CN114597429A

CN114597429A - Method for improving cathode flooding of miniature direct methanol fuel cell

Info

Publication number: CN114597429A
Application number: CN202210256538.3A
Authority: CN
Inventors: 赵振刚; 张帆; 张延辉; 张大骋
Original assignee: Kunming University of Science and Technology
Current assignee: Kunming University of Science and Technology
Priority date: 2022-03-16
Filing date: 2022-03-16
Publication date: 2022-06-07
Anticipated expiration: 2042-03-16
Also published as: CN114597429B

Abstract

The invention discloses a method for improving cathode flooding of a miniature direct methanol fuel cell, which comprises the following steps: 1) preparing a cathode collector plate by selecting foamed stainless steel, processing the cathode collector plate by using a laser cutting platform, and then polishing the surface of the cathode collector plate smoothly; 2) sequentially cleaning a cathode current collecting plate by using methanol, ethanol and deionized water, and soaking the dried cathode current collecting plate in a strong alkali solution for corrosion; 3) and (4) washing the corroded cathode collector plate by using deionized water, and finally putting the cathode collector plate into a drying box for drying, wherein the obtained cathode collector plate is applied to the mu DMFC. The foamed stainless steel cathode collector plate (CCC) is prepared by a KOH solution corrosion method, has better gradient wettability, can better discharge liquid water, slow down cathode flooding, reduce total impedance, improve stability, and has the advantages of simple process, high efficiency and low cost.

Description

Method for improving cathode flooding of miniature direct methanol fuel cell

Technical Field

The invention relates to the field of battery cathodes, in particular to a method for improving cathode flooding of a miniature direct methanol fuel battery.

Background

With the rapid development of the industry, fossil fuels are being playedThe intense consumption and the replacement of fossil energy by new energy become hot spots of research. The micro Direct Methanol Fuel Cell (DMFC) has the advantage of environmental protection, and the working principle of the micro direct methanol fuel cell is to generate electric energy by internal oxidation-reduction reaction. The reduction reaction on the cathode side can generate a large amount of water, and the water production speed on the cathode side is faster and faster along with factors such as methanol permeation, electroosmosis resistance, back diffusion, condensation and evaporation. When the rate of water formation exceeds the rate of discharge, excess water can severely block the gas passages, resulting in flooding. Once this phenomenon occurs, O₂The reaction sites are not uniformly reached and the reactants are starved on the cathode side. Then, the output of the μ DMFC has the disadvantages of large fluctuation and low power, and the performance and durability thereof are seriously affected. Therefore, improving flooding is a key in the μ DMFC technology.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a method for improving cathode flooding of a micro direct methanol fuel cell, and the method is characterized in that foam stainless steel is used for preparing CCC (cathode collector plate), and the gradient wettability CCC is prepared by analyzing the gas-liquid two-phase flow characteristics of the cathode side of a micro direct methanol fuel cell (micro direct methanol fuel cell) and adopting a KOH solution corrosion method to improve the flooding. This gradient wettability of CCC can guide the rapid draining of the liquid water on the cathode side of the DMFC. In addition, the preparation method greatly reduces the economic cost and has the advantages of easy operation and good effect.

In order to solve the technical problems, the technical scheme of the invention is as follows: a method of improving cathode flooding in a micro direct methanol fuel cell, said method comprising the steps of:

1) preparing a cathode collector plate by selecting foamed stainless steel, processing the cathode collector plate by using a laser cutting platform, and then polishing the surface of the cathode collector plate smoothly;

2) sequentially cleaning a cathode current collecting plate by using absolute methanol, absolute ethanol and deionized water, and then soaking the dried cathode current collecting plate in a strong alkaline solution for gradient corrosion;

3) and (4) washing the corroded cathode collector plate by using deionized water, and finally putting the cathode collector plate into a drying box for drying, wherein the obtained cathode collector plate is applied to the mu DMFC.

Preferably, the cathode current collecting plate is soaked in a strong alkali solution in the following manner: the cathode collector plate is vertically arranged, and the bottom of the cathode collector plate is soaked in the solution, wherein the soaking depth is 1.5-2.5 mm.

Preferably, the cathode current collecting plate has a corrosion time of 4-6min in a strong alkali solution.

