Electrochemical gas sensor and electrochemical gas measurement method

Abstract

To provide an electrochemical gas sensor which exhibits stable and highly responsive output characteristics over a prolonged period of time.SOLUTION: An electrochemical gas sensor 10 is provided, comprising a casing 11 containing an electrolyte, and an anode 30 and cathode 35 arranged with the electrolyte interposed therebetween. The anode 30 contains tin or a tin alloy. A solid oxidizing agent is provided inside the casing 11 so that tin eluted into the electrolyte is oxidized by the oxidizing agent.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electrochemical gas sensor and the like. [Background technology]

[0002] Electrochemical gas sensors, such as galvanic cell gas sensors, have an anode and a cathode arranged with an electrolyte stored in a casing, and when the gas to be measured is reduced at the cathode, the current that flows between the electrodes is detected via lead members connected to the anode and the cathode. This type of electrochemical gas sensor has the advantages of being simple in configuration and capable of operating at room temperature, and is therefore used, for example, in oxygen sensors for checking for oxygen deficiency.

[0003] Lead (Pb) has been used as an anode material for many years. However, in order to avoid the use of harmful substances such as lead (Pb), electrochemical sensors using anodes other than lead (Pb), such as electrochemical gas sensors using tin or tin alloys as the anode, have been proposed (see Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6985533 Summary of the Invention [Problem to be solved by the invention]

[0005] According to unpublished research by the present inventors, it has become clear that when tin (Sn) is used for the anode, an increase in the concentration of tin ions (tin salt or tin hydroxide) in the electrolyte causes a phenomenon in which the output response speed of the electrochemical gas sensor to changes in gas concentration decreases and the output value fluctuates.

[0006] The present invention has been made in light of the above circumstances, and aims to provide an electrochemical gas sensor or the like that can obtain high response or high accuracy output characteristics over a long period of time when tin (Sn) is used as the main component of the anode surface. [Means for solving the problem]

[0007] To achieve the above object, the present invention provides an electrochemical gas sensor comprising a casing that contains an electrolyte, and an anode and a cathode that are arranged with the electrolyte interposed therebetween, wherein the anode contains tin or a tin alloy, a solid oxidizer is placed within the casing, and the tin that dissolves into the electrolyte is oxidized by the oxidizer.

[0008] In the electrochemical gas sensor described above, the oxidizing agent may be bismuth hydroxide.

[0009] In the electrochemical gas sensor described above, the oxidizing agent may be copper hydroxide.

[0010] In the electrochemical gas sensor described above, the oxidizing agent may be silver oxide.

[0011] The electrochemical gas sensor may be characterized in that the casing includes a filter portion that allows the electrolyte to move and restricts the oxidizing agent to move.

[0012] The electrochemical gas sensor may further include a container that contains the oxidizing agent, and the container may include the filter portion.

[0013] In the electrochemical gas sensor, the contact area of ​​the cathode with the target gas is 10 mm 2 The above may be a feature.

[0014] The present invention, which achieves the above object, provides an electrochemical gas measurement method, which comprises: accommodating an electrolyte in a casing; arranging an anode and a cathode with the electrolyte interposed therebetween; causing the anode to contain tin or a tin alloy; bringing a solid oxidizing agent into contact with the electrolyte; detecting the target gas by reducing the target gas at the cathode and detecting the target gas by the current flowing between the anode and the cathode; and oxidizing the tin dissolved in the electrolyte with the oxidizing agent. [Effects of the Invention]

[0015] According to the electrochemical gas sensor of the present invention, even though tin is used for the anode, it is possible to obtain high response or high precision output characteristics for a long period of time. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a cross-sectional view showing an example of the configuration of an electrochemical gas sensor according to an embodiment of the present invention. [Figure 2] 1 is a table showing oxygen indication output responses corresponding to changes in atmospheric oxygen concentration, which is a first verification result using the electrochemical gas sensors of Examples 1 to 3 and Comparative Examples 1 to 3. [Figure 3] Graph (A) is a graph explaining the definition of the 90% response time (seconds), and graph (B) is a graph showing the response characteristics corresponding to changes in oxygen concentration, which is the second verification result using the electrochemical gas sensors of Examples 1 and 2 and Comparative Examples 1 and 2. [Figure 4] 10 is a table showing changes in oxygen concentration indications in an air atmosphere over the number of days that serve as third verification results using the electrochemical gas sensors of Examples 4 to 9 and Comparative Examples 4 to 7. DETAILED DESCRIPTION OF THE INVENTION

[0017] First Embodiment DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Fig. 1 shows the overall configuration of a galvanic cell gas sensor which is an electrochemical gas sensor 10 according to a first embodiment of the present invention.

