Updated Look at the DCFC: the Fuel Cell Technology Using Solid Carbon as the Fuel
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Particle processing is a key to using solid particles in fuel cell applications. Selectivity with regard to reactivity, impurities, etc. is an important feature and the treatment of particle surfaces could greatly impact the performance of direct carbon fuel cells. Solid fuel particles will become increasingly important in the future. Present energy conversion systems for solid fuels are too inefficient. New energy conversion systems for solid fuels with higher energy conversion efficiencies are possible. Fuel cell technology is a key technology in these new conversion systems. The direct carbon fuel cell (DCFC) operates on carbon particles obtained from a variety of solid fuel feedstocks. The DCFC is the only fuel cell designed to directly oxidize carbon particles in a special anode chamber. The particles are generally graphite structure with high purity. The electrolyte used is the high temperature solid oxide, molten carbonate, or hydroxide electrolyte. Since a pure stream of CO2 is produced, the stream can easily be sequestered and disposed. Pure carbon dioxide produced as a by-product would also have a market in many industries. A well-defined technology roadmap identifying key research and development (R&D) issues is necessary to provide a framework for the development of these systems and to prevent entrenchment in inherently inefficient technologies. This review paper describes the direct carbon fuel cell and its system, how it works, the developmental status, the characteristics of the carbon particles needed, and the research and development issues for the technology.
KeywordsDirect carbon fuel cell Solid oxide Molten carbonate Reactive carbon
The DCFC has been under development for many years. This paper is an update of a previous evaluation . It was researched as early as 1896 , but an intense research and development is necessary to realize its potential. William W Jacques, a US electrical engineer and chemist, described a DCFC in 1896 in US Patent 555511 for a “Method of converting potential energy of carbon into electrical energy.” Jacques used coke electrodes in a molten NaOH electrolyte.
The high operating temperature is needed to achieve sufficient conductivity of the carbonate electrolyte, yet allow the use of low-cost metal cell components.
•3–6-μm pore size
•45 to 70% initial porosity
•0.20 to 0.5-mm thickness
•0.1 to 1 m2/g
•7 to 15-μm pore size
•70 to 80% initial porosity
•60 to 65% after lithiation and oxidation
•0.5 to 1-mm thickness
•γ − LiAlO2, α − LiAlO2
•0.1 to 12 m2/g
•0.5 to 1-mm thickness
•62 Li-38 K
•Tape cast 0.5 to 1-mm thickness
SOFC general materials used to both tubular and planar designs
•Tape cast sintered
•12.5 × 10−6cm/cm∘C CTE
•∼150 − μm thickness
•20 to 40% porosity
Doped lanthanum strontium manganite
•LSM; tape cast/sintering
•11 × 10−6cm/cm∘C CTE from room temperature to 1000∘C
•30 to 40% porosity
•Yttria-stabilized ZrO2 (YSZ) (8 mol percent Y2O3)
•10.5 × 10−6cm/cm∘C CTE from room temperature to 1000∘C
•30 to 40 − μm thickness
•Various SS metals 10 × 10−6cm/cm∘C CTE
The materials for the anode listed in Tables 1 and 2 are for use when the fuel is hydrogen. Special anode materials selective for carbon oxidation are available and will be discussed later. In addition to hydrogen, carbon monoxide (CO) and other hydrocarbons such as methane (CH4) can be used as fuels. It is feasible that the water gas shift reaction involving CO (CO + H2O = H2 + CO2) and the steam reforming of CH4 (CH4 + H2O = 3H2 + CO) in the high temperature environment of SOFCs produce H2 that is easily oxidized at the anode. The direct oxidation of CO in fuel cells is also well established. In the SOFC case oxidation of carbon could occurring at the surface is quite different than oxidation after vaporization of the carbon.
