Device and method for measuring effective diffusion coefficient of porous electrode in reducing atmosphere

Abstract

The invention discloses a device and a method for measuring the effective diffusion coefficient of a porous electrode in a reducing atmosphere, and belongs to the technical field of solid oxide fuel cells. The cathode of the solid oxide fuel cell is divided into four areas, a to-be-measured anode gap is polished between the area I and the area II, and the diffusion coefficient of the anode supporting layer of the solid oxide fuel cell is measured by adopting an oxygen pump method combined with a limiting current concept. According to the device disclosed by the invention, even if the device is difficult to seal in a high-temperature environment and the gas concentration measurement precision is not high, a stable and accurate diffusion coefficient can be obtained by adopting the method disclosed by the invention.

Description

Technical Field

[0001] The present invention belongs to the technical field of measuring the effective diffusion coefficient of porous electrodes, and in particular relates to a device and method for measuring the effective diffusion coefficient of a porous electrode in a reducing atmosphere. Background Art

[0002] As an efficient and clean energy conversion device, the performance of fuel cells is affected by polarization losses, which include concentration losses, ohmic losses, and activation losses. Concentration losses occur throughout the entire fuel gas and air transport process, of which the loss caused by gas passing through porous electrodes accounts for the majority of the concentration losses in the entire fuel cell stack. As fuel cells develop towards higher power densities, the impact of concentration losses becomes increasingly prominent, causing the cell output voltage to drop, which in turn affects overall efficiency. Concentration losses mainly come from the diffusion obstruction of the porous electrodes of fuel cells. Determining the diffusion coefficient of the electrodes is important for optimizing the electrode structure and reducing concentration losses.

[0003] Common diffusion coefficient measurement methods include the Loschmidt cell method, which primarily involves a diffusion coefficient measurement device based on an optimized Loschmidt diffusion cell design. A gas sensor is used to measure the concentration change over time, and Fick's second law is used to dynamically fit the effective diffusion coefficient of the material. This method requires high gas concentration measurement accuracy, and the sealing of the environmental chamber during high-temperature measurements is difficult, resulting in unstable and inaccurate diffusion coefficients. The WK diffusion cell method, which primarily involves a diffusion coefficient measurement device based on a modified Wicke–Kallenbach diffusion cell design, uses a concentration sensor to measure the steady-state concentration of gas on both sides of the porous material. The diffusion flux is calculated using the law of conservation of mass, and the diffusion coefficient D is calculated in conjunction with Fick's first law. This method requires high gas concentration measurement accuracy, and both the diffusion flux and concentration gradient rely on concentration measurements. The electrolytic cell method or oxygen pump method uses an oxygen pump cell to provide a known diffusion flux. The voltage data is substituted into the Nernst potential equation to determine the concentration inside the chamber. Finally, the diffusion coefficient D is calculated using Fick's first law. However, current limitations of this method include small concentration differences inside and outside the chamber, low test voltages, weak interference immunity, and slight leaks that significantly reduce voltage measurement accuracy. The limiting current method supplies a high-flow, low-concentration atmosphere. By applying a limiting current, the gas concentration at the electrode-electrolyte interface is reduced to zero, and the concentration gradient is determined. The diffusion flux is determined by the limiting current, and the effective diffusion coefficient D is calculated using Fick's law. This method cannot accurately determine whether the concentration at the porous medium interface is zero during testing, and there may be errors in the limiting current value, which may lead to an inaccurate effective diffusion coefficient. Summary of the Invention

[0004] The purpose of the present invention is to provide a device and method for measuring the effective diffusion coefficient of a porous electrode in a reducing atmosphere. Even if the device is difficult to seal and the gas concentration measurement accuracy is not high in a high temperature environment, the method can still obtain a stable and accurate diffusion coefficient.

[0005] To achieve the above-mentioned purpose, the present invention adopts a device for measuring the effective diffusion coefficient of a porous electrode in a reducing atmosphere, comprising a solid oxide fuel cell, a cathode flow channel plate and an anode flow channel plate; wherein the solid oxide fuel cell comprises an anode, an electrolyte and a cathode from bottom to top, and the solid oxide fuel cell is divided into N rectangular cathode areas of equal area from the cathode side, and the cathode areas are evenly distributed from left to right, and the electrolyte leaks between each area. An anode gap to be measured is polished between the first area and the second area, and the solid oxide fuel cell is sandwiched between the cathode flow channel plate and the anode flow channel plate, and both sides of the cathode flow channel plate are completely closed and airtight, and both sides of the anode flow channel plate are open, and the working gas hydrogen-water vapor mixture flows in from the side of the anode flow channel plate close to the anode gap to be measured and flows out from the side away from the anode gap to be measured.