[0018] The electrochemical gas sensor 10 detects, for example, oxygen as a gas to be detected, and includes a casing 11 that is a generally cylindrical container overall, an electrolyte held within the casing 11, a cathode 35 disposed at a gas inlet 22 formed in the casing 11, an anode 30 disposed within the casing 11, and a current-voltage conversion circuit board 90. For ease of explanation, the side of the casing 11 in the direction of the central axis where the electrolyte guide hole 12B is located is defined as the "front side," and the opposite side is defined as the "rear side."

[0019] (Casing) The casing 11 includes a main body 12 having a generally cylindrical shape with a bottom, and a front cap 13 attached to the front side of the main body 12. The main body 12 and the front cap 13 are made of a resin material such as ABS resin.

[0020] The main body 12 integrally includes a cylindrical peripheral wall 12C, a rear surface 12G covering the rear side of the peripheral wall 12C, and a front surface 12A covering the front side of the peripheral wall 12C. These components form an electrolyte storage space S within the main body 12, in which electrolyte is stored. An electrolyte filling hole 12F is formed in the rear surface 12G. The electrolyte filling hole 12F is sealed with a bolt plug 110 after the electrolyte storage space S is filled with electrolyte. A pressure adjustment hole 12E is formed in the peripheral wall 12C, and this pressure adjustment hole 12E is closed with a water-repellent pressure membrane (pressure adjustment membrane) 57. The front cap 13 is fixed to the front surface 12A of the main body 12 with a screw 120. An electrolyte guide hole 12B is formed in the center of the front surface 12A. The compressed air membrane 57 is made of, for example, soft vinyl chloride resin, and its peripheral edge is fixed to the peripheral wall 12C by, for example, heat welding, ultrasonic welding, adhesive, a fitting cap, or the like.

[0021] A ring-shaped step is formed on the front side of the electrolyte guide hole 12B, and the cathode 35 is positioned to cover the electrolyte guide hole 12B using this step. A water-repellent gas-permeable diaphragm 55 is positioned on the front side of the cathode 35. The gas-permeable diaphragm 55 liquid-tightly seals the electrolyte guide hole 12B. This prevents the electrolyte that flows out of the electrolyte guide hole 12B and contacts the cathode 35 from leaking to the outside, and at the same time, only gas that can pass through the gas-permeable diaphragm 55 can reach the cathode 35. Specifically, the gas-permeable diaphragm 55 covers a wider area than the electrolyte guide hole 12B on the further front side of the front surface 12A and is sandwiched between the front cap 13 and the front surface 12A of the main body 12. In this case, the ring-shaped seal member 16 presses the gas-permeable diaphragm 55 against the periphery of the electrolyte guide hole 12B. Gas-permeable membrane 55 selectively allows oxygen, which is the object of measurement, to pass through while limiting the amount of oxygen passing through to match the cell reaction, and is made of a fluorine-based resin such as FEP (tetrafluoroethylene-hexafluoropropylene copolymer) or PTFE (polytetrafluoroethylene). The thickness of gas-permeable membrane 55 is, for example, 13 to 25 μm.

[0022] The front cap 13 has a lid-side gas introduction through-hole 13B formed in the center, which constitutes the gas inlet 22. Here, the front cap 13 is fixed to the main body 12 with screws 120, but it may also be fixed by other means, such as crimping. The gas-permeable diaphragm 55 and the cathode 35 of the front cap 13 are pressed against the front surface 12A of the main body 12 via the ring-shaped seal member 16. As a result, the liquid-tightness of the front side of the electrolyte containing space S is ensured.

[0023] A first filter section 70 is disposed on the rear side of the electrolyte guide hole 12B. This first filter section 70 is made of a filter material that does not allow solid bismuth hydroxide to pass through but allows only the electrolyte to pass through. This first filter section 70 and the electrolyte guide hole 12B allow only the electrolyte to be guided to the cathode 35. Specifically, in this first embodiment, a nonwoven fabric is used as the first filter section 70. This first filter section 70 is fixed to the inner wall of the casing 11 by heat welding, ultrasonic welding, adhesive, a fitting cap, or the like.

