Enhanced CO Response of NASICON-based Gas Sensors Using Oxide-added Pt Sensing Electrode at Low Temperature Operation

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NASICON (Na₃Zr₂Si₂PO₁₂)-based solid electrolyte-type sensors equipped with various metal oxides (MO)-added Pt sensing electrode (SE, Pt(nMO) (n: MO additive amount in wt%) and Pt counter electrode (CE, Pt) on the same side of the NASICON disc were fabricated and their (Pt(nMO)/Pt sensors) CO-sensing properties were examined at 25–300°C. The Pt(15Bi₂O₃)/Pt sensor showed the largest CO response with a change in electromotive force to a positive direction (positive response) at 25°C, while the Pt(15CeO₂)/Pt sensor showed the largest negative CO response at 25°C. The CO response of the Pt(15CeO₂)/Pt sensor seems to be determined by mixed potential at the triple phase boundaries (TPBs) containing the electrochemical reactions of CO oxidation and oxygen reduction. X-ray photoelectron spectroscopy of the Pt(15Bi₂O₃) SE before and after exposure to CO indicated a slight reduction of Bi³⁺ after the exposure to CO. Therefore, the additional electrochemical reactions containing the reduction of Bi₂O₃ were anticipated to occur at the TPBs of the Pt(15Bi₂O₃) SE, which resulted in the large positive CO response of the Pt(15Bi₂O₃)/Pt sensor. Furthermore, the addition of 15 wt% CeO₂ to Pt CE of the Pt(15Bi₂O₃)/Pt sensor largely enhanced the magnitude of CO response and attained relatively excellent CO selectivity against H₂.