 0.83 V in 0.1 M potassium hydroxide solution. Furthermore, the electron transfer number (n) increased to 3.67 at 0.7 V vs. RHE when 0.1 M sodium hydroxide was used as the electrolyte. This study also concludes that the kinetic mechanism of the Co metal complexed in DPP tends to follow a 4-electron transfer pathway. These findings highlight the significant influence of the metal center on the ORR catalytic activity of porphyrin complexes in alkaline conditions. Density Functional Theory (DFT) calculations provided insight into the electronic and adsorption properties of these complexes, highlighting the unique of CoDPP’s LUMO delocalization on both the cobalt center and porphyrin ring, a small HOMO-LUMO energy gap (2.51 eV), and the highest electron affinity (2.94 eV). Additionally, CoDPP exhibited the strongest oxygen adsorption energy (-2.56 eV), correlating with its superior experimental ORR performance. These findings establish a clear relationship between the electronic structure, adsorption properties, and catalytic activity of MDPP complexes, positioning CoDPP as a promising candidate for ORR and offering guidance for designing advanced porphyrin-based catalysts for energy applications.