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

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.

A-site deficient chromite with in situ Ni exsolution as a fuel electrode for solid oxide cells (SOCs)

“…For this impedance feature, the DRT analysis suggests two processes (Peak#3 and Peak#4) at a frequency between $3-25 Hz that, by analogy with the behavior of MIEC materials reported by Adler et al, 62 are likely to be connected to each other and be characteristic of the response of the L65SCrN. A last process (Peak#5) at low frequencies between 0.1 Hz and 1 Hz was attributed to the gas losses where there is an overlapping of the RWGS reaction (catalytic conversion), 63 the diffusion in the electrodes and the gas conversion processes (within the gas channels). At frequencies above 10 3 Hz, it was not possible to t the EIS spectrum due to an artifact of the measurement.…”

Operational Aspects of a Perovskite Chromite-Based Fuel Electrode in Solid Oxide Electrolysis Cells (SOEC)

“…Moreover, the temperature variation showed a strong dependence of a process between 3 and 100 Hz for both cells, characterized by a decrement of the imaginary impedance upon a temperature increase. These processes are likely linked to electrochemical polarization and surface oxygen exchange processes for both fuel electrode types, which is consistent with previous studies of L65SCrN and Ni-CGO fuel electrodes. , Nevertheless, these processes could overlap with the gas conversion at ∼1 Hz. ,, Additionally, on porous electrodes, ion conduction, stoichiometry changes (chemical capacitance), and surface redox kinetics cannot be deconvoluted by individual frequencies. All of these phenomena make very difficult to precisely quantify the ASR fuel electrode term.…”

“…Figure shows imaginary impedance spectra for the (a) L65SCrN- and (b) Ni-CGO-based full cells at different temperatures. The characteristic frequencies of the charge transfer process on the LSCF oxygen electrode and the LSCF/CGO interfacial double-layer capacitance were identified to be between 100 Hz and 1 kHz. ,, A low-frequency peak, which is generally associated with the gas conversion in our setup, was identified at ∼3 Hz. A slightly lower frequency (∼1–3 Hz) of the gas conversion was observed for the L65SCrN electrodes, which is in agreement with our previous work with similar L65SCrN fuel electrodes .…”

Experimental Investigation of In‐plane Performance Variation on Anode Supported Solid Oxide Fuel Cells Using Segmented Cathodes and Reference Electrodes

“…Thus, it can be said that the performance variation on the planar cell is due to the impedance in the very low frequency range (1–0.01 Hz), and the additional voltage drop in the outlet region was due to gas conversion impedance . This may be as large as the second contribution (∼35%) to the total cell resistance in a channel type setup .…”