Preferably, the strong alkali solution is a KOH solution, and the concentration is 1.0-1.5 mol/L.

Preferably, the water stable contact angle of the foamed stainless steel is 115-130 degrees, and the pore diameter of the micropores is 15-25 μm.

The invention has the following characteristics: the present invention utilizes foamed stainless steel to prepare a cathode current collector plate (CCC). By analyzing the gas-liquid two-phase flow characteristics of the cathode side of the mu DMFC, the CCC with gradient wettability is prepared by adopting a KOH solution corrosion method to improve flooding. And four types of CCC were obtained by varying the etch time. They are not corroded CCC (N-CCC), 1min corroded CCC (1-CCC), 5min corroded CCC (5-CCC) and 9min corroded CCC (9-CCC), respectively. Thereafter, the μ DMFC of CCC with different wettability gradients was tested under ambient conditions. The effect of wettability modification on the performance of μ DMFC was analyzed by alternating current impedance spectroscopy (EIS) and discharge voltage. Experimental results show that the 5-CCC has the best gradient wettability, can better discharge liquid water and slow down cathode flooding. At 80mA/cm²The discharge voltage of the 5-CCC mu DMFC is improved by 33.33 percent compared with the discharge voltage of the N-CCC mu DMFC when the discharge is carried out for 1h under the current density of (1). In contrast, the mu DMFC discharge voltage of 1-CCC was 27.28% higher than that of N-CCC, and the mu DMFC discharge voltage of 9-CCC was 23.33% higher than that of N-CCC. The total resistance of 5-CCC μ DMFC is lowest and the stability is better when the discharge is performed for a long time.

Compared with the prior art, the invention has the following beneficial effects: the foamed stainless steel cathode collector plate (CCC) is prepared by a KOH solution corrosion method, has better gradient wettability, can better discharge liquid water, slow down cathode flooding, reduce total impedance, improve stability, and has the advantages of simple process, high efficiency and low cost.

Drawings

FIG. 1 is a schematic diagram of a μ DMFC;

FIG. 2 is a diagram of the structure of CCC in mm;

FIG. 3 shows the wettability modification of CCC, (a) soaking pattern, (b) zone gradient;

FIG. 4 is a SEM of a cathode collector, (a) before and (b) after wettability modification;

FIG. 5 is a diagram of a test system for a μ DMFC;

FIG. 6 is a wettability test of untreated CCC;

FIG. 7 shows the wettability test of 1-CCC, (a) the alpha region; (b) a beta region; (c) a gamma region;

FIG. 8 is a wettability test of 5-CCC, (a) the alpha region; (b) a beta region; (c) a gamma region;

FIG. 9 shows the wettability test of 9-CCC, (a) the alpha region; (b) a beta region; (c) a gamma region;

FIG. 10 is a photograph of the flooding of the cathode, (a) a stainless steel mu DMFC, (b) a foamed stainless steel mu DMFC;

FIG. 11 is a polarization curve of a foam stainless steel μ DMFC at a methanol solution concentration of 1mol/L to 5 mol/L;

FIG. 12 is EIS of μ DMFC for different gradient wettability CCC before discharge;

FIG. 13 is an EIS of μ DMFC at different gradient wettability CCC after discharge;

fig. 14 shows the discharge voltage results of μ DMFC.

Detailed Description

The technical solutions of the present invention will be described in further detail with reference to the drawings and specific examples, but the present invention is not limited to the following technical solutions.

Example 1

Gas-liquid two-phase flow at cathode side of mu DMFC

The μ DMFC is composed of a CEP (cathode end plate) 1, a CCC (cathode collector plate) 2, an MEA (membrane electrode) 3, a TG (rubber mat) 4, an ACC (anode collector plate) 5, an AEP (cathode end plate) 6, an LSC (liquid storage chamber) 7, and an LIH (liquid injection hole) 8, as shown in fig. 1.

The material of CCC is foamed stainless steel and the flow field is of a hole type, as shown in fig. 2. Under the normal working condition of the mu DMFC, a method for improving the performance of the mu DMFC is found by analyzing the gas-liquid two-phase flow characteristic of the cathode side of the mu DMFC.

The flow regime of the fluid within the CCC is related to permeability according to darcy's law.

