Electrical characterization of thermomechanically stable YSZ membranes for micro solid oxide fuel cells applications

doi:10.1016/j.ssi.2009.12.019

Solid State Ionics

Volume 181, Issues 5–7, 11 March 2010, Pages 322-331

https://doi.org/10.1016/j.ssi.2009.12.019 Get rights and content

Abstract

Yttria-stabilized zirconia free-standing membranes were fabricated by pulsed laser deposition on Si/SiO₂/Si₃N₄ structures for developing silicon-based micro devices for micro solid oxide fuel cell applications. Their mechanical stability under working conditions was evaluated satisfactorily by applying thermal cycling to the membranes. Membranes mechanically stable at operating temperatures as high as 700 °C were obtained for deposition temperatures in the range between 400 and 700 °C. Thermomechanical behavior as measured by X-ray microdiffraction was correlated with the evolution of the microstructure with the temperature from TEM analysis, comparing as-deposited and post-deposition annealed membranes. Electrical properties of both yttria-stabilized zirconia films and membranes were studied by DC conductivity and impedance spectroscopy, respectively. A difference of almost one order of magnitude was measured between bulk and stressed films while conductivities close to the bulk were observed for YSZ membranes. Values of area specific resistance of 0.15 Ωcm² were measured at temperatures below 450 °C for 240 nm thick YSZ membranes deposited at 600 °C and annealed at the same temperature for 2.5 h.

Introduction

Power supply of portable electronic devices has become an active research field because of the proliferation of devices such as laptops or mobile phones. Due to their long life time, high power density and integrability, micro batteries and micro fuel cells appear as promising power generators to cover these needs. Among others, the most promising types of fuel cells for portable applications are the so-called micro-polymer electrolyte fuel cells (µPEMFCs) and, after recent advances, micro-solid oxide fuel cells (µSOFCs) [1], [2]. In particular, µSOFCs present advantages like a high efficiency in energy conversion, large energy density and the capability of operation using different fuels (including hydrocarbons). Moreover, miniaturization of SOFCs by both reduction of the electrolyte thickness and integration into Micro Electro Mechanical Systems (MEMS) have been shown as an effective strategy for reducing the operating temperature and subsequently the thermal response time (warm-up and cycling), the energy consumption and the materials reactivity. Recent successful experiences from ETH Zurich [3], [4], Stanford [5], [6], [7] and Harvard [8] have triggered increasing interest in this field, showing the potential of micro SOFC at temperatures below 500 °C. For a comprehensive report on the current status of development of micro-SOFCs refer to the extensive review by Evans et al. [2].

As previously mentioned, one of the key points for the development of micro-SOFCs is the reduction of the electrolyte thickness to the sub-millimetre range to trivially decrease the electrolyte contribution to the total resistance. In addition, both the reduction of the electrolyte thickness to values comparable to the grain size and the reduction of this grain size to the nanoscale have been proposed as strategies for increasing the ionic conductivity based on thin films [9], [10] and surface exchange [5]. Although big efforts have been devoted to understand these and other effects of nanoscale on ionic transport there is still an ongoing controversy over it. This is mainly due to the fact that thin films cannot be prepared without a substrate, which intrinsically difficult any type of characterization because of both the stress induced by the substrate and the forced in-plane geometry of the measurement. In particular, electrical, mechanical and (micro) structural characterization presents significant problems [11], [12], [13], [14].

In the case of micro-SOFCs, comprehensive understanding of the ionic conduction across the electrolyte as well as the evolution with the temperature of the (micro) structure and mechanical stress of the involved thin films is of the utmost importance for the final performance of the device. Moreover, in all of the reported micro-SOFCs [2], the active area is indeed restricted to the free-standing part of the thin film; therefore, special emphasis should be put in its specific characterization. However, very few papers have been devoted to this although strong differences are expected between attached thin films and suspended areas, e.g. in the stress state [15], [16], [17] and therefore the electrical behaviour [12], [18]. For instance, it is well-known that important residual stresses are present in PLD fabricated YSZ membranes depending on different deposition conditions [14], [15] but the effect of this on the thermomechanical stability, crystallinity or electrical behaviour of the membrane remains still unclear.

