Design of High Performance Intermediate Temperature Fuel Cells Based on BaCe0.8Y0.2O3/Pd Heterointerfaces

Abstract

The current PCFC performance lags far behind the SOFC performance even at the intermediate temperature (IT) range of 400-600°C, even though proton-conducting perovskite type oxides, such as BaCe0.8M0.2O3 (M=Y, Gd, Yb etc.) exhibit higher ion conductivity than the oxide ion conducting oxides used as an electrolyte of SOFCs. One of major reasons for the deteriorated performance is large cathodic interfacial polarization due to a lack of suitable cathode for PCFC operation. Meanwhile, exceptionally high power output has been reported for a hydrogen membrane fuel cell (HMFC) in the temperature range of 400-600°C, which consists of a thin proton conducting ceramic electrolyte supported on a dense hydrogen permeable metal anode. Hence, it is of fundamental and of technological interest to investigate the cathode and anode interfacial polarization of HMFC in order to clarify the mechanism for the high efficiency power generation. In the present study, we successfully fabricated high efficiency hydrogen membrane fuel cells based on BaCe0.8Y0.2O3- d thin film with hydrogen permeable Pd solid anode and conventional La0.6Sr0.4Co0.2Fe0.8O3 cathode. BaCe0.8Y0.2O3- d (BCY) thin films were fabricated by RF sputtering with a single BaCe0.8Y0.2O3- d target or cosputtering with BaCe0.8Y0.2O3- d and Ce0.8Y0.2O2 double targets. The sputtering conditions of sputtering power, process atmosphere, target-substrate distance, substrate temperature and post-annealing temperature were optimized to prepare a highly crystalline film having the same composition as the target composition. The hydrogen membrane fuel cells were fabricated by depositing a BCY thin film of 1 mm thickness on a hydrogen-permeable Pd anode by RF sputtering in optimal condition. The Pd foil (0.05 mm thickness, Tanaka Co.) was used as a solid anode. The foil (12 ´ 12) was polished with alumina particles (1.0 μm diameter) and was cleaned by sonication in acetone and pure water before deposition. La0.6Sr0.4Fe0.8Co0.2O3 (LSCF) button electrode (5 mmÆ) was deposited on the BCY films as a porous cathode by screen-printing with a commercial LSCF paste (NexTech) and subsequent heating with heat-gun for 2 min. The performance of BCY thin film base HMFC was evaluated by measuring the current-voltage (I-V) relation and electrochemical impedance spectra. I-V and I-P characteristics of HMFC consisting of 1 mm thick BCY thin film were measured at 520, 550, 570 and 600°C (Fig. 1). The performance is drastically improved at higher temperature; the cell can gives maximum power density of 1050 mW cm-2 and OCV of 1.08 V at 600°C. This value is higher than the champion data of the maximum power density of PCFCs [1]. The power output is very stable and the current keeps about 1300 mA cm-2 at 0.7 V for a few hours. The impedance spectra of HMFC were replicated with an equivalent circuit build by series connection of cathode charge transfer elements and anode mass transfer elements. The contribution of the mass transfer in Pd bulk was found to be relatively small in comparison to cathode polarization and ohmic loss in normal fuel cell atmosphere. Moreover, the cathodic charge transfer resistance of HMFC was found to be smaller than those of the recent PCFC systems by 25 times. The current results demonstrated that the HMFC retained relatively large gas-proton-electron triple boundary zones near interface between BaCe0.8Y0.2O3- d electrolyte and porous La0.6Sr0.4Co0.2Fe0.8O3- d cathode. The I-V characteristics of the HMFC was measured by supplying wet Ar to LSCF side and dry Ar to Pd side, exhibiting rectification, where negative bias on Pd is the reverse bias. These indicate that BCY/Pd heterointerface works as a Schottky junction of n-type BCY and large work function Pd metal. [1] Kim, J.; Sengodan, S.; Kwon, G.; Ding, D.; Shin, J.; Liu, M.; Kim, G. ChemSusChem 2014, 7, 2811.