(Invited) Rate-Determining Step for Oxygen Reduction Reaction on Oxide Cathode in SOFC and Its Interpretation Based on Band Energy Diagram
The measurements were made from 400 to 600˚C in pure O2 gas flow and the oxygen partial pressure (Po2) dependence of σe 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, REl, middle frequency impedance, RMF, and low frequency impedance, RLF were extracted for further analyses. Although the origin of RMF 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 RMF of all the samples does not show the Po2 dependence, RLF clearly exhibits 1/2 power dependence for all the cathode materials from LC/LSC to PrOx system. The σe values of PrOx 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 (O22-) adsorbed on the oxide cathode surface and Vö diffusing from cathode/electrolyte interface. Therefore, σe is proportional to the probability for bi-dentate immobile O22- to meet with Vö, expressed by the concentration product of [O22-][Vö]. Poor performance of PrOx can be attributed to a low [O22-] 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 O22-. 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 O22- and Vö, O22- splits and the third electron is donated to decompose O22- into OoX and O-ad. The observation above justify our conclusion from the experiments that the reaction between O22- 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 O22- and cathode oxide surface. A higher EF of cathode material contributes to easy transfer of electrons, which is assumed to be represented, as a zeroth approximation, by the energy separation between EF and the maximum occupied partial DOS of O2p orbital in the valence band, ΔE, which corresponds to the basicity of the oxides. The σe for ORR observed shows excellent proportionality with ΔE, suggesting an important role of O22- in the cathodic ORR.