(Invited) Oxygen Reduction Reaction at Cathodes on Proton Conducting Oxide Electrolytes: Contribution from Three Phase Boundary Compared to Bulk Path

So far, the oxygen reduction reaction for cathodes on proton conducting electrolytes such as acceptor-doped BaZrO₃ is less well understood than for cathodes on oxide ion conductors. Two groups of reaction pathways can be hypothesized: (i) without involvement of oxygen vacancies (which are, in contrast to cathodes on oxide conducting electrolytes, not required for the overall reaction on proton conducting electrolytes), (ii) with involvement of oxygen vacancies as catalytically active center, where the vacancies are restored at the end of the reaction by water desorption. Reaction rates for different steps being rate determining are derived, considering also the dependence of point defect concentrations on both pO₂ and pH₂O. These predictions are compared to experimental findings.

By thermogravimetry, a proton concentration in the range of 0.1-1mol% could be detected in Ba_0.5Sr_0.5Fe_0.8Zn_0.2O_3-d (BSFZ) under humid conditions.[1] Impedance measurements on pore-free BSFZ and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-d (BSCF) microelectrodes  show that their proton conductivity suffices for the oxygen reduction reaction to proceed by the "bulk path" on the whole surface area.[2] In the analysis of these measurements on gas-symmetrical cells, the influence of the perceptible hole conductivity of the BaZr_1-xY_xO_3-x/2 proton conducting electrolyte in high pO₂must properly be accounted for.[3]

The measured pO₂ and pH₂O dependencies indicate that molecular oxygen species and oxygen vacancies are involved in the rate determining step. Thus, it appears that at least for perovskites such as BSFZ and BSCF with high oxygen vacancy concentration, the early steps of oxygen reduction are similar for cathodes on oxide conducting and proton conducting electrolytes, and differ only in the later stages (oxygen incorporation and transfer to the electrolyte vs. water desorption).

[1] D. Poetzsch, R. Merkle, J. Maier, Phys. Chem. Chem. Phys. 16 (2014) 16446

[2] D. Poetzsch, R. Merkle, J. Maier, in preparation

[3] D. Poetzsch, R. Merkle, J. Maier, J. Power Src. 242 (2013) 784