JPH03195965A - Electrochemical sensor - Google Patents

Abstract

PURPOSE:To increase an effective length which takes part in electrochemical reactions in a working electrode and improve a sensor sensitiveness by forming projections and recesses on and in, respectively, the surface of at least the working electrode, arranging electrodes such that counter electrodes are positioned as opposed to a plurality of the sides of the working electrode while surrounding it as a center and reference electrodes are positioned as sandwiched by the working electrodes and the counter electrodes therebetween. CONSTITUTION:A detecting gas which is transmitted and reaches the reaction portion 20 of a working electrode causes electrochemical oxidation and reduction reactions on the surface of the projected and recessed reaction portion 20. Then, ions generated by the reactions are transmitted via the electrolyte layer 5 of the opposed portion of the working electrode 2 and a counter electrode 3. That is, the substantial effective area of the reaction portion 20 which takes part in the electrochemical reactions has a wide range along the three-sides of the reaction portion 20. Therefore, an effective length which takes part in the electrochemical reactions on the working electrode 2 is increased. As a result, a sensor sensitiveness can be improved.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an electrochemical sensor, and more specifically, an electrolytic type electrochemical sensor that detects or quantifies a specific gas component using an electrolytic reaction. It is related to.

[Conventional technology]

Using electrochemical redox reactions, gases in the atmosphere,
For example - carbon oxides, hydrogen, alcohols, nitrogen oxides,
Many electrochemical sensors that detect sulfur oxides and the like have been reported so far. - This type of gas sensor is used in the field of industrial gas concentration detectors and the like because it has high gas sensitivity for boat fishing.

In recent years, in electrochemical sensors, gas sensors using solid polymer electrolytes such as sulfonated perfluorocarbons as electrolytes for ion (proton) conduction provided between electrodes have been studied.
It is disclosed in Japanese Patent No. 5293 and the like.

The basic structure of this sensor is the same as previous gas sensors that used a liquid electrolyte, including a sensing electrode, a reference electrode, and a counter electrode, but it uses a solid electrolyte instead of a liquid electrolyte. It is characterized by the fact that As described above, electrochemical sensors using solid electrolytes have the advantage that sensor elements that are smaller and cheaper can be manufactured than those using liquid electrolytes.

Furthermore, the inventors changed the structure of the sensor element from the opposed electrode structure in which the sensing electrode, the reference electrode, and the counter electrode face each other with a solid electrolyte in between, as in the prior art described above, to the planar electrode structure. In other words, by creating a structure in which three electrodes are formed on the same plane of one substrate and a solid electrolyte is placed on top of them, thin film formation technology and printed circuit formation technology currently used in the semiconductor manufacturing field etc. , we thought that photolithography technology could be used and that it would be possible to promote miniaturization and simplification of manufacturing, so we developed such a planar electrode configuration, that is, a planar electrochemical sensor, and first applied for a patent application. Patent applications have been filed in No. 62-316419, etc.

[Problem to be solved by the invention]

However, in the case of a planar electrode configuration using a solid electrolyte, the ionic conductivity of the solid electrolyte is very low, and the working electrode and counter electrode are arranged side by side on the same plane. There is a problem in that ions generated by the electrochemical redox reaction often cannot be sufficiently transferred to the counter electrode.

Among these, problems related to the arrangement of the working electrode and the counter electrode will be explained. For example, FIG. 6 shows the structure of a conventional electrochemical sensor, in which a working electrode 2, a reference electrode 4,
Counter electrodes 3 are arranged in order and covered with a solid electrolyte 5. In this case, since the ion conduction from the working electrode 2 to the counter electrode 3 is mainly carried out in the parts facing each other, the effective area of the working electrode 2 that is involved in the substantial electrochemical reaction is indicated by the X mark in the figure. , it is limited only to the vicinity of one side near the counter electrode 3. Therefore, the detection gas sensitivity cannot be obtained to a value commensurate with the area of the working electrode 2, and the sensitivity ends up being lower than the designed value.

As a solution to the above problem, it has been considered to increase the thickness of the solid electrolyte layer 5. That is, the portion of the solid electrolyte layer 5 that forms the ion conduction path between the working electrode 2 and the counter electrode 3 is made thick so that ion conduction can occur quickly. However, simply increasing the thickness of the solid electrolyte layer 5 between the working electrode 2 and the counter electrode 3 only improves the ion conduction of the opposing portion of the working electrode 2 and the counter electrode 3.
cannot effectively utilize the entire surface for electrochemical reactions. Therefore, it is necessary to thicken the entire solid electrolyte layer 5 between the working electrode 2 and the counter electrode 3 and covering the top of the working electrode 2, and to set a wide ion conduction path from the entire surface of the working electrode 2 to the counter electrode 3. .

