Controlling the Nitrogen Content of Metal-Nitrogen-Carbon Based Non-Precious-Metal Electrocatalysts via Selenium Addition

Non-precious metal electrocatalysts based on pyrolysed metal-nitrogen-carbon (MNC) are viewed as an inexpensive replacement for platinum-based electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells. One of the drawbacks of all the reported procedures to synthesize MNC electrocatalysts is the inability to control the nitrogen content. Since the type of nitrogen present (pyridinic, pyrrolic, quaternary/graphitic) and their proportions both play a seminal role in deciding the ORR activity of the electrocatalyst, it is important to carefully study the effect of nitrogen content on electrocatalyst properties. In this study, selenium was used as a ligand to replace the nitrogen coordinated to the iron atom in the electrocatalyst, thereby imparting control on the nitrogen content. Upon introducing 14 at% of selenium, the N content of the catalyst dropped to 3.7 wt% and the ORR activity reached a maximum of 7.2 mA cm−2 at 0.8 V vs. RHE. We demonstrated the need for iron to complete the active site: upon complexing the iron site with bipyridine, ethylene diammine and oxalic acid in 1N H2SO4, the overpotential toward the ORR increased by ∼60 mV, ∼140 mV and ∼140 mV respectively at 2 mA cm−2.