Diffusion and conversion impedance in solid oxide fuel cells

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.

Section: Resultsmentioning

confidence: 86%

Kinetic Studies on State of the Art Solid Oxide Cells: A Comparison between Hydrogen/Steam and Reformate Fuels

Njodzefon¹,

Graves²,

Mogensen³

et al. 2016

“…The gas conversion resistance (change in Nernst potential) will approximately for fuel utilization lower than 10% and higher than 90% be highly nonlinear and increase drastically. [31][32][33] The consequence is that in particular the peak power density at 750 • C in Figure 7 will be somewhat lower than the 1.56 Wcm −2 , which the linear extrapolated curve indicate.…”

Section: Resultsmentioning

confidence: 99%

Towards High Power Density Metal Supported Solid Oxide Fuel Cell for Mobile Applications

Nielsen

Persson

Muhl

et al. 2018

For use of metal supported solid oxide fuel cell (MS-SOFC) in mobile applications it is important to reduce the thermal mass to enable fast startup, increase stack power density in terms of weight and volume and reduce costs. In the present study, we report on the effect of reducing the Technical University of Denmark (DTU) SoA MS-SOFCs support layer thickness from 313 μm gradually to 108 μm. The support layer thickness decrease in the DTU co-sintering MS-SOFC fabrication route results in an increased densification of the support layer and a slight decrease in performance. To mitigate the performance loss, two different routes for increasing the porosity of the support layer and thus performance were explored. The first route is the introduction of gas channels by puncturing of the green tape casted support layer. The second route is modification of the co-sintering profile. In summary, the cell thickness and thus weight and volume was reduced and the cell power density at 0.7 V at 700 • C was increased by 46% to 1.01 Wcm −2 at a fuel utilization of 48%. All modifications were performed on a stack technological relevant cell size of 12 cm × 12 cm. Solid oxide fuel cell (SOFC) is attractive due to the excellent power generation efficiency and fuel flexibility. The conventional ceramic electrolyte and anode supported SOFC (AS-SOFC) is limited to stationary power generation applications as a result of the difficulty of quick startup and the high operating temperatures of 700 to 1000 • C. If quick startup of a SOFC is realized, mobile applications may be considered. To enable this, it is important to shorten the startup time and reduce the heat cycle performance degradation and increase stack power density both in terms of weight and volume. As an approach to solving these problems, metal supported SOFCs (MS-SOFCs) have attracted attention. The development of this new generation of SOFC is currently in progress. DTU Energy's MS-SOFC technology is based on co-sintering of laminated tape casted electrolyte, anode and support layers in a reducing atmosphere. This implies that the sintering shrinkage of the different layers must be matched sufficiently so that the mechanical stresses originating from any mismatch in sintering shrinkage of the individual layers can be absorbed by the cell structure. If the cell structure is unable to absorb the mechanical stresses, the cell will crack. Perfect matching of sintering shrinkage of the layers is practically very difficult as e.g. the electrolyte layer needs to be completely dense and thus gastight, while the anode and support layers in contrast needs to be highly porous to allow sufficient gas transport.An overview and recent progress of MS-SOFCs have been reviewed in Ref. 1. The company Ceres Power founded in 2001 is at present the organization, which most effectively has demonstrated up scaled large cell sized >80 cm 2 MS-SOFC stack technology. This is followed by consortia involving the company Plansee. At last MS-SOFC stacking has been demonstrated within consortia consi...

“…4a and 5͒, but at the same time the summit frequency stays constant at 1 Hz. For spectrum B1 the gas-conversion resistance dominates the lowfrequency arc ͑Table II͒, even though other processes ͑i.e., gas diffusion 24,39 ͒ are observed to contribute to the arc. Also, in test B EL the low-frequency arc increases ͑and subsequently decreases͒ at constant summit frequency ͑Fig.…”

Section: Eis At Ocv: Changes In the Nimentioning

confidence: 99%

“…40 The analysis is based on an expression of the combined gas-conversion and gas-diffusion impedance in a plug-flow setup, recently derived by Jacobsen et al 39 Analysis combining cell voltage, in-plane voltage differences, and EIS.-In this subsection, we discuss the advantages of combining complementary measurement techniques. The cell voltage provides a measure of the performance of the entire cell and gives easily accessible data and an overview for comparison with other cell tests, but cell voltage measurements lack the possibility of giving electrode-specific information or revealing unevenness in the distribution of resistances in the electrodes.…”

Section: Eis At Ocv: Changes In the Nimentioning

confidence: 99%

Advanced Test Method of Solid Oxide Cells in a Plug-Flow Setup

Jensen

Hendriksen

Mogensen

2009

Self Cite

This paper describes a case study of two electrolysis tests of solid oxide cells [Ni/yttria-stabilized zirconia (YSZ)–YSZ–lanthanum strontium manganite (LSM)/YSZ] tested in a plug-flow setup. An extensively instrumented cell test setup was used, and the tests involved measurements of the cell impedance at open-circuit voltage and under current load, the cell voltage, and the in-plane voltage in the electrodes. From the cell-voltage measurements it was evident that a significant passivation of the cells occurred over the first ∼10days . Thereafter, the cells reactivated at constant electrolysis conditions. From measurements of the in-plane voltages in the electrodes and impedance spectra obtained during the electrolysis operation, we derive information about the resistance distributions in the Ni electrodes and describe how these distributions evolve over time. Impedance spectra at open-circuit voltage before and after electrolysis testing at various gas compositions were used to show that the Ni electrode was affected by the electrolysis operation, whereas the LSM electrode was not.