Solid Oxide Fuel Cells
Solid Oxide Fuel Cell Component Fabrication
Equipment is in place at the CFCC for fabricating complete laboratory-scale (1 to 3 cm^2) and sub-commercial scale (25 to 100 cm^2) solid oxide fuel cells that allow a wide range of material and electrochemical studies to be undertaken. High-temperature sintering ovens and processing furnaces, tape casting, screen printing and cold pressing equipment are available for fabricating SOFCs from a wide variety of state-of-the-art materials using an array of fabrication processes. Equipment of this kind has been used to good effect in fabricating fuel cells having stabilized zirconia or doped ceria electrolytes, transition metal/electrolyte cermet anodes and complex metal oxide cathodes. A class 10,000 clean room is available for pre-test assembly and post-test disassembly of SOFC components.
Solid Oxide Fuel Cell Testing and Characterization
The CFCC is equipped to test SOFC ranging from milliwatt laboratory-scale single cells to 500 W short stacks. CFCC can bring to bear a variety of analytical equipment for real-time analysis of fuel cell performance including GC-mass spectrometry and AC-impedance spectrometry. A wide range of state-of-the-art post-test materials analysis methods are available through CFCC partners, Colorado School of Mines and the National Renewable Energy Lab. Facilities are also available for long-term endurance testing of fuel cells, up to 500 W output, operating on hydrogen or hydrocarbon reformate and for testing complete systems, up to 5 kW, operating on natural gas.
This reactor is designed to provide an anode environment closely approximating that of an SOFC. The anode is sandwiched between two co-flowing gas channels. One channel carries a typical SOFC fuel while the other carries a mixture of CO2 and/or steam, the expected products of the electrochemical oxidation reactions. The species in these two channels diffuse through the porous anode and undergo reforming. The products of the reforming reactions are monitored in the exit gas channels with a mass spectrometer. These data can then be compared to a combined kinetics/transport model to provide a direct measure of the extent of on-anode reforming under SOFC conditions. The system is described in Applied Catalysis A, 295, 40–51 (2005).
System Integration: kW-Scale Solid Oxide Fuel Cells
Geothermic fuel cells (GFCs) present a novel, ambitious and potentially transformative new application of solid-oxide fuel cell (SOFC) technology. Solid-oxide fuel cells efficiently convert the chemical energy of fuels into electricity. Their high operating temperature (~800 °C, 1500 °F) make SOFCs attractive for combined heat and power (CHP) applications. As the SOFCs generate electricity, their waste heat is harnessed and used to heat hot water. Such combined heat and power systems demonstrate outstanding efficiencies (85%), reducing fuel consumption and pollutant formation.
GFCs present a unique combined-heat-and-power platform. In the GFC concept shown in Figure 1, high-temperature SOFCs are placed within an oil-shale formation located hundreds of meters below the earth’s surface. While generating electricity “down hole,” the heat released by the fuel cells is shed into the surrounding oil-shale formation. The heat retorts the oil shale to form a mixture of oil, hydrocarbon gas and carbon-rich shale coke. These valuable products are then withdrawn from the formation using conventional collector wells.
The U.S. Geological Survey estimates that over four trillion barrels of oil are trapped in the Rocky Mountain region. In contrast to liquid shale oil (or “tight oil”) trapped within porous geology, oil shale is a sedimentary rock that contains organic matter called kerogen.
The world’s first GFCs were designed and built by Delphi Powertrain Systems and extensively tested at the Colorado Fuel Cell Center. Experimentation included laboratory operation of GFC modules and down-hold operation of an integrated GFC system. During the down-hole demonstration, nine 1.5-kWe solid-oxide fuel cell stacks were installed within a clay formation at the Colorado School of Mines campus (Figure 2). Fueled by municipal natural gas, this GFC was integrated with a natural gas fuel processor, a reactive-gas preheater, and ancillary balance-of-plant and diagnostic components at an outdoor test site.
This new application of solid-oxide fuel cell technology in unconventional oil and gas processing presents a potentially transformative technology for accessing the world’s vast oil-shale reserves, while offering a large, high-volume market opportunity for SOFCs.
Figure 1: Illustration of the GFC concept. High-temperature solid-oxide fuel cells are placed within an oil-shale formation located hundreds of meters below the earth’s surface. While generating electricity “down hole,” the heat released by the fuel cells is shed into the surrounding oil-shale formation. The heat retorts the oil shale to form a mixture of oil, hydrocarbon gas and carbon-rich shale coke. These valuable products are then withdrawn from the formation using conventional collector wells.
