(Invited) Rate-Determining Step for Oxygen Reduction Reaction on Oxide Cathode in SOFC and Its Interpretation Based on Band Energy Diagram

It is well known that the rate-determining step (RDS) of highly reactive oxide cathodes like La_1-xSr_xCoO₃ (LSC), La_1-xSr_xCo_1-yFe_yO₃ (LSCF), and Ba_1-xSr_xCo_1-yFe_yO₃ (BSCF), for solid oxide fuel cells (SOFCs) is the surface reaction of oxygen reduction reaction (ORR), shown as 1/2O₂(g) + Vö -> O_o + 2h, while its elementary step is still under discussion to date.  The authors focus on the surface reactivity, defined as interfacial conductivity, σ_e, which is inverse of area-specific resistance, using dense thin film cathode.  In the present study, σ_e was estimated using conventional triple-electrode cells composed of thin dense cathodes with additional thin surface modified layer, either polycrystalline pellet of Gd-doped CeO₂ (GDC) or single crystalline yttria-stabilized zirconia (YSZ) with GDC barrier layer, and porous platinum anode. Various type of cathodes of LaCoO₃ (LC) and LSC with variable (La+Sr)/Co ratios as well as pure and BaPrO₃ coated PrO_x, which shows stoichiometric composition of Pr₆O₁₁ at low temperatures below ≈480˚C in high oxygen pressure regime.  The compact cathode films of LC/LSC with typically 500 nm thick coated with surface modification layer of 0, 10, 20 and 40nm are sputtered at RT, followed by a post heat-treatment at 600 ˚C.  PrO_x is selected to examine the influence of electronic band structure by comparison with LC and LSC systems, since PrO_x is known as a mixed Vö and electron conductor in contrast to Vö and electron hole dominance in nonstoichiometric LC and LSCs. 

The measurements were made from 400 to 600˚C in pure O₂ gas flow and the oxygen partial pressure (Po₂) dependence of σ_e was examined at 600˚C from 10^-2 to 1 atm.  All the impedance data show three distinct semicircles in the impedance spectra, and specific types of resistance, corresponding to the electrolyte impedance, R_El, middle frequency impedance, R_MF, and low frequency impedance, R_LF were extracted for further analyses.  Although the origin of R_MF is not well understood, it is reasonable to assume as the impedance that occurs out of cathode region; presumably an interfacial impedance between cathode and electrolyte.  Although R_MF of all the samples does not show the Po₂ dependence, R_LF clearly exhibits 1/2 power dependence for all the cathode materials from LC/LSC to PrOx system.  The σ_e values of PrO_x were 3-5 and 1-2 orders of magnitude lower than LSC and LC, respectively, which were greatly improved by adding merely 1nm thick surface layer of BaPrO₃, presumably due to increased surface basicity.  The rate equation with square root dependence of Po₂ can be interpreted that the RDS is the reaction between the peroxide ion (O₂^2-) adsorbed on the oxide cathode surface and Vö diffusing from cathode/electrolyte interface.  Therefore, σ_e is proportional to the probability for bi-dentate immobile O₂^2- to meet with Vö, expressed by the concentration product of [O₂^2-][Vö].  Poor performance of PrOx can be attributed to a low [O₂^2-] due to low basicity.

The ab initio molecular dynamics were employed to observe the surface reaction of oxygen molecules on NiO with and without Vö at the surface.  NiO was selected as a model system and GGA functional without spin correlation were employed.  O₂ molecules are adsorbed on NiO surface in step-by-step manner from mono-dentate O₂^- to bi-dentate O₂^2-.  The increase of coordination number of O₂ immediately increase the number of donated electron, maintaining one electron per coordination as average, and no further electron donation occurs without a help of Vö.  Upon meeting of O₂^2- and Vö, O₂^2- splits and the third electron is donated to decompose O₂^2- into Oo^X and O^-_ad.  The observation above justify our conclusion from the experiments that the reaction between O₂^2- and Vö is the RDS for ORR. 

A combination of O1s XAS and XPS measurements were made to estimate the electronic structure of oxide cathode materials to understand the energy level alignment between adsorbed O₂^2- and cathode oxide surface.  A higher E_F of cathode material contributes to easy transfer of electrons, which is assumed to be represented, as a zeroth approximation, by the energy separation between E_F and the maximum occupied partial DOS of O2p orbital in the valence band, ΔE, which corresponds to the basicity of the oxides.  The σ_e for ORR observed shows excellent proportionality with ΔE, suggesting an important role of O₂^2- in the cathodic ORR.