In situ investigations of gas-solid interfaces in solid-state electrochemical systems by FTIR spectroscopy

For the first time in the SOFC field, a new characterization technique— in situ potential-dependent FTIR emission spectroscopy (pd-FTIRES), was developed in this work. It can be joined with various electrochemical techniques to carry out in situ studies of oxygen reduction at SOFC cathodes, which will eventually lead to the design of more active cathodes. By using in situ pd-FTIRES, evidence has been found that two, possibly three distinct di-oxygen species, in both superoxidic ($O-2$) and peroxidic ($O2-2$) forms, are present on the electrode surface under actual SOFC operating conditions. Broad spectral features are assigned to the polarization-induced changes in the optical properties of the electrode surface layer. This is the first report of infrared electro-emission effect. Five different cathode materials, Sm_0.5Sr_0.5CoO_3−δ (SSC), La_0.6Sr_0.4CoO_3−δ (LSC), La _0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF), La_0.6Sr_0.4FeO_3−δ (LSF), and Pt, have been studied under similar conditions using in situ pd-FTIRES. While the mechanisms of oxygen reduction at all perovskite cathodes studied seem to be similar, with the reduction of superoxide ions ($O-2$) being the rate-determining-step (rds), the oxygen reduction mechanism on the Pt electrode appears to be different: superoxide ions are not observed from the pd-FTIRES spectra. The catalytic activities of these perovskite cathodes as determined from the pd-FTIRES spectra seem to follow the order of SSC > LSC > LSCF > LSF. On the other hand, the interfacial resistances of symmetrical cells (electrode/electrolyte/electrode) based on these cathodes, as measured using impedance spectroscopy under the same conditions for in situ pd-FTIRES, seem to be in the order of LSCF < SSC < LSC < LSF << Pt. Oxygen reduction and evolution processes are studied using in situ pd-FTIRES by changing the direction of DC polarization. Further, interference fringe patterns are observed in FTIRES spectra as the thickness of electrode is reduced, providing useful information about the optical thickness of the electrode.