In this way, although the ion conductivity is improved, the sensing gas passes through the thick solid electrolyte layer 5 and the working electrode 2
Since the detection gas has to reach the surface of the working electrode 2, the diffusion of the detection gas to the surface of the working electrode 2 becomes extremely poor, and there is a problem that the detection gas sensitivity is reduced.

Therefore, an object of the present invention is to solve the problems of the prior art as described above, and to spread the gas over a wide range of the working electrode without inhibiting gas permeation from the solid electrolyte layer to the working electrode surface.
By increasing the effective area involved in gas detection,
An object of the present invention is to provide an electrochemical gas sensor with improved detection gas sensitivity.

[Means to solve the problem]

An electrochemical sensor according to the present invention that solves the above problems has a working electrode disposed on the same surface of an insulating substrate.

In an electrochemical gas sensor element in which a counter electrode and a reference electrode are provided, and a solid electrolyte layer is provided covering each electrode and the electrodes in between, at least the surface of the working electrode has irregularities, and various arrangements are made with the working electrode in the center. A counter electrode is arranged at a position facing the plurality of sides of the working electrode, and a reference electrode is arranged at a position sandwiched between the working electrode and the counter electrode.

The materials used for the insulating substrate, electrodes, and solid electrolyte are the same as those for ordinary electrochemical sensors.

The finer the irregularities formed on the surface of at least the working electrode, the more the surface area of the electrode can be increased, but they are set to an appropriate size, taking into account ease of manufacture. The shape of the unevenness can be a rectangular parallelepiped, cylinder, hemisphere, or any other shape, and the unevenness can be arranged at equal intervals vertically and horizontally, arranged in a staggered pattern, or arranged with partially changed spacing. You can also do it. The unevenness may be created by processing the surface of an electrode formed on a flat insulating substrate into an uneven shape, but it is also possible to form fine unevenness on the surface of the insulating substrate and then form a thin conductive layer along the unevenness. If it is made into an electrode, the same unevenness as that of the insulating substrate will be formed on the surface of the electrode.

Since the working electrode is directly involved in the electrochemical reaction of the sensing gas, if the unevenness is formed only on the working electrode, the purpose of increasing the surface area of the working electrode and improving the reaction of the sensing gas is not achieved. However, if unevenness is also formed on the counter electrode and the reference electrode, the functions of each electrode can be enhanced. In order to facilitate the processing of the unevenness, the entire surface of the insulating substrate may be formed with unevenness.

Next, various arrangements will be explained.

First, the working electrode is arranged with its shape, area, etc. set so as to ensure a sufficient effective area for the detection gas to cause an electrochemical reaction. With this working electrode at the center, a counter electrode is arranged at a position facing the plurality of sides of the working electrode. For example, if the working electrode is in the form of a strip, it should be placed at a position opposite all three sides or two longitudinal sides of the three sides excluding one end that will serve as the terminal for connection with an external circuit. Place the opposite pole. In order to provide a counter electrode facing all three sides of a strip-shaped working electrode, a counter electrode may be provided for each side, but if a counter electrode is provided in a U-shape surrounding the three sides of the working electrode, The structure is simple and wiring connection to external circuits is easy. If the shape of the working electrode changes, the shape and arrangement of the sides will also change, so the shape and arrangement of the counter electrode placed opposite it will also change. When the working electrode has curved sides, the counter electrode is also arranged in a curved shape.

The reference electrode serves as a reference for setting the potential of the working electrode. For this purpose, it is preferable to provide the reference electrode in the ion conduction path from the working electrode to the counter electrode, so the reference electrode is provided at a position sandwiched between the working electrode and the counter electrode.

The solid electrolyte layer is formed to cover the various types arranged as described above and between them. As the material for the solid electrolyte layer, the above-mentioned perfluorosulfonate polymer, polystyrene sulfonic acid, polyvinyl sulfonic acid, and other materials similar to those used in ordinary electrochemical sensors can be used. The solid electrolyte layer should be formed so as to completely cover each type of top, and in places where it covers unevenness such as the working electrode, it should be covered to the extent that part of the tip of the protrusion is exposed above the solid electrolyte layer. You can also do it.

[For production]

When irregularities are formed on the surface of the working electrode, the total surface area including the irregularities increases even if the planar area of the working electrode is the same. As a result, the substantial effective area in which the detection gas undergoes an electrochemical reaction increases, making it possible to increase sensor sensitivity. Further, since ion conduction is performed through the gaps between the unevenness, ion conduction is also performed satisfactorily. If irregularities are formed on electrodes other than the working electrode, the surface area of each electrode increases, so that the function of each electrode can be improved.

