Effect of protonated amine molecules on the oxygen reduction reaction on metal-nitrogen-carbon-based catalysts

Abstract

Non-precious metal electrocatalysts based on pyrolyzed metal, nitrogen, and carbon (MNC) are viewed as an inexpensive replacement for platinum-based electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells. One of the enduring issues in the field of MNC catalysis is identifying the exact active structure responsible for the ORR. Many ORR active sites have been proposed recently, such as transition metal coordinated to (i) four pyrrolic nitrogens, (ii) four pyridinic nitrogens, and (iii) four pyridinic nitrogens interacting with one protonated nitrogen; among these, the latter is viewed as the most promising active site for the ORR. In this study, we have synthesized a manganese-based MNC catalyst (MnN sub(x)/C). EPR and X-ray absorption fine structure (XAFS) analysis indicated the presence of four nitrogens around the Mn(II) ion. The ORR performance of an MnN sub(x)/C catalyst was recorded in the presence of the disodium salt of ethylenediaminetetraacetic acid (EDTA-Na sub(2)), ethylene diamine (ED), and combination of ED and acetic acid (AA). The presence of EDTA-Na sub(2) and ED + AA in the electrolyte solution maximizes the availability of protonated amine-N around the catalyst. As a consequence, we noticed significant improvement in the ORR kinetics in H sub(2)SO sub(4) (10 sup(-4), 10 sup(-6) N) and NaOH (10 sup(-6) to 10 sup(-2) N) electrolyte solutions. The improvement in the onset potential for the ORR ranged between 80 and 160 mV as the pH was changed from 4 to 12. Based on XAFS data and ORR polarization in the presence of EDTA-Na sub(2) and ED + AA, we believe that the MnN4 moiety interacting with the protonated amine is the most probable active site contributing to ORR activity in the H2SO4 (10 sup(-4), 10 sup(-6) N) and NaOH (10 sup(-6) to 10 sup(-2) N) electrolyte solutions.