JPH10228915A - Phosphoric acid fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable proper measurement of the internal distribution of the electrode potential and grasp of the running conditions despite the attached positions difference of the fuel electrode and the air electrode. SOLUTION: In the center of the porous layer 3 of a unit cell, being arranged an air electrode 1 and a fuel electrode 2 on both the surfaces of the phosphoric acid contained porous layer 3, one end of three canaliculi 4A, 4B, 4C containing the phosphoric acid are inserted and other ends are connected to the porous layer 3A, 3B, 3C with the referring electrodes 5A, 5B, 5C being connected liquidly to the each referring electrode with the phosphoric acid at the position of the porous layer 3 where both the air electrode 1 and the fuel electrode 2 are arranged on the porous layer, the electrode potential is measured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a phosphoric acid fuel cell provided with a reference electrode for actually measuring an electrode potential and a current density of a power generation part in a unit cell under operating conditions.

[0002]

2. Description of the Related Art Phosphoric acid type fuel cells are generally constructed by laminating a plurality of unit cells having a porous layer holding phosphoric acid between an air electrode and a fuel electrode via a separator. , 200
In the vicinity of ° C., the operation is performed by supplying an oxidizing gas to the air electrode to cause an oxidation reaction, and supplying a fuel gas to the fuel electrode to cause a reduction reaction. At the time of operation, a difference in gas concentration, a difference in temperature, and the like occur in the plane of the unit cell, so that a portion having a different electrode potential between the fuel electrode and the air electrode exists in the same plane. On the other hand, since hot concentrated phosphoric acid, which is a strong acid, is used for the electrolyte, if the electrode potential difference between adjacent parts increases, a local battery is formed, which causes corrosion of the constituent members. In particular, at the air electrode, the electrode potential increases as the current density decreases, and a high potential portion easily exceeding the corrosion potential of carbon, 800 mV (based on a reversible hydrogen electrode), is generated. Corrosion of the carbon material as a constituent material, deterioration of the catalyst, and the like are likely to occur.

Therefore, in order to avoid such a serious failure due to corrosion of the electrode and to find out the optimal cell structure and operating conditions of the phosphoric acid type fuel cell which can be operated stably for a long period of time, the electrode under the operating conditions must be used. It is necessary to accurately grasp the potential distribution and the current density distribution, and the following method is employed. <Measurement of Electrode Potential Distribution> FIG. 7 is a basic configuration diagram showing a conventional method of measuring an electrode potential distribution of a unit cell. As shown in the figure, in this configuration, in the unit cell having the porous layer 3 holding phosphoric acid between the air electrode 1 and the fuel electrode 2, the air electrode 1 and the fuel electrode 2
The porous layer 3 is larger than the porous layer 3 and two reference electrodes 5G and 5 are provided outside the cell power generation section sandwiched between the air electrode 1 and the fuel electrode 2.
H is arranged. In this configuration, if there is a potential distribution in the electrode surface, a current flows through the power generation portion and the peripheral portion of the surface due to the formation of the local battery, and at this time, two reference voltages are generated by the voltage drop generated in the electrolyte around the cell. Since a potential difference occurs between the electrodes 5G and 5H, the potential difference in the plane is indirectly known by measuring the potential difference. <Measurement of Current Density Distribution> FIG. 8 is a basic configuration diagram showing a conventional method for measuring a current density distribution. As shown, in the unit cell composed of cathode 1 and the anode 2 and the porous layer 3, measures the voltages V ₁ across provided terminals on the outer surface of the air electrode 1 and the fuel electrode 2 by voltmeter 8A In addition, terminals are provided on the outer surface of the separation plate 7 located on both sides of the unit cell in opposition to the above terminals, and a voltage V ₂ at both ends is measured by a voltmeter 8B. The current density J at the terminal setting position is obtained by 1). By measuring V ₁ and V ₂ at a plurality of points in the plane and calculating the current density J, the in-plane current density is obtained. In the equation (1), R is the resistance value of the separator.