[0024] In the first embodiment, the contact area of ​​the cathode 35 with the gas to be measured is 10 mm 2 It is preferable to set it to 50 mm or more, and more preferably 2 More than this, preferably 100mm 2 This improves the detection sensitivity of the gas to be measured, making it possible to measure, for example, oxygen concentrations of 30,000 ppm or less with high accuracy, and preferably, oxygen concentrations of 10,000 ppm or less. In the present invention, the contact area of ​​the cathode 35 with the gas to be measured is not particularly limited, and can be set appropriately depending on the measurement range and the purpose of the measurement. For example, if the specification is to measure an oxygen concentration range that can determine an oxygen deficiency state for humans (for example, a concentration range that includes an oxygen concentration of 20.9% in air), it is sufficient that the contact area is equal to or larger than the minimum contact area required by the specification. Specifically, for the purpose of determining an oxygen deficiency state, the electrode area (contact area with the gas) is, for example, 0.5 mm 2 It is preferable that the above is set.

[0025] A recessed sealed space 12H is formed on the outside of the rear surface 12G of the main body 12. This sealed space 12H is filled with resin (including adhesive) and sealed during the assembly process. This sealing structure creates a liquid-tight environment on the rear side of the electrolyte solution containing space S.

[0026] (cathode) The cathode 35 is a disk-shaped metal member whose front side is in contact with the gas to be measured and whose back side is in contact with the electrolyte. There are no particular limitations on the cathode 35 as long as at least the electrode surface has a structure that allows current to be generated by oxygen reduction. For example, metals such as gold, silver, platinum, and titanium are preferably used for the electrode surface.

[0027] In this first embodiment, the cathode 35 is formed by plating (plating) the surface of a metal substrate made of stainless steel with gold or silver. The cathode 35 has a convex shape facing outward (toward the front side) to suppress deformation due to internal pressure fluctuations. One end of a cathode lead member 42, which serves as the cathode wiring, is connected to the cathode 35. Note that the cathode lead member 42 may be formed by simply extending a portion of the substrate (wire) constituting the cathode 35. The cathode lead member 42 passes through the electrolyte containing space S and reaches the rear surface 12G of the main body 12. The cathode lead member 42 is further disposed to penetrate the rear surface 12G and pass through the sealed space 12H, and a terminal 90B is provided at its end. A portion of the cathode lead member 42 near the terminal 90B is connected to a cathode-side input terminal (not shown) of a current-voltage conversion circuit board 90 disposed within the sealed space 12H. As a result, the vicinity of the end of cathode lead member 42 is embedded in the resin filled in sealed space 12H, but terminal 90B at the end is exposed to the outside. Cathode lead member 42 is made of a material that does not react when in contact with the electrolyte, for example, a metal wire having at least a surface made of a precious metal such as gold, silver, or platinum. In the first embodiment, a stainless steel wire having a silver (Ag) plated surface is used.

[0028] (anode) The anode 30 disposed in the electrolyte containing space S is a columnar metal member. At least the surface of the anode 30 that comes into contact with the electrolyte is made of tin (Sn) or a tin alloy. In particular, pure tin is used in this first embodiment. When a tin alloy is used, for example, a general lead-free solder material (e.g., Sn-Ag-Cu, Sn-Ag, Sn-Ag-Bi-Cu-Ni, Sn-Ag-Bi-In, Sn-Ag-Cu-Bi-In, Sn-Ag-Cu-Bi-Ni, Sn-Ag-Cu-Ni, Sn-Ag-Cu-Sb, Sn-Ag-Ni-Co, Sn-Bi, Sn-Bi-Ag, Sn-Cu, Sn-Cu-Ni, Sn-Cu-Ni-P-Ge, Sn-Sb, Sn-Ag-Cu alloy, Sn-Cu alloy, Sn-Ag alloy, etc.) can be used.

[0029] An anode support part 28 is formed on the rear surface 12G of the main body 12. The anode support part 28 is, for example, a through-hole that penetrates the rear surface 12G and communicates with the sealed space 12H. A columnar anode 30 is fitted into this through-hole, causing a portion (on the rear surface side) of the anode 30 to protrude into the sealed space 12H. The sealed space 12H is filled with resin, thereby fixing the anode 30 to the anode support part 28.

[0030] An anode lead member 48, which serves as the anode wiring, is provided on a portion (rear side) of the anode 30 that protrudes into the sealed space 12H. When the anode 30 is held by the anode support 28, one end of the anode lead member 48 and the anode 30 are welded to each other, thereby electrically connecting them. The anode lead member 48 is disposed so as to pass through the sealed space 12H, and a terminal 90A is provided at its end. A portion of the anode lead member 48 near the terminal 90A is connected to an anode-side input terminal (not shown) of a current-voltage conversion circuit board 90 disposed in the sealed space 12H. As a result, a portion of the anode lead member 48 near its end is embedded in the resin filled in the sealed space 12H, but the end terminal 90A is exposed to the outside. The anode lead member 48 is formed of a metal wire, for example, stainless steel. When the anode lead member 48 has a wiring structure that comes into contact with the electrolyte, for example, a metal wire having at least a surface made of a noble metal such as gold, silver, or platinum is used.