[0006] As a further solution of the present invention, N=4, which are zone I, zone II, zone III, and zone IV, respectively. The electrolyte leaks between zone I and zone II, between zone II and zone III, and between zone III and zone IV, respectively, and an anode gap to be measured is polished between zone I and zone II. In order to avoid local hydrogen-oxygen reaction interfering with the H2 diffusion path, the electrolysis zone is set in zone III and zone IV away from the anode gap to be measured. Therefore, zone III and zone IV are the electrolysis zones of the solid oxide fuel cell. In order to make the test voltage accurately reflect the concentration gradient at the anode gap, zone I close to the anode gap to be measured and away from the electrolysis zone is selected as the voltage measurement point.

[0007] As a further solution of the present invention, the device can directly measure the effective diffusion coefficient of the anode layer of the finished solid oxide fuel cell in the working state.

[0008] A method for measuring the effective diffusion coefficient of a porous electrode in a reducing atmosphere employs an oxygen pump method incorporating a limiting current concept to measure the diffusion coefficient. The oxygen pump method incorporating a limiting current concept comprises the following steps: introducing a hydrogen-water vapor mixture through an anode flow channel plate; allowing hydrogen and water vapor on the anode side to diffuse toward the cathode side through a slit in the anode to be measured under the action of concentration and pressure differentials; electrolyzing one region in the electrolysis zone; measuring the open circuit voltage of each of the other three regions; and recording the voltage after diffusion reaches a steady state.

[0009] The oxygen produced in the electrolysis zone on the cathode side reacts with hydrogen to generate water. The partial pressure of H2 on the cathode side decreases, while the partial pressure of H2O increases. The fluxes of H2 and H2O diffusing through the anode cracks due to the partial pressure gradient on the anode crack to be measured are In steady state, the flux of H2 and H2O through the anode gap to be measured and the flux of O2 produced by electrolysis The relationship between the current I is:

[0010]

[0011] Where F is Faraday's constant, according to Fick's law, It can be expressed as:

[0012]

[0013] Where D is the diffusion coefficient, A is the area of ​​the anode gap to be measured, is the concentration of H2, R is the ideal gas constant, T is the temperature, P total is one atmosphere pressure, x is the thickness of the diffusion layer, is the volume fraction of H2; simplifying the formula, we can get:

[0014]

[0015] Where, δ is the thickness of the anode to be measured, I is the electrolysis current, is the volume fraction of H2 on the anode side. Since the flow rate on the anode side is large, the hydrogen concentration is not affected by diffusion, so is a constant value, equal to the volume fraction of H2 on the initial anode side, is the volume fraction of H2 on the cathode side;

[0016] The measured circuit voltage E can be calculated using the Nernst equation:

[0017]

[0018] in, and The oxygen partial pressures at the cathode and anode are respectively, and the equilibrium constant K of the equation 2H2(g)+O2(g)=2H2O(g) is used. eq have to,

[0019]

[0020] Where, is the partial pressure of H2O, is the partial pressure of O2, is the partial pressure of H2; Substituting equation ⑤ into equation ④, we get

[0021]

[0022] Where, is the volume fraction of H2O on the cathode side, is the volume fraction of H2O on the anode side, and the total pressure on both the positive and negative sides of the battery is P total=1atm,

[0023]

[0024] Rearrange the formula to get

[0025]

[0026] Substituting formula ⑧ into formula ③ can obtain the effective diffusion coefficient D; when the electrolysis current is close to the limiting current I limit When the hydrogen partial pressure on the cathode side approaches 0, equation ③ is simplified again to:

[0027]

[0028] Directly bring in the limiting current I limit The diffusion coefficient can be obtained by the value.

[0029] As a further solution of the present invention, the flow rate of the hydrogen-water vapor mixture introduced into the anode flow channel plate is 2-3 L / min, and the content percentage of hydrogen-water vapor is: 25%-75%:75%-25%.

