Fiber-optic hydrogen sensors


The Hydrogen sensor project of VU University Amsterdam is part of the  "Sustainable hydrogen program" of ACTS.

Goal of the project is the development of sensitive, selective and fast indicator materials for optical fiber hydrogen sensors. These coatings will be based on the "switchable" metal-hydride materials discovered at the condensed matter group of the VU. These "switchable mirrors" are based on materials involving alloys of rare earths, magnesium and some transition metals, whose optical proportions depend drastically on the amount of absorbed hydrogen. Fiber optics as read-out is selected, as it is intrinsically explosion-safe and easy to implement in a multi-sensor safety system.
Switchable mirrors:

Rare-earth or transition metal based Mg alloys undergo a transition from metal to semiconductor when hydrogen is absorbed in the lattice. As a result the alloy changes optically from reflective to transparent or in some cases to a light absorbing black state.

A mirror consisting of a Mg70Ti30 thin film layer After applying a hydrogen containing gas the mirror changes into a light absorbing Mg70Ti30Hx layer.

Our goal is to develop a "metal-hydride switchable mirror" which is used as a safety detector in a future hydrogen economy.

hydrogen car
A typical application for (fiber-optic.) metal-hydride based sensors. Multiple sensors could monitor optically the safety at several locations in a hydrogen powered car.

We aim to detect 10% of the lower explosion limit in air, (which is 4%) within seconds and with an optical change of a factor 10. The detector should allow repeated use.
Device architecture:

Our best hydrogen detection material is Mg70Ti30. This alloy forms a strongly light absorbing metal-hydride when hydrogen exceeds the equilibrium hydrogenation pressure of 0.4 mbar. The ideal architecture of the sensing layer and its optical response is calculated from the dielectric constants of the used metals [1].

Calculated effect of the layer thickness on the optical response of the metal-hydride film (Mg70Ti30-Hx) as a micro-mirror hydrogen detector. The optical response vs wavelength in the whole visible spectrum for a Mg70Ti30 layer of 60 nm, during hydrogen loading.
The Mg-Ti alloy layer is covered by a 30 nm thick Pd layer which promotes the hydrogen uptake of the detector and prevents the layer from oxidation.

Typical configuration of  a fiber-optic micro-mirror hydrogen sensor, based on a metal-hydride. The  catalytic Pd layer can be covered with an extra  protective coating.

Device preparation:

The Mg-Ti detection layer and the Pd cap layer are deposited on freshly cleaved multi mode glass fibers. By using glass fibers no electrical  leads are needed near the sensing point in a potentially explosive environment. In a safety application multiple fiber detectors can be read by a single set of light source and detector. Deposition of the layers is done by magnetron sputtering in argon. The composition and thickness are verified with a stylus profiler and Rutherford Backscattering Spectrometry.

 Argon-plasma magnetron sputtering apparatus for depositing thin metal films on glass fibers  SEM image of a cleaved end of a glass fiber, after the deposition of the "switchable-mirror" layers.


We connect the detector to a standard bifurcating fiber which guides light from a white light source to the indicator layer and guides the reflected light to a CCD spectrometer. A low cost readout can be build from a red LED, a CD-player beam splitter and a PIN diode.

Setup for characterizing the hydrogen detectors. Light is guided via a bifurcator to the fiber end. The reflected light is measured by a CCD spectrometer. The environment of the fiber end can be temperature controlled. A typical response of a Mg70Ti30 based detector. When a small concentration hydrogen is mixed with the buffer gas (Argon) the reflection of the detector layer drops by a factor 10 within seconds. 

The detector regenerates to its original state when the hydrogen concentration drops below the equilibrium pressure of the hydride. The unloading rate of the detector increases when oxygen is present. The large optical change upon hydrogen loading allows the use of alarm levels in a hydrogen detector application, which improves the stability and reaction speed of the application.

The hydrogen detector maintains its optical properties and reaction speed after many hydrogen loading / unloading cycles.

Over 100 stable cycles are measured, which is more than enough for a safety device. Oxidation of the detection layer due to cycling stress can be reduced by using a thin NbOx or AlOx layer between the Pd layer and the Mg70Ti30 layer.

Device improvements:

The detection of low concentrations hydrogen is within seconds. The detector functions (unlike commercially available detectors) well in oxygen poor environments like argon glove boxes and over a large temperature range.

The hydrogen detector is characterized for low concentrations hydrogen in a broad temperature range; it still operates below the freezing point of water.

Oxygen en carbon oxides in the environment of the detector can lower the sensitivity and the kinetics due to surface reactions on the catalytic Pd layer. These effects are reduced by alloying the Pd layer with for example Ag [2]. A hydrophobic organic coating on top of the Pd layer prevents the detector from degradation by moist when the detector is stored or used for a long period in air.

By alloying the catalytic Palladium layer with Silver it becomes less sensitive for oxygen and carbon molecules on the surface of the detector. The left side of the mirror is coated with a hydrophobic coating, which prevents the metal from degradation by the moist in the air.


A series of fiber optic hydrogen detector prototypes together with a small readout system is currently spread among 10 hydrogen research laboratories in Europe. They will test the behaviour of the detector in a variety of conditions like in argon glove boxes or in polluted environments.

First prototype of the fiber-optic hydrogen detector. Readout box for the field test of the fiber-optic hydrogen detector.

So far we observe a reproducible detection of pure hydrogen in air during a test period of several months. Our research will now focus on developing a sensor for quantitative reading of the hydrogen concentration in air.

First results of the metal-hydride based fiber-optic hydrogen detector field test. The detector is tested for a period of months in air, several times a small cloud of hydrogen was released near the fiber-end. The drop in reflection in the unloaded state was caused by a drift in the readout box.


[1]. M. Slaman, B. Dam, M. Pasturel, D.M. Borsa, H. Schreuders, J.H. Rector, R. Griessen, Fiber optic hydrogen detectors containing Mg-based metal hydrides, In press by Sensors & Actuators B: chemical 123, Issue 1, 10 April 2007, Pages 538-545

[2]. M. Slaman, B. Dam, H. Schreuders, R. Griessen, optimization of Mg-based fiber optic hydrogen detectors by alloying the catalyst, Int. J. Hydrogen energy 33 (2008) 1084-1089.

[3]. Patent: Protective coating for metal-hydride based devices, filed in march 2006, Inventors: B. Dam, H. Schreuders, M. Slaman and M. Pasturel (owned by ACTS); WO2007126313, NL1031708, (P6007119NL).

[4]. Patent: Optical switching device, filed in September 2005, Inventors: B. Dam, R. Griessen, W. Lohstroh, M. Pasturel and  M. Slaman (owned by ACTS); WO2007049965, NL1030299, EP1952195, CA2627651, AU2006306870, US2008291452, PCT/NL2006/050268 (P6003851NL).

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