[0004]

J = (V ₂ −V ₁ ) / (2R) (1)

[0005]

In order to find out the optimum cell structure and operating conditions, the conventional phosphoric acid fuel cell is constructed as described above, and measures the in-plane electrode potential distribution and the current density distribution. Has been measured. However, these measurement methods also have the following disadvantages, and there is a problem that accurate measurement results cannot always be obtained. <Measurement of Electrode Potential Distribution> In the conventional technique, since the reference electrode is provided outside the power generation part, the electrolyte potential of the power generation part and the electrolyte potential of the reference electrode are different, and the reference electrode is configured to disturb the surrounding electrolyte potential. You will be affected. Therefore, even in the case of a cell having a small area, a low gas utilization rate, and almost no in-plane distribution of the electrode potential, a potential difference may occur between the reference electrodes, and the reference electrode serves as a reference for evaluating the electrode potential. There is a drawback that an unfortunate situation occurs.

FIG. 9 is an explanatory diagram showing the cause of the potential difference between the reference electrodes installed around the cell when the cell has no in-plane distribution in the configuration of FIG. In an open circuit, the electrode potential of both the fuel electrode and the air electrode is determined by the gas concentration, so if the gas concentration is uniform in the plane, the electrode potentials will be equal over the entire surface. When flowing, polarization of η _a occurs at the fuel electrode and η _c occurs at the air electrode.

At this time, the fuel electrode 2 and the air electrode 1
If the ends are offset, the existence of the opposite electrodes of the two electrodes
Existing part, that is, the part 2a of the anode 2 and the cathode 1
1a portion is a power generation portion, a portion where the counter electrode does not exist,
That is, the portion 2b of the anode 2 and the portion 1b of the cathode 1
Non-power generation part (actually smaller than the part where the counter electrode exists)
Current flows). Polarization due to power generation in the power generation part
But the non-power generation part is in the same state as the open circuit.
Polarization does not occur, and a potential difference occurs by the amount of polarization.
A local battery is formed in the pole. Below, the air electrode 1
And reference electrode when fuel electrode 2 protrudes.
Explanation of the effect of the reference electrode potential on the measurement results
I do. (Effect when air electrode protrudes) Air electrode 1 protrudes
If there is, that is, if 1b exists, the power generation unit
The potential of the minute 1a is compared with the potential of the non-power generation portion 1b,
1 polarization (η _c) Only lower
Electron current 12 flows through the electrodes from power section 1b to power generation section 1a.
Porous with flowing, equal amount of proton current 11 holding phosphoric acid
It flows through the quality 3 from the power generation part to the non-power generation part. Follow
Therefore, comparing the electrolyte potentials of the power generation part and the non-power generation part,
Due to the voltage drop, the electrolyte potential is lower in the non-power generation area.
You.

For this reason, when the air electrode protrudes, the air electrode potential decreases as the current density increases.
Since the potential of the reference electrode 5G also decreases with the electrolyte potential, the decrease in the air electrode potential is apparently reduced, and the increase in the fuel electrode potential is measured to be apparently large. (Effects when the fuel electrode protrudes) When the fuel electrode 2 protrudes, that is, when the fuel electrode 2b is present, the potential of the power generation portion 2a is compared with the potential of the non-power generation portion 2b to reduce the potential of the fuel electrode 2. Since the polarization (η _a ) increases by the power generation, the electron current 12 flows from the power generation portion 2a to the non-power generation portion 2b in the electrode.
Flows, and an equal amount of proton current 11 flows from the non-power-generating portion to the power-generating portion in the porous body 3 holding phosphoric acid. Therefore, comparing the electrolyte potentials of the power generation part and the non-power generation part,
Due to the voltage drop, the non-power generation portion has a higher electrolyte potential.