[0030] As a further solution of the present invention, during electrolysis, the current is set to increase from a small current gradient to a limiting current. By adopting a large current electrolysis, the cathode oxygen partial pressure is increased and the leakage of the device is slowed down. During the high temperature reaction of 720°C, the oxygen concentration on the cathode side is increased, the oxygen concentration gradient inside and outside the device is reduced, the leakage of the device is reduced, the open circuit voltage is accurately measured, and a stable and accurate diffusion coefficient is obtained.

[0031] As a further solution of the present invention, the voltage in zone I reaching 0.3V to 0.7V is used as a standard to determine whether the electrolysis current has reached the limiting current of the device.

[0032] As a further solution of the present invention, the calculated diffusion coefficient curve will have a platform area near the limiting current, and the diffusion coefficient in the platform area is the measured diffusion coefficient of the anode crack to be tested.

[0033] Compared with the existing technology, the present invention uses a regional solid oxide fuel cell for measurement. Under the conditions of a high temperature of 720°C, where the device is difficult to seal and the gas concentration measurement accuracy is low, the limiting current concept and the oxygen pump method are combined. By gradually increasing the electrolysis current to the limiting current, the oxygen leakage of the device is effectively reduced, and the problem of the diffusion coefficient changing with the current due to device leakage is solved. The anti-interference ability of the device is improved, the diffusion coefficient platform area is measured, and a stable and accurate diffusion coefficient is obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1It is a schematic structural diagram of the device for measuring the effective diffusion coefficient of the porous electrode in reducing atmosphere according to the present invention.

[0035] Figure 2 This is the oxygen pump method combined with the limiting current concept in the present invention, electrolysis zone III, anode 2L total flow 25% H275% H2O, non-electrolysis zone voltage diagram.

[0036] Figure 3 This is the oxygen pump method combined with the limiting current concept in the present invention, electrolysis zone III, anode 2L total flow 40% H260% H2O, non-electrolysis zone voltage diagram.

[0037] Figure 4 This is the oxygen pump method combined with the limiting current concept in the present invention, electrolysis zone III, anode 2L total flow 60% H240% H2O, non-electrolysis zone voltage diagram.

[0038] Figure 5 This is the oxygen pump method combined with the limiting current concept in the present invention, electrolysis zone III, anode 2L total flow 75% H225% H2O, non-electrolysis zone voltage diagram.

[0039] Figure 6 This is the oxygen pump method combined with the limiting current concept in the present invention, electrolysis zone IV, anode 2L total flow 25% H275% H2O, non-electrolysis zone voltage diagram.

[0040] Figure 7 This is the oxygen pump method combined with the limiting current concept in the present invention, electrolysis zone IV, anode 2L total flow 40% H260% H2O, non-electrolysis zone voltage diagram.

[0041] Figure 8 This is the oxygen pump method combined with the limiting current concept in the present invention, electrolysis zone IV, anode 2L total flow 60% H240% H2O, non-electrolysis zone voltage diagram.

[0042] Figure 9 This is the oxygen pump method combined with the limiting current concept in the present invention, electrolysis zone IV, anode 2L total flow 75% H225% H2O, non-electrolysis zone voltage diagram.

[0043] Figure 10 This is the oxygen pump method combined with the limiting current concept in the present invention, electrolysis zone III, anode 3L total flow 25% H260% H2O, non-electrolysis zone voltage diagram.

[0044] Figure 11 This is the oxygen pump method combined with the limiting current concept in the present invention, electrolysis zone III, anode 3L total flow 40% H240% H2O, non-electrolysis zone voltage diagram.

[0045] Figure 12This is the oxygen pump method combined with the limiting current concept in the present invention, electrolysis zone III, anode 3L total flow 60% H225% H2O, non-electrolysis zone voltage diagram.

[0046] Figure 13 It is the diffusion coefficient under different working conditions measured by the oxygen pump method combined with the limiting current concept in the present invention.

[0047] In the figure: 1. Solid oxide fuel cell, 2. Anode, 3. Electrolyte, 4. Cathode, 5. Region I, 6. Region II, 7. Region III, 8. Region IV, 9. Anode gap to be tested, 10. Cathode flow channel plate, 11. Anode flow channel plate. DETAILED DESCRIPTION

[0048] The present invention will be further described below with reference to the accompanying drawings.