This work evaluates and throws light on some of these aspects by electrically characterizing yttria-stabilized zircona (YSZ), the state-of-the-art electrolyte material for SOFCs, using an approach based on free-standing membranes that allow a direct access to cross-plane properties. Self-supported YSZ membranes on Silicon-based microplatforms were fabricated for this goal using Pulsed Laser Deposition (PLD). This study puts particular emphasis on the critical point of the stress evolution of the membrane with the temperature, i.e. the thermomechanical stability, and the correlation of this mechanical evolution with the electrical behaviour and microstructural changes. In-plane and cross-plane conductivity of the membrane were evaluated by DC and AC measurements while the (micro) structural characterization was carried out by transmission electron microscopy (TEM), micro X-ray diffraction (micro-XRD) and micro Raman analysis. The here presented microplatforms have been also discussed as the initial step for a future integration of micro-SOFCs into MEMS.

Section snippets

YSZ-free standing membranes micro fabrication

The technological flow of the free-standing membranes fabrication process is shown in Fig. 1 and detailed in reference [15]. Before depositing YSZ some micromachining steps were required. Single crystal (100)-oriented p-type silicon wafers of 100 mm diameter and 300 µm thick were thermally oxidized to obtain a 100 nm thick layer of SiO₂. Afterward, low pressure chemical vapor deposition (LPCVD) was used to grow 300 nm thick layers of Si₃N₄ (Fig. 1a). A photolithographic step on the back side of the

Results and discussion

First of all, it is important to remark that the fabrication process of the YSZ self-supported membranes is highly reliable (see an image of the released membrane in Fig. 3a). In order to quantify this it is possible to define a survival rate (SR) as the ratio between the total number of fabricated membranes and the membranes broken after the deposition (the statistic was carried out over more than one hundred membranes). Membranes deposited at T_s = 200, 400, 600 and 700 °C yielded a high survival

Conclusions

Free-standing YSZ membranes of thickness between t = 60–240 nm and side length in the range of a = 50–820 µm were fabricated on Silicon based microplatforms in order to evaluate their suitability for working as electrolytes in low-to-intermediate temperature micro SOFCs. High density, homogeneity and excellent thermomechanical stability at post-deposited annealing temperatures as high as 700 °C were observed for a range of deposition temperatures of T_s = 400–700 °C. This high deposition temperature has

Acknowledgments

This investigation has been supported by the Spanish Ministry of Science and Education (MAT-2008-04931, CSD-2008-023 and TEC-2007-64669 projects) and the “Generalitat de Catalunya” (2009-SGR-228). A.T., N.S would like to thank the financial support of the postdoctoral program “Ramón y Cajal” (MICINN) and A.C to the postdoctoral program “Juan de la Cierva” (MICINN).

References (44)

A. Evans et al.
J. Power Sources
(2009)
A. Bieberle-Hutter et al.
J. Power Sources
(2008)
A.C. Johnson et al.
J. Power Sources
(2009)
X. Guo et al.
Acta Mater.
(2005)
J.H. Joo et al.
Solid State Ionics
(2006)
I. Kosacki et al.
Solid State Ionics
(2005)
J.L.M. Rupp et al.
Acta Mater.
(2007)
R.W. Knoll et al.
Thin Solid Films
(1984)
P. Gao et al.
Thin Solid Films
(2000)
J.A. Thornton et al.
Thin Solid Films
(1989)

C.R.M. Grovenor et al.

Acta Metall.

(1984)

D. Ruddell et al.

Thin Solid Films

(2003)

M. Yashima et al.

J. Phys. Chem. Solids

(1996)

H. Tomaszewski et al.

Thin Solid Films

(1996)

A. Nagata et al.

Vacuum

(2002)

A.P. Caricato et al.

Appl Surf. Sci.

(2005)

Y. Estrin et al.

Acta Mater.

(2001)

A. Lakki et al.

J. Eur. Ceram. Soc.

(2000)

S. Heiroth et al.

J. Eur. Ceram. Soc.

(2010)

D. Nikbin

Fuel Cell Rev.

(2006)

P. Muecke et al.

Adv. Funct. Mater.