[0031] (Bismuth hydroxide) Bismuth hydroxide 80, which serves as a solid oxidizer, is disposed within the electrolyte storage space S. That is, the electrolyte of this first embodiment contains bismuth hydroxide 80 in its liquid form. The bismuth hydroxide 80 exists in the electrolyte in a particulate state. More specifically, the bismuth hydroxide is contained within a container 82. The container 82 includes a cylindrical or rectangular tubular main body 84 made of resin or the like, and a pair of second filter members 86 arranged to cover the openings at both ends of the main body 84. The second filter members 86 are made of a porous or mesh-like filter material that allows only the electrolyte to pass through but not the solid bismuth hydroxide. Specifically, in this first embodiment, a nonwoven fabric is used as the second filter member 86. The second filter member 86, containing the bismuth hydroxide 80 therein, is fixed to the opening of the main body 84 by thermal welding, ultrasonic welding, adhesive, a fitting cap, or the like. The pair of second filter members 86 form a flow path within the container 82 through which the electrolyte can pass. The container 82 is preferably fixed to the casing 11 by a connecting part or the like (not shown).

[0032] In the first embodiment, the case where bismuth hydroxide 80 is contained in the container 82 is exemplified, but the present invention is not limited to this, and the container 82 may be omitted and the bismuth hydroxide 80 may be dispersed in the electrolyte solution.

[0033] (Current-voltage conversion circuit board) The current-voltage conversion circuit board 90 includes a conversion circuit that converts the current flowing between the cathode 35 and the anode 30 into a voltage. The conversion circuit has a structure in which, for example, a resistor and a temperature-compensating thermistor are arranged in series between the cathode 35 and the anode 30. The voltage output is output from a pair of terminals 90A and 90B formed on the current-voltage conversion circuit board 90. Note that, as long as the voltage output can be detected, either the cathode lead member 42 or the anode lead member 48 itself may be used as the terminal 90A or 90B.

[0034] The current-voltage conversion circuit board 90 is disposed within the sealed space 12H between the cathode lead member 42 and the anode lead member 48. Therefore, when the sealed space 12H is filled with resin, the anode lead member 48, a portion of the cathode lead member 42, and the current-voltage conversion circuit board 90 are embedded in the resin.

[0035] Although not specifically shown here, the casing 11 and the current-voltage conversion circuit board 90 can be housed in a metal exterior body. This exterior body can be divided into a first exterior body and a second exterior body that are electrically insulated from each other, and by connecting one terminal 90A to the first exterior body and the other terminal 90B to the second exterior body, the exterior body itself can be used as an electrode terminal.

[0036] (electrolyte) The electrolyte is an aqueous solution containing an alkali metal as a main component. The electrolyte contains, for example, sodium hydroxide (NaOH) or potassium hydroxide (KOH), and is a strong alkaline aqueous solution. In the first embodiment, the electrolyte contains potassium hydroxide (KOH).

[0037] The electrolyte preferably contains water and a liquid substance having a humidity control effect and / or an anti-freezing effect (hereinafter referred to as an adjusting liquid). For example, the adjusting liquid may contain ethylene glycol (C2H6O2).

[0038] The concentration of potassium hydroxide, which is the main component of the electrolyte solution in this embodiment, is preferably 1 mol / L or more in terms of molar concentration relative to the solution (water and adjusting solution).The concentration of the main component is preferably 10 mol / L or less relative to the solution.

[0039] Furthermore, the electrolytic solution of this first embodiment contains bismuth hydroxide (is in contact with bismuth hydroxide). The amount of bismuth hydroxide to be brought into contact with the electrolytic solution is not particularly limited. For example, 10 mg or more is preferable, and 100 mg or more is more preferable. Here, 600 mg of bismuth hydroxide is contained.

[0040] The electrolyte solution preferably further contains a surfactant, such as polyoxyethylene (10) octylphenyl ether.

[0041] <Operation of First Embodiment> In the electrochemical gas sensor 10 of the first embodiment, oxygen that has passed through the gas inlet 22 and the gas-permeable diaphragm 55 is reduced at the cathode 35, and the following electrochemical reaction occurs between the cathode 35 and the anode 30 via the electrolyte.