[0049] like Figure 1 As shown, a device for measuring the effective diffusion coefficient of a porous electrode in a reducing atmosphere includes a solid oxide fuel cell 1, a cathode flow channel plate 10, and an anode flow channel plate 11; the solid oxide fuel cell 1 includes, from bottom to top, an anode 2, an electrolyte 3, and a cathode 4, and the solid oxide fuel cell 1 is divided from the cathode side into several rectangular cathode regions of equal area, and the cathode regions are evenly distributed from left to right. In this embodiment, the cathode regions are divided into four rectangular cathode regions, namely, region I 5, region II 6, region III 7, and region IV 8. Electrolyte 3 leaks between region I 5 and region II 6, between region II 6 and region III 7, and between region III 7 and region IV 8, and an anode gap 9 to be measured is polished between region I 5 and region II 6. The solid oxide fuel cell 1 is sandwiched between a cathode flow channel plate 10 and an anode flow channel plate 11. Both sides of the cathode flow channel plate 10 are completely sealed and airtight, while both sides of the anode flow channel plate 11 are open. The working gas, hydrogen-water vapor mixture, flows into the anode flow channel plate 11 from the side close to the anode gap (9) to be tested and flows out from the side away from the anode gap (9) to be tested.

[0050] In order to avoid local hydrogen-oxygen reaction interfering with the H2 diffusion path, the electrolysis zone is set in zone III 7 and zone IV 8 away from the anode gap to be measured. Therefore, zone III 7 and zone IV 8 are the electrolysis zones of the solid oxide fuel cell 1. In order to make the test voltage accurately reflect the concentration gradient at the anode gap, zone I 5 close to the anode gap to be measured and away from the electrolysis zone is selected as the voltage measurement point.

[0051] The device can directly measure the effective diffusion coefficient of the anode layer of a finished solid oxide fuel cell under working conditions.

[0052] A method for measuring the effective diffusion coefficient of a porous electrode using an SOFC device employs an oxygen pump method incorporating a limiting current concept to measure the diffusion coefficient. The oxygen pump method incorporating a limiting current concept comprises the following steps: a large flow of a hydrogen-water vapor mixture is introduced into an anode flow channel plate 11; hydrogen and water vapor on the anode side diffuse toward the cathode side through anode slits 9 to be measured under the action of concentration and pressure differentials, electrolyzing one region of the solid oxide fuel cell 1 during electrolysis; measuring the open circuit voltage of each of the other three regions; and recording the voltage after diffusion reaches a steady state.

[0053] When the oxygen pump method combining the limiting current concept is used, when oxygen is produced by electrolysis on the cathode side, oxygen reacts with hydrogen to form water. The partial pressure of H2 on the cathode side decreases, while the partial pressure of H2O increases. The flux of H2 and H2O diffusing through the anode crack caused by the partial pressure gradient on the anode crack 9 to be measured is In steady state, the flux of H2 and H2O through the anode gap 9 to be measured is equal to the flux of O2 produced by electrolysis. The relationship between the current I is:

[0054]

[0055] Where F is Faraday's constant, according to Fick's law, It can be expressed as:

[0056]

[0057] Wherein, D is the diffusion coefficient, A is the area of ​​the anode gap 9 to be measured, is the concentration of H2, R is the ideal gas constant, T is the temperature, P total is one atmosphere pressure, x is the thickness of the diffusion layer, is the volume fraction of H2; simplifying the formula, we can get:

[0058]

[0059] Where, δ is the thickness of the anode to be measured, I is the electrolysis current, and F is the Faraday constant. is the volume fraction of H2 on the anode side. Since the flow rate on the anode side is large, the hydrogen concentration is not affected by diffusion, so is a constant value, equal to the volume fraction of H2 on the initial anode side, is the volume fraction of H2 on the cathode side;

[0060] The farther the non-electrolysis zone is from the electrolysis source and closer to the hydrogen source, the lower the voltage is. That is, the voltage in Zone I is the lowest. Because the anode gap 9 to be measured is the hydrogen source, the hydrogen concentration difference here is the smallest, and the theoretical Nernst potential is the lowest. Therefore, Zone I, with its lowest voltage, is selected as the voltage measurement point. The concentration difference across the anode gap 9 to be measured is calculated when electrolysis reaches stability at different currents.

