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.

Sodium metal batteries have potentially high energy densities, but severe sodium-dendrite growth and side reactions prevent their practical applications, especially at high temperatures. Herein, we design an inorganic ionic conductor/gel polymer electrolyte composite, where uniformly cross-linked beta alumina nanowires are compactly coated by a poly(vinylidene fluoride-co-hexafluoropropylene)-based gel polymer electrolyte through their strong molecular interactions. These  beta alumina nanowires combined with the gel polymer layer create dense and homogeneous solid-liquid hybrid sodium-ion transportation channels through and along the nanowires, which promote uniform sodium deposition and formation of a stable and flat solid electrolyte interface on the sodium metal anode. Side reactions between the sodium metal and liquid electrolyte, as well as sodium dendrite formation, are successfully suppressed, especially at 60 °C. The sodium vanadium phosphate/sodium full cells with composite electrolyte exhibit 95.3% and 78.8% capacity retention after 1000 cycles at 1 C at 25 °C and 60 °C, respectively.

While FeN 4 species are widely suggested as the active sites of noble-metal-free Fe−N−C oxygen reduction reaction (ORR) electrocatalysts, the ORR mechanism, particularly the rate-determining steps (RDSs) at the Fe centers, and the likely contribution of co-existed C−N active site remain disputed. Moreover, the dynamic structures of the FeN 4 active sites during ORR electrocatalysis also remain elusive. By in situ (isotope-labeled) Raman spectroscopy of molecular Fe phthalocyanine (FePc) model catalysts and pyrolyzed Fe−N−C catalysts, we achieve direct, simultaneous spectral identification of the ORR intermediates/RDSs at different active sites under different pH conditions, from which their intrinsic activities and ORR mechanisms can be quantitatively decoupled. Besides the single-atomic Fe−N x site, two kinds of C−N sites were pinpointed and clarified as separate active sites in pyrolyzed Fe−N−C catalysts, showing different ORR intermediates (*O 2 − and *OOH) and RDSs. Furthermore, from the FePc model catalyst, we reveal a pH-dependent structural switching of the FeN 4 between planar and non-planar structures during ORR electrocatalysis, which provides important insights into their pH-dependent ORR activity (RDS) and stability.