Figure 2: Installation of the GFC within the earth at the Colorado School of Mines campus.
Through years of steady technological advancement, proton-conducting ceramic materials are approaching a technology readiness level that rivals more well-established oxygen-ion conductors and polymer-electrolyte membranes. Researchers at Colorado School of Mines are the nation’s leaders in developing protonic ceramics for a number of applications:
- Fuel cells for efficient electricity generation (PCFCs);
- Electrolyzers for energy storage (PCECs); and
- Membrane reactors for fuels synthesis (PCMRs).
Doped barium cerate-zirconate electrolyte materials (BaCexZr1-x-yYyO3-d) are the focus of these efforts. Devices based on these materials have demonstrated impressive performance at intermediate temperatures, often 200–300 °C below that of equivalent oxygen-ion conductors. Such lower-temperature operation can dramatically reduce device degradation rates, decrease the cost of stack and balance-of-plant materials, reduce thermal-cycling stresses and enable a wider range of integration options. Additionally, protonic-ceramics can be harnessed to generate pure hydrogen streams under electrolysis operation.
Such protonic-ceramic materials have demonstrated mixed ionic-electronic conductivity; these properties can be exploited for developing cost-effective, novel solutions and new technologies. We are currently developing fuel cell stacks based on BaCe0.2Zr0.6Y0.2O3-d (BCZY26). These protonic-ceramic fuel cells have demonstrated remarkable fuel flexibility. The effort includes a tenfold scale up in cell size, and integration into a stack package. Our three-cell stack recently achieved a power density of 180 mW cm^–2 at 0.78 V under methane fuel at 550 °C. The team is now engaged with an industrial developer to demonstrate a 0.5 kW stack based on the same technology. This effort is supported by award number DE-AR0000493. The program runs from 2015 through 2020.
We have observed more pronounced electronic conductivity under electrolysis operation. This is likely due to cerium reduction within the BCZY material under reverse bias. By reducing the cerium content to below the percolation threshold, we have confirmed great improvements in Faradaic efficiency, reaching 95% at 120 mA cm^2 and 600 °C.
We are also exploring the use of protonic ceramics for electrochemical synthesis of ammonia as a means of renewable energy storage. In this program, protonic-ceramic devices are infiltrated with ruthenium-coated calcium aluminate catalysts to convert nitrogen and water vapor into ammonia. This effort is supported by award number DE-AR0000493 and spans 2017–2020.
The market for distributed generation of electricity is vast. With over 12 million distributed-generation units and 200 GW of domestic capacity, the DG market provides a robust growth opportunity for quiet, high-efficiency fuel cells. A compact (< 25 kW), efficient, propane-powered fuel cell would provide compelling value across a number of markets spanning recreation, defense, aerospace and the developing world. The agriculture sector possesses widespread infrastructure for an ammonia-fueled protonic-ceramic fuel cell electric generator for remote power. Finally, as evidenced by FuelCell Energy’s Direct FuelCell® platform, large markets exist for MW-scale renewable or natural-gas-fueled generators. The technical goals targeted in this follow-on program mark critical milestones in development of kW- and MW-scale fuel-cell generators to fit these evolving market opportunities, while also positioning the technology as a fuel-flexible product.
Figure 1: Illustration of a natural-gas-fueled proton-conducting ceramic fuel cell. These fuel cells are now under development at the Colorado Fuel Cell Center.
High-Temperature Flow Reactor
A reactor assembly capable of temperatures up to 850 °C is equipped with the necessary mass flow controllers and liquid feed pumps to provide kinetics data for both gas-phase and catalytic reactions that are relevant to SOFC operation. The composition of the reactor effluent can be quantitatively measured with a gas chromatograph. This information provides some of the data required for validation of our gas-phase and catalytic mechanisms. The system is configured so that the product stream can be fed directly to the SOFC test stand. The system is described in detail in J. Phys. Chem A. 108, 3772-3783 (2004).
Catalytic Stagnation-Flow Reactor
In typical catalytic reactors (e.g., monoliths, packed beds, porous-foam supports), it is difficult, if not impossible, to directly observe the catalyst surface. Therefore, we have developed a stagnation-flow reactor that enables direct experimental observation of the catalyst surface and the adjacent gas-phase boundary layer. Microprobe mass-spectrometric sampling is used to measure species profiles in the stagnation-flow boundary layer.