(2008)

H. Huang et al.

J. Electrochem. Soc.

(2007)

Cited by (69)

Feasibility evaluation of low-temperature deposited thin-film electrolyte with successive post-annealing for solid oxide fuel cells
2024, Journal of Power Sources
During the past decade, thin-film-based solid oxide fuel cells (TF-SOFCs) have demonstrated remarkable performance at a low operating temperature of ∼500 °C by reducing ohmic resistance with vacuum-deposited thin electrolytes. However, a high-temperature deposition process at approximately 700 °C is employed in TF-SOFCs, which is only available at a laboratory scale and is not appropriate for commercial deposition equipment. To address this issue, we investigate the feasibility of depositing electrolytes at relatively low temperatures (≤300 °C) accompanied with post-annealing. A TF-SOFC comprising a trilayer thin-film electrolyte, i.e., yttria-stabilized zirconia (YSZ) sandwiched between gadolinia-doped ceria (GDC), fabricated at low-temperature deposition with subsequent post-annealing is selected and tested. Based on a thorough examination using X-ray diffraction and scanning electron microscopy, the appropriate growth conditions for the YSZ and GDC thin-film layers are selected. Through the optimization, we successfully create a low-temperature deposited 750 nm-thick GDC/350 nm-thick YSZ/150 nm-thick GDC (CZC) trilayer electrolyte exhibiting comparable performances with that deposited at 700 °C. High performance of a single cell, a peak power density of 890 mW/cm² at 500 °C, is achieved. This result suggests the potential of fabricating high-quality TF-SOFCs with commercial equipment.
Corrosion of structural components of proton exchange membrane water electrolyzer anodes: A review
2023, Journal of Power Sources
Proton exchange membrane (PEM) water electrolysis is one of the low temperature processes for producing green hydrogen when coupled with renewable energy sources. Although this technology has already reached a certain level of maturity and is being implemented at industrial scale, its high capital expenditures deriving from the utilization of expensive corrosion-resistant materials limit its economic competitiveness compared to the widespread fossil fuel-based hydrogen production, such as steam reforming. In particular, the structural elements, like bipolar plates (BPP) and porous transports layers (PTL), are essentially made of titanium protected by precious metal layers in order to withstand the harsh oxidizing conditions in the anode compartment. This review provides an analysis of literature on structural element degradation on the oxygen side of PEM water electrolyzers, from the early investigations to the recent developments involving novel anti-corrosion coatings that protect more cost-effective BPP and PTL materials like stainless steels.
Zigzag or spiral-shaped nanostructures improve mechanical stability in yttria-stabilized zirconia membranes for micro-energy conversion devices
2019, Nano Energy
Free-standing solid-state ion conducting thin film membranes are key components in micro-energy conversion devices such as micro-solid oxide fuel cells or electrolyzers. Through this work, we explore the design and fabrication of thin film architectures with either straight, zigzag or spiral-shaped columnar grain nanostructures of 8 mol% doped Yttria stabilized zirconia (8YSZ) in order to modify the ceramics elastic properties and mechanical stability for MEMS integration. We report that the zigzag and spiral-shaped nanomorphologies' can be engineered with a ∼44% reduced elastic modulus. Ultimately, this results in an increased fabrication yield when the thin ionic conductor thin film structures are turned into free-standing membranes as required for different micro energy converter applications. Raman spectroscopy reveals that the symmetry is lowered by the existence of monoclinic distortions in the cubic phase which modifies the elastic moduli of films with straight columnar structures. Fundamentally, we show here evidence that for yttria-stabilized zirconia modifications in membrane nano-architectures and strain can lead to phase changes, which agrees well with findings published in the 1970s based on applied external stress's on macroscopic structures (i.e. pellets). The influence of the change in nano-morphology on the cross-plane ionic conductivity is minor. The oxygen ion conducting thin film nanomorphology design exhibits potential to optimize grain connectivity and tortuosity by growth as either columnar, zigzag or spiral-shaped morphologies, in order to obtain membranes with controllable phases and elastic moduli for micro-energy conversion devices.
Portable Early Market SOFCs
2016, High-Temperature Solid Oxide Fuel Cells for the 21st Century: Fundamentals, Design and Applications: Second Edition
