Heteroatom doped atomically dispersed Fe 1 -NC catalysts have been found to show excellent activity toward oxygen reduction reaction (ORR). However,t he origin of the enhanced activity is still controversial because the structurefunction relationship governing the enhancement remains elusive.Herein, sulfur(S)-doped Fe 1 -NC catalyst was obtained as amodel, which displays asuperior activity for ORR towards the traditional Fe-NC materials. 57 Fe Mçssbauer spectroscopy and electron paramagnetic resonance spectroscopyr evealed that incorporation of Si nt he second coordination sphere of Fe 1 -NC can induce the transition of spin polarization configuration. Operando 57 Fe Mçssbauer spectra definitively identified the low spin single-Fe 3+ -atom of C-FeN 4 -S moiety as the active site for ORR. Moreover,DFT calculations unveiled that lower spin state of the Fe center after the Sd oping promotes OH* desorption process.T his work elucidates the underlying mechanisms towards Sd oping for enhancing ORR activity, and paves aw ay to investigate the function of broader heteroatom doped Fe 1 -NC catalysts to offer ageneral guideline for spin-state-determined ORR.
An effective and universal strategy is developed to enhance the stability of the non‐noble‐metal M–Nx/C catalyst in proton exchange membrane fuel cells (PEMFCs) by improving the bonding strength between metal ions and chelating polymers, i.e., poly(acrylic acid) (PAA) homopolymer and poly(acrylic acid–maleic acid) (P(AA‐MA)) copolymer with different AA/MA ratios. Mössbauer spectroscopy and X‐ray absorption spectroscopy (XAS) reveal that the optimal P(AA‐MA)–Fe–N catalyst with a higher Fe3+–polymer binding constant possesses longer FeN bonds and exclusive Fe–N4/C moiety compared to PAA–Fe–N, which consists of ≈15% low‐coordinated Fe–N2/N3 structures. The optimized P(AA‐MA)–Fe–N catalyst exhibits outstanding ORR activity and stability in both half‐cell and PEMFC cathodes, with the retention rate of current density approaching 100% for the first 37 h at 0.55 V in an H2–air fuel cell. Density functional theory (DFT) calculations suggest that the Fe–N4/C site could optimize the difference between the adsorption energy of the Fe atoms on the support (Ead) and the bulk cohesive energy (Ecoh) relative to Fe–N2/N3 moieties, thereby strongly stabilizing Fe centers against demetalation.
Oxygen evolution reaction (OER) is an obstacle to the electrocatalytic water splitting due to its unique four‐proton‐and‐electron‐transfer reaction process. Many methods, such as engineering heterostructure and introducing oxygen vacancy, have been used to improve the catalytic performance of electrocatalysts for OER. Herein, the above two kinds of regulation are simultaneously realized in a catalyst by using unique ion irradiation technology. A nanosheet structured NiO/NiFe2O4 heterostructure with rich oxygen vacancies converted from nickel–iron layered double hydroxides by Ar+ ions irradiation shows significant enhancement in both OER and hydrogen evolution reaction performance. Density functional theory (DFT) calculations reveal that the construction of NiO/NiFe2O4 can optimize the free energy of O* to OOH* process during OER reaction. The oxygen vacancy‐rich NiO/NiFe2O4 nanosheets have an overpotential of 279 mV at 10 mA cm−2 and a low Tafel slope of 42 mV dec−1. Moreover, this NiO/NiFe2O4 electrode shows an excellent long‐term stability at 100 mA cm−2 for 450 h. The synergetic effects between NiO and NiFe2O4 make NiO/NiFe2O4 heterostructure have high conductivity and fast charge transfer, abundant active sites, and high catalytic reactivity, contributing to its excellent performance.
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