Numerical Model of Co-Electrolysis in Solid Oxide Cells with Ni/Gd-Doped Ceria (CGO) Fuel Electrode
Authors: Koksharov Andrey , Taubmann Julian , Riegraf Matthias , Costa Rémi , Latz Arnulf , Jahnke Thomas 
Published: July 2021

Solid oxide electrolysis cells (SOECs) are expected to play a crucial role in the conversion of electrical energy from sustainable sources into chemical compounds. The co-electrolysis of atmospheric carbon dioxide with steam is seen as a potential source of synthesis gas [1], which can be further processed with Fischer-Tropsch synthesis to renewable liquid fuels [2]. A key step during co-electrolysis is the energy conversion at the cermet fuel electrode through electro-reduction. This electro-reduction is commonly believed to take place at the triple phase boundary (TPB) between nickel, oxygen ion conducting ceramics and gas [3]. Recent studies show that, due to the high electro-catalytic activity and mixed ionic and electronic conductivity (MIEC) of the ceria phase, the electrochemically active electrode area on Ni/Gadolinium-doped ceria (CGO) electrodes further extends to the double phase boundary (DPB) between CGO surface and gas phase [4,5,6]. However, it is still unclear whether DPB or TPB pathways dominate the electrochemical reduction in industrially used porous electrodes [5], particularly after structural changes as a consequence of Ni degradation such as coarsening and migration [6].

The proposed numerical model of a commercial electrolyser cell aims to reveal main kinetic mechanisms responsible for the electro-reduction at the Ni/CGO fuel electrode and to identify possibilities for the cell optimisation. The cell model includes: detailed gas and surface kinetics, detailed gas and ion transport. Cross-section images from scanning electron microscopy helped to identify 7 layers with different material properties. The model allows to differentiate the following aspects: charge transfer at the LSCF/CGO/air interface, charge transfer at CGO/fuel and Ni/CGO/fuel interfaces, charge transfer between CGO-surface and CGO-bulk, charge transfer between YSZ and CGO, as well as gas transport/conversion.

For the model validation, electrochemical experiments were performed including current-voltage curves and electrochemical impedance spectroscopy with a full cell in the 3 operating modes H₂O electrolysis, CO₂ electrolysis and co-electrolysis. In order to avoid the overlap of electrochemical contributions and gas transport processes, a cell with a small active electrode area was operated with large fuel gas flow rates. Various operating temperatures (780 to 860 °C), fuel compositions (30 to 85% H₂O, 30 to 85% CO₂) and oxygen contents (1 to 100% O₂) were used to identify the electrochemical mechanisms. Possible reaction pathways are investigated and electro-reduction at DPB and TPB is compared.

References:

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