Direct Growth of Carbon Nanotubes Doped with Single Atomic Fe–N₄ Active Sites and Neighboring Graphitic Nitrogen for Efficient and Stable Oxygen Reduction Electrocatalysis

Abstract

Single atomic Fe-N x moieties embedded on a high surface area carbon (Fe-N/C) represents one of the most promising nonprecious metal electrocatalysts for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells. While significant progress has been made in the preparation of Fe-N/C catalysts with high-density Fe-N x sites, key structural descriptors determining the intrinsic activity of the Fe center remain elusive, and effective ways to enhance the intrinsic activity are still lacking. Moreover, most Fe-N/C catalysts developed to date are built on carbons with rather low graphitization degree, which suffer from relatively severe carbon corrosion and thereby poor stability. The direct growth of carbon nanotubes doped with high-density Fe-N x sites neighbored with graphitic-nitrogen-rich environment is reported here, which are successfully applied as a both active and stable ORR electrocatalyst in fuel cells. Combining both experiments and density functional theory calculations, it is revealed that the neighboring graphitic nitrogen can effectively induce higher filling degree of d-orbitals and simultaneously decrease on-site magnetic moment (namely, lowered spin) of the Fe center, which can optimize the binding energies of ORR intermediates and thereby substantially enhance intrinsic ORR activity.

Impact of Surface Functionalization on the Intrinsic Properties of the Resulting Fe–N–C Catalysts for Fuel Cell Applications

“…[ 42 ] But it is equal to other Fe–N–C catalysts containing CNTs, even though our loading was less than half. [ 11 ] Indeed, it is difficult to prepare firm catalyst layers from the nanotube containing catalysts. Based on this, we assume that with further optimization of the electrode preparation process, a further improvement will be possible, that eventually then reflects the same trend as found in RRDE.…”

“…[ 10 ] Xia et al prepared intentionally nanotube‐containing Fe–N–C catalysts with good performance in acidic and alkaline electrolyte. [ 11 ]…”

Microenvironment modulation of single-atom catalysts and their roles in electrochemical energy conversion

“…Heteroatom tethering, which provides anchoring sites for fixing metal atoms through MSI, has also attracted great research interests. In carbon-based supports, control over the content of heteroatom such as N, P, S, O, or B provides immense scope for tuning the microenvironments of the single-atom sites within the functionalized carbons (23,31,(53)(54)(55). For instance, Liang and co-workers developed a sulfur-tethering strategy for the synthesis of noble metal SACs with high metal loading up to 10 wt % (33), and the results indicate that the doping of sulfur endows mesoporous carbon with high-density anchoring sites for bonding metal atoms via the strong metal-sulfur interactions.…”

Single Cobalt Sites Dispersed in Hierarchically Porous Nanofiber Networks for Durable and High‐Power PGM‐Free Cathodes in Fuel Cells

“…[6,10,15,49] Carbon corrosion is also detrimental to catalyst stability by destroying MN 4 active sites and reducing electrode conductivity. [51,52] It also leads to the collapse of the TPBs and water flooding in the cathode, resulting in severe proton and bulk gas transport resistance. [8] The M-N-C catalysts derived from ZIF-8s generally have a lower degree of graphitization due to the use of limited metal doping and lower peak pyrolysis temperature, which primarily provides abundant defects and N dopants to accommodate high-density single metal sites, but with lower corrosion resistance.…”