Both the papers that you shared have only 8 pages. Can you please clarify which page are you talking about in these papers. From initial look, both have given a certain doping level, until which the activation energy increases and then, not so much, because the reaction mechanism changes!!!

Waqas Hassan Tanveer page numbers are mentioned already in the question.

LSC-GDC...page 2165 (page 6)

LSCF-GDC...page 32 (page 6)

Thank you for highlighting the pages, Nin Nan

Here is a possible explanation to your question.

Ceria, when used as an MIEC in electrodes, under reducing conditions (anode) doped ceria is partially reduced from Ce4+ to Ce3+ . This induces n-type electronic conductivity, which can lead to partial internal electronic short circuits, and this problem increases with increasing temperatures. See Fig. (Fuel Cell Fundamentals by R O Hayre) where the electronic conductivity at the anode side will be greater than the ionic conductivity for temperatures greater than about 550C.

The disadvantages of doped ceria can be partially solved by adopting a multilayer approach where, for example, a GDC layer facing the cathode is combined with another solid electrolyte (e.g., YSZ) facing the anode. However, multilayer cells also have performance problems due to formation of reaction products with low conductivity at the interface between the electrolyte layers, as well as the mismatch in thermal expansion between the electrolyte layers, which can result in microcracks.

So these reaction products, or change in reaction mechanisms for Doped Ceria at the cathode side is what is mentioned in the papers you shared. And it seems like they have come up with an optimal doping concentration of 30% for lowest ASRs for doped ceria used at the cathode side.

Hope this answers your query

regards

Waqas