(Invited) Determination of Effective Reaction Area in a Mixed-Conducting SOFC Cathode

Solid oxide fuel cell (SOFC) is one of the most promising next-generation energy conversion devices having high energy conversion efficiency. For its full-scale commercialization, oxygen reduction kinetics at the SOFC cathode, which frequently governs the performance of SOFC, should be improved. In order to achieve high performance of the SOFC cathode, it is necessary to optimize its material and structure. When a porous electrode consisting of a mixed ionic-electronic conducting oxides, such as (La,Sr)CoO₃ and (La,Sr)(Co,Fe)O₃, is used as an SOFC cathode, electrochemical oxygen reduction reaction occurs not only at the electrode/electrolyte interface but also on the surfaces of the oxide particles. In other words, the area where the electrode reaction occurs, i.e.the effective reaction area, expands to a certain distance from the electrode/electrolyte interface. It is believed that the effective reaction area in a porous mixed-conducting oxide electrode is controlled by the resistance of the surface reaction at the oxide surfaces and the resistance of the oxide ion diffusion in the oxide bulk. Based on this idea, many numerical simulations have been carried out to evaluate the effective reaction area in SOFC electrodes. However, as far as we know, no experimental evaluations of the effective reaction area have been done so far.

From the backgrounds mentioned above, in this work, we tried direct observation of the effective reaction area in a SOFC cathode by using experimental manners. The (La,Sr)CoO₃ was chosen as a mixed conducting oxide. For quantitative evaluation, the patterned thin film electrode, which was a kind of a columnar electrode modeling the porous electrode, was used. The in situ micro X-ray absorption spectroscopy (XAS) and the ¹⁸O/¹⁶O isotope exchange measurements under polarization were applied to determine the effective reaction area. Electrochemical impedance spectroscopy measurements were also performed with the same patterned thin film electrode, and the results were compared with those obtained by the XAS and the isotope exchange measurements.

A patterned La_0.6Sr_0.4CoO_3-d (LSC) dense film was fabricated on a Ce_0.9G_0.1O_1.95(GDC) electrolyte by means of lithographic techniques. The LSC film was prepared by the pulse laser deposition (PLD) and its thickness was approximately 400 nm. The porous platinum electrodes were used as a reference and a counter electrodes.

XAS measurements were carried out at the beam line BL37XU, SPring-8, JASRI, Japan. In this beam line, the incident X-ray from the synchrotron can be focused into micro or sub-micro meter size. The beam size used in this work was typically 0.8 x 0.5 mm. By using the focused X-ray beam, XAS measurements with the high special resolution of less than 1 mm became possible. During the cathodic polarization, the oxygen potential is reduced at the effective reaction area. Such an oxygen potential decrease accompanies the partial reduction of oxides, and the amount of the oxygen potential drop varies with the distance from the electrode/electrolyte interface depending on the amount of the reaction current. In this work, the partial reduction of the LSC electrode was evaluated as a function of the distance from the electrode/electrolyte interface by using in situ micro XAS measurements at the Co K-edge, and then the effective reaction area was determined.

In ¹⁸O/¹⁶O isotope exchange measurements, the cathodic bias was applied to the patterned electrode after the pre-annealing at a desired temperature and p(O₂) at least for 1 hour, and then ¹⁸O₂ was introduced for 5 min. The cell was quenched down to room temperature immediately after the bias was terminated. The distributions of ¹⁸O and ¹⁶O were investigated by the secondary ion mass spectroscopy (SIMS) measurements.

All the measurements were performed in the temperature range of 873-1073 K and the oxygen partial pressure p(O₂) range of 10^-4-1 bar under cathodic polarization of -0.1-0.25 V. At 873 K, both XAS and oxygen isotope exchange measurements suggested that the electrochemical oxygen reduction reaction takes place in the area of approximately 20 μm from the electrode/electrolyte interface under p(O₂) of 1 bar and cathodic bias of -0.2 V. The effective reaction area tended to expand with increasing temperature, decreasing p(O₂), and increasing cathodic bias. The results of electrochemical impedance spectroscopy measurements will be also given at the presentation.