[0042] (electrochemical reaction) Cathode reaction: O2 + 2H2O + 4e - →4OH - Anode reaction: 2Sn → 2Sn 2+ +4e -

[0043] This electrochemical reaction generates a current corresponding to the oxygen concentration between the anode 30 and the cathode 35. This current (electrons) is guided from the anode 30 to the outside of the electrolyte containing space S via the anode lead member 48, and flows to the cathode lead member 42 and the cathode 35 via a resistor and a temperature-compensating thermistor in the current-voltage conversion circuit board 90. The current is converted into a voltage signal in the current-voltage conversion circuit board 90, and a voltage signal serving as the sensor output is output between terminals 90A and 90B. This voltage signal is converted into an oxygen concentration using a well-known method to provide an oxygen concentration indication value (%).

[0044] At this time, divalent tin ions (Sn 2+ In an unpublished demonstration experiment conducted by the present inventors, it was found that in the case of a conventional electrochemical gas sensor, divalent tin ions (Sn 2+ However, when bismuth hydroxide is brought into contact with the electrolyte, as in the electrochemical gas sensor 10 of this embodiment, divalent tin ions (Sn 2+ ) is oxidized to tetravalent tin ions (Sn 4+ ) and the deterioration of the response speed performance of the electrochemical reaction is suppressed.

[0045] Next, we will explain the expected effect of bringing the electrolyte solution into contact with bismuth hydroxide. Please note that the following explanation is based on the inventor's assumptions and may differ from the actual facts.

[0046] (Estimation of the effects of bismuth hydroxide) Divalent tin ions (Sn 2+ ) itself may induce a decrease in the response speed of the electrochemical reaction. 2+ ) is estimated to be in the following state in the alkaline electrolyte: Sn 2+ +4OH - →[Sn(OH)4] 2- Therefore, in this embodiment, bismuth hydroxide 80, which can function as an oxidizing agent, is added to the electrolyte in advance, so that the electrolyte and bismuth hydroxide 80 are always in contact with each other. As a result, the bismuth hydroxide 80 oxidizes the divalent tin ions (Sn 2+ ) and promotes the following reaction: 2Bi(OH)3+3[Sn(OH)4] 2- →2Bi↓+3[Sn(OH)6] 4- As a result of this reaction, Bi becomes a precipitate in a part of the bismuth hydroxide 80 in the container 82, and the color changes to black. At the same time, divalent tin ions (Sn 2+ ) is displaced (oxidized) to the tetravalent side. As a result, the concentration of divalent tin ions decreases, and the deterioration of the response speed of the above electrochemical reaction is suppressed.

[0047] In this embodiment, bismuth hydroxide is used as the solid oxidizing agent to be brought into contact with the electrolyte, but the present invention is not limited to this, and other solid oxidizing agents that can oxidize tin can be used. For example, bismuth nitrate, copper hydroxide, silver oxide, etc. can also be used.

[0048] (Function of filter material) In the electrochemical gas sensor 10 of this embodiment, the double structure of the first filter part 70 and the second filter part 86 prevents the bismuth hydroxide from coming into direct contact with the cathode 35 or the anode 30. If the bismuth hydroxide 80 adheres to the cathode 35 or the anode 30, the potential of these electrodes may change, possibly causing fluctuations in the output value. In terms of the structure of the electrochemical gas sensor 10, it is preferable to avoid potential fluctuations on the cathode 35 side, which comes into contact with the gas to be measured, as much as possible. Therefore, it is preferable to provide at least one filter part between the bismuth hydroxide 80 and the cathode 35.

[0049] <Examples 1 to 3> Three electrochemical gas sensors 10 of the first embodiment were prepared, each having a silver-plated cathode 35, a tin-plated anode 30, and an electrolyte solution with the following composition: The powder of bismuth hydroxide 80, purity 3N (99.9%), CAS: 10361-43-0, manufactured by Kojundo Chemical Laboratory Co., Ltd., was used, and 600 mg was placed in the container 82. The contact area of ​​the cathode 35 with the gas to be measured was set to approximately 430 mm 2 was set to.

[0050] (Electrolyte composition) Potassium hydroxide: 1N ※ *Nominal (N) is the molar concentration (mol / L) multiplied by the valence of the acid or alkali.

[0051] <Comparative Examples 1 to 3> Electrochemical gas sensors of Comparative Examples 1 to 3 were prepared, which had the same configuration as Examples 1 to 3, except that bismuth hydroxide was not placed in the electrolyte solution.

