Ultra-thin film solid oxide fuel cells utilizing un-doped nanostructured zirconia electrolytes Changhyun Ko * , Kian Kerman, Shriram Ramanathan Harvard School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA article info Article history: Received 15 February 2012 Received in revised form 13 April 2012 Accepted 16 April 2012 Available online 21 April 2012 Keywords: Micro-solid oxide fuel cell (m-SOFC) Thin film electrolyte Zirconia (ZrO 2 ) Platinum Ultra-violet oxidation Portable energy abstract Aliovalently-doped zirconia (ZrO 2 ) systems such as yttria-stabilized ZrO 2 (YSZ) have been explored as ionic conductors for solid oxide fuel cells (SOFCs) owing to their high ionic conductivity and exceptional mechanical and chemical stability. Thin film micro-SOFCs (m-SOFCs) with free-standing membranes create an opportunity for reduced temperature operation with the need to engineer the various materials components. In this study, we have fabricated m-SOFCs composed of nominally pure ZrO 2 electrolytes (down to sub-10 nm thickness) prepared by room temperature photon-assisted oxidation of Zr precursor metal films and nanoporous Pt electrodes and report on fuel cell performance up to w500  C in hydrogen. The m-SOFCs exhibit maximum power density of w33 mW cm 2 with open circuit voltage of w0.91 V at 450  C. The electrolyte thickness-dependent functional properties of the m-SOFCs are dis- cussed in detail along with thermo-mechanical stability and microstructural studies. The results could serve as a benchmark to understand doping effects in designing thin film fast-ion conducting zirconia- based electrolytes for low temperature fuel cell operation. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Solid oxide fuel cells (SOFCs) have several potential desirable attributes including high energy conversion efficiency and fuel flexibility [1e3]. Reduction in their operating temperature from typical values of w800  C down to 300e500  C could enhance the scope of their applications into the mobile energy space [4]. Utilizing ultra-thin film electrolytes in the sub-100 nm range enables a reduction in Ohmic resistance and is one approach to realizing low temperature SOFCs [5,6]. This miniaturized SOFC based on thin film structure is called micro-SOFC (m-SOFC). Much research is needed in synthesis of thin film nanostructured oxides with controlled microstructural defects and their role in device characteristics. Similarly, interpretation of electrical properties of such ultra-thin layers is a formidable problem and is worthy of research itself. Cation-doped zirconia such as yttria-stabilized zirconia (YSZ) has been incorporated into the majority of SOFCs as an electrolyte due to high ionic conductivity arising from oxygen vacancies as well as excellent mechanical and chemical stability. ZrO 2 has three different structural polymorphs namely cubic fluorite, tetragonal, and monoclinic structures. The high temperature cubic phase is stabilized by aliovalent acceptor doping such as Ca, Mg, Sc, and Y. For each dopant, the doping concentration range for stabilizing the cubic phase varies and is temperature-dependent (for example, the optimal Y 2 O 3 doping level for stabilizing cubic phase in bulk ceramic form is 8e9 mol%) [7,8]. Particularly for m-SOFCs, YSZ thin films are synthesized at lower temperatures (<600  C) limiting the size of grains to nanoscale range. How such nanostructuring influences electrical conduction in oxides is an interesting area of study and further their impact on fuel cell performance is of importance. Recently, Jung et al. observed that nanocrystalline film shows higher ionic conductivity than bulk counterparts and its peak conductivity is achieved at much lower Y doping level in comparison to bulk YSZ ceramics. They argued that it is due to the formation of metastable cubic phase with smallest defect interac- tion leading to the largest ionic mobility among the ZrO 2 phases [9]. Brossmann et al. found that grain boundary oxygen-ion diffusion is faster than bulk diffusion by 3e4 orders in magnitude in mono- clinic un-doped ZrO 2 ceramics with 70e300 nm grain size by tracer diffusion and secondary ion mass spectroscopy [10]. Eder and Kramer extracted both electronic and ionic conductivities of nanocrystalline monoclinic ZrO 2 bulk samples treated under oxidizing and reducing environments by impedance spectroscopy techniques [11]. The authors observed that the conductivity is increased in the reduced samples likely due to the formation of oxygen vacancies. The m-SOFC structure is a model system to study this problem in an application context in the intermediate * Corresponding author. Tel.: þ1 617 497 4746; fax: þ1 617 495 9837. E-mail addresses: changhyun.ko@gmail.com, cko@fas.harvard.edu (C. Ko). Contents lists available at SciVerse ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour 0378-7753/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2012.04.034 Journal of Power Sources 213 (2012) 343e349