In various arrangements, the working electrode is placed in the center, the counter electrode is placed in a position facing multiple sides of the working electrode, and the reference electrode is placed in a position sandwiched between the working electrode and the counter electrode. Ion conduction occurs between multiple sides of the pole and the counter electrode, increasing the effective length involved in the electrochemical reaction at the working electrode. As a result, the resensor sensitivity will be increased.

〔Example〕

Next, embodiments of the present invention will be described below with reference to the drawings.

As shown in FIGS. 1 and 2, a working electrode made of PT is formed on the surface of a silicon substrate with an insulating layer made of Sing or the like, or an insulating substrate 1 made of various synthetic resins or ceramics. 2, a counter electrode 3, and a reference electrode 4 made of Au. Each electrode is formed by ordinary film forming means such as sputtering or vacuum deposition. The working electrode 2 has a reaction part 20 formed in a strip shape at the center of an insulating substrate l, and a terminal part 21 for connection to an external circuit, one end of which is bent into a small L shape following the reaction part 20. We are prepared. The counter electrode 3 includes a U-shaped reaction section 30 provided so as to surround three sides of the reaction section 20 of the working electrode 2, and a U-shaped reaction section 30 that is arranged adjacent to the terminal section 21 of the working electrode 2 following the end of the reaction section 30. , is provided with a terminal portion 32 whose one end is bent into a small L shape. The reference electrode 4 is the reaction part 20 of the action 8i2 and the reaction part 3o of the counter electrode 3.
Following the reaction section 40, one end is bent into a small L shape in the same manner as the terminal sections 21 and 31 of the working electrode 2 and the counter electrode 3. A terminal portion 41 is provided. The reaction portion 40 of the reference electrode 4 is formed halfway up the side of the reaction portion 20 of the working electrode 2 . To give an example of specific dimensions of the insulating substrate 1 and the electrodes 2 to 4, it is preferable to provide the working electrode reaction section 20 and the counter electrode reaction section 30 with an interval of about 50n to 21 mm on the insulating substrate 1 of the 10XIO Dragon. preferable.

Next, as explained in detail in FIG. 3, a large number of fine irregularities are formed on the surface of the working electrode reaction section 200.

In addition, in FIG. 2, in order to make the diagram easier to understand, it is shown that the convex part 24 is formed on two sides of the plate-shaped working electrode reaction part 20, but in reality, the convex part 24 is As shown in FIG. 3, the working electrode reaction portion 20 having an uneven shape is formed by forming unevenness on the surface of the insulating substrate 1 and forming a film of electrode material along the uneven surface. The shape of the unevenness is such that rectangular parallelepiped-shaped protrusions 24 are arranged at equal intervals vertically and horizontally. The dimensions of the protrusion 24 vary depending on the overall structure of the sensor, but usually the width of the protrusion 24 and the distance a between the protrusions 24 are a, b = 3 to 10μ, and the height h of the protrusion 24 is But, k
It is carried out at approximately t = 2 to 5μ. In order to form irregularities on the surface of the insulating substrate 1, they can be formed by digging out portions other than the portions corresponding to the convex portions by means such as etching.

A rectangular frame 10 made of an insulating material such as an organic polymer is fixed on the insulating substrate 1 so as to surround the reaction sections 20 to 40 of each type. Various terminal parts 21 to 41 are arranged outside the frame 10. A solid electrolyte layer 5 made of an ion-conductive polymer or the like is filled inside the frame 10 so as to cover the tops of and between the electrode reaction parts 20 to 40. In the illustrated embodiment, the unevenness of the working electrode 2 is completely covered, but a portion of the convex portion 24 of the working electrode 2 may be exposed above the solid electrolyte 5.

As a material for the solid electrolyte layer 5, for example, sulfonated perfluorocarbon (trade name: Nafion, manufactured by DuPont) is used as a polymer solid electrolyte. The thickness of the solid electrolyte layer 5 is, for example, approximately 1 to 50 μm.

The solid electrolyte layer 5 may be formed by, for example, dissolving sulfonated perfluorocarbon in ethanol, applying it to the inner part of the frame 10 by a solution casting method, and drying it.

