Table 3.3 Summary of main features of CO electrolyzers or co‐electrolyzers.
| MCEC | SOEC | PC‐SOEC | ||
|---|---|---|---|---|
| Anode | Reaction | CO32− → 1/2O2 + CO2 + 2e− | O2− → 1/2O2 + 2e− | 2H2O → 4H+ + O2 + 4e− |
| Materials | Nickel based | LaxSr1−xMnO3 + Y‐stabilized ZrO2 (YSZ) | BCZY, SCZY | |
| Electrolyte | Charge carrier | CO32− | O2− | H+ |
| Materials | Molten carbonates | Solid: Y2O3–ZrO2, Sc2O3–ZrO2, MgO–ZrO2, CaO–ZrO2 | Solid: BCZY, BZY, BCY, proton conductors | |
| Cathode | Reaction | H2O + CO2 + 2e− → H2 + CO32− 2CO2 + 2e− → CO + CO32− | H2O + 2e−→O2− + H2 CO2 + 2e− rarr; CO + O2− | 4H+ + 4e− → 2H2 CO2 + 2H+ + 2e− → CO + H2O |
| Materials | Nickel‐based | Ni‐YSZ | Ni cermets | |
| Operation temperature | 600–800 °C | 600–1000 °C | 600–1000 °C | |
3.5.1.2 CO2 Co‐electrolysis
Electrochemical CO2 reduction has gained importance in the field of energy storage and conversion, and the catalysts and electrolytes influence not only the catalytic activity and selectivity of the reaction, but also on the CO2 reduction mechanism to different species [107].
High temperature is desired for the CO2 electrolysis (Figure 3.6), and among the electrolysis systems, molten carbonate electrolyzer cells (MCECs), SOEC, and PC‐SOEC are favored for the CO2 direct valorization by electrolysis (Table 3.3).
Because CO2, H2O, and H2 are involved in the reactions, the system is further complicated when the co‐electrolysis takes places, a scheme with all the species and reactions involved is represented in Figure 3.7.
Among the Co‐electrolyzers at high temperatures, solid oxide cells (PC‐SOEC and SOEC) present two main advantages. First, all components in the cells are solids and the risk of liquid leakage is avoided. Secondly, high temperature facilitates the electrolysis because the kinetics and thermodynamics are favored.
Different materials and configurations can be used for the electrolyte and electrodes in solid oxide electrolyzers. The selection of the most suitable material will depend on the operation conditions, such as temperature or gas atmosphere. The type of electrolyte selected (protonic or ionic conductor) will set the reactions in each electrode. All electrolytes must possess some characteristics to ensure a good performance [108]. The electrolyte must be chemically, morphologically, and dimensionally stable in both atmosphere of the cell (oxidizing and reducing) and for all range of operation conditions. In addition, in order to minimize the ohmic losses in the cell performance, the electrolyte should have a good ionic or protonic conductivity in the cell operation conditions. The electronic conductivity should be as small as possible to avoid electron leakage in the electrolyte and the resulting low Faraday efficiency. Likewise, the porosity in the electrolyte has to be negligible to avoid gas leakage in the cell and consequently low performance. Finally, thermal expansion coefficient (TEC) should match with the adjacent components of the cell to avoid problems such as cracks and delamination. Moreover, TEC should be unalterable with oxygen partial pressure and temperature changes.
Figure 3.7 Schematic representation of the CO2 and Co‐electrolyzer systems.
Regarding electrodes, some features have to be accomplished as well. Both, the oxidant and fuel electrodes have to be chemically, morphologically, and dimensionally stable in the working atmospheres and temperatures. To improve cell performance, the electronic conductivity has to be as large as possible. Additionally, oxygen ion or proton conductivity is required to extend the triple phase boundary (the point where electrons, gases, and ions are in contact and the electrochemical reaction takes place) along the whole electrode surface. Electrodes should have enough porosity to allow fast gas transport from/to the active reaction sites. TEC of electrodes should match the electrolyte and adjacent components along the operation conditions. Furthermore, electrodes have to match with other components in the operation and fabrication conditions. Finally, electrodes must exhibit enough catalytic activity (low polarization) for the different reactions that take place in the active sites, such as oxygen reduction reaction, hydrogen oxidation reaction, water splitting reaction, CO2 reduction, etc.
Other characteristics are also desirable for all components of the cell as the low cost, low toxicity, and manufacturability.
In the past years, several studies about PCEC have been published. Steam and CO2 co‐electrolysis was performed by Ruiz‐Trejo and Irvine using BaCe0.5Zr0.3Y0.16Zn0.04O3−δ around 500 °C, obtaining promising results [109, 110]. Recently, Bausá et. al. reported co‐electrolysis at 700 °C using a BaCe0.2Zr0.7Y0.1O3−δ
