Electrochemical Detection of Hydrogen Using Two-Dimensional Carbon Nanosheets
Authors: Miyamoto Azumi , Kuwaki Yuta , Hatakeyama Kazuto , Armando Quitain , Sasaki Mitsuru , Matsumoto Yasumichi , Kida Tetsuya 
Published: February 2020

1. IntroductionTowards a smooth transition to a hydrogen energy society, it has become necessary to develop hydrogen production routes with cheap and lower environmental impacts. There have been many attempts to produce hydrogen by new methods including water electrolysis, biomass conversion, water splitting by sunlight, etc. For safety use of hydrogen, in-situ detection of hydrogen is also important. So far, a large number of hydrogen sensors have been developed for hydrogen leakage detection. Among them, resistive-type gas sensors are promising because of its high sensitivity. However, their major drawback is high energy consumption because of the high temperature operation. To tackle this problem, we tried to detect hydrogen using a room-temperature operable proton-conducting membrane. Here, we focused on graphene oxide (GO), two-dimensional carbon nanosheets. GO films have been demonstrated to show proton-conducting properties at room temperature, depending on synthesis temperature [1]. They have been successfully applied to fuel cell applications. One advantage of GO membranes over conventional Nafion membranes is that they show proton conductivity even under lower humid conditions because of the presence of water in the interlayer. The proton conductivities can also be improved by doping sulfate anions [2].
In this study, we studied the hydrogen sensing properties of GO membranes using a concentration-type cell. Then, their sensitivities to ethanol and carbon monoxide were examined to demonstrate the wide applicability of GO membranes to sense combustible gases.
2. ExperimentalWe produced a self-supporting membrane from a graphene oxide suspension that was made from graphite powders by vacuum filtration. By sandwiching the membrane with platinum meshs, we fabricated a sensor device with a concentration-type cell structure. Electromotive forces (EMFs) of the device were measured as sensor responses using an electrometer. In this measurement, synthetic air was introduced into one side of the membrane as a reference gas. Meanwhile, hydrogen, CO, and C2H5OH in ppm concentrations (200-1000 ppm) in air were introduced into the opposite side of the membrane, and changes in electromotive force were measured. We also investigated an effect of the surface state on the sensor response by coating the surface with platinum or conductive carbon.
3. Results and discussionElectromotive force of the graphene oxide (GO) membrane was found to respond monotonically to changes in the partial pressure of combustible gases. Good sensor response to hydrogen was observed at room temperature. However, the sensor response to carbon monoxide was not observed when the humidity was very low. It is suggested that the presence of water vapor is essential for the reaction of carbon monoxide to proceed on graphene oxide according to the following reaction:
CO + H2O　↔ CO2 + 2H+ + 2e-

We found that the sensor response became faster when a platinum/carbon electrode was coated on the membrane surface, suggesting that the electrochemical reaction was accelerated by the Pt electrode. On the other hand, for a conductive carbon electrode, the sensor response became larger but drastically slow. It is probable that careful control of the electrode microstructure would improve the sensor performance. The GO sensor also showed good responses to ethanol at room temperature even in a dry environment, suggesting that water is not required in the ethanol oxidation reaction as shown below:
C2H5OH + 1/2O2 ↔ 2CO2 + 6H+ + 6e-

In summary, we demonstrate that GO membranes exhibit good sensing properties to combustible gases, indicating that surface electrochemical reactions efficiently proceed on the surface of proton-conducting GO even at room temperature.
4. References1. K. Hatakeyama, H. Tateishi, T. Taniguchi, M. Koinuma, T. Kida, S. Hayami, H. Yokoi, and Y. Matsumoto, Chem. Mater., 26, 5598-5604 (2014).
2. K. Hatakeyama, Md. S. Islam, M. Koinuma, C. Ogata, T. Taniguchi, A. Funatsu, T. Kida, S. Hayami, Y. Matsumoto, J. Mater. Chem. A, 3, 20892-20895 (2015).