Δ p in the formula (1)_lpWhere the pressure drop of the fluid is taken as the equation,

for flow quality,. DELTA.L is the distance, p_lIs density, μ_lTo dynamic viscosity, r_effThe effective radius of the wick, K is the permeability of CCC.

The capillary pressure of the CCC is calculated as follows,

Δ p in the formula (2)_cpCapillary pressure provided for CCC,. sigma.H₂Surface tension of O, θ is the contact angle. Maximum flow of water in CCC when the liquid flow pressure drop equals the capillary pressure

The calculation of (a) is as follows,

as can be seen from equation 3, decreasing the contact angle increases the flow rate of water, thereby discharging the water accumulated in the flow field in time.

The air mainly flows in the hole-type flow field, the flowing state of the air can be expressed by the Hagen Poiseuille equation,

Δ p in the formula (4)_apIs the pressure drop of air, mu_aThe viscosity of the air is used as the viscosity of the air,

and D is the effective diameter of the hole-type flow field, and is the average flow velocity of air. Further, the oxygen flow rate is calculated as follows,

in the formula (5)

Is the flow rate of oxygen in the flow field, rho_aIn terms of air density, wt% is the mass fraction of oxygen. As can be seen from Eq 5, the flow rate of oxygen is proportional to the fourth power of the effective diameter of the flow field. If the drainage of the CCC is not timely, the hole-shaped flow field is blocked by redundant water, so that oxygen cannot smoothly reach the cathode catalyst layer.

In general, wetting modification of the CCC of foamed stainless steels accelerates H₂And (4) discharging O. This can prevent the pore-type flow field from being blocked by water generated from the cathode, thereby improving the performance of the μ DMFC.

The method for modifying the gradient wettability of CCC comprises the following steps:

1) CCCs of foamed stainless steel were machined with a laser cutting platform (type 6060L-1000W) and the surface of these CCCs was then ground smooth.

2) Sequentially cleaning CCC with absolute methanol, absolute ethanol and deionized water, and then soaking the dried CCC in a 1mol/L KOH solution for gradient corrosion. The CCC was placed vertically in a manner such that the bottom was immersed in the solution to a depth of 2mm, as shown in FIG. 3 (a).

3) And (4) placing the treated CCC in deionized water for washing, and finally placing the CCC in a drying box for drying.

During the treatment, only a portion of the bottom of the CCC is immersed in the strong alkaline solution. The alkali solution will then climb up under the action of capillary forces. However, as the height is increased, the mass of the solution will be less and less, resulting in less and less corrosion due to gravity and viscous forces. For convenience of analysis, CCC after modification was divided into three regions, α, β, and γ, on average, as shown in fig. 3 (b).

From the SEM images, it was found that the surface of the CCC before the wettability modification had only a few micro scratches, compared to the appearance of a layer of nanostructures after the wettability modification, as shown in fig. 4. According to the analysis of a Wenzel model,

cosθ_w＝R_AF cosθ， (6)

in the formula (6) < theta >_wIs a Wenzel contact angle, R_AFRoughness of the wetted area. It can be seen that the etched nanostructures on the CCC surface increase the surface roughness and thus reduce the contact angle.

The method for modifying wettability can not only increase capillary force of the foamed stainless steel, but also provide an additional capillary gradient force. The water drainage capability of the foamed stainless steel cathode collector plate can be greatly improved, so that the mu DMFC can not be flooded under high-strength working conditions.

Testing system of mu DMFC

The test system consists of a direct current electronic load, an electrochemical workstation and an incubator, as shown in figure 5. The test system is mainly used for testing EIS, discharge and polarization curves of the mu DMFC. Before the test, the μ DMFC was activated to obtain the maximum power density. The temperature of the oven was set at 25 ℃ at the time of the test. Then, the μ DMFC was placed in an incubator, and the positive electrode and the negative electrode of a direct current electronic load were connected to the cathode and the anode of the μ DMFC, respectively. The electrochemical workstation is connected with the mu DMFC according to the requirement of actual test.

Results and analysis

Wetting of CCC at different treatment times

The contact angle of N-CCC was 120.905 deg., as shown in FIG. 6. This is mainly because the air inside the foamed stainless steel lowers the surface energy, resulting in a contact angle of more than 90 °, i.e. exhibits hydrophobicity. After different time wettability modifications, the alpha, beta, and gamma regions of CCC all have different degrees of hydrophilic tendencies as shown in fig. 7, 8, and 9.