[0061] The measured open circuit voltage E in zone I can be calculated using the Nernst equation:

[0062]

[0063] in, and The oxygen partial pressures at the cathode and anode are respectively, and the equilibrium constant K of the equation 2H2(g)+O2(g)=2H2O(g) is used. eq have to,

[0064]

[0065] Where, is the partial pressure of H2O, is the partial pressure of O2, is the partial pressure of H2; Substituting equation ⑤ into equation ④, we get

[0066]

[0067] Where, is the volume fraction of H2O on the cathode side, is the volume fraction of H2O on the anode side, and the total pressure on both the positive and negative sides of the battery is P total =1atm,

[0068]

[0069] Rearrange the formula to get

[0070]

[0071] Substituting formula ⑧ into formula ③ can obtain the effective diffusion coefficient D; when the electrolysis current is the limiting current I limit When the hydrogen partial pressure on the cathode side approaches 0, equation ③ is simplified again to:

[0072]

[0073] Directly bring in the limiting current I limit The diffusion coefficient can be obtained by the value.

[0074] The flow rate of the hydrogen-water vapor mixture introduced into the anode flow channel plate 11 is 2-3 L / min, and the content percentage of hydrogen-water vapor is: 25%-75%:75%-25%.

[0075] Due to the limited airtightness of the device, leakage of external oxygen into the device during the measurement process is unavoidable. Due to the difficulties in experimental operation and sealing when directly measuring concentration using a hydrogen sensor in a high-temperature environment, it is impossible to directly measure the hydrogen concentrations Cin and Cout on the inside and outside of the test sample. This method uses the Nernst potential to measure hydrogen concentration. The concentration gradient on the entire closed cathode side results in molecular diffusion-dominated diffusion, resulting in different concentration values ​​measured from C1 to Cn. Compared to the traditional oxygen pump method, while setting up multiple measurement points, a concentration measurement point C1 is first established on the side of the test sample away from the electrolysis zone. This concentration measurement point is located away from the main diffusion path, and its concentration is closer to the hydrogen concentration Cin within the test sample. Forced convection is used outside the anode crack to ensure that the concentration outside is more stable than the inlet gas concentration. When using the oxygen pump method with the limiting current concept, oxygen leakage is more likely to enter the sealed, unvented cathode side than to the anode side, which is fed with a large flow of mixed gases of varying concentrations. This results in an open-circuit voltage higher than the theoretical value and a diffusion coefficient lower than the theoretical value. However, as the electrolysis current increases, the oxygen concentration on the cathode side gradually increases, the driving force for external oxygen to leak into the cathode side gradually decreases, the amount of oxygen leakage gradually decreases, the cathode side potential is less affected by leakage, the battery voltage tends to the theoretical value, the diffusion coefficient gradually tends to the theoretical value and remains stable, and a diffusion coefficient platform area appears. The diffusion coefficient in the platform area is the measured diffusion coefficient of the anode crack 9 to be tested. Figure 13 shown.

[0076] The electrolysis current is determined to have reached the device's limiting current when the voltage in zone Ⅰ 5 reaches 0.3V to 0.7V.

[0077] The specific embodiment of the measuring method of the device for measuring the effective diffusion coefficient of the porous electrode in a reducing atmosphere is as follows:

[0078] The test cell is made of a 10cm×10cm flat anode-supported SOFC cell. The anode is made of NiO-8YSZ (8% Y2O3-ZrO2) with a thickness of about 500μm, the electrolyte is made of dense 8YSZ electrolyte with a thickness of about 15μm; the cathode is made of GDC and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-σ (LSCF) mixture, about 30μm thick, the cathode material is divided into four sections, evenly spaced and coated on the NiO-8YSZ (8% Y2O3-ZrO2) || 8YSZ half-cell; the battery is divided into four mutually insulated areas according to the cathode, such as Figure 1 As shown, between the insulating sections of region I5 and region II6, the electrolyte is ground away to form an anode gap 9 with a width of 1 mm and a length of 50 mm. The anode gap 9 to be tested is a porous anode. The cathode flow channel plate 10 is made of stainless steel SUS430.

[0079] Example 1: A 25% hydrogen-75% water vapor mixture, a 40% hydrogen-60% water vapor mixture, a 60% hydrogen-40% water vapor mixture, and a 75% hydrogen-25% water vapor mixture are introduced into the anode of a solid oxide fuel cell 1 at 2 L / min, and the electrolysis of zone III 7 of the solid oxide fuel cell 1 is performed. The electrolysis circuit measures the open circuit voltage of zone I 5, zone II 6, and zone IV 8 that are not electrolyzed. Under the action of concentration and pressure differences, hydrogen and water vapor on the anode side diffuse through the anode gap to be measured to the cathode side. The open circuit voltage of the non-electrolysis zone is measured, as shown in FIG. Figures 2 to 5 shown.

