Identification of the Catalytically Dominant Iron Environment in Iron- and Nitrogen-Doped Carbon Catalysts for the Oxygen Reduction Reaction

Abstract

For large-scale utilization of fuel cells in a future hydrogen-based energy economy, affordable and environmentally benign catalysts are needed. Pyrolytically obtained metal-and nitrogen-doped carbon (MNC) catalysts are key contenders for this task. Their systematic improvement requires detailed knowledge of the active site composition and degradation mechanisms. In FeNC catalysts, the active site is an iron ion coordinated by nitrogen atoms embedded in an extended graphene sheet. Herein, we build an active site model from in situ and operando 57 Fe Mossbauer spectroscopy and quantum chemistry. A Mossbauer signal newly emerging under operando conditions, D4, is correlated with the loss of other Mossbauer signatures (D2, D3a, D3b), implying a direct structural correspondence. Pyrrolic N-coordination, i.e., FeN 4 C 12 , is found as a spectroscopically and thermodynamically consistent model for the entire catalytic cycle, in contrast to pyridinic nitrogen coordination. These findings thus overcome the previously conflicting structural assignments for the active site and, moreover, identify and structurally assign a previously unknown intermediate in the oxygen reduction reaction at FeNC catalysts.

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

“…. These QS values are in line with those recently experimentally reported in situ for HS Fe(II)N 4 sites (with IS values > RHE in O 2 -free acidic electrolyte34 ). The tted parameters of the Mössbauer spectra of FePc-CNT acquired at different potentials (OCP, -0.4, -0.8 V vs. RHE, in CO 2 saturated electrolyte) in the present work are listed in Supplementary Table3, and the corresponding spectra are shown in Fig.4a-c.…”

“…The low utilization of FeN 4 sites implies that operando spectroscopic changes are only partly related to surface sites, complexifying the analysis of the operando structure of active surface sites. More research efforts are therefore required towards the development of model Fe-based catalytic systems featuring a monostructure and high electrochemical accessibility of the single-metal-atom sites, and the application on such model catalysts of operando characterization techniques with atomic scale sensitivity 33,34 .…”

Substituent Effects in Iron Porphyrin Catalysts for the Hydrogen Evolution Reaction**

“…The electronic structures were evaluated with single point calculations using the same settings as the Mössbauer predictions, but with the OLYP density functional [60b,61] and excluding dispersion corrections. This approach was verified on structurally and electronically similar test cases, as documented in the Supporting Information of Ref [6f] . From the optimized geometries and frequency calculations Gibbs free enthalpies were obtained and corrected using the electronic energies obtained from the OLYP calculations.…”

“…Intense structural characterization has shown the active center in MNC catalysts to be constituted of single metal atoms incorporated into an amorphous carbon structure by coordination of N donor atoms, which are covalently doped into the carbon layers, see Figure 1a for a schematic depiction. The configuration of the N atoms surrounding the metal center and the geometry of the ligand sphere are still under debate, [6c,9b,10] but most suggestions for the active site structure have a square planar nitrogen coordination environment in common [5b,6f,7a,9b,10a,b,11] …”

What is the Real Origin of the Activity of Fe–N–C Electrocatalysts in the O2 Reduction Reaction? Critical Roles of Coordinating Pyrrolic N and Axially Adsorbing Species

“…Hutchison et al demonstrated the effect of axial ligation on ORR by multilevel computations. Specifically, Ni et al found that the active site of Fe–N–C catalysts for ORR is the iron site coordinated with pyrrolic N by Mössbauer spectroscopy, which is consistent with our computations.…”