Solid oxide fuel cells (SOFCs) normally operate at considerably high temperatures (>700°C) to facilitate ionic charge transport and electrode kinetics [1
]. Encountered by issues such as limited material selection and poor cell durability, many researchers have tried to reduce the operating temperature [3
]. However, lower operating temperature led to a significant sacrifice in energy conversion efficiency due to the resulting increase in ohmic and activation losses [1
There are roughly two ways to minimize the ohmic loss surging at lower operating temperatures. One is to reduce the thickness of the electrolyte, and the other is to synthesize materials with higher ionic conductivities. First, the strategy to reduce in electrolyte thickness has been carried out by many research groups [6
]. Shim et al. demonstrated that a fuel cell employing a 40-nm-thick yttria-stabilized zirconia (YSZ) can generate a power density of 270 mW/cm2
at 350°C [11
], while Kerman et al. demonstrated 1,037 mW/cm2
at 500°C from a 100-nm-thick YSZ-based fuel cell [12
]. Another approach of minimizing ohmic loss is using electrolytes with higher ionic conductivities. Gadolinium-doped ceria (GDC) has been considered as a promising electrolyte material due to its excellent oxygen ion conductivity at low temperatures [13
]. However, the tendency of GDC being easily reduced at low oxygen partial pressures makes its usage as a fuel-cell electrolyte less attractive because the material will have a higher electronic conductivity as it is reduced. For this reason, many studies have been performed to prevent electronic conduction through GDC film by placing an electron-blocking layer in the series [15
]. Liu et al. demonstrated the electron-blocking effect of a 3-μm-thick YSZ layer in a thin-film fuel cell with a GDC/YSZ bilayered electrolyte [18
]. If the GDC electrolyte thickness was reduced down to a few microns, another problem emerges, i.e., oxygen gas from the cathode side starts to permeate through the thin GDC electrolyte [13
]. For the reasons mentioned, the application of a protective layer is essential for GDC-based thin-film fuel cells. Recently, Myung et al. demonstrated that a thin-film fuel cell having a 100-nm-thick YSZ layer deposited by pulsed laser deposition onto a 1.4-μm-thick GDC layer actually prevented both the reduction of ceria at low oxygen partial pressures and oxygen permeation across the GDC thin layer [20
]. For the development of large-scale thin-film fuel cells, an anodic aluminum oxide (AAO) template has been considered as their substrate due to its high scalability potential. However, commercially available AAO templates have a considerably rough surface unlike silicon-based substrates, which have been used for conventional thin-film fuel cells. For this reason, atomic layer deposition (ALD) technique was employed to deposit a highly conformal and dense YSZ layer to minimize uncontrolled pinholes and/or morphological irregularities.
In this report, we demonstrate a prototypical, AAO-supported thin-film fuel cell with a bilayered electrolyte comprising a GDC film and a thin protective YSZ layer. The radio frequency (RF)-sputtered GDC layer with excellent oxygen ion conductivity is used as the primary electrolyte layer, while the YSZ layer deposited by ALD technique prevents the reduction of ceria at low oxygen partial pressure and oxygen permeation across the GDC thin layer. To investigate the effect of ALD YSZ layer as a protective layer, the electrochemical performance of a GDC/YSZ bilayered thin-film electrolyte fuel cell is compared with that of a single-layered GDC-based thin-film fuel cell.