Diode-Type Hydrogen Sensor Employing Pd-coated Pt Electrodes and Anodized Titania under Gaseous Atmosphere and Its Application to Oil-Deterioration Monitoring
Authors: Hyodo Takeo , Yamanaka Shumon , Yamaguchi Kano , Kawashima Kentaro , Shimizu Yasuhiro , Ueda Taro 
Published: December 2024

Numerous efforts have been directed to developing highly sensitive and selective H2 sensors, because H2, which is the most promising alternative to fossil fuels, is highly flammable and explosive in a wide concentration range (4–75% in air). However, general gas sensors (e.g., semiconductor-type and catalytic combustion-type gas sensors) cannot operate under O2-free atmosphere, because the effective reaction of H2 with negatively-charged oxygen adsorbates on the oxide surface is essential for the large H2 response. The diode-type gas sensors using noble-metal (N) sensing electrodes and an anodized titania film (N/TiO2 sensors) show extremely large H2 response even under oxygen-free atmosphere as well as relatively good selectivity to other inflammable gases, compared with other types of gas sensors [1–4], and thus the diode-type gas sensors have been quite promising as a H2-sensing device for the next generation. Typically, the titania films are fabricated by the anodization of a polished Ti plate at a current density of 50 mA cm−2 in 0.5 M H2SO4 aqueous solution at 20°C, and a pair of N films is deposited on the surface of both the titania film and the Ti plate by magnetron sputtering. The H2-sensing properties of the sensors are generally measured under a forward-bias voltage of 100 mV (N(+)–TiO2–Ti(−)) to H2 balanced with air or N2 (e.g., 5–8000 ppm) under dry or wet atmosphere at elevated temperatures (e.g., 250°C). The use of a Pd film as the N-sensing electrode resulted in quite large H2 response, and the alloying of Pd with Pt for the sensing electrode effectively improved the H2 response and the long-term stability. However, the H2 response of these sensors was too sensitive to a change in oxygen concentration. On the other hand, a Pt/TiO2 sensor showed lower H2 response, and the optimal coating of the Pt-sensing electrode with a slight amount of Au drastically enhanced the magnitude of H2 response in air especially under humidified atmosphere, due to a decrease in the amount of oxygen adsorbates on the Pt electrode. In addition, the sensor also showed quite excellent H2 selectivity against other inflammable gases such as methane, propane and propene. Recently, a thermally oxidized titania film has also attempted to be utilized as an oxide film of diode-type gas sensors to improve the operation stability under any condition.
The N/TiO2 sensors can monitor the deterioration of oils, too. The deterioration of oils increases its acidity, which decreases the performance and lifetime of various machines and instruments. The acidity is generally evaluated as total acid number (TAN, the amount of KOH in milligrams, which is needed to neutralize all acid components in 1 g of oil) by batch process such as titration, and thus the new monitoring devices is required to measure TAN inline in real time. We have already demonstrated that ion-selective field-effect transistor (IS-FET)-based devices coated with a cation-conducting polymer can monitor TANs of deteriorated oils [5]. In addition, we have recently attempted to monitor the deterioration of oils by the diode-type N/TiO2 sensors [6]. For example, the Pt/TiO2 sensor clearly showed nonlinear current (I)–voltage (V) characteristics not only under gaseous atmosphere but also in the oils at 30°C, and the magnitude of current monotonically increased with an increase in TANs of the oils at forward-bias voltages of +0.8 V and +1.0 V. These results obviously indicate that the output of the Pt/TiO2 sensor is well correlated to TANs of the deteriorated oils.

T. Hyodo, T. Yamashita, Y. Shimizu, Sens. Actuators B
207 (2015) 105–116. doi: 10.1016/j.snb.2014.10.005.

T. Hyodo, W. Sakata, T. Ueda, Y. Shimizu, ECS Sens. Plus
1 (2022) Article No. 013602. doi: 10.1149/2754-2726/ac5b9f.

T. Hyodo, T. Okusa, W. Sakata, T. Ueda, Y. Shimizu, ACS Sens.
8 (2023) 51–60. doi: 10.1021/acssensors.2c01702.

T. Ueda, T. Kurano, T. Hyodo, M.i Hamano, H. Taka, Y. Shimizu, Chem. Sens.
39(B) (2023) 22–24.

T. Hyodo, M. Yuto, H. Tanigawa, M. Tsuruoka, H. Sakamoto, T. Ueda, K. Kamada, Y. Shimizu, ECS Trans.
98(12) (2020) 59–66. doi: 10.1149/09812.0059ecst.

S. Yamanaka, T. Ueda, M. Kasai, T. Hyodo, and Y. Shimizu, .Abstract of The 19th International Meeting on Chemical Sensors (IMCS2023), Oct. 4–8, Changchun, China (2023) p. 19.