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Reversible solid oxide cell
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Reversible solid oxide cell
A reversible solid oxide cell (rSOC) is a solid-state electrochemical device that is operated alternatively as a solid oxide fuel cell (SOFC) and a solid oxide electrolysis cell (SOEC). Similarly to SOFCs, rSOCs are made of a dense electrolyte sandwiched between two porous electrodes. Their operating temperature ranges from 600 °C to 900 °C, hence they benefit from enhanced kinetics of the reactions and increased efficiency with respect to low-temperature electrochemical technologies.
When utilized as a fuel cell, the reversible solid oxide cell is capable of oxidizing one or more gaseous fuels to produce electricity and heat. When used as an electrolysis cell, the same device can consume electricity and heat to convert back the products of the oxidation reaction into valuable fuels. These gaseous fuels can be pressurized and stored for a later use. For this reason, rSOCs are recently receiving increased attention due to their potential as an energy storage solution on the seasonal scale.
Reversible solid oxide cells (rSOCs), as solid oxide fuel cells, are made of four main components: the electrolyte, the fuel and oxygen electrodes, and the interconnects.
The electrodes are porous layers that favor the reactants diffusion inside their structure and catalyze electrochemical reactions. In the single technologies like SOFCs and SOECs, the electrodes serve a single purpose, hence they are called with their specific names. The anode is where the oxidation reaction occurs, while the cathode is where the reduction reaction takes place. In reversible solid oxide cells, on the other hand, both modalities can occur alternatively in the same device. For this reason, the generic names of fuel electrode and oxygen electrode are preferred instead. On the fuel electrode the reactions involving the fuel oxidation (SOFC modality) or the reduction of the products to produce the fuel (SOEC modality) takes place. On the oxygen electrode, oxygen reduction (SOFC modality) or oxygen ions oxidation to form oxygen gas (SOEC modality) takes place.
State-of-the-art materials for rSOCs are those used for SOFCs. The most common fuel electrodes are made by a mixture of nickel, that serves as electronic conductor, and yttria-stabilized zirconia (YSZ), a ceramic material characterized by high conductivity to oxygen ions at elevated temperature. The most popular oxygen electrode materials are lanthanum strontium cobalt ferrite (LSCF) and lanthanum strontium chromite (LSC), perovskite materials able to catalyze oxygen reduction and oxide ion oxidation reactions.
The electrolyte is a solid-state layer placed between the two electrodes. It is an electric insulator, it is impermeable to gas flow but permeable to oxygen ions flow. Hence, the main properties of this component are the high ion conductivity and the low electrical conductivity. When the rSOC is operated in SOFC mode, oxygen ions flow from the oxygen electrode to the fuel electrode, where the fuel oxidation occurs. In SOEC mode, the reactants are reduced in the anode with the production of oxygen ions, which flow towards the oxygen electrode. The most widespread material for electrolytes is YSZ.
The interconnects are usually made of metallic materials. They provide or collect the electrons involved in the electrochemical reactions. In addition, they are shaped internally with gas channels to distribute the reactants over the cell surface.
The most common tool to characterize the performances of a reversible solid oxide cell is the polarization curve. In this chart, the current density is related to operating voltage of the cell. The usual convention is the one of positive current density for the fuel cell operation, and negative current density for the electrolysis operation. When the rSOC electrical circuit is not closed and no current is extracted or supplied to the cell, the operating voltage is the so-called open circuit voltage (OCV). If the composition of the gas in the fuel electrode and the oxygen electrode are the same for both modalities, the polarization curve for the SOEC mode and the SOFC have the same OCV. When some current density is extracted or supplied to the cell, the operating voltage starts to diverge from the OCV. This phenomenon is due to the polarization losses, which depend on three main phenomena:
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Reversible solid oxide cell
A reversible solid oxide cell (rSOC) is a solid-state electrochemical device that is operated alternatively as a solid oxide fuel cell (SOFC) and a solid oxide electrolysis cell (SOEC). Similarly to SOFCs, rSOCs are made of a dense electrolyte sandwiched between two porous electrodes. Their operating temperature ranges from 600 °C to 900 °C, hence they benefit from enhanced kinetics of the reactions and increased efficiency with respect to low-temperature electrochemical technologies.
When utilized as a fuel cell, the reversible solid oxide cell is capable of oxidizing one or more gaseous fuels to produce electricity and heat. When used as an electrolysis cell, the same device can consume electricity and heat to convert back the products of the oxidation reaction into valuable fuels. These gaseous fuels can be pressurized and stored for a later use. For this reason, rSOCs are recently receiving increased attention due to their potential as an energy storage solution on the seasonal scale.
Reversible solid oxide cells (rSOCs), as solid oxide fuel cells, are made of four main components: the electrolyte, the fuel and oxygen electrodes, and the interconnects.
The electrodes are porous layers that favor the reactants diffusion inside their structure and catalyze electrochemical reactions. In the single technologies like SOFCs and SOECs, the electrodes serve a single purpose, hence they are called with their specific names. The anode is where the oxidation reaction occurs, while the cathode is where the reduction reaction takes place. In reversible solid oxide cells, on the other hand, both modalities can occur alternatively in the same device. For this reason, the generic names of fuel electrode and oxygen electrode are preferred instead. On the fuel electrode the reactions involving the fuel oxidation (SOFC modality) or the reduction of the products to produce the fuel (SOEC modality) takes place. On the oxygen electrode, oxygen reduction (SOFC modality) or oxygen ions oxidation to form oxygen gas (SOEC modality) takes place.
State-of-the-art materials for rSOCs are those used for SOFCs. The most common fuel electrodes are made by a mixture of nickel, that serves as electronic conductor, and yttria-stabilized zirconia (YSZ), a ceramic material characterized by high conductivity to oxygen ions at elevated temperature. The most popular oxygen electrode materials are lanthanum strontium cobalt ferrite (LSCF) and lanthanum strontium chromite (LSC), perovskite materials able to catalyze oxygen reduction and oxide ion oxidation reactions.
The electrolyte is a solid-state layer placed between the two electrodes. It is an electric insulator, it is impermeable to gas flow but permeable to oxygen ions flow. Hence, the main properties of this component are the high ion conductivity and the low electrical conductivity. When the rSOC is operated in SOFC mode, oxygen ions flow from the oxygen electrode to the fuel electrode, where the fuel oxidation occurs. In SOEC mode, the reactants are reduced in the anode with the production of oxygen ions, which flow towards the oxygen electrode. The most widespread material for electrolytes is YSZ.
The interconnects are usually made of metallic materials. They provide or collect the electrons involved in the electrochemical reactions. In addition, they are shaped internally with gas channels to distribute the reactants over the cell surface.
The most common tool to characterize the performances of a reversible solid oxide cell is the polarization curve. In this chart, the current density is related to operating voltage of the cell. The usual convention is the one of positive current density for the fuel cell operation, and negative current density for the electrolysis operation. When the rSOC electrical circuit is not closed and no current is extracted or supplied to the cell, the operating voltage is the so-called open circuit voltage (OCV). If the composition of the gas in the fuel electrode and the oxygen electrode are the same for both modalities, the polarization curve for the SOEC mode and the SOFC have the same OCV. When some current density is extracted or supplied to the cell, the operating voltage starts to diverge from the OCV. This phenomenon is due to the polarization losses, which depend on three main phenomena:
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