catalyst used, syngas is typically produced by the electrochemical reactions of steam (H 2 O) and CO 2 reduction on the cathode side and the oxidation reaction of O 2 ions on the anode side, as shown in Figure 1. Alternatively, syngas is produced via the electrochemical reduction of H 2 O to H 2, followed by a homogeneous/ heterogeneous rWGS reaction, which occurs with a fast kinetic rate at the cathode side at high temperatures (above 720°C). During operation in co-electrolysis mode, the SOEC is supplied with electrons that are used in the hydrogen electrode to split H 2 O and CO 2 into H 2 , CO, and O 2 ions. The O 2 ions then move through the electrolyte into the O 2 electrode, where the electrons from the ions are collected, and O 2 gas is released. The mixture of H 2 and CO molecules is collected at the H 2 electrode and extracted as syngas, which can be utilised in methanol synthesis for the production of low-CI methanol and other value-added chemicals, or for the production of synthetic fuels via FT synthesis. Among various operating parameters, current density and temperature are the two most important. It is advantageous to operate SOECs at high temperatures and high current density to increase efficiency and decrease production cost. The performance and durability of SOEC stacks have been investigated for co-electrolysis of steam and CO 2, with operating temperature between 650 and 850°C and current density up to -0.8 A/cm2. High-temperature co-electrolysis of CO 2 and steam is a promising method to produce syngas by making use of renewable energy and CO 2 as a sustainable feedstock. This is a versatile method to sustainably produce tailored syngas compositions. While co-electrolysis is a very promising pathway for production of low-CI syngas, it faces a number of challenges, including the need for efficient catalysts, high energy input, and cost-effective scale-up. The German Aerospace Center (DLR) Institute of Engineering Thermodynamics is a leading research institute carrying out valuable studies on the development of technology for the co-electrolysis of CO 2. Sunfire, Topsoe, Elcogen, SolydEra, and thyssenkrupp nucera, in association with Fraunhofer IKTS, are among the world’s leading SOEC electrolyser companies that offer high-temperature co-electrolysis
e-
e-
e-
Air (optional)
H + CO
e-
e-
½O
e-
=
e-
O
½O
e-
Enriched air or O
CO + H O
Cathode: CO (g) + 2e-
CO(g) + O - H (g) + O O (g) + 4e
H O(g) + 2e -
Anode:
2O
Figure 1 Co-electrolysis process
a complex porous system. Like any other electrolyser, an SOEC electrolyser has three main components: the anode, the cathode, and the electrolyte. The anode and cathode have relatively high porosity, which allows gases to pass through them. The cathode side receives water (steam) molecules, while the anode generates O 2 from the recombination of O 2 ions. The electrolyte, between the anode and cathode, is dense and conducts O 2 ions from the cathode to the anode. Captured CO 2 could be delivered, along with steam, to the solid oxide electrolyser and reduced to the H 2 and CO gas mixture (syngas) utilising renewable power. A schematic diagram of the process of co-electrolysis operation is shown in Figure 1 . The overall reaction occurring inside the co-electrolyser is given in Equation 4 :
(Eq. 4)
H 2 O + CO 2 → H 2 + CO + O 2
As depicted in Figure 1, CO 2 and steam are supplied to the cathode side of the co- electrolyser. At the cathode-electrolyte interface, H 2 O and CO 2 are reduced to H 2 and CO, which diffuse back through the fuel electrode. O 2 ions then pass through the electrolyte and oxidise to O 2, or they enrich the air supplied as a flushing stream to the anode side of the electrolyser. Depending on the operating temperature and
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