Reversible solid oxide cell
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 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.
For this reason, rSOCs are recently receiving increased attention due to their potential as an energy storage solution on the seasonal scale.
[1] The electrodes are porous layers that favor the reactants diffusion inside their structure and catalyze electrochemical reactions.
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.
Various chemistries can be considered when dealing with reversible solid oxide cells, which in turn can influence their operating conditions and overall efficiency.
electrolysis is equal to 1.48 V. 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.
[6] 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.
During the SOFC operation, the composition of the gas in the fuel electrode moves towards the boundary line of the fully oxidized region, increasing its oxygen content.
[7] 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.
[3] Reversible solid oxide cells are receiving increased attention as energy storage solutions for the weekly or the monthly scale.
In this regard, hydrogen storage is a promising alternative, since the produced fuel can be compressed and stored for months.
Moreover, the possibility to operate both the fuel oxidation and the electrolysis on the same device is beneficial on the capacity factor of the system, helping at reducing its specific investment cost.
For this reason, in reversible operation, the thermoneutral voltage poses significant limitations in achieving high roundtrip efficiencies.
It has been demonstrated that increasing the yield of methane in the electrolysis operation can substantially decreases the thermoneutral voltage and heat demand of the reaction.
In these conditions, the rSOC can even be operated in exothermic mode at reduced voltages, permitting to produce additional heat at high temperature.
[9][10] One of the most challenging aspects in designing large rSOC systems for energy storage purposes is the thermal integration.
The latter requirement can be avoided if the rSOC is operated with an exothermic reaction in SOEC modality, with a negative effect on the roundtrip efficiency.
Thermal energy storage typologies and heat transfer fluids that have been considered for this purpose are those used for Concentrated solar power (CSP) technologies.
Diathermic oil can be used to store heat at relatively low temperature (for instance, 180°C) and exploited for water evaporation.
[12] If carbonaceous chemistries are employed, the beneficial effect of methane synthesis inside the cell can be exploited to reduce the heat request of the electrolysis mode.
The set of these component is regarded as balance of plant (BOP), and may comprehend pumps, compressors, expanders or fans, needed for fluid circulation and processing inside the system.
[11][14] On the other hand, systems exploiting the beneficial effects of methane formation, either inside the rSOC or in external reactors, can reach rountrip efficiencies in the order of 70% and beyond.
