ELECTRONIC STRUCTURE, MAGNETISM AND DYNAMICS
X-ray Spectroscopy of (Ba,Sr,La)(Fe,Zn,Y) O3-δ Identifies Structural and Electronic Features Favoring Proton Uptake, G. Raimondi (a), F. Giannici (b), A. Longo (c,d), R. Merkle (a), A. Chiara (b),
M.F. Hoedl (a), A. Martorana (b) and J. Maier (a), Chem. Mater. 32, 8502-8511 (2020); https://doi.org/10.1021/acs. chemmater.0c02655. (a) Max Planck Institute for Solid State
Research, Stuttgart (Germany) (b) Università di Palermo (Italy) (c) ESRF (d) CNR-ISMN, Palermo (Italy)
 R. Zohourian et al., Adv. Funct. Mater. 22, 1801241 (2018).  F. Giannici et al., Chem. Mater. 23, 2994-3002 (2011).  A. Longo et al., ACS Catal. 10, 6613-6622 (2020).  M.F. Hoedl et al., J. Phys. Chem. C. 22, 11780-11789 (2020).
PRINCIPAL PUBLICATION AND AUTHORS
XAFS has been a well-established technique for the study of fuel cell oxides for more than a decade . Figure 87 shows the edges of all measured compositions in different iron oxidation states. For most of the explored compositions, the iron retains a formal oxidation close to +3, typical of the reduced samples, even after the oxidation treatment. This is particularly true in barium- rich and/or Zn,Y-doped compositions that show a higher proton uptake. Such a behaviour underlines the synergy of local distortions and electronic structure. The extended XAFS analysis of the inter-octahedral Fe-O-Fe bonds indicates that they are linear for undoped and oxidised samples, while they are buckled for reduced and doped samples.
Alongside the more commonly studied core states of the metal, the electronic states of oxygen have recently been the subject of growing interest in the materials science community thanks to the experimental and theoretical developments around inelastic hard X-ray scattering. The hard X-ray inelastic spectrometer at beamline ID20 makes it possible to study the low-energy valence states of oxygen in the bulk of the materials, avoiding any surface effects which would predominate in
the more commonly employed soft X-ray regime . This combination of XAFS and XRS gives unprecedented insight into the interplay between different dopants and chemical conditions in the fuel cell cathodes.
In doped barium ferrites, the O K-edge spectra (Figure 88) give direct evidence of a significant degree of hole transfer from Fe to O and related covalency in the the Fe-O bonds. This was predicted by DFT calculations . Such a hole delocalisation is more pronounced in the undoped oxidised compositions, where the linear geometry of the bond favours the overlap of the O(2p)-Fe(3d) orbitals (peaks labeled A,B). In doped and reduced samples, on the contrary, the Fe-O-Fe bonds are buckled, hindering the hole transfer. Less charge transfer in turn increases the lattice oxygen basicity, and therefore the proton uptake. Since protons are incorporated in the material via a solid-state acid-base reaction, increasing oxygen basicity favours the protonation reaction. This mechanism explains why the locally distorted reduced and doped compositions are the ones showing high proton concentrations, providing tools for the rational design of cathode materials to be used in protonic ceramic fuel cell devices.
Fig. 88: Oxygen K-edge spectra. a) Undoped sample,
oxidised vs. reduced. b) Reduced sample, doped
vs. undoped. A proposed scheme of the electron
charge transfer for oxidised, reduced and doped samples
is depicted below.