Hydrogen Gas Sensor Based on Proton-Conducting Clathrate HydrateThis work was supported by the National Research Foundation of Korea (NRF) grant (NRL program: R0A-2005-000-10074-0(2009)) and WCU program: R31-2008-000-10055-0 funded by the Ministry of Education, Science, and Technology (MEST)

Among alternatives to fossil fuels, H2 gas is considered to be one of the most promising energy resources owing to its advantages of being nonpolluting and abundant in nature. In particular, fuel cells are currently receiving much attention for utilization of H2 gas in the development of both stationary and mobile power generators. H2 gas, however, inevitably has some safety concerns owing to the rapid energy conversion above its lower explosive limit of 4% in air. Accordingly, effective H2 gas monitoring systems need to be developed to allow the safe application of various H2-based energy devices. To this end, a number of analytical techniques have been developed in recent years to detect H2 gas and determine its concentration. Among them, electrochemical detection methods including both amperometric and potentiometric detection systems have received much interest. In these approaches, H2 in inert gas (i.e. N2 and Ar) or air is readily detected through the variation in current or voltage, even at low H2 concentrations, and the systems can be simply designed for any dimensions. These electrochemical sensors are generally composed of anodic and cathodic electrodes, where electrochemical reactions occur, and a proton conductor to receive and transport protons generated from the anode. However, for real applications, the main shortcomings of conventional electrochemical sensor systems, namely, the complicated fabrication procedure, poor stability, and high cost, should be overcome. As a potential method, we designed and tested a clathrate hydrate based hydrogen sensor. Clathrate hydrates have been explored as a potential solid proton conductor because of their relatively high conductivities even at low temperature. In particular, Me4NOH·5H2O has attracted much interest as a potential proton conductor because of its relatively high melting temperature (68 8C). In comparison with widely used solid proton conductors such as polymer film and ceramic-based materials, the real interest is placed on the technical and functional advantages that the icelike Me4NOH·5H2O offers. In this study, we seek to answer the following key issues: 1) Can we simplify the preparation procedure by directly using the raw reagent Me4NOH·5H2O itself, without any complex reactions or further treatments? 2) Is it possible to easily tailor the sensor to desired dimensions by synthesizing a bulk solid conductor by a crystallization process from a liquid state at room temperature? 3) Is the adopted clathrate hydrate material more cost-effective than conventional materials, including nafion products in particular? Even though these three main issues have been resolved, significant technical difficulties arising from the icelike features of clathrate hydrates remain for the fabrication of sensing devices. In particular, the catalytic electrode deposition procedure on the conductor is carried out in the organic solvent phase, which induces dimensional instability for Me4NOH·5H2O. In full consideration of this problem, we suggest a new approach to fabricate a H2 electrochemical sensor based on a clathrate hydrate. Utilizing the proposed approach, we successfully detected H2 gas below the lower explosive limit in the amperometric mode using Pt catalyst loaded carbon electrodes. Notably, the suggested method does not require a deposition procedure of the Pt catalyst onto the conductor in the organic phase. The fabricated sensor assembly in the present study is shown schematically in Figure 1. Well-dispersed Pt black in organic phase was coated with a brush uniformly on the surface of a carbon electrode, which was used as the anode. A pristine carbon electrode was used as the cathode. These two electrodes were connected to an ammeter. Liquid Me4NOH·5H2O was placed between the two electrodes, and subsequently solidified at room temperature for use as a proton conductor. As illustrated by the overall fabrication