[0052] (First Verification) The electrochemical gas sensors of Examples 1 to 3 and Comparative Examples 1 to 3 were placed in a 100% nitrogen atmosphere (hereinafter referred to as the nitrogen atmosphere) and left to stand for 24 hours to stabilize the output. After that, the atmosphere was switched to an atmosphere in which 10,000 ppm of oxygen had been added to the nitrogen atmosphere (hereinafter referred to as the evaluation atmosphere), and the output was measured. The results of a graph showing the change in output of electrochemical gas sensor 10 over time, starting from the timing of switching from the nitrogen atmosphere to the evaluation atmosphere, are shown in Figure 2. For ease of evaluation, in the graph of Figure 2, the output value (mV) after 5 minutes has elapsed has been converted (normalized) to an oxygen concentration of 10,000 ppm.

[0053] As can be seen from Examples 1 to 3, the output value of the electrochemical gas sensor 10 of the first embodiment stabilizes after 5 minutes and becomes substantially constant after 15 minutes. Furthermore, the electrochemical gas sensors 10 of Examples 1 to 3 exhibit almost no variation between individual sensors. On the other hand, the electrochemical gas sensors of Comparative Examples 1 to 3 tend to stabilize their output value after 25 minutes, but do not achieve a completely constant output value even after 30 minutes. Furthermore, the electrochemical gas sensors of Comparative Examples 1 to 3 exhibit variation between individual sensors.

[0054] (Second Verification) As shown in Figure 3(A), the difference (A-B) (mV) between the stable output voltage A (mV) in a nitrogen atmosphere (oxygen reading 0%) and the output voltage B (mV) when a reference time (here, 5 minutes) has elapsed since switching from the nitrogen atmosphere to the test atmosphere (oxygen reading 10,000 ppm) is defined as 100%, and the time T90 from the switching point until the output voltage (mV) rises to 90% of the difference (A-B) is defined as the "90% response time (seconds)." Using the electrochemical gas sensors of Examples 1 and 2 and Comparative Examples 1 and 2, we measured how this 90% response time (seconds) changed over the number of days, and the results are shown in Figure 3(B).

[0055] It can be seen that the electrochemical gas sensors 10 of Examples 1 and 2 always maintained a stable 90% response time (seconds) of 20 seconds or less even after 80 days had passed. Therefore, even if bismuth oxide was continuously in contact with the electrolyte, the response performance did not deteriorate over time. On the other hand, the electrochemical gas sensors of Comparative Examples 1 and 2 already had a 90% response time (seconds) of about 50 seconds at the beginning of the experiment, which exceeded 120 seconds after about 20 days had passed, and thereafter fluctuated significantly between 120 and 180 seconds.

[0056] As can be seen from the above verification, the electrochemical gas sensor 10 of the first embodiment has extremely high response performance and hardly deteriorates over time. In particular, it can be seen that the response performance is extremely high even when the oxygen concentration of the measurement gas is 30,000 ppm or less.

[0057] Second Embodiment Next, a galvanic cell gas sensor will be described, which is an electrochemical gas sensor 10A according to a second embodiment of the present invention. Most of the internal structure of the electrochemical gas sensor 10A according to the second embodiment is the same as or similar to that of the electrochemical gas sensor 10 according to the first embodiment. Therefore, the same reference numerals will be used to describe these parts and members, and illustrations and descriptions will be omitted. The following description will focus on the differences from the first embodiment.

[0058] (copper hydroxide) In the electrochemical gas sensor 10A, copper hydroxide 80A, which serves as a solid oxidizing agent, is placed in the electrolyte containing space S. That is, the electrolyte contains copper hydroxide 80A. This copper hydroxide 80A exists in the electrolyte in the form of particles.

[0059] (Estimation of the effects of copper hydroxide) Divalent tin ions (Sn 2+ ) itself may induce a decrease in the response speed of the electrochemical reaction. 2+ ) is estimated to be in the following state in the alkaline electrolyte: Sn2+ +4OH - →[Sn(OH)4] 2- Therefore, in the second embodiment, copper hydroxide 80A, which can function as an oxidizing agent, is added to the electrolyte in advance, so that the electrolyte and copper hydroxide 80A are constantly in contact with each other. As a result, the copper hydroxide 80A oxidizes the divalent tin ions (Sn 2+ ) and promotes the following reaction: Cu(OH)2+[Sn(OH)4] 2- →Cu↓+[Sn(OH)6] 2- As a result of this reaction, Cu precipitates in part of the copper hydroxide 80A in the container 82, and at the same time, divalent tin ions (Sn 2+ ) is displaced (oxidized) to the tetravalent side. As a result, the concentration of divalent tin ions decreases, and the deterioration of the response speed of the above electrochemical reaction is suppressed.