The development of small-scale SOFCs for several applications is described. Sensors represent the lowest power application, but phone chargers are now seen as a potential market near 10 W. In larger power sectors, such as CHP and APUs which require above 100 W, the need for more rapid start-up, improved ceramic processing and increased power density is clear. Invention was most rapid in 1994 when the first mSOFC patent was filed and the system design formulated, followed by building a 1000-micro-tube stack in 1996. The rapid start-up under 10 s for 2 mm diameter tubes was impressive, but the BOP was then the time limiting factor. Recent advances include the move to anode support, coextrusion with 10 μm of YSZ electrolyte and the use of LSCF as the cathode which could operate at 700 °C. The resulting micro-tubes were assembled into stacks using wires for interconnection. In 1990, it was thought impossible that SOFCs could power phone chargers or fly in aircraft because their weight and volume were too great. Now, mSOFCs are competing with PEMFCs powered by methanol because propane and butane are accepted, cheap and widely available fuels. mSOFCs can beat PEMFCs in extended flight UAVs despite their denser ceramic components and thick thermal insulation because propane is a fuel with higher energy density than pressurised hydrogen gas stored in a heavy cylinder. The future prospect for portable SOFC products now looks promising in a range of product areas.
Is it possible to design a portable power generator based on micro-solid oxide fuel cells? A finite volume analysis
2015, Journal of Power Sources
Citation Excerpt :
Although all the existing works represent an excellent first approach to understand the complexity of the system, an explicit design of the whole μ-SOFC PG, including the geometry and experimental data from the individual elements, is required to advance in the optimization of the stacking configuration, the heat management strategy and the steady-state and transient regimes. In this paper, a vertical stack of the whole μ-SOFC PG is proposed on the basis of experimental works previously published by the authors (details on the selection of materials and individual architectures can be found elsewhere [18–22,28–31]. By employing a three-dimensional thermo-fluidic model and finite volume analysis, a μ-SOFC power generator of 1Wel output fuelled with ethanol is studied in steady and dynamic conditions.
The inherent limited capacity of current battery technology is not sufficient for covering the increasing power requirements of widely extended portable devices. Among other promising alternatives, recent advances in the field of micro-Solid Oxide Fuel Cells (μ-SOFCs) converted this disruptive technology into a serious candidate to power next generations of portable devices. However, the implementation of single cells in real devices, i.e. μ-SOFC stacks coupled to the required balance-of-plant elements like fuel reformers or post combustors, still remains unexplored. This work aims addressing this system-level research by proposing a new compact design of a vertically stacked device fuelled with ethanol. The feasibility and design optimization for achieving a thermally self-sustained regime and a rapid and low-power consuming start-up is studied by finite volume analysis. An optimal thermal insulation strategy is defined to maintain the steady-state operation temperature of the μ-SOFC at 973 K and an external temperature lower than 323 K. A hybrid start-up procedure, based on heaters embedded in the μ-SOFCs and heat released by chemical reactions in the post-combustion unit, is analyzed allowing start-up times below 1 min and energy consumption under 500 J. These results clearly demonstrate the feasibility of high temperature μ-SOFC power systems fuelled with hydrocarbons for portable applications, therefore, anticipating a new family of mobile and uninterrupted power generators.
Solid-oxide fuel cells
2015, Epitaxial Growth of Complex Metal Oxides
Fuel cells convert chemical energy directly into electricity with high efficiency. Solid-oxide fuel cells (SOFCs) have the highest performance and durability. Traditional SOFCs operate at temperatures above 600 °C, and lowering the temperature to 400–500 °C is the key to new applications, for example, miniaturized systems with fast start/stop cycling and self-sustaining operation. A low-temperature SOFC is possible to achieve by tailoring the various cell components with thin-film technology. This chapter shows advantages and limitations in the use of thin films for SOFCs, in light of recent progress in the field.

View all citing articles on Scopus

View full text

Electrical characterization of thermomechanically stable YSZ membranes for micro solid oxide fuel cells applications

Abstract

Introduction

Section snippets

YSZ-free standing membranes micro fabrication

Results and discussion

Conclusions

Acknowledgments

J. Power Sources

J. Power Sources

J. Power Sources

Acta Mater.

Solid State Ionics

Solid State Ionics

Acta Mater.

Thin Solid Films

Thin Solid Films

Thin Solid Films

Acta Metall.

Thin Solid Films

J. Phys. Chem. Solids

Thin Solid Films

Vacuum

Appl Surf. Sci.

Acta Mater.

J. Eur. Ceram. Soc.

J. Eur. Ceram. Soc.

Fuel Cell Rev.

Adv. Funct. Mater.

J. Electrochem. Soc.