The water droplets in the alpha region of 1-CCC (1 min CCC etched) were in suspension and had a contact angle of 119.038 deg. at all times. Its beta region shows a phenomenon of slow permeation. At 20s, the contact angle dropped to 103.00 °. In contrast, the water droplets in the γ region completely penetrated, and the time taken for the entire permeation process was 20 s. This is because, in the case of CCC having hydrophilic property, liquid water is spontaneously drawn into the capillary pores by the driving force generated by the capillary effect once it contacts the wall surface. Therefore, it can be seen that the γ region of 1-CCC is more hydrophilic than the β region and the α region. The KOH solution can climb upwards along the foam stainless steel CCC under the action of capillary force to perform gradient corrosion on the CCC. But the soaking time is too short, resulting in a lower quality of the KOH solution climbing. Thus, the β region and the α region exhibit hydrophobicity. For 5-CCC (5 min CCC), the water droplets in the alpha region are in suspension, compared to the water droplets in the beta and gamma regions, which are completely permeable. And the penetration times of water droplets in the β region and the γ region were 15.2s and 4.8s, respectively. This suggests that as the CCC soak time increases, the mass of the climbing KOH solution increases, resulting in complete penetration in the beta region. For 9-CCC (9 min CCC), the alpha, beta and gamma regions all exhibited full penetration. And their permeation times were 3.9s, 2.3s and 1.7s, respectively. This indicates that the KOH solution will climb up a significant amount over a long soak time, resulting in severe corrosion of the entire foamed stainless steel CCC. Therefore, the whole of 9-CCC is in a hydrophilic state.

The CCC after these wettability modifications was graded as shown in Table 1. The 1-CCC is 119.038 degrees < -1 degree >, the 5-CCC is 120.500 degrees < -1 degree, and the 9-CCC is <1 degree < -1 degree. It is clear that 9-CCC is almost free of gradient wettability. The hydrophilicity of the gamma region of 5-CCC is preferred over the gamma region of 1-CCC because water droplets in the gamma region of 5-CCC penetrate more rapidly. Thus, 5-CCC has the optimal gradient wettability. Further, the gradient wettability generates a gradient force which can draw the liquid water to move directionally. This is because the wall surface has different hydrophilicity and the adhesion between the droplet and the wall surface is also different. As the adhesion increases, the contact area between the droplet and the wall surface increases, and the droplet moves in a direction of high hydrophilicity by the adhesion.

TABLE 1 wettability testing of CCC at different treatment times

	1min	5min	9min
				Alpha region	119.038°(20s)	120.500°(20s)	<1°(3.9s)
Beta region	103.000°(20s)	<1°(15.2s)	<1°(2.3s)
				Gamma region	<1°(20s)	<1°(4.8s)	<1°(1.7s)

Cathode flooding result test of DMFC

During discharge, water molecules are generated on the cathode side of the μ DMFC. The accumulation of these water molecules over time can form liquid water, affecting the performance of the μ DMFC. To verify it, at 50mA/cm²Under the current condition, the mu DMFC is discharged for 3 h. Their flooding phenomenon is shown in fig. 10. It is evident that the liquid water on the cathode side of the stainless steel DMFC is mostly concentrated in the pore type flow field. However, the liquid water of the foamed stainless steel μ DMFC is absorbed by capillary force, so that little liquid water is present in the pore-type flow field. The foamed stainless steel only adsorbs water on the surface of the gas diffusion layer and does not change the water content in the cathode catalyst layer. The water absorption of the foamed stainless steel does not affect the hydraulic pressure inside the MEA. Therefore, the foamed stainless steel can be better used for water management research of the mu DMFC.

Polarization curve testing was performed on the foamed stainless steel μ DMFC in order to find the optimal methanol solution concentration required for discharge. As shown in FIG. 11, the maximum power of the foamed stainless steel μ DMFC at a methanol solution concentration of 4mol/L can be up to 15.5 mW. Thus, in a 4mol/L methanol solution, 80mA/cm²Under the condition, the mu DMFC with different gradient wettability CCC is stably discharged for a long time, and the discharge time is 150 min.