The boundary-layer profiles can be used to infer critical information about the global catalytic chemistry at the catalytic stagnation surface. A new fully enclosed reactor has been developed, which enables the use of hazardous gases and sub-atmospheric operation. More information may be found in Applied Catalysis A, 255:279-288 (2003).
Detailed Kinetic Modeling
The high-temperature SOFC environment creates the potential for extensive chemical transformations to occur prior to the electrochemical oxidation event. A proper understanding of these reactions is essential to be able to determine the composition of the mixture near the three-phase-boundary and thus available to participate in the charge-exchange reactions. We have developed mechanisms that accurately describe the gas-phase kinetics of a variety of hydrocarbons and oxygenates under SOFC conditions. We are using data from separated-anode experiments to extend existing microkinetic models to describe the on-anode reforming kinetics as well as the reforming and CPOX kinetics in external fuel-conditioning reactors.
Solid-Oxide Fuel Cell Modeling
We have developed a computational framework for modeling chemically reacting flow in solid-oxide fuel cells (SOFC). Depending on materials and operating conditions, SOFC anodes afford a possibility for internal reforming or catalytic partial oxidation of hydrocarbon fuels. An important element of the models is the capability to represent elementary heterogeneous chemical kinetics in the form of multi-step reaction mechanisms. Porous-media transport in the electrodes is represented with a Dusty-Gas model. Charge-transfer chemistry is represented in a modified Butler-Volmer setting that is derived from elementary reactions, but assuming a single rate-limiting step. The models are applied in planar and tubular geometries. The models are described in J. Electrochem. Soc., 152:A2427-A2440 (2005).
Partial Oxidation and Catalytic Combustion Modeling
We have developed modeling capabilities to characterize chemically reacting flow in partial oxidation, reforming, and catalytic-combustion processes. Physical geometries are typically defined-channel monoliths or porous-foam supports. The channel models are based on a boundary-layer formulation (Catalysis Today, 59:47-60, 2000). The porous-foam models use a Dusty Gas formulation to model Knudsen and Ordinary diffusion as well as Darcy flow. All the models consider detailed reactions mechanisms to represent elementary heterogeneous chemistry on the catalyst surfaces. Recent efforts concentrate on metal-substituted hexaaluminate catalysts in porous-foam supports.
Other Labs and Services on Campus
Ceramics Processing Laboratory
The Colorado Center for Advanced Ceramics houses a variety of ceramics processing instrumentation available to the CFCC. This includes numerous controlled atmosphere and air sintering furnaces, a Thermal Technologies vacuum hot press (2000ËšC 25 tons, capability to run in Ar or N2), a cold isostatic press, uniaxial presses, ball mills and powder preparation facilities and a plasma etcher.
Electron Microscopy Lab
The Electron Microscopy Laboratory is equipped with three scanning electron microscopes (Quanta 600 with heating stage to 1500°C and an in-situ Ernest-Fullam tensile stage; JEOL FEG SEM with EBSP and photolithography capabilities installed 3/2006; JEOL JXA-840 SEM with EDS;) and two transmission electron microscopes (Philips CM200 STEM with EDS and heating stage to 1500°C; Philips EM400 TEM). The laboratory is fully equipped with TEM sample preparation facilities including dimplers, ion beam thinners, and vacuum coaters.
Thermal Analysis Laboratory
Thermal analysis equipment available at the Colorado Center for Advanced Ceramics includes a Netzsch simultaneous thermal analyzer, a Netzsch high temperature dilatometer, a Netzsch differential scanning calorimeter, a Netzsch thermal conductivity tester, a Seiko TG/D and Cahn TGA both interfaces to a Fisons mass spectrometer, a Fourier transform infrared spectrometer and a controlled atmosphere microbalance.
Mechanical Testing Facilities
The Metallurgical and Materials Engineering Department houses numerous mechanical testing facilities, including 4 MTS computer-controlled, servo-hydraulic uniaxial testing units, 2 MTS screw-driven uniaxial testing units, fracture toughness fixtures, impact testers, fatigue testing for bending and torsion, hot and cold rolling mills, numerous hardness testers, a MTS XP Nanoindenter.
In addition to the Electron Microscopy Lab, we have capabilities in AFM. Extensive use is made of X-ray diffraction and small angle x-ray scattering. Extensive use is made of the Mines NMR facility which has three spectrometers from 200 to 400 MHz. A thermo-nicolet IR spectrometer equipped with ATR and a heatable variable atmosphere cell is avaialvle to the CFCC personnel and several Raman spectrometers.