In the AFC mode, the hydroxide ion approach which occurs at 100–500 °C is believed to have several advantages—lower cost, lower temperature materials of construction, more electrochemically active carbon, and greater production of CO2, although hydroxide reacts with carbon dioxide.
For the hydrogen reaction, the half-cell electrochemical reactions are
Electrode development in circulating electrolyte AFCs (hydroxide electrolyte) has concentrated on (1) multi-layered structures with porosity characteristics optimized for flow of liquid electrolytes and gases (H2 and air) and (2) catalyst development. Most developers use noble metal catalysts; some use nonnoble catalysts. Spinels and perovskites are being developed in an attempt to lower the cost of the electrodes. Development of low-cost manufacturing processes includes powder mixing and pressing of carbon-based electrodes, sedimentation and spraying, and high-temperature sintering. AFC electrolyte development has been restricted to KOH water solutions with concentrations ranging from 6 to 12 normal. Still, use of less expensive NaOH has been considered. Minimal cost advantages appear to be far outweighed by performance reductions due to wetting angle and lower conductivity. However, NaOH as an electrolyte increases the lifetime of electrodes when CO2 is present, because sodium carbonate, although less soluble than potassium carbonate, forms much smaller crystals, which do not harm the carbon pores.
2 Performance Potential of DCFC
3 DCFC Developmental Status
There have been several DCFC developers—Lawrence Livermore National Laboratory (LLNL) and Contained Energy (MCFC), SRI (MCFC + SOFC), SARA (hydroxide), and CellTech (SOFC). There has been work around the world at other organizations such as Stanford University, Akron University, Virginia Polytechnic Institute and State University, University of Colorado, Technical University Delft, Brown University, Duke University, AIST, and NEDO that has occurred over the years .
The LLNL and Contained Energy design utilizes an MCFC cathode, a nickel or high-fired graphite anode, and porous ceramic separator. The anode reaction is carbon and carbonate ions forming CO2 and electrons. Deashing processes, such as those developed by UCC Energy (Sydney, Australia), the New Energy and Industrial Technology Development Organization and South African Synthetic Oil Limited, could yield fuel at competitive costs (about $2–3 per million BTU). LLNL has demonstrated the technology in small, experimental cells with areas between 3 and 60 cm2. LLNL has tested a five-cell, self-feeding bipolar stack using pure fine carbon. LLNL reports that in repeated tests, the cells deliver up to 0.1 W/cm2 continuously and are 80% efficient at 80 mW/cm [3, 4, 6, 7, 8, 9].
The reported performance  of the SARA design is average output power in Mark III-A of 12–20 W (over 540 h) with peak output power (sustained for 5 to 10 s) 35–50 W. The concepts have achieved 300 mV at 40 A in Mark III-A and 100 mA/cm2 in Mark III-A 250 mA/cm2 in Mark II-D. The efficiency was 60% measured in the non-optimized Mark III. They project the maximum efficiency of the DCFC at ~ 85–90% and the practical DCFC plant efficiency at ~ 70–75%.
The SRI design combines advances in SOFC and MCFC technologies with the concept of using a circulating liquid-molten salt anode that incorporates the carbon-containing fuel. SRI is examining various molten salts, including molten carbonate, and using a YSZ electrolyte and a lanthanum strontium manganite cathode. A variety of fuels have been tested, including biomass, coal, tar, coke containing 6% sulfur, acetylene black, plastic, and mixed waste. Using conventional coal without pretreatment, SRI has achieved power densities > 100 mW/cm [3, 5].
It is hardly expected that the DCFC development will precede the state of development of its base AFC, MCFC, or SOFC technologies. There state of readiness appears eminent, however.
4 Solid Carbon Particle Fuel Production and Quality Requirements
The problem of the application of fossil fuels for powering fuel cells is to process the hydrocarbons in fossil fuels to produce elemental hydrogen and/or solid carbon. This can be accomplished by thermal cracking (decomposition) and pyrolysis processes. According to recent reports , the carbon formed in the plasma reactor is of a quality suitable for the molten carbonate cell type DCFC.