EIS

In order to further investigate the effect of CCC with different gradient wettability on the performance of the μ DMFC, the present invention performed ac impedance tests on the μ DMFC before and after discharge, respectively, as shown in fig. 12 and 13. Before discharge, the contact resistances of the μ DMFC of the different gradient wettability CCCs were substantially the same, with a value of 0.61 Ω. However, the charge transfer resistance of N-CCC μ DMFC is minimal. This is because the surface of CCC is corroded after the wettability modification, which lowers the conductivity thereof, resulting in an increase in the charge transfer resistance. After a long discharge time, the EIS of the mu DMFC with different gradient wettability CCC is obviously changed. Their total impedance and mass transfer impedance increase significantly. This is because the water present at the cathode can flood the catalytic layer, increasing the concentration loss and resistance to oxygen transfer. In addition, the contact resistance of the μ DMFC of the different gradient wettability CCC was slightly decreased to a value of 0.50 Ω, as shown in table 2. This is mainly because the relative humidity of the reactant gases increases after a long discharge and further increases the water content in the film. The membrane hydration level affects the conductivity of the membrane. Improving the hydration level of the membrane can improve the mobility of the mass, thereby improving the conductivity of the membrane.

TABLE 2 contact resistance before and after discharge for different treatment times CCC

	N-CCC	1-CCC	5-CCC	9-CCC
					Contact resistance before discharge	0.61Ω	0.61Ω	0.61Ω	0.61Ω
Total impedance before discharge	2.91Ω	2.95Ω	2.81Ω	2.91Ω
					Contact resistance after discharge	0.50Ω	0.50Ω	0.50Ω	0.50Ω
Total impedance after discharge	4.03Ω	3.51Ω	3.40Ω	3.50Ω

In the low frequency region, the curve radius of 5-CCC μ DMFC is the smallest, and the curve radius of N-CCC (without infiltration) μ DMFC is the largest. The curve radii of the 1-CCC μ DMFC and the 9-CCC μ DMFC are substantially the same, as shown in FIG. 13. Meanwhile, the total impedance of the 1-CCC μ DMFC and the 9-CCC μ DMFC is substantially the same. The total impedance of the 5-CCC μ DMFC is the smallest compared to the N-CCC μ DMFC being the largest. This means that the cathode flooding at μ DMFC varies for different gradient wettabilities CCC. The 5-CCC gradient wettability is optimized to effectively direct water toward the CCC end. Thereby freeing the flow field channels and microporous channels of CCC and providing more oxygen to the cathode side of the μ DMFC. However, the capillary gradient force of 1-CCC and 9-CCC is relatively poor, the drainage function cannot be well played, and the oxygen transmission rate of the cathode side cannot be effectively increased. N-CCC has no capillary attraction and cannot pull water in CCC towards the ends. The accumulated water produced by the reduction of oxygen cannot be removed from the cathode in a timely manner, which can hinder the transfer of oxygen and lead to increased cathode polarization.

Overall, the total impedance of the μ DMFC after a long discharge time is significantly increased, and the magnitude of the increase in the mass transfer impedance is much larger than that of the charge transfer impedance. This shows that the mass transfer impedance has a much greater effect on the performance of the DMFC than the charge transfer impedance has on the performance of the DMFC in the case of long discharge. The oxygen transmission channel can be increased by utilizing the wettability gradient force for drainage, and the mass transfer rate is improved. The method has important significance for improving the performance of the mu DMFC.

Discharge voltage

The discharge test was performed for a long period of time by adding 2ml of methanol solution to different types of μ DMFCs, and the discharge voltage was as shown in fig. 14. At 1h of discharge, the discharge voltage of N-CCC at μ DMFC was 0.09V. The discharge voltage of the mu DMFC of 5-CCC is improved by 33.33 percent compared with the discharge voltage of the mu DMFC of N-CCC. In contrast, the 1-CCC μ DMFC discharge voltage was 27.28% higher than the N-CCC μ DMFC, and the 9-CCC μ DMFC discharge voltage was 23.33% higher than the N-CCC μ DMFC. This is mainly because, during long discharge, a large amount of liquid water generated from the cathode blocks the micro-pore channels of CCC, resulting in a reduced oxygen transmission path. However, the wettability-modified CCC effectively draws the water generated on the cathode side to the end, and the water in the microporous channel becomes granular. This can release the microporous channels of CCC, greatly increasing the drainage rate and oxygen transmission rate.