For this reason, when the fuel electrode 2 protrudes, the fuel electrode potential increases as the current density increases.
Since the potential of the reference electrode 5H also increases with the electrolyte potential, the increase in the fuel electrode potential is apparently small, and the decrease in the air electrode potential is apparently large. Further, in a configuration in which the protruding portions of the fuel electrode and the air electrode are simultaneously present in the cell, or in the case of a configuration in which the protruding portion exists unevenly, as inferred from the above description,
Since a non-constant potential difference occurs between the reference electrode 5G and the reference electrode 5H, the reference electrodes 5G and 5H thus arranged cannot be used as a reference for measuring the electrode potential. <Measurement of Current Density Distribution> In the conventional method having the configuration shown in FIG. 8, the voltage drop in the separation plate 7 is measured at a plurality of measurement points.
Although the in-plane distribution is calculated by calculating the current density, since the electrode base material and the carbon-made separator constituting the object to be measured are made of an excellent electric conductive material, the measured voltage drop is macroscopic. And a local measurement is essentially impossible. Therefore, the obtained distribution is limited to the relaxed current density distribution, and there is a problem that an accurate current density distribution cannot be obtained.

An object of the present invention is to solve the above-mentioned disadvantages of the prior art, and to provide an electrode potential distribution in a unit cell plane even when there is a misalignment between the disposed fuel electrode and the air electrode. An object of the present invention is to provide a phosphoric acid fuel cell in which the current density distribution in a plane is appropriately measured without being appropriately measured or alleviated by constituent members, and the operating state is accurately grasped.

[0011]

In order to solve the above problems, the present invention provides a porous layer holding phosphoric acid as an electrolyte between an air electrode for performing an oxidation reaction and a fuel electrode for performing a reduction reaction. In a phosphoric acid type fuel cell in which sandwiched unit cells are stacked via a separator, (1) phosphoric acid, or a porous body containing phosphoric acid, a mixture of phosphoric acid and powder, or phosphoric acid Consisting of a thin tube filled with the fibers and a reference electrode for potential measurement liquid-junction at one open end thereof, and the other open end of the thin tube,
At least one means for measuring the electrode potential is inserted into the porous layer of the power generation area where the air electrode and the fuel electrode overlap each other, and the phosphoric acid inside the thin tube is liquid junctioned with the phosphoric acid in the porous layer. I will prepare it.

(2) A thin tube filled with phosphoric acid or a porous material containing phosphoric acid, a mixture of phosphoric acid and powder, or a fiber holding phosphoric acid, and a liquid junction at one open end thereof And the other open end of the thin tube is inserted between the fuel layer and the porous layer of the power generation area where the air electrode and the fuel electrode overlap, and the inside of the thin tube is First potential measuring means in which phosphoric acid is liquid-juncted with phosphoric acid in the porous layer, and a thin tube also containing phosphoric acid, and a reference for potential measurement liquid-junction at one open end thereof And the other open end of the thin tube is inserted between the porous layer in the power generation area and the air electrode, and the phosphoric acid inside the thin tube is liquid-juncted with the phosphoric acid in the porous layer. Having at least one pair of electrolyte potential difference measuring means comprising second potential measuring means. That.

According to the configuration described in the above (1), the reference electrode is liquid-juncted with the phosphoric acid in the porous layer of the power generation portion via the phosphoric acid contained in the thin tube.
The potential of the reference electrode is maintained at the same potential as the electrolyte potential of the power generation part. Therefore, even when the fuel electrode and the air electrode sandwiching the porous layer are misaligned, the electrode potential can be measured without being affected by the disturbance of the electrolyte potential around the cell. By measuring, the in-plane electrode potential distribution can be accurately grasped.

Further, if the configuration is made as in the above (2), the point of the porous layer is determined from the potential difference between the reference electrode of the first potential measuring means and the reference electrode of the second potential measuring means. Is measured, the current density is determined without being affected by the electrode substrate or the separator having good electric conductivity as in the related art. Therefore, when a plurality of pairs of these potential measuring means are provided in the plane, the current density distribution in the plane can be accurately grasped.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described based on embodiments. <Embodiment 1> FIG. 1 is a basic configuration diagram showing a configuration for measuring an electrode potential distribution of a unit cell according to a first embodiment of the present invention.
(A) is a plan view, and (b) is a cross-sectional view taken along the line XX of (a). FIG. 10 is a basic configuration diagram showing a configuration for measuring the electrode potential distribution of a conventional unit cell used for comparison.
(A) is a plan view, and (b) is a cross-sectional view of the ZZ plane of (a).

