Advanced Ceramics / Glass / Raw and Processed Materials

SPECIAL SECTION/ADVANCED CERAMICS & GLASS: Flexible Ceramic Membranes for Green Applications

April 1, 2011
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The huge global demand for clean and efficient energy, stable food, and clean water and air will spawn boundless opportunities for fuel cell technologies.



Our planet is changing as a result of higher levels of carbon dioxide (CO2) in the atmosphere. According to the National Oceanic and Atmospheric Administration (NOAA), which has measured the CO2 level since 1960, the current level of 390 ppm represents a 26% increase over that of 1960.1

Similar increases over the next 50 years will bring the CO2 level to nearly 500 ppm. Since a direct relationship exists between the CO2 level and the earth's temperature variations, many expect that a level exceeding 450-500 ppm will be extremely uncomfortable-if not life changing-for humans and other species.

In Hot, Flat, and Crowded, Thomas Friedman states that the earth's six billion people are split into three distinct groups: one billion live very comfortably with an abundance of energy, food, clean water and air; two billion are about to join those comfortable one billion; and the remaining three billion have little to none of these four basic needs.

Today's unserved energy markets, particularly in the emerging economies of China, India, Africa and much of South America, will provide tomorrow's opportunities. The huge global demand for clean and efficient energy, stable food, and clean water and air will spawn boundless opportunities for new production and distribution solutions. Today's commercialization efforts for fuel cell technology and other advanced energy methods will be an important part of the overall solution.

Figure 1. Cross-section of a dense, 40 µm thick electrolyte coated with a thin anode and cathode on opposing sides.

Electrolyte-Supported Fuel Cells

ENrG Inc. worked with Corning Incorporated for five years to integrate ultra-thin, flexible ceramic sheets into the fabrication of electrolyte-supported fuel cells (ESCs). ENrG established a state-of-the-art fabrication line to focus on the printing, handling, sintering and testing of the thin and robust fired electrolyte (U.S. patent number 5089455).

In 2008, ENrG licensed the membrane technology for fabrication of the thin electrolyte and, in the following year, successfully demonstrated a proprietary green tape process to produce highly flexible 25-40 µm thick fired electrolytes. (Electrode inks were developed with Heraeus Inc.) Also in 2009, ENrG introduced its Thin E-Strate™ and ThinESC™ (thin electrolyte-supported cell) products in Europe and provided substrate samples and initial button cells to system manufacturers for evaluation.

Thin E-Strate™ is so durable it can take heat from a blow torch and flex afterwards.
Flexible ceramic membranes are required for today's new high-temperature fuel cell systems. These systems, operating at > 700°C, subject the membrane to an extremely harsh environment. The tri-layer membrane, a combination of thin electrolyte with electrodes on both sides, produces energy through an electrochemical process that combines hydrogen and oxygen into water across the membrane.

Over a 5000-40,000 hour operating life, these membranes typically lose their electrochemical ability to produce energy. To achieve a broader level of market acceptance, a high-temperature fuel cell system must retain > 80% of its original capacity at 40,000 hours of life.

The system-level power performance degradation is directly attributable to the fuel cell stack and the membrane in each electrochemical cell of the stack. The thicker (100-150 microns) and typically weaker membranes currently in use can microcrack or completely fracture over time. They can also become electrochemically poisoned due to contaminants from other system components, or the fuel and/or air streams.

Microcracking or fracturing is the direct result of the thermo-mechanical issues that result from cycling the system and membranes from room temperature to the > 700øC operating temperature while the system is sealed within a metal cassette. Electrochemical contamination results from the effects of longer-term operation within a steam atmosphere, which can lead to chemical contamination from the many materials within the system, including the Cr within the stainless steel components.

Potential Applications

The role of ceramic materials and flexible ceramic membrane manufacturing processes is becoming increasingly important in the cleantech market space and its many applications. In the past several years, cleantech has garnered a tremendous increase in funding and venture capital attention. In some cases, key system components such as solid oxide fuel cell (SOFC) stacks are nearly entirely composed of ceramic materials and could not function without them.

Ceramic membranes provide functions across the cleantech landscape, including ion transport, thermal management, catalysis of gases and liquids, power generation, energy storage, hydrogen purification and storage, generation of light, and energy from waste processes, among many others. The high performance of ceramic membranes makes them strong candidates to fill critical technology gaps in many applications and industries, including advanced nuclear energy, air pollution control, solar energy, biofuels and biorefining, carbon sequestration, high-capacity energy storage, the hydrogen infrastructure, solid-state lighting, thermoelectric, thin-film solar, waste-to-energy, water remediation, harsh environment sensors, and wind power.

Increased production volumes and their resulting cost reductions are a prerequisite for the commercialization of many clean energy technologies. Using the same technology, component and/or manufacturing process to meet the requirements of many cleantech applications will help reach cost targets and accelerate commercialization. Electrolyte-supported SOFCs typically use a zirconia-based membrane for the substrate. This same membrane product has volume opportunities outside of SOFC applications in areas such as sensors (gas or vibration), gas separation, gas generation and electrolysis, to name a few. The same properties required for the membrane in electrolyte-supported SOFCs are often the same properties of value in other markets.

