Reversible solid oxide cell
A reversible solid oxide cell is a solid-state electrochemical device that is operated alternatively as a solid oxide fuel cell and a solid oxide electrolysis cell. 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.
Technology description
Cell structure and working principle
Reversible solid oxide cells, 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 or the reduction of the products to produce the fuel takes place. On the oxygen electrode, oxygen reduction or oxygen ions oxidation to form oxygen gas 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, a ceramic material characterized by high conductivity to oxygen ions at elevated temperature. The most popular oxygen electrode materials are lanthanum strontium cobalt ferrite and lanthanum strontium chromite, 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.
Polarization curve
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. 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:- the activation losses, predominant at very low current densities;
- the ohmic losses, increasing linearly with the current density;
- the concentration losses, occurring at very high current density, when the reactants inside the electrode get depleted.
Other than the open circuit voltage, another fundamental theoretical voltage can be defined. The thermoneutral voltage depends on the enthalpy of the overall reaction taking place in the rSOC and the number of charges that are transferred within the electrochemical reactions. Its relationship with the operating voltage gives information about the heat demand or generation inside the cell.
During the electrolysis operation:
- if, the reaction is endothermic;
- if, the reaction is exothermic.
Chemistry
Various chemistries can be considered when dealing with reversible solid oxide cells, which in turn can influence their operating conditions and overall efficiency.Hydrogen
When hydrogen and steam are considered as reactants, the overall reaction takes this form:where the forward reaction occurs during SOFC mode, and the backward reaction during SOEC mode. On the fuel electrode, hydrogen oxidation takes in SOFC mode and water reduction takes plain SOEC mode:
On the oxygen electrode, oxygen reduction occurs in SOFC mode and oxide ions oxidation occurs in SOEC mode:
The thermoneutral voltage for steam electrolysis is equal to 1.29 V.
Carbonaceous reactants
Differently than low-temperature electrochemical technologies, rSOCs can process also carbon containing species with reduced risk of catalyst poisoning. Methane can be internally reformed on the Ni particles to produce hydrogen, similarly to what happens in steam reforming reactors. Subsequently, the produced hydrogen can undergo the electro-oxidation. Moreover, when working in SOEC modality, water and carbon dioxide can be co-electrolyzed to generate hydrogen and carbon monoxide to form syngas mixtures with various composition.The reactions taking place on the oxygen electrode are the same considered for the hydrogen/steam case. Even if characterized by much slower kinetics with respect to the one involving hydrogen and steam, the direct electro-oxidation of carbon monoxide or the direct electro-reduction of carbon dioxide can be considered as well:
The thermoneutral voltage of the
One useful way to depict the cycling between SOFC and SOEC mode of the rSOC operation with carbonaceous reactants is the C-H-O ternary diagram. Each point in the diagram represents a gas mixture with a different number of carbon, hydrogen or oxygen atoms. When dealing with the operation on reversible solid oxide cells, three distinct regions can be distinguished in the graph. For different operating conditions, distinct boundary lines between these regions can be drawn. The three regions are:
- the carbon deposition region: gas mixtures lying in this region are characterized by compositions that are prone to carbon deposition on the fuel electrode;
- the fully oxidized region: this region is characterized by gas mixtures that are fully oxidized, hence they cannot be used as fuels in the rSOC;
- the operating region: this region is characterized by gas mixtures that are suitable for the rSOC operation.
Ammonia
An alternative chemistry for rSOCs is that one involving ammonia conversion to hydrogen and nitrogen. Ammonia has great potential as hydrogen carrier, due to its higher volumetric density with respect to hydrogen itself, and it can be directly fed to SOFCs. It has been demonstrated that ammonia-fed SOFCs operate through successive ammonia decomposition and hydrogen oxidation:Ammonia decomposition has been demonstrated to be slightly more efficient than simple hydrogen oxidation, confirming the great potential of ammonia as a fuel other than an energy carrier.
Unfortunately, ammonia cannot be directly synthesized on the fuel electrode of a rSOC, because the equilibrium reaction
is completely shifted towards the left at their higher than 600 °C working temperature. For this reason, for clean ammonia production, hydrogen production via electrolysis must be coupled with nitrogen production from air with hydrogen oxidation and subsequent water separation.