Electrolyte‐Dependent Electrochemical Properties of Polycrystalline Cerium Oxide Films

Cerium oxide–based materials have attracted significant attention for electrochemical applications because of their redox properties. However, the selection of a suitable electrolyte for a specific electrochemical reaction must ensure sufficient ionic conductivity while preserving the electrode surface in its initial state throughout operation. In this article, a polycrystalline compact cerium dioxide (pCeO ₂ ) film deposited on glassy carbon by non‐reactive magnetron sputtering was studied as an electrode material to elucidate the influence of electrolyte composition on its electrochemical properties. Electrochemical measurements were performed in neutral electrolytes—phosphate‐buffered saline (PBS), citrate buffer and potassium chloride (KCl)—using cyclic voltammetry and chronoamperometry (CA), while surface chemistry was characterised by X‐ray photoelectron spectroscopy and resonant photoelectron spectroscopy. Electrochemical cycling in PBS induced the surface reduction of pCeO ₂ , which is partially attributed to the phosphate‐mediated stabilisation of Ce ³⁺ species that suppresses re‐oxidation. The electrochemical response in PBS was strongly concentration‐dependent: low concentrations (≤10 mM) led to a progressive increase of the integrated charge, whereas higher concentrations (100 mM) caused electrode deactivation. A possible mechanism for surface passivation by phosphates is proposed. In citrate buffer, prolonged cycling promoted surface reduction, likely accompanied by citrate accumulation on the electrode surface. In contrast, the pCeO ₂ electrode showed a markedly lower current density in KCl. These results demonstrate that the electrochemical properties of pCeO ₂ films are strongly influenced by electrolyte type, concentration and cycling conditions, providing important guidance for optimising the stability and functionality of ceria‐based materials in electrochemical sensing and catalytic applications.