In FIG. 1, a unit cell has air electrodes 1 on both sides of a porous layer 3 having a thickness of 300 to 500 μm holding phosphoric acid.
And a fuel electrode 2, and three thin tubes 4A containing phosphoric acid are provided at the center in the thickness direction of the porous layer 3 where the air electrode 1 and the fuel electrode 2 overlap. One end of each of 4B and 4C is inserted. These thin tubes 4A, 4B, 4C
Is connected to a reference electrode 5 composed of a reversible hydrogen electrode.
Porous layer 3 holding phosphoric acid provided with A, 5B, 5C
A, 3B, 3C. That is, the reference electrodes 5A, 5B, and 5C are liquid-juncted to the porous layer 3 at the open ends through the phosphoric acid contained in the thin tubes 4A, 4B, and 4C. Further, in the present configuration, in order to test the characteristics when the positions of the air electrode 1 and the fuel electrode 2 are relatively shifted, the air electrode 1 is arranged to be shifted by 1.8 mm in the direction of the reference electrode 5B.

The unit cell of the conventional system shown in FIG. 10 is also formed by arranging an air electrode 1 and a fuel electrode 2 on both sides of a porous layer 3 having a thickness of 300 to 500 μm holding phosphoric acid. Three reference electrodes 5F, 5G, 5H are arranged on the surface of the porous layer 3 located outside the power generation part. In this unit cell, the fuel electrode 2 and the air electrode 1 were used to test the characteristics.
Are shifted by 1.0 mm.

FIG. 2 shows the I of the cell of this embodiment shown in FIG.
In the -V and IE characteristic diagrams, the characteristic A in the figure is the cell voltage, and the characteristics B, C, D, and E are the air electrode potential, the fuel electrode potential, and the potential of the reference electrode 5B with respect to the reference electrode 5A, respectively. , The potential of the reference electrode 5C. FIG. 11 is a diagram showing IV and IE characteristics of the conventional cell shown in FIG. 10. In FIG. 11, characteristic A is a cell voltage, and characteristics K, L, M, and N are reference electrode 5F, respectively. , The air electrode potential, the fuel electrode potential, the potential of the reference electrode 5G, and the potential of the reference electrode 5H.

As seen from the characteristics M and N, in the conventional method, a potential difference is observed between the reference electrodes, and the current density is 300 mA / cm ^2.
, A potential difference of 114 mV occurs. On the other hand, in the method of the present embodiment, no significant potential difference is observed between the reference electrodes as seen in the characteristics D and E, despite the large amount of displacement between the two electrodes. That is, if the potential is measured using the means of the present embodiment, the electrode potential can be measured properly without being affected by the disturbance of the electrolyte potential around the cell power generation portion caused by the displacement of the electrode. Therefore, by arranging a plurality of these measuring means and performing measurement, it is possible to properly measure the electrode potential distribution in the plane of the cell, and a situation in which a local battery is formed in the plane due to the gas concentration distribution and temperature distribution. If they do, they will be accurately grasped. <Embodiment 2> FIG. 3 is a basic configuration diagram showing a configuration for measuring an electrode potential distribution of a unit cell in a second embodiment of the present invention.
(A) is a plan view, and (b) is a cross-sectional view taken along the line YY in (a). The unit cell is formed by disposing an air electrode 1 and a fuel electrode 2 on both sides of a porous layer 3 having a thickness of about 100 μm, which is equivalent to that of a normal power generation cell holding phosphoric acid. Capillary tubes 4D and 4E containing phosphoric acid are inserted between the porous layer 3 and between the air electrode 1 and the porous layer 3, and are arranged in a liquid junction with the phosphoric acid of the porous layer 3 at the end openings. Have been. The other ends of the thin tubes 4D, 4E are connected to porous layers 3D, 3E provided with reference electrodes 5D, 5E, each of which is a reversible hydrogen electrode, and the reference electrodes 5D, 5E are connected to the phosphor layers incorporated in the thin tubes 4D, 4E. At the open end, it is liquid-juncted with the phosphoric acid of the porous layer 3 via an acid. The porous layers 3 of the thin tubes 4D and 4E
The open ends in the middle are arranged at positions where they are substantially overlapped with each other, that is, at positions where the interval δ shown in the drawing is very small. Therefore, the air electrode potential is not affected by the voltage drop of the porous layer 3 with respect to the reference electrode 5D, and is not affected by the voltage drop of the porous layer 3 with respect to the reference electrode 5E. Fuel electrode potential can be measured.

