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PRINCIPAL PUBLICATION AND AUTHORS

Origin of the unusual ground state spin S = 9 in a Cr10 Single Molecule Magnet, J. Rubín (a), A. Arauzo (a), E. Bartolomé (b), F. Sedona (c), M. Rancan (c), L. Armelao (c,d), J. Luzón (e), T. Guidi (f,g), E. Garlatti (h), F. Wilhelm (i), A. Rogalev (i), A. Amann (j), S. Spagna (j), J. Bartolomé (a), F. Bartolomé (a), J. Am. Chem. Soc. 144, 12520-12535 (2022); https:/doi.org/10.1021/jacs.2c05453 We acknowledge financial support from the Spanish Agencia Estatal de Investigación, through project PID2020-115159GB-I00 / AEI/10.13039/501100011033 and Aragonese project RASMIA E12_20R (co-funded by Fondo Social Europeo) (a) Instituto de Nanociencia y Materiales de Aragón (INMA), Zaragoza (Spain) (b) Escola Universitària Salesiana de Sarrià (EUSS), Barcelona (Spain) (c) Università di Padova, Padova (Italy) (d) Consiglio Nazionale delle Ricerche (CNR), Roma (Italy) (e) Centro Universitario de la Defensa (CUD), Zaragoza (Spain) (f) University of Camerino, Camerino (Italy) (g) ISIS facility, Oxfordshire (UK) (h) Università di Parma, Parma (Italy) (i) ESRF (j) Quantum Design Inc., San Diego (USA)

REFERENCES

[1] D.M. Low et al., Chem. Eur. J. 12, 1385 (2006). [2] S. Sharmin et al., Appl. Phys. Lett. 86, 032507 (2005).

Oxygen X-ray Raman scattering contributes to the understanding of fuel cell cathode materials

Oxides exhibiting hole, oxygen vacancy and proton conductivities are key materials for protonic ceramic fuel and electrolysis cells (PCFCs/PCECs). Their hydration properties depend strongly on the covalency of Fe-O bonds. Measurements of the O K-edge using hard X-ray Raman scattering elucidates electronic features from Fe3d-O2p hybridised orbitals.

A PCFC/PCEC can generate electricity (or produce hydrogen) via electrochemical reactions, and operates at lower temperatures (400-600°C) compared to conventional solid oxide fuel cells [1]. Since the electrolyte conducts protons, it is necessary that the air electrode also has some proton conductivity (Figure 90a). The reaction kinetics at the air side (= cathode in fuel cell mode) is typically the bottleneck

that needs to be improved, e.g. by tuning the cation composition of barium ferrate perovskites. Doping with oversized cations such as Zn or Y has proven to strongly increase the proton uptake [1]. The physical-chemical origin of this increased protonation was ascribed to local lattice distortions [2]. These affect the hybridisation of the Fe3d-O2p orbitals by decreasing the covalency of the Fe-O bonds, thus favouring the proton attachment via an acid- base reaction.

Probing the O K-edge using hard X-ray Raman scattering (XRS) gives direct access to the electronic features related to Fe-O bond covalency. O K-edges of undoped barium lanthanum ferrate (BLF) and Zn(Y)-doped (BLFZn/BFY) barium ferrate, nominally containing Fe3+ or Fe4+(reduced or oxidised samples), were measured at beamline ID20.

The data were analysed using the FDMNES code. Two semi-empirical parameters, screening and dilatorb, were introduced for modelling the ionicity of the oxygen atoms and the extent of Fe4+→O2- hole transfer [3,4]. Higher dilatorb and lower screening values indicated a higher degree of hybridisation of the transition metal and oxygen orbitals for the BLF sample. This can be related to the observed lower proton uptake of BLF compared to BLFZn and BFY (Figure 90b).

O K-edge simulation results are reported in Figure 91a. While for the oxidised samples it was necessary to introduce screening and dilatorb, the reduced samples could be modelled without these parameters, because they contain almost no holes (i.e., Fe4+→O2- charge transfer is absent). The agreement of experiment and simulation is good for BLFred and BLFZnred.

Fig. 90: a) Sketch of a PCFC cathode illustrating the need for proton conductivity in the cathode material. b) Proton uptake at 250°C for selected BLF, BLFZn, BLFY samples, data from [1].