[0080] Among them Figure 2 As shown, when a 25% hydrogen-75% water vapor mixture is introduced into the anode at 2 L / min, zone III 7 of the solid oxide fuel cell 1 is electrolyzed; when the electrolysis current is less than the limit value, taking 1 A as an example, the H2 volume fraction on the cathode side is calculated using formula ⑧:

[0081]

[0082] Substitute the result into formula ③

[0083]

[0084] When the electrolysis current is 2A, the voltage in zone Ⅰ 5 reaches the limit value, and it is substituted into formula ⑨

[0085]

[0086] Similarly, when a 40% hydrogen-60% water vapor mixture is introduced, when the electrolysis current is less than the limit value, taking 1A as an example, the H2 volume fraction on the cathode side is calculated using formula ⑧ is 0.215, and the effective diffusion coefficient D is 2.28×10 -5 m 2 / s; When the electrolysis current is 3.5A, the voltage in zone Ⅰ reaches the limit value. Substituting it into formula ⑨, the effective diffusion coefficient D is 3.69×10 - 5 m 2 / s.

[0087] When a 60% hydrogen-40% water vapor mixture is introduced and the electrolysis current is less than the limit value, taking 1A as an example, the H2 volume fraction on the cathode side is calculated using formula ⑧ is 0.376, and the effective diffusion coefficient D is 1.88×10 -5 m 2 / s; When the electrolysis current is 5A, the voltage in zone I 5 reaches the limit value. Substituting it into formula 9, the effective diffusion coefficient D is 3.52×10-5 m 2 / s.

[0088] When a 75% hydrogen-25% water vapor mixture is introduced and the electrolysis current is less than the limit value, taking 1A as an example, the H2 volume fraction on the cathode side is calculated using formula ⑧ is 0.5, and the result is put into formula ③, and the effective diffusion coefficient D is 1.69×10 -5 m 2 / s, when the electrolysis current is 6.5A, the voltage in zone I 5 reaches the limit value, and when it is substituted into formula 9, the effective diffusion coefficient D is 3.66×10 -5 m 2 / s.

[0089] Example 2: A 25% hydrogen-75% water vapor mixture, a 40% hydrogen-60% water vapor mixture, a 60% hydrogen-40% water vapor mixture, and a 75% hydrogen-25% water vapor mixture were introduced into the anode of a solid oxide fuel cell 1 at a flow rate of 2 L / min. The electrolysis current was gradually increased and then decreased. The IV region 8 of the solid oxide fuel cell 1 was electrolyzed. Under the action of concentration and pressure differences, hydrogen and water vapor on the anode side diffused through the anode gap 9 to be measured to the cathode side. The open circuit voltage of the non-electrolysis region was measured, as shown in FIG. Figures 6-9 shown.

[0090] Among them, when 2L / min of 25% hydrogen-75% water vapor mixture is introduced, when the electrolysis current is less than the limit value, taking 1A as an example, the H2 volume fraction on the cathode side is calculated using formula ⑧ is 0.131, and the result is put into formula ③ to obtain the effective diffusion coefficient D of 3.55×10 -5 m 2 / s; When the electrolysis current is 2.5A, the voltage in zone I 5 reaches the limit value. Substituting it into formula 9, the effective diffusion coefficient D is 4.22×10 -5 m 2 / s.

[0091] A 40% hydrogen-60% water vapor mixture is introduced at a flow rate of 2 L / min. When the electrolysis current is less than the limit value, taking 1 A as an example, the H2 volume fraction on the cathode side is calculated using formula ⑧. is 0.253. Substituting the result into formula ③, the effective diffusion coefficient D is 2.87×10 -5 m 2 / s, when the electrolysis current is 3.5A, the voltage in zone I 5 reaches the limit value, and when it is substituted into formula 9, the effective diffusion coefficient D is 3.69×10 -5 m 2 / s.