[0060] <Third embodiment> Next, a galvanic cell gas sensor that is an electrochemical gas sensor 10B according to a third embodiment of the present invention will be described. Most of the internal structure of the electrochemical gas sensor 10B according to the third embodiment is the same as or similar to that of the electrochemical gas sensor 10 according to the first embodiment. Therefore, the same reference numerals will be used to describe these parts and members, and illustrations and descriptions will be omitted. The following description will focus on the differences from the first embodiment.

[0061] (silver oxide) In the electrochemical gas sensor 10B, silver oxide 80B, which serves as a solid oxidizing agent, is placed in the electrolyte containing space S. That is, the electrolyte contains silver oxide 80B. This silver oxide 80B exists in the electrolyte in the form of particles.

[0062] (Estimation of the effects of silver oxide) Divalent tin ions (Sn 2+ ) itself may induce a decrease in the response speed of the electrochemical reaction.2+ ) is estimated to be in the following state in the alkaline electrolyte: Sn 2+ +4OH - →[Sn(OH)4] 2- Therefore, in the third embodiment, silver oxide 80B, which can function as an oxidizing agent, is added to the electrolyte in advance, so that the electrolyte and silver oxide 80B are constantly in contact with each other. As a result, the silver oxide 80B oxidizes divalent tin ions (Sn 2+ ) and promotes the following reaction: Ag2O+[Sn(OH)4] 2- +H20→2Ag↓+[Sn(OH)6] 2- As a result of this reaction, Ag precipitates in part of the silver oxide 80B in the container 82, and at the same time, divalent tin ions (Sn 2+ ) is displaced (oxidized) to the tetravalent side. As a result, the concentration of divalent tin ions decreases, and the deterioration of the response speed of the above electrochemical reaction is suppressed.

[0063] <Examples 4 to 9> Two electrochemical gas sensors 10 of the first embodiment were prepared as Examples 4 and 5. The powder of bismuth hydroxide 80, purity 3N (99.9%), CAS: 10361-43-0, manufactured by Kojundo Chemical Laboratory Co., Ltd., was used, and 1.15 mmol was placed in a container 82. The contact area of ​​the cathode 35 with the gas to be measured was set to about 11 mm 2 The electrolyte composition was set to contain 5 mol / L of potassium hydroxide. As Examples 6 and 7, two electrochemical gas sensors 10A of the second embodiment were prepared. Copper hydroxide 80A powder, CAS: 20427-59-2, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., was used, and 1.15 mmol was placed in the container 82. The contact area of ​​the cathode 35 with the gas to be measured was set to about 11 mm 2The electrolyte composition was set to contain 5 mol / L of potassium hydroxide. Two electrochemical gas sensors 10B of the third embodiment were prepared as Examples 8 and 9. Silver oxide 80B powder, CAS: 20667-12-3, manufactured by Kanto Chemical Co., Ltd., was used, and 1.15 mmol was placed in the container 82. The contact area of ​​the cathode 35 with the gas to be measured was set to about 11 mm 2 The electrolyte composition was set to potassium hydroxide at 5 mol / L.

[0064] <Comparative Examples 4 to 7> As Comparative Examples 4 and 5, electrochemical gas sensors were prepared that had the same configuration as Example 4, except that no oxidant was placed in the electrolyte. Also, as Comparative Examples 6 and 7, conventional lead anode-type electrochemical gas sensors were prepared that used lead (Pb) as the anode material.

[0065] (Third Verification) The electrochemical gas sensors of Examples 4 to 9 and Comparative Examples 4 to 7 were placed in an air atmosphere (indoors) with an oxygen concentration of 20.9%, and their outputs were measured. Starting from the first day of measurement, the changes in the output of the electrochemical gas sensors over the number of days were plotted as a graph, and the results are shown in Figure 4. For ease of evaluation, the graph in Figure 4 converts (normalizes) the output value (mV) on the first day to an oxygen concentration of 20.9%, and displays the fluctuations in the oxygen concentration readings over the number of days that have passed since then.

