(Invited) Design of Three-Dimensional Reaction Sites of Solid-Electrolyte Gas Sensors for Sensitive Detection of VOCs
Authors: Ueda Taro , Hyodo Takeo 
Published: December 2024

The exhaled breath of patients contains a higher concentration of specific volatile organic compounds (VOCs) than that of healthy peoples, and thus the development of highly sensitive gas sensors for these gases enables the early detection of these diseases. Among the various gas sensors, we focused on solid-electrolyte gas sensors consisting of yttria-stabilized zirconia (YSZ) as an oxide-ion conductor, and we previously reported that the toluene response of YSZ-based gas sensors improved by the addition of CeO2 to the Au sensing electrode (SE) [1].
The toluene-response mechanism of the YSZ-based gas sensors is explained on the basis of the mixed potential theory. The sensor output voltage (E) is the potential difference between the SE and the counter electrode (CE). The SE potential of the sensor negatively shifted by the concurrent electrochemical reactions of toluene oxidation (eq. 1) and oxygen reduction (eq. 2) at the interface between YSZ and SE (triple-phase boundaries, TPBs) with the same rate upon exposure to toluene balanced with air.
C7H8 + 18O2− → 7CO2 + 4H2O + 36e− (1)
2O2− → O2 + 4e− (2)
On the other hand, a given amount of toluene is oxidized during gas diffusion in the SE, which decreases the concentration of toluene reaching the TPBs. Since this reduces the toluene response of the sensor, thin-film SEs were fabricated in the thickness range of 30–250 nm by using spin-coating method employing a solution containing Au and Ce components in order to increase toluene concentration at the TPBs. However, the toluene response of the fabricated sensors largely increased with an increase in the SE thickness when the additive amount of CeO2 was relatively high [2–3]. The obtained results suggested that the electrochemical reactions of the toluene proceeded not only at the interface (SE/YSZ/gas) but also at the interface between SE and gas (SE(Au/CeO2)/gas) in the SE, because CeO2 worked as an oxide-ion conductor at elevated temperatures.
In this study, we focused on the design of three-dimensional reaction sites of YSZ-based gas sensors for sensitive detection of VOCs. We fabricated Au SEs containing a sufficient amount of CeO2 (n) by spin-coating method. The obtained SEs were denoted as Au-nCeO2(x) SEs (n: an additive amount of CeO2, 24, 32 (wt%), x: the number of spin-coating cycles, 5–15).
Figure 1(a) shows the response transient to various concentrations of toluene of the Au-24CeO2(5) sensor in dry air. The E value decreased upon exposure to toluene, and the magnitude of the response (ΔE) negatively increased with an increase in the toluene concentration. Figure 1(b) shows the thickness (spin-coating cycle) dependences of ΔE to 50 ppm toluene of the fabricated sensors in dry air. The ΔE value of the Au-24CeO2(x) sensor increased with a decrease in the SE thickness, and the Au-24CeO2(5) sensor showed the largest response (ca. 256 mV). In contrast, the ΔE of the Au-32CeO2(x) sensor increased with an increase in the SE thickness. In addition, we performed electrochemical impedance measurements in order to quantify the number of TPBs, and we discussed the effects of the number of TPBs on the toluene response. The details of this will be explained in the presentation.
References
[1] Ueda, H. Abe, K. Kamada, S. R. Bishop, H. L. Tuller,T. Hyodo, Y. Shimizu, Enhanced Sensing Response of Solid-Electrolyte Gas Sensors to Toluene: Role of Composite Au/Metal Oxide Sensing Electrode, Sensors and Actuators B, 252, 268‒276 (2017); https://doi.org/10.1016/j.snb.2017.05.172
[2] Oide, T. Ueda, K. Kamada, T. Hyodo, Y. Shimizu,Toluene-Sensing Properties of Mixed-Potential Type Yttria-Stabilized Zirconia-Based Gas Sensors Attached with Thin CeO2-Added Au Electrodes, Analytical Sciences, 36, 287‒290 (2020); doi.org/10.2116/analsci.20C003
[3] T. Ueda, N. Oide, K. Kamada, T. Hyodo, Y. Shimizu,Improved Toluene Response of Mixed-Potential Type YSZ-Based Gas Sensors Using CeO2-Added Au Electrodes, ECS Sensors Plus, 1, 013604 (2022); doi.org/10.1149/2754-2726/ac63d2

Figure 1