High‐Performance Micro‐Solid Oxide Fuel Cells Fabricated on Nanoporous Anodic Aluminum Oxide Templates

Abstract

Micro‐solid oxide fuel cells (μ‐SOFCs) are fabricated on nanoporous anodic aluminum oxide (AAO) templates with a cell structure composed of a 600‐nm‐thick AAO free‐standing membrane embedded on a Si substrate, sputter‐deposited Pt electrodes (cathode and anode) and an yttria‐stabilized zirconia (YSZ) electrolyte deposited by pulsed laser deposition (PLD). Initially, the open circuit voltages (OCVs) of the AAO‐supported μ‐SOFCs are in the range of 0.05 V to 0.78 V, which is much lower than the ideal value, depending on the average pore size of the AAO template and the thickness of the YSZ electrolyte. Transmission electron microscopy (TEM) analysis reveals the formation of pinholes in the electrolyte layer that originate from the porous nature of the underlying AAO membrane. In order to clog these pinholes, a 20‐nm thick Al2O3 layer is deposited by atomic layer deposition (ALD) on top of the 300‐nm thick YSZ layer and another 600‐nm thick YSZ layer is deposited after removing the top intermittent Al2O3 layer. Fuel cell devices fabricated in this way manifest OCVs of 1.02 V, and a maximum power density of 350 mW cm−2 at 500 °C.

Pt–Al2O3dual layer atomic layer deposition coating in high aspect ratio nanopores

“…The increased surface-to-volume is an advantage for surface governed phenomena, such as molecular transport in nanofluidics, largely governed by the surface charges via the electrical double layer (EDL) [6][7][8]. The increased effective surface area improves surface limited phenomena, such as overpotential losses in electrodes [9,10] and the increased line length of the triple phase interface allows increasing the current density in fuel cells [11] or the sensitivity in gas-and biosensors [12][13][14][15][16]. In addition to these intrinsic features, embedding functional capabilities in or on these materials will further enhance their interest.…”

Solid-oxide fuel cells

“…In this design, the porous substrate, besides being used as robust support, is tailored to deposit a dense substrate on top and it is often also functionalized, that is, to be used also as anode. This tailoring is necessary for improving the current collection capabilities of the thin-film electrode in direct contact with it (typically the anode) (Kwon et al, 2011;Noh et al, 2014b). Moreover, this design makes it possible to easily enlarge the FC active area.…”

“…The fabrication of thin-film electrolytes has effectively permitted reducing the associated resistance losses through them, which was first identified as the main source of performance loss on conventional SOFCs. By using thin-film technology, powerdensity target values have been obtained on micro-SOFCs operating at much lower temperatures than those needed on standard high-temperature SOFCs, that is, <500 C. Several publications from MIT (Baertsch, et al, 2004;Fleig et al, 2004;, Harvard University (Kerman et al, 2011a;Lai, Xiong, Tsuchiya, Johnson, & Ramanathan, 2009;Tsuchiya et al, 2011), and Stanford University Shim et al, 2007;Su & Prinz, 2012) in the United States; ETH Zurich (Bieberle-H€ utter, et al, 2005Evans et al, 2009) and IMB-CNM/IREC (Garbayo, Esposito, et al, 2014; in Europe; and KIST (Kwon et al, 2011;Noh et al, 2014b) in Korea have already presented very promising results for micro-SOFC devices, with power densities of >100 mW/cm 2 operating at temperatures as low as 350 C (Shim et al, 2007). Figure 15.10 shows a compilation of these power density results published up to now.…”

“…This especially presents good thermal compatibility with the FC thin films, but it requires several deposition layers of buffering, to obtain the optimal microstructural "furniture" for the deposition of the dense electrolyte layer. Other substrates, such as porous metallic Ni or anodized Al 2 O 3 , have also been proposed in the literature but no further developments were reported on these types of substrates (Kang, Su, Park, Saito, & Prinz, 2006;Kwon et al, 2011). Table 15.4 presents a summary of substrate materials and their relation to the designing approach.…”

“…This first design, usually known as m-SOFC, is based on the use of silicon microfabrication technology for the fabrication of accessible thin free-standing membranes, Kwon et al (2011) where all the functional components of the SOFC are integrated. The use of silicon as support is extremely useful due to the wide series of techniques already developed and optimized for its micromachining.…”