FIGS. 4 and 5 show I-I of the cell of this embodiment.
FIG. 4 shows the V, IE characteristics when the gas concentration is high,
FIG. 5 shows the characteristics when the gas concentration is low. In the figure, the characteristic A is the cell voltage, and the characteristics F, G, and H are the air electrode potential, the fuel electrode potential, and the potential of the reference electrode 5E, respectively, with reference to the reference electrode 5D. As shown in the figure, when the gas concentration decreases, the air electrode potential decreases, the fuel electrode potential increases, and the cell voltage decreases. However, the potential of the reference electrode 5E changes even when the gas concentration changes. No behavior was observed, indicating a behavior proportional to the current density. In the configuration of the present embodiment, since the potential difference between the reference electrode 5E and the reference electrode 5D is the difference between the electrolyte potential on the fuel electrode side and the electrolyte potential on the air electrode side, the value of the measured potential of the reference electrode 5E is It is presumed that this is due to a voltage drop in the porous layer 3 holding phosphoric acid.

To confirm this, the voltage drop was measured by the current interrupt method. FIG. 6 illustrates the voltage drop (J) measured by the current interrupt method and the potential value (H) of the reference electrode 5E. As can be seen from the figure, the difference between the value of the potential of the reference electrode 5E and the value of the voltage drop measured by the current interrupt method is in good agreement with about 6 mV when the current density is 300 mA / cm ^2. This indicates that the value of the measured potential of the reference electrode 5E is due to a voltage drop in the porous layer 3 holding phosphoric acid.

That is, as in the present embodiment, a potential formed by a thin tube containing phosphoric acid is provided between the porous layer holding phosphoric acid and the fuel electrode and between the porous layer holding phosphoric acid and the air electrode. If a probe is inserted and a potential difference between the probes is measured, a voltage drop proportional to the current density is directly measured, so that the current density can be known from this value. Therefore, if a plurality of pairs of reference electrodes each having a potential probe formed of a thin tube containing phosphoric acid are installed in the cell and the potential difference is measured, the in-plane current density distribution can be directly measured, and the gas density can be measured. If a situation occurs in which a local battery is formed in the plane due to the concentration distribution and the temperature distribution of, the temperature can be accurately grasped.

[0023]

As described above, according to the present invention, (1) Since the phosphoric acid type fuel cell is configured as described in claim 1, the positions of the disposed fuel electrode and air electrode are determined. Even in the case where there is a shift, the electrode potential distribution in the plane of the unit cell is properly measured, and a phosphoric acid fuel cell in which the operating state is accurately grasped can be obtained.

(2) Further, the phosphoric acid type fuel cell is claimed in claim 2
If it is configured as described in the above, it is possible to obtain a phosphoric acid type fuel cell in which the in-plane current density distribution is appropriately measured without being relaxed by the constituent members, and the operating state is accurately grasped. Become.

