Single-Atom Iron-Based Electrocatalysts for High-Temperature Polymer Electrolyte Membrane Fuel Cell: Organometallic Precursor and Pore Texture Tailoring

Abstract

The oxygen reduction reaction (ORR) activity of platinum (Pt)-based catalyst is not satisfactory in high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) operating with a phosphoric acid-doped polybenzimidazole (PBI) membrane because of the low immunity of expensive Pt-based catalysts toward phosphate ions. Therefore, finding inexpensive and phosphate-tolerant ORR electrocatalysts is highly demanded in HT-PEMFCs. It is reported that Fe and N co-functionalized carbon (Fe−N− C) material is highly immune to phosphate anions, which makes it a good candidate for HT-PEMFC. In this work, highly micro- and mesoporous Fe−N−C catalysts are synthesized for the first time via a simple pyrolysis of organometallic ethylenediaminetetraacetic acid (EDTA)−Fe complexes prepared at different weight ratios of iron salt to EDTA. The organometallic EDTA−Fe complex is a complete single precursor for Fe and N as well as C and has never been used for the preparation of Fe−N−C catalysts before. This approach allows for the simultaneous optimization of both structural and functional properties of the Fe−N−C catalysts by simply varying the amount of iron salt, which plays both as an active species and as a template for pore generation. The Fe−N−C catalyst is then further optimized by simply adding silica sol solution to the initial precursor before carbonization followed by ammonia treatment to induce more mesopores and micropores as well as to further increase nitrogen doping, respectively, in the final carbon framework. This results in improved mass transfer and leads to the formation of more efficient ORR active sites. Interestingly, the Fe species are found to be present mainly as single-atom Fe species and also Fe particles over the N-doped carbon support, suggesting that the EDTA−Fe complex is an effective medium for generating atomic distribution of Fe in the carbon framework. The resulting single-atom Fe catalyst has been tested as an ORR electrocatalyst in HT-PEMFC, and the optimized catalyst shows a high peak power density of 260 mW cm−2 and a current density of 1260 mA cm−2 at 0.2 V. The high performance is likely correlated with the highly porous nature, the presence of efficient active sites associated with single-atomic Fe− Nx, and the immunity to phosphate adsorption of the iron nitrogenous catalysts despite extremely harsh fuel cell working environments. © 2020 American Chemical Society