Graphite particles are commercially made from the sooting of methane; however, graphite is also available from coal, coal coke, petroleum coke, etc. Sulfur, chlorine, sodium, and trace metals are components of coal known to present problems to catalytic surfaces such as that of the fuel cell anode. Fouling, erosion, blockage of active catalyst sites, and chemical reaction are known problems occurring with coal ash. In addition, with coal, the production of an aromatic or cyclic carbon compound rather than an active graphite solid particle is a problem inherent with the use of coal feedstock. What happens to the contaminants in coal with or without coal cleaning is an issue. Coal contains volatile components, so the DCFC system must be designed so that the coal can be introduced to the hot fuel cell without the explosive release of volatile components.
The cost of the chemical clean-up of coal particles to a low-ash, clean carbon has been estimated at $60/ton . Lower cost carbon particle clean-up systems obviously need to be developed. The estimated cost of oil agglomeration and advanced flotation are $17 and $12/ton of clean coal, respectively. Depending on the carbon content of the beneficiated coal, this could be in the economic range and competitive with other carbon formation processes providing the carbon content was high and the carbon reactive. For the MCFC-type DCFC, the anode chamber can be a carbonate melt similar to the MCFC electrolyte. This permits actually cleaning of impurities in the melt. For a low ash content carbon or coal, this could be a batch process.
Nanoparticles of carbon if produced could be highly reactive. These hold great promise for subsequent development. LLNL [6, 7] has examined the relationship between the nanostructure of carbons and their electrochemical reactivity in molten salts by correlating significant differences in the 3D atomic structure of carbon fuels with their electrochemical reactivities. The more disordered the carbon atoms, the more easily they yield electrons. DCFC performance depends on disorder more than on carbon purity or even aggregate surface area. Extremely fine (10–1000 μm) carbon particles with a high degree of structural disorder on the 30–100-nm scale work well. Fine carbon particles, when mixed with molten carbonate to form a dense paste or slurry, operate like rigid electrodes when the melt is brought into contact with an inert metallic screen or graphite current collector.
5 DCFC Systems
Thermal cracking (decomposition) and pyrolysis processes can occur in a variety of reactors. There could be a good match between the hydrogen plasma black reactor (HPBR) and the DCFC for producing electric power and/or hydrogen and maximizing the power cycle efficiency . The HPBR combined with the DCFC and SOFC in a combined power cycle is an example of an integrated power plant for use with the DCFC which a high-power cycle efficiency. The HPBR produces hydrogen and CO for the SOFC and carbon for the DCFC in an efficient manner. In the SOFC, oxygen ion is transmitted through the SOFC ceramic membrane and oxidizes the CO and H2 to CO2 and H2O with the production of DC power to complement the direct oxidation of carbon in the DCFC.
LLNL has designed a DCFC system that incorporates a self-feeding cell that can be refueled pneumatically with cleaned coal. The DCFC incorporates a pyrolysis step that produces hydrogen (which can be used as fuel for a conventional fuel cell or as a chemical reactant), low-molecular-weight hydrocarbons and a nonagglomerating char that could potentially be reactive and conductive and useable in the DCFC. One of the most efficient systems incorporates fuel cells and turbines—the fuel cell turbine hybrid. Hybrids are the highest efficiency power plants yet devised. Hybrids are combinations of energy conversion devices. Many combinations of energy conversion devices are possible, but the combination of the high-temperature fuel cell with the gas turbine is a very high efficiency one [14, 15]. In the direct fuel cell turbine hybrid concept, the fuel cell replaces the turbine combustor. The turbine expands the high-temperature fuel cell exhaust.