[0092] A 60% hydrogen-40% water vapor mixture is introduced at a flow rate of 2 L / min. When the electrolysis current is less than the limit value, taking 1 A as an example, the H2 volume fraction on the cathode side is calculated using formula ⑧ is 0.404. Substituting the result into formula ③, the effective diffusion coefficient D is 2.15×10 -5 m 2 / s, when the electrolysis current is 5.5A, the voltage in zone I 5 reaches the limit value, and when it is substituted into formula 9, the effective diffusion coefficient D is 3.87×10 -5 m 2 / s.

[0093] A 75% hydrogen-25% water vapor mixture is introduced at a flow rate of 2 L / min. When the electrolysis current is less than the limit value, taking 1 A as an example, the H2 volume fraction on the cathode side is calculated using formula ⑧. is 0.57. Substituting the result into formula ③, the effective diffusion coefficient D is 2.35×10 -5 m 2 / s, when the electrolysis current is 7A, the voltage in zone I 5 reaches the limit value, and when it is substituted into formula 9, the effective diffusion coefficient D is 3.94×10 -5 m 2 / s.

[0094] Example 3: A 25% hydrogen-75% water vapor mixture, a 40% hydrogen-60% water vapor mixture, and a 60% hydrogen-40% water vapor mixture were introduced into the anode of a solid oxide fuel cell 1 at a flow rate of 3 L / min, respectively. Zone III 7 of the solid oxide fuel cell 1 was electrolyzed. Under the influence of concentration and pressure differences, the anode hydrogen and water vapor diffused through the anode gap 9 to be measured toward the cathode. The open circuit voltage of the non-electrolysis zone was measured, as shown in Figure 3. Figures 10-12 shown.

[0095] Among them, when 3L / min of 25% hydrogen-75% water vapor mixture is introduced, when the electrolysis current is less than the limit value, taking 1A as an example, the H2 volume fraction on the cathode side is calculated using formula ⑧ is 0.109, and the result is put into formula ③ to obtain the effective diffusion coefficient D of 2.99×10 -5 m 2 / s, when the electrolysis current is 2.5A, the voltage in zone I 5 reaches the limit value, and when it is substituted into formula 9, the effective diffusion coefficient D is 4.22×10 -5 m 2 / s.

[0096] A 40% hydrogen-60% water vapor mixture is introduced at a flow rate of 3 L / min. When the electrolysis current is less than the limit value, taking 1 A as an example, the H2 volume fraction on the cathode side is calculated using formula ⑧. is 0.215, and the result is put into formula ③ to obtain the effective diffusion coefficient D of 2.28×10-5 m 2 / s, when the electrolysis current is 3.5A, the voltage in zone I 5 reaches the limit value, and when it is substituted into formula 9, the effective diffusion coefficient D is 3.69×10 -5 m 2 / s.

[0097] A 60% hydrogen-40% water vapor mixture is introduced at a flow rate of 3 L / min. When the electrolysis current is less than the limit value, taking 1 A as an example, the H2 volume fraction on the cathode side is calculated using formula ⑧. is 0.376. Substituting the result into formula ③, the effective diffusion coefficient D is 1.88×10 -5 m 2 / s, when the electrolysis current is 5A, the voltage in zone I 5 reaches the limit value, and when it is substituted into formula 9, the effective diffusion coefficient D is 3.52×10 -5 m 2 / s.

Claims

1. A device for measuring the effective diffusion coefficient of a porous electrode in a reducing atmosphere, characterized in that: The invention comprises a solid oxide fuel cell (1), a cathode flow channel plate (10) and an anode flow channel plate (11); wherein the solid oxide fuel cell (1) comprises an anode (2), an electrolyte (3) and a cathode (4) in order from bottom to top; the solid oxide fuel cell (1) is divided into N rectangular cathode regions of equal area from the cathode side; the cathode regions are evenly distributed from left to right; the electrolyte leaks between the regions; an anode gap (9) to be tested is polished between the first region and the second region; the solid oxide fuel cell (1) is sandwiched between the cathode flow channel plate (10) and the anode flow channel plate (11); both sides of the cathode flow channel plate (10) are completely sealed and airtight; both sides of the anode flow channel plate (11) are open; a working gas hydrogen-water vapor mixture flows in from the side of the anode flow channel plate (11) close to the anode gap (9) to be tested and flows out from the side away from the anode gap (9) to be tested.

