CO2 and H2o Co-Electrolysis without H2 on Solid Oxide Cell
Authors: Ha Su , Dewa Martinus 
Published: May 2020

Co-electrolysis of water and CO2 into H2 and CO (syngas) using a solid oxide electrolysis cell (SOEC) is a current research interest in the clean energy field. H2 is often added during the co-electrolysis reaction as a reductive gas. Removing H2 from the system can decrease the capital cost of the process.
Nickel (Ni)/yttria-stabilized zirconia (YSZ) cermet is a state-of-the-art electrode material for solid oxide cells due to its good catalytic performance and electrical conductivity1,2. Researchers have been debating the importance of H2 during co-electrolysis on Ni/YSZ cell. While the effect of adding H2 during the co-electrolysis reaction has not been intensively studied, almost all electrolysis research on Ni/YSZ cells operate in reducing atmospheres (usually by adding H2) to prevent oxidation3. There is a very limited published data available showing co-electrolysis without H2 since many studies claim that co-electrolysis without H2 will rapidly deactivate the cell especially if Ni/YSZ is used as the cathode. Our experimental data, however, showed that Ni/YSZ electrolyte supported cell has a decent IV performance of -400 mA/cm2 at 1.5 V and was stable even without H2 after 24 h under co-electrolysis conditions without H2 at 800°C. The disadvantage of using Ni/YSZ is that it requires activation by reducing nickel oxide with H2 during its start up.
Spinel oxide (AB2O4) has been recently investigated as a cathode material in SOECs. Spinel oxides are also known for their catalytic activity towards various reactions including CO2 decomposition4, NOx reduction5, and water-gas shift6. Other advantages of using spinel oxides as cathode materials compared to Ni/YSZ are that spinel oxides are electrically conductive in its oxide form and therefore do not requires the activation step by H2 during its startup. Wu et. al reported that CuFe2O4 has the best co-electrolysis IV performance compared to the other spinel oxides7,8. Our experimental data shows that CuFe2O4 spinel electrolyte supported cell without H2 activation shows a slightly better performance compared to a Ni/YSZ cell of -475 mA/cm2 at 1.5 V. However, it shows a lower stability due to delamination and incompatibility issues with the YSZ electrolyte. Applying an additional La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM) barrier layer successfully solved this stability issue, but electrochemical performance is compromised (-275 mA/cm2). Therefore, optimization of the design and fabrication method for the CuFe2O4 cell needs to be conducted to get both good IV performance and stability.

Figure Caption:

Fig 1. (a) Initial SOEC IV performance under co-electrolysis condition (50% CO2 and 50% H2O) at 800°C for electrolyte supported button cells with 3 different cathode materials. (b) The corresponding plots of long-term stability test under constant current after 24 h.

References:

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8. K. T. Wu, J. Matsuda, A. Takagaki, and T. Ishihara, ECS Transaction, 91, 2425–2432 (2019).

Figure 1