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Segmented-in-series SOFC (SIS-SOFC) stacks have recently evolved into an important contender along with the more common tubular and planar designs. In the segmented in series solid oxide fuel cell (SIS-SOFC), many cells are connected in series on insulating porous ceramic supports. FCT has identified and highly developed a proprietary segmented-in-series design that combines the advantages of planar and tubular SOFC stacks. Key features of our design include the use of flattened tube supports and cell widths that are much smaller than normal SIS-SOFCs. The stack design offers following enabling features to today’s SOFC stack technology.
- FCT’s SIS-SOFCs provides output power density comparable or better than state-of-art planar SOFC stacks, without problems of significant interconnection related losses or difficulties in sealing.
- High power density: The small cell widths greatly reduce electrode lateral resistance providing high power per cell area. This and the flattened tube design without separate interconnects allow a volumetric power densities are comparable to or better than a state-of-art planar SOFC stacks.
- The interconnect is integral to the cell arrays, eliminating problems with separate metallic interconnects and pressure-contact resistance losses encountered in planar stacks;
- The tubular design allows for easy sealing compared with planar SOFCs ;
- Mechanical Strength and Toughness: The unlike many other designs, FCT’s SIS-SOFC cell and stack allows optimizing the support material for mechanical strength separately from electrodes. partially-stabilized zirconia, and ribbed support provide extremely robust ceramic element.
- Processing: Simple low-cost processes such as screen printing; is used to fabricate the active layers on flattened tube.
- High voltage: Large number of series-connected cells on each individual module makes it possible to easily reach relatively high output voltages and simplifies electrical power management for smaller power generators.
Novel Fuel Flexible Internal Reforming Technology
In order for fuel cell technologies to be widely used, infrastructure necessary to produce, store and transfer hydrogen fuel needs to be built, which could take decades. For current fuel cell systems to use today’s hydrocarbon fuel, they need to resort to relatively complicated fuel processing componentry, which would make the system too large and inefficient than for many current applications. Thus a “fuel-flexible” stack that can be directly operated under various hydrocarbon fuels would be quite desirable for accelerating the commercialization of SOFC system.
FCT develops a proprietary internal reforming technology to develop a fuel flexible segmented-in-series SOFC modules and stack. We have already demonstrated SOFC operation under octane fuel, which is the main component of gasoline with power density that is comparable to that of planar stacks.
Low cost Hydrogen / syngas Generation using Electrochemical Partial Oxidation
An important necessity in realizing hydrogen economy is to drive down the hydrogen production cost in such that it is competitive with the cost of today’s hydrocarbon fuels. In collaboration with Northwestern University, FCT is working on a new approach to produce syngas ( H2 + CO) from natural gas, by electrochemical partial oxidation.
Methane reforming is typically used to produce syngas (H 2 +CO), an important feedstock for the production of various chemicals including methanol and liquid hydrocarbons. Syngas can also be converted to H 2 , another important industrial feedstock and a potential fuel. In collaboration with Northwestern University FCT is working on electrochemical partial oxidation, where syngas and electricity are co-produced in a solid oxide fuel cell (SOFC) that operates with methane as
the fuel. Due to the high value of the electricity produced, this approach has the potential to produce hydrogen/syngas more cost effectively than other methods.
In methane electrochemical partial oxidation (EPOx), a SOFC is operated with the natural gas (i.e. methane) flow rate and total stack current adjusted such that there are 1 oxygen ions (O 2 -) pumped across the cell per methane molecule, the stoichiometry of partial oxidation:
CH 4 + O2- = CO + 2 H 2 + 2e-. (1)
Under appropriate conditions, the SOFC produces both syngas (H 2 +CO) and electricity. This is quite different from conventional fuel cells, where the fuel is almost fully oxidized. For the case of a direct-methane SOFC, for example, the methane feed rate would be 3-4 times lower, such that the reaction is closer to complete oxidation:
(1/4) CH 4 + O 2- = (1/4) CO 2 + (1/2)H 2 O + 2e-. (2)
Comparison of reactions 1 and 2 shows that for a given sized SOFC, the same amount of electricity (2e-) is produced, but the products are different. This difference between EPOx and SOFC mode of SOFC operation is illustrated in the figure on right side.
EPOx provides two key advantages. First, the SOFC provides pure oxygen such that there is no dilution of the syngas by nitrogen, different than partial oxidation with air. Second, the co-production of electricity and syngas improves the economics of both syngas production and fuel cell electricity production. In particular, when SOFC costs drop below $400 per kilowatt, they are expected to become commercially viable. Considering this and current price of electricity, this means that there is no additional cost for the production of syngas/hydrogen for EPOx SOFC, other than the cost of the additional feedstock natural gas.
Thus, EPOx can produce syngas/hydrogen at a lower cost than any other technology. This is critical for alternative fuels such as hydrogen and gas-to-liquids fuels (where natural gas is converted via syngas to clean synthetic liquid fuels) to reach a competitive cost level.
The implementation of EPOx has been limited because direct methane operation often leads to fouling of the anodes with deposited carbon. Thus, non-standard SOFC materials, e.g. Pt for the anode, have been used in order to avoid coking, but are too expensive for practical devices. To date, SOFC power densities and syngas production rates were relatively low, and coking was often observed. We demonstrated a stable methane EPOx operation of SOFCs without coking, yielding both high electrical power densities and high syngas production rates. We also present results of thermodynamic calculations that provide estimates of expected EPOx operating conditions and productivity. Taken together, these demonstrate the feasibility of EPOx SOFCs. |
