Development of Submicron YSZ Electrolyte Fuel Cell Structure of 25 mm2 on Nanoporous Anodized Alumina

Abstract

Intermediate temperature thin-film fuel cells have received a great deal of attention as a novel fuel cell system owing to their high energy density at operating temperatures lower than 500 °C. Also, recent advances in micro/nanoscale fabrication process and analysis technology facilitated various approaches regarding the architectures of thin-film fuel cells. Meanwhile, porous templates have been suggested as supports in order to overcome the mechanical limitations of the membranes with thicknesses of only tens or hundreds of nanometers. In terms of activation overpotential, oxygen reduction reactions are more dominant than hydrogen oxidation reactions. Thus, many researches on thin-film fuel cells have been concentrated on the design of cathodes and the improvement of interfaces between electrolytes and cathodes rather than anodes. In the porous substrate based thin-film fuel cells, however, the anode is directly deposited on the array of nano-pores, so that the effects of the deposition conditions on the morphological properties of the electrode and eventually on the fuel cell performance can be thoroughly studied. It was revealed that the anode structure greatly affects the electrochemical reactivity and mass transfer rate by determining distribution of triple-phase boundaries (TPBs) and porosity in the electrode. Previous researches on the porous substrate based thin-film fuel cells have greatly contributed in preventing pinhole defects in the electrolytes. Recently, it has been reported that open circuit voltages (OCVs) higher than 1 V could be achieved by applying atomic layer deposition (ALD) to fabrication of nano-thin electrolyte membranes. The yttria stabilized zirconia (YSZ) electrolyte thin-films produced by ALD reportedly shows improved electrochemical characteristics compared to those produced by traditional processes. In this study, fabrication and characterization of thin-film SOFCs supported by the porous substrates were examined in terms of electrolytes deposition techniques, which were sputtering and ALD. The results of electrochemical evaluations showed that the fuel cell power density could be more than doubled by employing the ALD YSZ electrolyte rather than the sputtered YSZ. Moreover, it also showed that the ALD YSZ interface at cathode side could mitigate the degradation of porous Pt electrode, eventually improving the durability of the thin-film fuel cells based on the porous substrates. The porous substrate based structures have been proposed for scale up of micro fuel cells by improving thermos-mechanical stability of the thin-films. In this study, the thin-film fuel cells with active area of 25 mm2 were successfully demonstrated. Furthermore, even though the scaling up of nano-thin electrode caused additional ohmic losses, the total power output generated by a single cell was about 25 mW, which is enhanced by ~18.5 % compared to the previous study reported by Tsuchiya et al. (the total power of 21.1mW and the active area of 13.5 mm2). This study examined the effects of the design of anode structures and the deposition technique of electrolytes on the electrochemical performance and the long-term durability in the thin-film fuel cells supported by the porous substrates.