Technical Communication An optical hydrogen sensor based on a Pd-capped Mg thin film wedge V. Palmisano a, *, M. Filippi a,b , A. Baldi a , M. Slaman b,a , H. Schreuders a , B. Dam a a Department of Chemical Engineering, Technical University Delft, Julianalaan 136, 2628 BL Delft, The Netherlands b Department of Physics and Astronomy, Condensed Matter Physics, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands article info Article history: Received 22 June 2010 Received in revised form 1 September 2010 Accepted 1 September 2010 Available online 22 September 2010 Keywords: Optical hydrogen sensor Hydrogen safety Thin film Switchable mirror abstract We report the proof of concept of a thin film device with a one-to-one relationship between the H 2 partial pressure and the lateral progression of a visible optical change along a thin film multilayer 70 mm long. The device basically consists of a sensing Mg layer with a thickness gradient. It exploits the thickness dependence of the hydrogenation thermo- dynamics of Pd-capped Mg thin films. The optical change of the Mg layer during the metalehydride transition can be detected both in reflection and in transmission. This optical sensor allows a continuous measurement of hydrogen partial pressure in the range between 200 and 4000 Pa. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. 1. Introduction The development of high-performance hydrogen sensors is crucial for the successful implementation of hydrogen as a sustainable energy carrier. Hydrogen is characterized by a wide flammability range (4e75 vol%) and a relatively low ignition energy (only 0.017 mJ at room temperature), while its presence is undetectable by human senses. Conventional sensors [1e3], i.e. catalytic, electrochemical and semi- conducting metal-oxide sensors use electrical leads which may induce sparks and provide a cause for ignition in hazardous atmospheres. Optical sensors based on fiber optics allow working in an explosive environment thanks to the possibility of separating the sensing point from the electrical readout. Various strategies have been proposed which basically infer the hydrogen concentration from a change in the optical response, e.g. the interference pattern [4], the frequency of the optical signal [5], or in the intensity of the optical signal [6,7]. Following the discovery that due to the absorption of hydrogen some metals expe- rience a metaleinsulator transition characterized by a large change in optical reflectance or transmittance [8,9], our group in Amsterdam developed a fiber optic hydrogen detector based on a sensing layer made of Mg 0.70 Ti 0.30 [10]. This detector satisfies the requirements of having a high contrast, small dimensions, low cost and being safe to operate. However, it only indicates whether the hydrogen pressure is above or below a certain threshold level, which is given by the equilibrium pressure of the Mg 0.7 Ti 0.3 detection layer. * Corresponding author. Tel.: þ31 15278 2676; fax: þ31 15278 7421. E-mail address: v.palmisano@tudelft.nl (V. Palmisano). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 35 (2010) 12574 e12578 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.09.001