In the discharging process, the performance reduction rate of the mu DMFC of the N-CCC is the fastest, and the performance reduction rate of the mu DMFC of the 5-CCC is the slowest. The rate of performance degradation was substantially the same for 1-CCC μ DMFC and 9-CCC μ DMFC. This is mainly because the liquid water on the cathode side of the DMFC increases as the redox reaction proceeds. Liquid water impedes the transport of oxygen to the MEA, resulting in increased polarization losses. The distribution of the local current density of the μ DMFC becomes very uneven. The gradient wettability is proper, and the mu DMFC with high drainage speed shows better stability.

The performance of a DMFC depends on the accumulation and distribution of liquid water. If the liquid water is not drained from the cathode side in time, the diffusion efficiency of the gas is significantly reduced. This causes a rapid decrease in output performance of the μ DMFC and an unstable output power. Which in turn affects the lifetime of the DMFC.

In conclusion, the invention prepares the CCC with gradient wettability by analyzing the gas-liquid two-phase flow characteristics of the cathode side of the mu DMFC and adopting a KOH solution corrosion method to improve the flooding. Four different types of CCC were obtained by varying the erosion time. Under the condition of normal temperature, the mu DMFC with different gradient wettability CCC is subjected to operation test. The effect of the wettability modification on the performance of the μ DMFC was analyzed by EIS and discharge and the conclusions are as follows.

The gradient of wettability for 1.5-CCC is 120.500 ° -1 °. It is clearly superior to 1-CCC and 9-CCC. The gradient wettability CCC can be prepared by using a KOH solution corrosion method. The gradient wettability can generate gradient force which can draw liquid water to move directionally.

2. The liquid water on the cathode side of the stainless steel mu DMFC is mostly concentrated in the hole-type flow field. In contrast, little liquid water is present in the pore-type flow field on the cathode side of the foamed stainless steel DMFC. Foamed stainless steel is suitable for use in preparing CCC and for cathode water management in μ DMFC, improving cathode flooding problems.

3. The total impedance of the 5-CCC μ DMFC is minimal after long discharge compared to the maximum for the N-CCC μ DMFC. The 5-CCC gradient wettability is optimized to effectively direct water toward the CCC end. Thereby freeing the flow field channels and microporous channels of the CCC and providing more oxygen to the cathode side of the DMFC.

4. At the time of discharging for 1h, the discharge voltages of the 1-CCC mu DMFC, the 5-CCC mu DMFC and the 9-CCC mu DMFC are respectively improved by 27.28 percent, 33.33 percent and 23.33 percent compared with the N-CCC mu DMFC. The μ DMFC of the gradient wetting CCC shows better stability and higher discharge voltage.

The above description is only an example of the present invention, and any modification, equivalent replacement, improvement, etc. made without departing from the spirit and scope of the present invention should be included in the protection scope of the present invention. Further, it is intended that the appended claims cover all such variations and modifications as fall within the scope and boundaries of the appended claims or the equivalents of such scope and boundaries.

Claims

1. A method for improving cathode flooding in a micro direct methanol fuel cell, said method comprising the steps of:

1) selecting foamed stainless steel to prepare a cathode current collecting plate, processing the cathode current collecting plate by using a laser cutting platform, and then polishing the surface of the cathode current collecting plate smoothly;

2. The method of improving miniature direct methanol fuel cell cathode flooding as set forth in claim 1, wherein said cathode collector plate is immersed in a strong alkaline solution in a manner selected from the group consisting of: the cathode collector plate is vertically arranged, and the bottom of the cathode collector plate is soaked in the solution, wherein the soaking depth is 1.5-2.5 mm.

3. The method of improving micro-dmfc cathode flooding as recited in claim 1, wherein said cathode collector plate is etched in a strong alkaline solution for a period of 4-6 minutes.

4. The method of improving micro-dmfc cathode flooding as recited in claim 1, wherein said strongly alkaline solution is KOH solution at a concentration of 1.0-1.5 mol/L.

5. The method of improving micro direct methanol fuel cell cathode flooding as recited in claim 1 wherein said foamed stainless steel has a water stable contact angle of 115 ° -130 ° and a pore size of 15-25 μm.