Mapping a thermodynamic stability window to prevent detrimental reactions during CO2 electrolysis in solid oxide electrolysis cells

Preventing Ni oxidation and carbon deposition, regarded as two major issues for CO2 electrolysis with conventional Ni-based cathodes, is a critical challenge for the development of solid oxide electrolysis cells (SOECs). However, the origin of these two detrimental reactions remains unclear. Here, we unveil that the Nernst voltage, in relation to the operating voltage applied to the SOEC, is the determining factor in controlling these electrochemical reactions. This insight enables the establishment of a reaction phase diagram that identifies the temperature-dependent operating voltage window where both Ni oxidation and carbon deposition can be avoided. We construct this reaction phase diagram based on the results of experiments and a thermodynamic model, and we validate it at a typical reaction condition (cathode: CO2, 1 atm; anode: air, 1 atm; temperature: 800 °C). As a result, pure CO2 electrolysis in Ni-based cathode supported SOECs is successfully demonstrated without Ni oxidation and carbon deposition.