Why is Co such a good catalytic element in the context of SOFC and batteries, specially in the first case?
For example, La0.6Sr0.4Co0.8Fe0.2O3-δ is more electrochemically active for the oxygen reduction reaction than La0.6Sr0.4Co0.2Fe0.8O3-δ.
I understand both Co and Fe are 3+ in LSCF (DOI:10.1016/j.ssi.2011.01.014), maybe the 4th ionization energy is saying that Co needs less energy to give another electron and become 4+, and this would explain a little?
Maybe there is an more electronic structure-like explanation? For example, I understand LSF is a hopping kind of conductor (e- hopping between Fe3+ and Fe4+), while LSC is more a free electron kind of conductor...
I leave some data that I think might be useful:
Co [Ar] 3d74s2
oxidation states : 3, 2, 0, -1
Electron affinity (kJ.mol−1) = 63.873
1st Ionisation energy (kJ.mol−1) = 760
2nd Ionisation energy (kJ.mol−1) = 1648
3rd Ionisation energy (kJ.mol−1) = 3232
4th Ionisation energy (kJ.mol−1) = 4950
Fe [Ar] 3d64s2
oxidation states : 6, 3, 2, 0, -2
Electron affinity (kJ.mol−1) = 14.569
1st Ionisation energy (kJ.mol−1) = 762
2nd Ionisation energy (kJ.mol−1) = 1561
3rd Ionisation energy (kJ.mol−1) = 2957
4th Ionisation energy (kJ.mol−1) = 5287
Sorry, but I only know for batteries. See slide 11 on this presentation: https://www.umicore.com/storage/group/powering-ahead-denis-goffaux.pdf (open in Explorer, Chrome doesn't work very well). In a typical NMC battery, Co gives a longer life and more power to the battery. And easier to produce.
For more details, I could refer you to my colleagues @ Umicore
It is a matter of the metal-oxygen bond strength. The one for Fe-O is higher than the one associated with the Co-O bond, which means that Co enables the creation of more oxygen vacancies within the structure. And the oxygen vacancies are the active sites for ORR at SOFCs.
The role of Co for batteries is avoiding Li to occupy the sites of Ni, and vice versa, which stabilizes the layered structure.
Thank you Jan Tytgat I downoaded the file and I'm looking at it.
Thank you Sergio Mollá for your reply. I think I understand what you mean, nevertheless I still have a few doubts following this train of thought... I´ll try to write the down in case you find it interesting.
In the case of SOFC cathode materials, this would mean that having more Co in the material increases O-vacancies, which is equivalent to say that ORR active sites increase. This actually agrees with the oxygen vacancy concentration found for example in La0.6Sr0.4Co0.8Fe0.2O3-δ (3-δ = 2.94 at 700°C) and La0.6Sr0.4Co0.2Fe0.8O3-δ. (3-δ = 2.97 at 700°C) *, and it also agrees with the higher ORR activity of La0.6Sr0.4Co0.8Fe0.2O3-δ.
I also found an article (DOI: 10.1002/adfm.201200615) where they show that in MoO3 (an insulator, not a perovskite) the presence of oxygen vacancies creates electronic states between the valence and the conduction bands, and the density of states in this energies increases with increasing oxygen vacancy concentration.
I was thinking that if this were the case for perovskites (meaning the increase in density of states at or near Ef) such as LSCF maybe the higher amount of Co in the lattice might not only increase the active ORR sites but also decrease the activation energy for electronic transfer. I don´t know if this makes any sense.
In any case, the reason for all this would still be the Co (Fe) -O bond strength you mentioned.
* DOI: 10.1007/s10008-003-0488-3
Yes, and related to the M-O bond strength we can find the reducibility of the metal. Since Co in Co-O is easier to reduce than Fe in Fe-O, when Co is reduced, i.e. from Co3+ to Co2+, oxygen vacancies are created to compensate for it. So not only the vacancies found when the material is synthesized are important, then, during ORR, more vacancies are induced electrochemically. The easier a material allows to generate vacancies electrochemically, the more active it will be.
Great, Sergio Mollá , thank you very much for your explanation, now things start to make sense. I'll now continue having all this in mind!
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