This application claims the benefit of priority under 35 U.S.C. §119 of German Patent Application DE 10 2010 021 977.0 filed May 28, 2010, the entire contents of which are incorporated herein by reference.
The present invention pertains to an electrochemical gas sensor for detecting ozone or nitrogen dioxide.
A gas sensor for determining SO2 or H2S, which contains a measuring electrode, which has carbon nanotubes, is known from DE 10 2006 014 713 B3. The electrolyte contains a mediator compound based on transition metal salts, with which selective determination of the desired gas component is possible.
The mediator compounds are compounds that have at least one more group, selected from among hydroxyl and acid groups, besides at least one acid group. The mediator compound is, in particular, a carboxylic acid salt, which has, besides the carboxylic acid group, at least one hydroxyl group, preferably at least two hydroxyl groups, and/or at least one more carboxylic acid group. Suitable compounds are also tetraborates, such as sodium tetraborate or lithium tetraborate. Transition metal salts, especially Cu salts, of such mediators make possible the selective determination of SO2.
A measuring device described in US 2005/0230 270 A1 contains a microelectrode array consisting of carbon nanotubes to detect substances in liquid or gaseous samples.
The basic object of the present invention is to provide a gas sensor for detecting ozone or nitrogen dioxide.
According to the invention, an electrochemical gas sensor is provided for detecting ozone or nitrogen dioxide in a gas sample. The electrochemical gas sensor has a measuring electrode containing carbon nanotubes and a counterelectrode in an electrolyte solution. The electrolyte solution has lithium chloride or lithium bromide.
According to another aspect of the invention, a method of electrochemical gas sensing is provided. The method comprises providing an electrochemical gas sensor with a measuring electrode formed of carbon nanotubes (CNT) an electrolyte, which contains lithium chloride or lithium bromide in an aqueous solution and a counterelectrode. The measuring electrode and the counterelectrode are in contact with the electrolyte. The method further comprises detecting ozone or nitrogen dioxide with the electrochemical gas sensor.
It was surprisingly found that the gases ozone and nitrogen dioxide can be detected at a high sensitivity with a measuring electrode consisting of carbon nanotubes (CNT) combined with an aqueous electrolyte, which contains lithium chloride or lithium bromide, while changes in temperature and humidity have only a minor effect on the measured signal.
The reaction equations are:
O3+2e−+2H+→O2+H2O
NO2+2e−+2H+→NO+H2O.
Measuring electrodes manufactured from carbon nanotubes (CNT) are stable over a long time and can be integrated in existing sensor constructions in a simple manner. Carbon nanotubes are structurally related to the fullerenes, which can be prepared, e.g., by evaporating carbon according to a laser evaporation method. A single-walled carbon nanotube has, for example, a diameter of about one nm and a length of about a thousand nm. Besides single-walled carbon nanotubes, double-walled carbon nanotubes (DW CNT) and structures having multiple walls (MW CNT) are known as well. The layer thickness of the electrode material in the finished electrode is in a range of 0.5 μm to 500 μm and preferably 10-50 μm in measuring electrodes made of carbon nanotubes (CNT).
A measuring electrode manufactured from multiwall carbon nanotubes (MW CNT) yields especially good results.
Due to their manufacture, carbon nanotubes are provided with metal atoms, e.g., Fe, Ni, Co, including the oxides thereof, so that such carbon nanotubes on measuring electrodes possess catalytic activities. It proved to be advantageous to remove these metal particles by acid treatment.
The carbon nanotubes are advantageously applied to a porous carrier, a nonwoven material or a diffusion membrane. The carbon nanotubes are fitted together here by self-aggregation or with a binder. PTFE powder is preferably used as the binder.
It is especially advantageous to manufacture the carbon nanotubes from a prefabricated film, a so-called “buckypaper.” The measuring electrode can then be punched directly out of the buckypaper. Large quantities can be manufactured in this manner in a cost-effective manner.
The measuring cell has openings, which are provided with a membrane permeable to the analyte and otherwise seal the measuring cell towards the outside. The electrochemical cell contains at least one measuring electrode and a counterelectrode, which may be arranged coplanarly, plane-parallel or radially in relation to one another and are each flat. In addition to the counterelectrode, a reference electrode may be present. A separator, which maintains the electrodes at spaced locations from one another and is impregnated with the electrolyte, is located between the plane parallel electrodes.
The electrode materials used for the reference electrode may be precious metals such as platinum or iridium, carbon nanotubes or an electrode of a second type, which consists of a metal that is at equilibrium with a poorly soluble metal salt.
The counterelectrode preferably consists of a precious metal, e.g., gold, platinum or iridium/iridium oxide or carbon nanotubes or a consumable electrode consisting of silver, lead or nickel.
Alkali or alkaline earth metal halides, preferably chlorides or bromides, which are preferably hygroscopic in an aqueous solution, are used as supporting electrolytes.
The pH value of the electrolyte is preferably stabilized with a buffer. Especially advantageous formulas are an aqueous LiCl solution or an aqueous LiCl solution with saturated calcium carbonate CaCO3 as a solid solute, as well as an aqueous LiBr solution, or an aqueous LiBr solution with saturated calcium carbonate CaCO3 as solid solute. Calcium carbonate is used as a pH stabilizer for the electrolyte solution. Other alkaline earth carbonates, such as magnesium carbonate or barium carbonate, which are also expressly covered by the scope of protection, are also suitable for use as pH stabilizers as an alternative.
An advantageous use of an electrochemical gas sensor, which has a measuring electrode consisting of carbon nanotubes (CNT) and a counterelectrode in an electrolyte, which contains lithium chloride or lithium bromide in aqueous solution, is in the detection of ozone or nitrogen dioxide in a gas sample. Preferred materials for the measuring electrode are multiwalled carbon nanotubes (MW CNT). Especially preferred electrolytes are, besides the aqueous LiCl solution, an aqueous LiCl solution with saturated CaCO3 as a solid solute or an aqueous LiBr solution with saturated CaCO3 as a solid solute.
An exemplary embodiment of the gas sensor according to the present invention is shown in the figures and will be explained in more detail below. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.
In the drawings:
Referring to the drawings in particular,
While specific embodiments of the invention have been described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.
Number | Date | Country | Kind |
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10 2010 021 977.0 | May 2010 | DE | national |