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.
In this work, we present a comprehensive study on the role of metal species in MOF-based Me-N-C (mono- and bimetallic) catalysts for the hydrogen evolution reaction (HER). The catalysts are investigated with respect to HER activity and stability in alkaline electrolyte. On the basis of the structural analysis by X-ray diffraction, X-ray-induced photoelectron spectroscopy, and transmission electron microscopy, it is concluded that MeN sites seem to dominate the HER activity of these catalysts. There is a strong relation between the amount of MeN sites that are formed and the energy of formation related to these sites integrated at the edge of a graphene layer, as obtained from density functional theory (DFT) calculations. Our results show, for the first time, that the combination of two metals (Co and Mo) in a bimetallic (Co,Mo)-N-C catalyst allows hydrogen production with a significantly improved overpotential in comparison to its monometallic counterparts and other Me-N-C catalysts. By the combination of experimental results with DFT calculations, we show that the origin of the enhanced performance of our (Co,Mo)-N-C catalyst seems to be provided by an improved hydrogen binding energy on one MeN site because of the presence of a second MeN site in its close vicinity, as investigated in detail for our most active (Co,Mo)-N-C catalyst. The outstanding stability and good activity make especially the bimetallic Me-N-C catalysts interesting candidates for solar fuel applications.
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