Electrochemical detection of volatile organic compounds using a Na ₃Zr₂Si₂PO₁₂/Bi₂Cu _0.1V_0.9O_5.35 heterojunction device

A fast sodium ion conductor, NASICON (Na₃Zr₂Si ₂PO₁₂), has been widely used for gas sensor applications. In this study, we demonstrate that a device combining NASICON with an oxygen-ion conductor of BiCuVOx (Bi₂Cu_0.1V_0.9O _5.35) can electrochemically detect volatile organic compounds (VOCs), such as ethanol, formaldehyde, and toluene. The sensing electrode made of BiCuVOx was attached onto a sintered NASICON disk at high temperature to produce an interfacial layer that had a different morphology and composition from those of NASICON and BiCuVOx, as observed by scanning electron microscopy-energy dispersive X-ray spectroscopy analysis. The device in which NASICON was fitted with the BiCuVOx-based electrode was found to efficiently detect VOCs in ppm concentrations. The sensor signal (electromotive force) exceeded 100 mV in response to 10 ppm HCOH at 400 °C, demonstrating the high sensitivity of the device. It also exhibited a relatively quick response, reproducible and stable sensor signals, and high selectivity to VOCs. The sensor responses followed behavior typical for mixed-potential-type gas sensors based on oxygen-ion conductors. It was thus suggested that the electrochemical oxidation of VOCs with oxide ions took place at the interfacial oxygen ion-conductive layer that was formed by the reaction of NASICON with BiCuVOx.