The density, high strength and chemically inert properties of thin 3YSZ substrates result in gas-tight membranes that are excellent choices for severe environment and/or high-temperature applications. These ultra-thin membranes have the ability to flex in and out of the plane of sealing such that they stress relief during system operations. Other applications can benefit from the membranes' ionic conductivity, superior mechanical properties and low thermal mass. Functional coatings are applied using thick or thin film, spraying, direct writing, or other standard deposition methods.

A 40 µm thick membrane folded into nearly a figure eight.

Flexible and Robust

While most people think about ceramics in terms of brittle, inflexible materials that easily break if dropped or hit, ceramics can be formulated and processed as thin membranes that yield flexible, translucent sheets that can nearly be rolled around the circumference of a pencil. These membranes with coated electrodes are desirable in the production of highly efficient, low-resistant SOFC systems. Since they can be produced in thin supports, thereby reducing the resistance and increasing the power output, they help reduce the system cost by reducing the number of cells in the stack (or producing more power for the same number of cells in the stack).

The market price for micro-combined heat and power (CHP) energy systems in the U.S. is $1000/kilowatt (using the residential market as the benchmark). The price is higher in Europe, but much smaller systems are required there. Current SOFC systems are beyond this figure by a factor of 8-20x.

Significant design improvements and manufacturing cost reductions are required to reach the < $1000/kilowatt cost target. The fuel cell stack is approximately 40-60% of the system cost; the remainder is the balance of plant. The demonstration of new manufacturing technology that significantly reduces the cost of cell manufacturing while increasing the surface area on thin membranes is key to reaching these price figures.

Critical system components in a SOFC system include a fuel reformer, fuel cell stack, controls and power conditioning. The fuel cell stack is the heart of the system, serving as the source of electricity and heat. Key SOFC technology challenges include the demonstration of stable long-term performance, the need for larger area cells/stack designs that can withstand thermal cycling, and cost reductions for stack components and stack assembly.

While many companies focus on improving the stability of their longer term stack performance, few have focused on ways to simultaneously achieve thermal cycle survivability, stable performance and low manufacturing costs. Thermal cycle survivability is an often overlooked "make-or-break" requirement for many CHP applications. Without thermal cycling tolerance, SOFC-based systems must be left on at all times, even in the summer when little or no waste heat is needed, or while the building occupants are away on vacation. This significant waste of energy detracts from the positive aspects of SOFC technology.

Table 1 shows the thin membrane properties for ENrG's Thin E-Strate™ and ThinESC™ technology, as well as potential applications for this technology. The ability to relieve stress through flexing provides thermal shock tolerance and the cycling durability required for CHP applications. As a "substrate," this thin ceramic sheet is strong enough for handling and additional processes such as applying electrodes and current conductors through an inexpensive screen-printing process.

Figure 1 shows a cross-section of a dense 40 µm thick electrolyte coated with a thin anode and cathode on opposing sides. The electrolyte technology is based on 3YSZ. Most electrolyte-supported fuel cell designs are based on 8YSZ, 10YSZ, ScSZ, YbSZ and other exotic materials. While the latter materials have higher oxygen ionic conductivity and therefore higher power performance, they are much weaker than 3YSZ. The minimum thickness of these electrolytes is about 110-150 µm, and durability has been sacrificed for performance.

One key to increasing the power performance of the lower ionic conductivity of 3YSZ is to fabricate as thin a membrane as possible, thereby reducing electrical resistance. The 2009 introduction of the 40 µm thick electrolyte (~65 µm overall with electrodes) was a major first step; prototype quantities were achieved through single caster runs. The next target is 25 µm thick electrolytes in 2011, and thinner beyond that. With the 40 µm thick electrolyte and inks developed so far, a power density of 250 mW/cm2 at 850°C has been achieved; it is expected that the power density will exceed 400 mW/cm2 by mid-2011.

Summary

SOFC technology has a ways to go before reaching the target 40,000 hour lifetime for commercialized systems. Many system manufacturers now require early adopters to absorb the cost of a second stack within the 40,000 hour lifetime warranty of a purchased system.

Ultra-thin-film ceramic membranes offer unique design and performance advantages that make them well-suited to meet the demanding requirements of many new energy and environmental technologies in the emerging cleantech market space. Low-resistance, ultra-thin membranes that can be tailored to many different functions, and applications can aid in solving global energy and environmental challenges.

For more information, contact ENrG Inc. at 155 Rano St., Suite 300, Buffalo, NY 14207; call (716) 390-6740; e-mail jolenick@enrg-inc.com; or visit www.enrg-inc.com.

Reference

1. Earth System Research Laboratory, Global Monitoring Division, www.esrl.noaa.gov/gmd/ccgg/trends/.

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