2. The device for measuring the effective diffusion coefficient of a porous electrode in a reducing atmosphere according to claim 1, characterized in that: N=4, which are zone I (5), zone II (6), zone III (7), and zone IV (8). Electrolyte (3) leaks between zone I (5) and zone II (6), between zone II (6) and zone III (7), and between zone III (7) and zone IV (8), and an anode gap (9) to be tested is polished between zone I (5) and zone II (6); zone III (7) and zone IV (8) are electrolysis zones of the solid oxide fuel cell (1), and zone I (5) is a voltage measurement point.

3. The device for measuring the effective diffusion coefficient of a porous electrode in a reducing atmosphere according to claim 2, characterized in that: The device can directly measure the effective diffusion coefficient of the anode layer of a finished solid oxide fuel cell under working conditions.

4. A method for measuring the effective diffusion coefficient of a porous electrode in a reducing atmosphere according to any one of claims 1 to 3, characterized in that: The diffusion coefficient is measured by an oxygen pump method combined with a limiting current concept. The oxygen pump method combined with a limiting current concept comprises the following steps: a hydrogen-water vapor mixture is introduced into an anode flow channel plate (11), hydrogen and water vapor on the anode side diffuse toward the cathode side through a measured anode gap (9) under the action of concentration difference and pressure difference, one area in the electrolysis zone is electrolyzed, the open circuit voltage of each of the other three areas is measured, and the voltage after diffusion to a steady state is recorded; The oxygen generated in the electrolysis zone on the cathode (4) side reacts with hydrogen to generate water. The partial pressure of H2 on the cathode side decreases, while the partial pressure of H2O increases. The fluxes of H2 and H2O diffusing through the anode cracks due to the partial pressure gradient on the anode crack (9) to be measured are respectively In steady state, the flux of H2 and H2O through the anode gap (9) to be measured is equal to the flux of O2 produced by electrolysis. The relationship between the current I is: Where F is Faraday's constant, according to Fick's law, It can be expressed as: Wherein, D is the diffusion coefficient, A is the area of ​​the anode gap (9) to be measured, is the concentration of H2, R is the ideal gas constant, T is the temperature, P total is one atmosphere pressure, x is the thickness of the diffusion layer, is the volume fraction of H2; simplifying the formula, we can get: Where, δ is the thickness of the anode to be measured, I is the electrolysis current, is the volume fraction of H2 on the anode side. Since the flow rate on the anode side is large, the hydrogen concentration is not affected by diffusion, so is a constant value, equal to the volume fraction of H2 on the initial anode side, is the volume fraction of H2 on the cathode side; The measured open circuit voltage E is calculated using the Nernst equation: in, and The oxygen partial pressures at the cathode and anode are respectively, and the equilibrium constant K of the equation 2H2(g)+O2(g)=2H2O(g) is used. eq ,have to, Where, is the partial pressure of H2O, is the partial pressure of O2, is the partial pressure of H2; Substituting equation ⑤ into equation ④, we get Where, is the volume fraction of H2O on the cathode side, is the volume fraction of H2O on the anode side, and the total pressure on both the positive and negative sides of the battery is P total =1atm, Rearrange the formula to get Substituting formula ⑧ into formula ③ can obtain the effective diffusion coefficient D; when the electrolysis current is the limiting current I limit When the hydrogen partial pressure on the cathode side approaches 0, equation ③ is simplified again to: Directly bring in the limiting current I limit The diffusion coefficient can be obtained by the value.

5. The method according to claim 3, characterized in that The flow rate of hydrogen-water vapor mixed gas introduced into the anode flow channel plate (11) is 2-3 L / min, and the content percentage of hydrogen-water vapor is: 25%-75%: 75%-25%.

6. The method according to claim 3, characterized in that During electrolysis, the current is set to increase from a small current gradient to the limiting current. By adopting large current electrolysis, the cathode oxygen partial pressure is increased and the leakage of the device is slowed down. During the high-temperature reaction at 720°C, the oxygen concentration on the cathode side is increased, the oxygen concentration gradient inside and outside the device is narrowed, the leakage of the device is reduced, the open circuit voltage is accurately measured, and a stable and accurate diffusion coefficient is obtained.

7. The method according to claim 3, characterized in that The voltage of zone I (5) reaching 0.3V to 0.7V is used as the standard to determine whether the electrolysis current reaches the device limiting current.

8. The method according to claim 3, characterized in that The calculated diffusion coefficient curve will have a platform area near the limiting current, and the diffusion coefficient in the platform area is the measured diffusion coefficient of the anode crack (9) to be tested.

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