Diffusion and Gas Conversion Analysis of Solid Oxide Fuel Cells at Loads via AC Impedance

Abstract

Impedance measurements were conducted under practical load conditions in solid oxide fuel cells of differing sizes. For a 2 cm 2 button cell, impedance spectra data were separately measured for the anode, cathode, and total cell. Improved equivalent circuit models are proposed and applied to simulate each of measured impedance data. Circuit elements related to the chemical and physical processes have been added to the total-cell model to account for an extra relaxation process in the spectra not measured at either electrode. The processes to which elements are attributed have been deduced by varying cell temperature, load current, and hydrogen concentration. Spectra data were also obtained for a planar stack of five 61 cm 2 cells and the individual cells therein, which were fitted to a simplified equivalent circuit model of the total button cell. Similar to the button cell, the planar cells and stack exhibit a pronounced low-frequency relaxation process, which has been attributed to concentration losses, that is, the combined effects of diffusion and gas conversion. The simplified total-cell model approximates well the dynamic behavior of the SOFC cells and the whole stack.

In-Situ Dynamic Characterization of Energy Storage and Conversion Systems

“…Finite diffusion element (FDE) O, or sometimes called porous bounded Warburg , was established based on W. Its application is extended to a rotating disk electrode (RDE), where diffusion occurs over the Nernst Diffusion Layer (NDL), that is a diffusion layer with finite thickness. The expressions for ZO [30] are listed in Table 2. Figure 4d [27] shows a typical Nyquist plot of an O element.…”

“…Four fuel cell groups consisting with different number of single cells (12,24,36, and 47 cells respectively) in the stack #308 were tested at the temperature of 26ºC and a current load of 0.2 Adc without the embedded system controller [30]. A desired ac voltage signal of 10 mV was set for the cell groups in hybrid mode.…”

“…There were totally five sets of impedance tests of the stacks published with different scales listed below [19,30,48]. The impedance data of the stacks were collected at different current levels without embedded system controller in most of the tests.…”

“…The single planar fuel cells in the stack #308 were tested separately at the temperature of 26ºC under varying current loads of 0.2 Adc, 0.5 Adc, and 1.0 Adc, without the embedded system controller [30]. The single cells are numbered from the anode side to the cathode side.…”

“…The Ballard Nexa TM fuel cell system [19,30,48] was connected to the measuring circuit ( Figure 2 The tests were conducted on three PEM stacks numbered #308, #515, and #881, identical to each other. There were totally five sets of impedance tests of the stacks published with different scales listed below [19,30,48].…”