Second International Symposium «Safety and Economy of Hydrogen Transport»
IFSSEHT-2003
NEW MATERIALS FOR HYDROGEN SENSORS BASED ON ELECTROACTIVE POLYMERS AND CARBON NANOTUBES
V. V. Abalyaeva1, O. N. Efimov1, A. L. Gusev2
1 Institute of Problems of Chemical Physics RAS, Chernogolovka, Moscow region, 142432 Russia Phone/fax: 7-095-5221887, e-mail: efimov@icp.ac.ru 2 Russian Federal Nuclear Center — All-Russian Scientific Research Institute of Experimental Physics (RFNC-VNIIEF), Sarov
Because of the rapid development of hydrogen power engineering, the development of materials and sensors for detection and monitoring of hydrogen in closed shells on its storage either in liquid or adsorbed state has become urgent. Though there are many conventional hydrogen sensors, the interest in the design of efficient and simple to operate devices is steadily growing. Such devices are amperometric sensors based on the reaction of hydrogen with Pd. The advantage of these sensors is the linear relationship between the current and the hydrogen concentration. Palladium as a thin foil and palladium coatings on conducting supports (carbon, nickel grid) are conventionally used as working electrode materials. However, the materials are degraded because of high hydrogen solubility in palladium and resulting changes in volume. Promising materials to be used in amperometric sensors are electroactive polymers, which possess electronic and ionic conductivity and high permeability to hydrogen. The most appropriate method is Pd inclusion in the polyaniline (PAn) matrix:
1-Y.
where Y depends on oxidation state and X is defined by a polymer chain length, which can consist of hundreds and thousands of repeated units. Polymer oxidation is accompanied by the addition of protons to nitrogen atoms (Y decreases) and the compensation of positive charge appearing in the polymer chain by PdCl3- intercalation in the polymer matrix. In electrochemical reduction of PAn the anions are reduced to high dispersed metal. This provides lower consumption of noble metal and allows one to use non-noble metal or glassy carbon supports as electric contacts.
Specific requirements of hydrogen sensors in cryogenic plants are high sensitivity at low temperatures and stable operation in vacuum. Therefore, we chose a solid polymer electrolyte based on polyvinyl alcohol and phosphoric acid.
Two techniques of palladium insertion in the polymer matrix were used to prepare a chemically modified electrode (CME).
In the electrochemical method, PdCl2 was added to the electrolyte (0.4 M aniline sulfate in 0.1 M H2SO4) and the synthesis was performed by potential scanning
(Fig. 1, curve 3). At potential scanning from -0.15 up to +0.75 V (with respect to the AgCl reference electrode) the growth of PAn coating on the Ti electrode surface was observed. The coating comprised Pd, and the PdCl3-anion was involved as a counter ion in the positively charged polymer matrix. On reverse potential scanning to cathode potentials the salt was reduced in the polymer bulk to high dispersed metal.
In the second case polyaniline was electrosynthesized with no PdCl2 in the electrolyte and the synthesis was accomplished by keeping the polymer at -0.15 V. This resulted in the synthesis of the polymer in the reduced form. On impregnating such an electrode with the PdCI2 solution, the salt was chemically reduced to metal as a result of its reaction with the polymer (Fig. 1, curve 2).
i, mA
0,12
0,10
0,04
0,02
y A
- 9
/
■
■
3
■ JC*
0 20 4 0 6 0 8 ) 1E 0 %
Fig. 1. Current value as a function hydrogen concentration in the inter-electrode space
The prepared CME (PAn- Pd) was assembled with a counter electrode, namely, a titanium plate. The solid polymer electrolyte was sandwiched between them. The electrode assembly was introduced in a teflon tube, which the H2-Ar mixture was passed through. The dependence of current on H2 content was measured at 0.5 V between the electrodes. The dependencies were linear at 10-50 0C and 0-100% of H2.
We showed the possibility of applying palladinized multi-walled carbon nanotubes (MWNT) grown on titanium nitride plates as working electrodes. The coatings were deposited by hydrocarbon pyrolysis in the presence of an Fe-containing catalyst. Two types of electrodes were used: those with preliminary deposition of PAn on
Hydrogen detectors
MWNT followed by the impregnation with the PdCI2 solution and the electrodes with direct palladium insertion into MWNT (Fig.1, curve 1). The study of the surface of electrode 1 using an X-ray microanalyzer showed that the felt-like coating consists of twisted bundles of tubes 0.1-0.2 mm in diameter, and Pd is located mainly on the bundles. The Pd content was controlled by varying impregnation time. The analysis of the dependence of hydrogen concentration on current shows that the electrodes of the 2nd type are much more active than the PAn-Pd ones. Activity of the MWNT-Pd electrodes impregnated for 24 hours was approximately two orders of magnitude higher that that of the PAn-Pd ones.
Thus, new materials based on electroactive polymers and carbon nanotubes are promising ones, which can be used in the design of efficient easy operating hydrogen sensors.
The work was supported by International Science and Technology Center (Project No 1580).
Fig. 2. Pd-K3-MNT Sensor surface image Pd-coated cords are seen
ISJAEE Special issue (2003)
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