Elucidating the Structural Composition of an Fe–N–C Catalyst by Nuclear‐ and Electron‐Resonance Techniques

Abstract

Fe-N-C catalysts are very promising materials for fuel cells and metal-air batteries.This work gives fundamental insights into the structural composition of an Fe-N-C catalyst and highlights the importance of an in-depth characterization. By nuclear-and electron-resonance techniques,weare able to show that even after mild pyrolysis and acid leaching, the catalyst contains considerable fractions of a-iron and, surprisingly,i ron oxide.O ur work makes it questionable to what extent FeN 4 sites can be present in Fe-N-C catalysts prepared by pyrolysis at 900 8 8Ca nd above.T he simulation of the iron partial density of phonon states enables the identification of three FeN 4 species in our catalyst, one of them comprising as ixfold coordination with end-on bonded oxygen as one of the axial ligands.

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

“…For all catalysts, the Mössbauer spectra are dominated by the so‐called D1 doublet that can have different origins as FeN 4 sites of different oxidation and spin states, as well as iron and iron oxide clusters. [ 22 ] In addition to this doublet, FeNC contains three sextets and a second doublet D2. Here, the large quadrupole splitting of D2 points to a large electric field gradient as typically found for ferric intermediate spin species.…”

Establishing reactivity descriptors for platinum group metal (PGM)-free Fe–N–C catalysts for PEM fuel cells

“…69 It is important however to note that the Mössbauer signature of high-spin Fe(III)N 4 sites and nanosized ferric oxides is similar, the latter leading to a sextet spectral component only at very low temperature (o60 K) and/or in the presence of an external magnetic field. 14 The minor presence of nanosized ferric oxides can therefore not be entirely excluded from room temperature Mössbauer…”

“…The latter are defined so that fuel cell stacks with PGM-free ORR catalysts become cost-and performancecompetitive with PGM-based catalysts, even for the highly demanding automotive application. [4][5][6][12][13][14][15][16][17][18][19][20][21][22] The most prominent example of PGM-free ORR electrocatalysts for acidic medium is the family of iron-(or cobalt-) and nitrogen-doped high surface area carbon matrix, typically referred as ''Fe-N-C'' catalysts, with atomically-dispersed Fe cations coordinated with nitrogen atoms as the recognized most active sites. 5,[23][24][25][26][27][28][29][30][31][32] Unlike PGM-based single atom catalysts, where the atoms exist in a carbon matrix as a sole atoms 33,34 or dimeric compounds, 35 iron generally has to be coordinated with hetero atoms.…”

“…16,69 Advanced experimental analytical techniques such as 57 Fe Mössbauer spectroscopy and high resolution STEM-EELS microscopy have now qualitatively proven the existence of such sites in active Fe-N-C materials. [13][14][15]26,60,[70][71][72][73] A serious hurdle in the rational improvement of the catalytic activity of Fe-N-C catalysts, however, has been the lack of suitable methods that accurately enumerate the electrochemically accessible Fe-N x sites (SD). Even for model Fe-N-C materials comprising only Fe-N x sites, the SD value cannot be accessed with the sole knowledge of the total Fe content, due to the location of a significant fraction of Fe-N x sites not only on the surface but also in the bulk of the carbon matrix.…”

Heinz Maier‐Leibnitz Awards 2020

“…This includes the development of new synthesis routes as well as spectroscopic characterization methods including in situ Mössbauer spectroscopy. In a Communication in Angewandte Chemie , Kramm and colleagues investigated an Fe–N–C catalyst by resonance techniques, and in a Communication in Chemistry—A European Journal they explored active sites in doped Co‐based hydrogen evolution catalysts …”