Metal oxides for high-temperature electrolyzers

High-temperature electrolyzers (HTE) or solid-oxide electrolyzers (SOE) are electrochemical devices able to produce valuable fuels in a highly efficient manner. In comparison to low-temperature electrolyzers, they can generate a variety of fuels such as hydrogen, syngas, or methane when employing steam and carbon dioxide as input species. In addition, HTEs are capable to produce pure oxygen. The main global reaction for high-temperature steam electrolysis is: (9.1)H2O→H2+O2 This reaction occurs as a two-step reaction, consisting of the two following half-reactions:(9.2)H2O+2e−→H2+O2−(9.3)O2−→1/2O2+2e− The first half-reaction (9.2) occurs on the steam electrode and the cathode, respectively, whereas the second half-reaction (9.3) occurs on the oxygen-electrode or anode. The main charge carrier in high-temperature electrolyzers is oxygen ion O2−, as can be seen in the reactions mentioned. When considering pure steam electrolysis, a great advantage of the HTE technology in comparison to low temperature liquid water electrolyzers is their better efficiency and reduced input of electrical energy required for the steam dissociation as the temperature increases because of enhanced kinetics and favorable thermodynamic at higher operating temperatures. In this case, a part of the electricity required can be replaced by heat [1–3]. In general terms, high temperature solid-oxide cells (HT-SOC) can be operated in a reversible mode as reversible solid-oxide cells (rSOC), which means: (i) when operating them in a solid-oxide fuel cell (SOFC) mode, they generate electricity and heat utilizing a fuel, and (ii) when supplying them with electricity and steam or carbon dioxide, they can generate valuable fuels in a solid-oxide electrolysis cell (SOEC) mode. For this purpose, only one unit is required, which is a great advantage when comparing them with other technologies that require two separate units.