Combined Deconvolution and CNLS Fitting Approach Applied on the Impedance Response of Technical Ni∕8YSZ Cermet Electrodes

Abstract

A high-resolution impedance study of the hydrogen oxidation in Ni/8YSZ ͑yttria-stabilized zirconia͒ cermet anodes has been realized in consideration of a broad range of operating conditions ͑temperature and partial pressure of fuel gas components H 2 , H 2 O, N 2 , He͒. A major problem in this respect concerns the origin and physical interpretation of empirical equivalent circuits used to fit the experimental data. We applied a two-stage approach for the evaluation of the impedance data: ͑i͒ at first, by the deconvolution of a distribution function of relaxation times ͑DRT͒, four different processes and their characteristic relaxation times have been identified. Two processes at frequencies Ͻ1 kHz represent a gas-conversion process or, respectively, a gas diffusion, whereas two processes at higher frequencies ͑2-30 kHz͒ are associated with the electro-oxidation of hydrogen at active sites, including the charge transfer reaction and the ionic transport. ͑ii͒ Subsequently, the last mentioned processes were fitted to a "transmission line" model describing the electronic and ionic transport properties of the Ni/8YSZ cermet. The high resolution of the DRT combined with the numeric accuracy of the complex nonlinear least square ͑CNLS͒ fit enabled us to determine ͑i͒ the effective ionic conductivity of the Ni/8YSZ cermet, ͑ii͒ the spatial extension of the electrochemically active area adjacent to the electrolyte/electrode interface, and ͑iii͒ the charge transfer resistance and its thermal activation energy.

Ni/YSZ electrodes structures optimized for increased electrolysis performance and durability

“…Furthermore, plots of distribution of relaxation times (DRT) are provided. The DRT plots are used to illustrate development of impedance over time and to highlight frequency ranges for different processes contributing to the total impedance response from the cells [25,26]. To enable a quantitative analysis of the IS complex-non-linear-least-squares (CNLS) method was used to fit an equivalent circuit model to the data.…”

“…The (RQ)Ni/YSZ,TPB at 1-4 kHz is ascribed to the electrochemical process at the triple phase boundaries (TPBs). These contributions to the overall impedance of the cell can also be described in more detail via a transmission line model (TLM) for the impedance response [26,28,29]. The impedance response from the LSC/CGO oxygen electrode has been approximated by a Gerisher-type response in the CNLS fitting routine in line with the work by Hjelm et al [15]; having a characteristic frequency of 100 Hz.…”

Thermoneutral Operation of Solid Oxide Electrolysis Cells in Potentiostatic Mode

“…Furthermore, plots of distribution of relaxation times (DRT) are provided. The DRT plots are used to illustrate development of impedance over time and to highlight frequency ranges for different processes contributing to the total impedance response from the cells (7)(8). To quantitatively break down losses, complex-non-linearleast-squares (CNLS) method was used to fit an equivalent circuit model to the IS data measured at OCV.…”

Performance Factors and Sulfur Tolerance of Metal Supported Solid Oxide Fuel Cells with Nanostructured Ni:GDC Infiltrated Anodes

“…This homogenized way of looking at the MIEC porous electrode as one phase with a given electronic and ionic conductivity can conveniently be described by a transmission line and has proven itself to be highly applicable. Examples include MIEC material or MIEC composite SOFC cathodes, [17][18][19] the MIEC Ni:YSZ composite SOFC anode, 20,21 MIEC infiltration based SOFC anodes 6 and in general MIEC electrodes. [22][23][24] For further details and justification of this type of transmission line modelling approach the reader is referred to the provided references.…”