Potential‐Induced Spin Changes in Fe/N/C Electrocatalysts Assessed by In Situ X‐ray Emission Spectroscopy

Abstract

The commercial success of the electrochemical energy conversion technologies required for the decarbonization of the energy sector requires the replacement of the noble metal-based electrocatalysts currently used in (co-)electrolyzers and fuel cells with inexpensive, platinum-group metal-free analogs. Among these, Fe/N/C-type catalysts display promising performances for the reduction of O 2 or CO 2 , but their insufficient activity and stability jeopardize their implementation in such devices. To circumvent these issues, a better understanding of the local geometric and electronic structure of their catalytic active sites under reaction conditions is needed. Herein we shed light on the electronic structure of the molecular sites in two Fe/N/C catalysts by probing their average spin state with X-ray emission spectroscopy (XES). Chiefly, our in situ XES measurements reveal for the first time the existence of reversible, potential-induced spin state changes in these materials.Platinum-group metal (PGM-) free catalysts of the M/N/Ctype (whereby M corresponds to a 3d transition metal) hold great potential as inexpensive replacements for conventional, noble-metal-based materials. Originally developed as electrocatalysts for the oxygen reduction reaction (ORR, relevant to fuel cells [1,2] and metal-air batteries [3][4][5] ), these materials have recently been successfully employed in other catalytic processes, like CO 2 -electroreduction [6,7] or the oxidation of benzene to phenol. [8,9] Nevertheless, their commercial implementation requires further improvements in their activity and stability [1,10] that call for a better understanding of the reactions mechanism and their relation to the operando oxidation-, spin-state and orbital configuration of the Ncoordinated metal sites (M-N x ) regarded as their active centers. [11][12][13][14] These properties have been generally assessed using Mçssbauer [12,15,16] and X-ray absorption [17][18][19] spectroscopy (MS, XAS), which are highly sensitive techniques but also suffer from certain drawbacks. MS, on the one hand, allows distinguishing the chemical environment of the metal species present in iron-based M/N/C catalysts, which have been shown to be the most ORR-active among this material class; however, their heterogeneous composition results in complex spectra requiring a careful deconvolution. The latter is occasionally complemented by the assignment of spin-and oxidation-states to these deconvolution components, based on similarities between the Mçssbauer parameters of the Fe-N x sites in these Fe/N/C catalysts and those of compounds with resembling but better defined M-N 4 centers (like porphyrins or phthalocyanines). [12] However, the long-range structures and electronic properties of these reference compounds are likely to differ from those of the catalysts active sites, thus making a full conclusive analysis difficult. Furthermore, the combination of the low temporal resolution of MS and the low metal contents of M-N x sites (typically < 2 wt. %) in M/N/C catalysts le...

Time‐Resolved Potential‐Induced Changes in Fe/N/C‐Catalysts Studied by In Situ Modulation Excitation X‐Ray Absorption Spectroscopy

“…The two catalysts included in this study are synthesized following previously reported methods based on Fe‐porphyrin versus MOF precursors, for DW21 versus Fe0.5, respectively (see Experimental Section for details). Additionally, both samples have been thoroughly characterized in our recent work, [ 59,60 ] in which ex situ, room temperature MS, and EXAFS spectroscopy showed that their Fe‐speciation predominantly consists of FeN x C y sites. Specifically, the catalysts’ MS spectra (see Figure S1a,b, Supporting Information, and fitting parameters in Table S1, Supporting Information) can be fitted with three doublets, each assigned to a kind of molecular FeN x C y site.…”

“…Further details on the latter subjects can be found in previous publications. [33,59,60,95,96] Spectroscopic Characterization: XAS spectra were recorded at the SuperXAS beamline of the Swiss Light Source (Villigen PSI, Switzerland) operated at 400 mA and 2.4 GeV. [97] A 2.9 T bending magnet provided a polychromatic beam that was first collimated by a Si-coated mirror, subsequently monochromatized by a Si(311)-monochromator, and then focused by a Rh-coated mirror yielding a spot size on the sample of ≈1 × 0.2 mm 2 .…”

“…Further Material Characterization: Both catalysts were extensively characterized in previous publications. [33,59,60] A detailed description of the procedures followed to synthesize both catalysts and test their electrochemical ORR-activity can be found in the Supporting Information. Additional spectroscopic procedures applied to obtain the results presented herein are specified in the following:…”

“…To overcome this limitation, in this work we have employed modulation excitation (ME-) XAS for the in situ study of two Fe/N/C-catalysts predominantly featuring FeN x C y -sites [36,59,60] but with substantially different ORR-performances. A thorough analysis of the spectra's pre-and rising-edge features, particularly sensitive to the FeN x C y -sites' local geometry and electronic configuration, accompanied by extended X-ray absorption fine structure (EXAFS) analysis unveiled that a fraction of the sites in both catalysts undergo a transition from five-/six-to fourfold coordination upon potential decrease.…”

Electrocatalysis with Single‐Metal Atom Sites in Doped Carbon Matrices

“…Absorption of X-rays by the transition metal atoms leads to the ejection of photoelectrons, and formation of excited core-hole states. cently that XES is capable of tracking in situ the changes of oxidation state and spin state of single Fe atoms in ORR conditions [119].…”