Atomically Dispersed Iron with Densely Exposed Active Sites as Bifunctional Oxygen Catalysts for Zinc–Air Flow Batteries

Abstract

coordinative dopants (N, O, S, P, etc.) [8] and metal-support interactions. [9] Spectroscopic studies and computational simulations suggest that MN x related species are the catalytic sites for the ORR or/and OER. [8c,10] Although some MN x C catalysts have already shown high catalytic activity for ORR or/and OER in threeelectrode-system, their exposed active sites are still too little to meet the requirements of practical applications in batteries. [11] Increasing atomic MN x moieties' density and utilization rate is a well-known approach to improving catalytic performance but remains challenging. [12] Currently, high-temperature pyrolysis is a necessary procedure to obtain atomic MN x moieties on carbon matrix but usually leads to the migration and aggregation of metal atoms driven by the high surface energy offered by the calcination process. [8d] Additionally, isolated metal atoms have inherently high surface free energy and tend to form clusters and nanoparticles, which have low activity for ORR or/and OER, and need to be removed by post acid treatment. [13] Although researchers have made a lot of attempts to increase the metal loading, the content of the atomically dispersed metal is usually below 3 wt%. [7c,12b,14] For example, Shui et al. prepared FeNC catalysts with single Fe atom content of 2.78 wt % by pyrolysis of SiO 2 -protected zeolitic imidazolate frameworks-8 (ZIF-8). [12b] Lin et al. prepared FeN-doped graphene nanosheets (FeN/ GNs) with 2.16 wt% Fe content through a ZIF-8 thermal melting strategy. [14a] Wu et al. reported a two-step doping and adsorption process to obtain atomically dispersed MnN 4 sites on partially graphitic carbon (MnNC) with the Mn content up to 3.03 wt%. [7c] Additionally, the utilization rate of M-N x moieties will dramatically reduce in high metal loading catalysts. Many M-N x moieties will be buried in carbon matrixes. Exposed site density (SD e ) has been a predominant factor in describing the active sites density and mass transfer ability of M-N x -C catalysts. SD e is the number of exposed active sites normalized to the mass of catalysts. Shui et al. display that when Fe content increases from 0.28 to 2.78 wt%, the utilization rate of FeN x moieties reduces from 43.5 to 11.4%, obtaining a maximum exposed active site density of 3.4 × 10 19 sites g −1 . [12b] Jia et al. report a chemical vapor deposition method to synthesize FeNC with the recorded exposed active site density of 1.92 × 10 20 sites g −1 . [15] Unfortunately, the exposed active site density is still not enough to efficiently catalyze ORR and OER.Here, we report a double steric hindrance strategy to synthesize single-atom iron catalysts with the recorded iron loading Atomically dispersed iron embedded carbon is a promising bifunctional catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), but its exposed iron sites must be increased. Herein, the authors propose a double steric hindrance strategy by using zeolitic imidazolate frameworks-8 as the fi...

Unraveling the Electronic Structure and Dynamics of the Atomically Dispersed Iron Sites in Electrochemical CO2 Reduction

“…With further decreasing the potential to −0.8 V vs RHE, a non-negligible pre-edge peak (A) at the lower energy of 7112.5 eV was observed, which can be assigned to a distorted square planar structure with axial ligands. 48,49 The peak A is more intense at −0.8 than at −0.4 V vs RHE and disappears after removing the potential (labeled as AFT in the figure), suggesting that the peak A is most likely related to a catalytic intermediate. To determine the exact structure of the catalytic intermediate, we used the FDMNES calculation method to simulate the XANES spectra obtained at OCP and −0.8 V vs RHE.…”

Coordination Engineering Induced d-Band Center Shift on Single-Atom Fe Electrocatalysts for Enhanced Oxygen Reduction

“…In recent years, owing to advantages such as a high safety factor, cost effectiveness, and a high energy density, rechargeable zinc–air batteries have attracted immense attention from universities and enterprises. , Their energy conversion efficiency depends considerably on the cathodic oxygen reduction reaction (ORR). One of the issues is the slow kinetics of the ORR process. , Development of high-performance ORR electrocatalysts is clearly the key to promoting this process. Thus far, Pt-based materials are the most advanced and commercialized ORR catalysts.…”

Recent Developments of Atomically Dispersed Metal Electrocatalysts for Oxygen Reduction Reaction†

“…Designing suitable synthetic routes to tune the metal doping process and the morphological structure of the matrix to obtain SACs with high loading or high utilization of active sites are the two mainstream strategies at present. [ 22 ] Given the rapid development of SACs for ORR in theoretical research and the urgent needs in practical applications, it is necessary to compile and review the existing research promptly to understand the recent research hotspots and directions, highlighting the urgent issues, and bring guidance and inspiration for further research.…”