Although the DCFC is capable of 100% energy conversion and 100% fuel utilization at any temperature, this efficiency and utilization will probably not be realized. Therefore, one would want to operate at in a hybrid system to take advantage of fuel cell underutilizations, irreversibilities, and inefficiencies. The high exergy exhaust from the higher temperature DCFC is quite suitable to perform useful expansion work in a gas turbine, if it is not needed for process requirements of the system design as, e.g., coal processing.
The plant costs of various power plants have been projected . In this comparison, DCFC long-term compares very favorably having the lowest plant cost projection. Specific applications for DCFC systems may exist in low-cost power, combined heat and power, and peak shaving/load-management applications in which DCFCs help utilities to balance supply and demand during periods of high electricity demand. However, these cost comparisons are difficult to analyze because of the not precisely documented supply limit of the respective technologies (esp. coal and flue gas quality/preparation) but these data give some hope that DCFC has a chance to become a competitive future technology. For DCFC systems to be competitive, however, they must present an attractive alternative not only to today’s technologies, but to technologies that will be available when DCFCs are eventually commercialized.
6 Research and Development Needs
The major development required is for the anode and anode chamber, for all the modes of the DCFC. The reactor strategy—batch or flow—depends on the purity of the solid graphite particles and thus the supplied coal quality.
US DOE-funded research has been conducted on V2O5-Pt catalysts for carbon oxidation anodes in DCFCs at the University of Akron and on carbon ionic conductors for use as electrolytes in novel DCFCs at Duke University .
Wetting characteristics of various types of carbon, in melts of variable basicity (cation composition and OH-added) and temperature
Spectroscopic characterization of molten carbonate in reducing medium, in contact with carbon, to establish identity of stable and semi-stable species
Molecular dynamics calculations to establish the relative stability of carbon-carbonate-CO-CO2 configurations in the melt
Monte-Carlo (stochastic) modeling of assemblies consisting of cells representing carbon particles, gas bubbles (CO2, CO), and carbonate melt. Buoyant forces, interfacial forces, and electrochemical reactions to be accounted for, cell to cell
Cell and stack area and height scale up, heat removal, performance degradation as a result of electrolyte contamination, and electrolyte purification and re-injection processes are also critical issues which need to be addressed for all DCFC technologies
Other special R&D requirements vary with the developers technology and system development schemes identified in this paper
Research continues in many areas of direct carbon fuel cells [19, 20, 21, 22, 23, 24]. Interest will continue since despite this release of carbon dioxide, the direct carbon fuel cell is more environmentally friendly than traditional carbon burning techniques. Due to its higher efficiency, it requires less carbon to produce the same amount of energy.
The fuel requirements are yet to be fully determined, with only limited studies investigating the effect of impurities and fuel composition on the overall fuel cell performance. There are no defined specifications for an ideal fuel; however, fuel properties which could potentially improve the performance of the fuel cell include high electrical conductivity, low crystallinity, small particle size, friable particles, high surface area, and low ash. Mineral processing technology is required to selectively separate with respect to these properties. DCFCs are less sensitive to other fuel properties that are critical for combustion, such as moisture and thermal content. In terms of reactivity with fuel impurities, systems with molten components generally will have significantly more stringent fuel requirements as even small levels (less than 1%) of ash will accumulate within the anode chamber and react with the molten metal or carbonate components, leading to solidification of some components and rapid degradation in cell performance. Because the entire system of the anode side of DCFC is a connected and interacting process line for coal from pre-processing to CO2 separation and coal transport fluid recovery (e.g., tin), it is necessary to clearly define the conditions at all component borders and their influence on the entire system behavior.
In theory, the DCFC holds the highest promise of any fuel cell type, but other types are also interesting. An intense period of research would be required to develop the anodes and anode chamber for various designs. Carbon particle reactivity, impurity level, and cost will play major roles in any eventual DCFC commercialization. It can hardly be expected that the DCFC development will precede the state of development of its base AFC, MCFC, or SOFC technologies.
Compliance with Ethical Standards
Conflict of Interest
The authors declare that they have no conflict of interest.
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