Operation-state-regulated reaction pathway selection in mixed ionic conductor–based protonic ceramic electrolysis cells

Protonic ceramic electrolytes represented by BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ (BZCYYb) exhibit both protonic and oxide-ion conductivity at intermediate temperatures; however, the mechanisms governing ionic transport selection during practical electrochemical operation remain unclear. In this study, a protonic ceramic electrolysis cell employing BZCYYb as the electrolyte was constructed. While maintaining an identical material system and device architecture, two fundamentally distinct electrochemical operating modes were achieved within the same device by synergistically regulating the polarity of the applied electric field and the reactant composition at the fuel electrode. Under the proton-dominated water electrolysis mode, the cell exhibits characteristic intermediate-temperature electrolysis behavior. In contrast, upon polarity reversal and the introduction of CO at the fuel electrode, the system transitions to a CO electrochemical oxidation mode involving oxide-ion participation, accompanied by corresponding changes in electrochemical behavior and product characteristics. Thermodynamic analysis combined with polarity-reversal control experiments demonstrates that the electrochemical potential boundary can selectively reinforce a specific reaction pathway, leading to a clear operation-state-dependent dominant mechanism. These findings indicate that the dominant charge carrier type and the spatial location of electrochemical reactions in mixed ionic conductors are not determined solely by intrinsic material properties or gas atmospheres, but can instead be regulated through electrochemical potential boundaries established under specific operating states. As a result, mixed ionic conduction can be transformed from a passive material attribute into a functional characteristic that is actively selectable and exploitable, providing new design insights for realizing multiple reaction pathways and switchable functionalities within a single electrochemical device.