SOFC Internal Process

Repeating from earlier, an equilibrium fuel cell directly generates an electrical potential difference, V, proportional to the work function, the 

Gibbs potential, the units of which are related to the electrical potential by Nernst s equation  

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Batawi E, Weissen U, Schuler A, Keller M, and Voisard C. Cell Manufacturing Processes at Sulzer Hexis. In Yokokawa H, Singhal SC, editors. Proceedings of the Seventh International Symposium on Solid Oxide Fuel Cells (SOFC-VII), Pennington, NJ The Electrochemical Society, 2001 2001(16) 140-147. 

Construct and evaluate a novel, planar SOFC stack of - 20 kWe that internally reforms natural gas and delivers an efficiency of > 50% (LHV). The stack should be capable of manufacture using a viable (if not yet proven) process, and with materials costs of less than US 300/kW 2005... 

Another Ni-based solid oxide fuel cell (SOFC) electrode was developed on which a YSZ (yttria-stabilized zirconia) cermet and Lanthanum chromite were deposited by a slurry coating method. It was also suggested that a plasma spraying process can be used for the cermet deposition on the electrodes. The following reactions are expected to take place in a fuel cell employing a natural gas source, where internal reforming takes place on the Ni-YSZ electrode ... 

High temperature fuel cells (MCFC and SOFC) allow for the reforming process to take place inside the fuel cell stack which lowers the requirement for cell cooling and reduces cost due to absence of the external reformer vessel. A future SOFC application could also be the production of hydrogen (and electricity) by internal reforming of natural gas where more H2 as a component of the synthesis gas is produced than can be converted electrochemically into electricity the heat losses from the fuel cell operation would be used as the endothermal heat source for the reforming step [65]. 

A 1 kW SOFC prototype based on tubular cells was designed, manufactured and tested in 1989. The cell had a working surface of 63 cm2 and a mean power 12.5 W or 0.2 W/cm2 at 900°C. 16 cells were assembled into a stack. The prototype consisted of 6 stacks. The prototype was fed with methane internal partial oxidation was used for fuel processing. The prototype worked at fuel utilization of 0.9 and had efficiency about 40%. 

Di Carlo, A., Borello, D. and Bocci, E. (2013) Process simulation of a hybrid SOFC/mGT and enriched air/steam fluidized bed gasifier power plant. International Journal of Hydrogen Energy, 38 (14), 5857-5874. 

In this chapter a detailed CFD study of the chemical and electrochemical processes in an internally reforming anode supported SOFC button cell was carried out. Detailed models for chemistry, electrochemistry and porous media transport have been implemented into the commercial CFD code FLUENT with the help of used defined functions (UDF). Simulation results were compared with experimentally reported data. The comparisons lead to the conclusion that precise calculation of surface carbon formation is critical for the accurate prediction of OCVs for hydrocarbon fuels with very low H2O content, and that Nemst equation may not be valid for the calculation of OCV for a fuel composition such as the one considered here. Anodic overpotentials showed remarkable difference from expected behavior. 

The conversion processes proceed at elevated temperatures close to those of SOFC operation. Therefore, an important aspect of the work on SOFCs has been the attempt to build unified plants combining conversion processes and fuel cell operation. In this way, the heat from the fuel cells could be transferred directly to converters, with a much lower loss of thermal energy. This combination has been called internal reforming. 

We can easily measure the composition of exhaust gases from cells such as the SOFC and from this we can evaluate our models, but from a practical point of view it is very difficult indeed to monitor what exactly is happening within high-temperature fuel cell stacks. The mechanism of internal reforming is subject to much debate and there are conflicting models in the literature. Certainly there is mnch to learn about the fundamental processes at the heart of the fuel ceU. 

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