Renewable energy resources such as solar energy, wind energy, hydropower or geothermal energy have attracted significant attention in recent years. Renewable energy sources have to match supply with demand, therefore it is essential that energy storage devices (e.g., secondary batteries) are developed. However, secondary batteries are accompanied with critical problems such as high cost for the limited energy storage capacity and loss of charge over time. Energy storage in the form of chemical species, such as H₂ or CO₂, have no constraints on energy storage capacity and will also be essential. When plentiful renewable energy exists, for example, it could be used to convert H₂O into hydrogen via water electrolysis. Also, renewable energy resources could be used to reduce CO₂ into CO and recycle CO₂ and H₂O into sustainable hydrocarbon fuels in solid oxide electrolysis (SOE).