[Brief description of the drawings]

FIG. 1 is a basic configuration diagram showing a configuration for measuring an electrode potential distribution of a unit cell according to a first embodiment of the present invention, where (a) is a plan view and (b) is a cross section taken along the XX plane of (a). Figure

FIG. 2 is a diagram showing IV and IE characteristics of the cell according to the first embodiment;

3A and 3B are basic configuration diagrams illustrating a configuration for measuring an electrode potential distribution of a unit cell according to a second embodiment of the present invention, wherein FIG. 3A is a plan view, and FIG. 3B is a cross-section along the YY plane of FIG. Figure

FIG. 4 shows a graph of I- at a high gas concentration in the cell of the second embodiment.
V, IE characteristic diagram

FIG. 5 is a graph showing I- at a low gas concentration in the cell of the second embodiment.
V, IE characteristic diagram

FIG. 6 is a comparison diagram of a potential difference between reference electrodes of the cell of the second embodiment and a voltage drop measured by a current interrupt method.

FIG. 7 is a basic configuration diagram showing a conventional method for measuring an electrode potential distribution of a unit cell.

FIG. 8 is a basic configuration diagram showing a conventional method for measuring a current density distribution of a unit cell.

FIG. 9 is an explanatory view showing a cause of a potential difference between reference electrodes in a conventional method for measuring an electrode potential distribution of a unit cell.

10A and 10B are basic configuration diagrams illustrating a measurement configuration of an electrode potential distribution of a conventional unit cell used for comparison, in which FIG. 10A is a plan view, and FIG. 10B is a cross section taken along the ZZ plane of FIG. Figure

FIG. 11 shows a conventional unit cell I used for comparison.
-V, IE characteristic diagram

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Air electrode 2 Fuel electrode 3 Porous layer 3A, 3B, 3C, Porous layer 3D, 3E Porous layer 4A, 4B, 4C Capillary tube 4D, 4E Capillary tube 5A, 5B, 5C Reference electrode 5D, 5E Reference electrode 6 Power generation area 7 Separation plate 8A, 8B Voltmeter 10 Equipotential line 11 Proton flow 12 Electron flow

Claims (2)

[Claims] A unit cell comprising a porous layer holding phosphoric acid as an electrolyte sandwiched between an air electrode for performing an oxidation reaction and a fuel electrode for performing a reduction reaction via a separator. In a fuel cell, a thin tube filled with phosphoric acid or a porous body containing phosphoric acid, a mixture of phosphoric acid and powder, or a fiber holding phosphoric acid, and a potential measurement liquid-junction at one open end And the other open end of the thin tube is inserted into the porous layer of the power generation area where the air electrode and the fuel electrode overlap, and the phosphoric acid inside the thin tube is contained in the porous layer. A phosphoric acid-type fuel cell comprising at least one means for measuring an electrode potential formed by liquid junction with phosphoric acid. 2. A phosphoric acid in which unit cells each comprising a porous layer holding phosphoric acid as an electrolyte sandwiched between an air electrode for performing an oxidation reaction and a fuel electrode for performing a reduction reaction via a separator. In a fuel cell, a thin tube filled with phosphoric acid or a porous body containing phosphoric acid, a mixture of phosphoric acid and powder, or a fiber holding phosphoric acid, and a potential measurement liquid-junction at one open end And the other open end of the thin tube is inserted between the fuel layer and the porous layer of the power generation area where the air electrode and the fuel electrode overlap, and phosphoric acid inside the thin tube is removed. A first potential measuring means which is liquid junction with phosphoric acid in the porous layer, and phosphoric acid, or a porous body containing phosphoric acid, a mixture of phosphoric acid and powder, or a fiber holding phosphoric acid. Filled capillary and potential measurement liquid junction at one open end And the other open end of the thin tube is inserted between the porous layer and the air electrode of the power generation area, and the phosphoric acid inside the thin tube is mixed with the phosphoric acid in the porous layer and the liquid. A phosphoric acid fuel cell comprising at least one pair of electrolyte potential difference measuring means comprising a second potential measuring means which is entangled.