Comparison of a solid oxide cell with nickel/gadolinium‐doped ceria fuel electrode during operation with hydrogen/steam and carbon monoxide/carbon dioxide

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Performance of a solid oxide cell (SOC) depends on the operating environment. Regarding single cell tests with ideal contacting (gold, platinum, nickel meshes) and inert flow fields (Al2O3), performance is limited by intrinsic losses in the cell. Contact losses and poisoning effects are minimized. In a SOC-stack with metallic interconnectors, performance is affected by contact resistances, chromium (Cr) evaporation, and limitations in gas supply. Here, 1 cm² single cells were tested with a stack-like contact applying metallic flow fields made from three different steel grades (Crofer 22 APU, AISI 441, UNS S44330) with and without a cerium-cobalt PVD-coating. Cell performance and losses were analyzed by IV-characteristics, impedance spectroscopy, and DRT analysis. For all uncoated interconnectors, significant performance losses due to increased contact losses and air electrode polarization were observed, which is attributed to Cr-oxide scale formation on the metallic interconnectors and Cr-poisoning of the air electrode as revealed by scanning electron microscopy-energy-dispersive X-ray spectroscopy. A CeCo-coating leads to similar oxide scales irrespective of the substrate material. Moreover, with the coating the electrochemical performance drastically improved due to decreased contact losses and an effective blocking of Cr-evaporation leading to a cell performance close to the ideal case for all three steel grades

Section: Resultsmentioning

confidence: 99%

Impact of CeCo-Coated Metallic Interconnectors on SOCs Towards Performance, Cr-Oxide-Scale, and Cr-Evaporation

Grosselindemann,

Reddy,

Störmer

et al. 2024

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“…1 Based on an analysis of impedance measurements and polarization curves at various product compositions Wolf et al proposed a boundary for the onset of the electrochemical CO 2 reduction on Ni/YSZ fuel electrode supported cells at a steam content of 30%, while above this value the CO 2 reduction predominantly happens via the RWGS. 2 On the other hand, electrodes with mixed ionic electronic conducting (MIEC) materials like nickel/gadolinia doped ceria (Ni/ GDC) can have a different affinity towards the electrochemical CO 2 reduction 3 and thus could show a different boundary for its onset in co-electrolysis conditions. Generally, a meaningful analysis of separate activation resistance contributions of Ni/GDC electrodes can be difficult due to the very different impedance characteristics compared to Ni/YSZ, especially as often a poor separation of gas diffusion and electrode activation in the impedance spectrum is observed.…”

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confidence: 99%

“…Generally, a meaningful analysis of separate activation resistance contributions of Ni/GDC electrodes can be difficult due to the very different impedance characteristics compared to Ni/YSZ, especially as often a poor separation of gas diffusion and electrode activation in the impedance spectrum is observed. [3][4][5][6][7][8] In the first part of this study, results from a comprehensive parameter variation on symmetrical cells with Ni/GDC electrodes are shown to identify characteristic frequency ranges of gas diffusion, surface-reaction, and bulk/interface resistance contributions under H 2 /H 2 O/CO/CO 2 -atmospheres. These variations include: a variation of the fuel gas composition, addition of small amounts of H 2 S to the gas phase, temperature variation, and a change of inert diluent from nitrogen to helium.…”

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confidence: 99%

“…As an increasing amount of CO and CO 2 decreases the effective gas diffusion coefficient and the charge transfer resistance for the CO-CO 2 redox couple is higher, both seem to be reasonable. 3 Bulk processes (i.e. charge transport in the bulk and at resistive interfaces) on the other hand are found at higher frequencies.…”

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confidence: 99%

“…In the case of Ni/ GDC electrodes an overlap of activation polarization and gas diffusion polarization is frequently reported in literature. [3][4][5][6][7][8] This is often attributed to the high chemical capacity of GDC, resulting in activation polarization contributions appearing at lower frequencies.…”

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confidence: 99%

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Electrochemical Characterization of Nickel/Gadolinia Doped Ceria Fuel Electrodes under H₂/H₂O/CO/CO₂-Atmospheres

Esau,

Grosselindemann,

Sckuhr

et al. 2024

Modelling of the co-electrolysis process requires understanding of the underlying reaction pathways under H2/H2O/CO/CO2-atmospheres. These include the electrochemical steam reduction/hydrogen oxidation, the electrochemical CO2 reduction/CO oxidation and their coupling via the catalytic (reverse) water gas shift reaction ((R)WGS). The assumption of a very fast RWGS and therefore neglectable electrochemical CO2 conversion is commonly used to model the co-electrolysis process. In contrast, previous studies on Ni/GDC fuel electrodes suggest that the electrochemical conversion of CO / CO2 can be present in H2/H2O/CO/CO2-atmospheres. To deconvolute surface-related and non-surface-related processes in the impedance response we present results from a complex variation of operating parameters for process identification by the use of electrochemical impedance spectroscopy and the subsequent impedance analysis by the distribution of relaxation times. A physically meaningful equivalent circuit model, based on a single channel transmission line, is then derived. The model enables quantification of the surface reaction resistance under varied C/H-ratios. From a kinetic analysis it is shown that the electrochemical H2/H2O conversion is dominant for y_CO+y_(CO_2 )≤ 50% and electrochemical CO/CO2-conversion onsets from y_CO+y_(CO_2 )≥ 60%.

Solid Oxide Cell Reactor Model for Transient and Stationary Electrochemical H₂O and CO₂ Conversion Process Studies

Sedeqi,

Santhanam,

Riegraf

et al. 2024

The ability of high-temperature solid oxide cell (SOC) electrochemical reactors to efficiently convert atmospheric carbon to high value chemicals for industrial and energy storage applications via CO2 and co-electrolysis makes them a key technology for active carbon utilisation. However, due to additional operational risks from thermochemical reactions on thermal management, limited experimental capacity, and relative novelty, CO2 and co-electrolysis lag behind steam electrolysis in large-scale adoption. Here, a 1D+1D SOC model based on fundamental first principles considering three-dimensional heat transfer was improved via a unique method for representing co-electrolysis electrochemistry, solving with low computational effort. Validation against experimental data for two compositions and pressures, showed high levels of accuracy with respect to characteristic cell voltages, temperatures, and outlet compositions. The model also showed CO2 reduction during co-electrolysis mainly occurred via reverse water gas shift, while CO2 electrolysis still accounted for up to 35% of the total share. Pressurised co-electrolysis operation promotes exothermic methanation, causing pronounced heating of the reactor, consequently reducing the isothermal current density. Therefore, low to moderate pressurisation is likely most suited for coupling with downstream synthesis processes to avoid the installation of unnecessarily large systems and associated high costs.