[0066] In the case of the electrochemical gas sensors of Examples 4 to 9, the fluctuation in the oxygen concentration readings was less than plus or minus 1% (more specifically, 0.5% or less) over the period up to 50 days, providing measurement accuracy that is acceptable for practical use. In particular, in the case of the electrochemical gas sensors of Examples 6 to 9, the oxygen concentration readings tended to become more stable after 20 days had passed from the first day, suggesting that calibration and use after 20 days could improve measurement accuracy. Furthermore, the electrochemical gas sensors of Examples 4 to 9 provided measurement results that were comparable to those of the conventional lead anode-type electrochemical gas sensors of Comparative Examples 6 and 7.

[0067] On the other hand, the electrochemical gas sensors of Comparative Examples 4 and 5 showed a tendency for the oxygen concentration readings to gradually decrease with the number of days that had passed. This is due to the anodic reaction (2Sn → 2Sn 2+ +4e - ) by the divalent tin ion (Sn 2+ It was presumed that this was due to the increase in the amount of elution of ) in relation to the number of days that had passed.

[0068] In the electrochemical gas sensor 10 of this embodiment, both the first filter portion 70 and the second filter portion 86 are provided, but the present invention is not limited to this, and the sensor may have a structure in which either the first filter portion 70 or the second filter portion 86 is provided. Also, both the first filter portion 70 and the second filter portion 86 may be omitted.

[0069] The structure of the electrochemical gas sensor shown in this embodiment is an example, and other structures may be used in the present invention. Furthermore, the electrochemical gas sensor of the present invention is not limited to a structure having two electrodes (two-electrode type), and may be a structure having three or four electrodes (three-electrode type, four-electrode type).

[0070] In the present embodiment, a case where a through-hole is formed in the main body 12 as the anode support portion 28 has been exemplified, but the present invention is not limited to this. For example, a recess may be formed inside the main body 12 as the anode support portion 28, and the anode 30 may be fitted into this recess. Furthermore, the anode 30 may be fixed to the main body 12 using a resin such as an adhesive as the anode support portion 28. Furthermore, in this modified example, a case where the anode 30 is fixed to the molded main body 12 afterwards has been exemplified, but the present invention is not limited to this. For example, the anode 30 may be insert-molded when manufacturing the main body 12, and the anode 30 may be held in the main body 12 by this insert-molded structure.

[0071] The present invention is not limited to the above-described embodiment, and it goes without saying that various modifications can be made without departing from the spirit of the present invention. [Explanation of symbols]

[0072] 10, 10A, 10B Electrochemical Gas Sensor 11 Casing 12 Main Unit 12A front 12B Electrolyte guide hole 12C Peripheral wall 12G rear 13 Front cap 13B Lid side gas inlet hole 16 Sealing material 22 Gas intake 28 Anode support part 30 Anode 35 Cathode 42 Cathode lead member 48 Anode lead material 55 Gas-permeable membrane 57 Pressure Membrane 70 First filter section 80 Bismuth hydroxide 82 Container 84 Main body 86 Second filter section 86 90 Current-voltage conversion circuit board 90A terminal 90B terminal S Electrolyte storage space

Claims

1. An electrochemical gas sensor comprising a casing for accommodating an electrolyte, and an anode and a cathode disposed with the electrolyte interposed therebetween, the anode contains tin or a tin alloy; A solid oxidizer is disposed within the casing; The tin dissolved in the electrolyte is oxidized by the oxidizing agent. Electrochemical gas sensor.

2. 2. The electrochemical gas sensor according to claim 1, wherein the oxidizing agent is bismuth hydroxide.

3. 2. The electrochemical gas sensor according to claim 1, wherein the oxidizing agent is copper hydroxide.

4. 2. The electrochemical gas sensor according to claim 1, wherein the oxidizing agent is silver oxide.

5. 2. The electrochemical gas sensor according to claim 1, wherein the casing includes a filter portion that allows the electrolyte to move and restricts the oxidizing agent to move.

6. a container for containing the oxidizing agent therein; 6. The electrochemical gas sensor according to claim 5, wherein the container includes the filter portion.

7. The contact area of ​​the cathode with the target gas is 10 mm ２ 2. The electrochemical gas sensor according to claim 1, wherein the above-mentioned is true.

8. The casing contains an electrolyte; an anode and a cathode are arranged so that the electrolytic solution is interposed therebetween; The anode contains tin or a tin alloy, and a solid oxidizing agent is brought into contact with the electrolyte, When the target gas is detected by a current flowing between the anode and the cathode by reducing the target gas at the cathode, the tin dissolved in the electrolyte is oxidized by the oxidizing agent. Electrochemical gas measurement method.

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