Publication date: 10th April 2024
This paper develops a model of a gadolinium-doped ceria (GDC) as a ceramic mixed ionic-electronic conducting (MIEC) electrolyte membrane in a membrane reactor. In applications such as fuel cells, the membrane is polarized via the chemical potentials associated with a fuel and oxidizer on opposite sides of the membrane. The membrane’s primary purpose is to conduct oxygen ions, with electrons being delivered to an external circuit. In fuel cells, because the electronic leakage through the membrane, which increases greatly above 600 ˚C, represents an efficiency loss, GDC-based cells are operated at relatively low temperature (T < 600 ˚C) where the electronic conductivity is relatively low.
The present application uses feed streams of H2O and CO that are separated by a GDC membrane and without any external electrical power. The purpose is the produce H2, via an electrochemically assisted process. The exothermic water-gas-shift reaction (H2O + CO = H2 + CO2, DH = 42 kJ/mol) provides the chemical potential needed to drive the membrane reactor. Unlike in fuel cells, electronic conduction (via a reduced-ceria small polaron) is an advantage for increasing charged-defect fluxes across the GDC membrane. This favors relatively high temperature operation.
The GDC membrane is sandwiched between porous composite cermet electrodes (e.g., Ni-YSZ). The oxygen conduction is represented as oxygen vacancies and the electronic conduction via reduced-ceria small polarons. The H2O reduction and CO oxidation are electrochemical charge-transfer processes that proceed at three-phase boundaries within the composite electrodes. Within the electrode structure, electrons are mobile in electrode phase (e.g., Ni), oxygen vacancies are mobile in the electrolyte phase (e.g., YSZ), and gases are in the pore phase. Nickel also serves as a heterogeneous catalyst.
The presentation develops the electrochemical and transport theory to model the process and uses computational simulation to predict performance. Some of the underpinning theory and model formulation were documented in the context of a GCD-based fuel-cell [1]
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