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PEM Sensors
While in a fuel cell the membrane electrode assembly (MEA) is used to support the conversion of chemical energy to electrical energy through a reduction-oxidation reaction, LANL?s chemical detection system uses the MEA to foster an electron transfer reaction triggered by the presence of certain chemical agents.
There are two requirements for the assembly to work in this way: first, an external power supply must provide energy to drive the reaction; and second, there has to be a compound present capable of accepting electrons from or donating electrons to the electrodes. The resulting current flow is directly proportional to the amount of the electroactive compound present, and is the basis for quantitative analysis.
Electronics The power supply used to drive electron transfer reactions in electrochemical cells is called a potentiostat, of which there are many commercially available. These instruments are usually interfaced with a desktop computer, resulting in an overall package that is upwards of 100 pounds in weight and several cubic feet in volume. For this reason, researchers have had to develop a miniaturized version of this component.
Work is currently underway to demonstrate a robust, self-contained potentiostat roughly the size of a deck of playing cards and weighing less than 8 ounces. The unit is intended to have an electrical current output range of several orders of magnitude, programmable software, and an integrated alarm to signal the presence of a chemical agent. The unit may also be able to interface with a hand-held computer (like a Palm Pilot) for data collection and analysis.
Chemistry For the detector to respond electrochemically to the presence of a chemical agent, a substance reactive to that agent must be present in the electrode.
The detector's responsiveness to a particular chemical agent is dependent on the presence in the electrode of molecules electrochemically reactive to that agent. The investigation of these molecules is especially challenging because experiments with actual chemical warfare agents are logistically impossible. Instead, researchers use compounds of similar chemical composition, but with lower level s of toxicity. An example is oxydemeton methyl, which is used as a substitute for the nerve agent VX.
Solid-State Sensors Los Alamos researchers are also interested in fabricating solid-state sensing devices for rapid, compact detection of G-type nerve agents (like sarin, soman, and GF.) These sensors will detect nerve agents according to the following steps:
adsorption of the agent from the atmosphere into a thin-layer reactive medium,
catalytic decomposition of the agent within the reactive medium, liberating fluoride ions (F-),
detection of the fluoride ion at the surface of a detector underneath the reactive layer.
Substantial attention has been paid to the fluoride-ion-selective thin film electrode, traditionally the least reliable component in this type of sensor. Researchers have seen promising results using a substrate of oriented silicon, which closely matches the thermal expansion properties of the lanthanum fluoride coating.
Research Objectives
- Test and fine-tune new miniaturized potentiostats
- Continue the characterization of nerve agent surrogates in aqueous solutions using commercially available electrodes
- Optimize the fabrication process of solid-state sensors to improve durability and response reproducibility
- Improve fluoride ion release reaction kinetics through modified polymeric surface structures
Project Goals
- Fabricate a second-generation detector
- Determine the commercial mass production costs of the
refined miniature potentiostat
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