Effect of Acid Leaching on the Catalytic Activity of Co-Salen/C Non-Precious Metal Catalysts for Oxygen Reduction Reaction

To overcome the barrier of  high cost caused by the exclusive use of Pt-based catalysts, the development of non-precious  metal catalysts (NPMCs) to replace Pt in polymer electrolyte membrane fuel cells has become the goal of intensive research in recent years.^1,2 Some of these NPMCs have shown remarkable catalytic activity towards ORR. Among these NPMCs, carbon-supported transition metal/nitrogen ( M/N/C, M = Fe, Co, Mn, etc.) materials have gained increased attention due to their promising catalytic activity and high durability.^3-5 However, the role of transition metal playing in the catalysts’ active sites is still a subject of controversy. In order to further clarify the nature of the active sites of NPMCs, in this, with Co(SO₄)₇H₂O as the metal precursor and N,N-bis (salicylidene) ethylenediamine (Salen) as the nitrogen precursor, carbon-supported non-precious metal catalysts, Co-Salen/C, were synthesized using a facile thermal annealing approach. The catalysts were heat-treated at different temperatures (from 600- 1000^oC) to optimize oxygen reduction reaction (ORR) activity. To clarify the significance of metal (Co) content for the ORR enhancement, the catalyst was further processed by acid leaching.  

The electrocatalytic activity and electron transfer mechanism were demonstrated in oxygen-saturated alkaline electrolyte by cyclic voltammetry (CV), linear sweep voltammetry (LSV) as well as rotating disk electrode (RDE) techniques. Scanning electron microscope-energy dispersive spectrometer (SEM-EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) measurements were used to identify the structure and composition of the catalysts.