Characterization of Proton-Conducting Y(CrO4)1-X (PO4) x As a Cathode Material of Proton-Conducting Ceramic Fuel Cells

Abstract

It has been reported that proton conducting oxide such as BaCeO3 and BaZrO3 have high proton conductivity at intermediate temperatures, IT, around 400-600°C. Therefore, proton-conducting ceramic fuel cells (PCFC) attract much attention as IT-operating fuel cell system (IT-FC). Nonetheless, the current PCFC performance lags far behind the SOFC performance even at the ITrange of 400-600°C. One of the reasons for the deteriorated performance of PCFC is the lack of suitable cathode materials. Generally, oxide ion-electron mixed conductors such as La0.6Sr0.4Co0.8Fe0.2O3- δ(LSCF) are used for the PCFC cathode, but these reveal large overpotential since forming the limited reaction zone due to the mismatch of ionic carriers with electrolyte. Accordingly, there is motivation to develop the proton-electron mixed conductors which can be used in cathode conditions of fuel cells. Previously, we found that Y(CrO4)1-x (PO4) x solid solution (YCP) have proton-electron mixed conductivity in wet air atmosphere. Hence, it is motivated to use YCP as additives of cathode materials of PCFC in order to extend the reaction zone. In this work, we examined the fuel cells using composite cathode of YCP and LSCF. BaCe0.7Zr0.1Y0.2O3- δ(BZCY) was used as proton conducting electrolyte. Bulk electrolyte cell were constructed with a BZCY disc which were prepared by solid state reactive sintering (SSRS) method. The electrolyte precursor powder was prepared by mixing proper amount of BaCO3, CeO2, ZrO2, and Y2O3 according to the desired stoichiometry with the addition of 2.0wt.% NiO as a sintering aid. This mixture was ball-milled for 48 h and uniaxially pressed under 20 MPa for 1 min and then cold-isostatic-pressed under 100 MPa for 1 min. Finally, the green pellets were calcined at 1400°C for 18 h so as to obtain bulk electrolyte disc (2 mmd, 9 mmf). YCP powders were synthesized by pyrolysis of the mixed Y-Cr-P hydroxide precursor at 700°C for 3h in O2. The hydroxide precursors were prepared by coprecipitation method with (NH3)H2PO4, Cr(NO3)3 and Y(NO3)3. YCP powders dispersed in a-Terpineol were mixed with LSCF paste (NexTek) in the volume ratio of YCP:LSCF = 1:1. YCP-LSCF mixed pastes were screen printed on the side of BZCY electrolyte as a cathode and Pt paste as anode was applied to the opposite faces so as to form Pt|BZCY|YCP5/LSCF cell. Au wire was attached along the side of BZCY disc as a reference electrode by using Au paste. To investigate cathode polarization characteristics, the complex impedance measurements were carried out with the three-electrode cell. The XRD pattern indicated that a single orthorhombic perovskite phase of BZCY was prepared by the SSRS method. The density of the sintered pellets was about 90%. The bulk electrolyte cell with the YCP5/LSCF cathode gives OCV over 1.0 V and maximum power density about 40 mW cm-2 at 600°C. The complex impedance measurements indicate that the cells reveal two impedance arcs according to cathodic polarization in Nyquist plots. The semicircle appearing in high frequency region (104-102 Hz), SH, can be assigned to the charge transfer reaction between the electrolyte and the cathode and one in low frequency region (102-10-1 Hz), SL, can be attributed to the dissociative oxygen adsorption reaction. SH of the cell with YCP-LSCF composite cathode was smaller than that of the cell with LSCF cathode. On the other hand, the SL of the composite cathode cells was larger than that of the LSCF cathode cells. These results indicate that the interfacial charge transfer reaction is promoted by adding proton-conducting YCP because of the extension of the electrode-electrolyte-air three phase boundary.