Abstract
Electrochemical reaction kinetics at the electrodes of Solid Oxide Cells (SOCs) were investigated at 700°C for two cells with different fuel electrode microstructures as well as on a third cell with a reduced active electrode area. Three fuel mixtures were investigated – hydrogen/steam and model reformate fuels–hydrogen/carbon-dioxide and hydrogen/methane/steam. It was found that the electrode kinetics at the fuel electrode were exactly the same in both reformates. The hydrogen/steam fuel displayed 5–7% faster kinetics than the reformate fuels. 19% faster kinetics were recorded for the cell with a finer microstructure. The measured gas conversion impedance was compared with models in literature for both the 16- and the 2 cm2 cells. The continuously stirred tank reactor (CSTR) AC model approximated the overpotential of the smaller cells (2 cm2) with greater accuracy in the current density range 0–0.5 A/cm2 while the plug flow reactor (PFR) model although derived for the case of zero DC bias predicted the 16 cm2 cell ASR better than the zero bias CSTR model. Furthermore, the gas conversion impedance in the hydrogen/steam fuel split into two processes with opposing temperature behavior in the reformate fuels. By using a 87.5% smaller active electrode area the gas conversion impedance was diminished in the hydrogen/steam fuel at (the same absolute) high fuel flow rates. In both reformates, the second and third lowest frequency processes merged into a single process as the gas conversion was reduced. The SOC with finer electrode microstructure displayed improved kinetics.
Highlights
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Whereas utility scale wind and solar generation generally occurs at sites far away from the cities and requires large open areas, the Solid Oxide Cell (SOC) technology is more compact both for utility scale and distributed generation and can be generated much closer to where it is needed, avoiding losses associated with power transport
When operated as a Solid Oxide Fuel Cell (SOFC) the SOC converts the chemical energy in hydrogen as well as hydrocarbon fuels directly into electricity (Eq 1)
Summary
General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Kinetic Studies on State of the Art Solid Oxide Cells: A Comparison between Hydrogen/Steam and Reformate Fuels. The Solid Oxide Cell (SOC) is an energy conversion technology that, when included in the energy mix, can alleviate some of the challenges faced by wind and solar technologies such as intermittent availability. When operated as a Solid Oxide Fuel Cell (SOFC) the SOC converts the chemical energy in hydrogen as well as hydrocarbon fuels directly into electricity (Eq 1). The same SOC can be operated in a reverse mode as a Solid Oxide Electrolysis Cell (SOEC) in which case hydrogen or syngas is produced by electrochemical reduction of steam or steam/carbon-dioxide mixture respectively (Eq 2). The heat can either be used to operate gas or steam turbines or used directly for district heating
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