The gas detection action in the electrochemical sensor having such a structure will be explained. The detection gas that has passed through the solid electrolyte layer 5 and reached the working electrode reaction area 20 is exposed to the uneven working electrode reaction area 2.
An electrochemical redox reaction occurs on the surface of 0. Ions generated by the reaction are conducted through the solid electrolyte layer 5 at the opposing portion of the working electrode 2 and counter electrode 3. That is, ion conduction is performed from all three sides of the working electrode reaction section 20 toward the opposing electrode reaction section 3. Therefore, the working electrode reaction section 2
The substantial effective area involved in the electrochemical reaction of 0 is the first
As shown by the X mark in the figure, this is a wide range along three sides of the working electrode reaction section 20. Comparing this with the conventional example shown in FIG. 6, it can be seen that the effective area is much wider.

Next, FIG. 4 shows an embodiment having a different electrode structure from the embodiment described above.

In this embodiment, counter electrodes 3 having strip-shaped reaction sections 30 are provided on the left and right sides of the working electrode 2, respectively. That is, the center side of the U-shaped counter electrode reaction section of the above embodiment is removed, and the terminal sections are connected to both sides. In the case of this actual tS example, the working electrode reaction section 20
Since ion conduction is carried out from both sides in the longitudinal direction to the opposing counter electrode reaction section 30, the effective area is the range along both sides in the longitudinal direction of the working electrode reaction section 20, as shown by the X mark in the figure.

Next, the results of measuring the gas sensitivity characteristics of the electrochemical sensor having the above structure will be explained. Using air containing 1100 pp of CO gas as the detection gas,
When the set potential of the working electrode 2 was set to 0.50 V, the output current flowing between the working electrode 2 and the counter electrode 30 was measured.

The sensor structures shown in FIGS. 1 and 4 were used, and for comparison, similar measurements were also conducted on a sensor with a conventional structure shown in FIG. 6. In either structure, the geometric planar area of the working electrode 2 is the same, 1
8 mm''. The measurement results are shown in FIG.

In the figure, Example 1 is a case of a sensor having the structure shown in FIG. 1, Example 2 is a case of a sensor having a structure shown in FIG. 4, and Comparative Example is a case of a sensor having a structure shown in FIG.

As is clear from the graph in FIG. 5, the detection output of the embodiment of the present invention is much higher than that of the comparative example, and the sensor sensitivity is improved. The excellent effectiveness of the chemical sensor was demonstrated.

〔Effect of the invention〕

As described above, in the electrochemical sensor according to the present invention, first, the surface area of the working electrode is increased by forming irregularities on the surface of at least the working electrode, and the area where the sensing gas undergoes an electrochemical reaction is increased. As a result, sensor sensitivity improves. In addition, by arranging the counter electrode at a position facing multiple sides of the working electrode, parts of the working electrode that have not been used in the past can also participate in the electrochemical reaction, allowing ions to flow between the working electrode and the counter electrode. The wider conduction path also results in improved sensor sensitivity. By synergistically adding the above-mentioned two effects, the practical effective area for carrying out the electrochemical redox reaction of the detection gas at the working electrode increases significantly, and the sensor sensitivity becomes extremely high.

Moreover, simply forming irregularities on the surface of the electrode does not increase the overall size of the sensor at all. Furthermore, even if the area of the counter electrode increases, the utilization efficiency of the working electrode improves, so the sensor sensitivity is improved by more than the increase in the area of the counter electrode, and the size of the sensor can be reduced by the increase in sensor sensitivity. You can also do it. Therefore, it is possible to provide an electrochemical sensor that has much better sensitivity than a conventionally structured sensor even though it has the same size.

[Brief explanation of drawings]

Fig. 1 is a plan view showing an embodiment of the present invention, Fig. 2 is a sectional view, Fig. 3 is an enlarged perspective view showing the uneven structure of the working electrode, and Fig. 4 is a plan view showing an embodiment of the present invention.
The figure is a plan view showing another embodiment, FIG. 5 is a graph showing measurement results of sensitivity characteristics, and FIG. 6 is a plan view of a conventional example. 1... Insulating substrate 2... Working electrode 20... Reaction part 3... Counter electrode 30... Reaction part 4... Reference electrode
40...Reaction part 5...Solid electrolyte layer Fig. 1

Claims (1)

[Claims] 1. In an electrochemical gas sensor element in which a working electrode, a counter electrode, and a reference electrode are provided on the same surface of an insulating substrate, and a solid electrolyte layer is provided covering each electrode and between the electrodes, at least the surface of the working electrode is provided with irregularities. The arrangement of each electrode is such that the working electrode is in the center, the counter electrode is placed opposite multiple sides of the working electrode, and the reference electrode is placed between the working electrode and the counter electrode. An electrochemical sensor characterized by: